Detectors selection criteria

Another aspect of cost reduction would be solvent economy. The need to preferentially select inexpensive solvents and employ the minimum amount of solvent per analysis would be the third performance criteria. Finally, to conserve sample and to have the capability of determining trace contaminants, the fourth criterion would be that the combination of column and detector should provide the maximum possible mass sensitivity and, thus, the minimum amount of sample. The performance criteria are summarized in Table 1. Certain operating limits are inherent in any analytical instrument and these limits will vary with the purpose for which the instrument was designed. For example, the preparative chromatograph will have very different operating characteristics from those of the analytical chromatograph. 

Reliable analytical methods are available for determination of many volatile nitrosamines at concentrations of 0.1 to 10 ppb in a variety of environmental and biological samples. Most methods employ distillation, extraction, an optional cleanup step, concentration, and final separation by gas chromatography (GC). Use of the highly specific Thermal Energy Analyzer (TEA) as a GC detector affords simplification of sample handling and cleanup without sacrifice of selectivity or sensitivity. Mass spectrometry (MS) is usually employed to confirm the identity of nitrosamines. Utilization of the mass spectrometer s capability to provide quantitative data affords additional confirmatory evidence and quantitative confirmation should be a required criterion of environmental sample analysis. Artifactual formation of nitrosamines continues to be a problem, especially at low levels (0.1 to 1 ppb), and precautions must be taken, such as addition of sulfamic acid or other nitrosation inhibitors. The efficacy of measures for prevention of artifactual nitrosamine formation should be evaluated in each type of sample examined. 

The main criterion for the detector is that it gives a response specifically for tin, rather than the compound. This is because of the detection limits required for environmental analysis, which are not attainable with non-specific detectors. The most popular detection methods for organotin analysis are FPD using a tin selective filter, MS, MIP-AES, and ICP-MS. Recently, HPLC-MS methods based on atmospheric ionization including electrospray, ion spray, or chemical ionization have been used to detect the organotin species. These methods have the advantage that molecular information concerning the analyte is available, rather than just atomic information. 

McNair and Miller have characterized the FID based on the following criteria (63). The FID is selective for all carbon-containing compounds. The FID has a minimal detectable amount of 10 " g. A linear dynamic range of 10 enables, for a 1-pL injection volume, a detector response that is linear from 1 g of hydrocarbon all the way down to 1 pg of hydrocarbon. The next criterion is that of good baseline stability over time and this translates into minimal baseline drift. Also, the effect of a change in flow rate or temperature on baseline stability is an important characteristic of FIDs. Finally, it is... 

Two appendixes are associated with, but not part of, the Standard. The first supports the selection of the detection criterion and the definition of the minimum accident of concern. The second appendix provides sample calculations of the radius of coverage for a detector. 

The most popular GPC detector is the differential refractometer. It is a concentration-sensitive detector that measures the difference in refractive index (ARI) between the eluent (the flowing solvent) and the sample solution. It is a universal detector that will respond to any polymer with a significant refractive index difference from the solvent. So another consideration when selecting a solvent, besides being a good solvent for the polymer, must be a refractive index that will provide a significant ART The solvent/polymer combinations listed in the Appendix fulfill this criterion. 

Identification of substances by comparison with spectral libraries is no longer possible. The relative intensities (ion ratios) of the selected ions serve as quality criteria (qualifiers) (1 ion no criterion, 2 ions l criterion and 3 ions=>3 criteria ). This process for detecting compounds can be affected by errors through shifts in retention times and compromised peak area determination caused by the matrix. In residue analysis, it is known that with SIM analysis false positive findings occur in about 10% of the samples. Positive SIM data are confirmed in the same way as positive results from classical GC detectors by running a complete mass spectrum of the analytes suspected. Confirmation of positive results, and statistically of negative results as well, is required by international directives either by full scan, MS/MS, ion ratios or HRMS. 

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