Tips for Testing Anions in Soil

Complete Article from Edition 51 APG eNewsletter

Article by Joseph Roush, QC Chemist at APG

The most common anions tested in soil are fluoride, chloride, bromide, nitrate, phosphate, and sulfate. These analytes are typically tested by ion chromatography (IC). This article will give several tips that may prove useful in such testing. One possible issue associated with this method is low recovery values for phosphate and fluoride. The technology used for all the information in this article is an ion chromatograph with chemical suppression.

The first step in testing the anions in soil is the extraction process. Typical extractions involve combining the soil sample with a solvent, followed by some type of agitation. Extraction times range from one to twelve hours by sonication, shaking or tumbling. This process often gives recoveries in the ninety percent range for chloride, bromide, nitrate, and sulfate.  However, recoveries for phosphate and fluoride frequently present in the range of twenty to forty percent. This is partially due to the large amount of iron found in soil. Excess iron in the sample will cause a complex with phosphate and fluoride, which reduces extraction efficiency. 

Results for phosphate and fluoride can be improved by practices such as extracting the samples into an eluent matrix instead of standard laboratory water. Experimentation has shown a 3.2mM sodium carbonate and 1.0mM sodium bicarbonate solution to be effective in raising responses for most anions, with markedly higher increases for the troublesome analytes. Another possibility is to remove iron after the agitation process with solid phase extraction materials available from many industry suppliers. 

A high iron level in analysis samples also creates a concern for contamination in the IC system. After the extraction, there is a large amount of soluble iron in the sample. When the soluble iron comes into contact with the carbonate/bicarbonate eluent, it is changed to insoluble iron and will bind to many different parts of the IC system. There are several places that iron is likely to build up, the most likely being the place where the iron would first come into contact with the eluent: the injection port. Iron is also likely to build up on inline filters, tubing, and columns.

There are several good indicators that iron buildup may be a problem. IC plumbing and filters will appear rust colored if there is significant iron buildup. A second indicator is the recovery of phosphate. Phosphate recoveries tend to decrease noticeably through subsequent analyses, while other analytes show greater consistency. Additionally, a significant increase in relative standard deviation values for phosphate, compared to the other analytes, should serve as a warning light; particularly when combined with the other symptoms noted. Table 1 shows a typical data set with iron contamination. 

Table 1: Iron Contamination

There are several ways these problems can be dealt with. For example, routinely checking the condition of the filters will help keep most of the excess iron from making it to the column. Guard columns are also a good idea. They will trap many contaminants, including iron, and will increase the lifetime of your column. For IC’s with chemical suppression, it is possible to use oxalic acid dihydrate in the suppressor regenerant. This is specifically used to bind excess iron in the system, and can be made at a concentration of 100mM. It is always a good idea to check with the instrument manufacturer to make sure the addition of the oxalic acid won’t harm the system. 

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