Laboratory Procedures

A fundamental question is how large should the samples sent to the laboratory be. The answer to this question depends on the procedures used by the laboratory. A state-of-the-art commercial laboratory procedure includes the following steps:

A portion of the sample material sent to the laboratory is weighed, dried, and reweighed to determine the moisture content. The limiting factor at this stage of the procedure is usually the size of the laboratory's drying oven. A portion of the dried material is ground into particles of 1/8 to 1/4 in.

A portion of the ]/&-to-1/4-in material is finely ground into as close to a powder as possible. For flexible plastic, dry ice must be added prior to fine grinding to make it more brittle.

The actual laboratory test is generally performed on 0.5 to 3g of the finely ground material, depending on the type of test and the specific equipment and procedures.

Variations on this procedure include the following:

Most laboratories do not have equipment for grinding inorganic materials such as glass and metal. In combustion testing, this material is removed from the sample prior to grinding, then weighed and reported as ash. For metals testing, metal objects can be cut up by hand or drilled to create small pieces for testing. Glass and ceramics are typically crushed. Many laboratories do not have fine grinding equipment, so they perform tests on relatively coarse material.

In addition to using different methods for preparing waste for testing, laboratories use different test methods.

The more sample material the laboratory receives, the more material they must exclude from the small quantity of material that is tested. The real question is not how large the samples should be but how field and laboratory personnel should share the task of reducing samples to a gram or two. For practical purposes, the maximum quantity sent to the laboratory should be the quantity the laboratory is prepared to spread out and mix in preparation for selecting the material to be dried. The minimum quantity should be the quantity the laboratory is prepared to dry and grind up.

Composite laboratory samples are typically accumulated in plastic trash bags, then boxed for shipment. An alternative is to accumulate the samples in 5-gal plastic buckets with lids. Plastic buckets are more expensive than plastic bags but have several advantages:

Plastic buckets (and their lids) are easier to label, and the labels are easier to read. Adding material to plastic buckets is easier. The lids, which are lifted only when material is added to the buckets, prevent moisture loss during the active sampling period.

Sample material can be compacted in plastic buckets if it is pushed down around the inside edge. The buckets can be used as shipping containers. The buckets can be reused if the laboratory ships them back.

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