Gas-Liquid Chromatographic Evaluation and GasChromatography/Mass Spectrometric Application of New HighTemperature Liquid Crystal Stationary Phases for Polycyclic Aromatic Hydrocarbon Separations George M. Janini," Gary M. Muschik, James A. Schroer and Walter L. Zielinski, Jr. NCI Frederick Cancer Research Center, P.O. Box B, Frederick, Md. 2 170 1
This paper is the fourth in a series dealing with application of high-temperature liquid crystals as stationary phases for gas-liquid chromatography (GLC). N,N'-Bis(p-phenylbenzylidene)a,cu'-bl-p-toluidine (BPhBT) with a nematic range of 257 to 403 OC, and N,N'-bis( p-hexyloxybenzylidene) a,a'-bi-p-toluidine (BHxBT) with a smectic range of 127 to 229 OC and a nematic range of 229 to 274 OC were synthesized and their use as stationary phases for GLC separations of polycyclic aromatic hydrocarbons (PAH) was investigated. A 3-ft X 2-mm i.d. column packed with 2.5% BPhBT on 100-120 mesh HP Chromosorb W showed insignificant noise levels when operated at 275 'C in a gas chromatography/mass spectrometry (GC/MS) system. When the GC/MS instrument was operated In the selective ion-monitoring mode, the detection limit for benzo[a]pyrene was ca. 4 ng. Isothermal BPhBT column operation at high temperatures (270-290 "C) has permitted the analsis of hlgh molecular weight PAH (having 22-24 carbons), providing, for the first time, basellne separation of some of the 5-7 ring PAH geometric isomers.
Some of the most efficient experimental carcinogens known a t present are key members of the chemical class commonly referred t o as polycyclic aromatic hydrocarbons (PAH) ( I ) . They occur in such diverse sources as atmospheric particulate matter, tobacco smoke, processed food, high-boiling petroleum distillates, and other environmental situations. T h e potential adverse effect of these chemicals on human health is, therefore, a matter of growing international concern (2). Clearly, the lack of practical separation methods is a serious obstacle to advancing the understanding of the carcinogenicity of PAH and their metabolites. Airborne particulate matter can contain over 100 different PAH compounds ( 3 ) ;and 150 PAH were isolated from both tobacco and marijuana smoke condensates ( 4 ) .Not all PAH are established carcinogens, yet the separation of carcinogenic from noncarcinogenic PAH components of the same molecular weight using conventional chromatographic methods is difficult (5,6).However, it was recently demonstrated that novel GLC baseline separations of isomeric 3-5 ring PAH components were achieved on the nematic liquid crystal N,N'-bis(p-methoxybenzy1idene)a,u'-bi-p-toluidine (BMBT) (5). This liquid crystal also provided unique resolution of underivatized 3ulP-hydroxy steroid epimers of the androstane and cholestane classes (7), polychlorinated biphenyls (8),and methoxybenzanthracene isomers (9). In all cases, the separations obtained were superior to those generated on conventional GLC liquid phases; however, it was noted that BMBT exhibits measurable column bleed over prolonged operating periods a t elevated temperatures (5, IO). In an effort t o overcome this limitation, a new liquid crystal (BBBT, the bis-p -butoxy homolog of BMBT) was synthesized and shown to provide the same unique separations with significantly diminished column bleed ( I O ) . It
has been recently observed, however, that analysis of high molecular weight PAH (having 22-24 carbons) on either of these liquid crystals resulted in excessive solute retention and broad elution peaks, even a t the maximum permissible operating temperatures. This paper reports the synthesis of N,N'- bis(p-phenylbenzylidene)-a,a'-bi-p-toluidine (BPhBT) and N,N'- bis(phexyloxybenzy1idene)-a&-bi-p-toluidine (BHxBT) as new high temperature liquid crystals compatible with GCIMS systems. This study extends the usefulness of liquid crystals as GLC phases for the analysis of 5-7 ring PAH.
EXPERIMENTAL Materials. BPhBT (patent disclosure filed) was prepared in 73% yield by refluxing a 2:l molar ratio mixture of p-phenylbenzaldehyde (Aldrich) and cup'-bi-p-toluidine (Eastman Kodak), respectively, in absolute ethanol for several days. The yellow product formed was collected by filtration of the hot suspension and washed with hot ethanol. p-Phenylbenzaldehyde was purified by extraction with 5% sodium carbonate and recrystallization. cup'-Bi-p-toluidine was used without further purification. BHxBT (patent disclosure filed) was similarly synthesized in 87% yield using p-hexyloxybenzaldehyde (Eastman Kodak) in place of p-phenylbenzaldehyde. Elemental analyses results were within 0.3%of calculated values for both products. Phase transition temperatures measured by differential scanning calorimetry were 257 "C (solid-nematic) and 403 "C (nematic-isotropic) for BPhBT, and 127 "C (solid-smectic I), 203 "C (smectic I-smectic 111, 229 "C (smectic 11-nematic), and 276 "C (nematicisotropic) for BHxBT. Mesomorphic phase changes were visually observed by inspection of the melt on a Fisher-Johns melting point apparatus. Dibenzanthracenes and dibenzpyrenes were obtained from Duke Standards Co. (Palo Alto, Calif.). All other PAH, glass-distilled chloroform (used for the preparation of the packing material) and benzene (used for the preparation of PAH solutions) were obtained from sources previously identified (5). Apparatus and Procedure. A Hewlett-Packard 7610 gas chromatograph equipped with a dual flame ionization detector was used. Chromatograms were generated on a 1-mV full scale strip chart reA full scale. Helium corder using an electrometer setting of 4 X carrier gas flow was regulated by a calibrated Brooks 5840 dual mass flow controller. Columns were 2-mm i.d. borosilicate glass. Temperatures reported were those indicated on the instrument dial. Air flow rate was maintained at 300 ml/min, while the hydrogen flow was usually kept about 5 ml/min lower than the carrier gas flow rate. The packing material (2.5 w t % on 100/120 mesh HP Chromosorb W) was prepared by the solvent slurry method, fluidized drying with nitrogen, and resieving to 100/120 mesh. BHxBT was completely soluble in chloroform, and therefore amenable to standard coating procedures. BPhBT was only slightly soluble in chloroform, but formed a fine dispersion which was slurried with the solid support. A mixed liquid phase was prepared by slurrying the support with equal amounts of BPhBT and BHxBT in chloroform. The solvent was then evaporated using a vacuum rotatory evaporator, and the packing was resieved to 100/120 mesh. Sample injection volumes were 1-2 pl. To verify the thermal stability of these liquid phases, the theoretical plate count of chrysene was determined and found to be reproducible to within 5% over a period of 100 h of continuous operation at 275 "C, using three different 6-ft X 2-mm i.d. glass columns packed with 2.5% (w/w) each of BPhBT, BHxBT, and 1:l BHxBT/BPhBt on 100/120 mesh HP Chromosorb W. The number of theoretical plates generated
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
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Table I. Retention Times for Four Members of the "Benzpyrene Fraction" on (a) BPhBT and (b) 1:l BPhBT/BHxBT Mixed Phase in t h e Temperature Range 250-275 " C a Retention times, minb Column temperature, "C
Benzo[h]fluoranthene (a) (b) 3.87 1.59 4.67 4.59 1.86 2.65 4.06 3.11 3.52 2.78 3.01
20250 255 260 265 270 275
Benzo[e]pyrene (a) (b) 4.55 1.94 5.33 2.19 5.23 2.96 4.59 3.37 3.96 3.05 3.41
Perylene (a)
Benzo[a]pyrene (a) (b) 6.38 2.65 7.90 3.23 7.75 4.71 6.74 5.46 5.79 4.87 4.92
(b) 5.47 6.49 6.35 567 4.79 4.11
2.36 2.67 3.67 4.22 3.78
a Columns: 4-ft X 2-mm i.d. glass. Packings: 2.5% (w/w); flow rate, 30 ml/min.; injector and detector, 280 "CbRetention times are averages of three determinations within f 1%.
NEMATIC
N
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i
~
ISOTROPIC
~
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\
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1.9
!75
T("C)
I
I .E I O % T ( ~ K )
Figure 1. Dependence of the separation factor ( a )on column temperature for separation of chrysenelbenz [alanthracene on BHxBT Column: 6 4 X 2-mm id. glass. Packing: 2.5% (w/w). Injector and detector: 240
'C; flow rate, 30 mllmin.
by the columns used in this study (2-ft to 6-ft) for chrysene ranged from 1200 to 2000. Operation of the BPhBT and the 1:l BPhBT/ BHxBT columns at 275 "C beyond 100 h resulted in a gradual deterioration of performance as witnessed by some loss of retention and resolution. Visual inspection indicated a gradual darkening of the yellowish color of the packing material near the inlet end of the column while the detector end of the column appeared to be unaffected. The use of approximately 1-inchlong plugs of a preconditioned (5-6%) SE-30 packing at both ends of the columns seemed to retard column deterioration with no adverse effect on column performance. Analyses by gas chromatography-mass spectrometry were carried out on a Finnigan 3300 mass spectrometer equipped with a Finnigan 6000 data system. The mass spectrometer was directly interfaced to the column outlet of a Varian 1400 gas chromatograph. Ultra-high purity helium was used as the GLC carrier gas (at a flow-rate of 28 ml/min) and also as the chemical ionization charge-exchange reagent gas (at an ion source pressure of 300 pm). The emission current and electron energy were maintained at 0.50 mA and 70 eV, respectively. The temperatures of the injector port, transfer line, and analyzer were kept at 250, 275, and 120 "C, respectively. The data system dataacquiring parameters were optimized for maximum sensitivity for mass fragmentography operation.
RESULTS A N D DISCUSSION T h e principal objectives of this study included the preparation and evaluation of BPhBT and BHxBT as GLC liquid phases for GLC and GC/MS application t o P A H geometric isomers which are otherwise difficult to separate on conventional GLC liquid phases. Figure 1and Table I illustrate the effect of liquid crystalline state on the variation of solute selectivity offered by these two liquid crystals in the temperature domains at which they may be useful for GLC separa1880
I
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I
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I
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Time, minutes
Chromatogram of a mixture of 16- to 18-carbon PAH compounds on BHxBT at 215 "C Figure 2.
Column and condltions: Same as Figure 1
tions. Figure 1 shows the temperature dependence of the separation factor ( a ) for the solute pair chrysene/benz[a] anthracene on BHxBT. In the BHxBT smectic I region (127-203 "C; not shown in Figure l),the separation was poor and t h e peaks were broad. I n contrast, the BHxBT smectic I1 region (203-229 "C) effects a solute retention behavior comparable to that observed in the nematic region (229-274 "C). T h e separation factor for chrysene/benz[a]anthracene approached a maximum a value in the vicinity of 215 "C, above which a decreased with increasing temperature. In contrast t o the sharp changes in a observed for the smectic I-smectic I1 and nematic-isotropic transitions, t h e change accompanying the smectic 11-nematic transition was less abrupt, indicating only a minor change in the solution properties of the liquid crystalline matrix for this latter transition. T h e heat of transition for the smectic 11-nematic transition (1.81 kJ/mole) was considerably lower than that associated
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
5
7
9
11
Time, minutes
Figure 4. Total ion current chromatogram of 20-carbon pentacyclic arenes on BPhBT
Column: 3-ft X 2-mm i.d. glass. Packing: 2.5% (w/w). Conditions: Oven, 275 O C ; flow rate, 28 ml/min. Scan rate: 200 AMU/s. Range: m/e 40-350 100
0
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4 6 Time, minutes
8 Oh
M.W. 252
-
I
Chromatogram of a mixture of 18- to 21-carbon PAH compounds on 1:l BHxBTIBPhBT mixed phase Figure 3.
Column: 4-ft X 2-mm i.d. glass. Packing: 2.5% total (w/w). Conditions: Oven, 265 O C . Injector and detector: 275 O C ; flow rate, 30 ml/min
with either the smectic I-smectic I1 (5.24 kJ/mol) or nematic-isotropic (7.32 kJ/mol) transitions. T o illustrate the usefulness of BHxBT in its smectic I1 region, the separation of five 18-carbon and two 16-carbon PAH is shown in Figure 2. This separation compares favorably with that reported previously in the nematic region of BMBT (Figure 3 in Ref. 5 ) .Furthermore, BHxBT columns operated a t 215 "C have no discernible bleed, allowing samples in the order of 10 ng of benz[a]anthracene to be analyzed. This exhibited selectivity and associated sensitivity are of utmost importance for the quantitative measurement of this compound, a recognized carcinogen and a major constituent of the total PAH fraction in urban airborne pollutants ( 3 ) . The temperature dependence of solute retention for four members of the so-called "benzpyrene fraction" on BPhBT and on the 1:l BPhBT/BHxBT mixed phase is presented in Table I. Below 255 OC for the BPhBT and 250 "C for the 1:l BPhBTDHxBT columns, the separations were poor and the peaks were broad (data not presented in Table I) limiting the use of these phases to higher temperatures. Following the BPhBT solid-nematic transition (257 "C), an increase in retention is observed, concomitant with enhanced solute selectivity. Solute retention and selectivity both attain optimum values a t 270 OC which decrease with further increase in temperature. Since BHxBT is structurally similar to BPhBT and both are mesomorphic, similar PAH retention behavior was observed on the 1:l BPhBT/BHxBT mixed phase. The temperature of maximum retention and optimum selectivity on the mixed phase is, however, depressed to 255 "C. Expansion of the restricted nematic ranges of mesomorphous stationary phases by the use of mixtures of liquid crystals has been described (11-13). Further studies on BPhBT binary mixtures with other high-temperature liquid crystals may, therefore, result in phases having various expanded nematic temperature ranges. While the high solid-nematic phase transition temperature of BPhBT restricts its utility to the separation of narrow so-
50
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250
300
mle
Flgure 5. Helium chemical ionization mass spectrum of benzo[a]pyrene. MS conditions were as stated under Experimental
lute molecular weight ranges, nonetheless, BPhBT is useful for the separation of 4-7 ring PAH solutes. Figure 3 shows the separation of a mixture of some four- and five-ring PAH. The baseline separation of benzo[a]pyrene from benzo[e]pyrene as well as from the other PAH in a relatively short analysis time is yet unmatched by other separation techniques (6), including GLC using other liquid crystalline phases ( 4 1 0 ) . The shorter analysis time required to effect PAH separations on these new liquid crystals (BPhBT and BHxBT) resulted in narrower peak widths and greater solute concentrations per unit time, allowing smaller samples to be analyzed. The low bleed levels and high efficiency characteristics observed in this study for BPhBT and 1:l BPhBT/BHxBT packed columns enabled their application as liquid phases in GC/MS systems. I t is well recognized that GLC is often limited by the lack of standard reference materials and principally offers retention data and detector responses for sample identification. Solute confirmation can often be directly assisted, however, with the use of combined GC/MS. Moreover, the application of selective ion monitoring (SIM) or mass fragmentography has substantially enhanced the sensitivity and selectivity of GC/MS systems. A mixture of four pentacyclic arenes (including benzo[a]pyrene) was analyzed on a BPhBT column at 275 "C with the GC/MS system in the helium chemical ionization mode. The data were accumulated on the MS data system a t a scan rate of 200 AMU/s for the range m/e 40-350. Figure 4 shows the computer-reconstructed chromatogram with no background correction, and Figure 5 shows the mass spectrum of benzo[a]pyrene taken from the G U M S run. The base peak in the mass spectra of all components separated in Figure 4 was the molecular ion ( m / e 252). The spectra were identical and agree with reference standards ( 1 4 ) . Using the SIM mode a t m/e 252, it was possible to detect a sample injection of 4 ng of benzo[a]pyrene. Further optimization of the spectrometer parameters should allow de-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976 * 1881
300
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mle
M.W. 588
>305 X 10
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Flgure 6. Solid probe mass spectra of (a) BPhBT and (b) BHxBT
1"" 30
A
I
In 50
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150 m/e
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280
B
I, 0
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Figure 7. Mass spectral background at 275 OC for (a) BPhBT and (b) BPhBTIBHxBTGC conditions: (a) 3 4 X 1-mm i.d. glass column of 2.5% BPhBT; (b) 6-ft X 2-mm i.d. glass column of 2 . 5 % total loading of 1:l BPhBT/BHxBT. MS conditions were as stated under Experimental
4
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Time, minutes
300
Figure 9. Chromatogram of five 24-carbon arenes on BPhBT at 290 OC Column: 2-ft X 2-mm i.d. glass. Packing: 2.5% (w/w). Conditions: injector and detector, 290 OC; flow rate, 50 rnl/min. Compounds in order of elution are: 4,5,6,7-dibenzpyrene (dibenzo[def,p]chrystene),4,5,7&dibenzpyrene, coronene, 2,3,6,7-dibenzpyrene (benzo[rst-Ipentaphene), and 1,2,6,7-dibenzpyrene
tection of less than 1 ng. When the mass spectrometer was operated in the normal mass scanning mode (mle 40-350) the detection limit for benzo[a]pyrene was approximately 30 ng. At this level of sample (ca. 30 ng) a peak a t m/e 285 (the major mle peak from the liquid phase bleed) had a comparable abundance to the mle 252 peak from benzo[a]pyrene. The mass numbers and relative intensities of the peaks in the mass spectra resulting from column bleed appears to vary with column ageing. Figure 6 shows the solid probe mass spectra of ( a )BPhBT and ( b )BHxBT. The background mass spectra resulting from the bleed of a BPhBT column and a 1:l BPhBTlBHxBT column (both operated a t 275 "Cafter 12 h of column conditioning) is presented in Figure 7. No background was observed for either column above rnle 300. As it is often the case (15), the column bleed spectra differ conFigure 8. Chromatogram of five 22-carbon arenes on BPhBT at 270 siderably from the solid probe spectra of the same liquid OC phases . Column: 2-ft X 2-mm i.d. glass. Packing:2.5% (w/w). Conditions:injector and Some of the highest molecular weight PAH (containing 22 detector, 280 'C: flow rate, 30 ml/min. Compounds in order of elution are: carbons and above) are recognized potent carcinogens (1617). 1,2,3,4-dibenzanthracene (dibenz[a,c]anthracene), II12-benzperylene (ben1,2,5,6-dibenzanthracene(dibenz[a,h]anthracene), Analytical difficulties have previously been encountered for zo[gbi]perylene), their separation and measurement. Capillary column gas 1,2,7&dibenzphenanthrene (picene), and 2,3,6.7-dibenzanthracene (pentachromatography has provided a certain degree of resolution cene) 1882
ANALYTICAL CHEMISTRY, VOL. 48, NO.
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( 4 ) ,yet dibenzanthracenes (22-carbon pentacyclic arenes) and dibenzpyrenes (24-carbon hexacyclic arenes), among others, were essentially unresolved. Both packed and surface-coated open-tubular columns were also found to be inadequate for the resolution of these compounds ( 3 ) .Furthermore, their analysis on other liquid crystals has been unattractive because of prolonged solute retention, yielding broad elution peaks near the maximum practical column temperatures. Separation of such solutes appears feasible, however, on BPhBT columns. An illustration of the baseline separation of five 22-carbon pentacyclic and hexacyclic arenes is given in Figure 8. Figure 9 shows the separation of five 24-carbon hexacyclic and heptacyclic arenes. As expected, solutes having larger lengthto-breadth ratios are retained longer. The GLC separations and GC/MS application reported here should be useful for direct application to PAH carcinogenesis research and for the evaluation of human exposure to PAH components.
LITERATURE CITED (1) A. Dipple, Excerpta Med. lnt. Congr. Ser. 350, Vol. 2. Chemical and Viral Oncogenesis (1974). (2) /ARC lnt. Tech. Rept., No. 71/002 (1971). (3) R. C. Lao, R. S. Thomas, H. Oja, and L. Dubois, Anal. Chem., 45, 908
(1973).
(4) M. L. Lee, M. Novotny, and K. D. Bartle, Anal. Chem., 48, 405 (1976). (5) G. M. Janini, K. Johnston, and W. L. Zlelinski. Jr., Anal. Chem., 47, 670 (1975) and references therein. (6) M. Dong, D. C. Locke, and E. Ferrand, Anal. Chem., 48, 368 (1976) and references therein. (7) W. L. Zielinski, Jr., K. Johnston, and G. M. Muschik. Anal. Chem., 48,907 (1976). (8) Analabs Tech. Bull., North Haven, Conn., 1975. (9) J. C. Wiley, Jr., C. S. Menon, D. L. Fisher, and J. E. Engel, TetrahedronLett., 33, 281 1 (1975). (IO) G. M. Janini, G. M. Muschik, and W. L. Zielinski, Jr., Anal. Chem., 48,809 119761.
(11) H. Keiker, B. Scheurle, and H. Winterscheidt, Anal. Chim. Acta, 38, 17
(1967). (12) M. J. S. Dewar, J. P. Schroeder, and D. C. Schroeder, J. Org. Chem., 32, 1692 (1967). (13) J. P. Schroeder, D. C. Schroeder, and M. Katsikas, "Liquid Crystals and Ordered Fluids", J. F. Johnson and R. S. Porter, Ed., Plenum Press, New York and London, 1970, p 169. (14) E. Stenhagen, S. Abrahamsson, and F. McLafferty, "Registry of Mass Spectral Data", Vol. 2. John Wlley and Sons, New York, 1974, pp 1526-1527. (15) D. M. Taylor, "Flnnigan Applications Tips", No. 54, Sunnyvale, Calif., 1974. (16) /ARC lnt Tech. Rept. Ser., Vol. 3 (1973). (17) E. L. Wynder, E. A. Graham, and A. B. Croninger; CancerRes., 13, 855 (1953).
RECEIVEDfor review May 17,1976.Accepted August 9,1976. Research sponsored by the National Cancer Institute under Contract N01-CO-25423 with Litton Bionetics, Inc.
Quantitative Determination of the Monomer Composition in Hexafluoropropylene/Vinyl idene Fluoride Copolymers by PyroIysis-Gas Chromatography Julian T. Blackwell
E. 1. du Pont de Nemours & Company, Elastomer Chemicals Department, Du Pont Experimental Station,
This paper demonstrates that the Curie point pyrolyzer provides sufflciently reproduclble temperature and rapid temperature rlse time to permlt quantitative analysis of the monomer composltlon in hexafluoropropylene (HFP)/vinylldene fluoride (VF,) copolymers through gas chromatographic analysis of the pyrolysis products. The polymer composition Is calculated from the relative amounts of monomer regenerated and the trlfluoromethane produced during pyrolysls. A calibration curve was obtained using samples whose compositions were measured by lSFNMR as standards and a least square flt calculated. The reproduclbility of the pyrolysis step, achieved by the Curie polnt pyrolyzer, enables us to determine the monomer composition with a reproducibillty of fI%. The monomer composition of unknown HFP/VF2 copolymers can now be calculated, based on the callbration.
In pyrolysis-gas chromatography, structural information is obtained by thermally fragmenting the sample ip a stream of carrier gas and analyzing the pyrolysis products by gas chromatography. Quantitative polymer analysis from pyrolysis chromatograms or "pyrograms" is a function of the reproducibility of pyrolysis temperature, rapid temperature rise time, and sample uniformity (1-6). The Curie point pyrolyzer as developed by Simon and coworkers ( I , 6, 7) offers advantages in all three areas. In the Curie point pyrolyzers, high temperature pyrolysis
Wilmington, Del. 19898
is achieved by introducing a ferromagnetic wire in a radiofrequency (rf) magnetic field. At its Curie temperature or Curie point, the wire loses its ferromagnetic properties. This temperature is quite reproducible and is a function of the composition of the wire. Ferromagnetic alloy wires with Curie temperatures between 200 and 1000 "C are commercially available. Simon and Buhler have shown that 0.5-mm diameter cylindric ferromagnetic wires can reach their Curie temperature in less than 30 ms.
EXPERIMENTAL Apparatus. The coupling of the Curie point pyrolyzer to the Hewlett-Packard 5750 (HP-5750) gas chromatograph is shown schematically in Figure 1and a breakdown of the pyrolysis chamber in Figure 2. The .magnetic induction coil was positioned as close to the injection port as possible. The solenoid gas valve is operated by a switch on the control panel. The HP-5750 was equipped with a gas sampling valve and the solenoid valve was connected in place of the sample loop. With the gas sample valve open, the solenoid valve can be used to divert the carrier gas flow either directly through the injection port or through the pyrolysis chamber. The pyrolysis chambers supplied by the manufacturer were found to be unsuitable and replacements were fabricated. The needles used were 24-gauge Huber point hypodermic needles from which the syringe fitting was removed. These were joined to 3-mm 0.d. quartz tubes and sealed with Torr Seal, an epoxy resin used in vacuum systems. Sample Preparation. A 10% solution of each hexafluoropropylene (HFP)/vinylidene fluoride (VF2) copolymer was prepared by dis-
ANALYTICAL CHEMISTRY, VOL. 48,
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