Precision Measurement

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

The average value of the rephcates is reported along with the standard deviation, which reflects the variabihty in the measurement. Large standard deviations relative to the average measurement indicate the need for an action plan to improve measurement precision. This can be accomphshed through more rephcate measurements or the elimination of the source of variation, such as the imprecision of an instmment or poor temperature control during the measurement. 

Capillary viscometers are useful for measuring precise viscosities of a large number of fluids, ranging from dilute polymer solutions to polymer melts. Shear rates vary widely and depend on the instmments and the Hquid being studied. The shear rate at the capillary wall for a Newtonian fluid may be calculated from equation 18, where Q is the volumetric flow rate and r the radius of the capillary the shear stress at the wall is = r Ap/2L. 

Although the dew-point method may be considered a fundamental technique for determining humidity several uncertainties occur in its use. It is not always possible to measure precisely the temperature of the polished surface or to eliminate gradients across the surface. It is also difficult to detect the appearance or disappearance of fog the usual practice is to take the dew point as the average of the temperatures when fog first appears on cooling and disappears on heating. 

Equation (3) merely sums the two peaks to produce a single envelope. Providing retention times can be measured precisely, the data can be used to determine the composition of a mixture of two substances that, although having finite retention differences, are eluted as a single peak. This can be achieved, providing the standard deviation of the measured retention time is small compared with the difference in retention times of the two solutes. Now, there is a direct relationship between retention volume measured in plate volumes and the equivalent times, which is depicted in Figure 6. 

These levels are ditfieuir or even imjxjssible fo achieve in certain cases wirh the equip-menr normally available. When they cannot be achieved, indicate the actual measuring precision... 

Particle Collection Particles in the extracted partial volume flow are retained in the collector filter. Tlie particle mass emitted is determined by the weight difference of the filter before and after the collection. Factors crucial to the measuring precision and the smallest measuring range of particle concentration are ... 

Each protective system has its own pattern of capital and running costs. Choosing the optimum alternative requires care because some of the factors can be measured precisely while others cannot, and the costs arise at different points in time. 

To measure the conductivity of a solution it is placed in a cell carrying a pair of platinum electrodes which are firmly fixed in position. It is usually very difficult to measure precisely the area of the electrodes and their distance apart, and so if accurate conductivity values are to be determined, the cell constant must be evaluated by calibration with a solution of accurately known conductivity,... 

The end of the 1980 s saw the application of TIMS to U-series measurement (Chen et al. 1986 Edwards et al. 1987 Goldstein et al. 1989). This represented a major technological advance. Analysis time was reduced from one week to several hours, sample sizes for many carbonate or volcanic rock samples decreased from -10-100 pg to 0.1-1 ug U or Th, measurement precision improved from percent to permil levels, and for the decay series, the dating range was extended from 350,000 years to... 

Closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement (Precision under repeatability conditions). Repeatability may be expressed quantitatively in terms of suitable dispersion characteristics. 

A simple statistical test for the presence of systematic errors can be computed using data collected as in the experimental design shown in Figure 34-2. (This method is demonstrated in the Measuring Precision without Duplicates sections of the MathCad Worksheets Collabor GM and Collabor TV found in Chapter 39.) The results of this test are shown in Tables 34-9 and 34-10. A systematic error is indicated by the test using... 

Measuring Precision without Duplicates (Youden/Steiner) ... 

Flame AAS can be used to measure about 70 elements, with detection limits (in solution) ranging from several ppm down to a few ppb (and these can be enhanced for some elements by using a flameless source). Both sensitivity and detection limits (as defined fully in Section 13.4) are a function of flame temperature and alignment, etc. The precision of measurements (precision meaning reproducibility between repeat measurements) is of the order of 1-2% for flame AA, although it can be reduced to <0.5% with care. The accuracy is a complicated function of flame condition, calibration procedure, matching of standards to sample, etc. 

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