Mercury sampling

Another interesting area of apphcation is where flow systems are used in conjunction with the flow-injection approach to select particular components from a sample, i.e. organic or inorganic mercury samples, or to preconcentrate particular elements of interest. McLeod... 

A series of mercury mass balances was obtained at a coal-fired power plant by comparing the volatile and particulate mercury in the stack gas stream to the mercury initially in the coal, corrected for the mercury adsorbed and retained by the various ashes. These data were used to determine the fate of the mercury in the combustion process and to check the accuracy of the volatile mercury sampling procedure (gold amalgamation). The bottom ash had the lowest mercury concentration of the ash samples collected, and the mercury concentration increased as one proceeded through the ash collection system from the initial mechanical ash to the electrostatic ash. The mercury recovered in the various ashes represented about 10% of the total mercury introduced in the raw coal. 

After the firing was completed, the permanganate solution was analyzed by the same procedure used for the volatile mercury samples. Mercury in coal samples from the National Bureau of Standards and the United States Bureau of Mines were analyzed simultaneously as controls. At least duplicate analyses were performed on all samples. Empty crucibles were fired into acidic permanganate solutions by an identical procedure to obtain solutions for blank determinations. 

Precision is good, and agreement with the outside laboratories is acceptable, considering the fact that many of the samples in the comparison have sub-ppb mercury concentrations that are just above the stated minimum detection limit of 0.1 ng/g for the technique. Since these samples were run over the course of a year, some did not benefit from accumulated experience with reagent blanks and the special handling required for low ppb mercury samples. 

Once it had been determined that instrument response is independent of mercury species or matrix, any stable mercury sample, even a coal SRM, can be used to verify instrument performance on a daily basis. The solid samples seem to be far more stable than any mercury standard in a hydrocarbon matrix that we have prepared to date. 

As the cold-vapor mercury sample is already in the atomic state, there is no need of an atomizer, per se. The vapor, transferred directly from the cell or desorbed as a plug from a heated amalgamation trap, is commonly swept into a moderately heated (resistance wound heating to 200°C) 10 cm quartz T-tube located within the optical beam of a conventional AA spectrometer. Attenuation of an intense electrodeless discharge lamp line source at 253.7nm is used as a measure of the absorption. Alternatively, dedicated continuum source AA-based spectrometers fitted with long path absorption cells (30 cm) are frequently used to increase sensitivity and detection limit. 

The sampling method employed for the collection of natural gas mercury samples was the Modified EPA 30 method. This method follows the principles of the standard... 

ASTM D 6350 98 Standard Test Method for Mercury Sampling and analysis in natural gas by Atomic fluorescence Spectroscopy... 

For measuring the conductivity and thermoelectric power, the molybdenum cell incorporates a 6-bore alumina tube leading from the cell into the cold closure structure via the capillary. Four bores contain molybdenum wires that serve as electrodes for a conventional four-terminal conductivity measurement with an external potentiometer. Two more wires consisting of platinum run from the center of the cell to the cold end of the alumina tube. These wires together with the sample form a mercury-platinum thermocouple the absolute thermoelectric power of mercury can be derived from the measured e.m.f. In the configuration illustrated in Fig. 7.2, the conductivity and thermoelectric power samples are located in the center of the density sample, and therefore the three quantities are measured on adjacent portions of the same bulk mercury sample. 

Fig. 7.6. Autoclave with optical access for reflectivity measurements on fluid mercury (Hensel, 1987). One end of the sapphire window is open to the laboratory atmosphere while the other is in contact with the fluid mercury sample at high temperature and pressure. A similar system with windows at each end may be used for transmission studies.

The effect of compressibility of mercury, sample container, sample and residual air with increasing pressure. 

In order to calculate the true volume intrusion of mercury into the pores of a sample, a correction must be made to account for the compression of mercury, sample cell and sample [69]. The usual procedure is to carry out a blank experiment in the absence of a sample or with a non-porous sample [70]. During the course of calibration measurements on non-porous nylon it was found that a normal blank correction procedure led to erroneous mercury penetration volumes [71]. In particular it was found that the shape of the intrusion curve varied with the size of the sample. 

If 100. J of heat energy is applied to a 25-g sample of mercury, by how many degrees will the temperature of the mercury sample increase See Table 3.2. 

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