6.2.2.5 Particle size reduction

Particle size reduction can facilitate more representative subsampling.If the contaminant is present as a solid particulate, particle size reduction through sample grinding can facilitate more representative subsampling by reducing the range of particle sizes and the maximum size present. The determination of the particle size reduction technique and maximum target particle size should be determined during project planning and is part of DQO development. It should be noted that the maximum particle size has a significant effect on the FE. See the discussion in Section 2 and Hyperlink 7 on Gy sampling theory regarding the relationship between particle size, uncertainty, subsample size, and FE. Select the appropriate grinding process and equipment to achieve the maximum particle size that was determined in project planning (see Section 3). Many common options are described in USEPA guidance (Gerlach and Nocerino 2003).

Depending on project DQOs, particle size reduction may or may not occur in combination with particle size selection (see Section 6.2.2.6). Examples of when particle size reduction may be appropriate after particle size selection include, but are not limited to, metals at small arms ranges, clay target fragments at skeet ranges, lead-based paint chips, munitions constituents, etc.

Extended high-speed milling can elevate sample temperature due to friction. The thermal stability and volatility of the contaminant(s) should be considered when choosing equipment and a grinding scheme. For example, USEPA SW-846 Method 8330B for nitrocellulose-based propellant residues, specifies a 2-minute (or longer) cool-down period between five 60-second grinding intervals to maintain acceptable temperatures and minimize loss of volatile energetic contaminants.

Milling is not recommended for general purpose application for organic contaminants at this time (other than energetics, USEPA SW-846 Method 8330B), as it has not been extensively evaluated. Loss of organic COCs may occur due to increased temperatures during milling, as stated above. Additionally, excessive milling may lead to destruction of organic contaminants, as demonstrated with the mechanochemical dehalogenation (or mechanochemical destruction) soil remediation process. See Reference Guide to Non-Combustion Technologies for Remediation of Persistent Organic Pollutants in Stockpiles and Soil (USEPA 2005) for additional information. The usefulness of particle size reduction by milling for organic contaminants is usually small because the larger mass (10–30 g or more) normally extracted and analyzed and the particulate “nugget” effect is often minimal. However, “nuggets” can and do occur for specific organic contaminants; e.g., soil analyzed for PAHs at skeet ranges can exhibit a nugget effect due to the deposition of clay pigeon fragments. In such cases, the advantages and limitations of milling for organic contaminants should be evaluated during project-specific systematic planning.

Milling is recommended for ISM metals analyses. Grinder types and the applicability for the processing of metal particulates are still being evaluated. Care must be taken to avoid the loss of fines during the grinding and transfer process. Milling may also increase some measured metals concentrations when metallic content in the center of the larger particles is subsequently released through particle size reduction and included the metals analytical process. The potential improvement in precision and increase in measured metals concentrations should be considered during project-specific systematic planning when determining if milling is appropriate. In some instances it may be possible to meet precision DQOs without milling by increasing the metals subsample size to 10 g. See Section 6.3.3 for further discussion.

Grinder surfaces that contain the contaminants of interest or compounds that may interfere with the analysis of the contaminants should be avoided. If metals are contaminants, then compare the composition of any metal-containing grinding surfaces with the target analyte list. For example, high chromium steel puck mills should not be used when total chromium is a contaminant. Ceramic, agate, tungsten carbide, or low chromium steel grinding components would be more appropriate. The grinder should be checked for contamination by processing a suitable blank material to demonstrate that the grinder does not release contaminants at the concentrations of interest. Confirm that the laboratory has the proper grinding equipment during project setup.

When grinding has been selected as part of the ISM DQOs, the entire conditioned ISM sample is ground. Splitting an unground ISM sample with high heterogeneity due to the “nugget” effect can lead to nonrepresentative subsampling. If the milling equipment is not large enough to process the entire sample at once, then mill smaller portions of the sample and then recombine and mix after the milling step (see Section 6.2.2.4). The milling equipment listed below is not an exclusive list of equipment capable of meeting ISM quality objectives; it is an example list of equipment that has been used successfully in the past.

Mortar and pestle grinding can be accomplished with either manual or automated systems. The large automated systems are recommended because of increased capacity, better reproducibility, and reduced likelihood of repetitive-stress injuries. The sample contact materials can be steel, ceramic, or others depending on the contaminants. The sample is loaded into a heavy walled bowl. The sample is crushed between the bowl wall and the pestle by manually pushing the pestle or spinning the bowl with a fixed pestle in an automated system.

Rotary pulverizers can reduce particle size from approximately 6 mm to <100 μm. The distance between the grinding plates determines the final particle size. Dry sample is fed into the chute, and ground sample is collected in a hopper beneath the grinding plates.

Ball mills consist of both high-speed and low-speed systems. Typically, the sample is placed in a container along with a grinding medium and shaken rapidly or tumbled slowly. The grinding medium (e.g., steel or agate balls, ceramic cylinders) crushes the sample particles. High-speed systems consist of high-strength containers and high-speed shakers; thus, they can provide more reproducible reduction to <100 μm particle sizes. Typical grinding time for high-speed systems is a few minutes. The low-speed systems typically consist of single-use cans, a grinding medium, and a low-speed tumbler or roller. Roller mills or paint can shakers are common examples. Typical grinding times are several hours; however, excessive overgrinding should be avoided due to possible analyte loss.

Dish and puck mill (shatter box) grinding is described in USEPA SW-846 Method 8330B (USEPA 2006c). The sample is loaded into the dish with puck inserted. If the dish is not large enough to process the entire sample at once, then grind smaller portions of the sample and then recombine and mix after the grinding step. The grinding cycle time and cooling period (if necessary) depend on the analytes of interest. An example grind cycle consists of 1 minute of grinding and at least 2 minutes without grinding to allow the dish and sample to cool. This process may be repeated two to four more times, depending on the materials to be ground. The cooling part of the cycle reduces internal temperatures and hence thermal degradation of the analytes. USEPA SW-846 Method 8330B (energetics) recommends a final particle size of <75 μm. The optimal grinding conditions and final particle size for other contaminants might be different than those described in USEPA SW-846 Method 8330B for energetics. Performance for other contaminants should be demonstrated with reference materials or other “known” samples.