In this video, we will prepare the quinine sulfate standard used for calibration of the EXO fDOM sensor.
Quinine sulfate in acid solution fluoresces similarly to dissolved organic matter and is a good surrogate for fDOM calibration. Before beginning the quinine sulfate standard prep, please read and follow all the safety instructions in the MSDS documentation for each chemical. You should wear safety glasses, gloves, and a lab coat.
You will need one one-liter volumetric flask, one 100-liter volumetric flask, a small beaker, two labeled glass amber bottles for storage, a micropipette with a 300 µL capacity, a transfer pipet, a scale, a spatula, solid quinine sulfate, and 0.05 M sulfuric acid.
There are multiple forms of quinine commercially available; each form has stability and water solubility challenges, but we find quinine sulfate dihydrate to exhibit the best stability and solubility properties that fit our calibration needs. We will use solid quinine sulfate dihydrate with high purity, over 99%. And 0.05 M sulfuric acid. You can source these from a lab supplier.
First, we will weigh 0.100 g of solid quinine sulfate dihydrate. Tare the scale with a small beaker and carefully transfer the quinine sulfate. Try to maintain precision. Any error you introduce in these steps will be reflected in your final standard solution and your calibration results. Add a small amount of acid; 50 mL is fine. Swirl to completely dissolve the solid. Then transfer the solution to a clean 100 mL volumetric flask. Rinse the beaker with 0.05 M acid and add the rinsate to the 100 mL flask. Then dilute the solution to the 100 mL mark with 0.05 M sulfuric acid. Use a transfer pipet to make an accurate volume.
Cover the flask and mix well by inverting three times. This solution is 1000 mg/L quinine sulfate or 0.1%. Store it in a labeled glass amber bottle in the refrigerator at 4 degrees Celsius, and you can use it for up to six months.
Now we will dilute this solution to a 300 µg/L calibration standard. In a clean one-liter volumetric flask, add 0.05 M sulfuric acid to about 90% of the volumetric mark. Use a micropipette to transfer 300 µL of the 1000 mg/L solution to the flask, and pipet up and down to rinse the quinine sulfate from the pipet tip. Then fill the flask to the one-liter graduation with 0.05 M sulfuric acid. Use a transfer pipet to get an accurate volume. Cover the flask and invert three times. This solution is 300 µg/L quinine sulfate, also known as 300 QSU or 100 RFU. Transfer the solution to a glass amber bottle and store it in the refrigerator at 4 degrees Celsius. This dilution should be used within five days of preparation and discarded after use. Refer to the MSDS documentation for proper waste disposal.
Keep in mind that quinine fluorescence can be degraded by copper and chloride, so make sure to remove any copper film or guards and the central wiper from your sonde and make sure the sonde is thoroughly rinsed and dry before fDOM sensor calibration. Also, thoroughly clean the quinine sulfate from the sensors after calibration. Do not reuse the standard. You can make a new 300 µg/L solution when needed from the 1000 mg/L stock. Just a reminder, you can store the 1000 mg/L stock in the refrigerator for up to six months.
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