Quality Assurance is what we do to get the right answer for our purpose FITNESS FOR PURPOSE Use objectives: state purpose for which results will be used. Specifications: How good should the numbers be and what precautions are required in the procedure? 1. Sampling Representative Samples 2. False Positives and False Negatives 3. Choose method properly: a. Selectivity to be able to distinguish analyte from other species in the sample b. Sensitivity is the capability to respond reliably and measurably to changes in analyte concentrations c. Reference Materials, Standards, Blanks and Recovery 4.Performance test samples for method validation. Assessment: process of collecting data to make sure that specifications are achieved (Control Charting)
Quality Assurance is what we do to get the right answer for our purpose FITNESS FOR PURPOSE Reference Materials SRM Blanks 1. Method Blank 2. Reagent Blank 3. Field Blank Spike Recovery. A known amount of analyte is added to the sample matrix to test if the response to a sample is the same as expected from a calibration curve. (Good recovery is usually 90% 110%) %recovery = C spiked sample C unspiked sample C added
When one develops a new method, one needs to validate it and declare several properties and characteristics of the said method: 1. Selectivity 2. Accuracy use of Primary standards and SRMs 3. Precision Evaluation of SDs and Unvertainties 4. Linearity Linearity of the response vs. concentration 5. Range Linearity, accuracy and precision are acceptable 6. Robustness Is not affected by small changes in conditions (solvent, ph, temperature, instrument) 7. Limit of Detection Minimum amount of analyte which can be distinguished from zero. 8. Limit of Quantification Minimum amount of analyte which can be accurately quantified.
1. External Calibration 2. Standard Addition 3. Internal Standards y = m x + b where y is signal, x is concentration
1. External Calibration The ferric oxide contaminant in a solution was determined by evaporating the water in a 250 ml sample to near dryness and treating the residue with 5 ml HCl. The treated sample was transferred quantitatively to a 50.00 ml volumetric flask. Before dilution to volume, 5 ml EDTA solution was added. The following were the obtained absorbances for a three replicate run and the calibration standards (the conc are in µg/ml Fe) Samples: blk 0.203; A 0.355; B 0.356; C 0.359 Calibration Standards: blk 0.201; 0.10 ppm 0.292; 0.20 ppm 0.378; 0.30 ppm 0.467; 0.40 ppm 0.554
1. External Calibration Samples: blk 0.203; A 0.355; B 0.356; C 0.359 Calibration Standards: blk 0.201; 0.10 ppm 0.292; 0.20 ppm 0.378; 0.30 ppm 0.467; 0.40 ppm 0.554 a. What is the regression coefficient (r)? b. What is the average concentration of Fe 2 O 3 in the sample? Evaluate the precision of the method (in relative %). c. A quality control sample was prepared by adding 0.10 ppm Fe into an aliquot of the sample. Calculate the % recovery of the spike if the absorbance was 0.447.
2. Standard Addition + Std 1 Unknown Signal 1 Unknown Signal 0 [Unknown] [Unknown]+ [Standard] = Signal 0 Signal 1
2. Standard Addition + Std 3 Unknown Signal 3 + Std 2 Unknown Signal 2 + Std 1 Unknown Signal 1 Unknown Signal 0
2. Standard Addition Example. Successive Standard Additions of 1.00 ml of 25.0 mm ascorbic acid were made to 50.0 ml of orange juice. Prepare a calibration curve to find the concentration of ascorbic acid in the orange juice. TOTAL Volume of added standard Detector Response 0.00 1.66 1.00 2.03 2.00 2.39 3.00 2.79 4.00 3.16 5.00 3.51
2. Standard Addition Detctor Response vs added Volume 4 y = 0.3726x + 1.6586 R² = 0.99974 3 2 1 0 6.00 4.00 2.00 0.00 2.00 4.00 6.00 1
2. Standard Addition The Chromium in an aqueous solution was determined by pipetting 10.00 ml of the unknown into each of the 50.00 ml volumetric flasks. Different volumes of stock standard containing 12.2 ppm Cr were added to each of the flasks. The following were the obtained absorbances: Unknown, ml Stock, ml Absorbance 10.00 0.00 0.201 10.00 5.00 0.292 10.00 10.00 0.378 10.00 15.00 0.467 10.00 20.00 0.554
3. Internal Standard C Unknown + C IS Signal Unknwon Signal IS C Known + C IS Signal Known Signal IS Signal of Analyte Concentration of Analyte = F Signal of IS Concentration of IS
3. Internal Standard Example. A mixture of 52.4 nm analyte (X) and 38.9 nm standard (S) gave the relative response of 0.644/1.00. A second solution containing an unknown quantity of X plus 742 nm S had a relative response of 1.093/1.000. Find [X] in the unknown
3. Internal Standard A hair sample is to be analyzed for the marijuana metabolite, 9 hydroxycannobinol. A 0.1432 g hair is mixed with chloroform to extract the metabolite. The extract is mixed with tridecane (1.0 ppm final concentration) internal standard and is injected into the gas chromatograph. What is the amount of 9 hydroxy cannobinol in the original hair sample?
3. Internal Standard Std Conc IS Conc Area Area IS (ppm) (ppm) Methamphetamine 0.00 1.00 0 5735 0.25 1.00 12657 5756 0.50 1.00 24007 5693 1.00 1.00 48963 5715 1.50 1.00 73275 5680 Area Area IS Methamphetamine Sample 43756 5831