ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2333
Modification of a Gas Chromatographic Inlet for Thermal Desorption of Adsorbent-Filled Sampling Tubes William K. Fowler,” Christina H. Duffey, and Herbert C. Miller Southern Research Institute, 2000 Ninth A venue South, Birmingham, Alabama 35205
Two techniques are described for modification of a gas chromatographic inlet to permit the thermal desorption of adsorbent-type sampling tubes directly into the carrier gas stream. Examples of chromatograms recorded with the aid of these techniques are presented to illustrate typical performance characteristlcs as observed in actual use. For many purposes, these schemes compare favorably with existing thermal desorption devices, and their inherent simplicity and low cost represent distinct advantages in many applications.
have been implemented, susceptibility to leakage of carrier gas during operation, and exposure of sample vapors to potentially reactive or catalytic surfaces. A need for an intrinsically simple gas chromatographic inlet-desorption system capable of circumventing all of the above limitations was identified in our laboratory, and consequently a number of possible inlet modification procedures were evaluated. Two of these have been adopted in our laboratory with good success and are described in this paper.
T h e detection and quantification of trace contaminants in air via their collection on adsorbent media has received considerable attention from researchers in recent years. The method has been employed for the analysis of a diverse array of volatile and semivolatile environmental pollutants (1-69, including several inorganic gases (&IO), a t parts-per-billion levels or lower. In addition to air pollution analysis, the technique has also been utilized for the collection and subsequent analysis of eluting gas chromatographic fractions ( I I). It is especially compatible with gas chromatography in the analysis step and, in principle, is applicable to any atomic or molecular vapor. In most applications, a small sampling tube is packed with a suitable adsorbent substrate which is usually, but not always (12),a conventional chromatographic packing material. After the tube has been appropriately conditioned, a known quantity of sample (e.g., polluted air) is passed through it to trap the species of interest on the sorbent bed. The adsorbed substances are then desorbed by elution with a solvent or by thermal means into a flowing gas stream. The latter procedure is generally preferred for ultra-trace analysis by gas chromatographic methods because it enables virtually 100% of the adsorbed material to be introduced into the gas chromatograph a t once (although this can become a disadvantage if a procedural error or instrumental failure causes the introduced sample to be lost). In contrast, a solvent employed for stripping of the adsorbate must undergo a risky, time-consuming concentration step to permit the introduction of most of the collected material into the gas chromatograph in a single injection. Apparatus for the thermal desorption of sampling tubes directly into the analytical column of a gas chromatograph can be purchased from commercial sources or fabricated inhouse. However, commercially available equipment is often prohibitively expensive for those who wish merely to engage in limited experimentation or who expect to use the method only occasionally. Moreover, available designs for systems amenable to construction in the machine shop or laboratory (13-28) are generally complex and may require costly parts and labor as well as tools, machines, and expertise not readily accessible to many investigators. Additional drawbacks to these alternatives may include difficulty in manipulation, the necessity for lengthy “cool-down” periods between heating cycles, limited compatibility with conventional gas chromatographs, the existence of a “cold spot” in the sample transfer line, difficulty in returning to the original inlet configuration on the gas chromatograph once the requisite modifications
Inlet Modification Techniques. The two injection port modification schemes are depicted in Figures l a (Method A) and l b (Method B), respectively. The figures illustrate the requirements for 3-mm 0.d. (2-mm i.d.) standard Pyrex glass sampling tubes and 3-mm 0.d. analytical columns, but the systems can be adapted to accommodate other sizes and types of tubing as well. Method A is compatible with injection ports that allow passage of a linear sampling tube from the aperture of the septum retaining face (with the septum retaining nut removed) straight out through the column attachment fitting (normally0.25-in. male Swagelok) at the rear of the block. An additional requirement for compatibility is that the analytical column possess sufficient flexibility to allow a relocation of the point of its attachment t o the rear of the injection port. In the design utilized for this work, a liner was inserted into the injection port to minimize the air space between the injection port inner wall and the sampling tube and to help maintain the sampling tube at or near the center of the injection port chamber during insertion. The liner consisted of a length of standard 0.25-in. 0.d. copper tubing, but experiments have indicated that stainless steel or even glass tubing may serve as well. The glass sleeve of Figure l a (0.25-in.0.d. standard wall Pyrex) provided mechanical support for the reducing union and compression for the silicone O-ring (0.125-in. id.). Also, it thermally isolated the temperature-limiting O-ring from the injection port. Vespel ferrules were used in the connecting fittings at both ends of the reducing union, and the union was drilled out to 0.125-in. i.d. Compression on the O-ring (and therefore the tightness of the 0-ring-sampling tube seal) was regulated by appropriate adjustment of the glass sleeve during tightening of the fitting. Both ends of the sleeve were fire-polished; a slight, natural constriction introduced by the fire-polishing process helped to maintain the concentricity of the sampling tube. The sleeve was made as short as possible to minimize the “dead’ volume between the sorbent bed and the analytical column. Carrier gas was supplied to the sampling tube through a 3-mm 0.d. flexible (i.e., Teflon) tube. Attachment to the sampling tube was accomplished by a 0.125-in. Swagelok union (not shown in Figure 11, utilizing a Teflon ferrule at the sampling tube end. A toggle valve was installed in the carrier gas line between the head pressure regulator of the gas chromatograph and the sampling tube and was used to stop the carrier gas flow when sampling tubes were changed. The normal carrier gas inlet on the injection port block was not used. Method B (Figure lb), a complementary alternative to Method A, is suitable for use with injection ports that do not allow protrusion of the sampling tube from both the front and rear of the block. It is also compatible with inflexible (e.g.,glass or glass-lined) analytical columns, since the geometry of the column connection at the rear of the block is not altered. In this scheme, the seal between the front and rear of the sampling tube was accomplished with a pair of conventional gas chromatographic septa, perforated to accept the tube. The po-
EXPERIMENTAL
0003-2700/79/0351-2333$01 .OO/O
Q 1979 American Chemical Society
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ANAL YTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
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Figure 1. (a) Schematic diagram of an injection port modified according to Method A. (b) Schematic diagram of an injection port d i f i e d according
to Method B tential for loss of sample by diffusion away from the primary carrier gas stream at the junction between the sampling tube and the analytical column was found to be much greater with this design. Consequently, a secondary supply of carrier gas (ca. 5 mL/min) was admitted via the normal block supply inlet (see Figure Ib) to minimize sample losses. Diffusional losses were further minimized by insertion of a liner similar to the one discussed for Method A but made of glass. Attachment of the primary carrier gas supply line to the sampling tube was performed as described previously for the other system. Desorption Procedure. The procedure for desorption of sampling tubes was the Same for both inlet modification schemes. The sampling tube was fastened to the carrier gas line while the toggle cut-off valve was in the “off’ position. With a Teflon ferrule in the connecting fitting, attachment of the sampling tube was readily performed when the nut was merely loosened on the union. Fire-polishing the end of the sampling tube greatly facilitated this operation. A finger-tight connection sufficed in nearly every instance where 3-mm 0.d. sampling tubes were employed. The sampling tube was inserted into the heated injection port until its downstream tip contacted the end of the analytical column. The toggle valve was then immediately switched to the “on” position to initiate the flow of carrier gas. After all components had eluted, the toggle valve was returned to the “off’ position in preparation for removal and replacement of the sampling tube. Sampling Tube Construction. To obtain chromatograms of bis(2-chloroethy1)thioether (the vesicant sulfur compound known as “mustard gas,” or simply “mustard”), sampling tubes were constructed from 115-mm lengths of 3-mm o.d., 2-mm i.d.
Pyrex glass tubing. One end of the tube was fire-polished, and a 45-mm length of capillary tubing was inserted into the other end. The capillary tubing was passed through to the fire-polished end, where it was retained by the natural constriction attributed to the fire-polishing process. A small plug of silanized glass wool was then inserted into the 3-mm 0.d. tube so as to rest against the end of the capillary tubing near the center of the outer tube. This arrangement effectively held the sorbent bed securely within the central portion of the 3-mm 0.d. tube against the sudden pressure changes experienced during insertion and removal of the sampling tube in the modified injection port. After addition of approximately 5 mg (ca. 12-mm column) of 35/60-mesh Tenax-GC (Enka, N.V., Amheim, Holland), another plug of glass wool was positioned a t the top of the sorbent bed. The unglazed end of the tube was then fire-polished to complete the construction of the sampling tube. Samples were introduced into and desorbed from the end that contained the capillary tubing. Sampling tubes used for analysis of polynuclear aromatic hydrocarbons (PAHs) were fabricated similarly except that 6-mm o.d., 4-mm i.d. Pyrex glass tubing (of somewhat shorter length) was employed instead, and no inner sleeve of tubing was included for retention of the sorbent. The sorbent bed consisted of an unweighed, 30-mm long section of 80/100-mesh Gas-Chrom Q (Applied Science Laboratories, Inc., State College, Pa.). The newly constructed sampling tubes were conditioned by heating them overnight in an appropriately modified injection port a t their respective working desorption temperatures and carrier gas flow rates (refer to the gas chromatographic analysis conditions subsequently specified).
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2335
1
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Figure 2. Chromatograms resulting from the desorption of a series Sampling Tube Spiking Procedure and Solution Preparation, For all work reported here, sampling tubes were "spiked" with standard solutions of the appropriate substances by injection of microliter quantities of the solutions directly into one end of the sampling tube while drawing air through it. During this process, the solvent evaporated and passed through the sorbent bed essentially unretained, while the solutes were adsorbed on the sorbent. The solution of PAHs consisted of 350 pg/mL of anthracene, 650 pg/mL of fluoranthene, and 680 pg/mL of chrysene in dichloromethane,prepared by serial dilution from a stock standard. The bis(2-chloroethy1)thioether solution contained 3 pg of the compound per milliliter of chloroform and was also prepared by serial dilution from a stock standard. All chemicals and solvents were of reagent grade except mustard, which was supplied by the U S . Army and was of unknown purity. Instrumentation and Analysis Conditions. Our work with mustard was performed on a Tracor Model 275-HA gas chromatograph equipped with a sulfur-specificflame photometric detector (FPD) and an injection port modified according to Method A. The analytical column consisted of 1.8 m of 3-mm o.d., 1.5-mm i.d. TFE Teflon tubing packed with 2% SE-30 and 5% Carbowax 4000 on 60/80-mesh Chromosorb 750. The carrier gas (N,) flow rate was 25 mL/min, and the column temperature was 140 "C. The 3-ng mustard samples were desorbed at an injection port temperature of 175 "C. The resulting detector response was plotted on a strip-chart recorder whose chart speed was 0.25 in./min. A Perkin-Elmer Model 3920 gas chromatograph employing a flame ionization detector (FID) was utilized for all work involving PAHs. The analytical column-a 0.3-m section of 6-mm o.d., 4-mm i.d. Pyrex tubing-was packed with 1% OV-17 on 60/80 mesh Gas-Chrom Q. The column was maintained initially at 150 "C for 2 min after insertion of the sampling tube, then programmed to 250 "C at 16 "C/min, where it was held for approximately 4 min prior to cool-down. The flow rate of helium in the primary carrier gas stream was 35 mL/min, while the flow rate of the secondary stream (see Figure Ib) was ca.5 mL/min. The injection port temperature was 250 "C and the recorder chart speed 0.25 in./min.
RESULTS A N D DISCUSSION Representative examples of chromatograms recorded during
0: sorbent
tubes, each spiked with 3 ng of bis(2-chloroethyl)thioether
the evaluation of these inlet modification schemes are presented here to illustrate typical performance characteristics as observed in real applications. Figure 2 shows the chromatographic traces resulting from the successive desorption of five different spiked sampling tubes in a GC inlet modified according to Method A. The mustard peaks in each case are reasonably sharp and free from tailing effects. The slight variation in peak height was found to be due chiefly to the quadratic response of the sulfur-specific FPD and to nominal differences in individual sampling tube characteristics. Thus, reproducibility is considerably improved if a single sampling tube is employed for all determinations and if a detector with a linear response is used. Several hundred milliliters of air was passed through each tube during the spiking process; consequently, there is little or no evidence of the solvent in the front of the chromatograms. The result of a standard injection of a solution of the PAHs into an unmodified injection port is compared in Figure 3 to the analogous chromatogram obtained by desorption of a spiked sampling tube in the same injection port after modification according to Method B. Peak broadening and a concomitant loss of resolution are discernible but are not severe, and may be attributed at least in part to the very high boiling points of these compounds (340 to 450 "C). Also evident in these chromatograms are shifts in both the relative and absolute retention times of the sample components. This phenomenon is observed commonly in thermal desorption experiments, particularly when working with high-molecularweight compounds, and is due primarily to the added retention of the adsorbate in the sorbent bed. However, the effect is highly reproducible under fixed operating conditions and therefore presents little difficulty in most applications. When normalized t o a given sample size, the peak areas resulting from thermal desorption of the PAHs (Figure 3) agreed with those resulting from the liquid injection to within 30% of the latter. Although no definitive assessment of the experimental error was undertaken, it is possible that much of the error stemmed from the assumption that the solutes and the solvent were volatilized to the same relative extent
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2336
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are occasionally extinguished, and thermal conductivity detectors may be damaged or destroyed. However, conditions of hydrogen and air flow into the FPD usually can be chosen so as to eliminate any extinguishment problem without sacrificing a great deal of sensitivity. Interestingly, no such difficulty has been experienced in our laboratory with the FIDs of four different instruments. Finally, it is recognized that certain chromatographic column packings may not be able to withstand repeated interruption of carrier gas flow (and exposure to air) while a t an elevated temperature. No cases of deterioration in column performance have been noted, however, after extensive use of a considerable variety of column packings in the thermal desorption mode. I t should be pointed out that peak widths, and therefore chromatographic efficiencies, may be acutely influenced by a number of factors, including the distribution of the vapor influx rate into the sampling tube over the sampling interval. Accordingly,the use of peak height as an approximate measure of quantitative response should be avoided in quantitative analysis involving sorbent sampling systems of this type. In practice, the septa, O-rings, and Teflon ferrules employed in the configurations described here have sustained hundreds of repetitive desorptions of sampling tubes without requiring replacement. The time required to change sampling tubes in the modified injection port is approximately 10 to 20 s and depends primarily upon the dexterity of the operator.
LITERATURE CITED
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THERMAL DESORPTION
2
4
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8
10
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Figure 3. Chromatograms produced by liquid injection and by thermal desorption, respectively, of a mixture of polynuclear aromatic hydrocarbons
from the syringe needle in the hot injection port during injection of the liquid solution. In view of the unusually large difference between the boiling points of the solutes and that of the solvent, this assumption may not have been valid. None of the above-mentioned chromatographic characteristics appeared to be affected critically by the injection port temperature. The compounds of Figures 2 and 3 were also desorbed a t an injection port temperature of 200 "C (instead of 175 and 250 "C, respectively), rendering chromatograms nearly indistinguishable from those depicted in the figures. Extensive utilization of these inlet modification techniques in our laboratory has revealed only a few limitations with regard to their general applicability. The restriction upon the injection port and column oven temperatures imposed by the silicone O-ring (200 "C maximum) in a t least one instance necessitated the use of a lower injection port temperature than was desired. Nevertheless, even at the lower temperature, the results proved satisfactory for the intended application. Another potential limitation is that the detectors of some instruments cannot tolerate an interruption in the flow of carrier gas under certain conditions of operation. Specifically, FPDs
(1) E. D. Pellizzari, "Development of Analytical Techniques for Measuring Ambient Atmospheric Carcinogenic Vapors", Research Triangle Institute, Research Triangle Park, N.C. Environ. Prot. Agency (U.S.), Rep. EPA-600/2-75-076, November 1975. (2) B. Versino. M. de Groot. and F. Geiss, Chromatographia, 7(6), 302 (1974). (3) E. D. Pellizzari, J. E. Bunch, and B. H. Carpenter, Environ. Sci. Techiml., 9(6), 552 (1975). (4) E. D. Pellizzari, J. E. Bunch, and B. H. Carpenter, Environ. Sci. T8chm/., 9(6), 556 (1975). (5) J. Adams, K. Menzies, and P. Levins, "Selection and Evaluation of Sorbent Resins for the Collection of Organic Compounds", Arthur D. S.),Rep. EPALittle, Inc., Cambridge, Mass.,Environ. Prot. Agericy (U. 60017-77-044, April 1977. (6) A. D. Snyder, F. N. Hodgson. M. A. Kemmer, and J. R . McKendree. "Utility of Sola Sorbents for Sampling Organic Emissions from Stationary Sources", Monsanto Research Corp., Dayton, Ohio, Environ. Prot. Agency (U.S.), Rep. EPA-600/2-76-201, July 1976. (7) B. E. Saltzman and W. R. Burg, Anal. Chem.. 49(5), 1R (1977). (8) D. G. Taylor, Ed., "NIOSH Manual of Analytical Methods", 2nd ed., Vols. 1-4, National Institute for Occupational Safety and Health, Cincinnati, Ohio, NIOSHPub. Nos. 77-157-A (Vol. I , April 1977), 77-157-6 (Vol. 2, April 1977), 77-157-C (Voi. 3, April 1977), 78-175 (Vol. 4, August 1978). (9) E. V. Ballou, Ed., "Second NIOSH Solid Sorbenis Roundtable". National Institute for Occupational Safety and Health, Cincinnati, Ohio, (NIOSH) 76-193, July 1976. (10) H. K. Dillon, W. J. Barrett, and P. M. Eller, Am. Ind. Hyg Assnc. J , 39(8), 608 (1976). (11) J. L. Wtiak, G. A. Junk, G. V. Calder, J. S. Fritz, and ti, J. Svec, J . Org. Chem.. 38117). 3066 (19731. (12) R. G. Lewis,'A.'k. Brown, andM. D. Jackson, Ami. Chem., 49(12), 1668 (1977). (13) W. A . Aue, C. R. Vogt, and D. R. Younker, J . Chromatcgr., 114(1), 184 (1975). (14) J. W. Russell, Environ. Sci. Technol., 9(13), 1175 (1975). (15) T. A. Beliar, M. F. Brown, and J. E. Sigsby, Jr., Anal. Chem., 35(12), 1924 (1963). (16) J. S.Parsons and S. M i n e r , Environ. Sci. Technol., 9(12), 1053 (1975). (17) W. V. Ligon, Jr., and R. L. Johnson, Jr., Anal. Chern., 48(3), 481 (1976). (18) G. A. Eiceman, L. R. Field, and R. E. Sievers, Ami. Clem., 50(14), 2152 (1978).
RECEIVED for review June 14, 1979. Accepted September 7, 1979. The work presented in this paper was performed in part under U.S. Army Contract No. DAAKll-77-C-0086 and in part under U S . Environmental Protection Agency Contract No. 68-02-2272. The authors thank both sponsors for granting permission to publish this work.
