Element selective chromatograms

The AED employs a microwave-induced He plasma to dissociate eluted analyte molecules to their component atoms and excite them to emit at characteristic wavelengths. This is very similar to the mechanism in the argon plasma inductively coupled plasma source (cf. Section 7.3.1). A spectrometer with a diode array detector (Figure 7.26b and c) isolates and measures the intensity of sensitive emission lines unique to each element. Depending on the relative sensitivity and proportion of atoms in the molecules, separate element response channels may display peaks in several element-selective chromatograms. These data may be combined with retention... 

Fig. 15. Cation-exchange chromatography with detection by ICP-AES. Element-selective chromatograms for rare earth impurities in CejOj. Cerium elutes at 30 min. Reproduced with rermission (Yoshida and Haraguchi 1984).

Element-selective detectors. Many samples, e.g. those originating from environmental studies, contain so many constituent compounds that the gas chromatogram obtained is a complex array of peaks. For the analytical chemist, who may be interested in only a few of the compounds present, the replacement of the essentially non-selective type of detector (i.e. thermal conductivity, flame ionisation, etc.) by a system which responds selectively to some property of certain of the eluted species may overcome this problem. 

Temperatures reached are such that all elements can be detected by their emission lines. By choosing a line specific for one element, a selective chromatogram can be obtained for all compounds that contain this element. It is thus possible to search for classes of compounds corresponding to the association of many elements. This approach eliminates interferences due to sample matrices. 

GC analysis with element selective detectors and GC/MS analysis is usually performed with the same sample aliquot and under similar chromatographic conditions, for example, with the same type of stationary phase. This is done to achieve accurately comparable retention times on both instruments. This comparison in particular may prove its value when background materials interfere in a GC/MS run and complicate interpretation of the total ion chromatogram (TIC). 

By choosing a specific line of an element, a selective chromatogram can be obtained of all of the compounds eluted that contain this element. This is the domain of speciation analysis, the search for classes of compounds corresponding to an association of several elements. This approach avoids interferences due to the sample matrices. It is disappointing that this method of identification of organic... 

Selenomethionine is the predominant selenium moiety in plants whereas selenocysteine is probably formed from glycine and selenite in mammalian tissues. Analytical approaches are typically based on degradation of the original matrix to these and other amino acids followed by their determination. For this purpose, GC with element-selective or mass spectro-metric detection provides a valuable alternative to HPLC due to improved sensitivity. Selenoamino acids are derivatized with isopropylchloroformate and bis(p-methoxyphenyl) selenoxide, and with ethyl chloroformate. Figure 3 shows comparative selenium-specific GC-AED chromatograms for ethylated extracted enzymatic yeast hydrolyzates of archived selenized yeast employed in the Clark trial , a reference fresh selenized yeast and reference selenomethionine. 

Element-specific chromatograms using an atomic emission detector (AED) have provided information on the types of additives in a variety of polymer extracts. The high resolution of capillary GC and the selectivity and sensitivity of the AED complemented mass... 

Fig. 229. Element selective gas chromatogram with methylcyclopenta-dienylmanganese GC-MED system. Column Temperature indicated on chromatogram.

Nowadays, atomic absorption spectrometry (AAS) systems are comparatively inexpensive element selective detectors, and some of the instruments also show multi(few)-element capability. There are flame (F AAS), cold vapor (CV AAS), hydride-generating (HG AAS), and graphite furnace (GF-AAS) systems. However, the use of AAS-based detectors for on-line speciation analysis is problematic. F AAS is usually not sensitive enough for speciation analysis at "normal" environmental or physiological concentrations and sample intake is high (4—5 ml/min), which complicates on-line hyphenations with LC an auxiliary flow is necessary. CV AAS and HG AAS use selective derivatization for matrix separation and increased sensitivity for the derivatized species. But, the detector response is species dependent and interferences can be a problem. GF AAS requires only a few microliters of sample and provides low detection limits, between 0.1 and 5 gg/1. Matrix interferences are widely eliminated by Zeeman correction and matrix modifiers nevertheless, quantification errors were reported as atomization temperature does not exceed 2900°C. The most critical problem in respect to speciation analysis is the discontinuous measiuement because of the temperature program operation employed, which takes a few minutes. Therefore, GF AAS is unsuitable for on-line hyphenations as chromatograms carmot be monitored with sufficient resolution. 

Figure 13 Element-selective SPE-GC-AED chromatograms and full-scan SPE-GC-MS chromatogram of 7 ml of wastewater (C, El, S, P, Cl and Br traces). The sample was spiked with bromophos-ethyl (no. 6) at the 1.86 pg/L level. The sample analyzed by GC-MS was also spiked with some n-alkanes. For peak assignment, see Table 8. The bars indicate the peak height of a (hypothetical) compound containing one of each hetero-atoms at the 0.2 pg/L level. Numbers indicate wavelength in nm. (From Ref. 95.)...

By and large, most currently employed approaches for the accurate and precise quantitation of TBT and other butyltins rely on GC with some type of element selective or specific detection technique. Most of these use flame photometric detection (FPD) with a tin specific filter at GOOnm emission. Though somewhat selective for tin containing species, it is not 100% specific for tin alone. Thus, the combination of GC with FPD and DCP appeared to be a very reliable and practical approach to obtain one and/or two selective chromatograms from one or two injections of a fish or shellfish extract. 

None of the detectors previously described yield any information as to the nature of the compound eluted. At most they are selective. Compounds identification proceeds with the use of an internal calibration based on retention times or requires the knowledge of retention indexes (cf. paragraph 2.10). When the chromatogram is very complex, a confusion of identity could occur. To counteract this, several complementary detectors could be associated (Figure 2.17), or a detector able to convey structural information based on spectroscopic data, or elemental composition of the analytes. The retention time and specific characteristics for each compound could then be known. These detectors lead to stand-alone analysis techniques for which the results depend only on the ability of the column to separate properly the constituents of the sample mixture. 

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