Anal. Chem. 1986, 58.3054-3058
3054
librium with HzO of isotopic composition 6H20,6ls0, is the measured 6ls0 of COz in the equilibration vessel after equilibration, P O , is 6 l g 0 of COP in equilibrium with the original H 2 0 before isotopic exchange, cy = (1sO/160)~o,/ (180/160)H,o at 25 "C, also, = 6lSO of COP used prior to equilibrium, and p is the gram atom ratio of oxygen in the HzO to that of the COS,the values of aC02-H20 were determined and are listed in Table 11.
CONCLUSIONS Determinations of the fractionation factor for oxygen-18 between C02 and H 2 0 results in a value of 1.04144 f 0.00008 (20) by the chlorine trifluoride technique and 1.04146 f 0.000 19 (2a) from the guanidine hydrochloride technique. Combining the chlorine trifluoride with the guanidine hydrochloride data here and from ref 3 results in a fractionation factor of 1.04145 f 0.000 15 ( 2 0 ) . This confirms our earlier published value of 1.04143 and strengthens the case for a full reevaluation of aC02-H20 with normalization to V-SLAP/ V-SMOW. Registry No. lSO, 14797-71-8;COz, 124-38-9;guanidine hydrochloride, 50-01-1; chlorine trifluoride, 7790-91-2. LITERATURE CITED Clayton, Robert N., unpublished work, 1975; cited in Friedman, Irving: O'Neil, James R. Geol. Surv. Prof. Pap. (US'.) 1977, 440-KK. Staschewski, D. Phys. Chem. Ber. 1964, 66, 454. Dugan, Joseph P., Jr.; Borthwick, James: Harmon, Russell S.: Gagnier, Michelle A.; Glahn. Jeanne E.: Kinsel, Erick P.; MacLeod, Sarah; Viglino, Janet A. Anal. Chem. 1985, 57, 1734. Coplen, Tyler B.: Friedman, Irving: O'Neil, James R. Wafer-Resour , Invest. ( U S .Geol. Surv.) 1984, 84-4136. O'Neil, James R.; Epstein, Samuel J. Geophys . Res. 1966, 71, 4955.
(6) Matsuhisa, Yukihiro; Matsubaya, 0.; Sakai, Hitoshi Mass Spectrosc , 1971, 19, 124. (7) O'Neil. James R.; Adami, Lanford H.: Epstein, Samuel J. Res. U . S . Geol. Surv. 1975, 3 , 623. (8) Friedman, Irving; O'Neil, James R. Geol. Surv. Prof. Pap. ( U . S . ) 1977, 440-KK. (9) Brenninkmeijer, Carl August: Kraft, Peter: Mook, W. G. Isot. Geosci. 1983, 1, 181. (10) Deines, Peter. Int. J. Mass Spectrom. I o n Phys. 1970, 4 , 263. (11) Mook, W. G.; Grootes, P. M. Int. J. Mass Spectrom. Ion Phys. 1973, 12, 273. (12) Coplen, Tyler B.; Kendall, Carol: Hopple, Jessica. Nature (London) 1983, 3 0 2 , 236. (13) Cohn, Mildred; Urey, Harold C. J. Am. Chem. SOC. 1938, 60, 679. (14) Epstein, Samuel; Mayeda, Toshiko K. Geochim. Cosmochim. Acta 1953, 4, 213. (15) Borthwick, James: Harmon, Russell S. Geochim. Cosmochim. Acta 1982, 4 6 , 1665. (16) Clayton, Robert N.: Mayeda, Toshiko K. Geochim. Cosmochim. Acta 1963, 27, 43. (17) Stewart, Michael K. J. Geophys. Res. 1975, 80, 1133. (18) Craig, Harmon: Gordon, L. Proceedings of Conference on Stable Isotopes in Oceanographic Studies and Paleotemperatures Spoleto 1965, 9. (19) Ehhalt, D. H.; Knott, K. Tellus 1965, 77, 389. (20) Stewart, Michael K. I n t . J. Appl. Radiat. Isot. 1981, 3 2 , 159. (21) Bigeleisen, Jacob: Perlman. M. L.; Prosser, H. C. Anal. Chem. 1952, 2 4 , 1356. ( 2 2 ) Friedman, Irving: O'Neil. James R. Geol. Surv. Prof. Pap. (U.S.) 1977, 440-KK. (23) Gonfiantini, Roberto "IAEA Report of the Advisory Group Meeting on Stable Isotope Reference Samples", 1984; 77 p. (24) Craig, Harmon Geochim. Cosmochim. Acta 1957, 12, 133.
RECEIVED for review March 7,1986. Accepted August 4,1986. Financial support for this work was provided, in part, from National Science Foundation Grants EAR 82-18380 and EAR82-18378 to R. S. Harmon and from the Institute for the Study of Earth and Man at Southern Methodist University.
Versatile Microreactor and Extractor Athula B. Attygalle' and E. David Morgan* Department of Chemistry, University of Keele, Keele, Staffordshire ST5 5BG, Great Britain
The Keele mlcroreactor is a versatlle device of extremely small volume for manlpulatlon by preparatlon, reaction, derivative formation or extractlon, and, in some cases, filtration of samples before analysls, whereby the product, derivatlve, flltrate, or extract is obtained in a volume of no more than 5 or 10 pL. Representative examples of Its use are given. Reactions can be carrled out in anhydrous condltlons, under moderate pressure, and above or below amMent temperature. The extracting solvent can have density elther greater or less than that of water.
Organic functional group analysis by gas chromatography (GC) or high-pressure liquid chromatography (HPLC) often first requires chemical derivatization. At the end of the derivatization reaction, water, acid or alkali, and an organic extracting solvent are frequently added to produce two phases. The nonpolar phase containing the organic products, free of spent reagents, is used for chromatographic examination. Microanalytical chemists, especially those working on natural products, such as pheromones, or on forensic materials, or environmental samples are often confronted with the problem of derivatizing very small samples with a minimum of loss. Present address: I n s t i t u t e for Organic Chemistry 11, Univeristy of Erlangen-Nurnberg, Erlangen, West Germany.
The conventional practice is to extract into a larger volume of solvent, at least 100 ML,and examine only a small fraction (usually about 1WL)of it by GC or HPLC. Either sensitivity is lost or the analyte must be concentrated by evaporation of solvent, which is slow and inefficient. Solvent removal can result in substantial material losses. Ma et al. (I) have shown in the derivatization of (Z)-3-decenoic acid as much as 80% of the material was lost either to the nitrogen stream or by adsorption on the walls when 1mL was reduced to 2 KL. We have found that evaporation of a dilute ether solution of a pheromone which has bp 75 OC at 15 mmHg gave an inactive residue and all the activity in the distillate (2). Furthermore, large volumes of solvents introduce more impurities. Therefore a method of derivatization, using small solvent and sample volumes compatible with the sample size requirement of GC and HPLC is important to microanalytical chemists. Such a method should cut down on manipulation and transfers and avoid a concentration step. Poole and Schuette have reviewed the numerous methods available for sample preparation before capillary gas chromatography, by extraction and concentration, and have summarized the problems associated with them ( 3 ) . In the present study, a microreactor with an extractor of extremely small volume was developed to carry out chemical reactions or separations, leaving the products in a small volume of solvent (5-10 KL),which is particularly useful for GC or HPLC.
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screw cap
upper chamber
neck ( 5 ~ 1 )
i
4.5crn. bottom chamber
5crn.
I
Flgue 1. Sectkml view of I h w Keele miaoreacm showhg Uw p s t k sealing cap, the F’TFE-lined siiiwne seal, the upper chamber, the neck region of volume 5 pL. and the bottom chamber of volume 75 pL.
EXPERIMENTAL SECTION Apwratns and Chromatographic Conditions. A CarbErha HRGC Fractovap 4160 gas chromatograph with an on-column injection system and a flame ionization detector was used with a Pye Unicam:Spectra Physics DP 101 computing integratm for quantitative analysis. A 25 m X 0.32 mm fused silica capillary column with a chemically bonded OV-1 stationary phase of 0.4-pm film thickness was used for the analysis. Helium was used as the carrier gas a t a flow rate of 2 mL/min. Gas Chromatography/Mass Spectrometry. A Pye 104 gas chromatograph linked through a glass jet separator to an AEI MS 12 mass spectrometer was used under the following conditions: trap current, 10 p A electron energy, 70 eV; accelerating voltage, 8 kV; multiplier voltage, 1.5 X 10 kV, source temperature, 140 OC. A 1% OV-101 on Chromorb W, 100-120 mesh column was used with helium carrier gas a t 20 mL/min. Keele Microreactor. A microreactor cum extractor was designed as illustrated in Figure l. The design has essentially three features: a bottom chamber of wider diameter (4 mm i.d. x 6 mm height) and volume approximately 75 pL: a narrow capillary neck (1mm i.d. X 10 mm height) with volume approximately 5 pL; a wider upper chamber (6 mm i.d. X 22 mm height). The bottom chamber has a coneshaped end for efficient recovery of sample using a syringe needle. The design is based on that of Wheaton V-Vials (Wheaton Scientific, MiUviUe, NJ) has similar overall dimensions, with the same screw thread and uses the same plastic caps and PTFGrubber sandwich septa. The heavy glass base and flat bottom enable the reactor to stand on the laboratory bench without using a rack or stand. Reagents or samples can be added or removed, where necessary, by piercing the septum with a syringe needle. The Keele microreactor is availahle commercially as the WheatonKeele mimoreactor (Wheaton Scientific, Milville, NJ, and through their world-wide distribution network). After the microreactor is used for reaction or extraction, the required liquid layer is drawn into the narrow capillary neck, from which it can be efficiently and almost completely removed for analysis. If the lighter’ liquid phase kequired, more of the lighter phase is forced into the neck. If the heavier phase is required, the mined phases are transferred to the upper chamber and allowed to separate, and then air is withdrawn from the bottom chamber with a syringe until the required liquid fills the neck. Except when the reactor is used with the sealing cap on (e.g., when using air- or moisture-sensitive reagents) liquids are conveniently added to the bottom chamber with a variable volume pipet (Eppendorf, Hamburg, West Germany) with the disposable plastic tips modified to take a glass melting point tube drawn out to a fme capillary which can reach through the neck of the reactor (Figure 2). A 1-cm portion of the disposable tip was removed to provide an opening slightly smaller than the outer diameter
Figure 2. Adaption of a variablevolume micropipet with disposable plastic tips, to u s e wim organic solvents. and paticularly wim W e Keele microreactor. The glass capillary is drawn out to a finer diameter (5t mm 0.d.) and at least 20 mm long so that n can reach to the bottom of the microreactor.
of the melting point capillary (approximately 1.9 mm 0.d. and 1.3 mm id.). The glass capillary was pushed through the end of the plastic disposable tip to give a gas tight seal. The organic liquids only came into contact with the glass capillary. Extraction Efficiency. In order to measure the efficiency of the device in recovering chemical substances from an aqueous medium, a number of compounds of different polarities were examined. Water, (50 pL), hexane (10 pL), and one of the following representative compounds was placed in the bottom chamher using a variable volume pipet, modified as in Figure 2: (n-5-decenyl acetate, 2,3,54rimethyIpyrazine, pentadecane, 6pentadecene, methyl 9-oxc-(2E)-decenoate,and 3-octanol (Figure 3A). The liquids were withdrawn from, and flushed hack into, the bottom chamber four to five times using a glass syringe (100 pL, SGE) fitted with a 7-cm steel needle (Figure 3B). Finally the liquid mixture was placed with the syringe in the upper chamher and the two layers were allowed to separate for a few minutes (Figure 3C). The syringe needle (0.23mm 0.d.) was placed in the bottom chamber and air was withdrawn until the upper hexane layer filled the neck region (Figure 3E). The hexane layer was removed with a 5-pL syringe suitable for on-column injection (5 pL, SGE) fitted with a fine needle (0.23 mm 0.d.). The volume of the organic layer was measured and an aliquot (0.5 pL) was injected on-column into the gas chromatograph with the oven temperature programmed from 50 to 300 “C a t 15 “C/min. Preparation of Pentafluorobenzyl Derivatives of Carboxylic Acids. Aqueous tetrabutylammonium hydroxide solution (10 pL, 0.1 M, Aldrich), aqueous sodium hydroxide (IO pL, 0.1 M), the carboxylic acid (100 ng) in hexane (IO pL), and pentafluorobenzyl bromide (2 pL, Aldrich) were placed in the bottom chamber of the Keele microreactor with a variable volume pipet modified as described above (Figure 2). The reactants were drawn up and down into a glass syringe (100pL, SGE) several times to ensure thorough mixing. The reaction mixture was placed back in the bottom chamher and the vial was closed with a PTFE liner
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
and boron trifluoride-methanol reagent (20 pL, Aldrich) were added to the bottom chamber of the microreactor. It was sealed with a PTFE liner and a screw-cap and heated at 90 "C in an oven for 10 min. Water (20 pL) and hexane (10 pL) were added and the reaction mixture was mixed thoroughly, the upper organic layer was separated and examined by GC as described above. The same carboxylic acids as used for pentafluorobenzylation were used. Attempted Extraction of Drugs from Clinical Fluids. Paracetamol (p-hydroxyacetanilide) was added to human blood (40 pL) or calf serum (40 pL) and attempts were made to extract the drug into hexane (20 pL) by the procedure of drawing in and out of a syringe (100 pL, SGE). Microfiltration. The narrow neck of the microreactor was fitted with a glass wool plug and a suspension of two crushed ant gasters in hexane (50 pL) was added to the upper chamber. The liquid was filtered into the bottom chamber by withdrawing air from the bottom chamber by a syringe needle inserted through the glass wool plug.
nn
E
d
d
C
U D
E
Figwe 3. Extraction technique with the Keele microreactor: (A) water or reaction medium (a) (50 pL) placed with the extraction solvent, hexane (b) (10 pL), in the bottom chamber; (B) the liquids are wWrawn from and flushed back into the bottom chamber using a syringe (c) (100 pL); (C) the liquid layers are placed in the upper chamber and allowed
to separate into layers, here the organic layer (d) is on top; (D)the syringe needle is placed in the bottom chamber and air is withdrawn until the upper liquid layer (d) fills the neck region; (F) the upper layer (d) is ready for withdrawal by a syringe.
and screw-cap. The vial was shaken for 20 min at room temperature. When the reaction was complete, water (20 pL) and hexane (10 pL) were added and the organic layer was removed and examined by capillary gas chromatography as described above. The acids used for the derivatization were benzoic acid, decanoic acid (caproic acid), dodecanoic acid (lauric acid), tetradecanoic acid (myristic acid), and hexadecanoic acid (palmitic acid). Methoxymercuration-Demercuration of Alkenes. Anhydrous mercuric acetate (50 pg) and the alkene (200 ng) in methanol (50 pL) were placed in the bottom chamber of a dry Keele microreactor. The vial was closed, covered with black tape, and shaken for 24 h. The vial was cooled in ice and a minimum quantity of finely powdered sodium borohydride was added until no more reaction was visible. The reaction mixture was acidified with a minimum quantity of glacial acetic acid. Water (20 pL) and hexane or diethyl ether (10 p L ) were added, the phases were mixed and the organic layer was separated for GC examination. Methylthiolation of Alkenes. The derivatization was performed by placing the alkene (100ng in 10 pL of hexane), dimethyl disulfide (10 pL, Aldrich), and ethereal iodine ( 2 pL, 0.25 M) in the bottom chamber of the microreactor. The vial was capped and shaken overnight at room temperature. Hexane (20 pL) and aqueuos Na&03 (25 p L , 0.3 M) were added and the phases mixed. The organic layer was separated and examined as described before. The alkenes used were 6-pentadecene, 8-heptadecene, 9-nonadecene, 9-heneicosene, 9-tricosene (all cis-trans mixtures), and (Z)-5-decenyl acetate. Esterification of Carboxylic Acids with Boron Trifluoride-Methanol. carboxylic acid (100 ng) in hexane (10 pL)
RESULTS AND DISCUSSION Most derivatization reactions used in GC or HPLC require a solvent extraction step. Conventional extraction methods, such as using a separating funnel, require milliliter quantities of solvent while only a few microliters are sufficient for the chromatographic examination. The recent method described by Cais and Shimoni ( 4 ) shows high percentage recoveries (above 80% ( 5 ) )but requires at least 2 mL of liquid. Grob et al. (6) have used low solvent volumes in a ratio of 200 pL of solvent to 900 mL of water in the extraction of organic substances from water samples. This method has been improved to give better recoveries by Murray (7) by developing an extraction flask. A similar extraction appartus is now commercially available (J & W Scientific, Inc., Rancho Cordova, CAI. The method developed by van Rensberg and Hassett (8) can be used to extract 1 to 50 mL with solvent volumes of 20 pL and greater. None of the above methods and devices is suitable to perform a derivatization reaction prior to the extraction step, especially when the reaction requires heating. The present method allows derivatization and extraction in the same container with the minimum of manipulations and transfers. The Keele microreactor can be sealed with a Teflon liner and screw cap and heated up to 200 "C in an oven without any loss of material. V-Vials (Wheaton Scientific, Milville, NJ, and their agents) are frequently used in a similar manner for microderivatization reactions, but the present design of V-Vials is not suitable to extract the products into a few microliters of a solvent lighter than water. For less exacting work, a microreactor with lower chamber of volume 250 pL and middle chamber of 10 pL would be useful and easy to manipulate. In the procedures now described the aqueous solution (about 50 pL) is extracted with a solvent such as hexane (10 pL) by pumping the mixture in and out of a glass syringe (100 gL). This process ensures the breakage of the liquid mass into tiny droplets causing an intimate mixing of the two phases. This facilitates the mass transfer procedure to take place in a highly efficient manner. The liquid mass is allowed to settle in the upper chamber. Surface tension in the narrow neck prevents the liquids from running into the lower chamber. A fine syringe needle (0.23 mm 0.d.) is inserted to the bottom chamber and air is withdrawn until the desired organic liquid layer enters the neck region (Figure 3). An advantage of this procedure over the other conventional methods is that this method can be used with solvents either heavier or lighter than water. The efficiency of the extraction process was determined by using six compounds of different polarities. Each of the substances chosen is found as a pheromone in nanogram quantities in various insects and so represents a range of problems that might be encountered. The results are shown
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Table I. Recoveries from Distilled Water Using the Microextraction Procedure linear retention index
total amt used, ng
1361 974
80 75
1500
85
1480
90
1532 983
85
1
substance 2-5-decenyl acetate 2,3,5-trimethylpyrazine pentadecane 6-pentadecene methyl 9-oxo-2E-decenoate 3-octanol a
70
mean percentage recovery of extraction f standard deviation" 1st extract 2nd extract 3rd extract 50.8 f 1.3 45. 5 f 3.5 61.7 f 1.6 63.7 f 1.4 54.7 f 1.5 41.7 f 1.0
20.2 f 0.9 18.7 f 1.3 17.4 f 1.4 18.2 f 1.2 22.4 f 0.9 19.9 f 1.6
8.9 f 0.8 11.8 f 1.2
7.8 f 0.3 9.6 f 0.4 6.9 f 0.5 11.3 f 0.8
Mean of five separate extractions.
in Table I. The substances were extracted into 10 pL of hexane. Of that, 5-7 p L were recovered, the remainder being accounted for by dissolution in the aqueous phase and by evaporation. Typical recoveries of the compounds tested were 40-65% for a single extraction and nearly 90% for three consecutive extractions (Table I). The modification made to disposable syringe tips (Figure 2) has a number of advantages. The use of conventional glass microliter syringes to add accurately small volumes of reagents to the bottom chamber is discouraged, because it is very difficult at the levels of detection being used to clean a syringe sufficiently after use to avoid cross-contamination. Hence variable volume pipets with disposable plastic tips are preferred. But the commercially available tips are too large to enter the bottom chamber and are not suitable for organic liquids that wet them. By use of drawn-out melting point melting point tubes fixed to the plastic tips these problems can be overcome. Furthermore, plastics are best avoided in microanalysis because plasticizer contaminations are so easily introduced. A few derivatization reactions were performed to demonstrate the usefulness of the reactor. These have been chosen to display a range of conditions, many others are available. Pentafluorobenzylation of Carboxylic Acids. Pentafluorobenzyl bromide is a useful reagent to derivatize phenols (9),mercaptans (9),sulfonamides (IO),and carboxylic acids (1) for subsequent GC analysis using electron capture detection. The reaction is fast and quantitative. The pentafluorobenzyl derivatives of carboxylic acids have good gas chromatographic characteristics. The derivatives can be identified easily by their mass spectra obtained by gas chromatography/mass spectrometry (GC/MS). The mass spectra are simple and always show the molecular ions (about 10%). All derivatives except that of benzoic acid showed the base peak a t m / z 181 due to the pentafluorotropylium cation. Although 100 ng of carboxylic acid was used for convenience, a few nanograms were sufficient for flame-ionization detection. The sensitivity can be increased to picogram levels when an electron capture detector is used. The thorough mixing of the liquid layers using the 100-pLsyringe must be carried out under a well-ventilated hood because pentafluorobenzyl bromide is lachrymatory. The method can be extended for the analysis of mercaptans and phenols in water which affect palatability of water (9). As only very small volumes are required, the cost of sample transportion can be reduced. Similarly, the method can be used to study sulfonamides by gas chromatography (GC) (10). Methoxymercuration-Demercuration of Alkenes. Methoxymercuration-demercuration has gained popularity as a method for locating the position of double bonds in alkyl chains. Essentially it produces addition of the elements of methanol to the double bond. Abley et al. (11)first described the application of GC/MS to the methoxy derivatives obtained from the reaction. The addition can occur in two ways resulting in two derivatives for a single monounsaturated com-
pound. The two methoxy derivatives are not usually separated by GC. As a result a mixed spectrum is obtained. The fragmentation of the derivatives in the mass spectrometer results in preferential cleavage on either side of the methoxy group to yield four intense and characteristic fragment ions. From the mass of these ions, the original double bond position can be deduced. Many applications of this technique have been reported (12-15). The main drawback of the conventional procedure is the final solvent extraction step. The methoxy derivatives produced are partitioned between water and ether (15) or hexane (14) before the nonaqueous layer is examined by GC. The extraction step required 50 pL or usually more of the nonaqueous solvent to allow convenient withdrawal of the upper layer. By the present modification of the procedure using the Keele microreactor, 200 ng of the alkene was conveniently used for derivatization and the product extracted into 5 pL of hexane. A number of alkenes in the C,, to C, mass range were used for testing the method. Methylthiolation of Alkenes. The addition of dimethyl disulfide to alkenes is a more recent method used to locate the double bond position in alkenes (16, 17). Dimethyl disulfide gives a single derivative, which, in the mass spectrometer, cleaves preferentially across the carbon-carbon bond between the CH,S substituents leading to two major fragment ions. These give intense peaks in the mass spectrum that can be employed to locate the double bond position in the alkene. In the procedure described by Buser et al. ( I 7) the adducts are finally extracted into 200 pL of hexane and concentrated to 20-50 wL. By use of the present extraction technique the sensitivity of the method was improved manyfold. The products were isolated in 5-7 pL of hexane and the concentration step was not necessary. Although 100 ng of an alkene was used here, samples of as little as 10-20 ng of alkene can be examined easily. Although not yet popularly used, methylthiolation has a number of advantages over the more widely used methoxymercuration-demercuration method (18). Esterification of Carboxylic Acids w i t h Boron Trifluoride-Methanol. The use of BF3-methanol is a convenient method to prepare methyl esters of carboxylic acids (19). The method is simple and fast. The only disadvantage in the conventional procedure is the final solvent extraction step. The sensitivity of the method was substantially increased by using the Keele microreactor. Because the products were extracted into 5-7 pL of hexane, samples containing 10-100 ng of carboxylic acid were conveniently esterified and examined by GC. Carboxylic acids of Cl0to CI6 range and benzoic acid were used as the test mixture. Attempted Extraction of Drugs from Clinical Fluids. The extraction of drugs from biological fluids can be an important use of micromethods. However, the natural emulsifiers in blood plasma and serum caused the formation of persistent emulsions when mixed with the organic phase by
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pumping in a microlitre syringe. These emulsions were not easily broken by standing or centrifuging. This method is therefore not suitable for such fluids, a preliminary extraction or cleanup would be necessary or gentler means of contacting the phases. Microfiltration. The Keele microreactor can also be used for microfiltration. If the narrow neck of the device is plugged with a very small amount of glass wool, and a liquid suspension placed above it, the clear liquid can be filtered off into the lower chamber, if a microlite syringe is inserted through the plug and air is withdrawn from the lower chamber. The clear liquid is removed with the syringe. The method is only suitable for larger volumes (above 500 pL). With smaller volumes of liquid, the losses through wetting of the glass wool plug become important. Further examples of the use of the device in small sample analysis are in preparation, e.g., ref 20.
Cais, M.; Shimoni, M. Ann. Clin. Biochem. 1981, 78, 317-323. Peleg, I.; Vrornen, S. Chem. Ind. (London) 1983, 615-616. Grob, K.; Grob, K., Jr.; Grob, G. J. Chromatogr. 1975, 706,299-315. Murray, D. A. J. J. Chromatogr. 1979, 777, 135-140. van Rensburg, J. F. J.; Hassett, A. J. HRC CC,J . High Resolut. Chromato&. Chromatogr . Commun. 1982, 5 , 574-576. Kawahara, F. K. Anal. Chem. 1988, 40, 1009-1010. Ehrsson. H. J . Chromatogr. 1975, 707, 327-333. Gyllenhall, 0.; Abley, P.; McQuillin, F. J.; Minnikin, D. E.; Kusamran, K.; Masker, K.; Polgar, N. Chem. Commun. 1970, 348-349. Blomqulst, G. J.; Howard, P. W.;McDavld, C. A,; Remaley, S.; Dwyer. L. A.; Nelson, D. R. J. Chem. Ecol. 1980, 6 , 257-269. Michaelis, K. Z . Naturforsch., 6 1981, 368, 402-404, Vostrowsky, 0.; Vostrowsky, 0.; Michaelis. K.; Bestmann, H. J. Justus Liebigs Ann. Chem. 1981, 1721-1724. Baker, R.; Bradshaw, J. W. S.;Speed, W. Experientia 1982, 3 8 , 233-234. Francis, G. W.; Veland. K . J. Chromatogr. 1981, 279, 379-384. Buser, H.; Arn, H.; Guerin, P.; Rauscher, S. Anal. Chem. 1983, 5 5 , 818-822. Billen, J. P. J.; Evershed, R. P.; Attygalle, A. B.; Morgan, E. D.; Ollett. D. G. J . Chem. Ecol. 1986, 12, 669-685. Metcalfe, L. D.; Schmitz, A. A. Anal. Chem. 1981. 3 3 , 363-364. Ollett, D. G.; Attygalle, A. 8.; Morgan, E. D. J. Cbromatog. 1988, 367, 207-212.
LITERATURE CITED Ma, M.; Hummel. H. E.; Burkholder, W. E. J. Chem. Ecoi. 1980, 6 , 597-60 1. Cammaerts-Tricot, M. C.; Morgan, E. D.; Tyler, R. C. J , Insect Physiol. 1977, 23 421-427. Poole. C. F.; Schuette, S. A. HRC CC,J. High Resoiut. Chromatogr. Chromatogr. Commun, 1983, 6 , 526-549.
RECEIVED for review March 15, 1985. Resubmitted July 25, 1986. Accepted July 25,1986. The authors thank the Nuffield Foundation for financial support and the Royal Society for a grant for the purchase of GC equipment.
Correlation of Surface Acoustic Wave Device Coating Responses with Solubility Properties and Chemical Structure Using Pattern Recognition David S. Ballantine, Jr. Geo-Centers, Inc., 4710 Auth Place, Suitland, Maryland 20746 Susan L. Rose and Jay W. Grate* Chemistry Division, Naval Research Laboratory, Washington, D.C. 20375-5000 Hank Wohltjen Microsensor Systems, Inc., P.O. Box 90, Fairfax, Virginia 22030
Twelve surface acoustic wave device coatings were exposed to 11 chemical vapors and responses were correlated with solubility properties and coating structure to determine possible vapor/coating interaction mechanlsms. Hydrogen bonding ability Is implicated as a signlflcant vapor/coatlng interactlon mechanism. Pattern recognltlon schemes applied to the preliminary data aided In soluMlity property/response correlations. Principal component analysis demonstrated good separation of dffferent classes of chemical vapors tested. Hierarchical clustering provlded additional evidence of the correlations between solubHity properties and the observed clustering. I n addition, pattern recognition methods were used to determine potential selectivity of an array detector using these coatings. Learning techniques show that one-fourth of the sensor can adequately separate compounds of interest from chemically similar interferences.
Surface acoustic wave (SAW) devices exhibit great potential as small, very sensitive chemical sensors. The principles of
operation have been described in detail ( l ) ,but they are essentially mass-sensitive detectors. They consist of a set of interdigital transducers that have been microfabricated onto the surface of a piezoelectric crystals. When placed in an oscillhtor circuit, an acoustic Rayliegh wave is generated on the surface of the cfystal. The characteristic resonant frequency of the device is dependent on transducer geometry and the Raleigh wave velocity. Small mass changes or elastic modulus changes on the surface perturb the wave velocity and are readily observed as shifts in this resonant frequency. The extreme sensitivity of these devices makes them attractive as potential gas sensors. The 112-MHz dual SAW devices routinely used in our laboratory, for example, have a theoretical sensitivity of >17 Hz/(ng/cm*). Considering that the active area of the device covers 0.17 cm2 and assuming a signal to noise ratio of 3, this sensitivity results in a minimum detectability of about 0.2 ng ( I ) . The ultimate performance of a SAW device as a chemical sensor is critically dependent on the sensitivity and selectivity of the adsorbent coating applied to the surface of the piezoelectric crystal, However, no systematic investigation of
0003-2700/86/0358-3058$01.50/00 1986 American Chemical Society
