In this study, miniaturized portable/online
UVC-LED spectrophotometers
have been developed for fast detection of nitrate nitrogen and dissolved
organic matter, using 235 and 275 nm light-emitting diodes (LEDs)
as the light sources and GaN-based photodiodes as UV detectors. The
application of the wide-band gap GaN-based photodiodes eliminated
the influences of parasitic visible emission of UVC-LEDs. Using Orange
G as the test absorbing compound, the portable UVC-LED spectrophotometer
showed high linear upper detection limits and a negligible stray light.
The portable spectrophotometer showed good linearity for NO3
––N concentration in the range of 0–15
mg/L (R
2 > 0.99), while a loss of sensitivity
was observed at higher NO3
––N
concentration. This nonlinearity was attributed to the intrinsic stray
light arising from the incomplete overlap between the 235 nm LED emission
spectra and the nitrate absorption spectra. With the empirical correction
coefficient α being determined from natural water and municipal
wastewater samples, the UVC-LED spectrophotometers have comparable
accuracy to the standard spectrophotometric method. The results of
size exclusion chromatography further elucidated that it was the fraction
of humic substances that determined the correction coefficient α
for the UV spectrophotometric detection of nitrate in natural water
and municipal wastewater samples.
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