The HPLC column
This is also known as the stationary phase. This is the workhorse of the HPLC machine, is made from one of a variety of substances (often silica) and is very compact in nature. The silica particles are functionalized by long carbon chains. The carbon chains are apolar and therefore the longer the chain, the more apolar the column becomes. Columns containing 18-carbon chains are commonly used and are known as C18 columns.
The HPLC sample
Sample types vary greatly depending on the field and the type of compounds in question. HPLC can be used to analyze compounds in biological specimens (urine, blood, saliva, and muscle), environmental samples, medicinal chemistry (drugs), and microbiology (toxins produced by fungi and bacteria).
Injection of sample
Samples are injected into the HPLC column. This used to be carried out manually, meaning that some poor soul had to sit by the HPLC machine for hours on end injecting each sample with a syringe, sometimes all night long!
Luckily, newer models have an automatic injector, reducing the manual input and allowing higher throughput. Modern machines are equipped with software allowing the user to input a list of samples, how much and in what order they should be injected. So you can enjoy your lunch while your HPLC runs itself!
The mobile phase
This is really just a mixture of water and an organic solvent (usually acetonitrile or methanol). The mobile phase gets it name because it moves through the column and at the same time elutes (or flushes out) the compounds from the column.
Compounds are often eluted along a concentration gradient. If you’re anything like me, concentration and gradient are two words you hate to see coming together in a sentence! It just means that the percentage of water in the mobile phase decreases over time, while the percentage of the apolar solvent increases simultaneously. This means that the mobile phase gradually becomes more apolar. Don’t worry too much about gradients for now, as they will appear again in a follow-up article.
The HPLC run
HPLC can be performed in a number of modes. The most commonly used method is known as reversed-phase (RP-HPLC) and this is what I describe here. In this mode, compounds are separated starting with the most polar, and ending with the apolar compounds. For all modes, a high-powered pump moves the sample and the mobile phase through the column. A typical run can take between 10-60 minutes.