Title: Detectors
1Detectors for Gas Chromatography
2Flame 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.
3Flame Ionization Detector
very robust and reliable.
4Nitrogen 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.
5Flame 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.
6Flame Photometric Detector
7Electron 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.
Its an excellent detector for molecules
containing an electronegative group such as Cl
or F etc. (or derivitized molecules).
8Electron Capture Detector
9Atomic 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.
10Atomic Emission Detector
11Photoionization 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
compounds- substituted aromatics and cyclic
compounds
12Photoionization Detector
13Thermal 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. TCDs 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-TCDs 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.
14Thermal Conductivity Detector
15Thermal Conductivity Detector
Representative Thermal Conductivity Values, 100 oC
16GC Detectors Sensitivities and Ranges