Recovery, spike

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... 

Assuming that the spike recoveries are normally distributed, what is the probability that any single spike recovery will be within the accepted range ... 

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... 

A spike recovery for the analysis of chloride in well water was performed by adding 5.00 mb of a 25,000-ppm solution of Ck to a 500-mL volumetric flask and diluting to volume with the sample. Analysis of the sample and the spiked sample resulted in chloride concentrations of 183 ppm and 409 ppm, respectively. Determine the percent recovery of the spike. 

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. ... 

With a prescriptive approach to quality assessment, duplicate samples, blanks, standards, and spike recoveries are measured following a specific protocol. The result for each analysis is then compared with a single predetermined limit. If this limit is exceeded, an appropriate corrective action is taken. Prescriptive approaches to quality assurance are common for programs and laboratories subject to federal regulation. For example, the Food and Drug Administration (FDA) specifies quality assurance practices that must be followed by laboratories analyzing products regulated by the FDA. 

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. 

Construct a property control chart for the following spike recovery data (all values are for percentage of spike recovered). 

Using Control Charts for Quality Assurance Control charts play an important role in a performance-based program of quality assurance because they provide an easily interpreted picture of the statistical state of an analytical system. Quality assessment samples such as blanks, standards, and spike recoveries can be monitored with property control charts. A precision control chart can be used to monitor duplicate samples. 

Another important quality assessment tool, which provides an ongoing evaluation of an analysis, is a control chart. A control chart plots a property, such as a spike recovery, as a function of time. Results exceeding warning and control limits, or unusual patterns of data indicate that an analysis is no longer under statistical control. 

Answer the following questions if the limits for a successful spike recovery are 10%. 

The following data were obtained for the repetitive spike recoveries of field samples. ... 

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. 

Maintenance of the integrity of matrices used for tield fortification samples is of the utmost importance to the tield investigator since cross-contamination of the matrix prior to tield fortification could lead to tield spike recoveries for matrices of a questionable nature. The matrices to be used for tield fortification samples must be maintained in a pristine state prior to use in the tield. Inadvertent contamination of the tield fortitication matrices will invalidate any tield fortification samples which are prepared. Extreme care must be taken to ensure that these matrices stay free of any residue of the test substance. 

Table 5 presents data on field spike recoveries. Recoveries of chlorpyrifos from "low" spiked substrates in the California studies ranged from 62.5 to 126%, and recoveries of chlorpyrifos from "high" spiked substrates ranged from 93.7 to 133%. In the Florida, Arizona, and Michigan studies, all recoveries ("low" and "high") ranged from 61 to 158%. The field recoveries cited above were found to be reasonable and within the range of field recoveries seen in many worker exposure studies. 

Table 5 Chlorpyrifos Field Spike Recoveries, Chlorpyrifos Worker Exposure Study...

Laboratory analytical recoveries and field spike recoveries were acceptable for all substrates encountered in this series of studies. Calculated penetration factors were similar to penetration factors reported in the literature. 

Note For this hypothetical exercise, Samples 1-3 have a target value of 3.60% absolute whereas Samples 4-6 are Spiked Recovery Samples with target values of 3.40 (No. 4), 3.61 (No. 5), and 3.80 (No. 6). 

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)... 

Table 34-4 Individual sample analysis precision for hypothetical spiked recovery samples ...

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 34-6 Individual sample analysis accuracy using Spiked Recovery study...

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

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

ENTER TRUE VALUES FOR EACH ROW (SPIKED RECOVERY SAMPLES) ... 

Spiking recoveries by the above procedure carried out on standard solutions of triphenyltin chloride in various types of water ranged from 74% at the 4 xg/l tin level (relative sd 8.9%) to 93.6% at the 2 mg/1 tin level (relative sd 4.2%). 

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. 

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