Analyzer Basic

Photometry

Photometry Measurement Non-Dispersive Gas Analysis

• Molecules convert Absorbed Energy into Oscillatory or Rotational Energy
• Absorbing Capability Requires a Dipole Moment
• Each Component Absorbs at specific Wavelengths
• Preconditions for IR active Components
• Move of Electrons inside the Atoms to higher Levels
• Requires higher Energy in Visible or Ultraviolet  Range
• Selectivity via Detector, Filters or Narrow-band Beams

Absorption of Radiation

 Advantage of NDIR / UV / VIS Technology

• Several Components absorb in NDIR Region
• Some Components Show Cross Interference's  due to overlap in Absorption  of Radiation
• Especially Hydrocarbons, H₂O and CO₂ may cause cross Interference's
• NDUV or NDVIS is an Alternative Method to NDIR
• SO₂, NO₂ and Cl₂ Show less or no  Interference's in NDUV / VIS Region 

Lambert-Beer Principle - Non-Linear Curve

Paramagnetic – PMD

Paramagnetic Technology

Magnetic Susceptibility

All gases exhibit some form of magnetic susceptibility.  This susceptibility causes the gas to be attracted or repelled by a magnetic field.  Oxygen exhibits a very strong paramagnetic susceptibility with respect to most other gases, and therefore is drawn to a magnetic field.  Most other gases are weakly diamagnetic (repelled from a magnetic field).

Based on this fact, the paramagnetic detector consists of a test body filled with N₂ and suspended in a non-uniform strong magnetic field.  A mirror is attached to the center of the test body and a light source/photocell combination is used to measure the movement of the test body.   The final part of the detector is a current loop used to pull the test body back to its “null” position.

• When sample is introduced to the device, the oxygen present in the sample is drawn to the magnetic field.  The buoyancy of the test body causes it to be displaced out of its null position in the field.
• The light focuses a beam on the mirror of the test body and is reflected to the photocells.  When the test body moves, the photocells register an imbalance and drive a restoring current to pull the test body back to its null position.
• The intensity of this restoring current is directly proportional to the concentration of oxygen present.  By measuring this current, we can accurately derive the oxygen concentration of the sample.

Paramagnetic Detector

Thermal Conductivity Detection – TCD

Thermal conductivity detector

• One of the most important detectors.
• The detector consists of a metal block with four cells. The sample will flow through two of them and the other two will be the reference.
• Platen- or Wolfram-filaments in a Wheatstone bridge circuit.
• Electrical power will go through each filament and will heat up the filament.
• The resistance is depending on the thermal conductivity of the gas, which will flow through the cells. 
• Changes of the gas composition will cause changes in the temperature and resistance of the filaments of the cell.
• The equilibrium of Whetstone bridge circuit will be disturbed.
• The temperature of the sample cells will be stabilized on a higher temperature level if the gas has less thermal conductivity than the reference gas. The temperature of the reference filaments will not change. This temperature difference causes an voltage discrepancy. The voltage discrepancy will be amplified and can be measured with an instrument (PC, printer etc.)
• The resulting signal is proportional to the concentration of the component.
• With the TCD one can measure all species  of organic and inorganic  % to high ppm levels
• Hydrogen, Helium, Argon or Nitrogen as carrier

 Flame Ionization Detection - FID

How does the FID detector work?

• A hydrogen flame is generated in a burner
• Sample gas with hydrocarbons is introduced into the hydrogen flame
• Hydrocarbon molecules are ionized in the hydrogen flame
• Ions formed produce an electrical current
• Current is proportional to the carbon count  in the hydrocarbon molecules

FID Burners

Flow Diagram - 400A

Chemiluminescence Detection – CLD

Chemiluminescence - Theory of operation

• Nitrogen oxide (no) reacts with ozone (o₃) to form nitrogen dioxide (no₂) in an excited state. 
• No₂ immediately reverts to ground state by emitting a photon of light ==> chemiluminescence. 
• Light intensity is proportional to the concentration of no/nox in the sample gas. 
• The silicon photo diode detector measures light intensity and generates an output signal.

CLD Detector Technology

 Theory of Operation (CLD)

• Nitrogen oxide (no) is measurable in a gas stream by its chemiluminescence
• Reaction properties:                              

     NO + O₃ = NO₂* + O₂          (1)

     NO₂* = NO₂ + hV (Photon) (2)

• Emission of photon = chemiluminescence