Results for ILC 6

In the most recent FIRMS Network Inter-Laboratory Comparison of stable isotope measurements three test materials were distributed to 31 laboratories worldwide. The materials distributed were:

glycine
4-nitroacetanilide and
sodium nitrate

Laboratories were requested to perform 10 measurements for each apposite element (δ2H, δ13C, δ15N, δ18O) of each analyte.

Overall 27 laboratories submitted results. Not every laboratory submitted ten replicate results and some reported results for a selection of samples or element.

Carbon Results

For δ13C measurements a target standard deviation of 0.2 permil was assigned, based on previous ILC results (doi:10.1016/j.scijus.2008.12.002).

Three laboratories attained z-scores greater than 3 for the analysis of both glycine and 4-nitroacetanilideresults which should be considered as unsatisfactory.

Three laboratories attained z-scores greater than 2 for the analysis of 4-nitroacetanilide, results which should be considered as questionable.


Nitrogen Results

For δ15N measurements a target standard deviation of 0.2 permil was assigned, based on previous ILC results (hyperlink: doi:10.1016/j.scijus.2008.12.002)

Four laboratories attained z-scores greater than 3 for the analysis of glycine or 4-nitroacetanilide, results which should be considered as unsatisfactory. Two laboratories returned unsatisfactory results for both analytes.

Three laboratories attained z-scores greater than 2 for the analysis of glycine and one laboratory for the analysis of 4-nitroacetanilide, results which should be considered as questionable. Overall, four laboratories returned questionable or unsatisfactory results for both analytes.

Results for the analysis of the sodium nitrate sample presented a similar picture to ammonium nitrate, analysed in the previous ILC. One laboratory attained a z-score greater than 2 whilst seven laboratories attained z-scores greater than 3, representing over a third of the total returns.

These data clearly indicate a fundamental problem with the isotopic analysis of nitrates, materials of significant forensic interest. It is envisaged that future FIRMS ILCs will continue to address such issues.

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