Source of imprecision

The study of tautomerism using H NMR spectroscopy is simple when the tautomers give separate signals (84B2906) otherwise, interpolation methods need to be applied, which entail several sources of imprecision [83JPR(325)238]. A paper reports the observation of two NH signals for the N-labeled tautomers of 3(5)-methyl-5(3)-phenylpyrazole (45) in toluene-dg at 190 K (Scheme 15) [92JCS(P2)1737]. 

Inadequate regulation of atomizer temperature Is a major source of Imprecision In electrothermal atomic absorption spectrometry. The programmed heating of electrothermal atomizers can be achieved by five different methods, depending upon the electrical or physical parameters which are monltorled during... 

Clearly, precision studies should mirror the operating conditions used during the routine use of the method. Individual sources of imprecision, such as change of operator or instrument type, can be studied but two types of overall precision are commonly estimated, i.e. repeatability and reproducibility. 

HPLC has more or less supplanted GC as a method for quantifying drugs in pharmaceutical preparations. Many of the literature references to quantitative GC assays are thus old and the precision which is reported in these papers is difficult to evaluate based on the measurement of peak heights or manual integration. It is more difficult to achieve good precision in GC analysis than in HPLC analysis and the main sources of imprecision are the mode of sample introduction, which is best controlled by an autosampler, and the small volume of sample injected. However, it is possible to achieve levels of precision similar to those achieved using HPLC methods. For certain compounds that lack chromophores, which are required for detection in commonly used HPLC methods, quantitative GC may be the method of choice, for analysis of many amino acids, fatty acids, and sugars. There are a number... 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

Figure 18-6 shows measured spectrophotometer errors. Electrical noise was only modestly dependent on sample absorbance. The largest source of imprecision for A < 0.6 was irreproducible positioning of the cuvet in the sample holder, despite care in placing the cuvet. The resulting error curve reaches a minimum near A = 0.6. In Section 5-2, we learned that the detection limit for an analytical procedure is determined by the reproducibility of the measurement. The less noise, the lower the concentration of analyte that can be detected. 

Sources of imprecision are an insufficient number of particles and steps, too large a step length, and inaccuracies in ik other than the position of its nodes. Errors in the nodal surfaces of k translate into positive errors in the calculated energy, so an upper bound is obtained. Ideally, the exact solution k of the Schrodinger equation is provided within the loges bounded by the (approximate) nodal surfaces. 

Homogeneous immunoassay (HOIA) does not require physical separation of the free and antibody-bound antigen because the measured physical signal derived form the antibody-boimd, labeled material may be significantly different from that of the unbound entity. There may be an enhancement or an inhibition of enzyme activity upon binding of the antibody to the antigen. HOIA are simple to perform and automation can be carried out easily. Elimination of the separation step avoids a major source of imprecision in the assay. However selectivity may be compromised since interfering substances are not eliminated in the separation step. 

Precision (as discussed previously) is the ability of the assay to give the same result when repeated multiple times either within the same run, or from day to day (i.e., between runs). Precision data are usually collected early in the evaluation at a new method, since the quality of the method depends on the level of precision. If precision is poor, more elaborate validation experiments must be postponed until the sources of imprecision are identified and controlled. 

Sources of imprecision in laboratories screening for congenital hypothyroidism Analysis of nine years of performance data. Clin Chem 1989 35 1701-5. 

In order to achieve the highest level of accuracy, special care must be taken in setting up mass spectrometric instrumentation prior to measurement of isotopic ion abundance ratios, as a long stabilisation period may be necessary in order to achieve the highest precision. A major source of imprecision is the instability of the instrument and the electronic signal level must be correctly set and not be subject to drift. In the case of the latest computer controlled instruments such checks can be carried out automatically. 

An alternative way of approaching the decision function in inspection is through the notion of fuzzy sets. Indications of defects such as wear, corrosion, scratches, stains, and quality of cloth have imprecisely defined criteria, and judgments cannot often be represented by a single precise number (Karwowski et al. 1990 Watson et al. 1979). The framework of fuzzy sets (Zadeh 1965) provides us a way of dealing with the category of problems where an absence of sharply defined criteria of class membership is the source of imprecision. Thus, inexact knowledge can be represented, and so can situations where membership in sets cannot be defined purely on an yes/no basis. 

For these reasons, it is critical that when short- and long-term precision are evaluated, you know all the potential sources of imprecision and drift. It is therefore important that you choose either a matrix that is representative of your samples or one that will genuinely test the instrument out. Typical sample matrices include the following ... 

Spurious contamination of samples with extraneous lead is in many cases a significant source of imprecision in measurements of low blood lead levels (<5 fig/dL). This source of imprecision is not unique to measurements of... 

S.E. Jewell, W.J. Slazyk, S.J. Smith, and W.H. Hannon, Sources of Imprecision in laboratories screening for congenital hypothyroidism analysis of nine years of performance data, Clin Chem 35(8) 1701-1705(1989). 

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