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Chemical Stability and Compatibility - Mercury stability in different matrices
I am writing with regards to some issues I am having with a standard that contains: Ag: 10 ug/ml, Al: 125 ug/ml, As: 10 ug/ml, Ba: 10 ug/ml, Ca: 200 ug/ml, Cd: 10 ug/ml, Co: 10 ug/ml, Cr: 100 ug/ml, Cu: 100 ug/ml, Fe: 200 ug/ml, Hg: 25 ug/ml, K: 200 ug/ml, Mg: 200 ug/ml, Mn: 20 ug/ml, Mo: 40 ug/ml, Na: 200 ug/ml, Ni: 100 ug/ml, Pb: 200 ug/ml, Sb: 20 ug/ml, Se: 10 ug/ml, Sn: 25 ug/ml, Ti: 25 ug/ml, Tl: 10 ug/ml, V: 100 ug/ml, Zn: 100 ug/ml, The standard is in a 5% HNO3 / trace HF / trace tartaric acid matrix and is packaged in a polypropylene bottle. I have tried multiple matrices when preparing my diluted calibration standards: 5% HNO3 did not work as the concentration of mercury was too high at low ppm (2.5 ppm). The only matrix that worked okay for mercury was a 5% solution made of 4 mL HNO3 and 1 mL of HCl in a 100 mL of water. This was the only solution that was stable for mercury, however Na wouldn’t work at low ppm (1 ppm) as recovery was 10% for our checks. I am out of ideas and we are trying to use a calibration that works for all the elements. I was wondering if you would have any ideas of a matrix that would work at high and low concentrations for all the elements and that keeps them stable?
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We would be concerned about the long term stability of Hg in the reported matrix due to the packaging material (plastic) as well as the presence of tartrate, which could possibly reduce the Hg. If Hg reduction to the metal (Hg°) is occurring, then false high results can be observed due to the increased volatilization of Hg during nebulization (this is assuming that you are using ICP as the analytical method). More information regarding Hg stability can be found at http://inorganicventures.com/mercury-chemical-stability. All of the elements mentioned are stable in 10% (v/v) HCl though Ag will be photosensitive (more information regarding Ag stability can be found at http://inorganicventures.com/silver-chemical-stability). However, with regard to a HCl matrix, if Tl is present in the +1 valence state (Tl+1) then it can also form insoluble chloride complexes and experience stability issues. The issue of insoluble Tl-chloride can be managed by using Tl+3 (our recommended approach), and hopefully the product documentation states which form of thallium is present; thallium nitrate indicates thallium is present as Tl+1 whereas thallium oxide indicates Tl+3 is the form in solution. If the matrix you mention (4% HNO3 / 1% HCl) works well for Hg, we’d be concerned that the HCl content is not high enough to keep insoluble AgCl complexes from forming, in addition to the above concerns regarding Tl. The issues you report for Na should not be related to any of the matrices discussed; Na is chemically stable in HNO3, HCl, HF and mixtures of these acids, though any sample preparation should avoid glassware due the presence of HF, which can leach Na and contaminate the solution (the opposite of the effect you are observing). The solution is compelling for a number of reasons, as it combines elements that can be unstable in HNO3 at low concentrations when stored in plastic (Hg), with elements that can be incompatible with HCl (Ag, Tl). We’d be happy to discuss in greater detail the possibilities for creating stable mixtures of these elements. |