DETECTORS USED IN GAS CHROMATOGRAPHY AND HPLC BY P.RAVISANKAR. VIGNAN PHARMACY COLLEGE VADLAMUDI GUNTUR ANDHRA PRADESH INDIA.

1. DETECTORS USED IN
GASCHROMATOGRAPHY
&
HIGH PERFORMANCE LIQUID
CHROMATOGRAPHY
prof. Ravisankar
Vignan Pharmacy college
Valdlamudi
Guntur Dist.
Andhra Pradesh
India.
banuman35@gmail.com
00919059994000

2. Contents:
• Definition
• Ideal properties of a detector
• Detectors used in GC
- concentration dependent detectors
- mass flow dependent detectors
• Detectors used in HPLC
- selective detectors
- universal detectors

3. Detectors:
• The detector senses the presence of the individual
components as they leave(elute) the column. The detector out
put after amplification is traced on a recorder.
• The duration of the intervals is usually a single second or even
less than that.
• Hence the detector is considered to be the brain of the
instrument.
• The detector converts a change in effluent into an electric
signal that is recorded by data system

4. Ideal properties of a detector:
The detectors used in both GC and HPLC should have
following ideal properties:
1) High sensitivity.
2) Good stability and reproducibility.
3) A linear response to solutes.
4) Negligible base line noise.
5) Should be inexpensive.
6) Capable of providing information on the identity of solute.
7) A temperature range from room temperature to at least
4000c.
8) A short response time independent of flow rate.
9) High reliability and ease of operation.
10)The detector should be non-destructive.
11)Response independent of mobile phase composition.

5. Detectors used in GC:
Detection devices for a GC must respond rapidly and
reproducibility to the low concentrations of the solutes emitted
from the column.
Concentration dependent detectors:
- Thermal conductivity detector(TCD)
- Electron capture detector(ECD)
- Argon ionization detector
- Helium ionization detector
Mass flow dependent detectors:
- Flame ionization detector(FID)
- Nitrogen phosphorous detector(NPD)
- Flame photometric detector(FPD)

6. • In addition to these other detectors used are:
- Thermionic detectors
- Photoionization detectors
- Atomic emission detectors
- Sulfur chemiluminescence detector
• The most widely used detectors are TCD, FID, ECD.
Thermal conductivity detector(TCD):
• TCD was one of the old detectors for GC, is still widely in use.
• It is also known as katharometer and hot wire detector.
• Principle in TCD is change in thermal conductivity of gas
stream.
• Thermal conductivity of most of the samples is lesser than
most commonly used carrier gases like H, and He.

7. • The thermal conductivity of He is 6-10 time greater than that
of the most organic compounds.
• It is simple, inexpensive, non-selective, accurate, and
non-destructive type.
Thermal conductivity detector

8. • Since the detector response depends upon the difference in
thermal conduction between sample and carrier gas, a large
difference is essential.
• An increase in temperature of the detector causes a change in
the resistance of thermistor and this resistance gives a
measure a measure of thermal conductivity of gas.
• TCD consists of a temperature controlled metal block in which
two cylindrical chambers are present, which consists of two
filaments made up of platinum or tungsten.
• Both the filaments are connected to the arms of Wheatstone
bridge arrangement.
• Resistance of filaments are constant as only the carrier gas is
passed through them, once the effluent passes through them
the change in conductivity is seen and is recorded.

9. Flame ionization detector(FID):
• FID is the most widely used and generally applicable detector
for GC.
• With an FID the effluent from the column is directed into small
air-hydrogen flame.
• Most of the organic compounds produce ions and electrons
when pyrolyzed at the temperature of air-hydrogen flame.
• Detection invovles monitoring the current produced by
collecting this charge carriers.
• A few hundred volts applied between the burner tip and a
collector electrode located above the flame causes the ions
and electrons to move towards the collector.
• The resulting current is then measured with high-impedance
picoammeter.

10. • The ionization of carbon compounds in the FID is not fully
understood, although the number of ions produced is roughly
proportional to the number of reduced carbon atoms in flame.
• Because the FID responds to the number of carbon atoms
entering the detector per unit time.
• The detector is insensitive towards non-combustible gases
such as H2O, CO2, SO2, CO.
• Functional groups like carbonyl, alcohol, halogen, and amine
yield fewer ions or none at all in a flame.
• These properties make the FID a most useful general detector
for the analysis of most organic samples, including those
contaminated with water and oxides of nitrogen and sulfur.

11. • The FID exhibits a high
sensitivity (10-13g/s),
large linear response
range(107), and low noise.
• It is generally rugged and
easy to use
Disadvantages:
• It destroys sample.
• It requires additional
Gases and controllers.

12. Electron-capture detector(ECD):
• It is most widely used detector for environmental samples
because it selectively responds to halogen containing organic
compounds.
e.g.: pesticides, polychlorinated biphenyls
• In ECD, the sample eluate from a column is passed over a
radioactive β emitter, usually nickel-63.
• The ECD is selective in its response.
• An electron from the emitter causes ionization of carrier gas
and the production burst of electrons.
• In the absence of organic species, a constant standing current
between a pair of electrodes results from this ionization.
• In the presence of organic molecules containing
electronegative functional groups that tends to capture
electrons, hence the current decreases.

13. • Compounds such as halogens, peroxides, quinones and nitro
groups are detected with high sensitivity.
• The detector is insensitive to functional groups such as
amines, alcohols and hydrocarbons.
• An important application of ECD is for the detection and
quantitative determination of chlorinated insecticides.
• The advantage of ECD is that it does not alter the sample.

14. Electron capture detector

15. Flame photometric detector(FPD):
• FPD is widely used in the analysis of air and water pollutants,
pesticides, coal hydrogenation products
• It is selective towards compounds containing sulfur and
phosphorous.
• In this detector the eluent is passed in to a low temperature
hydrogen-air flame, which converts part of phosphorous in to
HPO species that emits bands of radiation at about 510 and 526
nm.
• It also converts sulfur in to S2 which emits a band at 394nm.
• Suitable filters are used to isolate appropriate bands and their
intensity is recorded photometrically.
• FPD is used to detect elements like halogens, nitrogen and
metals like tin, chromium, selenium and germanium.

16. Flame photometric detector

17. Photoionization detector:
• In this the molecules eluting from GC column are photoionized
by ultra-violet radiation from a 10.2 eV hydrogen or a 11.7 eV
argon lamp.
• This source ionizes species with an ionization potential below
the lamp energy.
• compounds with a higher ionization potential do not absorb the
energy and thus they are not detected.
• The ions and electrons produced by Photoionization are then
collected at pair of biased electrode.
• The detector is most sensitive for aromatic hydrocarbons and
organosulfur or organophosphorous.

18. Photo ionization detector

19. Atomic emission detector(AED):
• In AED, the effluent from GC column is introduced in to a
microwave induced plasma(MIP), an inductively coupled
plasma(ICP) or a direct current plasma.
• Among these MIP has been most widely used and is available
commercially.
• The MIP is used in conjunction with diode array or charge
coupled device atomic emission spectrometer.
• The plasma is sufficiently energetic to atomize all of
elements in a sample and to excite their characteristic atomic
emission spectra.
• Hence, the AED is an element selective detector.

20. • The sample in this case consists of gasoline containing small
concentrations of methyl tertiary butyl ether(MTBE), as well as
several aliphatic alcohols in low concentrations.
• By using oxygen(777nm) rather than carbon(198nm) in lamp
we can obtain a chromatogram peaks for alcohols and for
MTBE which are readily identifiable.

21. Mass spectrometry detectors:
• one of the most powerful detectors for GC is the mass
spectrometer.
• The combination of GC with Mass spectrometry is known as
GC-MS.
• Currently, nearly fifty instrument companies offer GC-MS
equipment.
• Currently, capillary columns are invariably used in GC-MS
instruments and no other separators are needed.
• Thermal degradation of components can be difficulty in GC-
MS, lowering the temperature can minimize degradation.
• However the mass spectrometer can be used to identify
decomposition products, which can lead to chromatographic
modifications that solve degradation problem.

22. • The most common ion sources used in GC-MS are electron-
impact ionization and chemical ionization.
• The most common mass analyzers are quadrapole and ion-
trap analyzers.
• In GC-MS, the mass spectrometer scans the masses
repetitively during a chromatographic experiment.
• GC-MS instruments have been used for identification of
components present in natural and biological systems.
e.g.: water pollutants, breath components and drug
metabolites.
• MS can also be used to obtain information about incompletely
separated components.
• It is extremely powerful tool for identifying components in the
mixtures.

23. • Several other types of GC detectors are useful for detection of
specific components, they are thermionic detectors and sulfur
chemiluminescence detectors.
• Thermionic detector is selective towards organic compounds
containing phosphorous and nitrogen.
• The response to a phosphorous atom is approximately ten
times greater than a nitrogen atom and 104-106 times larger
than to a carbon atom.
• Sulfur chemiluminiscence detector is specific to the sulfur
atom.
• It is based on the reaction between certain sulfur compounds
and ozone.
• This detector has proven particularly useful for the
determination of the pollutants such as mercaptans

24. Detector Type Support gases Selectivity Detectability Dynamic range
Flame ionization
(FID)
Mass flow
Hydrogen and
air
Most organic
compounds.
100 pg 107
Thermal
conductivity (TCD)
Concentration Reference Universal 1 ng 107
Electron capture
(ECD)
Concentration Make-up
Halides, nitrates,
nitriles, peroxides,
anhydrides,
organometallics
50 fg 105
Nitrogen-
phosphorus
Mass flow
Hydrogen and
air
Nitrogen, phosphorus 10 pg 106
Flame photometric
(FPD)
Mass flow
Hydrogen and
air possibly
oxygen
Sulphur, phosphorus,
tin, boron, arsenic,
germanium, selenium,
chromium
100 pg 103
Photo-ionization
(PID)
Concentration Make-up
Aliphatics, aromatics,
ketones, esters,
aldehydes, amines,
heterocyclics,
organosulphurs, some
organometallics
2 pg 107
Hall electrolytic
conductivity
Mass flow
Hydrogen,
oxygen
Halide, nitrogen,
nitrosamine, sulphur
- -

25. Detectors used in HPLC:
• The function of the detector used in HPLC is to monitor the
mobile phase it emerges from column.
• The detectors used in HPLC are of majorly two types:
selective detectors(solute property):
• Absorbance detectors
• Fluorescence detectors
• Electrochemical detectors
• Mass spectrometric detectors
Universal detectors(bulk property):
• Refractive index detectors
• Evaporative light scattering detectors

27. UV-Visible Absorption Detectors:
• In this a Z-shape flow through cell for absorption
measurements on eluents from a chromatographic column.
• To minimize extracolumn band broadening, the volume of
such a cell should be kept as small as possible, typically 1-10µl
• Many absorption detectors are double-beam devices in which
one beam passes through eluent cell and other beam is
reference beam.
• Matched photoelectric detectors are used to compare the
intensities of the two beams.
• Single beam instruments are also used, were the intensity
measurements of the solvent system are stored in a computer
memory and used for the calculation of absorbance.

28. UV absorption detectors with filters:
• These are used earlier with a mercury lamp as the source.
• Most commonly, the intense line at 254nm was isolated by
filters, by substitution of filters lines at 250, 313, 334, and
365nm can also be used.
• Because of this reason, these type detectors are restricted as
most of the organic and inorganic species exhibit broad
absorption bands.
UV absorption detectors with scanning capabilities:
• A detector with scanning spectrophotometer with grating
optics is most widely used.
• Several operational modes can be chosen with these
detectors.

29. • For example, the entire chromatogram can be obtained at
single wavelength or different wavelengths can be chosen for
a single peak.
• When entire spectra are desired for identification purposes
the flow of eluent can be stopped for a sufficient period to
permit scanning the wavelength region of interest.

30. Fluorescence detectors:
• Fluorescence detectors for HPLC are similar in design to the
fluorometers and spectrofluorometers.
• In most, fluorescence is observed by a photoelectric
transducer located at 900 excitation of beam.
• The simplest detectors use mercury excitation source and one
or more filters to isolate a band of emitted radiation.
• More sophisticated instruments use a xenon source and a
grating monochromator to isolate the fluorescence radiation.
• Laser-induced fluorescence is also used because of its
sensitivity and selectivity.
• An inherent advantage of these detectors is their high
sensitivity, hence used in LC for separation and determination
of components of samples that fluoresce.

31. • Fluorescent compounds can be obtained by treating with
reagents that form fluorescent derivatives.
• For example, dansylchloride forms fluorescent compounds
with primary amines, secondary amines, amino acids and
phenols, hence widely used for detection of amino acids in
protein hydrolyzates.
Fluorescent detector

32. Refractive-index detectors:
• The ability of a compound or a solvent to deflect light provides
a way to detect it.
• RI is a measure of molecule’s ability to deflect light in a
flowing mobile phase in a flow cell relative to a static mobile
phase contained in a reference cell.
• The amount of deflection is proportional to concentration.
• The RI detector is considered as a universal detector but it is
not very sensitive.
• To achieve high sensitivity, in practice solvents are selected
that have a very high or very low refraction index.
• The detection limit is in the range of 10-6-10-8 g/ml.

33. Refractive-index detector

34. Electrochemical detectors:
• This is based on amperometry, voltammetry, coloumetry, and
conductometry.
• Electron transfer processes offer highly selective and
sensitive method.
• Easily adaptable for use with micro columns.
• As background noise is dependent on mobile phase conditions,
it is difficult to utilize these detectors with gradient elution
separations.
• It is of two types:
Amperometric detection:
• Fixed potential is applied to the electrode (glassy carbon) and
a solute which will oxidize at that potential yields an output
current

35. Coulometric detection:
• 100% of the solute species is converted, which offers
advantages of no mobile phase flow dependence on signal and
absolute quantitation through Faraday’s law.

36. • ELSD can outperform traditional detectors when analysing non-
chromophoric samples by HPLC.
• Traditional HPLC detectors such as UV and RI have limited
capabilities.
• UV and RI are not compatible with a wide range of solvents
RI detection is not gradient compatible
• Different analytes produce different UV responses depending on
their extinction co-efficient.
• ELSDs can detect anything that is less volatile than the mobile
phase
• ELSD is universal and compatible with a wide range of solvents
Evaporative Light Scattering Detector:

37. • Three steps are involved in detection:
- nebulization
- mobile phase evaporation
- detection
Nebulization:
• Column effluent passes through
nebulizer needle.
• It mixes with the nitrogen gas.
• Dispersion of droplets are formed.
Mobile phase evaporation:
• The above formed droplets are
passed through a heating zone.
• Mobile phase evaporates
from the particles.

38. Detection:
• Sample particles pass through an optical cell.
• Sample particles interrupt laser beam and scattered light.
• Photodiode detects the scattered light.
• ELSD is an effective replacement or a perfect complement to
existing LC detectors.

39. Conclusion:
Detector is the key element that is present in any
device that is used for the identification and estimation of any
compound. It detects in a faster rate i.e. with in seconds hence
it is considered as brain if the instrument. With out a detector
no one can analyze the compound. Hence, it attains such an
importance in the field of analysis.

40. References:
• Instrumental analysis by Skoog, Holler, and Crouch.
• Instrumental methods of chemical analysis by B. K. Sharma.
• Pharmaceutical analysis by David G. Watson.
• Principles of instrumental analysis by Douglas A. Skoog and
Donald M. West.
• Basic gas chromatography by Harold M. McNair and
James M. Miller.

41. THANK YOU