Detection Systems

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Detection SystemsDetection Systems

y Electron Capture Detectors

one of the most widely used detectors

for environmental samples because this

detector selectively responds to

halogen-containing organic compounds,

such as pesticides and polychlorinated

biphenyls.

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y Electron Capture Detectors

the sample eluate from a column is

passed over a radioactive -emitter,

usually nickel-63.

highly sensitive and have the advantage

of not altering the sample significantly.

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Electron Capture DetectorElectron Capture Detector

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Electron Capture DetectorElectron Capture Detector

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y Mass Spectrometry

one of the most powerful detectors in

gas chromatography.

measures the mass-to-charge ratio

(m/z) of ions that have been produced

from the sample.

Combination of gas chromatographyand mass spectrometry is called GC/MS.

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GC/MS DeviceGC/MS Device

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Block diagram of a mass spectrometer

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Ionization

source

Inlet

Analyzer

(Vacuum System)

Ion Detector

Data System


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Detection Systems

Mass Spectrometry

Inlet Systems

Samples put must be pure because thefragmentation occurs causes the mass

spectrum of mixtures to be very difficult

to interpret.


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Detection Systems

Mass Spectrometry

Inlet Systems

Gas chromatography is an ideal way tointroduce mixtures because components

are separated from the mixture prior to

introduction to mass spectrometer.


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y Mass Spectrometry

Ionization Sources

x converts gas phase sample molecules into

ions

x can convert gas phase sample molecules

into ions (or, in the case of electrospray

ionization, move ions that exist in solutioninto the gas phase)

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Ionization Sources for Molecular Mass Spectrometry

BASIC

TYPE

NAME &

ACRONYM

METHOD

OFIONIZATION

TYPE OF

SPECTRA

Gas

Phase

Electron

Impact (EI)

Energetic

electrons

Fragmentation

patterns

Chemical

Ionization

(CI)

Reagent

gaseous ions

Proton adducts,

few fragments

Desorptio

n

Fast atom

bombardmen

t (FAB)

Energetic

atomic beam

Molecular ions &

fragments

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Ionization Sources for Molecular Mass Spectrometry

(cont.)

BASICTYPE NAME &ACRONYM METHOD OFIONIZATION TYPE OFSPECTRA

Desorption

(cont.)

Matrix-

assisted laser

desorption/ioni

zation

(MALDI)

High-energy

photons

Molecular

ions, multiply

charged ions

Electrosprayionization

(ESI)

Electric fieldproduces

charged

spray, w/c

desolvates

Multiplycharged

molecular

ions

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y Mass Spectrometry

Analyzers

x separates the ions according to their m/z

values.

x sorts the ions by their masses by applying

electromagnetic fields

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Common Mass Analyzers for Mass Spectrometry

BASIC TYPE ANALYSIS PRINCIPLE

MagneticSector

Deflection of ions in a magnetic field.

Double

focusing

Electrostatic focusing followed by magnetic

field deflection.

Quadrupole Ion motion in dc and radio-frequency fields.

Ion Trap Storage of ions in space defined by ring

and end cap electrodes.

Ion-cyclotron

resonance

Trapping of ions in cubic cell under

influence of trapping voltage and magnetic

field.

Time-of-flight Equal kinetic energy ions enter drift tube. 14


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Other Types of DetectorsOther Types of Detectors

y Thermionic Detector nitrogen and phosphorus-containing

compounds produced increase currents in a

flame in which an alkali metal salt isvaporized .

Widely used for organophosphorus

pesticides and pharmaceutical compounds.

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y Electrolytic-conductivity or Hall detector compounds containing halogens, sulfur or

nitrogen are mixed with a reaction gas in a

small reactor tube the change in conductivity in the active

compound present is measured.

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ElectrolyticElectrolytic--Conductivity DetectorConductivity Detector

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y Photoionization Detector

molecules are photoionized by ultraviolet

radiation the ions and electrons produced are then

collected with a pair of biased electrodes,

and the resulting current is measured

Used for aromatic and other molecules that

are easily photoionized.

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PhotoionizationPhotoionization DetectorDetector

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GASGAS--SOLID CHROMATOGRAPHYSOLID CHROMATOGRAPHY

y Based on adsorption of gaseous

substances on solid surfaces.

y Useful for the separation of species that

are not retained by gas-liquid columns.

y Performed with both packed and open

tubular columns (also called porous-layer

open tubular columns or PLOT columns).

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Detection Systems