The primary choice is Isocratic or Gradient.

An Isocratic pump does nothing except pump! It pumps the same solvent continuously throughout the analysis. You have to mix up the eluent as per the HPLC Method, and the pump simply pumps it through the system. For some methods this is OK, and if so, an isocratic pump is preferable because it is much easier to use and costs less money! However if you need to be able to pump a solvent gradient  (or may do in the future), you should buy a gradient pump now.

A Gradient pump will pump 2, 3 or 4 different solvents and mix them on-line. It can also change the ratios of each as the run progresses. This is known as a solvent gradient. This is required if the range of polarities of the components in the sample mixture is too wide. Using isocratic elution, some would come out all together with the solvent front, while others would still eluting from the column even after an hour or more. The solution is to start with a weaker eluent, and gradually increase the strength of the eluent until all the peaks are eluted.

A gradient pump is much more versatile, and even allows you to mix isocratic eluents on line automatically. However it needs to be programmed, and hence is inherently more complicated to use. If you are not sure though, it is probably best to buy a gradient pump because you cannot upgrade one to the other.

Two solvents is enough for most applications, although a third is often useful for flushing at the end, eg water to remove buffer salts or a strong solvent to clean the column.

If you choose a gradient pump, you will also need a degasser. This takes out dissolved air from the eluent components. Air is much more soluble in some solvents than others, and when they mix, bubbles are formed. Air is one of the things that pumps like least, and it is very important that air does not enter the pump. When the pump mixes a gradient, the air is released inside the pump (causing immediate problems with flow rate) so a degasser is essential with gradient elution. If you select a gradient option below, a degasser will automatically be selected, although you will have the option to remove it later if you feel really bad about it!

Next we need to know what flow rates you require.

Most analytical HPLC is run at around 1-2ml/min. For this sort of work, a 5 or 10ml/min pumphead is ideal. If this is what you will need, select one of the Analytical pump options below.

However if you wish to work with microbore columns (maybe because you will be working with Mass Spec detection, or with very small sample sizes) then your flow rates are likely to be in the region 1-500ul/min (ie 0.001-0.5ml/min) and this is asking a lot of an analytical pumphead. It is important that a pump can pump the same flow rate constantly and from one day to the next, so that retention times remain constant, so using the pump when it is almost stopped is pushing it to the limit. In this case we use a microflow pumphead (max 3ml/min or less) or possibly a syringe pump. If this is what you require, choose a microflow pump.

Should you need to scale up for a preparative application, you will possibly need flow rates higher than 10ml/min, and in this case you should choose a preparative pump, or a standard pump with a preparative pumphead. Using such a pumphead, many pumps can go to 40ml/min. If you need more than this, you will need a preparative pump, and the flow rates can go to 100, 200 or even 500ml/min. For any of these options, select a Preparative pump option below.

Isocratic Pump, Analytical