Precision and Sources of Error

The precision of enantiomeric excess determined by GC is high over the whole range from 0 (racemic) to 99.9% (nearly enantiomerically pure). At a high enantiomeric excess, the minor enantiomer should preferentially elute as the first peak in order to facilitate correct integration.

Despite the great success of GC for determining enantiomeric excess, potential sources of error should be considered:

• decomposition of the sample during chromatography (the enantiomer which spends a longer time in the column will be lost preferentially, causing an error in enantiomeric excess);

• coelution of impurities accidentally increasing peak areas;

• enantiomerization causing peak distortions (plateau formation);

• peak distortions caused by inadequate instrumentation;

• nonlinear detector response.

In general, the error in enantiomeric excess due to decomposition of the analyte can be reduced if the difference of the residence time in the column is minimized for both enantiomers. This goal may be realized by using short columns, high pressure drops, elevated temperatures and CSPs exhibiting only small separation factors a. A rather frequent cause for the deviation from the expected 1:1 ratio for the racemic mixture consists of the coelution of impurities. This interference can be recognized by determining the enantiomeric excess on two columns coated with CSPs of opposite chirality. The verification of the ideal 1 : 1 ratio of a racemic mixture is always recommended in enantiomer analysis by chromatography. It may also be used to test integration devices.

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Solar Panel Basics

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