Internal quality control activities.pptx

Control activities

1. Internal quality control activities

2. Introduction INTERNAL QUALITY control activities are a powerful tool to increase the competitiveness of the laboratory Organization Infrastructures Equipment Analytical methods Periodical revisions Preventive and corrective actions IT IS GENERALLY ACCEPTED THAT THIS ACTIVITIES ARE BEING WELL PERFORMED ( but supervising them is not enough ) Internal quality control activities are required INTERNAL QUALITY CONTROL “Set of procedures performed by the laboratory staff to conitnuously control the measurement results , to decide whether these measurements are reliable or not ” Accuray and precision can be assessed using the internal quality control

3. Introduction It is based on statistical procedures to predict changes in the analytical system Therefore , REQUIREMENTS FOR THE INTERNAL QUALITY CONTROL ACCORDING ISO 17025:2005 Use of controls , certified reference materials ( pure substances or matrix ) Repetitions of analysis using the same method or an alternative method Repetition of the analysis of samples previously analyzed Verification of the results obtained for different parameters of an analyzed sample

4. ANALYSIS OF BLANK SAMPLES ANALYSIS OF CONTROL SAMPLES ANALYSIS OF BLIND SAMPLES CORRELATION OF THE RESULTS WITH THOSE OBTAINED FOR OTHER PARAMETERS OF THE SAME SAMPLE They consist basically in: Internal quality control activities

5. 1. ANALYSIS OF BLANK SAMPLES AIM : “ To detect errors due to positive interferences ” ( errors from negative interferences cannot be detected using this procedure ) Different types of blanks can be used : Reagent blank : we obtain information about the contribution of the reagents to the analytical signal Procedure blank : It includes the contribution of all the components of the analytical signal Matrix blank : It includes the contribution of the sample matrix ( we need an identical matrix without analyte ) THESE SAMPLES ARE USED: when reagents are not pure enough Internal quality control activities

6. 2. ANALYSIS OF CONTROL SAMPLES It consists in the periodic analysis of samples that contain the analyte For routine analysis , these results are shown in a control chart If the results have not been obtained in routine analysis , the t test must be used to compared them with other previously obtained . Use of certified reference materials ( Requirements ) Availability of CRM during the verification period The homogeneity of the material needs to be very high to take aliquots of it Stable enough ( according to that stated in the certificate ) DRAWBACKS: high cost , unlimited amount Use of control samples : The are often samples of a known content , prepared by the laboratory staff , but they cannot be considered as certified reference materials Use of synthetic samples : they mimic the composition of natural samples . Theu are usually mixtures of standards Internal quality control activities

7. 3. ANALYSIS OF BLIND SAMPLES Internal quality control activities It consists of the analysis of samples of known content ( not at the moment of the analysis , which are processed on a routina basis together with other samples Blind samples : The staff knows that they are blind samples Double blind samples : laboratory staff does not know that they are analyzing blind samples The comparison with the reference value using the t-test 4. CORRELATION OF RESULTS BY COMPARING THE VALUES FOR DIFFERENT PARAMETERS OF THE SAME SAMPLE Eg .: Content of metabolites of ethanol

8. PERFORMANCE OF MEASUREMENTS TO FIND OUT THE FUNCTIONING OF THE SYSTEM Mean value Control limits CONTROL OF THE ANALYTICAL SYSTEM Without detection of outliers ( Type b e rror) Wrong detection of points out of control ( Type a error) If limits are too small there is a high probability of type a errors If limits are wide there is a high probability of type b errors NUMBER OF SAMPLES USED TO DEFINE THE CONTROL CHART FOR A SPECIFIC ANALYTICAL SYSTEM ( duplicate analysis of 20-30 samples ) Use of plots where the x-axis is the time so that the evolution of the analytical system is monitored Control charts BUILDING A CONTROL CHART

9. CENTRAL VALUE CRM: the certified value No CRM: The mean value of the series VARIANCE Range : The most suitable parameter for duplicate measurements is the mean range (R) _ Mean variance Control charts BUILDING A CONTROL CHART

10. CENTRAL VALUE FOR BLANK SAMPLES Selection of representative blanks Analyte blanks ( zero concentration of analyte ) at the beginning and at the end of each session Plotting the measurement of blank performed (a previous conversion to concentration units is not needed ) Control charts BUILDING A CONTROL CHART

11. Control charts INTERPRETATION OF CONTROL CHARTS FOR MEAN VALUES They can be also used when extraction procedures are involved to check the efficiency of the extraction by using control samples Based on either Western Electric or the Westgard rules Usually, some actions have to be done on the system when: 1 point is out of control limits 2 (2 out of 3) subsequent points are out of warning limits 7, (8, 9,10) de 11 subsequent points are on the same side of the central value 6 (7) subsequent values with increasing and decreasing tendencies 4 (8) subsequent values at both sides of the central line close to the warning limits

12. ESTIMATION OF THE DISPERSION OF DATA The use of CRM is not needed, only stable and homogeneous samples CONTROL CHARTS WITH MEASUREMENTS OF STANDARD DEVIATION Central value : standard deviation obtained from the variances of the analyzed groups Limits : CONTROL CHARTS WITH MEASUREMENTS OF THE RANGE Central value : Mean range (R) of the ranges of every group Limits : Obtained at significance levels of 0,05 y 0,003 which are obtined by multiplying the mean value by tabulated D factors Control charts

13. They are more sensitive than Shewhart control charts to detect out of control situations Control charts BUILDING A CONTROL CHART CUMULATIVE SUM CONTROL CHARTS (CUSUM)

14. BUILDING A CONTROL CHART Control charts CUMULATIVE SUM CONTROL CHARTS (CUSUM) It is necessary to establish the scale for the control chart Every division corresponds to

15. Recomendación de la IUPAC (Wood et al., 1995) Type of sample Control of accuracy Control of precision Blanks Short series (n<20) Analysis performed frequently Similar sample matrix and concentration levels To analyze the control sample once To analyze half of the samples using duplicates To analyze a blank in each series Long series (n>20) Analysis performed frequently Similar sample matrix and concentration levels To analyze a control sample out of 10 To analyze at least five samples in duplicates To analyze a blank every 10 samples Short or long series Analysis performed frequently Wide range of concentration assayed To analyze two controls ( one at low concentrations and another at high ones ) To analyze at least five samples as duplicates To analyze a blank every 10 samples Ad-hoc analysis It depends on the nature of the sample All samples will be analyzed as duplicate To analyze blanks Practical aspects of the internal quality control

16. When out of control situations happen , we should : Cancel the delivery of results from the series of analysis out of control Assess which series could have been affected Find the causes that have originated the error Correct the problem by changing the variables involved and documenting the changes properly Analyze the samples affected by using control samples Practical aspects of the internal quality control

17. The results could be provided if : Practical aspects of the internal quality control The problem which was originally detected corresponded to a control sample The problem has only affected to control samples The problem only affects to the samples in a certain concentration interval

18. LITERATURE Quality Assurance in Analytical Chemistry . E. Prichard, V. Barwick , Wiley, 2007 M. Thompson, R. Wood, Harmonized guidelines for internal quality control in analytical chemistry laboratories . Pure & Appl. Chem., Vol. 67, No. 4, pp. 649-666, 1995