Laboratory fortified blank

With each batch of samples processed, one laboratory fortified blank should be analyzed. 

The usual QC sample program for routinely analyzed sample sets consists of laboratory blanks, laboratory-fortified (or spiked) blanks, and matrix spikes (in EPA terminology), as described in Section 11.2.9. The laboratory blank is a sample prepared with laboratory water. The laboratory-fortified blank is laboratory water with a known amount of radioactive tracer added. This tracer usually is the same radionuclide that is being analyzed. The matrix spike is a replicate of an actual sample to which a known amount of radioactive tracer has been added. At selected intervals, all are processed in the laboratory and counted exactly as are the routinely analyzed samples. 

Laboratory-fortified blanks and matrix spikes both test the analyst s ability to obtain the expected result. The extent to which the net radionuclide concentration of the fortified blank (corrected for yield and radioactive decay) deviates from the expected value for the tracer radionuclide concentration is a measure of analytical bias. Any consistent deviation from the expected value should be investigated to eliminate the cause. Typical causes are the wrong counting efficiency, an analytical problem with interchange between carrier and tracer, unreliable yield determination, or erroneous tracer radionuclide concentration. 

Accumulated results from laboratory-fortified blanks and matrix spikes are a measure of both precision and bias. To determine precision, numerous replicate values can be examined and the standard deviation can be calculated. The mean value is compared to the expected tracer concentration to determine bias. For this application, the results collected at different periods must be seen to belong to the same set, i.e., there is no obvious temporal difference among measurements due to analytical or measurement problems. The calculated standard deviation value can be compared to the suitably propagated components for counting and for the rest of the analytical process. Causes of unexpectedly large or small values of the standard deviation should be examined. 

Prepare four replicate fortified blanks consisting of noncontaminated dialysis water, urine, and/or blood spiked at 50 and 500 pg/L with the target compounds using a concentrated stock solution. The methanolic standard stock solution can be prepared in the laboratory from pure compounds as described in Section 29.2.1. Analyze the fortified blanks according to the procedure described in this unit. The mean value should be within a range of 70%-130% of the true value, and %RSD (relative standard deviation) should be <20%. 

Matrix-fortified laboratory blanks consisting of solvent and reagent blanks to determine levels of bias due to matrix effects or analytical method problems. 

CCa is obtained by analyzing at least 20 blank materials per matrix fortified with the analyte(s) at the permitted limit. The concentration at the permitted limit plus 1.64 times the corresponding standard deviation equals the decision limit (a = 5 percent). CC/3 is obtained by analyzing at least 20 blank materials per matrix fortified with the analyte(s) at the decision limit. The value of the decision limit plus 1.64 times the standard deviation of the within-laboratory reproducibility of the measured content equals the detection capability (/3 = 5 percent). 

Standard ISO/IEC 17025 2005 requires a laboratory to have quality control procedures for monitoring the validity of tests and calibrations undertaken. This means that laboratories must perform internal performance-based quality control checks in accordance with Section 5.9 of ISO/IEC 17025 2005 as it applies to every test, technology, and/or parameter within their scope(s) of accreditation in order to demonstrate compliance with accreditation requirements. Reference or fortified material containing known amounts of analyte, at or near the permitted limit or the decision limit (a non-compliant control sample) as well as compliant control materials and reagent blanks should preferably be carried through the entire procedure simultaneously with each batch of test samples analyzed. Ideally, the control samples should also be very similar to test samples and stable over time. The laboratory should maintain a sufficient amount of control material to last for a significant time period (preferably a number of years) and at suitable analyte concentrations. 

I. Validation package from the originating laboratory. All analytical data submitted should represent five replicates. These should include analyses of five controls (blank matrices, preferably from different sources to check for relative matrix interferences and possibly matrix effects, Section 5.3.6a), analyses of fortified controls (spiked blank matrix) at the regulatory limit specified for the drug... 

---