Effect of spectral resolution on the measurement of monoaromatic hydrocarbons by DOAS.
The excellent response characteristics and detection sensitivity with much lower operational cost and the capability to discriminate between the isomer of some monoaromatic hydrocarbons (MAHCs) make differential optical absorption spectroscopy (DOAS) a powerful tool to trace concentration variation of MAHCs. But due to the similarity in chemical structure, those MAHCs have the similar overlapped characteristic absorption structures, which make the selection of instrumental parameter critical to the accurate detection of MAHCs. Firstly, the spectral resolution used in DOAS system determines the nonlinear absorption of O2 and the mass dependence of characteristic absorption structure; thereby it determines the effect of elimination error of O2 absorption in the atmospheric spectra for the detection of MAHCs. Secondly, spectral resolution determines the differential absorption characteristics of twelve MAHCs representing major constituents in technical solvents used in the automobile industry and the interference of spectral overlapping. Thirdly, the spectral resolution determines the sensitivity, time resolution and linear range. So the spectral resolution range with the best ratio of signal to noise is used to determine the most suitable spectral resolution range, as well as the spectral resolution range that ensure the characteristic absorption structure of MAHCs and the minimization of O2 absorption interference. Finally, 0.15-0.16 nm (FWHM: full width at half maximum) is assumed to be closest to the optimum spectral resolution and it is confirmed by the results of practical measurement of MAHCs by DOAS.
UV-absorption cross sections of a series of monocyclic aromatic compounds
Abstract Aromatic hydrocarbons are of increasing importance as components of urban volatile organic compounds. Besides gas-chromatographic techniques, the detection of aromatic hydrocarbons by Differential Optical Absorption Spectroscopy (DOAS) is a promising alternative technique. The absolute calibration of DOAS-instruments is straightforward: Only the absorption cross sections of the species of interest and the instrument function are required. In this work we present measurements of high-resolution gas phase UV-absorption cross section spectra of benzene (C 6 H 6 ), toluene (C 7 H 8 ), p-, m-, o -xylene (C 8 H 10 ), phenol (C 6 H 5 OH), benzaldehyde (C 7 H 6 O) and the cresol isomers (C 7 H 8 O), p-, m- and o -cresol in the wavelength interval between 230 and 290 nm. The spectral resolution is 0.11 nm (FWHM), except for o - and m -cresol, where the resolution is 0.32 nm (FWHM). These spectra can be used to calibrate DOAS-instruments. From these absorption cross sections typical detection limits for the atmospheric measurements with Long-Path-Differential Optical Absorption Spectroscopy are calculated to be in the range of a few ppt (phenol), to a few 10 ppt (cresol, benzaldehyde, benzene, p -xylene), up to 200 ppt ( o -xylene). The attainability of these detection limits is demonstrated by measurements of atmospheric phenol and benzaldehyde.
The status of FAO and codex alimentarius developments on vegetable proteins
The FAO is interested in increasing world protein supplies both by advising and assisting developing countries to increase their production of staple foods and by encouraging the development of technology to produce protein concentrates both from vegetable and animal sources to supplement diets which are low in protein content. Recent revisions of the amount of protein required in the diet in relation to total per capita energy requirements and the difficulties encountered in introducing novel foods into established dietary patterns has led FAO to concentrate on increased agricultural productions in developing countries as the primary means of combatting malnutrition. In the meantime, in North America and Europe, advances in the technology of processing vegetable proteins has given highly acceptable products simulating meats which are being used as meat extenders and meat analogues. The consequent increase in vegetable protein production has revealed a need for international, agreement on standardization, and it has been agreed that this will be undertaken by the Joint FAO/WHO Food Standards’ Programme. The program operates chiefly through the Codex Alimentarius, Commission, an international governmental body, the purpose of which is to protect consumers against health hazard in food and against fraud. The Commission, which has 116 member governments, controls an intensive program of work covering the composition, labeling, additives, contaminants, pesticide residues, hygiene, sampling and analysis aspects of foods and has established a procedure which allows member governments ample opportunity to reach agreement on food standards and to accept them for incorporation into national legislation. The Codex Commission, at its 12th session, decided to establish a Committee on Vegetable Proteins with the following terms of references: “to elaborate definitions and world-wide standards for vegetable protein products deriving from soybeams, cottonseed, groundnuts, cereals and from other vegetable sources as they come into use for human consumption, and to elaborate guidelines on utilization of such vegetable protein products in the food supply system, on nutritional requirements and safety on labeling and on other aspects as may seem appropriate.” The question of which country will host the Committee is still under discussion.