DNAx20SampleHandling - Sample Handling Considerations for...

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1 Sample Handling Considerations for Biological Evidence and DNA Extracts Theresa F. Spear California Department of Justice California Criminalistics Institute Introduction This article will make recommendations for handling biological evidence from sample collection to sample storage. Depending upon the type of biological evidence, it may be very difficult to detect certain types of samples. Saliva and semen stains are often not visible. Even bloodstains may be difficult to find if they are on a dark substrate. Although specialized, forensic light sources are sometimes helpful in visualizing these stains, they cannot always be relied upon to find all biological stains. It is especially difficult to detect saliva or semen on some dark colored fabric with any of the widely used forensic light sources. Thus, a stereomicroscope or chemical mapping techniques may be required to improve the chances of finding all biological evidence. After a sample has been detected, steps must be taken to insure that the sample integrity is maintained and the chances of contamination are minimized. Sample Collection in the Field Once a sample has been located and a decision has been made to take the sample, issues arise as to how to collect, dry and package it for transportation to the laboratory. Considerations relevant at this stage include the ability to obtain as much sample as possible, to minimize degradation and finally to insure that samples are not inadvertently contaminated with other biological samples. The standard recommendation for collecting biological evidence is not to remove the stain from an object but rather to collect the object with the stain. The advantages of this strategy are that the entire stain is obtained, it is not necessary to collect an “unstained control” sample and there are no further manipulations required that might negatively impact the sample. If the stain is on a smooth, non-porous surface (i.e. it can be easily “flaked” off), it will be necessary to protect the stain from contact with other objects. Depending upon the nature of the evidence, a stain can be protected by immobilizing the evidence item in a cardboard box (e.g. with pieces of wire) or by taping a piece of paper over the stain (if this will not destroy other evidence, such as fingerprints). Provided that the stain can be adequately protected, this is the optimum collection procedure. Given that some stains are found on immovable objects, it is not always possible to collect the object with the stain. Some samples will need to be collected in the field. If the entire object cannot be collected then the next best way to collect biological evidence is to remove the stain by cutting it out (e.g. from a piece of carpet) . Remember to use clean scissors and to cut out an “unstained” control. Scissors or tweezers can be cleaned by rinsing with clean water and then drying with tissue. Repeat this cleaning process twice prior to each sampling.
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2 It is not necessary to clean tools with bleach. Improperly used, bleach could destroy
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DNAx20SampleHandling - Sample Handling Considerations for...

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