Spectrometric Detection: UV-Vis, FTIR and Mass Spectrometry

UV-Vis measures attenuation of light across selected wavelengths. Absorbance is A = log10(I0/I), where I0 and I describe the relevant reference and transmitted intensities. The blank and optical setup must suit the measurement; cloudiness and unwanted absorbing material can distort the signal.

For an appropriate system, the Beer–Lambert relation is A = εbc. Molar absorptivity ε has units L mol⁻¹ cm⁻¹, path length b is in centimetres and concentration c is in mol/L. Changing the solvent, pH or chemical form can change the relevant spectrum and absorptivity.

Linearity is demonstrated over the method's working range. An absorbance of 1.0 is not a universal physical cutoff for every instrument or analyte. Stray light, chemical equilibria, detector behaviour and concentration-dependent effects can cause departures from the ideal relationship.

A calibration curve with suitable standards is commonly more defensible than borrowing an absorptivity from an unrelated solvent system. Matrix selectivity and preparation recovery still require assessment. A numerical concentration calculated from an overlapping absorbance can be precise arithmetic applied to the wrong signal.

Clinical assays for paracetamol, salicylate or haemoglobin derivatives use defined chemistries and instrument-specific procedures. A single wavelength from a teaching table should not be used to choose treatment. Cyanmethaemoglobin methods estimate total haemoglobin; they should not be described as direct specific carboxyhaemoglobin assays.

Multiwavelength co-oximetry can differentiate relevant haemoglobin forms within the instrument's capability. Ordinary pulse oximetry and a calculated blood-gas oxygen saturation are different measurements. Suspected dyshemoglobinaemia requires appropriate clinical interpretation and a suitable assay.

FTIR records an interferogram and transforms it into an infrared spectrum. Absorptions reflect molecular vibrations and their environment. Spectra may be displayed as absorbance or transmittance against wavenumber; the acquisition range depends on the instrument and sampling setup.

ATR uses an internal-reflection element and an evanescent field to examine material contacting its surface. Contact pressure, crystal material and sample properties affect the spectrum. The shallow sampling region is useful for some specimens but may not represent the bulk of a layered or heterogeneous exhibit.

Transmission preparations, including appropriate pellets or cells, remain useful for selected applications. ATR does not automatically require no preparation or replace every other infrared method. Compare spectra using suitable sampling modes or justified correction procedures.

Functional-group bands help interpret a spectrum, while the lower-wavenumber fingerprint region often provides additional discrimination. Band positions are influenced by chemical structure and conditions. A list of approximate functional-group frequencies is a starting point, not a unique identification algorithm.

Mixture spectra can contain overlapping features. Their relative intensities are not invariably a simple direct readout of component mass fractions, particularly with ATR contact and penetration effects. Dominant excipients may hide a low-level toxic ingredient in a tablet.

A library search generates candidates. Review diagnostic bands, mismatches, sampling quality and plausible alternatives against appropriate references. There is no universal 0.95 match-score threshold. Separation, particle examination or complementary testing may be required when a mixture prevents a defensible conclusion.

FTIR can support quantitative analysis in validated applications, but a routine library comparison does not itself quantify a drug in blood. Keep identification of an exhibit separate from biological exposure, dose and clinical effect.

A mass spectrometer generates ions, separates or filters them according to mass-to-charge ratio and records a response. Sample introduction, ionisation, analyser configuration and acquisition settings determine which information is obtained. Chromatography can separate components before they reach the source.

GC–MS commonly uses EI spectra for reference comparison. The method must suit analyte volatility, stability and any derivatisation. An EI library hit can be valuable, but background, coelution, weak signals or related compounds can produce misleading rankings.

In a triple-quadrupole LC–MS/MS experiment, the first mass filter selects a precursor, a collision region supports fragmentation, and the final filter monitors selected products. A precursor-to-product pair is a transition. MRM allows targeted acquisition but does not include every compound merely because it is present in the vial.

High-resolution instruments can distinguish closely spaced masses and measure accurate mass. Mass accuracy and resolving power are different characteristics. Both depend on instrument performance, calibration and experimental conditions rather than one universal five-parts-per-million guarantee.

Accurate mass, isotope patterns and fragmentation can support candidate formulae and structures. Isomers can share the same elemental formula and similar fragments. An unknown absent from a library may remain tentatively characterised until additional evidence or a reference material supports a stronger identification.

Non-targeted and retrospective analysis are limited by what was extracted, chromatographically separated, ionised and acquired. Data cannot recover a target that decomposed before analysis or was excluded from the recorded mass range. Preserve the distinction between a candidate annotation and a confirmed forensic result.

Electron ionisation transfers energy through electron–molecule interactions in the gas phase. EI spectra often contain substantial fragmentation; the molecular ion can be weak or absent. Reference libraries commonly use defined electron-energy conditions, so comparison requires attention to how the spectrum was acquired.

Electrospray produces gas-phase ions from a charged liquid spray. Depending on the analyte and conditions, spectra can contain protonated or deprotonated ions, adducts, multiply charged species and in-source fragments. Positive ESI does not invariably yield only one [M+H]+ peak.

Many basic drugs respond well in positive mode and acidic compounds may suit negative mode, but the actual response must be tested. A compound being soluble in a reversed-phase LC method does not guarantee useful electrospray ionisation or adequate sensitivity.

APCI uses vaporisation and atmospheric-pressure ion–molecule chemistry, typically initiated by a discharge. It can suit some compounds that respond differently under ESI. Analyte volatility, thermal stability and solvent conditions affect its usefulness.

APCI is not universally immune to matrix effects, and ionisation techniques should not be ranked as a simple fixed ladder of softness or evidential strength. Choose and validate the source for the target range. A source change can alter response, fragmentation and the identification information available.

Quantification relates measured response to standards of known concentration. Many MS assays use an analyte-to-internal-standard response ratio. The standard should behave suitably during the relevant preparation and measurement; a stable-isotope label does not automatically compensate for every source of error.

Calibration levels, model and weighting are selected and evaluated for the method. Five to seven points, 1/x² weighting and a fixed concentration range are not universal requirements. Check calibration performance together with independent controls and the suitability of the case specimen.

A detection limit and quantification limit have different meanings. LOD is not necessarily one-third of LOQ, and neither is a fixed range for all drugs. Report a result only within the capability demonstrated for the matrix and analytical conditions.

Identification review can include retention, diagnostic ion or spectral evidence, ratios and reference comparisons. Requirements differ with acquisition mode and applicable standards. Two shared transitions are not sufficient in every case, while a different validated MS mode may provide adequate evidence without that exact format.

Investigate poor internal-standard response, contamination, carryover or a failed control before interpreting a specimen. A correct-looking chromatogram does not excuse a quality failure. Record any repeat preparation, dilution or additional testing and retain the underlying data.

SAMHSA federal workplace-testing requirements govern a defined programme and specimen scope; they are not a universal hierarchy for post-mortem or clinical toxicology. Use the standard relevant to the examination rather than borrowing a cutoff or transition rule from another programme.

A useful report identifies the specimen, relevant methods, findings, units and limitations. Distinguish a presumptive screen, an identified target and a quantitative result. A finding below a validated quantitative range should not be presented as an exact concentration.

The United States ANSI/ASB 113 identification standard has a defined toxicology scope and does not prescribe routine identification criteria for every volatile, metal or gas. UKIAFT guidance addresses laboratory quality in the UK. India and other jurisdictions apply their relevant scientific and evidential requirements.

Accreditation concerns a laboratory's assessed activities and scope; it is not an individual certification automatically held by every analyst. Neither accreditation nor an instrument model predetermines how a court will assess a particular report. Legal requirements vary by jurisdiction and proceeding.

For biological results, the wider interpretation still depends on timing, treatment, metabolism and specimen condition. A urine finding alone cannot establish the concentration in blood at an earlier event, impairment, lack of consent or the identity of a person who administered a substance.

Related reading: Chromatographic Techniques: TLC, HPLC, HPTLC and GLC.

Related reading: Modern Extraction: SPE, SPME and Accelerated Solvent Extraction.

Related reading: Gases and Volatile Poisons: CO, Cyanide, Methanol, Ethanol.