Sampling of Stains and Deposits 1 Basis for Sampling

Sampling decisions should flow directly from the questions in a case. The selection of sam- ples should be governed first by significance, then by specificity and utility. Significance is defined as the potential to narrow down hypotheses. Specificity is defined as the potential to narrow sources. Utility or usefulness is defined in this context as the potential to pro- duce interpretable results (Cwiklik and Gleim, 2009).

In Case Example 4.6, automotive paint smeared on the shoulder of a person struck by a car was instrumental in leading police to investigate the case as a homicide rather than a hit-and-run. Numerous multilayer paint chips were also observed on the clothing, only some of them with the same type of topcoat as the smeared paint. The multilayer chips had far greater potential to narrow a source vehicle for the paint, but only the less specific smears could narrow down hypotheses regarding the person being struck. The primary criterion for sampling selection should be significance. An important secondary criterion is specificity. In the case example, once the topcoat color and type was known, the mul- tilayer paint chips with that type of topcoat could further narrow the source vehicle by means of the undercoats and primer.

If a potentially significant stain or deposit is of poor quality, it should be tested anyway, unless there is another stain or deposit that can provide the same level of information in terms of source, activity, and offense, as discussed in Section 4.2.4 (Cook et al., 1998). If the question is one of source, the more significant stain or deposit of lesser quality provides a reference for selecting additional samples that can provide better source-level information. Again, this is illustrated in Case Example 4.6, in which a smear of a paint topcoat was the reference for selecting multilayer paint chips for further testing.

A third consideration in selecting among potentially significant sample areas is the likelihood of getting useful results. An adequate sample is of sufficient size to permit the requisite testing and of sufficient quality to permit clear interpretation. Clear inter- pretation means that when an answer is obtained, the analyst knows which question it answers. A sample of sufficient quality truly represents the material of interest and does not include anything extraneous. Extraneous material is contamination. However, in the case of a mixed deposit or a co-deposit, a sample should represent the mixture. A sample should permit clear attribution; for example, whether the results are from the deposit or

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the background, from which layer or part of a mosaic, and, when possible, from which component of a mixture (Cwiklik and Gleim, 2009).

Techniques for sampling depend on the type of material and the anticipated method of testing. For stains and deposits specifically, techniques for sampling are of several types, as described in the following sections.

4.4.2 Preliminary Sampling

Preliminary sampling using thin clear sticky-tape, (i.e., tape lifts) was mentioned earlier regarding areas where deposits would be expected but are not observed on direct examina- tion. This method is also useful for visible deposits that are difficult to evaluate in situ or for deposits that would benefit from a quick examination at higher magnification. The piece of tape can be placed on a microscope slide, sticky side up, and the deposit examined using reflected light, either with a stereomicroscope or at higher magnification. The tape sample can also be mounted directly in a refractive index oil and covered with a cover slip. (Please see Chapter 8 for discussion.) To lift a visible sample from the garment, the nonsticky side can be wrapped around the end of an object, like a ballpoint pen with the tip retracted, narrow and hard but not sharp, and pressed against the deposit. This removes a small concentrated portion without tearing the tape or damaging the substrate fabric. The tape should be marked and initialed, even if it is on a microscope slide (Figure 4.19).

Preliminary samples can also be obtained using forceps, for fibers and larger particles, or for smaller or finer deposits, sharpened tungsten needle probes. A small amount of material can thus be collected and transferred directly to a microscope slide, onto a drop of distilled water or refractive index oil. This method is superior to the tape-lift method and is recommended for use by trace evidence examiners because it removes so little mate- rial that it barely alters the evidence. If the needle is tipped with a bit of adhesive, it can be swept across powdery deposits, thereby collecting a small sample that can be transferred to a microscope slide (Teetsov, 2002). (Please see Chapter 8 for a more detailed explanation.)

4.4.3 Crusts and Films

Crusts and films can be detached from the substrate using forceps and placed onto a micro- scope slide or piece of hard paper that is directly adjacent. Impacted fibers can behave like crusts and films and should be collected accordingly. If this technique does not work because the film firmly adheres to the substrate, it can be cut from the fabric using a razor blade. A portion of adequate size should be left in place for any reexamination. The sample

can be placed in a clean piece of hard paper and protected with a double three-way fold (a paper bindle). Sticky-notes such as Post-it® _{Notes can serve as both the sampling surface}

and the packaging (because they can be folded into a bindle after sampling), and they are additionally useful in providing the sample with a not-very-sticky anchor for further examination with a stereomicroscope or high-magnification reflected light microscope (Figure 4.20). Remember that the stickiness is due to an adhesive, which may be detected in FTIR or GC-MS (gas chromatography–mass spectrometry) analysis of any organics.

4.4.4 Caked Deposits and Heterogeneous Agglomerates

Caked and agglomerated deposits tend to crumble when removed and are thus best trans- ferred using an implement with sufficient surface area to contain the material. A spatula or razor blade is usually suitable. A sticky-note or microscope slide should be placed directly adjacent to the deposit to receive the sample as well as any particles that may fly off during sampling. The samples can be placed in paper bindles and the bindles taped or placed into a secure plastic bag or paper envelope, securing any powders or small particulates from migrating out of the paper bindle. If an envelope is used, the corners should be taped.

4.4.5 Powdery Deposits

Powdery deposits, whether dense or sparse, can usually be sampled by placing a piece of paper on the adjacent fabric and tapping or gently scraping a portion of the deposit onto the paper. The paper can be folded into a bindle and taped or placed into another con- tainer. If insufficient material is recovered from a light deposit, a supplemental sample can be collected on clear sticky-tape, on a sticky-note, or on an adhesive-tipped needle probe.

Figure 4.20 Particles and crusts removed from a thick blood deposit were placed on a sticky- note for preliminary examination, and the sticky-note was sealed in a zip-lock style plastic bag.

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The sample on thin clear tape can be mounted in situ and is best employed for transmit- ted light microscopy. Sticky-notes use a weaker adhesive that is more suitable for FTIR, GC-MS, or high-magnification reflected light microscopy. A sample collected on an adhe- sive-tipped needle probe can be mounted directly on a microscope slide and the adhesive dissolved. The latter technique is superior but requires more skill.

4.4.6 Stains

It is usually best to excise a portion of a stain. Disposable razor blades work well. Sometimes the examiner is not permitted to perform destructive testing, which would preclude cut- ting the item. In this case, one can place a petri dish (or if one is careful, a large microscope slide) beneath the stained area and drip a suitable solvent onto it, allowing the solvent absorbed from the first drop to evaporate before applying the next drop. A concentrated sample of the stain should remain on the petri dish and can be analyzed. This method should also be considered when the item must be returned intact, or to avoid causing hard- ship to the owner by damaging the item.

4.4.7 Viscous Deposits

Viscous deposits can often be sampled by scraping off a portion. It is best to place the sam- ple directly onto a microscope slide. A plastic container is not a good choice because the components of the viscous material, even if water soluble, may soften the plastic or retain some of the plasticizer, compromising some types of analyses.