1. Don't you think that when using potentiometry for
HPLC or CE you are not making use of the
selectivity of ISE's, which is usually treated as one
of the main advantages of potentiometry?
2. When using polymeric membrane ISE's with HPLC
or CE you have to be carefull choosing the eluent (to
make sure you are not destroying the membrane)
Could you please expand on it?
3. High potentials (voltage) used in CE generate an
electric field. Don't you have problems with the
influence of this electric field on potentiometric
measurements?
4. How do you check the selectivity of your
materials?
We worked with relative responses, rather than with
selectivity coefficients Ki,jpot
The latter values (Ki,jpot) refer to the Nicolski-Eisenmann
equation, describing the potential response of the analyte ion i
in the presence of an interfering ion j. For the simple case of
two monovalent ions, this eqn. reads:
E = Const. - 0.059/n log(ci + Ki,jpot.cj)
Determination of classical selectivity coefficients? cfr
Bakker, Pretsch and Buhlmann, Anal. Chem. 2000, 72, 11271133.
detection limits (molar conc., 20µL injected)
analyte
Clenbuterol
Mabuterol
Cimaterol
Carbuterol
Bufuralol
Practolol
Alprenolol
Oxprenolol
p-Oxprenolol
Bisoprolol
Celiprolol
Bevantolol
Tolamolol
Propranolol
Nadolol
Tertatolol
Carazolol
UV 230nm
2.6 x 10-7
2.9 x 10-6
3.0 x 10-7
1.7 x 10-7
3.5 x 10-7
1.7 x 10-7
2.2 x 10-6
1.0 x 10-6
0.7 x 10-6
3.5 x 10-7
1.6 x 10-7
1.3 x 10-6
1.3 x 10-6
2.7 x 10-8
2.3 x 10-7
1.2 x 10-6
4.4 x 10-7
Borate
mol/L
6.7 x 10-7
2.2 x 10-7
2.5 x 10-5
8.5 x 10-5
2.8 x 10-7
5.4 x 10-5
6.7 x 10-7
1.7 x 10-6
1.6 x 10-6
1.3 x 10-5
1.7 x 10-5
2.2 x 10-6
1.4 x 10-5
2.1 x 10-7
5.9 x 10-5
3.5 x 10-7
6.0 x 10-4
OctylCD
mol/L
1.1 x 10-7
5.6 x 10-8
4.8 x 10-6
7.7 x 10-5
2.2 x 10-8
6.2 x 10-5
3.6 x 10-7
7.7 x 10-7
7.5 x 10-7
3.7 x 10-6
1.6 x 10-5
1.0 x 10-6
1.3 x 10-6
1.0 x 10-7
2.1 x 10-5
1.2 x 10-7
2.0 x 10-5
Calix
mol/L
1.7 x 10-6
3.4 x 10-5
3.6 x 10-5
2.4 x 10-4
1.5 x 10-6
1.9 x 10-4
3.0 x 10-7
5.8 x 10-7
3.5 x 10-7
2.7 x 10-6
6.8 x 10-5
1.6 x 10-6
2.0 x 10-5
5.3 x 10-7
1.0 x 10-4
1.8 x 10-6
2.1 x 10-4
5. You usually combine potentiometric detection with
CE or HPLC. What happens if you apply your
sensors without CE/HPLC? Is the sensitivity too
small to detect small concentrations of compounds
such as neurotransmitters etc?
6. What are the advantages of applying
potentiometry to CE/HPLC in comparison with the
other detection systems?
HPLC/UV/ one of the most successful combinations ever. Engineers
HPLC/MS: big success. Engineers
HPLC/conductivity: successful in ion-chromatography. Engineers
HPLC/amperometry: limited succes. PAD success. Chemists
HPLC/fluorimetry: limited succes. Engineers
CE/UV (direct or indirect): low sensitivity because of short light path.
Successful. Engineers
CE/conductivity: works but not used. TSP engineers CE/amperometry:
uncertain. Lunte, Dovichi, Chemists
Potentiometry: Future in hands of organic chemists. Enormous development
potential.
7. Do you think only charged compounds can be
detected (measured) in potentiometry?
Neutral compounds too. Nobody did it.
8. As you said, in a typical "liquid" membrane,
adsorbtion and extraction take place. Are those
phenomena required (wanted) or not? How do they
influence the results?
Future will tell how high the interaction energy can be for optimal
results. Strong interaction gives low detection limits, but maybe also
slow kinetics. Let's try.
9. What about the stability of these sensors? How
many times can they be used without any
conservation? When enzymes are used, when are
they useless for further application?
Very stable. LogP of components still important. Months of use.
Membrane-substrate electrode sensitive to detergents
Enzymes linked to supports: very stable
Enzymes in contact with substrate electrodes: unstable. Used mostly in
throw-away sensors (explain glucose).
10. What are the limits for electrode dimensions?
CE works well with 100 micrometer
Physiologists work for decennia with 1 um. Liquid ion exchanger.
Faraday cage.
11. What is the role of kinetics of complex formation
of a receptor with the analyte on the detector
response and on its other properties?
Lets find out
12. Is the potential of a reference electrode really
constant?