Resonance-induced sensitivity enhancement method for conductivity sensors
Methods and systems for improving the sensitivity of a variety of conductivity sensing devices, in particular capacitively-coupled contactless conductivity detectors. A parallel inductor is added to the conductivity sensor. The sensor with the parallel inductor is operated at a resonant frequency of the equivalent circuit model. At the resonant frequency, parasitic capacitances that are either in series or in parallel with the conductance (and possibly a series resistance) is substantially removed from the equivalent circuit, leaving a purely resistive impedance. An appreciably higher sensor sensitivity results. Experimental verification shows that sensitivity improvements of the order of 10,000-fold are possible. Examples of detecting particulates with high precision by application of the apparatus and methods of operation are described.
1. A method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector, comprising the steps of:
providing a capacitively-coupled contactless conductivity detector operable at an operation frequency W 0 , said capacitively-coupled contactless conductivity detector comprising at least two electrodes disposed about a closed channel of an HPLC apparatus and spaced apart from each other, said capacitively-coupled contactless conductivity detector having a capacitance C W between the sensing electrode and a solution in said closed channel, a parasitic capacitance C P between said at least two electrodes, and a solution resistance R S between the at least two electrodes;
providing a inductor having all inductance L S and an internal serial resistance of R LS in parallel electrical connection with said capacitively-coupled contactless conductivity detector, said inductance L S selected to provide a substantially purely resistive impedance when in parallel combination with said capacitively-coupled contactless conductivity detector at said operation frequency W 0 ;
measuring with said capacitively-coupled contactless conductivity detector a signal relating to an analyte-bearing fluid situated in said closed channel;
analyzing said signal with an analysis module to extract a parameter of said analyte-bearing fluid; and
recording said parameter in a memory for future use;
thereby providing a capacitively-coupled contactless conductivity detector that exhibits enhanced sensitivity at said operation frequency W 0 as compared to said capacitively-coupled contactless conductivity detector without said parallel inductor.
2. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 1 , wherein said capacitively-coupled contactless conductivity detector and said parallel inductor are fabricated on a monolithic substrate.
3. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 2 , wherein said monolithic substrate comprises silicon.
4. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 1 , wherein said inductor is an active inductor.
5. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 1 , further comprising the step of adding a series resistance to said parallel combination of said capacitively-coupled contactless conductivity detector and said parallel inductor.
6. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 5 , wherein said step of adding a series resistance comprises adding a negative resistance.
7. A method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector, comprising the steps of:
providing a capacitively-coupled contactless conductivity detector, said capacitively-coupled contactless conductivity detector comprising at least two electrodes disposed about a closed channel of an HPLC apparatus and spaced apart from each other, said capacitively-coupled contactless conductivity detector having a capacitance C W between the sensing electrode and a solution in said closed channel, a parasitic capacitance C P between said at least two electrodes, and a solution resistance R S between the at least two electrodes;
providing a inductor having an inductance L S and an internal serial resistance of R LS , in parallel electrical connection with said capacitively-coupled contactless conductivity detector, said inductance L S selected to provide a substantially purely resistive impedance when in parallel combination with said capacitively-coupled contactless conductivity detector;
operating said combination of said capacitively-coupled contactless conductivity detector and said parallel inductor at or close to a frequency W 0 given by
W
0
=
1
2
(
1
C
P
C
W
′
2
L
S
2
R
S
2
(
-
C
P
L
S
2
-
C
W
′
L
S
2
+
C
W
′
2
R
S
2
(
L
S
-
C
P
R
L
S
2
)
+
-
4
C
P
C
W
2
L
S
2
R
S
2
(
-
L
S
+
(
C
P
+
C
W
′
)
R
L
S
2
+
(
C
W
′
L
S
(
L
S
-
C
W
′
R
S
2
)
+
C
P
(
L
S
2
+
C
W
′
2
R
S
2
R
L
S
2
)
)
2
)
to measure a signal relating to an analyte-bearing fluid situated in said closed channel;
analyzing said signal with an analysis module to extract a parameter of said analyte-bearing fluid; and
recording said parameter in a memory for future use;
thereby providing a capacitively-coupled contactless conductivity detector that exhibits enhanced sensitivity at or close to said operation frequency W 0 as compared to said capacitively-coupled contactless conductivity detector without said parallel inductor.
8. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 7 , wherein said capacitively-coupled contactless conductivity detector and said parallel inductor are fabricated on a monolithic substrate.
9. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 8 , wherein said monolithic substrate comprises silicon.
10. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 7 , wherein said inductor is an active inductor.
11. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 7 , further comprising the step of adding a series resistance to said parallel combination of said capacitively-coupled contactless conductivity detector and said parallel inductor.
12. The method of increasing the sensitivity of a capacitively-coupled contactless conductivity detector of claim 11 , wherein said step of adding a series resistance comprises adding a negative resistance.
13. A capacitively-coupled contactless conductivity detector having increased sensitivity, comprising:
a capacitively-coupled contactless conductivity detector operable at an operation frequency W 0 , said capacitively-coupled contactless conductivity detector comprising at least two electrodes disposed about a closed channel of an HPLC apparatus and spaced apart from each other, said capacitively-coupled contactless conductivity detector having a capacitance C W between the sensing electrode and a solution in said closed channel, a parasitic capacitance C P between said at least two electrodes, and a solution resistance R S between the at least two electrodes; and
a inductor having an inductance L S and an internal serial resistance of R LS in parallel electrical connection with said capacitively-coupled contactless conductivity detector, said inductance L S selected to provide a substantially purely resistive impedance when in parallel combination with said capacitively-coupled contactless conductivity detector at said operation frequency W 0 ;
thereby providing a capacitively-coupled contactless conductivity detector that exhibits enhanced sensitivity at said operation frequency W 0 as compared to said capacitively-coupled contactless conductivity detector without said parallel inductor.
14. The capacitively-coupled contactless conductivity detector of claim 13 , wherein said capacitively-coupled contactless conductivity detector and said parallel inductor are fabricated on a monolithic substrate.
15. The capacitively-coupled contactless conductivity detector of claim 14 , wherein said monolithic substrate comprises silicon.
16. The capacitively-coupled contactless conductivity detector of claim 13 , wherein said inductor is an active inductor.
17. The capacitively-coupled contactless conductivity detector of claim 13 , further comprising a resistance in series with said parallel combination of said capacitively-coupled contactless conductivity detector and said parallel inductor.
18. The capacitively-coupled contactless conductivity detector of claim 17 , wherein said series resistance comprises a negative resistance.