Calibration Standards - Xylem EXO Series User Manual

Calibration
5.10

Calibration Standards

Quinine Sulfate Solution for fDOM Sensor
A
Before using a quinine sulfate reagent (solid or solution) or sulfuric acid reagent, read the safety instructions
provided by the supplier. Take extra precautions when making dilutions of concentrated sulfuric acid, as this
reagent is particularly dangerous. Remember that only trained personnel should handle chemicals.
Preparation
Use the following procedure to prepare a 300 μg/L solution of quinine sulfate (300 QSU) that can be used to
calibrate the EXO fDOM sensor for field use:
1. Purchase solid quinine sulfate dihydrate with a high purity (>99%). (Recommended supplier: Fisher Scientific
item #6119-70-6.) Purchase 0.1 N (0.05 M) sulfuric acid, to avoid the hazards of diluting concentrated
sulfuric acid to make this reagent. (Recommended supplier: Fisher Scientific item # AA35651K7.)
2. Weigh 0.100 g of solid quinine sulfate dehydrate and quantitatively transfer the solid to a 100-mL
volumetric flask. Dissolve the solid in about 50 mL of 0.05 M (0.1 N) sulfuric acid (H
the solution to the mark of the volumetric flask with additional 0.05 M sulfuric acid, and mix well by
repeated inversion. This solution is 1000 ppm in quinine sulfate (0.1%).
3. Transfer 0.3 mL of the 1000 ppm solution to a 1000 mL volumetric and then fill the flask to the top
graduation with 0.05 M sulfuric acid. Mix well to obtain a solution that is 300 μg/L (300 QSU).
4. Store the concentrated standard solution in a darkened glass bottle in a refrigerator to retard
decomposition. The dilute standard prepared in the previous step should be used within 5 days of
preparation and should be discarded immediately after exposure to EXO's metal components.
Degradation of quinine fluorescence by copper
Exposure of the dilute quinine sulfate solution to any copper-based component of the EXO sonde and sensors
(primarily the wiper assembly) will begin to degrade the solution significantly within minutes. Thus, perform
your calibration as quickly as possible on immersion of the sensors into the quinine sulfate solution. Discard
the used standard. When quinine sulfate standards are required in the future, perform another dilution of the
concentrated solution.
Effect of temperature on fluorescence
The intensity of the fluorescence of many dyes shows an inverse relationship with temperature. This effect
must be accounted for when calibrating the EXO fDOM sensor with Rhodamine WT. Enter the QSU
calibration value from the table below that corresponds to the temperature of the standard.
Temp (˚C)
QSU
30 96.4
28 97.3
26 98.2
24
99.1
22
100
20
100.9
|
78
pg
Temp (˚C)
QSU
18 101.8
16 102.7
14 103.6
12
104.6
10
105.5
8
106.4
SO ), dilute
2 4