evolved gas analysis: tga/ftir - ta instruments...wire will do – an electric guitar string works...

TAINSTRUMENTS.COMTAINSTRUMENTS.COM

Evolved Gas Analysis: TGA/FTIR

Michael Bradley, Ph.D, M.B.A. ; Senior Manager,

Product Applications – ThermoFisher Scientific

James Browne, Applications Scientist

TA Instruments


EGA Analysis: TGA/FTIR

•Evolved Gas Analysis is a powerful tandem for

today’s analytical chemist

•Sometimes referred to as ‘hyphenated

techniques’, the two most common

combinations with TGA are FTIR and Mass

Spectrometry

•In today’s discussion, we present the basics of

TGA/FTIR analyses.


EGA Analysis: TGA / FTIR; General

Experimental Considerations

•No special sample preparation is necessary – Simply run a TGA experiment.

•Normal ramp rates work well (10 °C / min).

•Dynamic heating generally results in better data than an isothermal hold depending on the volatility of the analyte.


EGA ANALYSIS: TGA / FTIR

General Tips for Obtaining Good Data


EGA Analysis – TGA/FTIR; General

Experimental Considerations

•Be sure the TGA is clean

•Periodically disconnect the spectrometer transfer line and run an isothermal in air or oxygen at 1000 °C.

•This is especially important if you are running olefinic or oily samples.

•Clean the adapter port, make sure it is not clogged. Any stiff, thin wire will do – an electric guitar string works well. The thin ‘E’ string is good for the EGA adapter on the Discovery a wound ‘D’ string is good for the FTIR transfer line.

•Be careful cleaning the EGA adapter – it is very hot and will give a serious burn. Also be careful running a cleaning wire into the transfer line – it will come out very hot.

• If necessary, follow the procedures in the help menu for more thorough cleaning of the furnace.

•Disconnect or re-route the flow from the adapter when running samples that are not TGA/FTIR samples.


EGA Analysis: TGA/FTIR – Know Your

Background

•Begin each experiment by obtaining a single-beam spectrum and a 100% line

�N.B. – A 100% line is simply a background and absorbance spectrum scan. You should see no evidence of obvious absorbances. Positive absorbances probably mean that residual sample remains, negative absorbances mean that the background is changing and probably improving.

•Check with the spectrometer manufacturer for any information regarding expected signal to noise performance, etc.

•Check the single beam spectrum between runs, be certain there is no residual sample remaining. Run a 100% line if you are not sure.

•Check with the spectrometer manufacturer for cleaning procedures for the gas cell, windows, etc.


TGA/FTIR – Single Beam Spectrum


TGA/FTIR – 100% Line


TGA/FTIR – 100% Line / Single Beam Overlay


TGA/FTIR – Use Spectral Tools to Your

Advantage – i.e. Subtract Interferences

•One of the most common by products of the TGA experiment is water vapor.

•Because of the ubiquitous nature of water vapor, and its spectral properties, it often interferes with making positive identifications of evolved gases.

•Fortunately, there is a simple solution:

�Collect and store a background spectrum of water vapor

�Obtain a purged background spectrum and open the furnace and collect the lab atmosphere.

�Store it and use it to subtract from your sample spectra and also correct for over-subtractions of water that can occur occasionally.

�FTIR software packages provide for spectral subtractions –check the directions for your particular software.


EGA Analysis: TGA/FTIR – Water Vapor and

CO2


TGA/FTIR – Water Vapor Minus CO2 – Asymmetric Stretching

and Wagging Regions Set to a Straight Line.


Example: Unknown Pharmaceutical


Example: Unknown Pharmaceutical – Higher

Temperature


Example – Unknown Decomposition Product

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

Ab

so

rba

nc

e

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)

NIST Reference of Cyclopentanone

Unknown

Unknown minus water


Example: Analysis of Polyphenylene Oxide

•Engineering polymer

�Heat resistant

�Good tensile properties

�High surface energy –

paintable



Gram-Schmidt

Reconstruction

will often

resemble

derivative of

the mass loss

with respect to

temperature


473°C


Peak intensity

at 473 °C


0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

0.065

0.070

0.075

0.080

0.085

Ab

so

rba

nc

e

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)


Peak intensity

at 473 °C


-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

0.14

Ab

so

rba

nc

e

1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)


Reference Spectrum in

Red


0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

Ab

sorb

ance

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)


Spectrum at 650 °C


-0.002

-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

0.011

Ab

so

rba

nc

e

500 1000 1500 2000 2500 3000 3500 4000

Wavenumbers (cm-1)


Spectrum at 650 °C

Over-subtraction of

background corrected


EGA Analysis: TGA/FTIR - Summary

•TGA combined with FTIR is a very powerful analytical tool.

�Plastics

�Pharmaceuticals

�Fuels

�Biomass

�Hazardous Decomposition Products

•The search capabilities of ThermoFisher’s Mercury and

Spectra software make FTIR spectroscopy very accessible

to all analytical chemists.

•Simple sample preparation


Thank You

The World Leader in Thermal Analysis,

Rheology, and Microcalorimetry

evolved gas analysis: tga/ftir - ta instruments...wire will do – an electric guitar string works...

Documents