Anal. Chsm. 1982, 5 4 , 1196-1198
1196
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 (18) Evans, T. E., Dow Chemical, unpublished data.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),
--- -
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 subJect to US. Copyright. Publlshed 1962 by the American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1197
c ) Lake Michigan Surface Water (12 pg Mg mL-')
a) Grand River Surface Water (19pg Mg mL-')
-/-?./-UVDOM
I
c
w
i?
z
b ) Grand River S,ediment Pore Water (69 pg Mg mL-' ) 1
d) Lake Michigan Sediment Pore Water (12119 MgmL-1)
i
E
8
d
UVDOM
L
O
+- - 0
-+
Elution Volume (mL)
Flgure 1. DWSEC chromatograms for Mg and ultraviolet absorbing organic matter for filtrates (0.5 mL) of Grand River and Lake Michigan surface and sediment pore waters sampled in May 1981. The recorder output for Mg respanse was attenuated (X2) for the river samples. The reverse-tailing phonomenon for Mg peaks was caused in part by the large injection volumes (0.5 mL). UVDOM peaks with asterisks were injection artifacts and did not represent UVDOM in the samples. iable-wavelengthultraviolet absorbance detector (wavelength set at 254 nm) equipped with an Altex spectrophotometer flow cell. The flow cell outlet was interfaced, with small-diameter Tygon tubing, to the cross-flow nebulizer of a Jarrel Ash 975 ICAP detector for specific metal analysis. Hardware and operating parameters for the HPI,C/ICAP interface have been described (13). The mobilie phase, deionized distilled water, was pumped through the chrlomatographic system and introduced into the ICAP system at a rate of 1.0 or 1.5 mL min-'. Surface waters and sediment pore waters were collected from the Grand River, MI, and Lake Michigan and filtered through a clean HA-WP 0.45 pm pore size Millipore filter or centrifuged, to remove particles. One-half-millilitersamples were injected directly into the liquid chromatograph jfor UVDOM and metal analysis. To estimate the quantity of metals removed from samples by the stationary phase, we examined the percentage of injected metals recovered from a TSK 2000 SW column (previously equilibrated by several injections of river water). Thirty milliliters of column eluate was collected after each 0.5-mL sample injection and then analyzed for the two metals. A second 0.5-mL aliquot of each sample was diluted to 30 mL with distilled water and analyzed directly to establish the quantity of metals injected. Samples for this experiment consisted of combinations of Grand River filtrate (sampled in October 1981) and a prepared solution containing 50 pg of calcium mL-' as Ca(NO& and 20 pg of magnesium mL-l as MgS04 in distilled water.
RESULTS AND DISCUSSION When Grand ]Riveror Lake Michigan filtrates were analyzed by DWSEC/ICAP, three peaks resulted for both magnesium and calcium (Figure 1 and 2). Although retention times varied, similar separations were obtained with both the TSK 2000 sw and T 8 K 3000 sw columns. These results suggest occurrence of a t least three forms of each metal in these natural waters. The identical retention times for magnesium and calcium pealks in the same samples suggest that the metal4 are speciated similarly. When calcium was injected as Ca(NO,), (in distilled water or coinjected with river water), the "free metal ion" eluted with metal peak 2 and before metal peak 3 (Figure 2). [Retention times of components fractionated by DWSEC are affected somewhat by the composition of the dis,solved sample; for example, metal retention times were slightly different for the mixed sample (Figure 2c) than for the undiluted river water filtrate (Figure 2a).] The
I
15
I
30
Elution Volume (mL)
Flgure 2. DWSEC calcium chromatograms for (a) 0.5 mL Grand River filtrate sampled in May 1981 (total Ca = 51 pg of Ca mL-'), (b) 0.5 mL of 50 pg of Ca (as Ca(NO,),) mL-' distilled water, and (c) 0.25 mL of Grand River flltrate plus 0.25 mL of 50 pg of Ca (as Ca(NO,),) mL-' distilled water.
elution of the "free metal ion" before the third calcium peak indicates that elemental molecular weight was not the primary factor controlling apparent molecular size and elution volume. Since the apparent size of the ionic Ca2+ion can be increased by a hydration layer, it could elute before less ionic calcium compounds or complexes. Other column-solute interactions such as sorption or coulombic attraction would likely extend, rather than shorten, the elution volume of the free ion but may have contributed to the relatively long retention time of metal peak 3. UVDOM in these samples was distributed among three major peaks (a fourth small late-eluting peak was observed in some samples). [In contrast to the DWSEC results, UVDOM peaks were not resolved when 0.1 M NaCl was used as the mobile phase (unpublished data).] The second and third UVDOM peaks coeluted with the first and second metal peaks. Interestingly, the major organic peak (UVDOM peak 1) did not coelute with metals and one of the major metal components (metal peak 3) did not coelute with major UVDOM peaks in river water. We did not attempt to define the composition of component peaks, but our chromatographic evidence suggests that two of the three metal peaks could be associated with UVDOM. Metal carbonates provide a reasonable explanation for the presence of metal peak 3. Formation of CaC03 crystals is commonly observed in these waters (14). Successful speciation of weak metal complexes by DWSEC apparently depends on having a stationary phase with relatively low binding strength for the metals. A strong interaction between metals and the stationary phase, such as observed for Sephadex Ci-10 size exclusion columns in a distilled water matrix, can dissociate weak complexes by competing for the metals (15). This dissociation was thought to hinder potential fractionation of intact magnesium complexes on Sephadex gels (15). Although TSM columns are less susceptible to sorption interactions than Sephadex polymers (16), we a t times observed apparent removal of ions from injected water samples. For example, less calcium was recovered from a prepared Ca(NO3I2solution than from a river water sample (Figure 2). Potential metal uptake by a TSK 2000 sw column was examined by comparing levels of calcium and magnesium eluted (in 30 mL) to the amounts injected with combined samples of river water and standards. Six samples injected into a column (previously subjected to numerous injections of river water) yielded mean (fstandard deviation) metal recoveries of 105 A 13% , of that injected, for calcium and 82 f 7 % for
1198
Anal. Chem. 1982, 5 4 , 1198-1200
magnesium. The percent recovery of calcium and magnesium appeared to be independent of the relative composition of the standard solution and river water mixtures. This suggests a lack of selective sorption for the metals by the column. A subsequent injection of EDTA solution (0.5 mL of 0.1 M EDTA) stripped 340 pg of calcium and 100 pg of magnesium from the column and indicated that metals from previous water samples had accumulated on the column. These levels were about 10-fold higher than those observed in the untreated river water filtrate (34 pg of Ca and 9 pg of Mg per injection). After EDTA treatment, the recoveries of metals from five subsequent river water injections ranged from 30 to 80% of injected levels for calcium and from 40 to 90% for magnesium. These data suggest that columns should be equilibrated with samples, similar to those to be analyzed, for effective recovery of metals. A small amount of metal bleed, observed for calcium (0.15 f 0.04 pg of Ca mL-l eluate, N = 4) and magnesium (0.05 f 0.02 pg of Mg mL-l eluate, N = 4) after several equilibration injections of river water, indicated the weak nature of the column-metal association. The above chromatographic results demonstrate that DWSEC with specific detectors can fractionate and measure forms of dissolved elements in natural water filtrates. Separation mechanisms for metal components by DWSEC appear to involve both molecular size and component polarity. When sample components are ionized, and salts and buffers are absent from the mobile phase, attraction or repulsion of ions by the column can affect DWSEC fractionation behavior (15). Selective enlargement of ions by hydration may also affect resolution of constituents. Because anions are generally larger in size than metal cations, size fractionation of metal components by DWSEC probably depends strongly on the nature of the complexing anions. The identical speciation patterns we observed for calcium and magnesium suggest that the anions complexing the two metals in our water sample had the same composition.
ACKNOWLEDGMENT We thank J. M. Malczyk for preparing sample filtrates, D. W. Morse, J. E. Grimes, and W. R. Burns for help in sample collection, A. M. Corbin for technical assistance in the laboratory, and B. J. Eadie and D. Scavia for reading the manu-
script. The Columbia National Fishery Research Laboratory, U.S. Fish and Wildlife Service, Columbia, MO, provided the ICAP detector.
LITERATURE CITED (1) Uden, P. C.; Quimby, 9. D.; Elllott, W. G. Anal. Chim. Acta 1978, 101, 99-109. (2) Gast, C. H.; Kraak, H.; Poppe, H.; Maessen, E. J. M. J. J. Chromatogr. 1979, 185,549-561. (3) VanLoon, J. C. Anal. Chem. 1979, 51, 1139 A-1150 A. (4) Parks, E. J.; Brinkman, F. E.; Blair, W. R. J. Chromatogr. 1979, 185, 563-572. (5) Morita, M.; Uehlro, T.; Fuwa, K. Anal. Chem. 1980, 52, 349-351. (6) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1223-1227. (7) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1227-1231. (8) Taylor, L. T.; Hausler, D. W.; Squires, A. M. Science 1981, 213, 644-646. (9) Synder, L. R.; Klrkland, J. J. "Introduction to Modern Liquid Chromatography, Second Edltlon"; Why-Interscience: New York, 1979; 663 PP. (10) Gelotte, 9. J. Chromatogr. 1960, 3, 330-342. (11) Stumm, W.; Morgan, J. J. "Aquatlc Chemistry an Introduction Emphasizing Chemical Equilibria in Natural Waters"; Wiley-Intersclence: New York, 1970; 583 pp. (12) Urano, K.; Katagurl, K.; Kawamoto, K. Water Res. 1980, 14, 74 1-745. (13) Frayley, D. M.; Yates, D.; Manahan, S. E. Anal. Chem. 1979, 51, 2225-2229. (14) Strong, A. E.; Eadie, 9. J. Limnol. Oceanogr. 1978, 23, 877-887. (15) Koropchak, J. A.; Coleman, G. N., University of Georgia, Athens, GA, personal communlcatlon. (16) Saito, Y.; Hayano, S. J. Chromatogr. 1979, 177,390-392.
Wayne S. Gardner* Peter F. Landrum Great Lakes Environmental Research Laboratory/NOAA 2300 Washtenaw Avenue Ann Arbor, Michigan 48104 Dennis A. Yates Environmental Trace Substances Research Center and Department of Chemistry University of Missouri Columbia, Missouri 65211 RECEIVED for review September 24, 1981. Accepted February 22, 1982. The Columbia National Fishery Research Laboratory, U.S. Fish and Wildlife Service, Columbia, MO, provided partial research support for D. A. Yates. This is Contribution No. 274 of the Great Lakes Environmental Research Laboratory.
Comparison of Definitions of Response Times for Copper(I I) Ion Selective Electrodes Sir: Response time is one of the most significant parameters to characterize the nature of ion-selective electrode (ISEs). However, no universally accepted definition of the response time is available. This situation has caused considerable problems in the discussion of the response time (1). Various definitions have so far been proposed ( 2 , 3 ) . t , is defined as time required for the ISE potentials to reach a% of its equilibrium potential after a step change in sample concentration (activity). As a, 50,90,95, and 99 have been proposed. Also, t* was defined in 1975 by IUPAC as the length of time required for the ISE potential to become equal to its steady value within 1 mV (4). I t should be noted that t9,,was recommended later in 1978 (5). The reason for this may be due to the fact that t* gives different values for univalent and divalent ISEs when the electrode response is described by a single exponential or a single hyperbolic equation, even if the value oft, is theoretically the same for both uni- and divalent
ISEs. However, it should be noted that it is not yet sufficiently clarified whether or not the ISE response is represented simply by the equation having one time constant, but rather it has been revealed that multiexponential or hyperbolic terms are needed to simulate many of response curves of ISEs (6). In the latter case, even t , may lose its theoretical background. From a practical point of view, it is difficult to apply these definitions when the final electrode potential is not readily determined. Such is the case with rather slow response time. Actually, the response times have been reported on various ISEs, which cover a very wide time scale ranging from 10 ms to even hours (6). For example, the solid membrane ISEs do not always exhibit response times of 10 ms to second order, but they often give response times of the order of minutes or more (6-11). On the basis of the above mentioned background, we also defined a quantity t ( A t , AE)as a response time. The size of
0 1982 Amerlcan Chemlcal Society 0003-2700/82/0354-1198$01.25/0
