Page:NIOSH Manual of Analytical Methods - 3702.pdf/3

 ETHYLENE OXIDE: METHOD 3702, Issue 2, dated 15 August 1994 - Page 3 of 4 CALIBRATION AND QUALITY CONTROL: 2.

Calibrate the GC daily in the field. a. Prepare bag standards by adding a known volume of EtO to a known volume of clean air in a bag. This creates a standard of known concentration in ppm (µL EtO/L air). (1) Evacuate a 5- to 10-L bag completely by drawing the air out with a large (1- to 2-L) syringe. (2) Draw clean air (or oxygen or nitrogen) from a supply cylinder into the syringe for measured transfer into the bag. Alternately, if a clean air supply is not available, draw room air through charcoal sorbent into the syringe. Repeat until the bag contains 5 L of air. (3) Add a known amount of pure EtO or standard EtO mixture to the bag by means of gas-tight syringe. Example : Using a gas-tight syringe, take 50 µL from a cylinder of pure EtO and inject it into 5 L of air to create a 10 ppm standard. Alternately, 200 µL of a 27% v/v EtO mixture (i.e., 88/12 w/w Freon 12 and EtO) can be added to the 5 L of air to obtain a 10.8 ppm standard. (4) Allow the bag to equilibrate, with occasional kneading, for at least 5 min. b. Analyze aliquots of various sizes to establish a calibration graph (steps 4 and 5). Analyze three or more replicates at each point. NOTE: On a high instrument attenuation (low sensitivity), injections of 0.2, 0.4, 0.6, 0.8 and 1.0 mL might be possible. This would correspond to injections of 2, 4, 6, 8, and 10 nL. On a more sensitive attenuation, injections of 0.02, 0.04, 0.06, 0.08 and 0.10 mL would be typical. Results will vary from instrument to instrument, and from time to time on the same instrument. c. Plot nL EtO vs. peak height or area. This plot should be a straight line. d. Periodically throughout the day, check calibration by repeating some of these injections of standards. Ideally, each sample would be bracketed, before and after, with injections of standards, although this is seldom practical. 3. Check the sampling equipment to prevent contamination. a. Use different syringes for sampling and for standard preparation. Identify each syringe with a unique number. NOTE 1: So called gas-tight syringes appear to be so only when new. Leakage results in injections less than indicated, resulting in inaccurate reporting of the actual concentration. NOTE 2: Even after more than a dozen purges, a syringe used to transfer pure ethylene oxide continues to elute small amounts of EtO. b. Bag sampling. (1) Check inertness and impermeability of the entire sampling train. This is best accomplished in the laboratory before going to the field. Purge a bag of the type to be used for sampling with clean air, and then fill it with clean air and an amount of EtO to create a concentration in the range of interest. Take replicate samples from that bag and inject them into the GC. Measure peak heights. Pump the contents of this bag through a sampling train of the type to be used in the field, into a second bag. Analyze replicate samples from the second bag immediately after transfer and hourly thereafter. If the concentrations found immediately after transfer are equal to those prior to transfer, the sampling train is not altering the EtO concentration (i.e., is inert). If the concentrations do not change over time, the bags are inert. (2) Consider any residual analyte which may remain in a bag after use, since practicality dictates reuse of bags. One or two purges with clean air are usually sufficient to remove residual EtO unless the concentration of the previous sample was extremely high. Analyze a sample of "clean" air taken from the bag during its final purge. If no measurable EtO is found, the bag is ready for reuse. NOTE: Some bags continue to show traces of analyte, even after several purges, and NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94