740
Anal. Chem. 1987, 5 9 , 740-744
Poly(4=vinylpyridine)=CoatedGlassy Carbon Flow Detectors Joseph Wang,* Teresa Golden, a n d Peng T u z h i Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003
The performance of a thin-layer flow detector with a glassy carbon electrode coated with a film of protonated poly(4vinylpyrtdine) Is descrlbed. Substantial Improvement in the selectivity of amperometric detectlon for liquid chromatography and flow injectlon systems is observed as a result of excluding cationic species from the surface. The detector response was evaluated wlth respect to flow rate, solute concentration, coating schetne, film-to-flim reprodudblllty,and other variables. Despite the increase In diffusional redstance, low detection limits of ca. 0.04 and 0.10 ng of ascorbic acid and uric acid, respectlvely, are maintained. Protection from organic surfactants can be coupled to the charge exclusion effect by usJng a bilayer coatklg, wlth a ceUukse acetate film atop the poly( 4-vlnylpyridlne) layer. Appiicablilty to urlne sample is demonstrated.
One of the most important applications of faradaic electrochemistry is its use for sensitive measurements in flowing streams. In particular, amperometric detectors have been extremely useful in conjunction with liquid chromatography and flow injection systems. The focus of most work on electrochemical detection has been the development of new cell designs or electrode materials, new detection modes, and new applications. The sophistication currently available on modification of electrode surfaces has been rarely utilized for flow analysis. Proper functionalization of the detector surface can offer significant analytical advantages. For example, detection of irreversibly oxidized species can be enhanced by use of electrodes modified with electrocatalytic moieties (1-4). Electroinactive anions can be detected based on the repetitive doping-undoping of polypyrrole-modified electrodes (5). The discriminative properties of polymeric layers hold great promise for increasing the selectivity and stability of amperometric detectors. For example, films of cellulose acetate cast on the detector surface have selective permeability for small molecules (6, 7). Hence, interferences due to large electroactive species or nonelectroactive surfactants are eliminated. Charge exclusion is yet another discriminative property of polymeric layers that can be utilized in flow analysis. For example, we have demonstrated recently that electrodes coated with negatively charged perfluorinated polymeric films offer highly selective response for cationic neurotransmitters in the presence of otherwise interfering anionic or neutral oxidizable species (8). In this paper we describe the behavior and the use of a glassy carbon flow detector coated with a layer of poly(4vinylpyridine) (PVP). The cationic polyelectrolyte PVP was one of the first polymers used for modifying electrode surfaces (9). Much research effort has been directed to the characterization of the transport and electrostatic binding of multiple-charged anions a t electrodes coated with PVP. The ability of PVP to bind counterionic reactants has been exploited for preconcentration of analytes from dilute solutions (9, 10). Lowering the overvoltage for certain analytes, via electroactive reagents immobilized in PVP, is yet another analytical advantage of PVP-modified electrodes (11, 12).
Work aimed at exploiting the unique properties of PVP films to improve electrochemical monitoring of flowing streams has not been reported. The present study demonstrates that permselective positively charged PVP coatings offer significant selectivity improvements in a manner (charge exclusion) analogous to that of negatively charged perfluorinated films. Hence, an additional separation step, performed in situ at the detector surface, permits effective discrimination against cationic interferents. These characteristics of PVP-coated amperometric detectors are discussed below. EXPERIMENTAL SECTION Apparatus. The “home-made”flow injection system and the Bioanalytical Systems Model LC-303 liquid chromatograph were described previously (7). A Biophase CI8 column (25 cm X 4.6 mm) was employed. A glassy carbon thin-layer electrochemical detector (Model TL-5, Bioanalytical Systems) was used. The Ag/AgCl reference electrode and stainless steel auxiliary electrode were located in a downstream compartment. An EG&G PAR Model 174 polarographic analyzer was used in the flow injection experiments. A sample loop of 20 fiL was used. A fiie micrometer (Welch Scientific Co.) was used to measure the thickness of the dry PVP coating. Reagents. Stock solutions of dopamine, epinephrine, norepinephrine, acetaminophen, propranolol, uric acid, chlorpromazine, caffeic acid, desipramine, lidocaine, catechol, (Sigma), oxalic acid (Eastman), and ascorbic acid (Baker) were prepared fresh each day. Solutions of poly(4-vinylpyridine) (Polysciences) were prepared by mixing 0.2 g of PVP in 50 mL of methanol. Cellulose acetate (39.8% acetyl content) was purchased from Aldrich. A 0.05 M phosphate buffer (pH 5.5), prepared from K,HP04 and KH2P04,using phosphoric acid for acidification, was used as electrolyte and carrier in the flow injection experiments. The chromatographic mobile phase was a 0.05 M chloroacetic acid solution adjusted to pH 4.7 with phosphoric acid. The mobile phase contained 15 mg/L sodium octyl sulfate and 150 mg/L disodium ethylenediaminetetraacetate. The urine sample was obtained from a healthy volunteer,filtered by passing through a glass filter (10-15 pm porosity), and diluted with the mobile phase solution. Procedure. Prior to the coating the amperometric response of the bare surface was evaluated. For this purpose, the electrode was polished with 0.05 pm a-alumina particles, rinsed with double-distilled water, and sonicated in a water bath for 1 min. The same surface was modified by dipping for 30 s in a stirred 0.4% (w/v) methanol solution of the polymer. The electrode was then lifted out, and solvent was allowed to evaporate from the surface during a 10-min period (while the electrode was covered with a beaker). The resulting coating had thickness of 11 p m . The bilayer electrode assembly was prepared by syringing 10 pL of the 5% cellulose acetate solution (in a 1:l mixture of acetone and cyclohexanone) on top of the PVP layer; this was followed by 40-min hydrolysis in a stirred 0.07 M KOH solution. Amperometric detection was performed by applying the desired working potential (in the plateau region) and allowing the background current to decay to a steady-state value. At the end of a day’s work the PVP film was wiped off the surface with a soft tissue wet with acetone. RESULTS AND DISCUSSION This basis for the practical utility of the PVP-coated detector in flow analysis is its discrimination against cationic electroactive species. Such discrimination is attributed to charge repulsion associated with the net positive charge of the
0003-2700/87/0359-0740$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
Table I. Permeability of the PVP Coating toward Various Solutes under Flow Injection Conditions solute
ip,c/ip,b
solute
ip,c/ip,b
chlorpromazineb uric acidb
0.54
lidocaine' catechol' ascorbic acidb oxalic acidd hydroquinonec caffeic acid' propranolold
0 0.90 0.73 0.44 0.84
dopamineb desipramine'
acetaminophen' norepinephrineb epinephrineb
0.63 0.03 0.05 0.70
0.05 0.04
A
C
S
D
741
I
0.63
0.08
"Conditions: flow rate, 1 mL/min; electrolyte and carrier, 0.05 M phosphate buffer (pH 5.5). Applied potential, +0.7 V; solute concentration, 1 X M. 'Applied potential, +0.9 V; solute concentration, 1 X M. dApplied potential, + L O V; solute concentration, 3 X M. polyelectrolyte (at pH values lower than 6). Table I shows the film permeability toward various oxidizable solutes, under flow injection conditions. The ratio between the current at the film-coated electrode over that at the bare electrode, ip,c/ip,b,is used as a measure of the permeability. Distinct changes in the permeability, based on solute charge, are observed. Cationic species, such as dopamine, epinephrine, norepinephrine, desipramine, or propranolol, yield very low iP,Jip,b values, ca. 0.03,0.04, 0.05,0.05, and 0.08, respectively. Hence, contributions from easily oxidizable cationic species can be minimized without lowering the operating potential. On the other hand, facile transport through the PVP film is observed for anionic (counter ions) or neutral species, with the response approaching that of the bare electrode (ip,&/i,,b values ranging from 0.44 to 0.90). Thus, high sensitivity is maintained for numerous analytes (see below). Note, the distinct differences in permeability between catechol and the structurally similar catecholamines (as attributed to the protonated amine moiety). The nonzero permeability values obtained for cationic species indicate that the coating is sufficiently porous to allow slow diffusion of co-ions. A similar effect was observed by Oyama and Anson (13) for the transport of ferrous ions through a protonated PVP layer. Indeed, diffusion of co-ions is quite a common phenomenon in ion-exchange polymers (13);penetration of various anions through negatively charged perfluorinated films has been reported (8, 14). The response time of polymer-coated detectors is another important parameter when monitoring dynamic flowing streams. Because transport through the film is a major contributor to the total diffusional resistance, slower response characteristics are expected a t coated electrodes. Figure 1 shows characteristic flow-injection current-time profiles for 1X M acetaminophen at the bare electrode (A) and at electrodes coated by depositing 5 (B), 10 (C), and 15 (D) pL of the PVP solution (about 5, 10, and 15 pm thickness, respectively). The bare and coated electrodes exhibit rapid increase and decrease of the current. For example, the response times to reach 90% of the maximum signal are 2.6 s at the bare electrode and 2.6, 2.7, and 3.0 s at the 5-, lo-, and 15-pL coatings, respectively. The peak widths (at 0.6Cm,) are 4.5 (bare), 4.6 (5-pL coating), 4.8 (10-pL coating), and 5.3 s (15-gL coating). The fast response indicates rapid replenishment of the solution from the surface, as desired for dynamic flow systems. Note also the difference in peak size (and hence permeability) of the different coatings, as expected from the more facile transport through the thinner film. Experimental conditions were changed to evaluate their effect on the film permeability. Among these were the composition of the PVP solution, the coating scheme, or the pH of the phosphate buffer electrolyte. The rejection of dopamine was used as a measure of the film performance. The largest
1
I
mln
Tlme
Figure 1. Flow injection detection peaks for 1 X M acetaminophen using the bare (A) and PVP-coated (B-D) glassy carbon electrodes. Electrodes were modified wCh 5 (B), 10 (C), and 15 (D)pL of PVP solution. Flow rate 1.OmL/min; applied potential, +0.90V. Electrolyte and carrier, 0.05M phosphase buffer (pH 5.5).
(97%) rejection was obtained by dipping the electrode for 30 s in a stirred 0.4% methanol solution of PVP and by using a pH 5.5 electrolyte. Such conditions were used in most subsequent work. Surprisingly, an increase in film permeability was observed using more acidic phosphate buffer solutions, despite the increased extent of protonation of the pyridine groups. Measurements of the film thickness, upon exposure to more acidic solutions, were performed to further elucidate the above behavior. Thickness values of 11,8, and 6 pm were obtained by use of phosphate buffer solutions of pH 5.5,4.0, and 2.5, respectively. Hence, it appears that the overall resistance to transport of co-ions depends on the charge and thickness of the film, with the pH 5.5 solution offering the best compromise for maximum rejection. To examine the flow-rate dependence, chromatographic peaks for uric acid were recorded at different flow rates, ranging from 0.3 to 1.8 mL/min (30 ng injected; applied potential, +0.6 V). Unlike the response of a bare glassy-carbon thin-layer detector that increases upon increasing the flow rate, the PVP-coated detector exhibited a decrease of signal with increased flow rate. Such behavior was observed at PVP coating of different thickness. A t the 5-pm-thick film the peak current decreased from 18 nA at 0.3 mL/min to 15 nA at 1.8 mL/min; a sharper decrease in peak current, from 13 to 8 nA, was observed over the same flow-rate range using the 10-pmthick coating. Other polymer-coated detectors, based on cellulosic (7) or Nafion (8)films yield a negligible dependence on flow rate, as transport through the film becomes the major contributor to the total diffusional transport. The reason for the unique flow-rate dependence of PVP-coated detectors remains to be elucidated. The potential of the permselective transport characteristics of PVP-coated glassy carbon detectors for selective amperometric detection in flow injection systems is illustrated in Figures 2 and 3. An additive response is observed at the bare electrode (A) when mixtures of oxidizable compounds are concerned. Hence, selective detection of chlorpromazine or ascorbic acid in the presence of desipramine and epinephrine, respectively, is not feasible (without a separating column). In contrast, the PVP-coated electrode (B) effectively excludes the positively charged desipramine and epinephrine from reaching the surface; as a result, the chlorpromazine and ascorbic acid response is not affected by the presence of these compounds (even at excess levels, compare (a) and (c)). The PVP-coated electrode exhibited similar improvements in selectivity using other mixtures of electroactive species. For example, measurements of 2 X low5M uric acid were not affected by the presence of 4 X M dopamine. Because
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987 I
;
B
l
C
t a
b
c
-
a 2 mln
b
C
a
U ~
~~
Tlme
Flgure 2. Flow injection detection of chlorpromazine in the presence of desipramine, at the bare (A) and PVP-coated (B) electrodes; (a) 2 X loq5M chlorpromazine; (b) same as (a) but after addition of 2 X 10-5 M desipramine; (c) same as (b) but after addition of 2 X 10-5 M desipramine. Conditions were as in Figure 1, using a dip-coated
electrode (B) and an applied potential of +0.8 V.
A
I I I
B 10
0 I60 nA
I
b
C
2mln
30
Flgwe 4. Chromatograms for diluted (150) urine sample, obtained (A) at the bare electrode and (B) at the coated electrode. Peak identities are as follows: (1) ascorbic acid. (2)uric acid. Applied potential was 4-0.60V, mobile phase, 0.05 M chloroacetic acid solution adjusted to pH 4.7, and flow rate, 1 mUmin.
I
a
20
t ,mi"
a
b
C
I
Tlme
Figure 3. Flow injection detection of ascorbic acid in the presence of epinephrine, at the bare (A) and PVP-coated (B) electrodes: (a) 2 X M ascorbic acid; (b) same as (a) but after addition of 2 X M epinephrine; (c) same as (b) but after addition of 2 X M epinephrine. conditions were as in Figure 1, using a dip-coated electrode
I'
(B) and an applied potential of +0.7 V.
co-ions do transport at some extent through the PVP film, only certain concentration ratios of the two mixture components can be tolerated. Assays of more complex samples require a preceding chromatographic separation. Liquid chromatography with amperometric detection can also benefit from the additional separation step, performed in situ, at the PVP-coated electrode. For example, Figure 4 compares chromatograms for a diluted urine sample obtained at the bare (A) and PVPcoated (B) electrodes. The exclusion of cationic species provides simplification of chromatograms recorded amperometrically. More than 15 peaks, of variable sizes, are observed at the bare electrode compare to 10 at the coated one. Hence, measurements of anionic species, e.g., ascorbic acid and uric acid (peaks 1 and 2, respectively) are greatly improved as interferences due to partially coeluting components are minimized. The distinct changes in peak size (bare vs. coated electrodes)-representing the solute characteristic permeability through the PVP coating-can be used to assess the peak identity. This can be achieved by comparing chromatograms of the two electrodes recorded separately or simultaneously in a dual-electrode operation. In the latter case, dual response ratios (with equipotential operation) can be used for peak identification.
i
r 0
I 20
1 10
I 30
t ,m i n Flgure 5. Chromatograms for injection of 1 ng of ascorbic acid (l), uric acid (2),and 2 ng of norepinephrine (3)and epinephrine (4) obtained (A) at the bare electrode and (B) at the coated electrode. Other conditions are given as in Figure 4.
The inherent sensitivity characterizing amperometric detection is maintained at the PVP-coated electrode, when oxidizable anionic species are concerned. Figure 5 compares bare (A) and coated (B) electrodes chromatograms for a mixture containing 1ng of ascorbic acid and uric acid (as well as 2 ng of norepinephrine and epinephrine, deliberately added to demonstrate the charge-exclusion effect). The decrease in the ascorbic acid and uric acid peaks, expected from the in-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 5, MARCH 1, 1987
I
Table 11. Effect of Solution Organic Content on the Permeability of PVP Coating toward Dopaminea
*
organic component
0%
methanol acetonitrile
0.03
propanol
0.03 0.03
743
4 0 nA
ip,c/ip,b
10%
25%
50 %
0.14 0.31 0.22
0.78 0.98 0.82
0.92 1.00 0.95
Conditions: as in Figure 3B; dopamine concentration, 1 X M. *Compositionof carrier and sample solutions (v/v). crease in diffusional resistance a t the coated electrode, is accompanied by similar noise levels. Hence, some decrease in signal-to-noise characteristics is observed at the coated electrode. Such changes, however, are not a matter of major concern as subnanogram levels can be detected (detection limits of 0.04 ng of ascorbic acid and 0.10 ng of uric acid, compare to 0.03 and 0.05 ng, respectively, at the bare electrode; SIN = 3). The concentration dependence of the PVP-coated detector was evaluated by using liquid chromatography and flow injection systems. First, eight successive injections of uric acid solutions of increasing amount (5-40 ng) were used under the chromatographic conditions of Figure 4. The peak response increased linearly with increasing amounts of uric acid; the slope of the resulting calibration plot corresponded to a sensitivity of 1.43 nA/ng (correlation coefficient, 0.999; intercept, -1.2 nA). Linearity was observed also in 16 successive flow injections of hydroquinone solutions of increasing concentration, 5 X lo+ to 8 X loa M (sensitivity, 20.6 nA/104 M; correlation coefficient, 0.999; intercept, -4.5 nA; conditions as in Figure 1 using a dip-coated electrode). One drawback for chromatographic applications is the loss of film integrity in solutions containing large fractions of organic solvents. This is indicated from the increased permeability of dopamine in solutions of increasing organic content (Table 11). While solutions containing 10% of methanol, acetonitrile, or propanol yield a relatively good rejection of dopamine (ip,c/ip,b values ranging from 0.14 to 0.31), facile transport of dopamine (ip,c/ip,b of 0.73 to 1.0) is observed in solutions with higher (2550%)organic composition. Two important points should be noted however. First, while the increase in dopamine transport upon increasing the organic content is obvious, the film still maintains preference toward anionic analytes. For example, the same film that yields a ip,c/ip,b value of 0.14 for dopamine in the 1O:go (v/v) methano1:phosphate buffer solution resulted in a ip,c/ip,b value of 0.61 toward uric acid. Second, the permeability values reported in Table I1 were obtained over a 60-min period of 120 injections; i.e., exposure of the film to the organic-containing solutions does not result in a gradual degradation. The data of Table I1 indicate that predominantly aqueous solutions should be used to maintain the advantages discussed in this study. In addition, the sensitivity of the film permeability to small changes in organic content (or pH) poses limitations on use with gradient elution. The film-to-film precision was evaluated by estimating the permeability (ip,c/ip,b) of six different coatings. For flow inM acetaminophen such series jection measurements of 1 X yielded a mean permeability value of 0.70, a range of 0.60-0.81, and a relative standard deviation of 12% (conditions, as in Figure 1). Such changes in permeability when different coatings and surfaces are used cause no real problem as the trend in permeability for various solutes is maintained according to their charge. With approximately one out of ten coatings, a larger than usual permeability to cationic species was observed. Flow injection measurements of dopamine were used as a rapid test of the film performance. Films with ip,c/ip,b
L
I
The
Figure 6. Detection peaks at PVP (A) bilayer (B) coated electrodes: repetitive injections of a 1 X 1Od M acetaminophen solution, containing 200 mg/dL gelation; conditions as in Figure 1; hydrolysis time (B),40 min; operating potential, +0.90 V. values larger than 0.05 were removed, the electrode was then coated again and retested. We investigated the utility of the PVP film as a protective layer aimed a t minimizing electrode poisoning effects in the presence of surface-active organic materials. Cellulosic films have been used as effective barriers against surfactant adsorption in flow analysis (6, 7). As shown in Figure 6A, plain PVP coatings do not offer similar protection capability; a gradual decrease in the acetaminophen response (up to 35%) is observed in the presence of 200 mg/dL gelatin. Hence, the surfactant appears to interact with the PVP coating, gradually blocking access of acetaminophen to the carbon surface. The situation can be improved by using a bilayer electrode assembly, with a cellulose acetate film atop of the PVP layer (Figure 6B). No loss of electrode activity is observed as the outer cellulosic layer effectively excludes the gelatin. Thus, a bifunctional operation (simultaneous rejection of electroactive cations and nonelectroactive surfactants) can be achieved by using the PVP/cellulose-acetate bilayer configuration. Such operation is accompanied with a sensitivity loss, resulting from the additional mass-transport resistance. Lastly, we attempted to couple the discriminative properties of PVP coating with electrocatalysis to further enhance the selectivity and stability of amperometric measurements. For the latter function, the ability of Fe(CN)t- attached to the PVP to electrocatalyze the oxidation of ascorbic acid was examined. Unfortunately, no lowering of the overvoltage was observed in hydrodynamic voltammograms for ascorbic acid a t the Fe(CN),4-/PVP-coated electrode, compared to analogous voltammograms recorded at the bare and plain-PVPcoated electrodes.
CONCLUSIONS The experiments described above confirm the expectations placed in the utility of PVP-coated electrodes for amperometric monitoring of flowing streams. In particular, enhanced selectivity is obtained in flow injection and liquid chromatography systems as a result of excluding cationic species. Dual-electrode detection schemes, using bare and PVP-coated electrodes, can be used for obtaining additional information of peak identitylintegrity. A drawback for various liquid chromatography applications would be the loss of film integrity in mobile phases with high organic-modifier concentrations. The development of PVP coating with decreased solubility in organic solvents (similar to a recent development of perfluorinated polymers (15)) will overcome this deficiency. The extension of this approach for the incorporation of the other permselective polymers appears to provide a promising route for flow analysis. Other functionalities, e.g., catalytic or protective, may be coupled with the discriminative prop-
744
Anal. Chem. 1987, 59, 744-748
erties of such polymers to further enhance the detection capabilities. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Habert, M. K.; Baldwin, R. P. J . Chromatogr. 1985, 345, 43. Wang, J.: Freiha, B. Anal. Chem. 1984, 56, 2266. Santos, L. M.; Baldwin, R. P. Anal. Chem. 1986, 58, 846. Marko-Varga, G.: Appelquist. R.: Gorton, L. Anal. Chim. Acta 1988, 179, 371. Ikariyama, Y.; Heineman. W. R. Anal. Chem. 1986, 5 8 , 1803. Sittampalam, G.,Wilson, G. S. Anal. Chem. 1983, 55, 1608. Wang, J.; Hutchins, L. D. Anal. Chem. 1985, 5 7 , 1536. Wang, J.; Tuzhi, P.; Golden, T. Anal. Chim. Acta. in press. Oyama. N.: Anson, F. C. J . Am. Chem. SOC.1979, 101, 3450. Cox, J. A.: Kulesza, P. J. Anal. Chlm. Acta 1983, 154, 71.
(11) Facci, J.; Murray, R. W. Anal. Chem. 1982, 5 4 , 772. (12) Geno, P. W.: Ravichandran, K.; Baldwin, R. P. J . Nectroanal. Chem. 1985, 183, 155. (13) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192. (14) Rubinstein,'I.; Bard, A. J. J . Am. Chem. SOC. 1981, 103, 5007. (15) Moore, R. B.; Martin, C. R. Anal. Chem. 1966, 58, 2569.
RECEIVED for review August 21, 1986. Accepted November 11,1986. This work was supported by the National Institutes of Health (Grant No. GM 30913-03) and the American Heart Association. T.G. acknowledges the support of Battelle Pacific Northwest Laboratory for the award of a Summer Fellowship during the course of this work.
Determination of Volatile Constituents of Chinese Medicinal Herbs by Direct Vaporization Capillary Gas ChromatographyIMass Spectrometry Chen Yaozu,* Li Zhaolin, Xue Dunyuan, and Qi Limin
Department of Chemistry, Lanzhou University, Lanzhou, People's Republic of China
Analyses of the volatile constituents of Chinese medlclnal herbs have been accomplished via direct vaporiratlon GC/MS using sample slres of 3-15 mg of pulverlred herb samples subjected to heatlng at 250 OC for 60 s In a specially constructed vaporlror. By this new technique, three Chinese medicinal herbs have been analyzed and the results compared with those obtalned by tradltlonai steam distillatlon extractlon methods. Good agreement between the two techniques has been observed. Direct vaporitation, however, offered several distinct advantages. The method was rapid wlth a rnlnlmal sample preparatlon step, employed a small sample slre, and avoided the potential Interferences from lmpurltles derived from the solvents during extractlon and distillation.
Many Chinese medicinal herbs contain volatile essential oils as their effective pharmacological ingredients. In conventional methods of analyzing for essential oils, steam distillation and/or extraction with various organic solvents is often employed prior to the determination step. These methods involve extensive analytical manpower and are time-consuming. Besides, during such chemical treatments some constituents may be lost and the sample may suffer from potential contamination by impurities from the solvents employed. More recently, a method developed for the volatile monomers and additives of rubberlike polymeric samples involved a technique described by Hu ( I , 2) as chromatopyrography. In this basic principle, a vaporization chamber has been specially constructed in this laboratory to permit direct analysis by GC/MS. In this paper, we report the analysis of micrcquantities of three major Chinese medicinal herb samples by this technique of direct vaporization and compare the results with those obtained by the conventional steam distillation extraction methods.
EXPERIMENTAL SECTION Samples. The Chinese medicinal herbs Atractylodes macrocephala koidz (I),Zingiber officimleRoscoe (11),and Amomum globosum Lour (111) were all purchased from local markets and properly identified by Zhang Guoliang of the Department of Biology, Lanzhou University. Herb samples for direct vaporization were first pulverized to pass through a 60-mesh screen. Essential oil extracts for the same herbs were also obtained by steam distillation followed by extraction with ether. Gas Chromatography. Analyses were performed on a Shimadzu Model 9A gas chromatograph equipped with a flame ionization detector (FID) at a operating temperature of 260 "C and with nitrogen as carrier gas at a flow rate of 60 mL/min; hydrogen and air flow rates were 40 and 400 mL/min, respectively, with nitrogen being used as a makeup gas at a flow rate of 40 mL/min. Columns employed for the analysis of herb samples are as follows: for samples I and 111, a 30-m SE-54 capillary column; for sample 11, a 35-m PEG-2OM capillary column. Operating conditions for analysis are as follows: for herb sample I, from 120 to 140 "C at a rate of 1.5 OC/min, then to 230 O C at a rate of 2 OC/min; for herb sample 11, from 70 to 200 OC at a rate of 3 "C/min; for herb sample 111,from 65 to 160 "C at a rate of 2 OC/min, then to 230 "C at a rate of 10 OC/min. Mass Spectrometry. Mass spectra were obtained on a Finnigan MAT 312/S188 magnetic scanning mass spectrometer equipped with an electron impact (EI) source. Instrument parameters used were as follows: ionizing voltage, 70 eV; source temperature, 220 "C; emission current, 300 wA; scanning speed, 1 s/decade. Vaporization Chamber. The specially constructed device is illustrated in Figure 1 and the schematic shown in Figure 2 illustrates the overall instrumental layout. The herb powder (3-15 mg) in a platinum boat is placed onto the metal spoon (Figure 1,item 26) at the end of the operation rod which is then inserted into the front of the vaporization chamber and locked into place. When the temperature of the furnace and block heater have reached 250 O C , the ball valve is opened slowly and the operation rod is pushed forward to allow the sample boat to lie just outside the quartz heating tube. After a short time the sample boat on the spoon is then pushed into the center of the quartz tube. The volatile constituents of the sample then vaporize instantly and
0003-2700/87/0359-0744$01.50/0 1987 American Chemical Society
