Reproducibility in measurement

Table 5 compares the reproducibility in measuring line position for eight cyclohexane lines. The pixel position and standard deviation for five replicate measurements are shown for each line. Five sets of measurements are shown, with the spectrum shifted by a fraction of a pixel between datasets. For the sharp, intense 802-cm line, the reproducibility in measuring peak position is comparable to that for atomic lines. For a broad, weak line like the 1347-cm line, the reproducibility is, as one would expect, considerably poorer. Estimates of the S/N ratio for each line are provided in Table 5. The noise in a spectrum was taken as the standard deviation of the baseline in a 350-pixel zone. The peak height was used as the signal for this estimate.

Table 5 Signal-to-Noise Ratio and Reproducibility in Measuring Positions of Cyclohexane Lines... 

Lack of reproducibility in measurement is especially noticeable for the label attached to the terminal carbon position, which might be explained by the difficulty of accommodating such spin-labelled lipids. However, it was found that good reproducibility can be attained for DPPC samples, when they were annealed at 5 °C for 12 h after preparation. ... 

Hot tack strength is the abiUty of a heat-seal layer to hold together while molten, before the seal cools and sets up. This is a technically important property which is difficult to measure reproducibly in the laboratory. Owing to the reinforcing effect of ionic bonding on melt strength, ionomer sealing layers provide superior performance in a wide spectmm of appHcations. 

The use of peroxyacids, including PMSA, makes it possible to improve photometric method of nickel determination - to increase selectivity, accuracy and reproducibility of measurements. Peroxyacids as oxidants ai e used for nickel determination in aluminium and copper alloys, natural waters, stomatological products. 

The precise impact experiment offers both versatility and the highest of precision. In typical experiments, the projectile is faced with an impactor that is either the same material as the sample to be impacted or a standard material whose properties are known and reproducible. In a symmetric impact configuration the impactor and sample are the same upon impact, precisely one-half of the impact velocity is imparted to the sample. As the impact velocity can be readily measured to an accuracy of 0.1%, this condition provides the most accurately known and best characterized condition achieved in shock-compression science. 

It is often assumed that the measurements taken with a calibrated device are accurate, and indeed they are if we take account of the variation that is present in every measuring system and bring the system under statistical control. Variation in measurement systems arises due to bias, repeatability, reproducibility, stability, and linearity. 

The peak width increases with injection volume. Therefore this parameter has to be fixed for comparative measurements. It has become the custom to inject low molecular weight test samples in very small volumes at very high concentrations, occasionally even as pure compounds. This extreme is not recommended as it is more important to inject a constant sample amount, reproducibly, in a precisely kept volume. Typical GPC injections are between 50 and 200 /a1. It is better to inject a larger volume of a lower concentration polymer solution. GPC units are often not designed for injection volumes lower... 

The resisitivites p corresponding to the DC conductivities results reproduce the measured strong dependence of p on the composition correctly. The CP results agree with the experiments very well. The only exception is the composition with 20% sodium where the relative deviation is about 100% however, the absolute deviation is small. 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

This is a method which is very attractive in principle and which has been applied to yield approximate barriers for a number of molecules. There are, however, difficulties in its use. In the first place, it is not easy to measure the intensities of microwave lines with accuracy. There are unsolved problems of saturation, reflections in the wave guide, and variation of detector efficiency with frequency which are presumably reponsible for the fact that measurements made with ordinary wave guide spectrometers are not very reproducible. In addition, both the spectral lines may be split into components by tunnelling from one potential minimum to another and this splitting, even though it is not resolved, can alter the apparent intensity. Furthermore, it is often difficult to find pairs of lines such that neither is obscured by Stark lobes from the other. 

In the Trauzl-block test the expl is loaded into a Pb block at a loading d near 1.0g/cc and stemmed or sealed in with a screw cap. Theoretically these measurements should correlate well with practical conditions because initial and final states in the measurement and in practice should be similar. Unfortunately the Trauzl method is not very reproducible (in part because of rupture of portions of the block by reflected shocks). Consequently no further discussion of the Trauzl-block measurements will be made in this article. Interested readers are referred to Vol 2, B266—95 and Ref 5 for Trauzl block data for many expls... 

Atmospheric emissions of sulphur dioxide are either measured or estimated at their source and are thus calculated on a provincial or state basis for both Canada and the United States (Figure 2). While much research and debate continues, computer-based simulation models can use this emission information to provide reasonable estimates of how sulphur dioxide and sulphate (the final oxidized form of sulphur dioxide) are transported, transformed, and deposited via atmospheric air masses to selected regions. Such "source-receptor" models are of varying complexity but all are evaluated on their ability to reproduce the measured pattern of sulphate deposition over a network of acid rain monitoring stations across United States and Canada. In a joint effort of the U.S. Environmental Protection Agency and the Canadian Atmospheric Environment Service, eleven linear-chemistry atmospheric models of sulphur deposition were evaluated using data from 1980. It was found that on an annual basis, all but three models were able to simulate the observed deposition patterns within the uncertainty limits of the observations (22). 

Prinn et al (136) have compiled a record of CH3CCI3 measurements taken throughout the world since 1978. This record was used in conjunction with a two-dimensional transport model to calculate longitudinally averaged [HO ]. These results are reproduced in Table II. These values can be compared with a global annual average of 6.5 (+3,-2) x 10 cm by Volz et al (137) derived from CO. 

Example 2. Equilibrium constants of the reaction of twenty substituted dinitromethanes with formaldehyde have been measured (57) in the range 10-50°C. The isokinetic relationship is valid for only nine of them, as revealed in a preliminary graphical treatment using the plot of log Kjo versus log Kio( 163) the pertinent values of logK are reproduced in Table I. The values of x = T" were transformed according to eq. (36a) with... 

The diad fractions for the low conversion experiments only are reproduced in Table II. The high conversion data cannot be used since the Mayo-Lewis model does not apply. Again diad fractions have been standardized such that only two independent measurements are available. When the error structure is unknown, as in this case, Duever and Reilly (in preparation) show how the parameter distribution can be evaluated. Several attempts were made to use this solution. However with only five data points there is insufficient information present to allow this approach to be used. 

Ideally one would wish to remove the need for statistics by directly and reproduce-ably measuring a single bond only. One problem with the measurement of specific individual bond energies is that it is extremely difficult, even with a tip of small radius, to isolate a single bond species between the tip and the sample. To form a single bond in a controlled way requires the cantilever to be stiffer than the maximum force gradient experienced during the approach, but stiffer levers exhibit less sensitivity. If multiple bonds are formed, then it can be difficult to make an independent calculation of the contact area and hence the number of bonds involved. 

Precision. The choice of 400 mg of soil is arbitrary. It was chosen in order to keep the entire cleanup step within a 2 cc vial. A test of replication was done on one field sample contaminated with about 20,000 ppm of 1254. Three samples of 50, 54, and 54 mg were weighed into separate vials, extracted, and then diluted 1 1000 into hexane in a separate vial. The three chromatograms are shown in Figure 7. Two peaks were quantified to demonstrate how reproducible a measurement can be, even in a field sample. 

BCR Analytical Approach for the Certification of PAHs in Natural Matrix CRMs Prior to the certification analyses for the CRM, each participating laboratory has to prepare standard solutions of the analytes to be determined from certified reference compounds (purity >99.0 %) to calibrate their instruments for response and response linearity (multiple point calibration), detection limit, and reproducibility. In the case of PAH measurements, reference compounds of certified purity are used as internal standards, which are not present at a detectable concentration in the matrix to be analyzed (e.g. indeno[i,2,3-cd]fluoranthene (CRM 267), 5-methylchrysene (CRM 081R), benzo[f ]chry-sene (CRM 046), picene (CRM 168), and/or phenanthrene-dio). 

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