032
Anal. Chem. 1904, 56, 832-834
ITSLTURATES
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Figure 2. SFC chromatogram of diesel fuel: column, 25 cm X 4.6 mm i.d., 5 p m Alltech silica; conditions, 2 mL/min CO, 30 OC, 180 bar, 120 mL/min hydrogen, 600 mL/min air.
Flgure 1. Diagram of modified detector. Flame tip is 31 mm X 6 mm 0.d. X 2 mm i.d.
for excess high pressure COP Instead, the hydrogen is introduced through the original air line and flows up around the COz jet through the center of the tip nut. Air is supplied through two new tubes in the lower part of the detector body. A new flame tip% attached to the tip nut to carry the hydrogen and to serve as the polarizing electrode. The stainless steel top and quartz bottom of this tip are cemented together and the tip is attached to the tip nut with zirconium cement 29 (Sauereisen Cement Co., Pittsburgh, PA). Dead volume in the detector base is reduced by coupling the column effluent to a piece of fused silica capillary (0.2 mm id.) which leads directly into the COz jet. The electronics remain unchanged.
RESULTS AND DISCUSSION Keeping the mobile phase under pressure until injection into the flame eliminates the spiking due to fog particles which was observed in an earlier attempt to use an FID as a detector for a depressurized SFC effluent (11). Sufficient density is maintained to elute 2000 molecular weight polyisobutylene. The sample stream is split at the end of the fused silica capillary. The amount entering the flame (typical value: 0.7 mL/min) depends on the size of the orifice in the C 0 2jet and the mobile phase pressure. Operating pressure may be varied to adjust the split ratio to a limited degree, but once the new flame tip is cemented to the tip nut, adjustment of the orifice size becomes impractical. Injection of too much COz into the flame results in base line instability and noise. Insufficient flow of COZ can cause peak splitting and inversion. We attribute this to overheating of the flame tip, with the response due to a standing emission current and flame cooling effects (12,13). In operation, our flame is larger than that which would be used in this detector for gas chromatography, but this does not appear to have any
adverse effect on the collector electrode. Figure 2 shows a chromatogram of a diesel fuel. The operating conditions indicated can be listed on the chromatograph terminal. Quantitation of saturated hydrocarbons in such a compound-type separation is difficult in conventional HPLC (14). Refractive index response is molecular weight dependent, and volatility is a problem with transport type FID's. The ability to perform compound class separations by SFC and to detect volatile solutes with this FID enables rapid hydrocarbon type analysis of petroleum products, which will be the subject of a separate paper (15). While we have not optimized the detector design for ultimate sensitivity, it does seem to offer excellent performance compared to other nonselective HPLC detectors. The response is linear over at least 3 orders of magnitude. Signal to noise of approximately 10 was determined for a 100 ng injection of biphenyl. Use of this detector in microbore or capillary SFC, where the whole effluent could be introduced to the flame, may offer advantages for trace analysis. Registry No. Carbon dioxide, 124-38-9.
LITERATURE CITED (1) Gere, D. Science 1983, 222, 253-259. (2) Nieman, J. A,; Rogers, L. B. Sep. Sci. 1975, 10, 517-545. (3) Schmitz, F. P.: Klesper, E. Po@. Bull. 1981, 5 , 603-608 (4) Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L.; Springston, S.R.; Novotny, M. Anal. Chem. 1982, 54, 1090-1093. (5) Gere, D.; Board, R.; McManigill, D. Anal. Chem. 1982, 54, 736-740. (6) Rawdon, M. G.; Norris, T. A. Am. Lab. (Fairfield, Conn.), in press. (7) Arpino, P. J.; Guiochon, G. Anal. Chem. 1979, 51,682A-701A. (8) McFadden, W. H. J . Chromatogr. Sci. 1979, 17,2-16. (9) Bartmann, D. Ber. Bunsenges, Phys Chem. 1972, 76, 336-339. (IO) Vitzthum, 0.;Hubert, P.; Barthels, M. US. Patent 3827859, 1974. Keller, R . A. Science 1988. (11) Giddings, J. C.: Myers, M. N.; McLaren 162, 67-73. (12) McWiiliam, I. G. J s Chromabgr. 1970, SF, 391-406. (13) Schaefer, B. A. J. Chromatogr. Sci. 1977. 15,513-519. (14) Drushel, H. V. J . Chromatogr. Sci. 1983, 21, 375-384. (15) Norrls, T. A,; Rawdon, M. G., unpubllshed work, Texaco Research Center, Beacon, NY, 1983.
RECEIVED November 21, 1983. Accepted January 9, 1984.
Simultaneous Determination of Inorganic Anions and Cations by Ion Chromatography with Ethylenediaminetetraacetic Acid as Eluent Manabu Yamamoto, Hirofumi Yamamoto, and Yuroku Yamamoto* Department of Chemistry, Faculty of Science, Hiroshima University, Higashi-Senda, Nakaku, Hiroshima 730, Japan Susumu Matsushita,* Nobuyuki Baba, and Tetsuo Ikushige Central Research Laboratory, Toyo Soda Manufacturing Co., Ltd., 4560, Shin-Nanyo, Yamaguchi 746, Japan The use of single column ion chromatography is becoming more common and it has been applied for the analyses of part per million levels of inorganic anions and cations in a variety of aqueous solutions. In these studies, anion exchange resin
is used for anion separations (1-4) and cation exchange resin for cation determinations ( 5 ) . For some cation ion-exchange studies, organic and inorganic complexing reagents were used to separate metal ions as chelates ( 6 ) or complex anions (7).
0003-2700/84/0356-0832$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
Table I. Adjusted Retention Times anions Rt/min cations cI' 3.4 CaZ H,PO,' 3.6 Mg2 NO,' 3.8 NO, 5.0 S0,Z14.2 +
+
Rt/min 7.4 9.0
Conditions for separation of metal ions as chelates of complex anions in the ion exchange study must be modified for ion chromatography, because of the great differences in capacities of exchange resins used in ion chromatography and those used in ion exchange. However, one might expect that the modified ion exchange procedure is applicable to ion chromatography and that the simultaneous determination of metal ions as anionic complex ions and inorganic anions would be possible by use of a conductivity detector. In the present study, such a procedure was demonstrated by using 1mM EDTA solution as eluent and a silica-based anion exchange column.
EXPERIMENTAL SECTION An ion chromatographicsystem of a Toyo Soda Model HLC-601 ion chromatography was described in the previous paper (2). A Toyo Soda Model CM-8 conductivity detector was used with a range of 5 ,uS/cm. The sample loop volume was 0.1 mL and the flow rate of 1.0 mL/min was employed. The separation column, 4.6 mm i.d., 50 mm long, made of Teflon, was packed with porous silica-based anion exchanger of TSK gel IC-Anion-SW (Toyo Soda, particle size 5 zk 1 pm, capacity 0.1 mequiv/g) (4). The column and the conductivity detector were maintained at 27 f 1 "C. EDTA eluent (1 mM) was prepared by dissolving ethylenediaminetetraacetic acid disodium salt in deionized water and adjusted to pH 6.0 with 0.1 M NaOH. The 1000 ppm stock solutions of C1-, H2P04-, NOz-, Sod2-,Mg2+,and Ca2+ were prepared by dissolving the guaranteed reagent of KCI, KH2P04, KN02, KNO,, KzS04,MgC12,and CaClz with deionized water, respectively. Stock solutions of MgEDTA and CaEDTA were prepared in a manner similar to Mg(EDTA).2H20 and Ca(EDTA).4H20, respectively. RESULTS AND DISCUSSION Table I shows the retention times of C1-, HzP04-, NOz-, NO3-,SO,2-, Ca2+,and Mg2+. All of these ions were eluted within 15 min. The calibration curves of these ions by peak height measurements were linear up to the concentration of 20 ppm. The detection limits defined as the concentration corresponding to twice the value of noise of the base line were 0.04 ppm for C1-, 0.05 ppm for NOz-, NO3-,Mg2+,and Ca2+, and 0.1 ppm for HzP04-and Sod2-. The retention times observed for Ca2+and Mg2+injected as metal cations and those injected as EDTA chelate anions were not significantly different. Therefore, an alkaline earth metal (M2+)forms a chelate anion (M(EDTA)2-) immediately upon contact with EDTA eluent and is then separated according to the following exchange reactions. In this case, the H2EDTA2- and HEDTA3- anions were assumed to present nearly equimolar concentrations in the eluent at pH 6.0 M(EDTA)2-
+ RN(H2EDTA)2- += RN(MEDTA)2-
M(EDTA)2-
833
+ H2EDTA2-
+ RN(HEDTA)3- +RN(MEDTAl2-
+ HEDTA3-
Using the EDTA eluent, Mg and Ca chelate anions were eluted between NO3-and SO," as shown in Table I and also in Figure 1 and Figure 2. Negative peaks of chelate anions might be caused by the lower conductivity of the chelate anion relative to that of the EDTA eluent anion. Chromatograms for the determination of these ions in tap water and seawater are given in Figure 1 and Figure 2, re-
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Figure 1. Chromatogram of tap water.
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Flgure 2. Chromatogram of offshore seawater of 100-fold dilution. Table 11. Analytical Results of Water Samples (in @g/mL) sample ClNO; Sodz- Ca" rainwater 2.5 1.1 0.38 0.9 tap water 4.6 0.9 0.43 7.7
Mg2+ 0.5 0.9
spectively. I s these cases elution behavior of Na+, K+, Ca2+, and Mg2+was checked by atomic absorption spectrometry. Fractions were collected manually for each 20-s period over the entire course of chromatogram and were then subjected to atomic absorption analysis. I t was confirmed that Mgz+ and Ca2+could not be found except each corresponding peak in the chromatogram. Na+ and K+ were coeluting in the pseudopeak and were not resolved from each other. The analytical results for tap water and rainwater are given in Table 11. These confirm that the present procedure is promising for the rapid and simultaneous determination of part-per-million levels of these species in environmental water samples. The result of seawater analysis was given in Figure 2. When the seawater was injected without dilution, the large chloride peak completely masked other peaks. However MgZ+,Caz+,
834
Anal. Chem. 1984, 56,834-835
and S042-could be determined by a 100-fold dilution of seawater. H2P04-and NOT were coeluting just after C1- (Table I). They were observed as a small shoulder in Figure 1and Figure 2 but could not be determined separately. It is under study to separate these ions. Comparing this ion chromatographic method with the colorimetric and titrimetric methods currently used to determine these ions, one finds that the ion chromatographic method is so simple and rapid that it is quite preferable for the simultaneous determination of part-per-million levels of metal cations and anions for environmental analyses of water samples. Registry No. Mg, 7439-95-4;Ca, 7440-70-2; C1-, 16887-00-6; NO
