Technical Note – Suppressor Study IONUS Ion Chromatograph

Our ion chromatograph is designed with high flexibility allowing it to operate with  common brands of chromatography columns without restriction.

Recently, we have added a new suppressor to our product portfolio which like its predecessor operates using chemical regeneration. The following results present partially a comparative performance evaluation of various chromatography columns tested with the previous and the new suppressor with a particular focus on assessing the capabilities of the newly introduced suppressor.

Materials and methods

To prepare the analytical solutions precise amounts of the target anions were weighed to produce an individual 1000 mg/Kg standard when dissolved in ultra pure water. These known masses were then dissolved in ultrapure water (UPW) to create individual solutions of 1000 mg/Kg concentrations.

We used standards of 1000 ppm (Inorganic Ventures) mostly certified and based on sodium.

Using the prepared individual 1000 mg/Kg standards, a mixed stock working standard was prepared. The stock standard was used to produce various concentration ranges by weight to develop the calibration curve for the project. The exact mass values recorded in the laboratory worksheets can be provided upon request.

The focus of these investigations was on the determination of inorganic anions: Fluoride, Chloride, Nitrite, Bromide, Nitrate, Phosphate, and Sulfate. We use a 1 ppm standard for Fluoride, Chloride, and Nitrate, and a 2 ppm standard for Phosphate and Sulfate.

Operating Conditions of the Method:

A total of two first-generation (1+2) and four next-generation suppressors (A, B, C, D) were available for the tests.  In the following, the suppressors of the first generation are referred to as Supp, while the new generation suppressors are denoted as Supp². Because we proved by multiple injections on multiple columns that the suppressors were reproducible not all suppressors were tested with every chromatography column in every instance.

For each series of measurements five replicate measurements were performed and the relative standard deviation (RSD %) was calculated to assess precision and reproducibility. The results are summarized in the following table sheets. To maintain clarity chromatograms are not shown here but will be gladly provided upon request.

Table 1: Summary of the results Relative Standard Deviation (RSD / %) of the suppressor comparison using the Hamilton PRP-X100 column (4 × 50 mm). The flow rate was set to 1 mL/min with an eluent consisting of 2.2 mM sodium carbonate, 2.8 mM sodium bicarbonate and 0.1 mM ammonium thiocyanate. The column temperature was maintained at 40 °C throughout the measurements and the injection volume 20 µL.

As this column is already used in our final quality control testing, it was chosen as the starting point for the suppressor evaluation to provide a well-established reference point. The data regarding Supp² for Nitrite is affected by the carbonate dip behind the Nitrite peak. This will have a varying impact on the Nitrite integration. The integration of Bromide and Nitrate is elevated due to shared baseline integration.

The data produced from this shared baseline is impacted by the broader peak areas produced by the PRP-X100.  The PRP-X100 is a durable, resilient anion column, and when used with the Suppressor² produces reproducible results. However, some of the analytes are not baseline resolved, due to the column, not the Suppressor, thus the % RSD is higher.

Table 2: Summary of the results Relative Standard Deviation (RSD %) of the suppressor comparison using the memSep300HC (4 × 250 mm). The flow rate was set to 1 mL/min with an eluent consisting of 2.0 mM sodium carbonate, and 0.05 mM ammonium thiocyanate. The column temperature was maintained at 40 °C throughout the measurements and the injection volume 20 µL.

The deviation for fluoride is slightly higher and more variable as fluoride elutes immediately after the water dip. This causes slight shifts in the initial peak position, which in turn affects data acquisition accuracy.

Table 3: Summary of the results Relative Standard Deviation (RSD %) of the suppressor comparison using the Shodex Si504E (4 × 250 mm). The flow rate was set to 0.7 mL/min, with an eluent consisting of 3.2 mM sodium carbonate, 1.0 mM sodium bicarbonate and 0.05 mM ammonium thiocyanate. The column temperature was maintained at 40 °C throughout the measurements and the injection volume 20 µL.

The data for Nitrite is affected by the carbonate dip behind the Nitrite peak. This will have varying impact on the Nitrite integration. The data for Nitrite is effected by the carbonate elution though hardly noticed visually, it is a variable that can impact the resolution and ultimate area counts of Nitrite on this column.

Table 4: Summary of the results Relative Standard Deviation (RSD %) of the suppressor comparison using the memSep2FA (4.6 × 250 mm). The flow rate was set to 1 mL/min with an eluent consisting of 3.2 mM sodium carbonate, 1.0 mM and sodium bicarbonate. The column temperature was maintained at 40 °C throughout the measurements and the injection volume 20 µL.

Other manufacturers typically specify a 3% relative standard deviation (RSD) for chloride, based on a 10 ppm standard and a 50 µL injection loop. In our measurements RSD values for chloride and most other analytes remained below this threshold, even when using a 1 ppm chloride standard and a 20 µL injection volume. Only Nitrite showed higher variability likely due to carbonate interference.

Conclusions

The comprehensive study and the comparison of various chromatography columns demonstrated a clear improvement in performance when using the new suppressor. Instances where the RSD values were higher could be attributed to separation, related factors rather than the suppressor itself.

While further optimization of parameters such as column choice, eluent composition or flow rate may help reduce variability in those cases, this was not the focus of the present study.