2. bab 9 gas chromatography

8/16/2019 2. Bab 9 Gas Chromatography

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Kromatografi Gas

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Tipe Kromatografi Gas

Ada tiga tipe kromatografi gas, berdasarkan jenis fasa diamnya:

1. Gas-Cair

– Fasa diam cair yang teradsorb padapermukaan padat

– Merupakan tipe yang umum

2. Gas-padat

– Fasa diam: berupa zat padat.

3. Gas- Fasa Terikat – Fasa diam adalah suatu material organik yang

terikat dengan suatu permukaan padat 2


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Prinsip Kerja

• Sample (solute) dilarutkan dalam suatupelarut

• Sample and solvent diuapkan

• Uap solvent dan solute dibawa melewatikolom oleh suatu gas yang inert (fasagerak)

– Note: fasa gerak tidak boleh berinteraksidengan komponen yang akan dipisahkan

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Prinsip Kerja

• Pelarut melewati kolom

• Pemisahan terjadi karena ada interaksiantara solut dengan fasa diam

• Sample terdeteksi oleh detektor.

(e.g. thermal conductivity, flame ionization,thermionic or electron capture detectors)

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Komponen Dasar

1. Gas Pembawa (fasa gerak)• He, Ar, N2 and H2

2. Sistem Injeksi Sampel

3. Kolom Kromatografi• Contains stationary phase

• Many configurations

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Komponen Dasar

4. Oven• Isothermal or temperature programmed

5. Detector

• FID, TC, ECD, MS, etc.

6. Pencatat• Chromatography software (strip chart and

integrator package)

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Example GC System

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Filters/Traps

Ai r

H y d r o g en

G a s C ar r i e

r

Column

Data System -

Chromatography

Software Package

on PC

Syringe/Sampler

Inlets

Detectors

Regulators

H

RESET


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Sample Injector• Purpose: to introduce

sample as “plug” at thehead of the column – Effects band broadening

• Injector typically 50 °Chotter than oven

• Sample is “flashevaporated” andexpands into gasexpansion chamber

• Injection volumes aresmall – Capillary columns ~ 1 µL

– Packed columns 1-20 µL10


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Teknik Injeksi sampel

1. Injeksi Split: Teknik injeksi dalam kolom kapiler

dengan jumlah sedikit sampel yang masuk

dalam kolom, 0,1 – 1 % dari total sampel yang

diinjeksikan. Sisa sampel yang tidak digunakanakan terbuang

Keuntungan: mengurangi kondensasi dalam

permulaan kolom, bahkan dapat tidak terjadi

kondensasi sama sekali. Karena jumlah sampelsedikit, waktu elusi lebih cepat


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2. Injeksi splitless

• Teknik injeksi dalam kolom kapiler dengan jumlah sampel lebih banyak yang masuk

dalam kolom karena tidak ada yang

terbuang (80%)-Keuntungan: sesuai untuk analisis sampel

runut (sedikit)

3. Injeksi on-columnInjeksi untuk sampel yang sangat volatil,

sehingga dapat diinjeksikan dalam kolom


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Detektor

• Sangat banyak jenis detektor, pemilihan

detektor dapat didasarkan pada analit

yang akan dipisahkan dan sensitifitasnya.

• Characteristics dari detektor yang ideal

yaitu:

– Sensitivity – wide range (~107)

– Stabil

– Mempunyai range suhu yang panjang(sampai 400 °C)

– Merespon dalam waktu yang singkat

– Tidak merusak sampel 13


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Detektor Konduktivitas Termal• Detection Principle: analyte

gases have different thermal

conductivities than carrier gases

• A platinum, gold or tungstenwire (or a thermister) is placedin the exit gas stream from the

column• A constant voltage is applied to

heat the wire

• Temperature/Resistance of the

wire is proportional to thethermal conductivity of thesurrounding gas

• Double detector system: onedetector in carrier gas and onein the carrier + analyte 14


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Detektor Ionisasi Nyala

• Organic analytes arepyrolyzed in an air/H2 flame

• Ions are produced in theplasma around the flame

– proportional to number ofcarbons present

• Positive voltage is applied

to collector; negative tothe flame body

• Ions migrate to collectorproducing a current(signal)

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Air/H2Flame

Rubinson and Rubinson (2000)


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Thermionic or Nitrogen/Phosphorus

Detector• Similar to an FID

• Flame flows aroundan heated rubidium

silicate bead

• Heated bead forms aplasma (600-800 °C)

• Sensitive to P and N

• Ion current for P & N> 104 times that of C

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Electron Capture DetectorHow it Works

• Column effluent is passed over a ß-

emitter – Tritium or 63-Ni

• The carrier gas is ionized

• A burst of e- is produced with each radioactivedecay

• Potential is applied between the collector(anode) and the detector body (cathode)

• Produces a constant background current

• The current flow decreases in the presence ofanalyte molecules

– The analyte captures the emitted electrons

Detector Characteristics• Sensitive to molecules containing

electronegative functional groups (e.g. Cl- )

• Non-linear response to analyte concentration

17N2 or Ar/CH4 (carrier gas) + ß- ----> N+n (ionized carrier gas) + e- (burst)


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Detector Fotoionisasi

• An UV source

ionizes all the

molecules in the

column effluent

• Ions produced

are collected

resulting in acurrent flow

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Detektor GC yang lain:

• Mass Spectrometer

– GC-MS “hyphenated” system

– Used for identification purposes

• Atomic Emission Detector

– GC-AED

• Sulfur Chemiluminescence Detector

(SCD)

• Fourier-Transform Infrared (FTIR)

– Used with polar molecules19

arac er s cs o as


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arac er s cs o asChromatography Detectors

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DetectorType

SignalGenerator

SignalProduced Advantages Disadvantages

Sensitivity(g solute/mL

of carriergas)

ThermalConductivity

Thermal conductivityof analyte gas (usuallyless than that ofcarrier H2 or He

Change in the electricalresistance of wire in analytestream (measure i & R atconstant applied voltage; orchange in temperature whenthermister used)

Universal detector Simplicity

Large linear dynamicrange

Non-destructive

Sensitive to allcompounds

Low sensitivity Often can’t use with

capillary columns

~ 10-8

g

(10-100 ppm)

FlameIonization

Ionization of analyte inH2/air flame

Ion current Very sensitive

Large linear dynamicrange (`10

7)

General purpose detector Responds to reduced

carbon in analytes

Insensitive to H2O, CO2,SO2, and NOx

Destructive

~ 10-13

g

Thermionic Ionization of analyte inH2/air flame

Ion current Selective towards organiccompounds containing Por N compared to C (~10

4

to 106 X)

Selective towards organiccompounds containing P orN compared to C (~10

4 to

106 X)

Similar to theFID, but for Nand P

ElectronCapture

Reduction in theionization of a carriergas (e.g. N2) by aradioactive (ß-

emitter) source,usually 63-Ni

Ion current; decreases in thepresence of organicmolecules which can captureelectrons

Selective towards organiccompounds containingelectronegative functionalgroups (e.g. halogens,peroxides, quinines, andnitro groups)

Non-destructive

Very sensitive

Selective towards organiccompounds containingelectronegative functionalgroups (e.g. halogens,peroxides, quinines, andnitro groups)

Insensitive to amines,alcohols, hydrocarbons,

Small linear dynamic range(~100)

~ 10-15

MassSpectrometer

Ionization of analyte Analyte ions; discriminationbased on mass to chargeratio of analyte ions

Universal detector

Good for complex organicmixtures

Speed

High sensitivity Compound identification

Cost

Complexity

Ease of use

~ 10-12


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GC Columns

There are two basic types of GC columns1. Packed Columns

– Used in early gas-liquid chromatography

– Typically made of glass, Teflon and aluminum

– Length typically 2-3 m; ID ~3 mm

– Filled with a material called a “solid support”; thematerial which holds the stationary phase

– Solid supports have

• Large surface areas (>1 m2/g)

• Good strength characteristics• Inert (w.r.t. the solutes)

• Uniformly wetted

– Most Common Solid Support Material – Diatomaceous Earth

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GC Columns

2. Open Tubular (or Capillary) Columns

– Now made mostly of fused silica

• Permits bending into coils for easy handling

– Have small IDs (0.1-1 mm) and long lengths

(10-30 m)

– Uses same stationary phases as packed

columns – Stationary phase is coated on the inside of

the column

• Wall-coated tubular (WCOT)

• Support-coated tubular (SCOT) 22


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Silanization of Solid

Support Material

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Si – O – C – Cl

CH3

CH3

Si – O – C – OCH3

CH3

CH3

+ CH3OH

Si – OH Si – O – C – Cl

CH3

CH3

Cl – Si – Cl

CH3

CH3

+ + HCl

Support Dimethylchlorosilane (DMCS)

Silanization

Reaction

Wash withmethanol

Active AdsorptionSite

Silanized Support


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Fasa Diam pada GC

• Sifat:1. Low volatility

2. Thermal Stability

3. Chemical inertness4. Solvent characteristics that optimize k’ and α

• Pemilihan fasa diam sering didasarkan

karena polaritasnya• solute dan fasa diam harus mempunyai

polaritas yang sama “Like dissolves like”

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Jenis-jenis Fasa Diam

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Stationary

Phase

Common

Name Polarity Common Applications

Cycloparaffin Squalane Non-polar Hydrocarbons

Polydimethylsiloxane

OV-1,

SE-30

Non-polar General purpose non-polar phase;hydrocarbons; polynucleararomatics; drugs; steroids; PCB’s

Poly(phenylmethyldimethyl) siloxane(10% phenyl)

OV-3,

SE-52

SlightlyPolar

Fatty acid methyl esters; alkaloids;drugs; halogenated compounds

Polyethyleneglycol

Carbowax Polar Free acids; alcohols; ethers;essential oils; glycols

Poly(phenylmethyldimethyl) siloxane(50% phenyl)

OV-17 ModeratelyPolar

Drugs; steriods; pesticides; glycols

Poly(dicyanoalkyldimethyl) siloxane

OV-275 HighlyPolar

Polyunsaturated fatty acids; rosinacids; free acids; alcohols


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Struktur Fasa Diam

• Polydimethyl siloxane (all R= CH4) is a common“backbone” for creatingdifferent stationary phases

• Replacing methyl groupswith other groups changesits polarity and separationcapabilities

– Phenyl – C6H5 – Cyanopropyl – C3H3CN

– Trifluropropyl - C3H6CF326

Polydimethyl siloxane

R – Si – O – Si – O – Si – R

R

RR

R R

R

n


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2. bab 9 gas chromatography

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