Anal. Chem. 1987, 59, 541-543
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Indirect Photometric Chromatography of Cations and Amines on a Polymer-Based Column David L. McAleese Analytical Research, BASF Corporation Chemicals Diuision, Wyandotte, Michigan 48192 Indirect photometric chromatography (IPC) is a rapidly growing technique for the determination of “light-transparent”
sample ions. IPC was developed as a single column alternative to the conventional dual column eluent-suppressed ion chromatographic (IC) technique (1,2).IPC basically involves the selective displacement of transparent sample ions from an ion-exchange column by light-absorbing mobile phase ions. The sample ions are detected as inverted absorbance peaks on the recorder plot. Only a few studies have been generated on the determination of cations and amines by IPC. Silica-based cationexchange columns have been utilized for the separation of inorganic cations, alkylamines, and alkyl quaternary ammonium compounds (1,3-5). Polymer-based cation-exchange columns (packed by the analyst) have been used for the separation of inorganic cations and alkylamines (1,6). The methods have usually employed an aqueous copper sulfate solution as the mobile phase and UV detection in the 220250-nm range. The methods are less sensitive than IC, and the chromatograms generated by the copper(II) displacing ions are essentially no different than IC separations in terms of elution order and peak resolution. For this reason, these IPC methods are less attractive than conventional IC techniques. Mobile phases containing aromatic quaternary ammonium displacing ions have been used for the separation of alkylamines and alkyl quaternary ammonium compounds on a silica-based column; however, the column limits the IPC technique to a useful pH range of 2-7 for eluents and samples. The purpose of the present investigation is to demonstrate the utility of an improved IPC procedure for the determination of inorganic cations, alkylamines, cycloalkylamines, and alkyl quaternary ammonium compounds. The method employs a simple mobile phase and a polymer-based cation-exchange column that is commercially available and not restricted by pH considerations. Since the column is manufactured for eluent-suppressed IC use, this study presents a unique opportunity to compare separations by both techniques on the same column.
EXPERIMENTAL SECTION Chemicals. Benzyltrimethylammoniumchloride,hydrochloric acid, and tetramethylammonium hydroxide were obtained from Aldrich (Milwaukee, WI), Mallinckrodt (Paris, KY), and Eastman Kodak (Rochester,NY), respectively. AU chemicalswere reagent grade. Eluenta and standards were prepared with deionized water from a Millipore Milli-Q water purification system. Benzyltrimethylammonium chloride solutions were filtered twice through a 0.45-pm filter and degassed. Instrumentation. The liquid chromatography system consisted of a Dionex Model 2020i ion chromatographequipped with a dual piston pump, a 100-pL injection loop, a Bio-Rad Model CM-8 conductivity detector for IC measurements, a Waters Lambda-Max Model 480 variable wavelength UV detector for IPC measurements, a Hewlett-Packard 3357 Laboratory Automation System for peak integration, and a Hewlett-Packard 7470A printer/ plotter. Chromatographic Conditions. A Dionex HPIC-CG2 cation-exchange guard column (4.6X 50 mm) and a Dionex HPICCS2 cation-exchange separator column (4.6 X 250 mm) were employed for IPC determinations. These columns contain a surface-sulfonatedstyrene-divinylbenzenecopolymer resin. The eluent consisted of an aqueous 0.0075 M benzyltrimethyl0003-2700/87/0359-054 1$015010
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Flgure 1. (A) 1% separation of the following: (1) sodium, 19.9 ppm; (2) potassium, 36.2 ppm; (3) ammonium, 16.8 ppm. (9)IC separation of the following: (1) sodium, 2.5 ppm; (2) potassium, 5.3 ppm; (3) ammonium, 10 ppm; (4) lithium, 0.50 ppm.
ammonium chloride solution which was pumped at a flow rate of 0.5 mL/min. The UV absorbance detector was set at 275 nm and 0.05 absorbance unit full scale (AUFS). A Dionex HPIC-CG2 guard column, a Dionex HPIC-CS2 separator column, and a Dionex cation fiber suppressor column were used for IC analyses. The eluent consisted of an aqueous 0.010 M hydrochloric acid solution which was pumped at a flow rate of 1.0 mL/min. The suppressor column regenerant consisted of an aqueous0.040 M tetramethylammnonium hydroxide solution which was gravity fed at a flow rate of 4 mL/min. The conductivity detector was set on the 20-ps scale.
RESULTS AND DISCUSSION The separation of monovalent inorganic cations by IPC and IC is illustrated in Figure 1. For the IPC determination, two separator columns were connected in series to achieve the desired base line separation. Despite the extra column, the IC technique produced better separation of the cations with just one column. Lithium coeluted with ammonium by IPC (not shown), but was well separated from the other cations by IC. A comparison of the chromatograms also shows a reversal in the elution order of potassium and ammonium. The separation of alkylamines by IPC and IC is shown in Figure 2 . Both mobile phases were low enough in pH that the amines eluted in their protonated form. Once again, the ion chromatogram displayed better peak resolution, but both 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
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F W e 2. (A) IPC separation of the following: (1) trimethylamine, 38.5 ppm; (2) dimethylamine, 31.5 ppm; (3) methylamine, 21.8 ppm. (B) I C separatlon of the following: (1) trimethylamine, 29 ppm; (2) dimethylamine, 5.0 ppm; (3) methylamine, 1.6 ppm.
techniques produced base line peak separation with only one column. A comparison of the chromatograms shows that the order of elution was completely reversed for these compounds. The analogous situation was observed in the determination of ethylamine, diethylamine, and triethylamine. The reversal can be attributed to differences in the solvation of the amines in the mobile phases employed. It was interesting to find that trimethylamine, dimethylamine, and methylamine were base line separated by IPC on just two guard columns with an aqueous mobile phase consisting of 1mM benzyltrimethylammoniumchloride and 20% v/v methanol. The effect of methanol on column selectivity has been noted previously for IC determinations (7). In the IC mode, however, the presence of methanol in the mobile phase adversely affects the separation of these amines by preferentially solvating the later eluting compounds. The use of methanol or other organic solvents in the IPC mode can be exploited since the amines normally elute in the order of decreasing molecular complexity for each homologous series. The separation of alkanolamines and cycloalkylaminesby IC and IPC is illustrated in Figure 3 . In this case, the IPC technique produced by far the better separation and faster analysis time. A reversal in the elution order of the piperidinium ions was also observed. The detection limits of the various cations and amines by each technique are shown in Table I . These values were based on a 100-pLinjection and a signal-to-base-line noise ratio of 3. An interesting feature of IPC is the universal nature of its detection mode. The peak area of an ion is not dependent on the ion injected but only on its molar amount (1). This is reflected in the roughly equivalent nmol/mL detection limits listed for these ions. It is evident that for most low molecular weight cations and amines, eluent-suppressed ion chromatography is more sensitive than IPC. However, the IPC
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Flgure 3. (A) I C separation of the following: (1) ethanolamine, 30.1 ppm; (2) diethanolamine, 60.5 ppm; (3) piperidlne, 196 ppm; (4) N methylpiperidine, 200 ppm; (5) cyclohexylamine, 299 ppm. (B) 1% separation of the followlng: (1) ethanolamine, 51 ppm; (2) diethanolamine, 98 ppm; (3) plperkllne, 184 ppm; (4) N-methylpiperiine, 175 ppm; (5) cyclohexylamine, 314 ppm.
Table I. Detection Limits of Cations and Amines'
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und = not determined. technique is definitely favored for the separation of the more structurally complex amines. The utility of IPC is further demonstrated in the analysis of a commercial product containing a proprietary alkylamine and alkyl quaternary ammonium compound. The alkyl quaternary ammonium compound serves as the active agent of the aqueous-based material, and the alkylamine is present as an impurity. As opposed to ion chromatography, the IPC technique was successfully used for the assay of both compounds in product samples. Since some of the quality control laboratories were only equipped with fixed wavelength UV detectors, the chromatographic conditions given in the experimental section were modified so that a fixed wavelength UV detector could be accommodated. The concentration of benzyltrimethylammonium chloride in the mobile phase was reduced to 0.003 M and the eluent flow rate was increased
Anal. Chem. 1907, 59, 543-544
to 1.5 mL/min. The compounds were detected by indirect absorbance at 254 nm. With these conditions, both compounds eluted within 10 min after injection and were base line resolved. The sample was simply diluted in deionized water to an appropriate level prior to injection. The 3 mM benzyltrimethylammonium chloride concentration satisfied the “optimum absorbance” requirement of IPC by producing a base line absorbance of 0.75. It is known from classical spectrophotometry that the most accurate measurements are obtained when the absorbance is in the 0.2-0.8 range. For this reason, it is important to the accuracy of IPC to monitor the eluent under conditions where its absorbance is in this range (1). The measured absorbance of the 7.5 mM benzyltrimethylammonium chloride solution used for the other work in this study was 0.21 at 275 nm, but was much greater than 1 at 254 nm. The alkyl quaternary ammonium compound and the alkylamine were quantitated by the external standard method. The peak areas of the standards were linear over the range of concentrations investigated, 10-3000 ppm. The precision of the IPC method was evaluated with 10 separate determinations of the alkyl quaternary ammonium compound in a product sample. The relative standard deviation of the results was h0.6%. The accuracy of the method was determined by the analysis of product samples containing known amounts of the alkyl quaternary ammonium compound. The experimentally determined concentrations differed from the known levels by less than a relative deviation of *1.5%. The method has been in daily use in research and quality control laboratories for the past 1.5 years without failure. It has replaced tedious gravimetric and titration procedures, both of which were susceptible to interferences. This IPC method is attractive because of its simplicity, ease of operation, and lack of restrictions. Unlike the copper(I1) displacing ions, the mobile phase ions are not subject to precipitation as a function of pH. The eluents, samples, and
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standards require no pH adjustment since the polymer-based column is stable over the entire pH range of 0-14. For silica-based columns, mobile phases containing aromatic quaternary ammonium compounds must be adjusted to an acidic pH in order to preserve the integrity of the packing material. It should be noted, however, that the presence of hydronium ions in these mobile phases is undesirable because the hydronium ions can compete with the aromatic quaternary ammonium ions for exchange sites on the column. Separations could be adversely affected since these displacing ions do not elute compounds in the same order. It was found with the polymer-based column that mobile phases containing similar levels of hydronium and benzyltrimethylammonium ions produce very unstable base lines which are not suitable for chromatographic determinations. In conclusion, this IPC method yields separations which differ from IC and other IPC separations in both the order of elution and peak resolution. Furthermore, separations can be improved by the addition of organic solvents to the mobile phase. What the IPC method lacks in sensitivity is compensated for by separations that are not possible by conventional eluent-suppressed IC techniques. In this way, IPC is shown to serve as a complementary technique to IC and not just a single column alternative.
LITERATURE CITED (1) (2) (3) (4) (5)
Small, Hamish; Miller, Theodore E. Anal. Chem. 1982, 54, 462-469. Cochrane, R. A.; Hlllman, D. E. J. Chromatogr. 1982, 241, 392-394. Larson, J. R.; Pfelffer, C. D. Anal. Chem. 1983, 55, 393-396. Larson, J. R.; Pfeiffer, C. D. J . Chromafogr. 1983, 259, 519-521. Iskandaranl, Ziad; Miller, Theodore E. Anal. Chem. 1985, 5 7 , 1591-1594. (6) Sithole, Bishop E.; Guy, Robert D. Analysf (London) 1986, 7 1 1 , 395-397. (7) Buechele, R . C.; Reutter, D. J. J . Chromatogr. 1982, 240, 502-507.
RECEIVED for review July 29,1986. Accepted October 30,1986.
Liquid Chromatographic Assay of Dithionlte and Thiosulfate K. J. Stutts Central Research, The Dow Chemical Company, Midland, Michigan 48674 Thiosulfate is a known degradation product of sodium dithionite (a bleaching agent) and is detrimental in the paper processing industry. Determination of thiosulfate in dithionite streams is important because of the corrosive nature of this material to many metals including some types of stainless steel. The generally accepted upper limit of sodium thiosulfate concentration in the aforementioned industry is about 4% (w/w) with respect to sodium dithionite ( I ) . The bleaching solutions are typically 1-10% (w/w) sodium dithionite. Thus, stringent limits of detection (about 40 ppm for sodium thiosulfate should suffice for dithionite solutions) are not absolutely necessary. Several assays have been developed for thiosulfate and dithionite, but all of these have their shortcomings. Sodium dithionite of commerce is typically 85% (w/w) and purification by recrystallization is exceedingly difficult. Extreme air sensitivity requires great precautions in sample preparation. For these reasons, it has been preferable that the method of assay be absolute rather than by a calibration curve or an internal standard. This has lead to the widespread acceptance of titrimetric methods. Classical titrimetry (2-7) 0003-2700/87/0359-054380 1.50/0
can give quite reproducible results for dithionite and methods are available for thiosulfate (and sulfite) in the presence of dithionite. However, one titrimetric procedure for each species is necessary. Although tedious, this appears to be the industry standard and the most reliable method available. Samples of sodium dithionite below lo00 ppm are difficult to quantitate by these methods. Polarographic methods me well-known (8-13) for oxo-sulfur species. However, simultaneous quantitation of SZOs2-and Sz042-is extremely difficult because of sulfide interference and the dynamic equilibrium which exists between dithionite and the radical anion of sulfur dioxide (9, 14-16). Raman analyses (17,181 have been demonstrated but the cited detection limits are poor (>350 ppm). Ion chromatographic assays for thiosulfate and sulfite have been published (e.g., ref 19). However, chromatographicassay of dithionite has not appeared in the literature, probably because of its instability to oxidation. I have found that a satisfactory method for quantitative analysis of dithionite and thiosulfate can be accomplished via an offshoot of ion chromatography which uses UV detection. 0 1987 American Chemical Society
