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Pulsed-Flame Photometer: A Novel Gas Chromatography Detector Sergey Cheskis,Eitan Atar, and Aviv Amirav' School of Chemistry, Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
techniques such as the electrolytic conductivity (Hall) deThe pulsed-flame photometer detector (PFPD) k based on a tector? atomic emission detector? and chemiluminescence f ~ e r o u r c e a d ~ g a s f l o w r a t e t hcannotsustain at detectors.6~7The FPD is excellently reviewed by Dreesle+ a contlnuor#flame operation. Thus, the ignited flame prop and by Farwell and Barinaga.9 We have recently described agates back to the comburtlble gas mlxture source and Is a novel approach10.11 for the design of flame detectors that self-terminated after the comburtlblo gas mhtture Is burnt. ~ c o n t h u o u r ~ f l o w c r e a t e s a d d l t l o ~ l ~ I n a p e r k disk based on a pulsed-flame operation instead of the conventional continuous-flamemode. The pulsed-flame detector is fa$hbn. The mainfeaturethat characterizesthe pulwd-flame based on a flame source and combustible gas flow rate that photometer detoctor k the puked nature of the emitted light. cannot sustain a continuous-flame operation. Thus the Tbw domain information k addd to the heteroatom speclflc ignited flame propagates through the detector and selfomkrlon with Um.-d.pondent omkrlon bdng observed. I n terminates after the combustible gas mixture is burnt. thk paper we describe the PFPD and evaluateitsperformance Typicallythe combustiblegases H2 and air are mixed together as a gas chromatograph detector. The main advantages of in a small flame chamber and flow to a continuously heated tho PFW Include Improved detectionwnalthflty for sulfur and Ni/Cr wire igniter. The ignited flame then propagates back phorphorw, much higher rd.ctlvlty against hydrocarbon to the gas source and is self-terminated once all of the mokcukr, lower gas conrunptbn, reduced emisebn quench combustible gas mixture present in the combustion path is Ing, addltknd tomporal Infonnatkn, and the aMllty to detect consumed. The continuousgas flow removesthe combustion adecthfdy other heteroatom8 such as nitrogen or the products and creates additional ignition after one or a few rknultaneouadotocth of sulfur and carbon. Pdntsfor further hundred milliseconds in a periodic fashion (Le., 1-10 Hz). In examination are the emkrlon dependence on the columnflow comparison with the continuous-flamephotometric detector, rate and combwtlon cell surface effects on the puked flame the pulse-flame detector is characterized by (a) reduced flow rate of gases which is necessary to allow flame propagation, omkrlon. The mlnhnum detection levels achieved are 2 X (b) small gas accumulation chamger with a flame-arresting g S/s, 1 X lo-" g P/s, 5 X 10-l2 g N/o and 6 X 10-l1 design, and (c) continuous or pulsed periodic flame ignition g C/R Th.PFPDkuMffWOd by tho Injectionof large source. In addition, the processing electronics are modified of solvent lncludlng aromatic solvents, and both sulfur and to work with pulsed signals. In this paper we describe our phosphorus can be detected and quantlfled at rub-ppb pulsed flame photometer detector and evaluate its perforconcentratkn Ievek. The sulfur concentratlon dependence mance in the sulfur, phosphorus, carbon-, and nitrogenof the PFPD k quadratic, and Its response is Independent of selectivedetection modes using capillarygas chromatography the structure of the sulfurtontalnlngmolecule. The detector (GC). response t h e Is fasl enough to closely resemble the flame In describing our results we try to establish and discuss lonlzation detector for both sulfur and phosphorusmolecules how the pulsed flame operationconsiderablyimproves (orders resolved chromatographicallyon a narrow-boro cdumn. The of magnitudes) the achievable minimum detected amount, opwatlon of the PFPD on either 8, P, N, C rlnglaeloment increases the detection selectivity against hydrocarbon molchanneborS P, S C, P N,or S P Nmuitklement ecules, reduces hydrogen gas consumption,and increases the dotectlon modes k described and presented. information content available for the simultaneous deter-
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mination of multiple heteroatoms in GC analysis.
INTRODUCTION The flame photometer detector (FPD) is a member of the flame-based detector family which also includes the flame ionization detector (FID), the flame atomic absorption detector (FAA), the thermoionic ionization detector (TID), the flame infrared emission detector (FIRED),etc. The FPD was first described by Draeger and Draegerl and Brody and Chaney2 more than 26 years ago. There have been relatively few improvementsin the detector design since the first design of Brody and Chaney, with the notable exception of the dualflame photometer detector.3 The FPD is by far the most popular commercially available detector for the selective measurement of sulfur compounds in spite of emerging new (1) Draeger, H.; Draeger, B. W. German Pat. 1,133,918, 1962. (2) Brody, S.;Chaney, J. J. Gas Chromatogr. 1966, 4, 42; U.S. Pat. 3,489,498, i970. (3) (a) Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1978,50,339. (b) Patterson, P. L. Anal. Chem. 1978,50,345. 0003-2700/93/0365-0539$04.00/0
PULSED-FLAME PHOTOMETER DETECTOR A schematic diagram of the pulsed FPD is shown in Figure 1. The pulsed FPD main structure (1)is mounted on a GC adapter base (2) which is connected to the GC-FID mount (3). In Figure 1the FID mount is of a Carlo Erba Model Vega 6000 GC, but the same pulsed FPD was also mounted (with (4) (a) Hall, R.C. J. Chromatogr. Sci. 1974,12,152. (b) Dmhel, H. V. J. Chromatogr. Sci. 1983,21, 375. (5) Uden, P. C.;Young, Y.; Wang, T.;Cheng, Z. J. Chromutogr. 1989, 468,319. (6) Benner, R. L.;Stedman, D. H. Anal. Chem. 1989,61,1268. (7) Shearer,R.L.;ONeal, D. L.; Rios, R.; Baker, M. D. J. Chromatogr. Sci. 1990, 28, 24. (8)Dressler, M.Selective Gas Chromatographic Detectors; Elsevier: Amsterdam, 1986. Barinaga, C. J. J. Chromatogr. Sci. 1986,24,483. (9) Farwell, S. 0.; (10) Amirav,A. Pulsed Flame Detector Method and Apparatue. Israel Patent Application No. 95617, Sept 1990. U.S.Pat. 5153673, October 1992, European and Japan Patent Applications submission Aug 1991. .. U.S. Application is approved. (11) Atar, E.; Cheskis, S.;Amirav, A. A d . Chem. 1991,63, 2061. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 5, MARCH 1 1993
I; Flgurr 1. Schematicdiagram of the pulsed FPD. The optical detection system Is not drawn to scale. (1) main structure,(2) OC adapter, (3) GC-FID mount, (4) combustion cell holder, (5) quartz combustor, (6) Igniter, (7) window, (8) quartz rod light guide, (9) color glass filter, (10) photomuitlpller, (11) auxiliary screw valve, (12) Igniter and heater electrlcal feedthroughs, (13) external material sampling Inlet, (14) hydrogen Inlet, (15) alr Inlet.
different adapters) on a Varian Model 3600 GC and on a Hewlett-Packard Model 5890A GC. The performance of the pulsed FPD is identical on these three gas chromatographs. Hydrogen is fed from the combustor holder (4) which is connected to the GC in a similar manner as the FID flame tip. The combustor holder is longer than the FID flame tip and is not electricallyisolated. It possesses severalholes and an externalair flow restrictor to induce proper mixing between the air flowing along the outside of the combustor holder and hydrogen flowing inside the combustor holder. The GC column (not shown) is inserted 1-2 mm below the combustor (5) nozzle. The flame is ignited by the Cr/Fe/Al alloy igniter (6) (high-temperature Kanthal AF 0.25-mm wire) and propagates to and through the combustor (5) where it selfterminates. The pulsed flame emitted light passes through asapphire window (7),through aquartz rod (5-mmdiameter) (8), which serves as a light guide,and through an optical filter (9), and is finally detected by a photomultiplier (10). An auxiliary screw valve (11)serves to control and increase the air concentrationin the hydrogen-rich mixture near the igniter for easier ignition. The internal diameter of the pulsed-flame detector structure is 4.6 mm while the outer diameter of the quartz combustor (5) is 4.1-4.3 mm. Thus, the combustible gas mixture is split so that portion of it is flowing with the GC column eluted gas to the combustor through ita 0.4-0.6mm nozzle hole while the other portion (which is not ignited) is flowing around the outside of the combustor. The design tolerances optimizes the pulsed flame brightness for the optical detection system. While the gases flowing inside the combustor with the GC carrier gas and eluted compounds are fiiing the combustor, the outer flow mixed with the combusted gases of the previous pulse fills the volume above the combustor and initiates the pulsed flame ignition. The screw valve (11) also facilitates a fine tuning of the relative gas mixture inside the combustor and around it (in the small gap
between the 4-4.3-mm quartz combustor 0.d. and the detector house 4.6-mm i.d.). If the outer gas flow rate is too high, the pulsed flame will propagate inside the combustor only every other pulse. The screw valve is adjusted to allow the outer gas flow rate to be close to but lower than this critical flow rate. The pulsed flame igniter is effectively shielded in this structure, and ita red emitted light is not observed by the photomultiplier in the sulfur and phosphorus detector modes. The igniter filament is mounted on a ceramic tube with twin holes and is connected to the electrical feedthroughs (12). A platinum or Pt/Rh alloy can also be used for ignition, but since it can also initiate undesirable catalytic combustion, the Kanthal AF wire is preferred. A pulsed capacitor dischargecan also induce ignition,but continuous DC heating provides a simpler ignition method. Sample molecules can be fed via an external inlet (13) in order to study the time dependence of the pulsed flame and ita emission spectra. Under normal GC detection this opening (13) is sealed by a screw. The pulsed FPD structure (1)is separately heated by a 25.4-mm-long, 6.35-mm-diameter cartridge heater located in a hole in the structure (not shown). Typical detector temperatures are 150 OC in the sulfur mode and 200-210 OC in the phosphorus detection mode. The 150 OC lower limit is set to avoid water condensation and to speed up solvent evaporation if the solvent briefly extinguishes the pulsed flame. In the phosphorus mode the higher temperature is required to avoid peak tailing due to slow desorption of phosphorus oxide products from the combustor walls. The pulsed flame structure, GC adapter, light guide holder, and combustor holder are all made of stainless steel 316 or 304. The combustor holder was either made of stainless steel or was made from aluminum. The gasket rings between the GC and the GC adapter (2), or between the GC adapter (2) and the detector structure (1)are also made from aluminum. The detector is connected to the adapter via three screws (not shown) to the groove in the GC adaptor, but the screws can be loose since a constant small leak does not significantly affect the detector performance. The photomultiplier is not drawn to scale and is much longer and wider than is shown. It is separated from the hot PFPD body by a 8-mm-o.d., 6-mmi.d., 60-mm-longstainlesssteel tube which serves to thermally isolate the photomultiplier from the PFPD body which can be operated from 150 to 450 "C. The most critical element in the detector is the quartz combustor (5). The total combustor length is 16-16.3 mm, ita outer diameter is 4.Ck4.3 mm, and the inner diameter can range from 1.8 to 3.0 mm depending on the quartz tube used. The combustion cell length is 10-12 mm with a ca. 0.5-mm nozzle near ita base. The base section is used to looselymount the quartz combustor on the combustor mount (4). A simple tube, without aflamearresting nozzle, can also be used. The initial quartz cleanliness is of extreme importance to the overall performance, and semiconductor-grade quartz is preferred. Pyrex can be used for sulfur detection, but the PFPD performance is considerably degraded with it in the phosphorus mode. A good cleaning procedure we found employs agitation in a 3:l HCVHN03 mixture followed by the application of several triple-distilledboiling water cycles (wateror methanol should be self-drained on a piece of paper). Finally the base section of the combustor is vertically mounted on a slowly spinning hand drill, and the top of the combustor is flame treated at about 1000-1200 OC to eliminate or reduce red pointa which appear on the combustor surfacein the flame (butane/oxygen micro torch flame) (Little torch kit item L-03034-00 of Cole Parmer). The base section of the combustor is not heated in the flame. The combustor can also be deactivated by SiClz(CH& (sylon)or by boric acid.12 The flame treatment yields the best results, but dust particles should be totally avoided.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 5, MARCH 1, 1993
Short HF cyclesto remove outer layers can also be attempted followed by the flame treatment, and this HF procedure provides the fastest combustor preparation way. Even a highly etched combustor can be used, but only one or a few minutes concentrated HF treatment is required. A new detector body after arrival from the machine shop requires carefulcleaning with organic solventsand diluted nitric acid, including in an ultrasonic cleaner, in order to eliminate combustor contamination. Once the combustor is ready, it usually does not deteriorate for a long time provided that the detector house is clean and so long as certain organometallic compounds are not used, which may require metal cleaning by HCl/HN03 solution. The detector works reliably for over a month continuously without any apparent change in its performance. Typical total gas flow rates (inside the combustor and around it) are 8 mL/min of hydrogen and 12 mL/ min air with pulse repetition rate of 2-4 Hz. At higher gas flow rates the repetion rate can be increased to over 10 Hz. The pulse repetition rate is stable within about 1%and is slightly increased with the detector temperature. It is also inversely dependent on the combustor and igniter volume. The photomultipliers used are Hamamatsu R647-04 and R1463-01head on ‘/2-in. tubes at typically 500-850 V and 106 current to voltage conversion gain. The filters used were a 25-mm-diameter, 2-mm-thick BG-12 for sulfur (and carbon) (340-460nm), BG-39 GG-495for phosphorus (495-600nm) and RG-9 for nitrogen (730-800 nm). All these filters are colored glass filters from Schott and not interference filters since the trade off of part of the extra selectivity for extra sensitivity is desirable with the pulsed FPD. For the flame spectroscopy a 160-mm-long quartz light guide is used instead of the 75-mm light guide used for GC detection so that a Jarrell Ash 0.20-m scanning monochromator could be used outside the Carlo Erba Vega 6000 GC. The support electronicsincludesa 2.5 V, 3.0 A dc power supply for the igniter, a 0-1kV high-voltageregulated power supply for the photomultiplier, and a 0-130 V ac Variac for the 50-W cartridge heater of the detector (weuse 150-W240-V Hotwatt heaters). The processingelectronicsincluded the Model 425A scan delay and the Model 9415 linear gate (gated integrator) of Ortek-Brookdeal and a simple 20-MHzdigital oscilloscope CS-8010 of Kenwood. Since a commercial gated integrator that can operate in the low time scales of 2-20-ms gate width and 0.1-10-ma gate delay are presentlynot available,a “sample and hold”gated integrator was constructed. When combined with a Systron & Donner 100C pulse generator (that replaced the Ortek-Brookdealgated integrator and scan delay units) slightly improved performance is observed. The electronic time constant in all of the experiments is 0.3 8. A Le Croy 9400 is used for signal averaging in the measurements of pulsed-flame emission time profiles. Triggering is usually achieved on the pulsed-flame background emission which servesas an “internal trigger”or with a photodiode (not shown in Figure 1)which serves as an “external trigger” in flame emission spectra measurements. Alternatively the use of a PC with an ADC card with appropriate software can replace the gated integrator-oscilloscope combination(workis under progress in this direction).
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PULSED-FLAME EMISSION TIME AND SPECTRA The most important feature of the pulsed flame is that the flame emission is pulsed, and thus, new time domain information is available which can be used to enhance the detection sensitivity and selectivity. In Figure 2 the char(12)Lewis,B.;Elbe, G. Combustion,Flames and Expositions of Gases; Academic Press: New York, 1987.
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Figure 2. PFPD emission time dependence of sulfur (S2’) (solid line) obtainedat 395 nm through a monochromatorand that of OH’ obtained at 312 nm (dashed trace) which is Identical to that of hydrocarbon emission such as CH’ or C2*. Detector temperature was 150 ‘C and very hydrogen rich flame conditionswere used. Tetrahydrothiophene was introduced as the source of sulfur. Independent dlode triggering was used with a LeCroy 9400 signal averager oscilloscope. The combustor cell length was 10 mm. The viewing window is 5 mm In diameter and is centered on the combustor (see Figure 1).
acteristic temporal light emission behaviors encountered in the sulfur detection mode is shown. Note that the emission is pulsed and the pulses are short. When the monochromator is tuned to 312 nm the pulsed OH* emission is obtained as shown in the dashed trace and is of about 2-ms duration. The emission duration depends on the flame velocity and the viewing opening diameter near the window and light guide (5 mm). The flame velocity depends on th Hdair ratio and is slower at the H2-rich conditions of sulfur detection. The flame velocity is also dependent upon the detector temperature and increases at higher detector temperatures. It is important to note that the observed emission time behavior of OH* and hydrocarbon related emission of CH* and Cz* are all identical with our time resolution (
