Apparatus for measuring outgassing of volatile materials from an object

An apparatus for measuring material outgassed from an object. The apparatus has a chamber containing the object, a condensed material detector (e.g. a quartz microbalance), a heater for heating the object and chamber, and a cooler for cooling the detector. The chamber is sealed from the ambient atmosphere and the detector is located within the chamber. The chamber may contain a vacuum or a gas at ambient atmospheric pressure. Material outgassed from the object is distributed throughout the chamber by vapor transport and is incident upon the detector, where it condenses. Since the detector is the only cooled surface in contact with the vapors, it collects nearly all the outgassed material. This provides high sensitivity to outgassing. The chamber may also include a mechanical stirring device for aiding vapor transport, or may be oriented so that a thermal convection current is established. Preferably, the chamber includes a snout for thermally isolating the detector and chamber so that a steep temperature gradient exists close to the detector surface. Alternatively, the detector has a coating with a high affinity for outgassed materials of interest. The chamber, detector, and object are at nearly the same temperature and the outgassed materials preferentially condense on the detector due to the high affinity of the coating for the outgassed materials.

FIELD OF THE INVENTION 
This invention relates generally to devices for measuring the amount of 
volatile material that is outgassed from an object. More particularly, the 
invention relates to devices that measure the amount of outgassed volatile 
material by measuring the amount of the volatile material condensed on a 
surface. 
BACKGROUND OF THE INVENTION 
The measurement of the outgassing characteristics of materials is important 
in the construction of many delicate devices. Aboard satellites and other 
spacecraft, lenses, filters, windows, and other optical components must 
remain clean. Contamination of these components can be caused by 
outgassing of volatile materials from nearby parts. For example, plastics 
and lubricants are known to outgas in vacuum. The outgassed materials are 
deposited on nearby surfaces. The deposited materials damage components 
with sensitive surfaces. Therefore, it is desirable to minimize the amount 
of material outgassed in proximity to sensitive components. The amount of 
outgassing present is usually limited by cleaning parts and by carefully 
selecting the materials of which the satellite is made. 
Computer hard drives can also be damaged by outgassing and recondensation 
of volatile materials. Volatile materials outgassed from internal hard 
drive components recondense on the magnetic data storage surfaces. These 
contaminants can be swept up by the slider that carries the read-write 
head, causing it to crash into the disk surface, resulting in failure of 
the drive and loss of data. The contaminants can also alter the frictional 
forces between the disk and the slider, possibly preventing the disk from 
rotating. Therefore, it is desirable to minimize the amount of outgassed 
volatile materials in the drive. This is accomplished by only using 
low-emission materials in the manufacturing of parts and by carefully 
cleaning parts prior to assembly. In selecting the materials and/or parts 
that can be used in the hard drive, it is necessary to measure the 
outgassing characteristics of candidate parts and materials. 
Measurement of outgassing characteristics has typically been performed by 
placing the part in question in a vacuum chamber and evacuating the 
chamber. A pressure or mass detector is also placed within the chamber. 
Material outgassed from the part is detected by the sensor. A disadvantage 
of measuring outgassing in a vacuum is that it is time consuming to 
measure outgassing of many different objects because the vacuum must be 
reestablished each time a different object is placed within the vacuum 
chamber. 
A problem with other techniques for measuring outgassing is that they do 
not intrinsically integrate the quantity of outgassed material. They must 
electronically integrate signals to obtain the total amount of outgassed 
material, which is less accurate. Further, they do not provide a sample of 
the outgassed material for chemical or physical analysis. These problems 
are present in outgassing detectors that detect the concentration of 
outgassed material in the vapor or gas phase and do not collect the 
outgassed material. 
U.S. Pat. No. 4,561,286 to Sekler et al. discloses a piezoelectric 
contamination detector that compensates for changes in temperature that 
can otherwise interfere with contamination measurements. Sekler does not 
disclose the use of the device in a chamber for measuring the amount of 
material outgassed from an object. 
U.S. Pat. No. 4,735,081 to Luoma et al. discloses a detector for detecting 
vapors in gaseous fluids, such as air. The air is passed over a detector 
having a crystal oscillator with a coating selected to absorb the vapor of 
interest. Luoma does not disclose the measurement of outgassed materials 
from an object in a closed chamber. 
U.S. Pat. No. 5,408,864 to Wenman discloses a method of measuring the 
amount of gas desorbed from a solid. The solid is placed in a chamber with 
a specially designed gas. The pressure in the chamber is measured as a 
function of temperature. The absorptive properties of the solid are then 
determined from this measurement. Wenman's method requires accurate 
measurement of pressure inside the chamber. Wenman's method does not use 
condensed material detectors. 
U.S. Pat. Nos. 4,781,358 and 4,719,073 to Langan disclose an apparatus and 
method for monitoring parts in a sintering furnace. Langan monitors the 
outgassing of the parts as they are being sintered. Langan flushes the 
furnace with gases as the parts are sintered. 
U.S. Pat. No. 5,287,725 to Zhao et al. discloses an apparatus for detecting 
volatile material on the surface of a semiconductor wafer. The wafer is 
placed in a vacuum chamber and heated while the walls of the chamber are 
cooled. The apparatus is rather inefficient, detecting only a small 
portion of the material evaporated from the wafer. 
OBJECTS AND ADVANTAGES OF THE INVENTION 
It is a primary object of the present invention to provide a device for 
measuring the amount of outgassing from an object that: 
1) does not require operation in a vacuum environment, and can be operated 
at atmospheric pressure; 
2) assures that a large portion of the material outgassed from the object 
is detected, and therefore is naturally integrating, which provides a high 
sensitivity and accurate calibration for the outgassed material; 
3) collects the outgassed material, making it available for further 
chemical or physical analysis; 
4) can be used to measure the quantity of material on the surface of an 
object; and 
5) is relatively simple in design, construction and operation. 
These and other objects and advantages will be apparent upon reading the 
following description and accompanying drawings. 
SUMMARY OF THE INVENTION 
These objects and advantages are attained by an apparatus having a chamber 
containing an object to be measured for outgassing, a condensed material 
detector, and a means for establishing a temperature difference between 
the object and the detector so that the object is hotter than the 
detector. The detector is exposed to the interior of the chamber so that 
material outgassed by the object is incident on the detector. The detector 
is cooler than the object so the outgassed material may condense on the 
detector. The detector may be cooled by a refrigerator, and the object may 
be heated by an electric heater. 
The chamber may contain a gas at any pressure from vacuum to atmospheric 
pressure or above. The outgassed material may reach the detector by 
diffusion. The chamber may or may not be sealed from the ambient 
atmosphere. If the chamber contains a gas at a pressure greater than about 
1 Torr, the apparatus may be oriented so that thermal convection currents 
are established by the temperature difference to increase the rate of 
transport of outgassed material to the detector. The apparatus may also 
include a stirring device for increasing the rate of transport of the 
outgassed materials from the object to the detector. The gas in the 
chamber may be any relatively inert gas, such as nitrogen, noble gases, or 
even air. 
The apparatus preferably includes a snout that extends from the chamber to 
the detector. A tip of the snout close to the detector is preferably made 
to be thermally insulating. 
The chamber is preferably made of weakly adsorbing material such as glass, 
polytetrafluoroethylene, stainless steel, or gold-plated copper. 
Preferably, the object and the chamber are in thermal contact so that they 
are at the same temperature. Also preferably, the temperature difference 
between the object and detector is in the range of about 50-150 degrees 
Celsius. Also, it is preferred for the chamber to be hotter than the 
detector. 
The apparatus may also include an additional collecting surface for 
collecting outgassed material, so that the detector does not collect all 
the outgassed material from the object. 
Also, the detector may have a coating with a high affinity for outgassed 
material from the object. 
Alternatively, the present invention includes an embodiment where the 
detector and the object are at nearly the same temperature (i.e. within 50 
degrees Celsius). The detector has a coating with a high affinity for 
outgassed material from the object and so collects the outgassed material 
without needing to be held at a temperature lower than the temperature of 
the object.

DETAILED DESCRIPTION 
The present invention provides an apparatus for measuring the amount of 
material outgassed from an object. The measurement is performed by causing 
the outgassed material to travel by vapor transport from the outgassing 
object to a condensed material detector, where the outgassed material 
collects. The outgassed material preferentially collects on the detector 
because the detector is held at a temperature lower than the object, or 
because the detector is selected to have a higher affinity for the 
outgassed material, or both. The vapor transport can occur over a very 
wide range of pressures (e.g., from vacuum to above atmospheric pressure) 
and is preferably aided by thermal convection or mechanical stirring. 
FIG. 1A shows a specific preferred embodiment of the present invention. An 
object 20 to be measured for outgassing is located within a chamber 22. 
The object is in good thermal contact with a heater 24 that heats the 
object 20. The chamber contains a gas 26 at atmospheric pressure. The gas 
26 is circulated throughout the chamber 22 by a stirring device such as a 
fan 28. A snout 30 has an opening directed toward a condensed material 
detector such as a surface acoustic wave device or resonant quartz 
microbalance 32. The microbalance 32 is located on a heatsink 34 such that 
the microbalance is maintained at a temperature cooler than the 
temperature of the object 20. The heatsink may be water cooled, air 
cooled, or cooled by an active refrigerator 52. The heatsink may also have 
a purge heater 50 for periodically heating the microbalance 32 in order to 
purge collected material. Preferably, the chamber 22 is also heated in 
addition to the object 20. 
The snout 30 and heatsink 34 preferably do not form an airtight seal. A gap 
36 between the snout and heatsink may be a few thousandths of an inch 
(e.g. about 0.001-0.003 inches). Therefore, the gas 26 in the chamber is 
at ambient atmospheric pressure. However, the chamber is not substantially 
open to the ambient atmosphere. The chamber tends to maintain the same gas 
within it during operation of the apparatus. The gas 26 may be pure air, 
dry nitrogen, or any other relatively inert gas. The apparatus may include 
a supply of gas so that the gas 26 in the chamber 22 may be periodically 
purged. Also, the apparatus may be purged by opening the chamber to the 
atmosphere while heating. The gas 26 may be purged between outgassing 
measurements of different objects 20, for example. 
Preferably, the chamber is made of a relatively inert material such as 
stainless steel, TEFLON, or glass. These materials have the beneficial 
property of having surfaces that have a low affinity for many common 
outgassed materials. Copper can also be used, but should be gold-plated to 
suppress oxidation at elevated temperatures. The chamber must be made of 
materials that can tolerate elevated temperatures above about 100 degrees 
Celsius. 
In operation, the heater 24 heats the object 20 and the heatsink 34 cools 
the microbalance 32. The object 20 is hotter than the microbalance 32. 
Preferably, the heater 24 also heats the chamber 22 and snout such that 
the snout 30, chamber 22, and object 20 are all hotter than the heatsink 
34 and microbalance 32. Material outgassed from the object 20 is condensed 
on the cool microbalance 32, where it is detected and measured. The 
outgassed material does not collect on the chamber surfaces 35 because the 
chamber 22 is hot and is weakly adsorptive. The snout 30 causes a steep 
temperature gradient to exist close to the microbalance 32. The steep 
temperature gradient helps the outgassed material to be preferentially 
deposited on the microbalance 32. 
Preferably, a tip portion 31 of the snout 30 is made of a thermally 
insulating material such as quartz so that heat is not conducted between 
the snout 30 and heatsink 34. Also preferably, the tip portion 31 has a 
thin thickness of less than 0.01 inches. The snout 30 has a thickness 60 
of about 0.05 inches. The thin thickness of the tip 31 provides a high 
thermal resistance between the heatsink 34 which is cold and the chamber 
22 and snout 30 which are hot. The high thermal resistance of the tip 31 
helps provide a steep temperature gradient just above the surface of the 
microbalance 32. The snout 30 should be as short as possible while still 
maintaining the steep temperature gradient. 
The fan 28 circulates the gas 26 around the object 20 and helps to reduce 
the time necessary for the outgassed material to travel from the object 20 
to the microbalance 32. Volatile material outgassed from the heated object 
is mixed into the circulating gas 26. The volatile material is condensed 
on the surface of the microbalance 32 and is thereby detected and 
measured. The volatile material is not deposited on the inside surfaces 35 
of the chamber 22 because the chamber is held at an elevated temperature 
and is made of a weakly adsorptive material. Almost all the volatile 
material outgassed from the object is deposited on the microbalance 32. 
Alternatively, molecular diffusion (with no active mixing by fan 28) of 
outgassed material from the object 20 to the microbalance 32 can be relied 
upon for transport, but is not preferred since it results in a slow time 
response. 
The low affinity of the chamber surfaces 35 for the outgassed material 
results in the outgassed material preferentially condensing on the 
detector. Preferably, the chamber has a low surfaces area for the volume 
enclosed, and has a sufficiently low affinity for the outgassed material 
such that at least about 80% of the material outgassed by the object is 
collected on the detector (the rest is collected on the surfaces 35 of the 
chamber). Also it is preferable for the residence time of the outgassed 
material on the surfaces 35 (determined in large part by the material of 
the chamber surfaces) to be shorter than the time scale of the measurement 
(i.e. shorter than about 10-60 minutes). 
Since almost all the material outgassed from the object 20 is deposited 
onto the microbalance 32, the apparatus of the present invention is 
readily calibrated and is sensitive to small quantities of material 
outgassed from the object 20. The microbalance is also naturally sensitive 
to small amounts of deposited material. In a specific example, the 
microbalance has a resonant frequency of about 6 MHz and changes in 
resonant frequency by 100 Hz for each microgram of material deposited on 
the microbalance surface. The apparatus can therefore detect substantially 
less than 1 microgram of material outgassed from the object. 
The heater 24 and heatsink 34 produce a temperature difference between the 
object 20 and microbalance 32. The temperature difference results in a 
difference in the degree of saturation of the vapor of the outgassed 
material at the object and microbalance. In particular, the degree of 
saturation of the outgassed material at the microbalance is high enough 
for condensation to occur. The temperature difference must be selected to 
result in outgassing at the object 20 and condensation at the microbalance 
32. As a specific example, for organic materials, the partial pressure of 
the outgassed material at the temperature of the microbalance can be about 
10.sup.-2 Torr and at the temperature of the object, the partial pressure 
can be greater than about 1 Torr. These conditions will result in organic 
materials being outgassed from the object 20 and condensed on the 
microbalance 32. It is noted these partial pressures will not be reached 
in actual practice because the gas 26 is being actively mixed. These 
partial pressure differences can be used to select temperatures for the 
object and microbalance for a given species of outgassing material. If the 
vapor pressure of the material to be detected is known as a function of 
temperature, then the object and microbalance temperatures can be selected 
to result in outgassing and condensation of the material at the object and 
microbalance, respectively. For many volatile materials, temperature 
differences in the range of about 50-150 Celsius are appropriate. In order 
to avoid condensation of water vapor, the detector temperature should be 
at room temperature or above. In the absence of water vapor, the detector 
could be cooled below room temperature. 
After a measurement is performed, the microbalance is heated by the purge 
heater 50 while the chamber is open, being pumped on, or being purged with 
gas, and the volatile outgassed materials are evaporated from the 
microbalance 32. It is known in the art that microbalances yield 
inaccurate measurements when a large amount of material is deposited on 
the microbalance surface. Heating the microbalance prepares it to perform 
another measurement by causing the condensed material to evaporate. This 
heating process can also be used to characterize the outgassed material. A 
thermal analysis spectrum of the outgassed material can be obtained by 
heating the microbalance slowly with the chamber open (or while vacuum 
pumping on the chamber) and monitoring the signal loss as a function of 
temperature. For a mass-sensitive detector (e.g. the microbalance), this 
spectrum will provide a means for thermal gravimetric analysis. The 
temperature at which material leaves is a function of how strongly the 
material is bound to the microbalance. For example, lower vapor pressure 
materials will evaporate at a higher temperature. The thermal analysis 
spectrum may also be a useful way to distinguish small amounts of 
outgassing material in the presence of large amounts of another material 
that evaporates at a different temperature. It is noted that this method 
of generating a thermal analysis spectrum is effective with other types of 
real-time condensed material analysis. Examples include mass spectrometers 
used in temperature-programmed desorption (TPD) measurements and 
microbalances used in thermal gravimetric analysis (TGA) measurements. 
Also, the temperature of the object and chamber can be increased gradually 
during the outgassing process. This results in different species of 
outgassed material to be outgassed at different times. Therefore, the 
different species can be distinguished. This method is useful for 
detecting the presence of a small amount of one outgassed species in the 
presence of another. 
It is noted that the time response of the apparatus is affected by the 
volume enclosed by the chamber 22. In order to have a fast time response, 
the chamber should be constructed to fit the object 20, with a minimum of 
excess volume. 
FIG. 1B shows a closeup view of the snout 30 and snout tip 31. The snout 
has a wall thickness 60 and a length 62. If the snout 30 is in thermal 
contact with the heatsink 34, the temperature of the microbalance 32 may 
rise to the point where the outgassed material no longer condenses on the 
microbalance 32. In addition, the snout 30 itself may be cooled to a point 
where outgassed material is condensed on the snout surfaces. Both effects 
result in a reduction of detected material and a corresponding loss of 
sensitivity of the apparatus. For these reasons, the snout 30 and snout 
tip 31 should be made of material with a low thermal conductivity. Use of 
a thermally insulating snout tip 31 also results in a steep thermal 
gradient close to the microbalance 32. Preferably, the thermal conductance 
of the snout tip 31 is less than about 0.01 watt/Kelvin. This low 
conductance of the snout tip 31 can be accomplished by using a thin wall 
thickness to impede heat flow. The thickness 61 of the snout tip 31 is 
less than about 0.01 inches, and is more preferably less than about 0.002 
inches. Possible materials for the snout 30 and snout tip 31 include 
quartz and stainless steel. The length 62 of the snout 30 should be as 
short as possible while still allowing a large temperature difference 
between the chamber and microbalance 32. 
Alternatively, the tip portion 31 and snout 30 are the same thickness. In 
this case, the snout 30 and tip 31 should both be made of a thermally 
insulating material. 
The present invention includes the possibility of sealing the chamber 
airtight against the ambient atmosphere (i.e. forming an airtight seal 
between the heatsink 34 and snout 30). Such an embodiment is shown in FIG. 
2. Here, the snout 30 is made of a material with a low thermal 
conductivity such that the heatsink 34 is not excessively heated by the 
snout 30. Alternatively, only the snout tip 31 is made of a thermally 
insulating material (e.g., quartz). The pressure of the gas 26 inside the 
chamber can be greater or less than ambient pressure. Preferably, the 
pressure of the gas 26 inside the chamber is sufficient to enable the fan 
28 to circulate the gas 26. The gas pressure should be at least about 1 
Torr (and is preferably greater) for the fan to be useful in this regard. 
FIG. 3 shows an embodiment in which the shout 30 and heatsink 34 are sealed 
airtight and the chamber contains a vacuum provided by a vacuum pump 46. 
Outgassed material from the object 20 typically follows straight-line 
paths 48 (i.e. molecular flow), which generally intersect the inside 
chamber surface 35. The outgassed material incident upon surfaces 35 is 
re-outgassed from the surfaces 35 (which are hot) and is eventually 
incident upon the microbalance 32, where the outgassed material condenses. 
It is noted that operating the apparatus with a vacuum quickens the 
response time because the outgassed material takes less time to reach the 
detector. The response time quickens as pressure is reduced until the mean 
free path of particles in the chamber becomes comparable to the size of 
the chamber. An apparatus with a small chamber volume will have a faster 
minimum attainable response time (all other factors being equal). 
It is also noted that, for maximum sensitivity, the vacuum pump 46 should 
not be operated while a measurement is being performed. Preferably, the 
chamber is pumped out, the vacuum pump is sealed off from the chamber 22, 
and then the measurement is performed by heating the object. Most 
preferably in this embodiment, the chamber volume is small and the pump 46 
is fast so that the chamber can be rapidly evacuated, resulting in a 
minimum of outgassed material being removed by the pump 46. This is of 
particular concern when the chamber is maintained at a high temperature, 
because objects will be heated immediately after being placed in the 
chamber. 
Preferably, the entire apparatus is placed inside a vacuum, so that the 
chamber contains a vacuum without being sealed vacuum tight. This is 
preferable because it allows for a small gap between the snout 30 and 
heatsink 34 so that they are better thermally insulated from one another. 
Also shown in FIG. 3 is the option of actively heating substantially the 
entire chamber 22 with heaters 24. The chamber 22 may be made of a 
thermally conductive material such as copper so that the entire chamber 
surface 35 is hot. If copper is used for the chamber, the inside surface 
35 should be plated with an inert material such as gold to prevent 
oxidation. In the embodiment where the chamber contains a vacuum, it is 
preferred to heat the entire chamber so that the outgassed material does 
not recondense on the chamber surfaces 35. 
FIG. 4A shows an embodiment in which the chamber 22 is filled with gas 26 
and circulation of the gas 26 is provided by thermal convection 54. 
Thermal convection is assured because the apparatus is oriented so that 
the microbalance and cool heatsink 34 are above the hot object 20 and 
heaters. Due to the convection 54, no fan 28 is required for a relatively 
fast time response to be achieved. The convection efficiently cycles the 
gas 26 containing outgassed material past the microbalance 32, where it 
condenses. FIG. 4B shows another embodiment that relies on convection for 
transport. Here, the microbalance 32 and heatsink 34 are disposed to the 
side of the heater 24 and hot object 20, causing the convection 54 to be 
established. 
FIG. 5 shows an alternative embodiment of the present invention in which 
the snout 30 is not used. Here, the heatsink is in contact with the 
heaters 24. The surface area of contact between the heaters 24 and 
heatsink 34 should be minimized. Also, a layer of insulating material 70 
can be disposed between the heatsink 34 and the heaters 24. However, if no 
snout 30 is used, it is preferable to have an air gap in place of the 
insulating material 70. An air gap will provide good thermal isolation if 
the chamber is located above the heatsink. This is because no convetion 
currents will be formed by locating the hot chamber above the cold 
heatsink. It is preferable to use the snout in the present invention 
because it helps to maintain the temperature difference between the 
microbalance 32 and the rest of the apparatus. 
It is known in the art that the performance of condensed material detectors 
can be adversely affected by excessive buildup of condensed material. For 
example, microbalances behave nonlinearly when overloaded with condensed 
material and may even cease functioning altogether. Similarly, 
optical-based detectors such as ellipsometers or spectrophotometers may 
perform poorly if performing measurements on a surface with large amounts 
of condensed material. For these reasons, it is desirable in some 
circumstances to attenuate the amount of material condensed upon the 
condensed material detector. FIG. 6 shows an embodiment of the present 
invention providing this feature. A microbalance 32 is made substantially 
smaller than an exposed surface 58 of the heatsink 34. The exposed surface 
58 acts as an additional collecting surface. The exposed surface 58 will 
therefore compete with the microbalance 32 for outgassed material from the 
object 20. The exposed surface 58 and microbalance 32 together will 
capture all of the outgassed material from the object 20. The amount of 
material captured by each will depend upon the temperatures of the 
microbalance and heatsink 34 and the adsorption/absorption characteristics 
of the heatsink and microbalance. It is noted that the function of the 
exposed surface 58 an also be provided by any cooled surface (additional 
collecting surface) exposed to the interior of the chamber 22. 
If the microbalance and additional collecting surface (e.g. exposed surface 
58) have similar temperatures and adsorption/absorption characteristics, 
then the relative amounts of material captured by each may be proportional 
to their relative surface areas. In the case where the relative amounts 
are not proportional to the relative surface areas due to the particular 
transport properties of the chamber, the proportionality factor can be 
determined by a calibration process. In this way, overloading of the 
microbalance can be prevented while still allowing relatively accurate 
outgassing measurements. 
It is noted that the microbalance 32 is only a specific example of a 
condensed material detector. FIG. 7 shows an embodiment in which a 
substrate 72 such as a glass slide is in contact with the heatsink and a 
light beam 74 performs measurements on the outgassed material that 
condenses on the substrate 72. The snout 30 is made of transparent 
material. The optical measurement can include ultraviolet, visible, or 
infrared absorption or ellipsometry. Also, with different geometrical 
arrangements, light can be passed through the substrate 72 to perform 
measurements. Further, the substrate 72 can be removable from the 
apparatus. 
If the pressure in the chamber is lower than about 0.01 Torr, it is 
possible to use an electron or ion spectroscopy technique, such as Auger 
spectroscopy, UV or X Ray photoemission, or secondary ion mass 
spectrometry, to monitor the build up of material on the substrate 72 
(e.g. a glass slide). Light and charged particle spectroscopies have the 
advantage of being able to provide some identifying information about the 
material condensed, and to do so in real-time. To avoid the problem of 
large amounts of condensed material buildup, which might render some 
techniques inoperable (e.g. oscillators by quenching the oscillation, and 
optical detectors by absorbing all the light) the outgassed material can 
be condensed over a relatively wide area. This option is described with 
reference to FIG. 6. 
It is not necessary to use a condensed material detector that responds in 
real time (e.g. microbalance 32) to the collection of outgassed material. 
Alternatively, the substrate 72 is removed from the apparatus after a 
measurement, and the condensed material is extracted for some other type 
of analysis, e.g. liquid or gas chromatography. These approaches could 
also be combined. For example, the microbalance 32 or substrate 72 that 
has monitored the mass collected during the measurement is removed after 
the measurement, and the chemical identity of the condensed materials on 
the microbalance subsequently determined by chromatography or 
spectroscopy. 
It is noted that the condensed material detector (e.g. the microbalance 32 
or substrate 72) can have a coating of a material that has a high affinity 
for outgassed materials to be detected. In this way, the detector can be 
made to be less dependent upon being properly cooled. In cases where 
optical, or electron, or ion spectroscopy is used, the coating can be 
applied to the substrate 72 at which the optical, or electron, or ion 
spectroscopy apparatus is directed. 
FIG. 8 shows another alternative embodiment of the present invention in 
which a large temperature difference between the microbalance 32 and 
object 20 is not necessary. In this embodiment, the snout 30 is not 
needed. The object 20 must be heated to a temperature that is sufficient 
to cause outgassing, but the microbalance does not necessarily need to be 
cooled. The object and the detector are at nearly the same temperature. 
The surface of the microbalance 32 has a coating 76 to which the outgassed 
material has a high affinity. The affinity can take the form of a physical 
or chemical interaction, and it may occur at the top most molecular layer 
of the detector (surface adsorption), by diffusion in into the bulk of the 
detector (absorption), or by collecting in the pores of a porous solid. 
Also, an acoustic wave device or substrate 72 may have a coating to 
improve the collection properties. In the case of a microbalance with a 
surface coating, the collected material changes the effective mass of the 
oscillator, changing its frequency. It may also change the Q (quality 
factor) of the oscillator by causing damping. 
The coating 76 must have an affinity for outgassed material that is greater 
than the affinity of the object 20 for the outgassed material. For the 
purposes of this instrument, by affinity we mean the tendency of the 
material to bind or be adsorbed to the object or coating. This depends on 
the mathematical product of the outgassing material's surface (for 
adsorption) or volume (for absorption) with the equilibrium constant for 
adsorption or absorption from the gas phase. For this instrument to work, 
the affinities of the coating and object should be such that at 
equilibrium, the amount of material adsorbed or absorbed on the coating is 
substantially greater (preferably &gt;5 times more) than the amount of 
material adsorbed or absorbed on the object. The amount of material 
absorbed/adsorbed on the object and coating is dependent upon the relative 
surface areas in addition to the relative affinities of the object and 
coating. 
Also, the affinity of the coating 76 for the outgassed material should be 
greater than the affinity of the chamber surface 35 for the outgassed 
material. The entire apparatus (including the microbalance 32) can then be 
held at substantially the same temperature and the coating will absorb 
most of the outgassed material. However, it is generally preferable not to 
heat the microbalance as heat tends to liberate the outgassed material 
from the coating 76. As in the above embodiments, the chamber 22 can 
contain a gas or vacuum. The fan 28 is preferred but not necessary. Since 
a strong temperature gradient is not used, it is generally not possible to 
cause convection currents as shown in FIGS. 4A and 4B. 
The coating 76 must have an affinity for outgassed material that, is 
greater than the affinity of the object 20 for the outgassed material. By 
the term "affinity", we mean adsorption and/or absorption energy. A 
coating with a strong affinity will have a high binding energy with the 
outgassed material. The affinity of the coating 76 for the outgassed 
material should be greater than the affinity of the chamber surface 35 for 
the outgassed material. The entire apparatus (including the microbalance 
32) can then be held at substantially the same temperature and the coating 
will absorb most of the outgassed material. However, it is generally 
preferable not to heat the microbalance as heat tends to liberate the 
outgassed material from the coating 76. As in the above embodiments, the 
chamber 22 can contain a gas or vacuum. The fan 28 is preferred but not 
necessary. Since a strong temperature gradient is not used, it is 
generally not possible to cause convection currents as shown in FIGS. 4A 
and 4B. 
Many different materials can be used for high-affinity coatings [1]. 
Specific examples of coatings include various polymers, [2] self-assembled 
monolayers, [3,4] organometalic materials, [5] and cavitands, [6]. 
Coatings can be selected systematically based on a knowledge of the 
chemical and physical properties of the target compound. [1] Reference can 
be made to the following publications concerning the selection and design 
of high-affinity coatings: 
1. J. W. Grate and M. H. Abraham, "Solubility interactions and the design 
of chemically selective sorbent coatings for chemical sensors and arrays" 
Sensors and Actuators B, 3 (1991) 85-111. 
2. K. D. Schierbaum, A. Gerlach, M. Haug, and W. Gopel, "Selective 
detection of organic molecules with polymers and supramolecular compounds; 
application of capacitance, quartz microbalance and calorimetric 
transducers" Sensors and Actuators A, 31 (1992) 130-137. 
3. K. Matsuura, Y. Ebara, and Y. Okahata, "Guest selective adsorption from 
gas phase onto a functional self-assembled monolayer immobilized on a 
super-sensitive quartz crystal microbalance" Thin Solid Films, 273 (1996) 
61-65. 
4. X. C. Zhou, L. Zhong, S. F. Y. Li, S. C. Ng, H. S. O. Chan, "Organic 
vapor sensors based on quartz crystal microbalance coated with 
self-assembled monolayers" Sensors and Actuators B, 42 (1997) 59-65. 
5. X. A. Battenberg, V. F. Breidt, and H. Vahrenkamp, "Synthesis and test 
of organometallic materials as sensitive layers on quartz microbalance 
devices" Sensors and Actuators B, 30 (1996) 29-34. 
6. P. Nelli, E. Dalcanale, G. Faglia, G. Sberveglieri, and P. Soncini, 
"Cavitands as selective materials for QMB sensors for nitrobenzene and 
other aromatic vapors" Sensors and Actuators B, 13-14 (1993) 302-304. 
The present invention is useful in any application where it is desired to 
measure the amount of material outgassed from an object. For example, the 
present invention can be used to estimate the surface area of an object, 
where the surface area is proportional to an amount of adsorbed material 
(which can be caused to outgas). Also, the present invention can be used 
to measure the amount of volatile material deposited on an object. The 
amount of lubricant coating a screw, for example, can be measured using 
the present invention. Also, the amount of material outgassed by parts 
(e.g. plastic, metal, glass, or composite parts) can be measured. This is 
particularly useful in selecting materials and components for 
contamination-sensitive applications, such as parts for use in data 
storage hard drives, hermetically sealed contamination-sensitive devices, 
or in satellites. In such applications, it is important to have accurate 
information about the outgassing characteristics of component parts. 
It will be clear to one skilled in the art that the above embodiment may be 
altered in many ways without departing from the scope of the invention. 
Accordingly, the scope of the invention should be determined by the 
following claims and their legal equivalents.