GAS STANDARDS CONTAINING HALOGENATED COMPOUNDS FOR ATMOSPHERIC MEASUREMENTS

A microgravimetric procedure was previously developed for the preparat ion of accurate cylinder gas standards for volatile organic compounds in the nanomole/mole (10(-9) mol/mol; parts per billion, ppb) range. T he present study evaluated the use of this procedure for preparing suc h standards in the picomole/mole (10(-120) mol/mol; parts per trillion , ppt) range with respect to accuracy and stability. The resulting gas standards were intercompared using gas chromatography with electron c apture detection (GC-ECD). Regression analysis of the GC-ECD responses versus the corresponding gravimetric concentrations showed excellent agreement among groups of the standards for each compound. The standar ds were evaluated over a period of almost 3 years and were found to ex hibit continued stability and accuracy. These primary gas standards we re used to accurately characterize the concentration of halogenated vo latile organic compounds at the ppt level in two different batches of compressed gas cylinder mixtures prepared commercially. The results sh owed small but real differences in the concentrations of each compound between different cylinders in a given mixture, demonstrating the dif ficulty in preparing a batch of cylinders containing an identical mixt ure at extremely low concentrations.