Respiratory Gas Moisture Separator System for Mass Spectrometer Monitoring Systems

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.

The single drawing is a schematic diagram illustrating the respiratory gas moisture separator of the invention.

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.