216-223 LCE April04

The human sense of shape and pattern recognition can discern subtle nuances among groups of visual cues that no computer system can reproduce faithfully. Yet when it comes to measuring and evaluating chromatograms, analysts put a great deal of faith into their computerized data-handling systems. During the course of one year, a practising chromatographer might look at 10 000 or more peaks. After some time, an observer develops a finely tuned sense of what constitutes a good or bad peak shape, which peaks will be detected and measured correctly by the data system, and an overall idea of how the observed peaks indicate the instrumentation’s operational health. This sense is formalized in system suitability software that determines an array of chromatogram metrics from test analyses and compares them with goals or minimum performance levels. These programs confirm the suitability of individual chromatography systems to perform specific analytical tasks, often on a daily basis. Such software relies upon accurate designation of target peaks and parameters when set up. Otherwise, it is bound to perform poorly and can fail to find incipient problems. Chromatographers who use datahandling or suitability software (or who make such measurements themselves) should have a thorough understanding of the various metrics that are extracted from a chromatogram. Without this awareness, they relinquish control over the quality of their results. They are using a tool without a working knowledge of its functions and limitations. This month’s “GC Connections” examines the basic measurements of a peak’s size and shape for the purpose of assessing and monitoring chromatographic separations over a period of time. The calculations presented here represent some of the most commonly used metrics of this type. Other related calculations are found throughout various commercially available performance-monitoring systems as well as in individual laboratories’ quality control and assurance (QC–QA) procedures. This article is intended to aid in the general understanding of these calculations, and it does not purport to present computations that are any more or less appropriate than others.