Characterization of a pressurized C5-C16 hydrocarbon gas calibration standard for air analysis

A compressed gas standard containing parts-per-billion (ppb) amounts of the volatile hydrocarbons methylpentadiene (isoprene, 540 ppb) and isooctane (259 ppb) and a series of less volatile C12-C16 n-alkanes (n-dodecane, 349 ppb; n-tridecane, 340 ppb; n-tetradecane, 202 ppb; n-pentadecane, 271 ppb; and n-hexadecane, 308 ppb) was prepared by a one-step (no further gas dilution) microgravimetric method. The gravimetric mixing ratios were confirmed by referencing to a capillary diffusion method. The cylinder was heated to 75 degrees C to minimize condensation losses of analytes to the cylinder walls. Mixing ratios were monitored over a 2.5-year period. Some initial analyte losses (approximately 3 to 20%) were observed for the heavier C14-C16 n-alkanes. Subsequently, analyte loss rates were found to be in the range of <1.0% per year for n-dodecane to n-hexadecane. The developed guidelines for preparation, storage, and retrieval of these semivolatile analytes enable the use of compressed gas standards for calibration and method development purposes in the environmental gas-phase analysis of these and related compounds.