Known samples

Preferably the transferring lab provides a sample which has already been analyzed, with the certainty of the results being known (41). This can be either a reference sample or a sample spiked to simulate the analyte. An alternative approach is to compare the test results with those made using a technique of known accuracy. Measurements of the sample are made at the extremes of the method as well as the midpoint. The cause of any observed bias, the statistical difference between the known sample value and the measured value, should be determined and eliminated (42). When properly transferred, the method allows for statistical comparison of the results between the labs to confirm the success of the transfer. 

The moisture content of cmde sulfur is determined by the differential weight of a known sample before and after drying at about 110°C. Acid content is determined by volumetric titration with a standard base. Nonvolatile impurities or ash are determined by burning the sulfur from a known sample and igniting the residue to remove the residual carbon and other volatiles. 

Should the additional component compositions be required to fully understand the unit operation, the laboratory may have to develop new analysis procedures. These must be tested and practiced to establish reliabihty and minimize bias. Analysts must sribmit known samples to verify the accuracv. 

Known samples should also be run to verify the accuracy and precision of the routine methods to be used during the unit test. Poor quality will manifest itself as poor precision, measurements inconsistent with plant experience or laboratory history, and disagreement among methods. Plotting of laboratory analysis trends wiU help to determine whether calibrations are drifting with time or changing significantly. Repeated laboratory analyses will establish the confidence that can be placed in the results. 

If the random errors are higher than can be tolerated to meet the goals of the test, the errors can be compensated for with rephcate measurements and a commensurate increase in the laboratory resources. Measurement bias can be identified through submission and analysis of known samples. Establishing and justifying the precision and accuracy reqrtired by the laboratory is a necessary part of estabhshing confidence. 

In a molded polymer blend, the surface morphology results from variations in composition between the surface and the bulk. Static SIMS was used to semiquan-titatively provide information on the surface chemistry on a polycarbonate (PC)/polybutylene terephthalate (PBT) blend. Samples of pure PC, pure PBT, and PC/PBT blends of known composition were prepared and analyzed using static SIMS. Fn ment peaks characteristic of the PC and PBT materials were identified. By measuring the SIMS intensities of these characteristic peaks from the PC/PBT blends, a typical working curve between secondary ion intensity and polymer blend composition was determined. A static SIMS analysis of the extruded surface of a blended polymer was performed. The peak intensities could then be compared with the known samples in the working curve to provide information about the relative amounts of PC and PBT on the actual surface. 

Resistivity. Control of the resistivity of the mud and mud filtrate while drilling may be desirable to permit better evaluation of formation characteristics from electric logs. The determination of resistivity is essentially the measurement of the resistance to electrical current flow through a known sample configuration. Measured resistance is converted to resistivity by use of a cell constant. The cell constant is fixed by the configuration of the sample in the cell and is determined by calibration with standard solutions of known resistivity. The resistivity is expressed in ohm-meters. 

A data set containing measurements on a set of known samples and used to develop a calibration is called a training set. The known samples are sometimes called the calibration samples. A training set consists of an absorbance matrix containing spectra that are measured as carefully as possible and a concentration matrix containing concentration values determined by a reliable, independent referee method. 

Sherman compares calculated and observed intensities for a number of known samples in some of which the enhancement components predominate over the intensities by direct excitation. The agreement obtained is usually within a few per cent, and this would be satisfactory even for considerably simpler problems. To be sure, the calculations do not give concentrations from measured intensities. But the fact that intensities can be satisfactorily calculated from known concentrations means that absorption and enhancement effects are thoroughly understood, and that x-ray emission spectrography is on a firm foundation. 

The working curves in Figure 7-8 were obtained on known samples,... 

Validation of extraction procedures is frequently lacking. A good assessment of quality assurance implies that the extraction recoveries are verified, e.g. by spiking of standard addition. A major drawback is that the spike is not always bound the same way as the compounds of interest. For the development of good extraction methods, materials with an incurred analyte (i.e. bound to the matrix in the same way as the unknown), which is preferably labelled (radioactive labelling would allow verification of the recovery), would be necessary. Such materials not being available, the extraction method used should be validated by other independent methods, e.g. by verification against known samples and by use of a recovery SPC chart. A mere comparison of extraction methods is no validation. 

Using the temperature and electron density as fitting parameters, within a range established from measurements from known samples, the ratio nx /nx may be obtained and the concentration of element x calculated using... 

Where XA is the concentration of element A and dI /dXA is the absolute sensitivity factor of element A in matrix M. A calibration curve is established using known samples for each element in each matrix. Unknown concentrations may then be determined provided all instrumental factors and experimental conditions are held constant, which conditions may be difficult to fulfil. 

The flasks used had narrow mouths, whereas the Laug technique specifies widemouthed beaker flasks. The small opening reduced air exchange and moisture condensed on the insides of the flasks. This condition is not good for flies and probably accounts for the mortality observed in the checks. However, this effect had not shown up 5 hours after the tests were started, and by that time most of the flies in the flasks containing insecticides were down. These tests are qualitative rather than quantitative, but the data indicate that the chemically determined quantities of the various insecticides in the fat extract were nearly equal in toxicity to known samples at the same level. 

Photochemistry of Model Compounds. Preliminary photochemical studies have been carried out on l,3-diphenoxy-2-propanol (3)8 as a model compound for bisphenol A-epichloro-hydrin condensates 1. The utilization of 3 as a model compound for thermal degradation of 1 has been reported. Irradiation (254 nm) of 3 in acetonitrile (N2 purge) provides two major volatile products, which have been identified as phenol and phenoxyacetone (4), by comparison of retention times (gas chromatography) with known samples. A possible mechanism for... 

The first one we mention is the question of the validity of a test set. We all know and agree (at least, we hope that we all do) that the best way to test a calibration model, whether it is a quantitative or a qualitative model, is to have some samples in reserve, that are not included among the ones on which the calibration calculations are based, and use those samples as validation samples (sometimes called test samples or prediction samples or known samples). The question is, how can we define a proper validation set Alternatively, what criteria can we use to ascertain whether a given set of samples constitutes an adequate set for testing the calibration model at hand ... 

Orientation Determination. While polarized edge studies, together with a known sample orientation, can provide information about the electronic structure of the absorber, one can also use polarized edges to probe ordered systems of unknown orientation. This sort of approach was used in a study of B adsorbed on graphite (26,27). In this case, the orientational dependence of an edge transition was used to calculate the degree of orientational purity of the graphite surface. 

Was the method tested with known samples to show that it was suitable in... 

The gaseous atmosphere was then vented through a trap at -78° (to remove most of the benzene vapor) into an evacuated vessel. Samples were removed by gas-tight syringe and injected into a Hewlett-Packard 5790 gas chromatograph, equipped with a U ft, 1/8 in Porapak P column and a flame ionization detector. Use of known samples of hydrocarbons (methane and ethane) established that the minimum detectable amounts of product by this procedure were about 0.5-1 0 % (based on starting Nb complex). Several of the reactions (Mo(CO)g, W(C0)g and Ru (CO) p) gave small amounts (around 1-2 %) of these alkanes only with Cr(C0)g was a substantial yield of hydrocarbon product consistently observed (see below). 

As an alternative to modifying the X spacing dynamically as the simulation progresses, we can attempt to define a more elaborate X dependence for the force field that takes into account known sampling issues for the system we are considering. X dependence can be introduced to the potential function in many different reasonable ways. The most common is to linearly scale the parameters that define the potential function with X. So, for example, force constants, equilibrium internal coordinates and non-bonded parameters are defined as60... 

MLR is based on classical least squares regression. Since known samples of things like wheat cannot be prepared, some changes, demanded by statistics, must be made. In a Beer s law plot, common in calibration of UV and other solution-based tests, the equation for a straight line... 

If it is established that a measuring device provides a value for a known sample that is in agreement with the known value to within established limits of precision, that device is said to be calibrated. Thus, calibration refers to a procedure that checks the device to confirm that it provides the known value. An example is an analytical balance, as discussed above. Sometimes the device can be electronically adjusted to give the known value, such as in the case of a pH meter that is calibrated with solutions of known pH. However, calibration can also refer to the procedure by which the measurement value obtained on a device for a known sample becomes known. An example of this is a spectrophotometer, in which the absorbance values for known concentrations of solutions become known. We will encounter all of these calibration types in our studies. 

Retention distance (or time) is normally used to aid the identification of a component of a mixture, provided that a known sample of the component has been subjected to separation under identical conditions. Because of the variations that can occur in the retention time due to technical factors, e.g. fluctuations in flow rate, condition of the column, the relative retention or selectivity factor (a) is sometimes used. This expresses the test retention time as a ratio of the retention time of another component or reference compound when both are injected as a mixture ... 

The accuracy of an analysis can be determined by several procedures. One common method is to analyze a known sample, such as a standard solution or a quality control check standard solution that may be available commercially, or a laboratory-prepared standard solution made from a neat compound, and to compare the test results with the true values (values expected theoretically). Such samples must be subjected to all analytical steps, including sample extraction, digestion, or concentration, similar to regular samples. Alternatively, accuracy may be estimated from the recovery of a known standard solution spiked or added into the sample in which a known amount of the same substance that is to be tested is added to an aliquot of the sample, usually as a solution, prior to the analysis. The concentration of the analyte in the spiked solution of the sample is then measured. The percent spike recovery is then calculated. A correction for the bias in the analytical procedure can then be made, based on the percent spike recovery. However, in most routine analysis such bias correction is not required. Percent spike recovery may then be calculated as follows ... 

Suitable quality control can be to analyse a known sample in each batch or after every five unknown samples. 

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