1. Dr. Vrushali Sachinkumar Tambe
Professor (Department of Pharmaceutical Chemistry)
PES Modern College of Pharmacy (for Ladies), Pune,
India
Detectors used in Gas
Chromatography
3. Flame ionization detector (FID)
• It is a Mass sensitive detector
• Principal: At normal temperature and pressure, gases are insulators.
When electrically charged molecules are produced in gas, it becomes
conductors. If proper voltage is applied to electrodes, all of the ions will be
collected, so that final current will be proportional to the number of ions
between the electrodes.
• Working: Hydrogen-air flame burns with the production of relatively few
ions, but when organic compound burns, ion production increases. If such
a flame is places between two electrodes having applied voltage of 100-
300V, an ion current will be produced when an organic compound burns in
the flame. The ring or cylindrical electrode is positively charged while the
flame jet is a negative electrode.
• The gaseous eluents from the column is mixed with hydrogen and air and
all are burned on the jet's tip. After the fuel (H2) and oxidant (O2 in air) are
begun, the flame is lit using an electronic ignitor, actually an electrically
heated filament that is turned on only to light the flame.
• The gaseous products leave the detector chamber via the exhaust. The
detector housing is heated so that gases produced by the combustion
(mainly water) do not condense in the detector before leaving the
detector chimney.
4. • The most sensitive gas chromatographic detector for
hydrocarbons.
• With a linear range of 106 to 107
• Limits of detection in the low picogram or femtogram range
• Water and other inorganic compounds can be used as
sample solvent
Limitations
Molecules that contained only carbon and hydrogen
respond best in this detector but the presence of
"heteroatoms" in a molecule, such as oxygen, decreases
the detector's response.
• It destroys the sample.
Factors affecting performance:
• Effect of carrier gas: Helium, nitrogen or argon is used.
• Effect of hydrogen and air flow rate
• 3 gas flow variables: hydrogen, air and carrier gas
5. • Hydrogen and air flow rates are varied to
improve the response. Air flow rate is 10 times
higher than the hydrogen. Hydrogen flow rate
is 25-30ml/min while air flow is 300 ml/min.
With decrease in flow rate, the sensitivity to
heteroatms increases.
6.
7. Thermionic detector/Nitrogen phosphorus detector (NPD)
• It is a highly sensitive but specific detector that gives a
strong response to compounds that
contain nitrogen and/or phosphorus. Although its
construction is similar to that of the FID, it operates on an
entirely different principle.
• Principle:-
• FID is modified to permit the thermal ionisation of an alkali
metal salt by the flame. This has been accomplish by
suspending a salt coated wire in flame or placing cylinder
filled with salt or compressed disc of the salt on top of
flame jet. Flame containing alkali metal atom elicit
increased response for organic compound containing N, P, S
or halogen depending upon the salt selected, detector
designed and gas flow rate salt tends to reduce response
to carbon.
8. Proposed mechanism –
• Gas phase reaction when alkali metal present in the flame
• Solid phase reaction where active intermediate are formed which then react with
the surface of salt to form a volatile compounds of the alkali metals
• Photoevaporation based on theroy that photons are emitted by the flame are
absorbed by alkali metal salt causing it to evaporate.
• The essential component of the NPD sensor is a rubidium or cesium bead contained
inside a small heater coil. Helium is used as the carrier gas that after leaving the
column, is mixed with hydrogen and passed into the detector through a small jet. The
cesium or rubidium bead is situated in a wire coil that is heated by a current passing
through it. The coil, containing the bead, is situated above the jet, and the helium-
hydrogen mixture passes over it. When there is no solute being eluted from the
column, the heated alkali bead emits electrons by thermionic emission that are
collected at the anode and, thus, produce a constant ion current. When a nitrogen or
phosphorus containing solute is eluted from the column, the emission of electrons is
increased and the current collected at the anode is increased. The general sensitivity
of the NPD is about 10-12 g/ml for phosphorus and 10-11 g/ml for nitrogen.
• Alkali metal salts used: aluminium oxide, sodium sulphate, lithium chloride,
rubidium chloride
• Gas flow rate: 3-6ml/min for hydrogen, 200ml/min for air, 10-100ml/min for carrier
gas
9.
10. Flame Photometric Detector
• Samples eluting from a GC column are passed through a low temp hydrogen-rich
flame. The decomposed sample then enters into a second flame that supplies the
energy for exciting these fragments. Upon relaxation light is emmited.
• The specific emission that results after application of an S or P filter, is converted
into a current and amplified by the photomultiplier tube. This current is the
detector signal.
• Some FID's use a dual stacked jet arrangement providing two vertically positioned
flames:
• The lower flame produces highly reduced species from all the organic material
eluting from the column, many of which emit at wavelengths that would either
obscure or quench the required emissions from the P or S compounds.
• The second flame is an optimized flame which excites P or S containing radicals to
HPO* and S2* species respectively. These produce very characteristic emission
spectra, giving maximum emission intensities at wavelengths of 510 and 526 nm
for phosphorus and 394 nm for sulphur. The emitted radiation is monitored by the
photomultiplier and the resulting current is amplified.
• Other FPD designs use single flames where the two zones are created by the
supply of additional fuel gas higher in the detector compartment.
Useful for
• Analysis of air and water pollutants, pesticides and coal hydrogenation products.
• Selective for compounds containing sulphur and phosphorus
• Is also used for halogens, nitrogen, tin, chromium, selenium and germanium
11.
12. Photoionization detector
In a photoionization detector high-energy photons, typically in the vacuum
ultraviolet (VUV) range, break molecules into positively charged ions. As
compounds enter the detector they are bombarded by high-energy UV
photons and are ionized when they absorb the UV light, resulting in ejection
of electrons and the formation of positively charged ions. The ions produce
an electric current, which is the signal output of the detector. The greater the
concentration of the component, the more ions are produced, and the
greater the current. The current is amplified and displayed on an ammeter or
digital concentration display.
13.
14. Atomic emission detector (AED)
• Effluent is mixed with MIP (Microwave induced plasma), inductively
coupled plasma (ICP) or Direct current plasma (DCP). The energetic
plasma atomize all the elements in the sample and excite the
characteristic emission spectra. AED is element selective detector.
MIP is used in conjuction with diode array or charged coupled
devices.
• Atomic Emission Detector or GC-AED is used in the analysis of
gasoline, diesel, oil, environmental pollutants in soil, water and
effluent, and Volatile Organic Compounds (VOCs) in water.
15.
16. Sulphur chemiluminscence detector
• It employs a dual plasma burner to achieve high
temperature combustion of sulfur-containing
compounds with hydrogen and air to form sulfur
monoxide (SO). A photomultiplier tube detects
the light produced by the chemiluminescent
reaction of SO with ozone. This results in a linear
and equimolar response to the sulfur
compounds, without interference from most
sample matrices.
• Useful for determination of pollutants like
mercaptans
17. Nitrogen Chemiluminescence Detector
• Nitrogen Chemiluminescence Detector (NCD) is a
nitrogen-specific detector that produces a linear and
equimolar response to nitrogen compounds. This is
accomplished by using a stainless steel burner to
achieve high temperature combustion of nitrogen
containing compounds to form nitric oxide (NO). A
photomultiplier tube detects the light produced by the
subsequent chemiluminescent reaction of NO with
ozone. Because of the specificity of the reaction,
complex sample matrices can be analyzed with little or
no interference.
• Useful for organic nitrogen compounds and inorganic
compounds like ammonia, hydrazines, HCN and
nitrogen oxides.
18. Thermal Conductivity Detector
It is concentration sensitive and universal detector
The thermal conductivity detector, TCD, can be
based on one of two concepts: the hot wire made
up of platinum, gold or tungsten or the thermistor.
The hot wire based detector (or katharometer) is
the most common. The flow of pure carrier gas
(reference stream) through the wires removes heat
at a specific rate. When the gas mixture is added,
there is a decrease in the rate of heat removal,
which raises the temperature of the wire. The data
processing unit reads and records the resistance
change resulting from the change in temperature.
19.
20. • When an analyte elutes and the thermal conductivity of the column effluent is
reduced, the filament heats up and changes resistance. This resistance change is
often sensed by a Wheatstone bridge circuit which produces a measurable voltage
change. The column effluent flows over one of the resistors while the reference
flow is over a second resistor in the four-resistor circuit.
• Since all compounds, organic and inorganic, have a thermal conductivity different
from helium, all compounds can be detected by this detector. The TCD is often
called a universal detector because it responds to all compounds.
Factors affecting performance
• Nature and flow rate of carrier gas: select carrier gas with thermal
conductivity significantly different from that of analyte (Helium or hydrogen)
• If hydrogen is used, it react with analyte catalysed by hot wire filament.
• Air or oxygen should not enter the cell, as it results in oxidation of filament
• The sensitivity is inversely proportional to the flow rate, increase in flow rate
decreases the contact time.
• The temperature of detector is maintained at or above the column to avoid
the condensation of analyte and consequent fouling of the detector. Increase
in the temperature of the detector body, decreases the sensitivity by
increasing the temperature of the flowing gas stream, decreases its capacity
to remove heat from the filament, therefore detector body should be
operated at lowest practical temperature when maximum sensitivity is
desired.
21. GC-IR
• Earlier, GC fractions were collected using cold trap and
non-destructive detectors were used to indicate their
appearance. The composition of each fraction can be
checked by NMR, IR and electroanalytical methods.
• The spectrometer head contains an interferometer,
which modulates the IR source beam. The modulated
beam is focused into a heated "light pipe" cell, through
which the separated GC components are directed. An
interferogram is generated when the IR signal is
received at the detector, and this signal is fed to the
computer for Fourier transformation and further
processing. Since IR spectrometry is nondestructive,
the GC effluent can also be directed from the light pipe
outlet to a conventional GC detector.
22.
23. GC-MS
Mass Spectrometer (MS) detectors are most powerful of all gas
chromatography detectors. In a GC/MS system, the mass spectrometer
scans the masses continuously throughout the separation. When the
sample exits the chromatography column, it is passed through a
transfer line into the inlet of the mass spectrometer. The sample is
then ionized and fragmented, typically by an electron-impact ion
source. During this process, the sample is bombarded by energetic
electrons which ionize the molecule by causing them to lose an
electron due to electrostatic repulsion. Further bombardment causes
the ions to fragment. The ions are then passed into a mass analyzer
where the ions are sorted according to their m/z value, or mass-to-
charge ratio. Most ions are only singly charged.
The Chromatogram will point out the retention times and the mass
spectrometer will use the peaks to determine what kinds of molecules
are exist in the mixture. The figure below represents a typical mass
spectrum of water with the absorption peaks at the appropriate m/z
ratios.
24. One of the most common types of mass analyzer in GC/MS is the quadrupole ion-trap
analyzer, which allows gaseous anions or cations to be held for long periods of time by
electric and magnetic fields. A simple quadrupole ion-trap consists of a hollow ring
electrode with two grounded end-cap electrodes as seen in figure #. Ions are allowed
into the cavity through a grid in the upper end cap. A variable radio-frequency is
applied to the ring electrode and ions with an appropriate m/z value orbit around the
cavity. As the radio-frequency is increased linearly, ions of a stable m/z value are
ejected by mass-selective ejection in order of mass. Ions that are too heavy or too
light are destabilized and their charge is neutralized upon collision with the ring
electrode wall. Emitted ions then strike an electron multiplier which converts the
detected ions into an electrical signal. This electrical signal is then picked up by the
computer through various programs. As an end result, a chromatogram is produced
representing the m/z ratio versus the abundance of the sample.
25. GC/MS units are advantageous because they
allow for the immediate determination of the
mass of the analyte and can be used to identify
the components of incomplete separations.
They are rugged, easy to use and can analyze
the sample almost as quickly as it is eluted. The
disadvantages of mass spectrometry detectors
are the tendency for samples to thermally
degrade before detection and the end result of
obliterating all the sample by fragmentation.
26.
27. Hall electrolytic conductivity detector
• Conductivity detectors for GC can only be used for those
species which can be converted to a strong acid or base
after an appropriate post column reaction. This is normally
carried out by pyrolysis or catalytic oxidation/reduction in a
low volume flow-through furnace which also receives a
supply of oxygen or hydrogen, depending on the mode. The
catalyst is usually a nickel wire at 500-1000 °C, and organic
compounds are subsequently converted to small molecular
weight fragments:
• Oxidation:
nitrogen (N) --> NO2 (low yield)
sulphur (S) --> SO2/SO3
Reduction:
halogen (X) --> HX
nitrogen --> NH3
sulphur --> H2S
28. • In the ELCD, the reaction products are swept
from the reactor into a specially designed cell,
where they are mixed with a solvent such as
water or a low molecular weight alcohol; a
separator where the liquid phase is separated
from the insoluble gases and an electrical
conductivity cell. The electrical conductivity is
measured differentially, i.e. the difference
between the conductivity of the pure solvent
and the solvent +reactants is monitored.
29.
30. Electron-capture Detectors
• Electron-capture detectors (ECD) are highly selective detectors commonly
used for detecting environmental samples, pesticides as the device
selectively detects organic compounds with moieties such as halogens,
peroxides, quinones and nitro groups and gives little to no response for
all other compounds.
• The simplest form of ECD involves gaseous electrons from a radioactive
Beta emitter in an electric field. As the analyte leaves the GC column, it is
passed over this Beta emitter, which typically consists of nickle-63 or
tritium. The electrons from the Beta emitter ionize the nitrogen carrier gas
and cause it to release a burst of electrons. In the absence of organic
compounds, a constant standing current is maintained between two
electrodes. With the addition of organic compounds with electronegative
functional groups, the current decreases significantly as the functional
groups capture the electrons.
• The advantages of ECDs are the high selectivity and sensitivity towards
certain organic species with electronegative functional groups. However,
the detector has a limited signal range and is potentially dangerous owing
to its radioactivity. In addition, the signal-to-noise ratio is limited by
radioactive decay and the presence of O2 within the detector.
