(a) Thermal Conductivity Detector (TCD)
The thermal conductivity detecor measures changes in thermal conductivity in the carrier gas.
It contains a heated wire (or filament) surrounded by the flow of carrier gas. When an analyte
elutes from the column and enters the TCD, it changes the thermal conductivity of the gas
mixture, causing a change in the temperature of the heated wire. This change affects the
resistance of the wire, resulting in a measurable electrical signal. TCD is a universal detector,
meaning it can detect a wide range of compounds, but is less sensitive compared to other
detectors.
(b) Flame Ionization Detector (FID)
The flame ionization detector operates on the principle that most organic compounds produce
ions when burned in a hydrogen flame. The analyte is introduced into the flame along with a
flow of hydrogen gas. The combustion of the organic molecules produces ions and electrons,
resulting in a measurable current. The number of ions produced is proportional to the amount
of analyte, making FID very sensitive and widely used for detecting hydrocarbons and other
organic compounds. It is not suitable for gases that do not combust.
(c) Electron Capture Detector (ECD)
The electron capture detector is based on the principle of ionization of a gas by a radioactive
source, typically nickel-63. The detector uses a carrier gas that is ionized, allowing for a
current to flow. When electronegative compounds (such as halogens, nitriles, and certain
functional groups) enter the detector, they capture electrons from the ionized gas, resulting in
a reduction of the current. The decrease in current is proportional to the concentration of the
analyte, making ECD a highly sensitive detector for certain non-polar compounds.
(d) Thermionic Detector (TID)
The thermionic detector works similarly to other detectors but is specifically sensitive to
nitrogen and phosphorus-containing compounds. It uses a heated filament that emits electrons
when heated. When nitrogen or phosphorus compounds pass through the detector, they
enhance the ionization process, resulting in an increased current. This makes TID particularly
useful for detecting these specific elements in complex mixtures.
(e) Photoionization Detector (PID)
The photoionization detector operates on the principle of photoionization, where a light
source (usually ultraviolet light) is used to ionize the molecules of the analyte. When volatile
organic compounds (VOCs) enter the detector, they can absorb photons from the UV light
and become ionized. The produced ions lead to a measurable current that is proportional to
the number of ionized molecules, making PIDs effective for detecting aromatic and
unsaturated compounds.
(f) Nitrogen-Phosphorous Detector (NPD)
The nitrogen-phosphorous detector is a type of thermionic detector specifically designed to
detect compounds containing nitrogen and phosphorus. It utilizes a tungsten-based filament
that, when heated, generates a thermionic emission that increases in presence of nitrogen or
phosphorus compounds. This sensitivity makes NPD a vital tool in analyzing pesticides,
drugs, and other nitro- or phosphorous-containing compounds.
Chromatogram Description
• X-axis: Represents time, usually in seconds or minutes.
• Y-axis: Represents detector response (such as current, voltage, or peak area)
proportional to the concentration of compounds.
• Peaks: Each peak corresponds to a different compound. The retention time (the time
at which a peak appears) can help identify the components based on their unique
chemical properties. The area under each peak is related to the concentration of the
respective compound and can be quantified.
• Baseline: Represents the detector response in the absence of analytes.
Types of Flow Meters Used in GC
1. Mass Flow Controller (MFC): Measures and maintains the mass flow of the carrier
gas, providing high precision and automatic control.
2. Rotameter: A variable area flow meter that measures flow rates visually using a float
in a tapered tube, typically used in applications where lower precision is acceptable.
3. Differential Pressure Flow Meters: Measures the pressure drop across a restriction
to infer flow rate, commonly used in larger systems.
4. Coriolis Flow Meters: Measures mass flow directly based on the Coriolis effect,
offering high accuracy for various applications.
Using a 210 nm wavelength in GC or HPLC offers several benefits for separation,
quantification, and identification. This wavelength is particularly effective for detecting
compounds with conjugated double bonds, such as aromatic compounds and certain
pharmaceuticals. It provides good sensitivity for these types of analytes and can enhance the
selectivity of the method. Additionally, using UV detection at this wavelength helps in
quantifying compounds accurately due to a lower noise level in the spectra. Overall, it
improves the reliability and clarity of the results in various analytical workflows.
