The Instrumental Precision

As already outlined, the maximum measurable total absorption value strongly depends on the instrumentation, that is to say on the model of the spectrophotometer in use. Generally for the modem common spectrophotometers in research 

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With the exception of the clathrate framework model, all these hypotheses appear to be qualitatively consistent with the available X-ray diffraction data on liquid water. It is argued by some investigators, however, that there are still significant inconsistencies between the most sophisticated statistical thermodynamic models for liquid water and the most sophisticated X-ray and neutron diffraction measurements [734-736]. The interpretation of these data from different experiments, using the concept of pair-correlation functions, shows discrepancies that are considered significant in terms of the instrumental precision, and the definitive answer seems not yet available [737J. 

To determine the instrument precision the same sample was measured six times in succession. The percentage standard deviation was determined by manual integration with use of the software WIN-NMR to be 0.79% for PC for SPH, which is near the limits of quantitation, the percentage standard deviation was found to be 3.1%. 

An example of precision criteria for a gas chromatographic method would be that the instrument precision (RSD) should be 1%, and the repeatability of the method would be 2%. 

During testing a depth resolution of 50-80 micron and a lateral resolution of 20-40 micron was achieved. The spatial resolution was limited not mainly hy source or camera properties, but by the accuracy of compensation of the instrumental errors in the object movements and misalignments. According to this results a mote precision object rotation system and mote stable specimen holding can do further improvements in the space resolution of microlaminography. 

Precision The precision of a gas chromatographic analysis includes contributions from sampling, sample preparation, and the instrument. The relative standard deviation due to the gas chromatographic portion of the analysis is typically 1-5%, although it can be significantly higher. The principal limitations to precision are detector noise and the reproducibility of injection volumes. In quantitative work, the use of an internal standard compensates for any variability in injection volumes. 

The performance of a measuring instrument can be expressed in several ways. The precision or accuracy describes the instrument performance in a general and qualitative sense. Thus, these expressions cannot be characterized using numbers. 

The precision of a thermal anemometer is dependent on the instrument quality and the conditions of use. A general rule is, the lower the measured ve locity, the higher the inaccuracy and vice versa. When measuring very low indoor velocities, around 0.1 m s, the relative error can be as high as 100% and not much lower than 30%. Low velocities are extremely difficult to measure with accuracy. 

The measurement range of a vane anemometer is typically between 0.3 and 30 m s E It may start rotating with slightly lower velocities, but due to the characteristic curve having a small nonlinear part in the low-speed end, the useful range is narrower. The actual precision depends on the quality of the instrument however, the inaccuracy may vary between 1% and 5% of the scale. The larger the vane, the higher the accuracy. 

For solutions which do not follow Beer s Law, it is best to prepare a calibration curve using a series of standards of known concentration. Instrumental readings are plotted as ordinates against concentrations in, say, mg per lOOmL or lOOOmL as abscissae. For the most precise work each calibration curve should cover the dilution range likely to be met with in the actual comparison. 

The method, obviously, is subjective, the precision and speed of the match depending upon the observer and his experience. Results on foods have usually been expressed in terms of color disks, which are different for each product and which must be carefully standardized. [Conversions to standard colorimetric systems of notation can be made (12), provided suitable colorimetric data are available for the disks used.] Furthermore, instruments suitable for the most precise work by this method are not at the present time commercially available. 

For work of the highest precision, it is highly advisable to carry through an analysis of variance together with suitable tests of significance, not only to establish what the precision is, but also to uncover individual sources of error so that they can be made less serious. How this is done for instrumental and manipulative errors has been demonstrated in this chapter. 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

The instruments generally yield very precise measurements (E, CV ... 

Sodium and Potassium. For the electrolytes, sodium and potassium the flame pho meter is the instrument of choice (29). This instrument permits readily the dilution of the serum 200 fold, for analysis, using an internal lithium standard. Most instruments require 1 ml for analysis. It is therefore practicable to measure out 3pi and dilute it to 1 ml. This is best done with a sampler-diluter of high precision. The tip of the diluter needs to be a drawn out polyethylene tip, or the 5 pi will not be measured with any degree of accuracy. 

Seligson s group (95) has published a similar turbidimetric procedure but used nephelometry to measure continuously the effect of lipase on the light scattering of an olive oil emulsion. The instrumentation and approach is the same as that described above for the nephelometric determination of amylase. The method according to the authors is fast and precise with good specificity and sensitivity. The short time required for analysis makes it suitable for emergency use. The technical simplicity permits this method to be easily automated, and it appears to be the lipase method of choice. 

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