Near-infrared chlorophyll measurement

The phenomenon of fluorescence has been synonymous with ultraviolet (UV) and visible spectroscopy rather than near-infrared (near-IR) spectroscopy from the beginning of the subject. This fact is evidenced in definitive texts which also provide useful background information for this volume (see, e.g., Refs. 1-6). Consequently, our understanding of the many molecular phenomena which can be studied with fluorescence techniques, e.g., excimer formation, energy transfer, diffusion, and rotation, is based on measurements made in the UV/visible. Historically, this emphasis was undoubtedly due to the spectral response of the eye and the availability of suitable sources and detectors for the UV/visible in contrast to the lack of equivalent instrumentation for the IR. Nevertheless, there are a few notable exceptions to the prevalence of UV/visible techniques in fluorescence such as the near-IR study of chlorophyll(7) and singlet oxygen,<8) which have been ongoing for some years. 

The primary electron donor of PS II was detected as a flash-induced absorption change attributable to chlorophyll a oxidation [160]. Its bleaching maximum is close to 680 nm and it is thus designated P-680 (it is also called Chi an). The fluorescence of the light-harvesting chlorophylls interferes with measurements at this wavelength, thus many kinetic studies of P-680 have been done by measuring the smaller broad absorption increase at around 820 nm [161,162]. This broad absorption in the near infrared is probably responsible for the fact that P-680 is a quencher of chlorophyll fluorescence. 

Phytoplankton blooms can be monitored also from space. The oceanographic ocean color sensors installed on satellites measure the visible and near-infrared spectral range to identify optically active water constituents, such as chlorophyll, yellow substance, and suspended matter, and to perform a required atmospheric correction. The polar orbit and the swath of most oceanographic sensors permit a daily coverage of the Baltic Sea with a spatial resolution of 1 km, or better. The synoptic character and the repeating rate allow studies of the spatial and temporal development of the phytoplankton nearly in real time. 

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