Anal. Chem. 1002, 64, 2425-2428
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Determination of Inorganic Halogen Species by Liquid Chromatography with Inductively Coupled Argon Plasma Mass Spectrometry Valeri V. Salov; Jun Yoshinaga, Yasuyuki Shibata, and Masatoshi Morita’ National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305, Japan
An analytlcal procedure based on Inductively coupled argon plasma mass Spectrometry and hlgh-performance llquld chromatography Is developed for a determlnatlon of SIXlnorganlc halogen anions IOs-,Br03-, Ct-,CIOs-, B r , I-. Absolute detectlon lhnlis for I, Br, and CI species are 25 pg, 0.8 ng, and 36 ng, respectlvely. The InJectlonvolume can be changed in the Interval 1-25 pL. The method has been applied to the analyds of drlnklng water, “MIso” soup, and human urlne. Three unldenttfled halogencompounds are detected In human urlne by the developed method. Llmlis of detectlon and a sensHlvtty of the procedure are discussed.
INTRODUCTION Halogens are important elements in the chemical industry. Many industrial materials contain and are produced from halogen compounds. Halogen compounds are also important in life processes as osmolytes, for maintaining ion balance, hormonal function, etc. Trace analysis of halogen species is thus important for industrial purposes, for understanding geochemical cycles of the elementa and for the understanding of their nutritional and toxicologicalimplications. Recently developed inductively coupled plasma mass spectrometry (ICP-MS) is known to be an extremely sensitive method for the analysis of metals1-8 and nonmetallic elements.1s7p9 Bromine and iodine are elements that can be sensitively determined by ICP-MS.10 Combining ICP-MS or atomic emission spectrometry with modern liquid chromatographic techniques is an effective method of speciation a n a l y s i ~ . l ~ The ~ J ~ present paper is devoted to the application of high-performance liquid chromatography (HPLC) with ICP-MS detection to the trace determination of halogen species in aqueous samples. EXPERIMENTAL SECTION Reagents. Potassium iodide, iodate, bromide, bromate, chloride, chlorate, and perchlorate (“G” grade) were obtained from Kanto Chemical Co. Inc., Japan. Potassium periodateand ammonia solution (“AAS” grade) were obtained from Wako Pure Chemical Industries, Ltd., Japan. Tetramethylammonium hydroxide (TMAH) and malonic acid (‘GR”grade) were supplied ~~~
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scientist from Vemadaky Intitute of Geochemistry and Analytical Chemistry, ul. Kosygina 19,117975GSP-1,Moscow, Rueeia. (1)Thompson, J. J.; Houk,R. S. Anal. Chem. 1986,58, 2541-2548. (2)Gray, A. L. Spectrochim. Acta, Part B 1986,40B,1525-1537. (3)Douglas, D. J.; Houk, R. S.Prog. Anal. At. Spectrosc. 1985,8,1-18. (4)Faasel, V. A. Freseniw’ 2.Anal. Chem. 1986,324,511-518. (5)Vaughan, M. A.;Horlick, G. Appl. Spectrosc. 1986,40,434-460. (6)Houk, R. S. Anal. Chem. 1986,58,97-105. (7)Date, A. R.;Cheung, Y. Y.;Stuart, M. E. Spectrochim. Acta,Part B 1987,42B,3-20. (8)Houk,R. S.; Thompson, J. J. Mass Spectrom. Reu. 1988,7,425461. (9) Shibata, Y.; Morita, M. Anal. Sci. 1989,5,107-109. (10)Olivares, J. A.; Houk, R. S. Anal. Chem. 1986,58,20-25. (11)Morita, M.;Uehiro, T. Anal. Chem. 1981,53,1997-2ooO.
by Nacalai Tesque, Inc., Japan. The reagenta were used without further purification. Other reagents were of the highest grade commercially available. Standard Solutions. Stock solutions (0.0500 M) for each anion were prepared. A working Standard Solution (mixture of 5x M IO3-, I-, 5 x M BrOa-, Br-, and 5 X M C1-, C103-) was prepared each week from stock solutions. Sample Preparation. A sample (8 mL) of “Miso” soup was centrifuged and filtered (Minisart NML, 0.45 pm, Sartorius GmbH, Germany). Other samples were used without any preparation. HPLC. A Perkin-Elmer 410 Bio LC system was used with HPLC columnsGS-220MIGS-220HIand GS-220(gel-permeation, 7.6-mmi.d. X 100,250,and 500mm, respectively;Asahi Chemical Industry Co., Ltd., Japan). The column outlet was connected by a short Teflon tube (i.d. 0.25 mm) to the inlet of the pneumatic nebulizer. The eluentflow rate was 1mL/min. Samples (aqueous solutions) were injected with a 25-pL syringe (Hamilton). ICP-MS. The operation conditions of the ICP-MS (PMS2000, Yokogawa Electric, Japan) were as follows: Ar flow rate nebulizer 0.78 L/min, auxiliary 0.5 L/min, plasma 13 L/min; sampling height, 4.5 mm from top of the induction coil; power, 1.3kW;spraychamber coolingtemperature,5 “C.@The following isotopes were monitored lZ7I+,78Br+,3sCl+. Calculations. The chromatogram peak areas and retention times were calculated by using PMS-2000 Yokogawa software and an NEC PC-9801RA computer with an Intel 80386 CPU. Special simulation and statistics programs in C were developed by means of Borland C++ software (Borland International).The PC-8041 computer with Intel 80386 CPU (Sharp Co.) was used for these calculations.
RESULTS AND DISCUSSION Separation of Halogen Anions. Iodine is the most interesting halogen for ICP-MS since it can be very sensitively determined. The chromatographic behavior of 103- and Ihas been described.12-14 In order to establish appropriate separation conditions, an ion chromatographic procedure12 was examined. By this method, however, the adsorption isotherm for I- already showed a nonlinear shape at M. It was also noted that the retention time was long (more than 30 min) and depended on the concentration. For these reasons, it was considered not appropriate to apply these chromatographic conditions to low-concentration analysis. During the search for good separation conditions, it was found that the use of an Asahipak GS-220 gel permeation column with 0.025 M TMAH, 0.025 M malonic acid buffer solution adjusted to pH 6.8 by NH40Hg made it possible to separate I03-and I-. Using a 500-mmlength column, the totalanalysis time was about 25 min. In order to reduce retention times, 36 TMAH-malonic acid-ammonia buffer solutions with different compositions and 3 columns with different lengths (12)Oikawa,K.;etal.Zon Chr0matography;KyoritsuShuppan:Tokyo, 1988 (in Japanese). (13)Mullins, F. Analyst 1987,112,665-671. (14)Ito, K.; Shoto, E.; Sunahara, H.J. Chtomatogr. 1991,549,266212.
0003-2700/Q2/0364-2425$03.00/0 0 lQQ2 Amerlcan Chsmlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, I992
Table I. Retention Times ( 8 ) of Investigated Species, p = 0.95. sDecies GS-220M n GS-220 n 6 104166.1 f 0.4 166.1 f 0.1 32 103180.4f0.5 23 781f 13 2 BrO2186.4 f 0.6 12 829 f 5 3 c1207.7 f 0.5 12 c103210.9 f 0.8 23 958 f 4 3 Br304.8f0.2 32 1462f6 3 I199.5f 1.4 7 notdetected 3 I compound 701 f 12 3 Br 171.4 f 0.7 21 748 3 Br compound-1 compound-2
BKG(Cl-36)~10-~
cp*
I
i
I
10
cps
I
a The total extra column delay 7.9 f 1.6 s is not subtracted from the retention time.
Count 1,'
5E +04
' ,#
. I
0.32
35
0.64
0.96
1.28
1.60,
ml/min
Flgwo 2. Dependence of the background count from pure water on
the flow rate.
5E+04 1
0.0
374.7,
s
Retention time Flgure 1.
I I
I
Separation of six halogen anions (GS-220M, 10 pL of the
mixed solution containing 5 X B r , and 5 X M IO3-, I-).
M Ct-, C103-, 5 X
M BrOa-,
were investigated. The retention mechanism of the 7 anions under investigation was considered to be an ion pair/ hydrophobic interaction. For the simultaneous and quick (5-min) determination of six anions (IO3-, Br03-, C1-, C103-, Br-, I-), 0.05 M malonic acid + 0.0375 M TMAH buffer solution and a GS-220M column were used (Table I, Figure 1).These conditions were a compromise between the number of determinants and the analysis time. The total analysis time could be reduced to 2.5 min, if it was required to determine 1 0 3 - and I- only. Background. The background count directly influences the limit of detection in ICP-MS. It depends on many factors includingpossible ultratrace concentrationsof halogen species in pure water and reagents, spectral interference by molecular ions and photons, memory effects, and so on. The background count from pure water depends on the flow rate, as shown in Figure 2. The data were obtained by use of the Yokogawa PMS-2000 instrument's own peristaltic pump. In the present work, it was observed that the background standard deviationdepended linearly on the total count. One equation could be applied to all three halogens: S D B(cps) ~ = (0.094 0.004)Bkg (cps) - (68 f 197); n = 18; r = 0.997; SO = 332 (the intercept is simply zero).
*
Background signals decreased in the order 37Cl>> 35Cl>> 81Br > 79Br > lZ7I. When we consider that the isotope abundance of 3Wl is higher than 37Cl and that of '9Br is almost the same as 81Br,spectral interferences at mle = 37 and 81 can be present. Pure water or 1N nitric acid were used to wash the ICPMS tube connectionsto suppress memory effects. There was no difference between them if the concentration of the iodine species was less than 5 X le5M. Chromatographic memory effects tended to appear if the columns were not used for several days. In this case 3-4 h of blank operation was needed to wash the columns. CalibrationCurves. Calibration curves for six ion species were straight lines. The chemical form of the element had no influence on the ICP-MS response; i.e. the response (peak area) was the same for the element within each of the following pairs (IO3-, I-), (BrOa-, Br-) and (Cl-, c103-). The data for each pair were therefore combined, and three calibration curves for I, Br, C1were obtained (the second strings for I and Br and a string for C1in the Table 11). With 60 pointa covering large concentrationintervals (0.0002-0.5 nmol) the calibration curve was linear with a small negative intercept for I. This was critical for low-concentrationdeterminations. To obtain more accurate resulta, it was better to use different calibration curves for low and high concentrations. The situation was the same for Br. For iodine the concentration range (0.00020.03 nmol) of the calibration was better suited to trace-level determinations. The injection volume could be changed within the range 1-25 p L without altering retention times and peak widths. Sensitivity. The measure of the sensitivity is the slope of the calibration curve. Other conditions being equal, it depends on the number density of the detected ions in the ~1asma.l~ The degree of ionization a! can be used for relative sensitivity estimations. The a values for 79 elements at T = '7500 K, and electron density ne = 1015 cm-3 have been calculated6 from the Saha eq~ati0n.l~Expected relative sensitivities for I, Br, and C1 did not match the observed (15)Boulos, M. M.; Barnes, R. M. In Inductively Coupled Plasma Emission Spectroscopy; Boumana P. W. J. M., Ed.; John Wiley & Sone: New York, 1987; Part 2, pp 289-352. (16) Korn, G. A.; Korn, T.M.Mathematical Handbook for Scientists and Engineers; McCraw-Hill Book Co.: New York, 1968.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
2427
Table 11. Used Calibration Line Parameters, Y (Peak Area Count) = bX (mol) + a. D = 0.95. a
10-126
n
r
so
interval, nmol
248 & 169 -1777 & 1308 50 & 290 -1531 f 779 21 f 2382
1030 & 20 1174 f 8 62& 1 67.6 f 0.4 6.05 & 0.09
42 60 30 42 24
0.999
436 4583 539 2168 4243
0.0002-0.03 0.03-0.5 0.002-0.5 0.65 2-3000
atom
I Br c1 a
n
LOO0 0.999 LOO0 0.999
number of points, r = correlation coefficient, SO = standard deviation from line.
Table 111. Analyzed Samdes
IV
sample drinking water drinking water home Japanese soup 'Miso" human urine
V
human urine
VI
human urine
no. I I1 I11
analysis time 7:32 p.m. 1050 a.m.
91.12.18
0 1 5 p.m.
2 2 0 p.m.
91.12.18
9:30 a.m.
11:20 a.m.
91.12.17
1005 p.m.
1:lO p.m.
91.12.17
8 1 5 p.m.
8 3 0 p.m.
date
National Institute for Environmental Studies, Tsukuba Tsukuba, Ibaraki 305 Institute Restaurant girl 1year and 5 months boy 3 years and 2 months man 31 years
sensitivities quantitatively, but there was a qualitative agreement. Limits of Detection. Limits of detection may be determined by peak height or peak area. The conventional way of defining the detection limit from the peak height is to use S/N(signal height to noise level) = 2. Very small peaks with 2q3k height from I, Br, and C1can be detected if these elements are present in absolute amounts 0.2 pmol, 10 pmol, and 1 nmol and 25 pg, 0.8 ng, and 36 ng respectively. Peak area detection limits could be determined as peak width x Q Q B L ~(at flow rate 1mL/min, Q B L ~for I, Br, and C1 were 73,449, and 10 550 cps, respectively; Figure 2 and the above-mentioned QBkg (Bkg) dependence). By using 12 s for peak width (in this experiment, a condition was selected in which 30% of the total time was spent monitoring each of the three elements while the remaining 10% was spent switching the mass condition from one element to the next), we obtained a detection limit peak area count of 788 (I), 4849 (Br), and 113 940 (Cl). This corresponded to detection limits for I, Br, and C1 of 0.52 pmol, 77 pmol, and 19 nmol and 66 pg, 6.2 ng, and 0.67 pg, respectively. Calibration line parameters can be used for the estimation of detection limits.17 This method gave values between those described above. Application. Several samples were analyzed (Tables IIIV). Drinking water contains a small amount of 1 0 3 - but not I-. Iodate may be present as the result of purification of water (chlorination). The dominant existence of 103- is also found in German mineral water.18 In soup prepared from drinking water (sample 111)we found I- but not I03-. Long heating with yorganic" compounds may cause the reduction of IO3- I-. The total amount of iodine in sample I11 is more than that in sample I. Bromide was found in high concentrations. The excess amount of Br- was considered to come from food materials. The concentrations of halogen species in human urine can depend on the water and food consumed, the time of the day the sample was taken, urine volume, and many other factors. Br- and I- concentrations in urine in this experiment were
-
(17) Doerfel, K. Statistika u analiticheskoi Khimii; Moskva: Mir: Moskva, 1969 (in Russian). (18) Heumann, K. G.; Seewald, H. Freseniw' Z . Anal. Chem. 1985, 320,493-497.
91.12.17 91.12.18
time of sampling 7 1 5 p.m. 800 a.m.
place or man
next day
Table IV. Concentrations of Different Species in the Analyzed Samples, n = 6, p = 0.95
I(20)
I1 (20)
4.1 f 0.5 (5.2 f 0.6) 2.5 f 0.5 (3.2 f 0.6)
I11 (5)
not found
IV (5)
not found
V (5)
not found
VI (5)
not found
not found not found
0.918 f 0.001 ((3260 f 4) X 10-5) not found not found 0.780 f 0.001 ((2734 f 4) X 1Oa) 9.3 f 1.8 7.38 f 0.07 150 f 2 (5.90 f 0.06) (5.33 f 0.08) (12 f 2) 107 f 2 33 f 2 6.35 0.05 (42 f 1) (5.08 f 0.04) (3.81 f 0.06) 26 f 2 78f1 4.87 f 0.02 (34 f 2) (3.90 f 0.02) (2.78 f 0.04) 17.59 f 0.06 270 f 4 83 f 1 (106 f 1) (14.07 f 0.05) (9.6 f 0.1)
*
Table V. Concentrations (ppb) of Unidentified Compounds in the Analyzed Samples, p = 0.95 sample Br Br no. I compound compound (amt,rrL) Compound n first second I V ( 2 5 ) notdetected 3 60.8& 10 184f7 total IV (25) 2.0 f 0.5 8 357 & 40 285 & 48 V(6) notdetected 6 1611 f 20 VI(5) notdetected 6
n
column
3 GS-220 4 GS-220M 6 GS-220M 6 GS-220M
Table VI. Molar Concentration Relations for Some Species sample
I11
IV V VI
Cl-:Br2.03 X 1.69X 1.60X 1.53X
10' 10' 10.' 10'
Br-3794 192 187 212
C131.62 X 3.24X 3.00X 3.25X
lo6 106 lo5
106
Br-:(unknown, 1 + 2) not found 16.5 17.1 8.7
similar to reported data.'g It is interesting to note that the ratio of C1-, Br-, and I- seems rather constant (Table VI). Unidentified Compounds. With the GS-220M column, a peak was observed close to the bromate peak in urine samples IV-VI (Table I). From statistical analysis of repeated retention time measurements, it was shown that the peak was not bromate. This was further checked and proved by (19) Iyengar, G. V.; Kollmer, W. E.; Bowen, H. J. M. The Elemental Composition of Human Tissues and Body Fluids; Verlag Chemie: New York, 1978.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 20, OCTOBER 15, 1992
Count Br‘”’ 79
1
Count a
b Brio’
73
e 0
374 7
749 3
llZ4 B
1448
7,
s
0 .‘a
Retention time
Figure 3. Separation of Br compounds (05220, 25 pL): (a)sample IV; (b) sample I V containing 7.5 X M added BrOs-).
use of a longer column. With a long column, the “unknown” peak was shown to be composed of at least two different compounds (Figure 3). When the injection volume was increased to 25 pL,a very small but definite “unknown” peak appeared in the iodine window (Figure 4). Concentrations were estimated on the basis that an “unknown” molecule contained one iodine or bromine atom (Table V). The identification of these compounds is a subject of further study. The present method w i l l have a wide range of applications in the fields of biological and environmental science.
374.’7,
s
Retention time Figure 4. Chromatogram of the sample I V (QS220M, 25 pL).
ACKNOWLEDGMENT V.S. thanks the Science and Technology Agency of Japan, Japan Research Development Co., and Japan International Science and Technology Exchange Center for financial support in the form of a postdoctorate research fellowship. We thank Dr. N. Furuta for discussion about plasma equilibria and Dr. T. Uehiro for the consultations dealing with the NEC graphics adapter.
CONCLUSION Inorganic halogen species can easily be determined by ICPMS coupled with liquid chromatography. The sensitivity of the ICP-MS detector is very high and greater than that of other detectors12-14for Br and I. Halogens are selectively determined by ICP-MS. They can be rapidly separated and analyzed with minimal preparation procedures.
RECEIVED for review March 10, 1992. Accepted June 22, 1992. Registry No. I&-, 15056-35-6;10~-, 15454-31-6;BrO~-, 16641454;Cl-,16887-00-6; CIOs-, 14866-68-3;Br-,24959-67-9;I-, 2046154-5;HzO, 7732-18-5.
