Light-emitting diodes (LEDs) have been popular in the field of miniaturized absorbance detection for many years, due to their short warm-up times, low power requirements, stable output, small footprint, and long lifetime. However, a single LED can’t deliver multi-wavelength detection due to the narrow bandwidth of an LED emission. In response, many research groups described the utility of multiple LEDs for use in multi-wavelength detection, although in many cases this has also involved the use of portable spectrometers and complex optics, resulting in high-costs.

Here we are introducing a novel approach for achieving 'low-cost and small-footprint' multi-wavelength detection using multiple LEDS, which employs a micro controller with a field programmable gate array to generate short and separated micro pulses for three DUV LEDs, along with data acquisition and processing via a custom-made program and a suitable algorithm. The pulse signal generation and data processing have been accomplished simultaneously without any lag using the micro controller. The detection system was designed by connecting the outputs of three DUV-LEDs (λ = 235, 250, and 280 nm) via a solarisation resistant trifurcated optical fiber to a 12-μL flow cell. An ultra-fast responding photodiode with a spectral response over the DUV region and an active area of 1.1x1.1 mm²was connected to the optical-fiber-compatible flow cell.

For evaluation, the first generation of the new detection system has been successfully coupled with a standard LC and utilized for the simultaneous monitoring of pharmaceuticals at different wavelengths without compromising the analysis sensitivity. Paracetamol, caffeine, and aspirin were simultaneously determined at 250, 280, and 235 nm, respectively, using a chromatographic system composed of C18 column and methanol: 50 mM potassium dihydrogen phosphate solution adjusted to pH 3.3 (43:57) as mobile phase. The linearity and sensitivity of the method utilizing the novel detection system were evaluated. A regression correlation coefficient (r) of 0.9999 was achieved over a range of 0.1-3.2 µg/mL for paracetamol, 0.4-6.4 µg/mL for caffeine, and 0.8-12.8 µg/mL for aspirin. The quantitation limits (LOQs) were 0.10 µg/mL, 0.38 µg/mL, and 0.66 µg/mL for paracetamol, caffeine, and aspirin, respectively. The repeatability of the multi-wavelength determination has shown a relative standard deviation < 3 % among all drugs.

Brett Paull

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