Choosing an HPLC column: which stationary phase suits your analysis?
From C18 to HILIC: a practical selection model based on polarity, molecular size and pH
Choosing an HPLC column is the most important decision in your method development. The column determines the separation mechanism, the retention time, the resolution and ultimately whether your analysis produces reliable data. Yet in practice, the choice is often made out of habit rather than on the basis of the properties of the analytes.
This article gives you a systematic selection model. You will learn which stationary phase suits which class of substance, why pore size is decisive for large molecules, and what to do when a standard C18 gives insufficient separation.
⏱ approx. 10 minutes reading time
The rule of thumb
Start with a C18 reversed-phase column. It covers the vast majority of analyses of small organic molecules. Only if the separation gives insufficient resolution should you choose a different stationary phase or a different separation mechanism.
Step 1: determine the polarity of your components
The polarity of the substances to be separated determines which chromatographic mechanism you need. This is the first and most decisive question when choosing an HPLC column.
| Mode | Suitable for | Stationary phase | Mobile phase |
| Reversed phase (RP) | Non-polar to moderately polar. By far the most widely used. | Non-polar: C18, C8, phenyl | Polar: water with methanol or acetonitrile |
| Normal phase (NP) | Polar substances that separate poorly on RP | Polar: silica, cyano, amino | Non-polar: hexane, dichloromethane |
| HILIC | Very polar compounds: sugars, amino acids, metabolites | Polar: amino, amide, silica | High percentage of acetonitrile with aqueous buffer |
| Ion exchange | Ionisable substances: acids, bases, salts | Charged groups on a support | Buffer with variable ionic strength or pH |
| Size exclusion (SEC) | Separation by molecular size: polymers, proteins | Porous network without interaction | Isocratic, adapted to the sample |
Step 2: choose the stationary phase
Within reversed phase, there are several stationary phases, each with its own selectivity. The choice determines how strongly your analytes are retained and which secondary interactions occur.
| Stationary phase | Characteristic | Choose this when |
| C18 (ODS) | Universal, highest retention for hydrophobic substances | Standard starting point for small organic molecules |
| C8 | Shorter chain, less retention than C18 | Substances are retained too long on C18; faster analyses desired |
| Phenyl | Offers pi-pi interactions in addition to hydrophobic retention | Aromatic compounds that co-elute on C18 |
| Cyano (CN) | Intermediate polarity, usable in RP and NP | Moderately polar substances; switching between modes without changing columns |
| Amino (NH2) | Polar, suitable for hydrogen bonding | Sugars, carbohydrates and HILIC applications |
| Biphenyl | Stronger pi-pi selectivity than phenyl | Structural isomers and aromatics that are difficult to separate |
When C18 is not sufficient
Changing the column is not always the answer. There is a lot of difference between C18 stationary phases: end-capping, silica purity and bonding density give distinctly different selectivity. First try a different C18 from another manufacturer before switching to a different phase type.
Step 3: take the chemical properties of the analytes into account
Ionisable substances
Acids and bases change charge with the pH of the mobile phase. A charged molecule has hardly any retention on a non-polar phase. You have two options: suppress the charge by adjusting the pH (pH at least 2 units below the pKa for acids, or above it for bases), or choose an ion exchange column that makes use of the charge.
Molecular size and pore size
The pore size of the column material must match the size of your analytes. If the pores are too small, large molecules cannot penetrate the pore network and the effective surface area is lost.
| Molecule type | Pore size | Explanation |
| Small organic molecules (<5 kDa) | 80 to 120 Ångström | Standard for pharmaceutical and chemical analyses |
| Peptides | 200 Ångström | Intermediate position; depends on chain length |
| Proteins and large biomolecules | 300 Ångström and larger | Larger pores are essential for accessibility |
Step 4: determine the column dimensions
Length, diameter and particle size determine the balance between resolution, analysis time, solvent consumption and system pressure.
| Parameter | Larger or longer | Smaller or shorter |
| Column length | Better separation, longer analysis time, higher pressure | Faster analysis, less resolution |
| Internal diameter | Higher loading capacity, more solvent | Less solvent, higher sensitivity, more sensitive to dead volume |
| Particle size | Lower back pressure, less efficiency | Higher efficiency (N), much higher back pressure |
Particles of 3 to 5 micrometres are standard for classic HPLC. Particles of 1.7 to 2.7 micrometres deliver considerably higher efficiency but require a UHPLC system that can handle the associated back pressure.
Take care with narrow columns
The smaller the internal diameter, the greater the effect of dead volume outside the column. A 2.1 mm column is considerably more sensitive to extra-column band broadening than a 4.6 mm column. With narrow columns, always optimise your fittings, capillaries and detector cell first
Step 5: check the pH stability of the column
The column must be resistant to the pH of your mobile phase. Exceeding the pH range causes hydrolysis of the bonded phase or dissolution of the silica backbone, resulting in irreparable loss of performance.
| Column type | pH range | Remark |
| Classic silica | Approx. pH 2 to 8 | Rapid degradation occurs outside this range |
| Hybrid silica | Approx. pH 1 to 12 | Organic-inorganic hybrid backbone, more widely applicable |
| Polymer columns | pH 1 to 14 | Full pH freedom; usually lower efficiency than silica |
| Acid-resistant C18 variants | pH 0.5 to 8 | Specifically for low-pH methods |
Guard column: a small investment, a big effect
A guard column protects the analytical column against strongly retained contaminants and particles from the sample. The rule of thumb: use the same packing as in the analytical column, so that the selectivity does not change. Alternatively, an inline filter can be used when only particle protection is needed.
A guard column costs a fraction of an analytical column and considerably extends its life, especially with complex matrices such as plasma, food or environmental samples.
Frequently asked questions about choosing an HPLC column
What is the difference between C18 and C8?
C18 has an alkyl chain of 18 carbon atoms, C8 of eight. The longer chain of C18 is more hydrophobic and gives stronger retention of non-polar substances. Choose C8 when analytes are retained too long on C18, or when you want a shorter analysis time without drastically changing the mobile phase.
Can I use a C18 column for gradient elution?
Yes. Gradient elution, in which the composition of the mobile phase changes during the run, is a standard technique with C18 columns. It enables controlled elution of a wide range of compounds with differing polarities within a single analysis.
Why do my sugars not separate on a C18 column?
Sugars are very polar and have hardly any retention on a non-polar C18 phase; they elute with the dead volume. Choose a HILIC column or an amino column, where the polar stationary phase forms hydrogen bonds with the hydroxyl groups of the sugars.
How do I know when my column needs replacing?
Look out for three signals: decreasing efficiency (theoretical plate number N), increasing peak tailing (asymmetry factor above 2.0) and shifting retention times with an unchanged method. First flush with a strong solvent; if the problem persists, replacement is the most direct solution.
