Stability testing results for stock solutions to standardize calibration of field fluorescence sensors

Optical sensors measuring fluorescence of dissolved organic matter (fDOM) are increasingly used in water quality studies because they provide proxy measurements for a variety of contaminants and constituents of concern including metals and wastewater effluent. Similarly, sensors measuring fluorescence of chlorophyll (fChl) and phycocyanin (fPC) have gained popularity to measure phytoplankton concentration, biomass, and even primary productivity. Optical sensors require calibration using a calibration standard solution which is diluted from a working stock solution. However, preparation of stock solutions is challenging because they are difficult to prepare accurately, need to be prepared in a laboratory setting, and the final diluted calibration standards need to be used immediately because they degrade quickly. Consequently, technicians are not always confident with their preparation technique or the accuracy of diluted calibration standards. As such, there is a critical need for working stock solutions to be prepared and verified in a laboratory by experienced personnel so field fluorescence sensors measuring fDOM, fChl, and fPC can be standardized across the entire agency. Before the USGS National Field Supply Service (NFSS) can provide working stock solutions to sensor users, a laboratory stability study was conducted to determine the shelf-life and ideal storage conditions for stocks prepared using quinine sulfate (used for fDOM sensor calibration) and rhodamine (used for fChl and fPC calibration). Working stock solutions were prepared at the start of the experiment and stored in a variety of materials (glass, high density polyethylene, and low density polyethylene) at two temperatures (4 and 25 degrees Celsius). Benchtop and sensor measurements were made approximately weekly on calibration standard solutions that were prepared from the working stock solutions to test stock stability over a 3-month period from September 2022 to February 2023.