chromotgraphy and aas notes

8/2/2019 Chromotgraphy and Aas Notes

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Qualitative Analysis What chemical species are present in a substance or sample

Quantative Analysis How much of each chemical species is present in a substance or

sample

Physical Properties Properties that can be observed or measured without changing

composition of the matter( hence sampe element/ compound is

present before and after a physical change)

e.g appearance, texture, density, solubility, colour, malleability

Chemical Properties Properties that become evident during a chemical reaction (

chemical properties cannot be determined by viewing or touching

substance)

e.g reactivity with other chemicals, flame tests, pH, heat of

combustion

Key knowledge

How does chromatography work? Types of chromatography What each type can do?

Quantitative Qualitative

What substances can each type analyse? What do the results look like? What do the results mean? How do we analyse to results?

CHROMOTOGRAPHY

Q. Why use

ChromotographyIts used in pharmaceutical research, detection of bannedsubstances, forensic work eg. ( detection of explosives)

Chromatography Form of chemical analysis Where unknown substance is compared to observations

made with known substances

Under same conditions In order to separate components of a mixture By using two separate phases

Stationary Phase Solid or liquid substance Coated onto solid surfaceNOTE : should posses large surface area

Mobile phase Solvent used Which is Gas or liquid

That moves through the stationary pahseOrigin The line slightly above the initial solvent level , on whichsamples are placed it is drawn with a lead pencil.

Solvent Front The level reached by the mobile phase when the chromatogram is removed.

Adsorbed Attached to the stationary phase ( analogylike climbing a

ladder you to not absorb into you just interact with it)

Desorbed Dissolved back into the mobile phase

NOTE :Separation is caused by adsorption to the stationary phase by making and breaking

weak bonds with the surface of stationary phase ( weaker the bonds, faster component

moves over stationary phase) and desorption back into the mobile phase.


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Principle of Separation

Some substances are more strongly attracted to the stationary phase than othersSubstances that have stronger

intermolecular forces

it is more strongly adsorbed to the stationaryphase

it spends more time attached to the stationaryphase -

- the substance moves more slowly up the plateSubstances that are more soluble in

the mobile phase

spend more time dissolved in the mobile phase move more rapidly up the plate

Separate according to

polarity number and type of functional groups/ atoms as larger molecules move slower and

have slower retention times.

Revision of Intermolecular Bonding

Dispersion forces

( also referred to as instaneousdipole. Van der walls)

Attraction between (negative) electrons in onemolecule and (positive) nuclei in neighbouringmolecules

Most important between non-polar molecules Increases with molar mass (more electrons)

Dipole Interactions Attraction between polar molecules Increases with number of polar groups

Hydrogen Bonding Attraction molecules with -OH or -NH groups (NOF)

Strongest form of intermolecular bondingQ. What are two things

the separation of sample

components based on?

How strongly each component adsorbs onto thestationary phase

How readily each component dissolves into the mobilephase

And separation is caused by the different rates ofmovement as they are carried through stationary phase

by the mobile phse

Q. How does the solvent

carry the sample up the

plate?

Sample molecules move up the plate by adsorbing by attaching

to the stationary phase and desorbing by dissolving back into the

mobile phase.

Uses of Chromatography

- Identification : to help identify components by seperature mixtures- Purify substance : to purify sample containing mixture of substances- Check purity checking to see sample is pure- Confirming presence of component in mixture- Concentration : help identify concentration of components in sample


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Types of Chromatography Techniques

Paper Chromatography Qualitative only

only used to identify the component and is not quantitative as

it doesnt indicate the concentration of sample.Thin Layer

Chromatography(TLC)

High Performance Liquid

Chromatography(HPLC)

Qualitative and Quantative

Qualitativeretention time helps identify component ( will have

same retention time if run under same conditions

Quantativeconcentration of component can be worked out

by peak area ( given)

Gas- Liquid

Chromotography ( GLC)

Paper Chromatography & Thin layer and (TLC) Qualitative : because it is only used to identify the component and is not quantitative as

it doesnt indicate the concentration of sample.

Based on how far components move up the plate (Rf) Used to separate small amounts of substances Components undergo repeated transfer of particles back and forth between thestationary phase and mobile phase and separate according to the differences in

attraction between mobile and stationary phase( The greater the attraction to mobile

phase, faster move up)

Components made visible if not visible ( e.g fluorescing under UV light, or under certainchemicals)

Stationary Phase Mobile Phase

Paper

ChromatographyHigh quality absorbent paper Appropriate solvent that will

easily dissolve the sample

Thin Layer

Chromatography (

TLC)

Fine absorbent powder coated

onto plastic or glass sheet.

e.g silica gel ( SiO2) , alumina(

aluminium oxide (Al2O3),cellouse

Q. Why is a lid place on top of

containing holding chromatogram

- Reduce rate of evaporation of mobile phase- Protects chromatogram from temperature

changes, contamination

- Which may alter Rf valuesQ. Why must the stationary phase and

origin be perfectly horizontal?

- Solvent front travels evenly up the stationaryphase

Must be above solvent so sample does notdissolve in the solvent

Draw in grey lead pencil as ink can runand move the origin hence wont know

where the substance startedincorrect Rf

Measure to middle of the spot

NOTE :better quality will have finer dots ( high

resolution

Mark the solvent straight away and centres asquickly as possible after chromatogram is

removed

NOTE: solvent can still move after removed


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Rf values: identifying samples

Enables us to compare between No two compounds chromatograms have identically during chromatography Sample will different Rf values when conditions are changed e.g length of coloumn,

temperature, solvent, stationary phase

Rf = Distance Moved by particular dot as measured FROM THE ORIGIN

Distance moved by solvent front as measured FROM THE ORIGIN

NOTE : must write FROM THE ORIGIN

Example : Calculate the Rf value for sample A and use the table to identify the sample

Distance moved:

- Sample A : 35mm- Solvent from : 60mm

Rf = 35/60

= 0.58

Rf

0 - Component remains at origin- No affinity for mobile phase

0-1 - Some affinity for both the mobile phase and stationaryphase

1 - Component located at solvent front- Low affinity to stationary phase

Q. Why would you want the solvent front to run for

longer hence be closer to the top of the paper ?

To achieve better separation

Q. When can Rf values be compared? When the mobile and stationary phase are

the same

For Rf values to be the same between chromatograms consistency of

Size of sample Choice of solvent Complete saturation of vapor in which paper is developed


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Qualitative/

Quantative

Stationary

phase

Mobile

phase

Advantages Limitations

Paper

chromatographyQualitativ

e

High

Quality

Paper

Water

based or

polar

- Large samples- Cheap- Easy & simple to

use

- can be storedfor future

- better resolutionof polar mixtures

- Corrosivematerials

cannot be

used

- not as sensitiveas TLC

- - componentscan streak

Thin Layer

Chromatography

( TLC)

fine

powder

(e.g.

Al2O3 or

silca gel)

on a

glass or

plastic

plate

usually

an

organic

(non-

polar)

solvent

- Faster & moresensitive ( used

on smaller

samples)

- Corrosivematerials can be

used as

absorbent layer

made up of

inorganic

substances

- Better on nonpolar mixtures

- more choice ofstationary

phases

- delicate plateshard to handle

- colorlessmixtures have

to be colored

cannot be

stored for

future

examination as

colored is not

permanent

- both cannot separate complex mixtures containing similar compouds


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Two- way Chromatograms Uses two different solvents on the same sample Allows for better separation of complex mixturesMethod

1. Place sample spot on corner of chromatogram2. Run the chromatogram as normal in the first solvent3. Turn the chromatogram on its side and run in the second solvent

Example

The amino acids present in a sample of fruit juice can be detected by thin-layer

chromatography. To achieve better separation of the complex mixture of substances

present in the juice, a two-way chromatogram was prepared.

1.a) Calculate the Rf value for each component in solvent 1.

b) Attempt to identify each component based on solvent A data.

2.a) Calculate the Rf value for each component in solvent B.

b) Use this data to further identify each component.

Q. Why has the chromatogram been run intwo solvents?

Two-way chromatogram produces betterseparation of components of complex

mixtures. No single solvent can separate all

amino acids.

4. Draw the chromatogram as it would have appeared after being run in solvent Aonly.


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COLUMN CHROMOTOGRAPHY Qualitative : can only identify substance glass or metal column packed with alumina or silca gel sample carried by mobile liquid phase under influence of gravity or pressure eluent goes to spectrometer and retention time is recordedColumnChromatography

Used to separate mixtures into individual components By separating via differing rates of adsoption and desorption

NOTE

Rt value does not depend on concentration of sample Rt is characteristic of component under conditions it is performed in Longer columns produce better resolution ( more separation occurs) Polar vs non polar will determine rateRetention time dependence factor Type of bonding (e.g can be hydrogen bonding, inic, dispersion forces ) with stationary

phase

Affinity for mobile phase

Normal Phase Chromatography : With a Polar Stationary phase and non polar mobile phase (

ASSUME

Same length compound , different

homolgous series

- Least polar chains eluted first- Polar compounds are in column for longer time

Same homologous series, different

chain length

Shorter chain eluted first ( less dispersion forces)

Reverse Phase Chromatography : With Non polar stationary phase and polar mobile phase

Same length compound , different

homolgous series

- Most polar chains eluted first- Non- polar compounds are in column for longer

time

Same homologous series, different

chain length

Shorter chain eluted first ( less dispersion forces)


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High Performance (or pressure) Liquid Chromatography (HPLC)

Components

Stationary Phase - solid or viscous liquid coating

- chosen to give good separation- small particles: large surface area

(allows frequent adsorption and desorption, giving better separation)

Mobile Phase A solvent pumped through under high pressure.

Detector - UV is absorbed by any molecules in the eluent

- results are presented as a chromatogram

Q. Compare

HPLC to GlC

In Gas liquid chromatography the sample will need to get into gaseous

form and we do this by heatingusually lower molecular weight and

smaller of 300 of molar units or less , hence boiling points is important and

also if it the sample is heat sensitive it will decompose in GLC

Lowest retention time: A

Lowest concentration : lowest peak areaIf this was HPLC : will have a non polar coloumn and polar solvent

Most polar :A

Q. Why do larger

molecules have a

longer Rt?

Stronger dispersion forces give the sample a greater affinity for the

stationary phase: stronger adsorption if a solid stationary phase, or

greater solubility if a l iquid stationary phase.

NOTE : affinity to stationary phase means strength of adsoption to the solid stationary phase

or solubility in a liquid stationary phase

Components :

Stationary phase Mobile phase Sample Detector ( eyes or device )From the graph in recorder you

want to know

1. Retention time - thenumber of compounds,

identify the component


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Qualitative Analysis

Rt (retention time)

Time taken for the substance to pass through the machine which is

used to identify molecules by comparing its Rt value to that of a known

standard

Quantative

Analysis

Area under peak

To calculate the amount of a substance:

1. Measure the peak area (or peak height) on the chromatogram.

2. Compare to the area produced by a series of standard solutions

using a calibration graph.

Uses

HPLC is generally used to identify organic compounds, especially those with a large MR:

hormones proteins pharmaceuticals food components

Units measured in

Ppmparts per million

Ppbparts per bilion

1ppm = 1mg/L1ppm = 1 microgram/L

Example Question

A herbal tea extract was analysed using HPLC, The chromatogram is shown below.

Q. How many components are evident? Four components, corresponding to the four

peaks.

Q. What was the Rt of the component that

was bound least strongly to the stationary

phase. Explain your answer.

The component with an Rt of about 1.4

minutes was bound least strongly to the

stationary phase. The less strongly a

component is bound the more time it spends

in solution with the mobile phase and themore rapidly it will move through the

chromatograph.

Q.Which component was present in the

highest amounts? Explain your answer.

The peak with an Rt of about 2.9 minutes was

present in the highest concentration. The

area under this peak is the largest.


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Gas Chromatography

NOTE : nitrogen is the common carrier gas

Mobile

phase

gas ( usually nitrogen gas )carrier gas

Sample Vapourised as it is injected into the GC.

Why? Injection point very close to oven or inside the oven want it in quickly

Flame

Ionisation

Detector

Current produced as sample burns producing ions.

Stationary

phase

Gassolid Chromatography:

porous beads

Gasliquid Chromatography:

beads coated with a high boiling point liquid hence is very thick and

VISCOUS ( e.g. like oil and honey)

Factors affecting retention time

High Temperature decreases Rt Particles have greater energy less

affinity for the mobile phase

NOTE : - Peaks can be shorter and wider

but important that PEAK AREA IS THE

SAME

High pressure / flow

rate

decreases Rt Mobile phase carries sample through

more rapidly

More polar stationary

phase with a non-

polar sample

decreases Rt Particles have less affinity for the mobile

phase hence will move quicker

Longer column increases Rt Further to travel

BONUSmore distinct separationbetween two components which

are short distance apart

MINUStakes more time

Increased solubility of

the sample in the

stationary phase:

increases Rt Greater affinity: more time spent dissolved

in liquid stationary phase so less time

mobile phase


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Uses

GC can be used for organic substances which can be vaporized withoutdecomposing: with MR up to about 300 molar mass. Larger molecules must be

analysed using HPLC.

GC is the most sensitive technique ( more sensitive than HPLC however GLC cannotbe used for many substances ) : detects as little as 10-12 g.

Limitations Is not sutiable for some substances which are vaporised or degraded when heated (

even caramalized in the case of honey)

In gas chromatography

Q.Why is the column

packed with very fine

particles?

The column is packed with very fine particles to increase the surface

area increases the rate of adsorption and desorption in each of the

stationary phase. This, increasing the degree of separation.

Q. Why is the injection

port heated?

The injection port is heated to vapourise the sample. The sample is

carried through the GC as a gas.

Q. Discus the relative

advantages of HPLCand GC. What types

of substance is each

used for?

GC and HPLC are both used for organic compounds. GC is the most

sensitive, but can only be used for substances which can bevapourised without decomposing: MR < 300. HPLC is used for larger

or less stable molecules.

A mixture of liquid alkanes is analysed using a GC. Identify

the peaksusing your knowledge of inter-molecular forces.

Explain your answer

The four alkanes analysed are:

decane: C10H22

hexane: C6H14

octane: C8H18

pentane: C5H12

Pentane comes out first less molar mass less dispersion

forces less interaction travels faster slower retention

time

A: pentane, B: hexane, C: octane, D: decane.

The larger molecules have stronger dispersion forces so will spend more time bound to the

stationary phase therefore will take longer to pass through the GLC and gave a longer Rt.

Exercise: Concentration of Ethanol in Wine

The ethanol (b.p. 78C) sample of old cask wine is analysed using GC. Four standards and

the test sample were prepared as shown ion the table below. Propanol (b.p. 97C) was used

as a reference to enable comparison where different volumes of solution may be injected

into the GC.


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Talk about set up of standards & dilution factor for test sample 50 100 dilution factor of two.

Q. Why do we need to know about boiling

point?

Want the coloumn slightly higher than boiling

point so substance stays in gaseous form

Five chromatograms

Need propanol as variable amount is injected into machine: obvious by inspection.

Q. Why are the putting in 20ml ? As a reference standard

- Non linear results due to sensitivity of the dectorchanging

Allows for comparison of ratio of peak heights

1. Complete the table by calculating the peak area ratio: ethanol propanol

-


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Q. Calculate the concentration of the

original wine sample.

Original wine sample was 50 mL, final diluted sample

was 100 mL. Dilution factor = 100/50 = 2

Concentration of original wine = 2 x 5.05 = 10.1 %

Q. The label on the wine cask stated

the concentration to be 11.5%. Give a

reason for the difference (apart from

a simple mistake performing theanalysis).

Ethanol is oxidised into ethanoic acid (to produce

vinegar).

Q. The analysis was conducted at

110C. Why was this temperature

chosen rather than 60C?

All components of the samples must be gases: the

temperature must be above their boiling points.

Q. Why is it bad to store wine upright The cork will dry out hence causing the cork to

shrink , then bacteria and yeast and fungi go into

the win they multiply after drinking the wine. So

CH3CH20H ( ethanol ) due to drunken party yeast as

it is oxidised CH3COOH ( ethanoic acid )


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terms: mobile phase, stationary phase, adsorption, desorption, Rfvalues, retention

time

Gas chromatography samples are analysed by converting them into gases.These gases are then passed through a column packed with special materials

to achieve separation.

HPLC works in a similar fashion to GC, except that the mixture to be analysedremains as a liquid.

GC is used when substances are easy to vaporise and HPLC for mixturesharder to vaporise or for mixtures that might be damaged by high

temperatures.

Retention time is the time from injection until the component is detected inthe column. Components are identified by their retention times.

GC and HPLC can be calibrated for quantitative analysis by obtainingreadings from a number of standards and then plotting a calibration curve.

GC is used for substances with relative molecular mass of up to 300 that arevolatile and organic.

HPLC can be used for substances with relative molecular mass over 300,including substances that are unsuitable for GC because they decompose

when heated. HPLC can also be used to isolate and purify substances.


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SPECTROSCOPYKey Knowledge

Principles and applications of

spectroscopic techniques and

interpretation of qualitative and

quantitative data from atomic absorption spectroscopy (AAS), infrared spectroscopy (IR), mass spectroscopy, nuclear magnetic resonance

spectroscopy (NMR),

and visible and ultravioletspectroscopy (visible-UV);

Matching analytical technique/s to a

particular task.

Basic Principles

1. Atoms or molecules absorb and emit electromagnetic radiation of specific energies.2. Atoms or molecules undergo a change when they absorb electromagnetic radiation.3. Different parts of the electromagnetic spectrum affect different parts of the atom or

molecule.

Spectroscopy use of light or electromagnetic radiation for analysis

Electromagnetic

radiation

Form of energy Consists of electric and magnetic fields That travel at the speed of light

e.g light, radiowaves, xrays ( difference is that have different amount of

energy found in the photons)

Photon Single unit of electromagnetic radiation Consisting of mass- less participle travelling In a wave like motionNOTE : each photo contains a certain amount ( quantam ) of energy

Q. What allows the atoms or molecule move

to a higher energy level in spectroscopy

techniques?

In each technique the atom absorbs a

specific quantam of energy which allows the

atom to move to a higher energy level

Q. What is the difference between the

movements of atoms to a higher energy

level compared with molecules?

- In atoms the movement is of the electrons

to a higher energy level(electronic energy

levels, E = hv)

- where as in molecules the electorns move

to a higher energy level but the movement

of molecules also increases to a highervbrational, rotational and nucleur spin

energy ( these energy levels are quantified

fixed values)

Properties of Electromagnetic Radiation


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Wavelength Describes distance between any two consecutive identical pointson a wave

Frequency (Hz) the number of complete cycles of the wave that pass a given point in a second

NOTE :

frequency (v) of a photon is inversely proportional to wavelength ( hence whenfrequency increases , wavelength decreases

energy of photon (E) is inversely proportional to wavelength ( shorter the wavelengthmore energy)

the energy of photon (E) is directly proportional to its frequencyThe Electromagnetic Spectrum

radiation from each part has a specific frequency and wavelength coloured light is in the visible spectrum (note : different colors consist of different

energies or wavelengths)

Using radiation in spectroscopy

atoms and molecules absorb and emit specific wavelengths of electromagneticradiaton

radiation interacts with atoms and molecules in different ways the part of the atom or molecule that is affected depends upon the wavelength or

energies it is subjected to

Example

Visible / UV radiation Valence electrons move to higher energy levels

Infrared Radiation ( not enough to move valence electrons )

- Molecules themselves move to higher energy levels- By causing changes to covalent bonds in molecules

Depending on strengthstretch, rock, bend, twist like a spring

Radiowave radiation - can change direction of spin- moving to higher nucleur spin energy

In summary

- different parts of electromagnetic radiation affect different parts of an atom indifferent ways

Technique Part of Spectrum Used Part of Structure Affected by

supplied radiation

Flame Tests Visible Valence electrons in metal

atomsAtomic Emissions Spectroscopy

( AES)

Atomic Absorption

Spectroscopy

(AAS)

Ultraviolet and Visible

Colorimetry

(Visible Spectroscopy)

Visible Electrons in molecules

UVVisible Spectroscopy Ultraviolet and Visible

Infrared Spectroscopy ( IR) Infrared Bending and stretching of

bonds in molecules

Nuclear Magnetic Resonance

Spectroscopy ( NMR)

Radiowaves Nuclear spin states (

nucleons in molecules)


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TYPES OF SPECTSCOPIC ANALYSES

1. Analyse energies emitted by a substance ( e.g flame tests,AES)2. Analyse energies absorbed by a substance ( e.g AAS)

Qualitative Measuring absorbance as function of wavelength

- to identify substancee.g

- Infrared- UV Visible Spectroscopy

Quantative Measuring how much energy of specific wavelength

is being absorbed by substance

- Used to determine amount of substance- Colorimetry- AAS- UV visible spectroscopy

Flame Test Heat is used to excite valence electrons so they jump to higher energy levels hence electrons

are in excited state (absorbed energy is used to overcome the forces of attraction

between valence electrons and the positively charged nucleus)

they return to lower energy levels they release specific quantam of energy in the formof photons

which have discrete wavelengths that are characteristic of the element henceallowing us to identify the metal cation

note : ionisation occurs when electron leaves atom and electrons cant go half way

between shells

Flame Test Spectroscopy technique Allowing for identification of certain metal cations in

compound

Qualitative can help identify an element but is not accurate as many elements have

same colour

eg. both sodium and calcium emit a yellow/orange colour in the flame test.

Limitations

Can only be used to identify some metal species, many metal atoms may notproduce coloured flame because

o Insufficient heat to excite electrons and cause them to move to higher energylevel

o Not enough sample was vaporised for distinct flame colour to be observed Many metals produce similar coloured flames cant identify definitively Sample destroyed in process Some metals may not produce radiation in the wavelength of visible spectrum Not quantative hence amount of metal cannot b determined Most samples not pure , flame colour will be a combination of more than one species

Q. How can these

disadvantages be

overcome

- Use a hotter flame so sufficient energy is available to exciteelectrons in wider range of elements

- Use a spectroscope so light passes through prism and properup into discrete wavelengths ( atomic emission spectrum)

Advantages

Quick & can confirm presence of a metal in sample


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Atomic Emissions Spectroscopy and Spectroscopes Very similar to flame test BUT uses the slit to focus a narrow beam of emitted light and a prism to separate the this

light into its individual wavelengths.

Qualitative Analysis : can be used to identify elements in sample when the emissionspectra is compared to spectra of KNOWN samples PRODUCED UNDER SAMEEXPERIMENTAL CONDITIONS.

Q. Why do different elements produce

different wavelengths of light?

Different elements possess different energy

levels, hence the wavelengths of radiation

emitted as electrons move to lower energy

level will differ.

Q. What do spectral lines on emission spectra

represent?

They represent the energies released as

excited electrons in metal atoms movefrom

excited states to lower energy levels.

Q. What is the cause for the arrangement of

lines on an emission spectrum characteristic

of element?

Different elements have different numbers of

protons hence will have different energy

levels.Q. Why can they be used to identify different

atoms?

Every atom has different electron energy

levels, and so has a unique emission

spectrum.

Q. What improvement makes AES a more

useful technique than flame tests?

Prism produces spectrum: identifies elements

more accurately

Spectroscope Optical instrument with a slit (to allow passage of light to instrument) and optical lens.

Used to produce and examine spectra of light- Radiation from sample source passes through slits and then passed through prism- Different wavelengths are bent or refracted through different angles causing

incoming light to split into components

Above : The emission spectra of four elements are below, these spectra are individual to

each element.

Limitations of atomic emission spectroscopy

Only few atoms have electrons excited by heat of flame ( most remain in groundstate) cant detect presence of small quantities


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Atomic Absorption Spectroscopy

Q, What is the difference

between emission and

absorption spectroscopy?

In emission sample is heated and light from excited stateemits light when it returns to ground state

Where as in absorption light is put through a sample anddiscrete units of energy are removed these units of energyremoved correspond to the amount of energy needing to

excite the valence electrons.

Qualitative : can determine identify of element as the spectrum produce is the same Quantative : can determine concentration of element ( the amount absorbed

determines on concentration then unabsorbed light is compared with original

ongoing light

Black lines( represent wavelengths absorbed) on a coloured background (wavelengths remaining )

Many applications

Mercury levels in fish: fish must be purated, added liquid diluted to test levels Iron levels in blood : dilute blood Copper in a sample of ore from a mine: copper must be dissolved it

NOTE : emission and absorption spectra are not mirror images this is to do with the number of

pathways in emission spectra compared to absorbing once.

Q. What is the difference between Atomic

Absorption spectroscopy and Atomic

Emission Spectroscopy

AAS analyses light absorbed where as AES

analyses light emitted

Process

Atoms will ABSORB light if the energy of light is exactly that required to promote anelectron from its normal energy level to a higher energy level.

As each element has a unique absorption spectrum, each element to be analyzedrequires its own light source that will emit light of the correct wavelength.

The light used is composed of the element and when vaporized it produces light ofthe correct wavelength for the analyze of the particular element.

Unabsorbed light read by spectrometer ( black lines represent absorbedwavelengths)


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Operation of an Atomic Absorption Spectrometer

- solid line emits light constantly used a reference

Q. Why do we want to pulse

light?

Can be used to compared to solid line

When light is pulsed

Both emitted and transmitted light are measured.

Only emitted light is measured

NOTE : Transmitted light is the difference between light ON and light OFF.

Interpreting the datastandard solutions, calibration graphs

A calibration (standard) curve for an A.A.S. analysis

Q. Why did the student have to perform the

dilution?

The absorbance of Sample A did not fall on

the calibration graph. A dilution was required

to produce a solution with a lower

absorbance.

Advantages of Atomic Absorption Spectroscopy

Very sensitive :can detect up to ppm/ ppb More elements : Wide range of elements can be detected More accurate than AES Qualitative and Quantative


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QUANTATIVE SPECTROSCOPY ANALYSIS- Radiation of specific frequency is passed through a saple and some of this radiation is

absorbed or transmitted by species e.g AAS and UV Visible spectroscopy

- Radiation is absorbed by ground state electrons in atoms or molecules- That radition NOT absorbed reaches detector

Spectrometer Measures amount of radiation That is absorbed or transmitted by chemical species

ABSORBANCE = Initial Radiation Intensity Final Radiation Intensity

NOTE :

- The amount of radiation absorbed is directly proportional to concentration of solution( ONLY AT LOW CONCENTRATIONS more concentrated substances will absorb more,

hence higher absorbance reading)

STANDARD CURVES / CALIBRATION CURVES At low concentrations the absorbance of substance is directly proportional to the

concentration of substance in solution

Method

1. Dilute the sample so that is able to transmit some light / radiation2. Colorimetry only : solutions that are not coloured may need to be coloured by mixing

chemical under investigation with another substance

3. Select appropriate wavelength of radiation for analysis4. Measure the amount of radiation absorbed by sample5. Measure the amount of radiation that is absorbed by known concentration so

component being analysed under same conditions as the sample

6. Plot absorbance of each standard solutions against concentration ( standard curve/calibration curve)

7. Use curve to determine the concentraton of sample, NOTE : dilution factorsNOTE :

- Line of best fit > need to pass origin- At higher levels of concentration the absorbance- concentration relationship

becomes non linear

- Do not extrapolate graph at high concentrations ( but you can at low )- Sample and known concentrations must be under same conditions

Examples of spectroscopy techniques which use standard curves

Colorimetry Atomic Absorption Spectroscopy UV- Visible Spectroscopy

Q. 25 Why do we calibrate spectrometers

each time they are sued to analyse a

sample?


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COLORIMETRYQ.How do you know by looking at two

glasses of cordial which one is more

concentrated?

The colour in B is darker in more concentrated

than A

Q. Why is it darker? B looks darkerIt contains more pigment molecules and they

absorb more light.

Q. Why is it red? the colour is darker as white light shines through

it is absorbing lights which are not red

and since there is more pigment molecules

more light is absorbed.

Objectivewe can see results with eyes, but using instruments is more accurate Uses a colorimeterBasic Principles

1. The greater the concentration of the solution, the greater the intensity of the colour.2. The colours absorbed are the complement of the colours we observe.( opposite side

of the colour wheel)

Solution must be coloured : To measure copper ion concentration in CuCl2 + NaSO4=CuSO4 ( coloured )

Colorimetry the process of comparing the intensity of a coloured solution with a set of

standards of known concentration

NOTE : SOLUTION MUST BE COLOURED !

Quantative concentration of unknown solution can be determined NOT QUALITATIVE

cannot identify the solution.

Basic Principles

NOTE : substance canot absorb radiation of the same colour only complementary colours


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Colorimetry Instruments

Purpose of lamp the source of light

Purpose of slit to focus a beam of light

Purpose of color filter to pre select the colour we want to go through our solution (

usually the complementary colour )

Purpose of cell to contain test sample

Purpose of detector reads how much light has been absorbed

Method

Make standard solutions graph a calibration curve

Use it to work out unknown concentration.Q. Would you try make up 0.05 M solution

from powder?

- Weigh out too small an amounthence high chance of error.

Hence we use multiple dilutionsQ. Why do graphs tail off at high

concentrations?

- The graph is less accruate after itstops going linear ( higher

concentrations)

- Should aim to get diluted sample inthe linear section

- If the sample is in non linear section dilute it to get into linear area

Q. What is the difference between AES and

AAS compared with colorimetry and UV

visible spectroscopy

- AES and AAS measure absorbance ofdiscrete wavelegnths

- Colorimetry measures absorbance ofa range of wavelengths

Q. What determines what colour filter is

chosen?

The filter selects the band of wavelengths

which are most strongly absorbed by

coloured solution/


24/24

Ways to work out dilution

1. 25 ml and put into 250 ml flask for 1/10 dilution. ( example)2. Can also use c1v1=c2v2

Revison of concentrations

Concentration (n = c x V) Molarity (M) = mol/L ppm (parts per million), ie 1 g per million g, it also similar to mg/L for aqueous solutions ppb (parts per billion), ie similar to g/L %w/v, ie gram per 100mL %w/w, ie gram per 100g %v/v, ie mL per 100mL

To convert M (mol/L) to g/L, it is converting mol to g, therefore X by molar mass

To convert g/L to M (mol/L), by molar mass

Density (d = m/V ie g/mL) for aqueous solutions density is 1 g/mL, ie 1 mL = 1 g

Dilution (moles are constant) C1V1 = C2V2

Limitations

Low concentrations : Not capable of testing very low concentrations Accuracy of Results :Results are not as accurate as AAS Visible Light Only : Restricted to substances that can absorb visible light, as not all

substances can absorb visible light

Coloured Solutions : Solutions must be coloured or be able to be coloured More than one substance : More than one substances may absorb chosen wavelength

hence only suitable for pure substances.

Advantages of Colorimetry Simple Equipment and reagents are inexpensive Can be used to test variet of chemical species ( most metal cations and some simple

anions)

Same is not destroyed

chromotgraphy and aas notes

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