Cold Vapor Atomic Fluorescence Spectroscopy
Hallmarks of cold vapor atomic fluorescence spectroscopy (CVAFS)-based mercury analyzers include sub-part-per-trillion detection
limits and a much wider dynamic range than achieved by CVAAS; typically 5 orders of magnitude for CVAFS versus 2–3 for CVAAS.
CVAFS 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 approach with fluorescence detection can be as low as 0.02 ppt.
The US EPA has promulgated methods for each of these approaches; Method 245.7 (4) is for use without preconcentration and
1631 (5) is with preconcentration. These methods were developed to satisfy the need for quantitation at the National Recommended
Water Quality Criteria for mercury (6). These criteria are published pursuant to Section 304(a) of the Clean Water Act (CWA)
and provide guidelines for states to use in adopting water quality standards that ensure ambient waters are safe to fish in,
and subsequently, that fish are safe for consumption. Additional information on this subject is available at: http://water.epa.gov/scitech/swguidance/standards/current/index.cfm.
With CVAFS instruments a peristaltic pump is typically used to introduce sample and stannous chloride into a gas–liquid separator
where a stream of pure, dry gas (typically argon) is bubbled through the mixture to release mercury vapor. The mercury is
then transported in the carrier gas through a dryer and then either directly to the fluorescence cell or to the preconcentration
trap and then onto the fluorescence cell. With fluorescence the drying stage is quite important as water vapor and other molecular
species can interfere with the fluorescence measurement. Once in the detector, mercury vapor absorbs 254-nm light and fluoresces
at the same wavelength. Measurement of the fluorescence signal is usually made at 90° to the incident beam to minimize scattering
from the excitation source. The intensity of the fluoresced light is directly proportional to the concentration of mercury.
The concentration of standards and samples with this technique are typically 100–1000× lower than those used with CVAAS, demanding
much cleaner reagents. To ensure reagents are low in mercury, methods such as EPA Method 1631 describe techniques to remove
mercury from salts and some solutions.
Figure 2: An overview of cold vapor atomic fluorescence with gold amalgamation.
Direct Analysis by Thermal Decomposition
Hallmarks of the direct analysis approach include 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 laboratories
that analyze large numbers of solid samples, or that would simply rather not perform the digestion typically associated with
CVAAS and CVAFS, direct analysis may be ideal. It is worth noting that this approach also carries with it the benefit of generating
less acid waste than the solution-based techniques. However, direct analysis is not well suited for a laboratory whose need
is to run large numbers of samples already in aqueous solution. For liquid sample analysis, the detection limit available
using direct analysis is not typically comparable with those of CVAAS or CVAFS. This is primarily because of the relatively
small liquid volumes that are processed using direct analysis; typically less than 1 mL per sample. Consider, for example,
that the total mercury in 1 mL of a sample that contains 5 ppt (ng/L) of mercury is only 0.005 ng — this is right at the detection
limit for direct analysis. In contrast, 5 ppt is a concentration that is trivial to measure by CVAFS. However, with solid
samples the sensitivity difference is quite small since the digestion required to put the sample into solution introduces
a significant dilution.
Figure 3 shows an overview of the direct analysis technique. With this approach, a weighed sample is introduced into the decomposition
furnace with oxygen (or air) flowing over the sample. The furnace temperature is ramped in two stages; first to dry the sample
and then to decompose it. As the evolved gases are released, they are carried into a catalyst where further decomposition
occurs and elemental mercury is released. When the gas stream leaves the catalyst elemental mercury is captured on the surfaces
of a gold amalgamation trap. After the sample's mercury has been collected, the gold trap is heated and the accumulated mercury
proceeds to an atomic absorption detector for quantitation.
Figure 3: An overview of direct analysis using thermal decomposition.