Accurate mass alone cannot distinguish drugs that share the same molecular formula and exact molecular-ion mass. A postmortem blood method developed by Rosano and colleagues separated morphine from hydromorphone and codeine from hydrocodone chromatographically before quantitation. Its good overall performance depended on the complete analytical method, including separation, internal standards and identification criteria.
The study behind the distinction
The 2014 study developed a UPLC–MSE/time-of-flight method for 16 drugs and metabolites in postmortem blood and compared it with a routine UPLC–MS/MS method using nominal-mass detection. The panel included opioids and their metabolites, cocaine-related compounds, citalopram, dextromethorphan and diphenhydramine. It was a defined quantitative panel, not validation for every compound that might appear in a general drug screen.
For the casework comparison, discarded extracts from 159 medical examiner cases were de-identified and reanalyzed. The cases had initially screened positive for relevant analytes and undergone routine quantitative confirmation. Thus, the comparison used previously prepared extracts rather than two independent collections of blood. The paper did not provide a uniform anatomical collection site, death-to-collection interval or decomposition classification for this case series.
Two pairs that exact mass cannot resolve
The reported molecular ions for morphine and hydromorphone were both m/z 286.1443. Codeine and hydrocodone both had a reported molecular ion of m/z 300.1600. A narrower mass window cannot distinguish the members of either pair on that molecular ion alone.
The investigators optimized chromatographic conditions to separate these pairs because they also share some fragment-ion masses. For high-resolution quantitation, molecular-ion data were extracted within ±5 parts per million of exact mass. Identification incorporated retention time within 0.3 minutes of reference analysis, molecular and isotope information, and available diagnostic fragments. Low collision energy was 6 eV; the higher-energy acquisition ramped from 10 to 40 eV.
The implication for reviewing a report is specific: “high resolution” describes a useful measurement capability, but does not identify which combination of evidence distinguished the reported analyte from an isomer or interference.
What quantitative validation demonstrated
The validated calibration range was 25–3,000 ng/mL for all 16 analytes, with calibration R² above 0.99. The detection threshold was administratively defined at 10 ng/mL and checked in 12 analytical batches. The lower quantitation limit was 25 ng/mL. A detection between those limits therefore did not become a fully quantified result merely because the instrument supplied a numerical signal.
Extraction recovery exceeded 60% in the abstract’s summary, and matrix suppression or enhancement remained within 20%. Precision and bias were tested with 16 replicate measurements of control material at 40, 125, 1,600 and 2,500 ng/mL. Both methods met the study’s precision criterion of CV below 15% and bias criterion below 20%, although the high-resolution method showed greater variation for several analytes at low and high concentrations.
The comparison reported concordant findings for 253 positive and 2,269 negative analyte results in the 159 cases. These are analyte findings, not 2,522 separate decedents. The pooled quantitative regression was y = 1.002x + 1.523, with R² = 0.993. The authors reported no significant between-method quantitative difference. Twenty-two comparisons included a concentration between detection and quantitation limits, reported as below 25 ng/mL for at least one method.
Internal standards can complicate identification ions
Analyte-matched deuterated standards supported quantitation, but some lost their deuterium labels during fragmentation and produced ions shared with the corresponding drug. The authors identified this issue for several standards, including codeine-D3 and hydrocodone-D3. Testing D6 alternatives for those two analytes made the listed fragment ions available as distinct identifiers.
This was not evidence that internal standards should be omitted routinely. It showed why the laboratory must understand which ions remain independent identifiers under its chosen acquisition conditions. The proposed use of additional molecular-ion ratios also required further investigation and standardization, according to the authors.
Analytical agreement does not answer the biological question
Agreement supports measurement performance for this panel and workflow. It does not show that a measured postmortem concentration equals its antemortem value, or that a detected drug caused impairment or death. Those questions require specimen and case interpretation beyond analytical validation.
For related distinctions, see why authentic specimens matter in assay validation and what a negative autopsy screen can exclude.
Primary source
Rosano TG, Na S, Ihenetu K, Swift TA, Wood M. Multi-drug and Metabolite Quantification in Postmortem Blood by Liquid Chromatography–High-Resolution Mass Spectrometry: Comparison with Nominal Mass Technology. Journal of Analytical Toxicology. 2014;38:495–506. https://doi.org/10.1093/jat/bku066.
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