Constant flow pumps

Unfortunately, neither the computer nor the potentiometric recorder measures the primary variable, volume of mobile phase, but does measure the secondary variable, time. This places stringent demands on the LC pump as the necessary accurate and proportional relationship between time and volume flow depends on a constant flow rate. Thus, peak area measurements should never be made unless a good quality pump is used to control the mobile phase flow rate. Furthermore, the pump must be a constant flow pump and not a constant pressure pump. 

The function of the pump in hplc is to pass mobile phase through the column at high pressure and at a controlled flow rate. One class of pump (constant pressure pump) does this by applying a constant pressure to the mobile phase the flow rate through the column is determined by the flow resistance of the column and any other restrictions between the pump and the detector outlet. Another type (constant flow pump), generates a given flow of liquid, so that the pressure developed depends on the flow resistance. 

Two types of constant flow pump have been used in hplc. Fig. 2.2c shows a syringe type pump. 

Two alternative but much less common types of constant flow pump are... 

Mobile phases employed for the separations are housed in a cartridge and delivered to the LC columns through a set of binary HPLC pumps (Shimadzu Corporation), as shown in Figure 6.2. The pumps provide a flow rate accuracy of 2% or 2 fiL (whichever is greater) in constant flow pumping mode, with a flow rate precision of 0.3%. A degasser (two channels internal volume of 195 /.d. /channel) is also housed in the pump module employed to minimize the occurrence of air bubbles. 

Importantly, in a constant-pressure pump the flow rate will change if the flow resistance changes. Whereas in the constant flow pumps the changes in flow resistance are compensated duly by a change of pressure. Therefore, it is always advisable to use constant flow pump in HPLC determinations. 

Sample collected with low-volume sampler consisting of a constant flow pump and a cartridge containing polyurethane foam. Extract with diethyl ether in hexane. 

The constant-flow pumps is the most widely used in all common IC applications. Flow rate stability is an important pump feature that distinguishes pumps. For size exclusion chromatography, the flow rate has to be extremely stable. External electronic control is a very desirable feature when automation or electronically controlled gradients are to be run. 

Industrial Hygiene Air Sampling with Constant Flow Pumps... 

The other type of constant flow pump is a positive displacement syringe. It is pulseless but suffers from the limited volume it can deliver before refilling. The major type of constant pressure pump is a pneumatically driven syringe. 

Flow rate. The flow rate from the pump should be accurate, regardless of the system in which the pump is used. This means that the flow rate actually produced by the pump is the same as that dialled up on the front panel, and that this should not be affected by the rest of the HPLC system (for constant flow pumps, at least). The flow rate should be reproducible and practically free of pulsations. Pulsatile flow can limit the sensitivity of HPLC assays, resulting in a rhythmic variation in the apparent refractive index of the mobile phase flowing through the detector, which ultimately manifests itself in the chromatogram as baseline noise. Various means are employed to attempt to eliminate flow pulsation (see section 5.3). 

The vast majority of pumps currently used in analytical HPLC are of the constant flow variety. With this type of pump, changes in the chromatographic system, for instance those leading to variations in the back pressure experienced by the pump, are compensated for, and the flow rate remains constant this is an important factor in most analytical applications. Two major types of constant flow pump exist the reciprocating piston design and the syringe type. 

The HPLC system consisted of a Varian Model 5560 liquid chromatograph equipped with a constant-flow pump, a variable wavelength detector (Varian, Instrument Group/Walnut Creek, CA), a Rheodyne injector valve and an Alltech Spherisorb ODS column, 4.6 mm id x 25 cm (Alltech/Applied Science, IL). The mass spectrometer was a Finnigan Model 4600 quadrupole instrument with... 

The active air sampling system is located on the meteorological tower. A constant flow pump draws air through a four-stage series filter pack at 10.0 L/min. The filter holder is suspended face-down beneath a 40 cm diameter polyethylene funnel, 2.5 m above ground. 

Figure 3.11 Types of constant flow pumps in HPLC.

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