The mobile phase is the part of an HPLC method that analysts think about least and troubleshoot most. Baseline drift, ghost peaks, high back-pressure and poor sensitivity are frequently mobile-phase problems. This guide covers the properties that matter when selecting HPLC solvents, practical values for common solvents, and how to choose the right grade.
Property 1: UV cutoff
With UV detection, the solvent must be transparent at the detection wavelength. The UV cutoff is the wavelength at which a solvent's absorbance in a 1 cm cell reaches about 1 AU. In practice, you want to work well above it, because the absorbance rises steeply as you approach the cutoff and gradient methods amplify any mismatch between solvents A and B.
| Solvent | Approx. UV cutoff (nm) | Viscosity at 20 to 25 °C (mPa·s) | Notes |
|---|---|---|---|
| Water | 190 | 0.89 to 1.00 | Use freshly purified or HPLC-grade |
| Acetonitrile | 190 | 0.34 to 0.37 | Best low-UV transparency, low viscosity |
| Methanol | 205 | 0.55 to 0.60 | Protic, different selectivity from acetonitrile |
| Ethanol | 210 | 1.1 to 1.2 | Greener alternative, higher pressure |
| Isopropanol | 205 | 2.0 to 2.4 | Strong solvent, very viscous |
| Tetrahydrofuran | 212 unstabilised | 0.46 to 0.55 | BHT stabiliser raises UV cutoff. Forms peroxides |
| n-Hexane | 195 | 0.30 | Normal phase |
| Ethyl acetate | 256 | 0.43 to 0.45 | Normal phase |
| Dichloromethane | 233 | 0.41 to 0.44 | Normal phase. Check stabiliser |
| Acetone | 330 | 0.30 to 0.32 | Rarely used with UV detection |
Values are approximate and vary with grade, stabiliser and source. Consult the COA for the lot you use.
Property 2: viscosity and back-pressure
Column back-pressure is proportional to mobile-phase viscosity. Acetonitrile-water mixtures are much less viscous than methanol-water mixtures. Methanol-water shows a viscosity maximum at around 40 to 50% methanol, roughly double that of pure water, which matters on UHPLC systems and long columns. Raising the column temperature lowers viscosity and pressure.
Property 3: elution strength and selectivity
In reversed-phase HPLC, elution strength generally increases in the order methanol, acetonitrile, tetrahydrofuran, with isopropanol also strong. Solvents also differ in selectivity: methanol is a proton donor and acceptor, acetonitrile has a strong dipole, and THF is a proton acceptor. Changing the organic modifier is one of the most powerful ways to change peak order, which is why method developers often screen both acetonitrile and methanol.
Property 4: miscibility and buffer solubility
Buffers must remain soluble across the entire gradient. Phosphate salts can precipitate at high acetonitrile concentrations, often above 70 to 80% depending on concentration and counter-ion, blocking check valves and columns. Methanol generally tolerates phosphate better. Ammonium acetate and ammonium formate are more soluble in organic solvents.
Property 5: detector compatibility
UV and PDA
Low-wavelength work, below about 220 nm, favours acetonitrile, phosphate buffers and phosphoric or perchloric acid, which are UV-transparent. Trifluoroacetic acid (TFA) absorbs strongly below about 220 nm, causing gradient baseline drift. Matching TFA concentration in both solvents or adjusting wavelength helps.
Mass spectrometry
MS requires volatile additives: formic acid, acetic acid, ammonium formate, ammonium acetate or ammonium bicarbonate. Non-volatile buffers such as phosphate contaminate the source. TFA suppresses ionisation in electrospray and is best avoided or minimised. Solvents should be LC-MS grade, which controls metal ions that form sodium and potassium adducts.
Refractive index and ELSD
Refractive index detection requires isocratic methods with very stable composition. Evaporative light-scattering and charged aerosol detection need volatile mobile phases and low residue solvents.
Choosing the right grade
| Application | Recommended grade |
|---|---|
| Isocratic UV above 250 nm | HPLC grade (AR may suffice in some cases) |
| Gradient UV, or detection below 220 nm | Gradient or HPLC grade with specified low-UV absorbance |
| LC-MS and LC-MS/MS | LC-MS grade solvents and additives |
| UHPLC with sub-2 µm particles | UHPLC or LC-MS grade, finely filtered |
| Preparative HPLC | HPLC or preparative grade with low residue on evaporation |
Good practice with mobile phases
- Prepare aqueous buffers fresh, filter through 0.2 or 0.45 µm membranes, and discard after a few days to avoid microbial growth.
- Measure pH of the aqueous portion before adding organic solvent, and state this in the method.
- Measure solvent volumes separately before mixing. Mixing contracts the volume.
- Degas by helium sparging, vacuum or inline degasser.
- Never top up old mobile phase. Replace it.
- Store THF and ethers properly and test for peroxides after opening.
- Use dedicated, detergent-free glassware for LC-MS.
Water: the forgotten solvent
In reversed-phase HPLC, water is often more than half the mobile phase, yet it is frequently the least controlled component. Water from a purification system should meet ultrapure specifications, typically resistivity of 18.2 MΩ·cm at 25 °C and low total organic carbon, and should be drawn fresh rather than stored in open containers, where it absorbs organic vapours and carbon dioxide from the laboratory air. Bottled HPLC-grade or LC-MS-grade water is a practical alternative where system maintenance is uncertain, and it gives a consistent, documented starting point during method transfer between laboratories.
Troubleshooting mobile-phase problems
- Rising baseline in gradient: solvent B absorbs more than A at the wavelength. Balance additives or raise the wavelength.
- Ghost peaks in blank gradients: impurities in water or solvent concentrating on the column. Switch to gradient-grade solvents and fresh water.
- Pressure spikes: buffer precipitation or particulates. Check solubility at the highest organic percentage.
- Loss of MS sensitivity: non-volatile additives, detergents or low-grade solvents.
FAQs
Why does acetonitrile give lower back-pressure than methanol?
Acetonitrile-water mixtures are less viscous than methanol-water mixtures, which peak in viscosity at around 40 to 50% methanol.
Can I use phosphate buffer with LC-MS?
No. Phosphate is non-volatile and contaminates the ion source. Use volatile buffers such as ammonium formate or ammonium acetate.
What is the UV cutoff of methanol?
About 205 nm. For work below about 210 nm, acetonitrile is usually the better choice.