Mass Spectrometry: Principles, Ionisation and Mass Analysers

A mass-spectrometric system introduces material, forms or transfers gas-phase ions, controls their motion and records a mass-related signal. Neutral molecules, pre-existing solution ions and solid-surface material can require different introduction and ionisation arrangements. Not every source starts with the same neutral gas.

An analyser measures behaviour related to m/z, rather than neutral molecular mass alone. A singly charged ion near 250 Da has a numerical m/z near 250, whereas a doubly charged ion of that ionic mass has m/z near 125. Recovering the neutral mass requires the correct charge and ion composition.

Ion transport and analysis use pressure conditions suited to the instrument. Atmospheric-pressure sources transfer ions through differential pumping, while other sources operate under vacuum. Controlled collisions can be useful in designated regions, including collision cells; every collision is not an instrument fault.

Detection can involve collected ion current, secondary-electron amplification or induced image current. Some instruments count or integrate arrivals at a detector, while Fourier-transform instruments can measure ions' collective motion without first collecting them. They should not all be described as weighing and counting each molecule individually.

The data system controls acquisition and converts raw signals into interpretable records. Calibration, tuning and processing affect the result. Instrument modules are compatible only within their supported design; arbitrary sources and analysers are not interchangeable building blocks.

In common positive electron ionisation, electron removal can create a radical molecular cation, M+•. It can fragment before detection, and its intensity varies by compound and conditions. A molecular ion may be strong, weak or absent; there is no universal rule that it is always less than ten percent of the base peak.

A protonated molecule, [M+H]+, is not the same species as M+•. Sodium and ammonium adducts and multiply charged ions can also occur. The highest-m/z peak is not automatically the molecular ion because isotopes, adducts, contaminants or clusters can appear above an analyte-related peak.

The base peak defines 100 percent relative abundance in a suitably normalised spectrum. Its identity depends on the acquisition. A fragment can be most intense, but a molecular or adduct ion can also be the base peak. Absolute response and relative abundance answer different questions.

Mass differences can suggest neutral losses only when the ions' relationship and charge are appropriate. A nominal loss of 17 can correspond to ammonia or a hydroxyl radical, for example. Accurate mass, precursor chemistry and electron parity help distinguish alternatives; a gap alone does not assign a unique functional group.

Isotope distributions can help constrain elemental composition and charge. Chlorine- or bromine-containing ions can show characteristic patterns under suitable conditions. Mixtures and overlapping ions can distort them, so a pattern should be assessed with the full spectrum.

Toluene's familiar EI ions near m/z 92 and 91 illustrate intact and fragment signals. An m/z 91 signal occurs in many other compounds and can involve related ion structures. It is not independently a toluene identification or proof of a unique fragmentation pathway.

Electron ionisation commonly uses a 70 eV electron beam for reproducible reference-compatible spectra. The beam energy is not all deposited as molecular internal energy in every event. Fragmentation depends on the ion produced and its subsequent processes.

Chemical ionisation uses reagent-ion reactions to generate analyte-related ions. Positive or negative modes and different reagent conditions provide different information. It can help when an EI molecular ion is weak, but [M+H]+ is not the only possible CI product.

Electrospray forms charged droplets from solution and ultimately gas-phase ions through desolvation and related processes. Positive and negative modes can produce protonated, deprotonated, adducted or multiply charged species. The observed distribution depends on analyte, solvent, additives and source conditions.

Atmospheric-pressure chemical ionisation involves vaporisation and reagent-ion chemistry and can suit some less polar analytes. APPI uses photoionisation-related processes. A source's “soft” classification does not guarantee zero in-source fragmentation or intact ions for every compound.

MALDI uses a matrix and laser-driven desorption/ionisation and is commonly coupled to TOF. Laser wavelength and instrument arrangement vary. Microbial identification usually compares patterns from prepared biological material with an appropriate database; it does not mean a spectrum alone types every organism in a complex specimen.

Ambient-ionisation approaches can provide rapid information but still need controls for background, sampling and selectivity. “No preparation” is not a guarantee of unbiased sampling. Select a source for the actual compound and matrix rather than assign a single best source to an entire forensic discipline.

A quadrupole applies radio-frequency and direct-current fields to control stable ion trajectories. Appropriate settings transmit selected m/z intervals. Scanning builds a spectrum, while selected-ion monitoring focuses on particular intervals. Acquired range and resolution follow the actual instrument and method.

Triple-quadrupole instruments commonly select precursor ions, induce fragmentation and select product ions. They are useful for targeted measurements with suitable transition, retention and quality-control evidence. A transition is not necessarily unique to a compound, and untargeted substances may be missed.

Ion traps confine ions and can support sequential isolation and fragmentation, including MSn. Three-dimensional and linear geometries differ. Space-charge effects, isolation and acquisition settings can influence performance. No single analyser family has exclusive access to all useful structural information.

TOF measures ion flight time after controlled acceleration and timing. In the simplified model, flight time scales with the square root of m/z. Initial energy and position distributions affect resolution; reflectrons and other design features can compensate selected effects.

Orbitrap analysis relates ion oscillation frequencies to m/z through induced-current measurement. FT-ICR uses ion motion in a magnetic field with Fourier-transform signal analysis. Their resolving power depends on acquisition conditions and m/z, rather than a constant value across every scan.

Magnetic-sector arrangements provide other mass-analysis and isotope-ratio capabilities. Detector arrangements, simultaneous acquisition and resolution differ among designs. Instrument selection should reflect required measurement, throughput and practical performance rather than a fixed hierarchy or an unsourced price table.

Resolving power is commonly expressed as m/Δm with a stated peak-width or separation definition, such as full width at half maximum. At m/z 500 and resolving power 100,000, Δm is 0.005 in the same m/z scale. A claimed ability to resolve 0.001 at that mass would require different performance under the stated definition.

Mass accuracy concerns agreement of a measured value with the appropriate reference. A common signed error is 10⁶(measured − calculated)/calculated ppm. Calibration, drift, signal level, interference and peak assignment influence it. Extra decimal places alone do not establish accuracy.

A five-ppm tolerance does not universally leave one molecular formula. Candidate count depends on mass, allowed elements, charge, adduct and additional constraints. Even a uniquely supported formula can correspond to several structural isomers, requiring further discrimination.

Electron multipliers and microchannel plates amplify ion-arrival signals; Faraday collectors measure current without that amplification. Timing, gain and dynamic-range limits affect the intended application. A detector can saturate or respond differently across conditions.

Image-current detection measures ion motion rather than a destructive arrival count. Vacuum requirements depend on analyser design, desired observation time and gas load. A universal mean-free-path number or daily leak-check schedule should not replace the actual instrument procedure.

GC-MS, LC-MS/MS and high-resolution methods can provide valuable information in drugs, toxicology, fire debris and other examinations. Each method has a scope. A negative targeted result does not exclude every unknown substance, and a failed library search does not prove novelty.

Compare spectra acquired under compatible ionisation and fragmentation conditions. EI and tandem-MS libraries contain different types of reference information. A high match factor supports a candidate but is not a universal confirmation threshold or calibrated probability of correctness.

Evaluate relevant retention, ion or spectral evidence, controls and reference standards within demonstrated identification criteria. Isomers can require improved separation, complementary spectroscopy or other evidence. Accurate mass cannot replace all missing structural discrimination.

Quantification also needs an appropriate calibration and assessment of matrix effects, extraction, stability and carryover. Internal standards can help within their capability. A selective ion trace with a high signal does not guarantee an accurate concentration.

US, UK, Indian and other laboratories use different instrument configurations according to their scope and resources. Accreditation and legal admissibility do not follow automatically from an analyser name. Report the analytical basis, supported conclusion and relevant limits rather than label all MS evidence uniquely conclusive.

Related reading: Tandem Mass Spectrometry (MS/MS) and Spectral Interpretation.

Related reading: Nuclear Magnetic Resonance (NMR) Spectroscopy.

Related reading: Hyphenated Techniques: GC-MS, LC-MS and GC-FTIR.