Data generation chromatograms

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. 

Figure 8 UPLC-QTOF-MS base peak ion chromatogram obtained for the combined methanol extracts from soybean and Medicago truncatula (CV Jemalong A17). Separations were achieved using a Waters Acquity UPLC 2.1 x 100 mm, BEH C18 column with 1.7 pm particles, a flow of 600 jj.I min-1, and a linear gradient of 0.1% acetic acid acetonitrile (5 95 to 30 70 over 30 min). Mass spectra were collected on a Waters QTOFMS Premier. (Data generated by David Huhman.) Reprinted from M. Bedair L. W. Sumner, Trends Analyt. Chem. 2008, 27 (3), 238-250, Copyright (2008), with permission from Elsevier.

When an analytical sequence is run, it is important that reference standards are inserted throughout the run in order to assess the data quantitatively as well as to monitor system performance. All of the system suitability tests need not be appUed to all of the data generated at all stages. However, it is important to inspect the chromatograms for any anomalies. 

The relative retention time (RRT) is often calculated for peaks in chromatograms particularly when its identity is unknown. This can be compared to other sample data or data generated using standards or from a database generated under the same chromatographic conditions. An internal standard is added to the test solution and the following equation is used. RRT values can be calculated for both GC and HPLC analysis ... 

Ions at m/z 55, 60, 214 and 236 are observed but do some or all of these arise from the background and are present throughout the analysis, or are they present in only a few scans, i.e. are they from a component with insufficient overall intensity to appear as a discrete peak in the TIC trace An examination of reconstructed ion chromatograms (RICs) from these ions generated by the data system may enable the analyst to resolve this dilemma. The TIC shows the variation, with time, of the total number of ions being detected by the mass spectrometer, while an RIC shows the variation, with time, of a single ion with a chosen m/z value. The RICs for the four ions noted above are shown in Figure 3.15. These ions have similar profiles and show a reduction in intensity as analytes elute from the column. The reduction in intensity is a suppression effect. 

Using this technology, two or more complete mass chromatograms could be generated at the same time although the dwell time for each data point and the sampling rate (number of data points in time) are reduced as shown in Table 4.1. MUX and other multiple sprayer approaches require new interfaces and MS software. The multiple sprayer approach is only available in limited models of LC/MS instruments. 

Finally, a detection system is required at the opposite end of the column that will detect when a substance other than the carrier gas elutes. This detector can consist of any one of a number of different designs, but the purpose is to generate the electronic signal responsible for the chromatogram displayed on the data system screen and from which the qualitative and quantitative information is obtained. 

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