Standard recoveries

It is clear that neither NMEA nor NDPA is appropriate for an internal standard in NDMA determination if criteria are interpreted strictly, but both compounds have been used for this purpose. Addition of a nitrosamine, not normally present in the sample, is helpful in detecting any gross errors in the procedure, but the addition should not be considered to be internal standardization. Utilization of NMEA or NDPA to indicate recovery of NDMA can lead to significant errors. In most reports of the application of these "internal standards", recovery of all nitrosamines was close to 100%. Under these conditions, any added compound would appear to be a good internal standard, but none is necessary. NDMA is a particularly difficult compound for use of internal standardization because of its anomalous distribution behavior. I mass j ectrometry is employed for quantitative determination, H- or N-labeled NDMA could be added as internal standard. Because the labeled material would coelute from GC columns with the unlabeled NDMA, this approach is unworkable when GC-TEA is employed or when high resolution MS selected ion monitoring is used with the equipment described above. 

Spiked standards recovery reported, identification on aglycone RP-HPLC retention times mg/kg fresh weight Accept... 

Vanillin in MeOH and sulphuric acid was used for the derivatization of MON. Chicken tissue (muscle, liver, skin with adhering fat tissues) samples were homogenized with MeOH-water, NaCl was added to the supernatants, and MON was isolated and concentrated by liquid-liquid partition carbon tetrachloride and by SPE on the silica gel. Standard recoveries ranged from 82% to 96%. The method is specific for MON in the presence of closely related PETs— NAR and SAL. Lasalocid and other antibiotics, such as tylosin, nicarbazin, bacitracin, lin-comycin, and bambermycin, do not react in the system and therefore do not interfere (102). A similar method was also used for the determination of MON in bovine tissues and milk. The homogenization of milk was performed by using MeOH. Recoveries achieved were 79-88% with RSD values of 4.6-9.1% (103). 

Hollenbach et al. captured "Tc from standards or soil sample digestates on a TEVA-Resin column for on-line purification and preconcentration prior to ICP-MS determination.49 A wash solution of 0.5 M HN03 was used to remove interferences prior to elution with 8 M HN03 solution. A Re isotope, which behaves similarly to pertechnetate on TEVA-Resin, was used as an internal standard. Recoveries from the column varied from 97 to 99.5%, and columns could be reused over a hundred times. The use of the on-line column separation reduced detection limits by 10-fold and alleviated matrix and isobaric interferences compared to direct sample injection. This pioneering study adapted FIAS-200 and FIAS-400 FI systems to perform sample injection and extraction-chromatographic separations upstream from the ICP-MS. 

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. 

Laboratory QC data are classified as batch QC data and individual sample QC data. For all types of analysis, batch QC data originate from laboratory blanks, laboratory control samples, matrix spikes, and laboratory duplicates. Individual sample QC data in organic compound analysis are obtained from surrogate and internal standard recoveries. Matrix interference detection techniques (serial dilution tests, postdigestion spike additions, and MSA tests) are the source for individual sample QC checks in trace element analysis. (Chapter 4.4.4.5 addresses the trace element matrix interference detection techniques and the associated acceptance criteria.)... 

Surrogate standard recovery measures analytical accuracy for each individual sample. Approved methods for organic compound analysis usually recommend the surrogate standard selection. Similar to target analytes, multipoint calibration curves are prepared for surrogate standards. 

The importance of surrogate standard recovery in evaluating data quality cannot be overemphasized. Laboratories evaluate the efficacy of extraction of each individual sample based on the surrogate standard recovery. If batch QC checks are acceptable, but the individual sample surrogate standard recovery is not, the validity of sample results is questionable. Results of organic compound analysis performed without surrogate standards cannot be considered definitive. 

Similar to LCS recoveries, surrogate standard recoveries should be monitored by the laboratory and plotted as control charts. The EPA recommends the use of in-house laboratory control limits for surrogate standards recoveries for all organic compound analyses (EPA, 1996a). The exception is the CLP SOW, which specifies these limits for soil and water analysis. Unless affected by matrix interferences, surrogate standard recoveries normally have relatively narrow control limits, 65-135 percent for most organic compound analysis. (Many laboratories, however, default to arbitrary limits of 50-150 percent for GC analyses, instead of using statistical control limits.)... 

For GC/MS analyses, some laboratories use surrogate standard recovery limits from outdated versions of EPA Methods 8260 and 8270. These recovery limits, shown in Example 4.18, are fairly close to the statistically derived control limits at most laboratories and can be safely used in the evaluation of data quality. The surrogate... 

Example 4.18 Surrogate standard recovery limits for GC/MS methods... 

These surrogate standard recoveries are from the CLP SOW (EPA, 1999d). They are also acceptable for EPA Method 8260 and 8270. 

Surrogate standard Recovery as determined by laboratory control charts. 

Are surrogate standard recoveries in each sample acceptable ... 

Did matrix interferences or sample dilutions affect the surrogate standard recoveries and reporting limits ... 

GC/MS methods are the only published methods that include the surrogate standard recovery limit guidance. Similar to LCS, acceptance criteria for surrogate standard recoveries of all other organic analysis methods are the laboratory control limits. The limits for internal standard recovery in GC/MS analysis are specified by the method and cannot be changed by the laboratory. Acceptance criteria for matrix interference detection techniques in trace element analyses, discussed in Chapter 4.4.4.5, are also specified in the analytical methods. 

The chemist reviews results of each analysis and determines whether data qualification is needed. Typical deficiencies that turn definitive data points into estimated ones include insufficient surrogate standard recoveries the absence of second column confirmation and the quantitation performed outside the calibration curve. The chemist may even reject the data based on low surrogate standard recoveries. Example 5.6 shows how surrogate standard recoveries may affect the validity of analytical results. 

The laboratory control limits of the BFB surrogate standard in EPA Method 8021 are 56 to 143 percent. The chemist may use the following rationale for qualifying individual sample data with the surrogate standard recoveries outside these limits ... 

The surrogate standard recoveries for some samples may be outside the control limits due to sample dilutions or matrix interferences. Such samples are usually clearly identified in the Case Narrative or in the laboratory reports, and their data are not qualified. For example, if a surrogate standard recovery is below 10 percent due to dilution, the result will not be rejected and will not be qualified as an estimated value. The chemist, however, may request from the laboratory the raw data for this sample in order to verify whether the dilution was justified and the interferences were truly present. 

A surrogate standard recovery that is above the upper control limit is not necessarily an indication of a high bias in the sample result. It may indicate matrix interference, a preparation error, or a measurement error. 

Surrogate standard recovery report (concentration spiked, percent recovered, and recovery control limits) / / /... 

Surrogate standard recoveries within control limits ... 

The recovery has been assessed for this assay in the following way The response for spiked QC plasma samples (1.5, 80 and 160 ng/mL) was compared to the response obtained for a spiked mixture of acetonitrile/ 0.1 M sodium acetate solution (1 1, v/v). The recovery of the internal standards was assessed in the same way (50 ng/mL). Recoveries for simvastatin and simvastatin acid were found to be > 75 % and > 38 % at all concentration levels tested. For the internal standards, recoveries of 52% (lovastatin) and 57% (lovastatin acid) were determined. 

Internal Standard, Recovery Standard, and Cleanup Standard Solutions... 

Clean-up optimisation, capacity, baseline aspects, negative peaks, fractionation, memory effects, internal standard, recovery, etc... 

Fig. 2. Upper Chromatogram of an amino-acid standard mixture. Each peak is 50 pmol. Lower Chromatogram showing the dissolved free amino-acid composition after direct injection of 2.0 ml of seawater sampled from a depth of 2160 m in the Black Sea. The total concentration of DFAA is 419 nmole 1". The composition on a mole % basis is Asp 8.3, Thr 6.5, Ser 25.7, Glu 3.6, Gly 18.6, Ala 7.4, Val 1.6, He 1.9, Leu 2.7, Tyr 1.9, Phe 1.7, Orn 9.4, Lys 1.0, His 5.2, Arg 3.1. Norleu, internal standard, recovery 97%. Full scale sensitivity as in standard (upper). Courtesy of Garrasi et al. (1979).

Accuracy can be affected by all components of an assay. Generally, accuracy has to be determined by comparing results to a reference method. However, in most cases, only an indirect assessment is possible, and several methods are used. Including calibration standards, recovery studies and parallelism. 

Whilst these standards are mostly prepared simply within the solvent matrix used, variations on this theme may result in inclusion of these standards actually within the sample matrix. These procedures are often referred to as internal standardization, standard addition, or standard recovery, depending on the actual implementation of how the known analyte addition is used within the spectrophoto-metric method. These procedures either provide spectral calibration or method quality assurance data. For example, spectroscopic methods will often require pretreatment to remove the interfering sample matrix. Addition of a known amount of the test analyte spike (at the expected level) to a sample is... 

The recovery of an analyte in an assay is defined by the FDA in a strictly operational way as the detector response obtained Ifom an amount of the analyte added to and extracted from the biological matrix, compared to the detector response obtained for the true concentration of the pure authentic standard. Recovery pertains to the extraction efficiency of an analytical method within the limits of variability. Recovery of the analyte need not be 100 %, but the extent of recovery of an analyte and of the internal standard should be consistent, precise, and reproducible. Recovery experiments should be performed by comparing the analytical results for extracted samples at three concentrations (low, medium, and high) with unextracted standards that represent 100 % recovery (FDA 2001). In terms of the symbols used in Section 8.4, the recovery is thus defined as the ratio (R /R"), and is equivalent to determination of F provided diat no suppression or enhancement effects give rise to differences between R and R" and that the proportional systematic errors and 1 are negligible. The FDA definition of recovery also corresponds to that of the PE ( process efficiency ) parameter (Matuszewski 2003) discussed in Section 5.3.6a, since the former (FDA 2001) measures a combination of extraction efficiency and matrix effects (if any). 

Coenzyme A and its analogs have not yet been subjected to GLC separation. Only the indirect method based on the analysis of pantolactonehas been described and used for the evaluation of the degree of hydrolysis of CoA in HCl solutions (75). Pantolactone was extracted into dichloromethane and chromatographed on a polar stationary phase with methyl myristate as the internal standard. Recovery of the determination was 91% and the relative standard deviation was 3.6%. 

The internal standard should have chemical and physical properties as close to those of the analyte as possible. If these conditions are met, then anything that may affect the recovery of the analyte will be reflected by a proportional loss of the internal standard. Ideally, the recovery of the internal standard should be near 100%. If it is, the concentration of the analyte in the sample may be adjusted using the internal standard recovery data. Too great a loss of the internal standard should be an indication that the preparation of the sample was flawed and should be repeated. 

Fig. 11-40 Ejection device used for very fast sample and standard recovery after gamma photon irradiation...

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