Analyte recovery determination

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

An important feature of isotope dilution is that it is not necessary to recover all the analyte to determine the amount of analyte present in the original sample. Isotope dilution, therefore, is useful for the analysis of samples with complex matrices, when a complete recovery of the analyte is difficult. 

Spike Recoveries One of the most important quality assessment tools is the recovery of a known addition, or spike, of analyte to a method blank, field blank, or sample. To determine a spike recovery, the blank or sample is split into two portions, and a known amount of a standard solution of the analyte is added to one portion. The concentration of the analyte is determined for both the spiked, F, and unspiked portions, I, and the percent recovery, %R, is calculated as... 

The sfe of chlorpyrifos methyl from wheat followed by on-line Ic/gc/ecd has been investigated (93). Extraction profiles were generated to determine the maximum analyte recovery and the minimum extraction time. Using pure CO2, a 65% recovery of chlorpyrifos methyl spiked onto wheat at 50 ppb was reported. When 2% methanol was added to the CO2, the recovery from a one gram sample averaged 97.8% (n = 10, 4.0% RSD). 

Following this procedure urea can be determined with a linear calibration graph from 0.143 p.g-ml To 1.43 p.g-ml and a detection limit of 0.04 p.g-ml based on 3o criterion. Results show precision, as well as a satisfactory analytical recovery. The selectivity of the kinetic method itself is improved due to the great specificity that urease has for urea. There were no significant interferences in urea determination among the various substances tested. Method was applied for the determination of urea in semm. 

Accuracy (systematic error or bias) expresses the closeness of the measured value to the true or actual value. Accuracy is usually expressed as the percentage recovery of added analyte. Acceptable average analyte recovery for determinative procedures is 80-110% for a tolerance of > 100 p-g kg and 60-110% is acceptable for a tolerance of < 100 p-g kg Correction factors are not allowed. Methods utilizing internal standards may have lower analyte absolute recovery values. Internal standard suitability needs to be verified by showing that the extraction efficiencies and response factors of the internal standard are similar to those of the analyte over the entire concentration range. The analyst should be aware that in residue analysis the recovery of the fortified marker residue from the control matrix might not be similar to the recovery from an incurred marker residue. 

In addition, each workbook contained a summary table of all results and limit of detection (LOD) determinations. The table was organized with sample identifications in the left-hand column. Eor each analyte, the analytical result and the LOD appeared in adjacent columns, and analyte recoveries appeared above the results columns. The summary table was generated automatically from the analytical results in the individual worksheets, without operator intervention or re-entry of any information. 

Once test sites have been identified, control soil should be collected and returned to the laboratory. This soil is used to (1) verify soil texture and related properties, (2) ensure adequate analytical recovery of target analytes, and (3) determine the presence of potential background interferences in the soil. 

The precision of the assay for nonreduced samples was demonstrated by the evaluation of six independent sample preparations on a single day (repeatability) and the analysis of independent sample preparations on three separate days by two different analysts (intermediate precision). The RSD values for the migration time were 0.9%. The RSD values for peak area percent of the main peak and the minor peaks in the profile were 0.6 and 12.6%, respectively. The higher variability observed with the minor peaks was determined to be primarily related to the sample heating during preparation for the analysis. These results demonstrate that the use of uncoated fused-silica capillaries in combination with a sieving matrix can provide adequate precision and analyte recovery. 

SPMDs. Use of this type of blank is generally limited to laboratories that assemble SPMDs. If the numbers of SPMD-fabrication blanks are inadequate, SPMD-process blanks can be used to determine analyte recovery and the precision of the overall analytical method. Also, this type of QC sample can be used for other purposes, such as determining potential effects of storage or changes in batches or lots of SPMD materials. 

For the determination of the absolute analytical recovery (= R= 0 rg) of the compounds the peak heights of prepared samples were compared to the mean peak height from seven direct injections of 10 pi of the standard solution into the HPLC-system. 

Results. Tables m IV show the SFE-GC/MS results obtained for duplicate extractions of 14 PAHs plus pentachlorophenol from a EPA standard reference material soil sample. Table m lists the certified values of the analytes as determined by a standard method as well as the SFE recoveries for the individual fractions of sample 1, total recovery from SFE, and total percent recovery from SFE in relation to the certified values. Table IV shows the repeatability of the experiment by comparing total SFE recoveries from two identical sample extractions. 

Again the chemical/physical properties of the analytes will determine the collection and reconstitution rinse parameters. During the extraction step, the volatility of the analytes will determine the collection temperature or type of adsorbent material used for collection. If the analytes are volatile, then a cold trap or cooled collection solvent along with a low flow rate should be used. This is because the analytes are volatile and the expansion of the C02 can create aerosols or mechanically move the analytes past the collection device. Less volatile analytes can tolerate higher extraction flow rates and higher collection temperatures can be used. If an adsorbent trap is used for collection, the chemist can specifiy an appropriate adsorbent and rinse solvent for optimal recoveries for the analytes of interest. Flow rate and volume are parameters that also need to be specified. The flow rate and rinse volume are determined by the solublity of the analytes in the rinse solvent and the amount of material to be removed from the trap. 

Hill et al. used ICP-MS and FI with TRU-Resin column separations for determination of U and Th in natural waters.134 High analyte recoveries were found even in waters with high dissolved organic matter, which normally presents a problem if samples are concentrated using typical ion-exchange or chelating resins. Samples on the TRU-Resin column were washed with nitric acid as usual and eluted in ammonium bioxalate. 

Fortified field blanks to determine the effects that the matrix might have on analyte recovery. 

Ogner [1] has described an automated analyser method for the determination of boron-containing anions in plants. This is based on the formation of a fluorescent complex between these anions and carminic acid at pH 7. The plant tissues are ashed at 550 °C and the residue dissolved in 0.5 N hydrochloric acid prior to adjustment to pH 6-7 with sodium carbonate solution. The solution is excited at 470 nm and fluorescence intensities measured at 585 nm. Interferences by the reaction of some cations with carminic acid are overcome by passing the solution through an ion exchange column to exchange the cations for sodium ions. Analytical recoveries of boron anions were in the range 98-104%. The detection limit of the method was 5 xg/l boron. 

Tanaka et al. [ 16] have described a spectrophotometric method for the determination of nitrate in vegetable products. This procedure is based on the quantitative reaction of nitrate and 2-sec-butylphenol in sulfuric acid (5 + 7), and the subsequent extraction and measurement of the yellow complex formed in alkaline medium. The column reaction is sensitive and stable and absorbances measured at 418 nm obey Beer s law for concentrations of nitrate-nitrogen between 0.13 and 2.5 xg/ml. In this procedure, the vegetable matter is digested at 80 °C with a sodium hydroxide silver sulfate solution, concentrated sulfuric acid and 2-sec-butylphenol are added, and after 15 minutes of standing time the nitrated phenol is extracted with toluene. Finally, the toluene layer is back-extracted with aqueous sodium hydroxide and evaluated spectrophotometrically at 418 nm. The standard deviation of the whole procedure was 1.4%, and analytical recoveries ranged between 91 and 98%. 

Aqueous samples are extracted with methylene chloride using a separatory funnel or a continuous liquid-liquid extractor. Solid samples are extracted with methylene chloride-acetone mixture (1 1) by either sonication or Soxhlett extraction. The methylene chloride extract should be finally exchanged to hexane or iso-octane or methyl tert-butyl ether. The latter solvents should be mixed with acetone during solvent exchange. The extracts should then be cleaned up by Florisil. Often Florisil cleanup reduces the percent recovery of analyte to less than 85%. A preliminary screening of the extract should, therefore, be done to determine the presence of interference and the necessity of florisil cleanup. Gel permeation cleanup also lowers the analyte recovery and thus is not recommended. If a FPD is used in the GC analysis, the presence of elemental sulfur can mask the analyte peaks. In such a case, sulfur cleanup should be performed. Sample extraction and cleanup procedures are described in Chapter 1.5. 

The contribution made by the last two steps can be discovered by applying a field check sample, which is obtained by dividing the problem sample into two and spiking one subsample with a target analyte. Recovery is then determined under different conditions of light, temperature, pH, and so on, in order to select the best sample storage conditions. 

Internal standard calibration can be used to compensate for variation in analyte recovery and absolute peak areas due to matrix effects and GC injection variability. Prior to the extraction, a known quantity of a known additional analyte is added to each sample and standard. This compound is called an internal standard. To prepare a calibration curve, shown in Figure 4.6b, the standards containing the internal standard are chromatographed. The peak areas of the analyte and internal standard are recorded. The ratio of areas of analyte to internal standard is plotted versus the concentrations of the known standards. For the analytes, this ratio is calculated and the actual analyte concentration is determined from the calibration graph. 

The sample volume also has an effect on both the rate and recovery in SPME extractions, as determined by extraction kinetics and by analyte partition coefficients. The sensitivity of a SPME method is proportional to n, the number of moles of analyte recovered from the sample. As the sample volume (Vs) increases, analyte recovery increases until Vs becomes much larger than the product of K, the distribution constant of the analyte, and Vf, the volume of the fiber coating (i.e., analyte recovery stops increasing when KfeVf Vs) [41]. For this reason, in very dilute samples, larger sample volume results in slower kinetics and higher analyte recovery. 

When compared to a typical potency assay, this validation parameter is the most variable Eq. (15.4) represents the typical calculation that is used to determine the analytical recovery for the method. The spiked amount is based upon the predetermined acceptance limit (see Section 15.3 for different methods of calculating this value). 

Collection of metal complexes of the analytes on suitable adsorbing materials is often employed as an enrichment step in combination with flame methods. In a procedure proposed by Solyak et al. [20], five metals [Co(II), Cu(II), Cr(III), Fe(III), and Pb(II)] were complexed with calmagite 3-hydroxy-4-[(6-hydroxy-m-tolyl)azo]-naphthalenesulfonic acid and subsequently collected on a soluble cellulose nitrate membrane filter. In this way an effective separation from alkaline and alkaline earth metals was achieved, based on the differences in their complex formation constants and those of the transition elements. The experimental parameters were optimized for the quantitative recovery of the elements. After hot dissolution of the filter with HNO3, the analytes were determined by FAAS. Minimum detectable concentrations ranged from 0.06 pg l-1 for Cu to 2.5 pg l-1 for Cr. 

In order to provide information on recovery of the analytes of interest from the sample matrix, the laboratory must prepare a second aliquot of one sample of each matrix in each Sample Delivery Group (SDG) and spike it with the analytes at concentrations specified in Section 13. This aliquot is analyzed and the recovery of the spiked analytes is determined. 

Interferences are of particular importance for devices destined for continuous use in very complex matrices. Biosensors are tested for interferences not just from species that are expected to bind to or react with the particular chemical recognition agent employed the end use of the biosensor is considered, and components of that sample matrix are examined for potential interference. Test assays are conducted in the sample matrix, and compared with results obtained in simple buffers in order to determine analyte recovery. 

Hengen and Hengen developed a method for the determination of nicotine and cotinlne in 1 ml samples of plasma. Nicotine was extracted from alkalinized plasma (NaOH) with diethyl ether, and cotinlne from the same sample with dichlbromethane. Modaline was used as internal standard for nicotine, lidocaine for cotinine. Analytical recovery of nicotine added to the plasma was 80 - 6 %, for cotinine 95 - 5 56. The internal standards were directly added to the plasma to monitor extraction losses. The sensitivity was such that less than 0.1 pg of nicotine and 0.1 pg of cotinine could be detected per liter. Day-to-day reproducibility for nicotine was within 14 % and within 6 % for cotinine. Narrow peaks for the gas chromatographed compounds were obtained on the very stable SP-2250 column (3 %) on Supelcoport at 155°C for... 

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