We received a sample from a proficiency testing scheme for analysis of several analytes, mercury among them. Although the sample was synthetic, its matrix was made to resemble a industrial effluent.
We proceeded with the usual mercury analysis by Atomic Absorption with Cold Vapor generation (CVAA), doing the usual KMNO4 sample digestion abd further reduction of hydroxilamine. Our local regulatory method mandates use of sodium borohydride instead of stannous chloride. When adding it to the sample before reading absorbance, a black precipitate was formed. Result was quite low; however, what was notticed and very weird is that any further samples that were processed gave negative results or quite low recoveries when using the quality control solutions, even when using another vessel for borohydride addition. The entire system had to be washed in order to restore the correct readings.
Haven't found a quick answer for it. My initial guess is that the sample was high on sulfate or sulfur compounts so that sulfide ions were formed, which could generate the mercury sulfide precipitate that doesn't volatilize towards the quartz tube. That however doesn't explain why the following samples could be affected. I suppose there could be deposition of a certain substance in inner layers of the tube that conducts the gas to the quartz tube but here I can only speculate.
Sample was reported to contain 0,1 mg / L when our reading was of about 0,01 mg / L. Notorious metals we found in the sample where copper and iron at the 5 mg / L level and Zinc at the 10 mg / L level.
Anyone has had an issue like this when analyzing mercury? Is there a reactive able to supress this intereference?