Spiked recovery method

TESTING FOR SYSTEMATIC ERROR IN A METHOD COMPARISON TEST FOR A SET OF MEASUREMENTS VERSUS TRUE VALUE - SPIKED RECOVERY METHOD (COMPARE 71 WORKSHEET)... 

Spike recovery methods are generally used to measure the iodine-separation yield. Spikes used include stable iodine ( I), and the radioactive and If stable iodine is used, sufficient I is required to eliminate errors resulting from the presence of naturally-... 

Daskalakis and co-workers recently evaluated several procedures for digesting the tissues of oysters and mussels prior to analyzing the samples for silver. One of the methods used to evaluate the procedure is a spike recovery in which a known amount of silver is added... 

The most useful methods for quality assessment are those that are coordinated by the laboratory and that provide the analyst with immediate feedback about the system s state of statistical control. Internal methods of quality assessment included in this section are the analysis of duplicate samples, the analysis of blanks, the analysis of standard samples, and spike recoveries. 

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... 

Spike recoveries on method blanks and field blanks are used to evaluate the general performance of an analytical procedure. The concentration of analyte added to the blank should be between 5 and 50 times the method s detection limit. Systematic errors occurring during sampling and transport will result in an unacceptable recovery for the field blank, but not for the method blank. Systematic errors occurring in the laboratory, however, will affect the recoveries for both the field and method blanks. 

Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the laboratory spike is acceptable, then the poor recovery for the field spike may be due to the sample s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship between the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1. ... 

The first sample to be analyzed is the field blank. If its spike recovery is unacceptable, indicating that a systematic error is present, then a laboratory method blank. Dp, is prepared and analyzed. If the spike recovery for the method blank is also unsatisfactory, then the systematic error originated in the laboratory. An acceptable spike recovery for the method blank, however, indicates that the systematic error occurred in the field or during transport to the laboratory. Systematic errors in the laboratory can be corrected, and the analysis continued. Any systematic errors occurring in the field, however, cast uncertainty on the quality of the samples, making it necessary to collect new samples. 

If the field blank is satisfactory, then sample B is analyzed. If the result for B is above the method s detection limit, or if it is within the range of 0.1 to 10 times the amount of analyte spiked into Bsf, then a spike recovery for Bsf is determined. An... 

If the spike recovery for Bsf is acceptable, or if the result for sample B is below the method s detection limit or outside the range of 0.1 to 10 times the amount of analyte spiked in Bsf, then the duplicate samples Ai and A2 are analyzed. The results for Ai and A2 are discarded if the difference between their values is excessive. If the difference between the results for Ai and A2 is within the accepted limits, then the results for samples Ai and B are compared. Since samples collected from the same sampling site at the same time should be identical in composition, the results are discarded if the difference between their values is unsatisfactory, and accepted if the difference is satisfactory. 

In a performance-based approach to quality assurance, a laboratory is free to use its experience to determine the best way to gather and monitor quality assessment data. The quality assessment methods remain the same (duplicate samples, blanks, standards, and spike recoveries) since they provide the necessary information about precision and bias. What the laboratory can control, however, is the frequency with which quality assessment samples are analyzed, and the conditions indicating when an analytical system is no longer in a state of statistical control. Furthermore, a performance-based approach to quality assessment allows a laboratory to determine if an analytical system is in danger of drifting out of statistical control. Corrective measures are then taken before further problems develop. 

Data from several laboratories within the Interregional Research Project No. 4 (IR-4) in the USA have been evaluated for determining the values of MDL and MQL. These data have been presented in Table 1. The two-step procedure described in the EPA guideline was used to calculate the values of MDL and MQL. For the first step, the slope, intercept and RMSE values for the first three calibration curves of each study were separately calculated, then the IDL and IQL values calculated and the value of LQQ estimated for the method. These values were compared with the actual values of LLMV. The standard deviation of the spike recoveries at the LLMV (xllmv) was used to calculate the MDL and MQL. The values of LLMV were separately determined by the laboratory not using any of the methods described in this article. 

COMPARISON OF PRECISION AND ACCURACY FOR METHODS AND LABORATORIES USING THE GRAND MEAN FOR SAMPLES No. 1-3 (Collabor GM Worksheet), OR BY USING A SPIKED RECOVERY STUDY FOR SAMPLES No. 4-7 (Collabor TV Worksheet)... 

To compute the results shown in Table 34-6 for the Spiked Recovery samples, the accuracy of each set of replicates for each sample, method, and location can be individually calculated using the root mean square deviation equation as shown in equations 34-5 and 34-6 in standard symbolic and MathCad 7.0 notation, respectively. The standard deviation of each set of sample replicates yields an estimate of the accuracy for each sample, for each method, and for each location. The accuracy is calculated where each yt is an individual replicate measurement and The Spiked or true values (TV) are substituted for GM in equations 34-5 and 34-6. The accuracy is calculated for each sample, each method, and each location and N is the number of replicates for each sample, method, and location. The results found in Table 34-6 represent samples 34-4 through 34-6. Note Each sample had a True Value given by a known analyte spike into the sample. 

Table 34-8 Individual sample precision and accuracy for combined Methods A and B and Labs 1 and 2 - Spiked Recovery samples ...

Johnson and Van Emon [57] have described a quantitative enzyme based immunoassay procedure for the determination of polychlorinated biphenyls in soils and sediments and compared the results with those obtained by a gas chromatographic method. The soil is extracted with methanol, or Soxhlet extracted or extracted with a supercritical fluid. In the case of the latter two extractants good agreement was obtained between immunoassay and gas chromatographic methods. Spiking recoveries from soil achieved ranged from 104% (Aroclor 1248) to 107% (Aroclor 1242). Detection limits were 9pg kg-1 (Aroclor 1245) and 10.5pg kg-1 (Aroclor 1242). Chlorinated anisoles, benzenes or phenols did not interfere. 

Precision and accuracy Quantitative analysis by NMR is very precise with relative standard deviations for independent measurements usually much lower than 5%. The largest errors in NMR measurements are likely due to sample preparation, not the NMR method itself. If a good set of standards is available and all NMR measurements for the test and standard samples are performed under the same acquisition conditions, the quantitative results can be readily reproduced on different instruments operated by different analysts at different times. Therefore, good intermediate precision can also be achieved. An accurate quantitative NMR assay will require accurately prepared standards. The accuracy of an NMR assay can be assessed, for example, by measuring an independently prepared standard or an accurate reference sample with the assay. In many cases, a spike recovery experiment can also be used to demonstrate the accuracy of an NMR assay. 

The accuracy of an analysis can be determined by several procedures. One common method is to analyze a known sample, such as a standard solution or a quality control check standard solution that may be available commercially, or a laboratory-prepared standard solution made from a neat compound, and to compare the test results with the true values (values expected theoretically). Such samples must be subjected to all analytical steps, including sample extraction, digestion, or concentration, similar to regular samples. Alternatively, accuracy may be estimated from the recovery of a known standard solution spiked or added into the sample in which a known amount of the same substance that is to be tested is added to an aliquot of the sample, usually as a solution, prior to the analysis. The concentration of the analyte in the spiked solution of the sample is then measured. The percent spike recovery is then calculated. A correction for the bias in the analytical procedure can then be made, based on the percent spike recovery. However, in most routine analysis such bias correction is not required. Percent spike recovery may then be calculated as follows ... 

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

The accuracy of a method is defined as the closeness of the value obtained to known or accepted values. Accuracy can be determined in a number of ways, depending on the nature of the CZE method and availability of orthogonal techniques to compare results. If practical, spike recovery studies (i.e., testing to determine whether recovery matches the amount of a known analyte or impurity spiked) are good alternatives to orthogonal assay comparisons. ICH guidelines also allow method accuracy to be inferred, once specificity, linearity, and precision are established. 

Referee Laboratories and Spike Recovery Testing. Outside laboratories, with demonstrated performance records, can be used to evaluate the suitability of a candidate method when none of the other accuracy testing options is feasible. However, This technique provides a very weak form of accuracy assessment. Indeed, it provides a comparability check, not an accuracy measure. Similarly, spike recovery tests provide only weak evidence of method accuracy. Quantitative spike recovery only indicates that the added form of the analyte was recovered. If the added form responds differently toward sample preparation or detection the utility of spike recovery testing remains doubtful. 

The authors determined specificity using the known hydrolytic degradation products. The precision of spiked samples of these degradation products were determined and found to be acceptable (99.9 0.4%). Accuracy of the method was determined using spiked recoveries of piroxicam benzoate, and the recoveries were acceptable (99.1-100.5%). Assay precision n = 6, RSD = 0.4%) was in accord with recommended criteria [7]. Within-day precision was performed on two instruments on two separate days, and the overall intermediate precision was 1.0%. The method was linear over the expected analyte concentration range giving a regression line of 1 = 0.999. The detection (DL) and quantification levels (QL) were assessed, and the latter was determined as 0.185 pg/ml (ca. 0.04%). 

Accuracy. In the quantitative method that is used to measure the heavy metal quantity in the drug substance, the accuracy is usually represented by the recovery rate obtained from a spiked recovery test where lead is added to the samples. Since the heavy metals limit test specified in monograph specifications is a test where the intensity of coloring of the samples with sodium sulfide is compared with that of the control solution, it is necessary to confirm that heavy metal components can be detected fully in the process of test solution preparation. The Heavy Metals Limit Test in JP specifies four preparation methods for the test solutions. An appropriate method will be selected and used for further testing. The test method that gives the best recovery rate is to be adopted. The procedure is as follows ... 

The results of these interlaboratory studies are reported in USEPA Method Validation Studies 14 through 24 (14). The data were reduced to four statistical relationships related to the overall study 1, multilaboratory mean recovery for each sample 2, accuracy expressed as relative error or bias 3, multilaboratory standard deviation of the spike recovery for each sample and 4, multilaboratory relative standard deviation. In addition, single-analyst standard deviation and relative standard deviation were calculated. 

Cahill et al. [241] have developed a simple and sensitive analytical procedure for determining the concentration of trifluoroacetic acid in plant, soil, and water samples. The analysis involves extraction of trifluoroacetic acid by sulfuric acid and methanol followed by derivatisation to the methyl ester of trifluoroacetic acid. This is accomplished within a single vial without complex extraction procedures. The highly volatile methyl ester is then analysed using headspace gas chromatography. The spike recovery trials from all media ranged from a low of 86.7% to a high of 121.4%. The relative standard deviations were typically below 10%. The minimum detectable limit for the method was 34 ng/g for dry plant material, 0.20 ng/g for soil and 6.5 ng/1 for water. 

The matrix spike recovery may be defined in two different ways (1) one method determines the percent recovery only for the standard added to the spiked sample, as followed by U.S. EPA, and (2) the other method calculates the percent recovery for the combined unknown sample and standard. Spike recovery calculated by both these methods would give different values. 

Thus, control charts measure both the precision and accuracy of the test method. A control chart is prepared by spiking a known amount of the analyte of interest into 4 to 6 portions of reagent grade water. The recoveries are measured and the average recovery and standard deviation are calculated. In routine analysis, one sample in a batch is spiked with a known concentration of a standard and the percent spike recovery is measured. An average of 10 to 20 such recoveries are calculated and the standard deviation about this mean value is determined. The spike recoveries are plotted against the frequency of analysis or the number of days. A typical control chart is shown below in Figure 1.2.2. 

Air analysis for some of the individual pesticides of this class has been published by NIOSH. These pesticides include mevinphos, TEPP, ronnel, malathion, parathion, EPN, and demeton (NIOSH Methods 2503, 2504, 1450). In general, pesticides in air may be trapped over various filters, such as Chro-mosorb 102, cellulose ester, XAD-2, PTFE membrane (1 pm), or a glass fiber filter. The analyte(s) are extracted from the filter or the sorbent tube with toluene or any other suitable organic solvent. The extract is analyzed by GC (using a NPD or FPD) or by GC/MS. The column conditions and the characteristic ions for compound identifications are presented in the preceding section. Desorption efficiency of the solvent should be determined before the analysis by spiking a known amount of the analyte into the sorbent tube or filter and then measuring the spike recovery. 

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