Detectors general aspects

The usual definition of chromatography as a method of separation does not imply that a detector is involved. In TLC and PC, detectors are not required for qualitative analysis, and consequently, these simple techniques are used primarily for qualitative screening. The opposite is true for the column methods, and one normally assumes that a chromatograph includes a detector. This section deals with the general aspects of detectors like those used in GC and LC column chromatographs. 

General Aspects It is useless to fit a straight line through the experimental data if one knows that the response of the detector is not linear. The response of most detectors is not linear when considering the entire range. However, for most detectors a part of the response range is (nearly) linear. 

This book is concerned with the unique benefits of mass spectrometry for trace level quantitative analysis, so no detailed discussion of other chromatographic detectors is included except for occasional comparisons. However, some general aspects applicable to all detectors are discussed here, with special emphasis on how these generalizations apply to mass spectrometry (see also Section 6.2.3). 

The most important hardware items appeared to be the detectors themselves. The gas detection system gave frequent spurious alarms, and on both platforms the ultraviolet (UV) fire detectors were also prone to spurious activation from distant hot work for example, and had a limited ability to detect real fires. The tmreliability of these systems had a general effect on response time and would, overall, lengthen the time to respond. The second aspect which was related to hardware was fimction and performance testing of the emergency blowdown systems. It is critical that the workers believe the systems will work when required, and this can only be achieved by occasional use or at least fimction testing. 

In general, it can be said that, often of necessity, the detector cell may be relative large with a low aspect ratio and thus, would theoretically produce serious band dispersion. In practice the predicted dispersion is reduced by deign of the inlet and outlet tubes, as discussed above, to ensure maximum secondary flow in the cell and thus, minimize dispersion. The success of the procedure to reduce detector cell dispersion depends on the type of detector and the principle of detection. For example.it is far easier to design a low dispersion electrical conductivity cell than a low dispersion UV absorption cell. 

QUANTITATIVE ASPECTS. Peak areas are generally used (area = height x width at half-height). The peak area ratio sample to internal standard was plotted against the corresponding weight of serial amounts of standard compound. A standard curve must be obtained for each individual acid because of differences in detector response. 

One major aspect of quantitative analysis is sensitivity and dynamic range of linearity. Such data have been reviewed (2) for the gas density, thermal conductivity, and flame ionization detectors. Since response is a function of molecular weight in the gas density detector, it is difficult to make comparisons in a simple manner. In general, however, the sensitivity of the gas density cell is about twice that of comparable thermal conductivity cells and about one-tenth that of flame ionization detectors (when bleed of the column is limiting). 

Fast LC. Some very short columns that are only 3 to 5 cm long and are packed with very small particles (3 xm) have become popular. Sometimes referred to as 3 x 3 columns, they are less costly and give good separations with minimum consumption of mobile phase. The term fast or high-speed LC has been applied to their use at high flow rates around 4 mL/min. Figure 9.26 compares a conventional column with a 3 x 3 column for the separation of six explosives. In general, separations that need 4,000 or fewer plates can be separated with these columns in a minimum of time with conventional detectors. The instrument requirements are more severe, and special detectors may be needed some of the practical aspects of fast LC have been described by van der Wal.46... 

In flow analysis, multi-detection is generally accomplished by resorting to multichannel flow analysers, and each channel incorporates a different dedicated detector. Another possibility is to take advantage of multi-parametric techniques such as ICP-OES, anodic stripping voltammetry and UV—visible spectrophotometry with diode array detection. A deeper presentation of this aspect is outside of the scope of this monograph. 

Most GC detectors acquire a special analytical role in conjunction with capillary columns. The general sensitivity aspects were already discussed, while some unique examples will be demonstrated below. Suitability of any detector for capillary column work is determined by (a) small volume of the detection cell (b) sensitivity compatible with the conditions of separation and (c) a sufficiently fast detector response. Strictly considered, only a few detector types would qualify entirely. With the exception of flame detection devices, dead volumes of interconnecting lines and the detector itself are the most serious problem that can only be overcome or reduced by adding extra carrier gas at the column exit. This procedure may lead to some sacrifice in the detection sensitivity of the concentration-sensitive detectors. 

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