Volume 71, Issue 2

The term photocatalysis is one amongst several in a quagmire of labels used to describe a photon-driven catalytic process; a simple description of photocatalysis is proposed herein. Other labels such as quantum yield and/or quantum efficiency used in solid/liquid and solid/gas hetero-geneous photocatalytic systems to express process efficiencies have come to refer (incorrectly) to the ratio of the rate of a given event to the rate of incident photons impinging on the reactor walls and typically for broadband radiation . There is no accord on the expression for process efficiency. At times quantum yield is defined; often, it is ill-defined and more frequently how it was assessed is not described. This has led to much confusion in the literature, not only because of its different meaning from homogeneous photochemistry, but also because the description of photon efficiency precludes comparison of results from different laboratories owing to variations in light sources, reactor geometries, and overall experimental conditions. The previously reported quantum yields are in fact apparent quantum yields, i.e. lower limits of the true quantum yields. We address this issue and argue that any reference to quantum yields or quantum efficiencies in a heterogeneous medium is inadvisable until the number of photons absorbed by the light harvester (the photocatalyst) is known. A practical and simple alternative is proposed for general use and in particular for processes employing complex reactor geometries: the concept of relative photonic efficiency (x r ) is useful to compare process efficiencies using a given photocatalyst material and a given standard test molecule. A quantum yield can subsequently be calculated since f = x r f phenol , where f phenol denotes the quantum yield for the photocatalyzed oxidative transformation of phenol used as the standard secondary actinometer and Degussa P-25 TiO 2 as the standard photocatalyst. For heterogeneous suspensions (only), an additional method to determine quantum yields f is also proposed.

In the preceding article [Serpone and Salinaro, Pure Appl. Chem. , 71(2), 303-320 (1999)] we examined two principal features of heterogeneous photocatalysis that demanded scrutiny: (i) description of photocatalysis and (ii) description of process efficiencies. For the latter we proposed a protocol relative photonic efficiency which could subsequently be converted to quantum yields. A difficulty in expressing a quantum yield in heterogeneous photochemistry is the very nature of the system, either solid/liquid or solid/gas, which places severe restrictions on measurement of the photon flow absorbed by the light harvesting component, herein the photocatalyst TiO2, owing to non-negligible scattering by the particulates. It was imperative therefore to examine the extent of this problem. Extinction and absorption spectra of TiO 2 dispersions were determined at low titania loadings by normal absorption spectroscopy and by an integrated sphere method, respectively, to assess the extent of light scattering. The method is compared to the one reported by Grela et al . [ J. Phys. Chem ., 100, 16940 (1996)] who used a polynomial extrapolation of the light scattered in the visible region into the UV region where TiO 2 absorbs significantly. This extrapolation underestimates the scattering component present in the extinction spectra, and will no doubt affect the accuracy of the quantum yield data. Further, we report additional details in assessing limiting photonic efficiencies and quantum yields in heterogeneous photocatalysis.

Synopsis : ISO, IUPAC and AOAC INTERNATIONAL have co-operated to produce agreed protocols or guidelines on the "Design, Conduct and Interpretation of Method Performance Studies" [1] on the "Proficiency Testing of (Chemical) Analytical Laboratories" [2] and on "Internal Quality Control in Analytical Chemistry Laboratories" [3] . The Working Group that produced these protocols/guidelines was asked to prepare guidelines on the use of recovery information in analytical measurement. Such guidelines would have to outline minimum recommendations to laboratories producing analytical data on the internal quality control procedures to be employed. A draft of the guidelines was discussed at the Seventh International Symposium on the Harmonization of Quality Assurance Systems in Chemical Laboratory, sponsored by IUPAC/ISO/AOAC INTERNATIONAL, held in Orlando, USA, 4-5 September 1996. Proceedings from that Symposium are available [4] . The purpose of these guidelines is to outline the conceptual framework needed for considering those types of analysis where loss of analyte during the analytical procedure is inevitable. Certain questions cannot be satisfactorily addressed, and hence remain irreducibly complex, unless such a conceptual framework is established. The questions at issue involve (a) the validity of methods for estimating the recovery of the analyte from the matrix of the test material, and (b) whether the recovery estimate should be used to correct the raw data to produce the test result. The types of chemical analysis most affected by these considerations are those where an organic analyte is present at very low concentrations in a complex matrix. "Protocol for the Design, Conduct and Interpretation of Method Performance Studies", W. Horwitz, Pure Appl. Chem . 60 , 855- 864 (1988), revised, 67 , 331-343 (1995). "The International Harmonized Protocol for the Proficiency Testing of (Chemical) Analytical Laboratories", M. Thompson and R. Wood, Pure Appl. Chem. 65 , 2123-2144 (1993). (Also published in J. AOAC International 76 , 926-940 (1993). "Harmonized Guidelines for Internal Quality Control in Analytical Chemistry Laboratories", M. Thompson and R. Wood, Pure Appl. Chem . 67 , 49-56 (1995). "Quality Assurance for Analytical Laboratories", edited M. Parkany, Royal Society of Chemistry, London, UK, 1996.

The development by collaborative study of a standardised method for the determination of the stigmastadienes content in vegetable oils is described. The procedure involves the isolation of the unsaponifiable matter in the oil ands its fractioning on a silica gel column. The fraction containing the steroidal hydrocarbons is then analysed by capillary gas chromatography and the quantitative assessment made by the use of an internal standard /cholea-3,5-staddiene). The procedure is very sensitive allowing determination at levels of 0.01 - 4.0 mg/kg.

A description is given of the development by collaborative study of a standardised method for the determination of cadmium in crude and refined oils and fats by graphite furnace atomic absorption spectrometry. The procedure is rapid and sensitive allowing determination at levels of 1 to 10 μg/kg.