NIC Discrete-Direct-Purge Reducing Vaporization
In this technique, the mercury analyzer handles each sample in a Discrete, independent sample tube. Each sample is therefore isolated from surface contact with any other samples in the analytical batch to be measured.
The Direct-Purge technique is then used to extract and transfer the converted Hg0 from each sample tube and into the detector for measurement. Since only mercury vapor contacts the flow path, sample-to-sample memory effects and carryover from over-range samples are virtually eliminated.
How It Works:
- First, reductant (SnCl2) is automatically added into the sample tube containing the acid-digested sample solution. The sample tube is sealed, leaving a closed-loop flow path to the detector.
- The carrier gas is introduced to sparge (or purge) the solution, releasing the elemental mercury vapor from the solution and into the flow path, which then flows directly into the detector for measurement.
- Almost no sample-to-sample carryover or memory effect
- Carryover from over-range samples is greatly reduced
- Discrete technique only needs 200-300uL of reagent per sample
- Reduces hazardous mercury wastes to less than one liter per full day of operation
- Filtration of samples not required, as Direct-Purge technique can handle particulates in samples with no issues
Other Reducing Vaporization Techniques
- Commonly based on Flow-Injection or Continuous-Flow Techniques to introduce the full sample solution into the system to complete the chemical reaction for the analysis.
- The acidic sample solutions and reductant (SnCl2) enter the system via pump tubing with peristaltic pumping, which must be replaced often.
- Chemical reduction begins when both the sample solution and reductant are mixed, reducing Hg2+into Hg0, within a continuous flow of reagents.
- Prior to the detector, the liquid-gas phase separator or membrane separation device is needed to remove the liquid, allowing the Hg0gas to enter the detector.
Mercury is well-known for its strong affinity to absorb onto different materials and its high solubility in acidic reagents.
After the digestion and oxidization procedure, samples are usually very acidic. The acidic sample solutions and reductant, SnCl2, enter the system via pump tubing with peristaltic pumping. The acidic sample leaves a residue on the inner surface of pump tubing, creating possible active sites for mercury absorption from current mercury, which is easily passed on to the following samples.
Such phenomenon is inevitable with this technique and commonly exhibits as what is called the mercury memory-effect, experienced by many lab analysts. This effect is especially significant and severe when analyzing samples of varying mercury concentration levels.
Large multi-liter carboys of hazardous waste are produced daily from such flow-based techniques, creating the need for expensive waste disposal. Reduction of hazardous mercury wastes should be a priority for all mankind, and it is a priority of the Minamata Treaty.