ACETONE and METHYL ETHYL KETONE in urine: METHOD 8319, Issue 1, dated 28 October 2014 - Page 3 of 6
CALIBRATION AND QUALITY CONTROL: 9. Calibrate daily with at least six working standards covering the concentration range of the method (2 to 600 mg/L). NOTE: If the range of concentrations of the samples is known (or expected), the calibration curve range can be adjusted accordingly. a. Prepare a diluted stock solution by pipetting 1 mL of the concentrated stock solution into a 10-mL volumetric flask and filling to the mark with pooled urine. b. Prepare each working standard by diluting a known amount of the diluted stock solution prepared in Step 9a into enough pooled urine to make a total of 10 mL. NOTE: A second, more dilute stock solution in urine can be prepared, if desired, so that the lowest calibration standards are made using more easily-measured spiking volumes. c. Prepare at least one pooled urine blank by transferring 10 mL of pooled urine (the same pooled urine used for creating the working standards) into a vial. d. Process the 10 mL of each working standard and each pooled urine blank using the same procedure as for the samples (steps 5 through 8). e. Analyze the working standards, the pooled urine blanks, and the samples together. FIGURES 1 and 2 show representative chromatograms of blank and fortified urines. f. Prepare a calibration graph by plotting, for each working standard, the normalized analyte response (peak area of analyte divided by the peak area of the internal standard on the same chromatogram) on the y-axis vs. µg of analyte/mL of urine on the x-axis. The simplest model that adequately describes the data should be used but either a linear (mostly likely 1/x weighted because of the range of the calibration curve) or a quadratic model may be utilized in processing the analytical data. Because humans can endogenously produce both acetone and methyl ethyl ketone, the compounds may be detected in the pooled urine blanks. Before plotting the calibration graph, subtract the normalized analyte response of the pooled urine blank from the normalized analyte response of each working standard. The standard curve should have a coefficient of determination (r2) of equal to or greater than 0.98 to be acceptable for use. Furthermore, when each standard is substituted back into the calibration equation, the value should be within ±20% of the expected. 10. Prepare at least two levels of quality control (QC) samples by spiking both analytes in urine. These levels should be at approximately 10-fold the limit of quantitation (LOQ) and 200-fold the LOQ, but can be adjusted to better suit the anticipated levels of the sample set. Unspiked samples of the urine used to prepare the QC samples should be analyzed to determine the blank level and the true target level. QC samples should be analyzed with every batch such that they constitute 10% of the sample batch. 11. QC values should normally be within ±20% of the spiked values. If not, the batch is considered out of control, the batch data discarded, and corrective actions should be taken before more samples are analyzed. MEASUREMENT: 12. Set gas chromatograph according to manufacturer’s recommendations and to conditions given on p. 8319-1. 13. Set headspace autosampler according to manufacturer’s recommendations and to the following conditions: (NOTE: different types of headspace samplers may require alternative conditions and some of these might not apply.)
NIOSH Manual of Analytical Methods (NMAM), Fifth Edition
