An Experiment in Manual Multiple Headspace Extraction for Gas Chromatography An Undergraduate Laboratory Experiment Using Simple Equipment William C. Welch and Thomas G. Greco Millersville University of Pennsylvania, Millersville, PA 17551 Modern applications of chromatography, especially in environmental analvsis of ~rioritvoreanic ~ollutants.involve the determination of;olatil"e co£s. To reduce the influence of the s a m ~ l ematrix and to isolate the desired analyte, methods involving gas-liquid extraction and gas-phase analysis have become commonplace in laboratories that carry out this work routinely. Methods such as purge-and-trap or auto-injection headspace analysis require specialized, expensive instrumentation that is usually not available in small undergraduate chemistry departments. This experiment was developed to eive the underaaduate analvtical student ex~eriencein headspace gas chromatography and gas-liquid extraction in the absence of sophisticated extraction and sample-injection equipment. The experiment can be done in one or two laboratorv Deriods using- items commonlv found in undergraduate departments.
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Modifying Successful Methods Used in the Analysis of Environmental Samples This work is based on the method of multiple headspace extraction orieinated bv McAuliffe (1.2) . , . and used bv McNally and ~ r l (b3 , 4 )to determine the solubilities oivolatile oreanic com~oundsin water svstems. Kolb et. al. (5.6. 7) a n l ~ t t r (8j e modified the o r i h a l procedure and f& ther developed its applications to water, soil, and industrial samples. More recently, a variation of this technique has been reported by Stuart et. al. (9)for the determination of the volatile aromatic compounds of gasoline in moundwater and soil. This experiment has been modified from the above methods and can be used for many applications. This particular work focused on the analysis of several environmentally i m ~ o r t a ncom~ounds: t benzene. toluene. the xvlenes, and several chlorinited alkanes. Theory Gas-LiquidPartitioning and Successive Equilibrations Multiple headspace extraction involves the partitioning of a volatile compound between two phases in a sample vial. The phases used here are water and air. Successive equilibrations of a water sample with fresh portions of air are carried out. Each air heads~aceextract is analyzed by cas chromatography, the peak area becoming smaller fir each successive extract. If carried out exhaustively, the peak areas must be summed to get the total area, which is related to the original concentration. In practice, this is done by arithmetic extrapolation, which is described below. The distribution of a volatile compound between the gas and liquid phases is described by the distribution coefficient KO:
where [XI, is the analyte concentration in the gas phase and [XI, is that in the liquid phase (10). Arithmetic Extrapolation
It can be shown that aRer n equilibrations, where (LXI,), is the quantity of component X in the gas phase, and [XI, is the quantity of X present initially in the water sample. A log plot of detector response (log area) versus n will produce a straight line with a slope equal to log (K+ 1) and with K = (10"l0P" - 1).It should be noted that K i s not the "true" distribution coefficient because the slope includes experimental and instrumental parameters. The sum of the peak areas can be calculated as the sum of their geometric progression (5,8):
whereAl is the area from the first equilibration. This area sum can be used to establish a response factor (RF) for each analyte when standard solutions are analyzed.
RF = amount or concentration
CA. Unknown water samples are analyzed under the same conditions, especially temperature. Then the unknown concentration is calculated from its measured area sum and the RF for the compound of interest.
Experimental Standard Solutions Stock Standard Mixture A stock standard mixture is prepared by weight in a 50-mL volumetric flask that has been half-filled with methanol (Fisher, catalog no. A452-4, HPLC grade). After dilution to the mark with methanol, the final concentration of each com~oundshould be in the ranee of 1000-1500 mg1L. After mixing, split portions of stock solution are transferred to smaller vials (Wheaton, no. 224882) with Teflonlined screw-top caps and stored at approximately 4 T until needed. Care should be taken to ensure that no air is trapped in the vials.
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Aqueous Working Standard Solution An aqueous working standard solution is prepared from the stock, which has been allowed to come to room temperature before opening. A Drummond-type micropipet is Volume 70 Number 4 April 1993
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used to transfer a specific volume (typically 0.2 to 1mL) of stock solution into a clean, dry 500-mL volumetric flask threequarters filled with pure water (Fisher, catalog no. W5-4, HPLC grade). The stock solution should be injected directly into the water and not on the neck of the flask. After dilution to the mark with water, allow the sample to slt for up to 30 min ta ensure complete dissolution of the samole. Then invert the sealed flask three tlmes. Avoid excesslve shaking to reduce the loss of volatiles. Also. care must be taken to use small amounts of stock so~utidnso that the solubility of the compounds in water is not exceeded. Generallv. the workine standards are limpL 0of s t k k solution per 500 ited to the range of 16100 mL of water.
(Dl Cap the syringe with a clean tight-fitting septum (Fisher, red rubber, catalog no. 03225-5, flanged for serum battles), and agitate the air-water mixture for 5 min. Without changingthe volume ratio, allow the inverted, sealed svrinee to stand for an additional 5 min befare samolinc . the headspare. Clamping the inverted syllnge ro a ring scond-with the plunger resting on the baa+works well for this stage. (El Using a 5-mL glass, gas-tight chromatographic syringe (Hamilton no.1005RNL withdraw 0.5 mL and iniect into the eas chromatomanhie svstem. (Other inie&on valumcs may be approp&.e,dipending on rond;tlons.* tF) Expel the first arr headspare from the sample spngc. IG, Draw a fresh air hradspace into the syringe so rhnr it will have equal volume to the water in the syringe. ~
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~
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. ~ ~ ~ ~
Steps C through G are repeated until several (usually 3-4) successive headspace extracts have been chromato-
Unknown Solutions To obtain recovery data, standard control samples (USEPA Quality Assurance s a m ~ l e sWS879 and WP879. Supelco, citalog no. 4-8226) were prepared and analyzed in the same fashion as the standards. Their concentrations were adjusted to fall within the range of the working standard solutions. In addition, a blank sample of pure water was prepared and analyzed using the method described above. Unknown control samples may be prepared "in house" or from a variety of standard-control sample sources, such a s the USEPA, USNIST, and commercial suppliers (e.g., Fisher and Supelw). Care must be taken to prevent the loss of volatile compounds. Commercially available standard mixtures are also available for analysis, and instructions for handling and use are provided.
Extraction and Analysis The steps, as illustrated in Figure 1,are given below. (AI A 30-50-mL portion of the wurkin~aqueous sample is drawn into a 50-ml. glass, Luer-tip sjnnge (B-D Yale no.
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2305 or Osee. air-tieht. no. SOMAX-LL-GTJ.Avoid using the water in the ne& of the volumetric flask. (BI Inven thr syringe a n d expel exwss dutwn into a waste contamer, leavlng only 25 m L of snmplc in the syringe. (C) Pull the plunger hock to introduce an equal volume af aw above the sample
Chromatographic Conditions Two types of columns were used: 5%phenyl silicone on a fused-silicacapillary 25-m length, 0.52-mm diameter, and l-mm fihn thickness 118-in. diameter, stainless steel packed eolumn OVlOl coated, 64%length Any type of GC should work. We used a Perkin-Elmer Model 3920B equipped with flame ionization detectors. Both columns were set at the following conditions: attenuation = l x helium flow rate = 5 mL/min inject temperature = 200 'C detector temperature = 210 'C temperature program: initial: 50 'C (2 min) rate: 8 'Clmin final: 90 'C (2 min) Different Darameters and columns mav be used for other appl~cat~ons and systems A Hewlctt-Packard Model HP3394Aintemator was used to record chromatograms and determine i e a k areas. All ~ e a k sexceDt . those of dichloroethane and trichloroethane. are resolved and shown in Figure 2. The chlorinated eth: ane results could not be determined under these conditions, although their wmbined behavior was linear with respect to extraction number. ~
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Results and Discussion Chromatograms of blank samples showed no detectable analyte peaks. Figure 3 illustrates the log peak area vs. n behavior obtained from two standard solutions of benzene on the capillary column. The average response factor ob-
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stop Figure 1. Schematic representation of the multiple headspace extraction procedure. (A) Withdraw sample. (6) Invert. (C) Introduce air. (D) Seal headspace and equilibrate. (E) Sample the headspace, and injen into GC. (F) Expel headspace. (G) Introduce fresh air.
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Journal of Chemical Education
Fig~re2. Gas cnromatogram (mkmn 1) of a standard m MJre: ( 1 ) metnanol; (2) cnioroform: (3) 1.2-d;choroethane ano 1.1.1trichloroethane: (4) benzene; (51 IolJene; ana (6)pxy ene.
Table 2. Average Distribution Coefficients and RF Values. Standard Deviations are Shown.
Compound
No. of values of used
K
RF
(ppmEArea)
Benzene
8
2
0.22 f0.33
1.51 x 1 0 4 f 4 ~
Toluene
8
2
0.2 f 0.02
1.90~ lo4* 1 x lo4
p-Xyiene
8
2
0.26 f 0.05
2.46 x l o 4 f 6 x lo-'
Chloroform
2
2
0.14 f 0.33
1.92 x 1 0 4 + z x
Table 3. Recovery Data for Test Samples.
Sample Benzene -1
Table 1. Calibration Data for the Benzene Standards Using Data Obtained from the Plot Shown in Figure 3. tr
ppm
slope
K
Area ( I ) X Area
(min) (mgiL)
RF (ppm/Z Area)
2.93 32.4
4.09095 0.23 395630 2093900
1.55~
2.91
4.08593 0.22
1 . 4 7 lo4 ~
6.5
79162
440970
t is the retention time of benzene in minutes.
tained from this data is then used for concentration calculations for unknown samples. Table 1shows the calculated results of two benzene standard solutions. Table 2 displays the average value of each distribution coefficient K and RF for benzene, toluene, p-xylene, and chloroform. Table 3 displays the results for aqueous standard test samples of these compounds. The method appears to be quite good for the aromatic compounds, but more work or different chromatographic conditions may be needed for the halogenated alkanes. Conclusion Students carried out this experiment in the instrumental analysis course, and it was very well received. They acquired a better understanding of vapor-liquid equilibrium and valuable experience with headspace sampling and analysis via gas chromatography. This technique may be applied to a wide variety of samples, such as soils (9,111,crude oil, chicken feed and eggs, and pharmaceuticals (5).It requires no sophisticated or automated equipment, and reasonably accurate results can be obtained for aromatic compounds. The major potential sources of error are improper temperature control during the analysis period and improper preparation or handling of the solutions containing volatile analytes. Improvement should be obtained ifa 50-mL
ppm Measured
9.8 4.5 12 15 3.6 29.7 f 11.4 11 9 3.4 19.1 f 1.7 7 6 5.0 5.4 47.7 i 18.4
8.9 13 17 4 31.3 9 7 4 17.3 5 5 4 4 64.7
*
a
-zb Figure 3. Plot of log area vs. exiraction number n for two standard solutions of benzene.
Actual ppm
-3b
-4d Toluene -la -2 db
pxylene -la
-zb
-3O
m~ylene-4d ~Xylene--ld ChloroformC
'Sample 1 was prepared from EPA reference WP879 stock solutions. 95% confidence limlts are shown. b~am~les 2 and 3 were prewred in our laboratory The chloroform samplehas prepared from EPA referenceWS879 stock solution 4. The 95% confidence limit is shown. 'prepared from Supelco stock solution catalog no. 4-8226.
gas-tight syringe is used for the headspace extractions instead of the glass plunger type. Acknowledgment We thank Dr. Robert L. Grob. Villanova Universitv. for his valuable advice, Mr. ~ i l l i a mBates for his work onthe oreliminarv studies. and the Millersville Universitv Alumni ~siociation,bhich provided partial support f& this work, in the form of a Neimeyer Hodgson Student Research Grant. Llterature Cited
. . 8. Ettre,L.S.; Jrmes,E.;Todd.B. S.Chromut0gr0grphyNelus~ffer1864,IZ(I), 1-3. 9. Swart,J.D.;Roe,V.D.:Lscy.M. J.;Robbins,G.A.hI.Chm. 1988,61,2584--2585. 10. Gmb, R L. In Modprn Pmetiee ofGas ChmmuLogmphy.2nd ed.; Cmb, R. L.. Ed.; Wiley-lnteteience, 1985:Chapter 10.
11. Kiang,P. H.; Gmb, R L. J. Enuimn. Sei. H d t h , P a r r A 1986,.421~1~, 71-1W.
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