Sampling Accuracy

Accuracy The accuracy of a gas chromatographic method varies substantially from sample to sample. For routine samples, accuracies of 1-5% are common. For analytes present at very low concentration levels, for samples with complex matrices, or for samples requiring significant processing before analysis, accuracy may be substantially poorer. In the analysis for trihalomethanes described in Method 12.1, for example, determinate errors as large as +25% are possible. ... 

Theoiy related to material characteristics states that a minimum quantity of sample is predicated as that amount required to achieve a specified limit of error in the sample-taking process. Theoiy of sampling in its apphcation acknowledges sample preparation and testing as additional contributions to total error, but these error sources are placed outside consideration of sampling accuracy in theoiy of sample extraction. 

Some properties of OPs are given in Table 10.1 and some structures in Figure 10.1. There is some variation in the values quoted for the aforementioned properties in the literature, reflecting purity of sample, accuracy of method, etc. The foregoing are representative values, and are not necessarily the most accurate ones for the purest samples. 

Jet u 200 M drilling fluids required smearing of borehole walls in fine-grained soils and sediments causing sealing Diameter limitations fluid level (water and NAPL) difficult sampling accuracy limited produced fluids... 

The number of increments taken shall directly influence the sampling accuracy provided the bulk material comprises of varying particle sizes. 

Sampling accuracy Determine gravimetrically the average volume of water withdrawn from a tared vial filled with water after six 50- lL injections 50 2pL... 

Sample Data Table for evaluating autosampler sampling accuracy... 

FIGURE 9 Gravimetric data table showing the evaluation of autosampler sampling accuracy. 

Protein determination is necessary to estimate the amount of protein in the sample, to normalise against the protein concentration or during purification procedures. Depending on the amount of sample, accuracy and presence of interfering agents, one needs to decide on the method to be used. For accurate quantification, the sample protein is compared with a known amount of a standard protein which could either be the commonly used bovine serum albumin (BSA) or it could sometimes be immunoglobulin G (IgG). The various methods and their specifications are outlined below ... 

This actual resource savings is only one aspect of justification for the phased scheme. As the result of information developed in the survey phase, a significant improvement in sampling accuracy and completeness can be anticipated during later phases. It is very doubtful that equal data could actually be acquired in a direct approach due to the lack of the very necessary learning processes involved in difficult source sampling and analytical projects. 

Robustness. Examples of typical possible sources of variation in automated methods are homogenization speed, homogenization time, age of sample, accuracy of solvent dispense, and temperature variation. If all studies described in the method development have been performed, the robustness of the sample preparation has been demonstrated and does not require additional testing. Parameters in relation to the measurement technique may need to be considered and are covered in the relevant chapter. 

Quality assurance is what we do to get the right answer for our purpose. We begin by writing use objectives, from which specifications for data quality can be derived. Specifications could include requirements for sampling, accuracy, precision, specificity, detection limit, standards, and blank values. For any meaningful analysis, we must first collect a representative sample. In an analysis of standards, the method must produce a result that is acceptably close to the known value. A method blank contains all components except analyte, and it is taken through all steps of the analytical procedure. We subtract the response of the method blank from the response of... 

Any powder can be considered to be made up of two components, the fraction above and below a certain size and assumptions made as to the weights of the individual grains in each of the two components. Equation (2.2) may then be used to determine the sampling accuracy of a single powder. Furthermore, if the particles are counted instead of weighed, a more general equation is applicable [1] ... 

These additional steps increase the possibility of adventitious contamination. It has been demonstrated that the sampling efficiency of an impinger is dependent upon both system design and the particle sizes being sampled. Accuracy and reproducibility of results have been reported to be difficult, and particles... 

Accuracy indicates the closeness of the measurement to the true or accepted value and is expressed by the error. Figure 5-2 illustrates the difference between accuracy and precision. Note that accuracy measures agreement between a result and the accepted value. Precision, on the other hand, describes the agreement among several results obtained in the same way. We can determine precision just by measuring replicate samples. Accuracy is often more difficult to determine because the true value is usually unknown. An accepted value must be used instead. Accuracy is expressed in terms of either absolute or relative error. 

The automotive pressure sensor is characterized by its strict requirements. Tab. 7.3.2 gives sample accuracy requirements for automotive pressure sensors [2]. The pressure sensor used in the engine compartment is required to operate over a temperature range from -30 to 120 °C, and the pressure sensor for air-condi-... 

The amount of metallic nickel in catalysts was determined by a DTA method by Macak and Malecha (84). Nickel produced by the reduction of nickel oxide was reoxidized by oxygen and the AT of the oxidation reaction was determined by the apparatus. The maximum value of AT between the reactor for catalytic reaction and that with inert Si02 packing was proportional to the amount of nickel in the catalyst sample. Accuracy of the method was about 4%. 

Guild LV, Myrmel KH, Myers G, and Dietrich DF (1992) Bi-level passive monitor validation - a reliable way of assuring sampling accuracy for a larger number of... 

A constant amount of internal standard must be added with a high degree of precision to each sample. Accuracy is generally less important, and within limits, an internal standard does not need to be as pure as the analyte. Depending on the reasons for its inclusion, the internal standard, may be added at the beginning or at later stages in the assay procedure. 

B. Precision is demonstrated by the repeatability of analyses of replicate samples and replicate portions of the same sample. Accuracy is demonstrated by fortification recovery, calibration checks, blanks, and quality control samples (blind samples). 

The sensitivity of NAA for a particular element depends on the irradiation parameters (i.e., neutron flux, irradiation, and decay times), measurement conditions (i.e., measurement time, detector efficiency), nuclear parameters of the elements being measured (i.e., isotope abundance, neutron cross-section, half-life, and y-ray abundance). The accuracy of an individual NAA determination usually ranges from 10 to 10 ° grams per gram of sample. Accuracy of a NAA determination is usually between 2% and 10% of the reported value, depending on the element analyzed and its concentration in the sample. 

Centrifugal sampler Cyclones (1) Battery operated (2) Small (hand size) (1) Allows for spatially variable samples (2) Elimination of sample transfer loss (1) Sampling accuracy affected by vortexing (higher counts, false higher efficiency rating) (2) Counts should be done manually (use of strip) (3) Flow rate only quantifiable theoretically (4) Questionable accuracy of the collected sample (5) Limitation on collection of small particles onto the collection surface... 

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