ASTM D7455 Standard Practice for Sample Preparation of Petroleum and Lubricant Products for Elemental Analysis
5. Gross Sample Preparation
5.1 Collection of a meaningful and representative sample is often the most critical step in an analytical procedure. In trace element analysis, in particular, extreme care must be taken to avoid contamination of the samples during the sampling and all subsequent analysis steps. Good laboratory practices in this area can include:
5.1.1 The sample received by the laboratory for analysis needs to be stored in a designated specific storage location while awaiting analysis. This area, whenever possible, shall not contain materials with high levels of specific elements that could contaminate other samples requiring trace element analysis.
5.1.2 All laboratory equipment used specifically for trace element analysis need to be free of any source of contamination. This may require that specific equipment be used only for trace element analysis.
5.1.3 Use working surfaces that can be decontaminated easily if a spillage occurs. The atmosphere needs to be free of interfering gases and dust particles.
5.1.4 Wear clean, fresh, protective, and impermeable gloves for sample preparation for trace element analysis, appropriate for the materials being handled. Test the gloves to confirm that they do not contain interfering elements or elements of interest, since they may cause contamination. The development of clean area sample handling protocols is encouraged.
5.2 All laboratory samples should be collected in accordance with Practices D4057 and D4177. The personnel collecting the samples should be properly trained in sampling, since invalid sampling can lead to invalid results. The responsibility of the laboratory starts on receipt of the sample in the laboratory.
5.2.1 It would be useful for the laboratory personnel to assist the plant personnel in securing a representative contamination-free sample. Often inappropriate or unclean containers are used to collect the samples in plant or field. It would help if the laboratory can provide pre-cleaned sample containers to the plant personnel collecting the samples.
5.2.1.1 The clean container, lid, liner seal, and liner adhesive shall not contaminate the sample, and the sample shall not compromise the integrity of the container.
5.2.1.2 Consult Department of Transportation packaging guidelines or other appropriate sources, such as placing in a Hazmat-Pak, taping caps in place after securing tightly container/liner compatibility, judicious use of vermiculite as a packing material considering contamination as well as user safety from inhaled dust, etc.
5.2.2 To preserve sample integrity and prevent the loss of volatile components, which can be present in some samples, they shall not be kept uncovered any longer than necessary. Samples need to be analyzed as soon as possible after taking from the bulk supplies to prevent loss of volatile species or contamination.
5.3 Before taking an aliquot of the bulk sample, ensure that the sample is well-mixed and homogenous. This can be ensured, if necessary, by the use of an ultrasonic device or vigorously stirring with a magnetic or mechanical stirrer or shaker. Purge the sampling port lines on tanks properly before collecting the samples.
5.4 Care must be taken during the homogenization procedure that the sample is not contaminated with dust, particles, or by contact with other sources of metals.
5.4.1 Stabilized crude oils typically contain multiple phases, particulates, and volatiles. Decisions whether to separate the phases and analyze them separately, or homogenize the whole sample need to be made prior to analysis.
5.5 Many samples contain volatile components. Hence, it is prudent to keep the sample refrigerated until ready for analysis. This is particularly true for products such as gasoline, diesel, reformulated gasoline, jet fuel, etc. Vessel pressure integrity and permeability need to be considered.
5.5.1 Unless a given test method procedure states otherwise, refrigerated samples should be warmed to ambient temperature before taking an aliquot for analysis. During warming, condensation of moisture on the sample containers is a concern.
5.5.2 Potential overpressure hazard, just like heating a sealed container in an oven needs to be lessened by loosening the cap to prevent rupture. However, this can result in loss of light ends, depending on the material composition.
5.5.3 If the sample does not readily flow at room temperature, heat it to a sufficiently high and safe temperature to ensure adequate fluidity.
5.5.4 Great care needs to be exercised in heating the viscous samples prior to analysis. Changes in chemical composition, loss of volatile elements, etc. are causes for concern.
5.6 Fuel samples can contain particulate matter and free water. In order to be representative, always shake vigorously prior to taking a test specimen for analysis.
5.6.1 Samples that have been in transit for several days, idle in storage, or viscous may be placed in a heated ultrasonic bath to break up clusters of particles and to bring them back into suspension. The samples may be vigorously shaken with a power mixer after being in an ultrasonic bath and prior to pouring a test specimen for analysis. The bath temperature should be 60°C for very viscous fuels and below the flash point for non-viscous fuels. The total agitation time for a sample should be at least 2 min.
5.6.2 Impeller mixers can be a source of contamination as well. Air-driven motors (for example, oil or dust), bearing wear contaminants, leaching of metals from impellers, impeller abrasion contact with container, etc., are some of the points that need to be thought of as potential contamination sources.
5.6.3 From a safety perspective, electric motors present an ignition hazard with flammables. Long term use of ultrasonic baths can overheat samples. To prevent fire or explosion, an operator should always remain present during these preparations and never leave any of these operations unattended.
6. Contamination Control
6.1 Samples that are used for the determination of metals shall be scrupulously kept away from contact with metal containers and laboratory utensils. Wherever possible, plastic or other inert materials shall be used.
6.2 Reagents and solvents used in chemical processing of the samples need to be free of trace elements. For example, laboratory wipe tissues commonly used in many laboratories for wiping laboratory glassware has been reported to contain 48 ppm of zinc, and could be a potential source of contamination.
6.3 Trace elements in the sample can interact with the wall containers and subsequently will result in low biased results. On the other hand, if the sample matrix is reacting with the containers, metals will leach out in the sample leading to high biased results. However, in both cases, the results will not represent the true elemental concentration in the original sample.
6.4 Acid-washed glass containers lined with inert substances and polyethylene-polypropylene-PTFE, or both, containers are the best containers for storage of petroleum products and lubricants.
6.5 Materials used in dilutions, solubilizations, or adjusting pH, etc. need to be screened for possible contamination, especially in trace element analysis.
