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INSTRUMENTATION AND APPLICATIONS OF GAS

CHROMATOGRAPHY

Instruments; Carrier Gas Flow regulators and meters Sample injection system Columns & ovens Detectors

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INSTRUMENTATION

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SCHEMATIC DIAGRAM OF GAS CHROMATOGRAPH

GAS CHROMATOGRAPH COMPONENTS

Flame Ionization Detector

Column

Oven

Injection Port

top view

front view

CARRIER GAS

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The mobile phase gas is called the carrier gas and must be chemically inert.

Sample componet column detector mobile phase gas Helium ,argon ,nitrogen , carbon

dioxide and hydrogen also used. Selection of the best crrier gas very important

, because it effects both the column separation and detector performance .

The ratio of viscosity of diffusion coefficient should be minimum for rapid analysis that’s why H, He are prepared for a carrier gas .

Impurities in the carrier gas such as air water vapour and trace gaseous hydrocarbons can cause sample reaction, column character and affect the detector performance.

The carrier gas system should contains a molecular sieve to remove water and other impurities.

These gases are available in pressurized tanks. presure regulateres and flow meters are required to control the flow rate of the gas.

The gases are supplied from the high pressure gas cylinder , being stored at pressure up to 300psi

carrier gas should be better then 99.99%and 99.999% is often used

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Air inlet (detector))

H2 inlet (detector)N2 inlet(make-up gas)

He inlet

(carrier gas)

PROCESS FLOW SCHEMATIC

Carrier gas (nitrogen or helium)

Sample injection

Long Column (30 m)

Detector (flame ionization detector or FID) Hydrogen

Air

CARRIER GAS(MOBILE PHASE)

Requirements:

It should be inert and available at low cost High purity Easily available Less risk of explosion or fire hazards Pressure: -Inlet 10 to 50 psi -packed column 25 to 150 mL/min. - capillary column 1 to 25 mL/min.

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FLOW REGULATORES & METERES

Flow regulators are used to deliver the gas with uniform pressure or flow rate

Flow rates of carrier gas: – Linear flow rate (cm/s): u = L/tr – Volumetric flow rate (mL/min): u (π r2) L is length of column, tr is retention time, r is the

internal radius of column Flow rate depends on type of column – Packed column: 25-100 mL/min – Capillary column: 1 to 25 mL/min Flow rate will decrease as column T increases 10

Soap bubble flow meter

Aqueoussolution ofsoap ordetergent 11

soap bubbles formed indicates the flow rate.

Glass tube with a inlet tube at the bottom.

Rubber bulb-----store soap solution

When the bulb is gently pressed of soap solution is converted into a bubble by the pressere of a carrier gas &travel up.

SOAP BUBBLE METER

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inlet tube

SAMPLE INJECTION PORT

Calibated Microsyringes are used to inject liquid sample

Purge :volatile components are removed from sample by gentle heating

Rubber or silicone diaphragm(septum) Sample port T: 50°C Packed C: sample sizes-1 to 20 μL Capillary C: 10 to 30 mL splitter is used to deliver a fraction

of injection(1:50 to 1:500) Avaid over loading Slow injection & oversized samples

cause band spreading & poor resolution13

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Micro syringe

1. Wash a syringe with acetone by filling the syringe completely and ejecting the waste acetone onto a paper towel.  Wash 2-3 times.

2.  Remove air bubbles in the syringe by rapidly moving the plunger up and down while the needle is in the sample. 

3.Usually 1-2 mL of sample is injected into the GC. 

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COLUMN CONFIGURATIONS

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Two types of columns are used in gas chromatography, packed and open tubular or capillary.

Packed column length from less than 2 m to 5 m

Capillary columns from few m to 100 m They are constructed of stainless steel, glass,

fused silica, or Teflon.

COLUMN OVENS

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Column temperature is very important in GC

The column is ordinarily housed in a thermostated oven.

they are usually formed as coils having diameters of 10 to 30 cm.

The optimum column temperature depends upon the boiling point of the sample and the degree of separation required.

Roughly, a temperature equal to or slightly above the average boiling point of a sample results in a reasonable elution time (2 to 30 min).

COLUMN

Types of columns 1.packed columns 2. Open tubular or

capillary.

19 Capillary column- 30mPacked column-3m

PACKED COLUMNS

Packed columns are fabricated from glass, metal (stainless steel, copper, aluminum), or Teflon tubes that typically have

Lengths------ 2 to 3 m Inside diameters ------- 2 to 4 mm. These tubes are densely packed with a

uniform, finely divided packing material, or solid support, that is coated with a thin layer (0.05 m) of the stationary liquid phase.

In order to fit in a thermostating oven, the tubes are formed as coils having diameters of roughly 15 cm.

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OLDER PACKED COLUMNS

Older packed columns – uniform silica particles (150-250 μm)

required to ensure uniform path lengths usually 1/8” (3.2 mm OD, 2.2 mm I.D.)

diameter, 1 – 2 m length max flow rate about 1 mL/min or 8 cm/min The columns themselves were either glass or

stainless steel

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CAPILLARY (OR)OPEN TUBULAR COLUMNS

1.Wall-coated open tubular (WCOT) Capillary tubes coated with a thin layer of

stationary phase Old: stainless steel, Al, Cu, plastic, glass.2.Support-coated open tubular (SCOT) Inner surface of the capillary is lined with a

thin film (~30μm) of a support materials, like diatomaceous earth

Lower efficiency than WCOT, higher than packed column

3.Fused-silica open tubular column (FSOT): Physical strength, low reactivity, flexibility.

0.32 to 0.25 mm

COLUMN STATIONARY PHASES:

Packed liquid coated silica particles (<100-300 mm

diameter) in glass tube best for large scale but slow and inefficient Capillary/Open Tubular wall-coated (WCOT) <1 mm thick liquid

coating on inside of silica tube support-coated (SCOT) 30 mm thick coating

of liquidcoated support on inside of silica tube

best for speed and efficiency but only small samples 23

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THE STATIONARY PHASE

requirements are: Low vapor pressure Thermal stability Low viscosity (for fast mass transfer) High selectivity for compounds of interest

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DETECTORS Use: Detect the difference between a pure

carrier gas &eluted compound Ideal detector: High sensitivity to even small concentrtion linerity, ie, less response to low concentration

&proportional response to high concentration Large linear dynamic range Useful at a range of temperatures Good stability and reproducibility Rapid response time Easy to use Stable, Predictable response Inexpensive operation from RT to 400 oC

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TYPES OF DETECTORS

1. Thermal Conductivity Detector(TCD)2. Flame Ionization Detector(FID)3. Atomic Emission Detector(AED)4. Electron Capture Detector(ECD)5. Nitrogen Phosphoroes Detector(NPD)6. Photo Ionization Detector(PID)7. Flame Photometric detector(FPD)8. Electrolytic conductivity detector (Hall

detector) 9. Absolute Mass Detector(AMD)10. Thermionic Detector(TD) 28

FLAME IONIZATION DETECTOR(FID)

Most widely used, Air-H2 flameNumber of ions depends onnumber of reduced (methylene)carbons in moleculeThe positive ions will be attracted to the cylindrical cathode. Negative ions and electrons will be attracted to the jet anode. Organic compounds Produces ions and electrons pyrolyzed(temp of flame) burner tip and electrode.(fhv power)Ions &electrons move to ward the collectorless sensitive for non-hydrocarbon groups Insensitive to noncombustible gases(CO2, SO2, NO2 and H2O)Insensitive to functional group(carbonyl, alcohol, halogen and amine) 29

AD: High sensitivity, low noise ,wide lenear range, easy to use, fast response

DA: Destroy the sample

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THERMAL CONDUCTIVITY DETECTOR(TCD)

Thermal conductivity detector cell

Arrangement of the twin detectors

Element(platinum, gold or tungsten wire) is electrically heated at constant power

– Temperature depends on thermal conductivity (He & H)of surrounding gas.

Hydrogen and helium have higher thermal conductivity and carrier gas provide best sensitivity

Six times greater than the Organic compounds

Poorer sensitivity than FID, but more universal

Advantages: simplicity, large range, inexpensive,linearity is excelent.organic & inorganic species

DA: low sensitivity ng/mL31

ELECTRON CAPTURE DETECTORS (ECD)

The sample elute from a column is passed over a radioactive β-emitter(nickel-63) Selectively to halogen-containing organic sample ,like pesticides and, polychlorinated biphenylsNi-63: radioactive β-emitter-- electron --ionization of carrier gas (N2)High electronegative group (halogen,peroxide, quinones and nitro group) in thesample capture the electronHighly selective and sensitive,nondestructive Insensitive to amines, alcohols andhydrocarbonsAD: High sensitivity, analyse the polyhalogenated organic compoundsSmall linear range

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THERMIONIC DETECTOR(NITROGEN PHOSPHORUS DETECTOR)

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N or P containing organic compounds phosphorus atom is approximately ten times greater than to a nitrogen atom and 104 to 106 larger than a carbon atom.

Compared with the FID , the thermionic detector is approximately 500 times more sensitive to phosphorus-containing compounds and 50 times more sensitive to nitrogen bearing species.

Column effluant + H2 +air(hot gas)electrically hearted Rb2SiO4 (rubidium silicate)bead at 180 V plasma (600 – 800°C ) ions to determine compounds

useful for detecting and determining the many phosphorus-containing pesticides.

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Eluent(column) hellium(carrier) water cooled microwave cavity hellium plasma(high temp) characterstic atomic emission spectra gratting diode arry optical emmision spectrometer detect the element .

ATOMIC EMISSION DETECTOR

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Six elements detect simultaneously .Determine the hetero atoms(H,P,S,O),silicon , heavy metals(Pb , Hg),tin, arsenic ,copper ,iron.

UV light (10.2 eV H2 or 11.7 eV Ar lamp) photoionization of molecular current to flow between based electrodes

Most sensitive for Aromatic and S, P easily

photoionized molecules

Linear range is high

PHOTO IONIZATION DETECTOR(PID)

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FLAME PHOTOMETRIC DETECTOR (FPD):

S and P – compounds photomultiplier to view light of 394

nm for sulphur (H2 + air S2) measurement or 526 nm for phosphorus (H2 + air HPO species)

Filteres are used to isolate the appropriate bands

Intensity is recorded photometrically X-, N-, Sn , Cr, Se and Ge

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filteres

photomultiplier

H2 + air

Column effluent

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QUALITATIVE ANALYSIS:

Retention time data should be useful for identification of mixtures

Comparing the retention time of the sample as well as the standard

Checking the purity of a compound: compar the standard and sample

Additional peaks are obtained…..impurities are present….compound is not pure

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QUANTITATIVE ANALYSIS:

Direct comparison method: -comparing the area of the peak, peak

height, width of peak. Calibration curves: -standards of varying concentration are

used determine peak areas .o Internal standard method: -A known concentration of the internal

standard is added separately to the standard solution

-The peak area ratio of sample and internal standard….unknown concentration is easily determined .

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ELEMENTAL ANALYSIS

Determination of C,H ,O ,S and N . Determination of mixture of drugs Isolation and identification of drugs Isolation and identification of mixture of

components(amino acids ,plant extracts ,volatile oils)

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