Advisory Committee Chair
Jason G Linville
Advisory Committee Members
Elizabeth A Gerdner
Ray H Liu
Document Type
Thesis
Date of Award
2008
Degree Name by School
Master of Science (MS) College of Arts and Sciences
Abstract
In the testing of urine samples for illicit drugs by gas chromatography-mass spectrometry (GC-MS), a drop in internal standard response is an indicator of possible sample adulteration. In 2007, Dennis V. Canfield (Liu et al. 2007) developed a formula to calculate factor A, an empirically determined value for the lowest acceptable internal standard (IS) signal-to-noise (S/N) response. Using this method, Liu et al. were able to distinguish true negatives from samples showing interference by ibuprofen and hydrogen peroxide (H2O2) in the detection of 11-nor-9-carboxy-9-tetrahydrocannabinol (THCCOOH) in urine samples. To determine if Liu’s method can be universally applied, a set of samples containing THC-COOH, deuterated IS (THC-COOH-d3), and ibuprofen were prepared at concentrations similar to Liu et al., but tested under a different set of laboratory conditions, and therefore a new value for factor A. Two types of ratios were used to evaluate the IS response of the samples. The first approach was to divide the IS signal by the noise. Two sets of data were produced, from ratios calculated using corrected and uncorrected values, respectively. The second approach was to divide the uncorrected IS intensity by the uncorrected THC-COOH intensity. While the uncorrected IS S/N ratio and the uncorrected IS intensity to THCCOOH intensity methods provided some indication of ibuprofen interference, the corrected IS S/N ratio method proved the most effective at identifying negative, positive, and inconclusive samples. Further research is needed to more closely examine the transition from no, to partial, and to full interference by ibuprofen in such samples. It is recommended that samples which produce an IS S/N ratio very close to the cutoff value of factor A also be considered for follow-up testing, as they may be exhibiting partial interference. The factor A method could also be useful in identifying other types of chemical interference encountered in drug testing, in addition to that of ibuprofen or H2O2 on THC-COOH.
Recommended Citation
Waters, Laura S., "Comparison Of Two Methods For Differentiating Negative From Inconclusive GC-MS Test Results Using An Isotopic Analog Of The Analyte As The Internal Standard - 11-Nor-Δ9- Tetrahydrocannabinol-9-Carboxylic Acid Example" (2008). All ETDs from UAB. 3649.
https://digitalcommons.library.uab.edu/etd-collection/3649
