Photon Systems Instruments FMT150 - Page 40

© PSI (Photon Systems Instruments), spol. s r. o.
40
sensors that will be used in the experiment. Note that all the sensors submerged in the vessel will displace some
of the volume of the liquid. Determine the exact volume by weighing the water filled vessel and subtracting the
weight of the vessel with no water. Assuming density of 1 g.l
-1
for water, calculate the volume from the weight of
the water. Alternatively, pour water out of the vessel into a cylinder to measure the volume. In general, for 400
ml vessel when 2 sensors are used volume is around 475 ml, when 3 sensors are used the volume is around 465
ml. For 1,000 ml vessel, maximum volume with two sensors is around 1,200 ml.
• Section Correction table for CO
2
calibration on page 42 includes a table which lists the volume of standard
calibration solution that should be added to the vessel during calibration to obtain 20 µM solution of CO
2
. The
volume depends on the total volume of the vessel.
• Switch ON the PBR FMT150 and place the empty vessel in its holding place. Install the lid of the vessel with the
sensor and other ports closed off. Ensure the stirring bar is placed inside the vessel and the magnetic stirrer is
installed on the outside of the vessel. Fill the vessel with deionized water prepared overnight as described above.
Use the N
2
gas to bubble the solution and remove any CO
2
that may have entered the water during preparations
for the calibration (note that CO
2
is very soluble in water and extra precaution has to be taken to maintain the
water CO
2
free). Allow the N
2
gas to escape through one of the ports like the waste port (keep it open during
bubbling with N
2
). In the meantime, prepare additional 50 ml of water in falcon tube and bubble it through with
N
2
. This CO
2
free water is used for final removal of any residual air in the vessel prior the calibration itself.
• Connect the CO
2
electrode with PBR and start monitoring the raw data readings in the falcon tube (these will be
visible in the idle mode). Signal around 30,000 is in the range of background noise sensitivity of the CO
2
electrode.
Once the calibration constants are determined these values will be changed to provide dCO
2
readings in correct
units (µM or mg.l
-1
).
• Measure the CO
2
signal in the deionized water in the falcon tube. When the signal is stable in range around 30,000
remove the sensor and close the tube. Insert the CO
2
sensor into the PBR vessel (calibration vessel) and check it
for any gas bubbles that may be adhering to the membrane. Remove any bubbles by gently tapping the electrode
body with the fingers. Check the signal, when values around 30,000 are reached remove the N
2
gas supply tubing
(supplying N
2
gas at present) and pour the CO
2
free water from falcon tube into the vessel. The vessel must be
completely full. Unplug the waste tubing from the vessel and close all the inlets and outlets with LUER connectors.
• Define the calibration range required, preferably choosing concentrations which bracket the concentrations
expected from the sample solution. When calibrating for the first time, at least three calibration points should be
used. Table with volumes of stock calibration solution that should be added to the vessel to reach 20 µM dCO
2
concentration, based on maximum volume of the given vessel, are shown in Tab. 8. Please note that for example
1 ml of the calibration solution added to the vessel with volume of 465 ml results in final concentration 20 µM of
dissolved CO
2
(dCO
2
). If concentration of 100 µM dCO
2
is required 5 ml of calibration solution should be added to
the vessel with volume of 465 ml.
• When the background CO
2
sensor signal is stable pipette the given volume of the calibration solution into the
vessel through one of the port inlets. After addition of the calibration solution, the vessel should be tightly closed
to avoid any loss of CO
2
from the vessel to the surrounding air. Make sure that the magnetic stirring is continuously
operating during the time course of the calibration. 20 µM concentration of dissolved CO
2
corresponds
approximately to 590 ppm of CO
2
. Example of concentration conversion for the calibration solution is shown in
Tab. 7.
• Read out the CO
2
signal in software and wait until the equilibrium in the signal is reached (it takes about 4 hours).
Once the data reading has stabilized, record the final constant value and use it in preparing the calibration curve.
• When equilibrium is reached, add another amount of calibration solution to obtain next calibration step. For
example, if second calibration step with concentration of 50 µM dCO
2
is required, add another 1.5 ml of 20 µM
calibration solution if vessel with total volume of 465 ml is used for total of 2.5 ml required to obtain 50 µM dCO
2
.