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Paper Spray Testing of Fingerprints Shows Promise for Drug Testing

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NEW YORK (360Dx) – University of Surrey researchers have developed a paper spray mass spectrometry test for detecting cocaine in fingerprint samples.

The method, which was described in a study published this month in Clinical Chemistry, could prove an inexpensive and high-throughput approach for testing for cocaine and other drugs of abuse, said University of Surrey researcher Catia Costa, first author on the paper.

Paper spray mass spec is a relatively new technology, first described in 2010 by a team including Purdue University researcher Graham Cooks. The approaches involves treating a piece of paper containing the sample of interest with high voltage, which generates the ions that are then introduced to and analyzed by the mass spec.

The fundamental advantage of paper spray mass spec testing is its throughput, Costa said. Because the technique requires minimal sample prep and no chromatography, assays can be run very quickly, which, she noted, translates into lower costs per test. The assay developed in the Clinical Chemistry study takes four minutes per sample to run, and the authors noted that other paper spray assays can be run in less than a minute.

Using paper spray mass spec to analyze 239 fingerprint samples, Costa and her colleagues were able to detect cocaine, benzoylecgonine, or methylecgonine with a 99 percent true-positive rate and 2.5 percent false-positive rate, indicating that the technique could be a viable approach to testing for drugs of abuse.

Previous research has demonstrated the ability of paper spray mass spec to detect a variety of drugs in blood with good sensitivity, but, as Costa and her coauthors wrote, expanding the method to fingerprint samples provides advantages in terms of ease of sampling and traceability.

Blood and urine are still the most commonly used samples for drug testing, but interest is growing in using fingerprints as samples, particularly in settings such as courts, prisons, and rehab clinics. Fingerprints are easier and faster to collect and store than blood or urine samples and are in themselves an identifier of the sample giver, eliminating a potential source of error.

In theory, paper spray mass spec fits well with fingerprint sampling, as prints can be taken using the same piece of paper that is then subjected to mass spec analysis. Relatively little work has been done to determine whether paper spray mass spec can analyze such samples with sufficient sensitivity and accuracy, however.

The researchers collected fingerprints using Whatman Grade 1 chromatographic paper treated with silver nitrate, which allowed them to retain the subjects' fingerprint ridge details. They then analyzed the samples via paper spray mass spec, using a Waters Micromass QTOF 2 instrument for their analysis.

The approach showed less sensitivity than published methods using paper spray mass spec for detection of cocaine in dried blood spots, a finding the researchers said was potentially due to the lower sensitivity of the mass spec instrument they used, as well as to the fact that in the case of fingerprint analysis, internal standards must be added to the sample separately, which increases variability.

Nonetheless, the researchers were able to detect the drugs at sensitivities within established testing cutoffs.

The assay is not rigorously quantitative, Costa said, noting that this is a limitation of fingerprint-based testing, of which she said, "even different fingers from the same person can give different quantitative results." However, she that given the test's high selectivity, it "can be used as a semi-quantitative method." She added that she and her colleagues were also working to improve its quantitative performance.

Despite the assay's high selectivity, it did return a small percentage of false positives. The researchers found they were able to eliminate false positives by taking samples from two fingers per subject, though this also decreased the true-positive rate to 94 percent from 99 percent.

In actual practice, Costa said, subjects testing positive on the paper spray test would likely then undergo confirmatory testing using a highly specific and quantitative method.

Costa said she and her colleagues are now looking into the commercial potential of their assay to determine where in the testing workflow it might prove most useful. They are also working to test its performance in larger cohorts and expand the number of substances it tests for, she said.