Anal. Chem. 1982, 5 4 , 2406-2409
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Trace Gas Chromatographic Analysis by Use of Large Sample On-Column Injection with Bonded Phase Capillary Columns A. Zlatkis," F A . Wang, and H. Shanfleld Chemistry Department, Unlversity of Houston, Houston, Texas 77004
A new approach to trace gas chromatographlc analysls Is described whlch utlllzes on-column lnjectlon of large llquld samples (as much as 100 pL) onto bonded phase caplllary columns. Trace substances of Interest are adsorbed from the solvent at relatlvely low temperature durlng the first pass through the column, allowing the solvent portion to be removed selectlvely. Subsequently, the trace substances are thermally desorbed, trapped, and analyzed gas chromatographically by reverslng the column. I n this way it Is possible to achleve analyses at below the part per bllllon level, utlllzlng a slngle column wlth a conventlonal flame lonlratlon detector.
Trace level analysis in conventional capillary gas chromatography has generally been limited by the sample size which would still maintain satisfactory resolution. In a relatively few instances, it has been possible to utilize element-specific detection devices, but wide spectrum detectors such as the flame ionization detector (FID) have been limited typically to the part per million range due to sample size restrictions. The development of bonded phase (nonextractable or immobilized) fused silica open tubular columns (FSOT) has made it possible to employ liquid on-column injection techniques without loss of column performance. Blomberg et al. ( I ) have shown that the immobilized stationary phase is not displaced from the capillary surface by solvent or even water. Sandra et al. (2) have reported that film thickness, film homogeneity, and resolution were not affected in such bonded phase FSOT columns by extensive rinsing with both polar and nonpolar solvents. Work recently carried out in our laboratory showed that peak symmetry of aromatics was preserved in such a column with the solvent n-hexane in amounts as large as 10 pL, using an on-column injector constructed for this purpose (3)* The successful injection of samples as large as 100 pL would significantly improve trace component analysis. However, the solvent tends to swamp out component peaks. Grossly distorted chromatograms result with sample sizes of this order even when the "trace" components are present in relatively large amounts. The work which we report in this paper utilizes our oncolumn injector (3) to introduce sample volumes of up to 100 pL into bonded phase FSOT columns. This first step causes the liquid sample to pass through the column so as to adsorb the trace components of interest at relatively low temperature, permiting the stripped solvent to leave the column. The second step recovers these trace substances in a liquid nitrogen trap, and finally, analysis is carried out in the same column by operating it in reverse. In essence, this procedure amounts to a "heart-cutting" technique (4, 5).
EXPERIMENTAL SECTION The gas chromatograph employed in this work was an HP5830A GC unit equipped with a flame ionization detector, in conjunction with an HP18850A GC integrator. Sample injection was effected with our on-column injector (3) using a 1-mL needle syringe (32 gauge needle, 12 cm length, Hamilton Co., Reno, NV). Except where otherwise noted, on-column injection was done 0003-2700/82/0354-2406$01.25/0
at a column temperature of 60 "C. Following injection, the column was kept at this temperature for 2 min and then programmed for a 2 "C rise per minute. For most of the experiments concerned with quantitative analysis, the bonded phase FSOT column used was a DB-1 60 m X 0.32 mm i.d., 1.0 pm film (J & W Scientific, Inc., Rancho Cordova, CA). This is a nonpolar, hydrophobic, SE-30 cross-linked polymeric column phase. Integrity of column performance (in terms of retention time and Trennzahl number) was verified by running a Grob test mixture before and after the use of this procedure. A stock solution of n-nonane, n-decane (Fluka AG, Buchs SG, Switzerland), and n-undecane (Alltech Associates, Inc., Deerfield IL) was prepared in n-hexane (MCB Manufacturing Chemists, In., Dayton, OH), to a concentration of 5 ng/pL for each component. This was used to obtain retention time data and as a stock solution for dilution to approximately the 50 ppb level for other experiments. Solutions of the same concentrations in nhexane were also prepared for 2-octanone, 5-nonanone, and 2decanone (J. T. Baker, Phillipsburg, NJ). Analyses seeking trace impurities were also carried out on a 30 mg/mL solution of 1,2-dicyanobenzene in acetonitrile. Triplicate analytical runs were conducted in all cases, except for the analysis of dicyanobenzene in acetonitrile, where a l-pL sample injection was used in a single experiment and a 10-pL sample injection was used in one other. For these analyses a 50 m X 0.32 mm i.d., 0.5 pm fii, bonded phase methyl silicone FSOT column was employed (Quadrex Corporation, New Haven, CT).
RESULTS AND DISCUSSION Table I summarizes the analytical data for three runs with l-pL sample of the stock solution of n-nonane, n-decane, and n-undecane (5 ng/pL each) using on-column injection. This run served to provide retention time and area count data for these components. Figure 1was obtained when 100 pL of pure solvent n-hexane was injected into the column (conditions noted on figure). The solvent peak rose at 4.48 min and fell at 15.15 min. This information was applied to subsequent n-hexane solution runs. The stock solution of n-nonane, n-decane, and n-undecane was diluted 100-fold with n-hexane (0.05 ng/pL for each component), and 100 pL of this solution injected (on-column, conditions same as in Figure 1). During this operation, the column was not connected to the detector. Liquid was observed bubbling from the column outlet a t 6.01 min, and it ceased at 14.16 min. At the 15.15 min mark, the column outlet flow was directed through a 30 cm X 0.015 in. i.d. stainless steel empty capillary trap (using a heabshrinkable Teflon tube connection) immersed in liquid nitrogen. At this point, the oven temperature was raised to 150 OC to facilitate rapid desorption of trace substances from the column. After 20 min, the oven was cooled, and the column reversed, i.e., the outlet of the trap was connected to the injector and the end of the column formerly connected to the injector was connected to the detector. The liquid nitrogen flask was now removed, the oven closed, and the analysis started. Figure 2 shows the chromatograms obtained for two separate runs. Table I1 summarizes the analytical data for three runs. For further comparison, the chromatogram for 100 pL of the solvent hexane is shown in Figure 3. The latter reveals the large number of trace impurities in the solvent, including some n-decane and n-undecane. The relative standard deviations 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
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Table I. Analytical D,ata for 1W L On-Column Injection of n-Hexane Containing 5 ng of n-Nonane, n-Decane, and n-Undeaane n -undecane n-nonane n-decane 7
mean std dev re1 std dev (5,)
area count
retention time, min
area count
retention time, min
area count
retention time, min
3646 3737 3750 3711 56.7 1.53
21.68 21.71 21.71 21.70 0.017
3691 3779 3798 3756 57.1 1.52
29.67 29.69 29.69 29.68 0.012 0.04
3893 3986 401 2 3964 62.6 1.56
37.84 37.85 37.85 37.85 0.0058 0.02
0.08
~
Table 11. Analytical Data for 100 gL On-Column Injection of n-Hexane Containing 5 ng of n-Nonane, a-Decane, and n-Undecane
mean std dev re1 std dev (%) recovery (%)
n-nonane area retention count time, min
n-decane area retention count time, min
n-undecane area retention count time, min
4017 2602 3843 3487 772 22.1
4402 4096 3582 4027 414 10.3
4473 3562 3516 3850 540 14.0
21.88 21.65 21.90 21.81 0.14 0.64 94.2
29.78 29.73 29.86 29.79 0.065 0.22 107
37.87 37.89 37.85 37.87 0.020 0.053 97.1
r
rl
Flgure 1. Gas chromatcgrtim of 100 pL of n-hexane solvent only: 60 m X 0.32 mm i.d. DB-1 bonded phase FSOT column: on-column injectlon used, with column initially at 60 'C for 2 min, and then programmed up at 2 OC/min; carrier gas, helium; attenuatlon, 23.
observed in the 100-pL injection experiments (Table 11)were substantially larger than the 1pL data (Table I). Nevertheless, the percent recovery was over 90% for each of the standards used, with a maximum of 107% for n-decane. Figure 4 shows the chromatogram obtained with on-column injection of a 1-pL n-hexane stock solution containing 5 ng each of 2-octanone, 5-nonanone, and 2-decanone. This was used to obtain retention time and area count data for these components. Table I11 lists the analytical data obtained for three runs. Figure 5 is the chromatogram obtained with a 100-pL oncolumn injection for thew ketones (5 ng each present). In this case, trapping was comrnenced at the 15.10 min point. Table IV lists the analytical data for three runs. Recovery exceeds 90% for each of the standards used, with a maximum of 128% for 2-octanone. It is interesting to note in Figure 5A that solvent impurities observed in the solvent blank run (Figure 3) (at retention times of 18.47, 26.73, and 29.60 min) appear
Flgure 2. (A) Gas chromatogram of trace components (in 100 KL of nhexane) trapped after 15.15 min: 60 m X 0.32 mm i.d. D E 1 bonded phase FSOT column; trap and column at 60 OC for 2 min and then programmed up at 2 'C/min; carrier gas, helium; attenuation, 23. (B) Gas chromatogram of repeat of experlment in Figure 2A.
in this chromatogram, e.g., at retention times of 18.74, 26.84, and 29.63 min.
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Table 111. Analytical Data for 1pL On-Column Injection of n-Hexane Containing 5 ng of &Octanone, BNonanone, and 2-Decanone
mean std dev re1 std dev (%)
2-octanone area retention count time, min
area count
retention time, min
3110 3120 3134 3121 12.1 0.39
3372 3415 3416 3401 26.1 0.74
33.93 33.93 33.90 33.92 0.017 0.050
27.02 27.02 26.99 27.01 0.017 0.063
5-nonanone
2-decanone area retention count time, min 3023 3036 3051 3037 14.0 0.46
43.43 43.41 43.38 43.41 0.025 0.058
Table IV. Analytical Data for 100 p L On-Column Injection of n-Hexane Containing 5 ng of 2-Octanone, 5-Nonanone, and 2-Decanone 2-octanone 5-nonanone 2-decanone area retention area retention area retention count time, min count time, min count time, min
mean std dev re1 std dev (%) recovery (%)
4521 4103 3359 3994 589 14.7
27.22 27.17 27.08 27.16 0.071 0.26 128
34.10 34.06 33.99 34.05 0.056 0.16
3836 3140 3223 3400 380 11.2 100
3251 2714 2786 2917 291 9.98
43.57 43.54 43.47 43.53 0.051 0.12 96
Gas chromatogram of trace components In n-hexane solvent only, trapped after 15.15 min. Column and column conditions were the same as those given In Figure 2A Figure 3.
Figure 5. (A) Gas chromatogram of trace components (in 100 pL of n-hexane)trapped after 15.10 min: 60 m X 0.32 mm i.d. D E 1 bonded phase FSOT column; trap and column at 60 O C for 2 mln and then programmed up at 2 OC/min; carrier gas, helium;.attenuation,2'. (B)
Gas chromatogram of repeat of experiment in Figure 5A, except that trapping was started at the 15.80 mln point.
Figure 4. Gas chromatogram of 1 pL of n-hexane containing 5 ng each of 2octanone, 5-noanone, and Pdecanone: 60 m X 0.32 mm 1.d. DB-1 bonded phase FSOT column: on-column injection used, with column initially at 60 OC for 2 mln and then programmed up at 2 OClmin; carrier gas, helium; attenuation, 23.
Figure 5B is the result obtained when trapping was delayed to 15.80 min for the ketone standards. Retention times for each were closely reproduced. However, this relatively small difference in the time at which trapping commences results in a substantial difference in the recovery of components with lower retention times. The data reported for the hydrocarbons and ketones are
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
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tracton Collected
/ c f r a c t i o n collected
'Ime
Figure 6. (A) Gas chromatogram of 1-pL sample containing 30 pg of 1,2dicyanobenzene in acetonitrile: 50 m X 0.32 mm i.d. bonded
phase methyl silicone FSOT column; on-column injectlon used, wlth column initially at 70 O C (and then programmed up to 250 O C at 5 'C/min; carrier gas, hellum; attenuation, 2'. (B) Gas chromatogram of trace components trapped from 10 pL solution of 1,Pdlcyanobenzene in acetonitrile, lyirig between 3.4 and 16.5 mln, and following 18.88 min: 50 m X 0.32 mm i.d. bonded phase methyl silicone FSOT column. Column o eratlng conditions were the same as in Figure 9A. Attenuatlon was 2 .
F
at a concentration level of approximately 50 ppb. However, by operating the flame ionization detector at its maximum sensitivity (with two to one signal to noise ratio), it should be feasible to analyze for these substances below the part per
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billion level. With this technique, cumbersome (and often nonquantitative) preconcentration steps can be avoided in many analyses, e.g., analysis of priority pollutants. Alternatively, the use of concentration steps can achieve sensitivity levels to the parts per trillion or even lower. The sensitivity of this method could also be enhanced by the use of more sensitive detection systems embodying electron capture, flame photometry, or photoionization. Figure 6A shows the chromatogram for 1p L of a 1,2-dicyanobenzene solution in acetonitrile (30 mg/mL). The two large peaks correspond to acetonitrile (centered at about 3 min) and dicyanobenzene (centered at about 17 min). The impurity peaks lying before and after the dicyanobenzene are distorted and unresolved. In a following experiment using 10 pL, cold trapping was carried out for the trace substances between the acetonitrile and the dicyanobenzene peaks (approximately 3.4 min to 16.5 min). The cold trap was disconnected, the dicyanobenzene peak allowed to leave the column, and then the cold trap was reconnected to trap trace substances again (after approximately 18.9 min). The chromatogram of the trapped trace substances (obtained by reversing the column) is shown in Figure 6B. Many well-resolved peaks are now clearly shown. No attempt was made to identify or quantitate specific substances. This experiment establishes the potential feasibility of analyzing for trace impurities for one substance dissolved in another solvent. Thus, a dual elimination is involved: one for the solvent, the other for the principal solute.
LITERATURE CITED ( I ) Blomberg, L.; Bultjen, J.: Markldes, K.; Wannman, T. J . Chromafogr.
1982, 239, 51-80. (2) Sandra, P.; Redant, G.; Schacht, E.; Verzele, M. J . H@h Resolut. Chromafogr. Chromatogr. Commun. 1081, 4 , 411-412. ( 3 ) Wang, F A . ; Shanfleld, H.; Zlatkls, A. Anal. Chem. 1982, 5 4 , 1888-1 888. (4) Deans, D. R. Chromatographla 1988, 1 , 18-22. (5) Bertsch, W.; Hsu, F.; Zlatkls. A. Anal. Chem. 1978, 4 8 , 928-931.
RECEIVED for review June 28,1982. Accepted August 31,1982. This work was supported by the US.Army Research Office under Grant No. DAAG29-82-K-0054
