ASTM D7482 Standard Practice for Sampling, Storage, and Handling of Hydrocarbons for Mercury Analysis
7. Sampling Procedure
7.1 Employ the normal hydrocarbon sampling procedures necessary to obtain discrete and homogeneous samples. Either grab samples or composites from auto-samplers are allowed. Grab samples are preferred. When expecting particulates with adsorbed mercury or mercury droplets, iso-kinetic sampling is greatly preferred.
7.2 Wash the VOA vials with nitric acid, rinse with water, and dry.
7.3 Asample "set" is defined as three VOAglass vials. Each vial is individually wrapped with a plastic bag, and then the set of three vials are overpacked in an appropriate container for transport or shipping. Aset of three individually bagged sample vials may be bundled together and placed in a larger bag to separate them from other sample sets included in the same shipment. Adsorbant material may be packed around the bagged vials to further protect against damage during shipping and release of one or more of the sample vials if it ruptures.
7.4 One sample set is required for each sampling event. Fill three sample vials for each oil sampled as close to the same point in time as possible.
7.5 Sampling shall be done directly into the VOA vials if possible. If it is necessary to sample into one container and then transfer into the VOA vial, record the details of the original container and sampling method, as well as the details of the transfer. Auto-samplers are acceptable, but the container and plumbing materials and transfer details shall be documented in the notebook or Chain of Custody (COC) form.
7.6 Purge sample taps sufficiently immediately prior to sampling to remove any elemental mercury droplets that have accumulated in the piping, valve, or crevices. In some cases, significant amounts of elemental mercury have been found to accumulate in sample taps. Proper collection or recycling of the purge is important to prevent release of mercury and worker exposure.
7.7 It is imperative that persons conducting or supervising the sampling procedure understand the conditions that cause evaporative or other loss of mercury. While bulk mercury droplets are not highly volatile (vapor pressure <1 mmHg at 20°C) in many hydrocarbons, the dissolved mercury readily evaporates with an apparent vapor pressure similar to butane or pentane. Also, mercury can readily adsorb on many metal surfaces. Sample taps shall not contain process-contact materials composed of copper, zinc, tin, aluminum, brass, bronze, Monel or other alloys containing these metals.
7.8 To avoid loss of mercury vapor from the samples, proceed as follows:
7.8.1 Fill the vial as quickly as possible, filling the vial with, at most, 1-mL remaining.
7.8.2 Cap the vials immediately to minimize time of exposure to the open atmosphere.
7.8.3 In some cases, recapping the vial under a slow stream of nitrogen back in a laboratory hood has been valuable in preventing oxidation of elemental mercury during shipping.
7.9 COC Form - The COC form will accompany samples delivered to the laboratory. Record sample containers and plumbing materials information (especially for auto-samplers) in the permanent record or on the COC form. Note sampling details in the permanent record (notebook) and on the COC form, or both. At a minimum, record the following sample data on a permanent record and on the COC form, or both:
7.9.1 Description of the sample point (sample point number, process stream name, physical location, stream temperature and pressure, and the like).
7.9.2 Names and roles of persons in drawing the samples.
7.9.3 Description of the sampling procedure, including:
7.9.3.1 Exact start and end times.
7.9.3.2 Approximate time to fill individual vials.
7.9.3.3 Any subsampling or auto-sampling procedure notes.
7.9.3.4 Ambient conditions (like temperature and precipitation).
7.9.3.5 Physical appearance of the sample when taken and after cooling (for example, clear and bright, or cloudy, or presence of solids or visible mercury droplets).
7.9.3.6 Any problems encountered.
7.10 Allow samples to cool to ambient conditions prior to packing and shipping. Hydrocarbon samples for mercury analysis need not be cooled below ambient temperature. After the samples are at ambient temperature, examine the cap and septum. If cooling has produced a major distortion of the septum, loosen the cap to allow the head-space vacuum (or pressure) to equilibrate. Allow a minimum amount of air (or a slow sweep of nitrogen) ingress to eliminate the vacuum, then tighten the cap thoroughly.
7.11 Affix labels to each sample vial either before or after sampling each set. Clean the vial exterior to remove any oil. Labels shall include sample identification details and date of sampling, as well as the identity of the person responsible for collecting the sample.
7.12 Place each of the three vials in a separate plastic bag. The set of vials can also be overpacked into a larger resealable plastic bag.
8. Sample Storage and Stability
8.1 The best container for "total mercury analysis in crude oil" sample storage and stability is glass with PTFE-lined cap (see 6.1).
8.2 The following conditions should be observed for maintaining quality control of samples:
8.2.1 Use appropriate sampling containers as specified in this practice.
8.2.2 Avoid intermediate transfers of the samples from one container to another.
8.2.3 Do not allow excessive air contact during sampling. Mercury, particularly elemental mercury, can volatilize rapidly and be lost ifthe sampling flow is highly turbulent or has a long contact with the atmosphere.
8.2.4 Minimize or eliminate the sample container head space. Opinions vary on the best procedure. The most conservative approach is to leave a small head space bubble to allow for expansion, even though the septum capped vials specified in the practice generally have enough flexibility to handle thermal expansion during shipment.
8.2.5 Maintain a proper chain of custody.
8.2.6 Note the sample conditions on arrival at the lab and prior to analysis so as to document the excessive loss of hydrocarbons from previously full containers.
