Element Selective Detectors in Gas Chromatography Since the Nobel Prize winning work of A. J. P. Martin in 1952, gas chromatography has grown into one of the most powerful molecular sorting and identification techniques available to the analytical chemist. Most commonly, qualitative identification of an eluted species is based on its characteristic retention volume as indicated by an essentially nondiscriminating thermal conductivity or flame ionization detector. However, many samples, particularly those originating from environmental or biological investigations, contain so many constituent compounds that the resulting chromatogram is a complex maze of peaks. Often, the analyst is interested in only a few of these peaks and is faced with the problem of determining which they are and how he can eliminate interferences from nearby overlapping, or even obscuring, peaks. One method of peak discrimination is to employ a detector system which responds selectively or characteristically to some property of the eluted species such as its atomic or molecular spectral emission, its electrochemical activity in solution, its mass spectrum, its biological activity, or even its odor. Ideally, such a detector must be sensitive, quantitative, and have a rapid response time, but with these provisos the choice of selective principle may be largely up to the ingenuity of the analyst. For example, male gypsy moths can be employed as selective (in this instance, specific) detectors to identify the gas chromatographic peak corresponding to the female gypsy moth sex attractant. Such an arrangement clearly epitomizes the concept of selective detection. The most common selective detectors in use today respond to the presence of a characteristic element or group present in the eluted species. A
O-Rings —
—
Bulkhead Mirror Housing Exhaust Tube Windows
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Window Housing Radiator
-
Filter Housing Photomultiplier Housing
Igniter Plug
_ Ion Col lector
Igniter Button H 2 Inlet
Mirror Filter
LJ
- Burner - Og/Air Inlet
Heater
UliL Column Effluent
Figure 1. Schematic for Tracor FPD (5)
much higher degree of specific molecular identification can, of course, be achieved with on-line mass spectrometry (GC-MS) or Fourier transform infrared spectrometry (GC-FTIR), but the necessary instrumentation is expensive and not widely available to most gas chromatographers. Consequently, this article will deal only with element selective detectors, and the reader is referred elsewhere for discussions of GC-MS (1) and GCFTIR (2, 3). Flame Photometric Detectors (FPD) As its name implies, this detector is essentially a flame photometer. The eluted species passes into a flame (usually H2/O2) which supplies sufficient energy first to produce atoms and simple molecular species
1 1 8 4 A · ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER
1973
and then to excite them to a higher electronic state. The excited atoms and molecules subsequently return to their ground states with emission of characteristic atomic line spectra or molecular band spectra. By monitoring a selected emission wavelength, a phototube signal reproduces the chromatographic peak of interest. Flame photometric detection of a gas chromatographic effluent has an intrinsic sensitivity advantage over conventional flame photometry of solution samples (4). This is because all of the energy available in the flame can be utilized for the atomization and excitation processes since none is required for vaporization of the sample. Consequently, detection limits in the subnanogram range are often accessible. A schematic diagram of a typical
Report
David F. S. Natusch and Thomas M. Thorpe School of Chemical Sciences, University of Illinois, Urbana, III. 61801
Selective gas chromatographic detectors accrue advantages not found with nondiscriminating thermal conductivity or flame ionization detectors, especially in analyzing complex environmental or biological samples
FPD is presented in Figure 1. Com mercial FPD's employ a narrow band pass filter to isolate the appropriate analytical wavelength range (6). A mirror or lens focuses light from a large cross-sectional area of the flame onto the filter, thereby increasing sensitivity and reducing the influence of flame variations, but also apprecia bly enhancing the detection of flame background emission. Consequently, the mirror is eliminated from some recent detector designs. A variation on the detector construction illus trated in Figure 1 is employed in the Bendix FPD which utilizes fiber op tics to isolate the phototube from the flame. Although this is generally con sidered to be an advantage, problems have been encountered owing to hightemperature breakdown of the ce ment retaining the fibers (7). Better cements are now being used, but the initial section of fiber optics can readily be replaced by a rigid glass rod (7). Stability of the FPD is usually lim ited by flame flicker (8), and earlysystems were notorious for the ease with which the flame could be extin guished by large solvent injections or by high carrier gas velocities (4, 9). To obtain maximum reproducibility, therefore, the carrier and flame gas flow rates must be carefully con trolled, and where large sample vol umes are injected, it is often neces sary either to reignite the flame after solvent passage or to vent the solvent past the detector. The most highly developed FPD's are selective for phosphorus and for sulfur. These elements are detected by monitoring narrow band emissions from the simple molecular species HPO and S 2 at 526 and 394 nm, re spectively. Detector response to phos phorus compounds is linear; however, because of the presence of two sulfur
atoms in S2, the response to com pounds containing a single sulfur atom is proportional to the square of the compound concentration (6, 10). This characteristic provides a useful means of determining the number of sulfur atoms present in an eluted species, although caution should be exercised since the square relation ship often does not hold precisely (6, 10). The FPD response to sulfur and phosphorus atoms is commonly on the order of 10,000 times that elicited by hydrocarbons (9). For example, a thousandfold excess of methylphenylacetate eluted simultaneously with triethyl phosphate produces no inter ference in the detection of the latter species (6). Discrimination between sulfur and phosphorus is, however, less impressive. Thus, whereas phos phorus is detected about one-hun dredfold less sensitively than sulfur when the detector is operated in the S mode, sulfur is only detected four fold less sensitively than phosphorus for Ρ mode operation (9, 11). This differential cross response, which arises because the band spectra of HPO and S2 effectively overlap by different amounts within the band pass of the two filters, means that sulfur-containing species may act as interferents in the detection of phos phorus compounds. Such influences can, however, normally be overcome by suitable choice of column parame ters. Selectivity can, of course, also be improved by using a monochromator for wavelength discrimination in place of filters. Sensitivities of the FPD for sulfur and phosphorus are essentialy compa rable as indicated in Table I, and de tection limits are normally about an order of magnitude lower than can be achieved for these compounds with a flame ionization detector. Some rep
resentative detection limits are listed in Table II, although it must be rec ognized that at these low levels, de tection is generally limited by column adsorption (14). In addition to their utility as ele ment selective detectors for sulfur and phosphorus, dual FPD's can be employed to provide information about the relative numbers of sulfur and phosphorus atoms in a compound ( 15). In this application a single flame is viewed by two phototube heads, one selective for sulfur and the other for phosphorus, and the re sponse ratios, Rp/x~R7, determined for each peak. These response ratios range from 5.2 to 6.0 for compounds containing PS, from 2.8 to 3.3 for compounds containing PS2, and from, 1.7 to 2.3 for those containing PS3 (15).
The major field of application of GC-FPD systems has been in the de termination of pesticides and pesti cide residues containing sulfur and phosphorus. For such analyses, the high sensitivity and selectivity of the FPD (Table II) give it superiority over flame ionization or electron cap ture detectors. The FPD has also been used to detect gaseous sulfur compounds in air (14, 16), and a number of committed instruments utilizing the detector have been de signed for this purpose (16). The per formance of that designed by Stevens et al. (14) is particularly impressive for automated determination of H2S and SO2 in ambient air. Detection limits are 2 ppb for H2S and 4 ppb for SO2. Although not commercially avail able, flame photometric detectors have been designed to respond to a number of elements other than sulfur and phosphorus. They have been used to detect a variety of volatile metal salts and chelates (4, 17), organics (8), and silylated species (18) (Table
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973 · 1185 A
Table 1. Flame Photometric (Trimode),, Alkali Flame Ionization, and Electron Capture Detector Comparison (J 2)
Base line noise; f l a m e on, a m p s Detectability, g/sec
S-mode
P-mode
Parameters
Photomultiplieir d a r k c u r r e n t , amps Background c u r r e n t ; flame on, amps
Halogen-mode
AFID(CsBr)
4-10-"
4·10-9
4-10-a
6 ΙΟ-»
2· ΙΟ-»
3-10 '
5· 10-'»
2-10- 1 0
1-10-»
3-10- 1 2
1.1-10-" methyl parathion
8· 1 0 - " m e t h y l parathion
1.1-10" 1 0 a l d r i n
2.5-10- 1 3 m e t h y l parathion
6 - 1 0 - " a t o m i c CI
3-10"" atomic Ρ
1 . 3 - 1 0 - ' 3 a t o m i c Ρ 9-10- 1 2 a t o m i c S Linearity, slope of 106, slope 1.01 log-log plot response vs. concentration Response t o PSpecificity compounds ~150 times t h a t for S-comp o u n d s . No re s p o n s e for halo· genated com pounds Specificity factor, 10,000 (9) with respect to hydrocarbons Gas flows, m l / m i n Ν , GC column c a r r i e r 35 H2 —220
3-10-9
3
None, calibration s t r a i g h t line only, slope 1.1 Response to S-compounds ~ 4 0 times t h a t for P-comp o u n d s . No re s p o n s e for halogenated com pounds
5-10 , slope 1.01
—10 3
ECD
