Pergamon 0039o914o(~)01455q Talanta, Vol. 42, No. 4, pp. 605-612, 1995 Copyright © 1995 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0039-9140/95 $9.50 + 0.00 A MULTI-WAVELENGTH PHOTOMETER BASED ON LIGHT-EMITTING DIODES PETER C. HAUSER,* THUSITHA W. T. RUPASINGHE and NORMAN E. CATES The University of Auckland, Department of Chemistry, Private Bag 92019, Auckland, New Zealand (Received 14 July 1994. Revised 25 October 1994. Accepted 27 October 1994) Summary--The light originating from seven light-emitting diodes of different colours is guided, one at a time, into a measuring cell by means of a fibre optic coupler. Detection is carried out with photodiodes which are connected to a log-ratio amplifier yielding direct absorbance readings. Optical filters are used to narrow the emission band from blue light emitting diodes as these bands are relatively wide compared to those of the emitters of other colours. An inexpensive and compact multi-wavelength photometer covering the visible range is thus obtained, which in many cases can replace a conventional spectrophotometer for absorbance measurements. The performance for a range of commonly used photometric analytical procedures is described and compared to conventional measurements with a spectrophotometer. Light-emitting diodes (LEDs) produce light with bandwidths of typically 25 nm which is comparable to the combination of wideband sources with interference type optical filters. It is, therefore, possible to carry out photometry with LEDs without the use of wavelength discriminating devices, the use of LEDs in photometric detectors has often been re- ported ~-2t and its application in flow-through methods has been reviewed. 22'23 The LEDs are commonly combined with photodiodes which measure the light intensity after passage through the sample. Geometric arrangements reported include dip-type probes and flow-through cells. The detectors have usually been connected to operational amplifiers in the current follower configuration which yield an output voltage directly proportional to the photo-current and hence light intensity. This signal represents transmittance measurements, m value corre- sponding to absorbance can be obtained by connection to a logarithmic amplifier. This is more appropriate because it is this value that is directly related to concentration according to Beer-Lambert's law. Such devices have been reported more recently. ~72~ LEDs with colours ranging from the blue to the infrared are avail- able and, therefore, with the exception of the near-UV cover all the wavelength range that is *Author to whom correspondence should be addressed. commonly used for analytical spectropho- tometry. Absorbance bands for molecular ab- sorption spectroscopy are usually fairly wide, typically 100 nm, and it is therefore almost always possible to find a matching LED. A range of six LEDs of the colours blue, green, yellow, orange, red and near-infrared can thus in many cases replace a conventional visible- range spectrophotometer. However, most LED- photometers described to date have only employed a fixed wavelength for a predeter- mined purpose. A change of wavelength had to be effected by physically changing the light source. This limitation has been caused by the difficulty in coupling light from more than one source into a single detector cell. Bi-colour LEDs are available which contain two light emitting substrates in one body and have been used for a dual wavelength detector) 3 A tri- colour device has been constructed by using a green/yellow bi-colour LED and a number of red LEDs arranged concentrically around the bi-colour LED in order to pass red light into the cell as well) 4"~5The coupling of the light from two separate LEDs into a single cell with bifur- cated optical fibres for the purpose of correction for turbidity has recently been described, t6 Here a multi-LED photometer is described that em- ploys a fibre optic coupler to guide the light from up to seven LEDs into a single measuring cell. 605 TAL 42/4--H