Respiratory gas moisture separator system for mass spectrometer monitoring systems

Wet or saturated respiratory gas samples being monitored from one or more hospital patients are sufficiently dried for analysis by a medical mass spectrometer by a novel momentum separator in which the major part of the wet sample flows directly to an exhaust pump while a small sample for analysis is taken from the reverse angle tee connection in the momentum separator. This small sample containing water vapor, but without the heavier moisture droplets, is then heated to about 100.degree. C. to maintain the water in the vapor state prior to entering the spectrometer inlet leak.

BRIEF SUMMARY OF THE INVENTION 
In many modern large hospital facilities, the respiratory gases of 
patients, particularly those under intensive care, are continuously 
monitored and analyzed to provide advance warnings of possible respiratory 
difficulties. Various medical parameters, such as oxygen uptake, are 
conveniently and readily analyzed by the use of a medical mass 
spectrometer which continuously receives small samples of the combined 
gases, reduces the pressure through a molecular leak so that the combined 
gas molecules may be ionized by electron bombardment, and then subjects 
the ionized gas molecules to a magnetic field that segregates the various 
gas molecules according to their respective mass-to-charge ratios. Ion 
current collector cups are positioned to receive the ions from the 
particular gases of interest in the mixture and produce electrical 
currents proportional to the quantity of that gas admitted to the system. 
Intensive care patients are often confined to a separate hospital ward 
where their respiratory gas constituents may be continually monitored by a 
centrally located medical mass spectrometer which may, in the larger 
hospitals, be located as much as one hundred feet or more from a monitored 
patient. The patient may breathe through a canula or a mouthpiece 
connected to a flowmeter that gives an indication of the amount of 
respiratory gas flow inhaled and exhaled by the patient. Connected to the 
flowmeter is a small capillary tube, normally having an inside diameter of 
approximately 0.020", which may be five or six feet in length to connect 
to a wall-mounted connection. The connection is the termination of a fifty 
to one hundred foot length of tubing having a larger diameter with 
corresponding lower flow resistance that leads to an appropriate 
distributor valve and thence to the medical mass spectrometer. 
Normally, the respiratory gases exhaled by an intensive care patient have a 
very high moisture content. If these high moisture gases flow through the 
small relatively high resistance capillary tubing directly to the mass 
spectrometer, the warm moist gases will condense to form water droplets 
which, if permitted to enter into the very small spectrometer inlet leak, 
will cause the inlet and its associated tubing to be readily clogged, thus 
requiring repeated shutdown of the equipment to blow out condensed 
moisture. 
The object of this invention is to remove that moisture from the conduit 
prior to the time it enters the medical mass spectrometer inlet leak. 
Briefly described, the high moisture content of respiratory gases is 
reduced for medical mass spectrometer analysis by first rapidly reducing 
the pressure of the gas to hold the moisture in a vapor state and then 
directing the composite moist gas sample through a momentum separator in 
which the major part of the sample flows directly to an exhaust pump while 
a small sample for analysis is withdrawn through a reverse angle tee 
connection. This small gas sample, containing water vapor but with any 
heavier moisture droplets removed, is then passed through a heated section 
of the sample line maintained at about 100.degree. C. before entering the 
spectrometer inlet leak.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Illustrated in the drawing is a typical medical mass spectrometer 10 such 
as described in U.S. Pat. No. 3,824,390 to Magyar. Spectrometer 10 
includes an evacuated ionization chamber 12 through which an electron beam 
is passed to ionize gas molecules that are admitted to the chamber 12 from 
a sample inlet 14 which includes a conventional molecular leak which, 
without disturbing the composition of the gas sample, reduces its pressure 
to a level compatible with the degree of vacuum provided in the chamber 
12. The gas samples thus ionized are focused by an electrode and 
accelerated into an analyzing chamber 16 where the ionized gas molecules 
are subjected to a magnetic field that deflects the course of the ions 
into curved paths according to the respective mass-to-charge ratios of the 
various gas molecules therein. Collector cups, appropriately positioned 
within the analyzing chamber 16 to receive the deflected ions of each of 
the gases of interest, produce currents that are proportional to the 
quantity of the gas ions collected. 
A very small quantity of gas is admitted into the chamber 12 through the 
inlet leak 14. It is, therefore, apparent that, if the gas sample contains 
impurities, such as moisture droplets, a very small inlet leak may readily 
become clogged, thereby requiring shutdown of the equipment for 
maintenance. 
In hospital intensive care wards having facilities for monitoring the 
respiratory gasses of patients, the gas samples are generally withdrawn 
from a flowmeter which is connected to a breathing mouthpiece to measure 
the quantity of inhaled and exhaled gasses of the patient or withdrawn 
from other portions of the patient's breathing circuit. The flowmeter or 
breathing circuit includes a connector to which is connected a capillary 
tube, such as tube 18 in the drawings. Tube 18 normally has an inside 
diameter in the order of 0.020" and may have a length of approximately six 
feet to terminate in a wall connection 20. Also connected to the wall 
connection 20 is gas transmission tubing 22 which normally has a larger 
inside diameter of approximately 0.200" and a comparable lower flow 
resistance. The tubing dimensions are by way of example and other 
diameters may easily be employed. Tubing 22 extends from the wall 
connection 20 to an inlet valve 24 and thence to a momentum separator 26 
illustrated in cross section detail in the drawing. Momentum separator 26 
is comprised of a straight section of tubing 28 having a diameter 
substantially the same as that of the transmission tubing 22. The outlet 
30 of the tubing 28 is connected to a vacuum pump 31 and the air sample to 
be analyzed is withdrawn through a tee connection 32 which is joined to 
the straight section 28 at an angle substantially greater than 90.degree. 
from the outlet 30 so that flow into the connection 32 is substantially 
opposite to the flow of the main sample toward the vacuum pump 31. 
Therefore, as the moist gasses and condensed droplets are drawn rapidly 
through the straight section 28 by the vacuum pump 31, gas samples without 
the moisture droplets may be withdrawn through the tee connector 32. 
Tee connector 32 is connected to a heat exchanger schematically illustrated 
as containing several coils of tubing 36 in a closed oven 34 heated by 
resistance elements 38 which, in turn, are controlled by a heater control 
40. The heater control 40 maintains the temperature within the oven 34 at 
approximately 100.degree. C. to further maintain the water in the sample 
in a vapor state. The sample is then carried into the inlet 14 of the mass 
spectrometer 10. The vacuum system (not shown) associated with the 
spectrometer 10 withdraws the necessary samples from the very small inlet 
leak and the unused gas sample then passes through the exhaust tubing 42 
to the vacuum pump 31. 
In operation, moist respiratory gases are admitted into the capillary 
tubing 18 from a patient's breathing circuit at atmospheric pressure. The 
capillary tubing 18 has a flow resistance that will drop the pressure at 
the wall connection 20 to a lower value, typically one-half atmosphere. 
The rapid pressure drop tends to keep all moisture in the vapor state even 
though the temperature also drops. This lower pressure gas is then 
transmitted through tubing 22 and valve 24 to the momentum separator 26 
where any condensed moisture continues through to the vacuum pump 31 while 
the moist gas, without condensed droplets, passes through the tee 
connector 32 to the oven 34 before being admitted into the inlet leak 14 
of the mass spectrometer 10. The moisture separating system therefore 
reduces the effect and quantity of water arriving at the inlet leak and 
permits accurate gas analysis without the repeated need to dry and clean 
the tubing that transmits the gases to the spectrometer.