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ANALYTE SUBTRACTION
To measure the concentration of a sample using Analyte Subtraction method:
• Press
and then
to select
ISE Measure
mode for the selected channel.
• Select the Analyte Subtraction method (see ISE Setup > Reading Mode).
• Prior to starting an AS procedure, the ISE sensor must be calibrated with a minimum of two standards
containing ISA. The slope of the electrode will be used in all calculations involved in AS.
• If following an established procedure: Press
then edit the method variables and follow the
procedure below.
• Press
to set the method parameters. Press
/
to select next/previous parameter
to edit, then press
and use
or
to set the desired parameter value. Press
to save the modified value and then press
to exit method parameters edit menu.
• 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 factor). 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 micropipet, 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 Stir bar and place on a magnetic 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.
• Press
to set the method parameters. Press
/
to select next/previous parameter
to edit, then press
and use
or
to set the desired parameter value. Press
to save the modified value and then press
to exit method parameters edit menu.
• If developing a procedure: Before attempting Analyte Addition analysis, it is important to determine
which standard volume, concentration and sample size will produce the best results. As a general rule,
the standard must be less concentrated than the sample so the addition of sample will increase the total
Ion content of the beaker and change the mV value by at least 10 mV. For a positively charged Ion (i.e.
Sodium), the AA increases the mV. For a negatively charged Ion (i.e. Sulfide, Fluoride, Chloride), the AA
should decrease the mV. Start with a small trial. For example: Measure 50 mL of standard, 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 micropipet, 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 standard, 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 Addition mode.
• Volumetrically add standard to a clean beaker. Add Stir bar and place on a magnetic 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. The method parameters are also displayed on the LCD.
• 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 ISE sensor between samples.
• Press
, to modify the method parameters.
Note
: Press
at any time to stop the measurement and return to
ISE Measure
mode.
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