Flow rate precision

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. 

Propagated error assuming flow rate precision of... 

Qualification Approaches If an analytical instrument is comprised of different functionally discrete modules, a modular approach to qualification testing that focuses on the specific operations of the individual module can be suitable for certain aspects of some operational testing (such as the flow rate precision and accuracy testing of a HPLC pump and the temperature accuracy column compartment). 

One of the concerns to chromatographers is how good a flow-rate precision can be obtained in LC to ensure good laboratory practice. A partial answer to this question can be found in a paper entitled Precision of Contemporary Liquid Chromatographic Measurements (12), where it was reported that with an optimized system, retention times could be measured with a standard deviation of 0.1% or less. This deviation was determined both on samples run on the same day and on different days. The article states ... 

Many LC users are curious about the precision they can expect from an analysis. This question is often answered with specifications concerning flow rate precision, injection volume precision, and so forth. However, the precision of the analysis is influenced by all of the variables in the HPLC system, including operator error. Often within a laboratory, the precision of the LC analysis is within 1%. However, the larger question concerns the between-laboratory reproducibility of analyses. 

The numerous reasons which can account for various deviations from the ideal FFF retention theory were discussed in the corresponding sections. Here, additional problems are treated which can complicate FFF measurements and significantly distort the results obtained. General requirements for a successful FFF measurement include precise flow control and flow rate precise temperature measurement precise determination of t0 and tr correct relaxation procedure control of sample overloading and integrity and control of mixed normal and steric retention effects as well as wall adsorption control. Some of these complications cannot be avoided so one must correct for these effects, usually in a sem-iempirical and partially very complicated fashion. 

It should be able to deliver high flow-rate precision. 

The same experiment can be used for the assessment of flow-rate precision by evaluating the repeatability of retention times for the examined compounds, expressed again as RSD. Both terms, flow-rate and injection volume precision, affect the precision of the results of the analysis. Carryover problems can be identified in the wash chromatograms at maximum detector sensitivity. They can be checked by multiple analyses of high-concentration samples followed by blank injections. 

The valve positioner, which is usually contained in its own box and mounted on the side of the valve actuator, is designed to control the valve stem position at a prescribed position in spite of packing friction and other forces on the stem. The valve positioner itself is a feedback controller that compares the measured with the specified stem position and makes adjustments to the instrument air pressure to provide the proper stem position. In this case, the setpoint for the valve positioner can be a pneumatic signal coming from an l/P converter or the 4-20 mA analog signal coming directly from the controller. A valve with a deadband of 25% can provide flow rate precision... 

Depending on the physical equipment being used, it may or may not be easy to control flow rates precisely along more or less complex trajectories. However, even simple flow schedules, such as linear ramps or step-wise constant ramps, could be advantageous in exploring the X-T plane. Whether the additional complexity involved in... 

Pumps These are piston-type precision pumps. They pump the polymer in solution through the system. The pump must deliver the same flow rates throughout the time in order to maintain the same pressure inside the system. Any variation in flow rates affects directly the results. The pump also has to deliver the same flow rates independently of the viscosity differences. In addition, some detectors are highly sensitive to the solvent flow rate precision. Such constant flow is a critical feature of the instrument. 

In order to conduct microdialysis experiments, several other components are required. Syringe pumps are often used to control the perfusate flow rate. The pump has to be able to deliver flow rates precisely in the microliter per minute range. Tubing is needed to connect between the probe and the pump which drives the perfusion flow and, in some cases, between the probe and a sample collector as well. The total dead volume of tubing should also be maintained as small as possible to have better time resolution. The perfusion fluid is a medium resembling the composition of extracellular fluid with minimal or zero concentration of the molecules of interest. Dial-ysate exiting from the outlet of the microdialysis probe is usually collected in a vial for later analysis. It is also possible to coimect the outlet directly to an analysis instrument without using a collector, which is usually preferred, if possible, for its convenience and usually faster analysis results. 

Figure 16 Effect of pump flow rate precision. Each 1 % error in GPC ffow rate results in a 10% error in molecular-weight value.

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