Calibration standards frequently used

Most frequently, SEC with dextran-, pullulan-, or polystyrene calibration standards has been used to characterize the molecular properties of xylans. However, as for viscometric studies [108], a sufficient solvent ionic strength is a prerequisite for useful SEC measurements of charged polysaccharides, including glucuronoxylans [111-113]. An advantage of the SEC technique is that the presence of protein and phenolic components or oxidative changes can be detected by simultaneous ultraviolet (UV) detection. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

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

One of the major problems lies in the extent of interference from other constituents of a sample. This can often be obviated by a prior separation using chromatography or solvent extraction (p. 55) or by the use of masking agents (p. 40), pH control or changes in oxidation state. Standards should always be matched to the gross composition of the sample as closely as possible, and calibration curves frequently checked. The precision of absorption measurements has already been discussed (p. 361). 

The shock sensitivity of materials are also frequently evaluated in a drop weight test. A variety of drop weight testers have been developed, but the standard device used to evaluate the sensitivity of liquid propellants is test No. 4 of the Joint Army-Navy-Air Force Panel on Liquid Propellants (2). A sample of material (usually less than 0.1 gram) is confined in an enclosed volume, and a calibrated weight is allowed to... 

Development of Dosimeter Materials. - 4.2.1 Quantitative ESR and Intensity Standards of Mn1+ and CuS04.5H20. An ESR spectrometer consists of vacuum tube amplifier and a Klystron oscillator with thermal noise and frequency drift. Hence, an inherent standard of Mn2+ was used in ESR dating of carbonate stalactites,8 and patented as a standard in ESR radiation dosimetry in 1980.102 The standard sample of MgO with Mn2+ is frequently used for calibration of -factor and the magnetic field as well as for radiation dosimetry.103... 

Internal standard calibration is used when the changes in the analytical system are known to be frequent and substantial. To compensate for these changes, internal standards at known concentrations are added to all calibration standards, field samples, and laboratory QC samples prior to analysis. Internal standards are synthetic analogs of specific target compounds or compounds that are similar in nature to the target analytes and that are not found in environmental samples. Internal standard calibration is a requirement of GC/MS methods. Laboratories sometimes use it for GC methods as it significantly improves the accuracy of compound quantitation. 

Frequently, absolute accuracy cannot be established owing to the lack of a suitable calibration standard. For this reason, precision or repeatability can be more important than accuracy Because the measurement is being made continuously or repeatedly, we are more interested in changes in the reading than its absolute value. Precision is defined as the ability of an analyzer to produce the same output each time the same quantity of the component or property is being measured. The terms stability, reliability, and reproducibility are sometimes used synonymously with repeatability. However, the term reliability is also used to describe the instrument s "up time."... 

Gel filtration is frequently used to determine the molar masses of native proteins (Ch. 4.1.2.2). To obtain accurate results, it is important that there should be no nonspecific interactions between the protein and the gel matrix, and this can usually be assured by the right choice of buffers. The method is not very accurate, and it needs calibration with protein standards of known molar masses and of similar shape. If the protein has a characteristic activity that can be measured, e.g., an enzyme activity, then its molar mass can be determined using crude protein mixtures by assaying the eluate from a gel filtration column for activity. 

Frequent calibration of the instruments with the use of appropriate standards is required to make suitable allowances for decreases in the efficiency of the instruments. Such calibration standards are available from the National Institute of Standards and Technology (NIST). Other sources traceable to NIST standards through active program of participation in comparison measurements also provide such standards. The United States Pharmacopeia (USP) also provides nuclear decay data for new calibration standard. USP 24 lists t j2, energy of photons, and number of photons per disintegrations, for the following radionuclide standards ... 

Sample preparation is much the same as for other GD couplings to instruments. Metal or alloy discs, and compacted conducting samples, are the most frequently used. Solution samples have been examined by deposition onto graphite, aluminium and copper cathodes. Quantitative analyses are usually performed by using a calibration graph run from standard reference materials, a process that takes considerably more time than preparing a solution-based curve. Conversely, time is saved in the direct analysis of solids also, the inert atmosphere of a GD cell reduces the spectral interferences frequently encountered in flame atomizers. 

For soft ionization methods, the ealibration method is less standardized. A wide variety of reference componnds have been proposed for nse in electrospray LC-MS. Proper cahbration of the m/z axis is especially important in the analysis of proteins, becanse the error in the mass measnrement of a mnltiple-charge unknown is magnified by the number of charges at the unknown ion. Frequently used reference compounds for m/z calibration in electrospray MS are ... 

Measurements invariably involve errors and uncertainties. Only a few of these are due to mistakes on the part of the experimenter. More commonly, errors are caused by faulty calibrations or standardizations or random variations and uncertainties in results. Frequent calibrations, standardizations, and analyses of known samples can sometimes be used to lessen all but the random errors and uncertainties. In the limit, however, measurement errors are an inherent part of the quantized world in which we live. Because of this, it is impossible to peiform a chemical analysis that is totally free of errors or uncertainties. We can only hope to minimize errors and estimate their size with acceptable accuracy. In this and the next two chapters, we explore the nature of experimental errors and their effects on the results of chemical analyses. 

Reaction calorimeters are frequently calibrated using a known heat of a chemical reaction. No standard reaction is internationally accepted. For the measurement of heat capacities, drop calorimeters are frequently used and the calibration is made using a substance, the temperature dependence of which on heat capacity is known. As substances, metals like Cu, Ag, Au, and aluminum oxide in the form of sapphire are used. Calorimeters... 

When determining the concentration of a component in a mixture, an internal standard is a known concentration of a different substance that is added to the experimental sample. An external standard is a known concentration of the substance of interest. External standards are more commonly used. They are not added to the experimental sample they are analyzed separately. These standards are frequently used to evaluate potential bias in the results, which may be caused by poor instrument calibration, interference from other compounds, or loss of the substance during sample pre-processing. 

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