Instrumentation discrepancies

In addition to fulfilling the in-house requirements for quality control, state and local air monitoring networks which are collecting data for compliance purposes are required to have an external performance audit on an annual basis. Under this program, an independent organization supplies externally calibrated sources of air pollutant gases to be measured by the instrumentation undergoing audit. An audit report summarizes the performance of the instruments. If necessary, further action must be taken to eliminate any major discrepancies between the internal and external calibration results. 

The instrument has been evaluated by Luster, Whitman, and Fauth (Ref 20). They selected atomized Al, AP and NGu as materials for study that would be representative of proplnt ingredients. They found that only 2000 particles could be counted in 2 hours, a time arbitrarily chosen as feasible for control work. This number is not considered sufficient, as 18,000 particles are required for a 95% confidence level. Statistical analysis of results obtained for AP was impossible because of discrepancies In the data resulting from crystal growth and particle agglomeration. The sample of NGu could not be handled by the instrument because it consisted of a mixt of needles and chunky particles. They concluded that for dimensionally stable materials such as Al or carborundum, excellent agreement was found with other methods such as the Micromerograph or visual microscopic count. But because of the properties peculiar to AP and NGu, the Flying Spot Particle Resolver was not believed suitable for process control of these materials... 

Nowadays one very often fits data by means of a least-squares computer program. (Indeed, in Chapter 2 some data-fitting applications will be considered.) It is important to examine the residuals from the fits, to confirm that, with time, they lie randomly about the zero line. Examining them makes it is much easier to spot discrepancies. If the kinetic data are acquired with a computer-controlled instrument, then the data are already contained in a file that can be read by the fitting program. 

Commercial instruments have a reasonable balance between the recommended column size and the volume of the column and connecting tubing (XY). However, the theoretical plate number of a single column may give different values on different instruments, and even on replacement of the components and parts of a single instrument. Such discrepancies can be understood in terms of differences in the mechanics of the instruments and the design of their parts. 

Intermediate precision is defined as the long-term variability of the measurement process and is determined by comparing the results obtained when a method is run within a single laboratory over a number of days. Intermediate precision may reflect discrepancies in the results obtained by different operators, from different instruments, with different sources of reagents, with multiple lots of... 

This discrepancy is rather disturbing because the creep recovery experiments were obviously designed and tested with considerable care (186). Furthermore, as pointed out earlier, creep recovery is the most direct method for measuring J°. Most of the likely errors (such as inertial effects in the instrument or nonattainment of steady state) would tend to give values which are too small rather than too large. Similar but smaller effects have been observed by other methods in solutions of polyisoprene (167) and poly (a-methyl styrene) (187). In the latter... 

In the treatment of a rigid dumbbell, where the whole time-correlation functions (TCF) can be solved exactly, Stockmayer and Burchard21 disclosed the origin for the discrepancy between theory and experiments. They recognized that all measurements of the TCF can be carried out down only to a limiting minimum delay time. With common instruments, this lower limit lies at about 100 ns but the lowest time is often much higher under conditions such that the TCF should have decayed to e"2 at channel 8Q220). These experimental condition imply that only an apparent first cumulant is determined defined by... 

Having collected optimal quality data, first-rate data management is also critical. Many data that are collected can now be fed directly from the measuring instrument to computer databases, thereby avoiding the potential of human data entry error. However, this is not universally true. Therefore, careful strategies have been developed to scrutinize data as they are entered and once they are in the database. The double-entry method requires that each data set be entered twice (usually by two operators) and these entries compared by a computer for any discrepancies. This method operates on the model that two identical errors are probabilistically very unlikely, and that every time the two entries match the data are correct. In contrast, dissimilar entries are identified, the source (original) data located, and the correct data point entry confirmed. 

Overlap of proton absorptions is common, but absolute coincidence of nonequivalent 13C peaks is quite rare with a high-resolution instrument. Now, select the most likely molecular formula(s) from Appendix A of Chapter 1 for comparison and determine the index of hydrogen deficiency for each. In addition to difficulties caused by unresolved or overlapping peaks, discrepancies may appear between the selected molecular formula(s) and the H and 13C counts because of the presence of elements of symmetry. But this information also contributes to an understanding of the molecular structure. 

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