Examples multivariate chromatogram

In some cases, many different spectra (or chromatograms) of the same object are available. For inhomogenous objects, for example, several samples of different constitution can be taken. This allows to apply multivariate data processing techniques. When the signals of the compounds in the sample are specific and linearly additive, the number of compounds which contribute to the signal, can be determined by a Principal Components Analysis (PCA) " (see Sect. 3.2.1). Without knowing the identity of all compounds, which are present and without knowing their spectra, a calibration by partical least squares (PLC) allows to quantify the compounds of interest. 

Sequential signals are surprisingly widespread in chemistry, and require a large number of methods for analysis. Most data are obtained via computerised instruments such as those for NIR, HPLC or NMR, and raw information such as peak integrals, peak shifts and positions is often dependent on how the information from the computer is first processed. An appreciation of this step is essential prior to applying further multivariate methods such as pattern recognition or classification. Spectra and chromatograms are examples of series that are sequential in time or frequency. However, time series also occur very widely in other areas of chemistry, for example in the area of industrial process control and natural processes. 

As an example, consider a chromatogram in which a number of compounds are detected with different elution times, at the same time as a their spectra (such as UV or mass spectra) are recorded. Coupled chromatography, such as high-performance chromatography-diode array detection (HPLC-DAD) or liquid chromatography-mass spectrometry (LC-MS), is increasingly common in modern laboratories, and represents a rich source of multivariate data. These data can be represented as a matrix as in Figure 4.3. 

For multivariate signals, two or more variables are sampled for example, in diode array high-performance liquid chromatography (HPLC) these are wavelength and time. If a spectrum is sampled every 2 nm, between 200 and 398 nm, and the chromatogram is acquired at Is intervals between 5min and 6 min 59 s, then a total of 100 (wavelengths) x 120 (points in time) or 12000 datapoints are acquired. 

If the initial screening indicates a potential shelf life problem (for example, if SPME-MS-MVA predicted a shelf life of 10 days or less instead of the typical 15-17 days), then the chromatographic file (total ion chromatogram) could be subjected to further scmtiny in order to uncover the cause of the off-flavor. The .ms file, which contains all the information necessary to perform conventional MS identification of chromatographic peaks, could be e-mailed to a corporate research lab. The corporate lab could then perform more sophisticated analysis of the data. It could, for example, do further multivariate analysis investigations... 

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