Very often, laboratory specialists who analyze mercury in solid samples (such as contaminated soil, sorbents and sorbent traps, waste water, etc.) have to address the problem of high variations of mercury content in the samples that are analyzed. It is not unusual to have to clean the mercury analyzer or manually readjust the instrument’s settings/parameters in order to be able to switch from analysis of mercury contents at ppm level to ppb level. Not to mention a headache when the sample is unique (like analysis of sorbent traps), and the risk of getting an invalid result for a sample with unknown mercury concentration which might be out of the calibration range is at stake.
Commonly, when working with conventional gold-trap analyzers, the sample in such cases is homogenized and divided into several independently analyzed subsamples to avoid saturation of the analytical signal. Such an approach increases measurement error.
Analyzing samples with the high variation of mercury content has never been a problem for the users of Lumex Instruments RA-915M/Pyro-915+ analyzer due to a very wide dynamic range of the analyzer and linearity of the calibration in this range.
And now, with the introduction of the new algorithm for the RAPID software, even analysis of samples with the mercury content of thousands of ppm became extremely easy.
A novel approach for the analysis upper limit expanding (above 10,000 ng Hg in a sample) is based on atomizing temperature control depending on a rise/decay of analytical signal: Closed-Loop Control System (CLCS).
If the analytical signal exceeds a preset level L1 during analysis and the signal increases faster than a preset value R, the sample heating is automatically decreased and switched on when these values drop back below the prescribed levels. So in case of high mercury content in a sample, CLCS keeps the analytical signal between the specified levels L1 and L2, controlling the atomization temperature and thereby avoiding the signal overflow. For a sample with “common” concentration, when the signal does not exceed the L1 value, atomization follows standard temperature scenario with the overall time of analysis within 1 - 3 min.
The CLCS automated control shows excellent linearity of the calibration graph obtained in a range of three orders of magnitude (see an example below):
a) analytical signals for various masses of mercury* b) calibration graph*
*calibration with Mercury Ions Stock solution in the range of 40 – 40,000 ng Hg.
Some Lumex Instruments customers have already tested the new technique for the analysis of solid samples by thermal decomposition and direct analysis by AAS following the standard methods like, for example, EPA 7473, or ASTM 7622.
The technique is especially beneficial in combination with the autosampler for Lumex RA-915M and Pyro-915+ analyzer that was introduced last year. The modular setup creates an opportunity for further reduction of costs per analysis (already low due to the absence of any reagents or carrier gases).
The sampler can handle up to 75 ladles with samples that are automatically inserted into the first chamber of the atomizer, where the sample is heated at a temperature of 200 – 800°C (depending on the operation mode). The mercury compounds are evaporated and partially dissociated, forming elemental mercury. All the gaseous products formed are transported into the second chamber of the atomizer by a mercury-free filtered ambient air. Mercury compounds are dissociated entirely and the organic matrix of the sample is burnt out. Downstream from the atomizer, the air flow enters the analytical cell heated up to 700°C, and the mercury atoms are detected by the RA-915+/915M analyzer.
The RAPID software monitors and recognizes mercury release from the sample and using CLCS algorithm adjusts automatically for optimal decomposition temperature profile representing a new advance in handling high mercury content by automatic temperature profile adjustment and expanding the dynamic range of determined concentrations.