Thermo orion 410Aplus, Instruction Manual

Page: 120 / 129

Benchtop pH/ISE Meter Instruction Manual
Automatic Blank Correction 119
Appendix F
Automatic Blank Correction
The Models 710Aplus, 720Aplus, and 920Aplus have a feature known as
automatic blank correction. In this technique the meter decides whether
blank correction is the best measurement strategy by analyzing the
electrode response during a multipoint calibration. A separate blank does
not have to be run. Graphically, automatic blank correction is equivalent to
drawing a smooth curve through the lowest three points of the multipoint
calibration and extrapolating to zero concentration based on the assumption
of Nernstian electrode behavior.
Multipoint calibrations at the lower limit of detection are desirable when
the response of an ion selective electrode is non-linear and cannot be
characterized with a one or two point calibration. This is usually seen as a
low electrode slope. Generally the electrode is behaving in a Nernstian
manner but the effect of a blank is being observed. See Figure below.
10
-1
10
-2
10
-3
10
-4
10
-5
0.1 1 10 100 1000
10 - fold change
~56mV
electrode potential (mV)
-60
-40
-20
-0
20
40
60
80
100
120
140
150
160
Calibration Curve
(Electrode Potential vs. Concentration)