Analytical system for analyzing CO.sub.2 content of a fluid

A total carbon dioxide analyzer system has a reaction chamber that includes a tube and a piston mounted for sliding movement in the tube to change the volume of the chamber. A valve coupled to the chamber has a sample inlet port and a reagent inlet port, and is movable between a first state in which the sample inlet port is connected to the chamber, a second state in which the reagent inlet port is connected to the chamber, and a third state in which the reaction chamber is sealed. A system controller coordinately operates the valve and moves the piston to increase the volume of said chamber to draw sample into the chamber for mixing with an acid reagent in the chamber, and then seals the chamber and increases the volume of the chamber to facilitate the release of carbon dioxide. A transducer then senses the pressure of carbon dioxide in the reaction chamber.

This invention relates to analytical systems and processes and more 
particularly to systems and processes for the measurement of carbon 
dioxide in fluids. 
Carbon dioxide (in combined form and in dissolved form) is present in a 
wide range of fluids including natural waters, precious fluids such as 
plasma and serum, and excretory fluids such as urine. Carbon dioxide, 
being one of the main products of cell metabolism and constantly being 
formed in the cell organisms, is always available in the environment and 
inside cells. The bicarbonate buffer system is a main buffer system of 
cells and of plasma and of body fluids of living organisms. As such, the 
carbon dioxide content of such fluids will frequently provide useful 
diagnostic or analytical information. 
In serum and plasma, for example, the total carbon dioxide content includes 
bicarbonate, carbonic acid, and dissolved and protein-bound carbon 
dioxide, and may be expressed: 
EQU TCO.sub.2 =HCO.sub.3.sup.- +H.sub.2 CO.sub.3 +CO.sub.2 
Total carbon dioxide (TCO.sub.2), when considered with other sources of 
acid-base information such as pH, may be differentiated, through 
calculation, into its HCO.sub.3.sup.- and H.sub.2 CO.sub.3 components. 
Commonly, TCO.sub.2 is used semiquantitatively with clinical signs or 
other electrolyte values, particularly potassium and chloride levels, to 
provide insight into the nature of metabolic disturbances, the 
effectiveness of therapeutic measures, and to assist in the control of 
life support devices. TCO.sub.2 in venous serum and plasma normally ranges 
from 23 to 30 mmol/l. 
A number of different techniques have been employed to measure carbon 
dioxide content of fluids. The VanSlyke method, for example, involves the 
manual manipulation of a mercury column--an awkward and time consuming 
process. Other techniques that have been employed, including 
electrochemical electrode systems of the PCO.sub.2 type, differential pH 
systems, and infrared photometry systems, have limitations such as 
frequent calibration requirements, membrane maintenance problems, 
requirements of large sample size, and slow throughput. 
In accordance with the invention there is provided an analytical system 
that includes a variable volume reaction chamber with a valved inlet. Both 
the volume of the chamber and the condition of the valve are adjusted in 
coordinated manner by a system controller, and when the valve is open, 
liquids are flowed into and out of the reaction chamber by change of 
chamber volume. In operation, a sample to be analyzed and an acid reactant 
(that interacts with the sample to produce carbon dioxide) are flowed into 
the chamber by increase in chamber volume, and the controller then seals 
the chamber by closing the valve. Carbon dioxide is then generated: 
EQU CO.sub.3.sup.= +HCO.sub.3.sup.- +nH.sup.+ .revreaction.H.sub.2 CO.sub.3 
.revreaction.CO.sub.2 +H.sub.2 O. 
The volume of the sealed chamber is increased (reducing the pressure) and 
the mixture is stirred to enhance the release of dissolved carbon dioxide 
into the gaseous phase, and then the quantity of carbon dioxide in the 
chamber is measured. While various carbon dioxide sensing techniques may 
be utilized, including infrared photometry and hot wire anemometer 
techniques, a pressure transducer type of sensor is used in preferred 
embodiments. Preferably, the pressure transducer is connected to the 
reaction chamber by the valve after the degassing interval and the chamber 
volume has been reduced to a predetermined value (so that the pressure is 
less than ambient pressure and within the measurement range of the 
transducer). In the particular embodiment of the transducer is of the 
piezoelectric type and measures a range of 0 to 60 mmCO.sub.2 /liter (a 
pressure range of about 6-11 psia). This measurement under reduced 
pressure prevents the appearance of physical water in the transducer 
chamber. It will be apparent that other pressure dependent measurement 
arrangements such as the measurement of carbon dioxide as a function of 
the volume of the reaction chamber at a predetermined pressure may be 
used. 
While the invention is useful in measuring carbon dioxide content of many 
different fluids, including industrial wastes and industrial process 
fluids, in a particular embodiment total carbon dioxide in serum and 
plasma is determined in a system that utilizes a sample volume of less 
than fifty microliters. The reaction chamber includes a cylinder and 
piston arrangement, in which axial displacement of the piston changes the 
volume of the reaction chamber. A compact minimum "deadspace" valve 
arrangement mounted on the chamber cylinder has separate inlets for the 
acid reagent and for the diluted sample. The valve includes a seal block 
of plastic material with lateral porting from a bore in which a movable 
valve member is press fitted. The valve member has a plurality of through 
passages, each of which has a volume of less than twenty microliters. A 
stirring mechanism in the chamber is driven by an external electromagnetic 
actuator for enhancing release of carbon dioxide from the liquid. While a 
variety of acid reagents may be utilized, in this embodiment the acid 
reagent is lactic acid of sufficient strength to cause the final reaction 
mixture (diluted sample and reagent) to have a pH of less than 3.0. The 
lactic acid used interacts with the bicarbonate in the sample to produce 
carbon dioxide but does not precipitate the protein constituents of the 
sample. The acid reagent is also used for cleaning and is flushed through 
the line through which the sample is introduced in system cleaning 
sequence. 
The invention provides accurate analysis of carbon dioxide content of 
fluids in an analysis cycle of short duration and in an easily automated 
arrangement.

DESCRIPTION OF TICULAR EMBODIMENT 
There is shown in FIGS. 1 and 2 apparatus for measuring the carbon dioxide 
content of a fluid sample such as blood serum or urine. The apparatus 
includes a frame member 10 on which a valve assembly 14 is mounted. That 
valve assembly includes a housing member 16, a seal member 18, and a shaft 
member 20. Pressure transducer 24 is threadedly received in port 22 and 
projects from the upper surface of housing 16. 
A variable volume reaction chamber 30 is disposed below valve assembly 14 
and includes a cylindrical glass tube 32, the upper end 34 of which is of 
reduced diameter and extends through aperture 36 and is seated in a recess 
38 in seal member 18. Disposed within tube 32 is a piston assembly 40 that 
includes a head portion 42 in sealing engagement with the inner surface 44 
of tube 32, a shaft portion 46 and a coupling portion 48. The lower end 50 
of tube 32 is seated on support member 52 which is threaded in support 
flange 54 and adjusted such that the upper end 34 of tube 32 is firmly 
seated against seal member 18. 
A controller mechanism connected to piston coupling 48 includes coupling 
member 60 that is mounted for vertical movement along guide rod 62 and 
that carries nut 64 in engagement with lead screw 66. Bearing assemblies 
68, 70 in frame flanges 72, 74 support lead screw 66 for rotation, and 
stepping motor 76 (mounted on flange 78) drives lead screw 66 through 
coupling 80. A piston position indicator assembly includes tab 82 mounted 
for movement with coupling member 60, and cooperating sensor assembly 84 
mounted on frame 10. 
The position of shaft 20 of valve assembly 14 is controlled by stepper 
motor 90 which is supported on bracket 92 and connected to shaft 20 by 
means of coupling 94. A valve shaft position indicator assembly includes 
disc 96 and sensor assembly 98. 
Further details of valve assembly 14 may be seen with reference to the 
exploded view of FIG. 3. Valve housing 16 is an aluminum member that 
defines a rectangular through passage 100 that is about two centimeters on 
each side and about three centimeters in length. Formed in the upper wall 
of housing 16 (to the rear of transducer port 22) is through bore 102 that 
receives sample inlet coupling tube 104. Formed in a side wall of housing 
16 (forward of bore 102) are a through bore 106 that receives reagent 
inlet coupling tube 108, and (forward of bore 106) a vent port 110. 
Aperture 36 in the bottom wall of housing 16 receives the upper end of 
reaction chamber tube 32. 
Seal member 18 (of tetrafluoroethylene--Teflon) is received within 
rectangular through passage 100 of valve housing 16. The rectangular 
dimensions of seal member 18 are slightly greater (about 0.05 millimeter) 
than the rectangular dimensions of passage 100. Seal member 18 has a 
cylindrical through passage 112 about 0.6 centimeter in diameter, and an 
array of five passages, each about 11/2 millimeter in diameter, that 
extend radially from through passage 112: transducer passage 114 and 
sample passage 116 each extend upwardly to the upper surface of seal 
member 18; reagent passage 118 extends radially upwardly from through 
passage 112 and then transversely to a side wall of seal member 18; vent 
passage 120 extends radially to the same side wall; and reaction chamber 
passage 122 extends downwardly from through passage 112 to a coaxial 
recess 38 in which the upper end 34 of reaction tube 32 is seated. 
Stainless steel shaft 20 is received within passage 112 of seal member 18 
and has a diameter about 0.1 millimeter greater than that of bore 112. 
Shaft 20 has two annular grooves 126, 128; and four passages, each about 
one millimeter in diameter, are drilled through the shaft. As indicated in 
FIG. 4, a first passage 130 extends between ports 132 and 134 that are 
located in the same radial plane 136 and spaced 90 degrees apart; a second 
passage 138 extends between port 134 and port 140 which is in a second 
radial plane 142 axially spaced from plane 136, a third passage 144 
extends from a port 146 (located 180 degrees from port 140 in radial plane 
142) to a port 148 in radial plane 150; and passage 152 extends from port 
154 (located in radial plane 142 and offset 90 degrees from ports 140 and 
146) to port 156 in radial plane 158. Each of passages 138, 144 and 152 
extends through shaft 20 from one side to the other, that is the two ports 
of each passage are on opposite sides of the shaft. When shaft 20 is 
positioned within seal block 18, radial plane 136 is aligned with 
transducer passage 114, radial plane 142 is aligned with chamber passage 
122, radial plane 150 is aligned with reagent passage 118, and radial 
plane 158 is aligned with sample passage 116. 
Mounting plate 160 has a bore 162 through which shaft 20 extends; a coaxial 
recess 164 in its front face that receives an assembly of washers 166, 168 
and snap ring 170 that is seated in annular groove 126; and a coaxial 
recess in its rear face which receives bushing 172. Snap ring 174 is 
seated in annular groove 128 and washer 176 spaces bushing 172 from snap 
ring 174. 
Further details of the valve assembly and associated components may be seen 
with reference to FIGS. 5 and 6. As there indicated bolts 178 secure valve 
housing 16 to mounting plate 160, and bolts 180 secure mounting plate 160 
to frame 10. 
As shown in FIG. 5, housed in reaction chamber 30 on piston head 42 is 
magnetic stir bar 182 that is rotated by the electromagnetic drive 
mechanism 184 that is supported on flange 54 as indicated in FIGS. 1 and 2 
when piston head 42 is in its bottom position 42". 
A diagrammatic representation of the analysis system is shown in FIG. 7. 
Valve 14 is mounted on top of the variable volume reaction chamber 30 and 
transducer 24 is mounted on top of valve 14. Connected to reagent inlet 
108 via line 186 is a supply 188 of reagent which, in this embodiment, is 
one M lactic acid. Connected to sample inlet 104 via line 190 and probe 
192 is a spin cup 194 that is driven in rotation (bidirectionally) by 
motor 196 and shaft 198. Spin cup is of conical shape and has at its upper 
end an annular lip 200 which overlies annular waste chamber 202. Motor 196 
is controlled by system controller 204 (which is a microprocessor in a 
preferred embodiment) to rotate spin cup 194 briefly in opposite 
directions to achieve thorough mixing of sample and diluent; and to rotate 
cup 194 at high speed to expel its contents over lip 200 into waste 
chamber 202. System controller 204 also controls valve stepper motor 90 
and piston stepper motor 76. 
Sample line 190 (including probe 192) has a volume of about 190 microliters 
and the volume of reaction chamber 30 is varied by axial movement of 
piston 40. Piston head 42 moves between an upper position 42' in which the 
chamber volume is about 300 microliters and a lower position 42" in which 
the chamber volume is about 4200 microliters and spin bar 182 is 
positioned within the electromagnetic drive 184. Axial movement of piston 
40 is controlled by stepper motor 76 which drives lead screw 66, and 
sensor assembly 82, 84 indicates an axial index position of piston head 
42. 
Valve 14 is controlled by stepper motor 90 with an angular index position 
being indicated by sensor assembly 96, 98, and is indexed between a 
"sample" position (FIG. 8A), a "seal" position (FIG. 8B), a "sensor" 
position (FIG. 8C), a "discharge" position (FIG. 8D) which is the same as 
the FIG. 8A sample position, and a "reagent" position (FIG. 8E). In the 
"reagent" position, shaft port 146 is aligned with seal passage 122 so 
that reagent passage 118 and chamber passage 122 in seal member 18 are 
connected by shaft passage 144. Rotation of the shaft 90 degrees to the 
FIG. 8A position also (shown in FIG. 5) aligns shaft port 154 with seal 
passage 122 so that chamber passage 122 and sample passage 116 are 
connected by shaft passage 152. In this valve position, (as indicated in 
FIGS. 5 and 6) shaft passage 130 connects transducer passage 114 with vent 
passage 120 so that the transducer 24 is exposed to ambient pressure for 
equilibration. Indexing of the shaft 45 degrees to the FIG. 8B position 
places the system in a " degassing" mode in which reaction chamber 30 is 
sealed. Indexing of the shaft 45 degrees further (to the FIG. 8C position) 
aligns shaft port 140 with chamber passage 122 so that shaft passage 138 
connects reaction chamber 30 and transducer 24 and the system is in a 
"measuring" mode. 
In system operation, operation of motors 76, 90 and 196 are coordinated by 
system controller 204 to provide sequential analysis cycles. With reagent 
in the reaction chamber, piston 40 in its upper position 42', and valve 14 
initially in the position shown in FIG. 8A, a sample of the material to be 
analyzed (serum or urine) and a buffered diluent (at a sample:diluent 
ratio of about 1:7) are placed in spin cup 194 and mixed by bidirectional 
rotation of cup 194 under the control of system controller 204. Controller 
204 then operates stepper motor 76 to rotate lead screw 66 and drive 
piston 40 downwardly to increase the volume of reaction chamber 30 about 
four hundred microliters. This operation first flows reagent from line 190 
into chamber 30 followed by about two hundred ten microliters of the 
diluted sample from sample cup 194. 
Controller 204 then rotates valve shaft 20 45 degrees to the "degassing" 
position indicated in FIG. 8B in which reaction chamber 30 is sealed. 
After chamber 30 is sealed, controller 204 operates stepper motor 76 to 
drive piston 40 downwardly to the position 42" in which stir bar 182 is 
positioned in the electromagnetic drive field of the stir bar drive 
mechanism 184 and the volume of the reaction chamber is about 4200 
microliters (a gas volume of about 3500 microliters). Stir bar 182 spins 
rapidly to mix the diluted sample and reagent and the bicarbonate reaction 
produces carbon dioxide from its combined forms which, together with 
dissolved CO.sub.2, is released in this reduced pressure environment. 
After a "degassing" interval of about seven seconds under reduced pressure 
in the reaction chamber 30, controller 204 operates stepper motor 76 to 
raise piston 42 to the position shown in FIG. 7 to provide a gas volume of 
about 150 microliters above the degassed liquid sample. Stepper motor 90 
is then operated to rotate the valve shaft 45 degrees to the position 
shown in FIG. 8C in which passage 138 connects reaction chamber 30 to 
transducer 24. Transducer 24 senses the gas pressure in the reaction 
chamber (the pressure of the released carbon dioxide) and the resulting 
data is recorded (and/or displayed). 
Controller 204 then operates stepper motor 90 to index valve shaft 90 
degrees in the opposite direction (to return valve shaft 20 to the 
position of FIGS. 8A and 8D) in which valve passage 152 connects reaction 
chamber to line 190; and then operates stepper motor 76 to drive piston 40 
to its uppermost positon 42' to expel liquid from reaction chamber 30. 
Valve shaft 20 is next rotated to the FIG. 8E position in which the 
reaction chamber is connected to reagent line 186 and motor 76 then drives 
plunger 40 down to draw about one milliliter of lactic acid reagent into 
chamber 30. Valve shaft 20 is then indexed 90 degrees to the FIG. 8A 
position in which the reaction chamber is connected to the line 190 and 
transducer 24 is connected to the line 190 and transducer 24 is connected 
via vent passage 120 to atmosphere for equilibration. Plunger 40 is moved 
upwardly to eject reagent from reaction chamber 30 through line 190 into 
spin cup 194, and the spin cup is rotated at high speed by motor 196 to 
discharge the contents of cup 194 into waste chamber 202. 
In this condition, the system has completed an analysis cycle and is ready 
for the next cycle with line 190 filled with reagent. The analysis cycle 
with this embodiment has a duration of about 1/2 minute, and the system 
measures total carbon dioxide content of serum specimens of about 30 
microliters volume to a repeatability of about two percent. 
While a particular embodiment of the invention has been show and described, 
various modifications will be apparent to those skilled in the art, and 
therefore it is not intended that the invention be limited to the 
disclosed embodiment or to details thereof and departures may be made 
therefrom within the spirit and scope of the invention.