In Part I of this work, we developed a method for the detection of drugs of abuse in biological samples based on fast gradient elution liquid-chromatography coupled with diode array spectroscopic detection (LC-DAD). In this part of the work, we apply the chemometric method of target factor analysis (TFA) to the chromatograms. This algorithm identifies the target compounds present in chromatograms based on a spectral library, resolves nearly co-eluting components, and differentiates between drugs with similar spectra. The ability to resolve highly overlapped peaks using the spectral data afforded by the DAD is what distinguishes the present method from conventional library searching methods. Our library has a mean list length (MLL) of 1.255 and a discriminating power of 0.997 when both retention index and spectral factors are considered. The algorithm compares a library of 47 different compounds of toxicological relevance to unknown samples and identifies which compounds are present based on spectral and retention index matching. The application of a corrected retention index for identification rather than raw retention times compensates for long-term and column-to-column retention time shifts and allows for the use of a single library of spectral and retention data. Training data sets were used to establish the search and identification parameters of the method. A validation data set of 70 chromatograms was used to calculate the sensitivity (correct identification of positives) and specificity (correct identification of negatives) of the method, which were found to be 92% and 94%, respectively.
Bibliographical noteFunding Information:
The authors wish to acknowledge funding received from the Research Corporation (grant # RA-0344, SCR), and the National Institutes of Health (grant # 5R01GM054585-09, PWC). This work was also funded by the National Institute of Justice (PWC), through the Midwest Forensics Resource Center at Ames Laboratory under interagency agreement number 2002-LP-R-083. The Ames Laboratory is operated for the US Department of Energy by Iowa State University, under contract No. W-7405-Eng-82. The authors also graciously acknowledge Kate Fuller from the Minnesota BCA for her assistance with the blood extraction methods and Glenn Hardin from the Minnesota BCA for his assistance in developing the library of compounds used in this study.
- Factor analysis
- Gradient elution LC
- Library search
- Retention index