the start of a chromatographic run with the computer's own time-base to facilitate the recording of
retention times are both essential capabilities. Although a standard means of peak detection makes for a
high degree of uniformity in the treatment of successive chromatograms, the Gaussian nature of peaks,
the presence of 'tailing' or 'fronting' and the possibility of overlap cause additional problems. The
software must be able to distinguish between baseline or baseline drift and noise and the start of a
genuine peak. This is generally achieved by continuous monitoring of the slope and/or setting a
threshold value for the analogue signal from the detector. Slope sensing also allows peak maxima,
shoulders and troughs between partially resolved peaks to be detected.
Most computing integrators allow the user to select values for parameters such as threshold levels for
peak detection, sampling rate during the chromatographic run and the best geometrical/mathematical
procedure to be used for the calculation of peak areas. Sampling rates should be as high as 50 Hz or
more for sharp early eluting peaks and especially for those from a capillary GC column; later and
broader peaks from packed columns can be sampled at rates below 10 Hz. Sampling rates may be fixed
throughout a run or varied automatically with peak widths and under software control. A complete run
of twenty minutes will require many thousands of data points which can be stored in RAM for
immediate processing or transferred to disk.
Peaks are identified from absolute or relative retention times by comparison with data from previously
run standards stored in RAM or in libraries on disk. To take account of the variability of retention times
from successive runs, retention time windows are used. These are defined as being tR ± x% for a
standard, the unknown being positively identified if its retention time falls within the specified range.
The size of the window can be varied by the user to conform with the degree of certainty required.
Reference peaks can be selected for the calculation of relative retention times or as internal standards in
quantitative analysis (pp. 9, 114).
Raw data and results may be printed out and chromatograms replotted with retention times, peak
identities and quantitative information alongside each peak. With the increasing availability of high
density graphics, the VDU has become an important component of many computing integrators. Thus
data can be displayed in selected formats and complete annotated chromatograms presented as the data
are collected. As in the case of computerized IR spectrometry (p. 540) the VDU also enables immediate
enhancements and comparisons of two or three chromatograms to be made. Some systems include a
multiplexer (p. 535) to allow the sequential sampling of several chromatographs, and buffered
interfaces which can store complete chromatograms temporarily until such time as the computer can
transfer them into RAM or on to disk.