of the computer are, of course, specific for t h e particular computer used Driver routines for the graphical display are, however, of medium complexity, whereas programs generating vectors between different display points can be written in high level language. T h e hardware cost of the graphical display is modest. If the cost of a second-hand black-and-white television monitor is omitted, the total hardware cost for the system is less t h a n $500. With the aid of the schematic diagrams given in Figures 2 and 3, assembling time ought t o be less than 1 month.
Using the same approach as described above, the graphical display can be modified to operate in connection with a color television monitor. This would, however, increase the hardware cost by approximately a factor of five.
RECEIVEDfor review November 4, 1975. Accepted January 16, 1976. This work has been supported by the Danish Natural Research Council through a guest professorship t o Daniel Jagner.
Gas Chromatographic Multidetector Coupled to a Glass Capillary Column Milan Hhvnaz,' Walter Frischknecht,' and Lenka Cechova Givaudan Forschungsgesellschaft AG, Dubendorf, Switzerland
Sulfur- and/or nitrogen-containing compounds are sensorically important constituents of many flavor materials. Their presence in complex mixtures can be elegantly detected by means of the GC-MS coupling technique, provided t h a t well-defined mass spectra of the constituents are obtained. T h e application of glass capillary columns with their high resolving power, therefore, will in many cases be of advantage ( I , 2). In spite of high gas chromatographic resolution, trace compounds of interest may occasionally be overlapped by large neighboring components in the chromatogram, so t h a t only the mass spectrum of the dominating constituent is observed while the information concerning the trace is lost. Even from well-separated trace components hardly interpretable mass spectra may result, d u e t o t h e limited sensitivity of the GC-MS system or because of increasing interference from column bleed a t elevated working temperatures. In such cases, element-specific GC detectors can be employed advantageously to signal the presence of S- and/or N-containing substances. These detectors, u p t o now, were mostly operated in combination with packed GC columns; newer applications are covered in a recent paper by E t t r e e t al. ( 3 ) .T h e combination with capillary columns has been reported lately by Goretti e t al. ( 4 ) (S-mode F P D ) and
Figure 1. Schematic view of the splitting arrangement (1) Column outlet; (2) PTFE shrink tubing; (3) initial splitting: (4) final splitting: (5) purge gas inlet
Present address, EMPA, Dubendorf, Switzerland.
Hartigan e t al. ( 5 ) (N-mode AFID); Zlatkis e t al. (6) present the simultaneous use of FID and F P D (sulfur mode) in combination with a nickel capillary column without giving any technical details. Most recently, also a multiple detector system operated in combination with a packed column was described by McLeod e t al. (7). We set out to couple a glass capillary column t o a multiple detector arrangement consisting of a FID (serving as monitoring device), a sulfur detector, and a nitrogen detector, plus a sniffing port for the sensoric evaluation of column effluent. Since we desired simultaneous detection, the column effluent was split by means of a four-way splitter before entering the detectors. Each of these detectors re-
Figure 2. Photograph of a simple two-way end-split operated with a direct purge gas stream (A) Connection to glass capillary column: (B) purge gas line; (C) glass splitting chambre (inner volume less than 1 ,ut); (D) split stream toward the FID (through a glass-lined tubing sleeve): (E) split stream toward the extending sniffing port (split ratio FID: nose about 1:2)
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Figure 3. Schematic view of the split line's attachment to the socket body of the nitrogen detector (1) burner jet: (2)glass capillary tubing (0.3-mmd. i., about 0.9-mm 0.d.); (3) glass-lined metal tubing (Ys-in, o.d., about 1.3-mm i,d,); (4) brass G '14 in. to Swagelok union: (5) SS tubing (3-mm o.d., 1-mm i.d.) sold. into the union 4 for introducing the accessory N?: (6) Swagelok '/sin. nut: (7) short PtAr capillary tubing (0.1-mm i.d., 0.3-mm 0.d.): (8) fused glass pearl: (9) column outlet: (10) oven cabinet wall
quires a different combustion-gas composition for optimum performance; therefore, individually made-up gas streams were introduced into the effluent branches. The system described here in greater detail has already been operating quite satisfactorily for about 1year.
EXPERIMENTAL Apparatus. A Carlo Erba Gas Chromatograph Model 2300 equipped with a linear temperature programmer Model L T 210 was used. I t is designed as a dual-column apparatus with separate facilities for injection, inlet splitting, and flame-ionization detection and allows the installation and operation of glass capillary columns in compliance with the requirements established by Grob e t al. ( 8 ) in their state-of-the-art paper. The flame ionization detector (Carlo Erba Model 20) as well as the Nitrogen Phosphorus Selective Detector (9, I O ) of Carlo Erba (working with pressed KC1 ring as the alkali salt source and a variable-height polarization electrode) were used as commercially available. T h e sulfur detector (Tracor Flame Photometric Detector working with the 394-nm filter and the 750-V power-supply unit) was used in the modification of Gay e t al. (11) suitable for combination with glass capillary columns. Each of these detectors was operated with a separate electrometer (Carlo Erba Model 180). The detector signals were recorded (at 1 mV full scale) simultaneously on a Model 56 double-pen (FID and FPD) and on a Model QPD 33 single-pen (NPSD) recorder made by Hitachi Perkin-Elmer, respectively. All of the six heated holes leading through the top cover of the GC oven were used. The two original FID outlets were occupied by the FID and by the nitrogen detector, respectively; the two original inlet splitter holes were used for the inlet splitter itself and for the 938
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Figure 4. Chromatogram of simultaneous sulfur, nitrogen and FID traces from the separatlon of a total of 250 ng of an artificial mixture injected ( 1 ) n-heptan-14, (2) ethylpyrazine (3) 6-methyl-3-ethyl pyrazine (4) noctan-1-al (5) n-decane, (6) 3-methyl-2-vinyl thiazole. (7) 1,3-d1thiane,(8) 3,6-dirnethyl-2-ethylpyrazine (9) di-n-butyl sulfide, (10) di-+propyl disulfide, (11 ) n-nonan-14, (12) 1-(2-thienyl)propan-2-on (13) n-decan-2-one, (14) ndodecan-2-one, (1 5) n-pentadecane, (1 6) n-hexadecane For GC conditions see the text
sulfur detector, respectively, one of the two present injection ports served for sample injection, the other accommodated the effluent outlet line to the sniffing port and accessory gas supply lines described below The original pneumatic circuitry in the GC unit was used as far as possible, the accessory gas streams were adjusted by micro needle valves (Carlo Erba) or rotometer valves (Brooks), fed from pressure-controlled gas lines The glass capillary columns were supplied bv H J Jaeggi, Laboratory for Gas Chromatography, 9043 Trogen (Switzerland) Four-Way Column Effluent Splitter. A crucial element of the whole concept is the two-step four-way effluent splitting a t the glass capillary column outlet Its principle is schematically given in Figure 1, whereas Figure 2 shows a photograph of a simpler version of the splitter, which is ordinarily used in our laboratory for sniffing column effluents in parallel to flame ionizatlon detection The splitting proper was achieved by fusing appropriate lengths of a Pt/Ir-capillary tubing (12) (75% Pt/25% Ir, Institut fur Chromatographie, Bad Durkheim, Germany) of about 0 1 m m i d and about 0 3 mm o d into pieces of glass capillary tubing having the same dimensions as the glass column employed (1 e , about 0 3 mm i d and about 0 9 mm o d ) We used the Pt/Ir material instead of pure platinum ( 1 3 )capillaries for its superior mechanical strength,
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Figure 5. Simultaneous detection of sulfur and nitrogen compounds in a meat flavor analysis (for GC conditions see the text)
although fusing is more difficult than with platinum; using glass capillaries prepared from softer glass types and working with a n oxygen enriched butane microflame burner largely eliminates these difficulties. T h e first of the two splitting steps was realized by means of two metal capillaries of identical length. In this way, a splitting ratio is obtained which does not differ substantially from 1. In view of t h e usually low carrier gas flow rates prevailing through glass capillary columns (about 2-4 ml/min), we decided to make up the total gas flow between the first and the second splitting points by adding a purge gas; this was introduced through equal lengths of capillary
tubing fed from a common supply line (Figure 1).In this way, a sufficient mass flow into the ionization detectors was achieved, a t the same time minimizing post-column dead volumes in the connection lines. For the second splitting step, capillaries of different lengths have to be used in order t o arrive a t the desired splitting ratios (Le., about 25% to the FID, about 20% to the FPD, about 15% to the NPSD, and about 40% to the nose). These values are reproducible within 5% relative when the purge gas flow changes from onehalf to twice the usual amount of about 6 ml/min. Detector Connections and Gas Supply. T h e GC detectors emANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
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ployed differ fundamentally in their requirements regarding combustion-gas composition; moreover, the element-specific detectors, having been optimized in the past in combination with packed GC columns exclusively, are reported (IO, 1 4 ) t o work properly with carrier gas flows unachievable from capillary columns. Therefore, maintaining optimum operating conditions for each of the detectors used is as important a factor of the whole concept as is a properly functioning splitter arrangement. Whereas the delivery of individual combustion-gas ingredients for all of the GC detectors can largely be managed by the existing pneumatic circuitry of the GC apparatus, additional gas streams had to be introduced independently for making up the carrier (including the purge) gas flows coming from the respective splitter branches into the element-specific detectors. These detector make-up gas streams have an additional advantage of being free from any column bleed. The amounts of about 45 ml Nz/min for the FPD and about 40 ml Nz/min for the NPSD which are found to be sufficient cannot be simply introduced into the corresponding splitter branches, however, because there they totally stop the gas flow from the column. They were therefore directed around the splitter extension tube and made t o join the respective part of the column effluent just below the burner jet proper, as is shown schematically for the case of the NPSD in Figure 3 . The location of the holes in the top cover of the GC unit being given, the metal capillaries of the second stage splitters had to be extended by means of glass capillary tubing. These extension pieces are ending just below the burner jets of the ionization detectors; in the modified (II ) FPD, this extension tubing ends immediately below the insert orifice. The outlet to the sniffing port is made of glass-lined metal tubing (Scientific Glass Engineering Pty., Melbourne, Australia) of in. o.d. and 0.4 mm i.d. which is directly soldered to the Pt/Ir capillary. For better mechanical stability, the glass extension tubes leading to the burner jets were inserted into a short piece of a glasslined metal tubing of l/a in. 0.d. and 1.3 mm i.d.; this supporting sleeve was held in place by a Swagelok reducing unit which also served as a gas-tight seal at the detector entries. In the case of the FPD and of the NPSD, the reducing units were provided with a side arm for the introduction of the additional nitrogen necessary for proper combustion-gas composition as mentioned above. The gas mixtures in the respective detectors have the following composition: FID, 15 ml Ha/min and 240 ml/min air; FPD, 150 ml Hdmin, 70 ml/min air, 5 ml Oz/min, and 45 ml Nz/min; NPSD, 28 ml Hdmin, 230 ml/min air, and 40 ml NZ/min.
RESULTS A N D DISCUSSION I n Figure 4 the simultaneously recorded traces from FID, FPD, and NPSD show the separation of a n artifical mixture obtained from a wall-coated open-tubular glass column of 30-m length and 0.38 m m i.d. (OV-1 as stationary phase), operated between 60 and 150 "C with a linear temperature programming rate of 5 "C/min; t h e sample (0.5 ~ 1 ) was introduced by splitless (15) injection into the "cool" column. It consisted of the compounds given in the legend of Figure 4;t h e mixture was diluted with n-pentane to a total concentration of 0.5 pg/Wl. T h e peak heights on t h e sulfur trace correspond to about 3 ng/peak and those on the nitrogen trace t o about 2 ng/peak. As expected, 3methyl-2-vinyl thiazole produces peaks on both of the element-specific detectors installed, whereas t h e hydrocarbons and t h e oxygenated compounds are detected, at concentrations of about 7-15 ng/peak, by the FID only. In our experience, the NPSD shows about 400 times higher sensitivity for nitrogen-containing compounds than for hydrocarbons. T h e F P D is practically insensitive t o nonsulfur compounds. T h e practical use of t h e multidetector arrangement is demonstrated in Figure 5 where a part of t h e flavor ohtained from roasted meat was run on a wall-coated opentubular glass column of 50-m length and 0.36-mm i.d. (UCON 50 HB 5100 as stationary phase) within t h e temperature range of 60-180 "C with a linear programming rate of 4 OC/min. T h e correlation of all the detector traces with the simultaneously obtained olfactive evaluation of 940
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the effluent provides valuable information for further analytical work. T h e use of PTFE shrink tubing ( I ) connection on the split entrance permits rapid column interchange. From the signal/noise ratio, as shown in Figure 4, a detection limit of a few tenths of a nanogram per compound can be achieved without difficulty. T h e inhalation of toxic compounds from t h e column effluent should be avoided. An alternate design of the splitting device merits consideration. T h e four splitting arms of equal length would be inserted into the column exit and the desired splitting ratios to the various detectors would be obtained by introducing independently controlled purge gas streams directly at the detector entries. This concept, making use of t h e original idea (16) of a simple two-way flow-switch, would provide more elasticity in adapting the splitting ratios t o the requirements of a particular analytical problem favoring, e.g., sensoric evaluation of t h e effluent. This evidently is not the case for the splitter construction described here, where the splitting ratios are determined by t h e various lengths of metal capillary pieces. T h e alternate concept, however, requires a GC unit offering sufficient space for t h e pneumatic installations. T h e possibilities t o improve the rather poor selectivity value with the NPSD used, which acts primarily as a phosphorus (9, 10) selective detector, are quite limited; t h e other detectors of this kind behave similarly. Therefore, another principle of detection of nitrogen-containing compounds should be taken into consideration if selectivity comparable t o t h a t of the FPD is called for. Most promising in this regard is the electrolytic conductivity (17) detector in the refinement of Hall (18); unfortunately, there are no indications concerning t h e coupling of t h a t detector t o capillary columns u p t o now so t h a t further study is needed.
ACKNOWLEDGMENT We thank the Management of the Givaudan Companies for permission to publish this paper. T o L. Gay (Municipal Chemical Laboratory, Zurich, Switzerland) as well as B. Brechbuhler (Brechbuhler AG, Urdorf, Switzerland) we are indebted for valuable discussions, and to P. Meneguz of our staff for his technical assistance. Our acknowledgment goes further to F. Etzweiler of our MS Laboratory for his practical advice concerning the fusing of metal capillaries into glass.
LITERATURE CITED K . Grob and H. J . Jaeggi, Anal. Chem., 45, 1788 (1973). N . Neuner-Jehle. F. Etzweiler, and G. Zarske, Chromatographia,6, 21 1 (1973). R . Pialiucci. W. Averill. J. E. Purcell. and L. S. Ettre, Chromatoaranhia. - . 8, 16% (1975). G. Goretti and M. Possanzini, J. Chromatogr.,77, 317 (1973). M. J . Hartigan, J. E. Purcell, M. Novotny, M. L. McConnell, and M. L. Lee, J. Chromatogr.,99, 339 (1974). W. Bertsch, F. Shunbo, R. C. Chang, and A. Zlatkis, Chromatographia, 7. 128 (19741. H. A. McLeod, A. G. Butterfield, D. Lewis, W. E. J. Philips. and D. E. Coffin, Anal. Chem., 47, 674 (1975). K. Grob and H. J. Jaeggi, Chromatographia,5, 382 (1972). Short Notes 1974lNo. 2, pp 3-6 (Carlo Erba Strurnentazione S P A , Mila-
no, Italy).
G. R. Verga and F. Poy, J. Chromatogr., 116, 17 (1976). L. Gay and E. Brechbuhler. paper in preparation. R. Kaiser, "Grundlagen der Kapillar GC", Course Text, lnstitut fur Chromatographie. Bad Durkheim. Germany, 1974, p 7. F. Etzweiler and N . Neuner-Jehle, Chromatographia,6, 503 (1973). J. G. Eckhardt, M . B. Denton, and J. L. Moyers, J. Chromatogr. Sci., 13, 133 (1975).
K. Grob and G. Grob, J. Chromatogr. Sci., 7, 11970): Chromatoaraohia.5. 3 (1972).
584, 587 (1969); 8, 635
6. R . Deans, Chro-matographia,
1, 18 (1968). D. M. Coulson, J. Gas Chromatogr., 3, 134 (1965). (18) R. C. Hall, J. Chromatogr. Sci., 12, 152 (1974).
,
RECEIVEDfor review December 2, 1975, Accepted February 3,1976.
