Alkali flame ionization detection

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. 

AFID = alkali-flame ionization detection FID = flame ionization detection FPD = flame photometric detection GC = gas chromatography IGEFET = interdigitated gate electrode field-effect transistor ITMS = ion trap mass spectrometry MIMS = multiphoton ionization mass spectrometry MS = mass spectrometry... 

Determination of caffeine in soft drinks was undertaken using the aerosol alkali flame ionization detector.24 Soft drinks studied were Coke, Diet Coke, Pepsi, Diet Pepsi, Dr. Pepper, and Mountain Dew. A sample clean-up and concentration procedure is employed followed by GC separation with alkali flame ionization detection. Results showed that Coke, Diet Coke, Pepsi, Diet Pepsi, Dr. Pepper, and Mountain Dew contained 41 2, 52 2, 43 4, 35 9, 46 6, and 60 15 mg caffeine per 355-ml serving. These values compared favorably with levels reported in the literature. 

Nene S, Anjaneyulu B, Rajagopalan TG (1984) Determination of diethylcarbamazine in blood using gas chromatography with alkali flame ionization detection. J Chromatogr 308 334-340... 

Chauhan J, Darbre A, and Carlyle R. F (1982) Determination of urinary amino acids by means of glass capillary gas-liquid chromatography with alkali-flame ionization detection and flame lonizaton detection / Chromatogr 227, 305-321... 

Conte, E.D. and Barry, E.F., Gas chromatographic determination of caffeine in beverages by alkali aerosol flame ionization detection, Microchem. J., 48,372,1993. 

A modification of flame ionization detection involves the introduction into the flame of atoms of one of the alkali metals, e.g. potassium, rubidium... 

AFID = alkali flame ionization detector ECD = Electron capture detection EPA = Environmental Protection Agency FPD = flame photometric detection GC = gas chromatography GPC = gel permeation chromatography HRGC = high resolution gas chromatography NPD = nitrogen- phosphorus detection SPE = solid phase extraction device... 

The thermionic or alkali flame ionization detector (FID) is a variant of the FID that has enhanced sensitivity to nitrogen (N) and phosphorous (P) containing compounds. It is essentially a hydrogen flame around a pellet of an alkali metal salt with an electric potential across the flame. Organic compounds in the air are drawn into the flame and ionized. The alkali metal enhances the response to N and P-containing compounds and, since many chemical agents contain N and P, it is selective for their detection. This detector is rarely used without prior separation by GC. 

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. 

The nitrogen-phosphorus detector (NPD) is also called the alkali flame ionization detector (AFID), or thermionic NPD if no flame is used. The flame NPD is similar to the FID, but with an additional unit, usually a rubidium silicate bead, which is heated by an electrical current (Figure 2.9). When a compound enters the detection compartment, the ion current for compounds containing N or P increases. The mechanism for N detection may briefly described by the following ... 

An NPD basically is a modified FID detector with a rubidium or cesium chloride bead on a Pt wire inside a heater coil situated close to the hydrogen jet and the collector electrode. It is also called the alkali flame-ionization detector (AFID), see Figure 2.143. Nitrogen and phosphorus can be selectively detected. The sensitivity for the specific detection of nitrogen or phosphorus can be four orders of magnitude greater than that for carbon. 

Since GC instrumentation is available in most analytical laboratories, it has been the principal method of analysis for volatile A -nitrosamines. Many detectors have been coupled to GC for the detection of A -nitrosamines. The conventional flame ionization detector (FID) was initially used but was found to be limited for Ai-nitrosamines. Nitrogen-specific detectors such as the Alkali Hame Ionization (AFID), the Coulson Electrolytic Conductivity (CECD) and Hall Electrolytic Conductivity (HECD) are useful for routine screening. Although the HECD is the most selective, it is not specific to A-nitrosamines and an independent confirmation is necessary for each analysis. The efficiency of common GC detectors for the analysis of A-nitrosamines has been compared by several authors. 

Flame emission spectrometry is used extensively for the determination of trace metals in solution and in particular the alkali and alkaline earth metals. The most notable applications are the determinations of Na, K, Ca and Mg in body fluids and other biological samples for clinical diagnosis. Simple filter instruments generally provide adequate resolution for this type of analysis. The same elements, together with B, Fe, Cu and Mn, are important constituents of soils and fertilizers and the technique is therefore also useful for the analysis of agricultural materials. Although many other trace metals can be determined in a variety of matrices, there has been a preference for the use of atomic absorption spectrometry because variations in flame temperature are much less critical and spectral interference is negligible. Detection limits for flame emission techniques are comparable to those for atomic absorption, i.e. from < 0.01 to 10 ppm (Table 8.6). Flame emission spectrometry complements atomic absorption spectrometry because it operates most effectively for elements which are easily ionized, whilst atomic absorption methods demand a minimum of ionization (Table 8.7). 

Calcium. Calcium like barium is best determined by AAS, since flame emission suffers from background effects where other alkali metal are present. P CAM 173 does not recommend the use of nitrous oxide/acetylene instead of air/acetylene, although the former offers greater sensitivity and detection limit when 1000 ppm potassium is added to the standards and samples. The nitrous oxide/acetylene flame needs the potassium to minimize the ionization of Ca. 

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