Anal. Chem. 1982, 5 4 , 2427-2431
in the present study lends support to this suggestion. The role of C10, in the present study is that of a so-called “pairing-ion”reagent which produces on the adsorbent surface a charge density that depends on its bulk solution concentration and on the overalU ionic strength. Thus, adsorbed C10, transforms the adsorbent into a “dynamic cation exchanger” and, at the same time, imparts to the surface an electrical potential which varies as a result of changes in both surface charge density and bulk solution ionic strength. Studies in the field of so-called “ion-pair” chromatography should not focus solely on the “dynamic ion exchange” contribution to retention (13,32-34) but, should also evaluate the contribution of potential-dependent adsorption. In1 some cases other phenomena, such as ion pairing in the bulk solution, may also make some contribution to the overall retention of an ion. However, the retention of NBS- on QXAD apparently does not involve ion pairing,
Electrochemical Society: Pennington, NJ, 1981; p 16. Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. Deelder, R. S.; van den Berg, J. H. M. J. Chromafogr. 1981, 278, 327. Dlamond, R. M.; Whitney, D. C. I n “Ion Exchange: A Series of Advances”; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1966; Vol. 1, Chapter 8. Reichenberg, D. I n “Ion Exchange: A Series of Advances”; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1986; Vol. 1, Chapter 9. Feltelson, J. I n “Ion Exchange: A Series of Advances”; Marlnsky, J. A., Ed.; Marcel Dekker: New York, 1969; Vol. 2, Chapter 4. Gjerde, D. T.; Fritz, J. S. J. Chromatogr. 1979, 776, 199. Gjerde, D. T.; Fritz, J. S.;Schmuckler, G. J. Chromatogr. 1979, 786, 509. Gjerde. D. T.; Frltz. J. S. Anal. Chem. 1981, 53, 2324. Helfferich, F. “Ion Exchange”; McGraw-HIII: New York, 1962; Chapter 5. Shaw, D. J. “Electrophoresis”; Academlc Press: London, 1969; Chapter 2. Baum, R. G.; Saetre. R.; Cantwell, F. F. Anal. Chem. 1980, 52, 15. May, S.; Hux, R. A.; Cantwell, F. F. Anal. Chem. 1982, 54, 1279. Dye, J. L.; Nlcely, V. A. J. Chem. Educ. 1971, 48, 443. Bonner, 0. D. J. Am. Chem. SOC. 1955, 77, 242. Aveston, J.; Everest, D. A.; Wells, R. A. J. Chem. SOC. 1858, 231. Barlow, C. A., Jr.; MacDonald, J. R. I n “Advances in Electrochemistry and Electrochemlcal Engineerlng”; Delahay, P., Ed.; Interscience: New York, 1967; Vol. 6, Chapter 1. Barrett, J., Rohm and Haas Co., Philadelphia, PA, personal communication, 1982. Puon, S.;Cantwell, F. F. Anal. Chem. 1977, 49, 1256. Bidllngmeyer, B. A. J. Chromatogr. Sn’. 1980, 78, 525. Hearn, M. T. W. I n “Advances in Chromatography”; Giddings, J. C., Grushka, E., Cares, J., Brown, P. R., Eds.; Marcel Dekker: New York, 1980; Vol. 18, Chapter 2. Scott, R. P. W.; Kucera, P. J. Chromatogr. 1979, 175, 51. Knox, J. H.; Hartwlck, R. A. J. Chromatogr. 1981, 204, 3. Hung, C. T.; Taylor, R. B. J . Chromafogr. 1981, 209, 175.
LITERATURE CITED Cantwell. F. F.; Puon, Si. Anal. Chem. 1979, 57, 623. Rotsch, T. D.; Cahlll, W. R., Jr.; Pietrzyk, D. J.; Cantwell, F. F. Can. J. Chem. 1981, 59, 2179. “Amberlite XAD-2”; Technical Bulletin; Rohm and Haas Co.: Phlladelphia, PA, 1972. Schmuckler, G.; Goldstein, S. I n “Ion Exchange and Solvent Extraction”; Marinsky, .I. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1977; Vol. 7, Chapter 1. Gjerde, D. T.; Schmuckller, G.; Frltz, J. S. J. Chromatogr. 1980, 787, 35. Adamson, A. W. “Physlcal Chemistry of Surfaces”, 2nd ed.; Intersclence: New York, 1967; Chapter 4. Van Dolsen, K. M.; Vold, M. J. I n “Adsorptlon from Aqueous Solutions”; Webber, W. J., Matljevic, E., Eds.; American Chemlcal Society: Washington, DC, 1968; Chapter 12. Overbeek, J. Th. I n “Collold Science”; Kruyt, H. R., Ed.; Elsevier: New York, 1952; Vol. 1, Chapter 4. Grahame, D. C. Chem. Rev. 1947, 4 7 , 441. Cantwell, F. F. I n “Ion Exchange and Solvent Extraction”; Marinsky. J. A., Marcus, Y., Eds.; Marcel Dekker: New York, to be published. Buck, R. P. I n “Proceedings of the Symposium on Ion Exchange Transport and Interfacial Properties”; Yeo, R. S., Buck, R. P., Eds.;
2427
RECEIVED for review April 29,1982. Accepted August 13,1982. This work was supported by an Alberta Heritage Foundation for Medical Research postdoctoral fellowship to S.A., by the Natural Sciences and Engineering Research Council of Canada, and by the University of Alberta. Presented in part at the VI International Symposium on Column Liquid Chromatography, Philadelphia, PA.
Ion Interactiion Chromatography of Inorganic Anions on a Poly(styrene---divinylbenzene)Adsorbent in the Presence of TetraaIky lamimonium SaIts Zlad Iskandaranl and Donald
J. Pletrzyk”
Department of Chemistry, The University of Iowa, Iowa City, Iowa 52242
Two major equiilbria that contribute to the enhanced retention of inorganic anions on a Hamilton PRP-1 column (a poiy(styrene-divlnylbenzene) copolymer) In the presence of a tetraaikyiammonlum salt (R,lM+) are Identified as one involving retention of the R,N+ salt as a double layer on the stationary phase surface and the second as an anion seiectivlty between analyte anlons and thoso occupying the secondary layer of the double layer. The mobile phase variables are structure and concentration of RBI+ salt, mobile phase solvent compositton, type and concemtration of coanlon accompanying the R,N+ salt or introduced for Ionic strength control, and pH. Control of these parameters permits the quantltatlve separation of many complex mixtures of Inorganic anlons at concentrations In the parts per bliiion range. Inorganic monoand multivalent anions were studied.
Several reports (1-3) have shown that inorganic anions are 0003-2700/82/0354-2427$01.25/0
retained on bonded stationary phases when using tetraalkylammonium (R4N+)salts as mobile phase additives. These studies tended to focus on separations, rather than on the mode of retention or a detailed evaluation of elution variables, and represent an alternate to the more familiar two-column ion exchange liquid chromatographic (LC) technique (4)and its variations (5-7) known as ion chromatography. We have recently shown (8) that the enhanced retention of an organic anionic analyte on a nonpolar poly(styrenedivinylbenzene) copolymeric adsorbent, PRP-1, from a mobile phase containing a R4N+salt, its coanion, and a mixed solvent follows a double layer model and not an ion pair or solvophobic model; a review of these models is provided elsewhere (8). Two major equilibria influence the retention. One, which describes retention of the R4N+salt itself on the PRP-1 surface, leads to the formation of a double layer. The R4N+occupies the primary layer producing a positive charge at the stationary phase surface while the coanion occupies a diffuse secondary layer. The second major equilibrium is one that describes the 0 1982 American Chemical Society
2428
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
selectivity of one anion over another in the secondary layer. By manipulating the mobile phase conditions, which control these equilibria, it is possible to facilitate the separation of organic anions. Because of the nature of these interactions (dynamic ion exchange) and similarity to ion exchange, this type of LC was referred to as ion interaction LC. This report describes experiments which demonstrate that retention of inorganic anions on PRP-1 in a R4N+modified mobile phase also follows the double layer model. In addition, the mobile phase variables, which influence selectivity and resolution, are identified so that separations can be more readily optimized. Depending on detection inorganic anions a t a trace level can be quantitatively separated.
Table I. Retention of Monovalent Inorganic Anions on PRP-1 capacity factor, k ' , at CH,CNa % analyte 3 6% anion 17.5% 3 0% FZ ' H3'2HC0,c1NO,Br NO,Br0,MnO ,SCNc10,10,CN-
EXPERIMENTAL SECTION Reagents. Tetrapentylammonium bromide (TPeABr) and other R4N+salts were obtained from Eastman Kodak Chemical Co. and Pfaltz and Bauer, Inc. Conversion of TPeA+ into different anion forms was accomplished by anion exchange (8). Analytes were analytical reagent grade sodium or potassium salts. Chromatographic grade CH3CN was obtained from MCB. Water was treated by passing distilled water through a mixed bed ion exchanger, an activated charcoal column, and 2-wm stainless steel filters. Instrumentation. A Waters Model 202 LC was used. Detection was by either a Waters 254 nm UV, a Waters refractive index, a Tracor Model 970 variable wavelength, or a Wescan Model 213 conductivity detector. Peak areas were determined with a Spectra Physics 4100 microprocessor. The PRP-1 column, which is a macroporous, high surface area, PSDVB copolymeric adsorbent, was obtained prepacked from Hamilton Co. or slurry packed with bulk form 10 wm spherical PRP-1 particles. The column was 4.1 mm i.d. X 150 mm. Typical efficiencies at optimum mobile phase conditions were 18 000 plates/m. Procedures. Column conditioning procedures and a discussion of this requirement are provided elsewhere (8). Sample solutions of the inorganic salts (about 1.0 mg/mL) were prepared by dissolving weighed quantities of the salt in water in 6-mL Hypovials fitted with Hycar Septa (Pierce Chemical). Pressure-Lok Series B-110 1O-pL or 25-wL syringes (Precision Sampling Corp.) were used to inject 2- to 1O-wL samples. Flow rates were usually 1.0 mL/min and column inlet pressures were about 500 to 1000 psi. Mixed solvents used for the mobile phases are expressed as percent by volume. Where indicated ionic strength was maintained by adding known amounts of inorganic electrolyte. A water jacket was used to control column temperatures (25 "C).
3.64 3.94 4.05 4.37 4.86 5.13 5.81 4.43 4.61 12.0
1.68 2.50 2.52 2.63 2.81 2.91 3.31 2.68 5.72 13.1 11.4 b
8.79
1-
4.4 5
M TPeAF mobile phase a A CH,CN/H,O, 1.00 X at a flow rate of 1.00 mL/min using a conductivity detector. Not detected. Table 11. Retention of Divalent Inorganic Anions on PRP-1 analyte analyte anion k' a anion 1-
so,2-
so,*C,0,2S2-
6.48 12.2 7.88 12.5 14.7, 15.6
Cr 2O succinate molonate fumarate C0,2-
k' a
9.19 8.63 11.6 10.4
3.67
a A 3:" CH,CN:H,O solvent mixture containing 5.00 X M NaF at a flow rate of 1.00 lo-, M TPeAF, 5.00 x mL/min using a conductivity detector.
RESULTS AND DISCUSSION Retention of Inorganic Anions. The retention of a given R4N+salt as the analyte on PRP-1 follows an order according to its coanion. For common monovalent anions (8)this order is
Clod- > I- > NO3- > Br- > NO2- > C1-> citrate > formate > F- > OH- (1) and is similar to the anion selectivity order found on a typical strongly basic anion exchanger (9). If the anion is introduced as the analyte in the form of a sodium salt, its retention on PRP-1, which also follows eq 1,is observed when using a R4N+ salt modified mobile phase. Data demonstrating this are shown in Tables I and I1 for monovalent and divalent anions, respectively. Since the divalent anions are highly retained, a high CH3CN/H20 ratio, a low TPeAF concentration, and added NaF were used to keep k'values small enough so that they could be determined accurately in reasonable analysis times. The TPeA+ was used as the F salt because this anion provides a low eluting power (see eq 1and Table I). If a Br- salt, which is a stronger eluent anion, was used, K'values would be significantly reduced. Since I- was studied under the conditions in both Tables I and I1 it is possible to qualitatively compare the retention level of the divalent anions to the monovalent
9.10 12.8 13.6 15.3 19.5 22.3 29.1 17.9
s
anions. For the conditions used in Table 11, C032-coincided with a negative or vacant peak found when using a conductivity detector (for example, see Figure 4). Also, a double peak was found for the S2- sample used; the reason for this was not explored. The multivalent anions tend to be more highly retained than monovalent anions just as in the case of conventional anion exchange (9). Even though all anions are introduced as Na+ or K+ salts, the exact analyte charge is not always certain. Since there is no eluent pH control other than that provided by the added salt mixture (pH is about 9) and most multivalent anions are anions of weak acids, their actual charge will be determined by their ionization constants. Since PRP-1 is pH stable a more basic mobile phase (NaOH solution) can be used. The peak shapes for the multivalent anions (Table 11)tend to be larger than those observed for the monovalent anions (Table I). Mobile phase modifications did not significantly improve this property. At optimum mobile phase conditions monovalent analyte anions provided efficiencies of about 18000 plates/m while the divalent ones provided 10000 plates/m. It appears that the additional protonic equilibria may be the major cause of this broadening. As shown previously, when studying the retention of organic analyte anions on PRP-1 in a R4N+salt modified mobile phase (8), experimental evidence supporting a double layer model is provided by correlation of analyte retention to R4N+, coanion, and analyte concentration. Evidence confirming that inorganic analytes follow the same model is shown in Figures 1to 3. A detailed discussion of this model, approximations, and limitations is provided elsewhere (8). In Figure 1retention (l/k?, where NO3- and NO, are the analytes, increases linearly as the coanion, F- (added as NaF),
ANALYTICAL CHEMISTRY, VOL.
Table 111. Retention
54,NO. 14, DECEMBER 1982 2429
(if Several Inorganic Anions on PRP-1 as a Function of R,N+ Salt 10%U TBuAF
analyte anion
-
formate c1NO Br-
4.60 4.69 6.10 6.01 7.:lo
NO,-
20% TPeAF
20% TPeABr
30% THxABr
30% TrMeHxDACl
7.75 8.01 10.6 11.4 16.8
capacity factor,c k' 6.67 7.21 9.23 10.2 14.0
6.44 6.98 8.76 10.1 14.1
6.44 8.44 9.01 8.85 10.5
selectivity NO,-/BrNO 2-/ C1-
1.118 1.30 1.16 1.54
NO;/NO;
NO 3-/f orm ate
1.47 1.32 1.58 2.16
1.19 1.28 1.52 2.09
1.26 1.61 2.18
1.17 1.63
Where Bu = butyl, Pe = pentyl, Hx = hexyl, Me = methyl, T = tetra, and Tr = tri. A CH,CN:H,O solvent % CH,CN. M R,N' salt at a flow rate of 1.00 mL/min using a conductivity detector. mixture containing 1.00 x
lwc i t m p t h (NaF),M
Figure 1. Retentlon of inorganlc anions on PRP-1 as a function of coanion concentration. 'The mobile phase conditions are 1385 hA TPeAF, and added NaF at a flow rate of CH3CN:Hz0, 1.00 X 1.00 mL/min using UV detection.
o -3 0 l -2 5 " -20 " " " log TPeAF, M
Figure 3. Retention of inorganic anions on PRP-1 as a function of TPeAF concentration. The mobile phase conditions are 1:4 CH,CN:H,O with added TPeAF at a flow rate of 1 .OO mL/min using a conductivity detector.
0 18
0 10 OC8
002
004
analyte, mg
006
008
Figure 2. Retentlon of Inorganic anions on PRP-1 as a function of concentration. The mobile phase conditions are 1:3 CH,CN:H,O, 1.00X lo3 M TPeAF at a flow rate of 1.00 mL/min using UV-deteition.
concentration increases. Similarly, Figure 2 shows that l / k ' for analyte retention increases with an increase in analyte concentration. Both of these trends are consistent with the double layer model (8). Since the R4N+salt must always be accompanied by a coanion, it is not possible to experimentally measure the effect due only to the R4N+. Thus, a maximum k'value should be observed (8)as the R4NCsalt concentration increases. Figure 3 illus4tratesthe predicted retention maximum where retention of the analytes, NO,, Cl-, and formak, was determined as a function of TPeAF concentration. Optimization of Mobile Phase Conditions. The experimentally controllable mobile phase parameters are (1) type and concentration of the R4N+ salt, (2) choice of coanion, (3) type and concentration of' mixed solvent, and (4)ionic strength (achieved by adding salts of the same or different coanion). It should be noted that the significance of the coanion and its potential as a mobile phase variable was not recognized in other studies where inorganic anions were separated by using R4N+counterions and alkyl bonded stationary phases (1-3). The pH of the mobile phase can also be a variable since
many inorganic anions are basic. In these stuiies the mobile phase was weakly basic due to the electrolytes in the mobile phase thus ensuring that most analytes were in their anionic form. No detailed attempt was made to study retention over a wide pH range. Several of these mobile phase parameters are interrelated and consequently not all can be varied independently of the others. Generalizations regarding optimization of these parameters for inorganic analyte separations are briefly summarized in the following. An increase in the R chain length in the R4N+ Or a CH3CN decrease in the CH3CN/H20 mixture increases adsorption of the R4N+salt itself on the PRP-1 (corresponds to the number of charge sites on the PRP-1surface). An enhanced analyte retention correlates with these effects providing other variables are held constant. For a given solvent mixture retention of the inorganic anions increases as the R chain length increases. If the solvent mixture is adjusted so that analyte retention is similar when comparing different R chain lengths, the optimum selectivity (for the inorganic anions used in this study) is obtained when R = pentyl, particularly, when analysis time and solvent composition were also considered as part of the optimization. Data illustrating this are shown in Table 111. It should be noted that the pentyl group is not optimum for all organic anions (8). Previous studies on alkyl bonded stationary phases (1-3) using cetyltrimethylammonium bromide, octylaminemineral acid, and tetrabutylammonium hydroxide as counterions, respectively, did not focus on the role of the R4N+ structure in influencing analyte retention. Maximum analyte retention occurs when the TPeA+ salt concentration is about 2.5 X M (see Figure 3). An optimum concentration, however, must take into account the detector that is being used; for a conductivity detector the background conductance
2430
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
Table IV. Percent Recovery for BrBr- concn, ppm taken found 58 104 348 0.700 2.32 4.06
61.7 100 355 0.7 20 2.19 4.12
%
difference a t 6.4 -4.0 t 2.0 t 2.9 -5.6
tl.5
a The first three are for a conductivity detector while the second three are for a UV detector. mL
Separation of monovalent inorganic anions on PRP-1. The mobile phase conditlons are 17.5:82.5 CH,CN:H,O, 1.00 X lo-, M TPeAF at a flow rate of 1.00 mL/min using a conductivity detector; v = vacant peak. Sample size was 1 pL containing about 1 pg of each analyte. Flgure 4.
Flgure 5. Separation of inorganic anions on PRP-1. The mobile phase M TPeABr, 5.00 X lo4 M NaBr was 35:65 CH3CN:H,0, 5.00 X for A and B, 35:65 CH,CN:H,O, 1.0 X lo-, M TPeABr for C, and 3:7 CH,CN:H,O, 5.00 X M TPeAF, 5.00 X M NaF for D. Flow rate was 1.00mL/min and sample size was 1 pL containing about 1 pg of each analyte; v = vacant peak.
and minimum detection limits increase as the mobile phase electrolyte concentration increases. For this reason a 1.00 x M TPeA+ salt concentration was usually used even though the peaks are broader at R4N+salt concentrations below the maximum as shown in Figure 3; it was found that peak broadening increased as the R4N+salt concentration decreased below the maximum. A favorable CH3CN/H20 ratio is one that provides a k’of about 5 for retention of the TPeA+ salt itself on the PRP-1. If MeOH or EtOH is used (their concentration must be higher to give the same k’for TPeA+ salt retention as obtained in CH,CN), analyte peak shapes tend to be broader. For a given R4N+salt and mobile phase solvent mixture, eluting power can be varied by changing the coanion accompanying the R4N+salt. This eluting power follows the order indicated in eq 1 (see also Tables I and 11). Thus, F(similar to OH-) provides the weakest eluent anion (see Table I11 where TPeAF and TPeABr can be compared). Analyte retention can also be reduced by adding NaF or a salt that provides a stronger eluent anion to the TPeA+ salt-mobile phase mixture. Separations. The data in Tables I and I1 suggest that many different anion mixtures are separable by ion interaction chromatography on PRP-1. Several examples are shown in Figures 4 and 5. Modification of the mobile phase conditions is possible, as outlined previously, to reduce analysis times if mixtures are simpler than those shown in the figures. If the R4N+salt is omitted from the mobile phase, no retention of the inorganic analyte anions is observed. Figure 4 illustrates the separation of several common monvalent anions. If I- or C10, were included, their retention a t these conditions would occur well beyond that for NO,. The negative or vacant peak shown in Figure 4,and which has been observed in other LC studies involving counterions (10,11), is characteristic of the conductivity detector’s response
to the equilibria that occur as the analyte mixture migrates through the column (1.2). When the analyte sample enters and passes through the column, the equilibrium condition is such that the first peak (NaF) is due to the F- analyte added and the F equivalent to all other analyte anions that undergo an ion exchange with the adsorbed TPeAF sites. The vacant peak then represents a decrease in TPeAF concentration in the mobile phase because each of the analyte anions at equilibrium is retained as the TPeA+ (and eluted as the TPeA+ salt). Since their retention is higher than that of TPeAF (see eq l),the net number of TPeA+ sites on the stationary phase is increased equivalent to the decrease in TPeA+ salt in the vacant peak. If TPeAF is used as an analyte in a 17.5:82.5 CH3CNHz0mobile phase minus the TPeA+ salt, its retention time (as a positive peak) corresponds to the vacant peak. Also, when a TPeAF column breathrough is measured (8),the first evidence of the breakthrough corresponds to the time a t which the vacant peak occurs. If TPeABr rather than the F salt (Br- is a stronger eluent anion than F-) is used in the mobile phase in Figure 4,F- and C1- would be in the dead volume peak, NOz- would be next followed by the vacant peak, and finally the NO; peak. The vacant peak, which is due to the Br-, would interfere with the shape and resolution of the NOz--N03- peaks. It is therefore important to also consider the detector properties when choosing the eluent anions; in general, vacant peaks are not observed when using a UV detector unless the eluent contains an anion that adsorbs at the wavelength used for detection (12). Figure 5 illustrates the separation of more highly retained anions. To reduce and improve analysis times the TPeA+ concentration is decreased (reduces the number of sites), the stronger eluent anion Br- is used, NaBr is added, and the percent CH3CN is increased. Since a Br- eluent is used in Figure 5, the Br- analyte appears with the column void volume peak. In Figure 5B, the small unknown peak is due to the Sz02-sample. Quantitative Recovery. Figure 2 indicates that the analyte retention is indirectly proportional to analyte concentration. However, as analyte concentration is reduced below 0.005 mg/lO pL injected (see Figure 2), l / k ’ was shown to approach a constant value. This is consistent with the equation previously derived that correlates analyte retention to its concentration (8) since a t very low analyte column loadings the magnitude of other terms in the equation is the determining factor. Reproducibility and quantitative recovery at these low column loadings were verified by preparing both direct and internal standard calibration curves using Br- standards and conductivity and UV detection. All Br- standards were injected as 10-pL quantities and the mobile phase was 3:7 CH3CN:H20,1.0 X 109M TPeAF. For direct calibration by conductivity the Br- range covered 25 to 600 ppm and fits the relationship log peak area = 0.98 log (ppm of Br-) + 2.67 with a correlation factor of r = zkl.000. Detection below 20 to 30
Anal. Chem. I M P , 54.
ppm was difficult because of this detector's response to background electrolyte. For a UV detector the range covered 0.6 to 6 ppm Br- and followed log peak area = 0.44 log (ppm of Br-) + 4.78 with r = f0.9926. Detection well below 1ppm is possible; the minimum was not determined because of the quality of the water. Similar results were found for several other anions that were tested. For internal standard calibration, I- was the analyte and Br- was the internal standard. A linear calibration curve (conductivity detection) was found from 43 to 850 ppm Iwhich was the range studied. If a UV detector is used a linear calibration down to 8510 ppm was obtained. The Br- was maintained a t 290 ppm, the sample aliquot was 10 pL, and the mobile phase was 3:7 CH3CN:H20,1.00 X M TPeAF. Several separate Br- d u t i o n s covering the range of 0.7 to 460 ppm were prepared and chromatographed. Percent recoveries based on the two calibration curves were favorable with the error in recovery often better than1 f 4 to 5 % . Similar results were found with other anions tested. Minimum detection limits were not established because it was apparent that the quality of LC water available was the limiting factor. For the LC water in this Rtudy, Br- and NO3- were estimated to be about 500 ppb and 50 ppb, respectively.
2431-2433
2431
LITERATURE CITED Reeve, R. N. J . Chromatogr. 1979, 177, 393-397. Molnar, I.; Knauer, H.; Wilk, D. J . Chromatogr. 1980, 207,225-240. Skelly, N. E. Anal. Chem. 1982, 54, 712-715. Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. Pohl, C. A.; Johnson, E. L. J . Chromatogr. Sci. 1980, 78, 442-452. Gjerde, D.T.; Schmuckier, G.; Fritz, J. S. J . Chromatogr. 1980, 187, 35-45. Stevens, T. S.; Davls, J. C.; Small, H. Anal. Chem. 1981, 5 3 , 1488- 1492. Iskandaranl, 2.; Pletrzyk, D. J. Anal. Chem. 1982, 5 4 , 1065-1071. Helfferich, F. "Ion Exchange"; McGraw-HIII: New York, 1962; p 95. Denkert, M.; tiackzell, L.; Schill, G.; Sjogren, E. J . Chromatogr. 1981, 278, 31-43. Bldllngmeyer, B. A.; Demlng, S. N.; Prlce, W. P., Jr.; Sachok, B.; Petrusek, M. J . Chromatogr. 1979, 786, 419-434. Sachok, B.; Demlng, S. N.; Bidilngmeyer, B. A. J . Ll9. Chromatogr. 1982, 5 389-402.
RECEIVED for review April 26,1982. Resubmitted July 1, 1982. Accepted September 14,1982. Part of this work was presented a t the 17th Midwest Regional Meeting of the American Chemical Society, Columbia, MO, 1981, and part at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1982. This investigation was supported by Grant CHE 7913203 awarded by The National Science Foundation.
Comparison of Ion Chromatography and Titrimetry for Determination of Sulfur in Fuel Oils Puilgandla Viswanadham," Donald R. Smick, Jerome J. Plsney, and Walter F. Dilworth IBM System Products Division, Rochester, Minnesota 5590 1
A comparison was made, of Parr bomb ion chromatographic and titrimetric methods ito determine sulfur in fuel olis. The two methods are comparable in precision and accuracy. When rapld sulfur determination in a number of samples is desired, the titrimetry method appears to be an approprlate choice between the two methods.
Sulfur is a common constituent in many lubricating oils, fuel oils, and gasoline and is of concern because its oxidation products are corrosive atmospheric pollutants. Fuels derived from coal contain varying amounts of sulfur, depending upon the geographic location and geological history of the coal. A knowledge of the sulfur content in natural as well as synthetic fuels is important in both proper utilization and disposal. Several excellent meLhods for determining sulfur are available in the literature (1-3). One recent method utilizes ion chromatography and has been described by Mizisin et al. ( 4 ) and Butler et al. (5). In this study vve used an iodide titration method with a LECO DB64 Model 765-100 carbon sulfur analyzer as well at3 the ion chromatographic method of analysis for determining sulfur in fuel oils. The two methods are compared and the relative merits of each are discussed.
EXPERIMENTAL SECTION Titrimetric Method (6). A LECO carbon-sulfur analyzer was used for the titrimetric method. A known weight of the sample taken in a porcelain crucible was subjected to combustion in a stream of oxygen in an induction furnace, according to the 0003-2700/82/0354-243 1$01.25/0
procedure in the instruction manual (6). The liberated sulfur dioxide was titrated with iodine using starch as an indicator. The iodine for titration is generated according to the reaction IO3- + 51- + 6H' = 312 + 3Hz0
(1)
The reaction of SOz(g) and iodine is represented by SOz(g) + 12 + 2HzO = HzS04 + 2HI
(2)
Stable starch and KI solutions are added to a solution of HCl in a titration vessel. Addition of trace amounts of KI03 solution to this mixture liberates iodine, resulting in blue coloration. This blue coloration serves as the reference point. As the sulfur dioxide from the combustion products enters the titration vessel, iodine is converted into HI, thus decreasing the intensity of the blue coloration. The diminution of the blue color is detected by a photodetector that actuates the automatic buret to deliver requisite amounb of KIO:, solution for the restoration of the original color until all of the S02(g)is consumed. The amount of the KIO, solution delivered is related to the sulfur by eq 2 and the sulfur content is displayed on a digital readout. Ion Chromatographic Method. Determination of sulfur by the ion chromatographic method involved two procedures. Accurately weighed samples (0.4 to 0.8 g) were oxidized in the Parr bomb with 20 mL of a 0.003 M NaHC03 and 0.0024 M Na2C03 mixture as the absorbing solution. Subsequent to oxidation, the solution was quantitatively transferred to a 100-mL volumetric flask and made up to volume. The sulfur in the sample is oxidized to sulfate 2s (in fuel) + 302(g) = 2SO,(g) (3) SO2 + HzO = HZSO,
(4)
Na2CO3+ HzS04= Na2S04+ HzCO3
(5)
0 1982 American Chemlcal Society
