All chemicals were of analytical grades. Sodium hydroxide (50% solution, low carbonate), sodium bicarbonate, and sodium carbonate were from J.J Baker. Methanol and ethanol were of chromatographic grade and obtained from Fortrum. 18.2 MΩ water was used in all sample preparation. OnGuard-P and OnGuard-RP cartridges were obtained from Dionex. Certified anions standards were obtained also from Dionex.

Dionex DX 500 ion chromatograph with GP 50 gradient pump and ED 40 electrochemical detector with conductivity mode was used for the anion analysis. The instrument was configured with autosampler AS3500. AS11-HC 2 mm and AS14-HC 2 mm were used as analytical columns. Deionized 18.2 MΩ water were generated from Milli-Q system (Millipore).

Samples were introduced into the autosampler vials after passing through 0.45 µm filter and/or the OnGuard cartridges prior to analysis. The instrument scheduled with the appropriate method and data were processed and controlled by PEAKNET 5.1 supplied by Dionex. The flush solvent bottle of the autosampler was filled either with water, 10% methanol (by volume), or 50% methanol (by volume).

Methanol interference experiments were conducted using a DX-500 ion chromatograph using an AS11-HC 2 mm analytical column. The analyte anions were passed through AG11-HC 2 mm guard column before being introduced to the AS11-HC 2 mm analytical column. The eluates were then sent through a self-generating suppressor (ASRS-II) before being detected by the conductivity cell. The ionic eluent used in these experiments was 30 mM sodium hydroxide with a flow rate of 0.38 ml/min. On the other hand; the OnGuard interference was studied under the same conditions as used above except the AG14-HC 2 mm and AS14-HC 2 mm are used as the guard and analytical columns, respectively. The ionic eluent in this case was a mixture of sodium carbonate (3.5 mM) and sodium bicarbonate (1.0 mM) with a flow rate of 0.75 ml/min.

The injection of the sample in IC using a flush-solvent bottle for the autosampler that contains 100% water results in air bubbles in the injection valve and hence affects the sample injection volume leading to inaccuracy in the determination of ions. To eliminate this problem, water was replaced by 50% methanol, which has been used for many years in a number of laboratories. However, it was found in the current study that methanol interferes with the determination of anions especially with sulfate on AS11-HC 2 mm and interferes with fluoride on AS14-HC 2 mm analytical columns. Figure 1 displays the chromatogrms of a Dionex-standard containing fluoride, chloride, sulfate, nitrate and phosphate analyzed on the AS11-HC 2 mm analytical column, first using deionized water in the flush-solvent bottle (Chromatogram A) and then using 50% methanol (by volume) in the flushsolvent bottle (Chromatogram B). Inspection of

these chromatograms reveals that the baseline near sulfate peak is ill-defined (not stable) when 50% methanol is used resulting in overestimation of the sulfate concentration (Chromatogram B). In the process of identifying the source of the interference, and quantifying the overestimation in sulfate concentration, two sets of experiments were performed. First, different solutions of methanol (with different volume fractions ranging from 1% and 10%) were analyzed on the Dionex IC using the same chromatographic conditions used for analysis of the ions but using 100% of deionized water (18.2 MΩ) in the flush-solvent bottle. Results which are shown in Figure 2(a) indicates that methanol solutions (1% to 10% by volume) injected onto the IC have a peak with a retention time of 3.9 minutes which interferes with sulfate ion peak with retention time of 3.4 minutes under the same experimental conditions mentioned above. Results have also showed direct relationship between conductivity and volume fraction of methanol in the flush-solvent bottle (Figure 2(b)). A linear correlation was obtained until methanol concentration reaches 10% (by volume), Figure 2(b). These results explain the broadening of sulfate peak resulting from interference with methanol peak that is produced when traces of methanol in the flush-solvent bottle are injected along with the samples.

The second set of experiments which are performed to quantify the overestimation in sulfate concentration is analysis of deionized water samples (18.2 MΩ) using 0% methanol (Figure 3(a)), 10% methanol (Figure 3(b)), and 50% methanol (Figure 3(c)) in the flush-solvent bottle. Inspection of Figure 3 reveals that the overestimation in sulfate concentration when 50% methanol (by volume) used is about 1.8 ppm which is about 18% of the 10-ppm sulfate standard used. Figure 3(b) indicates low interference when 10% methanol is used; where the overestimation in sulfate concentration is only 0.1 ppm. In addition, no air bubbles were noticed in the injector syringe when using 10% methanol which minimizes the error resulting from inaccurate injection volume conesquential to using just water in the flush-solvent bottle.

To evaluate the interference resulting from OnGuard-RP and OnGuard-P cartridges, a Dionex standard containing fluoride, chloride, bromide, nitrate, phosphate and sulfate was analyzed on an AS14-HC 2mm analytical column with and without using OnGuard cartridges prior to analysis (Figure 4). Figure 4(a) demonstrates the chromatogram of the Dionex-standard when OnGuard-RP cartridge was used, while Figure 4(b) illustrates the chromatogram of the same standard without treatment prior to analysis, and Figure 4(c) represent the chromatogram using OnGuard-P cartridge. When OnGuard-RP cartridge was used, the underestimation in fluoride, bromide, nitrate, phosphate, and sulfate concentrations, as compared to the values without pretreatment, were found to be 0.45, 1.85, 1.93, 1.83, and 4.1 ppm, respectively. This is equal to 40.0%, 26.0%, 22.8%, 26.8%, and 18.0% of a standard containing 0.75, 2.5, 2.5, 2.5, and 5.0 ppm of fluoride, bromide, nitrate, phosphate, and sulfate, respectively. On the other hand, it was found that chloride was overestimated by 0.25 ppm in a standard containing 1ppm of chloride. When OnGuard-P cartridge was used, the underestimation in fluoride, chloride, bromide, nitrate, phosphate, and sulfate concentration was found to be more pronounced, as it is seen in Figure 4(c). The underestimation reached values of 0.3, 0.6, 0.45, 0.32, 1.37, and 3.05 of a standard containing 0.75, 1.0, 2.5, 2.5, 2.5, and 5.0 ppm of fluoride, chloride, bromide, nitrate, phosphate, and sulfate, respectively which is equal to 60.0%, 40.0%, 82.0%, 87.2%, 45.2%, and 39.0%. It is worth mentioning here that these experiments were conducted with 10% methanol in the flush-solvent bottle.