Anal. Chem. 1983, 55, 847-851
of any data randomly distributed about an expected function estimable with SF.
ACKNOWLEDGMENT The authors acknowledge statistical assistance of D. W. Gaylor and R. L. Kodell. LITERATURE CITED (1) Moler, G. F.;Delongchamp, R. R.; Korfmacher, W. A,; Pesrce, &. A,; Mitchum. R. K. Anal. Chem. 1883, 55.835-841. (2) Brownlee, K. A. "Statlstlcal Theory and Methodology in Science and Engineerlng", 2nd ed.; Wiley: New York, 1965. (3) Savitzky, A , ; Golay, M. J. E. Anal. Chem. 1984, 3 6 , 1627-1639. (4) Hastings, N. A. J.; Peacock, J. B. "Statlstlcal Dlstrlbutions"; Wlley: New York, 1975. (5) Edwards, T. H.; Wlllson, P. D. Appl. Spechosc. 1974, 28, 541-545. (6) Proctor, A.; Sherwood, P. M. A. Anal. Chem. 1880, 52, 2315-2321.
847
(7) Bromba, M. U. A.; Ziegler, H. Anal. Chem. 1981, 53, 1583-1586. (8) Mehta, R. V.; Merson, R. L.; McCoy, B. J. J. Chromatogr. 1874', 88,
+ -R
(9) (10) (11) (12)
Mok S. D.; Grushka, E. J. Chromatogr. 1976, 726, 191-204. VidaCMadjar, C.; Guiochon, G. J. Chromatogr. 1977, 142, 61-86. Mott, S. D.; Grushka, E. J . Chromatogr. 1978, 148, 305-320. Dondl, F.;Uettl, A.; Blo, G.; Bighl, C. Anal. Chem. 1081, 53, 496404.
for review June 28, lgg2* Accepted January 21, 1983. Presented in part at the 37th Southwest Regilonal Meeting of the ChemicalSociety, sari hbnii,TX, 1981, the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, 1982, and at the 184th National Meeting Of the American ChemicalSociety, Kansas city* :May 1982.
Measurement of Deuterium Oxide Elution Data in Reversed-Phase Liquid Chromatography with Microwave Induced Plasma Detection H. A. H. Bllllet,' J. P. J. van Dalen, P. J. Schoenmakers, and Leo De Galan Laboratorium voor Analytische Scheikunde, Technische Hogeschool Delft, Ja ffalaan 9, 2628 BX Delft, The Netherlands
The retentlon itlme of deuterated moblle phase components (e.g., D,O) provldes the most generally appllcable estlmate of the mobile phase volume In reversed-phase liquid chromatographlc (RPLC) columns. I t Is shown that detectlon of D,O with a refractive Index detector can be insensltive and unreliable. A specific detector for deuterium Is described that conslsts of a slrnpie interface for the partial evaporation of the column effluent and a mlcrowave Induced helium plasma for the excltatlon of deuterlum atoms. The accuracy of the retentlon data Is demonstrated for a varlety of moblie phase composltlons. The proposed system can be used for the speciflc detectlon of any volatlle deuterated compound. The data thus obtalned facilltate a falr dlscusslon on the appllcablllty of such compounds for the determlnatlon of hold-up times In RPLC columns.
Reversed-phase liquid chromatography (RPLC) has made a tremendous iimpact over the last 5 years, because it offers many practical (1)and fundamental (2) advantages over other liquid chromatlographic (LC) techniques. Almost exclusively, RPLC is undertaken with a nonpolar chemically bonded stationary phase and with a highly polar mobile phase mixture in which water is one of the major components. The structure of such a chemically bonded stationary phase and the retention mechanism underlying RPLC separations are still a matter of considerable dispute. An aspect of particular interest is the definition and estimation of the mobile phase volume (V,) in an RPLC column. In contrast to other chromatographicsystems, the mobile and stationary phases presently used in RPLC cannot be considered as mutually immiscible. It has been demonstrated that a considerable number of mobile phase molecules can be adsorbed on, or, more likely (3),absorbed in, the stationary
phase. When a binary mixture of water and an organic modifier is used as the mobile phase, the number of sorbed organic molecules can be measured ( 4 , 5 ) ,provided that no water molecules are present in the sorbed layer. If thiri assumption is correct, water molecules would then pass the column without exchange with the stationary phase and, hence, water would be very well suited as a V , marker. However, it will be very difficult to detect small variations in the water content of the mobile phase. Deuterium oxide, which closely resembles water, is, in principle, better detectable because its refractive index differs sufficiently from that of water. Berendsen et al. (6) made an extensive study of the ~possibilities for the determination of the dead volume in the RPLC system with binary methanol-water mixtures. They recommend two methods: the use of concentrated salt solutions (KBr or KI) and linearization of the logarithm of net retention time against carbon number for a homologous seTies. These methods suffer from the disadvantagethat they are not applicable to mobile phase mixtures other than methanolwater. For instance, high salt concentrationslead to demixing of acetonitrile-water and tetrahydrofuran-water mixtures. Other groups, especially those of Karger (7) and Halasz and Engelhardt (N), recommend the use of DzO because of its universal applicability. However, the detection of DzO by means of a refractive index detector involves two serious problems: the appearance of vacancy peaks; the poor detectability of D20 in certain mobile phase mixtures. The first problem has been extensively discussed by Karger and McCormick (7). They also made some suggestions for the elimination of vacancy peaks by properly diluting the injected DzO with methanol. In our experience this is indeed possible with water-rich binary mixtures (over 50%), but the remedy fails with methanol-rich mixtures, where the change in refractive index with concentration (see Figure 1)is much steeper.
0003-2700/83/0355-0847$01.50/0 0 1983 Amerlcan Chemical Society
848
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ANALYTICAL CHEMISTRY. VOL. 55, NO. 6. MAY 1983
:b"
"b" negative
caoillarv with
vacancy-Peak
u _-
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Figure 2. The interface between the LC apparatus and the MIP detector.
Figure 1. Variation of the refractive index with the composition of
methanol-water mixtures. The second problem, which is also illustrated in Figure 1, is more fundamental. The solid line shows the variation of the refractive index with the methanol-water ratio. The horizontal dashed linesrepresent the refractive indexes of pure methanol, water, and D20. Figure 1shows that two different binary methanol-water mixtures have refractive indexes identical with that of D20.Of course, we should not compare the refractive index of the mixture to that of pure D20 hut rather to a highly diluted solution of D20 in the mobile phase. The figure serves to demonstrate, however, that conditions can occur where a refractive index detector shows little or no sensitivity for D20 (compare Figure 3). The change in sign of the D,O peak causes yet another problem in the region around 60% methanol. The signal is so complex (overlap of a positive and negative peak), that it is almost imposaible to detect unambiguously the position of the D20peak and the methanol vacancy peak. All these problems can be overcome when the refractive index detector is replaced by an element-specific detector for deuterium. In principle, a mass spectrometer could be used, but for several reasons this is unattractive. The coupling of a mass spectrometer to a liquid chromatograph is difficult (9). The masa spectrometer is not very sensitive in the low mass region and is obviously an expensive solution. A more attractive alternative is the microwave induced plasma (MIP), which has been described for element-specific detection in gas chromatography in numerous publications covered by two recent reviews (IO,11). When operated in helium, the MIP offers unique possibilities for the excitation of nonmetals, such as carbon, hydrogen, and also deuterium. The isolation of specific atomic spectral lines is easily accomplished with modest equipment. The advent of the T&,,, cavity allows the generation of a helium MIP a t atmospheric pressure (12-14). T o our knowledge the coupling of the MIP to a liquid chromatograph has not been described in the literature. It must be expected that the introduction of all of the column effluent at a rate of 1 mL/min will lead to serious overloading and extinguishing of the plasma. In the system to be described in this paper a simple interface between the LC column and the MIP allows the introduction of an adjustable fraction of the column effluent into the plasma.
EXPERIMENTAL SECTION The Microwave Induced Plasma Spectrometer. The microwave plasma is generated in a quartz tube (7 mm o.d., 2 mm i.d.1 placed in the center of a cylindrical TMm cavity (13). The plasma operates on 15 W incident power, less than 1W reflected power, and a helium flow rate of 2.1 L/min. The helium is doped with 0.7% oxygen to prevent carbon deposits in the plasma tube. Radiation is detected axially from the open outlet of the quartz tube with a Jarrell Ash 0.5-m Ebert monochromator with a Linear dispersion of 1.65 nm/mm. Isolation of the deuterium line at 656.10nm from the hydrogen line at 656.28 nm can be accomplished with a slit width of 25 nm provided that wavelength modulation is used (15,16),which also helps to reduce the spectral background. Emission intensities were measured with a Hamamatsu R 446 photomultiplier, ampLified with a lock-in amplXer tuned to the fundamentalfrequency of the wavelength modulating quartz plate (117 Hz) and recorded on a strip chart recorder. Chromatographicdata could also be m l l d with an on-line coupled PDP 11/45 computer. Retention times were obtained from the first central moment of the eluted peaks. Interface between the MIP and the Liquid Chromatograph. The interface between the LC apparatus and the MIP consists of a four-way piece of glass tubing positioned vertically (see Figure 2 for dimensions). A short metal wire, which is welded to the end of the capillary outlet of the column, is mounted in the vertical axis of the g k piece. In this way, the column effluent leaving the capillary tube forms a film on the metal wire. The horizontal part of the iunction acts as the helium supply to the plasma. The fraction of the column effluent that is evaporated from the wire and transferred as vapor to the plasma can be controlled by the temperature of the helium and amounts to about 5%. It is important to maintain a stable film on the wire to be assured of a constant delivery to the plasma. With the present interface tbis implies a minimum mobile phase flow of 1.8 mL/min. A short piece of Teflon tubing, fitted inside the glass piece, guides the column effluent to the waste and minimizes at the enme time the dead volume of the interface. Liquid Chromatograph. The chromatographicequipment from Waters kssocates (Milford, MA) included a M 6000A pump, an U6K injector, and a M 401 refractive index detector. Measurements were made by injecting pure D20 (E. Merck, Darmstadt, GFR, art. 2919) on the LC column (25 cm, 5 mm i.d.). Columns were home packed with Nucleosil 10 C18 from Macherey-Nagel (Dijren, GFR)and ODS-Hypemil5jm materialfrom Shandon (Shandou Southern Products Limited, UK). Mobile phases were mixed from individually measured volumes of methanol, acetonitrile (both from J. T. Baker, Phillipburgh, NJ), tetrahydrofuran (U.C.B., Brussels, Belgium), and water (specifically treated with ion-exchange resins and earhon filters after distillation). RESULTS AND DISCUSSION Performance of the Interface. To evaluate the per-
Mobile phase lowwater
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983 601methanol
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Flgure 3. Chromatograms of D20 in different methanol-water moblle phases obtained wlth the refractive index detector (---) and the MIP The column (25 cm, 5 mm id.) was packed with Nudetector (-). cleosll 10 C18. ‘The flow rate was 1.8 mL/mln.
formance of the interface, the contribution to peak broadening and the residence time were measured by coupling the injector directly to the interface. We corrected the measured residence time for the small (0.2 s) contribution of the 10 cm capillary (250 hm i.d.) connecting tube. Using a flow rate of 1.8 mL/min, we found an average residence time of 1.39 s from the first central moment of the peak and 0.48 s as the square root of the second central moment (peak width). The contribution to peak broadening is thus less than 210% for unretained compounds in conventional LC columns (H[L N 0.05 cm2). In the present study, where peak width is relatively unimportant, no correction was made for this contribution. Of course, all residence times measured for LC columns1 were corrected for the detector contribution of 1.39 s. The mobile phase composition exerts no influence on the residence time in the interface. Measurements over the complete methanol-water range show a variation of less than 0.1 s, which is negligible in comparison to the retention times expected in common LC columns. With a continuous supply of D20-containingmobile phase, the base line stability of the signal is the saime as with pure mobile phases. Comparison of Refractive Index and MIP Detection of D20. Retention data for D20 published in the literature have all been measured with a refractive index (RI) detector (6, 7, 17). Figure 3 presents a comparison between RI detection data and retention data measured with the present specific detection system for the binary mobile phase methanollwater. In pure water two single coinciding signals are obtained. In a mobile phase containing 10% methanol the RI trace shows two well-separated peaks. The first one coincides with the MIP signal, the second one is due to the methanol vacancy as can be confirmed by the injection of H20. With increasing proportion of methanol the vacancy peak moves closer to the
f-
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Figure 4. Chromatogram of D20 in a ternary mobile phase (35/35/30, MeOH/ACN/H,C)) measured with the refractive index detector (- - -) and the MIP detector (-).
D20 peak and in 30% methanol the two peaks coincide in the RI signal. At still higher methanol content, the vacancy peak appears before the D20 peak and also changes sign. The example of 60% methanol demonstrates the difficulty in determining the D20 retention time from the RI trace. By contrast the MIP detector gives a single peak. It should also be noted that the size of the signal measured with the MIP detector is independent of the mobile phase composition. This is not true for the RI signal, because the difference in refractive index varies appreciably with the methanol content. In fact, in 80% methanol this difference is so small that the D 2 0 signal is barely discernible and the vacancy peak can easily be mistaken for the D20 signal. This inherent inaccuracy of the RI detector is emphasized in the final two chromatograms, where the true D20 peak is only visible in the MIP trace. The difference between the vacancy peak of the RI signal and the D 2 0 peak of the MIP detector (indicated as “error” in Figure 3) increases from 2.8 s in pure methanol to 4.7 s for 70% MeOH. Under the chromatographic conditions of F’igure 3, this correspondsto an error of 3 to 5% in column hold up time and an even larger error in the usually very small capacity factors measured at such high modiffier contents. Slaats et al. (17) have demonstrated the possible consequences of a wrong choice of the column hold up time. The detector performs equally well with solvents other than methanol/water. Figure 4 shows an example of the compdex signal observed with RI detection in a ternary mobile phase. By contrast the signal from the MIP detector is unequivocal and appears to offer unique possibilities to measure hold-up times in complex mobile phases. In binary acetonitrile-water solvents the MIP detector operates smoothly, but in tetrahydrofuran-water mixtures some difficulties were encountered. When the THF content is more than 60%, we observe a decrease in sensitivity for D20 and a deposition of carbon in the quartz plasma tube. Both phenomena can be attributed to the higher carbon content of THF in comparison to ACN and MeOH. Enhanced air cooling of the cavity, a more frequent change of the plasma tube, and an increase of the oxygen content of the helium gas to 4% allowed measurements up to 90% THF. In pure THF as well as in pure ACN the detector yielded broad and severely tailing peaks for D20, from which no
850
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
i
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Flgure 5. Elution time of D20 as a function of the composition of methanol-water mixtures, measured with specific MIP detection. The column (25 cm, 5 mm 1.d.) was packed with Nucleosil 10 C 18. The flow rate was 1.8 mL/min.
reproducible retention times could be obtained. This is probably due to adsorption of DzO on remaining silanol groups despite the fact that one of the packing materials used (Hypersil ODS) was end capped. The phenomenon is not observed in pure methanol and hence not due to poor detector performance. In addition to DzO other volatile deuterated compounds, such as CH30D and CD,OD, have also been detected easily with the MIP detector in these mobile phases. The response of the detector is roughly proportional to the deuterium content of the injected compound. The primary objective of this paper is to demonstrate the utility of the MIP detector in obtaining reliable retention data for D20. It is not our intention to demonstrate the superiority of DzO over other compounds to indicate the "true" hold-up time of LC columns. This question has been addressed by others (7,17)and the issue has not been settled. In fact, it may be doubted whether a column possesses a single hold-up time. Nevertheless, a sensible discussion can only be based on reliable data, such as the D,O retention times obtained with the present detector for different binary solvents in RPLC. Figure 5 shows retention data over the complete range of methanol-water mixtures obtained with Nucleosil 10 C18 as the stationary phase. The results confirm earlier data found by Slaats (17),Karger (7), and Berendsen (6), except in the composition range of more than 60% methanol, for which these authors used RI data, now known to be erroneous. The usefulness of D20 as a valid V, marker requires the absence of water in the stationary liquid layer, solvating the hydrocarbon ligands. The variation observed in DzO elution times with varying binary mobile phase composition may be due to variations in the actual stationary phase volume resulting from a varying amount of organic modifier adsorbed to the surface. A more realistic possibility, however, is that water takes part in the stationary solvation layer, in which case exchange with DzO would devaluate the latter as V, marker. The observed decrease in DzO elution volume, when small amounts of organic modifier are added to an initially pure aqueous mobile phase (Figure 6), could support the first possibility. However, the observation in Figure 5, where an initial increase of elution time is found, cannot be explained in this way. Figure 6 shows similar data for three different binary solvents on Hypersil ODS as the stationary phase. The very high retention measured in mobile phases containing more than 90% ACN or T H F must again be attributed to adsorption of DzO on remaining silanol groups. A similar observation has been reported by Slaats et al. (17). When we compare the data for methanol-water in Figures 5 and 6, we see that the column porosity as calculated from D20 elution data is influenced more by organic modifier
2,4 0
20
40
60
80
100500
-%modifier
Flgure 6. Elution time of D20 as a function of the composition of methanol-, acetonitrile-, tetrahydrofuran-water mixtures measured with the M I P detector. The column (25 cm, 5 mm 1.d.) was packed with Hypersil ODs. The flow rate was 1.8 mL/min.
solvation in the case of Nucleosil 10 C18 material (Figure 5) than in the case of Hypersil ODS (Figure 6). This suggests a significant difference in properties between the two phases. The specific surface area of the bare silica is much higher for Nucleosil (300 m2/g) than for Hypersil (180 m2/g),which also means a nearly 2-fold higher carbon content of the chemically bonded phase. k expected,the stronger modifiers ACN and THF give rise to more significant solvation layers than methanol, so that solvents containing ACN or THF produce lower D20 retention times than methanol-water (Figure 6). In fact, for Hypersil ODS the retention time of DzO varies remarkably little with the methanol content of the binary MeOH/HzO solvent. The fact that Hypersil is end capped, whereas Nucleosil is not, may be responsible for this observation. Remaining silanol groups are fewer and less accessible, so that the solvation layer remains more uniform even at high methanol or water contents. More detailed observations including materials of varying specific surface area and different bonded phases are needed to clarify the behavior of D 2 0 and other deuterated mobile phase compounds in RPLC. We have shown that such investigations can be performed more fruitfully with the MIP detector described in this study.
ACKNOWLEDGMENT The authors are indebted to J. Wakka for her enthusiastic cooperation in the project. Registry No. Deuterium oxide, 7789-20-0; deuterium, 778239-0.
LITERATURE CITED (1) Cooke, N. H. C.; Olson, K. J . Chromatogr. Sci. 1980, 18, 512. (2) Schoenmakers, P. J.; Billiet, H. A. H.; De Galan, L. Chromatographla 1982, 15, 205. (3) Berendsen, G. E.;Plkaart, K. A.; De Galan, L. J . Llq. Chromatogr. 1980, 3 , 1437. (4) Berendsen, G. E.; De Galan, L. J . Chromatogr. 1980, 196, 21. (5) Scott, R. P. W.; Kucera, P. J . Chromatogr. 1980, 196, 21. (6) Berendsen, G. E.; Schoenmakers, P. J.; De Galan, L.;Vlgh, Gy.; Varga-Puchony, 2.; Inzay, J. J . Llq. Chromatogr. 1980, 3 , 1669. (7) McCormick, R. M.; Karger, B. L. Anal. Chem. 1980, 52, 2249. (8) Karch, K.; Sebastian, I.; Hallsz, I.; Engelhardt, H. J . Chromatogr. 1976, 122, 171.
Anal. Chem. 1983, 55, 851-854 (9) Arpino, P. A ; Guiochon, G. J . Chromatogr. 1982, 251, 153. (10) Zander, A. I.; Hieftje, G. M. Appl. Spectrosc. 1982, 35, 357. (11) Carnahan, ,J. W.; Mulligen, K. J.; Caruso, J. A. Anal. Chim. Acta 1981, 130, 227. (12) Beenakker, C. I. M. Spectrochim. Acta, Parts 1976, 316, 483. (13) Beenakker, C. I. M. Spectrochim. Acta, Parts 1977, 328, 173. (14) Van Dalen, J. P.J.; De Lezenne Coulander, P.A,; De Galan, L. Spectrochlm. Alcfa, Pari 8 1978, 336, 299. (15) Snelleman, W.; Rains, T. C.; Yee, K. W.; Cook, H. D.; Menis, 0. Anal. Chem. 1070, 42, 394. (18) Van D a h , J. P. J.; De Lezenne Coulander, P. A.; De Galan, L. Anal. Chim. Acta 1977, 94, 1.
85 1
(17) Slaats, E. H.; Markovski, W.; Fekete, J.; Poppe, H. J . Chromatogr. 1981, 207, 299.
RECEIVED for review June 28, 1982. Resubmitted Decernber 20,1982. Accepted January 25,1983. This paper was presented at the Symposium Detection in High Perform,ance Liquid Chromatography, January 19827 Amsterdam, The Netherlands. 19-209
Determiination of Inorganic Anions by Ion Chromatography wiith Ultraviolet Absorbance Detection Rlchard J. Williams Allied Corporati&n, Chemical Research Laboratory, Morristown, New Jersey 07960
This work describes the appllcatlon of varlable wavelength UV detectlon In sorles with the normal conductlvlty detector, In ion chromatography for the determlnatlon of lnorganlc anions. This comblnatlon of detectors greatly Increases the amount of informatlon that can be collected on a glven sample. The appllcatlon of IJV detection has the foliowlng advantages: (1) ald In the identtflcatlonof unknown peaks, (2) use In resolving overlapplng peaks, (3) help In ellmlnatlng problems assoclated wlth the carbonate dlp, (4) reductlon of problems assoclated wlth ion excluislon In the suppressor column, (5) ablllty to detect anlons not normally detected by the conductlvlty detector, e.g., stilflde and arsenlte.
The technique of ion chromatography (IC) has gained wide acceptance for the analysis of inorganic ions in a variety of aqueous matrices (1-5). The technique involves separation of the ions of interest on a low capacity ion-exchange column followed by a suppressor column and a conductivity detector. The suppressor column consists of a high-capacity ion-exchange column opposite in type to the separator column. The sole purpose of the suppressor column is to decrease the background conductance of the eluent, usually by neutralization, so that the ionic species of interest can be detected with sufficient sensitivity by the Conductivity detector. Cation analysis is accomplished with millimolar acidic eluents, while anion analysis is accomplished with a mixed millimolar NaHC03/Na2C03eluent. Although UV detection is the most common form of detection in HPILC, it has been considered unsuitable for IC. This has come about from the widely held belief that most inorganic ions lack suitable chromophores for UV detection (6-8). This is not always the case, especially when dealing with inorganic anions. Buck et al. showed that many inorganic ions exhibit strong absorption below 220 nm (9). In a recent paper, Reeve was able to separate many of the common inorganic anionri on a cyane-bonded silica column using a spectrophotometer for detection a t 210-220 nm (IO). Leuenberger et al. determined nitrate and bromide in foodstuffs after separation on an amino-bonded silica column, using a UV detector at 210 nm (12). Bouyoucos and Armentrout combined a UV detector with the normal IC conductivity detector to monitor dialkylated organophosphorothionic acids which were obscured conductometrically by chloride (12). The 0003-2700/83/0355-085 1$01S O / O
author used the same combination of detectors to detect aromatc and unsaturated sulfonic acids by IC (13). The aim of this work was to examine the potential of combining a UV detector with the conductivity detector for the detection of inorganic anions separated by IC.
EXPERIMENTAL SECTION A Dionex Model 14 ion chromatograph (Sunnyvale, CA) was used throughout the study. Chromatograms were recorded on a Honeywell (Fort Washington,PA) dual pen strip chart recorder. The chromatographic conditions are summarized in Table I. All of the columns were obtained from Dionex. A 100-~L sample loop was used for all injections. A laboratory Data Control (Rivieria Beach, FL) Spectromonitor I1 variable wavelength UV-Vis detector was used along with the normal conductivity detector. All of the inorganic anions, as sodium or potassium salts, !were obtained commercially and used as received. Eluents were made from ACS reagent grade chemicals,except the 0.01 N HCl eluent which was made from Ultrex hydrochloric acid (J. T. Balker). Water was purified with a Milli-Q system (Millipore Corp.). RESULTS AND DISCUSSION The UV detector can be inserted into the system at two locations: (1) directly after the separator column (unsuppressed eluent, position l),(2) after the suppressor column (suppressed eluent, position 2). Figure 1shows the absorption spectra of both the unsuppressed and suppressed standard eluent, 0.003 M NaHC03/0.0024 M Na2C0,, and illustrates that the two positions are not equivalent. The suppressor column, besides decreasing the high background electrical conductance of the eluent, also decreases the high background absorbance of the eluent in the 190-220 nm region. This allows UV detection in the 190-220 nm region where most inorganic anions absorb most intensely. Thus, for maximum sensitivity, the preferred position of the UV detector is after the suppressor column. The suppressor column also decreases the background absorbance of the other common IC eluents such as 0.002 M Na2C03/0.002 M NaOH, 0.006 M Na2CO9, and 0.005 M NazB40,.10H20. Figure 2a shows a separation of seven common anions monitored using the conductivity detector. Figure 2b was obtained with the UV detector after the suppressor column. As is illustrated in Figure 2, nitrite, bromide, and nitrate absorb strongly in the UV, while fluoride, phosphate, and sulfate do not show appreciable absorption above 190 nm. Chloride absorbs weakly in the W region below 200 nm. Note that nonabsorbing anions are sometimes observed by the UV 0 I983 American Chemical Society
