1948
ANALYTICAL CHEMISTRY, VOL, 50, NO. 13, NOVEMBER 1978
chanical couple titrator required about 30 min to collect 35 to 40 equilibrium p H data points suitable for equivalence point determination; the optical couple system required about 20 min. Most of the time was spent in waiting for p H values to come to equilibrium. Computer control of the titration system allows rapid response to sharp equivalence points. Of approximately 35 data points collected in each titration of nitric acid with sodium hydroxide, 12 were within k0.4g of the equivalence point (total titrant delivery, 10 9). This allows precise determination of end points where the potential change is slight. The apparatus therefore should be applicable to a variety of titration systems, including complexation, oxidation reduction, and precipitation as well as acid base reactions. ACKNOWLEDGMENT T h e assistance of Hubert Priebe in the design and con-
struction of the syringe holder and spring assembly, and of Mark Ingram and Frank Dreissigacker in the design and construction of the electronic components, is gratefully acknowledged. LITERATURE CITED (1) W. E. Harris and B.Kratochvil, "Chemical Separations and Measurements, Background and Procedures for Modern Analysis", W. 8.Saunders, Philadelphia, Pa., 1974, p 160. (2) B. W. Renoe, K. R. O'Keefe, and H. V. Malmstadt, Anal. Chem., 48, 661 (1976).
RECEIVED for review May 2,1978. Accepted August 24, 1978. Research supported in part by the National Research Council of Canada and the University of Alberta. Financial support to R. G. by a National Research Council of Canada Scholarship is gratefully acknowledged.
Trace Determination of Subnanogram Amounts of Acrylonitrile in Complex Matrices by Gas Chromatography with a Nitrogen-Selective Detector R. S. Marano," S. P. Levine, and T. M. Harvey Ford Motor Company, Technical Services Department, Engineering & Research, Scientific Research Sfaff48121
T h e determination of acrylonitrile (ACN) in a variety of matrices has received attention in recent months because of its extensive industrial usage and its purported carcinogenic properties (1-3). The commonly used techniques for trace analysis of ACN depend on gas chromatographic (CC) separation followed by flame ionization detection (FID) ( 4 ) . Since the FID has poor specificity, it may be difficult to detect low levels of ACN in the presence of other organic species. For this reason, long GC analysis times or confirmation of peak identity by GC-mass spectrometry (MS) is required. A previously reported technique (5)utilizing a GC equipped with a nitrogen-phosphorus detector (NPD) (6) has been developed further in this laboratory. This technique offers significant advantages in terms of sensitivity and specificity of analysis. I t has been applied to the determination of trace levels of ACN in simulated air, water, and paint samples in order to illustrate the utility of this method. EXPERIMENTAL Apparatus a n d Operating Conditions. Hewlett-Packard Model 5710 and 5840 gas chromatographs were used. Each was equipped with a nitrogen-specific detector (Model 18789A Dual N-P FID). Two separate chromatographic conditions were used. Acetone solutions were analyzed on a 1.8 m x 2 mm i.d. glass column (on-column injection) packed with 1070 SP-1000 on 8O/lOO mesh Supelcoport. The column was operated at 80 "C with a helium flow of 30 cm'/min and an injection port temperature of 80 "C (analysis A). Aqueous solutions were analyzed on a 1.2 m X 2 mm i.d. glass column (on-column injection) packed with 60/80 mesh Chromosorb 102. This column was operated at 150 "C with a helium flow of 45 cm3/min and an injection port temperature of 100 "C (analysis B). In both cases, the NPD was operated at 300 "C under standard operating conditions of 2 to 3 cm3/min hydrogen, 50 cm3/min air, and 16 to 18 volts. Confirmation of acrylonitrile peaks was carried out on a combined Perkin-Elmer 910 GC/VG Micromass MM-16 mass spectrometer/Finnigan (INCOS) 2000 data system. Chromatographic analysis A (above) was utilized for GC/MS studies, with the MS operating under standard electron impact conditions. Samples a n d Analysis. Simulated air samples were taken hv drawing air containing ACN or ACN plus various organic 0003-2700/78/0350-1948$01.00/0
Research, Dearborn, Michigan
vapors through charcoal t .bes (SKC lot 106,100 mg + 50 mg back section) at a rate of 50-;30 cm3/min for up to 100 min. Tubes were capped and refrigerated until analyzed. Water samples were prepared by adding ACN to tap water. A simulated paint system was prepared by adding ACN to a sample of enamel paint. All solvents and ACN were reagent grade. In all cases, blanks were run to ensure the lack of interferences. Each charcoal tube was scored at the front end of the tube and broken. The glass wool and 100-mg portion of charcoal were transferred to a 1-mL vial (Reactivial, Pierce Chemical Co.), 0.5 mL of acetone was added, and the vial immediately sealed. The urethane foam separating the two sections of charcoal was discarded and the back section was desorbed in the same manner. The samples were desorbed (SKC Charcoal Developer) at room temperature for 30 min before analysis. Analyses were performed by injecting 1-2 pL of the acetone solution into the GC using a gas-tight syringe. For the simulated paint analysis, a 0.5-mL portion of an enamel paint that had previously been spiked with ACN was added to a 50-mL volumetric flask and water was added to volume. After vigorous mixing (Thermolyne MaxiMix), 2 pL of the aqueous phase was injected into the GC for analysis. Stock solutions of ACK were made for 4 pg/pL in acetone, 40 pg/pL in hexane, and 4 pg/pL in water. By successive dilution with the same solvent, appropriate standards were generated. Sealed vials containing charcoal and acrylonitrile were prepared at three levels of ACN. This was done by injecting the vials with 2.5 pL of standards containing 0.8 pg/pL, 8 pg/pL, and 40 Fg/pL of ACN in hexane. The charcoal was allowed to adsorb the standard for 24 h at room temperature after which time the vials were desorbed with 0.5 mL acetone. This simulated three sets of charcoal tubes which had adsorbed ACN at nominal levels of 2,20, and 100 p g . The desorption efficiency was determined by comparing the levels of ACN in these vials to that in vials prepared by injecting the standards into sealed vials containing only 0.5 mL of acetone. RESULTS AND DISCUSSION The sampling of ambient air for organic vapors using charcoal tubes is a well known technique ( 7 ) . I n fact, a National Institute for Occupational Safety and Health (NIOSH) procedure has been validat,ed for the collection and C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
5 0 3
1949
I 1 'r
ACETONE iDESORBlNO MEDIUM)
,
A,CRYLONITRILE
HYDROCARBON SOLVENTS ISIMULATED INDUSTRIAL ENVlRONMENTI
B
0
1 2 3 4 TIME W I N I
Figure 1. Chromatograms resulting from 1-pL acetone injections on SP-1000 column: (A) 8 nglpL standard, (6) extract from tube #5. Analysis A
analysis of ACN in air ( 4 ) . By using the NPD in place of a FID, the sensitivity of the NIOSH method is improved while the possibility of interferences from other organics is greatly reduced. An initial experiment was performed to assess the freedom from interference and the linearity of the method under simulated industrial use conditions. Air containing trace levels of ACN and a complex mixture of organic solvent vapors was drawn through seven charcoal tubes as previously described. The total ACN found on the tubes was in the range between 0.34 and 60.8 pg of ACN per tube. In each case >90% of the ACN was found on the front section of the tube. The precision of three replicate determinations for each tube was approximately 1% of the mean. ACN standards in acetone demonstrated linear detector response over the 0.8 to 500 ng/pL range, which is essentially equivalent to tubes containing 0.4 to 250 pg of ACN. In all cases, acetone sample extracts were analyzed along with standards of approximately the same concentration using GC analysis condition A (SP1OOO column). Figure 1gives typical chromatograms resulting from 1-pL injections of tube 25 (from the group of seven) and an 8 ng/pL standard. In this case, the charcoal tube extract is shown to contain 4.1 f 0.2 pg of ACN. Tube 21 was analyzed by this technique and the analysis was confirmed by GC/MS. This is illustrated in Figure 2. The GC-NPD analysis yielded a result of 60.8 i 0.7 pg ACN on tube z1 while the GC/MS analysis yielded a result of 61.5 pg ACN. One can appreciate the selectivity of the NPD from the examination of Figure 2. The potential interferences with the ACN analysis by high concentrations of organic solvent vapors is evidenced in the computer-reconstructed gas chromatogram from the GC/MS analysis. The ion current attributable to major ACN ions ( m / e 53 CH2=CHCN+., m/e 26 C2H2+.)is less than 0.1% of the total ion current. Nevertheless analyses by G C / M S and GC-NPD were in good agreement. I t was observed that the GC-NPD analyses could be repeated every 3 to 4 min because of the highly specific response of the detector. The results of recovery studies are given in Table I. These levels of ACN were chosen so as to correspond to levels expected to be found on actual sample tubes. The desorption step of the analysis appears to represent the limiting step in
/
i . . 23
Figure 2.
24
26
!2
27
2
e
-
2
Ion current chromatograms illustrating high level of hy-
drocarbon background relative to acrylonitrile in tube # 1
Table I. Recovery of Acrylonitrile from Charcoal acrylonitrile, p g desorption re1 std added found efficiency, % dev, %a 2 20 100 a
1.41 13.6 82.0
70.4 68.1 82.0 av 73.5
6.8 3.5 5.5 5.3
Four determinations.
terms of precision and accuracy. Recovery values obtained using acetone appear to be in the same range as those obtained by McCammon using methanol desorption ( 4 ) . The values thus obtained may then be corrected by considering the desorption efficiency of the method used. Preliminary results (Figure 3) were obtained for the determination of water spiked with ACN by direct aqueous injection into GC-NPD using chromatographic analysis condition B (Chromosorb 102). These conditions were also used for the analysis of the water extract of enamel paint spiked with ACN. Both tap water and the paint were spiked with ACN t o give concentrations of 5-10 ng/pL. The paint was extracted with water at a volume ratio of 100:1 (water/ paint). The detection limit was comparable to that quoted for acetone injections (10 pg/pL) and appears to offer significant improvement over that obtained in previous work ( 5 ) , yet in background matrices which are more complex. This was achieved by optimizing chromatographic conditions and detector operating parameters. A test was also run to analyze a methanol solution of ACN (analysis condition B, Chromosorb 102). The chromatogram appears identical to those shown in Figure 3. This permits the use of either acetone or methanol ( 4 ) elution of ACN on the charcoal tubes.
1950
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
for acetone and a Chromosorb 102 column for methanol or water. Neither carbon disulfide nor nitrogen-containing solvents can be used because of the extreme response and slow recovery of the detector to these solvents. The application of this technique to direct analysis of air samples without preconcentration on a charcoal tube should present no difficulty (although these tests have not yet been performed in this laboratory). A 1.0-mL gas sample injection of air containing 1.0 ppm of ACN will contain 2.3 ng of ACN. Since the detection limit of ACN is 10 pg per injection, direct air analysis should be applicable a t concentrations as low as 10 ppb ACN in air.
0 z
4
0,
A
'
< _
ACKNOWLEDGMENT The authors thank Michael D. Kelly for his technical assistance and suggestions.
LITERATURE CITED (1) (2) (3) (4) L0 1 2 3 TIME (MIN I
(5)
Figure 3. Chromatograms resulting from ~-I.LLwater injections on Chromosorb 102 column: (A) 8.8 n g l p L standard, (B) extract from spiked enamel paint
(6) (7) (8)
T h e NPD will yield a positive or negative response to the solvent depending on the bias voltage and the cleanliness of the crystal (8), thus necessitating the separation of the solvent from the ACN. This is accomplished using an SPlOOO column
R. J. Steichen, Anal. Chem., 48, 1398 (1976). M. E. Hall and J. W. Stevens, Jr., Anal. Chem., 49, 2277 (1977). J. F. Finklea, Am. Ind. Hyg. Assoc. J . , 38, 417 (1977). NIOSH method and data report No. S-156 supplied by C. S. McCammon; National Institute for Occupational Safety and Health, Cincinnati, Ohio, March 1978. C. A. Burgett, "The Gas Chromatographic Determination of Residual Acrylonitrile Monomer", Hewlett-Packard Note ANGC8-76 (1976). C. W. Rice, U S Patent 2,550,498 (April 24, 1951). "NIOSH Manual of Analytical Methods", HEW Publication No. (NIOSH) 75-121, U.S. Government Printing Office, Washington, D.C. 1974, P 8 CAM 127 (1975). H. B. Bente, "Evaluating and Optimizing the Performance of the Nitrogen-Phosphorus Detector", Hewlett-PackardNote 5950-3535 (February, 1977).
RECEIVED for review June 8, 1978. Accepted July 6, 1978.
Pepperbush Powder, a New Standard Reference Material Kensaku Okamoto, * Yuko Yamamoto, and Keiichiro Fuwa National Institute for Environmental Studies, P.O. Yatabe, Ibaraki, 300-2 7, Japan
T h e importance of standard reference materials for metal analysis has been recently well-recognized by scientists in many fields. The authors, in a cooperative study with NBS research groups, have performed some research for new biological reference materials, and t h e work includes t h e preparation of Japanese Tea Leaves (1) and "wet" Shark Meat samples (2). Pepperbush tree is known to accumulate Zn, Mn, Co, Ni, and Cd in t h e leaves ( 3 ) ;thus the elemental composition of pepperbush leaves is significantly different from that of other botanical reference materials such as Kale Powder ( 4 ) ,Orchard and Pine Needles Leaves (5), Tomato Leaves (6),Spinach (9, (8). We have, therefore, prepared pepperbush powder as a new biological standard reference material.
EXPERIMENTAL Sampling a n d Drying. The leaves of pepperbush (Clethra barbineruis), collected in September 1975, at Mikouchi in the Ashio district and free from stems, were washed with deionized water and dried in an air oven at 60 "C overnight. About 30 kg of the dry leaves were used in this work. Grinding and Sieving. The dry leaves (about 700 g) were ground for about 1 h in a ball-mill (95'70 Al2O3,7 L) which had been previously ground well with the leaves to minimize contamination with metals. The pulverized samples were placed on 0003-2700/78/0350-1950$01.OO/O
Table I. Homogeneity of Pepperbush Samples p g l g dry wt
maximum value minimum value average re1 std dev, %
Zn 345 325 337 1.5
Fe 172 161 166
2.2
Mn 2110 2020 2090 1.0
a set of sieves; a 50-mesh (297 pm) nylon sieve (top), a 60-mesh (177 wm) nylon sieve (middle), and a reservoir made of vinyl chloride (bottom), and vibrated mechanically for 15 min. Mixing. The powder which passed through a BO-mesh sieve was divided into two parts with a riffle sampler (JIS No. 2) made of vinyl chloride. The powder was piled up in two layers and again divided by passing through the riffle sampler. Pepperbush samples were homogenized by repeating this procedure ten times. Metal Determinations for Assessment of Homogeneity. Samples were dry-ashed as follows: One gram of pepperbush powder (dried at 105 "C to constant weight) was ashed in a platinum crucible at 450 "C overnight, dissolved in 2 mL of 6 M HC1 and made up to 100 mL with doubly-distilled water. Samples were also wet-digested with nitric acid and hydrogen peroxide as described previously (3). Zn, Fe, Mn, and Mg were determined by atomic absorption spectrometry using an air-acetylene flame. Co, Cd, Pb, and Cu were determined by atomic absorption 22 1978 American Chemical Society
