Pump Selection

Selecting a Pump

Labhut supply a wide range of pumps to suit many different laboratory applications. This is a short guide to ensure that you choose the correct one most suited to your application.

Flow rate

This is normally the primary choice for a pump. Typically the flow rates for a pump are loosely described as being for analytical use (0-10ml/min) or Prep/semi-prep  up to 100ml/min then flash (100ml/minand above).  Larger pumps use bigger pistons and this often means that the maximum pressure they can pump is reduced, (example – a 10ml/min Series II pump can pump at 6000psi, the 40ml/min model can reach 1500psi) and  the absolute accuracy of the low rate will similarly be effected.

The new HF pumps are specifically designed to pump at high flow rates with high pressures if the application demands this.

Very low flow rates are sometimes more accurate with a 5ml head or a twin head pump if precision is required

Pressure rating  and pressure reading

The next parameter considered after flow rate is the pressure rating. Long or thin columns require higher pressures for a given flow rate than shorter broader columns.  HPLC columns that use smaller particles also require higher pressures.  The maximum pressure of the pump is normally rated using Bar or psi (1bar = 14.5psi).

Not all pumps measure pressure. This requires a small device called a pressure transducer to be fitted inside the pump (typically this normally on the pulse damper but can be separate). The pressure measurement provides a useful diagnostic tool – if the pressure rises or falls it may be showing a blockage or a leak, and if it fluctuates it could indicate there is air in the system.

Also for HPLC the pressure can give an indication of how the column is ageing. To help with this all Cronus HPLC columns are all supplied with a test certificate showing the pressure at a given flow rate.

Typical pressures for a 25x0.46cm HPLC column are around 80-120bar.  

UHPLC columns require extra high pressures – and for these we recommend using the Ultra high pressure pumps.

PEEK or Stainless Steel Pump Heads

Pumps heads are available using two different types of material,  Stainless Steel and PEEK.

Stainless steel is recommended for water (must be pure) and organic solvents.  PEEK is recommended for solutions containing buffer salts or acids that may cause corrosion to steel.

Each pump also has an internal flow path to match the head type. These flow paths will be called 'inert' or 'bioclean' if the pump heads are supplied in PEEK.

Stainless steel is physically stronger than PEEK, and can therefore withstand higher pressures. So often pumps will have a higher pressure rating for a Stainless steel pump head than for PEEK. All UHPLC pumps are stainless steel

Pulsation

Pulsation normally refers to changes in the pressure when the pump is running.  For some systems this isn’t a problem, but it can affect HPLC performance (particularly if using a Refractive Index detector and it will also affect some other sensitive systems). 

Pulsation occurs when the piston inside the pump is making a fill stroke (backstroke). The flow temporarily stops (backflow is prevented by a check valve) and this causes a pulse in the pressure.

Pulsation is much more noticeable in a single piston pump compared to pumps that have two or more heads. In these systems the heads counter each others strokes so that one is always pumping whilst the other is making the backstroke.

Pulsation can also be reduced by using a pulse damper. These require pressures of more than 500psi to become effective and work by having a flexible membrane over a small solvent reservoir (this acts in a similar way to a spring and smooths  any pulses.  The pulse dampers have an upper pressure limit of around 5000 psi.

Gradient

Many applications require the use of a solvent gradient. This is where two solvents are mixed in a changing proportion over a period of time. For example a solvent mix may be required to begin at 20% A and 80%B and to finish at 80%A and 20%B (typically the proportion of the stronger solvent is increased).

There are two common ways to create a gradient – use a single pump with a proportioning valve that switches between the different solvent inlets to mix the solvents before they enter the pump giving a "low pressure" gradient - or to connect two more pumps together that mix the solvents together after they have left the pump (this is a high pressure gradient).

The lowest cost of forming a gradient is to use the Labhut B series pumps as these can be used on a standalone basis. High pressure gradients require two pumps and software control (PC or control unit) .

Constant pressure

Almost all pumps are constant flow pumps – this means that the piston will move at a constant speed to ensure the flow rate remains the same. This also means that the pressure in the system will vary to keep the flow rate constant.  Constant pressure pumps however will vary the flow rate to keep the pressure constant (these are not recommend for constant flow applications). The most common use for constant pressure pumps is to pack HPLC columns.

Maintenance

All pumps are made using well used technology and do not require any difficult annual maintenance. The piston seals are considered to be a consumable part and will need replacing every 4-18months depending on use and application. This a simple procedure that takes a few minutes.

It is highly receommended to use oly HPLC grade solvents and ensure that these are adequately filtered before entering the pump