Seals, pump piston

The characteristics of an equipment may change over time, e.g., LfV detector lamps lose intensity, or pump piston seals abrade, or short-term noise affecting the LOD is increased because of flow cell contamination. These changes will have a direct impact on the performance of... 

Fig. 4.31. A slush pump piston seal (1) steel core (2) cylindrical collar (3) rubber glands (4) seal (wiper ring) (5) aimular grooves (6, 7) fixtures...

If reconnecting the column fails to correct the leak, then other common areas to search for leaks are the detector flow cell, the pump (around the pistons), and the injector valve. The same process should be followed until the source of the leak is identified and corrected. New parts maybe required for example, the pump piston seals may have worn out. Always consult the manufacturer s user manual when attempting to replace any part. If in doubt, contact the manufacturer and ask for installation of the new part. It will be necessary to undertake system performance qualification on the replacement of any worn out or damaged part (see Chapter 8). 

The characteristics of an equipment may change over time, e.g., UV detector lamps lose intensity, or pump piston seals abrade, or short-term noise affecting the LOD is increased because of flow cell contamination. These changes will have a direct impact on the performance of the HPLC instrument. The frequency of performance tests will be determined by experience and is based on need, type, and history of equipment performance. Intervals between the checks should be shorter than the time the instrument drifts outside acceptable limits. New instruments need to be checked more frequently, and, if the instrument meets the performance specifications, the time interval can be increased. 

FIGURE 13.5 A photograph showing a pump piston (left), a pump seal (center), and a check valve (right). The pump seal is made to fit snugly over the piston, which is approximately l/4 in. in diameter. 

Comparison of the retention behaviour in 50% aqueous DMF and acetonitrile (Figure 4.3D) indicated that DMF is a selective modifier for the separation of alkanols ( , 1-6) and PAHs (11-16) and of aromatic and aliphatic compounds. Unfortunately, DMF is too aggressive as a solvent and attacks organic polymer piston seals in the pump and, therefore, highly concentrated solutions cannot be used as an eluent. 

Never let buffers sit immobile inside pumps or columns. They can precipitate and cause severe damage to the piston and the piston seal. 

LC pumps—replace or clean piston seals, pistons, in-line filter, solvent filter sinkers and check valves. 

Cautionary note A number of the HPLC methods listed in this chapter use phosphate buffers as solvents. It should always be remembered that phosphate salts are very abrasive to piston seals and pistons. If using these systems, always remember to wash out the aqueous pump thoroughly with water at the end of a run. If this operation is not carried out, the piston seal lifetime will be significantly reduced and in some cases, if left too long, pistons can stick and possibly fracture, which will be expensive to repair/replace. 

Ceramic materials, including sapphire, have been used extensively in HPLC pumps for more than 20 years as pistons and check valve components. These materials have also been used to construct heads because of their good chemical stability. The use of ceramics is limited, however, because of high cost and brittleness. Although many systems have one material as the primary construction material, the wetted surfaces of a pumping system can contain several other materials. Therefore, for material-sensitive applications, all the materials in the HPLC eluent flow path should be considered. Materials that may be encountered are polymeric materials for pump seals such as fluoropolymers, polypropylene, and Teflon sapphire pump pistons and check valve seats ruby check valve balls Kalrez, KelF, or ceramic washers and spacers polymer-based transducer components and in older systems connections and joints made with silver solder. 

Figure 3.5 Reciprocating-piston pump head showing the check valves, piston, seal, and pulse damper. (Adapted from Ref. 1 with permission.)...

Highly concentrated salts (> 0.2 M) and caustic mobile phases can reduce pump seal efficiency. The lifetime of injector rotor seals also depends on mobile phase conditions, e.g., operation at high pH. In some cases, prolonged use of ion pair reagents has a lubricating effect on pump pistons that may produce small leaks at the piston seal. Some seals do not perform well with certain solvents. 

Retention times tend to vary over time because of a number of causes, e.g., differences between batches of mobile phase, column performance and different columns, and ambient temperatures of laboratories. The RSD of retention time, especially important because it is used for peak identification, is influenced by 1) Pump flow 2) composition precision 3) mobile phase composition of solvent delivery systems and 4) column temperature. Imprecise retention time indicates problems within the HPLC system such as piston seals, check valves, etc. 

A rotary-piston pump is an oil-sealed, positive-displacement vacuum pump. The oil both lubricates the pump and seals the discharge from the suction side of the pump. As the piston rotates, gas enters a chamber, as shown in Figure 5.2. Then, the inlet port closes, and the gas is compressed in the chamber until the discharge valve opens, exhausting the gas to the atmosphere. Possible contamination of the oil with condensable vapors, usually water, is a problem. One way condensation can be avoided is by reducing the partial pressure of the condensable gases by allowing air to leak into the cylinder, which is called a gas ballast. 

Piston seals become worn and need to be replaced, e.g. once a year (they cannot be repaired). For this purpose the pump head is removed from the pump body then the seals become visible. They are removed with the special tool which was delivered together with the pump. Removing the seals with a screwdriver can scratch the pump head which causes leaking The new seals are brought in place with another special tool. It is necessary to follow the instructions exactly. One must not mix up the two different sides of a seal. A seal which is placed the wrong way must be removed and thrown away it cannot be used once again. The new seals need to be broken in with a low solvent flow and without a column connected. Water should be avoided to break in a new seal. Well suited solvents are isopropanol and methanol. 

In a special type of short-stroke piston pump the piston does not convey the mobile phase directly. The piston moves within an oil-filled channel, and oil and eluent are separated from each other by an elastic steel membrane. By this design the piston is not in contact with the mobile phase which may be aggressive on the other hand, the eluent cannot be contaminated by abrasives from the piston seal. 

Modern pumps have more features and better reliability and performance than earlier models because of better designs in seals, pistons, and check valves as well as innovations such as dual-piston in-series and piston seal wash.2 Performance at low rates can be improved by variable stroke mechanism, micro pistons, or active check valves. The fluidic components in more inert pumps for bio-purification or ion-chromatography are often constructed from titanium or polyetheretherketone (PEEK). Low-pressure mixing quaternary pumps have become standard equipment in research laboratories whereas high-pressure mixing pumps are popular for LC/MS, HTS, and micro LC applications. 

Common maintenance tasks that can be easily performed by the user are the replacement of solvent filters (sinkers), in-line filters, check valves, and piston seals. Most modern HPLC pumps are designed for easy maintenance, with front panel access to many internal components. Figure 10.1 is a diagram of a slide-out pump unit showing the two pump heads with check valves, the purge valve, the in-line filter, and other components. Procedures for replacing some consumable items are summarized below ... 

Piston Seal The piston seal replacement procedure can be more elaborate and is highly dependent on the particular pump model. A piston seal service kit from the manufacturer should be purchased and the procedure from the service manual should be followed closely for this operation. Typically, the piston should be fully retracted before dismantling the pump head and replacing the seal. A lower than expected system pressure and/or a leak behind the pump head are indications for the need to... 

Pressure too low Lower than expected system pressure is caused by leaks (piston seal, column connections, injector), pump malfunctions (lost prime, air bubbles in pump head, vapor lock, faulty check valves,broken piston), or inadequate solvent supply (empty solvent reservoir, plugged solvent sinker, bent solvent lines, or wrong solvent mixture). Problem diagnostics can be made by visual inspection for leaks and by monitoring the pressure reading of the pump. 

Fig. 4.6.5. Schematic representation of reciprocating pump. 1. motor driven piston (stroke height can be changed) 2, O-ring piston seal 3, diaphragm 4, pump head with back valves 5. eluent inlet 6. eluent outlet.

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