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Main Entry: quality control
Function: noun
: an aggregate of activities (as design analysis and inspection for defects) designed to ensure adequate quality especially in manufactured products
Quality control measure must be instituted for every sample matrix to confirm preparatory method performance. Although standard operating procedures governing ASC method development and verification require investigation into preparatory and analytical interferences, it is not realistic to identify all interferences involved in real world samples. The Standard quality assurance for ASC projects requires that each batch of 20 samples or every different matrix type be accompanied by one set of quality control parameters. A set of quality control parameters includes: four preparation blanks, one blank spike, one certified reference material (similar in matrix to the associated samples), one matrix duplicate, and one matrix spike and matrix spike duplicate. Most analyses are also accompanied by analytical duplicates and analytical spikes for additional information to account for analytical, not preparatory, interferences. Analytical duplicates and spikes should never be confused with matrix duplicates and spikes as they are a focused quality control measure for the analytical method only. ASC may also include a method of standard addition curve to more specifically identify biases associated with specific matrices.
All quality control measures not complying with the associated standard operating procedure must comply with corrective action measures. ASC does not believe that a singular quality control measure that does not meet the quality control objective necessitates reanalysis if it does not negatively impact the validity of the results. All clients are informed of any variances encountered in the preparatory or analytical procedures and the reasoning for the actions taken.
Common Quality Control Parameters
Preparation blanks are reagent water samples generated by the laboratory that are included in all sample preparatory procedures. Preparation blanks identify the background concentration for the preparatory method as well as the inherent variability at low levels. Preparation blanks also assist in the identification of the limit of detection for any given method. As variability increases, limits of detection will naturally increase as the confidence for reporting below the background concentration is compromised. Method detection limits are often represented by three times the standard deviation of the preparation blanks.
Applied Speciation and Consulting, LLC recognized both performance based method detection limits and 40CFR based reporting limits. Performance based detection limits conform to equations 1-3:
(1) Variance = ( (PB01 -PBavg )2 + (PB02 -PBavg )2 + (PB03 -PBavg )2 + ··· + (PBn -PBavg )2 )/(n-1)
(2) Standard deviation (s) = (variance)1/2
(3) Estimated Method Detection Limit (eMDL) = 3 * s
PB = Preparation Blank Concentration
avg = Average
Method 40CFR allows more latitude when generating method detection limits; however, it should be noted that method detection limits generated under 40CFR are not necessarily representative of instrument performance at the time of sample analysis. The method detection limit, in accordance with 40CFR is generated by 7 replicate analysis of either a known spiked standard or reagent water. The MDL is generated by multiplying the student's “t” value at a 99% confidence level with the standard deviation of the replicate results. In accordance with 40CFR, the method detection limit is only generated for analysis of different matrix types or if instrument performance varies significantly (check samples identify that the instrument can no longer detect within 10 times the method detection limit at any confidence level). It should also be noted that 40CFR specifies blank correction be applied to known spiked standard recoveries which may not comply with specific contracts.
Performance based detection limits can supply a better representation of instrument performance at the time of sample analysis. Generation of method detection limits using the 40CFR method can produce generalized accounts of the instrument's ability with given matrices but may not represent fluctuations in instrument performance over time.
Blank spikes (BS), or laboratory control samples (LCS), identify the performance of the preparation method on a clean matrix void of interferences. If a preparation method is not capable of extracting the analytes of interest from reagent water and solubilizing them for analysis the probability of acceptable efficiency on real world samples is minimal. The recovery for blank spikes is determined by the following equation: Recovery (%) = 100 * (measured concentration/certified concentration) |
Certified reference materials (CRMs) are standards rigorously tested by an outside source that represent a specific matrix type. Each batch of samples, or every different matrix type, must be accompanied by a certified reference material (if available) of similar constitution. For instance, the use of a soil CRM is not appropriate for determination of trace metals in tissues. The CRM sample accompanies the batch of samples through the preparatory and analytical procedure. The importance of CRMs cannot be understated as their recoveries represent the performance of the methods on real world samples. Low recoveries of CRMs often identify poor efficiency of the preparatory method to extract and/or solubilize the analytes of interest. It should be noted that CRMs are not available for all analytes and matrices, especially for trace metals speciation. Applied Speciation and Consulting, LLC always participates in intercomparison studies that generate these CRMs. |
Matrix duplicates (MD) identify the precision of the preparatory and analytical procedure. Precision is defined as the closeness of agreement between independent test results obtained under stipulated conditions (typically represented in the form of relative standard deviation). Matrix duplicates are sub-samples of a homogenous sample. Each matrix duplicate should accompany the initial sample throughout the preparatory and analytical process. Results for matrix duplicates should be identical to the initial sample. Poor precision for a matrix duplicate may denote heterogeneity or preparatory issues. Field duplicates are not representative of laboratory precision; rather, they represent field homogeneity and sample collection performance. ASC cannot guarantee comparability of field duplicates as they are a field quality objective. Precision for matrix duplicates conform to the equation stipulated below: Relative Percent Difference (RPD) = |(M-MD)/avg| * 100 where M and MD are the concentrations of the respective duplicates and avg is the average of M and MD results Larger projects may necessitate the analysis of multiple sets of matrix duplicates for a better representation of precision. For this instance, statistical analysis can be performed on the group of data in accordance with the preceding equation as defined below: s = [Σ x'(x-x')2 / Σ x']1/2 x = initial sample concentration x' = duplicate sample concentration |
The purpose of matrix spikes (MS) and matrix spike duplicates (MSD) is to identify method performance and precision. Matrix spikes are generated by the addition of a known amount of target analyte to a sub-sample. Unless the added target analyte is infused within a similar matrix, the ability of the matrix spike to represent method performance is limited; rather, matrix spikes often assist in the identification on chemical interferences inherent in the matrix. The efficiency of any method to dissolute an aqueous standard solution will always be significantly greater than a real world sample. With this in mind, ASC uses certified reference materials as an indication of method performance more than matrix spikes. Matrix spike duplicates serve the same purpose as matrix duplicates with the added complexity of an increase in concentration. Matrix spike and matrix spike duplicate accuracy and precision values are calculated by the following equations. Recovery = 100 * (measured concentration/certified concentration) (%) Relative Percent Difference (RPD) = |(MS-MSD)/avg| * 100 where MS and MSD are the concentrations of the respective duplicates and avg is the average of MS and MSD results |
The purpose of matrix spikes (MS) and matrix spike duplicates (MSD) is to identify method performance and precision. Matrix spikes are generated by the addition of a known amount of target analyte to a sub-sample. Unless the added target analyte is infused within a similar matrix, the ability of the matrix spike to represent method performance is limited; rather, matrix spikes often assist in the identification on chemical interferences inherent in the matrix. The efficiency of any method to dissolute an aqueous standard solution will always be significantly greater than a real world sample. With this in mind, ASC uses certified reference materials as an indication of method performance more than matrix spikes. Matrix spike duplicates serve the same purpose as matrix duplicates with the added complexity of an increase in concentration. Matrix spike and matrix spike duplicate accuracy and precision values are calculated by the following equations. Recovery = 100 * (measured concentration/certified concentration) (%) Relative Percent Difference (RPD) = |(A-AD)/avg| * 100 where A and AD are the concentrations of the respective duplicates and avg is the average of A and AD results |
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