Data Processing
Data from the chromatograph can be handled in two ways. it can be processed in real time and only the results of the calculations, such as retention time, peak height and peak area etc. are stored, the raw data being discarded. This is called "on the fly" processing and has the dubious advantage of providing results immediately after the peak is eluted. This procedure requires much less memory and, in the days when memory was limited and expensive, was an attractive alternative. Today when many megabytes of RAM are available together with gigabytes of disc storage, this procedure is rarely used. The alternative is to store each detector output value as it is received (usually on a hard disc), provide on-the-fly information and a chromatogram in real time and also process the results at the end of the analysis. This gives complete flexibility in data reduction, allows the chromatogram to be reconstructed in multiple forms, different algorithms selected for base line correction or peak area measurement, etc.
Data Analysis
The first function the software performs is peak detection. The start of the peak is identified by a significant change in detector output or by the rate of change of the detector output. If peak areas are being measured, the start of the peak defines the point where integration commences. In the simplest form integration involves summing the detector output from the peak start to the peak end.
The end of the peak is identified in the same way. The next important parameter to identify is the peak maximum. The simplest procedure is to subtract consecutive output values (S(n)) at (n) and (n–1) between the peak start and the peak finish and determine when,
S(n) – S(n-1) becomes negative.
The very simple example and would only work with no, or very little, noise. usually a smoothing routine is first carried out on the points proximate to the peak maximum and the peak identification is repeated. In this way an accurate measure the peak maximum can be obtained. Solute identity depends on the accuracy of the retention time measurement and the correct identification of the peak maximum is critical. The peak height is taken as the difference between the signal at the point of the peak maximum and that directly beneath the peak maximum on the baseline produced under the peak. The position of the start of the peak, the position of the peak maximum, the position of the end of the peak, the peak area and the peak height have now been calculated.
