98
Anal. Chem. 1987, 59, 98-101
Trace Enrichment and Separation of Metal Ions as Dithiocarbamate Complexes by Liquid Chromatography Hubertus I r t h , G . J. de Jong, U. A. Th. Brinkman, a n d R. W.Frei*
Department of Analytical Chemistry, Free University, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
A method is presented that Involves the slmuitaneous formation of metal dithiocarbamates and on-line preconcentratlon and, subsequently, separation of heavy-metal ions (Cd( I I), Pb(II), Hg(II), Cu(II), Co(II), Ni(II), BI(II1)) by reversed-phase liquid chromatography. A cetrlmlde-dithiocarbamate ion pair Is loaded onto a precoiumn packed with c18-bomled silica, and the InJected metal ions react Instantaneously with the dithiocarbamate to form stable complexes. These relatively apolar complexes are efficiently preconcentrated on the precdumn. They can be eluted on-line to a c18 separation column and, next, separated with an acetonitrile/ water gradient containing cetrimide and buffered to pH 6.8. The detection Is effected with a UV-vis diode-array detector which allows the determlnatlon of subnanogram amounts of metal Ions. The cleanup, preconcentratlon, and automation potentlal render the method suitable for multimetai analysis in complex samples. AppHcation of this principle to the trace analysis of Cu( I I ) and Hg( I I ) in drinking water Is demonstrated.
Older analytical methods for the determination of heavymetal ions include complexation with dithiocarbamates, usually with sodium diethyldithiocarbamate, followed by solvent extraction and subsequent colorimetric detection (1-4). Extraction techniques have also been widely used in sample preparation for atomic absorption spectrometry. In recent years a large number of papers concerning the determination of heavy-metal ions as dithiocarbamate complexes with high-performance liquid chromatography (HPLC) was published (5-1 7). Different dithiocarbamates, such as sodium and ammonium salts of diethyl-, pyrrolidine-, or hexamethylenedithiocarbamic acid, were found to be suitable as complexing reagents. They form strong and mostly neutral complexes with a large number of heavy metals such as Cu(II), Pb(II), Cd(II), Hg(II), Co(II), or Ni(I1) (18). The fast formation of generally stable complexes is the most important reason for the suitability of dithiocarbamates in analytical and especially chromatographic applications. For both normal-phase and reversed-phase separations of metal dithiocarbamates UV-vis detection is the most frequently employed detection method (5,6,10-17). Bond and Wallace (8) applied oxidative and reductive electrochemical detection in the determination of Co(II), Cu(II),Ni(II), Pb(II), and Cd(I1). Moreover, different derivatization techniques are described including simple off-line formation of metal dithiocarbamates with subsequent preconcentration on apolar phases or direct injection (5,6,10-14) and on-column derivatization adding the dithiocarbamate reagent to the HPLC mobile phase (8,15,19). The latter method allows the simple injection of the metal ions, and various problems due to irreversible adsorption of the metal chelates on surfaces (as, e.g., glass or Teflon) reported by Haring and Ballschmitter (6) are avoided. Nevertheless, there are some serious disadvantages connected with this method since photometric detection is only possible a t wavelengths higher than 310 nm due to strong absorption of the dithiocarbamate reagent in 0003-2700/87/0359-0098$01 S O / O
the low-UV range. Similarly, electrochemical detection requires the removal of dithiocarbamates before the detector using a suppressor column to avoid a high background signal of the detector (8). Another drawback is that trace-enrichment techniques become more difficult if cations instead of apolar metal dithiocarbamates must be preconcentrated, since then, stationary phases with cation-exchange groups or complexing ligands have to be used instead of simple hydrophobic surfaces. In this work a derivatization method is presented that includes simultaneous formation of metal dithiocarbamates and on-line preconcentration on a CI8 precolumn, which is loaded previously with a cetrimide-dithiocarbamate ion pair allowing the determination of Cd(II), Pb(II), Hg(II), Cu(II), Co(II), Ni(II), and Bi(II1) with reversed-phase HPLC. EXPERIMENTAL SECTION Reagents. All chemicals were of analytical grade purity. All 1 mM stock solutions of the metal ions were made of acetates and buffered at pH 4.5 with a 10 mM acetate buffer. Sodium N,Ndiethyldithiocarbamate (Na(DTC))and cetyltrimethylammonium bromide (cetrimide, CTAB) were obtained from E. Merck (Darmstadt, FRG). Acetonitrile was obtained from Baker (Deventer, The Netherlands). Aqueous solutions were made with doubly distilled water and filtered through Millipore (Bedford, MA) filters. Sodium diethyldithiocarbamate solutions were stabilized with 10 mM phosphate buffer at pH 7.0 to prevent hydrolysis. Diethyldithiocarbamate (DTC) complexes of the various metal ions were prepared by adding the stock solution of metal acetates to equal volumes of 2 mM solutions of Na(DTC) in 10 mM phosphate buffer (pH 7.0). Apparatus and Liquid Chromatographic System. All experimentswere carried out on a Hewlett-Packard (Waldbronn, FRG) LC 1090 equipped with a 1040 photodiode-array detector. The preconcentration pump was a Varian (Walnut Creek, CA) PCR-1 reagent pump. As analytical column, a 250- X 4.6-mm stainless-steel column packed with 5-jtm Spherisorb (Phase Separations, Queensferry, UK) ODS or a 200- X 2.1-mm stainless-steel column packed with 5-pm Hypersil (Shandon Southern Products, Cheshire, UK) ODS was used. The 2.0- X 4.6-mm-i.d. homemade (20) preconcentration column was hand-packed with a slurry of 5-pm Spherisorb ODS in methanol using a syringe. Injectionswere done with the LC 1090 injection system. Gradient elution was performed with acetonitrile containing 10 mM CTAB and an aqueous 10 mM phosphate buffer (pH 6.8) containing 2 mM CTAB. For further details see the text and the legends of the figures. Procedure for Simultaneous Derivatization and Preconcentration of Metal Ions. The cetrimide-dithiocarbamate ion pair, CTAB-DTC, was formed off-line by mixing equal volumes of equimolar (2 mM) solutions of cetyltrimethylammonium bromide and sodium diethyldithiocarbamate. Two hundred microliters (correspondingto 0.4 pmol dithiocarbamate) was injected into a solution of 2 mM cetrimide in 10 mM acetate buffer (pH 5.5) and pumped to the CI8 precolumn where the ion pair is adsorbed. Subsequently, 100 p L of the sample is injected onto the DTC-loadedprecolumn. For the preconcentration of larger volumes the sample is pumped directly to the precolumn, after a 2 mM Cetrimide concentration and pH value of 5.5 using 10 mM acetate buffer was adjusted. The metal dithiocarbamates that are formed and retained on the precolumn are eluted with a gradient of acetonitrile and water-containing cetrimide (10 mM and 2 mM, respectively) and buffered to pH 6.8 with a 10 mM phosphate buffer-to the analytical column, separated, and 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
QQ
Zn
sample o r flush sol u t i on Figure 1. Design of the on-precolumn derlvatlzation system: 1, HPLC pump (solvent A, acetonitrile, contalnlng 10 mM CTAB; solvent B, 10 mM phosphate buffer, containing 2 mM CTAB); 2, mixing chamber; 3, six-port injection valve; 4, derhratlzatlon precolumn; 5, analytical COC umn; 6,dlodegrray detector; 7, reagent pump; 8, cleanup precolumn; In the direct injection mode the cleanup column is removed and the sample is directly injected.
detected by means of a photodiode-arraydetector (scheme see Figure 1).
RESULTS AND DISCUSSION HPLC System. Most heavy-metal ions react rapidly and quantitatively with dithiocarbamates, but problems often occur in the HPLC separation of the chelates. The Pb(II), Cu(II), Ni(II), Hg(II), Co(II1) and Cr(V1) dithiocarbamates can be well-separated in reversed-phase systems using C8- or C18-bondedsilica with acetonitrile/water or methanol/water eluents (Co(I1) is rapidly oxidized to yield the very stable cobalt(II1) complex (21)). Other metal dithiocarbamates such as As(III), Sb(III), Ag(I), or Zn(I1) are, however, decomposed and form a broad peak or more than one peak (11). In our HPLC system Pb(DTC)2, Ni(DTCI2, Co(DTC)S, CU(DTC)~, and Hg(DTC)2gave symmetrical peaks, while the Cd(DTC)2peak was relatively broad and unsymmetrical and showed poor reproducibility. The chromatographic behavior of Cd(DTC)2could not be improved by changing the mobile phase or the pH value. The use of methanol instead of acetonitrile or of mixtures of methanol and acetonitrile caused a dramatic decrease of column efficiency. When the pH value of the mobile phase was adjusted to 4.5 by using an acetate instead of the phosphate buffer the Cd(DTC)2 peak even disappeared completely. One important reason for this behavior of Cd(DTC)2 in the chromatographic system could be its reaction with residual silanol groups on the C18-bonded phase. Sokolowski and Wahlund (22) described the interaction of amines (present in the ionized form) with free silanol groups of different C18-bondedsilicas, which led to strong peak tailing. They eliminated this peak tailing by adding a long-chain ammonium compound such as NJ-dimethyloctylamine or N,N,N-trimethylnonylammonium bromide to the eluent, which competes with the solute for the free silanol groups. In order to investigate whether the tailing of the Cd(DTCI2peak is also caused by the interaction with free silanol groups, cetrimide was added to the mobile phase. Since gradient elution was applied in order to separate the metal dithiocarbamates, it was difficult to maintain a constant CTAB concentration in the mobile phase due to the low solubility of CTAB in the aqueous phosphate buffer (maximum concentration, 2 mM). In a first step 1mM CTAB was added only to the aqueous 10 mM phosphate buffer (pH 6.8). Indeed a symmetrical Cd(DTC)2peak was obtained with nearly the same retention time as in the same chromatographic system without cetrimide. Resolution from Pb(DTC)2and Ni(DTC)2,however, was still rather low. In a next step 10 mM CTAB was added to the organic solvent, acetonitrile, and its concentration in the aqueous solvent was increased to 2 mM. Although the retention times of all the other metal dithiocarbamates remained about the same, the capacity factor of Cd(DTC)2 increased and this complex was now eluted between CO(DTC)~
1
2 8 4 T i m e (min)
0
Figure 2. On-precolumn derivatization of metal ions using Zn(DTC), as derivatlzatlon reagent: chromatographic system, analytical column, 200 X 2.1 mm, 5-pm Hypersll ODs; eluent, solvent A acetonitrile, solvent B 10 mM phosphate buffer, pH 6.8; gradient, from 0 to 75% solvent A In 4 min; flow rate, 0.4 mL/min; UV detection at 254 nm (0.1 AUFS); derivatization precolumn, 10 X 2.1 mm, loaded with 0.5-pmoi of Zn(DTC),; injection, 100 I.IL of 50 pM Hg(II), Co(II), Cu(II), Ni(I1).
and CU(DTC)~.Possibly an anionic as well as neutral cadmium dithiocarbamate exists which was already assumed by Bond and Wallace (7) who found that Cd(DTQ2 was completely retained on an anion-exchange (suppressor) column. Furthermore, electrochemical data suggested the existence of Cd(DTC)f. Therefore, the increase of the retention time at increasing CTAB concentrations may be related to ion pair formation of the anionic complex with cetrimide. A good separation of the seven metal dithiocarbamates could be obtained with a 9-min linear gradient from 60 to 25% water. Disulfiram, which is also visible in the chromatogram, is formed by oxidation of the reagent by air oxygen. System of Simultaneous Derivatization a n d Preconcentration of Metal Ions. Based on the principle of the on-column formation of metal complexes described by Smith (15,19)and Bond and Wallace (8)a derivatization system was developed that could overcome the disadvantages mentioned above (see introduction). Instead of adding a water-soluble dithiocarbamate to the eluent, the reagent was loaded as an apolar complex or ion pair onto a precolumn packed with CI8-bondedsilica. The metal complexes were formed by injection of the sample containing the metal ions onto this derivatization precolumn. If trace enrichment is required a larger volume of the sample can be introduced in the same way on the precolumn. (i) Zinc(I1) Diethyldithiocarbamate as Reagent. Firstly Zn(DT02 was used as the derivatization reagent. Zn(II), Mn(I1) and As(II1) possess low complex formation constants in the series of metal dithiocarbamates, and thus, Zn(DTC)2 will react via ligand exchange with all other heavy-metal ions of interest to form the corresponding metal dithiocarbamates (2). Moreover, Zn(DT(J2 is rather apolar, which allows its adsorption from aqueous solutions on C18-bonded silica. Figure 1 shows the design of a derivatization system based on the adsorption of Zn(DTC)2on a CI8 precolumn. Figure 2 shows a chromatogram of four metal ions, using this system. The Zn(DTC)2 peak did not overlap with the other metal (DTC) peaks. The metal ions were injected onto the derivatization precolumn using 2 mM CTAB in 10 mM phosphate buffer (pH 6.8) as flushing solution. The metal complexes formed were eluted to the separation column with a 4-min linear gradient from 100 to 25% water. It was observed that metal ions which form strong DTC complexes such as Cu(II), Ni(II), Co(II), or Hg(I1) react rapidly with Zn(DTC),; Cd(II),
100
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
Table I. Comparison of Peak Width for the Direct Injection of M(DTC) Complexes and On-Precolumn Derivatization"
H
16
12
8
Time (min)
4
M(DTC)
k'
direct injection
on-precolumn formation
Ni co
5.0 5.5 6.7 7.1
9.2 11.0 11.5 13.7
7.2 7.2 9.6 10.2
cu 0
Flgure 3. On-precolumn derivatization and separation of seven metal dithiocarbamates using CTAB-DTC as derivatization reagent: chromatographic system, analytical column, 250 X 4.6 mm, 5-pm Spherlsorb ODs;eluent, solvent A acetonitrile containing 10 mM CTAB, solvent B 10 mM phosphate buffer, pH 6.8, containing 2 mM CTAB; gradient, from 40 to 75% solvent A in 8 min; flow rate, 1.5 mL/min; UV detection at 254 nm (0.1 AUFS); derivatizatlon precdumn, 2.0 X 4.6 mm, loaded with 0.4 pmoi of CTAB-DTC; injection, 100 pL of 50 pM Bi(III), Cd(II), Pb(II), Ni(II), Co(II), Hg(II), and Cu(I1) (Ds = disulfiram).
Pb(II), or Bi(III), however, could not be determined with the Zn(DT02 system, probably due to the slow kinetics of the ligand-exchange reaction. (ii) Cetrimide-Dithiocarbamate Ion Pair as Reagent. In order to overcome the latter problem, Zn(DTC)2was replaced by an apolar ion pair of diethyldithiocarbamate and cetximide. The formation of this ion pair was already observed when cetrimide was added to the HPLC mobile phase, which caused a considerablyincreased retention for Na(DTC). This was also reported by Kirkbright and Mullins (9). The relatively low polarity of CTAB-DTC suggested its use-adsorbed on a Cla precolumn-as a derivatization reagent instead of the Zn(DT02 complex. The design of the derivatization system waa the same as shown in Figure 1. The ion pair was formed off-line by mixing 2 mM solutions of Na(DTC) and CTAB in a 10 mM phosphate buffer. The precolumn was loaded with about 0.4 pmol of CTAB-DTC by injecting 200 pL of the CTAB-DTC solution with a flushing solution containing 2 mM CTAB in 10 mM phosphate buffer (pH 6.8). After loading, the metal ions were injected onto the precolumn where they immediately react with the adsorbed dithiocarbamate ion pair to form the corresponding metal dithiocarbamates, which are retained completely on the Cla-bondedsilica because of their low polarity. When CTAB-DTC was used as the derivatization reagent, a rapid reaction was observed for all metal ions. Also, Cd(II), Pb(II), and Bi(III), which reacted only slowly with Zn(DTCI2, could be determined with good reproducibility; e.g., for Cd(DTC)2and Ni(DTC)2a relative standard deviation of 5.1% (n = 7), injection of 2.5 ppm) and 1.1% (n = 5, injection of 1.7 ppm), respectively, was obtained. The separation of seven metal dithiocarbamates shown in Figure 3 was obtained with derivatization on the precolumn. One hundred microliters of the sample was injected onto a 2-mm-long Clgbonded silica precolumn loaded with 0.4 pmol of CTAB-DTC. The derivatization on the precolumn led to narrower peak widths than the direct injection of an equal volume of off-line-formed metal dithiocarbamates (Table I). Because cetrimide was added to the mobile phase an excess of adsorbed CTAB-DTC gave a relatively narrow peak (whereas Na(DTC) injected in a reversed-phase system without cetrimide showed a relatively broad and tailing peak) and did not interfere with the separation of the metal dithiocarbamates. (iii) Stability of the DTC-Loaded Derivatization Column. In view of the preconcentration of large sample volumes,
Hg
aChromatographic system: analytical column, 250 X 4.6 mm Spherisorb ODs; derivatization precolumn, 2.0 X 4.6 mm Spherisorb ODs; eluent, solvent A acetonitrile containing 10 mM CTAB, solvent B 10 mM phosphate buffer (pH 6.8) containing 2 mM CTAB; gradient from 0 to 75% solvent A in 4 min; flow rate, 1.5 mL/min; injection, 10 pL. the stability of the adsorbed CTAB-DTC ion pair was investigated. CTAB-DTC (0.5 pmol) was adsorbed on a 2.0X 4.6-mm4.d. C18precolumn. The precolumn was flushed with different volumes of a solution containing 2 mM CTAB and 10 mM phosphate buffer (pH 6.8). Up to volumes of 25 mL no breakthrough of the CTAB-DTC ion pair was observed. This means that at least 25 mL of a sample containing trace levels of metal ions can be preconcentrated unless high concentrations of one or more metal ions which also form dithiocarbamate complexes are present. The use of a longer, e.g., 10 mm, precolumn will no doubt cause a substantial further increase of the breakthrough volume if this would be necessary. Since the CTAB-DTC ion pair-as the free dithiocarbamic acid (18)-is decomposed quickly at pH values below 3.6, it is important to buffer the sample solution at a pH of 5.5-6.0 before derivatization/preconcentration. The use of a phosphate buffer in the flush solution caused considerable memory effects, presumably due to the formation and precipitation of water-insoluble metal phosphates; therefore, it was replaced by a 0.01 M acetate buffer (pH 5.5). Determination of Cu(I1) in Drinking Water. With the described method drinking water samples were analyzed. Figure 4 shows three chromatograms obtained by trace enrichment of (a, top) 10 mL of fresh drinking water and, as blank, 10 mL of doubly distilled water, (b, middle) 10 mL of drinking water which stood for 12 h in a copper main, and (c, bottom) sample b spiked with 80 ppb Hg(I1). As one can see the copper concentration increased considerably when the drinking water was in the main for some hours (the copper concentration in samples a and b corresponds to 64 and 130 ppb, respectively). The metal ions have been identified by the diode-array detector due to their characteristic UV-vis spectra. If rather polluted samples have to be analyzed a second Cla-bonded silica or PRPl precolumn (23) can be inserted in front of the derivatization system to remove apolar sample constituents (see Figure 1). Only the polar and ionic compounds will reach the dithiocarbamate-loaded precolumn where the metal ions are converted into apolar complexes and retained on the Cla material. Retention of all other (polar) compounds will be negligible, and no interference with the final separation of the metal complexes will occur. Therefore, a simple UV (254-nm)detector can be used even for polluted samples.
CONCLUSION The on-precolumn formation and simultaneous preconcentration of metal dithiocarbamates have all advantages of in situ formation techniques,i.e., direct injection of metal ions, a noncomplicated but effective cleanup procedure, which might be attractive particularly in combination with a second (&-bonded silica or PRPl precolumn (23), and no losses due
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
Time (min)
+
:I
cu
:TAB -DTC
J b
a
12
16
Cu
Ds
4
In1
o
CTAB-DTC
101
toxic metal ions such as Pb(II), Cd(II), and Hg(I1) can be determined in the lower and sub-parts-per-billion range, with the restriction that the metal parts of the HPLC system can lead to a significant increase of blank values for metal ions such as, e.g., cobalt and copper (see Figure 4a). Compared to the determination of metal ions by ion chromatography the described method has the disadvantage that only a limited number of metal ions can be determined as dithiocarbamate complexes in reversed-phase chromatographic systems. However, a major advantage is that trace enrichment is not influenced by the total amount of cations as is the case for the preconcentration on ion-exchange phases. Ions like Na(I), K(I), Ca(II), or Mg(II), which often occur in neutral waters in high concentrations, do not disturb the derivatization and preconcentration on the CTAB-DTCloaded precolumn, whereas they interfere strongly with the enrichment of trace-metal ions on ion exchangers. Furthermore, detection of the metal dithiocarbamates can be done with a simple UV detector, whereas ion chromatographic techniques require postcolumn derivatization or less common detection methods as conductivity detection. In principle the present method can be extended to other dithiocarbamates that form ion pairs with cetrimide, such as hexamethylene- or pyrrolidinedithiocarbamate, thereby allowing the determination of metals such as Cr(II1) and Zn(II), which form unstable complexes with diethyldithiocarbamate. The use of cetrimide in the HPLC eluent led to narrower and more symmetrical peaks of Cd(DTC)2in the present system. Possibly this effect can also be observed for other metal dithiocarbamatm such as As(III), Sb(III), and Ag(I), which were reported to give problems during reversed-phase HPLC (11). Another aspect worth mentioning is the potential for automation as the derivatization precolumn can be loaded with CTAB-DTC by simply injecting this ion pair in a CTABcontaining flush solution prior to a sample injection. ACKNOWLEDGMENT The loan of the HPLC equipment and a diode-array detector by Hewlett-Packard is gratefully acknowledged. Registry NO. NaDTC, 14818-5;CTAB, 57-09-0; Cd, 7440-43-9; Pb, 7439-92-1; Hg, 7439-97-6; Cu, 7440-50-8; Co, 7440-48-4; Ni, 7440-02-0; Bi, 7440-69-9; H20, 7732-18-5. LITERATURE CITED
C
16
12
8
Time (rnin)
4
0
+
Figure 4. Determination of metal ions in drinking water with on-precolumn formation/preconcenttion using CTAB-DTC as derhratizatkm reagent: preconcentration of (a, top) 10 mL of fresh drinking water and 10 mL of doubly distilled water (dashed line), (b, middle) 10 mL
which stood for 12 h in a copper main, (c, bottom) 10 mL of sample b, spiked with 80 ppb Hg(I1). For the chromatographic condkions, see Figure 3.
to adsorption effects. In addition, the main disadvantages connected with the incorporation of the dithiocarbamate reagent in the mobile phase such as limited photometric or electrochemicaldetection potential and a more difficult trace enrichment can be avoided. With enrichment factors of 2500 (25 mL vs. 10 pL) and detection limits of 0.2-2 ng at 254 nm,
WlckboM, R. 2.Anal. Chem. 1058, 152, 256-269. Eckert, G. 2.Anal. Chem. 1057, 155,22-35. Bode, H. 2.Anal. Chem. 1055, 143, 182-195. Bode, H. 2.Anal. Chem. 1055, 144, 165-186. Drasch, G.; Meyer, L. V.; Kauert, G. Frezenius‘ 2.Anal. Chem. 1082, 31 1 , 695-696. Hiring, N.; Ballschmitter, K. Talanta 1080, 2 7 , 873-679. Bond, A. M.; Wallace, G. G. Anal. Chem. 1081, 53, 1209-1213. Bond, A. M.; Wallace, G. G. Anal. Chem. 1084, 56, 2085-2090. Klrkbrlght, G. F.; Mullins, F. G. P. Analyst (London) 1084, 109, 493-496. Schwedt, G. Chromatographia 1070, 12, 289-293. Schwedt, G. Chromatographie 1978, 1 1 , 145-148. O’Laughlin, J. W.; O’Brien, T. P. Anal. Lett. 1078, 10, 829-644. Moriyasu, M.; Hashlmoto, Y. Anal. Lett. 1078, 1 1 , 593-602. Liska, 0.; Lehotay, J.; Brandsteterova, E.; Guiochon, G.; Colin, H. J . Chromatogr. 1070, 172, 384-387. Smith, R. M.; Yankey, L. E. Analyst (London) 1984, 107, 744-748. Gaetanl, E.; Laureri, C. F.; Mangia, A. Ann. Chim. (Rome) 1070, 69, 181-187. Ichinokl, S.; Yamazakl, M. Anal. Chem. 1085, 5 7 , 2219-2222. Hulanickl, A. Talanta 1087, 14, 1371-1392. Smith, R. M.; Butt, A. M.; Thakur, A. Analyst (London) 1985, 110, 35-37. Goewie, C. E.; Nielen, M. W. F.; Frei, R. W.; Brinkman, U. A. Th. J . Chromatogr. 1084, 301,325. Smith. R. M.; Morarjl, R. L.; Salt, W. 0 . Analyst (London) 1081, 106, 129-134. Sokolowski, A.; Wahlund, K.-G. J . Chromatogr. 1980, 189, 299-316. Nlelen, M. W. F.; Frei, R. W.; Brlnkman, U. A. Th. J . Chromatogr. 1084, 317, 557-567.
RECEIVED for review May 27, 1986. Accepted September 3, 1986.
