Pump compositional accuracy

FIGURE 4 Table showing the solvent programming steps used for verifying pump compositional accuracy. This program is used specifically for the Waters Alliance 2695 system. 

It is recommended that OQ test the following on an HPLC system flow accuracy, pump compositional accuracy, pressure pulsations, column oven temperature accuracy/stability, detector noise/drift and wavelength accuracy, autosampler injection precision and carryover. 

Solvents from reservoirs are mixed prior to entering the single reciprocating pump. Compositional accuracy is achieved through solenoid valves that operate on a time cycle. To prevent a nonuniform aspiration cycle or siphoning, optical or electrical encoders can make the proper adjustment of solvent composition. A very short aperture time of the solenoid valve is required. 

Manufacturers publish their product s performance characteristics as specifications, which are often used by the customer for comparison during the selection process. Table 1 shows the specifications of an Agilent 1100 Series Quaternary Pump, which is quite representative of other high-end analytical pumps. Note pulsation is particularly detrimental to the performance of flow-sensitive detectors (e.g., mass spectrometer, refractive index detector). Differences in dwell volumes and composition accuracy between HPLC systems might cause problems during method transfers. 

Compositional accuracy [not applicable for single-solvent (isocratic) pumps]... 

FIGURE 4 An example of a pump step profile as part of the compositional accuracy test for reservoirs A and B. Reservoir A contains I % methanol/water and reservoir B contains solvent A with 0.5% acetone added. Courtesy of PerkinElmer LAS, Shelton, CT. 

Blank Pump Accuracy and Detector Linearity -Water Blank 1 1 Pump Composition 1 ... 

Experiments 2-3 are used for testing the pump s compositional accuracy. This test should be performed as in the procedure described earlier under Pump in section (j) of the OQ guidelines. This time the procedure is programmed as is part of a series of automated experiments with the entire HPTC system. 

Pump The pump should be capable of a flow rate between 0.50 and 5.00mL/min. The pump should be a quaternary gradient pump and have a compositional accuracy of +1.5% of the theoretical values for the four channels. The pump should have relative standard deviation (RSD) of <2.0% for six successive readings from a calibrated flowmeter. 

The criteria used to judge the performance of a low-pressure pump include compositional accuracy, ripple, flow accuracy, and pressure accuracy. Compositional accuracy is tested by following the American Society for Testing and Materials (ASTM) procedure E-19.09.07.5 In this procedure, two bottles of eluent are used, one containing 100% methanol (eluent A) and the other containing methanol with a low concentration of acetone... 

All of the pumps must be both accurate and reproducible in flow and composition. Accuracy requires careful control of both the temperature of the fluid and the supply pressure, along with comparable dynamic compressibility compensation schemes. Without dynamic compressibility composition, pumps delivering carbon dioxide will deliver different fractions of their set points, depending on the pressure. Total flow and composition will probably deviate from the set points differently at the two different scales. Accurate compressibility compensation assures scalable pumping. 

This chapter provides an overview of modern HPLC equipment, including the operating principles and trends of pumps, injectors, detectors, data systems, and specialized applications systems. System dwell volume and instrumental bandwidth are discussed, with their impacts on shorter and smaller diameter column applications. The most important performance characteristics are flow precision and compositional accuracy for the pump, sampling precision and carryover for the autosampler, and sensitivity for the detector. Manufacturers and selection criteria for HPLC equipment are reviewed. 

Operational specifications Pump precision of retention time <0.5% RSD Composition accuracy <1% absolute Detector noise, <+2.5 x 10"5 AU Auto sampler precision <0.5% RSD, <0.1 carryover System dwell volume <1 mL Instrumental bandwidth <40 pL (4o)... 

Through repetition of the same experiment and subsequent evaluation of the equilibrium data, the accuracy may be increased by averaging the values, if no systematic errors occur. Too high a deviation between equivalent measurements indicates problems in either the data evaluation or the experiment itself. In the latter case, it should be checked if the pumps deliver a constant flow rate and that the temperature is constant in the range of a few tenths of one centigrade. If the eluent consists of a fluid mixture, the influence of slight changes in eluent composition must be taken into account (Section 6.5.7.1). 

It is somewhat difficult to compare these predictions with experimental results since no really systematic experimental study has yet been published. This is due in part to difficulties in preparing mixtures of H2 and Fa of any desired composition and pressure and also to experimental limitations in the sufficiently rapid initiation of the pumping reaction. However, as far as the experimental information goes, it can be concluded that the efficiency is considerably lower than expected. For instance, in flash photolysis-initiated HF lasers a chemical efficiency of below 1% is usually found 101>. Two suggestions may be made to explain this discrepancy. One may look at it as either a chemical rate problem or a laser problem. In the first case, some unknown rate process must be assumed to reduce the build-up of excited HF. Since the formation and deactivation rates are known with some accuracy, this could only be excessive recombination or an unusually high rate of the reverse reaction 102>. Alternatively, parasitic oscillations or superradiance have been claimed to cause radiation losses in off-axis... 

Figure 5.3. An example of the proportioning accuracy of a reciprocating-piston pump in the composition range 0-10% B in 1% increments (A). The influence of the design of the solvent delivery system on gradient shape (B). The Broken line describes the set gradient and the solid line the gradient delivered to the column inlet. The dwell time is marked as tp.

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