(1) Select a model for the data;This continues until the fitting error is less than the specified error. One popular technique for doing this is called the NelderMead Modified Simplex. This is essentially a way of organizing and optimizing the changes in parameters (step 4, above) to shorten the time required to fit the function to the required degree of accuracy. With contemporary personal computers, the entire process typically takes only a fraction of a second to a few seconds, depending on the complexity of the model and the number of independently adjustable parameters in the model. The animation on the left (script) shows the working of the iterative process for a 2peak unconstrained Gaussian fit to a small set of x,y data. This model has four nonlinear variables (the positions and width of the two Gaussians, which are determined by iteration) and two linear variables (the peak heights of the two Gaussians, which are determined directly by regression for each trial iteration). In order to allow the process to be observed in action, this animation is slowed down artificially by (a) plotting stepbystep, (b) making a bad initial guess, and (c) adding a "pause()" statement. Without all that slowing it down, the process normally takes only about 0.05 seconds on a standard desktop PC, depending mainly on the number of nonlinear variables that must be iterated. This even works if the peaks are so overlapped that they bend into a single peak, as shown by Demofitgauss2AnimatedBlended.m, shown in the animation on the right, along with the algorithm's progress in reducing the fitting error, but it may take more iterations and it's more sensitive to any random noise in the data (which you can set in line 13).
(2) Make first "guesses" ("start" values) of all the nonlinear parameters (i.e. position and width of overlapping peaks; there is no need to guess the linear values, e.g. the amplitudes of the peaks);
(3) A computer program computes the model and compares it to the data set, calculating a fitting error;
(4) If the fitting error is greater than the required fitting accuracy, the program systematically changes the parameters and loops back around to the previous step and repeats until the required fitting accuracy is achieved or the maximum number or iterations is reached.
A simple example is fitting the blackbody
equation to the spectrum of an incandescent body for the
purpose of estimating its color temperature. In this case there
is only one nonlinear parameter, temperature. The
script BlackbodyDataFit.m
demonstrates the technique, placing the experimentally measured
spectrum in the vectors "wavelength" and "radiance" and then
calling fminsearch with the fitting function fitblackbody.m. (If a blackbody
source is not thermally homogeneous, it may be possible to model
it as the sum of two or more regions of different
temperature, as in example 3 of fitshape1.m.)
Another application is demonstrated by Matlab's builtin demo
fitdemo.m and its corresponding
fitting function fitfun.m, which model
the sum of two exponential decays. To see this, just type
"fitdemo" in the Matlab command window. (Octave does not have
this demo function).
Fitting peaks. Many instrumental methods of measurement produce signals in the form of peaks of various shapes; a common requirement is to measure the positions, heights, widths, and/or areas of those peaks, even when they are noisy or overlapped with one another. This cannot be done by linear leastsquares methods, because such signals can not be modeled as polynomials with linear coefficients (the positions and widths of the peaks are not linear functions), so iterative curve fitting techniques are used instead, often using Gaussian, Lorentzian, or some other fundamental simple peak shapes as a model.
The Matlab/Octave demonstration script Demofitgauss.m demonstrates fitting a Gaussian function to a set of data, using the fitting function fitgauss.m. In this case there are two nonlinear parameters: peak position and peak width (the peak height is a linear parameter and is determined by regression in a single step in line 9 of the fitting function fitgauss.m and is returned in the global variable "c"). Compared to the simpler polynomial leastsquares methods for measuring peaks, the iterative method has the advantage of using all the data points across the entire peak, including zero and negative points, and it can be applied to multiple overlapping peaks as demonstrated in in Demofitgauss2.m (shown on the left).
To accommodate the possibility that the baseline may shift, we can add a column of 1s to the A matrix, just as was done in the CLS method. This has the effect of introducing an additional "peak" into the model that has an amplitude but no position or width. The baseline amplitude is returned along with the peak heights in the global vector "c"; Demofitgaussb.m and fitgauss2b.m illustrates this addition. (Demofitlorentzianb.m and fitlorentzianb.m for Lorentzian peaks).
This peak fitting technique is easily extended to any number of
overlapping peaks of the same type using the same
fitting function fitgauss.m, which easily adapts to any number
of peaks, depending on the length of the firstguess "start"
vector lambda that
is passed to the function as input arguments, along with the
data vectors t and y:
1 function err = fitgauss(lambda,t,y) 2 % Fitting functions for a Gaussian band spectrum. 3 % T. C. O'Haver, 2006 Updated to Matlab 6, March 2006 4 global c 5 A = zeros(length(t),round(length(lambda)/2)); 6 for j = 1:length(lambda)/2, 7 A(:,j) = gaussian(t,lambda(2*j1),lambda(2*j))'; 8 end 9 c = A\y'; % c = abs(A\y') for positive peak heights only 10 z = A*c; 11 err = norm(zy');If there are n peaks in the model, then the length of lambda is 2n, one entry for each iterated variable ([position1 width1 position2 width2....etc]). The "for" loop (lines 57) constructs a n x length(t) matrix containing the model for each peak separately, using a userdefined peak shape function (in this case gaussian.m), then it computes the nlength peak height vector c by leastsquares regression in line 9, using the Matlab shortcut "\" notation. (To constrain the fit to positive values of peak height, replace A\y' with abs(A\y') in line 9). The resulting peak heights are used to compute z, the sum of all n model peaks, by matrix multiplication in line 10, and then "err", the rootmeansquare difference between the model z and the actual data y, is computed in line 11 by the Matlab 'norm' function and returned to the calling function ('fminsearch'), which repeats the process many times, trying different values of the peak positions and the peak widths until the value of "err" is low enough.
If you do not know the shape of your peaks, you can
use use peakfit.m or ipf.m to try different shapes to see if one
of the standard shapes included in those programs fits the data;
try to find a peak in your data that is typical, isolated, and
that has a good signaltonoise ratio. For example, the Matlab
functions ShapeTestS.m and ShapeTestA.m tests the data in its
input arguments x,y, assumed to be a single isolated peak, fits
it with different candidate model peak shapes using
peakfit.m, plots each fit in a separate figure window, and
prints out a table of fitting errors in the command window. ShapeTestS.m tries seven different
candidate symmetrical model peaks, and ShapeTestA.m tries six different
candidate asymmetrical model peaks. The one with the
lowest fitting error (and R2 closest to 1.000) is presumably the
best candidate. Try the examples
in the help files for each of these functions. But beware, if
there is too much noise in your data, the results can be
misleading. For example, even if the actual peak shape is
something other than a Gaussian, the multiple Gaussians model is
likely to fit slightly better because it has more degrees of
freedom and can "fit the noise". The Matlab function peakfit.m has many more
builtin shapes to choose from, but still it is a finite list
and there is always the possibility that the actual underlying
peak shape is not available in the software you are using or
that it is simply not describable by a mathematical function.
Signals with peaks of different shape types
in one signal can be fit by the fitting function fitmultiple.m,
which takes as input arguments a vector of peak types and a
vector of shape variables. The sequence of peak types and
shape parameters must be determined beforehand. To see how
this is used, see Demofitmultiple.m.
You can create your own fitting functions for any purpose; they are not limited to single algebraic expressions, but can be arbitrarily complex multiple step algorithms. For example, in the TFit method for quantitative absorption spectroscopy, a model of the instrumentallybroadened transmission spectrum is fit to the observed transmission data, using a fitting function that performs Fourier convolution of the transmission spectrum model with the known slit function of the spectrometer. The result is an alternative method of calculating absorbance that allows the optimization of signaltonoise ratio and extends the dynamic range and calibration linearity of absorption spectroscopy far beyond the normal limits.
The effect of random noise on the uncertainty of the peak parameters determined by iterative leastsquares fitting is readily estimated by the bootstrap sampling method, covered in a previous section, which randomly assigns weights of 0, 1, and 2 to the data points. A simple demonstration of bootstrap estimation of the variability of an iterative leastsquares fit to a single noisy Gaussian peak is given by the custom downloadable Matlab/Octave function "BootstrapIterativeFit.m", which creates a single x,y data set consisting of a single noisy Gaussian peak, extracts bootstrap samples from that data set, performs an iterative fit to the peak on each of the bootstrap samples, and plots the distributions (histograms) of peak height, position, and width of the bootstrap samples. The syntax is BootstrapIterativeFit(TrueHeight, TruePosition, TrueWidth, NumPoints, Noise, NumTrials) where TrueHeight is the true peak height of the Gaussian peak, TruePosition is the true xaxis value at the peak maximum, TrueWidth is the true halfwidth (FWHM) of the peak, NumPoints is the number of points taken for the leastsquares fit, Noise is the standard deviation of (normallydistributed) random noise, and NumTrials is the number of bootstrap samples. An typical example for BootstrapIterativeFit(100,100,100,20,10,100); is displayed in the figure on the right.
>>
BootstrapIterativeFit(100,100,100,20,10,100);
Peak Height Peak
Position Peak Width
mean:
99.27028 100.4002
94.5059
STD: 2.8292
1.3264 2.9939
IQR: 4.0897
1.6822 4.0164
IQR/STD Ratio: 1.3518
A similar demonstration function
for two overlapping
Gaussian peaks is available in "BootstrapIterativeFit2.m".
Type "help BootstrapIterativeFit2" for
more information. In both these simulations, the standard
deviation (STD) as well as the interquartile
range (IQR) of each of the peak parameters are
computed. This is done because the interquartile range is
much less influenced by outliers. The distribution
of peak parameters measured by iterative fitting is often
nonnormal, exhibiting a greater fraction of large deviations
from the mean than is expected for a normal distribution. This
is because the iterative procedure sometimes converges on an
abnormal result, especially for multiple peak fits with a large
number of variable parameters. (You may be able to see this in
the histograms plotted by these simulations, especially for the
weaker peak in BootstrapIterativeFit2).
In those cases the standard deviation will be too high
because of the outliers, and the IQR/STD
ratio will be much less than the value of 1.34896 that is expected for a normal distribution. In that
case a better estimate of the standard deviation of the
central portion of the distribution (without the outliers)
is IQR/1.34896.
It's important to emphasize that the bootstrap method
predicts only the effect of random noise on the peak
parameters for a fixed fitting model. It does not take into
account the possibility of peak parameter inaccuracy cased
by using a nonoptimum data range, or choosing an imperfect
model, or by inaccurate compensation for the
background/baseline, all of which are at least partly
subjective and thus beyond the range of influences that can
easily be treated by random statistics. If the data have
relatively little random noise, or have been smoothed to
reduce the noise, then it's likely that model selection and
baseline correction will be the major sources of peak
parameter inaccuracy, which are not well predicted by the
bootstrap method.
For the quantitative measurement of peaks, it's
instructive
to compare the iterative leastsquares method with
simpler, less computationallyintensive, methods. For example,
the measurement of the peak height of a single peak of uncertain
width and position could be done simply by taking the maximum of
the signal in that region. If the signal is noisy, a more
accurate peak height will be obtained if the signal is smoothed beforehand. But smoothing
can distort the signal and reduce peak heights. Using an
iterative peak fitting method, assuming only that the peak shape
is known, can give the best possible accuracy and precision,
without requiring smoothing even under high noise conditions,
e.g. when the signaltonoise ratio is 1, as in the demo
script SmoothVsFit.m:
True peak height = 1 NumTrials =
100 SmoothWidth = 50
Method Maximum
y Max Smoothed y Peakfit
Average peak height
3.65
0.96625 1.0165
Standard deviation 0.36395
0.10364 0.11571
If peak area is measured rather than peak height,
smoothing is unnecessary (unless to locate the peak beginning
and end) but peak fitting still yields the best precision. See SmoothVsFitArea.m.
It's
also instructive to compare the iterative leastsquares method
with classical
leastsquares curve fitting, discussed in the previous section, which can
also fit peaks in a signal. The difference is that in
the classical least squares method, the positions, widths, and
shapes of all the individual components are all known
beforehand; the only
unknowns are the amplitudes (e.g. peak heights) of the
components in the mixture. In nonlinear iterative curve
fitting, on the other hand, the positions, widths, and heights
of the peaks are all unknown
beforehand; the only
thing that is known is the fundamental underlying shape of the
peaks. The nonlinear iterative curve fitting is more
difficult to do (for the computer, anyway) and more prone to
error, but it's necessary if you need to track shifts in peak
position or width or to decompose a complex overlapping peak
signal into fundamental components knowing only their shape.
The Matlab/Octave script "CLSvsINLS.m"
compares the classical leastsquares (CLS) method with three
different variations of the iterative method (INLS) method for
measuring the peak heights of three Gaussian peaks in a noisy
test signal, demonstrating that the fewer the number of
unknown parameters, the faster and more accurate is the peak
height calculation.
Another comparison of multiple measurement techniques is presented in Case Study D.
Note: you
can rightclick on any of the mfile links on this page and
select Save Link As... to download them
to your computer, then place them in the Matlab path for use
within Matlab.
[Model errors] [Number of peaks] [Peak width] [Background correction] [Random noise] [Iterative fitting errors] [Exponential broadening] [Effect of smoothing]
Iterative curve fitting is often used to measure the
position, height, and width of peaks in a signal, especially
when they overlap significantly. There are four major sources
of error in measuring these peak parameters by iterative curve
fitting: model
errors, background
correction, random
noise, and iterative
fitting errors. This section makes use of the
downloadable peakfit.m function.
Instructions are here or
type "help peakfit". (Once you have peakfit.m in youjr path,
you can simply copy and paste, or drag and drop, any of the
following singleline or multiline code examples into the
Matlab or Octave editor or into the command line and press Enter
to execute it).
a. Model errors.
Peak shape. If you have the wrong model for your
peaks, the results can not be expected to be accurate; for
instance, if your actual peaks are Lorentzian in shape, but
you fit them with a Gaussian model, or vice versa. For example,
a single isolated Gaussian peak at x=5, with a height of 1.000
fits a Gaussian model virtually perfectly, using the Matlab
userdefined peakfit
function, as shown on the right. (The 5th input argument for
the peakfit function specifies the shape of peaks to be used
in the fit; "1" means Gaussian).
>>
x=[0:.1:10];y=exp((x5).^2);
>> [FitResults,MeanFitError]=peakfit([x' y'],5,10,1,1)
Peak#
Position
Height
Width Area
FitResults =
1
5
1
1.6651 1.7725
MeanFitError
= R2=
7.8579e07
1
The "FitResults" are, from left to right, peak number,
peak position, peak height, peak width, and peak area. The
MeanFitError, or just "fitting error", is the square root
of the sum of the squares of the differences between the data and the
bestfit model, as a percentage of the maximum signal in the
fitted region. R2 is the "Rsquared" or coefficient of
determination, which is exactly 1 for a perfect fit. Note the
good agreement of the area measurement (1.7725) with the theoretical area
under the curve of exp(x^{2}), which turns out
to be the square
root of pi, or
about 1.7725....
But this same peak, when fit with
the incorrect model (a Logistic model, peak shape
number 3), gives a fitting error of 1.4% and height and width
errors of 3% and 6%, respectively. However, the
peak area error is only 1.7%, because the height and
width errors partially cancel out. So you don't have to have a
perfect model to get a decent area measurement.
>> [FitResults,MeanFitError]=peakfit([x'
y'],5,10,1,3)
Peak#
Position
Height
Width
Area
FitResults =
1
5.0002
0.96652
1.762
1.7419
MeanFitError =1.4095
When fit with an even more
incorrect Lorentzian model (peak shape 2, shown on
the right), this peak gives a 6% fitting error and height,
width and area errors of 8%, 20%, and 17%, respectively.
>> [FitResults,MeanFitError]=peakfit([x'
y'],5,10,1,2)
FitResults =
Peak#
Position
Height
Width Area
1
5
1.0876
1.3139
2.0579
MeanFitError =5.7893
But, practically speaking,
Gaussian and Lorentzian shapes are so visually distinct that
it's unlikely that your estimate of a model will be that far
off. Real peak shapes are often some unknown combination of
peak shapes, such as Gaussian with a little Lorentzian or vice
versa, or some slightly asymmetrical modification of a
standard symmetrical shape. So if you use an available model
that is at least close to the actual shape, the
parameters errors may not be so bad and may in fact be better
than other measurement methods.
So clearly the larger the fitting
errors, the larger are the parameter errors, but the parameter
errors are of course not equal
to the fitting error (that would just be too easy). Also,
it's clear that the peak height and width are the
parameters most susceptible to errors. The peak positions, as you can see
here, are measured accurately even if the model is way wrong,
as long as the peak is symmetrical and not highly overlapping
with other peaks.
A good fit is not by itself proof that the shape function you have chosen is the correct one; in some cases the wrong function can give a fit that looks perfect. For example, this fit of a real data set to a 5peak Gaussian model has a low percent fitting error and the residuals look random  usually an indicator of a good fit. But in fact in this case the model is wrong; that data came from an experimental domain where the underlying shape is fundamentally nonGaussian but in some cases can look very like a Gaussian. As another example, a data set consisting of peaks with a Voigt profile peak shape can be fit with a weighted sum of a Gaussian and a Lorentzian almost as well as an with an actual Voigt model, even though those models are not the same mathematically; the difference in fitting error is so small that it would likely be obscured by the random noise if it were a real experimental signal. The same thing can occur in sigmoidal signal shapes: a pair of simple 2parameter logistic functions seems to fit this example data pretty well, with a fitting error of less than 1%; you would have no reason to doubt the goodness of fit unless the random noise is low enough so you can see that the residuals are wavy. Alternatively, a 3parameter logistic (Gompertz function) fits much better, and the residuals are random, not wavy. In such cases you can not depend solely on what looks like a good fit to determine whether the fit is model is optimum; sometimes you need to know more about the peak shape expected in that kind of experiment, especially if the data are noisy. At best, if you do get a good fit with random nonwavy residuals, you can claim that the data are consistent with the proposed model. Note: with the peakfit.m function, you can extract the residuals as a vector by using the syntax [FitResults,GOF,baseline,coeff,residual,xi,yi]=peakfit(....
Sometimes the accuracy of the model is not so important. In quantitative analysis applications, where the peak height or areas measured by curve fitting is used only to determine the concentration of the substance that created the peak by constructing a calibration curve, using laboratory prepared standards solutions of known concentrations, the necessity of using the exact peak model is lessened, as long as the shape of the unknown peak is constant and independent of concentration. If the wrong model shape is used, the R^{2} for curve fitting will be poor (much less than 1.000) and the peak heights and areas measured by curve fitting will be inaccurate, but the error will be exactly the same for the unknown samples and the known calibration standards, so the error will cancel out and, as a result, the R^{2} for the calibration curve can be very high (0.9999 or better) and the measured concentrations will be no less accurate than they would have been with a perfect peak shape model. Even so, it's useful to use as accurate a model peak shape as possible, because the R^{2} for curve fitting will work better as a warning indicator if something unexpected goes wrong during the analysis (such as an increase in the noise or the appearance of an interfering peak from a foreign substance). See PeakShapeAnalyticalCurve.m for a Matlab/Octave demonstration.
Number of peaks. Another
source
of model error occurs if you have the wrong number of peaks in your
model, for example if the signal actually has two peaks
but you try to fit it with only one peak. In the example
below, a line of Matlab code generates a simulated signal with
of two Gaussian peaks at x=4 and x=6 with peaks heights of
1.000 and 0.5000 respectively and widths of 1.665, plus random
noise with a standard deviation 5% of the height of the
largest peak (a signaltonoise ratio of 20):
>>x=[0:.1:10];y=exp((x6).^2)+.5*exp((x4).^2)+.05*randn(size(x));
In a real experiment you would not usually know the peak positions, heights, and widths; you would be using curve fitting to measure those parameters. Let's assume that, on the basis of previous experience or some preliminary trial fits, you have established that the optimum peak shape model is Gaussian, but you don't know for sure how many peaks are in this group. If you start out by fitting this signal with a singlepeak Gaussian model, you get:
>>
[FitResults,MeanFitError]=peakfit([x' y'],5,10,1,1)
FitResults
Peak#
Position Height
Width Area
1
5.5291 0.86396
2.9789 2.7392
MeanFitError = 10.467
The residual plot shows a "wavy"
structure that's visible in the random scatter of points due
to the random noise in the signal. This means that the fitting
error is not limited by the random noise; it is a clue that
the model is not quite right.
But a fit with two peaks yields much better results (The 4^{th} input argument for the peakfit function specifies the number of peaks to be used in the fit).
>> [FitResults,MeanFitError]=peakfit([x'
y'],5,10,2,1)
FitResults =
Peak# Position
Height Width Area
1
4.0165
0.50484
1.6982
0.91267
2
5.9932 1.0018
1.6652 1.7759
MeanFitError = 4.4635
Now the residuals have a random
scatter of points, as would be expected if the signal is
accurately fit except for the random noise. Moreover, the
fitting error is much lower (less that half) of the error with
only one peak. In fact, the fitting error is just about what
we would expect in this case based on the 5% random noise in
the signal (estimating the relative standard deviation of the
points in the baseline visible at the edges of the signal).
Because this is a simulation in which we know beforehand the
true values of the peak parameters (peaks at x=4 and x=6 with
peaks heights of 1.0 and 0.50 respectively and widths of
1.665), we can actually calculate the parameter errors (the
difference between the real peak positions, heights, and
widths and the measured values). Note that they are quite
accurate (in this case within about 1% relative on the peak
height and 2% on the widths), which is actually better than
the 5% random noise in this signal because of the averaging
effect of fitting to multiple data points in the signal.
But if going from one peak to two
peaks gave us a better fit, why not go to three peaks? If
there were no noise in the data, and if the underlying peak
shape were perfectly matched by the model, then the fitting
error would have already been essentially zero with two model
peaks, and adding a third peak to the model would yield a
vanishingly small height for that third peak. But in our
examples here, as in real data, there is always some random
noise, and the result is that the third peak height will not
be zero. Changing the number of peaks to three gives these
results:
>> [FitResults,MeanFitError]=peakfit([x'
y'],5,10,3,1)
FitResults =
Peak# Position
Height Width Area
1 4.0748
0.51617 1.7874 0.98212
2 6.7799
0.089595 2.0455 0.19507
3 5.9711
0.94455 1.53 1.5384
MeanFitError = 4.3878
The fitting algorithm has now tried to fit an additional lowamplitude peak (numbered peak 2 in this case) located at x=6.78. The fitting error is actually lower that for the 2peak fit, but only slightly lower, and the residuals are no less visually random that with a 2peak fit. So, knowing nothing else, a 3peak fit might be rejected on that basis alone. In fact, there is a serious downside to fitting more peaks than are actually present in the signal: it increases the parameter measurement errors of the peaks that are actually present. Again, we can prove this because we know beforehand the true values of the peak parameters: clearly the peak positions, heights, and widths of the two real peaks than are actually in the signal (peaks 1 and 3) are significantly less accurate than those of the 2peak fit.
Moreover, if we repeat that fit
with the same signal
but with a different
sample of random noise (simulating a repeat measurement of a
stable experimental signal in the presence or random noise),
the additional third peak in the 3peak fit will bounce around
all over the place (because the third peak is actually fitting
the random noise,
not an actual peak in the signal).
>> x=[0:.1:10];
>>
y=exp((x6).^2)+.5*exp((x4).^2)+.05*randn(size(x));
>> [FitResults,MeanFitError]=peakfit([x' y'],5,10,3,1)
FitResults =
Peak#
Position Height
Width Area
1
4.115
0.44767
1.8768 0.89442
2
5.3118 0.09340
2.6986 0.26832
3
6.0681
0.91085
1.5116 1.4657
MeanFitError = 4.4089
With this new set of data, two of
the peaks (numbers 1 and 3) have roughly the same position,
height, and width, but peak number 2 has changed substantially
compared to the previous run. Now we have an even more
compelling reason to reject the 3peak model: the 3peak
solution is not stable.
And because this is a simulation in which we know the right
answers, we can verify that the accuracy of the peak heights
is substantially poorer (about 10% error) than expected with
this level of random noise in the signal (5%). If we were to
run a 2peak fit on the same new data, we get much better
measurements of the peak heights.
>> [FitResults,MeanFitError]=peakfit([x'
y'],5,10,2,1)
FitResults =
Peak# Position Height
Width
Area
1
4.1601
0.49981 1.9108 1.0167
2
6.0585
0.97557 1.548
1.6076
MeanFitError = 4.4113
If this is repeated several times,
it turns out that the peak parameters of the peaks at x=4
and x=6 are, on average, more accurately measured by the
2peak fit. In practice, the best way to evaluate a proposed
fitting model is to fit several repeat measurements
of the same signal (if that is practical experimentally) and
to compute the standard deviation of the peak parameter
values.
In real experimental work, of course, you usually don't know the right answers beforehand, so that's why it's important to use methods that work well when you do know. The real data example mentioned above was fit with a succession of 2, 3, 4 and 5 Gaussian models, until the residuals became random. Beyond that point, there is little to be gained by adding more peaks to the model. Another way to determine the minimum number of models peaks needed is to plot the fitting error vs the number of model peaks; the point at which the fitting error reaches a minimum, and increases afterward, would be the fit with the "ideal combination of having the best fit without excess/unnecessary terms". The Matlab/Octave function testnumpeaks.m (R = testnumpeaks(x, y, peakshape, extra, NumTrials, MaxPeaks)) applies this idea by fitting the x,y data to a series of models of shape peakshape containing 1 to MaxPeaks model peaks. The correct number of underlying peaks is either the model with the lowest fitting error, or, if two or more models have about the same fitting error, the model with the least number of peaks. The Matlab/Octave demo script NumPeaksTest.m uses this function with noisy computergenerated signals containing a userselected 3, 4, 5 or 6 underlying peaks. With very noisy data, however, the technique is not always reliable.
Peak width constraints.
Finally, there is one more thing that we can do that
might improve the peak parameter measurement accuracy, and it
concerns the
peak widths. In all the above simulations, the basic
assumption that all
the peak parameters were unknown and independent of one
another. In some types of measurements, however, the peak
widths of each group of adjacent peaks are all expected to be
equal to each other, on the basis of first principles or
previous experiments. This is a common situation in analytical
chemistry, especially in atomic spectroscopy and in
chromatography, where the peak widths are determined largely
by instrumental factors.
In the current simulation, the
true peaks widths are in fact both equal to 1.665, but all the
results above show that the measured
peak widths are close but not quite equal, due to random noise
in the signal. The unequal peak widths are a consequence of
the random noise, not real differences in peak width. But we
can introduce an equalwidth
constraint into the fit by using peak shape 6 (Equalwidth
Gaussians) or peak shape 7 (Equalwidth Lorentzians). Using
peak shape 6 on the same set of data as the previous example:
>> [FitResults,MeanFitError]=peakfit([x'
y'],5,10,2,6)
FitResults =
Peak#
Position Height
Width Area
1 4.0293 0.52818
1.5666 0.8808
2 5.9965
1.0192 1.5666 1.6997
MeanFitError = 4.5588
This "equal width" fit forces all the peaks within one group
to have exactly the same width, but that width is determined
by the program from the data. The result is a slightly higher fitting
error (in this case 4.5% rather than 4.4%), but  perhaps
surprisingly  the peak parameter measurements are usually more accurate and more reproducible (Specifically,
the relative standard deviations are on average lower for the
equalwidth fit than for an unconstrainedwidth fit to the
same data, assuming of course that the true underlying peak
widths are really equal). This is an exception to the
general expectation that lower fitting errors result in lower
peak parameter errors. It is an illustration of the general
rule that the more you know about the nature of your signals,
and the closer your chosen model adheres to that knowledge,
the better the results. In this case we knew that the peak
shape was Gaussian (although we could have verified that
choice by trying other candidate peaks shapes). We
determined that the number of peaks was 2 by inspecting the
residuals and fitting errors for 1, 2, and 3 peak
models. And then we introduced the constraint of equal
peak widths within each group of peaks, based on prior
knowledge of the experiment rather than on inspection of
residuals and fitting errors. Here's another example, with
real experimental data from a measurement where the peak
widths are expected to be equal, showing the result
of an unconstrained fit and
an equal width fit; the
fitting errors is slightly larger for the equalwidth fit, but
that is to be preferred in this case. Not every experiment
can be expected to yield peaks of equal width, but when it
does, it's better to make use of that constraint.
Fixedwidth shapes. Going one step beyond equal widths (in peakfit version 7.6 and later), you can also specify a fixedwidth shapes (shape numbers 11, 12, 3437), in which the width of the peaks are known beforehand, but are not necessarily equal, and are specified as a vector in input argument 10, one element for each peak, rather than being determined from the data as in the equalwidth fit above. Introducing this constraint onto the previous example, and supplying an accurate width as the 10th input argument:
>> [FitResults,MeanFitError]=peakfit([x'
y'],0,0,2,11,0,0,0,0,[1.666 1.666])
FitResults =
Peak#
Position
Height
Width Area
1
3.9943
0.49537
1.666
0.8785
2
5.9924
0.98612
1.666
1.7488
MeanFitError = 4.8128
Comparing to the previous
equalwidth fit, the fitting error of 4.8% is larger here
(because there are fewer degrees of freedom to minimize the
error), but the parameter errors, particularly the peaks
heights, are more accurate because the width
information provided in the input argument was more accurate
(1.666) than the width determined by the equalwidth fit
(1.5666). Again, not every experiment yields peaks of known
width, but when it does, it's better to make use of that
constraint. For example, see Example 35 and
the Matlab/Octave script WidthTest.m
(typical results for a Gaussian/Lorentzian blend shape shown
below, showing that the more constraints, the greater the
fitting error but the lower the parameter errors, if the
constraints are accurate).
Relative percent error 
Fitting error 
Position Error 
Height Error 
Width Error 
Unconstrained shape factor
and widths: shape 33 
0.78 
0.39 
0.80 
1.66 
Fixed shape factor and
variable widths: shape 13 
0.79 
0.25 
1.3 
0.98 
Fixed shape factor and
fixed widths: shape 35 
0.8 
0.19 
0.69 
0.0 
Multiple
linear regression (peakfit version 9). Finally, note
that if the peak positions
are also known, and only the peak heights are unknown, you don't even need to
use the iterative fitting method at all; you can use the
easier and faster multilinear regression technique
(also called "classical
least squares") which is implemented by the function cls.m and by version 9 of peakfit.m as shape
number 50. Although multilinear regression results in fitting
error slightly greater (and R2 lower), the errors in
the measured peak heights are often less, as in this
example from peakfit9demo.m,
where the true peak heights of the three
overlapping Gaussian peaks are 10, 30, and 20.
Multilinear regression results (known position and
width):
Peak Position
Height Width
Area
1
400
9.9073
70 738.22
2
500
29.995
85 2714
3
560
19.932
90 1909.5
%fitting error=1.3048 R2= 0.99832
%MeanHeightError=0.427
Unconstrained iterative
nonlinear least squares results:
Peak Position
Height Width
Area
1
399.7
9.7737
70.234 730.7
2
503.12
32.262
88.217 3029.6
3
565.08
17.381
86.58 1601.9
%fitting error=1.3008 R2= 0.99833
%MeanHeightError=7.63
This demonstrates dramatically how different measurement
methods can look the same, and give fitting errors
almost the same, and yet differ greatly in parameter
measurement accuracy. (The similar script peakfit9demoL.m is the same thing
with Lorentzian peaks).
SmallPeak.m
is a demonstration script comparing all these techniques
applied to the challenging problem of measuring the height of
a small peak that is closely overlapped with and completely
obscured by a much larger peak. It compares unconstrained,
equalwidth, and fixedposition iterative fits (using
peakfit.m) with a classical least squares fit in which only
the peak heights are unknown (using cls.m).
It helps to spread out the four figure windows so you can
observe the dramatic difference in stability of the different
methods. A final table of relative percent peak height errors
shows that the more the constraints, the better the
results (but only if the constraints are justified).
The real key is to know which parameters can be relied upon to
be constant and which have to be allowed to vary.
Here's a a
screen video (MorePeaksLowerFittingError.mp4)
of a realdata experiment using the interactive peak fitter ipf.m with a complex experimental signal in
which several different fits were performed using models from
4 to 9 variablewidth, equalwidth, and fixedwidth Gaussian
peaks. The fitting error gradually decreases from 11%
initially to 1.4% as more peaks are used, but is
that really justified? If the objective is simply to get
a good fit, then do whatever it takes. But if the objective is
to extract some useful information from the model peak
parameters, then more specific knowledge about that particular
experiment is needed: how many peaks are really expected; are
the peak widths really expected to be constrained? Note that
in this particular case the residuals (bottom panel) are never
really random and always have a distinct "wavy"
character, suggesting that the data may have been smoothed
before curve fitting (usually not a good idea: see http://wmbriggs.com/blog/?p=195).
Thus there is a real possibility that some of those 9 peaks
are simply "fitting the noise", as will be discussed further
in Appendix A.
b. Background
correction.
The peaks that are measured in many scientific instruments are
sometimes superimposed on a nonspecific background or
baseline. Ordinarily the experiment protocol is designed to
minimize the background or to compensate for the background,
for example by subtracting a "blank"
signal from the signal of an actual specimen. But even so
there is often a residual background that can not be
eliminated completely experimentally. The origin and shape of
that background depends on the specific measurement method,
but often this background is a broad, tilted, or curved shape,
and the peaks of interest are comparatively narrow features
superimposed on that background. In some cases the baseline
may be another peak. The presence of the background has
relatively little effect on the peak positions, but it
is impossible to measure the peak heights, width, and areas
accurately unless something is done to account for the
background.
There are various methods
described in the literature for estimating and subtracting the
background in such cases. The simplest assumption is that the
background can be approximated as a simple function in the
local region of group of peaks being fit together, for example
as a constant (flat), straight line (linear) or curved line
(quadratic). This is the basis of the "autozero" modes in the
ipf.m, iSignal.m, and
iPeak.m functions,
which are selected by the T key to cycle thorough OFF,
linear, quadratic, and flat modes. In
the flat mode, a constant baseline is included in the
curve fitting calculation, as described above.
In linear mode, a straightline baseline connecting
the two ends of the signal segment in the upper panel will be
automatically subtracted before the iterative curve
fitting. In quadratic mode, a parabolic baseline
is subtracted. In the last two modes, you must adjust the pan
and zoom controls to isolate the group of overlapping peaks to
be fit, so that the signal returns to the local background at
the left and right ends of the window.
Example of an
experimental chromatographic signal. From left to right, (1)
Raw data with peaks superimposed on a tilted baseline. One
group of peaks is selected using the the pan and zoom
controls, adjusted so that the signal returns to the local
background at the edges of the segment displayed in the upper
window; (2) The linear baseline is subtracted when the
autozero mode set to 1 in ipf.m by pressing the T key;
(3) Fit with a threepeak Gaussian model, activated by
pressing 3, G, F (3 peaks, Gaussian, Fit).
Alternatively, it may
be better to subtract the background from the entire signal
first, before further operations are performed. As
before, the simplest assumption is that the background is
piecewise linear, that is, can be approximated as a series of
small straight line segments. This is the basis of the
multiple point background subtraction mode in ipf.m, iPeak.m,
and in iSignal. The user enters
the number of points that is thought to be sufficient to
define the baseline, then clicks where the baseline is thought
to be along the entire length of the signal in the lower
wholesignal display (e.g. on the valleys between the
peaks). After the last point is clicked, the program
interpolates between the clicked points and subtracts the
piecewise linear background from the original signal.
From left to right, (1) Raw
data with peaks superimposed on baseline. (2) Background
subtracted from the entire signal using the multipoint
background subtraction function in iPeak.m (ipf.m and iSignal have the
same function).
Sometimes, even without an actual
baseline present, the peaks may overlap enough so that the
signal never return to the baseline, making it seem that there
is a baseline to be corrected. This can occur especially with
peaks shapes that have gradually sloping sides, such as the
Lorentzian, as shown in
this example. Curve fitting without baseline
correction will work in that case.
In many cases the background may be modeled as a broad peak whose maximum falls outside of the range of data acquired, as in the realdata example on the left. It may be possible to fit the offscreen peak simply by including an extra peak in the model to account for the baseline. In the example on the left, there are three clear peaks visible, superimposed on a tilted baseline. In this case the signal was fit nicely with four, rather than three, variablewidth Gaussians, with an error of only 1.3%. The additional broad Gaussian, with a peak at x = 38.7, serves as the baseline. (Obviously, you shouldn't use the equalwidth shapes for this, because the background peak is broader than the other peaks).
In another realdata example of an experimental spectrum, the linear baseline subtraction ("autozero") mode described above is used in conjunction with a 5Gaussian model, with one Gaussian component fitting the broad peak that may be part of the background and the other four fitting the sharper peaks. This fits the data very well (0.5% fitting error), but a fit like this can be difficult to get, because there are so many other solutions with slightly higher fitting errors; it may take several trials. It can help if you specify the start values for the iterated variables, rather than using the default choices; all the software programs described here have that capability.
The Matlab/Octave function peakfit.m
can employ a peakshape input argument that is a vector of
different shapes, which can be useful for baseline
correction. As an example, consider a weak Gaussian peak on
sloped straightline baseline, using a 2component fit with
one Gaussian component and one variableslope straight line
('slope', shape 26), specified by using the vector [1 26] as
the shape argument:
If the baseline seems to be curved rather than straight, you can model the baseline with a quadratic (shape 46) rather than a linear slope (peakfit version 8 and later).x=8:.05:12;y=x+exp((x10).^2);
[FitResults,GOF]= peakfit([x;y],0,0,2,[1 26],[1 1],1,0)
FitResults =
1 10 1 1.6651 1.7642
2 4.485 0.22297 0.05 40.045
GOF =
0.0928 0.9999
If the baseline seems to be different on either side of the peak, you can try to model the baseline with an Sshape (sigmoid), either an upsigmoid, shape 10 (click for graphic), peakfit([x;y],0,0,2,[1 10],[0 0], or a downsigmoid, shape 23 (click for graphic), peakfit([x;y],0,0,2,[1 23],[0 0], in these examples leaving the peak modeled as a Gaussian.
If the signal is very weak compared to the baseline, the fit can be helped by adding rough first guesses ('start') using the 'polyfit' function to generate automatic first guesses for the sloping baseline. For example, with two overlapping signal peaks and a 3peak fit with peakshape=[1 1 26].
x=4:.05:16;
y=x+exp((x9).^2)+exp((x11).^2)+.02.*randn(size(x));
start=[8 1 10 1 polyfit(x,y,1)];
peakfit([x;y],0,0,3,[1 1 26],[1 1 1],1,start)
A similar technique can be employed in
a spreadsheet, as
demonstrated in CurveFitter2GaussianBaseline.xlsx
(graphic).
The downside to
including the baseline as
a variable component is that it increases the number
of degrees of freedom, increases the execution time,
and increases the possibility of unstable fits.
Specifying start values can help.
c. Random
noise in the signal.
Any experimental signal has a certain amount of random noise,
which means that the individual data points scatter randomly
above and below their mean values. The assumption is
ordinarily made that the scatter is equally above and below
the true signal, so that the longterm average approaches the
true mean value; the noise "averages to zero" as it is often
said. The practical problem is that any given recording of the
signal contains only one finite sample of the noise. If
another recording of the signal is made, it will contain
another independent sample of the noise. These noise samples
are not infinitely long and therefore do not represent the
true longterm nature of the noise. This presents two
problems: (1) an individual sample of the noise will not
"average to zero" and thus the parameters of the
bestfit model will not necessarily equal the true values, and
(2) the magnitude of the noise during one sample might not be
typical; the noise might have been randomly greater or smaller
than average during that time. This means that the
mathematical "propagation of error" methods, which seek to
estimate the likely error in the model parameters based on the
noise in the signal, will be subject to error (underestimating the error
if the noise happens to be lower
than average and overestimating
the errors if the noise happens to be larger than average).
A better way to estimate the parameter errors is to record multiple samples of the signal, fit each of those separately, compute the models parameters from each fit, and calculate the standard error of each parameter. But if that is not practical, it is possible to simulate such measurements by adding random noise to a model with known parameters, then fitting that simulated noisy signal to determine the parameters, then repeating the procedure over and over again with different sets of random noise. This is exactly what the script DemoPeakfit.m (which requires the peakfit.m function) does for simulated noisy peak signals such as those illustrated below. It's easy to demonstrate that, as expected, the average fitting error precision and the relative standard deviation of the parameters increases directly with the random noise level in the signal. But the precision and the accuracy of the measured parameters also depend on which parameter it is (peak positions are always measured more accurately than their heights, widths, and areas) and on the peak height and extent of peak overlap (the two leftmost peaks in this example are not only weaker but also more overlapped than the rightmost peak, and therefore exhibit poorer parameter measurements). In this example, the fitting error is 1.6% and the percent relative standard deviation of the parameters ranges from 0.05% for the peak position of the largest peak to 12% for the peak area of the smallest peak.
Overlap matters: The errors in the values of peak parameters measured by curve fitting depend not only on the characteristics of the peaks in question and the signaltonoise ratio, but also upon other peaks that are overlapping it. From left to right: (1) a single peak at x=100 with a peak height of 1.0 and width of 30 is fit with a Gaussian model, yielding a relative fit error of 4.9% and relative standard deviation of peak position, height, and width of 0.2%, 0.95%, and 1.5% , respectively. (2) The same peak, with the same noise level but with another peak overlapping it, reduces the relative fit error to 2.4% (because the addition of the second peak increases overall signal amplitude), but increases the relative standard deviation of peak position, height, and width to 0.84%, 5%, and 4%  a seemingly better fit, but with poorer precision for the first peak. (3) The addition of a third (nonoverlapping) peak reduces the fit error to 1.6% , but the relative standard deviation of peak position, height, and width of the first peak are still 0.8%, 5.8%, and 3.64%, about the same as with two peaks, because the third peak does not overlap the first one significantly.
If the average noise noise in the signal is not known or its probability distribution is uncertain, it is possible to use the bootstrap sampling method to estimate the uncertainty of the peak heights, positions, and widths, as illustated on the left and as described in detail above. The latest version of the keypress operated interactive version of ipf.m has added a function (activated by the 'v' key) that estimates the expected standard deviation of the peak parameters using this method.
One way to reduce the effect of
noise is to take more data. If the experiment makes it
possible to reduce the xaxis interval between points, or to
take multiple readings at each xaxis values, then the
resulting increase in the number of data points in each peak
should help reduce the effect of noise. As a
demonstration, using the script DemoPeakfit.m
to create a simulated overlapping peak signal like that shown
above right, it's possible to change the interval between x
values and thus the total number of data points in the signal.
With a noise level of 1% and 75 points in the signal, the
fitting error is 0.35 and the average parameter error is 0.8%.
With 300 points in the signal and the same noise level, the
fitting error is essentially the same, but the average
parameter error drops to 0.4%, suggesting that the accuracy of
the measured parameters varies inversely with the square root
of the number of data points in the peaks.
The figure
on the right illustrates the importance of sampling interval
and data density. You can download the data file "udx" in TXT format or in Matlab MAT format. The signal consists of two
Gaussian peaks, one located at x=50 and the second at x=150.
Both peaks have a peak height of 1.0 and a peak halfwidth of
10, and normallydistributed random white noise with a
standard deviation of 0.1 has been added to the entire signal.
The xaxis sampling interval, however, is different for the
two peaks; it's 0.1 for the first peak and 1.0 for the second
peak. This means that the first peak is characterized by ten
times more points than the second peak. When you fit these
peaks separately to a Gaussian model (e.g., using peakfit.m or
ipf.m), you will find that all the parameters of the first
peak are measured more accurately than the second, even though
the fitting error is not much different:
First
peak:
Second peak:
Percent Fitting
Error=7.6434% Percent Fitting Error=8.8827%
Peak# Position Height
Width Peak# Position Height Width
1
49.95 1.0049 10.111 1
149.64 1.0313 9.941
So far this discussion has applied
to white noise. But other noise colors have
different effects. Lowfrequency weighted ("pink") noise has a
greater effect on the accuracy of peak parameters
measured by curve fitting, and, in a nice symmetry,
highfrequency "blue" noise has a smaller effect on
the accuracy of peak parameters that would be expected on the
basis of its standard deviation, because the information in a
smooth peak signal
is concentrated at low frequencies. An example of this
occurs when curve fitting is applied to a signal that has been
deconvoluted to remove a
broadening effect. This is why smoothing
before curve fitting does not help, because the peak
signal information is concentrated in the low frequency
range, but smoothing reduces mainly the noise in the high
frequency range.
Sometime you may notice that the residuals in a curve fitting operation are structured into bands or lines rather than being completely random. This can occur if either the independent variable or the dependent variable is quantized into discrete steps rather than continuous. It may look strange, but it has little effect on the results as long as the random noise is larger than the steps.
When there is noise in the data (in other words, pretty much always), the exact results will depend on the region selected for the fit  for example, the results will vary slightly with the pan and zoom setting in ipf.m, and the more noise, the greater the effect.
d. Iterative fitting
errors.
Unlike multiple linear regression curve fitting, iterative
methods may not always converge on the exact same model
parameters each time the fit is repeated with slightly
different starting values (first guesses). The Interactive Peak Fitter
ipf.m makes it easy to test this, because it uses slightly
different starting values each time the signal is fit (by
pressing the F key in
ipf.m,
for example). Even better, by pressing the X key, the ipf.m function
silently computes 10 fits with different starting values and
takes the one with the lowest fitting error. A basic
assumption of any curve fitting operation is that the fitting
error (the rootmeansquare difference between the model and
the data) is minimized, the parameter errors (the difference
between the actual parameters and the parameters of the
bestfit model) will also be minimized. This is generally a
good assumption, as demonstrated by the graph to the
right, which shows typical percent parameters errors as a
function of fitting error for the leftmost peak in one sample
of the simulated signal generated by DemoPeakfit.m (shown in the
previous section). The variability of the fitting error here
is caused by random small variations in the first guesses,
rather than by random noise in the signal. In many
practical cases there is enough random noise in the signals
that the iterative fitting errors within one sample of the
signal are small compared to the random noise errors between
samples.
Remember that the variability in
measured peak parameters from fit to fit of a single sample of
the signal is not a
good estimate of the precision or accuracy of those
parameters, for the simple reason that those results represent
only one sample of the signal, noise, and background. The
sampletosample variations are likely to be much greater than
the withinsample variations due to the iterative curve
fitting. (In this case, a "sample" is a single recording of
signal). To estimate the contribution of random noise to the
variability in measured peak parameters when only a single
sample if the signal is available, the bootstrap method can
be used.
e. Selecting
the optimum data region of interest. When
you perform a peak fitting using ipf.m,
you have control over data region selected by using the pan
and zoom controls (or, using the commandline function
peakfit.m, by setting the center and window input arguments).
Changing these settings usually changes the resulting fitted
peak parameters. If the data were absolutely perfect, say, a
mathematically perfect peak shape with no random noise, then
the pan and zoom settings would make no difference at all; you
would get the exact same values for peak parameters at all
settings, assuming only that the model you are using matches
the actual shape. But of course in the real world, data are
never mathematically perfect and noiseless. The greater the
amount of random noise in the data, or the greater the
discrepancy between your data and the model you select, the
more the measured parameters will vary if you fit different
regions using the pan and zoom controls. This is simply an
indication of the uncertainty in the measured parameters.
f. A difficult case. As
a dramatic example of the ideas in parts c and d, consider this
simulated example signal, consisting of two Gaussian peaks of
equal height = 1.00, overlapping closely enough so that their
sum is a single symmetrical peak that looks very much like
a single Gaussian.
If there were no noise in the signal, the peakfit.m or ipf.m routines could easily extract the two equal Gaussian components to an accuracy of 1 part in 1000.
>> peakfit([x y],5,19,2,1)
Peak# Position Height Width AreaBut in the presence of even a little noise (for example, 1% RSD), the results are uneven; one peak is almost always significantly higher than the other:
Peak#
Position Height
Width Area
1
4.4117
0.83282
1.61 1.43
2
5.4022
1.1486 1.734 2.12
The fit is stable with any one
sample of noise (if peakfit.m
was run again with slightly different starting values, for
example by pressing the F
key several times in ipf.m),
so the problem is not iterative fitting errors caused by
different starting values. The problem is the noise: although
the signal is completely symmetrical, any particular sample of
the noise is not perfectly symmetrical (e.g. the
first half of the noise usually averages a slightly higher or
lower than the second half, resulting in an asymmetrical fit
result). The surprising thing is that the error in the peak
heights are much larger (about 15% relative, on average) than
the random noise in the data (1% in this example). So even
though the fit looks good
 the fitting error is low (less than 1%) and the residuals
are random and unstructured  the model parameters can
still be very far off. If you were to make another
measurement (i.e. generate another independent set of noise),
the results would be different but still inaccurate (the
first peak has an equal chance of being larger or smaller than
the second). Unfortunately, the expected error is not
accurately predicted by the bootstrap method,
which seriously underestimates the standard deviation of the
peak parameters with repeated measurements of independent
signals (because a bootstrap subsample of asymmetrical
noise is likely to remain asymmetrical). A Monte Carlo
simulation would give a more reliable estimation of
uncertainty in such cases.
Better results can be obtained in
cases where the peak widths are expected to be equal, in which
case you can use peak shape 6 (equalwidth Gaussian) instead
of peak shape 1: peakfit([x
y],5,19,2,6).
It also helps to provide decent first guesses (start) and to
set the number of trials (NumTrials) to a number above 1): peakfit([x,y],5,10,2,6,0,10,[4
2 5 2],0,0). The best case will be if the shape,
position, and width of the two peaks are known accurately, and
if the only unknown is their heights. Then the Classical Least Squares (multiple
regression) technique can be employed and the results
will be much better.
Appendix AE
illustrates one way to deal with the problem of excessive peak
overlap in a multistep script that uses firstderivative
symmetrization as a preprocess performed before iterative
leastsquares curve fitting to analyze a complex signal
consisting of multiple asymmetric overlapping peaks. This
results in better peak parameter accuracy, even though
the fitting error is no better.
For an even more challenging
example like this, where the two closely overlapping peak are
very different in height, see Appendix Q.
Fitting
signals that are subject to exponential broadening.
DataMatrix2 (figure on the right) is a computergenerated test signal consisting of 16 symmetrical Gaussian peaks with random white noise added. The peaks occur in groups of 1, 2, or 3 overlapping peaks, but the peak maxima are located at exactly integer values of x from 300 to 3900 (on the 100's) and the peak widths are always exactly 60 units. The peak heights vary from 0.06 to 1.85. The standard deviation of the noise is 0.01. You can use this signal to test curvefitting programs and to determine the accuracy of their measurements of peak parameters. Rightclick and select "Save" to download this signal, put it in the Matlab path, then type "load DataMatrix2" at the command prompt to load it into the Matlab workspace.
DataMatrix3 (figure on the left) is a exponentially broadened version of DataMatrix2, with a "decay constant", also called "time constant", of 33 points on the xaxis. The result of the exponential broadening is that all the peaks in this signal are asymmetrical, their peak maxima are shifted to longer x values, and their peak heights are smaller and their peak widths are larger than the corresponding peaks in DataMatrix2. Also, the random noise is damped in this signal compared to the original and is no longer "white", as a consequence of the broadening. This type of effect is common in physical measurements and often arises from some physical or electrical effect in the measurement system that is apart from the fundamental peak characteristics. In such cases it is usually desirable to compensate for the effect of the broadening, either by deconvolution or by curve fitting, in an attempt to measure what the peak parameters would have been before the broadening (and also to measure the broadening itself). This can be done for Gaussian peaks that are exponentially broadened by using the "ExpGaussian" peak shape in peakfit.m and ipf.m (or the "ExpLorentzian", if the underlying peaks are Lorentzian). Rightclick and select "Save" to download this signal, put it in the Matlab path, then type "load DataMatrix3" to load it into the Matlab workspace.
The example illustrated on the
right focuses on the single isolated peak whose "true" peak position, height,
width, and area in the original unbroadened signal, are 2800, 0.52, 60, and 33.2 respectively. (The relative standard
deviation of the noise is 0.01/0.52=2%.) In the broadened
signal, the peak is visibly asymmetrical, the peak maximum
is shifted to larger x values, and it has a shorter height
and larger width, as demonstrated by the attempt to fit a
normal (symmetrical) Gaussian to the broadened peak. (The
peak area, on
the other hand, is not much effected by the
broadening).
>> load DataMatrix3
The
large "wavy" residual in the plot above is a tipoff that the
model is not quite right. Moreover, the fitting error (1.2%)
is larger than expected for a peak with a halfwidth of 60
points and a 2% noise RSD (approximately 2%/sqrt(60)=0.25%).
Fitting to an
exponentiallybroadened Gaussian (pictured on the right)
gives a much lower fitting error ("Percent error") and a
more nearly random residual plot. But the interesting
thing is that it also recovers the original peak
position, height, and width to an accuracy of a fraction
of 1%. In performing this fit, the decay constant
("extra") was experimentally determined from the broadened
signal by adjusting it with the A and Z keys to give the
lowest fitting error; that also results in a reasonably good measurement of the
broadening factor (32.6, vs the actual value of 33). Of
course, had the original signal been nosier, these
measurements would not be so accurate. Note: When using
peakshape 5 (fixed decay constant exponentially broadened
Gaussian) you have to give it a reasonably good value for
the decay constant ('extra'), the input argument
right after the peakshape number. If the value is
too far off, the fit may fail completely, returning all
zeros. A little trial and error suffice. (Also, peakfit.m version 8.4 has two forms
of unconstrained variable decay constant exponentiallybroadened
Gaussian, shape numbers 31 and 39, that will measure
the decay constant as an iterated variable. Shape 31 (expgaussian.m) creates
the shape by performing a Fourier convolution of a
specified Gaussian by an exponential decay of
specified decay constant, whereas shape 39 (expgaussian2.m)
uses a
mathematical expression for the final shape so
produced. Both result in the same peak shape but are
parameterized differently. Shape 31 reports the peak
height and position as that of the original Gaussian before
broadening, whereas shape 39 reports the peak height
of the broadened result. Shape 31 reports the
width as the FWHM (full width at half maximum) and
shape 39 reports the standard deviation (sigma) of
the Gaussian. Shape 31 reports the exponential
factor an the number of data
points and
shape 39 reports the reciprocal of decay
constant in time
units. (See the script DemoExpgaussian.m
for a more detailed numerical example). For multiplepeak
fits, both shapes usually require a reasonable first guess
('start") vector for best results. If the exponential decay
constant of each peak is expected to be different and you
need to measure those values, use shapes 31 or 39, but the decay constant of all the
peaks is expected to be the same, use shape 5, and
determine the decay constant by fitting an isolated peak.
For example:
Peak Shape =
Exponentiallybroadened Gaussian
Autozero ON
Number of peaks = 1
Extra = 32.6327
Fitted range = 2640
 2979.5 (339.5) (2809.75)
Percent Error =
0.21696
Peak# Position
Height
Width Area
1
2800.1302
0.51829906 60.086295 33.152429
Comparing the two methods, the exponentiallybroadened Gaussian
fit recovers all the underlying peak parameters quite
accurately:
Position 
Height 
Width 
Area 

Actual peak parameters 
2800 
0.52 
60 
33.2155 
Gaussian fit to broadened
signal 
2814.832 
0.45100549 
68.441262 
32.859436 
ExpGaussian fit to broadened signal  2800.1302  0.51829906 
60.086295 
33.152429 
Other peaks in the same signal,
under the broadening influence of the same decay constant,
can be fit with similar settings, for example the set of
three overlapping peaks near x=2400. As before, the
peak positions are recovered almost exactly and even the
width measurements are reasonably accurate (1% or better).
If the exponential
broadening decay constant is not the same for all
the peaks in the signal, for example if it gradually
increases for larger x values, then the decay constant
setting can be optimized for each group of peaks.
The smaller fitting error
evident here is just a reflection of the larger peak
heights in this particular group of peaks  the noise is
the same everywhere in this signal.
Peak Shape = Exponentiallybroadened
Gaussian
Autozero OFF
Number of peaks = 3
Extra = 31.9071
Fitted range = 2206
 2646.5 (440.5) (2426.25)
Percent Error =
0.11659
Peak#
Position Height
Width Area
1 2300.2349 0.83255884
60.283214
53.422354
2 2400.1618 0.4882451 60.122977 31.24918
3
2500.3123
0.85404245
60.633532 55.124839
The residual plots in both of these examples still have some "wavy" character, rather than being completely random and "white". The exponential broadening smooths out any white noise in the original signal that is introduced before the exponential effect, acting as a lowpass filter in the time domain and resulting in a lowfrequency dominated "pink" noise, which is what remains in the residuals after the broadened peaks have been fit as well as possible. On the other hand, white noise that is introduced after the exponential effect would continue to appear white and random on the residuals. In real experimental data, both types of noise may be present in varying amounts.
One final caveat: peak asymmetry
similar to exponential broadening could possibly be the
result a pair of closelyspaced peaks of
different peak heights. In fact, a single
exponential broadened Gaussian peak can sometimes be fit
with two symmetrical
Gaussians to a fitting error at least as low as a single exponential broadened Gaussian
fit. This makes it hard to distinguish between these
two models on the basis of fitting error alone. However,
this can usually be decided by inspecting the other peaks
in the signal: in most experiments, exponential broadening
applies to every peak in the signal, and the broadening is
either constant or changes gradually over the length of
the signal. On the other hand, it is relatively unlikely
that every peak in the signal will be accompanied by a
smaller side peak that varies in this way. So, if a only
one or a few of the peaks exhibit asymmetry, and the
others are symmetrical, it's most likely that the asymmetry is due to closelyspaced peaks of
different peak heights. If all peaks have the same or similar asymmetry, it's more likely to
be a broadening factor that applies to the entire signal.
The two figures here provide an example from real
experimental data. On the left, three asymmetrical peaks
are each fit with two symmetrical Gaussians (six peaks
total). On the right, those three peaks are fit with one
exponentially broadened Gaussian each (three peaks total).
In this case, the three asymmetrical peaks all
have the same asymmetry and can be fit with the same decay
constant ("extra"). Moreover, the fitting error is
slightly lower for the threepeak exponentially broadened fit.
Both of these observations argue for the threepeak exponentially broadened fit
rather than the sixpeak fit.
Note: if your peaks are trailing
off to the left, rather that to the right as in the above
examples, simply use a negative value for the
decay constant (in ipf.n, press ShiftX and type a
negative values).
An alternative to this type of
curve fitting for exponential broadened peaks is to
use the firstderivative
addition technique to remove the asymmetry and then
fit the resulting peak with a symmetrical model. This is
faster in terms of computer execution time, especially for
signals with many peaks, but it requires that the
exponential time constant be known or estimated
experimentally beforehand.
The Effect of Smoothing before leastsquares analysis.
In
general,
it is not advisable to smooth a signal
before applying leastsquares fitting, because doing so
might distort the signal, can make it hard to evaluate the
residuals properly,
and
might bias the results of bootstrap sampling estimations
of precision, causing it to underestimate the
betweensignal variations in peak parameters.
SmoothOptimization.m
is a Matlab/Octave
script that compares the effect of smoothing on the measurements of peak height of a Gaussian peak
with a halfwidth of 166 points, plus white noise
with a
signaltonoise ratio (SNR) of 10, using three different methods:
(a) simply taking the single point at the center of the
peak as the peak height;
(b) using the gaussfit method to fit the top
half of the peak (see CurveFitting.html#Transforming),
and
(c) fitting the entire
signal with a Gaussian using the iterative method.
The results of 150 trials with
independent white noise samples are shown on the left: a
typical raw signal is shown in the upper left. The
other three plots show the effect of the SNR of the
measured peak height vs the smooth ratio (the ratio of the
smooth width to the halfwidth of the peak) for those
three measurement methods. The results show that the
simple singlepoint measurement is indeed much improved by
smoothing, as is expected; however, the optimum SNR (which
improves by roughly the square root of the peak width of
166 points) is achieved only when the smooth ratio
approaches 1.0, and that much smoothing distorts the peak
shape significantly, reducing the peak height by about
40%. The curvefitting methods are much less effected by
smoothing and the iterative method hardly at all. So the
bottom line is that you should not smooth prior to curvefitting,
because it will distort the peak and will not gain any significant
SNR advantage. The only situations where it might be
advantageous so smooth before fitting are when the noise
in the signal is highfrequency weighted (i.e. "blue" noise), where
lowpass filtering will make the peaks easier to see for
the purpose of setting the staring points for an iterative
fit, or if the signal is contaminated with highamplitude
narrow spike artifacts, in which case a medianbased prefilter
can remove the spikes without much change to the rest of
the signal. And, in another application altogether, if you
want to fit a curve joining the successive peaks of a
modulated wave (called the "envelope"), then you can
smooth the absolute value of the wave before fitting the
envelope.