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Summer 2009 Newsletter Digestion Techniques for Difficult Sample Matrices

When laboratories speak with clients regarding trace metals analysis, they usually ask what detection limit is required. Some labs may emphasize their instrument capabilities or address how they deal with spectral interferences caused by other metals in the sample. The question usually not asked is, "What is the sample matrix?" More simply stated, "What is the sample composition?" This is very important information for the analyst who will be digesting the sample, as the digestion technique used is crucial to the project's success.

If the sample is not digested properly, essential information may be lost. A digestion technique that is too mild will not completely destroy the matrix, and analytes of interest may not make it into solution, resulting in low values. Likewise, a technique that is too aggressive will destroy the matrix, but may also destroy the analytes of interest. This is especially true with lighter metals such as tin or antimony. Many of the samples we deal with at Chemir are complex and require multiple digestion steps, often utilizing customized techniques.

The most common digestion techniques involve the use of acids, either singly or in combination. These are usually fairly simple, yet effective techniques. Experience combined with research will generally give a strong indication of which acid(s) can be used successfully on a given matrix. Difficulties often arise when the acid(s) will digest the matrix but adversely react with the elements of interest or vice versa. Additionally, safety considerations must be taken into account prior to the use of specific acids. For example, some acids such as perchloric are excellent for digesting certain matrices, but have been known to react violently and even explode. Perchloric acid also requires special hoods in order to use it safely, and the use of incorrect equipment can lead to a laboratory explosion. Hydrofluoric acid is another dangerous acid that most of us are acquainted with. It is well known that this acid is very effective for the digestion of silicon containing compounds, but it is extremely dangerous to the analyst since it will leach the calcium from cells and bones if spilled.

Ashing is another technique frequently utilized for digestion. This involves burning away the sample matrix, leaving only ash. The ash is often dissolved in an acid solution and then the liquid is analyzed. This can be an effective technique, but care must be taken so that the sample is not ashed at too high a temperature. If this happens, some of the lighter elements can be destroyed, yielding poor results. Another difficulty is that some matrices are not amenable to ashing and may not completely burn away. In this case, the acids may not completely dissolve the remains and some of the elements of interest may not be dissolved into the acid solution. Best results are obtained by a skilled analyst using a calibrated muffle furnace. Once again, experience and research are key to the success of the technique.

Fusion of the sample is another technique used for difficult matrices. Although there are difficulties associated with these procedures, fusions often work well in cases where other techniques fail. This technique requires knowledge of which chemical compounds to use in which situations, the proper amounts of sample and fuseate to mix, the proper temperature, and the correct fusion vessel to use. A calibrated muffle furnace is essential because incorrect temperatures are likely to cause the method to fail. Many labs do not perform fusions since the technique is very labor intensive and can be quite costly. Many fusion methods require platinum crucibles that have a high initial cost and can be destroyed easily.

In many cases, samples cannot be digested with any single technique and will require a combination of two or more. As an example, one recent case involved the analysis of several elements contained in a ceramic coating. After a great deal of research, several unsuccessful attempts and one destroyed platinum crucible, a digestion sequence was determined. The final method involved twelve separate steps, six different acids and a fusion technique. Although difficult, the method yielded repeatable results and a satisfied client.

Knowing what the sample matrix is comprised of and what elements are required is essential. If a sample digestion is performed incorrectly, it could lead to the matrix not being dissolved or the elements of interest being lost. Chemir's approach is to ask the right questions before beginning, then developing a plan of the digestion techniques and their respective orders. The end result is reliable and accurate answers to issues involving trace metals analysis and identification.