Peak height chromatograms

Analytical information taken from a chromatogram has almost exclusively involved either retention data (retention times, capacity factors, etc.) for peak identification or peak heights and peak areas for quantitative assessment. The width of the peak has been rarely used for analytical purposes, except occasionally to obtain approximate values for peak areas. Nevertheless, as seen from the Rate Theory, the peak width is inversely proportional to the solute diffusivity which, in turn, is a function of the solute molecular weight. It follows that for high molecular weight materials, particularly those that cannot be volatalized in the ionization source of a mass spectrometer, peak width measurement offers an approximate source of molecular weight data for very intractable solutes. 

The mixture is identical in each example. The peaks are shown separated by 2, 3, 4, 5 and 6 (a) and it is clear that a separation of 6a would appear to be ideal for accurate quantitative results. Such a resolution, however, will often require very high efficiencies which will be accompanied by very long analysis times. Furthermore, a separation of 6o is not necessary for accurate quantitative analysis. Even with manual measurements made directly on the chromatogram from a strip chart recorder, accurate quantitative results can be obtained with a separation of only 4a. That is to say that duplicate measurements of peak area or peak height should not differ by more than 2%. (A separation of 4a means that the distance between the maxima of the two peaks is equal to twice the peak widths). If the chromatographic data is acquired and processed by a computer, then with modem software, a separation of 4a is quite adequate. 

This technique detects substances qualitatively and quantitatively. The chromatogram retention time is compound-specific, and peak-height indicates the concentration of pollutant in the air. Detection systems include flame ionization, thermal conductivity and electron capture. Traditionally gas chromatography is a laboratory analysis but portable versions are now available for field work. Table 9.4 lists conditions for one such portable device. 

A chroaatogreuB provides information regarding the complexity (numlser of components), quantity (peak height or area) and identity (retention par uleter) of the components in a mixture. Of these parameters the certainty of identification based solely on retention is considered very suspect, even for simple mixtures. When the identity can be firmly established the quantitative information from the chromatogram is very good. The reverse situation applies to spectroscopic techniques which provide a rich source of qualitative information from which substance Identity may be inferred with a reasonable degree of certainty. Spectroscopic Instruments have, however, two practical limitations it is often difficult to extract quantitative... 

Again we remember that each pulse of the line chromatogram (Fig. 4.6a), may be either a singlet or a multiplet peak, and the recorded peak height is proportional to the area value of the detected cluster. 

A nonlinear curve fitting procedure of the experimental (Eq. 4.28) to the theoretical (Eq. 4.27) 2D autocovariance function can serve to perform some fundamental characterization of the 2D separation. The total volume (Vy) and the peak height dispersion (/a() can be readily measured in the chromatogram, thus the number of components (m) and the peak widths (a, and ay) can be estimated (Marchetti et al., 2004). 

Figure 4 Chromatogram of Cr(VI) and Cr(III) in the range 1-5 Xg/L (correlation coefficients >0.999 based on peak height measurements). Cr(VI) appears at 2.8 min, Cr(III) appears at 4.6 min. (From Ref. 92, with permission.)...

Fig. 6.12 Chromatogram repeatability test (a) separation between toluene and decane (b) separation between decane and DMMP and (c) separation between decane and dodecane. The insets in parts (a) and (b) are the chromatogram for each individual gas component. Variation in peak height is due to the variations in SPME sampling time. Curves are sifted vertically for clarity. Reprinted from Ref. 27 with permission. 2008 American Chemical Society...

Suppose the sample containing the analyte also contains the electro-active substance Y that interferes with X in the chromatogram. When peak heights of Y and X are plotted versus corresponding working electrode potentials, voltammograms for X and Y are obtained simultaneously as shown in Figure 2-7. 

The data generated in GC experiments is called a chromatogram, with an example shown in Fig. 14.1. Each peak represents one component of the separated mixture. The retention time ( R) is indicative of the identity of the analyte the peak height or peak is related to the amount (mass or concentration, depending on the detector) of the compound that is present. The peak width is also important, as it provides a measure of the efficiency of the separation process and how many peak compounds the method is capable of separating. 

The direct comparison of a standard liquid-liquid extraction with a liquid-liquid extraction followed by MI-SPE demonstrated the enhancing character of the MI-SPE technique. In GC chromatograms, peak heights of sameridine extracted from spiked human plasma were increased by a factor of 5 after introducing an SPE step see Fig. 10. Besides that, the number of impurities was noticeably reduced by this selective extraction step [92 - 94]. Interestingly, in this case it was possible to use as template a structure which was a little different from that of the analyte (R2=methyl for the template and R2 = ethyl for the analyte sameridine). 

The relationship between the concentration of the solute and the peak produced in the chromatogram is, strictly speaking, only valid for peak area measurements, but in most instances it is more convenient to measure peak height. Such peak height measurements should only be used when all the peaks are very narrow or have similar widths. The tedium and lack of precision associated with non-automated methods of peak area measurements may be overcome using electronic integrators, which are features of most modern instruments. 

The chromatogram produced by gas chromatography (Procedure 3.2) is comparable to that produced by HPLC and all the considerations regarding peak height and area measurements, and internal and external standards are relevant. The... 

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