ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
339
Dual-Flame Photometric Detector for Sulfur and Phosphorus Compounds in Gas Chromatograph Effluents Paul L. Patterson,* Robert L. Howe, and Ahmad Abu-Shumays’ Varian Instrument Division, 2700 Mitchell Drive, Walnut Creek, California 94598
A new dual-flame photometric detector is described for use In the specific detection of phosphorus- and sulfur-bearing compounds In the effluent of a gas chromatograph. A first hydrogen-rich flame Is used to decompose samples from the gas chromatograph, and a second flame Is used to produce light emission from S2 and HPO molecules. Because sample decomposition occurs In the first flame, the light emission In the second flame exhibits a more uniform response with respect to sample concentration and composltion In comparlson to previous single-flame detectors. For those sulfur compounds tested, a pure square-law dependence of emission intensity vs. sample amount Is obtained In agreement with theoretical expectations for S2 emissions. A phosphorus detectivlty of 5 X g P/s and a sulfur detectlvlty of 5 X lo-” g S/s are achieved. Selectlvlty wlth respect to hydrocarbon emlsslons is 5 X l o 5 g C/g P in the phosphorus mode, and ranges from lo3 g C/g S to l o 6 g C/g S in the sulfur mode. The burner Is further capable of handling large volume solvent injections without extinguishing the flames.
The flame photometric detector (FPD) has seen widespread use in gas chromatography as a specific detector for sulfuror phosphorus-bearing compounds. This detector operates on the principle that phosphorus and sulfur compounds emit characteristic green and blue colorations, respectively, in hydrogen-rich, hydrogen-air flames ( I ) . I n the case of phosphorus, the green emission is due to the HPO molecule, and the intensity of this emission is known (2)to vary linearly with phosphorus atom flow into the flame. In the case of sulfur, the blue emission is due to the S2molecule, and the S2 emission intensity is theoretically expected (3)and is usually found (2) t o vary as the approximate square of sulfur atom flow into the flame. The most widely used version of the F P D has been that design first described by Brody and Chaney (2) in 1966. Brody and Chaney employed a diffusion flame in which a mixture of the gas chromatograph (GC) effluent and air was conveyed to the center orifice of a flame tip, and an excess of H2 was introduced from the outer periphery of the flame tip. With this burner and flow configuration, interfering light emissions from hydrocarbons occurred mainly a t the base of the flame in close proximity to the burner orifices, whereas sulfur and phosphorus emissions occurred in the diffuse upper portions of the flame. Brody and Chaney enhanced the selectivity of their detector by using an opaque shield about the flame base so as to prevent hydrocarbon emissions from reaching a photomultiplier tube viewing t h e flame from the side. Most of the known limitations ( 4 ) of the Brody-Chaney burner can be related to the fact t h a t a single flame is used to both decompose samples and produce the desired phosphorus or sulfur emissions. Samples in the GC effluent can include compounds of complex molecular structure, and such Present address, Varian Instrument Division, 61 1 Hansen Way,
Palo Alto, Calif. 94303.
0003-2700/78/0350-0339$01 .OO/O
compounds can change the flame environment and adversely affect the production of light. One well-known limitation of the Brody-Chaney burner is its susceptibility to solvent flameout (2,5). Solvent peaks corresponding to GC injections of more than a few ILL volume, momentarily starve the flame of oxygen and cause it to be extinguished. Burgett and Green ( 5 ) have reported t h a t an interchange of the H2 and air supplies to the burner can overcome this flameout problem. Such a gas stream interchange probably also negates the advantage of an opaque shield a t the flame base, because incoming hydrocarbons do not encounter oxygen-containing flame regions until further downstream of flame tip orifices. It is in the oxygen-enriched regions of the diffusion flame that hydrocarbon emissions are most prevalent. Another limitation of a single flame FPD is that the sulfur response is not always a pure square-law dependence on sulfur content. In fact, there have been reports (6-11) claiming the S response varies anywhere from a linear to a pure square-law response depending on such factors as the exact design of the burner, the flame gas flow rates, the concentration of sample, or the type of compound analyzed. A related problem is the known variation in sensitivity depending on the moleculGl structure of the sample being analyzed. Finally, it is also known that the sulfur or phosphorus light emissions in single flame detectors are severely quenched if there exists simultaneously a hydrocarbon background in the flame (12-16). Most of the above limitations can be overcome if the region of sample decomposition chemistry is removed from the region of optical emission. This is accomplished in the present work through the use of a dual-flame burner. One flame is used to achieve the decomposition of incoming samples, and a second longitudinally separated flame is used to produce the desired optical emission. There have been two previous reports of dual-flame burners used for sulfur or phoshorus detection. van der Smissen (17) employed an optically transparent Smithells-type (18) chimney to achieve two longitudinally separated H2-air flames. One flame burned in a H2-rich environment at a flame tip located inside the lower end of the chimney, and a second flame burned a t the chimney exit where the excess H2 combined with surrounding ambient air. Unlike the present work, van der Smissen viewed mainly the optical emission that occurred in the glow region between the two flames. A disadvantage of this burner was t h a t S and P emissions occurred at different spatial locations within this glow region, so that the optical viewing axis had to be relocated if a change from S to P modes of detection was desired. Also, t h e requirement for an optically transparent chimney meant that the burner was constructed of high temperature glass which typically has a very low thermal conductivity and contains significant alkali impurities. Consequently, both flames in the van der Smissen burner generally exhibited background colorations due to alkali impurities outgassed from hot glass walls, even with a cooling water jacket surrounding the chimney. Rupprecht and Phillips (12) also described the use of a dual-flame burner for the analysis of sulfur in hydrocarbon streams. In this case, an initial oxygen-rich flame was used @21978 American Chemical Society
340
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
FLAME TIP 2
FLAME 1
FLAME TIP 1
AIR 2
H,
-AIR
1
+
QC EFFLUENT
Flgure 1, Schematic diagram of the dual-flame burner
t o oxidize incoming hydrocarbons and sulfur compounds. S2 emission was then generated and detected in a second H2-rich flame located downstream of the oxidizer flame. A major disadvantage of this burner was the requirement of an elaborate ignition sequence in order to achieve successful ignition of both t h e oxidizer and detector flames. Like the van der Smissen burner, this burner was also constructed of glass, and alkali emissions were a source of interfering light in the flames. In addition, the region between t h e two flames constituted a sizeable dilution volume such that the S2 emission in the H2-rich flame exhibited very poor sensitivity. The dual-flame burner employed in the present work differs from previous burners by being mostly an all metallic structure. Consequently, interfering emissions from alkali impurities are largely eliminated. I n addition, the volume between the two flames is kept small so that excellent S and P sensitivity is still maintained when emissions in the second flame are viewed. I n the present burner, a simple on-off control of a n air source allows an easy conversion to a single flame mode, so t h a t advantages and disadvantages of singlevs. dual-flame operation can be readily evaluated. A companion paper (19) describes such a comparison of single- vs. dual-flame operation, especially with respect to hydrocarbon quenching in the flame. The present paper is concerned mainly with burner design and performance characteristics in t h e dual-flame mode. EXPERIMENTAL Instrumentation and Chemical Reagents. The detector described in this paper is the flame photometric detector manufactured by Varian (Palo Alto, Calif.). All chromatographic data were obtained using a Varian Model 3700 gas chromatograph. Chromatographic columns and operating conditions are indicated where used. Solvents used to prepare test and calibration samples were nanograde 2,2,4-trimethylpentane obtained from Mallinckrodt (St. Louis, Mo.) and 99% pure grade n-hexane obtained from Phillips (Bartlesville, Okla.). Thiophosphate pesticides were obtained from Polyscience (Niles, Ill.) as 1% solutions by weight in benzene. These thiophosphate samples were further adjusted to desired concentrations by dilutions in 2,2,4-trimethylpentane. All other sample mixtures were prepared by dilutions in either 2,2,4-trimethylpente or n-hexane using pure chemicals obtained from Chem Service (West Chester, Pa.). B u r n e r Design. A schematic illustration of the dual-flame burner is shown in Figure 1. A first flame is achieved by combining a source of air and hydrogen at a first flame tip such that the amount of H2 is in excess of that required for complete combustion of the air. The excess H2 is then combined with a second air source at another flame tip located downstream of the first flame. The two flames are ignited by igniting first the top flame, and then a mild flashback process ignites the lower flame.
Similarly, when solvent peaks come through the detector, the lower flame may be momentarily extinguished, but the top flame stays lit because of an abundant supply of air located at the outer periphery of the top flame. Flashback then causes the lower flame to automatically reignite after the solvent passes. The purpose of the first flame is to decompose complex chemical compounds coming from the GC and convert them into combustion products consisting of relatively simple molecules. In the case of sulfur and phosphorus compounds, a region of light emission characteristic of S2 and HPO molecules occurs in the volume between the first and second flames, and this emission indicates that sample decomposition is successfully achieved in the first flame. The purpose of the second flame is to regenerate light emission from Sz and HPO molecules by burning again the combustion products from flame 1. Although both flames produce light, only that light produced in flame 2 is viewed photometrically. This is because the optical emission environment in flame 2 is much less disturbed by incoming samples from the GC than is the light produced at the first flame. The flame tips and tower illustrated in Figure 1 were constructed of type 316 stainless steel, whereas the optical viewing window was Pyrex glass. Compared to high temperature glasses, stainless steel has a higher thermal conductivity and a negligible alkali impurity content (20). As a result, heat generated by the flames could be adequately dissipated by conduction such that neither flame 1 nor flame 2 exhibited any background flame colorations caused by materials outgassed from hot burner components. Flame tips 1 and 2 could probably also be constructed of high purity alumina ceramic, since that material has a thermal conductivity comparable to that of stainless steel. Some typical dimension of the burner components shown in Figure 1 were as follows: flame tip 1,i.d. = 1.5 mm, 0.d. = 3.2 mm; flame tip 2 , i.d. = 3.6 mm, 0.d. = 4.6 mm at the top to 11 mm at the bottom; tower id. = 13 mm; and separation between ends of flame tip 1 and flame tip 2 = 17 mm. All gas flows to both flames were completely controlled. On the Varian 3700 gas chromatograph, the control valves for H2and air supplies provide the dual capacity of flow regulation and flow shutoff. Once a gas flow was adjusted to its desired rate, that control valve could be turned off and on again without disturbing the flow rate setting. For optimum sulfur and phosphorus sensitivity and selectivity, the best flow rates were determined to be as follows: GC carrier gas, He = 30 mL/min; Hz = 140 mL/min; air 1 = 80 mL/min; and air 2 = 170 mL/min. The sum of the two air supplies was sufficient to consume approximately 7 0 4 of the H2 supplied to the burner. Hence, flame 2 as well as flame 1 was normally operated in a H2-rich environment. In a H2-air flame, the presence of large concentrations of H, 0, and OH flame radicals play an important role in both the sample decomposition chemistry and in the chemical reactions that result in light emission ( I , 3). An important consideration in a diffusion flame is the fact that the H2 and air do not mix instantaneously. Consequently, such a flame contains spatial regions that are locally rich in H radicals, and other spatial regions that are locally rich in 0 and OH radicals. Sample decomposition chemistry, the type of combustion products formed, and the spatial distribution of light produced can all have a strong dependence on whether the sample is introduced to the diffusion flame via the H2 gas stream or via the air gas stream. In the present burner, samples eluting from the GC enter flame 1mixed with air. Consequently, these samples initially are exposed to a flame region with an abundance of 0 and OH radicals, and shortly thereafter they encounter a flame region with a large excess of H atoms. The region between flame tip 1 and flame tip 2 can be described qualitatively as containing a mix of combustion products, unburnt H2, inert gases, and probably some residual H atoms. The presence of highly reactive H atoms in this interflame volume probably helps minimize sample losses due to absorptive interactions with the metal walls. The environment of flame 2 can be described as consisting of a central core region rich in H atoms, surrounded by an outer periphery region with an abundance of 0 and OH radicals. Since the desired S2 and HPO emissions are well-known to occur in H-rich flame regions, the flame tip geometry and gas flow rates were chosen to maximize the spatial extent of the inner core of flame 2, while minimizing the spatial extent of the outer periphery. In flame 2, both the
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978 EXIT TUBE
-
-WINDOW
1 - 1
I/"
'
'
'
I
'
"
'1
341
nrn I
IGNITOR-
F U M E 2---
AIR 2-
H2
-
U\\
PHOTOMULTIPLIER
-/ 1
-
1
Figure 2. Schematic diagram of the FPD burner and photometric viewing components
S2and HPO emissions occur in a well-defined inner conical region extending to approximately 7 mm above the top of flame tip 2. Interfering hydrocarbon emissions occur primarily in a halo-type region located at the outer periphery and in near proximity of the top of flame tip 2. In addition to dual-flame operation, the burner illustrated in Figure 1 can also be operated in a single-flame mode. This single-flame operation is achieved by simply turning off the air 1supply. Also, if so desired, an oxygen-rich emission environment can be achieved at flame 2 by decreasing the H2 flow and increasing the air 2 flow so that there is an excess of 02. Photometric Viewing. Figure 2 shows a schematic illustration of the burner and the photometric viewing components contained in the Varian FPD. The flame tips are surrounded by a cylindrical tower that is pneumatically sealed such that all gas flows to the burner can be measured at the top exit port of the tower. Light produced in flame 2 of the burner is viewed through the window in the side of the tower. A pair of optical lenses focuses an image of the flame through an optical filter and onto the photocathode of a photomultiplier tube. The lens arrangement shown in Figure 2 collects radiation from the flame at f/2.5 and provides a magnification near unity. The photomultiplier used in this detector is a side-viewing Hamamatsu R906 or an RCA 4552. The FPD in this work used an unconventional choice of filter for sulfur detection, and that bears some further explanation. To aid in this discussion, there are shown in Figure 3 the spectral distributions of light emitted from the S2molecule, from the HPO molecule, and from the CH and C2 radicals of a typical hydrocarbon. At the bottom of Figure 3, there is also shown the spectral response of the photomultiplier used, as well as the transmissions of the sulfur and phosphorus filters. Based upon high resolution spectral tracings of the S2 molecular emission bands, most prior FPD's have used a 394-nm interference filter for S detection. Typically such interference filters have spectral bandwidths of approximately 8-10 nm. The second spectral tracing in Figure 3 shows that at a resolution of 8 nm, the discrete emission bands of the Sz molecule merge and largely disappear. Hence, from the standpoint of maximizing the S signal, it really does not matter much whether the filter is centered precisely at 394 nm or at 380 or 400 nm. Also, the flame noise characteristics of the present detector were such that the signal-to-noise ratio remained the same whether the S emission was viewed at 360 nm or at 394 nm. Hence, the ultimate choice of S filter was based largely on the requirement for minimizing interferences from hydrocarbon and phosphorus emissions, and the spectral tracings of these interferences indicated that the S emission should best be viewed at wavelengths of 380 nm or less. The actual S filter used was a broad-bandpass, colored glass filter with peak transmission at 365 nm. For phosphorus detection, the filter used was a narrowbandpass interference filter peaked at 530 nm.
RESULTS AND DISCUSSION Effects of Gas Flow Rates. For a dual-flame burner with t h e dimensions described earlier, the sustained operation of both flames requires the H2 and air-flow rates t o be main-
,-P
FILTER
1
320
360
400
440
480
620
660
600
WAVELENGTH lnm]
Figure 3. Relative spectral distributions of light emitted from Sz and HPO molecules, and CH and C2 radicals of a typical hydrocarbon, S2 emission is shown at both high (1 nm) and low (8 nm) spectral resolution. Spectral response of photomultiplier tube and transmissions of S and P filters are indicated in bottom plot
tained within the following ranges: Hz, between 70 and 150 mL/min; air 1, between 70 and 120 mL/min; air 2, greater than 160 mL/min. In addition, the burner can accommodate GC carrier gas flows ranging u p to 100 mL/min. Either H e or Nz can be used as the GC carrier gas. However, H e is the preferred gas because it results in a slightly lower flame background and higher sample response than does N2. As expected, the F P D performance is strongly dependent on the relative magnitudes of H2 and air flows supplied to the burner. The set of flow rates described earlier in this paper were chosen to provide the best combination of sensitivity, selectivity, and signal-to-noise ratios while maintaining a positive combustion environment for both flames. A very convenient feature of the present burner is that the optimum sulfur and phosphorus responses are achieved with exactly the same set of gas flows. Consequently, a change from S mode to P mode of detection requires only a change in optical filter. Illustrated in Figures 4-6 are the relative variations of sulfur ( I s ) ,phosphorus ( I p ) , hydrocarbon (IC),and flame background ( I B )emission intensities as a function of variations in the H2, air 1, and air 2 flow rates. In generating the data in these figures, hexanethiol was used as the S compound, tributylphosphate was used as the P compound, and n-pentadecane was used as the hydrocarbon. Other sulfur, phosphorus, and hydrocarbon compounds studied exhibit similar dependencies on flow. All emission intensities have been normalized to unity response a t the normal operating flow rates. It is clear from these plots t h a t both sensitivity and selectivity of the FPD are strongly dependent on the H2 and air flows. In the present burner, the flame environment t h a t emits the observed light contains sizeable spatial gradients in chemical species and flame temperature along t h e line of viewing. However, in the core of flame 2, the sulfur and phosphorus species can be expected to be very well mixed with the unburnt H2 and other combustion products emanating from flame 1. As a result, it is possible to correlate the H2 dependencies of Is and I p with flame reaction mechanisms that have been previously proposed ( I , 3).
342
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978 NORMAL OPERATING FLOW RATE
1 l
PHOSPHORUS : ; M O' D E
0
d
'
;
06
0
--
l
)
I
)
l
I
I
I I
1
/
1
1
--
i
s--_
l
10 Y
B::
!
I
I
-
I~ :
-
I l
-
+
,
I
I
I
l
l
I
I
I
1
,
I
-
-
I
.
i
, - I " ' I
0
SO
BO
70
100
110
120
130
140
160
160
170
H1 FLOW RATE Iml/MINI
Figure 4. Variations in emission intensities of sulfur Is, phosphorus Ip, hydrocarbon I,, and flame background I , as a function of variations in H, flow. Other flows: He, 30 mL/min; air 1, 80 mL/min; air 2, 170 mL/min. All data normalized to unity at normal operating flow rate of 140 mL/min
190 200 210 AIR C2 FLOW RATE (ml/MIN)
180
erated by the chemiluminescent reaction, S2+ H
Is 10
230
Figure 6. Variations in emission intensities as a function of variations in air 2 flow. Other flows: He, 30 mL/min; HP, 140 mL/min; air 1, 80 mL/min. All data normalized to unity at normal operating flow rate of 170 mL/min
+ H2. Hence,
N O R M A L OPERATING FLOW RATE
220
a
s,*
a
[S,I[Hl'
+H
-
S2* (2)
Combinding relations 1 and 2 gives I s = [Hl4[SI,Z/[H2l3
(3)
Assuming small variations in H2 do not appreciably change flame temperature, and further assuming that equilibrium prevails in the core of flame 2, then the known (21) flame reaction, 3H2 + O2 = 2H + 2H20, yields the relation [HI 0: [H2]3/2.Substituting into relation 3 gives the prediction that Y
0:
SULFUR M O D E
z 0 6
:+ 70
IC OR Ig
where M is an inert third body; or 80
90
100
110
120
PO
AIR ill FLOW RATE ImWMINI
Figure 5. Variations in emission intensities as a function of Variations in air 1 flow. Other flows: He, 30 mL/min; H, 140 mL/min; air 2, 170 mL/min. All data normalized to unity at normal operating flow rate of 80 rnL/min
In the case of sulfur, it is generally believed that incoming sulfur-bearing compounds are decomposed irreversibly to yield H2S as a primary combustion product. In the present burner, this decomposition occurs largely in flame 1. The H2S then further reacts with H atoms in the hydrogen-rich flame to yield S2 molecules. Sugden et al. (3) have derived the following expression for the S2 concentration:
[%I
a
w2i3.
In the case of phosphorus, incoming phosphorus-bearing compounds are believed to be decomposed to yield P O as a primary combustion product. Again, in the present burner this decomposition occurs in flame 1. Then according to Gilbert ( I ) , either of the following two reactions could generate the HPO' emission: PO + H + M - + HPO* + M (4)
[slo2[HlZ/[H213
(1)
where [SIo represents the concentration of total added (monatomic) sulfur, [HI is the concentration of hydrogen atoms, and [H2]is the concentration of hydrogen molecules. Then according to Gilbert ( I ) , the Sz emission can be gen-
+
OH
+ H, +. HPO* + H,O
(5)
Again assuming equilibrium prevails in the core of flame 2, the flame reaction, H2 O2 = OH + OH then yields the relation [OH] 0: [H2]'/*. Hence, both reactions 4 and 5 predict that I p 0: [H2I3''. In Figure 7 there are plotted a graph of Is1I3vs. H 2 and a graph of vs. H2. Within the accuracy of the data, both graphs exhibit a straight line dependence a t H 2 flows above 105 mL/min. Although these power law fits of the data are not necessarily unique, the graphs in Figure 7 do indicate that the observed variations in S2and HPO emissions are qualitatively consistent with predictions based upon the probable reaction mechanisms occurring in the flames. Sample Response. Figure 8 shows the F P D response in both the phosphorus and sulfur modes. Plotted on logarithmic scales are the sample peak height response in nA vs. the heteroatom flow rate in ng P / s or ng S/s. Data are shown for three different substances. One substance contains only
+
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978 1 2
I
1
I
I
I
I
AI
10
09 08 vi
Bg
0 7
zc
OB
0.4
I]
0.3
1
51
i
I I
2
I
I
I
I
100
110
120
130
0
80
90
160
140
H, FLOW Iml M I N I
Flgure 7. Graphs of Ip2'3 and vs. H2 flow. Data normalized to unity at 140 mL/min
',I i
l
t
0
10'
10'
-
-
1
;
1
i
I
1
l
i
I&
1
4'
noise. At high sample concentrations, the upper limits of sample response correspond to the onset of self-absorption effects in the emitting flame. At these high sample amounts, the concentration of ground state S2 and HPO molecules becomes sufficient to cause some reabsorption of light emitted from excited S2* and HPO' molecules. The data shown in Figure 8 correspond to sample compounds having different chromatographic retention times. In comparing peak height responses of these different compounds, it is noteworthy to point out that the relevant parameter is heteroatom flow rate in ng P/s or ng S / s . In the case of phosphorus compounds, the P flow rate was determined by dividing the P weight in the GC sample peak by the peak width at half height. In the case of sulfur, the peak amplitude varies according to a square-law dependence, so the peak width at one-fourth the maximum peak height was used in the calculation of S atom flow rate. For phosphorus, the F P D sensitivity can be defined as follows: S, = Peak Height/P Atom Flow Rate For the data shown in Figure 8, S p = 20 nA/(ng P/s). Similarly, in the case of sulfur, the F P D sensitivity can be defined as follows:
TRIBUTYLPHOSPHATE
S, = Peak Height/(S Atom Flow Rate)Z
METHYL PARATHION
X HEXANETHIOL
I
I
1
i 10-5
343
-
For the data of Figure 8, SS = 2.0 nA/(ng S/s)2. Both S p and
SScan be varied by changing the gain of the photomultiplier tube through a change in its power supply voltage. However, the flame noise also changes accordingly, such that the ratio of sensitivity to noise stays approximately constant. A better indicator of FPD response is, therefore, the detectivity which is defined as follows: D, = 2
t
i I
1 i
X
noise/S,
and
D S= J 2
X
noise/Ss
I
10..
10-1
lo-'
10.'
1
10
10'
10'
HETEROATOM FLOW Ing P / s w or ng S seci
Figure 8. Plot of peak height response for P and S modes of detection. GC conditions: column, 200 cm X 6 mm 0.d. X 2 mm i.d., Pyrex, 5% OV-101 on 100/120 Chrom W; carrier flow rate, 25 mL/min He; detector temperature, 230 OC; injector temperature, 230 OC; column temperature, 220 OC for tributylphosphate, 230 OC for methyl parathion, and 110 OC for hexanethiol
phosphorus, one contains both phosphorus and sulfur, and one contains only sulfur. In the phosphorus mode, the data for tributylphosphate and methyl parathion exhibit exactly the same sensitivity except a t the very highest sample amounts. The P response has a slope of 1.0 on this logarithmic plot, meaning the P response is linear over approximately a range of lo5 in sample amount. In the sulfur mode, the data for hexanethiol and methyl parathion again exhibit exactly the same sensitivity except a t the very highest sample amounts. The S response has a slope of 2.0 on this logarithmic plot, meaning the S response of both sulfur samples follows a pure square-law dependence on sulfur amount. The sulfur response extends over a range of lo3 in sample amount. Sulfur compounds that have been analyzed and found to have a pure square-law dependence include the following: carbon disulfide, thiophene, hexanethiol, dodecanethiol, hydrogen sulfide, sulfur dioxide, sulfur hexafluoride, methyl mercaptan, disyston, methyl parathion, malathion, methyl trithion, and ethion. At low sample concentrations, the noise level indicated in Figure 8 is due to flame noise, and that is typically an order of magnitude greater than photomultiplier tube dark current
where noise is measured in nA, S p in nA/(ng P / s ) , D p in (ng P / s ) , Ss in nA/(ng S/s)', and D S in (ng S/s). For the data in Figure 8, Dp = 5 X
ng Pis
and
Ds = 5 x
ng S/s
The selectivity of the F P D is illustrated by the chromatograms shown in Figure 9. The sample analyzed contained 4000 ng of a C15 hydrocarbon, 20 ng of a sulfur-containing compound (dodecanethiol), 20 ng of a phosphorus-containing compound (tributylphosphate), and 20 ng of a compound containing both phosphorus and sulfur (methyl parathion). In the S mode, the chromatogram reveals the two S compounds along with a small peak for the C15, but there is no trace of the P compound. In the P mode, the chromatogram reveals the two P compounds with no trace of the S compound or the C15. F P D selectivity ratios can be determined by ratioing the phosphorus (Sp), sulfur (Ss),and hydrocarbon (SC) sensitivities with respect to each other. In the phosphorus mode, the Sp/Sc selectivity is greater than 5 X lo5 g Cig P or greater than lo6 atoms C/atoms P. This means that it takes more than one million C atoms to produce the same light signal as one P atom. The Sp/Ss selectivity is variable depending on the amount of interfering sulfur. This is because the S interference depends quadratically on S amount while the P signal varies only linearly with P amount. Hence, a t very low levels of sulfur, the Sp/Ssselectivity is 5 x lo4 g S/g P while at high
344
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
-60
fl1 " - H E X A N E
DODECANETHIOL 0 6 2 ng s / m c
S - MODE 16 x 10-'OA
4000 ng PENTADECANE
/,METHYL
-500
20 "9 PARATHION
ppb
n-HEXANETHIOL
0.24 ng S h e c
20 "I
P - MODE 32 I 10-SA
TRl0UTVLPHOSPHATE 0 47 ng ? / l a c
il w 0
1
2
3
4
6
MINUTES
Figure 9. Chromatograms showing analysis of a test sample containing pentadecane, dodecanethiol, tributylphosphate, and methyl parathion. Same sample analyzed in both the S and P modes of detection. GC conditions: column and carrier flow, same as Figure 8; detector temperature, 220 'C; injector temperature, 220 OC; column temperature, 210 O C
sulfur levels, the S p / & selectivity drops to about 50 g S / g P. In the sulfur mode, the & / S C selectivity varies from lo3 g C/g S a t low sulfur content to better than lo6 g C/g S a t high sulfur content. Again the variable selectivity is due to the fact that the S signal varies as the square of sulfur content while the C interference varies only linearly with C content. The Ss/Sp selectivity varies from 10 g P / g S at low sulfur content to greater than lo4 g P / g S at high sulfur content. Figure 10 provides an example of the capability of the present burner for handling large sample volume injections into the GC. The chromatogram shown represents the analysis of a 60-pL injection of n-hexane containing a 500-ppb trace of n-hexanethiol. The flames satisfactorily survive this large hexane solvent peak without being permanently extinguished. In addition, excellent sensitivity to hexanethiol is obtained even though this peak is not completely resolved from the tail of the hexane solvent peak. In Figure 10, the hexane solvent response consists of an initial off-scale peak followed by a smaller secondary peak. Such double-peaked solvent responses often occur in those instances where the flow rate of solvent into the burner is sufficient to momentarily extinguish the first flame. The secondary peak in the solvent response coincides with the reignition of the lower flame. During the time that the lower flame is out, the FPD response is of questionable quantitative value since the burner is no longer functioning in a dual-flame mode. However, it should be recognized that it is the rate of flow of solvent molecules into the burner that determines whether the first flame is extinguished rather than the total amount of solvent. For any given volume of solvent injected into the GC, the rate of flow of solvent into the burner will depend on GC operating parameters such as injector temperature, column temperature, and carrier gas flow rate through the column. Consequently, it is always possible to find a set of GC operating conditions which allows both flames
-
Flgure 10. Chromatogram illustrating analysis of a 60-wL injection of n-hexane containing a 500-ppb trace of n-hexanethiol. Gc conditions: column and carrier flow, same as Figure 8; detector temperature, 150 OC;
injector temperature, 130 O C ; column temperature, 120 O C
in the burner to continously function irrespective of the amount of solvent injected. In most cases, momentary flameout of the lower flame is a phenomenon that occurs only for solvent peaks a t very early retention times in the chromatogram.
ACKNOWLEDGMENT The authors thank Vincent Hornung for the design and construction of the electronics module used to operate the FPD.
LITERATURE CITED (1) P. T. Gilbert, "Nonmetals", in "Analytical Flame Spectroscopy", R. Mavrodineanu, Ed., Macmillan and Co., Ltd., London, 1970, Chap. 5. (2) S. S. Brody and J. E. Chaney. J . Gas Chromatogr., 4, 42 (1966). (3) T. M. Sugden, E. M. Bulewicz,and A. Demerdache. in "Chemical Reactions in the Lower and Upper Atmosphere", Proceedings of an International SymDosium, San Francisco, Calif., 1961, Interscience Publishers, New York, N.Y., 1961, pp 89-108. (4) S. 0. Farwell and R. A. Rasmussen, J . Chromatcgr. Sci., 14, 224 (1976). 15) C. A. Buraett and L. E. Green. J . Chromatoor. Sci.. 12. 356 11974). . , (6) A. 1. MizGy, J . Chromatogr. 'Sci., 8, 151 (7970). (7) R. K. Stevens, J. D. Mulik, A. E. O'Keefe, and K. S. Krost, Anal. Chem., 43, 827 (1971). (8) T. Sugiyama. Y. Suzuki, and T. Takeuchi, J. Chromatogr. Sci., 11. 639 (1973). (9) T. Sugiyama, Y. Suzuki, and T. Takeuchi, J . Chromatcgr.,77, 309 (1973). (10) J. G. Eckhardt, M. B. Denton, and J. L. Moyers. J . Chromatogr. Sci., 13, 133 (1975). (1 1) C. H. Burnett, D.F. Adams, and S. 0. Farweli, J . Chromatogr. Sci., 15, 230 (1977). (12) W. E. Rupprecht and T. R. Phillips, Anal. Chim. Acta, 47, 439 (1969). (13) S. G. Perry and F. W. G. Carter, The Chromatography of Sulphur Compounds". in "Gas Chromatography 1970", R. Stock, Ed., Proceedings of the 8th Internatonel Symposium on Gas Chromatography,Dublin, 1970, Institute of Petroleum, London, 1971, p 381. (14) T. Sugiyama, Y. Suzuki, andT. Takeuchi, J . Chromatcgr., 80, 61 (1973). (15) C. D. Pearson and W. J. Hines, Anal. Chem., 49, 123 (1977). (16) D. A. Clay, C. H. Rogers. and R. H. Jungers, Anal. Chem., 49, 126 (1977). (17) C. E. van der Smissen, U.S. Patent 3,213,747, Oct. 26, 1965. (18) A. Smithells and H. Ingle. Trans. Chem. Soc., 61, 204 (1892). (19) P. L. Patterson, Anal. Chem., 50, following paper in this issue. (20) F. Rosebury, "Handbook of Electron Tube and Vacuum Techniques", Addison-Wesley, Reading, Mass., 1965. (21) K. Schofield and H. P. Broida, "Flame Kinetic Studies," in "Methods of Experimental Physics", L. Marton. Ed., Vol. 7, Pari B, Academic Press, New York, N.Y., 1968.
RECEIVED for review August 22, 1977. Accepted November 4, 1977.
