Electron-capture detector advantage

One advantage of gas chromatography is the availability of detectors which respond specifically to certain types of compound. The best known are the electron capture detector for chlorine compounds and the flame photometric detector for nitrogen and phosphorus compounds. If one wants to detect very small molecules such as water or CSj, the standard flame ionisation detector must be replaced by a thermal conductivity detector. 

TFA esters have the particular advantage that an electron capture detector may be used, and with the large number of fluorine atoms per molecule of sugar high sensitivity may be achieved. It has been reported that the TFA derivatives are not stable and may not be stored prior to chromatography (88). These ester derivatives also result in multiple peaks related to anomerization and generally are more difficult to prepare than the corresponding trimethylsilyl derivatives. For these reasons there is little reason to choose them as derivatives when other materials can be used. 

Chlorinated silyl derivatives are mainly used for derivatizing compounds to optimize sensitivity when using electron capture detectors. The same precautions, advantages, and disadvantages apply as for TMS or DMS derivatives. 

The most common detectors for GC are the non-selective flame ionisation detector and thermal conductivity detector. For element speciation, selectivity is definitely advantageous, allowing less sample preparation and less demanding separation. Of the conventional GC detectors, the electron capture detector is very sensitive for electrophilic compounds and therefore has some selectivity for polar compounds containing halogens and metal ions. It has been used widely... 

Nota and Improta [813] determined cyanide in coke oven waste water by gas chromatography. The method is based on treatment of the sample with bromine and direct selective determination of the cyanogen bromide by gas solid chromatography using a BrCN selective electron capture detector. No preliminary treatment of the sample to remove interferences is necessary in this method, and in this sense it has distinct advantages over many of the earlier procedures. Bromine also oxidises thiocyanate to cyanogen bromide. Previous treatment of the sample with aqueous formaldehyde destroys thiocyanate and prevents its interference. 

A recent method of detection is electron capture negative ionisation (ECNI) as ionization technique in combination with GC-MS analysis. This method is advantageous because it offers a high sensitivity for compounds with four or more bromine atoms [36]. The sensitivity of ECNI for these compounds is approximately 10 times higher than with the use of an electron capture detector (ECD) [5]. In the analytical method which was developed to quantitate PCBs and PBBs in human serum, GC/ECD was used [30]. Because the response, and therefore the sensitivity, of the ECD depends on the position of the halogen on the biphenyl nucleus as well as the number of halogen atoms, it is necessary to run a standard for each compound to be determined [2], The use of narrow bore (0.15 mmi.d.) capillary columns is advised to obtain the required resolution [5]. 

The flame ionization detector (FID), which, for fluorinated compounds, has the advantage of a greater linearity range in spite of its lower sensitivity in comparison with the electron capture detector (BCD), has been extensively applied to organic metal chelates in particular. Comparisons of the different detectors were carried out (Table 1.2) For special analyses such as the determination of SO2 by reaction to SO2F2 with the radioactive F-isotope or by utilization of H-labelled 3-diketon-ates, radiometric measurements for detection are employed ... 

Electron capture detectors are highly sensitive and have the advantage of not altering the sample significantly (in contrast to the flame ionization detector, which consumes the sample). The linear response of the detector, however, is limited to about two orders of magnitude. 

At this point, the sample is analyzed by gas chromatography (GC), the analytical method of choice for volatile halogenated hydrocarbons. Information on the analysis of these samples by GC is presented in Section 6.2, with a discussion of the advantages and disadvantages of each method. The technique of Antoine et al. (1986) showed a 5% variance on a series of 2 ppb spiked samples, and the analysis had a linear response ranging from 0.5 to 50 ppb. Although infra-red spectrometry has less sensitivity than electron capture detectors (ECD), Hall electroconductivity detectors (HECD), and mass spectrometrlc detectors (MS), it has been used to quantify the levels of... 

The above considerations, when translated into the practice of GC detection and quantitation mean that detectors such as the electron capture detector or the photoionization detector will greatly benefit from the columns of reduced flow-rates (provided that the detection cells can be manufactured correspondingly smaller). Further advantages of capillary columns include considerably reduced bleeding rates during the high-temperature operation as well as the already discussed column inertness. These practical gains may frequently be decisive in practical applications. 

Concentrations of major carbohydrates in other physiological fluids are usually sufficiently high to permit a reliable profile analysis with the flame-ionization detector. Thus, the GC carbohydrate analyses have been described for plasma [412,413] as well as the seminal fluid from both normal and sterile men [164]. Several attempts have been made to relate the polyol concentrations in the human cerebrospinal fluid to certain pathological conditions [414-416]. If higher sensitivities are needed in the carbohydrate determinations, it is of advantage to consider perfluoroacyl derivatives and the electron-capture detector [402]. 

Of the many available detectors, the most common (Table 3) are thermal conductivity detector (TCD), flame ionization detector (FID), electron-capture detector (ECD), alkali-flame ionization detector (AFID or NPD), flame photometric detector (FPD), and mass selective detector. The TCD and FID are usually considered universal detectors as they respond to most analytes whereas the ECD, AFID, and FPD are the most useful selective detectors and give differential responses to analytes containing different functional groups. Note that this does not imply that the magnitude of the response of the universal detectors is constant to all analytes. The mass selective detector has the advantage of operation in either universal or selective detection mode whilst an infrared detector is a powerful tool for distinguishing isomers. 

Derivatization with halogenated compounds offers the advantage that the derivatives are subject to extremely sensitive detection with an electron capture detector (BCD). The disadvantage is that excess derivatization agent remains in the sample solution after the reaction, necessitating its removal prior to gas chromatographic determination because of the potential for interference during BCD detection. 

Because of its intermediate position, SFC combines several advantages of both GC and HPLC. One significant advantage of SFC compared to HPLC is the fact that besides the normal LC detectors (e.g., UV detector), sensitive universal or selective GC detectors (e.g., FID, electron capture detector) are applicable as long as supercritical carbon dioxide is utilized as mobile phase. Furthermore in packed-column SFC, the same variety of stationary phases (selectivities) can be used as in LC (in contrast to GC), with the additional advantage that the analysis time is significantly shorter. 

As mentioned before, a unique advantage of SFC is the fact that a wide variety of detection methods can be applied (see Table 11). Besides the traditional LC detection (e.g., UV) the use of GC detectors, especially, enables relatively sensitive, universal (FID), as well as selective [e.g.. electron capture detectors (BCD), thermoionic detectors (TID)] methods of detection. 

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