Monitoring the amounts of mercury released into the environment is an important requirement in a number of industries. Measurements are generally carried out using dedicated trace mercury analysers and a number of ASTM, EPA and ISO standard international methods have been established which prescribe the type of mercury analyser to be used and the measurement methodology for different sample types. The main mercury analyser techniques are cold vapour atomic absorption spectroscopy, (CVAAS) and cold vapour atomic fluorescence spectroscopy, (CVAFS).
Direct solid sampling
For many applications, the ability to analyse solid and sludge samples without further sample preparation is important. This is possible using a thermal decomposition CVAAS which liberates elemental mercury from solid samples by high temperature heating with oxygen. However for some applications gentle heating of the sample in air is adequate to release the mercury. The evolved gases are carried through a heated catalyst to produce free mercury while removing halogens, nitrogen oxides, and sulphur oxides. The remaining combustion products including elemental mercury (Hg0) are swept first through a dryer and then through a gold amalgamation trap before passing into an atomic absorption measurement cell.
Solution-based sample preparation methods
Some analytical standard methods prescribe pre-treatments to convert solid or liquid sample into a solution fro analysis. A series of chemical pre-treatment stages are performed to produce a sample solution in which all of the mercury present has been converted into Hg2+ ions. The resulting liquid sample is mixed with a solution of stannous chloride in the mercury analyser to reduce the Hg2+ to mercury vapour (Hg0). The mixed solution then enters a gas/liquid separator where argon gas is passed through it to carry the mercury vapour via a dryer to the measurement cell. Samples can be prepared in this way for CVAAS or CVAFS mercury analysers. In the case of CVAFS, the stream of dried mercury vapour may first be passed through a gold enrichment, or gold amalgamation trap. One or two stage gold enrichment captures and concentrates the elemental mercury, which can then be released into the measurement cell by heating, thus lowering the detection levels that can be achieved in the mercury analyser.
Cold Vapour Atomic Absorption Spectroscopy
The CVAAS measurement cell features a high intensity mercury lamp emitting UV light at 253.7 nm and an atomic absorption detector. By measuring the proportion of light absorbed by the mercury vapour in the sample, the mercury concentration can be calculated. Using solution based sample preparation, ultra-trace detection limits (< 0.5 ng/L IDL) can be achieved with a dynamic range from < 0.5 ng/L to > 500 μg/L and a linearity greater than 4 orders of magnitude. The direct heating approach offers a standard usable range of 0.001 ng – 1500 ng Hg, through appropriate selection of the mercury analyser’s operational parameters such as furnace/catalyst temperature and time and gas flows.
Cold Vapour Atomic Fluorescence Spectroscopy
Mercury vapor not only absorbs light at 253.7 nm but also fluoresces, re-emitting some of the light at the same wavelength. In the CVAFS measurement cell the fluorescent light is measured using a detector at 90 degrees to the incident beam to minimize detecting any scattered light from the excitation source The fluorescence emission is directly proportional to the concentration of mercury in the sample. Ultra-trace detection limits (< 0.05 ng/L IDL) can be achieved with a dynamic range from < 0.05 ng/L to > 400 μg/L and a linearity greater than 4 orders of magnitude. In the same way as fluorescence spectrophotometers have lower detection levels than UV-Vis spectrophotometers ('an introduction to fluorometry') CVAFS offers lower detection levels than CVAAS. This is because the fluorescence signal is measured directly and can easily be amplified with a low noise contribution because it is separate from the incident beam.
Mercury analysers from Lambda
Lambda is a leading supplier of analytical instruments and preparation equipment for a wide range of applications, including a choice of mercury analysers. If you would like more information, simply contact a member of the team today.