Reference standard check parameters

Satisfactory results for a method can only be obtained with well-performing equipment. Therefore, before an instrument is used to validate a method, its performance should be verified using universal standards (47). Special attention should be paid to the equipment specifications that are critical for the performance of the method. For example, if detection limit is critical for a specific method, the detector specifications for baseline noise and the response to the specified compounds should be checked. Furthermore, any reagent or reference standard used to determine critical validation parameters should be double-checked for accurate composition and purity. 

Silent-hours operation, which is commonly termed hands-off analysis, requires the automatic analysis to operate to a set protocol. For a fully automatic instrument to run in this manner, it will require a feedback system comparing the results with check cahbration standards. A calibration graph can be constructed from the analytical data, and the precision of this graph is easily evaluated. As the analyses proceed, the system can be monitored by reference to the check calibration standards. Should the performance remain within specification, the analyses can safely go on. The automatic instrument can then operate within the set protocols throughout the silent hours, taking full account of any variations in the instrument and its operating parameters. 

Analytical balances must be regularly checked against reference weights provided by the US National Institute of Technology and Standards (the old National Bureau of Standards). Other countries have similar calibration services. Some balances have an internal calibration standard, which periodically resets crucial software parameters that control the front-panel display. 

A calibration check of controllers should be made on a regular basis. The ISO 9000 standard reviews developing the frequency calibration checks. A visual examination should be made before proceeding with the check to determine that no damage exists. Some of the more common problems caused by a plant s hostile environment that can effect equipment such as sensors/transducers are noise interference, mounting holes (must be concentric and clean), installation, diaphragm considerations, and transducer calibration. Zero balance, full-scale sensitivity, and R-cal at 80% parameter reference points for calibration can be used. The sensor/transducer manufacturer provides these parameters.154... 

The dependence of molecular weight results on these parameters means that great care must be taken to obtain accurate initial calibration constants. Furthermore, standard reference materials are required to check system operation at regular intervals. 

Investigators who wish to check their measurement techniques against those of a standardizing laboratory can buy a sample of silicon powder from the U.S. Bureau of Standards. These samples, known as Standard Reference Material 640, were made available in 1974 [11.8]. The Bureau states the weighted average of the lattice parameter of this material to be 5.43088 A, with an estimated standard error of 3.5 X 10 A. 

Standard ISO/IEC 17025 2005 requires a laboratory to have quality control procedures for monitoring the validity of tests and calibrations undertaken. This means that laboratories must perform internal performance-based quality control checks in accordance with Section 5.9 of ISO/IEC 17025 2005 as it applies to every test, technology, and/or parameter within their scope(s) of accreditation in order to demonstrate compliance with accreditation requirements. Reference or fortified material containing known amounts of analyte, at or near the permitted limit or the decision limit (a non-compliant control sample) as well as compliant control materials and reagent blanks should preferably be carried through the entire procedure simultaneously with each batch of test samples analyzed. Ideally, the control samples should also be very similar to test samples and stable over time. The laboratory should maintain a sufficient amount of control material to last for a significant time period (preferably a number of years) and at suitable analyte concentrations. 

Having decided which parameters to plot, we need a mean value and a mean standard uncertainty for each, against which we can compare the check measurements. If the parameter is the activity of a reference source, we may be able to use the reference value or otherwise we must use the mean of measured values. The mean standard uncertainty must be calculated from the spread of measured values. If the parameter is activity, there is only limited merit in using the estimate of counting uncertainty provided by the spectrum analysis program. This will not include external sources of uncertainty thaL if we wish to control the overall measurement process, must be considered. It may be, for example, that if the source must be placed at the centre of the detector cap, different operators have a different perception of where that is. That additional uncertainty is part of the overall measurement uncertainty and should be taken into account. 

In the second step, a reference phase (RP) is selected, which, in the given case, is HP Zorbax SB Cjg (SB 100) with a maximum degree of alkyl derivatization. The correlation of all of the log oq p values with the log tt pp values of the reference column has been checked for all of the substances (see Fig. 6). The correlation of the aU log oq jjp-values with the log oq jjp-values of the reference column was checked for aU substances. When the reference phase is compared to a very similar stationary phase (e.g., HP Zorbax SB Cjg with only 90% alkyl derivatization) the correlation of the data adopts high values (r = 0.9996) as expected. However, when a phase like Inertsil ODS-3 is compared, the standard error of the correlation Sy was more than 7 times higher (r=0.9822). Especially the strongly basic substances (amitriptyline, nortriptyline, diphenhydramine and propranolol) and N,N -dimethyl acetamide scatter markedly from the linear correlation. These first correlation experiments enable the identification of all ideally hydrophobic compounds within the pool of test solutes. The selection criterion is a maximum deviation from the linear fit of 0.01 log a units. This can be fulfilled for 24 solutes from the complete set of 67 test substances. For these 24 compoimds, a pure hydrophobic retention mechanism is assumed. Hence, the relative retention of these compounds can be described by a phase parameter H (interpreted as hydrophobicity) and a related solute parameter, resulting in Eq. (8) for each solute i on a specific column P ... 

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