Anal. Chem. 1982, 54. 1191-1196
1191
Determination of Sub-Parts-per-Billion Levels of Acrylonitrile in Aqueous Solutions Tore Ramstad and Lawrence W. Nicholson' Analyiical Laboraiorles. Building 574, Dow Chemical Company, Mich@n Division, Mkfknd. Michigan 48640
A method combining steam dlstlilation, elevaied-temperature
purge-and-trap, and nHrogenapeclflc detection has been d e veloped for detection of low and sub-parts-per-bllllon levels of acrylonitrile In aqueous systems. Steam dlstlllatlon effects a sample cleanup and concentrates acrylonitrile. Combination of the elevated purge-and-trap technique wHh a thermal N-P detector provides Selective and sensitive detection. The method has been validated for acrylonitrile in water over the range of 10 ppt to 10 ppb. Several appllcaiions 01 the technlgue are Included.
Acrylonitrile (AN, 2-propenenitrile) is used in the manufacture of fibers, elastomers, resins, and latexes and in the synthesis of other chemicals, primarily adiponitrile and acrylamide. Evidence of AN toxicity can be found in the literature. AN has been shown to be toxic when ingested, inhaled, or applied to the skin of laboratory rats; tumorous and nontumorous effects were observed. AN is carcinogenic to rats at 35 ppm in drinking water; teratogenic effects have also been reported (1-4). Recent evidence obtained by Monsanto showed a no-effect level for neoplasms in rats at 1 and 3 ppm dose levels in drinking water (5). Acrylonitrile-based polymers used in food packaging materials are regulated by the Food and Drug Administration (FDA). The Occupational Safety and Health Administration (OSHA) regulates workplace exposure. AN is one of the Environmental Protection Agency's (EPA) Consent Decree Priority Pollutants. Therefore, there exists a need for improved methodology for the determination of AN. Several published methods describe the determination of AN in aqueous systems. Early methods relied on polarographic procedures (6-8). Titrimetric (9, 10) and spectrophotometric methods (11,12)have also been used. Azeotropic predistillation with methanol has been used to enhance AN detectability (13). In recent years, most reported procedures for AN utilize gas chromatography (GC) in the determinative step. Direct aqueous GC (14, 15) is an appropriate technique for "wellbehaved" waters (no matrix problems) where sensitivity requirements are modest (>-0.1 ppm). Static beadspace techniques (16,17)are more sensitive and more reproducible for low-level AN determinations in aqueous solutions. When a combination of static headspace and N-specific detection (18)was used for the determination of AN in aqueous food simulants, a detection limit of 1ppb and a relative precision of *lo% at the 5 ppb level were achieved. Preconcentration of Chromosorb 102 followed by thermal desorption (19) or delayed injection gas chromatography (20) are suitable methods for low parts per billion determinations of AN in water. AN adsorption on resins from large aqueous samples followed by solvent extraction is not practical because of AN'S water solubility and volatility (7.3%-at 20 "C and 100 mmHg at 23.6 "c. bD of 77 "'2). Recently, analysts have demonstrated the advantage of 0003-2700/82/0354-1191$01.25/0
Flgure 1. Distillation apparatus.
combining distillation with a suitable determinative step for several classes of organic compounds-the so-called volatile polar organics (VPOs)(21-23). Kuo et el. report a thorough investigation of distillation parameters (24). Direct injection of steam distillates permitted quantification of alcobols, ketones, and aldehydes at the parts per billion level. The authors concluded that the compounds' volatility relative to water was a useful parameter in determining the concentrations of the respective compounds in the distillate. Peters described a steam distillation device that can effect concentration factors of up to 200 for compounds whose azeotropes with water boil below 99 "C (25)(AN forms a 88:12 AN/H,O azeotrope boiling a t 71 "C). Earlier work in this laboratory revealed that AN could be determined at low parts per billion levels in water if the sample was purged at elevated temperature (unpublished). The dynamic nature of the purgeltrap technique (26) affords greater sensitivity than the conventional static headspace or other techniques discussed earlier. The method to be described combines distillation of an aqueous sample, elevated-temperature purge-and-trap, and use of N-specific detection to achieve high sensitivity and selectivity. EXPERIMENTAL SECTION Equipment and Procedure. Distillation. Water suitable for use in low-level fortifications or as a diluent was prepared by distilling demineralized water from a I-L round-bottom tlask and through a 25 cm X 25 mm 0.d. Vigreaux fractionation column, A forecut (ICtzO% of the initial volume) was discarded and then 0 1982 American Chemical Saciety
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1102
1
l'o
1
0.9
% 0.8
PurgeITrap Assembly
c
r -
0.7
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0.4
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0'
-* i? Chromatograph
1N.P Detector)
8
10
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16
18
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Purge Time iminl
Figure 4. Breakthrough curve for AN on Tenax-GC; 20 ppb AN purged at 40 cm3/min.
Figure 2. Purgeltrap apparatus used for the determination of acrylonitrile.
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100-
(98.51 (98 51
199.61
(99.91
(%
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100)
L 4 min
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Desorption Time (mini
0
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Volume of Distillate (mi)
Figure 3. Recovery of acrylonitrile vs. volume of distillate; 0.33 ppm distilled from 150 mL. appropriate volumes of distillate were collected in flasks, stoppered, and used shortly thereafter. Samples (50-200 mL) were distilled by using the apparatus shown in Figure 1. When necessary, diluent water was added to provide a volume of at least 100 mL. Certain samples were boiled under total reflux prior to collecting distillate. The condenser, a Liebig straight-throughtype, was connected via a spherical ground glass joint. Cargille boiling stones, 4-12 mesh, were used without pretreatment. Distillates were collected in (15 mL) Hypo-vials (Pierce). The receiver vessel, containing a small magnetic stirring bar and predistilled water, was surrounded by crushed ice. When the receiver was about three-quarters full of distillate, it was lowered (situated on a jack) so that the condenser tube was no longer submersed, the magnetic stirrer was turned on, and the vial fded to the brim. If the distillatewas not analyzed immediately, the vial was fitted with a Teflon-faced septum, capped, and crimped. No bubbles should be present. The distillate should be run within 1day, because even a crimp-capped vial is subject to contamination with AN from ambient sources (vide infra). All glassware was cleaned with demineralized water, methanol,
Figure 5. Desorption curve for Tenax-GC (several micrograms desorbed). Desorption temperature of 200 'C. and methylene chloride and dried before reuse. Wet glassware is more prone to AN contamination than dry glassware. Purge-and-Trap/GC. The purge/trap assembly was altered slightly to accept our purge vessels (27)-jacketed, slightly modified versions of the standard Bellar and Lichtenberg design (26). A schematic of the purge-trap system is presented in Figure 2. Thermostated water was circulated through the vessel jacket. A Hewlett-Packard (HP) 5710A GC with a N/P thermionic detector and a HP 7675A purge-and-trap unit were utilized in the analyses. The GC column, a 1.7 m X 2 mm i.d. 2.5% Oronite NIW on 60/80Carbopack B (28) (prepared in our laboratory), was operated at a flow rate of 25 cm3/min of He. Standards. AN (Aldrich Chemical Co.) standards were prepared in heated devolatilized ethylene glycol (EG). Standards were prepared in 4-dram vials, immediately transferred to 1-dram vials, fitted with Teflon-faced septa, and capped without intrusion of a bubble. EG is miscible in all proportions with water but does not purge from water.
RESULTS AND DISCUSSION Method Development. Several aspects of the method will be discussed initially.
ANALYTICAL, CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
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Table I. Results of Validation 1 0 PPb
run no.
plr h t
rec:
%
2
3
4
5
Number of Purges
Figure 6. Percenlt AN stripped under conditions of purge; 0.5 ppb AN subjected to subsequent purge cycles consisting of a 1Bmin purge conducted at 75 "C with a He flow of 40 cm3/min.
rec, %
174.0 102 1 2 175.2 103 3 169.0 99 4 170.6 100 5 158.6 93 ref 170.2 precision 99 + 7.8 % re1 std dev = 3.9
0.4 ppb
100 ppt
run no.
1
pk ht
34.0 102 34.1 103 34.2 103 34.8 105 ref 33.2 precision: 108 r 2.5 (2s) % re1 std dev = 1.2 1 2 3 4
0
run no.
pk ht
rec, %
1 85.0 104 2 79.8 91 3 86.0 105 4 83.0 101 5 81.5 991 ref 82.0 precision 101 i: 6.6 % re1 std dev = 3.3
run no.
pk h t
rec, %
1 35.7 106 2 35.8 106 3 31.0 92 4 32.6 96 ref 33.8 precision 100 i: 14 % re1 std dev = 7.1
25 ppt
run no. 1 2 3
/ I
L
ref
I
I 102
I 103
rec, %
12.3 130 10.0 105 9.5 100 9.5
blank . . * precision 112* 32 % re1 std dev = 14
(corresponds to 150 x 10'9g in 150 mll
10'
pk h t
104
PartsPer-Trillion (ppt) Acrylonitrile
Figure 7. Calibration curve for combined distillation-purge/trap.
Steam Distillation. In development of the method, direct aqueous GC/FID was used with the apparatus shown in Figure 1 to examine the recovery of AN in the distillation step. Analysis of distillate from a 150-mL solution of 0.33 ppm AN gave 89% recovery in the first 2 mL and virtually 100%after 12 mL (Figure 3). Low recoveries are experienced (15-20% loss) if the condenser is not submerged in water and the receiver cooled near 0 "C as shown in Figure 1. Purge-and-Trap. Volatile polar organics (VPOs) require more vigorous purge conditions than those normally used for less polar, slightly water-soluble organics. Increasing the purge gas flow rate above 20 cm3/min has little affect on purgeability of the latter; for AN, however, the effect is considerable. AN purgeability (and VPOs in general) may be substantially enhanced at elevated temperatures. For AN, both effects are a consequence of its water solubility-7.35% at 20 "C (29). Purgeability is enhanced 7.7 times at 75 "C relative to ambient (0.5 ppb purged). Previous experience in this laboratory has shown Tenax-GC to be a satisfactory adsorbent for AN. A breakthrough curve (Figure 4) was obtained with a purge rate of 40 cm3/min Ne for 1.3 cm3 of Tenax-GC adsorbent. Breakthrough occurs between 640 and 680 cm3. A purge time of 12 min was selected
run no.
pk h t
rec, %
1 2 3 ref
35.6 152 34.6 148 35.4 151 20.2 29.2 21.7 blank 1.0 precision 150 * 4.2 % re1 std dev = 1.4
a The recovery is of the distillation step and was obtained as follows: An amount of A N (X)was distilled from an aqueous solution into a (15 mL) Hypo-vial (actual volume 21 mL). Ten milliliters of distillate was purged. In the reference experiment, an AN solution of concentration X(10/21)/10mL was purged. To obtain the overall recovery including the purge step, multiply by 0.78.
because little gain is realized by purging longer than 12 min, and the chance of exceeding the breakthrough volume is precluded. Desorption. The rate of AN desorption from Tenax-GC was studied. The results, obtained under a "worst case situation", are shown in Figure 5. Complete desorption of AN from a 1.6 ppm solution required 8-10 min. At a more realistic concentration (0.5 ppb, not shown) AN was desorbed in a single 4-min period. The desorption time is important because desorbed compounds must be condensed into a narrow band on the front of the GC'column to avoid a loss of efficiency in the ensuing separation. Recovery of AN in the combined purge/trap experiment is shown in Figure 6. Approximately 78% of the AN is recovered in a single 12-min purge at 75 "C, only 10% at ambient temperature. Although recovery provides useful information, ita determination is not essential to the method because samples and standards are run identically. Linearity a n d Validation. The calibration curve (Figure 7) is the product of the individual steps comprising the procedure-distillation, purging, trapping, desorption, separation, and detection. T o obtain the curve, AN-free water (predistilled) was fortified with AN over the concentration range shown. At least three determinations were made per concentration (Table I). Quantitation was by peak height.
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 10 ppb AN
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Detector upset due to w l y witching
10 ppt AN
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25
1 ppt AN
iminl
/-
Figure 8. Representative chromatograms obtained In valldation/linearity study: (a) fortification at 10 ppb: (b) fortification at 10 ppt.
Linearity of the procedure extends to about 2 ppb (equivalent to 300 ng of AN in 150 mL where 10 mL of distillate was purged). Nonlinearity a t higher concentrations was due to detector nonlinearity. As is evident from Figure 7, the linear range covers about 21/2 decades. For samples whose concentrations of AN exceed the linear range, several options are available: (1)reduce the volume distilled to less than 150 mL; (2) purge less than 10 mL of distillate; (3) purge at a temperature lower than 75 “C; or (4)lower the bead voltage. Validation results (Table I) show the recoveries through the system at six concentrations. Recoveries reported are high for the two lowest concentrations (25 and 10 ppt). Uncertainty in the measurement of small peak heights may have been a contributing factor at 25 ppb; a similar explanation cannot be offered a t 10 ppt. Blanks contained negligible peaks at these two concentrations. Precision generally diminished as concentration decreased. Representative chromatograms obtained in the linearity/ validation study are shown in Figure 8. Negative drift, evident in the lower chromatogram, was typical detector behavior during sensitive detection. Chromatograms exhibiting similar behavior have been noted by others (17,30).This excursion results from the action of traces of water reaching the detector. Acetonitrile, apparent in the lower chromatogram, is a contaminant in laboratory air (vide infra). Detectability. Low-level AN determinations are difficult because of airborne AN contamination deriving from its use in liquid chromatography (LC); acetonitrile contains about 250 ppm AN (11). Special precautions (discarding a forecut and injecting the AN into boiling water) are required to achieve reliable low parts-per-trillion detection of AN (Figure 9).
Applications. Figure 10 shows chromatograms of two waters commonly used in the laboratory. Examples of public water are shown in Figure 11. Three of these waters yielded
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Figure 9. Chromatograms showing the high sensitivity achievable: (a) a blank: and (b) a 1 ppt spike of AN. Note that 400 mL of sample was required to obtain this sensltivity.
peaks at the retention time of AN; however, chromatograms were obtained on a single packing and no mass spectrometric or alternate verification was produced. A comment is in order with regard to the demineralized water vis-a-vis purified laboratory water (Figure 10). Demineralized water was passed through a commercial purification system, but the AN concentration of the water increased. The additional AN may stem from minute quantities of residual AN leached from AN-containing copolymer used in fabricating portions of the purification system. Apart from AN, however, the purified water shows fewer purgeable compounds that respond to the N / P detector than does demineralized water. Figure 12 shows chromatograms of AN fortified tomato and apple juices. Recoveries a t 0.5 and 0.8 ppb were 95-100%. Determination of AN and other monomers in real liquid foods can serve as a useful adjunct in migration studies that are generally conducted in food-simulating solvents. AN can be detected in a solid that can be dispersed or is penetrable by boiling water. The method accommodates less solid sample than liquid; consequently, detection limits are higher (Figure 13). The distillation step was modified slightly for solids. Suspended foods were boiled under total reflux before collection of distillate. Coconut packaged in an ANbased container revealed no AN at a detection limit of 2 ppb. The method described here has been developed specifically for low and sub-parts-per-billion determinations of AN in “clean” aqueous systems. Its applicability to highly conta-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
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Tomato Juice 150 mll DemineralizedWater 1150 mli 0.50 ppb Fortification of Tomato Juice Spike
BN L
A 5
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Recovery: 98%
(a1
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Recovery
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0.80 ppb Fortification of Apple Juice
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95% 199% at 0.20 ppb)
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lminl
Flgure 10. Chromatograms of two commonly used laboratory waters: (a) demineralized water; and (b) a reagent-grade water, Millibore Milli Q water.
Flgure 12. Chromatograms of (a) fortified tomato juice and (b) fortified apple juice. The recoveries (in the distillation step) are seen to be quantitative.
CH,CN
Diluent Wafer Blank
1150mll
Municipal Water 1150mli
B
(a1 t R AN
70 ppt AN
IJ
JI
Coconut 10591 t 2 ppb Spike
V LL.L 1 0
5
IO
15
I 20
25
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[mini
Flgure 11. Chrom,atograms of (a) a municipal water and (b) a plant influent water.
1 1I
-L
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18 (minl
20
22
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I
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Flgure 13. Chromatograms of (a) diluent water blank, (b) coconut (0.5 g) 150 mL of diluent water, and (c) coconut 2 ppb spike. AN was not detected in the coconut.
+
+
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Anal. Chsm. 1982, 5 4 , 1196-1198
mined waters has not been demonstrated. The distillation cleanup step for samples containing several parts-per-billion of AN may be unnecessary. For concentrated samples, sensitivity may be reduced by purging less than 10 mL of distillate and stripping at reduced temperatures.
LITERATURE CITED (1) Fed. Reglst. 1977, 42(ll Mar), FR 13546. (2) Fed. Reglst. 1978, 43 (06 Jul) (130),29135-29150. (3) A m . Ind. Hyg. Assoc. J . 1977, 38, 417-422. (4) Venitt, S.Mutat. Res. 1978, 5 7 , 107-109. (5) Food Chem. News 1081, (19 Jan), 44. (6) Bird, W. L.; Hale, C. H. Anal. Chem. 1952, 2 4 , 586-587. (7) Ponomarev, Y. P.; Annfrieva, T. L.; Shkorbatova, T. L. Vodosnabzh., Kanallz., Gldrotekh. Sooruzh. 1974, 77, 67-70. (8) Lezovic, A.; Slngliar, M. Petrochemla 1977, 77, 128-132. (9) Stefanescu, T.; Ursu, G. Mater. Plast. (Bucharest) 1973, 10, 330-334. (10) Wronski, M.; Smal, 2. Chem. Anal. (Warsaw) 1974, 19, 633-638. (11) Hall, M. E.; Stevens, J. W., Jr. Anal. Chem. 1977, 4 9 , 2277-2280. (12) Lawniczak, H. Blul. I n f . : Barwnlki Srodkl Pomocnicze 1977, 21, 87-91. (13) Danes, G. W.; Hamner, W. F. Anal. Chem. 1957, 2 9 , 1035-1037. (14) Markelov, M. A.; Semenenko, E. I. Plast. Massy 1978, 7, 57-59. (15) Harrls, L. E.; Budde, W. L.; Eichelberger, J. W. Anal. Chem. 1974, 46, 1912.
(16) Chudy, J. C.; Crosby, N. T. Food Cosmet. Toxlcol. 1977, 75, 547-551. (17) McNeal, T.; Brumley, W. C.; Breder, C.; Sphon, J. A. J. Assoc. Off. Anal. Chem. 197% 62, 4-6. (18) Evans, T. E., Dow Chemical, unpublished data. (19) Marano, R. S.;Levine, S. P.; Harvey, T. M. Anal. Chem. 1978, 5 0 , 1948-1950. (20) Melcher, R . G.;Caldecourt, V. J. Anal. Chem. 1980, 52, 875-881. (21) Crosby, N. T.; Foreman, J. K.; Palframan, J. F.; Sawyer, R. Nature (London)1972, 238, 342-343. (22) Alliston. T. G.; Cox, G. 5.; Kirk, R. S. Analyst . .(London) 1072, 9 7 , 915-920. (23) Chian, E. S.K.; Kuo, P. P. K.; Cooper, W. J.; Cowen, W. F.; Fuentes, R. C. Envlron. Scl. Technol. 1977, 7 7 , 282-265. (24) Kuo, P. P. K.; Chian, E.S. K.; DeWalle, F. B. Wafer Res. 1977, 7 7 , 1005-1011. (25) Peters, T. L. Anal. Chem. lg80, 5 2 , 211-213. (26) Bellar, T. A.; Llchtenberg, J. J. J . Am. Water Works Assoc. 1974, 66, 739-744. (27) . . Ramstad, T.: Nestrick. T. J.: Peters, T. L. Am. Lab. (Falrfleld,Conn.) 1981, 73 (7),65-73. (28) Langvardt, P. W.; Ramstad, T. J. Chromafogr. Scl. 1981, 79 (lo),
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538-542.-.
(29) "The Merck Index", 9th ed.; Merck & Co.: 1976;p 17. (30) Brown, M. E.; Breder, C. V.; McNeal, T. P. J. Assoc. Off. Anal. Chem. 1978, 6 7 , 1383-1368.
RECEIVED for review July 1,1981. Accepted January 29,1982.
CORRESPONDENCE Fractionation of Metal Forms in Natural Waters by Size Exclusion Chromatography with Inductively Coupled Argon Plasma Detection Sir: Techniques are needed to fractionate and measure species of metals at ambient levels in natural waters. Adaption of specific element detectors to monitor eluates of high-performance liquid chromatographic (HPLC) columns (1-3) offers a reasonable approach to speciate metal forms. Atomic absorption and inductively coupled argon plasma (ICAP) detectors interfaced with HPLC systems have fractionated and detected metal species from mixtures of known compounds (1-7) and environmental extracts (7,8). Because water is the most appropriate solvent for the hydrophilic constituents dissolved in natural waters, an aqueous matrix would be desirable for chromatographic separation and detection of metal forms in natural water samples. Ion exchange and aqueous size exclusion chromatography both involve aqueous mobile phases. The former requires elution with salt or buffer solutions for effective component resolution (9). Added solutes may cause undesirable sample matrix effects on metal speciation and on postcolumn metal analysis, particularly during gradient elutions. Size exclusion techniques provide rapid, gentle separations with a constant sample matrix but have generally been designed to separate large organic compounds (5, 9). Metal separations have been restricted to relatively high molecular weight metal-organic components (e.g., ref 5 and 6). Another approach for separation is to adjust chromatographic conditions to maximize chemical or apparent molecular size differences of dissolved components. Then, chemically different groups can be fractionated by size exclusion stationary phases even if molecular weights are similar. In the absence of salts or buffers (to reduce charge effects) sample components may be separated due to factors other than molecular weight. Distilled water may increase the apparent molecular size of ionic dissolved components due to
hydration layer formation or other hydrogen bonding or ionic repulsion mechanisms ( 1 0 , I I ) . Sample-column interactions, causing fractionation, may result from charge attractions or repulsions in the absence of salts or buffers in the mobile phase. Synthetic mixtures of organic compounds, having similar molecuIar weights but different polarities, have been resolved into chemically distinct groups by distilled water size exclusion chromatography (DWSEC) (12). Because component separations on size exclusion columns with distilled water are affected by chemical-physical interactions as well as component molecular size, DWSEC should also fractionate dissolved metal forms. We here interface DWSEC with inductively coupled argon plasma (ICAP) detection to fractionate and detect dissolved forms of magnesium and calcium in lake and river waters. ICAP is ideally suited to this application because it provides continuous metal monitoring for aqueous samples (2, 13) and accepts sample flow rates appropriate for DWSEC. By coupling DWSEC with ICAP detection, we hoped to define at least the minimum number of components of each metal existing in river and lake waters and determine if metal peaks would coelute with ultraviolet-absorbing dissolved organic matter (UVDOM) peaks. Coelution of the metal and UVDOM peaks would imply possible association of metals and organic compounds in the water. The absence of a UVDOM peak a t the retention time of a metal peak (or vice versa) would indicate that the component metal was not strongly associated with UVDOM.
EXPERIMENTAL SECTION A high-performanceliquid chromatograph was assembled from a Beckman Model llOA pump, a Rheodyne 7125 injector, a TSK 3000 sw size exclusion column (60 cm X 7.5 mm i.d.), or a TSK 2000 sw column (50 cm X 7.5 mm i.d.), and Hitachi 100-10var-
This article not subJectto US. Copyright. Publlshed 1962 by the American Chemlcal Society
