Spectroscopy and chromatography in VCE Chemistry, explained

Published 2026-05-26 · Updated 2026-05-26

The analysis strand of VCE Chemistry Unit 4 (How are carbon-based compounds designed for purpose?) answers a single question in several ways: given an unknown organic compound, how do we work out what it is, how pure it is, and how much of it is present? Each technique, mass spectrometry, infrared, nuclear magnetic resonance and chromatography, answers a different piece of that puzzle, and the real skill is knowing which tool does what.

This guide covers each technique in turn (what it measures and what you can read off it), then the task that ties them together: determining a structure from combined spectra. The golden rule throughout is that each technique tells you something different, so the most common exam question is choosing the right one or combining several.

What each technique tells you

Start with the map, then fill in the detail:

Keep this table in mind: most analysis questions are really asking "which row do I need?".

Mass spectrometry

Mass spectrometry ionises a molecule and breaks it into fragments, then sorts them by mass-to-charge ratio. Two features matter most for VCE:

• The molecular ion peak (M) is the peak at the highest mass-to-charge ratio (ignoring small isotope peaks). It corresponds to the whole molecule, so it gives the molar mass of the compound, the single most useful starting number in a structure problem.
• The fragmentation pattern comes from the molecule breaking into smaller pieces. The mass differences between peaks point to groups that were lost (for example a loss of 15 suggests a CH₃ group), which helps confirm parts of the structure. The tallest peak is the base peak, the most stable or most common fragment.

So MS mainly answers "how heavy is the whole molecule?", with fragments as supporting evidence.

Infrared (IR) spectroscopy

Bonds absorb infrared radiation at characteristic frequencies as they bend and stretch. An IR spectrum plots absorption against wavenumber, and particular functional groups produce characteristic absorption bands that you match against the table in the VCAA Chemistry Data Book.

The ones you use constantly:

• A broad band around 3200 to 3550 cm⁻¹ points to an O–H group (alcohol, or broader and lower for a carboxylic acid).
• A strong sharp band around 1700 cm⁻¹ points to a C=O group (aldehyde, ketone, acid or ester).

IR does not give the whole structure; it tells you which functional groups are present. The technique to reach for when a question asks you to confirm a functional group is IR.

NMR spectroscopy

NMR probes the environment of specific atoms. VCE uses two kinds.

Carbon-13 (¹³C) NMR tells you the number of different carbon environments: each chemically distinct carbon gives one peak, so the number of peaks equals the number of carbon environments, and the chemical shift of each peak (read against the data-book table) hints at the type of environment.

Proton (¹H) NMR is richer and reads on three levels:

• The number of peaks equals the number of different hydrogen environments.
• The area under each peak (integration) gives the ratio of hydrogens in each environment.
• The splitting of a peak follows the n+1 rule: a peak split into n+1 lines indicates n hydrogens on the neighbouring carbon. A triplet means two neighbours, a quartet means three.

Together, ¹H NMR tells you how the hydrogens are grouped and how those groups connect, which is the connectivity information that pins down a structure.

Determining a structure from combined spectra

This is the flagship task, and it works because the techniques are complementary. A reliable order:

1. MS gives the molar mass, and with a molecular formula, the degree of unsaturation.
2. IR identifies the functional groups present.
3. ¹³C NMR counts the distinct carbon environments.
4. ¹H NMR gives the number, ratio and neighbours of the hydrogen environments.
5. Assemble a structure consistent with all of the evidence, then check it back against every spectrum.

No single technique is enough; the answer is the structure that fits everything at once. The discipline of checking your proposed structure back against each spectrum is what separates full marks from a near miss.

Chromatography and HPLC

Chromatography separates the components of a mixture by how strongly each is attracted to a stationary phase versus a moving mobile phase. Components that cling to the stationary phase move slowly; those that prefer the mobile phase move quickly, so they separate out.

High-performance liquid chromatography (HPLC) is the quantitative version VCE focuses on:

• Each component leaves the column at a characteristic retention time, which helps identify it by comparison with known standards.
• The area of each peak is proportional to the amount of that component, so by running standard solutions of known concentration and building a calibration curve, you can read off the concentration of a component in an unknown sample.

So chromatography answers "what is in this mixture, and how much?", which the spectroscopic techniques alone cannot.

The other analytical tools

VCE also expects a working knowledge of the bench techniques that support analysis:

• Melting point is a quick check of purity: a pure substance melts sharply at a known temperature, while impurities lower and broaden the range.
• Distillation separates and purifies liquids by differences in boiling point.
• Iodine number measures the degree of unsaturation in a fat or oil (how many C=C double bonds), by how much iodine it reacts with.
• Redox titrations and volumetric analysis determine an unknown concentration by reacting it with a standard solution to a measured end point.

Common mistakes that cost marks

1. Using the wrong technique for the question. IR identifies functional groups; NMR gives carbon and hydrogen environments; MS gives molar mass; chromatography separates and quantifies.
2. Confusing the two NMR types. ¹³C counts carbon environments; ¹H counts and relates hydrogen environments.
3. Forgetting the n+1 rule when reading ¹H NMR splitting.
4. Treating the molecular ion peak as the base peak. The molecular ion is at the highest mass; the base peak is the tallest.
5. Proposing a structure from one spectrum instead of checking it against all of them.
6. Mixing up identification and quantification in HPLC: retention time identifies, peak area (against a calibration curve) quantifies.

How to prepare

Anchor everything to the "what does this tell me?" table, then practise the combined-spectra questions in the fixed order above, because that order stops you from over-reading one spectrum. Learn to read the IR and NMR data-book tables quickly, since the same tables are with you in the exam. Practise the n+1 rule and integration ratios until they are automatic.

The combined-spectra questions are hard to self-check, because a structure can look right yet contradict one of the spectra. Avocado is an AI-powered Chemistry tutor built specifically for the VCE study design, so you can work through spectra one technique at a time, attempt full structure-determination problems, and get specific feedback on exactly where a reading or assignment went wrong.

Frequently asked questions

What does each analytical technique tell you? Mass spectrometry gives the molar mass, infrared identifies functional groups, ¹³C NMR counts carbon environments, ¹H NMR gives hydrogen environments and their connectivity, and chromatography separates and quantifies a mixture.

What is the difference between ¹³C and ¹H NMR? Carbon-13 NMR counts the number of different carbon environments. Proton NMR counts the hydrogen environments and, through peak area and splitting, shows how many hydrogens are in each and how they connect.

What is the n+1 rule? In proton NMR, a peak split into n+1 lines indicates n hydrogens on the neighbouring carbon, so a triplet means two neighbours and a quartet means three.

How does HPLC measure concentration? Each component has a characteristic retention time for identification, and the area of its peak is proportional to its amount, so comparing against a calibration curve from known standards gives the concentration.

Why do you need more than one technique to determine a structure? Each technique gives only part of the picture, so the correct structure is the one consistent with all the spectra at once: molar mass from MS, functional groups from IR, and environments and connectivity from NMR.

Content aligned to the VCE Chemistry Study Design (Units 3 and 4: 2024–2027), Unit 4. Always confirm current study-design detail with your teacher and the VCAA website.