Solvent Delivery System

The high-pressure pump can operate at pressures from 500 to 5000 psi. The purpose of the pump is to deliver a precise, accurate, reproducible, constant, and pulse-free flow of mobile phase to the column. Three major classes of HPLC pumps are currently in use: constant-pressure pumps; syringe-based or displacement pumps; and constant flow pumps. Neither constant pressure or syringe-based pumps are easily adapted to gradient solvent delivery; hence, constant-flow pumps are by far the most common.

The majority of commercial high-pressure pumps available today are designed around a simple recip rocating piston pump. The rotational energy of a motor is transferred into the reciprocal movement of the piston by an eccentric cam or gear. The piston is driven in and out of a solvent chamber in the pump head, which typically has a volume of 10 to 100 |L. A pair of check valves control the direction of flow through the pump head. A piston seal keeps the mobile phase from leaking out of the pump head. On the intake stroke, the piston is withdrawn from the pump head, creating a low-pressure zone. The low pressure causes the inlet check valve (i.e. from the mobilephase reservoir) to open and the outlet check valve (i.e. delivery to the column) to close, allowing the mobile phase to fill the pump head. On the delivery stroke, the piston moves into the solvent chamber, which increases the pressure. The high pressure closes the inlet check valve and opens the outlet check valve, allowing the mobile phase to flow to the column. In a single-head reciprocating pump, the solvent chamber is delivering mobile phase to the column only half the time. The other half of the time it is being used to fill the solvent chamber. With a twin-head reciprocating pump, two pump heads operate simultaneously but at 180° out of phase with each other. As a result, mobile phase flows to the column 100% of the time. The twin-head design gives essentially pulseless flow as compared to the single-head design.

Many separations can be done isocratically, which means that solvent composition being delivered to the column is not changing in composition over the course of the separation. For more complex separations, gradient elution is required. Most commonly, gradient elution is performed by altering the proportion of the eluents over the course of the separation. In doing so, the early eluting compounds remain well resolved, while the more highly retained compounds elute quicker. Gradient elution is simply the programming, or changing of the solvent strength over the course of a separation. A gradient can be linear, convex, concave, stepped, or a complex sequence of each to achieve the desired separation. Computer-controlled pumping is required to generate a gradient flow. Three major approaches are used to produce a gradient flow: (i) controlled amounts of each eluent (up to four may be used) are metered into a mixing chamber before reaching the high-pressure pump; (ii) a proportioning valve, which is controlled by a microprocessor, regulates the amount of up to four eluents and the eluent mixture is sent to the high-pressure pump; and (iii) the outputs of multiple high-pressure pumps, which are controlled individually by a programming device, are mixed together in a high-pressure mixing chamber after the pump. Approaches (i) and (ii) are known as low-pressure mixing, and approach (iii) is high-pressure mixing. Low-pressure mixing is less expensive than high-pressure mixing since only one high-pressure pump is used versus the two or more required for high-pressure mixing, and the maintenance of one pump is much easier than the maintenance of two or more pumps. The main problem with low-pressure mixing is that it is more susceptible to bubble formation because the solvents are being mixed at atmospheric pressure. Hence, the common use of online degassers.

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Solar Panel Basics

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