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ANALYTICAL TECHNOLOGIESMeasuring anions and ammonium in bases and acids with ion chromatography(Second in a series)Jian-Ge Chen and ManLi Wu, Olin Research Center Because impurities in process chemicals can adversely affect semiconductor device surfaces, the ability to identify and measure such contaminants is important to the industry.1 As reported in the first article in this series, our laboratory recently developed a reliable and sensitive method for analyzing anionic contaminants at <10-ppb levels in 49% hydrofluoric acid using two-dimensional ion chromatography.2 However, the analysis of trace cationic impurities in concentrated acids and some anionic impurities in concentrated bases has been an especially difficult analytical challenge. Direct analysis of trace anions in concentrated bases has been limited by hydroxide concentration. When the behavior of anions in the presence of very large amounts of hydroxide was investigated, it was found that the analyte peaks and separation were changed as a result of hydroxide anion elution from the sample matrix.3 A similar problem is expected when analyzing cations in strong acids, where the hydronium ion matrix can interfere with the analysis. It has been reported elsewhere that low- to 30-ppb-level detection of trace anions and cations in concentrated bases and acids can be achieved with electrolytic sample pretreatment.4 A high-capacity electrochemical membrane suppressor electrolyzes water to create the acid or base required for neutralization of the concentrated base or acid. However, the literature indicated that high amounts of Cl— and SO42— can bleed out from the sulfonated membrane used in the neutralization unit and contribute to the background, which can increase the detection limits and affect the accuracy of trace-level analyses. It was also reported that, because of the diffusion of ammonia across the membrane during neutralization, the recovery of ammonium ions was only about 50%. In addition, the neutralizer unit could not be used with hydrochloric acid because chloride can be oxidized on the electrode surface, leading to the destruction of the neutralizer. This article describes two new ion chromatographic procedures for the parts-per-billion-level analysis of anions in concentrated bases and cationic ammonium in concentrated acids, respectively. Optimization of the techniques is described, along with the instruments' capabilities. Analyzing Anions in Concentrated Bases The ion chromatographic procedure developed for analyzing anions can measure chloride, nitrate, sulfate, and phosphate in 30% ammonium hydroxide (NH4OH) and 25% tetramethylammonium hydroxide (TMAOH) at levels below 10 ppb. The procedure eliminates OH— using an AutoNeutralization pretreatment unit (ANPU; Dionex, Sunnyvale, CA) prior to analysis by an analytical separation unit. This design enables analysis of trace anions with minimal matrix interference. In addition, the hydroxide-containing samples can be recycled to the ANPU until the desired amount of matrix is obtained. In order to minimize the system blank and to achieve reproducible and sensitive analyses, the system was optimized by examining such factors as column selection, sample injection volume, number of neutralization cycles, and size of the fraction to be directed from the ANPU to the analytical separation unit. With the optimized conditions, we were able to achieve detection limits of 5 ppb for chloride and sulfate, and 1 ppb for nitrate and phosphate. Figure 1 is a block diagram of the ion chromatographic system with the ANPU. The concentrated base is introduced directly into the pretreatment unit, where hydroxide ions are neutralized by electrochemically generated hydronium ions. The hydronium ions migrate across a cation-exchange membrane and combine with the hydroxide ions to form water. The water effluent that contains anions of interest is then fractionated and introduced onto a concentrator column in the analytical separation unit using an automated switching valve. Finally, the anions in the concentrator column are backflushed by a sodium hydroxide (NaOH) eluent onto a separator column for analysis.
Figure 1: Block diagram of the neutralization/ion chromatographic system for analysis of trace anions in concentrated bases. In the sample pretreatment unit, a guard column is used to remove any ionic contaminants from the water-mobile phase. After the sample is introduced into the sample loop, a valve is switched to allow it to enter the anion self-regenerating neutralizer (SRN). A six-port valve in the analytical separation unit directs the effluent from the SRN to the concentrator column. After an adequate collection time, the valve is switched to allow for the mobile phase to flush the retained anions into the analytical separation unit. During the system optimization project, all containers, flasks, and autosampler vials were pretreated before their first usage. Each flask was soaked overnight and rinsed thoroughly prior to injection. Samples were prepared and transferred with minimal operator contact of plasticware, glassware, and pipettes. All standard solutions used for the external calibration method for quantitation were prepared from 200-ppm Alltech standards and diluted with ultrapure Milli-Q water in 50-ml flasks. The concentrations of all standards ranged from 5 to 200 ppb.
Column Selection. As indicated in Figure 1, there are three types of columns in the analytical separation unit: a concentrator column, a guard column, and the analytical column. One criterion for choosing the concentrator column was that its maximum pressure had to be
Figure 2: Chromatograms of four anion standards obtained using different sets and gradient conditions: (a) concentrator A and analytical column 1; (b) concentrator A and analytical column 2; (c) concentrator B and analytical column 2; and (d) concentrator B and analytical column 2 optimized. Separation Optimization. Figure 2c also indicated that higher separation efficiency can sharpen the chromatogram peaks and thereby lessen the system's dependence on injection volume in achieving the desired sensitivity. When the chromatographic conditions were optimized, the detection limits of chloride, nitrate, and sulfate improved by a factor of two and the detection limit of phosphate improved by a factor of four, as seen in Figure 2d. ANPU Cleaning. According to the ANPU supplier's application note, the sulfonated membrane in the autoneutralizer constantly bleeds out approximately 6 ppb of chloride and 52 ppb of sulfate.5 This makes it very difficult to analyze 10-ppb levels of chloride and sulfate because of the large system blank. To overcome this difficulty, during this study the SRN was flushed with NaOH for several hours, followed by overnight flushing with deionized (DI) water to remove any residual anionic contaminants in the membrane prior to analysis. Sample-Loop Selection. To achieve the goal of analyzing all four anions at the <10-ppb level, the hydroxide matrix had to be removed with a minimum number of neutralization cycles in order to limit the interference caused by the system blank. For the analysis of highly responsive chloride and sulfate, small injection volumes were adequate to achieve low-parts-per-billion-level analysis. However, because of their lower conductivity response, larger injection volumes were needed for nitrate and phosphate. Although increasing the injection volume can provide a more sensitive analysis, it can also require a higher number of neutralization cycles. Therefore, we ran experiments to determine the optimal sample-loop volume. Sample loops of 150, 200, and 500 µL were tested, and it was found that the ANPU could neutralize up to 200 µL of 30% NH4OH and 500 µL of 25% TMAOH in two neutralization cycles. A 150-µL sample loop was chosen as optimal because it was the minimum volume that would give a linear dynamic anion range of 5—200 ppb using the optimized analytical separation system. Fractionation Volume. Since anion peaks are broadened after a sample passes through the ANPU, the peak profile was carefully monitored in order to determine the optimal fractionation volume for introduction into the concentrator column. To achieve the highest possible anion response as well as the lowest chloride and sulfate blanks, the optimal fraction would contain the majority of the anions, yet be of minimum volume. A solution of 200-ppm F— was used as a probe to monitor the peak profile, yielding the results seen in Figure 3. The fractionation volume between the two arrows in the figure consists of about 95% of the total peak area and was chosen as optimal. The total volume of this fraction was found to be significantly less than the 4 ml suggested in the supplier's application note for a 100-µL injection volume.5 Figure 4 shows a chromatogram of 30% NH4OH spiked with an anion standard mixture, which was obtained using the optimized conditions. The peak seen before the nitrate peak is carbonate, which is commonly found in bases.
Figure 3: Elution profile of 200-ppm F— after passing the ANPU.
Figure 4: Chromatogram of 30% NH4OH spiked with an anion standard mixture. In order to determine the levels of system blank peaks, a sample of Milli-Q water was injected. Using an optimized method, we found the system blank to contain only 2.7 ± 0.7 ppb of chloride (n = 18) and 3.6 ± 0.5 ppb of sulfate (n = 18). This is significantly lower than those described in the supplier's application note (6 and 52 ppb, respectively, for chloride and sulfate).5
Table I: Comparison of the standard addition method (SAM) and external calibration method (ECM) for the analysis of various anions in concentrated NH4OH and TMAOH. Anion Quantitation Technique. The external calibration method (ECM) is normally used for anion quantitation in ionic chromatography when the matrix is relatively clean. The attractive feature of ECM is its shorter analysis time when compared with the standard addition method (SAM). Table I compares test results for five samples that were analyzed using the two methods. As the table indicates, in most cases the results are in good agreement, although some ECM results are significantly higher. SAM was chosen for use with the ANPU/chromatograph system because of its better average spike recovery at the 10-ppb levels of all anions in 30% NH4OH and 25% TMAOH. As shown in Table II, the spike recoveries using SAM for chloride, sulfate, nitrate, and phosphate were 103.8 ± 4.5% (relative standard deviation), 101 ± 5.6%, 99.4 ± 4.6%, and 106.4 ± 6.1%, respectively.
Table II: Average spike recovery (n = 6) of anions at the 10-ppb level in 30% NH4OH and 25% TMAOH. Analyzing Cationic Ammonium in Concentrated Acids Since trace cationic impurities other than ammonium can be analyzed successfully by inductively coupled plasma— mass spectrometry (ICP-MS), our goal was to develop a sensitive ion chromatographic approach for the analysis of ammonium in such concentrated acids as 37% hydrochloric acid (HCl) and 98% sulfuric acid (H2SO4).6 The chromatographic procedure that resulted from our research employs two on-line neutralization columns to eliminate the matrix prior to the analytical separation. The difficulties of this approach are the regeneration of the neutralization columns after each sample injection and the separation of ammonium peaks from those of sodium contaminants released during column regeneration. In order to minimize these problems, factors such as injection volume, separation conditions, the neutralization column regeneration, and quantitation method were optimized. Under the optimized conditions, we achieved a detection limit for ammonium of 5 ppb. Figure 5 is a block diagram of the neutralization/ion chromatographic system for the analysis of ammonium in concentrated acids. After half-diluted samples are injected into the first part of the system, the hydronium ions are neutralized in two neutralization columns. Unlike the autoneutralizer described above, these neutralization columns will not allow ammonium to diffuse across the membrane. Following neutralization, the sample is directed onto a cation concentrator column before being introduced into the second part of the system for analytical separation. While the ammonium ions are being flushed into the analytical column, the neutralization and cation concentrator columns are regenerated to their original forms for the next run.
Figure 5: Block diagram of the neutralization/ion chromatographic system for analysis of trace ammonium in concentrated acids. During system optimization, six H2SO4 and two HCl samples were analyzed. Standards were prepared from 200-ppm Alltech ammonium solution using 50-ml flasks, and both the HCl and H2SO4 samples were half diluted with DI water in 25-ml Teflon containers. The ideal way to minimize sample handling and maximize analytical sensitivity would be to inject concentrated acids directly. However, there was severe corrosion of the autosampler when that was tried. The corrosion problem was greatly reduced with the half-diluted samples. Injection Volume. To ensure complete acid neutralization with minimum sample dilution, a conductivity detector was used to measure the matrix concentration of the effluent from the neutralization columns at injection volumes of 150—500 µL. It was found that at injection volumes above 175 µL the capacity of the two neutralization columns was exceeded. Thus, 175 µL was chosen as the optimal injection volume for analysis of half-diluted hydrochloric and sulfuric acids. Analytical Separation System Optimization. During regeneration, the neutralization columns can become contaminated with ionic sodium. Because the ammonium peak on a chromatogram is very close to that of sodium, it is very important to flush the neutralization system thoroughly with water before a new sample is injected. This is clearly shown in Figure 6. In the chromatogram at the top of the figure (6a), which was obtained after a 20-ml water rinse of the neutralization columns, the ammonium peak, where the arrow is pointed, is completely obscured by the presence of the large sodium peak. In contrast, in the middle chromatogram (6b), taken after a 60-ml water rinse, the ammonium peak is clearly detectable. The resolution between the sodium and ammonium peaks was further improved (Figure 6c) by using a different analytical column in conjunction with optimized mobile-phase conditions.
Figure 6: Comparison of chromatograms for ammonium analysis in 48% H2SO4using different amounts of rinsewater for neutralization columns and different column sets and mobile-phase conditions: (a) with a 20-ml water rinse, (b) with a 60-ml rinse, and (c) under optimized conditions. Arrows indicate ammonium peaks.
Figure 7: Representative chromatograms of ammonium analysis in 49% H2SO4 and 19% HCl. Arrows indicate the ammonium peaks. Ammonium Quantitation Technique. As was done for the anion analysis method, both ECM and SAM calibration approaches were studied. As the data in Table III indicate, the results for the two methods were in good agreement; therefore, either method can be used for the analysis. Using external calibration, the spike recovery in a total of six acid samples at the 40-ppb level was 108 ± 8.4%. Two representative chromatograms for the analysis of ammonium in concentrated HCl and H2SO4 are shown in Figure 7.
Table III: Comparison of the standard addition method (SAM) and external calibration method (ECM) for the quantification of 40-ppb ammonium in 98% H2SO4 and 37% HCl, including spike recovery percentages. Conclusion An autoneutralization/ion chromatographic procedure can achieve direct analysis of anionic contaminants in concentrated 30% NH4OH and 25% TMAOH with minimum sample preparation. Unlike with other, previously reported procedures, blank correction after system optimization is not required. The limits of detection, therefore, can reach the low- or sub-parts-per-billion range using this approach. Another neutralization/ion chromatographic system can be used for the analysis of low-parts-per-billion-level ammonium in 37% HCl and 98% H2SO4. By combining two neutralization columns and a concentrator column, the procedure achieves sensitive and reliable measurements. In addition, the columns in the neutralization unit can be regenerated automatically for continuous sample analysis. Acknowledgments The authors would like to acknowledge helpful comments from Pat Turley and Sonia Oberson at Olin's central analytical department. Olin Microelectronic Materials provided funding to support this project. References 1. Schafer H, and Budde KJ, "Study of Contamination Mechanisms at Silicon Surfaces during Wet Chemical Processes," presented at the International Ion Chromatography Symposium, Baltimore, September 1993. 2. Chen JG, and Wu ML, "Using Two-Dimensional Ion Chromatography to Measure Contaminants in Ultrapure Chemicals," MICRO, 15(1):31—37, 1997. 3. Smith ES, "Determination of Chloride in Sodium Hydroxide and in Sulfuric Acid by Ion Chromatography," Analytical Chemistry, 55:1427—1429, 1983. 4. Siriraks A, and Stillian J, "Determination of Anions and Cations in Concentrated Bases and Acids by Ion Chromatography—Electrolytic Sample Pretreatment," Journal of Chromatography, 640:151—160, 1993. 5. "Determination of Trace Anions in Concentrated Bases using AutoNeutralization Pretreatment/Ion Chromatography," Application Note #93, Sunnyvale, CA, Dionex, August 1994. 6. Volosin M, "Quality Control of High Purity Process Chemicals by ICP/MS," Spectroscopy, 7(4):44—47, 1992. Jian-Ge Chen, PhD, is a research scientist in the pharmaceutical R&D department of DuPont Merck (Wilmington, DE), with responsibility for developing analytical methods for evaluating novel pharmaceutics and supervising staff scientists. Prior to taking this position, he was a senior research chemist in the central analytical department at Olin Corp. (Cheshire, CT) specializing in trace analysis using microseparations, including multidimensional chromatography. He received his BS in chemistry (1990) from the State University of New York at Stony Brook and his PhD in analytical chemistry (1995) from the University of Pittsburgh. His publications include a review article, a book chapter, and more than 10 research papers. ManLi Wu, PhD, is a consulting scientist and supervisor for the chromatography group in the central analytical department of Olin Corp in Cheshire, CT. Her field of specialization is analysis of industrial chemicals and trace analysis of impurities using high-performance liquid chromatography, gas chromatography, ion chromatography, and other advanced techniques. Recently, she has been involved with GLP studies of biocides in the product chemistry and environmental fate areas. Wu received her BS from Tamkang University and MS from Tsing Hwa University, both in Taiwan, and her PhD in analytical chemistry from the University of Connecticut (1984). (Wu can be reached at 203/271-4298.)
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120 psi, which is the pressure rating of the neutralizer. The low-pressure concentrator suggested by the ANPU supplier had a maximum rating of about 80 psi; however, it was only available in a 4-mm format. In order to study the effect of varying diameters on column compatibility, we examined three different combinations of two 4-mm concentrators and two 2-mm analytical columns. The corresponding chromatograms for these three column combinations are shown in Figure 2(a—c). Based on these chromatograms, we concluded that not only is the column diameter an important criterion for choosing an appropriate system, the column packings are as well. As a result, the column combination that yielded the chromatogram shown in Figure 2c was chosen as optimal.
