False negatives in the modified ESR-based photosafety test (ESR-PT) caused by ultraviolet-C light

The modified electron spin resonance (ESR)-based photosafety test (ESR-PT) is a non-animal prediction test using ESR spectroscopy that is applicable to hydrophobic and colored chemicals and the results show high concordance with existing photosafety reference information. The modified ESR-PT is based on the detection of singlet oxygen and free radical photoproducts generated from chemicals in the presence of 4-hydroxy-2,2,6,6-tetramethyl-piperidine (4-hydroxy-TEMP). We obtained false-negative results caused by signal increment of the control solution, which is used as the denominator of the classifier in the modified ESR-PT, when we used a different type of lighting unit (type I) to the previously used type of lighting unit (type II). Spectral measurement of the irradiated light from the light sources revealed that the type I lighting unit emitted stronger UV-C light than the type II lighting unit. Consequently, as UV-C absorption of 4-hydroxy-TEMP (λ_max<210 nm) was confirmed, we repeated the modified ESR-PT using a type I lighting unit equipped with a UV-C cut filter, which led to an apparent decrease of the signal increment in the control solution, and all the false-negative-judged chemicals correctly tested positive but a false positive result was also noted. Therefore, installation of a UV-C cut filter in the lighting unit in modified ESR-PT appears to be a reliable solution for avoiding UV-C light-mediated false-negative results. However, it may also be necessary to reconsider the classifier used in ESR-PT to avoid obtaining false-positive results when using a UV-C cut filter.