Anal. Chem. 1995,67,167-173
Combined Pulsed Flame Photometric Ionization Detector Nitzan Tzanani and Aviv Amirav* School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel
The pulsed flame photometric ionization detector (PFPID) is designed for the simultaneous selective detection of molecules containing carbon, sulfur, phosphorus, or nitrogen atoms using gas chromatography (GC). The PFPID is based on a pulsed flame which propagates from the igniter through the ionization and photometric chambers consecutively. The flame is then self-terminatedand reignited after a few hundred milliseconds in a pulsed periodic fashion. Since the pulsed flame photometer works with a hydrogen-rich atmosphere, the GC eluting molecules are predominantly pyrolyzed and only slightly combusted in the initial pulsed flame photometric chamber. These products are further combusted in the second pulsed flame ionization chamber, where the combustible gas mixture is separately optimized. The resulting electrondions are collected by an electrically isolated collector positively biased to 10V. The addition of a pulsed flame ionization detector (PFID) to the pulsed flame photometric detector (PFPD) requires addition of negligible hardware (an isolated charge collector). It also requires the addition of a current-to-voltageconverter and a doublegated instead of a single-gated amplifier for simultaneous PFPD-PFID work. The PFPID can be operated separately as either a pulsed FPD or a pulsed FID or simultaneously as both at the same Hdair gas flow rates. The simultaneous selective analysis of sulfur and organic compounds in gasoline, kerosene, and diesel fuel is demonstrated and discussed in terms of the various operational parameters. The performance of the PFPID is analyzed and compared to that of other sulfur and carbon simultaneous selective detection schemes. Pulsed flame is a new concept for the operation of flame-based atomic and molecular detectors.lS2 It was successfully implemented and studied in detail in the pulsed flame photometric detector (PFPD), which served for the selective detection of sulfur, phosphorus, and nitrogen using gas ~hromatography.~The use of pulsed flame for FPD considerably improved the detection sensitivity and combined with a drastic improvement of the detection selectivity through time separation of the carbon from heteroatom flame emission. It also reduced the detector gas consumption and allowed the selective detection of other elements, such as nitrogen. nePFPD is also by having a carbon-selective channel which can be used for the simultaneous detection of sulfur (1) Amirav, A Pulsed Flame Detector Method and Apparatus. Israeli Patent 95617, 1992; US. Patent 5153673, 1992. (2) Atar, E.; Cheskis, S.; Amirav, A.Anal. Chem. 1991,63, 2061-2064. (3) Cheskis, S.; Atar, E.; Amirav, A. Anal. Chem. 1993,65, 539-555. 0 1994 American Chemical Society 0003-2700/95/0367-0167$9.O0/0
and carbon compounds eluting from a gas ~hromatograph.~ This carbon channel is based on the mutual emission time separation of the carbon and sulfur related flame emission. However, the PFPD carbon channel is less sensitive than the conventional flame ionization detector (FID). In addition, it can be used together with only the sulfur mode and not with the phosphorus- and/or nitrogen-selective detection modes (due to insufficient time separation of these elements). The PFPD carbon channel is also limited in its linear dynamic range and its power to discriminate against sulfur compounds. The time separation of undesirable sulfur emission from carbon emission is incomplete, and about 0.3%of the sulfur emission is still observed in the carbon channel, thereby limiting its selectivity against sulfur. In this paper we shall describe the coupling of a pulsed FPD with a pulsed FID to form a new detector called the “combined pulsed flame photometric ionization detector” (FFPID). In our design we have adhered to the following constraints: (a) the addition of pulsed FID (PFID) should not interfere with the PFPD performance and structure; (b) the PFID should require the addition of a minimum hardware to the existing PFPD structure; and (c) the PFPID should be mounted on a single GC detector oven and use the same gas supply as the PFPD. If these constraints are not met, one could always split the GC stream into a secondary FID at the price of reduced PFPD sensitivity and with the complication of needing additional space, a detector oven, pneomatics, and detector hardware. We note that the simultaneous FID-FPD ~ p e r a t i o n ~or- ~the combination of sulfur chemiluminescence selective detection and FID8 is a recognized desirable operational feature. Moreover, it can be generally claimed that the addition of any selective detector to FID is a major advantage, and several attempts along this line have been made, including the recent coupling of a nitrogen phosphorus detector (NPD) to a FIDSg Flame-based detectors such as the FID or FPD are usually destructive. In addition, the FID requires air-rich flame conditions, while the FPD requires hydrogen-rich flame conditions. Thus, under the hydrogen-rich conditions normally encountered in FPD, the flame ionization yield is reduced by 1 or 2 orders of magnitude*JO and the uniform FID response to most organic compounds seems questionable.
s,Anal,Chem, 1966,38,734-742, (5) Sevcik, J. Chromatographia 1971,4, 195-197. (6) Aue, W. A; Hill, H. N., Jr. Anal. Chem. 1973,45, 729-732. (7) CaStellO, G.; D’amato, G.; Nicchia, M., 1 . Chromatog. 1990, 521, 99-107. (8) Shearer, R L;ONeal, D. L;Rios, R; Baker, M. D.]. Chromufogr.Sn’. 1990, 28, 24-28. (9) Patterson, P. L. Simultaneous Detection of Oxygenates and Hydrocarbons in Gasolines with a Unique Tandem TID/FID Detector. Proceedings of PittCon 93, Atlanta, GA, 1993; Paper P55. (10)Gill, J. M.; Hartmann, C. H. 1.Gas Chromatogr. 1967,5,605-611. (4)
Analytical Chemistry, Vol. 67, No. 7, January
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3 Figure I. Schematic diagram of the pulsed flame photometric ionization detector. The detector body (1) is mounted on a GC adapter (2). The combustor holder (4) is placed on the FID base (3) in the normal way. The column is inserted through a flame arrestor nozzle ferrule (5) into the straight quartz combustor tube (6). The pulsed flame is ignited by the igniter (7) and propagates to the combustor (6). The emitted light is transferred through a window (8), a light guide quartz rod (9),and a colored glass filter (10) and is detected by a photomultiplier (1 1). The charge carriers are collected by the +10 V biased PFID collector (12). The PFID gases are enriched with air through the screw valve (13). The heater (not shown) and igniter are connected to their power supplies via a feedthrough (14), and the sample material can also be fed through a special opening (15) which is normally plugged.
Pulsed flame, unlike continuous flames, works with a premixed hydrogen-air gas mixture. In addition, under the PFPD hydrogenrich conditions, the sample molecules are mostly pyrolyzed and are not fully combusted. This suggests that the hydrogen-rich pulsed flame products can be further mixed after cooling with a fresh combustible gas mixture which is richer in air, and a second pulsed flame in the upper second chamber can serve for the pulsed FID of these molecules with a one pulse time delay. This conjectured basic concept of prior partial pyrolysis, although not proven, serves as the basis for our understanding and evaluation of the operation of the PFPID as described below. PFPID STRUCTURE AND OPERATION In Figure 1, the PFPID is schematically described. Actually, this is the same pulsed FPD described in rep, with some minor improvements and with the addition of the ion collector (12) for the PFID. The main structure (1) of the PFPID is mounted on a GC adapter base (2) connected to the GC-FID mount (3). In figure 1,the FID mount is of a Varian 3600 GC. Hydrogen is fed from the combustor holder (4),which is connected to the GC in a similar manner to the FID flame tip. The combustor holder is longer than the FID flame tip and is not electrically biased. It has several holes and two external air flow restrictors to induce proper mixing between the air flowing along the outside of the combustor holder and the hydrogen flowing inside the combustor holder (can be three flow restrictors for better mixing). The GC column is inserted through a nozzled combustor ferrule (5) about 168 Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
1-2 mm inside the straight quartz tube combustor (6). The main body is made of 316 stainless steel or Hastalloy C-276 (one detector) and the combustor holder and ferrule are made of Hastalloy C-276. We have found this new arrangement of the PFPD combustion cell to be better than the previous allquartz combustor, which is more difficult to prepare and clean. The pulsed flame is ignited by the igniter (7) (Kanthal AF 0.25 mm wire). It propagates through the ionization chamber, where an electrically insulated charge collector (12) (316 stainless steel) is positioned. This pulsed current collector is mounted on a BNC connector and is biased at +10 V (or -10 V). It is also vibrationally dampened by a tube of Vespel up to the ionization chamber. The pulsed flame further propagates to the photometric chamber and is self-terminated on the nozzled ferrule, which serves as a flame arrestor since the nozzle diameter (0.8 mm) is too small to allow Hz/air flame propagation. As the fresh combustible gas mixture pushes the combusted gases out to the igniter, the pulsed flame is reignited after a few hundred milliseconds in a pulsed periodic fashion. The light emitted from the pulsed flame passes through a sapphirewindow (8), a 5 mm diameter quartz rod (S), which serves as a light guide, and an optical filter (10). It is finally detected by a photomultiplier (11). An auxiliary screw valve (13) controls the flow of air to increase the air concentration in the previously hydrogen-rich mixture in the igniter region for easier ignition and better optimized ionization yield. It is important to note that, after the pulsed flame termination, while the GC column flow and combustible gas mixture flow fills the pulsed flame photometric combustor chamber, a separate “bypass”combustible gas mixture that flows around the flame photometric combustor chamber fills the upper ionization chamber. Thus, the ionization chamber accepts mostly products of the previous pulsed FPD chamber mixed with a fresh bypass combustiblegas mixture and also with additional air from a screw valve (13) at a separately optimized flow rate for the pulsed flame ionization yield. Since the pulsed FPD works with a hydrogen-rich Hz/air mixture, the GC eluting molecules are predominantly pyrolyzed and only slightly combusted. These products are therefore further combusted in the second pulsed flame, and the resulting ions are collected by the charge collector (12). Note that while the pulsed flame actually propagates first through the ionization chamber and then through the photometric chamber, the GC chromatogram of the PFID is delayed by exactly one pulse time interval from that of the PFPD. The PFPID is also heated separately and is connected to the igniter and heater power supplies through feedthrough (14). Sample molecules can also be introduced at the special opening (15), which is normally sealed by a screw. We note that the PFPID is somewhat similar in its operation concept to the doubleflame photometric detector.” This similarity holds provided that this detector would have to be operated with a hydrogen-rich first flame for the FPD and an airenriched second flame for the FID, whose collector would have to be integrated into the system. Both the PFID and PFPD pulsed signals are amplitied by current-to-voltage converters with an amplitication of lo6V/A for the PFPD (homemade) and 106-109 V/A for the PFID (Ithaco Model 1212 current ampliiier). Both signals are further processed either by two separate gated amplifers or by a doublegated (11) Patterson, P. L;Howe, R L.;Abu-Shumays,A Anal. Chem. 1978,50,339. Patterson, P. L. Anal. Chem. 1978,50, 345-348.
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Flgure 2. Time dependence of the chemical ionization signal. (A) Background signal without organic compounds. (B) Ionizationsignal with 5 ng injections each of octane after subtracting the background signal. (C) Pulsedflame background light emission measuredby the photomultiplier on the ionization time scale. The collector bias voltage was +10 V in traces A and B.
amplifier (homemade). When simultaneous double-gated amplification is used, the PFID signal serves to trigger both channels. A dual channel integrator (Spectra Physics Model 4400) provides simultaneous twochannel signal presentation and integration. One channel (either PFPD or PFID) is stored in the integrator memory and automatically plotted after the current chromatogram ends. A Carlo Erba Vega-6000 GC was used in the majority of the experiments, while recent studies were performed on a Hewlett Packard 5890 series I1 GC. OPERATIONAL FEATURES OF THE PFPID
The most important feature of the PFPID is that both the flame emission and ionization signals are pulsed. This exposes and provides new time domain information that can be used to enhance the detection sensitivity and selectivity. While the implications of time domain information for the PFPD were described in detail el~ewhere,~ we shall describe here the pulsed flame ionization data. In Figure 2, we show the pulsed background current signal obtained with the PFID without organic compounds (trace A), as well as the pulsed signal obtained when octane eluted from the GC (trace B). Trace B was obtained after subtraction of the background shown in trace A using a Le Croy 9400 digital oscilloscope signal averager so that it represents the pure carboninduced pulsed current. The background signal of the PFPD (trace C) clearly lags behind the PFID signal due to the finite flame propagation velocity. At this time we are not certain as to the origin of the PFID background signal. The peak background current signal is approximately 3 nA (-4 pC/pulse or 16 PA average current). This is much higher than we expected on the basis of the 2-3 PA dc background current obtained with a
conventionalFID with our H~/airgases at much higher flow rates. In addition, the time dependence of the current signal for octane is somewhat different. This fact strongly supports the idea that the background is not from ever-present organic impurities. One conjecture which we can invoke is that the background structure originates from a shock wave formed by the flame when it arrives at the main body wall and is forced to change its direction of propagation. This shock wave might result in the emission of alkali ions, induce a brief thermionic electron emission, or promote chemical processes such as surface-induced hydrogen recombination which might be responsible for the charged particle production. The octane signal is somewhat similar to the background signal but is also characterized by a time delayed “shoulder”which suggests that electrons or negative ions produced inside the photometric chamber are also collected. In Figure 3, both the octane and background signals are plotted versus the bias voltage on the charge collector. Each point of the octane signal is the average of three GC octane peak heights. Since the pulse flame propagation channel diameter is 2.7 mm and the collector diameter is 1.0 mm, a 10 V biasing voltage creates an electric field of -120 V/cm, which is similar to that used in conventional FID. However, typical voltage in FID is 150200 V, while with the PFPID, 8-10 V is enough for effective collection of charge carriers. We note that since 8-10 V is found in any electronic circuit, the PFPID does not require a special medium voltage power supply. We have found that the background and PFID signals respond differently to the collector bias voltage polarity. The ionization signal is higher, with positive bias voltage (electron and negative ion attraction), while the background is higher, with negative voltage bias. The increased signal with positive bias voltage is attributed by us to a higher efficiency electron attraction from the photometric combustor chamber (in comparison with positive ions), We are unable to provide a logical explanation far the observation of different bias voltage response of the background signal, but as a result, positive collector biasing is preferred for analytical applications. It is also mentioned that positive collector biasing may result in a slight distortion of the chromatographic peak shape due to charging of the electrically insulated quartz combustor. With negative voltage collector biasing, this minor Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
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agreement with experimental results obtained with continuous premixed flame~.’~J~ From Figure 4, it seems that the PFID can work with ions produced in a single hydrogen-rich photometric combustion chamber, and future experiments will attempt to explore this possibility.
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Figure 4. Dependence of the PFID response on the composition of the combustible gas mixture. Stoichiometric ratio composition is defined as 1. Three types of experiments were performed, and the results are normalizedto the total air plus hydrogen gas flow rate. (*) Experiments at the same pulsed flame frequency and different gas composition. (+) Experimentswith gradually increased hydrogen flow rate. (A)Experimentswith gradually increased air flow rate.
peak shape distortion is eliminated, since positive ions are not collected from the photometric combustor chamber. The collector is biased at +10 V from a power supply through a 100 MQ resistor, and the pulsed signal is coupled to the current amplifier through a 0.1 p F capacitor. In Figure 4, we plotted the dependence of the pulsed flame hyrocarbon-induced chemical ionization yield on the combustible gas composition. We define stochiometricratio as 2.5fi2/&,, where f i 2 and fair are the hydrogen and air flow rates, respectively. According to this definition, a value of 1 represents a stochiometric gas mixture composition. In obtaining the data presented in Figure 4, a closed combustor was placed in the photometric chamber, which was therefore inactive (quartz tube with 1 mm upper hole and the column passed 2 mm above this hole). Figure 4 shows the results of the PFID yield of a fresh combustible gas mixture which also included about 5% helium from the GC column. The following conclusions are obtained from Figure 4. (a) The PFID functions with premixed gas mixtures. (b) The PFID can work under a broad range of gas composition. No careful optimization of gas composition is required, and over the whole stable range of pulsed flame operation, the ionization yield is at least 40%of its maximum value and at least 60%of that value in the 1/5-1/1.2 HJair ratio range. (c) The PFID works efficiently with a stochiometic gas mixture. This observation is of considerable importance since it suggests that simple, lowcurrent electrolysis of water without Hz/Oz separation could be used to supply the PFID gases, eliminating the need for the FID gas bottles. However, this issue is irrelevant for the PFPID, as the PFPD requires a hydrogen-rich mixture. (d) The actual maximum pulsed flame ionization yield is obtained at 1.25 stochiometric ratio, which is a slightly hydrogen-rich condition. This finding is in 170 Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
ANALYTICAL PERFORMANCE OF THE PFPID In order to test the PFID, we prepared a methanol solution of seven molecules with diflerent functional groups or heteroatoms. Nonane was chosen as a simple hydrocarbon reference molecule and toluene as an aromatic hydrocarbon. Heteroatomcontaining molecules included pentanol(0) ,tetrahydrothiophene(THT) (S) , pyridine 0 ,chlorotoluene (CI), and dimethylmethylphosphonate (DMMP) (P). We have compared the FID and PFID chromatograms obtained with these compounds using a 25 m narrow bore column (HP1,0.2 mm i.d., 0.33pm film thickness) with a Hewlett Packard 5890 series I1 GC. It was found that, with the exception of DMMP, the pulsed FID relative response is similar to that of the FID. DMMP represents a unique molecule that may form long-lived stable negative ions that are separately collected in a “remote FID”.14 These ions may change the relative response, and it is estimated that they are not effectively collected by the standard FID. It whould also be mentioned that at detector body temperatures below 200-250 “C, pyridine, pentanol, THT, and DMMP exhibited various degrees of response reduction, presumably due to adsorption on the PFID chamber walls. Hastalloy C-276 is superior in this respect to 316 stainless steel. In Figure 5, the relative detection sensitivity is demonstrated by the comparison of chromatograms which show some noise. The same Hewlett Packard GC and column as above were used with the FID range being 0 and the integrator attenuation level 2. The extrapolated minimum detected amount (MDA) with the FID at a signal to root-mean-square noise ratio of 2 is about 0.6 pgC/s, which is close to the conventional values given for FID of 1 pgC/s. The observed signal to noise ratio is about 3.4 times worse in the PFID chromatogram,resulting in a MDA of about 2 pgC/s. While the values of MDA are accurate only within a factor of 2, the relative detection sensitivity is more accurate and the PFID is less sensitive than the FID of HP-GC by a factor of 3.4. We have found that the pulsed flame chemical ionization yield is approximately 15 mC/g of C in a pulsed FID which does not have a photometic chamber and which is optimized for current collection. This is similar to that of FID, though it was in the form of a pulsed current of about 3-4 A/g of C peak current. However, in the PFPID, the apparent chemical ionization yield is only 5 mC/g of C due to igniter dead volume and partial molecular combustion in the PFPD combustor chamber. Moreover, instabilities in the relatively high pulsed flame background signal contribute to a relatively higher noise level. In addition, in both cases the noise originated from electronic amplifier noise, which for the FID is about 1.0 x A While generally pulsed current amplification should provide lower amplitier noise by a factor of the square root of the duty factor, we failed to achieve any such improvement with our current amplifier using a 2 kHz bandwidth, and, as shown, our MDA is worse. The major reasons for this are the existence of a background signal as well as several (12) BoCek, P.; Jan&, J. Chromatogr. Rev. 1971, 15, 111-150. (13) Calcote, H. F.; Kurzius, S. C.; Miller, W. J. Proceedings of the 10th International SymPosium on Combustion; The Combustion Institute: F’ittsburgh, PA, 1965; p 605. (14) Patterson, P. L. J. Chromatogr. Sci. 1986, 24, 41-52.
ILMETHANOL 2.PYRIDINE 3.TOLUENE 4. T H T 5 . DMMP 6 . EROMOEENZENE
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Figure 5. Comparison of the minimum detected amount achieved with FID and PFlD when 0.2 pL methanol solutions were injected with a 50:l split ratio. Considering the concentration of 20 ppm (volume) of each of the indicated molecules and their densities, the sample amounts are 60-80 pg/molecule. The PFlD repetition rate was 3.3 Hz.
millivolts of 50 Hz noise and higher frequency “pickup” noise, especially from switch mode power supplies of the GC and from the laboratory environment. A major part of this noise comes through ground loops and we have also identified microphonics noise originating from the GC fan. The pronounced pulsed flame background precluded the use of lo9 V/A amplification due to signal saturation, so 108V/A amplification was used. Future work will be devoted to reducing the pulsed flame background as well as the amplilier noises. Signal processing with 5-25 Hz bandpass filter and ac-dc conversion seems promising. The simultaneous PFID-PFPD operation is demonstrated in Figure 6. The upper trace was obtained by the PFID simultaneously with the middle trace B, presenting the sulfur mode of the PFPD with a BG12 colored glass filter having a band-pass transmission range of 340-460 nm. The lower trace C was achieved with the PFPD, using a GG495 filter in the phosphorus mode simultaneously with a PFID chromatogram which is practically identical to trace A. In Figure 6, the following aspects are demonstrated. (a) The PFPD is a very selective detector. While the THT and DMMP are unobserved in the PFID chromatogram, no hydrocarbon including the solvent is shown in the
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Figure 6. Simultaneous two-channel detection with the combined pulsed flame photometric ionization detector. (A) PFlD chromatogram of the indicatedmolecules. (B) Sulfur-selectivePFPD chromatogram obtained simultaneously with trace A. (C) Phosphorus-selective PFPD chromatogram obtained simultaneously with a PFlD trace practically identical to trace A. Here, 0.2 pL aliquots were injected with a 40:l split ratio into a 15 m narrowbore column. The concentrationswere 10-3 for pyridine, toluene, bromobenzene, and decane, 10 ppm for THT, and 20 ppm for DMMP.
PFPD traces B and C. (b) The PFID and PFPD can work simultaneously as a PFPID, and each detection mode can be separately optimized. It should be mentioned that the PFPD has a carbon channel that can be operated simultaneously with the PFPD sulfur mode due to time separation of SZ*emission from that of CZ*and CH* emission. In Figure 7, we compare the PFID chromatogram A to that obtained by the PFPD in the carbon light emission mode (middle trace B), which was obtained simultaneously with the lower PFPD sulfur chromatogram C. Actually, these three chromatograms could have been obtained simultaneously, but we used only a doublegated amplifier each time. The PFPD carbon mode chromatogram is clearly noisier than that achieved with the PFID (MDA 2 100 pgC/s). In addition, the DMMP peak is also exhibited, and the pyridine peak is relatively amplified. These deviations occur since the HPO* emission is not well time separated from that of the carbon species and nitrogen also emits CN* light. In addition, the GC temperature changes the helium column flow rate, which affects the PFPD pulsed flame background emission. Thus, the PFID is much superior to the carbon mode PFPD and is a truly orthogonal carbon-selective detector that can be used simultaneously with the PFPD in its P and N modes as well. It should also be mentioned that if a consecutive PFPD, in its S, P, or N modes, and a PFPD in carbon mode is desired, then the PFPD carbon mode has the advantage that it Analytical Chemistry, Vol. 67, No. 1, January 7, 1995
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FUEL ANALYSIS WITH THE PFPID
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Figure 7. Comparison of the PFlD and the carbon mode of the PFPD with a methanol solution of the indicated molecules. (A) PFlD chromatogram. (8)PFPD carbon mode obtained simultaneously with the PFPD sulfur-selective detection mode. (C) PFPD sulfur-selective chromatogram. The experimental conditions are as in Figure 6 .
comes free without the addition of any hardware through a simple change of gate time and delay. APPLICATION OF PFPID FOR FUEL ANALYSIS One of the major applications of flame photometry and sulfurselective GC detection through other detection schemes is the analysis of sulfur compounds in gasoline and heavier fuels such as kerosene and diesel fuel. Sulfur compounds are usually odorants,can poison process catalysts, and upon combustion form S02, which is an environmentally damaging compound. While the total sulfur content can be obtained by nonchromatographic means, gas chromatography provides molecular informationwhich can be supplemented and complemented by a FID chromatogram. In Figure 8, the simultaneous sulfur-selective PFPD and PFID chromatograms of gasoline, kerosene, and diesel fuel are portrayed. The samples were taken from an Israeli Paz gas station. The actual amount injected into the 15 m long narrowbore column OB-1,0.25 pm film thickness) was 1 or 2 pg, and a Carlo Erba Vega-6000 GC was used. With this amount, no sulfur response quenching is observed (quenching can be observed by its time domain manifestation3),even with the first eluting methylthiopene, 172 Analytical Chemistry, Vol. 67, No. 7, January 1, 7995
TIME(min)
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Figure 8. Simultaneous PFlD and PFPD GC analysis of Israeli fuel samples. (A, top right) Gasoline 96 octane chromatograms. A 0.1 pL aliquot was injected with a 1OO:l split ratio (1 pg injection). The column was held at 38 "C for 1.5 min, followed by a 20 "Clmin temperature programming to 200 "C. (B, bottom left) Kerosene chromatograms. A 0.2pL aliquot was injectedwith a 1OO:l split ratio (2 pg injection). The column was held at 60 "C for 30 s,followed by a 15 "Clmin temperature programmingto 200 "C. (C, bottom right) Diesel fuel chromatograms. A 0.1 pL aliquot was injected with a 1OO:l split ratio (1 pg injection). The column was hekl at 80 "C for 30 s, followed by a 30 "Clmin temperature programming to 260 "C.
which almost co-elutes with toluene, the highest peak in the PFID chromatogram. The kerosene and diesel fuel samples clearly show that the sulfur compounds (unidentified) are heavier than the average hydrocarbon molecules. The PFPD selectivity is excellent, and no hydrocarbon response is observed, while the PFID sensitivity is sufficient to provide noise-free chromatograms with a 1pg fuel sample injected into the column. DISCUSSION OF AND THE MERIT OF A PFlD ADDED TO THE PFPD We have demonstrated that the PFPD can be coupled to a PFID with minor modifications. Thus, the PFPID can provide a simultaneous FID and S,P-, or N- (or any combinations thereof) selective chromatograms. The basic driving force behind this design is that the PFPID should combine the desirable features of FPD, FID, and NPD in one detector. While the nitrogenselective detection mode is considerably less sensitive (although more universal and quantitative) than NPD, and the PFID is less sensitive than FID, the PFPID provides by itself the essential information which is usually provided by these three major GC detectors combined. The PFPID possesses several advantages in comparison with other dual detector approaches: (a) Dual FPD-FID. The PFPD is more sensitive and selective than the FPD, and at the same time the PFID is more sensitive
I
than the FID operated under the hydrogen-rich conditions of the FPD with a small ring charge collector electrode.4-8 (b) PFPD and FID with a split column flow. While the FID is more sensitive than the PFID, the PFPD sensitivity is halved. However, the most important advantage of the PFPID is that it requires only a single detector base and oven, and the same pneumatics and gas flow valves serve both the PFPD and PFID. A single detector body is used with the minor addition of an electrically biased charge collector and its dedicated pulsed current amplifier. Thus, the PFPID is expected to be smaller in size and cheaper than the combination of the PFPD and a FID. (c) Sulfur chemiluminescencedetector (SCD) and FID. While the flameless SCD is more sensitive in sulfur detection than the PFPD,15 it does not have a simultaneous FID. The flame-based SCD does have a FID, which is less sensitive than the PFID,lG and the flame SCD is about as sensitive as the PFPD sulfur mode. The SCD linear dynamic range is higher, but the PFPD signal to noise ratio is higher due to the sulfur quadratic response. The (15)Shearer, R L. Anal. Chem. 1992,64,2192-2196. (16)Shearer, R L;O’Neil,D. L.; Rios, R; Baker, M. D. J. Chromatogr. Sci. 1990, 28,24-28. (17)Aue, W.A;Hastings,C . R J. Chromatogr. 1973,87,232-235. (18)Sun,X.Y.; Aue, W. A Can./. Chem. 1992, 70, 1129-1142. (19)Aue, W.A;Millier, B.; Sun, X. Y. Can. J. Chem. 1992, 70,1143-1155. (20)Sun, X. Y.;Aue, W. A Can. J. Chem. 1989,67,897-900. (21)Sun, X.Y.; Aue, W. A Mikrochim. Acta 1990,1, 1-6. (22)Aue, W.A; Mdlier, B.; Sun, X Y.Anal. Chem. 1990,62,2453-2457. (23)Sun,X Y.;Aue, W. A J. Chromatogr. 1989,467,75-84. (24)Aue, W.A;Sun, X. Y.; Millier, B. J. Chromatogr. 1992,606, 73-86.
PFPID seems simpler and smaller than the SCD and can also selectively detect other elements. (d) PFPD with dual carbon and sulfur modes. The PFID provides a truly orthogonal and more sensitive carbon detection channel, although it slightly complicates the detector structure and electronics. Fmally, it should also be noted that the PFPID Hz/air gas consumption is lower by about an order of magnitude in comparison with the aforementioned combinations a, b, and c. Future development efforts will be devoted to reducing the PFID amplifier noise to improve its MDA We shall also explore the use of PFPD as a universal heteroatomselective detector in view of the available old17 and r e ~ e n t l ~exploration -~~ results on the use of FPD as a selective organometallic molecule detector. ACKNOWLEDGMENT This research was supported by a grant from the Israel Science Foundation, administered by the Israeli Academy of Science and Humanities, and by a grant from the Israel Ministry of Science and Technology. Received’for review April 21, 1994. Accepted October 3,
1994.B AC940406S e Abstract
published in Advance ACS Abstracts, November 1, 1994.
Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
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