Flame Ionization Technology

All organic matter decomposes when burned in a hydrogen flame, and ions are generated for many organic molecules. The FID sums all chemicals contained in the sample that generated the ions in the hydrogen flame. Hame ionization detectors are insensitive toward noncombustible gases such as H2O, CO2, SO2, and NO. Therefore, these substances do not interfere with detection. Consequently, FID is quite often is coupled with a photoionization device to identify VOCs. 

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Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

Fig. 14.7. —Schematic diagram of a capillary flame ionization detector showing the gas flows and components. Reprinted with permission from the 6890 Gas Chromatograph Operating Manual. Copyright (2004), Agilent Technologies.

Detector Technology. The second advance in GLC is detector technology. Five detectors are used widely in toxicant detection the flame ionization (FID), flame photometric (FPD), electron capture (ECD), conductivity, and nitrogen-phosphorous detectors. Other detectors have application to toxicant analysis and include the Hall conductivity detector and the photoionization detector. 

From a practical point of view, SFC may allow the use of many different detection principles, including both typical LC detectors (UV-absorbance, fluorescence) and typical GC detectors (flame ionization, mass spectrometry). Also, capillary SFC seems to be well within the posssibilities of current technology, while capillary LC is not. 

Figure 31-8 A typical flame ionization detector. (Courtesy of Agilent Technologies. Palo Alto. CA.)...

Quantitative response in IMS is today acceptable in applications where IMS has been successful yet unacceptable compared to other detector technologies, such as flame ionization detectors or MSs. This is limited by kinetics of ion formation at the low end of response and by ion source saturation at the top end of the response curve and by matrix effects. Other technologies, such as electron capture detectors and the ion trap MS, shared a similar history and were engineered free of the limitations. There is no such advancement under way today in IMS. 

The flame ionization detector is the most commonly used for GC. It is considered an economical detector. The gas used for the detector is hydrogen and air. Signal out put of the flame detector is very sensitivity, Ipg/s, however the sensitivity also depends upon the sample preparation and the GC instrument. For example for benzene with the headspace GC-FID of one model is 0.02 ppm and 0.06 ppm for detection and quantitation limits, respectively which is more sensitive than the other model of the same manufacturer (Agilent technology, 2007). In quantitative analysis of a known compound by a GC, a MS detector can be used but usually avoided for routine analysis due to the running cost. 

ITigh-performance liquid chromatography (FIPLC) is routinely used for the compositional analysis of lipid classes, TAGs, and tocopherols. Flowever, there can be problems for the quantitative detection of lipids other than tocopherols because most lipids do not have a suitable chromaphore, and therefore cannot be detected spectrophotometrically. Evaporative technology such as nebulizing mass detectors and the quartz/flame ionization transport system has to be employed. 

GC detector technology has also developed in areas other than MS. Fourier-transform infrared spectrometry (FTIR) and atomic emission detection (AED) can both provide structural insights into eluted compounds. AED, in particular, allows analysts to move away from the carbon-based detection capabilities of the standard flame ionization detector to focus on other elements such as N, Cl, S, P, Br, and F allowing molecules with particular elemental constituents to be readily identified. 

Pollard, S. J., Hrudey, S. E., Fuhr, B. J. et al. (1992) Hydrocarbon wastes at petroleum-contaminated and creosote-contaminated sites - rapid characterization of component classes by thin-layer chromatography with flame ionization detection. Environmental Science and Technology, 26, 2528-34. 

Table 1 has proved to be useful to describe general trends with respect to field use of analytical instruments. Judgements with respect to the values entered in the matrix are those of the author based on evaluations of literature information and personal experience. GC-IMS [6] refers to a hand-held device based on a short column GC coupled to a CAM-based IMS instrument. A new technology, Transverse Field Compensation Ion Mobility Spectrometry [7], which has been subjected to only a limited amount of study has been included because of an indication of very good real time sensitivity. GC-FPD/FID refers to a gas chromatograph with a single column, a flame photometric detector (FPD), and a flame ionization detector (FID). GC-MS, a gas chromatograph - mass spectrometer, is well known. 

The second wave of electrochemical devices replaced flame photometry in many clinical laboratories by the ISE method in blood electrolyte (Na, K ) measurements. Improvement of electrochemical technologies played an important role in this development as well as safety considerations related to the use of flame photometry. The measurement of ionized calcium by potentiometry soon followed. The techniques for the potentiometric measurement of chloride, lithium, ionized magnesium, and total calcium are also available but need perfection mainly with respect to avoiding interferences. 

Gas Detection Principles. Some of the more common gas detection principles used in toxie gas monitoring include electroehemistry, electro-optical detection, solid state detection, mass spectrometry, moleeular (or flame) emission speetrometry,irrfrared speetrophotometry, ionization teeh-niques, and thermal conduetivity. Brief deseriptions of several different gas detection technologies are deseribed below. 

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