Flame detector, alkali

The non-polar chlorinated hydrocarbon pesticides are routinely quantified using gas chromatography (GC) and electron capture(EC) detection. Alternate detectors include electrolytic conductivity and microcoulometric systems. Organophosphate pesticides which are amenable to GC are responsive to either the flame photometric detector (FPD) or the alkali flame detector (AFD). Sulfur containing compounds respond in the electrolytic conductivity or flame photometric detectors. Nitrogen containing pesticides or metabolites are generally detected with alkali flame or electrolytic conductivity detectors. 

What distinguishes one alkali flame detector from the other is largely the way in which alkali is brought into the flame. The fact that the performance of the detector depends to a great extent on the physical and chemical structure of the alkali source may serve to explain many seemingly contradictory results. The recent, quite elaborate, review by Brazhnikov et al. (20) provides a good source of reference for different detector constructions. 

There are several commercial companies producing alkali flame detectors. In one version, two flames are stacked, one above the other. The lower, plain hydrogen-air flame bums the sample the combustion products are swept into the second flame which is doped with a sodium salt deposited on an electrically heated wire (31). The upper detector functions as alkali flame detector. Another modification uses a detector jet tip formed from fused salt the flame bums in contact with the salt surface (15, 32, 33, 34, 35). A third form uses an alkali-doped porous metal (36) or a platinum capillary filled with potassium hydroxide and carbon (37). When the capillary is heated to 900°—1000°, the carbon causes the grain boimdaries in the platinum to become enlarged, allowing alkali to diffuse slowly through it. The above remarks should be considered illustrative of detectors on the market and by no means comprehensive. Commercial firms, of course, must add the limitations of the patent situation to the diflBculties encountered in constructing these detectors. In fact, some detectors seem to be constructed more for the patent lawyer than for the analyst (38). 

No such limitations bind the residue chemist who wants to make his own. There are, in my opinion, two easy ways to produce a good alkali flame detector from a suitable commercial FID. Both are inexpensive but may take a little practice. Method number 1 was described by Giuffrida and Ives (39, 40). A 26-gauge platinum—iridium wire helix, coated with potassium chloride, is mounted on a detector jet tip such that it is in contact with the flame (41). Method number 2 stems from... 

Once a flame ionization detector has been converted to an alkali flame detector by the addition either of a coated spiral or a pellet, the residue chemist should bear in mind that each detector is a little different from the other and some tinkering with the flow rate and the electrode height (where possible) may be of great benefit. Some alkali flame detectors need an initial conditioning. Some types of pellets, including a commercially available one, are quite susceptible to contamination and the alkali surface has to be cleaned from time to time. The alkali salt itself, of course, should be a high-purity compound. 

As amply documented, the most important of the flow rates is that of hydrogen. The alkali flame detector response for phosphorus increases with increasing hydrogen flow. A higher hydrogen flow, of course, produces a bigger flame of higher temperature which is in contact with a... 

What type of performance can—or rather should— the residue chemist expect of the alkali flame detector Ten picograms of parathion should... 

The predominant use at the present time for the alkali flame detector is undoubtedly the analysis of phosphorus-containing materials. Although... 

Another potentially valuable but largely unexploited characteristic of the alkali flame detector is its linear response to the amount of heteroatom introduced into the flame. There has been some controversy in the literature on this point. Cremer (58), Karmen (57), Giuffrida (53), and others hold that the response is linear for the elements they investigated. 

The increased sensitivity and selectivity of the alkali flame detector for nitrogen-containing compounds, led to the development of an assay for caffeine in plasma, by Cohen et 41... 

A detailed study has been made456 of the application of the alkali flame detector to organosilicon, organotin and organolead compounds. 

The flame-based GC selective detectors derive their response from a specific flame emission (flame photometric detectors), or certain secondary ionization processes subsequent to the combustion in a flame (thermionic or alkali-flame detectors). Recent advances in the detector principles and their applications, as pertinent to biochemical uses, will now briefly be reviewed. 

GLC Determination of Caffeine in Plasma Using Alkali Flame Detector J. Pharm. Sci. 67(8) 1093-1095 (1978) ... 

Dressier et al have studied the response of alkali flame detectors to organotin compounds. 

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