In Vivo Measurement of Chlorophyll and the YSI 6025 Wiped Chlorophyll Sensor

Introduction Chlorophyll, in various forms, is bound within the living cells of algae, phytoplankton, and other plant matter found in environmental water. Chlorophyll is a key biochemical component in the molecular apparatus that is responsible for photosynthesis, the critical process in which the energy from sunlight is used to produce life-sustaining oxygen. In general, the amount of chlorophyll in a collected water sample is used as a measure of the concentration of suspended phytoplankton, the magnitude of which can signifi cantly aff ect the overall quality of the water. Th e use of the measurement of phytoplankton as an indicator of water quality is described in Section 10200 A of Standard Methods for the Examination of Water and Wastewater. Th e classical method of determining the quantity of chlorophyll at a particular site is to collect a fairly large water sample and analyze it in the laboratory. Th e procedure involves fi ltration of the sample to concentrate the chlorophyll containing organisms, mechanical rupturing of the collected cells, and extraction of the chlorophyll from the disrupted cells into the organic solvent, acetone. Th e extract is then analyzed by either a spectrophotometric method using the known optical properties of chlorophyll or by high performance liquid chromatography (HPLC). Th is general method is detailed in Section 10200 H of Standard Methods and has been shown to be accurate in multiple tests and applications as long as a competent laboratory analyst carries out the protocol. Th e procedure is generally accepted for reporting in scientifi c literature. Th is method is time-consuming, however, and usually requires an experienced, effi cient analyst to generate consistently accurate and reproducible results. It also does not lend itself readily to continuous monitoring of chlorophyll, and thus phytoplankton, since the collection of samples at reasonable time intervals, e.g., every hour, would be extremely tedious. YSI has developed the 6025 chlorophyll sensor for the determination of chlorophyll in spot sampling and continuous monitoring applications. It is based on an alternative method for the measurement of chlorophyll which overcomes these disadvantages, albeit with the potential loss of accuracy. In this procedure, chlorophyll is determined in vivo, i.e., without disrupting the cells as in the extractive analysis. Th e YSI 6025 chlorophyll sensor is designed for these in vivo applications and its use allows the facile collection of large quantities of chlorophyll data in either spot sampling or continuous monitoring applications. It is important to remember, however, that the results of in vivo analysis will not be as accurate as those from the certifi ed extractive analysis procedure. Th e limitations of the in vivo method are outlined below and should be carefully considered before making chlorophyll determinations with your YSI sonde and sensor. Some of the sources of inaccuracy can be minimized by combining the data from the YSI 6025 with data from extractive analysis of a few samples acquired during a sampling or monitoring study. However, the in vivo studies will never replace the standard procedure. Rather, the estimates of chlorophyll concentration from the easy-to-use YSI chlorophyll system are designed to complement the more accurate (but more difficult to obtain) results from more traditional methods of chlorophyll determination.