Flame Ionization Detector
Most common detector
Carbon atoms (C-C bonds) are burned in a hydrogen flame.
A small portion of carbon atoms are ionized (about 1 in 10,000),
The ions carry a charge from the flame to the walls of the detector
which surrounds the flame.The charge is electronically amplified
and sent to a recording device.
Flame Ionization Detector
very robust
and reliable.
Nitrogen Phosphorus Detector
The NPD uses a bead of a compound such as rubidium silicate
above a jet of H2. The bead is heated by an electric current
forming a plasma. Nitrogen and phosphorous react in the plasma
forming specific ions that carry a small current to the charged
collector. The NPD is electronically similar to the FID, but since
there is no flame, hydrocarbon ionization does not occur.
The response to N is 103 – 105 greater than response to C
The response to P is 104 – 10 5.5 higher than response to C.
The linear dynamic range is 104
It is only fairly reliable since the bead burns up
over time causing drift in the signal.
Flame Photometric Detector
The (FPD) is an element specific detector
Commercial instruments are limited to the detection of P and S
The analytes are burned in a H2 flame causing electrons to
move to an excited unstable state. When the electrons return
to the ground state, they emit a specific wavelength of light
526 nm for P and 394 nm for S
These wavelengths are monitored by a photomultiplier, amplified,
and turned into an electrical signal.
This detector is sensitive to 10-9g
Linear dynamic range for P of only 103 – 104.
For S, the response is non-linear.
Flame Photometric Detector
Electron Capture Detector
The ECD uses a radioactive source such as Ni63 which produces
Beta particles which react with the carrier gas producing free
electrons. These electrons flow to the anode producing an
electrical signal . When electrophillic molecules are present,
they capture the free electrons, lowering the signal. The amount
of lowering is proportional to the amount of analyte present.
It is sensitive down to 10-15 but the dynamic range is only about 104.
It’s an excellent detector for molecules containing an electronegative
group such as Cl or F etc. (or derivitized molecules).
Electron Capture Detector
Atomic Emission Detector
One of the newest gas chromatography detectors
Quite expensive compared to other GC detectors
The strength of the AED lies in the detector's ability to
simultaneously determine elements. It uses microwave energy
to excite helium molecules (carrier gas) which emit radiation
which breaks down molecules to atoms such as S, N, P, Hg, As, etc.
These excited molecules emit distinctive wavelengths which can
be separated by a grating and sent to the detector
(typically a photodiode array) which produces the electrical signal.
Atomic Emission Detector
Photoionization Detector
The photoionization detector (PID) uses a UV lamp
(xenon, krypton or argon) to ionize compounds. The ionization
produces a current between the two electrodes in the detector.
The detector is non-destructive and can be more sensitive than
an FID for certain compoundssubstituted aromatics and cyclic compounds
Photoionization Detector
Thermal Conductivity Detector
Consists of an electrically-heated wire or thermistor.
The temperature of the sensing element depends on the thermal
conductivity of the gas flowing around it. Changes in thermal
conductivity, such as when organic molecules displace some of
the carrier gas, cause a temperature rise in the element which is
sensed as a change in resistance.
TCD’s are often used to measure lightweight gasses or water
(compounds for which the FID does not respond).
The TCD is not as sensitive as other detectors but it is a
universal detector and is non-destructive.
Modern detectors called micro-TCD’s have very small cell volumes,
and new electronics that produce much higher sensitivities and wider
linear ranges. Due to its increased sensitivity, and the fact that it is a
universal non-destructive detector, it is again becoming more popular
for certain applications.
Thermal Conductivity Detector
Thermal Conductivity Detector
Representative Thermal Conductivity
Values, 100 oC
GC Detectors Sensitivities and Ranges