Fundamental Principles of GC/MS & Introduction to Shimadzu GC/MS

Customer Support CentreShimadzu Asia Pacific Pte. Ltd.

Singapore

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Outline

Objective: to understand how a GC/MS works

Topics:Gas chromatography basics Mass spectrometry basicsBasic GC/MS data

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What is GC/MS

The technique

GC/MS stands for Gas Chromatography Mass Spectrometry, an analytical technique which allows separation and identification of organic compounds in a sample

GC/MS analysis starts by the separation of the compounds by using gas chromatographic technique, which makes use of a polymeric material contained in a column (called ‘stationary phase’) and an inert gas (called ‘carrier gas’) flowing through the column

The separated compounds are detected and further analyzed by mass spectrometric technique. In mass spectrometry, a compound is ionized and the resulting ions are separated according to their mass-to-charge ratio, and the ion abundances are measured.

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What is GC/MS

The instrument

The instrument used for gas chromatographic mass spectrometric analysis is known as a gas chromatograph mass spectrometer (also called GC/MS)

Sample Analysis Report

SampleAnalysis

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Why we need GC/MS

The information

By using GC/MS, we can:Identify what compounds are contained in a sampleDetermine whether certain substances/compounds are present in a sampleDetermine the amount (% weight, % composition) of specified compounds in a sample

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Why we need GC/MS

Samples for GC/MS

Strictly speaking, samples for GC/MS are organic compounds withsufficient volatility (guideline: is in vapor form or can be turned to vapor at 400oC or less), sufficient thermal stability (does not decompose on heating)

In practice, some compounds which do not fulfill the criteria above can be made to fulfill the criteria by means of chemical reaction (‘derivatization’), thus making it possible for these compounds to be analyzed by GC/MS

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Why we need GC/MS

Typical applications of GC/MS:

Pesticide residues and pollutants in water, agricultural products, foodstuffOrganic solvents in packaging materials, ink, etc.Drugs of abuse in urine, blood, tabletsFatty acid contents in edible oils, fat, etc.Essential oil characterizationAllergens in fragrancePolymer characterizationetc…

Food industry Agriculture industry

Environmental field Forensic field

Biotechnology field Fragrance industry

Chemical industry

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Gas Chromatograph-Mass Spectrometer

Gas flow controller

Injection Port (Injector)

Column

Detector (MS)

Carrier Gas

Sample

Data System

• Separates mixture of compounds into individual components

• Interface between sample introduction device (e.g. syringe) and column

• Turns liquid sample into vapor by heating• Mixes sample vapor with carrier gas

• Controls the supply of gas(es) to the GC• Two types: manual flow controller and electronic flow controller

• Detects separated compounds and send signal to data system

• Controls GC system (more advanced, software-based system)• Converts signal from detector into human-readable format• Data analysis (process data)

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Mass Spectrometer

Ion source

Mass analyzer

Ion detector

• Turns sample molecules into fragment ions• Transmits ions to mass analyzer

• Separates ions according to their mass-to-charge ratio

• Measures abundance of ions

Sample

Vacuum system • Evacuates the system

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Gas Chromatograph-Mass Spectrometer

Injection port

Instrument controlData acquisitionData processingData storage

Control & DataSystem

GC OvenCapillary Column

Gas Chromatograph

Vacuum System

Ion SourceMass Analyzer

Mass Spectrometer

Detector

Gas

su

pply

(H

eliu

m)

Analyst

Interface

Detector of GC

Flow controller

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Shimadzu GC/MS

GCMS-QP2010GCMS-QP2010S

QP2010 QP2010SGC GC-2010 GC-2010

Ionization modeEI, CI, NCI (dedicated ion sources). NCI has simulated EI and CI.

EI only

Ion source temp. Variable 100 - 260 degC Variable 100 - 260 deg C

Mass range 1.5 - 1024 a.m.u. 1.5 - 900 a.m.u.

High vacuum system Dual TMP (65 L/s + 260 L/s)

Single TMP (65 L/s)

Low vacuum system Rotary pump Rotary pumpMaximum column flow rate

15 mL/min 2 mL/min

Sensitivity at installation (OFN 1 pg, m/z 272, EI)

S/N > 60 S/N > 30

Gas ChromatographMass Spectrometer

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GC Components of GCMS-QP2010GC Detector (optional)

Oven

Column

Injector

GC

Car

rier

G

as

Hyd

roge

n

Air

Mak

e up

Detector Gas(es) Flow Controller

Carrier Gas Flow Controller

Column

Oven

For GC/MS, carrier gas is normallyHelium

Electronic Flow Controllers

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MS Components of GCMS-QP2010

Ion Detector

Vacuum chamberIon source

Quadrupole mass analyzer

Direct Interface to GC

Dual TMP system

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Gas Chromatography Basics

Separation of compounds occurs inside the GC column

t2

Mixture

t1

Mobile (moving) phase

Stationary phase

t3

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Separation of compounds

When analytes are introduced into the column, the molecules distribute between the stationary and mobile phases

The molecules in the mobile phase are carried down the column

Those in the stationary phase are temporarily immobile and do not move down the column

M

S

M = mobile phase (carrier gas)

S = stationary phase

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Separation of compounds

The molecules in the mobile phase are carried down the column

Those in the stationary phase are temporarily immobile and do not move down the column

M

S

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Separation of compounds

Molecules in the mobile phase re-enter the stationary phase when they collide with the stationary phase

At the same time span, molecules leave the stationary phase and enter the mobile phase

M

S

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Separation of compounds

The molecules in the mobile phase are carried down the column

The process is repeated many many times inside the column

While the process is repeated, separation takes place

M

S

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Separation of compounds

All molecules of the same compound travel through the column at nearly the same rate and appear as a band of molecules (called sample band)

M

S

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Separation of compounds

Sample band of compound which is less ‘soluble’ in the stationary phase moves faster, because more of the molecules spend more time in the mobile phase (carrier gas)

M

S

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From Injection Port

To Detector

Retention Time = time spent by a compound inside the column

Column

Separation in column

From Injection Port

To Detector

Column

From Injection Port

To Detector

Column

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Chromatographic Data

Analysis time

Chromatogram

Intensity

Peak area (response)Depends on amount of sample

Retention time (measured from time of injection)Depends on compound and analytical conditions

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Goal of Gas Chromatography

No overlap between adjacent sample bands as they exit the column

Make each sample band travel at a different rateMinimize the width of the sample band

Analysis time

Good chromatography

Analysis time

Poor chromatography

Coeluting peaks

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Migration rates of compounds in column (1)

Different migration rates of compounds can be achieved if these compounds have different interaction strengths with the stationary phase

Weaker interactionStronger interaction

Stationary phase

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Migration rates of compounds in column (2)

Migration rate of compounds in column depend on:

Compound chemical structure

Stationary phase chemical structure

Column temperature

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Effect of compound chemical structure on migration rate

M

S

M

S

GC Column GC Column

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Effect of stationary phase on migration rate

M

S

M

S

GC Column 1 GC Column 2

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Effect of column temperature on migration rate

M

SLower T

M

SHigher T

GC Column GC Column

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Sample Band Width

Sample band width depends on:

Operating conditions

Dimensions of the column

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GC Parameters

Retention time (tR)Retention time of unretained compound (tM)Retention factor (k)Distribution constant (K)Phase ratio (β)Separation factor (α)Resolution (R)Number of theoretical plates (N)Height equivalent to a theoretical plate (HETP)Carrier gas linear velocity (v)

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Retention Time (tR)

The time an analyte takes to travel through the column

A measure of the amount of time an analyte spends in the column

Sum of the time spent in the stationary phase and the mobile phase

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Retention time of an unretained compound (tM)

The time an unretained compound takes to travel through the column

Unretained compound travels down the column at the same rate as the mobile phase (carrier gas)

Equivalent to the time a compound spends in the mobile phase

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Retention factor (k)

Another measure of retentionRatio of the amount of time a compound spends in the stationary and mobile phasesA measure of retention by the stationary phasePreviously called capacity factor, or partition factor

tR - tMk = tM

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Distribution constant (K)

Ratio of analyte concentration in the stationary phase and mobile phaseK is constant for a given compound, stationary phase, and column temperature

cSK =

cM

cS = concentration in stationary phasecM = concentration in mobile phase

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Phase Ratio (β)

The change in the phase ratio can be used to calculate the change in a compound’s retention, provided that the same stationary phase and column temperature (program or isothermal) are maintained

An increase in phase ratio results in a decrease in retention (k), since K is constant; and vice versa

rK = kββ =

2df

r = column radius (µm)df = film thickness (µm)

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Separation Factor (α)

A measure of the time or distance between the maxima of two peaks

α = 1 means the two peaks have the same retention and co-elute

α =k2

k1

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Resolution (R)

A measure of overlap between two peaks; the higher the resolution, the less the overlapSeparation (α) is only the distance between two peak maxima; resolution takes both α and the width of the peaks into accountBaseline resolution usually occurs at R = 1.50

Analysis time

Analysis time

Analysis condition 1

Analysis condition 2

Same α, different R

R = 1.18tR2 – tR1

wh1 + wh2R = 2

tR2 – tR1

wb1 + wb2

wh = peak width at half peak heightwb = peak width at base

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Number of theoretical plates (N) or Column Efficiency

Theoretical plates is a conceptTheoretical plates numbers are an indirect measure of peak width for a peak at a specific retention timeColumns with high N are considered to be more efficient than those with lower NA column with a high N will have a narrower peak at a given retention timeColumn efficiency is a function of:

Column dimensionsType of carrier gas and its average linear velocityCompound and its retention

tR

wb

tR

wh

N = 16N = 5.545

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Height equivalent to a theoretical plate (HETP or H)

Another measure of column efficiency

Small plate heights indicate higher efficiencyL

H =N

L = column length (mm)N = theoretical plates number

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Carrier Gas Linear Velocity (v)

Affects the chromatographic resolution (i.e. separation)

For each gas there is a linear velocity where optimum separation can be achieved (minimum HETP)

Van Deemter Curve

Linear Velocity (cm/s)

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