Analytical techniques for measuring mercury include cold vapor atomic absorption spectroscopy (CVAA), cold vapor atomic fluorescence spectroscopy (CVAF), and direct analysis by thermal decomposition. Each technique has advantages and disadvantages, I'll review each technique and provide tips for choosing the right one for various situations.
CVAA
In many parts of the world, CVAAA is still the most commonly used technique for measuring mercury. Hallmarks of this approach include detection limits in the single-digit parts-per-trillion (ppt) range, a dynamic range of 2-3 orders of magnitude, and an abundance of analytical methods that allow for the measurement of mercury in almost any sample matrix.
CVAF
Hallmarks of CVAF mercury analyzers include sub-part-per-trillion detection limits and a much wider dynamic range than achieved by CVAA; typically 5 orders of magnitude for CVAF versus 2-3 for CVAA. CVAF instruments are available in two configurations; one employing simple atomic fluorescence and one that employs gold amalgamation to preconcentrate mercury prior to measurement by atomic fluorescence. The detection limit via the simple fluorescence approach is about 0.2 ppt whereas using the preconcentration with fluorescence detection can be as low as 0.02 ppt.
Direct Analysis by Thermal Decomposition
Hallmarks of the direct analysis approach include the elimination of the sample digestion step, fast analysis times, and a detection limit of about 0.005 ng. Eliminating digestions means solid samples can typically be run in their native form. For labs that analyze large numbers of solid samples, or that would rather not perform the digestion associated with CVAA and CVAF, direct analysis may be ideal.
Which Technique is Right for you?
Do you need to comply with a regulatory method?
Selecting the right technique really depends on your anaytical needs. For some labs, the decision will be driven solely by the need to comply with a specific regulatory method. For example, if your lab is required to analyze samples using EPA method 245.1, then you will need to use the technique CVAA. If you are required to follow specific regulatory methods, visit our Hg selector tool, you may find the information included helpful.
If your lab is not constrained by a regulatory method, the driving force more than likely be such things as:
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The characteristics of your sample matrix (for example, is it a solid or a liquid)
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The detection limits you need to reach in that matrix
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Your preferences regarding digesting the sample or not
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Your budgetary constraints
By answering a few simple questions, you will be guided in the direction of the techique which is right for you.
Is your sample a liquid or a solid?If your sample is a liquid (i.e., wastewater or drinking water) then you will be best served by one of the chemical reduction techniques (CVAA or CVAF).
What are your detection limit requirements?
At this point, you can let your detection limit requirements drive your decisions; with the knowledge that CVAA will provide a detection limit of about 2 ppt and CVAF will provide a detection limit of about 0.2 ppt (or as low as 0.02 ppt with gold amalgamation). Unless you have a preference for CVAA, our recommendation is that you should consider CVAF. Its superior detection limits will allow you to report to lower levels and its wider dynamic range will be a real time saver from the perspective of not having to do as many sample dilutions.
Digest or not Digest
If your sample is a solid, you have the choice of digesting the sample, then analyzing it by CVAA or CVAF. Alternatively, you may be able to skip the digestion step and go with direct analysis by thermal decomposition.
For many, the simplicity of direct analysis is very appealing. For labs that already have digestion procedures in place the higher capital cost of direct analysis relative to CVAA (or CVAF), may drive the decision. Other factors, such as the sample homogeneity or volatility may be important considerations as well. Because direct analysis is limited to a relatively small quantity of sample (about 1 g), nonhomogeneous samples may be best dealt with by digesting a larger quantity of sample followed by analysis using CVAA or CVAF.