Is acetonitrile or methanol better for HPLC?

Neither is universally better. Acetonitrile has a lower UV cutoff (~190 vs ~205 nm) and lower viscosity (less backpressure), and it is aprotic. Methanol is cheaper and a proton donor, giving different selectivity. Many methods try both because they separate differently.

Why does acetonitrile give lower backpressure than methanol?

Acetonitrile is much less viscous (~0.37 cP vs ~0.54 cP), and methanol/water mixtures peak in viscosity around 40–50% organic, so methanol methods run at higher column backpressure.

Acetonitrile vs methanol in HPLC

The two workhorse reversed-phase solvents, compared head-to-head — and how to decide which one your method needs.

Compiled by Hemant RawatLast reviewed July 2026How we verify

Property	Acetonitrile	Methanol	What it means
UV cutoff	190 nm	205 nm	ACN lets you detect at lower wavelengths
Viscosity (25 °C)	0.37 cP	0.59 cP	ACN gives lower backpressure
Snyder polarity index	5.8	5.1	similar overall strength
Selectivity group	VIb	II	different groups → different selectivity
Proton character	aprotic	protic (H-bond donor)	the main selectivity difference
Boiling point	81.6 °C	64.7 °C
Water miscibility	miscible	miscible	both fully water-miscible
Cost	higher	lower	methanol is markedly cheaper

When to choose acetonitrile

Reach for acetonitrile when you need low-wavelength UV detection (its ~190 nm cutoff is the cleanest common choice), when backpressurematters (it's far less viscous, which helps on long or sub-2-µm columns), or for LC-MS, where it sprays and ionizes well. Its aprotic character often gives sharper peaks for many analytes.

When to choose methanol

Choose methanol when cost matters (it's much cheaper, which adds up on prep-scale or high-throughput methods), or when you want a proton-donor selectivity that resolves a pair acetonitrile can't. Its higher UV cutoff (~205 nm) and higher viscosity are the trade-offs.

The practical answer: try both

Because they sit in different Snyder selectivity groups(VIb vs II) at similar strength, acetonitrile and methanol frequently give different separationsof the same sample. Screening both is the standard first move in method development. Set the strength with the polarity index, then switch solvent type to shift which peaks move. See the full mobile-phase selection guide.

Sources

University of Toronto (TRACES) — Burdick & Jackson — Solvent UV cutoff table (absorbance = 1 AU, 1 cm cell)
Stenutz / L. R. Snyder — Solvent polarity index (P′) and selectivity groups
NIST — Chemistry WebBook — thermophysical properties (BP, density, refractive index)
PubChem (NIH/NLM) — Compound property records (physical constants, CAS, formula)
USP <467> / ICH Q3C — Residual Solvents — solvent classification (Class 1/2/3)

Values are compiled from public references and were last verified July 2026. See ourmethodologyfor how we source and verify. Always confirm critical values against primary references and the SDS.