Capillary electrophoretic separation of sugars in fruit juices using on-line mid infrared Fourier transform detection

Abstract

Fourier Transform infrared spectroscopy has been coupled to on-line capillary electrophoresis (CE) for the separation and detection of natural sugars in orange fruit juices. The CE separation electrolyte comprised 50 mM sodium carbonate buffer adjusted to pH 12.3 with NaOH. Galactose was selected as an internal standard. To ensure tight connections between the custom-made IR-transparent flow cell (optical path length was 15 µm) and the fused silica capillaries, commercially available O-rings were used. The scanner of the spectrometer was operated at a HeNe laser modulation frequency of 320 kHz, recording interferograms in a double-sided, forward–backward mode with 8 cm⁻¹ spectral resolution. For each spectrum 64 interferograms (512 for the background) were co-added and a Blackman–Harris 3-term apodization function was performed. A low-pass filter at 1828 cm⁻¹ was inserted in the IR beam to increase the light throughput in the spectral region of interest (1800 cm⁻¹–900 cm⁻¹). Using these features a new spectrum could be obtained every two seconds. Sucrose, glucose and fructose were structurally identified and quantified in orange juice samples. The limits of detection (3S/N) for all analytes were in the low millimolar range (0.7–1.9 mM) or, in absolute amounts, the low nanogram range (1.5–3.2 ng). The resolution ranged between 1.14 to 3.15 and the RSD of the proposed method was 1.8–4.4%.