Gas chromatography - specifically gas-liquid chromatography - involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid.

Gas chromatography Accessories

Detectors

The Gas Chromatography detector is capable of establishing both the identity and concentration of eluting components in the carrier gas stream. Before going further into the types of detectors it is essential to understand the nature of detectors and their desirable characteristics.

Non-selective - Responds to all compounds present in carrier gas stream except the carrier gas itself
Selective - Responds to range of compounds with a common physical or chemical characteristic
Specific - Responds to a single specific compound only

Detectors can also be grouped into concentration or mass flow detectors

Concentration Dependent

The response of such Gas Chromatography detectors is proportional to the concentration of the solute in the detector such as TCD. Dilution of sample with makeup gas will lower detector response.

Mass Flow Dependent

Signal is dependent on the rate at which solute molecules enter the detector such as FID. Response of such detectors is not affected by makeup gas flow rate changes.

Desirable characteristics of detectors:

Reproducible response to changes in eluent composition in carrier gas stream

• High sensitivity
• Large linear dynamic range
• Low noise
• Small volume to avoid peak broadening and resultant loss of resolution
• Preferably non – destructive

Gas Chromatography detectors

Flame Ionization Detector (FID)

• Mass sensitive detector
• Response depends on conducting power of ions or electrons produced on burning of organic compounds in the flame
• Selective detector but sample detected must be combustible
• Large linear dynamic range (107)
• No response to inorganic and permanent gases such as CO, CO2, NH3, CS2, N2, etc.
• It is the most widely used detector in Gas Chromatography

Thermal Conductivity Detector (TCD)

• Non-destructive universal detector
• Response depends on the thermal conductivity difference between the carrier gas and the eluted components
• Wide dynamic range (107 – % to ppm levels)
• Responds also to inorganic gases such as CO, CO2, NH3, CS2, N2, etc.

Electron Capture Detector (ECD)

• Operation based on absorption of β – rays emitted by radioactive source Ni63. Electrophoresis absorb the β – rays thereby reducing the current in the detector
• Specific detector, non-destructive in nature
• Linear dynamic range about 105
• Widely used in environmental analysis e.g. organ chlorine pesticide

Nitrogen Phosphorous detector (NPD)

• Ions migrate to the collector electron creating a current proportional to sample concentration
• Responds selectively to most organic compounds that contain phosphorus or nitrogen down to picogram levels
• Mass flow dependent detector
• Linear range about 106
• Useful for analysis of drugs and pesticides containing phosphorus

Liquid Chromatography

Liquid Chromatography (LC) is a chromatographic technique in which the mobile phase is a liquid.

Types of Liquid Chromatography:

• Techniques in LC are classified according to the method of solute separation
• Adsorption chromatography
• Partition chromatography
• Ion-exchange chromatography
• Affinity chromatography
• Size-exclusion chromatography

LC Detectors

As in GC, the choice of detector will depend on the analyte and how the LC method is being used (i.e., analytical or preparative scale)

• Refractive Index Detector
• UV/Vis Absorbance Detector
• Fluorescence Detector
• Electrochemical Detector
• Conductivity Detector

1.) Refractive Index Detector (RI)

Measures the overall ability of the mobile phase and its solutes to refract or bend light.

One of the few universal detectors available for LC

2.) UV/Vis Absorbance Detector

Measures the ability of solutes to absorb light at a particular wavelength(s) in the ultraviolet (UV) or visible (Vis) wavelength range.

Most common type of LC detector

Fixed Wavelength Detector

Fixed Wavelength Detector absorbance of only one given wavelength is monitored by the system at all times (usually 254 nm)

• simplest and cheapest of the UV/Vis detectors
• limited in flexibility
• limited in types of compounds that can be monitored

Variable Wavelength Detector

Variable Wavelength Detector a single wavelength is monitored at any given time, but any wavelength in a wide spectral range can be selected

• Wavelengths vary from 190-900 nm.
• More expensive, requires more advanced optics
• More versatile, used for a wider range of compounds

Photo Diode Array Detector

Photo Diode Array Detector operates by simultaneously monitoring absorbance of solutes at several different wavelengths.

• Uses a series or an array of several detector cells within the instrument, with
• Each responding to changes in absorbance at different wavelengths.
• Entire spectrum of a compound can be taken in a minimum amount of time
• Useful in detecting the presence of poorly resolved peaks or peak contaminants

3.) Fluorescence Detector

A selective LC detector that measures the ability of eluting solutes to fluoresce at a given set of excitation and emission wavelengths

Uses

Can be used to detect any compound that is ionic or weakly ionic

• high selectivity,
• low background signal
• limits of detection for a conductivity detector are ~ 10-6 M
• Typical applications
• food components
• industrial samples
• environmental samples
• Can be used with gradient elution
• constant ionic strength and pH of mobile phase
• background conductance of the mobile phase is sufficiently low

4.) Conductivity Detector

• Used in analytical applications of ion-exchange chromatography for the detection of ionic compounds
• detector measures the ability of the mobile phase to conduct a current when placed in a flow-cell between two electrodes
• current conducted within the cell will depend on the number and types of ions
• present in the mobile phase

Uses

Can be used to detect any compound that is ionic or weakly ionic

• high selectivity, low background signal
• limits of detection for a conductivity detector are ~ 10-6 M

Typical applications

• food components
• industrial samples
• environmental samples

Can be used with gradient elution

• constant ionic strength and pH of mobile phase
• background conductance of the mobile phase is sufficiently low

5.) Electrochemical Detector

Used to monitor any compound in the mobile phase that can undergo an oxidation or reduction

Uses

Can be used to detect any solute that can undergo oxidation or reduction

Detectors can be made specific for a given compound or class of compounds by properly choosing the conditions at the electrodes

• high selectivity
• low background signal

Limits of detection for a conductivity detector are ~ 10-11 M

• due to extreme accuracy with which chemical measurements,
• especially current measurements, can be made

Compounds that can be detected by reduction

• aldehydes
• ketones
• unsaturated compounds

Compounds that can be detected by oxidation

• phenols
• mercaptans (RSH)
• aromatic amines
• dihydroxy compounds