ASTM E2105-00

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Designation:E2105–00
Standard Practice for
General Techniques of Thermogravimetric Analysis (TGA)
Coupled With Infrared Analysis (TGA/IR)
This standard is issued under the fixed designation E2105; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E168 Practices for General Techniques of Infrared Quanti-
tative Analysis
1.1 Thispracticecoverstechniquesthatareofgeneralusein
E334 Practices for General Techniques of Infrared Mi-
thequalitativeanalysisofsamplesbythermogravimetricanaly-
croanalysis
sis(TGA)coupledwithinfrared(IR)spectrometrictechniques.
E473 Terminology Relating to Thermal Analysis
The combination of these techniques is often referred to as
E1131 Test Method for Compositional Thermal Analysis
TGA/IR.
by Thermogravimetry
1.2 A sample heated in a TGA furnace using a predeter-
E1252 Practice for General Techniques for Qualitative
mined temperature profile typically undergoes one or more
Infrared Analysis
weightlosses.Materialsevolvedduringtheseweightlossesare
E1421 PracticeforDescribingandMeasuringPerformance
thenanalyzedusinginfraredspectroscopytodeterminechemi-
of Fourier Transform Infrared (FT-IR) Spectrometers:
cal identity. The analysis may involve collecting discrete
Level Zero and Level One Tests
evolved gas samples or, more commonly, may involve passing
the evolved gas through a heated flowcell during the TGA
3. Terminology
experiment. The general techniques of TGA/IR and other
3.1 Definitions—For general definitions of terms and sym-
corresponding techniques, such as TGA coupled with mass
bols, refer to Terminologies E131 and E473.
spectroscopy(TGA/MS),aswellas,TGA,usedinconjunction
2 3.2 Definitions of Terms Specific to This Standard:
with GC/IR, are described in the referenced literature (1-4).
3.2.1 evolved gas, n—any material (or mixture) evolved
1.3 Some thermal analysis instruments are designed to
from a sample during a thermogravimetric or simultaneous
perform both thermogravimetric analysis and differential scan-
thermal analysis experiment. Materials evolved from the
ning calorimetry simultaneously. This type of instrument is
sample may be in the form of a gas, a vapor, an aerosol or as
sometimes called a simultaneous thermal analyzer (STA). The
particulate matter. For brevity, the term “evolved gas” will be
evolved gas analysis performed with an STAinstrument (5) is
used throughout this practice to indicate any material form or
similar to that with a TGA, and so, would be covered by this
mixture evolved from a sample.
practice. With use of a simultaneous thermal analyzer, the
3.2.2 evolved gas analysis (EGA), n—a technique in which
coupled method typically is labeled STA/IR.
the nature and amount of gas evolved from a sample is
1.4 The values stated in SI units are to be regarded as
monitored against time or temperature during a programmed
standard.
change in temperature of the sample.
1.5 This statement does not purport to address all of the
3.2.3 evolved gas profile (EGP), n—an indication of the
safety concerns, if any, associated with its use. It is the
total amount of gases evolved, as a function of time or
responsibility of the user of this standard to establish appro-
temperature, during the thermogravimetric experiment. In
priate safety and health practices and determine the applica-
TGA/IR, this profile is calculated from the infrared spectro-
bility of regulatory limitations prior to use.
scopic data recorded by application of the Gram-Schmidt
2. Referenced Documents reconstruction (GSR) algorithm (6,7). Because the GSR was
designed for use in gas chromatography coupled with infrared
2.1 ASTM Standards:
(GC/IR) analysis, the evolved gas profile has sometimes been
E131 Terminology Relating to Molecular Spectroscopy
erroneously called the evolved gas chromatogram.
3.2.4 functional group profile (FGP), n—an indication of
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
the amount of gas evolved during the thermogravimetric
Spectroscopy and is the direct responsibility of Subcommittee E13.03 on Infrared
experiment that contains a particular chemical functionality
Spectroscopy.
Current edition approved Sept. 10, 2000. Published November 2000.
The boldface numbers in parentheses refers to the list of references at the end
of this standard.
3 4
Annual Book of ASTM Standards, Vol 03.06. Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E2105
measured as a function of time or temperature. This profile is ever, in vapor phase evolved gas trapping, the sample integrity
calculated from the infrared spectroscopic data recorded by can be compromised by slow decomposition or by deposition
integrationoftheabsorbancesoverselectedspectralregionsas on the cell walls. A spectrum should be obtained initially
theexperimentprogresses.Typically,anumberofsuchprofiles within a short co-addition time to create a reference spectrum
are calculated in real-time.Additional profiles (using different to ensure the integrity of the spectrum obtained after long
spectral regions) can often be calculated after the experiment co-addition.
from the stored spectroscopic data. Because the software used
5.3 Evolved Gas Analysis Using a Flowcell—Another way
has similarities with that used for GC/IR analysis, the func-
to examine the gases evolved during a TGA/IR experiment is
tional group profile has sometimes been erroneously called the
touseaspeciallydesignedflowcell.Thisflowcellissituatedin
functional group chromatogram.
the IR beam of the infrared spectrometer. IR monochromators
3.2.5 hit quality index (HQI), n—the numerical ranking of
andfilterspectrometersaretypicallyusedtomonitoraspecific
infrared reference spectra against that of an analyte spectrum
frequencyrangeduringtheTGAexperiment.Ifafullspectrum
through the use of search algorithms that measure a compara-
is to be obtained with these IR devices, the evolved gas is
tive fit spectral data.
trapped via a stopped flow routine and the spectrometers are
3.2.6 specific gas profile (SGP), n—a special type of func-
permittedtoscantheinfraredspectrum.Incontrast,theFourier
tional group profile arises when the selected region of the
transform IR spectrometer permits the acquisition of the
spectrum contains absorbances due to a specific gas such as
completeIRspectruminbrieftimeframeswithoutimpactupon
ammonia or carbon monoxide.
the typical TGA experiment, that is, continuous spectral
collection without interruption of evolved gas flow or sample
4. Significance and Use
heating.
4.1 This practice provides general guidelines for the prac-
5.3.1 In the typical TGA/IR experiment, the evolved gas is
tice of thermogravimetry coupled with infrared spectrometric
monitored in real-time by the IR spectrometer. The temporal
detection and analysis (TGA/IR). This practice assumes that
resolutionrequiredduringaTGA/IRexperimentisontheorder
the thermogravimetry involved in the practice is proper. It is
of 5–60 s/spectral data acquisition event. If the full IR
not the intention of this practice to instruct the user on proper
spectrumistobeacquired,therapidityoftheTGAexperiment
thermogravimetric techniques. Please refer to Test Method
requires a Fourier-transform infrared (FT-IR) spectrometer to
E1131 for more information.
maintain sufficient temporal resolution. Such instruments in-
clude a computer that is capable of storing large amounts of
5. General TGA/IR Techniques
spectroscopic data for subsequent evaluation.
5.1 Two different types of TGA/IR techniques are used to
5.3.2 Some spectrometer data systems may have limited
analyze samples. These consist of discrete evolved gas trap-
software, or data storage capabilities. Such instrument systems
ping and use of a heated flowcell interface. It should be noted
are capable of recording suitable spectra during the TGA/IR
that only the latter technique allows for the calculation of the
experiment, but may not be able to calculate the evolved gas
evolved gas and functional group profiles.
and functional group profiles.
5.2 Evolved Gas Trapping Techniques—Evolved gas trap-
5.3.3 The flowcell is coupled directly to the TGA via a
ping techniques are the least elaborate means for obtaining
heated transfer line. Evolved gas components are analyzed as
TGA/IR data. In these techniques, the evolved gas is collected
they emerge from the transfer line. This technique typically
from the TGA furnace in discrete aliquots that are then
yields low microgram detection limits for most analytes (1).
analyzed. In use of such techniques, it is essential to monitor
Instruments that include the IR spectrometer, data system, the
the TGA weight loss curve to determine the time or tempera-
thermogravimetric analyzer, heated transfer-line, and heated
ture at which the effluent was captured. Vapor phase samples
flowcell are commercially available.
canbetrappedinaheatedlow-volumegascellattheexitofthe
5.3.4 It should be noted that any metal surface inside the
TGA, analyzed, then flushed out by the TGA effluent. When
TGA furnace, transfer line or flowcell assembly may react
thenextaliquotofinterestisinthegascell,theflowisstopped
with,andsometimesdestroy,specificclassesofevolvedgases,
again for analysis. This process can be made more convenient
forexample,amines.Thiscanresultinchangestothechemical
by designing the TGA temperature profile such that the
natureoftheevolvedgas.Consequently,itispossibletofailto
temperature is held constant while a trapped sample is being
identify the presence of such compound in the mixture. This
analyzed (ramp-and-hold method).Alternatively, fractions can
situation can sometimes be identified by comparison of the
betrappedinthecondensedphasebypassingtheTGAeffluent
TGA weight loss profile with the evolved gas profile.
through a solvent, a powdered solid, or a cold trap to yield
condensed phase material for subsequent analysis (8). Infrared 5.3.5 The infrared energy throughput of the flowcell should
spectrometry is performed with either a monochromator, a be periodically monitored since this indicates the overall
filter spectrometer or a Fourier transform spectrometer. See condition of this assembly. It is important that all tests be
also Practices E334 and E1252 for general techniques on conducted at a constant flowcell temperature because of the
microanalysis and qualitative practices. effect of the emitted energy on the detector (see 6.3.1). It is
5.2.1 Since the analyte of interest is static when employing recommended that records be kept of the interferogram signal
an evolved gas trapping technique, the spectrum can be strength, single-beam energy response and the ratio of two
recorded using a long integration time or increasing scan successive single-beam curves (as appropriate to the instru-
co-addition to improve the signal-to-noise ratio (SNR). How- ment used). For more information on such tests, refer to
E2105
PracticeE1421.Ifamercury-cadmium-telluride(MCT)detec- chosen to avoid both condensation and degradation of the
tor is being employed, these tests will also reveal degradation evolved gases. Typical working temperatures have a range of
of performance due to loss of the Dewar vacuum and conse-
150–300°C. The flowcell usually is held at a slightly greater
quent buildup of ice on the detector face. In general, when a temperature, ca. 10°C higher, to avoid condensation of the
lossoftransmittedenergygreaterthan10%ofthetotalenergy
evolved gas.
is found, cleaning of the flowcell is recommended.
6.1.1 The use of a TGA/IR system to analyze complex
5.3.6 Care must be taken to stabilize or, preferably, remove
materials, such as polymers or natural products, will result in
interfering spectral features that result from atmospheric ab-
carbonaceous material, high-molecular weight polymers and
sorptions in the IR beam path of the spectrometer. Best results
other high boiling materials accumulating in the transfer line
will be obtained by purging the entire optical path of the
and the flowcell.Aperiodic removal of these materials can be
spectrometer with dry nitrogen gas. Alternatively, dry air can
accomplished by passing air (or oxygen) through the hot line:
be used as the spectrometer purge gas; however, this will lead
however, the condensation of material will eventually yield a
to interferences in the regions of carbon dioxide absorption
reduction in gas flow.At this point, it is necessary to clean out
−1 −1
(2500 go 2200 cm and 720 to 620 cm ) due to the presence
the line before it clogs completely. Flushing the transfer line
of carbon dioxide in air. Further, commercially-available air
with one or more solvents, such as acetone, pentane or
scrubbers, that remove both water vapor and carbon dioxide,
chloroform may remove condensed materials. Alternatively,
provide adequate purging of the spectrometer. In some instru-
some commercial systems use a transfer line with a disposable
ments, the beam path is sealed in the presence of a desiccant,
liner that can be replaced.
but interferences from both carbon dioxide and water vapor
−1 6.2 Design of the Infrared Flowcell—The flowcell is opti-
(1900 to 1400 cm ) may be found. Similarly, the TGA
mized to give maximum optical throughput, to minimize
furnace, the transfer line and the gas cell interface are purged
decomposition and mixing of analyte gas stream, and to yield
with a gas that does not absorb infrared energy. Typically, this
linear infrared absorption. The flowcell dimensions are opti-
TGApurgegasisinert(nitrogenorhelium)andhasaflowrate
mized to accommodate a discrete volume and flow rate and
from 10 to 200 mL/min. Occasionally, oxidizing or reducing
provide sufficient optical pathlength for spectral data acquisi-
atmospheres,thatis,oxygenorhydrogenrespectively,areused
tion with reasonable temporal resolution. Preferably, the cell is
with the TGA to promote specific chemical reactions. When
heated to a constant temperature at or slightly higher than the
preparing for a TGA/IR experiment, the atmospheres within
temperature of the transfer line, ca. 10°C or higher; however,
the spectrometer and within the furnace and gas cell combina-
the maximum temperature recommended by the manufacturer
tionmustbeallowedtostabilizebeforespectraldatacollection
should not be exceeded. It must be noted that repeated
and the thermal experiment commence to minimize spectral
temperature changes to the cell and transfer line accelerate
interferences. Atmospheric stability for
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