Gas Chromatography: Detectors

The Ideal Detector

Adequate sensitivity - range 10^-18 to 10^-15 g analyte/s

good stability and reproducibility a linear response to analyte that

extends over several orders of magnitude

a temperature range from room temperature to at least 400 degrees C

The Ideal Detector

A short response time that is independent of flow rate

high reliability and ease of use similarity in response toward all

analytes of alternatively a higher predictable and selective response toward + classes of ananlytes

nondestructive of sample

Types of Detectors

Thermal Conductivity Detector Thermionic Detectors Atomic Emission Detector Flame Ionization Detector Electron Capture Detector

Thermal Conductivity Detector (TCD) Q. How does this work? Answer

Electronically measures the changes in the thermal conductivity of the gas.

The thermal conductivity changes due to the presence of the analyte

The Modulate Single Filament (TCD) A method to get rid of some noise ! How?

Both the analytical and reference gases are passed altrenately over a tiny filament

This is done at a set frequency, that sets the frequency of the output of the electrical signal

The amplitude fo the signal is proportional to the difference in the thermal conductivity of both gases

TCD

What is the best type of carrier gas for the TCD?

He and H have high thermal conductivity therefore they show the presence of other components of the gas because of the large drop in thermal conductivity

Other gases are not used in TCD

Advantages

simplicity large linear dynamic range ( about

10^5) general response to organic and

inorganic samples non-destructive character

Limitations

Low sensitivity (10^-8 solute/mL carrier gas)

Others exceed this by factor of 10^4 to 10^7

Thermionic Detectors (TID) How does this work?

The effluent is mixed with H Passes through a flame tip assembly

to ignite Hot gas flows around an electrically

heated rubidium silicate bead Heated bead forms a plasma What occurs in the plasma to produce

large amounts of ions for D and N containing molecules is not understood

TID

There is a large ion current that results

Advantages

Selective for organic compounds with P and N

This is useful for detecting pesticides that contain P

Atomic Emission Detection (AED) Eluent introduced into a microwave

- energized helium plasma coupled to a diode-array optical spectrometer

Plasma sufficiently energetic to atomize all of the elements in the sample and excite their characteristic atomic emission spectra

AED

Spectra observed using a spectrometer that employs a movable, flat diode array is capable of detecting omitted radiation between 170 to 1780 nm

The positional diode array is capable of measuring 2 to 4 elements at any given setting

Present software allows measurement of the concentration of 15 elements

Flame Ionization Detector Consists of a stainless steel burner

assembly installed in the detector compartment and a electrometer system in a separate unit adjacent to the gas chromatograph

Often it is installed in the tandem with the thermal conductivity cell

Effluent form the column enters burner base through millipore filters which remove contaminates

Flame Ionization Detector hydrogen mixed with gas stream at

bottom of jet and air or oxygen is supplied axially around the jet

hydrogen flame burns at the tip, which also functions as the cathode and is insulated form the body by a ceramic seal

Collector electrode is above the burner tip and is made of platinum

Flame Ionization Detector In series with flame gases is a

selection of resistors 10^7 to 10^10 ohms

vibrating reed electrometer used to provide sensitivities up to 5 x 10^13 Amps

Carbon counting device that produces a current proportional to number of ions or electrons formed in the flamed gases

Flame Ionization Detector Responds to all organic compounds

except for formic acid Response greatest with

hydrocarbons and decreases with substitution

Except for vapor of elements in Groups I and II, does not respond to inorganic compounds

Sensitivity high due to low noise level

Flame Ionization Detector Insensitivity to water, the

permanent gases, and inorganic compounds simplifies the resolution of components in analysis of aqueous extracts and in air pollution studies

Electron Capture Detector operates similar

to proportional counter for measurement of X-radiation

effluent form the column passes over a beta-emitter - I.e.) nickel-63 or tritium

ECD

electron from emitter causes ionization of carrier gas (N)

produces a burst of electrons standing current between

electrodes decreases in presence of organic species that capture the electrons

ECD

selective in its response and highly sensitive

Hewlett Packard makes one with a detection limit of less than 8 fg/sec for lindane

sensitive toward molecules with electronegative functional groups (halogens, peroxides, quinones, nitro groups)

insensitive towards amines, alcohols and hydrocarbons

ECD

important application: detection and determination of chlorinated insecticides

advantages does not alter the sample

significantly (in contrast to flame detection)

quick and easy relatively cheap

ECD

disadvantage : linear response range is usually limited to around 2 orders of magnitude