Pneumatic pump. HPLC

Liquid pressurizing systems can be classified into four types, namely (i) those which have no true pump—the pneumatic pumps (ii) those with reciprocating piston pumps, (iii) those with syringe type pumps, and finally (iv) those with a combination of the first and second systems, namely pneumatic amplifier pumps. Of these various types the first two (cheaper ones) will be discussed here because they are the more popular and because the syringe type pumps have been discontinued by most HPLC pump manufacturers in favour of the modern reciprocating pump. 

It should perhaps be noted here that these pneumatic pumps can however only operate for a limited time before re-filling of the solvent reservoir is necessary. Further, whilst they are excellent devices for operating HPLC columns in the isocratic mode they are not suitable for gradient mode elution. In spite of these drawbacks, the simplicity and cheapness of this type of eluant-pressurizing system make them very attractive to a number of laboratories and especially those on limited budgets. 

Compared to syringe type or reciprocating pumps, pneumatic amplifier pumps are very cheap. They tend to be rather difficult to dismantle for repairs, and some types are very noisy in operation. Because they do not provide a constant flow of mobile phase, they are not used much in analytical hplc. They can, however, operate at high pressures and flow rates and so are used mainly for packing columns, where high pressures are needed and variations in the flow rate through the column do not matter. 

Fig. 11.5. Diagram illustrating the components of an ESI source. A solution from a pump or the eluent from an HPLC is introduced through a narrow gage needle (approximately 150 pm i.d.). The voltage differential (4-5 kV) between the needle and the counter electrode causes the solution to form a fine spray of small charged droplets. At elevated flow rates (greater than a few pl/min up to 1 ml/min), the formation of droplets is assisted by a high velocity flow of N2 (pneumatically assisted ESI). Once formed, the droplets diminish in size due to evaporative processes and droplet fission resulting from coulombic repulsion (the so-called coulombic explosions ). The preformed ions in the droplets remain after complete evaporation of the solvent or are ejected from the droplet surface (ion evaporation) by the same forces of coulombic repulsion that cause droplet fission. The ions are transformed into the vacuum envelope of the instrument and to the mass analyzer(s) through the heated transfer tube, one or more skimmers and a series of lenses.

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

Constant pressure pumps utilise pneumatics or hydraulics apply the pressure required to force the mobile phase through the column, either directly or indirectly. Two main designs of constant pressure pump exist the pressurised coil pump, and the pneumatic pressure intensifier type. The pressurised coil pump is now all but redundant, but as it represents the most simple means possible of pumping at high pressure through an HPLC column it is described briefly. 

Other problems with pneumatic intensifier pumps include the fact that access to the high pressure seals for inspection and maintenance is usually quite restricted, by nature of their design. Because of the way they operate, the flow they produce is inherently highly pulsatile in nature and they also tend to be extremely noisy in use. For these reasons, pumps of this type are not used in general analytical HPLC. However, pneumatic intensifier pumps have found a niche in the packing of HPLC columns, where the intermittent nature of the function and their ability to deliver very high pressures compensate somewhat for their shortcomings in the analytical field. 

The most commonly used pump for HPLC is the reciprocating pump. This has a small cylindrical piston chamber that is alternately filled with mobile phase and emptied via back-and-forth movement of the piston. This produces a pulsed flow that must be damped. Reciprocating pumps have a number of advantages. They have a small internal volume, are capable of high output pressures, and they can readily be used for gradient elution. They provide constant flow rates, independent of solvent viscosity or column backpressure. Other pumps used are motor-driven syringe pumps and pneumatic (constant-pressure) pumps. 

Recently, a different approach to HPLC pumping was developed (Jensen 2004). Pressure in the system is generated by connecting laboratory air or nitrogen to a pneumatic amplifier that produces an amplification factor for pressure values up to 36 for example, a nitrogen supply at 100 psi can be amplified to deliver... 

Figure 4.22 Schematic of a pumping system based on a pneumatic gas pressure amplifier with microfluidic flow control via feedback from a sensitive flowmeter. In this way the flow rate is maintained regardless of changes in system back pressure or mobile phase viscosity, and changes in flow rates can be established rapidly and accurately, (a) A gradient system in which the mobile phase composition is controlled via flow rates of both mobile phase solvents, (b) A gradient system in which both back pressures and flow rates are monitored volume flow rates = k. (P(- -P ) and Ug = kg.(Pc - Pg) where k and kg are calibration constants, (c) A demonstration of the precision and accuracy with which controlled flow rates can be changed rapidly at total flow rates in the nL.min range, suitable for packed capillary HPLC. Reproduced from company literature (Eksigent 2005, 2006) with permission from Eksigent LLC.

In one approach, the microchip interface was constmcted from modified 1/16-inch high-performance liquid chromatography (HPLC) fittings [30]. It incorporates a freestanding liquid junction formed via continuous delivery of a flow of suitable solvent which carries the separation effluent through a pneumatically assisted electrospray needle located in front of the MS orifice. In some cases, stmctural features of microchips can be utilized as parts of the ion source (e.g., ESI emitter). Thus, the resulting coupled microchip-MS systems are more compact, and the delay time between the on-chip incubation and MS detection can be decreased. For example, a capillary nanoESI emitter was successfully incorporated into a microchip CE channel for on-line CE-MS analysis [31]. Such microchip-MS systems do not require the use of external pumps because analytes can be driven toward the ion source (ESI or nanoESI) by means of electroosmosis and electrophoresis [32]. 

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