1
Bigger is better: pushing the limit of TG and
TG/FTIR
Application Note
C-218
Abstract
In Thermogravimetry TGTG/
FTIR systems, there are many variables
that can affect the detection
limit of the system. It is demonstrated
in this paper that the larger the sample
size, the better the detection limit.
For TG/FTIR, with a certain kind of
TG/FTIR interface, the detection limit
is dependent upon the sample
holder volume of the TG system. It
is proved that, for a sample mixture,
the ratio of balance sensitivity to
standard sample holder volume should
be used to characterize the detection
limit of TG, while the standard sample
holder volume of the TG system
should be used to characterize that
of TG/FTIR.
Introduction
Thermogravimetry TG has widely
been used as a tool to detect material
composition. 1,2 When the sample’s
evolved gases are also of interest, an
evolved gas analyzer EGA unit will
be utilized. Among the options for
EGA, Fourier Transform Infra-Red
spectroscopy FTIRMass Spectroscopy
MS are two techniques
often used. Between TG/FTIR and
TG/MS, TG/FTIR is the more common
combination because of the relatively
simple coupling technique involved.
In a TG system, the important measured
parameters are weight, temperature
and time. Weight is the signal
which is the most important of these,
and much is made correctly of capacity
and sensitivity of the balance
used in the TG system. In FTIR, besides
the accuracy of the wavenumber,
another important feature is the
signal to noise ratio or sensitivity.
To measure small amounts of a component
in a sample, the detection limit
will ultimately decide if the measurement
can be made. The detection
limit is defined as the smallest concentration
or percentage of a components
which can be detected. Such
a definition has been widely used for
various analytical instrumentation,
and in general, the higher sensitivity,
the better the detection limits. However,
this term has not been applied
to TG or TG/FTIR, because sample
size will decide the amount of evolved
gasessample weight loss/gain,
and eventually the detection limit
for a sample.
It is the purpose of this paper to demonstrate
that the detection limit is
directly related to the sample size,
for both TGTG/FTIR.
Experimental
Experiments were performed on a
Synergy TG-FTIR system. The thermogravimetric
analyzer was a Cahn TG-
131 with a capacity of 100 grams
and maximum temperature of 1100 °C.
The FTIR was a Mattson RS-2. They
were coupled by a Cahn TG/FTIR
interface.
In order to show the effect of sample
size on TGTG/FTIR, two sets
of experiments were performed. One
set of experiments was performed
where 100 % of the interested component
was solid. Only the sample
sizes were decreased, along with volume.
Another set of experiments
was performed with liquid samples
which had different concentrations.
The sample volumes were kept about
the same.
Calcium oxalate monohydrate
CaC2O4
.H2O, from Aldrichcalcium
carbonate CaCO3, from Aldrich
were used as the solid samples. The
sample sizes ranged from 190 mg to
50 μg. Formaldehyde solution at
various concentrations, 0.0 % to
1.15 % diluted from 37 % formaldehyde
solution from Aldrich,
were used as the liquid samples. 1.0
± 0.1 mL about 1.0 gram of solutions
were used for experiments.
Samples were heated under a nitrogen
environment. Solid samples were
heated from room temperature up
to 1000 °C at a heating rate of 10 °C/
min. The temperature profile for
liquid samples was: an isotherm for
10 minutes at room temperature, a
ramp to 200 °C at a heating rate of
5 °C /min, followed by an isotherm
for another 15 minutes. The evolved
gases passed through a heated gas cell
where the FTIR spectra were collected.
The FTIR spectra were collected
continuously at a resolution
of 4 cm-1,the sampling scan number
for each spectrum was 64 about
11 seconds. The FTIR background
was collected at 256 scans, before
loading the sample. The transfer
linesthe FTIR cell of TG-FTIR
interface were heated at 250 °C to
prevent evolved gases from condensing.
ResultsDiscussion
Solidliquid samples are analyzed
for two different purposes, one is about
TG,another is about TG/FTIR.
Therefore, the following results and
discussion section is di-vided into
two parts, TGTG/FTIR.
The effect on TG
Figures 1a, 1b1c show TG
curves for CaC2O4
.H2O, CaCO3
and formaldehyde solution, respectively.
Calcium oxalate monohydrate
weight loss curve, Figure 1a,
shows three distinguishable weight
loss steps. Figure 1b shows the
typical weight loss curve for calcium
carbonate. In Figure 1c for the formaldehyde
solution, there is only one
weight loss step. One can see that
it is impossible to separate the weight
loss steps for waterformaldehyde
due to the high vapor pressure
of formaldehyde.
When the sample size for solid samples
was decreased, smaller amounts
of weight loss were observed. In this
Dun Chen, Thermo Fisher Scientific, Process Instruments, Newington, USA
2
Figure 1a: Curve of Weight change vs. temperature for Calcium Oxolate
Monohydrate
Figure 1b: Curve of Weight change vs. temperature for Calcium Carbonate
Figure 1c: Curve of Weight change vs. temperature for formaldehyde solution
case, the sensitivity of the balance
used in the TG system is very important.
Therefore, the least amount
of sample which can be measured
is dependent upon the sensitivity of
the balance.
In the above discussion, samples with
100 % of the interested component
were used. However, most experiments
involve samples containing a
mixture of components. For samples
of this type, the sample holder volume
should also be considered.
In order to demonstrate the importance
of sample holder volume and
balance sensitivity, calcium carbonate,
which has a density of 2.7 g/mL and
will loose 44 % of its weight as CO2,
is used as an example. Assuming the
balance sensitivity is 10 μg and
without considering the noise, the
smallest amount of 100 % calcium
carbonate which can be detected
based upon the weight loss signal
of CO2 is 23 μg 10 mg/44% or
0.0084 μL 23 μg/2.7 g/mL. If a sample
mixture contains 0.0001 % 1 ppm
of calcium carbonate, in order to obtain
the percentage of calcium carbonate
in a sample of this kind,
22.727 grams
10 μg/44% ＊ 0.0001% ＊ 1000000
or 8.4 mL 22.727 grams/2.7 g/mL
of the mixture must be used. This
will work fine, if the sample holder
is BIG enough AND the balance capacity
is large enough. Therefore,
for TG systems, the capacity and
sensitivity of the balance, as well as
the sample holder volume are important
parameters. Because the sample’s
density is often not high enough to
reach the balance’s capacity, the more
important parameter becomes the
volume of sample holder.
From the above discussion, it is proposed
that the detection limit of TG
systems should be evaluated by the
ratio of the balance sensitivity to
standard sample holder volume.
The effect on TG/FTIR
When evolved gases were analyzed
by FTIR continuously, it is possible
to qualifyquantify the evolved
gases. 3 Figures 2a, 2b,2c show
TG curves with corresponding timeevolved
IR traces for carbon dioxide
and formaldehyde. The frequency
windows used to construct timeevolved
FTIR traces were from
2200.00 to 2500.00 cm-1 for carbon
dioxide,from 2650.00 to 2856
cm-1 for formaldehyde. The net absorbance
at frequency of 2361.02 cm-1
CO22802.75 cm-1 formaldehyde
were used to construct the
time-evolved FTIR traces. It can be
seen that the gases evolved from samples
were given off during the sample
decomposition processes. For
CaC2O4
.H2O, CO should be monitored
during the second weight loss
step, however, beside CO, CO2 was
also observed. This was due to the
small amount of oxygen present in
the reaction gas, which reacted with
carbon monoxideformed carbon
dioxide.
3
Figure 3 shows FTIR spectra collected
during the experiments for
CaC2O4
.H2O 57 μg, CaCO3 0.252
mg,formaldehyde solution
1.0 % samples, respectively. Figure
3 represent the characteristic FTIR
spectra at maximum absorbance of
CO2formaldehyde. It can be
seen that CO2formaldehyde
peaks are well above the noise level.
This means that with even smaller
amounts of the interested component,
evolved CO2formaldehyde gases
can still be detected.
However, there are many factors that
will affect the detection limit for TG/
FTIR. The most important of them
is the type of interface used for coupling
the TGFTIR 4.
Figure 2a: TGIR curves for Calcium Oxalate Monohydrate
Figure 2b: TGIR curves for Calcium Carbonate
Figure 2c: TGIR curves for formaldehyde solution
Figure 3: FTIR spectra for the experiments
The conventional interface technique
is total flow coupling, in which all
gases vented from TG flow through
the FTIR gas cell. Under this type of
coupling, evolved gases from the sample
are diluted by the reaction and
purge gases, which will result in a
lower signal output. A more recent
design is the Thermo Synergy coupling,
in which a Sniffer tube is used
to withdraw gases right above the
sample. In this case, the evolved
gases,maybe a small amount of
reactionpurge gases, are analyzed
by the FTIR. This has resulted in a
greater signal outputwas presented
in a previous paper. 4 With both
TG/FTIR coupling systems, there
are other factors which will affect the
detection limit. These include the
length of the gas cell, the flow rate
of gas through the cell, the flow rate
of the reaction gas, the resolution of
the FTIR used for collecting the spectra,
the time used to collect each
spectrum, etc. Each of these plays
an important role in deciding the detection
limit of TG/FTIRneeds
to be optimized. Even under optimized
conditions, the detection limit is still
dependent upon the amount or concentration
of the interested gases
evolved from the sample. The more
of the gases evolved from the sample,
the better the detection limit. In other
words, the bigger the sample size,
the better the chance to detect the
evolved gases.
Therefore, the detection limit of TG/
FTIR is dependent upon the standard
sample holder volume of the TG
system.
4
Thermo Fisher Scientific
Process Instruments
USA
25 Nimble Hill Rd.
Newington, NH 03801
Tel. 603 436 9444
info.mc.us@thermofisher.com
www.thermo.com/cahn
C218_12.04.07
© 2007/01 Thermo Fisher Scientific·
All rights reserved · This document is
for informational purposes only and
is subject to change without notice.
Conclusions
For TG, the experimental results proved that the larger the sample size,
the better the detection limit. For TG/FTIR, there are many other variables
that can affect the detection limit. With one type of TG/FTIR interface,
the detection limit is dependent upon the sample holder volume of the TG
system. It is proved that, for a sample mixture, the ratio of balance sensitivity
to standard sample holder volume should be used to characterize the
detection limit of TG, while the standard sample holder volume of TG system
should be used to characterize the detection limit of TG/FTIR.
Reference
[1] W. W. WendlandtP. K. Gallagher, Thermal Characterization of
Polymeric Materials, Ed. E. A. Turi, Academic Press, New York, 1982.
[2] D. Dollimore, Analytical Chemistry, 1994, 66, 17R-25R.
[3] D. Chen, A. GreenD. Dollimore, Thermal Trends, Fall 1995, vol 2,
#4, PP 18-22.
[4] The Sniffer Interface, Cahn Instruments, Inc., Product Note, 1995. |
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