Compositional accuracy

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 Test all solvent lines at 2 mL/min with 0.1% acetone/water, step gradients at 0%, 10%, 50%, 90%, and 100%. Measure peak heights ofrespective step relative to 100% step. 1% absolute... 

Waters Symmetry CIS (150x3.9 mm i.d., 5 pm) used in most tests except wavelength accuracy and compositional accuracy tests. 

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. 

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. 

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. 

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. 

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. 

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. 

Compositional accuracy is the ability to generate the same solvent composition from run to run. 

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... 

The accuracy of the compositional information is not affected by the polymer architecture. Deviations from the true comonomer ratios are only possible if the detected property is dependent on the local environment. This is the case if neighbor-group effects exist. The possibility of electronic interactions causing such deviations is very small, because there are too many chemical bonds between two different monomer units. Other types of interactions, especially those which proceed across space (e.g., charge-transfer interactions), may influence composition accuracy [4]. 

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)... 

Case study 4 shows several examples of problems caused by equipment malfunctions and their subsequent diagnosis and solution. The first one involved a situation of poor retention time reproducibility of a gradient assay. It involved the analysis of a complex natural product, using a narrowbore column (2-mm i.d.) at 0.5 mL/min. System suitability test showed retention times to be erratic and could vary by 1-2 minutes without any obvious trends. Flow rate accuracy was found to be acceptable, however, the compositional accuracy test failed (see Chapter 9 on HPLC calibration). The tentative diagnosis was that of a malfunctioning of the proportioning valve. After its replacement, the retention time precision performance was re-established. 

In September of 1983, a request was submitted by the Laboratoiy and Chemical Services Committee of the American Gas Association, which outlined the need for a cahbration gas for natural gas analysis by chromatography which was certified by a recognized ceitilying agency. This request was undertaken by the Gas Research Instimte. The result is a program that will allow cahbration mixture suppliers to prepare a gas that will have a certified analysis traceable to a mixture certified by the National Bureau of Standards. This program with the NBS will create a common source to which calibration mixtures can be traced. This common source of traceability will create a reference point that can be used to resolve aity dispute between parties on questions of compositional accuracy. 

The radial composite accuracy system comprises nine accuracy grades of which grade 4 is the highest and grade 12 is the lowest. 

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