Course on Analytical Methods

Fig. 2.2. Representation of the techniques based on Electrons in – electron, ion, neutral and photon out LEED: Low Energy Electron Diffraction; HEED: High Energy Electron diffraction; RHHED: Reflected High Energy Electron Diffraction; ILEED: Ineleastic Low Energy Electron Diffraction; AES: Auger Electron Spectroscopy; EELS: Electron Energy Loss Spectroscopy; EIID: Electron Induced Ion Desorption; SEPSMS: Electron Probe Surface Mass Spectrometry; EID: Electron Induced Desorption; SDMM: Surface Desorption Molecular Microscope; CIS: Characteristic Isochromat Spectroscopy; APS: Appearance Potential Spectroscopy

Fig. 2.3. Schematic representation of the techniques that can be generated from Photon- in photon, neutral, electron or ion-out methodology. XPS: X ray Photoelectron Spectrroscopy; ESCA: Electrons Spectroscopy for Chemical Analysis

Fig. 2.4. Schematic representation of the techniques that can be generated from Ions-in ion-, neutral-, electron- or photon-out methodology. ISS: Ion Scattering Spectroscopy, SIMS: Secondary Ion Mass Spectrometry, INS: Ion Neutralization Spectroscopy, PIX: Proton Induced X ray emission

Table 2.1 Typical information that can be obtained employing surface analytical techniques and the possible limitations of these techniquesSurface Analytical technique

Typical applications

Signaldetected

Elementsdetected

Detection limits

Depthresolution

Imaging/Mapping possibility

Lateral resolution(Probe size)

Auger spectroscopy

Elemental analysis, depth profiling

Atomic scale roughness Li-U - 206nm yes 100 nm

Rutherford Back scattering (RBS)

Quantitative think film composition

Backscattered He atoms Li-U 1-10 at%(for Z<20)0.01-1 at % for X 20-70

2-20 nm yes 2 mm

Secondary Ion Mass Spectrometry

Dopant and impurity depth profiling, microanalysis

Secondary ions H-U ppb/ppm <5 nm yes <5 micron imaging<30 micron depth profiling

X-ray Photoelectron Spectroscopy

Surface analysis both inorganic and organic

Photoelectrons Li-U 0.01-1 at% 1-10 nm yes 10μm -2μ

X ray Fluorescence

Thin film thickness composition

X-rays Na-U 10 ppm - no 100μm

Low Energy Electron Diffraction

Surface structure adsorbate structure

Elastic back scattering of low energy electrons

Only geometry

submonolayer - yes Atomic dimensions

High Resolution Electron Energy Loss Spectroscopy

Structure and bonding of surface atoms and adsorbates

Vibrational excitation of surface atoms adsorbates by inelastic low energy electrons

All adsorbate molecuels

Sub monolayer - - Observation of direct adsorbate-adsorbent bond

Infra red absorption spectroscopy

Structure and bonding of adsorbates

Vibrational excitation of surface bonds

Adsorbates internal bonds

Sub monolayer - - -

Ion Scattering Spectroscopy

Atomic structure composition

Elastic reflection of inert gas ions

Any element mass dependent

- possible possible Atomic dimensions

Extended X ray Absorption Fine structure

Atomic structure of surface atoms and adsorbates

Interference effects in photo-emitted electron wave function in x-ray absorption.

Mostly all species

Intermediate- coordination

- possible Atomic dimensions

Thermal Desorption Spectroscopy

Adsorption energy

Thermally induced desorption or decomposition of adsorbates

All species Sub monolayer Not normally done

- No

Thermal in

• Electron out –Thermionic emission

• Neutral out - a number of techniques. Goes with the designation TPX – why differentials employed

• Ions out

• Photons out

Thermal in

• Simple technique

• Flash desorption

• Programmed desorption

• What are the advantageous of programmed desorption both T and t are variable – kinetics and thermodynamics are studied in simple experiments. What is the relevant of this handle.

THERMAL ANALYSIS

MASSTEMPERATURE

HEAT FLOW

THERMAL ANALYSIS (DTA)

OTHER PARAMETERSe.g. LENGTH

THERMODILATOMETRY (TD)

DIFFERENTIAL THERMAL

ANALYSIS(DTA)

THERMODILATOMETRY (TD)

Thermo-mechanical analysis (TMA)

DIFFERENTIAL SCANNING

CALORIMETRY(DSC)

Thermo-optical analysis(TOA)

Thermo-sonimetry

THERMO-GRAVIMETRY(TG)

Simple diagram for Thermal Analysis

Thermal Analysis refers to a number of methods that measure change in any property of a system with respect to temperature, when it is subjected to a controlled temperature variation.

What is Thermal Analysis?

Thermo gravimetric analysis (TGA)- Monitoring the change of weight as a function of temperature.

Differential Thermal analysis (DTA)-Change in thermal energy as a function of temperature. (exo- or endothermic).

Differential scanning calorimetry (DSC)- Change in heat as a function of temperature.

4. Thermomechanical analysis (TMA)- Change of dimensions. There are other techniques such as Electrothermal analysis,

Thermoacoustimetry and so on

THERMAL ANALYSIS TECHENIQUES

TGA-DTGThe first derivative of TG (rate of weight change) as a function of temperature is called DTG, i.e., the rate of weight change (dW/dt) as a function of temperature. This facilitates clear pinpointing of maximum weight change. DTA: In this technique the difference in temp. (T) between the sample and an inert reference material is measured as a function of temperature.

Exotherm T > 0 Endotherm T < 0 

DSC: In DSC instead of allowing a temperature difference to be developed, heat/energy is supplied to maintain same temperature of the sample and reference.  EGA : It is nothing but the analysis of volatile products released on heating, analyzed generally through a QMS or IR. (called as hyphenated techniques, TG-MS etc).

Exo

Endo

Principle behind various TA methods

It is useful to examine the behaviour of a sample by more than one thermal method while heating the sample in a programmed way.TG and DTA- SimultaneousTG and EGA-Coupled technique (TG-MS, TG-IR).TG-DTG (no extra cost involved)

Cost effectiveness is also one consideration to use simultaneous thermal measurement.

Fig1.3a

Multiple thermal techniques

1. Determination of thermal constants,Heat of fusion, specific heat , freezing point and melting point.The MP of pure metals (Au, Pb, Sn etc) is often used for calibration of DTA/DSC. The area under a melting endotherm is proportional to the latent heat of fusion of the sample.

2. Phase changes and phase equilibriaSolid to liquid phase change or liquid to gaseous state.

3. Structural changesSolid-solid transitions where a change in crystal structure occurs, it could be exo-or endothermic.

4. Thermal stabilityOne can monitor the thermal stability of an oxide, particularly stability of a porous material.

Applications of thermal methods

1. Determination of thermal constants,Heat of fusion, specific heat , freezing point and melting point.The MP of pure metals (Au, Pb, Sn etc) is often used for calibration of DTA/DSC. The area under a melting endotherm is proportional to the latent heat of fusion of the sample.

2. Phase changes and phase equilibriaSolid to liquid phase change or liquid to gaseous state.

3. Structural changesSolid-solid transitions where a change in crystal structure occurs, it could be exo-or endothermic.

4. Thermal stabilityOne can monitor the thermal stability of an oxide, particularly stability of a porous material.

Applications of thermal methods

Quantitative analysis (TGA)Plaster contains gypsum (CaSO42H2O), lime Ca(OH)2 and chalk

CaCO3.

Fig 1.4m

Applications of thermal methods contd

It is is a thermo balance consisting of

(a) High precision balance,(b) A furnace for achieving high

temperatures, e.g.., 1500 oC(c) A temperature programmer,(d) Data acquisition system.(e) Auxiliary equipment to

provide inert atmosphere

A – beamB – Sample cupC –Counter weightD – Lamp and photodiodeE – Coil

F – MagnetG – control amplifierH – Tare calculatorI – AmplifierJ – Data station

Thermo gravimetric analysis-Instrumentation

Gas outlet stopcoc

k

Cooling

Vacuum and purge gas tubing

Reactive and protective gas inlets

Vacuum connection and purge gas inlet

Balance : 1g,5g;1µ,0.1µgBalance : 1g,5g;1µ,0.1µg

GAS FLOW

Cross Section of TGA (Horizontal)

Gas outlet stopcoc

k

Cooling

Vacuum and purge gas tubing

Reactive and protective gas inlets

Vacuum connection and purge gas inlet

Balance : 1g,5g;1µ,0.1µgBalance : 1g,5g;1µ,0.1µg

GAS FLOW

Cross Section of TGA (Horizontal)

1. A thermo balance should provide accurate weight of the sample as a function of temperature. (capacity upto 1g,

typical sample in mg). Its reproducibility should be very high and also highly sensitive.

2. It should operate over a wide temperature range, say from RT to 1000/1500 oC.

3. The design of thermo balance should be such that sample container is always located within a uniform hot zone inside the furnace.

4. The sample container should be such that it does not react with the sample at any given temperature.

5. The balance should not be subject to radiation or convection effects arising from the proximity of the furnace.

6. It will be advantageous if thermo balance can be coupled to a GC or IR or to QMS.

Requirements of a TG balance:

Null point balance: As weight change occurs, the balance beam starts to deviate from its normal position, a sensor detects the deviation and triggers the restoring force to bring the balance beam back to the null position. The restoring force is directly proportional to the weight change.

Deflection balance: When balance arm is deflected by a change in weight, the relative illumination of photocells from light source changes due to the movement of shutter attached to the balance beam, resulting in flow of compensating current through one of the pair of photocells.

The current produced is proportional to the change in sample weight and after amplification is passed to the coil thus restoring it to its original position. There are two types of deflection balances, (i) Beam type and (ii) Cantilever type.

Types of Balances

Null point balance: As weight change occurs, the balance beam starts to deviate from its normal position, a sensor detects the deviation and triggers the restoring force to bring the balance beam back to the null position. The restoring force is directly proportional to the weight change.

Deflection balance: When balance arm is deflected by a change in weight, the relative illumination of photocells from light source changes due to the movement of shutter attached to the balance beam, resulting in flow of compensating current through one of the pair of photocells.

The current produced is proportional to the change in sample weight and after amplification is passed to the coil thus restoring it to its original position. There are two types of deflection balances, (i) Beam type and (ii) Cantilever type.

Types of Balances

1. Buoyancy effect of sample container

It is nothing but apparent gain in weight when an empty, thermally inert crucible is heated. It has three components;

(i) decreased buoyancy of atmosphere around the sample at higher temperatures;

(ii) the increased convection effect; and (iii) the possible effect of heat from the furnace on the balance itself.

Modern instruments take care of these factors. A blank run with an empty crucible is always preferable.

 Archimedes principle : any object, when wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object

The density of gases decreases with increasing temperature :e.g. Air : 25°C 1.29 mg/ml

225°C 0.62 mg/ml425°C 0.41 mg/ml

Sources of error in Thermogravimetry

2. Furnace and temperature effectsHeat from the furnace may cause convection. Magnetic and inductive interaction between certain samples and winding of the furnace. Thermocouple calibration

3. Other effectsTurbulence in the gas flow. Temperature measurement effects. Placement of the thermocouple. Quantity of sample used for analysis.Packing of the sample Container materials. Mostly Pt, Alumina crucibles are used.Gas flow to evacuate the decomposition products.

Sources of Error in Thermogravimetry

Decomposition of calcium oxalateStep-I CaC2O4.H2O CaC2O4 + H2OMW 146 128 18

Step-II CaC2O4 CaCo3 + COMW 128 100 28 Step-III CaCo3 CaO + CO2

MW 100 56 44

Applications of TGA

Ca, Sr and Ba are precipitated as monohydrated oxalates.In the first step, H2O is removed from all the three oxalates, while in the second carbonates are formed by losing CO. The third step, stable oxides are formed by losing CO2.

Applications of TGA in Quantitative analysis

Experimental conditions can alter the onset as well as the end of decomposition.

Shape/sharpness of curves change with heating rate.

Experimental conditions should be known for comparison of curves from different sources. Change of atmosphere influences the decomposition. Oxidation takes place in air, while decomposition takes place in its absence.

Factors affecting TGA curves

• High flow purge rates ? Not recommended, specially for vertical TGA balances due to more turbulence) :

• Better controlled atmosphere (inert) specially at higher temperatures

• Typical purge gas flow rate for small furnace : 60 ml/min for a vertical TGA , but can be increased up to 500 ml/min ( even 1000 ml/min ) in case of horizontal models, to rapidly purge the furnace without the use of vacuum.

GAS FLOW RATES

DTA: Gives information on heat change by measuring difference in temp. between sample and reference.DSC: Temperature of sample as well as reference is maintained same by supply of required heat to the sample/reference depending on exo- or endothermic change.

Peak areas: DTA: peak area (A) = K. H.m m- mass of sample, H- heat of reaction and K –constant

which depends upon sample geometry as well as thermal conductivity. K varies with the temperature, hence instrument has to be calibrated at each temperature.

DSC: Peak area can be calculated similar way as in DTA, however K is electrical conversion factor which does not change with temp for well designed equipment.

DTA and DSC