Analytical flame photometric detector

On-line measurements of the sulfur content of atmospheric aerosols have been made by removing gaseous sulfur species from the aerosol and then analyzing the particles for sulfur with a flame photometric detector (24) or by using an electrostatic precipitator to chop the aerosol particles from the gas so that the sulfur content could be measured by the difference in flame photometric detector response with and without particles present. These and similar methods could be extended to the analysis of size-classified samples to provide on-line size-resolved aerosol composition data, although the analytical methods would have to be extremely sensitive to achieve the size resolution possible in size distribution analysis. 

The impact of the flame photometric detector (FPD) resides in its simultaneous sensitivity and specificity for the determination of sulfur and phosphorus. It is inherently compatible with the FID and as such affords the analytical chemist a discriminating ability beneficial to many analyses. In 1966, Brody and Chaney published data on their design of an FPD (26)(Figure 5.18). 

Analytical Procedures. The extracts from exposure pads, hand rinses, and apple leaves were evaporated to dryness in the 40-45°C water bath, and the carbaryl residues were determined by the procedure of Maitlen and McDonough (4). In this procedure, the residues were hydrolyzed with methanolic potassium hydroxide to 1-naphthol which was then converted to the mesylate derivative by reaction with methanesulfonyl chloride. The carbaryl mesylate was quantitated with a Hewlett Packard Model 5840A gas chromatograph (GLC) equipped with a flame photometric detector operated in the sulfur mode. The GLC column was a 122 cm x 4.0 mm I.D. glass column packed with Chromosorb G (HP) coated with 5% OV 101. The column was operated at a temperature of 205°C with a nitrogen flow rate of 60 ml/min. 

It is obvious from the FTIR and NMR analyses of these extracts that in order to positively identify organosulfur structures we need an analytical technique that is sulfur selective. That is, a technique that responds to sulfur uniquely. One such technique, applicable to the problem in hand, is GLC-FID/FPD where the flame photometric detector is set in the sulfur selective mode. 

A more expensive alternative is to use standard AutoAnalyser type systems, based on multichannel peristaltic pumps, to pump samples and reagents and/or diluents at the desired rates to give automatic mixing at the desired ratio. Flame photometric detectors have been used for many years with AutoAnalysers, especially in clinical laboratories. Curiously, in the past, this approach has less often been routinely used in environmental analytical laboratories employing flame spectrometry, perhaps because an attractive feature of flame spectrometry is the speed of response when used conventionally. Over the past few years, however, there has been an increasing tendency towards fully automated, unattended operation of flame spectrometers. This undoubtedly reflects, at least in part, the improvements in safety features in modern instruments, which often incorporate a comprehensive selection of fail-safe devices. It also reflects the impact of microprocessor control systems, which have greatly facilitated automation of periodic recalibration. 

It may be mentioned that the concept of choosing a derivative with a particular detector in mind is quite frequently employed in residue analysis. And with the development of more diversified selective detectors, we are sure to see more of it. Thiophosphoryl derivatives of phenols for the flame photometric detector (59), nitrophenyl derivatives of amines and thiols (60) and brominated anilines for the EC detector (61), chloro-acetylated phenols for the microcoulometric detector (62), and many other examples (63) would be worth mentioning. The selectivity of a chemical reaction combined with the selectivity of a gas chromatographic detector can provide superior analytical eflBciency. 

The FPD is based on a German patent describing the emission obtained with phosphorus and sulfur compounds in a hydrogen-rich flame (64). Brody and Chaney developed this analytical method into a detector for gas chromatographic eflSuents (65) and predicted (correctly) its development in the years to come. Today, Tracor, Inc., markets it as Melpar flame photometric detector in single- and double-channel versions. 

Flame photometric detector (FPD) is a representative of optical detectors for gas chromatography. In FPD, column effluent is introduced to a hydrogen flame, which breaks analyte molecules into atoms. The temperature of the flame is sufficient to excite some atoms, especially sulphur and phosphorus. These excited atoms emit characteristic lights on return to the ground state. The light emitted by the element of interest is selected by a suitable bandpass filter and measured... 

FIGURE 10.8 Combustor and wall gas pathways in PFPD. (From Operator s Manual Model 5380 Pulsed Flame Photometric Detector, OI Analytical, Texas, 1997.)... 

One of the major advantages of SFC is its compatibility with both GC and HPLC detectors. GC flame detectors, such as the flame ionization detector (FID) [11,12], nitrogen thermionic detector [12,13], and flame photometric detector [14] have all been interfaced with SFC systems using a capillary restrictor which, while maintaining supercritical conditions in the column, also effectively decompresses the fluid to ambient pressure just before it enters the flame tip [10,15]. HPLC detectors such as ultraviolet and fluorescence detectors are employed when pure organic mobile phases or modified mobile phases are used. With these detectors, analytes are detected spectroscopically in a flow-through cell prior to decompression [16]. 

Zainullin RE and Berezkin VG (1991) Flame photometric detectors in chromatography A review. Critical Reviews in Analytical Chemistry 22 183-199. 

Element selective detectors Element selective detectors applicable in pesticide residue analysis include electron capture detector (ECD), electrolytic conductivity detector (ELCD), halogen-specific detector (XSD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD), pulsed flame photometric detector (PEPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED). To cover a wider range of pesticide residues, a halogen-selective detector (ECD, ELCD, XSD) in conjvmction with a phosphorus- (NPD, FPD), nitrogen- (NPD), and/or sulfur-selective detector (FPD, SCD) is commonly used. A practical approach is to spht the column flow to two detectors that reduces the number of injections however, the reduced amoimt of analyte that reaches the detector must be considered. 

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. 

A flame photometric detector measures optical emission from phosphorus and sulfur compounds. When eluate passes through a H2-air flame, excited sulfur- and phosphorus-containing species emit characteristic radiation, which is detected with a photomultiplier tube. Radiant emission is proportional to analyte concentration. 

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. 

Since the introduction of the flame photometric detector (FPD) (Brody and Chaney, 1966) and its first apphcation to marine DMS (Lovelock et al., 1972), gas chromatography with flame photometric detection (GC-FPD) has become the standard technique for the determination of dissolved DMS. Among other sulphur-selective detectors, thus far only the sulphur chemiluminescence detector (SCD) (Benner and Stedman, 1989 Shearer, 1992) has also been used for the determination of dissolved DMS (e.g., Ledyard and Dacey, 1994). However, detailed descriptions of this emerging analytical technique are still lacking. In contrast, the popularity of the FPD resulted in the pubUcation of a variety of methods for the determination of oceanic DMS (e.g., Andreae and Barnard, 1983 Leek and Bdgander, 1988 Turner and Liss, 1985), two of which have been compared during an inter-laboratory calibration (Turner et al., 1990). In the following, some principles of DMS determination by GC-FPD will be discussed, before the analytical procedure is described. 

In a pulsed flame photometric detector (PFPD), the combustion of hydrocarbon molecules is fast and irreversible, and heteroatom species such as S2, HPO, and HNO emit light after the flame is extinguished and thus under cooler temperatures. Consequently, their respective emissions can be electronically gated and separated from the hydrocarbon emission. Thus, PFPD can provide selectivity against hydrocarbon interference during detection analysis. PFPD sensitivity was reported to be superior to FPD. Moreover, N and As could be also detected. The PFPD is currently available for use in benchtop instruments, such as the MINICAMS from O. I. Analytical and other GC detector manufacturers. 

Flame photometric detector (FPD) 2 X 10 g of sulfur compounds, 9 X 10 g of phosphorous compounds 1 X 10 for sulfur compounds lx 10 for phosphorous compounds 10 to 1 by mass selectivity of S or P over carbon Hydrocarbon quenching can result from high levels of CO in the flame Self-quenching of S and P analytes can occur with large samples Gas flows are critical to optimization Response is temperature dependent Condensed water can be a source of window fogging and corrosion... 

Flame photometric detector, providing a mass flow dependent signal, the detector burns in a hydrogen-rich flame where analytes are reduced and excited. Upon decay of the excited species light is emitted of characteristic wavelengths. The visible-range atomic emission spectrum is filtered through an interference filter and detected with a photomultiplier tube. Different interference filters can be selected for sulfur, tin or phosphorus emission lines. The flame photometric detector is sensitive and selective. 

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