6.3.3    Inorganics

Metals

A number of issues should be considered when evaluating ISM metal samples:Sample processing considerations for inorganics cover analyte stability and subsample size for digestion.

  • While drying temperature and time are generally not an issue for most metals, they should still be considered if certain organometallic compounds, metal species, and/or Hg are analytes of concern, due to compound volatility.
  • Volatile metals, such as certain organometallic compounds, metal species, and/or Hg, require careful control to maintain acceptable grinding temperature (see the grind and cool technique described in USEPA SW-846 Method 8330B) or choose a grinding technique that does not elevate the sample temperature during the grinding process. Adjust the subsample size to 10 g if the final particle size is >0.25 mm.
  • Milling (grinding) is recommended for ISM metal samples (see Section 6.2.2.5). Dish puck mill–ground samples with particle size <0.25 mm can often meet DQOs with a 2 g subsample. Standard metal digestions are based on 1–2 g soil aliquots.
  • Alternatively, if milling is not required to meet project-specific objectives and/or not applicable for the selected COPCs, etc., the recommended minimum mass for unmilled (unground) ISM analysis is 10 g, based on an FE of <15% for a <2 mm sample particle size. If a larger FE meets project-specific DQOs, a smaller subsample mass may be acceptable. Laboratories may need to modify the standard metal digestion procedure or perform multiple digestions (e.g., five individual 2 g digestions) and combine the digestates to account for the increased soil mass. Sample digestion procedures that modify the conventional sample mass to reagent ratios and final sample volumes should be verified for recovery of the metal(s) of interest by successfully digesting a reference material using the modified sample mass to digestate ratio. Alternatively, demonstrate that the results of triplicate 10 g preparations meet the project-specific objectives when compared to thirty 1 g preparations using an ISM site-specific sample.
  • Grinding may release metals either naturally occurring or anthropogenic, such as arsenic, mercury, etc., from larger particles resulting in elevated results. These metal concentrations may not be available in the unground sample and, therefore, may not be applicable to the site-specific CSM and/or DQOs.
  • The grinding equipment may contribute metal concentrations to the ISM sample. This possible contribution is due to the metal composition of the grinding, crushing, or pulverizing apparatus. Common metals include chromium, cobalt, iron, manganese, nickel, tungsten, etc. Metallic composition analysis is usually available from the manufacturer. For example, avoid high chrome steel when low parts per million concentrations of chromium are of interest.
  • Malleable metals, such as lead or copper, may smear in grinding machinery. If a significant amount of larger particle size malleable metals are expected in ISM samples, additional sieving and fractional analysis should be considered or alternative sample preparation techniques may need to be investigated.

 

For additional information and considerations for inorganic ISM samples, see Implementation of Incremental Sampling (IS) of Soil for the Military Munitions Response Program (USACE 2009).