MASS SPECTROSCOPE/SPECTROGRAPH

Typical Number in Hospital: 2

Cost Bands: 5,6

References: 6,9

A mass spectrometer is an apparatus which separates a stream of charged particles (ions) into a spectrum according to their mass- to-charge ratios and determines the relative abundance of each type of ion present. Medical mass spectrometers include a sample-inlet chamber, an ionization chamber, a dispersion chamber and an ion-detection (collection) system.

A thin sampling tube may connect the instrument to the site of recording (e.g. a lung ventilator). Gas is drawn into the sampling inlet chamber by a rotary pump reducing the pressure to 10-20 mmHg. A small amount of gas passes through a porous plug into an ionization chamber which is evacuated to approximately 1 part in 10 million of a mmHg. A stream of electrons travelling between a heated filament and anode bombard the gas entering the ionization chamber to produce ions. These ions are focused and accelerated in an electric field to the dispersion chamber where they are sorted into different molecular components. The sorting or dispersion techniques may include a magnetic field, quadripole electric field, or measurement of time of flight. The ion collector system produces an output for each type of ion involved. The inclusion of a circuit which sums all components in conjunction with the individual elements allows calculation of molar fractions.

In general the range is restricted to molecular weights of 4 (helium) to 44 (carbon dioxide). Extended ranges are possible to monitor hexafluoride (mass 146) and halothane (mass 196). Interference occurs between some molecules so that oxygen and carbon dioxide cannot be measured in the presence of either ether or nitrous oxide. Carbon monoxide interferes with nitrogen and nitrous oxide, although these can be measured with infrared instruments.

At present mass spectrometers are in use for research applications but have potential for patient monitoring in intensive care units. The rapid response time permits multiplexing of the sampling gas tubes so that gas mixtures from several patients, or several points in the same breathing circuit, can be recorded simultaneously. It is also possible to measure gases dissolved in tissues or blood by using a non- thrombogenic catheter with a semipermeable membrane covering its tip. Gases are drawn out of solution and through the membrane under the action of the inlet pump.