Sample Preparation, Purification and Instrument Calibration

Preparation can lose, transform or introduce analytes. A validated selective assay should distinguish a target from relevant degradation products; it should not automatically label every surviving peak as the parent. Recovery experiments, stability studies and controls help assess these risks.

Dilution can reduce interference or bring a high result into range, but also raises the effective reporting limit in the original specimen. Include dilution factors and assess whether the resulting sensitivity still answers the case question. These limitations can matter even when the chromatogram appears clean.

Screening and confirmatory procedures both require appropriate quality controls. A screening method may prioritise speed or broad coverage, but its limitations must be understood and communicated. Scientific fitness for purpose and legal admissibility are separate questions.

A validation record describes the intended targets, matrices, ranges and performance requirements. Reviewers should be able to follow how those requirements were tested and what limitations remain. Missing evidence may weaken confidence; a validation file does not by itself determine a court outcome.

In a modern forensic toxicology workflow, preparation, calibration and detection all contribute to performance. Investigate their effects together rather than assuming one stage always dominates uncertainty.

Extraction is one possible preparation step. Selection considers target polarity, charge, volatility, stability, binding and matrix composition. Some methods use direct dilution or another supported preparation instead of extraction.

Liquid-liquid extraction (LLE) is the classical workhorse. The analyte partitions between an aqueous sample (blood, urine, viscera homogenate) and an immiscible organic solvent (dichloromethane, ethyl acetate, hexane, n-butyl chloride) chosen for its polarity match to the analyte. Acid-base manipulation drives the analyte into the right phase: alkalinise the aqueous phase to push basic drugs (morphine, cocaine, amphetamines) into the organic, acidify it to pull acidic drugs (barbiturates, salicylate) the same way.

Classical acid–base fractionation and salting approaches can be useful for selected targets. Liquid–liquid extraction depends on distribution and phase volumes, not merely survival in an organic solvent. Emulsions and binding can affect performance, and any corrective treatment must be assessed for analyte loss.

Solid-phase extraction (SPE) retains suitable analytes on a sorbent while other components are removed. Oasis HLB is a polymeric reversed-phase sorbent; MCX combines reversed-phase and strong cation-exchange retention, and MAX combines reversed-phase and strong anion exchange. C18 is another reversed-phase option. Select chemistry and conditions for the analyte and matrix.

A final elution in a small volume of suitable solvent releases retained analyte. Recovery and clean-up depend on the method and must be measured; they are not guaranteed by choosing SPE. Use devices within the validated procedure and control carryover.

SPME samples analytes onto a small extraction phase, commonly a coated fibre, through headspace exposure or direct immersion. GC applications often use thermal desorption; LC applications can use solvent desorption. The extracted fraction depends on coating, time, temperature and matrix conditions.

SPME can suit selected volatile and other targets, but is not required for routine blood-alcohol headspace analysis or every fire-debris examination. Fibre blanks, carryover, coating selectivity and quantitative conditions require evaluation. A small solvent requirement does not eliminate matrix-dependent response.

Pressurised solvent extraction uses elevated temperature to assist extraction while pressure maintains suitable liquid-solvent conditions. Soxhlet repeatedly contacts material with condensed solvent. Their suitability depends on target stability, specimen and the demonstrated extraction performance.

Neither approach guarantees exhaustive recovery of every bound target. Heating can increase extraction and co-extraction or cause transformation. Demonstrate recovery and any required cleanup for the actual method rather than assuming that faster equipment produces a better result.

Dilute-and-shoot reduces sample concentration before measurement, sometimes with additional centrifugation or filtration. It can be appropriate when sensitivity and matrix tolerance remain adequate. Dilution and transfer can still introduce adsorption, contamination or volumetric error; there is no automatic 100% recovery.

Sensitive LC-MS methods can support direct-dilution applications, but residual matrix may suppress or enhance ionisation. Suitable calibration and internal standards can help where their performance is demonstrated. Actual laboratory scope must be checked rather than inferred from a national institution’s name.

Cleanup, derivatisation, evaporation and reconstitution are optional method operations selected for a purpose. Not every SPE eluate or liquid extract requires all of them. Unnecessary operations can introduce loss or contamination as well as improve compatibility.

Centrifugation can separate precipitated material after protein precipitation under suitable conditions. The required force, duration and temperature depend on the preparation. Residual dissolved material can still cause interference after the sample appears clear.

Filtration can remove particles before injection, but membrane chemistry, pore size, solvent compatibility and analyte adsorption must be evaluated. A filter can remove analyte as well as particles; neither PTFE nor a specific pore size is a universal requirement.

Sorbent cleanup and size-based separation can reduce selected interference. Their selectivity must be assessed because a target may be removed with the matrix. A defatting phase cannot safely be discarded solely because the aqueous phase was acidified; some neutral or weakly ionised analytes can enter it.

Derivatisation changes a target’s chemical form to improve a defined analytical property, such as volatility, response or chromatographic behaviour. Silylation, methylation and acylation are examples. Reaction completeness, by-products, stability and reference comparability require evaluation; a derivative is not guaranteed to be thermally stable or interference-free.

Suitable silylation reagents can be used for selected compounds with reactive groups. Other derivatisation chemistries serve different targets or detector modes. Hazardous reagents require approved procedures and facilities; they are not interchangeable general recipes for every opioid, metabolite or acidic drug.

Reconstitution places a cleaned extract in a solvent compatible with the analytical method. Evaporation temperature and conditions must be selected for analyte stability and volatility; some analytes should not be evaporated to dryness.

Reconstitution volume contributes to the concentration factor, together with the original aliquot, transfers and any subsequent dilution. Calculate these factors explicitly. There is no universal tenfold concentration step in toxicology.

In reversed-phase LC, an injection solvent stronger than the initial mobile phase can distort peaks, particularly at larger injection volumes. Choose the reconstitution solvent and volume for analyte solubility, stability and chromatographic conditions; a fixed water–organic ratio does not suit every method.

1. Define the aliquot and controls
   Select representative material, record quantities and include appropriate blanks or controls.
2. Add an internal standard where required
   Use the validated addition point and assess which subsequent variation the standard can monitor.
3. Prepare for the measurement
   Apply supported dilution, extraction, cleanup or digestion for the actual target and matrix.
4. Derivatise only where needed
   Demonstrate reaction performance and use the approved procedure for the reagents and equipment.
5. Set the final measurement conditions
   Account for transfers, concentration and dilution. Use compatible solvent, vessels and any necessary filtration.
6. Review calibration and quality evidence
   Check the batch against the predefined criteria before interpreting specimen findings.

Calibration establishes a relationship between reference values and indications under specified conditions. Tuning optimises operating settings, adjustment changes a system’s response, and verification checks specified requirements. These activities can be linked in a procedure, but they are not synonyms.

Optical wavelength or wavenumber performance can be checked with appropriate reference filters, solutions or films. Holmium and polystyrene materials are examples, but band positions and tolerances depend on their certificates and measurement conditions. Check frequency follows the quality plan and equipment requirements, not one worldwide daily rule.

Mass-axis calibration and instrument tuning use references suitable for the instrument and polarity. PFTBA is used in some GC-MS systems, while LC-MS instruments use appropriate supported calibration mixtures. A tune or mass check does not establish quantitative analyte calibration in a specimen.

Atomic instruments require appropriate wavelength, background, response and other performance checks for their configuration. ICP-MS tuning may assess sensitivity, mass position, resolution and interference-related indicators. The required elements, concentrations and limits are instrument- and method-specific.

Quantitative calibration relates known quantities to measured responses over a supported range. Standards may be prepared in a suitable matrix or another justified medium. Processing calibrators with specimens can help model preparation effects, while independent controls assess performance. Calibration design follows the method rather than a fixed number of universal models.

External calibration means separate standards establish the response relationship; those standards can be solvent-based or matrix-matched. Internal-standard calibration uses a suitable response ratio and can be combined with matrix matching. Neither choice automatically removes all recovery differences or matrix effects.

Standard addition is the technique of measuring aliquots of the same sample after adding increasing known analyte amounts. Under an appropriate linear model, with volume changes accounted for, the negative concentration-axis intercept estimates the original concentration. It is not a remedy for every interference or extraction problem.

Bracketing standards are not a curve type: standards or quality controls distributed through a run can monitor drift. The frequency, acceptance limits and action after failure follow the method and laboratory procedure, rather than a universal ten-sample interval.

The acceptance criteria for the calibration curve are established for the method’s intended use and applicable guidance. Inspect residuals, back-calculated values, bias and precision; correlation alone does not demonstrate an adequate model. ISO/IEC 17025 does not impose a universal R² > 0.995 rule.

The lowest calibrator sets the bottom of the calibration range, but an LOQ also requires demonstrated acceptable performance. Quantify only within the validated range. For samples above range, validate dilution; for samples below range, additional concentration or another method requires demonstrated performance rather than an automatic rerun rule.

Method validation is the documented evaluation that a method meets its intended use for the stated analytes, matrices and range. The parameters and extent of revalidation depend on the method and changes introduced. A fixed list of nine tests does not cover every analytical purpose.

Bias compares measured values with accepted reference values, while precision describes variation among repeated measurements. Extraction recovery is a separate measure of analyte recovered through preparation. A low but consistent recovery may be usable if the complete method meets its performance requirements.

Within-run and between-run studies examine precision under relevant conditions. Replicate numbers and acceptance limits follow the validation plan and applicable guidance. Evaluate calibration residuals and back-calculated concentrations as well as any correlation coefficient.

LOD describes the amount that can be reliably distinguished and identified under the method; LOQ requires suitable quantitative bias and precision. Signal-to-noise or standard-deviation calculations may estimate candidate limits, but experimental verification is needed. A signal standard deviation must be related to calibration sensitivity before it is expressed as a concentration.

Selectivity concerns the extent to which the method measures or identifies the target despite relevant interference. Suitable blanks alone cannot establish it. Challenge the method with plausible interferents and consider whether chromatographic, spectral or other evidence distinguishes them.

Validation can include blank sources, internal-standard-only material and relevant interference challenges. Robustness examines the effect of deliberate small changes suited to the method. The variables and variation ranges must reflect actual operating conditions rather than a universal pH, temperature or flow checklist.

Compare pre-extraction and post-extraction spikes to examine extraction recovery, with suitable controls for matrix effects. Evaluate carryover using blanks after high-concentration samples or calibrators. Acceptance criteria must be defined for the method and intended reporting limits.

Reference materials support calibration, validation or quality control when their specified properties suit the task. A CRM is characterised by its certificate, values, uncertainty and traceability. It is not necessary to classify all laboratory references into four supplier-based classes.

A NIST SRM can provide useful reference values for its specified material and analytes. SRM 1577c is freeze-dried bovine liver; its certificate distinguishes assigned-value statuses and mass basis. It is not fresh human tissue, and every element listed on a certificate is not necessarily certified.

Blood, serum and tissue reference materials have different intended properties and analyte coverage. Read the exact current certificate and availability information. A serum reference for clinical constituents does not automatically contain certified drug concentrations suitable for a forensic drug panel.

Pharmacopoeial reference standards support specified compendial applications. Their assigned purpose, potency information and documentation must be checked before use in a different assay. In India, IPC standards are one possible source; laboratories in the US, UK and elsewhere also use appropriate pharmacopoeial, metrological or other qualified references.

Commercial solutions can be supplied with different grades and documentation. A brand name alone does not establish certified status or traceability to NIST. Check assigned concentration, uncertainty, solvent, stability, storage and intended use for the actual product.

In-house working standards and controls can be appropriate when their preparation, assigned values, stability and relationship to suitable references are documented. Diluting a CRM does not automatically make every resulting solution a new CRM. Maintain the calculation and uncertainty contributions relevant to the use.

A laboratory establishes quality-control arrangements appropriate to its work, including suitable blanks, reference materials and controls. Frequency and acceptance limits must follow applicable requirements and the validated procedure; there is no universal daily, monthly and annual schedule for every analytical method.

Proficiency testing and interlaboratory comparisons can assess relevant performance. Select suitable schemes and participation frequencies through the applicable requirements and risk-based quality plan. A single annual schedule or one named national toxicology programme is not universal.

Controlled records should identify the method, reference materials, QC design and actions after failures. These records allow a reviewer to assess the procedure actually used rather than assume that a published example describes the laboratory’s current practice.

Maintain traceable preparation records and relevant reference-material certificates so the analytical basis can be reviewed. Documentation requirements depend on the examination and applicable reporting framework; a specific CRM field is not a universal BSA Section 63 requirement.

Related reading: ISO/IEC 17025 and Quality Management in Forensic Laboratories.

Related reading: Method Validation, Measurement Uncertainty and Proficiency Testing.

Related reading: Chromatography Fundamentals: Paper, Column and TLC.