A gas-phase biosensor for environmental monitoring of formic acid: laboratory and field validation.
In order to encourage more exposure measurements to be performed, a formic acid gas-phase biosensor has been developed for this purpose. In the present paper, an enzyme based biosensor has been validated with respect to analyte selectivity and on-site use. To ensure that the sampler developed measures the compound of interest the biosensor was exposed to three near structural homologues to formic acid, i.e. acetic acid, methanol and formaldehyde. These vapours were generated with and without formic acid and the only compound that was found to have an effect on the performance of the biosensor, albeit a small one, was acetic acid. The field test was performed in a factory using formic acid-containing glue for glulam products. In parallel to the measurements with the biosensor a well defined reference method was used for sampling and analysing formic acid. It was found that the biosensor worked satisfactorily in this environment when used in a stationary position. It was also shown that the biosensor could determine formic acid vapour concentrations down to 0.03 mg m(-3).
Amperometric enzyme electrodes for the determination of volatile alcohols in the headspace above fruit and vegetable juices
AbstractWe have investigated two amperometric biosensors for the determination of volatile alcohols in the headspace of fruit juices and vegetable juices. One type of sensor is based on the use of alcohol dehydrogenase (ADH) and the detection of NADH (at +300 mV vs. Ag/AgCl with phenothiazine as redox mediator), and the other on the use of alcohol oxidase (AOx) and the detection of hydrogen peroxide (at +600 mV vs. Ag/AgCl). Samples were analyzed with the AOx-based biosensor by measuring the alcohol concentration in the headspace above the liquid without prior sorption or pre-concentration. The sensor has a linear response in the range 0.1–20.0 mM of alcohol (referred to the concentration in the liquid sample). It has excellent stability in that the signal decreases by 4.5 % only over a 60 h operational period. However, a comparison of the AOx-based biosensor with HPLC and an enzyme test kit revealed an overestimation of ethanol levels in juices by the biosensor due to the simultaneous detection of methanol present in the samples. A flow-through version of the biosensor placed at the exit of a HPLC system proved this assumption. In order to improve the specificity for ethanol, the ADH-based sensor was studied. While showing no cross sensitivity to methanol, its stability was rather limited, this making it not suitable for practical applications. Headspace analysis offers advantages such as high selectivity (because it can be interfered by volatile substances only) and the lack of contamination of the sensor by species in the liquid juice.
 FigureHeadspace analysis of alcohol in juices (tomato, grape, currant, apple) with an amperometric enzyme electrode: sensor set-up (left), calibration with ethanol samples (right)
The corrosion of metals by vapours from air-drying paints
Abstract The conditions under which organic vapours cause corrosion of bare metals are discussed. The corrosive effects of a series of air-drying paints at 100 per cent r.h. are evaluated. The drying-oil paints tested were shown to be corrosive as also were the samples of solvent acrylic and nitrocellulose paints. The epoxide, polyurethane and chlorinated rubber paints tested together with a solvent acrylic lacquer did not give rise to vapour corrosion effects. Analysis of the vapours evolved by the drying-oil paints have shown them to be mainly formic acid together with saturated fatty acids and aliphatic aldehydes which conformed closely to the theoretically predictable products of the oxidative breakdown of the unsaturated acid residues from which they were derived. The rate of release of volatiles other than formic acid decreased rapidly to very low levels. The rate of formic acid release is shown to decrease to a relatively constant rate which is capable of giving rise to vapour corrosion effects for at least 15 months after painting. A forced drying had little effect on the final steady rate of release of formic acid although it decreased the initial rate. The corrosion caused by the acrylic and nitrocellulose paints is attributed to acetic acid derived from the esters in the paint solvent.