Sampling arrangement for thermal gravimetric analyzer

A thermal gravimetric analyzer includes a reaction tube disposed within a heating element and including a sample container disposed therein. A reaction gas is introduced at a first end of the reaction tube and allowed to react with the sample, while a purge gas is introduced at a second, opposed end of the reaction tube to isolate a beam system, from which the sample and sample container are suspended, from the reaction gas. A Fourier transform infrared (FTIR) spectrometer as well as a mass spectrometer coupled to the reaction tube may be used to analyze the off-gas products of the reaction of the sample with the reaction gas. Sniffer tubes are used to provide off-gas samples to the two spectrometers, with each sniffer tube having a first inlet end disposed immediately adjacent the sample container and a second outlet end coupled to a sample inlet port of a respective spectrometer. The sniffer tubes increase off-gas product measurement sensitivity by reducing the total volume of gas analyzed and increasing the concentration of the off-gas products within the volume of the gas being analyzed.

FIELD OF THE INVENTION 
This invention relates generally to thermal gravimetric analyzers and is 
particularly directed to the sampling of the gases produced during 
reaction in a thermal gravimetric analyzer for spectral analysis. 
BACKGROUND OF THE INVENTION 
Combined thermogravimetry/mass spectrometry and thermogravimetry/Fourier 
transform infrared spectroscopy combine the direct measurement of weight 
loss as a function of reaction temperature with the use of spectroscopic 
detectors for the qualitative and quantitative determination of evolved 
volatile products to provide kinetic information about the specific 
reaction mechanisms. Although offering substantial advances in the areas 
of detection and analysis, the presence of a component at very low 
concentrations may be masked by higher concentrations of interferants. 
Additional steps such as collecting the products in a trap or on the head 
of a capillary column have been employed for increasing off-gas product 
detection sensitivity. However, these methods necessarily introduce 
additional time in the detection/analysis method and result in a loss of 
the time/temperature evolution data for the products analyzed. 
Unfortunately, it is frequently most desirable to obtain a time 
correlation of what is occurring in the reaction of the sample with the 
reaction gas introduced into the reaction tube. 
Another approach to obtaining a time correlation of the reaction process 
involves increasing the gas flow through the reaction tube while 
correspondingly increasing the sampling rate. This approach does not 
compromise the performance of the thermal system or the IR system, and 
avoids the prior art problems encountered with an excess pressure within 
the reaction tube which tends to reduce the stability of weight changes of 
the sample. Unfortunately, this approach renders it more difficult to 
detect the off-gas products by decreasing the signal-to-noise ratio at the 
detectors. 
The present invention addresses the aforementioned limitations of the prior 
art by providing a sampling arrangement for a thermal gravimetric analyzer 
which removes the off-gas products immediately adjacent to the sample 
holder and provides the thus removed concentrated off-gas products 
directly to mass and/or IR spectroscopic detectors. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved sampling arrangement for the spectral and mass analysis of 
off-gas products in a thermal gravimetric analyzer. 
It is another object of the present invention to enhance detection and 
measurement sensitivity and accuracy in a thermal gravimetric analyzer by 
sampling the off-gas products at a location immediately adjacent to the 
reaction site. 
Yet another object of the present invention is to increase detection and 
measurement sensitivity in a thermal gravimetric analyzer of reaction 
off-gas products by minimizing dilution of the off-gases by either the 
reaction gas or a purge gas. 
A further object of the present invention is to increase the 
signal-to-noise ratio in spectrographic apparatus for measuring the 
interaction between a sample material and its reaction environment. 
These objects of the present invention are achieved and the disadvantages 
of the prior art are avoided by an apparatus for detecting and measuring 
reaction of a reaction gas with a sample in a thermal gravimetric analyzer 
including a balance and spectrometer for determining characteristics of an 
off-gas produced by interaction of the reaction gas and the sample, the 
apparatus comprising a reactor container for receiving the reaction gas; a 
cup suspended from the balance and disposed in the reactor container for 
supporting and maintaining the sample in position in the reactor 
container; and a sampling arrangement disposed immediately adjacent the 
cup for receiving off-gas produced by interaction of the sample, 
temperature and the reaction gas and for providing the off-gas to the 
spectrometer for analysis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a simplified combined schematic and 
block diagram of a thermogravimetry/gas chromatography/mass 
spectrometrysystem 10 incorporating a sampling arrangement in accordance 
with the present invention. The thermogravimetry/gas chromatography/mass 
spectrometry system 10 includes a thermal gravimetric analyzer 11 
including a reactor tube 12 incorporating a sampling arrangement in 
accordance with the present invention. FIG. 2 is a partially cutaway 
lateral section view of reactor tube 12, while FIG. 3 is a partially 
exploded perspective view of the reactor tube incorporated in the thermal 
gravimetric analyzer 11. 
Reactor tube 12 is generally cylindrical and is typically comprised of a 
hard, rigid, transparent material such as quartz but could be made of 
nontransparent alumina. Disposed about reactor tube 12 is a heater element 
42. Disposed in a lower portion of reactor tube 12 and aligned with its 
longitudinal axis is a cylindrical volume reducer element 20 which is 
supported and maintained in position by a lower assembly seal 19 disposed 
on the lower end of the reactor tube 12. The lower assembly seal 19 is 
securely maintained in position on the lower end of the reactor tube 12 by 
means of a first clamp ring, or nut, 18. Extending through an aperture in 
the lower assembly seal 19 and up into a lower portion of the reactor tube 
12 is a thermocouple 22 which provides an indication of the temperature 
within the reactor tube 12 which is controlled by heater element 42. 
Disposed in an upper portion of the reactor tube 12 and aligned along its 
longitudinal axis is a baffle assembly 34. Baffle assembly 34 and the 
volume reducer element 20 are also preferably comprised of quartz but 
could be made of alumina. Baffle assembly 34 includes an elongated 
aperture extending the length thereof through which an extension wire 26 
extends. A lower end of the extension wire 26 is attached to a sample cup 
24 which contains the sample of the substance which reacts when 
temperature is applied and/or a reaction gas introduced into the reactor 
tube 12. An upper end of the extension wire 26 is coupled to a beam 
balance 28 for providing the weight of the sample within the sample cup 24 
as it reacts with the environment produced within the reaction tube 12. 
The lower volume reducer element 20 and the upper baffle assembly 34 serve 
to maintain the concentration of gases highest in the vicinity of the 
sample in the sample cup 24. 
The reaction gas is introduced in a lower portion of the reaction tube 12 
via a reaction gas inlet 29 as shown in FIG. 2. An inert purge gas 
typically comprised of a helium or nitrogen is introduced into an upper 
portion of the reaction tube 12 via the purge gas inlet 67. The purge gas 
isolates the beam balance 28 from the reaction gas and thus prevents 
damage to the beam balance by the reaction gas. 
Attached to an upper portion of the reaction tube 12 is an effluent gas 
adaptor 32. Gas adaptor 32 is coupled in a sealed manner to an upper 
portion of the reactor tube 12 by means of a second clamped ring 36. Gas 
adaptor 32 is further coupled in a sealed manner to a lower portion of the 
beam balance 28 by means of a third clamp ring 37. The effluent gas 
adaptor 32 includes first and second total flow vent tubes 60 and 62 
extending from a lateral wall thereof. The effluent gas adaptor 32 is 
preferably comprised of an inert, high strength metal such as stainless 
steel. 
In accordance with the present invention, disposed within reactor tube 12 
are first and second sniffer tubes 38 and 44. A first, lower end of each 
of the sniffer tubes 38, 44 is disposed immediately adjacent to, and 
slightly above, sample cup 24. A second upper end of the first sniffer 
tube 38 extends into the first total flow vent tube 60. Similarly, a 
second upper end of the second sniffer tube 44 is disposed in the second 
flow vent tube 62. Each of the first and second sniffer tubes 38, 44 is 
comprised of either Iconel, platinum, sapphire or a combination thereof in 
the disclosed embodiment, platinum being used in high temperature 
applications with temperatures as high as 1700.degree. C. Each of the 
first and second sniffer tubes 38, 44 samples the off-gas products 
immediately adjacent sample cup 24 and provides the sampled off-gas 
products to the first and second flow vent tubes 60, 62, respectively. 
As shown in FIG. 1, coupled to the first total flow vent tube 60 via a 
first off-gas withdrawal line 50 is an FTIR system 46 including an FTIR 
gas cell 48. A pump 54 coupled to the FTIR system 46 via a vacuum line 52 
draw the off-gas products from the reaction tube 12 via the FTIR system 
46. Similarly, coupled to the second sniffer tube 44 via the combination 
of the second flow vent tube 62 and a second off-gas withdrawal line is a 
mass spectroscopy system 66. FTIR system 46 provides an infrared 
spectroscopic analysis of the off-gas products, while the mass 
spectroscopy system 66 affords a molecular weight analysis of the off-gas 
products. 
Referring to FIG. 4, there is shown a sectional view of an FTIR adaptor 68 
for coupling the inventive sniffer tube 38 to an FTIR system and tube 44 
to a mass spectroscopy system. In the embodiment of the FTIR adaptor 68 
shown in FIG. 4, the reactor tube containing a sample cup 24 is coupled to 
both the FTIR system and a mass spectroscopy system as in the previously 
described embodiment. Attached to and extending from the FTIR adaptor 68 
is a sniffer tube vent tube 72 and a total flow vent tube 74. The total 
flow vent tube 74 encloses and supports the upper end of the sniffer tube 
44 and is coupled by means of a tube Tee union 76 (shown in dotted line 
form) to a vent and exhaust system which is not shown in the figure for 
simplicity. The sniffer tube vent tube 72 encloses and supports an upper 
end of sniffer tube 38. The lower ends of sniffer tubes 38 and 44 are 
positioned immediately adjacent to and slightly above the sample cup 24 as 
in the previously described embodiment. Both sniffer tubes 38, 44 remove 
the off-gas products from adjacent the sample cup 24 and provide them to 
the sniffer vent tubes 72 and 74. The sniffer vent tube 72 is coupled by 
means of a second tube union 78 (also shown in dotted line form) to an 
FTIR system (also not shown for simplicity) for spectral analysis of the 
off-gas products. The vent and exhaust system ensures an upward flow of 
the off-gas products within the reaction tube for delivery of the FTIR 
system for analysis. 
Referring to FIG. 5, there is shown a graphic comparison of the sensitivity 
in measuring off-gas products using the sampling arrangement of the 
present invention with prior art approaches. From the figure, it can be 
seen that at 3000 wavenumbers, the lowest curve labeled Total Flow, High 
Volume TGA indicates an absorbance of 1.63. This curve represents the 
measured results at a gas flow rate of 140 ml/min. These high gas 
dilutions decrease the signal to background noise ratio so as to mask the 
absorbance by the off-gas products and reduce measurement sensitivity. The 
next higher curve is labeled Total Flow, Low Volume TGA for a flow rate of 
35 ml/min. This relatively low absorbance signal is due to the smaller 
volume of the gas being sampled. The Total Flow, Low Volume TGA curve 
shows an absorbance value of 2.92 at 3000 wavenumbers. Finally, the 
uppermost curve labeled Sniffer, High Volume TGA shows a substantial 
increase in the measured absorbance at a value of 13.28 at 3000 
wavenumbers. This latter absorbance curve was derived using a sniffer tube 
in accordance with the present invention at a flow rate of 140 ml/min and 
indicates a sensitivity increase of an order of magnitude (10.times.) 
using the sampling arrangement of the present invention. The increased 
sensitivity of a sampling arrangement utilizing the sniffer tube of the 
present invention allows for a relaxation in the performance requirements 
of the detectors used in the chromatographic or mass spectroscopy system. 
There has thus been shown a sampling arrangement for use in a thermal 
gravimetric analyzer which substantially increases detection sensitivity 
of the off-gas products within the analyzer's reaction tube. By sampling 
the off-gas products immediately adjacent the sample container, the 
concentration of the off-gas sample is substantially increased allowing 
for a reduction in the volume of the gas analyzed and a relaxation in the 
sensitivity of the detectors in the detection and measuring apparatus. 
While particular embodiments of the present invention have been shown and 
described, it will be obvious to those skilled in the art that changes and 
modifications may be made without departing from the invention in its 
broader aspects. Therefore, the aim in the appended claims is to cover all 
such changes and modifications as that which falls within the true spirit 
and scope of the invention. The matter set forth in the foregoing 
description and accompanying drawings is offered by way of illustration 
only and not as a limitation. The actual scope of the invention is 
intended to be defined in the following claims when viewed in their proper 
perspective based on the prior art.