Analyzing the Peak Purity
Normalized Spectra Overlay

In a normalized spectra overlay, single spectra of the spectra recorded for the peak are extracted and are compared with each other. Usually, extraction is at the run time of the peak and at a specific peak height in the leading and trailing edges of the peak; for example, at 10% and 50% of the peak height. Comparing spectra that are normalized by the same method is a means of evaluating the peak purity.

The more closely the spectra match, the higher the possibility that the peak is pure. The following are indications for the impurity of a peak:

• The number of relative maxima and minima of two spectra does not match

• The relative maxima are clearly offset against each other

• The height of the relative maxima strongly deviates

However, please note the following:

• Especially near peak limits, spectra have an increased noise level due to the normalization procedure. This applies in particular to the UV range.

• In the case of very long peaks, baseline correction cannot completely compensate for detector drift.

• The spectra shape depends on the solution; that is, the spectra of a peak can only be compared if the mixing ratio was constant during detection.

• It is possible only to compare spectra with the (complete) absorption in the dynamic validity range of the Lambert Beer Law. If the peak maximum, including the baseline spectrum, is considerably above 1 AU, caution is called for.

 Tip:

The normalized spectra overlay via peak height is currently possible only on the spectra plot (also see Integration  The Spectra Plot). Within the PPA method, the "animated" extraction of spectra is possible. Pressing the CTRL key in the 3D field extracts the current spectrum. If the y-axis of the cross-wires is moved simultaneously, all spectra existing for this range are displayed in the spectra window. A different color indicates the extraction period in the chromatogram. This process can be repeated; for example, for different retention times within the peak width. If you release the CTRL key between the individual extractions, the spectra extracted per range are displayed in a separate color.