Investigation of a false positive codeine interference in an AACB/RCPAQAP urine toxicology survey sample

Introduction: A false positive codeine interference was suspected following review of AACB/RCPAQAP Urine Toxicology survey result (Cycle 29 - Sample 2). Although 27% of respondents reported the detection of codeine (median recovery 109 μg/L) using LC-MS based techniques, the high percentage of laboratories failing to detect codeine prompted an investigation into the presence of a potential interference. A participant also reported a similar interference in samples containing high concentrations of oxycodone using a LC-MS/MS technique. An investigation was subsequently undertaken using LC-QTOF-MS to identify the interfering substance.

Methods: Sample data obtained by data-independent acquisition (Waters G2-XS QTOF) for survey 29-02 was interrogated using QTOF software elucidation tools including elemental formula prediction and in silico fragmentation. Presumptive identification was achieved using accurate mass and molecular structure obtained from the Chemspider.com database.

Results: Untargeted data acquired from survey 29-02 indicated the presence of oxycodone and associated metabolites. A prominent signal was also observed at m/z 318.1695 co-eluting with the disputed codeine result. Using QTOF-MS elucidation tools, the co-eluting compound was presumptively identified as 6-oxycodol (C18H23NO4; m/z 318.1695), a minor metabolite of oxycodone. It was proposed that MS in-source fragmentation of 6-oxycodol, resulting in a subsequent water loss, would produce a fragment of similar mass to codeine (m/z 300.1587). Prediction of the formation of this isobaric fragment was also confirmed by in silico fragmentation analysis using the 6-oxycodol molecular structure. Also predicted was the formation of additional product ions commonly used in the LC-MS identification of codeine.

Conclusion: In-source fragmentation of 6-oxycodol was presumptively identified as a cause of false positive codeine interference in some LC-MS based methodologies. Resolution of this interference may involve modification of chromatographic separation or the use of unique product ion ratios. Acquisition of certified reference material is recommended for definitive confirmation of the interference.