Detection chromatographic detectors

Aqueous distillates are extracted, usually with dichloro-methane (DCM), concentrated to small volumes, generally in a Kuderna-Danish evaporator, and examined by gas chromatography (GC) using a specific detection system. Additional chromatographic cleanup may be required, depending on the complexity of the sample and specificity of the chromatographic detector. 

Advantages and disadvantages of chromatographic detectors and tandem systems are given in Table 4.13, whereas Table 4.14 gives a breakdown of the various detectors over the main chromatographic techniques. Simultaneous detection is possible, e.g. SCD/CLND. 

A powerful advantage of SFC is that more detectors can be interfaced with SFC than with any other chromatographic technique (Table 4.30). There are only a few detectors which operate under supercritical conditions. Consequently, as the sample is transferred from the chromatograph to the detector, it must undergo a phase change from a supercritical fluid to a liquid or gas before detection. Most detectors can be made compatible with both cSFC and pSFC if flow and pressure limits are taken into account appropriately. GC-based detectors such as FID and LC-based detectors such as UVD are the most commonly used, but the detection limits of both still need to be improved to reach sensitivity for SFC compatible with that in LC and GC. Commercial cSFC-FID became available in... 

The nature of a supercritical fluid enables both gas and liquid chromatographic detectors to be used in SFC. Flame ionization (FID), nitrogen phosphorus (NPD), flame photometric (FPD) GC detectors (p. 100 etseq.) and UV and fluorescence HPLC monitors are all compatible with a supercritical fluid mobile phase and can be adapted to operate at the required pressures (up to several hundred bar). A very wide range of solute types can therefore be detected in SFC. In addition the coupled or hyphenated techniques of SFC-MS and SFC-FT-IR are attractive possibilities (cf. GC-MS and GC-IR, p. 114 el seq.). 

Detection in 1C is strictly connected with the nature of eluents (composition, concentration), analytes and the sensitivity required. The ideal characteristics of a chromatographic detector are essentially the following (1) high sensitivity, (2) low cell dead volume, (3) linear relationship between concentration and signal, (4) stable and low background noise, (5) high speed of response, and (6) no signal drift. 

Detectors that respond to all classes of chemical compounds and that cannot discriminate between compounds of different chemical classes are called non-selective. The most widely used non-selective chromatographic detector is the FID, capable of detecting practically all of the organic compounds. 

The low resolution mass spectrometers used in EPA Methods 8260 and 8270 are not as sensitive as some of the selective chromatographic detectors (for example, the ECD) and for this reason are not capable of reaching the low detection limits that may be required for some DQOs. The mass spectrometer scans a large number of ion masses in a short period of time (for example, in EPA Method 8270, a mass range of 35-500 is scanned in 1 second) and dwells only briefly on each detected mass. In such full scan mode, the sensitivity of detection is traded for a wide range of detected ions. It is also affected by the background spectra (an equivalent of the electrical signal noise). 

Methods for the analysis of organic and organometallic compounds are discussed in this chapter. It has become evident that for the analysis of these two classes of compounds, the analyst can draw on a very similar repertoire of analytical techniques with respect to sample preparation, separation, and detection. Chromatographic and, in particular, hyphenated techniques are the workhorses of environmental water analysis. The various formats and technical realizations of mass spectrometers are the most versatile detectors. Their sensitivity and ability to provide structural information at the low and even sub-pg level are an asset and at the same time a prerequisite for (ultra)trace analysis in the aquatic environment. As further significant improvements in detector sensitivity are unlikely, the probable focus of attention in the future will again be on sample preparation. Here, the introduction of new approaches, techniques, and materials for sample preparation can be expected to make a significant impact in this field. 

The use of polymer-coated acoustic sensors as chromatographic detectors (GIX, HPLC) has also been demonstrated [1,43,218]. In such applications, a lack of selectivity fcH a given analyte is actually beneficial, since the function of the coated sensor is to detect each and every species passing the detector after preseparation by the chromatographic column (see Chapter 6). 

In some techniques the signal is recorded as a function of sample residence in the detector. This is the case for chromatographic detectors, for which the value of the signal and, in consequence, the detectability depend on the time of residence and rate of registration. In such cases, the sensitivity of the detector is given in units that... 

Fig. 2 An example chromatogram illustrating the determination of headspace oxygen by GC using a PLOT molecular sieve column with thermal conductivity detection. Chromatographic conditions were carrier gas helium (2mLmin ) oven temperature 26 C inlet 160 C, split mode, 10 1 split ratio, split flow of 20mLmin injector 160°C run time lOmin TCD detector 160 C. (From Ref. p. 41. Copyright 2002 Advanstar Communications Inc.)...

The identification and determination of essential oils in many natural samples have improved greatly with the use of more powerful analytical techniques, such as fast extraction methods, better chromatographic detectors, and hyphenation. This improvement in analytical parameters open a great future for the development of analytical methods for essential oil determinations, even at low limits of detection. 

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