Apparatus for analysis of organic material

A predetermined volume of a gaseous mixture of combustion products component is collected in a gas mixer up to a preselected pressure. A portion of the mixture is discharged successively through the tubular column and a sample chamber for a predetermined time period with the mixture being initially at the preselected pressure in the volume. Detection of delayed output of gas constituents from the column provides an analysis of constituents. The apparatus further comprises a furnace, a magazine for holding the plurality of test samples, a transfer device connected to the furnace for sequentially transferring a successive sample from the magazine to the furnace, and a furnace seal integral with the transfer device for temporal sealing of the furnace from ambient atmosphere after the sample is transferred to the furnace until subsequent transferral. A gas mixing system for homogenizing the gaseous mixture at a substantially constant temperature comprises a sealable vessel, an impeller rotatably mounted in the vessel for stirring the gases, wherein the impeller comprises a plurality of blades each formed of a material having a mass such that the total heat capacity of the blades and the thermal conductivity of the blade material are cooperative with the impeller rotation to maintain constant temperature during introduction of the gases.

This invention relates to apparatus and method for quantitatively analyzing 
gaseous mixtures, particularly mixtures containing the gaseous combustion 
products of organic samples. 
BACKGROUND OF THE INVENTION 
Systems are used analyzing organic samples by way of combusting a sample 
and analyzing the gaseous products. For example, U.S. Pat. No. 3,252,759 
(Simon) discloses a system in which gaseous combustion products are drawn 
into an enclosed reservoir that had been previously evacuated. The gas 
mixture is then released into a second evacuated vessel by way of a series 
of detectors that sequentially determine the amounts of such combustion 
product components as water and carbon dioxide in the mixture. Each 
detector comprises a pair of thermal conductivity measuring devices. The 
component being measured is removed from the gas mixture between the first 
and second devices in the pair, and the difference between the 
measurements, with calibration, provides the amount of the constituent. 
U.S. Pat. No. 3,698,869 (Condon) avoids alleged problems of the Simon 
system, with its vacuum requirements, by operating above atmospheric 
pressure. The combustion products are mixed with and forced under the 
pressure of an inert carrier gas into a reservoir. A pressure switch in a 
gas line leading out of the reservoir shuts a valve between the combustion 
train and the reservoir. The gas mixture from the reservoir is then passed 
into a "delay volume" in the form of a coiled tube. Time periods are 
successively allowed in both the reservoir and the "delay volume" to 
complete mixing of the gases. The mixture in the "delay volume" is then 
shut off from the reservoir and forced from the "delay volume" through a 
series of detectors at constant pressure by the carrier gas source and 
vented to atmosphere. The detectors are of the type disclosed in Simon and 
vented to atmosphere. Although the Condon system has proven to be quite 
practical and successful, there is still a substantial need for increased 
speed of operation, accuracy and simplification of operation. 
A modified technique is taught in "Frontal Gas Chromatography as an 
Analytical Tool" by Vlastimil Rezl and Jitka Uhdeova, American Laboratory, 
January 1976, Pages 13-26. A chromatographic column formed of a coiled 
tube containing gas adsorption material is substituted for the "delay 
volume" tube, and a single detector receives the flow output from the 
column. Gas components are successively adsorbed. The heights of the 
adsorption steps of the components displayed from the detector signal are 
used (again with calibration) to provide the quantitative analysis. Rezl 
et al. describe the technique in a system that involves constant pressure 
flow through the column, achieved with a dilution chamber containing an 
easily movable piston maintained under constant gas pressure from the back 
side. The Rezl system involves a substantial degree of complexity of gas 
lines and valving. 
Related concerns to improve operating efficiency exist with respect to 
introducing samples of solid material into a system for analysis. It is 
desirable to have an apparatus for loading the samples into the furnace 
with a minimum of time and dead volume of gas, and with a simple, 
efficient system for sealing from the ambient atmosphere. 
U.S. Pat. No. 4,055,259 (Sibrava) discloses a sample transport apparatus 
for conveying test samples horizontally from a source into a combustion 
chamber. Samples are initially contained in a motor driven rotary 
magazine. A sample is dropped from the magazine into an aperture in a 
motor-driven rotary transfer plate. The plate is rotated a half turn to 
drop the sample through a passage onto a sample transport member. The 
sample transport member rides horizontally into a conduit which extends to 
the furnace. The sample transport member is conveyed to the furnace by 
means of a motor-driven tape. A reversed procedure is used to withdraw and 
drop the solid remnants of combustion through a second rotary plate and a 
second passage out of the apparatus. Sealing of the various orifices is 
accomplished by means of O-rings on which the sample plates ride. 
Sample introduction into a vertically aligned furnace is disclosed in U.S. 
Pat. No. 4,525,328 (Bredeweg). A set of jaws is displaced horizontally to 
grab a sample, which is then moved over to the top of the furnace inlet 
and released by the jaws. 
The above-described transport devices are workable in varying degrees but 
suffer from the complexity of motors or interconnecting gears (or pulleys) 
and from the unreliability of sliding seals. 
Therefore, an object of the present invention is to provide an improved 
apparatus and method for analysis of gaseous mixtures. 
Another object is to provide a novel gas analysis apparatus having improved 
speed of operation, accuracy and simplification of operation. 
Yet another object is to provide an improved system for analysis of organic 
samples by analyzing gaseous combustion products. 
A further object is a novel system for rapid transfer of samples into a 
testing apparatus with reliable sealing against ambient atmosphere. 
SUMMARY OF THE INVENTION 
The foregoing and other objects of the present invention are achieved by 
apparatus for quantitatively analyzing a gaseous mixture for its 
components comprising volume means for collecting a predetermined volume 
of a gaseous mixture of a base component and at least one additional 
component, pressurized fluid means connected to force the gaseous mixture 
into the volume means up to a preselected pressure, a tubular column 
having an input end and an output end, the input end being in fluid flow 
relationship with the reservoir means, sample chamber means in fluid flow 
relationship with the output end of the tubular column, discharge means 
for discharging a portion of the mixture from the volume means 
successively through the tubular column and the sample chamber means for a 
predetermined time period with the mixture being initially at the 
preselected pressure in the volume means, extracting means in the tubular 
column for extracting a predetermined amount of the additional component 
from the mixture during the predetermined time period, and component 
detection means communicating with the sample chamber for detecting the 
amount of the additional component in the gaseous mixture in the sample 
chamber during the predetermined time period. 
The apparatus further comprises combusting means for sequentially 
combusting each of a plurality of rest samples to produce as combustion 
product at least one component of the gaseous mixture, the combusting 
means including a furnace, a magazine for holding the plurality of test 
samples, transfer means connected to the furnace for sequentially 
transferring a successive sample from the magazine to the furnace, furnace 
sealing means integral with the transfer means for temporal sealing of the 
furnace from ambient atmosphere after the sample is transferred to the 
furnace until subsequent transferral, and means for introducing oxygen 
into the furnace. The transfer means comprises a body with a drop section, 
a piston end, a body chamber therein extending horizontally from the drop 
section to the piston end, a body aperture extending from the body chamber 
upwardly through the body so as to be receptive of the sample, and a drop 
port in the drop section extending downwardly through the body from a 
point in the body chamber vertically and horizontally offset from the body 
aperture. A piston member is slidingly situated in the body chamber such 
as to have a first position and a second position. Guide means for guiding 
the sample coact with the piston member so as to be receptive of the 
sample from the body aperture when the piston member is at the first 
position and vertically aligned with the drop port when the piston is at 
the second position. Retaining means temporarily retain the sample in the 
guide means. Releasing means release the sample from the guide means 
through the drop port.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a block diagram of an apparatus for combusting a sample and 
quantitatively analyzing the gaseous mixture of combustion products for 
its components, according to the present invention. For accuracy and speed 
critical portions of the system are contained in an insulated oven (not 
shown) maintained at 82.degree. C. The apparatus includes a sample loader 
12 located above a combustion furnace 14 for loading a test sample into 
the furnace. The sample may be in the form of a pellet, a sample enclosed 
in tin foil or the like or other convenient form. For typical organic 
samples the furnace is maintained at about 950.degree. C. 
Oxygen is introduced into furnace 14 under pressure from an oxygen source 
16 through a line 33, an opened valve B and a gas line 32. Ignition occurs 
at the furnace temperature. Combustion products are passed over catalytic 
agents in the lower portion of the furnace and thence through a line 34 to 
a reduction oven 18 at 650.degree. C. containing copper to reduce oxides 
of nitrogen and remove all excess oxygen. At this stage the combustion 
products typically consist of water, carbon dioxide and nitrogen. 
With oxygen valve B closed, the gaseous combustion products are then 
partially mixed with and forced under the pressure of an inert carrier gas 
such as helium or argon, from a source 19 via line 37 and opened valve A 
and line 32, through line 35, valve D and line 36 into an enclosed sample 
vessel 20. Thorough mixing is effected and a preselected pressure of about 
one atmosphere above ambient is reached in the vessel. 
A pressure transducer 22 on sample vessel 20 detects when the preselected 
pressure is reached and generates an electrical signal which, through a 
controller 23 and electrical lines 50, 50' (shown dashed to distinguish 
from gas lines) closes carrier valve A and inlet valve D. After a short 
interval of further mixing of the carrier gas and the combustion products 
in the vessel, valve G is opened and the mixture is forced by its elevated 
pressure into and through a tubular column 24 via a line 38. Although 
shown foreshortened in FIG. 1 for clarity, the tubular column is 
conveniently a coiled length of tubing. Adsorbent material 26 in the 
column selectively determines the rate of passage of the individual 
combustion products in the gas mixture. 
The time variant gas composition then passes through a line 40 and into a 
sample chamber 28 containing a detector 30 that measures a physical 
property of the gas mixture that depends on its composition. Preferably 
the detector is a thermistor, measuring thermal conductivity. The mixture 
is subsequently vented through a line 42 to atmosphere. 
FIG. 2 shows the sequence of steps and the associated valve positions of 
the valves indicated in FIG. 1. During standby valve C feeding the inert 
carrier gas through a constrictor 31 and a gas line 44 is open and the 
helium is used to purge column 24 and sample vessel 20. Optionally valves 
D, E and F are also open for purging and the next step. This purging is 
maintained during all subsequent steps except analysis as described below. 
While the sample is loaded into sample loader 12 valve H is opened to vent 
the furnace 14 and oven 18. After loading, various valves are set for 
purging the entire system with helium, as indicated in FIG. 2 with 
continuing reference to FIG. 1. For example, vessel 20 receives helium 
from valve F via a line 46 and is vented via line 36 and valve E. Oxygen 
is then introduced into furnace 14 with valve B and vent valve H both 
open, while valves A and D are closed and the helium continues purging the 
rest of the system. 
For combustion the sample is dropped into the furnace where it is retained 
in a porous cup (not shown) of the type shown in aforementioned U.S. Pat. 
No. 4,525,328, oxygen valve B is shut off while venting furnace 14 and 
oven 18 through opened valves D and E, and purging of column 24 and sample 
chamber 28 continues through valve C. Combustion products are moved to 
sample vessel 20 with the carrier gas as valve A is opened and vent valve 
E closed. For the final step (analysis), all valves are closed except 
connecting valve G from vessel 20 to column 24. Purging is thus stopped 
and the pressure of the gases in vessel 20 forces the gases through column 
24 and sample chamber 28 at a highly reproducible rate during the analysis 
phase. 
FIG. 3 shows the type of signal output to be expected from the detector 
during the time period of the analysis step. Initially at phase "P" the 
signal reflects the property of the pure carrier gas. "Q" represents the 
mixture of the most mobile component (e.g. nitrogen) and carrier. "R" 
represents this mixture plus the next most mobile component (e.g. 
CO.sub.2). "S" represents the mixture of all the components which can 
traverse the column (e.g. carrier plus N.sub.2, CO.sub.2 and H.sub.2 O). 
Column 24 is formed as a coiled length of tubing of stainless steel. It 
should be between 10 and 100 cm in length, preferably between 30 and 90 
cm, for example 60 cm. The inside diameter is between 1 and 4 mm, 
preferably 1.5 and 2.5 mm, for example 2 mm. The adsorbent material 26 in 
the column is, for example, a porous polymer such as 80/100 mesh "Porapak" 
sold by Waters Associates. 
It is important that the total system volume for the mixture be minimized, 
to gain rapidity in making the analyses and maximize accuracy of the 
measurements. Thus, according to the present invention, the single tubular 
column 24 containing adsorbent 26 replaces a number of components used 
heretofore, namely a sample volume and two separate sample traps and 
multiple detector cells for removing and detecting individual gas 
components in sequence. Accuracy and speed are also obtained by the fact 
that baseline measurements are made during the analysis stage, replacing a 
separate two-minute baseline run that was necessary in the past. 
Advantages include the economy and gas circuit simplification afforded by 
a single detector cell over the multiple cell configuration of Condon and 
complexities of Rezl. The column is "regenerated" by purging with carrier 
gas whereas absorbing traps must be refilled periodically. Further 
improvements in speed and accuracy are effected through the use of certain 
other components according to the present invention, as described below. 
A rapidly operating sample loading device 12 that adds a minimum of volume 
to the system is shown in FIG. 4. An elongated body 52 has a piston end 
54, a body chamber generally comprising a central chamber 56 and a 
cylindrical cavity 58 extending generally horizontally from chamber 56 to 
piston end 54, a body aperture 60 extending from cylindrical cavity 58 
proximate chamber 56 upwardly through the body, and a drop port 62 
extending from chamber 56 downwardly through the body. Body aperture 60 is 
receptive of a sample 64 from above which is passed downwardly through 
port 134 in block 132. 
A piston member 66 is slidingly situated in cylindrical cavity 58 such as 
to have an inner position adjacent to chamber 56 and an outer position 
away from the chamber; FIG. 4 shows the piston at the outer position. A 
guide member 68 is attached to the inner end 70 of piston 66 such as to 
protrude into chamber 56. Guide member 68 has a vertical guide aperture 71 
therethrough that is alternatively aligned vertically with body aperture 
60 (as shown) to receive sample 64 when piston 66 is at the outer 
position, or with drop port 62 when the piston is at the inner position. 
A horizontally sliding sample-retaining door 72 is situated adjacently 
below guide member 68 and has a wall portion 74 for retaining sample 64, 
and a door aperture 76 adjacent to the wall portion extending vertically 
through the retaining door. As shown in FIG. 4A, the retaining door has a 
"T" shaped cross section and is slidingly supported in a track 78 in the 
underside of guide member 68. Wall portion 74 (FIG. 4) is located between 
door aperture 76 and piston 66. Retaining door 72 is positioned 
alternatively in a sample retaining position (shown in FIG. 4) or a sample 
drop position (not shown), each such position being with respect to guide 
member 68 such that wall portion 74 is aligned vertically with guide 
aperture 71 for the retaining position, and door aperture 76 is aligned 
vertically with guide aperture 71 for the drop position. 
A compressed spring 84 is positioned from a first hole 86 in piston 66 to a 
second hole 88 in door 72. Spring 84 presses mating surface 82 against a 
positioning pin 80 in a direction away from piston 66 such that retaining 
door 72 is normally maintained in the retaining position shown in FIG. 4 
by the spring. 
A door actuator 90 is used for sliding retaining door 72 to the drop 
position while piston 66 is at its forward position, thereby releasing 
sample 64 through door aperture 76 and thence through drop port 62. A 
retractable actuator rod 92 extends loosely through a bore 94 in body 52 
extending from chamber 56 in a direction generally parallel to and away 
from cylindrical cavity 58, rod 92 having an extended axis 96 which 
intersects retaining door 72. Thus rod 92 can be pushed against retaining 
door 72 to retract the door against spring 84 to the drop position and 
align door aperture 76 between guide aperture 71 and drop port 62, thus 
dropping sample 64 out of the loading device. 
Piston 66 and actuator rod 92 may each be moved by hand or a solenoid, 
motor or the like, but pneumatic linear actuators of the known or desired 
type are desirable for each of these functions. In the present embodiment, 
a piston actuator 98 is mounted to body 52 with a spacer bracket 100, and 
a rod actuator 102 is mounted to the body with a mounting collar 104. 
(Mounting screws are not shown.) 
Sample-receiving furnace 14 is sealed with an O-ring 103 under body 52 
below the drop port, affording a minimum of internal volume, by screws 105 
through a flange 107 on the furnace. 
Mounting collar 104 is coaxial with bore 94 and is sealed between body 52 
and actuator 102 with O-ring seals 106. An O-ring 108 seals an actuator 
flange 110 against rod 92 and allows the rod to slide therein, thus 
effectively providing sealing between the movable rod and the body. An 
annular cavity 112 is formed between collar 104 and rod 92. 
Means is provided for sealing the chamber with the piston when the piston 
is at its forward position. Inner end 70 of piston 66 has a peripheral 
annular surface 114 perpendicular to the common axis of the piston and 
cylindrical cavity 58. The cylindrical cavity is bounded at the chamber by 
a second annular surface 116 that is parallel to and facing first surface 
114 in alignment therewith. An O-ring seal 118 in an annular groove 120 in 
annular surface 114 (or in the second annular surface) effects a seal 
between chamber 56 and cavity 58 when piston 66 is at the forward 
position. 
A gas inlet orifice 122 is provided for introducing oxygen and, as 
required, helium (or argon) into the chamber, via gas line 32 (FIG. 1), 
annular cavity 112 and bore 94, when piston 66 is at the inner position at 
which time chamber 56 is fully sealed from the ambient atmosphere except 
for the drop port 62 entrance to furnace 14. 
A magazine loader 130 is mounted above body 52 for loading successive 
samples into body aperture 60. The magazine loader shown in top view in 
FIG. 5, as well as in FIG. 4, comprises a loading member 132 slidingly 
mounted on top of the body. The loading member has a vertical loading 
aperture 134 therethrough that is receptive of the next sample from above 
when loading member 132 is in a forward position (indicated by dashed line 
133). When member 132 is in the normal position loading aperture 134 is 
alignable with body aperture 60 to drop the next sample therethrough. 
A disk shaped magazine 136, a portion of which is also depicted in top view 
in FIG. 6, is mounted on an axle 137 and is positioned above loading 
member 132. The magazine has a plurality of equally spaced vertical 
magazine cavities 138 therethrough, the cavities being positioned 
arcuately near the rim 140 of the magazine disk. Each cavity (that has not 
yet supplied a sample) holds at least one of a plurality of samples 142. 
The magazine has successive loading positions and is movable to each of 
the successive loading positions. 
Continuing with reference to FIG. 4, a ring-shaped mounting plate 144, of 
outer diameter comparable to that of magazine 136 is fixed with respect to 
body 52 and is juxtaposed closely between sample magazine 136 and loading 
member 132 to retain the plurality of samples 142 in the respective 
magazine cavities 138. (Mountings for the plate and magazine, not shown, 
are of conventional design.) A plate opening 146 extends vertically 
through mounting plate 144 and is located with respect to the magazine to 
pass a sample through from an adjacent magazine cavity 138', the magazine 
being in a loading position. Plate opening 146 is oriented vertically 
above aperture 134 when loading member 132 is in its forward position 133. 
When member 132 is retracted, a sample received through opening 146 is 
dropped through aperture 60. 
Body 52 has an elongated pin opening 148 extending upwardly therethrough 
from cylindrical cavity 58. A pin 150 is threaded into piston 66 and 
extends radially upwards through pin opening 148. Loading member 132 on 
top of body 52 has a pin orifice 152 therein that allows member 132 to be 
engaged by pin 150. FIG. 5 shows a top view of the loading device without 
magazine or plate. A slot 154 in member 132 cooperates with a retaining 
screw 156 threaded into the top of body 52 to guide the loading member. 
Continuing with FIG. 4, a drive pawl 154 is attached to the top of the 
loading member 132 with screws 156 (also in FIG. 5) Ratchet teeth 158 are 
equally positioned arcuately under magazine 136 near rim 140 (FIG. 6) but 
radially in from plate 144, in actuating contact with the drive pawl. Upon 
actuation of piston 66 from the outer position to the inner position pin 
150, after a free motion, slides the loading plate such that drive pawl 
154 moves magazine 136 to a subsequent loading position to deposit a new 
sample through plate opening 146 and aperture 134 to rest on top of block 
52. When piston 66 is returned to its outer position pin 150 moves loading 
member 132 back so the sample is dropped through loading aperture 134 and 
body aperture 60. Thus the new sample is being loaded into the device 
while the previous sample 64 is being dropped into furnace 14. 
To accommodate many samples the magazine has a separation distance "S" 
between each successive magazine cavity 138 and also between each ratchet 
tooth 158 (or a multiple number thereof) that is less than the travel 
distance "T" of piston 66 between the outer position and the inner 
position. Pin 150 has a diameter "P". Pin orifice 152 is a slot with a 
dimension "O" in the direction parallel to cylindrical cavity 58 such that 
S+O=T+P. Upon actuation of the piston from the outer position to the inner 
position, the pin has some free motion in the pin opening so as to slide 
the loading plate a distance only equal to the separation distance "S". 
For example, the pin is 9.5 mm in diameter, the elongated dimension of the 
pin opening in the loading plate is 22 mm, the travel distance of the 
piston is 25 mm, and the separation between cavities in the magazine is 11 
mm and between ratchet teeth is 8 mm. 
In a further embodiment of the present invention a unique gas mixing vessel 
20, shown in FIG. 7, is used for homogenizing the gases at a substantially 
constant temperature while the gases are being introduced into the vessel 
at increasing pressure. In this embodiment a sealable vessel 160 is 
constructed of a casing 162 which includes a cover 164. A gas connection 
166 receives the gaseous combustion products from line 36 under the 
pressure of the carrier gas. While the pressure in vessel 160 is 
increasing up to a predetermined pressure, and for a short time 
thereafter, an impeller 168 mounted in the vessel stirs the gases. The 
impeller desirably has two (or more) blades 170 mounted with pins 171 on a 
shaft 172 that rotates in a ball bearing assembly 174. Each blade 
preferably has a twisted "S" shape such as to impart an axial component to 
gas motion in the vessel to efficiently and rapidly mix the gases. 
According to the present invention each blade 170 is formed of a material 
with a relatively high mass with a corresponding heat capacity and a 
thermal conductivity that are sufficiently high to effectively maintain 
constant temperature of the gas mixture in vessel 160 during 
pressurization. The purpose is to counter the tendency for the gases to 
increase in temperature during the inlet pressurization process. It is 
desirable to pressurize at a rate 1 atmosphere within 10 seconds, and it 
is highly desirable to maintain the temperature within 0.5.degree. C. 
during gas input so that the volume of gas can be sent to the column as 
soon as mixing is completed, without waiting for the temperature to 
settle. Thus the cycling time for the analyses is further shortened. 
The total heat capacity (i.e. specific heat times total mass) of the blades 
should be at least 30 times the total heat capacity of the gases in the 
vessel at the predetermined pressure. Thermal conductivity of the blade 
material should be at least 0.5 calories/sec .degree.C. cm relative to 1.0 
for pure copper. Copper is preferable, gold plated to minimize 
contamination. For example for a mixing vessel having a net volume (with 
impeller and associated components in place) of 300 cc, copper blades with 
a total mass of 35 gm is desirable. 
Continuing with FIG. 7, a PTFE plastic coated bar magnet 176 is mounted 
perpendicularly on shaft 172 inside vessel 160 proximate the lower end 
wall 178 of casing 162. A horseshoe magnet 180 with poles 182 aiming 
upwards is set on a vertical shaft 184 aligned with the impeller axis 186 
and is located below the vessel proximate end wall 178. A motor 188 drives 
shaft 184, and the outside magnet 180 is magnetically coupled to the 
inside magnet 170 for rotating same and thereby rotating blades 170 at 
high speed, probably between 1000 and 3000, e.g. about 1500 rpm. 
The gas connection components include valve D (and, as appropriate, valves 
E and G shown in FIG. 1) and tube section 36 connected between the valve 
and the vessel and, similarly, valve F and tube section 46 for a second 
gas connection 190. It is important that the tube sections 36, 46 
contribute only a very small volume to assure thorough mixing of the 
enclosed gases, e.g., less than 0.25% of the volume of vessel 160 
containing impeller 168, for example having a length of 30 cm each and an 
inside diameter of 1 mm. 
The top of the vessel casing comprises cover 164 held to a flange 192 with 
screws 194 and sealed against the flange with an O-ring seal 196. The 
cover includes the gas tube connections 166, 190. An opening 198 in cover 
164 provides for a differential pressure transducer 200 held over the 
opening with a yoke 202. The transducer may be of the conventional type, 
such as Model P612 sold by Kavlico Corp., Chatsworth, Calif. The yoke is 
attached to the cover with screws 203, and a thumb screw 204 presses the 
transducer against an elastomer face seal 206 on the cover. A buffer 
container 208 nominally at atmospheric pressure is connected via a tube 
210 to the transducer to provide a control pressure unperturbed by short 
term fluctuations in ambient pressure. A vent valve 212 allows resetting 
the buffer pressure. The close proximity of transducer 200 to opening 198 
in the vessel casing further minimizes extraneous volume for the mixing 
process. 
Transducer 200 is set to detect when the pressure from the input gases 
reaches a predetermined pressure, for example about one atmosphere above 
ambient and generally in the range of about half to 1.3 atmospheres. 
Referring also to FIG. 1, electrical signals through lines 50, 50' and 
controller 23 closes carrier gas inlet valves A and D when the vessel 
pressure reaches the predetermined pressure. 
While the invention has been described above in detail with reference to 
specific embodiments, various changes and modifications which fall within 
the spirit of the invention and scope of the appended claims will become 
apparent to those skilled in this art. The invention is therefore only 
intended to be limited by the appended claims or their equivalents.