This chapter discusses the procedure for P700 detection. The pigment designated P700 is the photoreactive center of photosystem I and as such is an obligate component in the overall photochemical transport of electrons from an appropriate donor to NADP. The pigment has been detected in all examined algae and green plants that are capable of growing autotropically. P700 occurs in low concentrations relative to the bulk chlorophyll(s) and is typically characterized by its main absorption bands around 430 and 700 nm. The location of these bands and the solubility properties of the pigment suggest that P700 is a form of chlorophyll a, segregated as a result of its unique environment within the photosynthetic apparatus. To measure P700 by means of light-dark spectroscopy, a monochromatic (∼700 rim) “detecting light” (Idet) is passed through a sample and monitored by a photocell. The change of transmission of Idetis observed upon illumination of the sample by a second “actinic light” (Iact) which is blocked from the photocell. Because this change is small, the detecting light must be stable and the noise in the photocurrent minimal. The signal-to-noise ratio is proportional to the square root of the number of quanta which arc “seen” by the photocathode and used for the determination. © 1980, Academic Press, Inc.
Marsho, T. V., & Kok, B. (1980).  P700 detection. Methods in Enzymology, 69(C), 280–289. https://doi.org/10.1016/S0076-6879(80)69027-2