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1 Radiometer Medical ApS, Åkandevej 21, DK-2700 Brønshøj, Tel: +45 38 27 38 27, www.radiometer.com RTC, December 2004 ABL800 FLEX Electrode measuring principle.

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Presentation on theme: "1 Radiometer Medical ApS, Åkandevej 21, DK-2700 Brønshøj, Tel: +45 38 27 38 27, www.radiometer.com RTC, December 2004 ABL800 FLEX Electrode measuring principle."— Presentation transcript:

1 1 Radiometer Medical ApS, Åkandevej 21, DK-2700 Brønshøj, Tel: , RTC, December 2004 ABL800 FLEX Electrode measuring principle

2 2 Agenda Parameters General construction Measuring principles The potentiometric measuring principle ­ Reference electrode ­ pH electrode ­ Electrolyte electrodes ­ pCO 2 electrode The amperometric measuring principle ­ pO 2 electrode ­ Metabolite electrodes Summary Electrode signal updating

3 3 Location The electrodes are placed in the electrode modules easily accessible under a window on the front of the analyzer

4 4 Parameters There are 10 electrode slots in the two electrode modules One electrode slot for the reference electrode Nine other electrode slots available for the following : pH, pCO 2, pO 2, cK +, cNa +, cCa 2+, cCl –, cGlucose, cLactate pH, pCO 2, pO 2, cK +, cNa +, cCa 2+, cCl –, cGlucose, cLactate

5 5 Electrode modules

6 6 General construction The term ‘electrode’ refers to whole sensor unit Cordless – limiting electrical noise Amplifiers positioned in each module to amplify the electrical signal Electrical contact Color-coded ring Electrolyte solution Electrode jacket (removable) Membrane Preamplifier

7 7 Two measuring principles The potentiometric measuring principle is used for The amperometric measuring principle is used for pH, pCO 2, cK +, cNa +, cCa 2+, cCl – pO 2, cGlucose, cLactate

8 8 The potentiometric measuring principle Electrodes measure a change in voltage due to a change in ion concentration across a membrane Sample Electrode Electrolyte solution V Reference electrodeElectrode Voltmeter Liquid junctionMembrane Electrolyte solution pH, pCO 2, cK +, cNa +, cCa 2+, cCl –

9 9 Measuring a potential Each link in the circuit exhibits a potential The only unknown potential is the one between the membrane and the sample, E Sample The potential of the whole circuit is measured, E voltmeter, and the unknown potential can be calculated:   EEEE SamplevoltmeterRefpH 

10 10 Reference electrode Provides a stable, fixed potential against which other potentials can be measured The potential at the reference electrode is not altered by sample composition A stable, fixed potential is obtained by maintaining constant conditions The reference electrode is used in the measurement of pH and the electrolyte parameters

11 11 Reference electrode – E1001 Electrode contact Electrolyte solution - A 4M sodium formate (HCOONa) adjusted to pH 5.5 with hydrochloric acid. Acts as salt-bridge solution that maintains an electrochemical contact between the coated Ag wire and the sample. Ag rod coated with AgCl Membrane Consists of three separate membrane layers: - Inner: Limits diffusion through the membrane and stabilizes the whole membrane system - Middle: Prevents protein interference - Outer: Reduces the interchange of sample or rinse solution and HCOONa solution Electrolyte jacket - The rubber ring seals the electrode in the jacket to prevent evaporation or leakage of the electrolyte solution

12 12 pH electrode Ag rod coated with AgCl - provides electrical contact to inner buffer solution - Ag/AgCl equilibrium maintains a stable potential Air bubble -allows expansion of solution at 37 °C Inner buffer solution - has a constant and known pH pH-sensitive glass membrane - changes in potential only due to changes in pH of sample

13 13 pH-sensitive glass membrane

14 14 Nernst equation, pH electrode Varying exchange of H + ions between sample and glass membrane gives rise to a change in potential (voltage) Change in potential can be converted to a change in concentration by the Nernst equation: The analyzer automatically converts activity (effective concentration) into concentration EE K n a   0 H log E = Measured potential E 0 = Standard potential K = Temperature-dependent constant n = Charge on ion (+1) a H+ = Activity of H +

15 15 Electrolyte electrodes Ion-selective-membrane electrodes The main difference between the electrolyte electrodes is the selectivity of the membranes with respect to which anions and cations can pass and how. The membranes are selective for a single ion species only. The membrane potential is determined against the Reference electrode

16 16 Electrolyte electrodes design Ag rod coated with AgCl - provides electrical contact to buffer solution - Ag/Ag + equilibrium maintains a stable potential Electrolyte solution Porous pin - absorbs electrolyte solution -holds PVC membrane PVC membrane - contains a K + ion exchanger Cellophane membrane - protects PVC membrane -prevents protein build-up -held in place by white plastic gasket Potassium is used as an example

17 17 Ion sensitive membranes varying K + exchange - changing potential - dependant on cK + in sample constant K + exchange - constant potential PVC membrane containing specific ion-carrying molecules Cellophane membrane K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ Electrolyte Solution Sample Other electrolyte electrodes have different ion carriers No ion-specific PVC membrane in Na electrode; instead the pin is made of special ion-carrying material

18 18 pCO 2 electrode Consists of a pH electrode and an internal reference electrode in one complete unit Reference electrode – Ag/AgCl Silicone membrane on nylon net - S ilicone is permeable to CO 2 -Nylon net traps electrolyte solution pH-sensitive glass membrane Air bubble Electrolyte solution - The bicarbonate electrolyte solution also contains glycerol to prevent collection of air bubbles in the jacket, thus improving electrode stability Electrode from above - The gold ring is the contact of the reference system in the electrode pH electrode – Ag/AgCl - Inner solution with known and constant pH

19 19 CO 2 pCO 2 measuring method Ê CO 2 permeates the silicone membrane and dissolves in the electrolyte solution trapped in the nylon net Ë Carbonic acid is produced, a pH change occurs Ì This pH change is measured by change in potential at the pH- sensitive glass membrane. The potential reading is converted into a pH value by the Nernst equation. Í The pH value is related to the partial pressure of CO 2 in the sample by the Henderson- Hasselbalch equation Nylon net with electrolyte solution Silicone membrane Glass membrane Sample COHOH HHCO   EE K n a   0 H log  pHpKlog HCO CO a    p

20 20 The amperometric measuring principle Electrodes measure the current produced during an electrochemical reaction at an electrode pO 2, cGlucose, cLactate Anode -oxidation Sample Applied voltage Cathode -reduction Electrolyte solution -buffered Ammeter -measures current

21 21 pO 2 and metabolite electrodes The pO 2 and metabolite electrodes are designed to measure the current produced during an electrochemical reaction The flow of electrons (the current) is proportional to the concentration of the substrate (the pO 2 /Glu/Lac) An electrochemical reaction: - a reaction where electrons are transferred A + + e -  A

22 22 pO 2 electrode design The pO 2 electrode is designed to measure the current produced during an electrochemical reaction Anode (+) – Ag rod coated with AgCl -oxidation of Ag Cathode (–) – Pt wire encased in glass -reduction of O 2 Polypropylene membrane - permeable to O 2 Electrolyte solution - provides electrical contact between anode and cathode Platinum black catalyst (thin black layer) - conversion of H 2 O 2 to H 2 O and O 2

23 23 pO 2 measuring method Oxygen from the sample diffuses across the membrane into the electrolyte solution and is reduced at the cathode Reduction of O 2 : ­ The H+ ions come from the electrolyte solution and e- comes from the silver anode H 2 O 2 produced from O 2 not completely reduced ­ This is then immediately decomposed and catalyzed by Pt black to O 2 which again is reduced at the cathode To complete the electrical circuit, Ag is oxidized at the silver anode O 2 -permeable membrane Electrolyte solution (contains H + ) Sample Pt black Pt cathode O4H + 4e - 2HO 22  O O+O 2222  Pt Ag Ag + e-e-  +

24 24 The reduction of oxygen produces a flow of electrons the size of the current, I, which is proportional to the amount of oxygen and measured by the ammeter in pico Ampere (pA) pO 2 measuring method I  pO2pO2 I = Sens(pO 2 ) x pO 2 + I 0 pA The current measured during this process is automatically converted to a pO 2 value by the analyzer

25 25 Metabolite electrodes The electrode is an amperometric electrode consisting of a silver cathode with a AgCl reference band and a platinum anode, all protected by an electrode jacket filled with electrolyte solution At the tip of the jacket is a multilayer membrane The electrodes are identical in design

26 26 Metabolite electrodes design Glucose and lactate electrodes are identical in construction, the major difference is the enzyme in the membrane layer Anode – Pt wire -oxidation of H 2 O 2 Multilayer membrane -outer layer permeable to glucose/lactate -middle enzyme layer (production of H 2 O 2 ) -inner layer permeable to H 2 O 2 Cathode – Ag rod coated with AgCl -reduction of Ag + Electrolyte solution - provides electrical contact between anode and cathode Glucose is used as an example

27 27 The metabolite membrane design Multilayered membrane ­ outer layer permeable to glucose/lactate ­ middle enzyme layer (production of H 2 O 2 ) ­ inner layer permeable to H 2 O 2 Inner layer Outer layer Middle layer

28 28 Metabolites – measuring method Glucose/lactate molecules are transported across the outer membrane to be converted by the enzyme to form H 2 O 2 The H 2 O 2 is then oxidized to oxygen and electrons, a current, which is measured by the ammeter Glucose+OGluconic acid+HO 222  Lactate+OPyruvate+HO 222  HO2H + +O+2e  The amount of current produced is proportional to the amount of H 2 O 2, which in turn is directly related to the amount of glucose or lactate in the sample

29 29 Metabolite membrane - outer layer Has pores of well-defined density and diameter to limit the amount of glucose/lactate entering the enzyme layer Outer side is treated to prevent protein build-up that could block the pores Glucose/lactate The sensor is not affected by hematocrit due to the outer low- porous membrane Blood sample Red blood cells Inner layer Outer layer Middle layer Outer layer

30 30 Metabolite membrane – middle enzyme layer The enzyme glucose or lactate oxidase is immobilized between the outer and inner layer The enzymes only catalyze the following reactions: Long lifetime of membrane No dependency on sample’s oxygen content Glucose+OGluconic acid+HO Lactate+OPyruvate+HO   Inner layer Outer layer Middle layer O 2 is supplied from the outer membrane to make the process independent of the O 2 content in the sample

31 31 Metabolite membrane – inner layer The H 2 O 2 from the enzyme reaction is transported across the inner membrane to the Pt anode for oxidation The inner membrane is an interference-limiting membrane. In less than 30 seconds the H 2 O 2 has diffused through the membrane and reached the Pt anode. Electrochemical substances that can interfere are delayed, e.g. Paracetamol-4-acetamidophenol, and will not pass through the membrane during the 30-second period Inner layer Enzyme layer E.g. Paracetamol -4-acetamidophenol Inner layer Outer layer Middle layer

32 32 Summary The following parameters are measured: ­ pH, pCO 2, pO 2, cK +, cNa +, cCa 2+, cCl –, cGlucose, cLactate Two measuring principles ­ ion-selective electrodes for pH, pCO 2 and electrolytes ­ measurement of current produced from reactions of O 2, Glu and Lac Multilayer interference-limiting membranes for glucose and lactate electrodes ­Long lifetime of membrane ­No dependency on sample’s oxygen content ­No interference from commonly known substances

33 33 Electrode updating Electrode signals are registered at second intervals during calibrations and measurements Updating starts when the sample/calibration solution has reached the measuring modules Duration and number of updatings (30) of electrode signals are predetermined The stability of the signals are evaluated before a calibration or a measurement result is accepted

34 34 pH electrode signal updating Seconds pH p 30 p1p1 p 20 p 21 Calibration pH upd.30 - pH upd.20  0.005

35 35 pO 2 calibration electrode signal updating Seconds pO2pO2 p 16 p 30 p1p1 p 24 p 25 Calibration pO 2 upd.30 - pO 2 upd.24  0.80

36 36 pCO 2 calibration electrode signal updating Seconds pCO 2 p 16 p 30 p1p1 p 24 p 25 Calibration pCO 2 upd.30 - pCO 2 upd.24  0.40

37 37 Radiometer Medical ApS, Åkandevej 21, DK-2700 Brønshøj, Tel: , Radiometer Training Center, December 2004


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