Accuracy of analysis

WDXRF data processing with increasing accuracy of analysis (from left to right) is summarised in Figure 8.7. XRF is used to identify/quantify fillers, stabilisers,... 

To establish the well drainage boundaries and fluid flow patterns within the TFSA-waterflood pilot, an interwell chemical tracer study was conducted. Sodium thiocyanate was selected as the tracer on the basis of its low adsorption characteristics on reservoir rocks (36-38), its low and constant background concentration (0.9 mg/kg) in produced fluids and its ease and accuracy of analysis(39). On July 8, 1986, 500 lb (227 kg) of sodium thiocyanate dissolved in 500 gal (1.89 m3> of injection brine (76700 mg/kg of thiocyanate ion) were injected into Well TU-120. For the next five months, samples of produced fluids were obtained three times per week from each production well. The thiocyanate concentration in the produced brine samples were analyzed in duplicate by the standard ferric nitrate method(39) and in all cases, the precision of the thiocyanate determinations were within 0.3 mg/kg. The concentration of the ion in the produced brine returned to background levels when the sampling and analysis was concluded. 

The percent spike recovery to measure the accuracy of analysis may also be determined by the EPA method often used in environmental analysis ... 

In the pharmaceutical industry, it is important that aU products are properly tested and vahdated prior to release for sale. One of the most important tests is the determination of tablet dissolution rate. In this procedure tablets are immersed in a suitable medium to mimic the action of the stomach and the release of the active ingredients monitored over a period of time. Automation of these procedures is obviously important for various reasons including cost, accuracy of analysis and for validation of the results according to good laboratory practice. 

If the composition of the sample matrix has high influence on the accuracy of analysis... 

The formation of a 1 1 complex was confirmed by examination of the removal of organic lead from the aqueous phase as a function of the reagent-to-organic lead ratio Cr/Cl at a temperature of 30°C. Figure 1 shows that removal of organic lead corresponds to what is calculated. For a molar reagent-to-lead ratio (Cr/Cl = 1), complete removal of organic lead is achieved. This demonstrates that, in the absence of NaCl, the complex is essentially insoluble in the aqueous phase, at least to within the accuracy of analysis, 0.1 ppm. 

We conclude this section on gas-phase oxidation catalysts with an illustrative catalytic study where it can be demonstrated that minor changes in preparation have a major effect on catalyst performance, and a very high degree of sophistication is demanded of the accuracy of analysis to detect those changes. 

The laboratory quality control program has several components documentation of standard operating procedures for all analytical methods, periodic determination of method detection levels for the analytes, preparation of standard calibration curves and daily check of calibration standards, analysis of reagent blank, instrument performance check, determination of precision and accuracy of analysis, and preparation of control charts. Determination of precision and accuracy of analysis and method detection limits are described under separate subheadings in the following sections. The other components of the quality control plan are briefly discussed below. 

U S. EPA s analytical procedures mention an 11-min purge with nitrogen or helium at a flow rate of 40 mL/min. and 4-min desorption at 180°C, backflushing the trap with an inert gas at 20 to 60 mL/min. A 5-mL sample volume is recommended for purging. A larger volume of sample may be required to obtain a lower detection level. Other conditions may be used if precision and accuracy of analysis are met. 

Aqueous samples are extracted with methylene chloride by liquid-liquid extraction. The extract is concentrated and then exchanged to hexane. Soils, sediments, and solid wastes are extracted by sonication or Soxhlett extraction. Samples should be spiked with one or more surrogate standard solution to determine the accuracy of analysis. Some of the internal standards mentioned above may also be used as surrogates. If only the PCBs are to be analyzed, hexane instead of methylene chloride may be used throughout. Oil samples may be... 

Like precision, accuracy depends on the nature of the analyte analytical, method and procedure, and the skill of the chemist performing analysis. It is also a function of concentration the lower the concentration, the lower the accuracy of analysis. 

To establish reasonable acceptance criteria for accuracy during planning, we should obtain statistical laboratory control limits from the laboratory that will perform analysis for the project samples. We should also be aware of matrix interferences in environmental samples that may reduce the accuracy of analysis. As part of QC procedures, to estimate the effects of matrix interference on accuracy, laboratories perform the accuracy determinations on environmental samples, known as matrix spike (MS) and matrix spike duplicate (MSD). These fortified samples enable the laboratory to detect the presence of interferences in the analyzed matrices and to estimate their effect on the accuracy of sample analysis. (In the absence of matrix interferences, an additional benefit from MS/MSD analysis is an extra measure of analytical precision calculated as the RPD between the two recoveries.)... 

Another tool that enables us to evaluate analytical accuracy of organic analyses is surrogate standards. These are compounds that do not naturally occur in the environment and that are similar in chemical nature and behavior to target analytes. In organic compound analysis, known amounts of surrogate standards are added to each sample prior to extraction. The comparison of surrogate standard recoveries to laboratory control limits permits the laboratory to monitor the efficacy of extraction and to measure the accuracy of analysis for each individual sample. 

An important consideration in sample preservation is the observance of holding time, which is defined by ASTM (ASTM, 1987) as follows The period of time during which a water sample can be stored after collection and preservation without significantly affecting the accuracy of analysis. ... 

The chemist understands that advisory acceptance limits stated in the SAP do not reflect the actual accuracy of analysis. The laboratory has determined the accuracy by using control charts for LCS data. The QC check sample data that are within these statistical limits indicate that the analytical process is in control. The chemist accepts the laboratory accuracy criteria and does not qualify the sample results, although the SAP acceptance criteria for LCS/LCSD have been exceeded. 

The accuracy of analysis of ribonuclease by the best sites of this study (Table V) can be compared to an early analysis by ion-exchange/ninhydrin [9] on a single hydrolysate, which has as good an error (4.4%). However, that analysis used mg quantities of material as compared to the pg quantities used at present. 

The SDS removal trap cartridges are able to remove up to 1 mg of SDS from a given sample. Use of these cartridges eliminates interference with reversed-phase separations and avoids column fouling which is commonly experienced when SDS is a part of the separation. Removal of SDS from biological samples also improves the accuracy of analysis by MS, AAA or sequencing. 

It is not difficult to see why so many different methods can be and have been appHed to the analysis of amino acids (see Table 1.1). Amino acids vary widely in reactivity, and when they are being derivatized undesirable side-reactions are often observed and high reaction yields are reproduced only with difficulty, which decreases the accuracy of analysis. Thus, in the mid-1960s GC was regarded mainly as a method for the qualitative analysis of amino acids [11]. 

The accuracy of analysis increases substantially if the compound used as the internal standard is chemically similar to the compound being determined. In a limiting case it is expedient to use as the internal standard the same compound as that being determined, but with a different isotopic composition. The combined use of GC and MS makes it possible to utilize this optimal variant. In recent years, especially in the combined GC—MS analysis of biologically active compounds, wide use has been made of compounds labelled with stable isotopes as internal standards [58], one of the optimal methods being the use of compounds having three or four C atoms. A positive feature of this method is the absence of isotopic effects in the course of the GC separation or in detection during chemical ionization used in MS [59]. Isotope effects also do not manifest themselves in the processes that result in the loss of the sample substance by... 

However, due to their mode of operation, pneumatic amplifier pumps have certain disadvantages. They are constant pressure rather than constant flow and therefore, as the elution volume is proportional to flow, fluctuations in the latter—due to, for example, partial column blockage or temperature change—can lead to poor precision and accuracy of analysis. The flow-rate is also dependent on solvent viscosity and coliunn back pressure. 

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