GC detectors

Coupling the column from the GC to a mass spectrometer provides a very powerful combination, GC-MS, which can identify and quantify almost all the compounds in a complex mixture such as an essential oil or perfume by reference to libraries of mass spectra of known compounds. Careful investigation of the mass spectrum can be used deductively to determine a possible structure for an unknown material using fragmentation theories to identify sub-structural components of the 

GC-Fourier transform infrared (GC-FI IR) spectroscopy is less frequently used than GC-MS, but involves a similar principle in which the outlet from the column is coupled to an infrared spectrophotometer. The technique currently suffers from a lack of library spectra, as the IR spectra taken in the vapour phase can be subtly different from condensed phase spectra or spectra collected using the well-established KBr disc method. 

Some separations can only be achieved by GC, and if it is necessary to isolate such a material, then preparative GC would be required. The flow from the column is momentarily directed to a cold trap as the desired compound elutes, which then condenses in the trap. The amounts that can be collected in this way are minute but a few hundred micrograms are sufficient for a H NMR or IR analysis. 

There are many GC detectors available although the flame ionisation detector remains the most widely used and the most widely applicable to quality control of pharmaceutical products. However, newer detectors such as the plasma emission detector for analysis of trace impurities or the GC-FTIR detector for the structural characterisation of components in mixtures are becoming increasingly important. 

Selectivity in a detector is most often required for sensitive bioanalytical methods where trace amounts of compounds are being analysed in the presence of interferants which are also present in the sample matrix. The properties of some commonly used detectors are summarised in Table 11.3. 

Compounds are burnt in the flame producing ions and thus an increase in current between the jet and the collector. Detects carbon/hydrogen-containing compounds. Insensitive to carbon atoms attached to oxygen, nitrogen or chlorine. In combination with capillary GC it may detect as low as 100 pg-10 ng. Wide range of linear response ca 10  

Responds to cooling effect of the analyte passing over the filament. Relatively insensitive to organic compounds in comparison to FID. It is a universal detector which can be used to determine water vapour. It is also nondestructive so that analytes can be collected after detection, if required. Used to determine water in some BP assays, e.g. water in the peptides menotrophin, gonadorelin and salcatonin 

Microwave-induced plasma atomic emission Plasma 

---

A universal GC detector in which the signal is a change in the thermal conductivity of the mobile phase. 

A nearly universal GC detector in which the solutes are combusted in an H2/air flame, producing a measurable current. 

Fisher s least significant difference a modified form of the f-test for comparing several sets of data. (p. 696) flame ionization detector a nearly universal GC detector in which the solutes are combusted in an H2/air flame, producing a measurable current, (p. 570)... 

Detectors. The function of the gc detector is to sense the presence of a constituent of the sample at the outlet of the column. Selectivity is the property that allows the detector to discriminate between constituents. Thus a detector selective to a particular compound type responds especially weU to compounds of that type, but not to other chemical species. The response is the signal strength generated by a given quantity of material. Sensitivity is a measure of the abiHty of the detector to register the presence of the component of interest. It is usually given as the quantity of material that can be detected having a response at twice the noise level of the detector. 

The dynamic range of LC detectors is usually considerably less than their GC counterparts which evinces more care in determining sample size in quantitative analysis. A GC detector may have a linear response over a concentration range of five or six orders of magnitude, for example, the flame ionization detector, whereas an LC detector is more likely to have a dynamic range of only three orders of magnitude and some detectors considerably less. 

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. 

On the other hand, if only specific GC detectors, e.g. the electron capture, nitrogen-phosphorus or flame photometric detectors, are tested, the argument of lack of GC method sensitivity is not acceptable. In most cases mass spectrometric detectors provide the sensitivity and selectivity needed. Unfortunately, tandem mass spectrometry (MS/MS) or MS" detectors for GC are still not widely used in official laboratories, and therefore these techniques are not always accepted for enforcement methods. 

Table 4.22 Lower limits of detection (LLD) of some GC detectors...

HPLC-QFAAS is also problematical. Most development of atomic plasma emission in HPLC detection has been with the ICP and to some extent the DCP, in contrast with the dominance of the microwave-induced plasmas as element-selective GC detectors. An integrated GC-MIP system has been introduced commercially. Significant polymer/additive analysis applications are not abundant for GC and SFC hyphenations. Wider adoption of plasma spectral chromatographic detection for trace analysis and elemental speciation will depend on the introduction of standardised commercial instrumentation to permit interlaboratory comparison of data and the development of standard methods of analysis which can be widely used. 

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

This effect of adding HC1 to the reaction cell has been used to advantage in distinguishing alkanes from alkenes and oxygenated hydrocarbons in a GC detector utilizing active nitrogen [56],... 

The most commonly used and widely marketed GC detector based on chemiluminescence is the FPD [82], This detector differs from other gas-phase chemiluminescence techniques described below in that it detects chemiluminescence occurring in a flame, rather than cold chemiluminescence. The high temperatures of the flame promote chemical reactions that form key reaction intermediates and may provide additional thermal excitation of the emitting species. Flame emissions may be used to selectively detect compounds containing sulfur, nitrogen, phosphorus, boron, antimony, and arsenic, and even halogens under special reaction conditions [83, 84], but commercial detectors normally are configured only for sulfur and phosphorus detection [85-87], In the FPD, the GC column extends... 

The NO + 03 chemiluminescent reaction [Reactions (1-3)] is utilized in two commercially available GC detectors, the TEA detector, manufactured by Thermal Electric Corporation (Saddle Brook, NJ), and two nitrogen-selective detectors, manufactured by Thermal Electric Corporation and Antek Instruments, respectively. The TEA detector provides a highly sensitive and selective means of analyzing samples for A-nitrosamines, many of which are known carcinogens. These compounds can be found in such diverse matrices as foods, cosmetics, tobacco products, and environmental samples of soil and water. The TEA detector can also be used to quantify nitroaromatics. This class of compounds includes many explosives and various reactive intermediates used in the chemical industry [121]. Several nitroaromatics are known carcinogens, and are found as environmental contaminants. They have been repeatedly identified in organic aerosol particles, formed from the reaction of polycyclic aromatic hydrocarbons with atmospheric nitric acid at the particle surface [122-124], The TEA detector is extremely selective, which aids analyses in complex matrices, but also severely limits the number of potential applications for the detector [125-127],... 

A stream-splitter may be used at the end of the column to allow the simultaneous detection of eluted components by destructive GC detectors such as an FID. An alternative approach is to monitor the total ion current (TIC) in the mass spectrometer which will vary in the same manner as the response of an FID. The total ion current is the sum of the currents generated by all the fragment ions of a particular compound and is proportional to the instantaneous concentration of that compound in the ionizing chamber of the mass spectrometer. By monitoring the ion current for a selected mass fragment (m/z) value characteristic of a particular compound or group of compounds, detection can be made very selective and often specific. Selected ion monitoring (SIM) is more sensitive than TIC and is therefore particularly useful in trace analysis. 

The ideal HPLC detector should have the same characteristics as those required for GC detectors, i.e. rapid and reproducible response to solutes, a wide range of linear response, high sensitivity and stability of operation. No truly universal HPLC detector has yet been developed but the two most widely applicable types are those based on the absorption of UV or visible radiation by the solute species and those which monitor refractive index differences between solutes dissolved in the mobile phase and the pure mobile phase. Other detectors which are more selective in their response rely on such solute properties as fluorescence, electrical conductivity, diffusion currents (amperometric) and radioactivity. The characteristics of the various types of detector are summarized in Table 4.14. 

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.). 

Even though detectors used for GC are generally more sensitive and provide unique selectivity for many types of samples, the available HPLC detectors offer unique advantages in a variety of applications. In short, it is a good idea to recognize the fact that HPLC detectors are favored for some samples, whereas GC detectors are better for others. It should be noted that mass spectrometric detectors have been used effectively for both GC and HPLC. 

As mentioned in Section 11.8.4, the parameters that are most important for a qualitative analysis using most GC detectors are retention time, tR adjusted retention time, t R and selectivity, a. Their definitions were graphically presented in Figures 11.16 and 11.17. Under a given set of conditions (the nature of the stationary phase, the column temperature, the carrier flow rate, the column length and diameter, and the instrument dead volume), the retention time is a particular value for each component. It changes... 

What does it mean to say that a GC detector is universal What does it mean to say that one GC detector is more sensitive and more selective than another ... 

---