Mass spectrometer mean free path

Effusion separator (or effusion enricher). An interface in which carrier gas is preferentially removed from the gas entering the mass spectrometer by effusive flow (e.g., through a porous tube or through a slit). This flow is usually molecular flow, such that the mean free path is much greater than the largest dimension of a traverse section of the channel. The flow characteristics are determined by collisions of the gas molecules with surfaces flow effects from molecular collisions are insignificant. 

Ions formed in the source of the mass spectrometer must reach the detector for them to be of any value. The average distance that an ion travels between collisions - the mean free path - at atmospheric pressure is around 10 m, and it is therefore unlikely that it will reach the detector under these conditions. Since the mean free path is inversely proportional to the pressure, reducing this to 10 torr will increase the mean free path to around 10 m, and thus allow ions to reach the detector of the mass spectrometer. 

Note The need for almost perfect vacuum, i.e., extremely long mean free paths, in FT-ICR mass spectrometers arises from the combination of high ion velocities of several 10 m s observation intervals in the order of seconds, and the effect of collisions on peak shape. 

The mean free path is the average distance a molecule travels before colliding with another molecule. The mean free path, X, is given by X = kT/ jr2 itP). where k is Boltzmann s constant, Tis the temperature (K), P is the pressure (Pa), and cr is the collision cross section. For a molecule with a diameter d, the collision cross section is ltd2. The collision cross section is the area swept out by the molecule within which it will strike any other molecule it encounters. The magnetic sector mass spectrometer is maintained at a pressure of 10-5 Pa so that ions do not collide with land deflect) each other as they travel through the mass analyzer. What is the mean free path of a molecule with a diameter of 1 nm at 300 K in the mass analyzer ... 

Table lisa conversion table for pressure units. In a mass spectrometer, the mean free path should be at least 1 m and hence the maximum pressure should be 66 nbar. In instruments using a high-voltage source, the pressure must be further reduced to prevent the occurrence of discharges. In contrast, some trap-based instruments operate at higher pressure. 

However, introducing a sample into a mass spectrometer requires the transfer of the sample at atmospheric pressure into a region of high vacuum without compromising the latter. In the same way, producing efficient ion-molecule collisions requires the mean free path to be reduced to around 0.1 mm, implying at least a 60 Pa pressure in a region of the... 

The separation methods furnish a given flow of eluate (liquid or gaseous) generally under atmospheric pressure. One way or another, the eluted substances must find their way into the mass spectrometer source, where a high vacuum is necessary. We saw in the Introduction that the mean free path could be evaluated according to the following equation, where the pressure p is expressed in Pa and the distance L is expressed in centimeters ... 

Figure 1 Principle components of a mass spectrometer. For mass spectrometry (MS) analysis of a sample, the neutral analyte(s) must first be ionized, positively or negatively, to allow manipulation by the magnetic and/or electric fields in the MS analyzer. Ions are sorted according to their mass to charge ratio (m/z), which is then plotted against their intensity to generate a mass spectrum. The flight path of the ions is evacuated to maximize the mean free path of the ions and to reduce the possibility of unfavorable interactions with residual air molecules.

In accordance with this rule the mean free path in a mass spectrometer pumped down to 10" torr is astonishingly long ... 

If a beam of electrons passing through the ion source of a mass spectrometer has sufficient energy, it will bring about the formation of positive ions along the path of the electron beam. A small, positive potential is normally applied to the repeller and causes the ions to drift toward the exit slit and into the ion gun and the analyzer. A typical source is shown in Fig. 1. Under ordinary operating conditions, the pressure in the ion source is of the order of 10" Torr and the voltage applied to the repeller is 1-10 V. At these conditions, the mean free path of the ions in the source is of the... 

Here, k = 1.38 10 J/K is Boltzmann s constant. Since the overall flight path L in a mass spectrometer must be smaller than the mean free path if the ions are to transit successfully, the operating pressure must be... 

Thus at a pressure of 10 torr and for r, o = 1 nm, the mean free path X 7 m, considerably larger than any reasonable dimension within an analytical mass spectrometer. Of course X decreases in inverse proportion to P. 

Briefly, the experiments involve the detonation of condensed explosives in a high-vacuum environment (Fig.l), where the products expand freely and cool adiabatically (to 3(X) K) in a very short time (2 p.s) after the shock arrival. Additional (extensive) expansion after the short freezing step allows the mean free path of the product molecules to increase into the free molecular flow regime so that a molecular beam of the products can be admitted without disturbance to a drift tube through a 1-mm-diam skimmer. As the frozen product beam is passing freely down the drift tube, a portion is ionized and its chemical composition is analyzed with a mass spectrometer. Further details are provided in another lecture at this conference (5). 

Our first results of the device promise a very efficient and simple mass spectrometer device Further improvements are expected if an RF or microwave stimulated plasma (ECR condition) is used It would allow to reduce the pressure in the plasma chamber and hence an increase of the mean free path in the separator The more extended separator length, then available, will positively influence the resolution of the mass spectrometer... 

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