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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the detection of defects in
selectively permeable barriers through which electrochemical sensors
communicate with a sample material to be analyzed and, more particularly,
to a method for checking the integrity of such a barrier associated with a
sensor incorporated in an electrochemical titrator.
2. Description of the Prior Art
Copending U.S. Pat. application Ser. No. 586,435 (now U.S. Pat. No.
4,003,705), by Buzza et al., and assigned to the assignee of the present
invention, describes an electrochemical analysis apparatus for measuring
both chloride and carbon dioxide in blood. The blood sample is reacted
with an acid reagent in a sample chamber to release carbon dioxide which
diffuses through a gas permeable membrane to a carbon dioxide sensor. The
sensor includes an alkaline reagent which undergoes a change in pH upon
reaction with the carbon dioxide and a pH measuring electrode arrangement
for measuring the resulting pH change to provide a measure of the carbon
dioxide concentration. Specifically, the pH signal is differentiated to
provide an instantaneous time rate of change of pH signal which is
measured to determine the carbon dioxide value.
For chloride detection, the aforementioned apparatus includes titration
apparatus comprising coulometric generator electrodes and amperometric
detector electrodes communicating with the sample chamber for titrating
the sample chloride. To this end the coulometric generator electrodes are
energized to generate silver ions which combine with the sample chloride
to precipitate silver chloride. The completion of the silver chloride
precipitation is detected by the amperometric detector electrodes and the
total coulometric current flow required to precipitate the silver chloride
provides a measure of the chloride originally in the sample. To ensure
that the measured coulometric current represents titration of the sample
chloride only, the coulometric generator is energized prior to sample
introduction to precipitate out chloride which may be present in the acid
reagent to establish an initial or base line chloride level in the sample
chamber prior to the introduction of each sample.
It is critically important that the selectively permeable membrane permit
the passage of carbon dioxide to the carbon dioxide sensor but reject all
other substances which could adversely affect operation of the carbon
dioxide sensor. Moreover, the membrane must provide electrical isolation
between the carbon dioxide sensor and the titration circuitry. If the
membrane includes pin holes or other defects which permit reagent leakage
through or around the membrane to the carbon dioxide sensor or which
permit electrical cross-talk between the carbon dioxide measuring and
chloride measuring systems, then erroneous sample measurements will
result. Typically, for adequate isolation a membrane should exhibit an
impedance of about 10.sup.9 ohms or more.
Several proposals have been advanced in the past for monitoring the
isolation integrity (i.e. impedance) of selectively permeable membranes in
electrochemical sensor arrangements. In one approach an a.c. signal is
superimposed across the membrane and a.c. detector and demodulation
circuitry is provided to monitor the a.c. signal transmitted by the
membrane. In another approach a separate monitoring electrochemical half
cell and associated monitoring circuitry is provided for developing a
signal across the membrane indicative of a fault in the membrane. While
each of these approaches is generally satisfactory for the purpose
intended, each requires the addition of independent and complex electrical
monitoring circuitry and components which reduces the overall
attractiveness of each for commercial implementation particularly for
combined electrochemical sensing and titration apparatus as described
above. As a result, a need exists for a simple and inexpensive technique
for checking the integrity of a selectively permeable barrier in such
apparatus.
SUMMARY OF THE INVENTION
The present invention resides in a new and improved method for checking the
impedance of a selectively permeable barrier associated with an
electrochemical sensor in combined sensing and titration apparatus. The
method overcomes the disadvantages of the prior approaches and does so in
a manner which is simple and straightforward in operation and readily
adapted for commercial implementation.
In accordance with a primary aspect of the present invention, applicants
have discovered that in combined titration and electrochemical sensing
apparatus, if the electrochemical sensor and the coulometric generator of
the titrator are both enabled during a period prior to introduction of
sample into the sample chamber, the signal then derived from the
electrochemcial sensor indicates the degree of isolation provided by the
barrier means separating the electrochemical sensor from the sample
chamber. In apparatus where the sample is reacted with a reagent
previously introduced into the sample chamber, the coulometric generator
means may be enabled to titrate a constituent of the reagent to an initial
or base line value before the sample is introduced, and the
electrochemical sensor is preferably enabled at this time to check the
barrier means.
Measuring circuitry coupled to the electrochemical sensor includes means
for differentiating the output signal from the sensor and threshold
detection means for measuring a predetermined threshold value of the
differentiated signal. The measuring circuitry generates a control signal
in response to a differentiated output signal above the predetermined
threshold value and the control signal is employed to inhibit further
operation of the analyzer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view, taken in a generally vertical
plane, through a sample cup employed in the electrochemical analysis
apparatus employed with the present invention and illustrates the mounting
of chloride determining electrodes and a carbon dioxide measuring sensor,
the sensor being partially cut-away, together with associated reagent
pumping systems.
FIG. 2 is a combined diagrammatic and electrical schematic diagram of the
apparatus for checking the isolation between the carbon dioxide and the
chloride measuring systems.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 of the drawings illustrates an electrochemical analysis apparatus of
the type described in aforementioned copending patent application Ser. No.
586,435 which represents one preferred apparatus for practicing the
present invention. The prior apparatus will be described to the extent
necessary for an understanding of the present invention and reference
should be made to the copending application for additional details
concerning the apparatus.
The analysis apparatus of FIG. 1 includes an analysis cell 10 formed from a
block of insulating material such as polymethylmethacrylate. A
cylindrical, vertically extending chamber 12, open at its upper end, is
formed in the block. A sample such as blood to be analyzed may be injected
into the chamber through the upper end thereof by means of a pipette or
burette in a conventional manner. The analysis cell is preferably formed
from upper and lower mating sections 14 and 16, respectively. Lower
section 16 has an upstanding, threaded, central hub 18 which is threaded
into upper section 14 to secure the two sections together. The lower
surface of the sample chamber is formed in the shape of an inverted cone
in the upper surface of the hub 18. A pair of passages 20 and 22 lie in a
generally horizontal plane in the lower section and intersect one another
at a right angle at the apex of the conical lower surface of the chamber
(passage 20 is disposed perpendicular to the plane of the figure). Both
passages communicate with the chamber 12 at the apex of the lower surface.
Acid reagent, such as sulfuric acid, from reservoir 24 is supplied to the
chamber 12 through passage 20 and the contents of the cell, including
reagent, sample and the like, are drained from the cell through passage 18
to a waste receptacle 26. Conventional pumping and valving arrangements 28
and 30 are employed to respectively deliver reagent to the cell and to
drain the contents of the cell. A conventional magnetic stirring element
32 positioned in the chamber is adapted to be rotated by a magnet 34
positioned below the cell 10 and rotated by means of a motor 36 to mix the
contents of the chamber 12.
Titration apparatus for titrating the chloride in the sample is embodied in
an electrode module 38 comprising amperometric anode 40, amperometric
cathode 42, and coulometric cathode 44 disposed in a horizontal bore in
the wall of the analysis cell 10 with the electrodes extending into the
sample chamber 12 through a reduced diameter section of the bore. The
structure of the electrode module 38 may take the form disclosed in our
copending U.S. Pat. application Ser. No. 595,207 filed July 11, 1975, (now
U.S. Pat. No. 4,007,105), by Buzza et al. and reference may be had to this
application for further details of the module 38.
The fourth and remaining electrode of the titration apparatus, coulometric
anode 46, is disposed at the bottom of the chamber 12 and is conically
configured to conform to the bottom chamber surface. In addition anode 46
has an opening 48 at the apex thereof aligned with the flow passage
thereat and through which the chamber is filled and drained. The base of
the conical anode 46 has a horizontally extending, circumferential lip 50
which abuts an O-ring 52 recessed in upper section 14 of the analysis cell
10 to provide a fluid seal between the two sections of the cell.
Electrical connection is made to the anode 50 through a passage (not shown)
in the lower cell section 16 by means of a conductor 46a. Corresponding
conductors 40a, 42a and 44a are connected to the remaining electrodes, and
the four conductors serve to connect the amperometric pair of electrodes
to a conventional amperometric detector (88, FIG. 2) and to connect the
pair of coulometric electrodes to a conventional coulometric generator
(90, FIG. 2).
The apparatus for detecting the carbon dioxide of the sample is identical
to that shown in the aforementioned copending patent application Ser. No.
586,435 and comprises first and second electrochemical sensors 54 and 54'.
Sensor 54 is illustrated mounted within a horizontal bore of the analysis
cell 10 and in communication with the sample chamber 12 thereof through a
selectively permeable barrier which, for the measurement of a gas such as
carbon dioxide, is a gas permeable, ion-impermeable membrane 56 which is
held in place over the end of the sensor by an annular retaining ring 58.
Suitable materials for membrane 56 are silicone rubber, polyethylene, or
polytetrafluoroethylene. Sensor 54' is identical in construction to sensor
54; however sensor 54' is not mounted in communication with chamber 12
through its corresponding membrane 56'. Rather, sensor 54' may be mounted
in a blind bore in cell 10 as taught in aforementioned application Ser.
No. 586,435.
Sensor 54 further comprises a conventional pH sensing electrode 60 spaced
slightly (on the order of 0.005 inches) from the membrane 56 to define an
electrolyte film space 62 between one end of the pH electrode and the
membrane. A pair of stainless steel tubes 64 and 66 extend longitudinally
within the sensor 54 and are in fluid communication with opposite sides of
the electrolyte film space 62.
A series electrolyte flow path is provided through both measuring sensors
54 and 54' from an alkaline electrolyte reservoir 68, containing potassium
bicarbonate for example, to a second waste receptacle 70. A conventional
pump and valving arrangement supplies reagent from reservoir 68 to the
inlet tube 64' of sensor 56'. The outlet tube 66' of sensor 56', in turn,
is connected to the inlet tube 64 of sensor 54. The outlet tube 66 of
sensor 54 is then connected to the waste receptacle 70. Electrolyte is
pumped from reservoir 68 along the series path through each sensor after
each sample measurement. As a result, the electrolyte film space 62 of
sensor 54 is purged of old reagent and replaced by new reagent which
rapidly equilibrates the pH electrode 60 in preparation for the next
sample injection and measurement.
A solution ground terminal 74, such as a piece of stainless steel, is
provided in the electrolyte flow path either between sensors 54 and 54' or
between snesor 54' and reservoir 68. The ground terminal 74 together with
output conductors from pH electrodes 60 and 60' are connected to supply
corresponding signals to the carbon dioxide measuring electronics.
FIG. 2 illustrates the apparatus of FIG. 1 in diagrammatic form and
includes, in block diagram form, the circuitry for measuring carbon
dioxide and for checking the isolation integrity of the selectively
permeable membrane 56 of sensor 54. In FIG. 2 reference numerals
corresponding to those in FIG. 1 have been utilized to identify the same
features.
The carbon dioxide measuring circuitry is identical to that described in
the aforementioned copending patent application 586,435 and comprises a
differential amplifier 76 having first and second input terminals 78 and
80 receiving the respective output signals from the pH electrodes 60 and
60'. The output of differential amplifier 76 is connected to
differentiator circuit 82 which differentiates the differential pH signal
from amplifier 76 to derive a signal at its output proportional to the
instantaneous time rate of change of pH. The output of differentiator
circuit 82, in turn, is connected to a peak pick and hold amplifier 84
which senses a maximum value of the time rate of change signal and derives
an output to a digital display 86. The differentiator and amplifier
circuits may take the form illustrated and described in "Glucose Analyzer
Service Manual", Beckman Instruction 83544-B), copyright 1970 by Beckman
Instruments, Inc.
In FIG. 2 the amperometric detector electrodes 40 and 42 are shown
connected to conventional amperometric detector circuitry 88 while
coulometric generator electrodes 44 and 46 are shown connected to a
conventional coulometric current generating source 90. Current source 90,
when activated, serves to liberate silver ions from coulometric anode 46
which combine with the chloride in the cell 12 to precipitate silver
chloride in a conventional manner.
In accordance with an important aspect of the present invention, the
coulometric current generating means and the electrochemical pH sensors
and associated measuring circuitry are activated prior to introduction of
a sample into the chamber 12 in order to check the isolation afforded by
membrane 56.
Coulometric generator is activated by closing the switch 92 in series with
current source 90 while the pH measuring system is activated by closing
switch 94 illustrated between amplifier 76 and differentiator 82.
Operation of switches 92 and 94 is under the direction of a control 96
which may be manually operated or programmed by well known logic
techniques to perform the membrane checking function.
In practice, prior to measuring a sample, the chamber 12 is drained and
filled twice with acid reagent from reservoir 24 in order to thoroughly
rinse the chamber. Thereafter, the chamber is filled with about one
milliliter of acid reagent into which a 10 microliter blood sample will be
subsequently injected. Prior to injection of the sample, however, control
96 operates to close switch 92 to enable the coulometric current generator
to titrate chloride present in the reagent to a predetermined base line
value which is repeatable for each sample to be measured. Typically the
coulometric generator is enabled to generate a current pulse for a period
of approximately eight seconds to perform this initial titration.
Normally, during the titration of the chloride present in the reagent, the
pH sensors are inactive. Thus, as far as the carbon dioxide sensing
portion of the analyzer is concerned, the reagent chloride titration
period has in the past in effect been "off-time". In accordance with the
present invention, however, control 96 further operates to close switch 94
and activate the pH measuring sensors 60 and 60' and associated measuring
circuitry 78-86 during the period of coulometric current generation prior
to sample introduction. If membrane 56 is defective, that is if pin holes
or other defects reduce the impedance of the membrane, an apparent .DELTA.
pH signal will be derived across pH electrodes 60 and 60', supplied as an
input signal to differential amplifier 76, differentiated by
differentiator 82, and supplied to peak pick and hold amplifier 84.
Peak pick and hold amplifier 84 further includes a conventional comparator
C for determining when the signal received by amplifier 84 exceeds a
predetermined value. The comparator C includes a first input terminal
which receives the signal from differentiator 82 and a second terminal
supplied with a threshold voltage level V.sub.TH establishing the minimum
voltage value indicative of a satisfactory membrane 56. When the voltage
from differentiator 82 supplied to the first input terminal of comparator
exceeds V.sub.TH, the signal at output terminal of the comparator changes
state to supply a control signal indicative of a defective membrane. The
control signal is supplied to display 86 to lock the peak output value of
the differentiator into the display in a conventional manner thus
indicating a defective membrane and the severity of the isolation
breakdown. In addition, the control signal is supplied to control 96 which
then inhibits further operation of the analyzer by opening switches 92 and
94 to prevent both the chloride and carbon dioxide measuring systems from
operating. Thereafter, the defective membrane is replaced.
If membrane 56 if found to be satisfactory, the sample is then injected
into the reagent filled chamber 12 by an operator and simultaneous
chloride and carbon dioxide measurements are taken for the sample.
Thus it is seen that an apparent .DELTA. pH response during coulometric
titration prior to sample introduction will really be an indication of an
isolation breakdown of membrane 56 due to pin holes or other defects in
the membrane. Differentiating the apparent .DELTA. pH signal to obtain the
rate of change of pH increases the detection sensitivity of such membrane
failure. This is because differentiating the pH signal essentially
amplifies the abrupt pH signal change caused by that portion of the
coulometric generator signal being transmitted through the membrane.
Furthermore, the level of the differentiated signal is independent of the
steady pH level which existed between the two pH electrodes prior to the
coulometric titration. This allows the establishment of an absolute fault
level based on the differentiated signal itself.
While a preferred embodiment of the invention has been illustrated and
described, it will be apparent that modifications may be made therein
within the scope of the appended claims.
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