Solvent delivery systems flow controllers

The solvent delivery system is one of the most important components of the liquid chromatograph since its performance directly affects retention time reproducibility d detector baseline stability [1,2,39,40]. As well as the pump, other components such as check valves, flow controllers, mixing... 

The assay was performed with a HPLC-system consisting of a Spectra-Physics (Spectra Physics, San Jose, CA 95134, USA) model SP8700 solvent delivery system used at a flow rate of l.Oml.min", a Kratos (Kratos Analytical Instruments, Ramsey, NJ 07446, USA) model 757 UV-detector, wavelength 260 nm, range 0.005 aufs, rise-time 1 second. Injections of extracts into a Zymark (Zymark Corporation Inc., Hopkinton, MA 01748, USA) Z 310 HPLC-injection station, equipped with an electrically controlled Rheodyne valve and a 20 pi sample loop, were performed by a Zymate II robot system. The Zymark Z 310 Analytical Instrument Interface was used to control the HPLC-injection station. 

Extraction System. The flow-through extraction system used in this study is shown in Figure 1. The system is operable up to 400 bar at 200°C. It consists of solvent delivery systems (Fluid 1, Fluid 2, Fluid 3), a flow-through reactor (FR), a set of separator traps (TP1, TP2), and the temperature and pressure control units. The reactor, traps, micrometering valves, and tubing connections are housed in a heated oven. 

The function of the solvent delivery system is to deliver the mobile phase (eluent) through the chromatograph, accurately and reproducibly. The solvent delivery system comprises the pump, check valves, flow control-... 

The solvent delivery system in this instrument uses a syringe-style, positive-displacement, continuous delivery design. Each syringe is motor driven and software controlled so that crossover-related flow phenomena are eliminated. Gradients can also be performed, and are mixed under high pressure to minimize the gradient delay. 

Modified instrumentation Solvent delivery system capable of accurate flow control down to 10 1 min" or less Small volume sample injection valves Small volume detector cells... 

Notes. (1) The nitrogen inlet system should be of the type suggested in Fig. 2.60 with a sufficiently great enough head of mercury or mineral oil in the escape valve to force the solvent from flask E to A. Control of solvent flow should be by stopcock F, and the outlet of the solvent delivery tube should be above the final level of solution. 

The injector system is often of the loop type. Here the main solvent delivery tube to the column top is by-passed in a loop, which may be isolated and depressurised, and injected with sample via a septum. After injection the liquid in the loop is released into the main solvent flow. The loop volume is of comparable capacity to the injection volume. Most instruments are designed for autosampling in the case of multiple analyses, the operation being controlled by the instrument software. 

Flow control loops are designed to maintain constant retention times by providing constant flow delivery from the pump, commonly by the use of an encoder wheel attached to the main motor drive. The encoder accurately measures the rotation of the motor extremely accurately to parts-of-a-degree of rotation, and it regulates the motor revolutions to as constant a value as possible. In a well-maintained pumping system, a constant motor speed will deliver a constant flow for a given solvent. In modem pumps a flow control loop is often combined with a pressure loop, to provide both... 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

---