Flame ionization detector response curve

Another difficulty in the gas chromatographic separation of amino acids is the choice of detector and it may be necessary to split the gas stream and use two different detectors. The flame ionization detector, which is commonly used, is non-specific and will detect any non-amino acid components of the sample unless purification has been performed prior to derivatization. In addition the relative molar response of the flame ionization detector varies for each amino acid, necessitating the production of separate standard curves. As a consequence, although gas chromatography offers theoretical advantages, its practical application is mainly reserved for special circumstances when a nitrogen detector may be useful to increase the specificity. 

Mass Spectrometry. The use of a quadrupole mass spectrometer as a GC detector for nonmethane hydrocarbon analysis has come of age in recent years. Development of capillary columns with low carrier gas flows has greatly facilitated the interfacing of the GC and mass spectrometer (MS). The entire capillary column effluent can be dumped directly into the MS ion source to maximize system sensitivity. GC-MS detection limits are compound-specific but in most cases are similar to those of the flame ionization detector. Quantitation with a mass spectrometer as detector requires individual species calibration curves. However, the NMOC response pattern as represented by a GC-MS total ion chromatogram is usually very similar to the equivalent FID chromatogram. Consequently, the MS detector can... 

Quantitative response in IMS is today acceptable in applications where IMS has been successful yet unacceptable compared to other detector technologies, such as flame ionization detectors or MSs. This is limited by kinetics of ion formation at the low end of response and by ion source saturation at the top end of the response curve and by matrix effects. Other technologies, such as electron capture detectors and the ion trap MS, shared a similar history and were engineered free of the limitations. There is no such advancement under way today in IMS. 

In the absence of full sets of standards PA concentrations in food or plant samples can be estimated from calibration curves constructed from one or a few available PAs, or less frequently PANOs. Other PAs can be measured against these external standards and described in terms of equivalents of the standard alkaloid. This approach is compromised without correction for the different response factors. In GC-MS the quantification of PAs against a different external standard is possible using flame ionization detection, although response factors differ between open-chain and cyclic esters, and the use of nitrogen specific detectors is preferred. 

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