358
Anal. Chem. 1987, 59, 358-362
way to view the relative contributions of various surface active sites before and after modifications. Registry No. Chloroform, 67-66-3; pyridine, 110-86-1; dichloromethane, 75-09-2; kaolinite, 1318-74-7;alumina, 1344-28-1. LITERATURE CITED Dash, J. G. Films on Solid Surfaces: The Physics and Chemistry of Physical Adsorption; Academic: New York, 1975;Chapter 9. Aboul-Gheit, A. K. J . Chem. Techno/. Biotechnol. 1979, 29,
480-486. Cooper, W. T.; Hayes, J. M. J . Chromafogr. 1984, 314, 111-125. House, W. A. I n A . Specialist Periodical Report: Colloid Science; Everett, D. H., Ed.; The Royal Society of Chemistry: London, 1983; Vol. 4;pp 1-58. Conder, J. R.; Young, C. L. Physiochemicai Measurements by Gas Chromatography;Wiley: New York, 1979. Hobson, J. P. Can. J . Phys. 1965, 43, 1934-1940. RudziAski, W.; Waksmundski, A,; Leboda, R.; Suprynowicz, Z.; Lason, M. J . Chromatogr. 1974, 92,25-32. Ross, S.;Olivier, J. P. On Physical Adsorption; Wiley: New York, 1964;Chapter 4. Jaroniec, M. Adv. Conoid Interface Sci. 1983, 18, 149-229. Jaroniec, M. Thin Solid Films 1983, 100,325-328. Dormant, L. M.; Adamson, A. W. J . Colloid Interface Sci. 1972, 38,
285-289. Dash, J. G. Films on Solid Surfaces: The Physics and Chemistry of Physical Adsorption; Academic: New York, 1975;p 119. Rohrschneider, L. J . Chromafogr. 1966,22, 6-22. McReynolds, W. 0.J . Chromatogr. Sci. 1970,8 , 685-691. Cerofolini, G. F. Chern. Phys. 1978,33, 423-434. Cerofolini, G. F. Thin Solid Films 1974, 23, 129-152. Cerofolini, G. F. Surf. Sci. 1971,24,391-403. Zel’dovich, Ya. Acta Physicochim. URSS 1935, 1 , 961-974. Conder, J. R.; Purneil, J. H. Trans. Faraday Soc. 1968, 64,
3100-31 11. (20) Conder, J. R. Chromatographia 1974, 7 ,387-394. (21) James, A. T.; Martin, A. J. P. Biochem. J . 1952, 5 0 , 679-690. (22) Leboda, R.: Sokdowkski, S. J . Colloid Interface Sci. 1977, 67, 365-374. (23) Waksmundzkl, A,; Jaroniec, M.; Suprynowicz, z. J . Chromatogr. 1975, 110. 381-384.
(24) Leboda, R.; Sokhwski, S.; Rynkowski, J.; Paryjczak, T. J . Chromafogr. 1977, 738,309-319. (25) Jaroniec, M. Surf. Sci. 1975, 5 0 , 553-564. (26) Sokobwski, S.;Jaroniec, M.; Cerofolini, G. F. Surf. Sci. 1975, 47, 429-439. (27) . . Jaroniec. M.: Sokdowski.. S.:. Cerofoiini. G. F. Thin Solid Films 1976. 31, 321-328. (28) Van Dongen, R. H. Surf. Sci. 1973,39,341-356. (29) Van Deemter, J. J.; Zuiderweg, F. J.: Klinkenberg, A. Chem. Eng. Sci. 1958, 5,271-289. (30) Cremer, E. Monatsh. Chem. 1961,92, 112-115. (31) Huber, J. F. K.;Keulemans, A. I.M. I n Gas Chromatography 1962; Van Swaay, M., Ed.; Buttersworth: London, 1962;pp 26-35. (32) Huber, J. F. K.; Gerrltse, R. G. J . Chromatogr. 1971, 5 8 , 137-158. (33) Dorn, W. S.;McCracken, D. D. Numerical Methods with Fortran I V Case Studies; Wiley: New York, 1972;Chapter 5. (34) Atkins. J. H. Anal. Chem. 1964,36, 579-583. (35) Brunauer, S.;Emmett, P. H.; Teller, J. J . Am. Chem. Soc. 1938,60, 309-3 - - . - 19 .- .
(36) Dubinin, M. M.; Radushkevich, L. V. Dokl. Akad. Nauk SSSR 1947, 55. . 327-329. ._ (37) Kaganer, M. G. Dokl. Akad. Nauk SSSR 1957, 176, 251-254. (38) Cerofolini, G. F. I n A . Specialist Periodical Report: Colloid Science, Everett, D. H., Ed.; The Royal Society of Chemistry: London, 1983; Vol. 4,pp 59-83. (39) Sing, K. S.W. I n Characterization of Powder Surfaces; Parfitt, G. D., Sing, K. S. W., Eds.; Academic: New York, 1976;p 31. (40) Keulemans, A. I. M.; Kwantes, A.; Zaal, P. Anal. Chim. Acta 1955, 13,357-372. (41) Snyder, L. J . Chromafogr. 1974,92, 223-230. (42) Snyder, L. R. Principles of Adsorption Chromafography; Marcel Dekker: New York, 1968;pp 166-167. (43) Babkin, Y.; Kiselev, A. V. Russ. J . Phys. Chem 1962, 36, 1326-1331. (44) Wade, W. H ; Hackerman, N. I n Advances in Chemistry Series, No. 43;Gould, R. F., Ed.; American Chemical Society: Washington, DC.
---
19AA. 333-3.11 ., nn rr
(45) Betts, T. J. J . Chromatogr. 1986, 354, 1-6. RECEIVED for review July 1, 1986. Accepted September 11,
1986.
N-Substituted A minoaI kyI suIfonates as Eluent s Membrane-Suppressed Anion Chromatography Knut Irgum
Department of Analytical Chemistry, University of UmeA, S-901 87 Umei, Sweden
Sodium salts of 2-amlnoethanesuifonic, 3-amino-I-propanesutfonk, and 3-amlno-2-hydroxy-l-propanesu#onlc acids wlth various N-substituents were used as eluents in membranesuppressed anion chromatography. Background conductlvlties as low as 0.4 /IS cm-’ were attained by uelng 10 mM eluent concentrations. Relative eluent strength varied, thus providing a number of eluents suitable for analysis of anions having both weak and strong lnteractlon with the quaternary ammonlum pdy(methacrylate) separator resin that was used.
The disclosure of ion chromatography by Small et al. ( 1 ) has brought about a renaissance in inorganic chromatography. For many anions, where tedious, insensitive, and often imprecise analysis methods were the only ones available, the impact of this new tool has been tremendous. The choice of eluents for use in anion chromatography is, however, still limited. In the suppressed mode, where a strong base anion exchange resin is used for separation, and a strong , a cation exchange membrane acid ion exchange resin ( l ) or surrounded by acid ( 2 ) ,is used for reducing the background conductivity of the eluent, only hydroxide, carbonate, and tetraborate have found widespread use. Of these, carbonate 0003-2700/87/0359-0358$0 1.50/0
is the only ion with sufficient eluting power to allow analysis of polyvalent anions, using what has until recently ( 3 , 4 )been considered as a reasonable eluent concentration. A drawback of carbonate as eluent is the rather high background conductivity, even after suppression. This has forced the use of postsuppression devices (5,6) to remove carbon dioxide from the effluent in order to increase the sensitivity for anions of weak acids (5) and to permit gradient runs (6). Hydroxide ion is useful as eluent for the analysis of early eluting anions, but in many applications, e.g., for samples containing phosphate species, the high pH makes it a less than ideal choice. The specifications that apply to a substance to be used as eluent in suppressed anion chromatography are strict. It should be available in high purity (or easily purified from foreign anions) at a modest cost. The eluent strength must be reasonably high without poisoning the column. I t should not cause artifact peaks as a result of reactions in the separator column or in the suppression step, nor should it poison or inhibit the suppressor device used. Suppressibility must be nearly total to minimize the “water dip” (2) and noise and to permit gradient runs. The potential of zwitterionic substances as eluents in suppressed anion chromatography should be apparent to anyone who has attempted the analysis of, e.g., glycine by that G 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
technique. Although there is no problem with elution, virtually no signal will appear, regardless of sample concentration. This is the best indication of the usefulness of this group of substances as eluents. Zwitterionic eluents have been used in suppressed anion chromatography earlier. Ivey (7) reported that residual conductivities as low as 5-10 pS cm-’ could be obtained when using salts of some N-substituted aminoalkylsulfonic acids, known as Good buffers (8,9), as eluents with a packed suppressor. Disodium glutamate has also been used by Small and Solc with packed bed suppressors (IO). The use of amphoteric substances with membrane suppressors has so far not been reported. This paper presents an evaluation of the eluent and suppression properties of some commercially available Nsubstituted aminoalkylsulfonates in a membrane-suppressed anion chromatographic system.
t
’ .‘, b
EXPERIMENTAL SECTION Reagents and Solutions. Eluent substances were purchased as free acids from Sigma (St. Louis, MO) and recrystallized from water/methanol according to the procedure of Good et al. (8). All other chemicals were reagent grade. Eluents were generally prepared as buffers with 10 mM sodium salt and 12.5 mM free acid, by partly neutralizing the free acid with NaOH before diluting to volume. Eluent flow rate was 1.0 mL/min. H2S04(12.5 mM) was used as suppressor regenerant. Deionized water (Milli-Q equipment; Millipore, Bedford, MA), having conductivity less than 60 nS cm-’, was used throughout. Equipment. The chromatographic system consisted of an Altex 110 A pump, a Valco six-port injector valve with 100-pL PTFE loop, a Waters Anion PW separator column (10-pm poly(methacry1ate) gel with quaternary ammonium functional groups; ion exchange capacity erroneously stated by the manufacturer as 30 & 3 mequiv/mL must be pequiv/mL), and a membrane suppressor constructed essentially according to Dasgupta (11). Length of the Nafion 811X (Du Pont, Wilmington, DE) perfluorosulfonated membrane tubing was 1.5 m and an Abulon Top (ABU, Svangsta, Sweden) 0.60-mm polyamide monofilament was used for filling the annular gap. The filled tube was coiled to an inside diameter of 0.7 mm before boiling to make the shape permanent (11). The various parts of the system were interconnected with 0.3 mm i.d. PTFE tubing. A peristaltic pump was used to pump the regenerant outside the suppressor membrane counter current to the eluent flow at a flow rate of 4 mL min-’. Effluent conductivity was monitored by using an LDC ConductoMonitor with a Model 7011 cell, while a Metrohm E 587 conductometer was used for measuring bulk conductivities. Abbreviations Used. Abbreviations used in this paper are as follows: ACES, 2 4 (2-amino-2-oxoethyl)amino)ethanesulfonic acid; taurine, 2-aminoethanesulfonic acid; AMPSO, 3-(dimethyl(hydroxymethyl)methylamino)-2-hydroxy-l-propanesulfonic acid; BES, 2-(di(2-hydroxyethyl)amino)ethanesulfonic acid; CAPS, 3-(cyclohexylamino)-l-propanesulfonicacid; CAPSO, 3-(cyclohexylamino)-2-hydroxy-l-propanesulfonic acid; CHES, 2-(cyclohexylamino)ethanesulfonic acid; DIPSO, 3-(di(2hydroxyethyl)amino)-2-hydroxy-l-propanesulfonicacid; HEPES, N-2-hydroxyethylpiperazine-N’2-ethanesulfonic acid; MES, 2(N-morpho1ino)ethanesulfonicacid; MOPS, 3-(N-morpholino)propanesulfonic acid; MOPSO, 3-~N-morpholino)-2-hydroxy-lpropanesulfonic acid; PIPES, piperazine-N,”-bis( 2-ethanesulfonic acid); POPSO, piperazine-N,N’-bis(2-hydroxy-3-propanesulfonic acid); PTFE, poly(tetrafluoroethy1ene); TAPS, 3-(tris(hydroxymethyl)methylamino)propanesulfonicacid; TAPSO, 3-(tris(hydroxymethyl)methylamino)-2-hydroxy-l-propanesulfonicacid; TES, 2-(tris(hydroxymethyl)methylamino)ethanesulfonic acid. RESULTS A N D DISCUSSION The suppression mechanism for the anion of a zwitterionic compound in a membrane suppressor is different from that occurring in a packed bed suppressor. In the packed bed, the anion of an amphoteric substance, like an ordinary a-amino acid, can be protonated into the positively charged ion by the highly acidic resin and retained by the same as such. This is the reason why disodium glutamate could be successfully
359
H+ HS04-
t ELUENT FLOW
MEMBRANE
REGENERANT FLOW
4
NaHS04
Na’
ELUENT FLOW
MEMBRANE
REGENERANT FLOW
Figure 1. Membrane suppressor reactions for aminocarboxylic and aminosulfonic acid salts exemplified by glycine and taurine.
employed in a packed bed suppressor system by Small and Solc, in spite of the low isoelectric pH (3.24) of that substance. In a membrane suppressor, the analogous reaction is transport of the positively charged eluent substance through the cation exchange membrane into the regenerant flow (cf. Figure 1). The dissociation of an amphoteric substance follows the scheme outlined in Figure 2. It is apparent from these equations that the isoelectric pH should be near neutral to minimize the residual conductivity that is caused by dissociation of the zwitterion into the anion and a proton or to the cation and a hydroxide ion. Water, though not a true zwitterion, matches this requirement perfectly, which is evident from the excellent suppressibility of alkali metal hydroxide eluents. Aminoalkanesulfonic acid anions are, contrary to aminocarboxylic acid anions, not protonated into the cation and retained by a strong cation exchange resin in the H+ form (cf., e.g., synthesis workup described in ref 12). The suppression reaction for aminoalkanesulfonates thus involves formation of the zwitterion only (7). Conductivity and pH of free aminosulfonic acids in aqueous solution are consequently governed by the amine pK alone, owing to the strong acidity of
360
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
Table I. Suppressibility of Aminoalkylsulfonate Eluents
substance"
pH
taurine AMPS0 BES CAPS CAPSO CHES DIPS0 MES MOPS MOPS0 TAPS TAPS0 TES
5.0 5.1 4.3 5.4 5.2 5.3 4.5 4.1 4.4 4.3 4.7 4.6 4.5 7.1
NaOH
free acidb conductivity, pS cm-' 1.4 1.8 10.5 0.8 0.9 1.0 7.1 31 10.6 10.5 9.6 5.1 7.2 0.06
eluent before suppressorc conductivity, PH pS cm-'
PH
9.4 9.5 7.7 10.7 10.3 9.9 8.2 6.7 7.7 7.3 9.0 8.1 8.0 12.0
4.4 4.1 3.8 4.3 4.4 4.4 4.1 3.7 4.0 3.9 3.6 4.0 4.1 3.7
715 600 620 700 620 640 600 620 620 620 600 590 620 2100
eluent after suppressorc conductivity: pS cm-' 1.6 2.9 9.0 1.0 0.4 1.1 8.0 28 8.3 12 30 6.1 5.7 2.1
water dip: pS
cm-'
-0.6 -1.7 -1.7 -0.9 -0.6 -0.7 -1.6 -4.1 -1.3 -4.0 -17.7 -2.4 -1.4 -0.1
PK2 8.74 9.0 7.15 10.40 9.6 9.55 7.6 6.15 7.20 6.95 8.40 7.7 7.50 14.17
Osee Experimental Section for a legend of abbreviations. b A t 12.5 mM concentration. For NaOH, values are pH and conductivity of water at 20 "C. Composition of eluents was 12.5 mM free acid + 10 mM NaOH, except in the case of NaOH, where the acid was omitted. Magnitude of negative peak caused by injection of deionized water. e Corrected for regenerant penetration.
NH2.CH2.COOH Neutral
Flgure 2. Dissociation paths for amphoteric zwitterions.
the sulfonic acid group. In order to achieve efficient suppression, this pK should thus be as high as possible. When Ivey tested some Good buffers with a packed bed suppressor, he was looking for a substance that could be used with a silica-based column and for the analysis of acid-instable compounds (13). He was therefore restricted to eluents with pH below 8. This restriction limits the degree of suppression attainable with eluent substances of this type, as significant dissociation of the zwitterion will take place. MES, which was the substance he used with most success, proved in fact to be among the least suppressible substances tested in this work (see Table I). It is, however, comparable to carbonate at the same concentration. In later work, h e y has found that by using salts of carefully purified HEPES, residual conductivities below 1FS cm-' could be reached in a packed suppressor system (13),this time relying on the amphoteric properties of the substance by protonation to the cation and capture of this by the suppressor resin, as seen with aminocarboxylic acids. Ivey concluded his paper (7) with some remarks on possible future eluent compounds. He suggested diaminodisulfonates as the most promising eluents. PIPES and P O P S 0 are the only commercially available substances in this group. Although they show a considerably higher eluent strength, they are not useful, as they give high residual conductivities (>500 WS cm-* for 5 mM disodium salts). The free acid double zwitterions are furthermore insoluble in water a t the concentrations needed. Crystallization may therefore take place in the suppressor if these substances are used. Hence the use of monoaminoalkylmonosulfonates only in this work. The constant search for polyvalent suppressible eluent substances has lost some of its rationale with the suppressors reported nowadays, capable of suppressing decimolar eluents
(3,4). Monovalent eluents actually have an advantage over their polyvalent counterparts, in that the eluent strength varies more strongly with concentration (14). This property makes them potent contenders for the forthcoming gradient IC techniques, provided they can be adequately suppressed. Additional buffering capacity, gained by the higher concentration these will have to be used at, will also expand the sample overload overhead (15). Suppression Efficiency. Inhibitory effects on the suppression reaction were not seen as the nitrogen substituents were varied, but a relation between residual conductivity and amine pK was evident (Table I). As expected, the most easily suppressed eluents are those with high amine pK; this limits the usefulness of these eluents in some applications. The experience from these experiments can, however, be transferred to aminocarboxylic acids. These can be effectively suppressed even if their amine pK is relatively Iow, as the isoelectric p H will be determined by the mean value of the amine and carboxylic acid dissociation constants. Work is in progress with a broad range of aminocarboxylic acids, including aminobenzoic and N- and C-substituted a-amino acids. Effect of N-Substituent Polarity on Retention Times. The polarity of the N substituent(s) has a strong influence on the relative eluent strength (Table 11). Of the commercially available Good buffers, only CHES, CAPS, and CAPSO have N substituents without hydroxy or amide groups. These are by far the strongest eluents of those tested, and nonpolar interaction between the N substituent and the polymer matrix (16) was suspected as being the reason for this. To examine this, TES, CAPS, and NaOH were tested with 5 M methanol added to the eluent to disturb interactions between the polymer matrix and the N-substituent part of the eluent anion. The eluent strengths of CAPS, and to a lesser extent NaOH, were decreased by methanol addition, while a slight increase was seen for TES (Table 111). Both observations indicate an interaction of the N substituent with the bulk polymer. Methanol will adsorb on the surface with the hydroxyl groups pointing outward. These groups attract the polar head of TES by hydrogen bonding but hamper the possibility for a nonpolar interaction between the cyclohexyl head of CAPS and the polymer surface. This leaves, however, the decrease in eluent strength for NaOH unexplained. No information is available on the exact constitution of the ion exchange groups in the separator column, but quaternary ammonium groups are usually attached to acrylic matrices by a short amide-bound alkyl spacer arm (17). A possible explanation
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
361
Table 11. Retention Values for Selected Anions Using Aminoalkylsulfonates and Hydroxide Ion as Eluents"
formate
F-
C1-
Br-
6.34 6.78 10.3 2.42 2.91 2.85 9.42 9.34 9.13 9.90 7.25 10.6 10.4
8.10 8.76 13.4 2.87 3.47 3.41 12.5 12.9 12.2 13.5 9.52 14.7 13.9
5.10 5.37 7.97 1.94 2.83 2.37 7.81 7.64 7.38 7.99 5.74 8.45 8.37
13.6 15.3 24.9 4.01 5.49 5.23 22.9 23.7 22.4 24.5 16.6 26.4 25.9
28.0 32.1 55.3 7.69 10.3 9.65 51.0 52.8 49.1 54.1 35.9 59.2 58.2
1.84
1.96
1.59
2.81
4.36
substanceb acetate taurine AMPS0 BES CAPS CAPS0 CHES DIPS0 MES MOPS MOPS0 TAPS TAPS0 TES NaOH
I-
partition ratio (12 qC NO< NO< HP04'-
NA NA NA 25.7 35.9 32.8
NA NA NA NA NA NA NA
20.4 22.9 38.8 5.91 7.79 7.37 35.7 37.4 34.8 38.4 25.5 42.7 40.2
36.9 42.3 73.3 9.54 12.9 11.9 67.7 71.0 65.4 72.4 47.2 82.2 76.0
3.57
5.41
13.5
NA NA NA
HMSA
SO-:
S203'- oxalate
NA
NA NA NA
NA NA NA
NA NA
21.6 22.6 18.6
NA NA NA NA NA NA
16.1 25.1 24.0
NA NA NA
19.3
27.4 46.2 43.1
NA NA NA NA NA NA NA
NA NA NA NA
NA
15.4 24.1 22.9
7.83
7.64
NA NA NA 20.8 31.7 29.5
NA NA NA
NA NA
NA NA
NA NA
NA
NA
NA
NA
NA
12.0
9.26
" Eluent composition was 12.5 mM free acid + 10 mM NaOH, except in the case of NaOH, where the acid was omitted. Temperature was 20 f 1 "C in all experiments. Test ion concentration was 0.1 mM except for fluoride, where 0.05 mM was used and for nitrite, hydrogen phosphate, iodide, and thiosulfate, where it was 0.2 mM. NA = not available. bSee Experimental Section for a legend of abbreviations. Corrected for dead volume with column detached (340 wL). Table 111. Retention Values for Selected Eluent Substances with Methanol Added to the Eluent" eluentb
CAPS in water CAPS in MeOH ratioc TES in water TES in MeOH ratio NaOH in water NaOH in MeOH ratio
acetate formate
C1-
Br-
I-
25.7 29.9
2.42 2.95
2.87 3.41
1.94 2.56
4.01 5.47
7.69 9.88
1.22
1.19
1.32
1.36
1.29
10.4 8.84
13.9 12.0
partition ratio ( k 9 NO2- NOC HP0:-
F-
8.37 7.64
25.9 22.0
58.2 45.8
0.85
0.86
0.91
0.85
0.79
1.84 2.08
1.96 2.39
1.59 1.92
2.81 3.34
4.36 5.41
1.13
1.22
1.21
1.19
1.24
1.16
NA NA NA 13.5 14.8 1.10
5.91 7.35 1.24 40.2 32.3
9.54 12.2 1.28 76.0 61.3
0.80
0.81
3.57 4.27
5.41 6.47
1.19
1.20
HMSA
21.6 25.1
16.1 28.0
1.16
NA NA NA
15.4 28.6
1.74
2.15
NA NA NA
NA 7.83 11.1
7.64 11.5
1.42
1.50
1.15
27.4 59.0
1.87
NA NA
19.3 22.2
S2032- oxalate
SO ':
NA NA NA 12.0 19.7 1.64
20.8 34.6 1.73
NA NA NA 9.26 13.5 1.45
" Eluent compositions and conditions as in Table 11, except that eluents were prepared in 5 M aqueous methanol instead of water. Values with eluent in water only are taken from Table 11. bSee Experimental Section for a legend of abbreviations. ck'(5 M MeOH)/k'(H20).
for the decrease in eluent strength for NaOH in the presence of methanol might be that it becomes less energetically favorable for the mobile and relatively strong ion pair of the hydroxide ion and the anion exchange group to move into the vicinity of the methanol-modified polymer surface. More research is, however, needed to explain this. Sample cations are reported to have effects in nonsuppressed ion chromatography, where these can be retained by cation exchange sites on a silica-based separator column (18). If eluent substance is adsorbed on the matrix of the separator column, it will act as cation exchange sites in the same way as in the ion interaction chromatographic methods (19). Sodium chloride (0.1 mM) and barium chloride (0.05 mM) solutions were therefore injected to check if spurious peaks or changes in retention time for chloride took place. No effect was seen. A few examples of separations that could be attained with these eluents on the equipment used are given in Figures 3, 4, and 5. I t is noteworthy that sulfite (HMSA) and sulfate could not be separated with these eluents on the column used. These ions were not separated by either the XAD-1-based or Vydac SC anion exchangers used in ref 14, whereas on a Dionex agglomerated separator material this separation is possible (20, 21). Oxidation of sulfite ion to sulfate can be the cause of this; the separator column is packed in stainless steel and metal ions are powerful catalysts for sulfite oxidation (22). It might have been possible to separate these ions had a strong eluent of this type with lower pH been available;
Formate
I
'$ t
0
5
T
uI
10
15
1, (min)
Flgure 3. Chromatogram of 0.05 mM F-, 0.2 rnM acetate, and 0.1 mM each of formate and CI-, using 6.25 mM taurine/5.0 rnM NaOH as eluent.
HMSA is subject to alkaline hydrolysis, but free HMSA is an oxidation-resistant singly charged ion (20,21),whose retention
362
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
Suppression characteristics are not affected more than expected from the differences in amine pK of the different compounds. New E l u e n t s T o Be Tested. The Good buffers are designed for buffering in biochemical systems. The risk of membrane penetration and protein denaturation by detergent action of aminoalkylsulfonates with nonpolar parts in the molecule has led to highly polar N substituents for all compounds, the cyclohexyl-types being the only exceptions (8). This is unfortunate in the current application, as the compounds with nonpolar N substituents are the best eluents. The investigations will therefore be continued with other aminoalkylsulfonic acids, especially synthesized for use as IC eluents. Further studies will be conducted with aminoalkylsulfonic acids carrying N-alkyl, N&-dialkyl, and N-aryl substituents. acids will also be Pyrrolidine- and piperidine-N-alkylsulfonic included in a future test, as will methylene- and ethyleneditaurine, the two latter being diamino diacids.
CIF- I
r
I
0
10
5
15
t, (min)
Figure 4. Chromatogram of halides using 12.5 mM CAPSllO mM NaOH as eluent. Sample contained 0.1 mM of each ion.
ACKNOWLEDGMENT Thanks are due to Michael Sharp for linguistic revision of the manuscript, to Anders Cedergren for valuable discussions, and to Karin Olsson for tracing the figures. Registry No. AES, 7347-25-3; AMPSO, 105140-24-7;BES, 66992-27-6;CAPS, 105140-23-6;CAPSO, 102601-34-3;CHES, 3076-05-9; DIPSO, 102783-62-0; MES, 71119-23-8; MOPS, 71119-22-7; MOPSO, 79803-73-9;TAPS, 91000-53-2;TAPSO, 105140-25-8;TES, 70331-82-7;HMSA, 14265-45-3;MeOH, 67-56-1; F-, 16984-48-8;C1-, 16887-00-6;Br-, 24959-67-9; I-, 20461-54-5; NO
