GENERATION AND CALIBRATION OF STANDARD GAS MIXTURES OR VOLATILE FATTY ACIDS USING PERMEATION TUBES AND SOLID-PHASE MICROEXTRACTION

Abstract

Volatile fatty acids (VFAs) are major components of odors associated with agricultural operations and livestockhousing, solid waste processing and disposal, industrial and municipal wastewater collection, and treatment systems.Emission estimation and assessment of odor control methods depend on reliable air sampling and analysis methods for VFAs.The objective of this research was to develop and test a method for continuous and reliable generation of standard gas mixturesfor VFAs based on permeation tubes. Standard gas mixtures for acetic, propionic, isobutyric, butyric, isovaleric, valeric, andhexanoic acids were generated with permeation tubes and monitored for a 100-day period. The gravimetric loss of VFAs fromeach tube was measured periodically and used to calculate the emission rate for each permeation tube. Emission rates wereas high as 2,011 ng/(min cm) for acetic acid to as low as 49 ng/(min cm) for isovaleric and hexanoic acids. The emission ratewas combined with the dilution flow rate to calculate standard concentrations. Five different concentrations for each VFAwere obtained by adjusting the dilution flow rate. Gas concentrations were monitored with DVB/Carboxen/PDMS 50/30 .mSPME fibers using triplicate 1 min extractions. Maximum standard gas concentrations ranged from 21.9 ppmv for acetic acidto 0.22 ppmv for hexanoic acid. Minimum standard gas concentrations ranged from 3.0 ppmv for acetic acid to 0.03 ppmvfor hexanoic acid. The relative standard deviations (RSDs) for all VFA concentrations ranged from .1.6% to .7.8%. Rapid(1 min) SPME extractions were sufficient to preconcentrate significant amounts of VFAs for separation on a GC-FID withoutderivatization. The SPME technology proved to be very useful for monitoring standard gas mixtures. Dilution gas flow ratedid not affect the emission rates from permeation tubes. In contrast, low acid levels affected permeation rates of the acidsfrom the tubes. The methodology described in this article could be used to generate and test standard gas mixtures for otherodorous gases.