Standard Preparation
Attach a gas cylinder of 10% nitrous oxide in nitrogen fitted with a flow control valve via a short length of neoprene tubing to a glass/PTFE sampling bulb fitted with a PTFE-backed rubber septum. Flow the 10% nitrous oxide through the sampling bulb for several minutes to purge traces of air. To prepare Calibration Standards, add 2 mL of DI water to a 12 mL Head Space Sampling Vial and cap with an inert PTFE/silicone rubber closure and crimp the aluminum closure cap. Add a known amount of the nitrous oxide mixture into the vial through the septum via gas-tight syringe. Predetermine the amount of headspace in the vial to determine the concentration of Nitrous Oxide in the headspace. Analyze right away to reduce loss.
SAMPLE PREPARATION
Note the badge serial and lot number on the back of the badge. Using a dissection tool or other small metal tool that will fit into one of the sampling holes, pop the sampling grid off. Add the molecular sieve pellets to a Head Space Sampling Vial (12 ml volume, typical), then quickly add 2.0 ml of DI Water to the vial. Immediately cap with an inert PTFE/silicone rubber closure and crimp the aluminum closure cap. Shake the Head Space Vial for several seconds and allow to stand for 10 minutes. Analyze immediately.
BLANK PREPARATION
Analyze an unexposed Monitor from the same manufacturing Lot as the Monitors being analyzed. Process the unexposed "BLANK" Monitor exactly as the Sample Preparations (above) and subtract the amount of the blank from the value obtained from the sample of the same lot. Report any significant BLANK values to the Quality Assurance group along with the Lot number of the Monitor analyzed.
QUALITY CONTROL SAMPLES
Due to the volatile nature of nitrous oxide, once the samples are prepared, they need to be analyzed right after sample preparation. So check all QC results against established acceptance criteria to make sure the analysis is in control before proceeding with the sample preparation and analysis.
At the beginning of each analytical batch, prepare two Lab Control Samples (LCS). To prepare an LCS, empty the molecular sieves from one unexposed Monitor into a head space sampling vial. Do not add water. Immediately cap with a inert PTFE/silicone rubber closure and crimp the aluminum closure cap. Add a know amount of the standard using a gas tight syringe and turn the vial upside down for 30 minutes. By that time the Nitrous Oxide will have been adsorbed by the molecular sieves. Turn the vial right side up and use pliers to remove the cap. Prepare like a normal sample.
Every ten shots prepare a Continuing Calibration Standard and verify the result is within acceptance limits.
CAPILLARY GAS CHROMATOGRAPHY(GC) ANALYSIS
Inject an aliquot of the Sample Preparation from each Monitor to be analyzed into a Gas Chromatrography System with Electron Capture Detector (GC/ECD) using the following conditions.
GC Column-Aluminum Oxide Porous Layer Open Tube (PLOT) Column
Column Size - 0.53 mm capillary x 30 meter
Injection Mode - Direct
Injector/Detector Temp - 200oC(inj)...250oC(det)
Column Temp - 35oC Isothermal
Injection Volume - 50.0 microliter (nominal)
CALCULATION
Calculate Exposure Level from Analyte Concentration as follows.
EXPOSURE LEVEL, ppm = ( VH X C ) / ( DE X SR X T )
Where
C = N2O Concentration of Sample Preparation (ppm v/v)
VH = Head Space Volume(ml) (10 ml accurately determined)
DE = De-Sorption Efficiency(fraction of extractn) (0.70)
SR = Monitor Sampling Rate (ml/min) (0.75)
T = Sampling Time (min)