Spectroscopic cell design

The spectrographic sample cell for the analysis of trace impurities in sample gases. The cell is provided with a gas inlet, a gas outlet and a volume for confining the sample gases for analysis. Two mirrors are provided, a first mirror proximate the gas inlet the second mirror proximate the gas outlet, the first mirror having a hole at its radial center. A conically-shaped cell profile is further provided such that the formation of vortices of sample gas is prevented in the cell upstream of or in the region where the analysis of trace impurities is to take place.

TECHNICAL FIELD OF INVENTION 
The present invention deals with a spectroscopic sample cell for the 
analysis of trace impurities in sample gases. The present cell design is 
particularly applicable for the diode laser absorption spectroscopy 
detection of trace levels of water and other impurities commonly found in 
high purity gases. 
BACKGROUND OF THE INVENTION 
Spectroscopy is commonly used as a tool for analyzing impurities in gases. 
Significant problems persist, however, when the impurity of interest 
interacts with the surface of the sampling system and of the cell which is 
generally used to contain the sample during analysis. An impurity which 
absorbs on or reacts with the surface may appear lower in concentration 
than it really is. Alternatively, if the system has previously been 
exposed to the same impurity so that the impurity is reversibly bound to 
sampling cell surfaces, then the system may contribute impurity to the 
gases being analyzed so that the same may appear higher in concentration 
than that actually contained in the sample under investigation. 
The detection of trace amounts of water in ultra high purity gases is 
particularly difficult. Background signals appear through moisture release 
from surfaces of the system. Moisture also tends to react with other 
components, and to adhere to surfaces. System considerations include 
spurious effects caused by cell wall absorption-desorption, atmospheric 
interference. 
Many commercial applications require specialty gas such as semiconductor 
nitrogen gas to have certified water levels below 10 ppb. Small of amounts 
of impurities in such process gases have been shown to drastically effect 
both the yield of final products, especially when used in severe 
conditions including high temperature operations. 
Accurate measurement techniques have been somewhat enhanced by taking 
precautions considered standard in trace moisture analysis, such as the 
use of high quality electropolished stainless steel for constructing the 
gas handling system while avoiding materials known to contribute large 
amounts of water vapor, such as certain polymers. In addition, the sample 
cell can be heated to remove absorbed water and by maintaining the cell at 
elevated temperatures, the speed of absorption/desorption processes can be 
increased and equilibrium reached more rapidly and with less sample gas 
being consumed. This procedure is somewhat inconvenient, however, and has 
never been demonstrated to be sufficient to reach trace moisture levels in 
the realm of 10 ppb. 
As noted in an article entitled Multipass Absorption Cell Design For High 
Temperature UHV Operation, R. D. Shaffer, et al., Applied Optics Vol. 28, 
No. 9, pp. 1710-13 (May 1, 1989), tunable diode lasers have been employed 
in a heated multipass absorption cells to give a noise equivalent 
concentration below 10 ppb. However, in this work, calibration was 
effected using ppm level moisture standards and a reduced pressure 
(10.sup.-3 torr) in the cell, whereas actual measurements are made at 10 
torr in the cell. No data are presented on background moisture levels at 
this pressure. Obtaining adequately low and reproducible moisture levels 
under measurement conditions may be considered the principal difficulty in 
most problems of moisture measurement. The noise equivalent concentration 
quoted by Schaeffer et al. is characteristic primarily of the performance 
of their optical system (and, indeed, indicates a very good performance) 
and does not characterize fully the performance of the cell itself. No 
effort was made by Schaeffer et al. to optimize the flow pattern in their 
cell. In such a design, multimirrors are employed within the spectroscopic 
sample cell to effectively establish the sample cell volume within the 
system. Sample gases are passed within the cell while the analytical light 
beam is reflected multiple times to define the probe volume. 
Spectroscopic techniques are frequently considered suitable for the 
analysis of reactive or corrosive gases because they rely on the 
interaction of a light beam, rather than any other physical probe, with 
the sample gas. This eliminates the possibility that an aggressive matrix 
gas will attack the sensor, producing erroneous readings or, at a minimum, 
reducing its lifetime. Spectroscopic techniques are also able to 
distinguish various impurities from one another on the basis of the 
wavelength of light at which interaction occurs with the impurity, so that 
no external separation means, (such as a chromatographic column) which 
might also be attacked by the matrix gas, is required. 
Notwithstanding the above discussion, it is necessary, when using 
spectroscopic analytical techniques, to have a gas cell which can contain 
the sample gas and which is equipped with windows capable of transmitting 
the light being used. As noted above, for maximum analytical sensitivity, 
it is frequently also necessary that the cell be equipped with mirrors so 
that the light may pass through the sample cell more than once. In 
general, it is necessary to place these mirrors inside the cell where they 
are exposed to the matrix gas. If the sample gas reacts with the cell or 
its components, a reduction in performance can result should the windows 
or mirrors be obscured by deposits or the reaction products vapors into 
the gas phase. In particular, if reaction with the matrix gas leads to 
particulate deposits or to an increase in surface area it can be expected 
that the cell will exhibit an increased tendency to adsorb water vapor and 
subsequently release it, giving rise to considerable interference effects. 
A solution to the above problems is generally approached by choosing cell 
and window materials which are relatively immune to the sample gas in 
question. For example, a corrosion resistant alloy such as Hastelloy is a 
good choice for a cell intended for HC1 analysis. However, such alloys are 
expensive and the windows, mirrors and/or sealing and mounting materials 
used to place them in the cell may still prove vulnerable to attack. In 
addition, this approach does not specifically address interference and 
background difficulties which may arise as a result of outgassing, 
desorption or other release of moisture or other volatile materials from 
the cell surfaces. Although, as previously noted, heated cells have been 
employed to reduce adsorption/desorption problems, this approach is not 
always sufficient and may accelerate destructive interactions between the 
sample gas and the cell. 
It is generally recognized that a cell with inlet and outlet at opposite 
ends is purged of impurities more efficiently than one in which both 
connections are at the same end, but this simple consideration is not 
sufficient to effectively purge all the important regions of the sample 
cell. 
It has been determined that accurate trace impurity measurement is 
exceedingly difficult in light of the interactions as noted above between 
the gas and walls of the spectrographic cell. Problems resulting from such 
interactions are particularly acute when stable vortices occur in the flow 
field. The sole mechanism for purging these vortices is diffusion, a 
characteristically slow process. As such, if no stable vortices were to 
occur in the flow field, the entire cell would be continuously purged by 
the same gas minimizing cell wall interaction and greatly enhancing the 
opportunity for accurate trace impurity measurement. 
It is thus an object of the present invention to provide a spectroscopic 
sample cell design which greatly enhances the opportunity for accurate 
trace impurity measurement.

SUMMARY OF THE INVENTION 
The present invention is directed to a spectroscopic sample cell for the 
analysis of trace impurities in sample gases. The cell is provided with a 
gas inlet, gas outlet and a volume for confining said sample gases for 
analysis. The spectroscopic sample cell is further provided with two 
mirrors, a first mirror proximate the gas inlet and a second mirror 
proximate the gas outlet. The first mirror is provided with a hole at its 
radial center. 
The spectroscopic sample cell is further characterized as having a 
conically-shaped cell profile substantially at the gas inlet having a 
diverging angle of less than approximately 30.degree.. The location and 
sizes of the mirrors and conically-shaped cell profile are such as to 
substantially prevent the formation of vortices in the sample cell up 
stream of or in the region where the analysis of trace impurities is to 
take place. 
DETAILED DESCRIPTION OF THE INVENTION 
As previously noted, it is critical that the gas sample - surface 
interactions in the spectroscopic analysis cell be minimized. This can be 
done by establishing a geometrical profile of the cell such that gas which 
flows through the cell being probed by analytical light beams, experiences 
no vortices upstream of the probe volume or within it. Without being bound 
by any particular theory of operation, it is hypothesized that if a vortex 
exist in the gas cell, the gas in that region will be relatively stagnant 
and will accumulate impurities released from the cell walls. Diffusion 
processes for purging the vortex will increase the response time of the 
cell. The presence of such a vortex may be tolerable providing it is down 
stream of the probe volume and therefore has a minimal influence on the 
analysis. 
Turning to FIG. 1, cell 10 is provided with confining side walls 11 
defining a spectroscopic sample cell having a substantially circular 
cross-section. Cell 10 is provided with a gas inlet 12 and outlet 13 at 
opposite ends of the cell. The invention is to a multipass cell employing 
a first mirror 18 proximate to gas inlet 12 and a second mirror 20 
proximate gas outlet 13. The mirrors reflect an appropriate light source 
which is caused to be reflected a multiple of times between said mirrors 
to define the probe volume. 
Unless the location and relative geometry of the various elements of the 
spectroscopic sample cell are carefully established, stable vortices can 
be created. The location and relative geometry need to be placed based 
upon the desired flowrate to be used during moisture analysis. In this 
regard, reference is made to FIG. 2 whereby spectroscopic cell 30 is shown 
whereby mirrors 31 and 32 have been somewhat haphazardly placed within the 
spectroscopic cell. As a result of boundary layer separation experienced 
by the sample gas through introduction at inlet 34, a stable vortex is 
shown by fluid dynamic simulation at 33. This obviously provides an 
unacceptable cell design. The design in FIG. 2 is by no means the worst 
which can be imagined with a similar configuration. Small variations in 
the hole size in mirror 31 and/or in its size can lead to formation of 
vortices between the mirrors, where their effect will be maximized. 
In light of the above discussions, simply providing a conically-shaped 
inlet and pair of mirrors located within the sampling cell alone is 
insufficient to provide the desired cell performance. As such, this basic 
configuration was subjected to numerical simulation of the fluid dynamics 
in the cell. In referring to FIG. 1, it was determined that the divergent 
angle .theta. be less than 40.degree. and preferably approximately 
10.degree. half angle, 20.degree. total, forming a nozzle 14 by diverging 
sidewalls 15 and 16. Divergence continues so that the nozzle expands to 
approximately 50% of cell diameter at which point the conically-shaped 
inlet abuts to the cell's full diameter at edge 17. 
First mirror 18, having a circular cross-section, is provided with a hole 
its radial center. In complying with appropriate design criteria, hole 19 
should have a diameter c less than inlet diameter a. Further, the diameter 
d of mirror 18 should be greater than the diameter j at the widest point 
of conically-shaped inlet 14. Finally, diameter d of first mirror 18 
should be such that free cell cross-sectional area at the plane at which 
first mirror 18 is located and which is unoccupied by said first mirror is 
greater than the cross-sectional area a at gas inlet 12. In other words 
a.sup.2 &lt; (f.sup.2 -d.sup.2). 
It is a design goal of the present spectroscopic sample cell that the first 
mirror is positioned and sized such that substantially all of the gas 
passing through the cell proceeds around the first mirror without 
separating into multiple stream paths. It is also intended that hole 19 in 
mirror 18 be sized to allow the passage of sufficient gas to pass there 
through in order to purge an area downstream of said first mirror as gas 
passes within cell 10 in the direction of arrow 21. These design criteria 
can be optimally met by providing first mirror 18 as being approximately 
4/7ths of the overall cell diameter f and providing second mirror 20 
having diameter e which is, in turn, approximately 5/7th of the cell 
diameter f. Appropriate purging behind first mirror 18 can be carried out 
without vortex formation by ideally sizing diameter c of hole 19 to be 
approximately 1/28th of the overall cell diameter f. In addition, it has 
been found that optimum results were achievable when first mirror 18 is 
spaced from the end of nozzle 14 at a distance h which is equal to 
approximately 1/4 cell diameter f. A preferred cell design embodiment is 
shown in FIG. 4 whereby cell 50 is provided with confining sidewalls 11 
defining a spectroscopic sample cell having a substantially circular 
cross-section. Cell 50 is provided with a collimated beam of light. The 
beam enters the cell through input window 59 and, after reflection from 
input mirror 54, is passed into the main body of cell 50 through aperture 
53 in first multi-pass mirror 52. The beam makes many passes through the 
cell volume, following a path determined by the curvature and separation 
of mirror 52 and 57 which are placed according to the design of Herriot. 
In this regard, the gas sample is introduced to cell 50 through inlet 55 
and through conically-shaped inlet 56. Subsequent to analysis, the gas is 
caused to pass from spectroscopic sample cell 50 through outlet 58 which 
is, in turn, functionally connected to appropriate pumping equipment (not 
shown). 
After the appropriate light beam makes its various passes through the cell 
volume, it exits the cell through the same aperture and window 59 and 
strikes a detector outside the cell (not shown). A variety of modulation 
and signal averaging techniques can be applied to the source and detection 
system to improve its sensitivity and robustness with respect to outside 
source interference. 
When appropriate design criteria are followed, the fluid dynamic simulation 
as shown in FIG. 3 presents itself noting the lack of any stable vortices 
as a result of gas being introduced at inlet 41 within cell 40. 
FIG. 5 illustrates the performance of the spectroscopic cell of FIG. 4 when 
used to detect a sudden change in moisture level. The data used to 
generate the graph presented as FIG. 5 were obtained using a nitrogen 
carrier gas. However, the same performance may be expected in more 
aggressive matrices. The time to reach the background level from 750 ppb 
is approximately 2 minutes which should be considered short compared to 
other spectroscopic methods and even compared with fast, non-spectroscopic 
methods. The background level demonstrated in FIG. 5 is approximately 40 
ppb but this is not considered the ultimate limit of the present approach 
which is also limited by other features particular to this embodiment 
which are capable of independent improvement. For example, the ambient 
moisture level of air outside the cell could be adjusted.