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Analysis of seven common anions according to ISO 10304-1

Figure 2 shows an overlay of six chromatograms presenting the separation of seven common anions. Very low % RSD for retention time (≤ 0.04 %) and peak area (≤ 0.45 %) for all anions demonstrate the high precision of the analysis system. The low-void-volume ICDS-40A suppressor reduces peak dispersion and improves the separation of fluoride from the water dip peak. The requirement of the ISO standard for resolution R (≥ 1.3) was met, calculated according to US Pharmacopeia (USP). This method is recommended for separation of seven common anions within relative short analysis times.

Linearity
The linear working concentration range used by conductivity detection was from 0.1 to 5 mg/L with the exception of bromide and nitrite, measured from 0.05 to 5 mg/L. The coefficient of determination R² was ≥ 0.9993 for all anions.

Sample analysis
The concentration of analyzed anions varied in samples, but all of them showed high contrast in concentration between phosphate, sulfate and partly nitrate to other anions. Figure 3 shows a representative example for analysis of drinking water collected from a water dispenser. The samples were therefore measured undiluted and diluted (1/40, v/v) to quantify all target compounds in the working concentration range. The content of highly concentrated phosphate, sulfate and partly nitrate was calculated from diluted samples. The concentration of residual anions was calculated from undiluted samples. Table 1 presents concentration of the anions in the drinking water samples analyzed.

Analysis of ten anions according to EPA 300.1

Figure 4 shows the separation results of ten anions. This method is recommended for analysis of three disinfection by-products, chlorite, bromate and chlorate, in the presence of seven common anions. All anions were eluted in less than 27.5 minutes. The material of the column used for this method had lower particle size compared to the column material applied for analysis of the seven anions discussed previously.

Although elution time is long, the advantage of this method is the baseline separation of ten analytes. Furthermore, the resolution of bromide and chlorate is higher even with larger injection volume. This method will also be useful for trace analysis of chlorate and bromide.

Linearity and Method Detection Limit (MDL)

To evaluate the method and overall system performance, linearity, method detection limits and reproducibility were also determined. MDL investigation was performed following the procedure described in EPA Method 300.1 by making seven replicate injections of MDL standards.

MDL standards were prepared by fortifying the blank with anion standards to a concentration 3 to 5 times that of the instrument detection limit. MDL was calculated as (t) x (S), where t is student’s t value for 99 % confidence level (t = 3.14 for seven replicates) and S is standard deviation from seven injections.

The results of linearity for calibration curves show that the system produces linear response with coefficient of determination R² greater than 0.999 for all ten anions in a broader concentration range. The calculated MDL were at low ppb range from 1.3 µg/L (fluoride) to 12.6 µg/L (chloride).

Reproducibility and Accuracy
This method also showed excellent reproducibility for retention time (< 0.7 % RSD) and peak area (< 0.42 % RSD). Table 2 shows that acceptable recoveries were obtained for all inorganic ions in three water matrices using both methods.