Data Acquisition and Handling

A full scan GC-UV analysis generates a considerable amount of data. These data, which are calculated as absorbance values, easily exceed 106 measuring points, when capillary column separation with spectral recording every 2 s is employed. However, the use of modern computer technology makes the handling of a large number of measuring values possible. Various calculations and mathematical functions can be applied to the measuring data. Besides the simple functions, interactive subtraction, baseline corrections, various orders of derivatives, rotation (90°), damping functions and cuts at selected wavelengths can be applied.

Figure 3 A 10 cm long gas flow cell for GC-UV measurements containing a built-in micro gas chromatograph and a light pipe a, Injector; b, injector chamber with carrier gas inlet; c, separation column; d, inlet from external GC; e, light pipe.

Nitric oxide /

180 200 220 240 260

Nitric oxide /

180 200 220 240 260

Wavelength (nm)

Figure 4 Results of cigarette smoke analysis obtained with the GC-UV method with the micro gas chromatograph, which is especially suitable for analysis of the most volatile compounds in cigarette smoke. The wavelength scale follows the x-axis, the absorbances the y-axis, and retention time the z-axes.

During an analysis the recording follows the axes shown in Figure 4 and, in order to monitor the chromatographic sequence, the data are turned 90° to get the retention time along the x-axes. Chromato-grams can now be studied at any chosen wavelength, or for the average absorptions within a chosen wavelength region.

The gas-phase UV spectra are well defined and most of them contain a fine structure. In order to enhance the spectral details, derivatives of the absorption spectra are preferably utilized. When forming chromatograms the negative values obtained upon derivation are not convenient to handle and, to avoid this problem, the absolute values of the derivatives can be used. Figure 5 shows the results from an analysis of a polychlorinated biphenyl (PCB, Arochlor

1248) sample. The presentation mode of the measured values is the same as for Figure 4, but the second derivatives with absolute values of the recorded absorption spectra are drawn for the retention times between 800 and 1600 s. The spectral details of the congeners and isomers concerning this group of related compounds appear quite clearly. Various types of graphical views can be carried out in order to suit certain presentations of results. One example of such a view is a contour plot, which is an effective way to treat the results. In Figure 6 a contour plot is created from the analysis of a petroleum product. The advantage of these type of presentations is that compounds with close retention times appear as nonsym-metrical contour lines, which makes it possible to 'see' the hidden peaks. Another advantage is that

Wavelength (nm)

Figure 5 The result from a retention time of 800 s up to 1600 s is shown in a three-dimensional plot of the absolute values of second derivatives of absorption spectra collected at the analysis of a PCB sample.

Wavelength (nm)

Figure 5 The result from a retention time of 800 s up to 1600 s is shown in a three-dimensional plot of the absolute values of second derivatives of absorption spectra collected at the analysis of a PCB sample.

compounds possessing the same functional group can often be directly shown.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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