In my last blog (“How Valid Is Your Data? - Part 1”) we discussed what happens when samples enter the laboratory. We covered spiking of samples, control blanks and relative percent difference in sample concentrations. That discussion led me to close Part 1 with a “reality check” example from my own experience analyzing beverages. While everything looked great from the outside looking in… a hidden issue needed to be resolved: spectra did not match the control blanks and/or calibration. This indicated the existence of a kinetics issue in the online reduction reaction used to achieve elemental Hg – specifically, method timing constraints.
In Part 2 of this blog, I am happy to report that I’ve successfully analyzed brewed coffee, red wine, milk, orange juice, brewed tea and soda pop. The original method guidance I was using was written for ICP, via microwave digestion. Sometimes H2O2 is used to enhance oxidation in microwave digestion and the method did in fact use H2O2. Knowing that dissolved oxygen would interfere with the reduction used in the CVAA technique, I assumed that the digestion process would entirely consume the H2O2. Well it didn’t, hence my spectra issues which affected the results!
After a review of the data and the issue, I opted to use a couple milliliters of sulfuric acid to attack any carbon bonds that may not be broken during the nitric acid digestion.
In the end, the method used hydrochloric, nitric and sulfuric acid to which was added the 50 ng/L pre-digestion and post-digestion spikes, and resulted in nearly perfect returns. One of the tricks I have learned over the years is to try to “complex” any Hg, before loss during the reagent addition steps. In most cases, a complex ion of Hg and chloride will form when given the opportunity. It follows, that the addition of HCl typically preserves the Hg in the sample, allowing for the accurate determination of Hg concentration via instrumental analysis.
It should be noted that I did have some additional challenges with the wine samples in particular. During the acid addition, the wine effervesced and created a foam which needed to be dealt with by modifications to the timing and volume of acid additions. But when all was all said and done; I believe that I now have a universal microwave method for beverages, with determination via CVAA, that provides excellent results in the low ng/L. I’ll soon present a webinar and application note detailing this study in beverage analysis for Hg, but until then, consume your beverage of choice and enjoy!
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