5A3b Chloride – 1:5 soil/water extract, ion chromatography (single-column with electronic suppression of eluent conductivity)

This variant of the ion chromatography technology described in 5A3a shares the same analytical capabilities, advantages and disadvantages over other analytical finishes mentioned in the preamble to that method. Anions in clear aqueous extracts within the mg/L concentration range separate in the order F^–, Cl^–, NO₂^–, Br^–, NO₃^–, HPO₄^2–, and SO₄^2–, with individual analyses able to be completed within 30 min using only a few mL of extract. No particles passing through the system should exceed 0.45 μm and best separations occur when sample pH values range between 5 and 9. The approximate detection limit is 0.1 mg/L, with an injection volume of 100 μL, which for Cl in a 1:5 soil/water extract equates to 0.5 mg Cl/kg of air-dry soil.

The application is applicable to the quantitative analysis of F^– only if extracts are known to be free (or made to be free) of interference from short-chain organic acids and carbonate. For example, solid-phase extraction cartridges and/or filtering devices that retain organic acids and pass inorganic anions are available to enable F^– analysis, typically with a low eluent flow rate. Note that ClO₄^– and Br^– coelute under the specified conditions. Refer to Australian Standard AS 3741 (Anon 1990) and Section 4110 of Section 4110 C of APHA (1998) for further details on analysis by ion chromatography. Further note that ClO₄^–, which is environmentally persistent, could be present at low concentrations via impurities in naturally occurring N fertilisers sourced from very dry locations and as an uncombusted residue from solid-state rockets (Thorne 2004).

Reagents

Reagent Water

Deionised or distilled water, analytically free of the anions of interest (≈18 megohm/cm resistivity) and containing no particles >0.20 μm.

Borate/Gluconate Concentrate

Combine 16.0 g sodium gluconate [CH₂(OH).(CH.OH)₄.COONa], 18.0 g boric acid (H₃BO₃), 25.0 g di-sodium tetraborate decahydrate (Na₂B₄O₇.10H₂O) and 125 mL glycerol (1,2,3-Propanetriol; C₃H₈O₃) in ≈600 mL reagent water. Mix well and make to 1.0 L with reagent water.

Eluent Solution [0.011 M borate, 0.0015 M gluconate, and 12% (v/v) acetonitrile]

Combine and mix 20 mL Borate/Gluconate Concentrate solution, 120 mL HPLC-grade acetonitrile (methyl cyanide; CH₃CN), and 20 mL HPLC-grade n-butanol (CH₃.(CH₂)₃.OH), and dilute to 1.0 L with reagent water. Ensure freedom from particulates by including an in-line filter before the separator column. To minimise the possibility of base-line drift, degas the eluent solution by bubbling with an inert gas such as helium or argon before use.

Chloride Primary Standard

1 L contains 1000 mg of Cl^–.

Prepare and store as in Method 5A3a.

Chloride Solution for Retention Time

1 L contains 4 mg of Cl^–.

Prepare and store as in Method 5A3a. The concentration can be increased to suit the particular equipment and column/s.

Anion Calibration Solutions

Prepare as in Method 5A3a. The concentration/s can be increased/varied to suit the particular equipment and column/s. These solutions should be prepared fresh daily if they contain NO₂^–, NO₃– and/or HPO₄^2–, otherwise prepare weekly.

High-range Calibration Solution

Dilute volumes of anion primary standards as specified in Table 5.2 with reagent water and make to 1.0 L in a volumetric flask. The concentration/s can be increased/varied to suit the particular equipment and column/s.

Intermediate-range Calibration Solution

Prepare as in Method 5A3a. The concentration/s can be increased/varied to suit the particular equipment and column/s.

Low-range Calibration Solution

Prepare as in Method 5A3a. The concentration/s can be increased/varied to suit the particular equipment and column/s.

Special Apparatus

Ion Chromatograph

Select/set up a complete isoconductive gradient ion system with an anion exchange and guard columns, plus syringes, conductivity detector, data recording and peak integrating system/s.

Anion exchange and guard columns

Refer to manufacturer’s recommendations.

Conductivity Cell

As per manufacturer’s recommendation.

Procedure

Prepare 1:5 soil/water suspensions as described in Method 3A1. Filter a suitable aliquot for same-day analysis using at least a 0.45 μm pre-washed membrane and preferably one of 0.20 μm.

Set up the IC in accord with manufacturer’s instructions. Specifications typically change over time. Equilibrate the system at constant temperature for approximately 15–20 min or until a stable baseline is obtained. Make other instrumental adjustments in accord with expected Cl^– concentration.

Determine the retention time for each anion by injecting a 2.0–3.0 mL aliquot of the Cl^– Solution for Retention Time. Use a syringe and inject into the sample entry port. If other anions are to be determined, their retention times should also be calibrated at this stage, with solutions containing only the anion of interest. Note the time required for the peak for each anion to appear on the chromatogram and record in minutes. These times will vary from one column to another, and are influenced by eluent flow rates and temperature. Retention times in the sequence F^–, Cl^–, NO₂^–, Br^–, NO₃^–, HPO₄^2–, and SO₄^2– may vary with instrument settings and solution concentrations.

Analyse the reagent water blank, the High-, Intermediate- and Low-range Calibration Solutions and unknown filtered soil extracts and calculate Cl^– concentrations as described in Method 5A3a.

Report water-soluble Cl^– (mg Cl/kg) on an air-dry basis.

Table 5.2. Solutions and dilutions for the high-range calibration solution containing anions of analytical interest.