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Anal. Chem. 1986, 58, 1273-1275
Table I. Furnace Conditionsn step
ramp, s
dry ash atomize cleanout
60
cool down
hold, s
temp, "C
10
150 1000 2100b 2700 20
20
1 0 1 1
4 4
14
L'vov platform, 4-s peak area integration, X10 scale expansion bInterna1 flow = O mL/min (argon). Table 11. Day-to-Day Imprecisions of Quality Assurance Materials (Mean f Standard Deviation ( % Relative Standard Deviation)) material
IDMS, rg/L
ZAAS, rg/L
RM 8419" SRM 90gb plasma pool
16 f 2 (12.5) 106 f 2 (1.8) 112 f 3 (2.7)
16 f 4 (25.0) 108 f 4 (3.7) 112 i 7 (6.2)
a Recommended value f estimate of uncertainty: 16 * 2 pg/L, from ref 5. bNot yet certified by NBS. Certification planned at 107 & 7 ua/L based on this work.
the accuracy of a new method. Imprecisions for both methods are established by the regular use of well-characterized reference materials.
EXPERIMENTAL SECTION ZAAS. A Perkin-Elmer 5000 atomic absorption spectrometer (Perkin-Elmer Corp., Norwalk, CT) utilizing a selenium electrodeless discharge lamp (196.0 nm) with a 2.0-nm slit width and equipped with Zeeman background correction and an AS-40 autosampler was used. The graphite furnace program is shown in Table I. A base-line correction was made immediately prior to atomization. The sample and standard diluent containing the matrix modifier were prepared by dissolving 1.0 g of Ni(N03)z.6Hz0and 2.0 g of Mg(NO3)~-6H20 (Spex Industries, Metuchen, NJ) in 100 mL of 0.2% Triton X-100 (Sigma Chemical Co., St. Louis, MO). Seronorm Protein, lot 103 (Accurate Chemical & Scientific Corp., Westbury, NY), which has been reserved (for commercial distribution by Accurate) by the IUPAC Subcommittee on Selenium, with an established selenium content of 93 i 7 Mg/L, was used as the standard (4). Aliquots of 100 pL of the standard, samples, and controls were diluted with 400 & of diluent. Bovine serum Reference Material (RM) 8149 (51, human serum Standard Reference Material (SRM) 909 (National Bureau of Standards, Gaithersburg, MD), and an internal human plasma pool were utilized as quality assurance materials. Blanks were monitored; however, no measurable selenium was detected. Deionized water (Millipore Corp., Bedford, MA) was used throughout,
IDMS. The apparatus and method have been fully described elsewhere by Reamer and Veillon (3). Again bovine serum RM 8419, human serum SRM 909, and the internal human plasma pool were used as quality controls. Blanks were monitored and results corrected when necessary. RESULTS AND DISCUSSION Means and imprecisions expressed as standard deviation and percent relative standard deviation for both techniques are shown in Table 11. The IDMS method is twice as precise as the ZAAS method. Thirty human plasma samples were analyzed for selenium by both methods. Regression analysis was carried out by using the Deming method ( 6 , 7 ) ,which takes into account the error in the reference IDMS method (ordinate) as well as the error in the experimental method, in this case ZAAS. This is in contrast to conventional regression analysis, which assumes no error in the ordinate. The results obtained are as follows: R = 0.987, SE = 7.8, slope = 1-01,and intercept = -0.39. In conclusion, the ZAAS method-in our hands-compares favorably with the definitive IDMS technique but is less precise. The method is rapid and accurate and permits blood plasma and serum selenium determinations with commercially available atomic absorption instrumentation.
ACKNOWLEDGMENT S. A. Lewis was and N. W. Hardison is a Research Associate, Georgetown University Hospital, Washington, DC. Registry No. Ni, 7440-02-0; Mg, 7439-95-4; selenium, 778249-2.
LITERATURE CITED "Selenium In Biology and Medicine"; Spallholz, J.
(1)
E., Martin, J. L., Ganther, H. E., Eds.; AVI Publishing Company, Inc.: Westport, CT,
(2)
Verllnden, M.; Deelstra,
1981.
H.;
Adrlaenssens,
E. Talanta 1981, 28,
637-646. (3) (4)
(5)
Reamer, D. C.; Velllon, C. Anal. Chem. 1981, 53, 2166-2169. Ihnat, M.; Thomassen, Y.; Wolynetz, M. S.; Velllon, C., submitted for publication In Acta Pharm. Toxicol, Velllon, C.; Lewis, S.A.; Patterson, K. Y.; Wolf, W. R.; Harnly, J. M.; Versleck, J.; Vanballenberghe, L.; Cornells, R.; O'Haver, T. C. Anal. Chem. 1985, 57, 2106-2109.
(6)
Demlng, W. E. "Statistical Adjustment of Data"; Wlley: New York,
1943. (7) Cornbleet, P. J.; Gochman, N. Clin. Chem. (Winston-Salem, N.C.) 1979, 25, 432-437.
RECEIVED for review November 4, 1985. Accepted December 30,1985. This research was supported in part by the National Cancer Institute Contract Y01-CN-40620. Specific manufacturer's products mentioned herein solely reflect the personal experiences of the authors and do not constitute their endorsement nor that of the Department of Agriculture.
Preparation of Carbon Dioxide for Oxygen-18 Determination of Water by Use of a Plastic Syringe Naohiro Yoshida* and Yoshihiko Mizutani Department of Earth Sciences, Toyama University, Gofuku, Toyama 930, J a p a n The COz equilibration method (1)is used generally for the
'*Odetermination of water, but it is tedious and time-consuming (2). Although several modifications ( 3 , 4 )have already been made to reduce the time and steps required for this 0003-2700/86/0358-1273$01.50/0
conventional method, they demand expensive equipment. The present study is intended to simplify the conventional COP equilibration method by use of a plastic syringe as an equilibration vessel without decreasing its precision. 0 1986 American Chemical Society
1274
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
30
0
-O....
0 0
h
0
s
0
0
0
' 1
v
0
1 I
0
I
I
I
I I
0
I
20
I
,
I I I
I I
i
l
,
,
,
I
I
5 TIME (hr)
10
II
100
Figure 2. Variation with time of the 6"O of CO, shaken with the working standard water, TTW. The broken line indicates the time required for the oxygen isotopic exchange equilibrium. 0
- E Figure l. Schematic diagram of the vacuum line for the collection and purification of CO,: (A) sample bottle for the collection of CO,; (B) muititrap 2; (C) multitrap 1; (D) cold finger; (E) sample inlet, which IS illustrated in detail; (F) three-way stopcock; (G) capillary glass tubing: (H) silicone rubber septum; (I) equilibrated CO, in the gaseous phase; (J) water sample.
t
I
-23.8
0
EXPERIMENTAL SECTION About 6 mL of water is drawn into a 20-mL polypropylene syringe (Terumo Co., Ltd., Tokyo, Japan) through a needle 0.8 mm in diameter and 38 mm long. After the removal of air bubbles, the volume of the water is adjusted to 5.0 mL. Then 7.0 mL of COz under atmospheric pressure is introduced into the syringe from a push-button can of standard COPfor gas chromatography, containing 99.9% COz at 8 atm (Gasukuro Kogyo, Inc., Tokyo, Japan), through an adapter equipped with a silicone rubber septum. The syringe is closed by sticking the needle into a small silicone rubber bung. The syringe is shaken at a few hertz in a water bath maintained at 25.0 f O . l "C. After more than 2 h, the syringe is taken out of the water bath. The syringe is connected to a preparation line by inserting the needle into a capillary glass tubing (2 mm i.d. and 5 cm long) through a silicone rubber septum, as shown in Figure 1. By pushing up the plunger, the gas phase in the syringe is transferred into the upper plastic syringe without disturbing the isotopic equilibrium. The gas in the upper syringe, which consists of COz, small amounts of water vapor, and air, is then introduced into the evacuated line between the three-way stopcock and the next stopcock. The cold finger is cooled with liquid Nz to freeze the COP The remaining air is evacuated through multitrap 1 cooled with liquid Nz. The COz condensed in the cold finger and multitrap 1 is collected in a sample bottle. The water vapor released is fixed in multitrap 2 cooled with dry ice-acetone. The oxygen isotopic ratio of the COzis measured on a VG Micromass 602E isotopic ratio mass spectrometer equipped with a double collector. To cancel out deviations caused by several factors, oxygen isotopic ratios of water samples are determined relative to a working standard of water. The working standard is analyzed once for every 20 samples.
RESULTS AND DISCUSSION The isotopic composition of COzin the push-button can was measured to be -27.81 f 0.01%0for 6I3C relative to standard PDB (5) and +14.06 f 0.02% for 6l80 relative to V-SMOW (6). These values were not influenced by the rate of withdrawal or by the quantity of COz remaining in the can throughout this study, indicating no isotopic fractionation of
coz.
The variation with time of the isotopic ratio of COzshaken with the working standard, Toyama tap water (TTW) for which 6ls0 is -10.02%0 relative to V-SMOW, is shown in
-28D 0
5
I
10
P 100
T I M E (hr) Figure 3. Variation with time of the 613C of CO, shaken with TTW.
Figures 2 and 3. Oxygen isotopic exchange equilibrium was established after more than 105 min as can be seen in Figure 2. The carbon isotopic ratio of C02,however, gradually increases with time as shown in Figure 3. This is due probably through the to the preferential escape of l2COz(also Cl6OZ) wall of the plastic syringe. T o estimate the rate and kinetic isotopic fractionation factors of diffusion through the wall of the plastic syringe, 5 mL of COzwas left under atmospheric pressure in the syringe without water for 6-76 h. The C02 loss observed was about 0.9 mL day-l, and the remaining COz was enriched in 13Cand l80revealing fractionation factors of 1.006 and 1.015, respectively. The result shown in Figure 2, however, clearly indicates that the rate of oxygen isotopic exchange is fast enough to maintain the exchange equilibrium despite the preferential loss of Cl6OZ. As shown in Figure 3,COzis enriched in 13C by 4.4%0after 96 h, so the change in the correction factor of 6lSO for I3C (5) is estimated to be less than 0.01%0after 24 h. The measurement of 613Cis unnecessary for all samples once the 613C value of COz equilibrated with standard water has been measured. Although the mass balance correction for the oxygen atomic ratio of water vs. COz(5)is necessary in the present method, it is only less than 0.02% fqr water samples having 6l80up to 10% different from the working standard. The equilibrium fractionation of oxygen isotopes in COz between the gaseous and aqueous phases (7)also causes a small difference in the absolute result between the present and the conventional methods. Such differences can be canceled out by referring to the working standard of which 6l80 was measured by the present method. Table I lists the results of oxygen isotope measurements of three water samples by the present and conventional methods. The reproducibility of the present method is within f0.04%0for 6 or 7 individual
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Table I. Measurements of ~ 3 ~ of ~ Three 0 Water Samples by 4 the Conventional and Present ,Methods PO,' L
sample
conventional method
JSWb TTW AAG'
-1.16 -10.02 -17.19
present method
f 0.04 (TI = 5) f 0.03 (n = 8) f 0.04 (n 5)
-1.18 -10.02 -17.22
f 0.04 (TI. = f 0.03 (n = f 0.02 (n =
6) 7) 7)
Statistic errors indicate the standard deviation (la). bDistilled Japan Sea water. Distilled Antarctic glacier. runs, which is identical with that of the conventional method. In addition, over 100 samples have been tested by the present and conventional methods and the differences have been found to be within fO.l%o (la). The agreement of the 6180 values measured by the two methods is sufficiently good within the experimental error. The present method is recommended for general use.
1275
ACKNOWLEDGMENT We thank Hitoshi Sakai and Harue Masuda of Ocean Research Institute, University of Tokyo, for providing the antarctic glacier sample. Registry No. leg,14797-71-8; HzO, 7732-18-5; COz, 124-38-9.
LITERATURE CITED (1) Epstein, S.; Mayeda, T. Geochim. Cosmochim. Acta 1053, 4 , 2 13-224. (2) Gonfiantini, R. "Stable Isotope Hydrology"; Gat, J. R.,Gonfiantlni, R., Eds.; IAEA Vienna, 1981; Chapter 4. (3) Roether, W. Int. J . Appl. Radlat. Isot. 1970, 2 1 , 379-387. (4) Chiba, H.; Sakai, H.; Yasutake, M. Pap. Inst. Therm. Spring Res. Okayama Univ. 1085, 56, 27-34. (5) Craig, H. Geochim. Cosmochim. Acta 1057, 72, 133-149. (6) Craig, H. Science (Washington,D.C.) 1081, 133, 1833-1834. (7) Vogel, J. C.; Grootes, P. M.; Mook, W. 0. 2. Phys. 1970, 230, 225-238.
RECEIVED for review August 21, 1985. Accepted November 27, 1985.
CORRECTION Determination of Double Bond Position in Conjugated Dienes by Chemical Ionization Mass Spectrometry with Isobutane R. E. Doolittle, J. H. Tumlinson, and A. Proveaux (Anal. Chem. 1985,57, 1625-1630). On page 1627, in Table 11, in the vertical column headed by [b]+ in the section of the table labeled R = CHzOCHO and in the horizontal row entitled 7,l; Z,Z, the entry that reads 155 (3) should read 115 (3). On page 1629, in Table 111, in the vertical column headed [a]+ in the section of the table labeled R = CHO and in the horizontal row entitled 4,8;Z$, the entry that reads 113 (98) should read 113 (48). The correction of the error on page 1629 has direct implications in the discussion (page 1628, paragraph 4) of patterns of the relative abundances of ions [a]+and [b]+ from the four pwible isomers of each conjugated diene since i t removes one of the exceptions to the observation that in the Z,E and E,Z isomers those ions [a]' and [b]+ from cleavage at or near the E bonds are in greatest abundance. This observation is under active investigation a t present.
