Mass Spectroscopy: Principles, Instrumentation and Forensic Applications
UGC-NET Paper 2 Unit II notes on mass spectroscopy: ionisation (EI, CI, ESI, MALDI), mass analysers, spectral interpretation and forensic applications.
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Mass spectroscopy (more strictly, mass spectrometry) is one of the four big instrumental topics in UGC-NET Forensic Science Unit II, sitting next to UV-Vis, IR and chromatography. NTA tests it because it is the workhorse behind drug identification, accelerant detection, explosives residue work and modern toxicology, and because the underlying ideas (ionisation, mass-to-charge separation, detection) give clean MCQ stems. The bullet looks small in the syllabus, but it carries weight in every cycle.
For Paper 2, treat mass spec as three connected blocks: how molecules are ionised (EI, CI, ESI, MALDI), how the resulting ions are separated by m/z (quadrupole, TOF, ion trap, sector, FT-ICR) and how the spectrum is read (molecular ion, base peak, isotope patterns, nitrogen rule). Once those three are locked, the forensic applications follow almost automatically. The deep-dive book chapters at the bottom carry the full physics and the tandem MS workflows.
- m/z (mass-to-charge ratio)
- The x-axis of every mass spectrum. Ions are separated by their mass divided by the number of charges they carry. For singly-charged ions, m/z equals the ion mass in daltons.
- Molecular ion (M+)
- The ion formed when one electron is knocked out of the intact molecule, with no fragmentation. Gives the molecular weight of the analyte.
- Base peak
- The most intense peak in the spectrum. By convention it is set to 100 percent relative abundance; every other peak is reported relative to it.
- Fragmentation
- Breakdown of the molecular ion into smaller charged and neutral pieces. The fragmentation pattern is the chemical fingerprint that identifies the compound.
- Nitrogen rule
- An organic molecule with an odd-numbered molecular weight contains an odd number of nitrogen atoms. A staple MCQ trick for narrowing down a structure from M+.
- EI (Electron Ionisation)
- Hard ionisation at 70 eV. Produces extensive fragmentation, ideal for library search. Standard ion source in GC-MS.
- ESI (Electrospray Ionisation)
- Soft ionisation at atmospheric pressure for polar and thermally fragile analytes in solution. Standard source in LC-MS for drugs and biomolecules.
- MALDI
- Matrix-Assisted Laser Desorption / Ionisation. A soft method that uses a UV-absorbing matrix to ionise large biomolecules. Almost always paired with a TOF analyser.
- TOF (Time-of-Flight)
- Mass analyser that separates ions by their flight time down a field-free drift tube. Theoretically unlimited mass range, very high speed.
Principle: ionisation, mass analysis, detection
Three stages, one vacuum, one spectrum.
A mass spectrometer does three things in a high vacuum, in this order:
- Ionise the sample. Neutral molecules cannot be steered by electric or magnetic fields, so the instrument first converts them into ions (positive or negative).
- Separate the ions by their mass-to-charge ratio (m/z). This is the job of the mass analyser.
- Detect each ion and record its abundance. The output is a plot of relative abundance against m/z, called the mass spectrum.
Everything else in mass spec is a variation on these three stages. NTA loves to test the order, so memorise it: ionisation, then analysis, then detection. A common distractor swaps the first two.
Two background facts that show up in MCQ stems:
- The whole flight path operates under high vacuum (typically 10^-5 to 10^-8 torr) so that ions reach the detector without colliding with air molecules.
- Mass spec is fundamentally a destructive technique. The analyte is consumed, unlike IR or UV-Vis where the sample survives. This matters in casework where the sample is precious (a single hair, a trace of explosive).
Ionisation methods (EI, CI, ESI, MALDI)
Hard versus soft, gas-phase versus solution-phase.
The ionisation source decides what kind of spectrum you get. The classic split is hard (lots of fragmentation, structural information, weak or absent M+) versus soft (intact molecular ion, little fragmentation, good for molecular weight).
| Method | Hard / Soft | Sample state | Typical energy / mechanism | Forensic application |
|---|---|---|---|---|
| EI (Electron Ionisation) | Hard | Gas phase (volatile, thermally stable) | 70 eV electron beam knocks out one electron | GC-MS drug screening, accelerant fire-debris, explosives |
| CI (Chemical Ionisation) | Soft | Gas phase | Reagent gas (methane, ammonia, isobutane) proton-transfers to the analyte; gives [M+H]+ | GC-MS where M+ is missing under EI; confirms molecular weight |
| ESI (Electrospray Ionisation) | Soft | Solution, atmospheric pressure | High-voltage spray produces charged droplets; ions emerge after desolvation |
Mass analysers (quadrupole, TOF, ion trap, sector, FT-ICR)
Five geometries, five trade-offs.
The mass analyser is the heart of the instrument. NTA tests both the principle of each and the typical instrument pairing.
| Analyser | Principle (one line) | Resolution | Typical pairing |
|---|---|---|---|
| Quadrupole (Q) | Four parallel rods with combined DC + RF voltages act as a mass filter; only ions of a chosen m/z reach the detector | Unit (low) | GC-MS, LC-MS, triple-quadrupole (QqQ) for MRM in toxicology |
| Time-of-Flight (TOF) | Ions are accelerated by a fixed voltage; lighter ions travel a field-free drift tube faster than heavier ions | High | MALDI-TOF for biomolecules, Q-TOF for accurate-mass screening |
| Ion Trap (3D / Linear) | Ions are trapped in a 3D RF field, then ejected sequentially by m/z; allows MSn experiments | Unit to medium | GC-ion-trap MS, LC-ion-trap MS for structural elucidation |
| Magnetic Sector | A magnetic field bends ion paths; radius of curvature depends on m/z |
Spectral interpretation basics
M+, base peak, isotopes, the nitrogen rule.
A forensic chemist does not solve every spectrum from first principles. The library does most of the work. But for NET MCQs you need the spectral interpretation grammar.
The molecular ion (M+). The highest-mass peak that corresponds to the intact molecule (allowing for isotopes). It gives the molecular weight. Under EI, M+ may be small or absent for fragile compounds (alcohols, amines), which is why CI or ESI is run alongside to recover the weight.
The base peak. The tallest peak. By convention it is set to 100 percent relative abundance. The base peak is often a stable fragment cation (a tropylium ion C7H7+ at m/z 91 in alkylbenzenes, an acylium R-CO+ in ketones).
Isotope patterns. Many elements show characteristic M+1 or M+2 peaks. Three to memorise:
- Chlorine gives a 3:1 ratio at M and M+2 (35Cl : 37Cl).
- Bromine gives a 1:1 ratio at M and M+2 (79Br : 81Br).
- Sulphur gives a small but distinct M+2 (about 4.4 percent) from 34S.
Spotting Cl or Br from the isotope pattern is a favourite MCQ. Carbamate pesticides, organochlorines and many explosives are flagged this way.
The nitrogen rule. An organic compound with an odd-numbered nominal molecular weight contains an odd number of nitrogens. Even M means zero or an even count of nitrogens. Heroin (369 Da, 1 N), cocaine (303 Da, 1 N), methamphetamine (149 Da, 1 N): all odd M, all odd N. This is one of the cleanest one-shot MCQ tools in mass spec.
Forensic applications
What the CFSL bench actually does with the instrument.
Mass spec is the most-used hyphenated technique in Indian forensic chemistry sections. Five high-yield application areas:
- Drug identification. GC-MS with EI is the legal gold standard for confirming narcotics under the NDPS Act. CFSL Hyderabad, CFSL Chandigarh and most state SFSLs run Agilent or Shimadzu single-quadrupole GC-MS instruments against the NIST library. For polar drugs that do not survive GC (LSD, designer cannabinoids), LC-MS with ESI is preferred.
- Accelerant / fire-debris analysis. Petrol, kerosene, diesel and lighter fluids have distinctive GC-MS profiles. The classifier looks for diagnostic ions of aromatic and alkane series (m/z 91, 105, 119 for the alkylbenzene series). ASTM E1618 is the international protocol Indian labs follow.
- Explosives. Both intact organic explosives (TNT, RDX, PETN, HMX) and post-blast residues are screened by LC-MS or GC-MS. Soft ionisation matters because nitroaromatics fragment badly under EI.
- Forensic toxicology. Triple-quadrupole LC-MS (tandem mass spectrometry (MS/MS)) in multiple-reaction-monitoring (MRM) mode is the workhorse for confirming drugs of abuse, prescription drugs and their metabolites in blood and urine. The two transitions per analyte give the specificity that a single quadrupole cannot.
- Ink, paint, polymer, soil and trace evidence. Pyrolysis-GC-MS turns non-volatile polymers into characterisable fragments. MALDI-TOF is used for some ink dye work.
India anchor. CFSL Hyderabad publishes GC-MS protocols for narcotics, viscera and explosive residue that most SFSLs treat as standard operating procedures. NFSU Gandhinagar runs LC-MS/MS, Q-TOF and MALDI-TOF instruments in its instrumental block, and trains MSc Forensic Science students directly on them. NDPS Act prosecutions in trial courts routinely depend on a CFSL GC-MS chromatogram and the matched NIST library hit as the central piece of scientific evidence.