Organic Compounds In Aqueous Samples Research Articles

Selective removal of water in purge and cold‐trap capmary gas chromatographic analysis of volatile organic traces in aqueous samples

AbstractThe design and features of an on‐line purge and cold‐trap pre‐concentration device for rapid analysis of volatile organic compounds in aqueous samples are discussed. Excessive water is removed from the purge gas by a condenser or a water permeable membrane in order to avoid blocking of the capillary cold‐trap. Synthetic mixtures covering concentrations ranging from tenths to tens of ppb's and different chemical classes are used to study the effect of various process factors on the efficiency and selectivity of water removal as well as on the purging recovery. The importance of the concentration of the solutes, the flow rate in conjunction with the volume of the purge gas, and the temperature of the condenser, the cold‐trap and the sample is emphasized. Theoretical models describing the purge process and the blocking of the cold‐trap agree fairly well with the highly reproducible experimental results (σ = 2–4%). Both the condenser and the Nafion membrane successfully remove water, although some compounds, dependent on volatility and polarity, are partly or completely lost. It is shown that non‐polar volatile organic compounds are efficiently enriched so that recoveries between 80–100% and a detection limit of 1 ppt can be obtained. The applicability of the system is illustrated on some examples.

Determination of volatile organic compounds in aqueous systems by membrane inlet mass spectrometry

The potential of a silicone rubber membrane, a simple mass spectrometry inlet system, for the direct determination of volatile organic compounds in aqueous samples has been re-examined. Greatest sensitivity (μg l −1 level) was found for volatile insoluble compounds, while decrease in volatility or increase in solubility, or both, appeared to reduce sensitivity. A novel correlation was demonstrated between the air-water partition coefficient for a compound and the molar enrichment factor for its transfer through the silicone membrane from aqueous solution. This relationship allows the prediction of the likely sensitivity of a compound to measurement by this technique from its physico-chemical properties. Some evidence was also obtained that elevated sample temperatures extend the range of compounds which can be readily determined.