Instrumental calibration

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

Random Measurement Error Third, the measurements contain significant random errors. These errors may be due to samphng technique, instrument calibrations, and/or analysis methods. The error-probability-distribution functions are masked by fluctuations in the plant and cost of the measurements. Consequently, it is difficult to know whether, during reconciliation, 5 percent, 10 percent, or even 20 percent adjustments are acceptable to close the constraints. 

Instrumentation Calibration may be required for the instruments installed in the field. This is typically the job of an instrument mechanic. Orifice plates should be inspected for physical condition and suitabihty. Where necessary, they should be replaced. Pressure and flow instruments should be zeroed. A prehminary material balance developed as part of the prehminary test will assist in identifying flow meters that provide erroneous measurements and indicating missing flow-measurement points. 

AFM through force or displacement modulation techniques. Numerous methods have evolved that take advantage of the greater sensitivity modulation techniques provide, allowing dissipative processes to be examined. However, evaluation of the probe/sample response requires care with test protocols and instrument calibration, as well as application of appropriate contact mechanics models only a few of these techniques have evolved into quantitative methods. 

Errors in instrumentation calibration have been reduced by 90 percent due to the elimination of multiple, outdated copies of calibration procedures/data sheets. An instrument mechanic can now get the current specification from a single, up-to-date source whenever a work order is received. The time wasted in searching multiple files has been eliminated. 

Requirements for standards used In macro- and microspectrofluorometry differ, depending on whether they are used for Instrument calibration, standardization, or assessment of method accuracy. Specific examples are given of standards for quantum yield, number of quanta, and decay time, and for calibration of Instrument parameters. Including wavelength, spectral responslvlty (determining correction factors for luminescence spectra), stability, and linearity. Differences In requirements for macro- and micro-standards are considered, and specific materials used for each are compared. Pure compounds and matrix-matched standards are listed for standardization and assessment of method accuracy, and existing Standard Reference Materials are discussed. 

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

Overall control is provided by the PDP-11/44, running DEC S RSX-llM operating system. RSX-llM is a multi-user multi-task operating system, and a number of other analytical instruments are interfaced to this computer system and are running concurrently. The automated Instron software is menu-driven because our experience has shown that menu-driven software is particularly effective for applications of this type. To perform either test the user accesses a main menu from which separate menus for instrument calibration, tensile tests, and flexure tests can be reached. The tensile and flexure menus have equivalent options the choices pertaining to automated testing are as follows ... 

Instrument calibration is done during the analysis of samples by interspersing standards among the samples. Following completion of the samples and standards, a linear calibration curve is estimated from the response of the standards using standard linear regression techniques. The calibration constants obtained from each run are used only for the samples quantitated in that run. Drastic changes or lack of linearity may indicate a problem with the detector. 

Stock solutions of approximately 1 mg mL were prepared by dissolving the appropriate amounts of the analytical standards in acetonitrile. Working standard solutions for fortification were prepared in volumetric flasks by appropriate dilutions of the stock solutions for each analyte or combination of analytes. During analysis, SCA is converted to DMS and HMS is derivatized therefore, the analytical standard solutions for quantitation and instrument calibration contained sulfentrazone, DMS and derivatized HMS. A measured volume of a standard solution containing sulfentrazone, DMS and HMS (prepared from stock solutions) was derivatized simultaneously with the samples. 

Material balances are also useful tools for the study of plant operation and trouble shooting. They can be used to check performance against design to extend the often limited data available from the plant instrumentation to check instrument calibrations and to locate sources of material loss. 

The molecular weight distribution (MWD) of the linear polyurethanes were determined by GPC. The solvent used was THF and the instrument calibrated by narrow MWD polystyrenes. Polymer BPUla... 

We test the hardness of polymers by applying an indenter to their surface with a known force and noting the depth to which the tip penetrates the sample. These tests typically fall into one of two categories. In the first, the depth of penetration is read directly from a dial on the instrument, calibrated in arbitrary hardness units. The farther the tip penetrates the sample, the lower is its hardness. The second type of test involves impressing a pyramidal indenter tip against the sample with a known force and measuring the depth to which it penetrates. In practice we measure the dimensions of the indentation and calculate the depth of penetration and compressive modulus based on the tip geometry. 

Multiple analysis, blind sample and round robin tracking and variance reporting Automatic tolerance verification and limit checking Instrument calibration scheduling and tracking... 

In principle, AHvap can be obtained from other sources and (M) can be calculated from Equation (33) without any a priori assumptions. In reality there are always thermal leaks and the instruments are calibrated with a known standard. If AV oc AT is the measured voltage difference and K the instrument calibration constant, (M)n is obtained as [20]... 

Solutions of pure chemicals of known concentration used for instrument calibration are frequently referred to as standard solutions . However, the term... 

Regular calibration and verification ensures that the parameters measured by a particular instrument can be related to a recognized standard. The frequency of instrument calibration may be quite varied, depending largely on the application. If, during the verification of instrument performance, it has been shown that the instrument stays in calibration for about three months, the calibration would be repeated at approximately two-monthly intervals. However, verification (system suitability) will be carried out each time samples are analysed. For some critical analyses, calibration may be performed for each batch of samples or, in an extreme case, for each separate sample. 

Instrument calibration procedures and performance checks are documented and available to users. 

Instrumental resolution, 23 132 Instrumentation. See also Instruments calibration of, 21 161 capillary electrophoresis, 4 633 composition measurement, 11 785 for fermentation, 11 36—40 flow rate, 11 781-783 flow visualization, 11 785-786 fluid mechanics, 11 781-786 food processing, 12 87-88 gas chromatography, 4 611 6 413-414 infrared spectroscopy, 14 225-228 23 137-138... 

As described in Section 13.3, using an instrument for a long period can result in variation in instrument calibration, or drift. This can be detected in solution analysis by the addition of a known amount of an internal standard to the sample solutions. This is impossible, however, for LA-ICP-MS analysis of a solid sample. One method of minimizing this is to simultaneously measure a... 

The instrument of Meyerhoff60) (Fig. 13) is based on the differential refractometer of Bodmann. It measures an image displacement which is proportional to (n - ii0) via an instrumental calibration constant the value of which must be established by means of standard solutions of known (n — %). Values of the latter quantity have been collated for several aqueous salt solutions, but relate mainly to X0 = 436, 546 and 589 nm. Calibration at 0 = 1086 nm and T = 20 °C may be effected with aqueous KC1 for which dn/dc (at c = 0) has a reported65) value of 0.1351 ml/g. This datum and other useful data relative to the increasingly used (laser source) wavelengths are assembled in Table 2. 

The ability to make analytical measurements depends intimately on the availability of well-defined standards and calibrants. Many measurements of analytes in seawater (such as dissolved organic carbon and dissolved organic nitrogen) cannot be compared among laboratories because of the lack of appropriate reference materials and blanks for instrument calibration and testing. Intercomparison exercises are critical (NRC, 1993, p. 75). 

Since certified reference materials for seawater nutrient analysis are currently unavailable, individual laboratories must prepare their own standard solutions for instrument calibration. Standard stock solutions are prepared at high concentrations (mM) so that they can be used for months without significant alterations in concentration. Working low-concentration standard solutions are unstable and need to be prepared daily by diluting stock solutions with distilled water or low-nutrient seawater. In this case, the accuracy of nutrient analysis at a given laboratory is highly dependent upon the accuracy of the daily preparation of the calibration solutions. 

Denise LeBlanc is the Group Leader for the Marine Sciences Group and the Manager of the Certified Reference Materials Program at the Institute for Marine Biosciences (IMB) of the National Research Council of Canada. The Certified Reference Materials Program manufactures instrument calibration standards and certified reference materials for shellfish toxins, PCBs, PACs, and trace elements in marine sediments, in biological tissues, and in seawater. Her experience resides in the manufacture and long-term production of reference materials. 

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