Principle of Operation of a Medical Mass Spectrometer

A mass spectrometer is an instrument that produces a stream of charged particles (ions) from a substance being analyzed, separates the ions into a spectrum according to their mass-to-charge ratios, and ascertains the relative abundance of each type of ion present.

The figure below shows a medical mass spectrometer system:

The above medical mass spectrometer system consists of: a sample-inlet assembly, an ionization chamber, a dispersion chamber and an ion-detection (collector) system. The sample-inlet assembly consists of a heated or unheated capillary tube that is approximately 0.25 mm inside diameter (ID) and sample-inlet chamber. Gas is drawn through this system by a rotary pump that reduces the pressure in the inlet chamber to about 10 to 20 mm Hg {1.3 to 1.7 kPa} absolute. A small amount of gas in the inlet chamber leaks by diffusion through a porous plug into the ionization chamber, which, along with the dispersion chamber, is evacuated to approximately 10^-7 mm Hg (10^-5 pa) by a high-vacuum, high-capacity pump. A stream of electrons traveling between a heated filament and an anode bombards the gas entering the ionization chamber and causes the molecules to lose electrons, in so doing producing positive ions. These ions are focused into a beam and accelerated by an electric field into the dispersion chamber, where the ion beam is sorted into its components on a molecular mass basis.

Various dispersion techniques can be used, that include: a magnetic field, a quadrupole electric field, or measurement of time of flight. The separated ion beams fall on the collector system that produces the output signal of the instrument.

In respiratory applications, it is important for a mass spectrometer to simultaneously produce separate continuous outputs for the several chemical species of interest. To achieve this, two techniques are employed: First method uses a single collector that is swept sequentially by the component beams at a high repetitive rate. Individual sample-and-hold circuits, each corresponding to a particular species, register the ion current as the beam for their respective species falls on the collector. The second approach employs multiple collectors, the positions of which can be adjusted so that each continuously receives the ions of only one of the components of interest.

The ion current measured by the collector is proportional to the partial pressure of the corresponding component in the gas mixture. Online signal processing, in conjunction with a suitable calibration procedure, makes it possible for the output to be expressed in terms of molar fractions.

Related: Spectrophotometry Instrumental Method of Analysis

The range of molecular weight that is sufficient for most respiratory measurements extends from 4 (He) to 44 (CO₂) atomic mass units. Extended ranges make possible the monitoring of gases such as sulfur hexafluoride and halothane. A number of key gases produce outputs for the same atomic units. Especially O₂ and CO₂ cannot be measured in the presence of N₂O. Additionally CO interferes with N₂. To measure CO and N₂O, infrared instruments can be used.

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