The different types of detectors used in Gas Chromatography (GC).

1. Presentation by
Ishita Bajpai,
M.Pharm
Pharmaceutics,AACP
*DETECTORS USED IN GAS
CHROMATOGRAPHY

2. *
 Common terms
 Definitions
Classification Of G.C Detectors
Working Of Commonly Used Detectors
Brief Note On Other Detectors

3. *
Detector:
As solutes elute from the column, they interact with the
detector. The detector converts this interaction into an
electronic signal that is sent to the data system.
Recovers chemical information from column effluent:
e.g. presence, concentration, mass, structure

4. Sensitivity:
 The signal output per unit concentration or unit mass of a
substance in the mobile phase entering the detector.
 Sensitivity of the detector is a measure of its ability to
discriminate between small differences in analyte concentration.
 Sensitivity of a detector is not the minimum amount that
can be detected.
This value is influenced by the chromatographic
conditions.
Early eluting peaks are usually sharp, whereas the ones
with long retention times are broad and sometimes difficult
to discern from the noise

5. Selectivity:
 A detector is specific when it elicits a response only to a single
sample component or to a limited number of components having
similar chemical characteristics.
 A selective detector allows one to see only components of interest
despite of their co-elution with any others.
For example, one analysis might require a detector that is selective
for chlorine-containing molecules.

6. Linearity:
Linearity is a difficult property to define and measure.
In chromatography, linearity usually refers to the response of the
detector.
A detector is linear if the output of a detector is given by the product
of a constant and the solute concentration (or, for a mass sensitive
detector, the mass of solute passing though it per unit time).
If a detector is declared to be linear, the linearity is usually limited to
a specific concentration range (or range of mass of solute passing
though it per unit time).

7. No practical detector is perfectly linear, but its response can approach
linearity and therefore, linearity needs to be measured.
One method is to assume that the output of a detector is proportional to
a power function of the concentration (or mass of solute passing though it
per unit time) and, thus, for a perfectly linear detector the exponent
would be unity.
The value of this exponent for a given detector has been termed the
response index of the detector and has been used to define the detector
linearity.
If the detector is to be considered sufficiently linear for accurate
quantitative analysis it should have a response index that lies between
0.97 and 1.03 over the concentration range stated.

9. High sensitivity.
Physically suitable
Capable of operable up to a maximum column
temperature of e.g.,3500c.
Ease of operation.

10. No response to undesired compound.
Response to compounds for which analysis is required .
An output signal which is a linear function of the
concentration of sample in the detector.
Linear response extending to high concentration.
No single detector meets all these requirements. Fortunately
thermal conductivity detector, flame ionization detector and
electron capture detector have these properties and hence
these are widely used .

11. *
 Based upon the physical properties
Commercial detector
Detector that respond to the concentration(in mole
fraction) of solute in the carrier gas.
Detector that respond to the mass flow rate of the solute
in moles per unit time

12. *
Thermal conductivity
Electron capture detector
Cross section detector
Argon ionization detector etc…

13. *
Helium ionization
Flame thermocouple
Prizoelectric adsorption detetor
Argon triode detector
Photoionisation detector

14. *
Thermal conductivity Detector
Electron Capture Detector
Argon Ionisation
Helium Ionisation etc

15. *
Flame Ionization Detector
Atomic Emission Detector
Catalytic Combustion Detector
Flame Photometric Detector

16. *
Uses H2/air flame to burn organic compounds
Sample effluent from the column is mixed with H2/air
and ignited electrically at a small metal jet
Number of ions produced ~ number of reduced carbon
atoms in flame ~ number of molecules
Electrode collects ions formed at the flame,
produces electrical signal
GC-FID requires 3 gases: He (carrier), H2 and air

18. In another arrangement two parallel plates are mounted above
the flame tip.
A potential of about 400v is applied across the two electrodes,
which lower the resistance b/w the electrodes and causes the
current to flow.The current arises from the ions and free
electron generated in a pure hydrogenair flame.
When ionizable materials from the column effluent enters the
flame and is burned, the current markedly increases.
 The FID is enclosed within a chimney so that it is unaffected
by draft and can be heated sufficiently to avoid condensation of
water droplet from the combustion process

19. An igniter coil and flame-out sensor are placed above the jet to
reignite the flame if it become extinguished.
The FID responds proportionately to the number of –CH2- group
introduced into the flame. For example, there is response to an
equimolar amount of butane is twice that to ethane.

20.  The insentivity of the FID to moisture and permanent gases
(CO,CO2. CS2, SO2, NH3, N2O, NO, NO2, SIF4 AND SICl4) is
advantages in the analysis of moist organic sample and in air
pollution studies studies when small trace of organic materials have
to be measured against these permanent gases as background.
Advantage
 Detector is extremely sensitive (μg) and background noise is low.
 Stable and insensitive to small changes in flow rate of carrier gas.
 Linearity is excellent.

21. Disadvantages:
 It is more expensive than thermal conductivity
detector.
 Destructive to the sample

22. *
Principle:
The thermal conductivity detector (TCD) uses a heated filament
placed in the emerging gas stream.
The amount of heat lost from the filament by conduction to the
detector walls depends on the thermal conductivity of the gas
phase.
 It is based upon thermal conductivity difference between carrier
gas and that of component.

23. Construction:
With a cavity in the metal block there extends a tightly coiled
filament constructed of tungsten metal, tungsten-rhenium
alloy, or tungsten sheathed with gold .
One element is placed in the gas stream of the sample
injection chamber and the other immediately beyond the
column.
 The two hot wires are placed in a wheat stone bridge
arrangement, such that an increase of sensitivity can be gained

24. WORKING:
The filament is heated to a constant temperature but less than
a dull-red condition by a regulated dc current supply.
Heat loss from the filament to the metal block is constant when
only carrier gas is flowing through the detector.
The thermal conductivity of hydrogen and helium are roughly
six to ten times greater than those of most organic compounds.

25. Thus the presence of even small amounts of organic
material causes a relatively large decrease in the thermal
conductivity of the column effluent.
The filament retains more heat, its temperature rises, and
its electrical resistance goes up.
The standard detector consist of four identical filament
mounted within one brass block .
The filament make up the arms of a Wheatstone bridge.

26. Two gas streams (column flow = carrier gas + sample,
reference flow = carrier gas) pass over two separate
temperature sensitive resistors (filaments)
Any imbalance between the pair of filament is recorded.
An intial baseline reading is established by passing only
carrier gas through both pair of filaments and setting of the
power supply.

28. Advantages:
Simplicity and easy to maintain.
 Non destructive character.
Measurement of major constituents of air (H2O, CO,
CO2, H2)
Disadvantages:
Low sensitivity.
 Biological samples cannot be analyzed.
Affected by temperature and flow rate.

29. *
In the electron capture detector (ECD) the column effluent
passes between two electrodes.
 One of the electrodes has on it surface a radioisotope that
emit high-energy electron (beta particle) as it decays.
These electrons bombarded the carrier gas (nitrogen),
resulting in the formation of plasma of positive ions, radical,
and thermal electron by series elastic and in elastic collisions.
This process is very rapid.(<0.1µsec)

30. The application of a potential difference to the electron-
capture cell allows the collection of the thermal electron that
constitute the detector standing current, or baseline signal,
when only carrier gas is passing through the detector.
Electrons generate an electrical current between a pair of
electrodes
When organic molecules containing electronegative
functional groups (F, Cl, Br etc.) are present the e-
are captured and reduce the measured current
M + e- → M-

31. *Radioactive source
include tritium
adsorbed in trianium or
scandium, and nickel-63
as a foil or plated on the
interior of the cathode
chamber.

32. ADVANTAGES:
It is highly sensitive even nanogram quantities can be
detected.
Halogenated compounds ,several pesticides etc can be
detected by this detector.
DISADVANTAGES:
 The compounds which doesn’t have affinity towards
electrons cannot be detected.

34. *
NITROGEN PHOSPHORUS DETECTOR (NPD):
Mechanism: Compounds are burned in a plasma surrounding a
rubidium bead supplied with hydrogen and air. Nitrogen and
phosphorous containing compounds produce ions that are
attracted to the collector. The number of ions hitting the collector
is measured and a signal is generated.
Selectivity: Nitrogen and phosphorous containing compounds
Sensitivity: 1-10 pg
Linear range: 104-106
Gases: Combustion - hydrogen and air; Makeup - helium
Temperature: 250-300°C

35. FLAME PHOTOMETRIC DETECTOR (FPD):
Mechanism: Compounds are burned in a hydrogen-air flame.
Sulfur and phosphorous containing compounds produce light
emitting species (sulfur at 394 nm and phosphorous at 526 nm).
A monochromatic filter allows only one of the wavelengths to
pass. A photomultiplier tube is used to measure the amount of
light and a signal is generated. A different filter is required for
each detection mode.
Selectivity: Sulfur or phosphorous containing compounds. Only
one at a time.
Sensitivity: 10-100 pg (sulfur); 1-10 pg (phosphorous)
Linear range: Non-linear (sulfur); 103-105 (phosphorous)
Gases: Combustion - hydrogen and air; Makeup - nitrogen
Temperature: 250-300°C

36. PHOTOIONIZATION DETECTOR (PID):
Mechanism: Compounds eluting into a cell are bombarded with
high energy photons emitted from a lamp. Compounds with
ionization potentials below the photon energy are ionized. The
resulting ions are attracted to an electrode, measured, and a
signal is generated.
Selectivity: Depends on lamp energy. Usually used for
aromatics and olefins (10 eV lamp).
Sensitivity: 25-50 pg (aromatics); 50-200 pg (olefins)
Linear range: 105-106
Gases: Makeup - same as the carrier gas
Temperature: 200°C

37. ELECTROLYTIC CONDUCTIVITY DETECTOR (ELCD):
Mechanism: Compounds are mixed with a reaction gas and passed
through a high temperature reaction tube. Specific reaction products
are created which mix with a solvent and pass through an electrolytic
conductivity cell. The change in the electrolytic conductivity of the
solvent is measured and a signal is generated. Reaction tube
temperature and solvent determine which types of compounds are
detected.
Selectivity: Halogens, sulfur or nitrogen containing compounds. Only
one at a time.
Sensitivity: 5-10 pg (halogens); 10-20 pg (S); 10-20 pg (N)
Linear range: 105-106 (halogens); 104-105 (N); 103.5-104(S)
Gases: Hydrogen (halogens and nitrogen); air (sulfur)
Temperature: 800-1000°C (halogens), 850-925°C (N), 750-825°C (S)

38. MASS SPECTROMETER (MS):
Mechanism:
The detector is maintained under vacuum. Compounds are
bombarded with electrons (EI) or gas molecules (CI).
Compounds fragment into characteristic charged ions or
fragments.
The resulting ions are focused and accelerated into a mass
filter.
The mass filter selectively allows all ions of a specific mass to
pass through to the electron multiplier.
All of the ions of the specific mass are detected. The mass
filter then allows the next mass to pass through while
excluding all others.

39. The mass filter scans stepwise through the designated
range of masses several times per second.
The total number of ions are counted for each scan.
 The abundance or number of ions per scan is plotted
versus time to obtain the chromatogram (called the TIC).
A mass spectrum is obtained for each scan which plots
the various ion masses versus their abundance or
number.

40. Selectivity: Any compound that produces fragments within
the selected mass range. May be an inclusive range of masses
(full scan) or only select ions (SIM).
Sensitivity: 1-10 ng (full scan); 1-10 pg (SIM)
Linear range: 105-106
Gases: None
Temperature: 250-300°C (transfer line), 150-250°C (source)

41. TCD and MS detect all
FID sensitivity is low for non-H
containing organics
ECD sensitive to halogen
compounds
NPD only detects N-, P-
containing compounds
PID uses UV, saturated
halogens not detected

42. *
http://www.chemistry.adelaide.edu.au/external/soc-
rel/content/gc-det.html.
Instrumrntal Methods Of Analysis by: Gray,Calvin,Bhatia107-124.
Instrumental Methods Of Analysis seventh edition by
Willard,Merritt,Dean,Settle Pge No. :552-560.
Skoog’s

43. *

44. *