ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
2395
AIDS FOR ANALYTICAL CHEMISTS Liquid Sample Introduction in Gas Chromatography John Chih-An
Hu
Quality Assurance Laboratories, Boeing Aerospace Company, Seattle, Washington 98 124
Although gas chromatography (GC) is highly sophisticated, the liquid sample injection port has continued to be a relatively weak link in the GC system. The conventional syringe injection is limited t o highly volatile or low viscosity samples and possesses problems associated with syringe and septum, such as ghost peaks, contamination of the authentic samples, and emission of toxic aerosol particles during needle withdrawal (I). Viscous liquid samples are usually injected by one of three basic procedures: (a) t h e sample is diluted with a suitable solvent and injected with a syringe, (b) the sample is directly injected with a syringe or with a "plunger in needle" type solid sampler, and (c) a special sample introduction device is used. T h e first procedure is tedious and time consuming. It causes solvent peak trailing, obscures early peaks, reduces sensitivity, and can modify the sample. In the second procedure the syringe is totally unsatisfactory because the viscous liquids cannot adequately be drawn into the syringe and much less quantitatively ejected from it. The solid samplers have t h e advantage of eliminating t h e dead volumes and allowing sampling by dipping instead of suction. But they still possess t h e hereditary problems associated with syringe and septum. They also have a metallic structure which tends to initiate thermal degradation of sample. The exact amount of sample cannot be measured directly and elaborate calibration is needed for quantitative work. T h e third method usually results in expensive highly specific procedures. Bowman and Karman devised a sealed tube loading and crushing system ( 2 ) . Hewlett-Packard manufactured a solid sample injector (Model 19012A, discontinued) and PerkinElmer introduced a capsule sampler (Model MS-41). Jenkins disclosed in a US.patent ( 3 ) a two-position, four-port loop type device for capillary GC. Hampton patented a complicated system for sample introduction and pyrolysis ( 4 ) . Ligon described a cartridge type device for t r a p desorption ( 5 ) . Peterson e t al. reported another device for t r a p desorption (6). Buser and Widmer introduced a capsule-insertion technique with a specially designed thorn (7). All these reported devices could be utilized to introduce viscous liquid samples to the gas chromatograph, but they are relatively complicated in construction and operation. H u reported a new approach to pyrolysis gas chromatography (8-101, namely, chromatopyrography ( 2 1 ) . One of the features of chromatopyrography is the creation of a new liquid sample introduction method. Chromatopyrography is specially designed for quick chemical characterization of the compounded rubberlike polymeric materials. It identifies the formulation and the polymer separately in a one-step/two-shot process. T h e composition of the volatile constituents which represents the characteristics of the formulation is analyzed in the first shot and the polymer is analyzed in t h e second shot. From the view point of liquid analysis, the first shot of the process is of special interest since the analysis of volatile constituents in a compounded rubber is essentially a liquid analysis with the liquids held in the polymer matrix in a solid form. As far as liquid analysis is concerned, the polymer 0003-2700/79/0351-2395$01.00/0
matrix may be viewed as a liquid sample holder. A logical extension of this concept is to create a liquid sample introduction method which emulates the polymer matrix as a sample holder. Liquid samples can be placed in a capillary tube held in a coil probe or coated on a ribbon probe surface. The loaded probe is introduced to the gas chromatograph and the liquid sample is analyzed in the same way as a rubber sample in the first shot of chromatopyrography. The use of a capillary tube is equivalent to the syringe method but does not exhibit any of the problems associated with the syringe and septum. This method is particularly suitable for viscous and heterogeneous liquid samples.
EXPERIMENTAL Apparatus. A FM 810 gas chromatograph (manufactured by Hewlett-Packard) was used. This instrument was equipped with a flame ionization detector and a FM 80 pyrolysis unit. The packed column (0.318 cm 0.d. x 3.3 m) with liquid phase SE-30 ( 5 % ) was installed. The injection port was preheated to and maintained at 270 "C. Helium flow rate was 30 mL/min. For hydraulic fluid analysis, the starting column temperature was 150 "C. The column temperature was programmed from 150 t o 300 "C at 20 "C/min and then kept at the upper limit (300 "C) until the chromatogram was completed. For wet paint solvent analysis, the starting column temperature was 30 "C. The column temperature was programmed from 30 to 200 "C a t 6 "C/min. For pyrolysis, the column temperature was programmed from 30 to 300 "C at 20 "C/min. The polymer was pyrolyzed with 12-A current (1000 "C) for If) s. Flow Path of Carrier Gas. The injection port has a labyrinthlike structure with three concentric cylinder:; as shown in Figure 1. The inner cylinder is the injector liner, the middle cylinder is the injector sleeve, and the outer cylinder is the injector barrel. The carrier gas enters the injection port near the front end of the injector barrel, travels inward through the outer annular space, then outward through the middle annular space, and enters the liner leading to the column with another reversal. of direction (Figure 1A). By the removal of septum, the flow of carrier gas inside the injector liner is stopped and no back flush exists in the liner. The carrier gas is released to the atmosphere at the open end of the injector barrel as shown in Figure 1B. Sample Loading and Measuring. The disposable, open ended capillary glass tubes were used as sample holders. The liquid sample was loaded through capillary action by letting the lower end of the capillary tube touch the liquid surface. The amount of sample could be reduced by lightly touching the lower end of the loaded capillary tube to a piece of tissue paper and withdrawing a controlled amount of sample. The exact amount of the sample was measured either by weighing or by volume measurement (the length of the liquid column inside a capillary tube) with a ruler, a measuring magnifying glass, or a measuring microscope depending on sample size and accuracy desired. Sample Tube Holder. The pyrolysis coil probe served as a sample tube holder. For wet paint analysis, the coil probe served the dual function of supporting the sample for solvent analysis and pyrolysis for polymer analysis. For homogeneous monomeric liquid analysis, no pyrolysis was involved and a simpler coil probe or other mechanical type support without electrical connections could be used. Figure 2A shows a simple coil probe made from nichrome wire, a l/a-in.diameter metal rod, and a nut. Such a low cost probe served the purpose satisfact.orily. The conventional d 1979 American Chemical Society
2396
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
A
B
Figure 3. Chromatograms of Hyjet I V by the syringe method (A) and by this method (5) SPRhlG
I Y J EC'OR
INJECTOR SLEEVE
LINER
G,ASS
1
I
HEATED' BLOCK
INJECTCR BAEiREL GAS
Figure 1. Carrier gas flow path
C4FlLWRY GMSS T L l B I
MiTALrVI?ECOlL
B
SAIIPLIHOLD% ICAPILL~RI GLASS nisi,
Figure 2. A coil probe (A) and the probe tip (6)
pyrolysis coil probe accommodated a 2-mm o.d. tube. For firm support of a smaller sample size, a smaller tube was placed inside the larger one as shown in Figure 2B. Sample Probe Insertion. After the coil probe was loaded with a sample, the septum cap was unscrewed but momentarily held in position with finger pressure. Simultaneously the septum cap was removed and the sample probe was inserted into the injector liner. Between septum removal and sample insertion, the carrier gas was released to the atomsphere at the uncovered end of the injector barrel. At this moment the sample probe was quickly placed into the interior of the injector liner where no sample loss due to back flush can occur. As soon as the probe nut was sealed, the forward flow was re-established and the chromatogram started to develop. Materials. An aviation hydraulic fluid, Hyjet IV, supplied by Chevron International Oil Co., Inc., was qualified to a Boeing Company Material Specification, BMS 3-11, Type 4. This fluid was analyzed by this capillary tube method and by the conventional syringe method with a Hamilton 701N syringe. A 1-gL sample was used to obtain each of the chromatograms in Figure 3. Two alkyd enamel wet paints were analyzed by chromatopyrography (11). The solvents of the wet paints were analyzed in the first shot of chromatopyrography and the polymer was analyzed in the second shot. These two wet paints were qualified to a Boeing specification, BMS 10-23. The yellow gloss enamel was a Type 1 material and was supplied by the Andrew Brown Division of Kopper Company. The green semigloss enamel was a Type 2 material and was supplied by Preservative Paint Com-
TIME (MINUTE) Figure 4. Chromatopyrograms of alkyd enamels: (A) yellow, ( 6 )green
pany. Approximately a 0.3-gL sample was used to obtain each of the chromatopyrograms in Figure 4.
RESULTS AND DISCUSSION T h e results are illustrated in Figure 3 and Figure 4. T h e X axis of these figures represents t h e retention time and the Y axis represents recorder response. Figure 3 shows two chromatograms of Chevron Hyjet IV, which were obtained with equal amounts of sample under identical conditions except for the sample introduction method. Chromatogram A was obtained with t h e conventional syringe method and chromatogram B was obtained using the capillary tube method. The quality of chromatogram B with respect to sensitivity and resolution is obviously better. This demonstrates that the capillary tube method has significant advantages over t h e conventional syringe method. Obviously, sample losses with the syringe method result in the reduced sensitivity and resolution. When the syringe is loaded with a hydraulic fluid, the liquid sample enters the syringe needle slowly. When t h e syringe is unloading, the sample leaves the syringe needle reluctantly. This sample cannot be completely emptied to the GC column, hut is divided into five uncontrollable fractions, t h e first is introduced to the GC column, the second is adsorbed on the walls of the injection port, the third is absorbed by the rubber septum, the fourth remains in the syringe needle and on t h e wall of the syringe cylinder, and the final fraction is sprayed out as aerosol particles and vapors ( 1 ) . Since the proportions attributed t o t h e five fractions vary between injections, t h e amount of t h e desired first fraction is not reproducible and the quantitation is, therefore, not reliable. The four fractions, which account for the sample losses, reduce the sensitivity,
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
cause ghost peaks, and create potential health hazards ( I ) . On the other hand, the capillary tube method has eliminated all these problems. I t also eliminates the cost of syringe cleaning or special cleaning equipment, syringe replacement, and sample preparation (separation and dilution). The cost of the capillary glass tubes is negligible. The capillary tube method is not only a better method, but it also results in substantial cost savings. An advantage of this method is that volatile trace contaminants in oil samples can be analyzed directly by eliminating solvent interference. Some volatile trace contaminants in aviation hydraulic fluid, for instance, increase viscosity to a degree which may be detrimental to the hydraulic system. It is important to quantitatively determine these trace contaminants. By using the capillary tube method, these trace contaminants, which would otherwise be obscured by the solvent peak, can be easily monitored. Figure 4 shows the results of a direct analysis of two wet paints by chromatopyrography. T h e conventional wet paint analysis was tedious and difficult. A systematic approach was recently reported by T. K. Rehfeldt and D. R. Scheuing (12). In their approach, liquid chromatography was used as the pivotal method. T h e complexity of the wet paint analytical task was illustrated by a colorful flow diagram (12) and it was a typically time consuming process. On the other hand, with this liquid sample introduction method, the complicated wet paint analysis can be clone easily, quickly, and economically by chromatopyrography. The heterogeneous paint sample is placed in the capillary glass tube without sample preparation, introduced to the gas chromatograph as explained, and analyzed by chromatopyrography. Figure 4A shows the chromatopyrogram of yellow paint and Figure 4B shows that of green paint. T h e curves on the left side of Figure 4 are the chromatograms of the solvents used in the formulations. Each peak in the chromatograms can be quickly identified by mass spectrometry. The whole pattern of the chromatogram rep-
2397
resents a specific formulation of the paint. Comparison of chromatogram A and chromatogram B patterns illustrates differences in paint formulations. Any alteration of the chromatogram pattern of a previously qualified paint indicates a formulation change. The curves on the right side of Figure 4 are the pyrograms of the polymer. These pyrograms are characteristic of the polymer structures. Each pyrogram pattern distinctly represents a polymer and can be positively identified. Since the curves on the right of Figure 4A and Figure 4B show the same pattern, both the yellow and green paints are characterized to contain the same alkyd polymer. The inorganic pigments which remain in the sample holder are, of course, available for further analysis if desired. The capillary tube method has widespread potential applications. By a fill-and-drain technique, the capillary tube may be used as a splitless introduction method for capillary GC. The method also can be applied in the petroleum industry for crude oil analysis.
LITERATURE CITED E. B. Sansone, H. Wolochow, and M. A. Chatigny, Anal. Chem., 49, 670 (1977). R. L. Bowman and A. Karman, Nature (London), 182, 1233 (1958). P. Jenkins, U S Patent 3,118,300, Jan. 21, 1964. W. C. Harnpton, U.S. Patent 3,186,801, June 1, 1965. W. V. Ligon, Jr., US. Patent 4,004,881, Jan. 25, 1977. H. Peterson, G. A. Eiceman, L. R. Field, and R. E. Sievers, Anal. Chem., 50, 2152 (1978). H. U. Buser and H. M. Widmer, J . High Reson. Chromatogr. Chromatogr. Commun., (4), 177 (Apr. 1979). J. C. A. Hu, 28th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, Feb. 28-Mar. 4, 1977, paper No. 258. J. C. A. Hu, Anal. Chem., 49, 537 (1977). J. C. A. Hu, U S . Patent 4,159,894, July 3, 1979. J. C. A. Hu, 30th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, Mar. 5-Mar. 9 1979, paper No. 092. T. K. Rehfeldt and D. R. Scheuing, Anal. Chem., 50, 980A (1978).
RECETC’ED for review May 14,1979. Accepted August 27,1979.
