ASTM D7482 Standard Practice for Sampling, Storage, and Handling of Hydrocarbons for Mercury Analysis
9. Contamination and Interference
9.1 Contamination is a significant issue whenever samples contain the target analytes at the parts-per-billion or low parts-per-million range. For mercury in hydrocarbon samples, a few simple steps should eliminate contamination and interference issues.
9.1.1 The primary source of contamination during this sampling procedure is usually the presence of accumulated mercury droplets that have come out of solution and accumulated in low-lying piping and sample taps. A single droplet can skew a mercury measurement by several orders of magnitude.
9.1.2 To avoid accumulation of mercury or mercury compounds (for example, mercury sulfide precipitates), sample taps shall not be located at the bottom of piping, but rather the taps shall angle upwards from the top or upper half of the piping. They shall be oriented to be self-draining back into the process.
9.1.3 To further reduce the potential for contamination, sample lines, whether draining from the bottom or top of the piping, shall be thoroughly purged prior to use. Exact purge volumes are difficult to specify but, at a minimum, shall be done with the valve wide open, and flushing at least ten times the volume of the sample piping and valve volume beyond the process line.
9.1.4 Ifpossible, visually examine samples or flush liquid to ensure that the flush liquid is running clean prior to sample collection.
9.2 Additional contamination issues arise from the use of improper sample containers. Improper containers can bias the sample results either high (for example, if a container has been previously used and the adsorbed mercury on the container walls not removed), or low (for example, use of uncoated steel or tin-lined steel which adsorb mercury from the sample).
9.3 In general, sample containers for mercury analysis shall not be reused unless specially cleaned and tested as a blank prior to sampling.
9.4 Uncoated metal container shall not be used for any sampling step, even for a short-term holding before transfer to the VOA vials. Stainless steel containers may be used, but they shall be cleaned and rinsed with a mild acid rinse and thoroughly dried before use. Epoxy-lined steel containers may also be used, and are preferred for larger volume (>1 L).
10. Pollution Prevention and Waste Management
10.1 Draw samples in accordance with both the facility requirements and applicable laws and regulations. Appropriate personal protective equipment shall be worn at all times. Return purge streams directly to the process, or collect for disposal or recycling.
10.2 Free elemental mercury, collected as a separate material or contained with the hydrocarbon sample, shall be treated with extreme caution. Personal exposure to vapors from free mercury droplets or puddles can exceed exposure limits. Appropriate personal protective equipment shall be worn by the sampling team whenever exposure is possible.
10.3 Elemental mercury can be collected (for example, using a mercury vacuum) for disposal, or rendered less hazardous by treating with various sulfur compounds or finely dispersed metal powders (for example, zinc, when activated by mild acid wash) to convert it to less-volatile and less toxic compounds. Facility procedures for responding to releases of elemental mercury shall be applied.
10.4 Disposal of mercury-contaminated materials, elemental mercury, or mercury compounds shall be referred to the facility hazardous waste coordinator.
11. Keywords
11.1 handling; mercury; mercury analysis; sampling; storage
