The identification of a particle as FDR is based on a combination of morphological and elemental composition criteria and also on the association of the particle with other particles present in the sample.

The following three particle compositions have thus far been observed only in FDR and are considered unique to it:

1. Lead, antimony, barium

2. Barium, calcium, silicon, with a trace of sulfur

3. Antimony, barium

The following five particle compositions are consistent with FDR but are not unique to it:

1. Lead, antimony

2. Lead, barium

3. Lead

4. Barium if sulfur is absent or only a trace

5. Antimony (rare)

The lead, antimony and the lead, barium particles, although not unique, have been found in few occupational residues and are therefore considered to be fairly characteristic.

Any particle, unique or consistent, may also contain one or several of the following and only the following elements: silicon, calcium, aluminum, copper, iron, sulfur, phosphorus (rare), zinc (only if copper > zinc), nickel (rare, only with copper, zinc), potassium, chlorine. The presence of some tin is a possibility in obsolete ammunition.

The compositions, shapes, sizes, appearance, and range of particle types should all be considered during interpretation. Particles that are individually consistent with FDR should not be found with otherwise similar particles that are in some way inconsistent with FDR.170

The particle classification scheme is the basis of the particle analysis method for FDR detection and identification. A revised particle classification scheme is presented in a later chapter.

Other work has shown that the elemental composition can vary between the inside and the outside of a particle, and using SEM and NAA the average amount of barium and antimony per particle was determined to be in the region of 0.2 to 20 ng for barium and 0.4 to 7 ng for antimony.

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