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Mass spectrometry is one of the most powerful analytical techniques available to chemists. It allows you to determine the relative molecular mass of a compound and, with high-resolution instruments, to establish its molecular formula. Understanding how a mass spectrometer works and how to interpret the data it produces is essential for Edexcel A-Level Chemistry.
A mass spectrometer operates in four key stages: ionisation, acceleration, deflection, and detection. Each stage plays a distinct role in separating and identifying the ions produced from a sample.
The sample is first vaporised (if it is not already a gas) and introduced into the ionisation chamber. Here, a beam of high-energy electrons is fired at the sample molecules. This is known as electron impact ionisation (EI).
When a high-energy electron strikes a sample molecule, it knocks out one of the molecule's own electrons, forming a positive ion with one unpaired electron — a radical cation:
M(g) + e⁻ → M⁺•(g) + 2e⁻
The ion M⁺• is called the molecular ion (or parent ion). It has the same mass as the original molecule (minus one electron, whose mass is negligible). In some instruments, electrospray ionisation (ESI) is used instead, which is gentler and particularly useful for large biomolecules.
The positive ions are accelerated by an electric field. Negatively charged plates attract the positive ions, accelerating them all to the same kinetic energy. Since they all have the same kinetic energy, ions with different masses will travel at different velocities — lighter ions move faster.
The accelerated ions pass through a magnetic field, which causes them to follow a curved path. The degree of deflection depends on the mass-to-charge ratio (m/z) of the ion. Lighter ions and ions with a higher charge are deflected more. Most ions produced by electron impact have a charge of +1, so m/z is effectively the mass of the ion.
Only ions with a specific m/z ratio will follow the correct curved path to reach the detector. The detector records the number of ions arriving (their relative abundance) at each m/z value. By varying the magnetic field strength, the spectrometer scans across a range of m/z values, building up a complete mass spectrum.
A mass spectrum is a bar chart with m/z on the horizontal axis and relative abundance (%) on the vertical axis. Two features are particularly important:
The peak at the highest m/z value (ignoring isotope peaks such as the M+1 peak) corresponds to the molecular ion — the intact molecule that has simply lost one electron. This gives you the relative molecular mass of the compound.
For example, if the molecular ion peak appears at m/z = 46, the compound has a relative molecular mass of 46. This is consistent with ethanol (C₂H₅OH, Mr = 46) or dimethyl ether (CH₃OCH₃, Mr = 46).
The tallest peak in the spectrum is called the base peak. It is assigned a relative abundance of 100%, and all other peaks are measured relative to it. The base peak represents the most stable (and therefore most abundant) ion formed during fragmentation. The base peak is not necessarily the molecular ion peak — in many spectra, a fragment ion is more abundant than the molecular ion.
The relative heights of the peaks reflect how many ions of each m/z value reach the detector. For elements with significant natural isotope distributions — such as chlorine (³⁵Cl and ³⁷Cl in a 3:1 ratio) and bromine (⁷⁹Br and ⁸¹Br in approximately 1:1 ratio) — characteristic patterns appear in the mass spectrum.
A compound containing one chlorine atom will show the molecular ion as two peaks: M⁺ and M+2, with relative intensities of approximately 3:1. A compound containing one bromine atom will show M⁺ and M+2 peaks of approximately equal intensity.
These isotope patterns are extremely useful for identifying whether halogens are present in an unknown compound.
Low-resolution mass spectrometry gives m/z values as whole numbers, which means different molecular formulae can give the same nominal mass. For example, both CO (Mr = 28.0) and C₂H₄ (Mr = 28.0) appear at m/z = 28 at low resolution.
High-resolution mass spectrometry (HRMS) measures m/z to four or more decimal places. Since the exact atomic masses of isotopes are not exact whole numbers (for example, ¹²C = 12.0000, ¹H = 1.00794, ¹⁶O = 15.9994), different molecular formulae give slightly different exact masses:
| Molecular formula | Exact mass |
|---|---|
| CO | 27.9949 |
| C₂H₄ | 28.0313 |
| N₂ | 28.0061 |
| CH₂O | 30.0106 |
High-resolution mass spectrometry can therefore distinguish between these and determine the molecular formula of an unknown compound, not just its molecular mass.
Mass spectrometry is often coupled with other techniques. GC-MS (gas chromatography–mass spectrometry) separates components of a mixture by gas chromatography and then identifies each component by its mass spectrum. This is invaluable in forensics, environmental chemistry, and pharmaceutical analysis.
The technique requires very small samples — often micrograms — and is extremely sensitive. However, it is a destructive technique: the sample is consumed during analysis.
Mass spectrometry provides the relative molecular mass from the molecular ion peak, fragmentation information from the pattern of lower m/z peaks, and (with high-resolution instruments) the molecular formula. It is the first technique you should consider when trying to identify an unknown compound, because it immediately tells you how heavy the molecule is.