Measurement procedures sample recovery

This consideration of the principles of diffusive sampling identifies a range of factors which may influence the performance of a diffusive sampler for monitoring VOC concentrations in indoor air. These factors will potentially be a source of error in such measurements and add to the overall uncertainty of the result given by the measurement procedure. In addition the amount of uncertainty will be influenced by other factors including amount and consistency of background contamination of sorbents, repeatability of analytical determination, formation of artifacts, stability of analyte on the sorbent, recovery of analyte during analyses and presence of interferents. 

Accuracy 2 Recovery, percent Xs+i Xi x 100 Sa Completeness 3a Analytical completeness, percent V X> -j— xlOO 1 xs+i—spiked sample measurement Xi—sample measurement sa—true amount added in the spiking procedure Xi—sample measurement V—number of valid sample measurements T—number of total sample measurements... 

In earlier work using this procedure, Sorrell [180] had shown the recoveries of six PAHs from distilled water to range from 61 to 91% and to average 78 8%. Subsequent measurements of the recoveries of 11 PAHs from seven raw or finished water samples taken over a nine-month period ranged from 53 to 116% and averaged 86 12%. 

A study was carried out into the potential recovery of plasticiser and solvent from waste PVC plastisols using a ceramic multi-bore crossflow tube filter. The procedure employed to perform the test sequence involved clean water flux measurement, media acclimatisation, optimisation trial, concentration run, cleaning trial and final water flux measurement. Permeate samples were analysed using gas chromatography and compared with standards of diisononylphthalate(DINP)/white spirit mixtures. The ceramic membrane successfully recovered a clear mixture of DINP and white spirit. 

The low concentration of radionuclides has stimulated near-universal application of an isotope dilution technique ( reverse isotope dilution ) that permits measurement of chemical recovery, termed yield. As commonly applied outside the radioanalytical chemistry laboratory, isotope dilution consists of the addition of a known amount of radionuclide tracer to a sample that contains its stable element, i.e., the natural mixture of stable isotopes of the element. The tracer is added at the beginning of the procedure and then measured in the separated and purified sample. If the added tracer and the stable isotopes of interest are assured at the beginning to be in the same chemical and physical form, then the fraction of recovered tracer represents the fraction of recovered stable isotope. 

A flow injection method based on the catalytic action of iodide on the colour-fading reaction of the FeSCN2+ complex was proposed and applied in order to determine iodine in milk. At pH 5.0, temperature 32°C and measurements at 460 nm, the decrease in absorbancy of Fe -SCN (0.10 and 0.0020 mol /I) in the presence of N02" (0.3 mol/ 1) is proportional to the concentration of iodide, with a linear response up to 100.0 pg/1. The detection limit was determined as 0.99 pg/1 and the system handles 48 samples per hour. Organic matter was destroyed by means of a dry procedure carried out under alkaline conditions. Alternatively, the use of a Schoninger combustion after the milk dehydration was evaluated. The residue was taken up in 0.12 mol/1 KOH solubilization. For typical samples, recoveries varied from 94.5 to 105%, based on the amounts of both organic matter destroyed. The accuracy of the method was established by using a certified reference material (IAEA A-11, milk powder) and a manual method. The proposed flow injection method is now applied as an indicator of milk quality on the Brazilian market (de Araujo Nogueira et al., 1998). 

In addition to the incomplete oxidation of some chemicals with the reflux methods, laboratory wastes generated from the standard COD tests contain hexavalent chromium, mercury, and silver metals, all of which are classified as hazardous wastes by the US Environmental Protection Agency and their disposal is regulated under Resource Conservation and Recovery Act. Consequently, various modifications of the standard procedures or alternative methods have been reported for the COD test. These include the replacement of hexavalent chromium, mercury, and silver metals microwave digestion automation and online COD measurement and electrochemical oxidation to measure the sample COD. 

An accurate extraction and measurement procedure to determine CBs in surface waters is presented in Ref. [46]. The procedure involved a 10 L batch liquid-liquid extraction directly from the sample bottle to prevent loss due to adsorption to the wall. Exhaustive extraction for recovery measurements was proposed, resulting in an extraction time of 10-45 h. A detection limit lower than 10 pg kg and a coefficient of variation of 3%-9% were obtained. 

To validate the analytical procedure recovery experiments are performed. To this end, the CRM is spiked with a known mass of the analytes at a variety of concentration levels (at least three different levels) and the concentrations measured are compared to the expected concentrations in at least three separate experiments. The extraction step has been shown to be a critical step in the analytical procedure and it may be responsible for poor recoveries. The efficiency of this step can be assessed either by repetitive extraction of the sample or by the addition of internal standards prior to the extraction step with the assumption that the latter actually represent the behavior of the analytes of interest. 

DP-6 over 3000 soil samples collected from several terrestrial field dissipation studies. The sample procedural recoveries using this method, conducted concurrently with the treated samples during soil residue analysis, are summarized in Table 5. This method was proven to be short, rugged, sensitive, and suitable for measuring residues in soil and sediment at levels down to 0.01 mg kg . The reproducibility of the methods also indicated acceptable method performance and, as a result, thousands of samples were analyzed using this methodology. 

For destructive measuring methods, a CRM would serve as a reference to check the recovery of a particular matrix removal procedure. This is especially important for open destructions at atmospheric pressure. Alternatively, isotope dilution methods may be used once isotopic equilibrium is established, loss of analyte does not affect the analysis result. Isotope dilution techniques are only available in a few specialised laboratories. Another type of problem is encountered in pressurised methods oxidising the matrix in a closed vessel or bomb. Due to the large amounts of gas (CO2, NO, SO2) evolving from samples with a high organic matrix content, an excessive pressure build-up occurs that prohibits the use... 

Regardless of the approach chosen, the procedure should be fully validated to assess whether the recovery of the analyte from the sample matrix is acceptable and to establish that the resulting aliquot submitted for measurement is free from any significant interferences (see Section 4.6). 

Matrix effect is a phrase normally used to describe the effect of some portion of a sample matrix that causes erroneous assay results if care is not taken to avoid the problem or correct for it by some mechanism. The most common matrix effects are those that result in ion suppression and subsequent false negative results. Ion enhancement may lead to false positive results.126 127 Several reports about matrix effects include suggestions on what can cause them and how to avoid them.126-147 While various ways to detect matrix effects have been reported, Matuszewski et al.140 described a clear way to measure the matrix effect (ME) for an analyte, recovery (RE) from the extraction procedure, and overall process efficiency (PE) of a procedure. Their method is to prepare three sets of samples and assay them using the planned HPLC/MS/MS method. The first set is the neat solution standards diluted into the mobile phase before injection to obtain the A results. The second set is the analyte spiked into the blank plasma extract (after extraction) to obtain the B results. The third set is the analyte spiked into the blank plasma before the extraction step (C results) these samples are extracted and assayed along with the two other sets. The three data sets allow for the following calculations ... 

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