Full width half maximum

I don't know what it means to get FWHM for three peaks together. Could you elaborate? Pictures help. Maybe the Igor experiment file.

John Weeks
WaveMetrics, Inc.
support@wavemetrics.com

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I suspect that you mean, you are co-adding three peaks to make one peak. You want the FWHM of the sum peak.

In this case, you have to measure the FWHM of the sum peak, it cannot be obtained from a theoretical equation. To measure it, assume your sum peak is in a wave called sumpeakwave. Then ...

* find the x-position of your sum peak (operation FindAPeak --> V_peakP)
* find the height of your sum peak (height = sumpeakwave[V_peakP])
* find the values in sumpeakwave that are just above half of the height
(operation FindLevels with level = height/2 --> xvalue1, xvalue2)
* calculate the FWHM = abs(xvalue2 - xvalue1)

--
J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAHuntsville

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I don't think it is possible to extract the FWHM in a mathematically meaningful way from a three-peak fit. If you just need an estimate of the FWHM of your enveloped shape, you can use FindLevel.

1) Find the height of your shape: Subtract the baseline from the maximum. In simple cases, this can be achieved by wavemax(fitresult)-wavemin(fitresult).

2) Use FindLevel to find the half maximum on the right and left side of your shape (make sure to use height/2 + baseline as input). Use the /R=(start, end) or /EDGE parameter to search for the two crossing values.

3) The distance between x1 and x2 will be your FWHM. Keep in mind that this is just a numerically estimated value.

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