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A wide array of swabs used for forensic collection can absorb trace material remarkably well, according to a new series of laboratory tests.

But less than half of the DNA absorbed by the tips is actually ever recovered off the swab material, according to a new study by Dutch researchers in the Journal of Forensic Sciences.

The cotton, foam, nylon flock, polyester and rayon swabs all showed high absorption capacity—but all gave up less than 50 percent of the DNA during an extraction process, according to the scientists. Most yielded just 20 to 40 percent, they added.

“It can be concluded that swabs are well designed to allow the pickup of a DNA sample from a surface,” they write. “However, subsequent extraction of the DNA from the swab is very low compared to the maximum amount of DNA obtainable in theory.”

The tests of the five kinds of swabs looked in-depth at their structure and properties.

Their minute structures were mapped using an electron microscope. The absorption capacity was tested by dunking into a tube with buffer fluid. The extraction efficiency was determined by pipetting a saliva sample directly onto the swab, and then vortexing it to see how much was extracted. The recovery efficiency was tested by moistening the swabs with buffer, and then swabbing a dried saliva sample off a glass slide.

Overall, the results of the hundreds of trials showed two superior swabs.

The nylon flocked (4N6FLOQSwab made and patented by COPAN, and distributed by ThermoFish Scientific and others) and the polyester Alpha swab (made by ITW-Texwipe) showed the greatest extraction efficiency.

But the nylon variety also showed the best recovery efficiency, at 47 percent. The recovery rate for the rest was generally between 20 and 40 percent, they added.

The look deep into the swabs' structures with the electron microscope showed that absorption, at least, was determined more by the tip dimensions and the fiber arrangement—and less about the material itself.

Recovery is a challenge because of DNA interacting with the swab material itself, they added.

“This is thought to be due to the chemical interaction of DNA molecules with the swab surface functional groups,” they write. “In some cases only a limited amount of DNA is present, such as in forensic touch samples. This may result in insufficient DNA becoming available for the determination of a DNA profile.”

But the hand guiding the swab may be as important as the material involved, according to a study reported by Forensic Magazine last fall. The University College London’s Centre for the Forensic Sciences found that experienced scientists consistently got better material yields from a wide range of surfaces, including plastic, metal, glass and other surfaces, as they recounted in Forensic Science International.

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