Bioanalytical Chemistry: Measuring Analytes in Biological Matrices
Core concepts
Bioanalytical chemistry quantifies drugs, metabolites, and biomarkers in matrices such as plasma, serum, urine, and tissue. Candidates must understand sample preparation (protein precipitation, liquid-liquid extraction, solid-phase extraction), chromatographic separation (reversed-phase LC is the default for small molecules), and detection (LC-MS/MS with electrospray or APCI ionization, or ligand-binding assays such as ELISA for macromolecules).
Key vocabulary: selectivity (ability to measure analyte amid interferences), matrix effect (ion suppression or enhancement in MS), calibration model (linear or quadratic, weighted, over the validated range), LLOQ (lower limit of quantification), carryover, recovery, and stability (bench-top, freeze-thaw, long-term, stock). Internal standards-stable-isotope-labeled analyte for MS, or a structurally similar analog-compensate for extraction and ionization variability.
Traps in questions
- Confusing accuracy (closeness to true value, assessed by recovery/QC bias) with precision (reproducibility, assessed by %CV).
- Assuming a high response means a good assay: matrix effects can inflate or suppress signal without affecting precision.
- Forgetting that calibrators are prepared in the same matrix (e.g., blank plasma) to mimic sample behavior.
- Mixing up LLOQ (lowest level meeting accuracy and precision criteria) with LOD (detection only, no quantitative criteria).
Rule of thumb
A common acceptance criterion: at least 67% (4 of 6) of calibration standards within ±15% of nominal (±20% at LLOQ), and QC samples within ±15%. Worked example: if a QC at 50 ng/mL reads 56 ng/mL, bias = (56−50)/50 × 100 = +12%, which passes a ±15% criterion.