3.4. Profiling Overlapped Regions in a Spectrum

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3.4 Profiling Overlapped Regions in a Spectrum

For all but the simplest spectra, you will need to deal with compounds whose signatures overlap. Approaches to profiling these overlapped regions are as varied as are the people profiling them, but the general guidelines below may help you to develop your own strategy. As with your more general approach to profiling spectra, a consistent strategy for profiling overlapped regions will produce more consistent results.

For more details on this and other profiling strategies, see “Tutorial and Sample Files”.

To profile compounds in an overlapped region of the spectrum

Identify likely compounds contributing to an overlapped region (see “Identifying Compounds”).
Evaluate each compound for the presence of additional, non-overlapped peaks elsewhere in the spectrum (see “Cluster Navigator”).
1. If they are present:
  1. Profile the non-overlapped peaks to help establish a concentration range for the compound (see “Concentrations and Compound Fits”).
  2. Look for clusters of peaks in the overlapped region with some clearly visible peaks. Such clusters occur mostly at the edges of the overlap region, but remember to look throughout the region, as some peaks stand out nicely between the peaks of another cluster.
  3. Use these partially-exposed peaks to establish the optimal frequency for the cluster.
2. If they are not present:
  1. Look for clusters of peaks in the overlapped region with some clearly visible peaks.
  2. Use these partially-exposed peaks to establish the optimal frequency for the cluster.
Once you have approximately profiled most of the groups in the overlap region, fine-tune the heights and frequencies of each cluster in the region to obtain an optimal shape. You can use the subtraction line to guide your modifications, as a well-matched cluster will leave a subtraction line that resembles a normal spectrum with the effects of the cluster removed.
In overlap regions, profile multi-peak clusters before single-peak clusters. Also, if urea is present, you may want to profile it last, to minimize its interference with other compounds in the spectrum.

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