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62
• If developing a procedure: Before attempting Analyte Subtraction analysis, it is important to
determine which sample volume, reactant volume and concentration, will produce the best results
and the way the reagent will react with the measured ion on a molar basis (stoichiometric fac-
tor). As a general rule, the reactant should contain the measured ion so the sample addition
will react with the ion and reduce the measured concentration of the sample. The change of the
mV value, before and after the sample addition, should be at least 10 mV. Start with a small
trial. For example: Measure 50 mL of reactant, add a magnetic stir bar and place on a stirrer,
add ISA (consult ISE manual) and place ISE electrode tip into the sample. Put instrument in
mV mode and record the observed mV. Using a micropipette, add a volume of the sample.
Start by adding 500 µL at a time (for example). Watch the change in mV. When you have
observed approximately a 10 mV change from the original value, calculate the total volume
added. Adjust sample and standard volumes proportionally to standard volumes that can be
measured with accuracy. Use volumetric pipettes for standard, ISA and sample addition.
• Press
then edit the procedure variables to the volumes determined in the prior step.
Procedure:
• Press
to enter Analyte Subtraction mode.
• Volumetrically add reactant to a clean beaker. Add a magnetic stir bar and place on a stir plate.
Stir standard. The method will prompt user to add ISA. Place ISE sensor tip into the solution
and a mV value will show on the display.
• Press
to take the first mV reading.
• When the reading is stable, press
to store the first mV reading. The second step of the
method will be displayed on the LCD in which the user is notified to add the Sample Volume
to the standard solution.
• Press
to take the second mV reading.
• When the reading is stable, press
to store the second mV reading. The ISE measurement
results will be displayed on the LCD.
• Press
to log the current results into an ISE Method Report. Press to return
to ISE Measure mode. Press to start another measurement. Rinse sensor between
samples.
• Press
to modify the method parameters.
Note: Press
at any time to stop the measurement and return to ISE Measure mode.
ISE MEASUREMENT (HI5222 only)
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