Measurement Of Molecular Oxygen

Question 1

Why and where can we find molecular oxygen?

Answer 1

1. In blood
2. In inhaled gases
   Most bound to hemoglobin

Question 2

To measure any substance, properties that allow its distinction from other compounds are exploited. What are those properties for oxygen? (4)

Answer 2

1. It can be chemically reduced- electrons added
2. It can serve as an oxidant- electrons removed
3. Paramagnetic (attracted to a magnetic field)
4. Can be made selectively permeable to certain membranes

Question 3

What are 3 methods of oxygen measurement?

Answer 3

1. Polarographic (or amerometric) electrode
2. Fuel cell
3. Paramagnetic sensor

Question 4

Oxygen can be electrochemically reduced according to the following equations (2)

Answer 4

Step 1: O2 + 2H2O + 2 electrons —-> H2O2 + 2OH-1

Step 2: H2O2 + 2 electrons —-> 2 OH- (water)

For each molecule of oxygen completely reduced 4 electrons are consumed*

Question 5

Definition of reduction and reductant?

Answer 5

1. Reduction: chemical terms means to contribute to electrons to
2. A compound that is reduced accepts electrons
3. A reductant contributes electrons

Question 6

Basic Features of an oxygen electrode?

Answer 6

In measuring O2 with the O2 electrode, O2 is consumed. Because oxygen is consumed, current is dependent on diffusion of O2 to the electrode

Question 7

Polarographic Electrode:

Answer 7

1. Differs from the pH electrode in that current flows through the circuit when oxygen is present
2. A small direct polarizing voltage (.6-.7V) is applied across the electrodes

Question 8

What happens with polarographic electrodes when voltage is too high or too low?

Answer 8

1. Too low of a voltage and oxygen is not reduced in proportion to its concentration
2. Too high of a voltage and other substances such a protein are reduced.
3. Current is produced in proportion to rate of diffusion of oxygen to electrode

Question 9

Problems with early oxygen electrodes: (2)

Answer 9

1. Large surface area of the electrode consumed a large quantity of oxygen
2. Protein was deposited on the electrode

Question 10

Clark’s solution:

Answer 10

1. Cover the electrodes with membranes to prevent protein deposition but allow oxygen in

Question 11

Other improvements to early oxygen electrodes:

Answer 11

1. Limit the surface area of the electrodes so less oxygen is consumed
2. Use voltage pulses to reduce oxygen consumption

Question 12

Clark Electrode:

Answer 12

1. Early sensors were bare wires and other reactions (reductions) took place at the electrode and caused interference.
2. Clark in 1953 covered the cathode with membrane permeable only to O2– called the Clark electrode

Question 13

Polarographic O2 Analyzer (4)

Answer 13

1. O2 in gas sample permeates a Teflon or polyethylene membrane and enters a potassium chloride (KCL) electrolyte soltuion
2. When a potential of .7 V from a battery is impressed across a platinum cathode and reference anode, current will flow through the circuit in proportion to the O2 availability
3. Measured current is thus linearly related to PO2 of the gas sample
4. Temperature compensation is required for accurate measurement

Question 14

Oxygen measurement by a fuel cell (Galvanic cell)

Oxygen reduction equation:

Answer 14

-Fuel cell analyzer is similar to an oxygen battery. It contains a noble metal cathode and a lead (Pb) anode and KOH electrolyte. A semipermeable membrane is exposed to the blood or expired gas. Oxygen is reduced as shown below:
At the cathode oxygen is reduced
- O2 + 2H2O + 4 electrons —–> 4 OH-

Question 15

Galvanic Cell: At the anode lead is _________ by OH ions?

Answer 15

1. Lead is oxidized by OH ions
   2Pb + 4 OH- —-> 2 PbO + 2 H2O + 4 electrons

Overall reaction
O2 + 2 Pb —–> 2 PbO

Question 16

Lead is _______ by hydroxyl ions and generates electrons

Answer 16

• oxidized
• A voltage develops that is proportional to the PO2.
• no polarization is required
• a voltmeter measures the electrical potential and displays a reading

Question 17

Fuel Cell O2 Analyzer

Answer 17

• Oxygen in gas samples permeates a membrane and enters a potassium hydroxide (KOH) electrolyte solution
• a potential is established between a lead (Pb) anode and noble metal cathode (Au) as oxygen is supplied to the anode
• measured voltage between electrodes is proportional to PO2 of the gas sample
• temperature compensation is required for accuracy

Question 18

Errors that may occur in blood oxygen measurment:

Integrity of the specimens (3)

Answer 18

1. Too much suction
2. Too much heparin
3. Air bubbles in blood

Question 19

Storage considerations:

Answer 19

1. Syringe should have no cracks and properly fitting plunger
2. Gases in blood will equilibrates in time with air in polypropylene syringes
3. High white cell count will increase aerobic respiration and decrease blood oxygen content
4. Bubbles if present, will increase oxygen dissolved in blood at lower temperature, such as 4C in ice

Question 20

Patient’s temperature— how is affect blood oxygen measurement?

Answer 20

1. If the patient’s temperature is different from 37 then analysis will not be accurate
2. More gas will be dissolved in cooler blood than warmer blood

Question 21

Transcutaneous Electrode:

Answer 21

1. Non invasive method to measure blood oxygen and carbon dioxide that use the same electrode technology as the CO2 and O2 electrodes
2. Electrodes are incorporated into an apparatus that allows the formation of an airtight seal with the skin and apparatus.
3. The apparatus contains a heating element that brings the skin temperature up to 43-43C
4. Blood flow to the skin increases and capillary O2 and CO2 diffuses to the surface where they are measured by the electrodes

Question 22

Paramagnetic Oxygen Sensor

Answer 22

1. Makes use of the fact that oxygen molecules are attracted to magnetic field (remember that molecular oxygen has 2 unpaired electrons)
2. In one system a stream of gas containing oxygen is put in an oscillating magnetic field
3. The oscillating magnetic field causes movement of the oxygen molecules
4. The oxygen movement causes the pressure of the gas to create as function of oxygen concentration

Question 23

Fluorescence:

Answer 23

1. Technology that can be used for measuring not only oxygen, but also pH and pCO2

Question 24

Fluorescence

Answer 24

• The emission of light or electromagnetic radiation by a substance as the result of absorption of energy of shorter wavelengths
• as an example is the excitation of a substance by UV light, and emission of visible light

Question 25

For measurement of oxygen, a fluorescent substance is bonded to a ______________ matrix and attached to the end of an optical fiber.

Answer 25

Hydrophilic

The dye is excited at 385 nm and the light is emitted at 515 nm. Oxygen functions to quench the fluorescence

Question 26

Ramen Gas Analyzer

Answer 26

1. Rascal II enables anesthetists to monitor physiologic gasses such as oxygen and CO2 as well as anesthetic gases

Question 27

Ramen Gas Analyzer: how it works

Answer 27

1. Based on a helium- neon laser, the Rascal II directs its beam into a sampling chamber
2. This excites the molecules of the gas, sending its electrons into a higher energy level.
3. When the electrons fall back into a lower energy level, they emit scattered light at a longer wavelength than the original beam
4. This change can be measured to identify the compound

Question 28

NOTE: in the Rame Gas Analyzer, the detector is __________ to the light beam

Answer 28

1. Perpendicular