Molecular leak

Cohen, J.P. and Schweighardt, F.K., Fueling nozzle with integral molecular leak sensor, U.S. patent Application, April 2006. 

Volatile or volatilizable compounds may be introduced into the spectrometer via a pinhole aperture or molecular leak which allows a steady stream of sample molecules into the ionization area. Non-volatile or thermally labile samples are introduced directly by means of an electrically heated probe inserted through a vacuum lock. Numerous methods of sample ionization are available of which the most important are electron impact (El), chemical ionization (CY), field ionization (FI), field desorption (FD), fast atom bombardment (FAB), and radio-frequency spark discharge. 

Fig. 11.2. Diagram of the components of an El source. Gaseous samples (gases or vaporized liquids or solids) are introduced into the ionization chamber using a reservoir with molecular leak, direct insertion probe or as an eluent from a GC column. The collimated stream of 70 eV electrons interacts with the neutral analyte molecules to generate stable radical cations (M+ ) and unstable radical cations (M+ ) that undergo dissociation reactions to form the characteristic fragment ions observed in many El mass spectra.

Batch inlet systems are in many ways the most convenient for gases and volatile, thermally stable liquids or solids, provided there is sufficient sample available. They consist of reservoirs, varying in volume from 20 ml to several liters, which are connected to the ion source via a molecular leak, usually a porous ceramic material, or a pinhole in thin gold foil or glass. The leak serves to reduce the pressure from 10 torr in the inlet system to torr in the ion source. Large ballast bulbs... 

For gases or volatile liquids, a small quantity of sample is injected with a microsyringe into a reservoir that is connected to the ionisation chamber via a minute orifice. Because the reservoir is maintained under vacuum and can also be heated, the sample is vaporised. This procedure is called a molecular leak or molecular pumping. 

Cr, NijCr alloys) were used as substrates. The investigation of the composition of the gas phase in the vicinity of the growing surface during deposition was performed by on-line mass spectrometry with an electron impact ionization quadripole mass spectrometer (QMS). Sampling of the gas phase was achieved via a 2 mm diameter stainless steel tube. The tube was positioned parallel to the flux with its entry 1 mm above the substrates. It was connected to the QMS through a molecular leak valve. 

For gases or volatile liquids a very small quantity of sample is introduced with a micro-syringe to a kind of reservoir linked to the ionization chamber via a very narrow channel. Under the effect of a high vacuum that is maintained in the reservoir, the compound is sucked up and vaporized. This procedure is known as a molecular leak or molecular pumping. For continuous monitoring of gases and volatile compounds in the vapour state or dissolved in a liquid such as water, the sample can be diffused through a porous membrane. 

A very useful innovation in sample introduction systems is the use of a gas chromatograph coupled to a mass spectrometer. In effect, the mass spectrometer acts in the role of detector. In this technique, known as gas chromatography-mass spectrometry (GC-MS), the gas stream emerging from the gas chromatograph is admitted through a valve into a tube, where it passes over a molecular leak. Some of the gas stream is thus admitted into the ionization chamber of the mass spectrometer. In this way it is possible to obtain the mass spectrum of every component in a mixture being injected into the gas chromatograph. 

This method of sample introduction is useful for gases and for liquids with sufihciently high vapor pressures. The gas or vapor is allowed to expand into an evacuated, heated vessel. The sample is then leaked into the ionization source through pin holes in a gold foil seal. This is sometimes termed a molecular leak inlet. Vacuum pumps control the system so that the pressure in the ionization source is at the required torr. 

These are used on most commerdal process mass spectrometers. Often aim long capillary of 10-100 pm inner diameter is suffident to provide the necessary pressure drop. Deactivated fused siHca is the most common capillary material, since it is reasonably inert and does not exhibit significant memory effects with most sample streams. Heating the capillary further reduces memory effects and pluggage due to condensation. Capillaries can also be made of other materials such as stainless steel or nickel if silica is problematic. Molecular leaks (pinhole orifices) are sometimes used, either in conjunction with or in place of the capillary. Porous frits, either of sintered glass or metal, are sometimes used to avoid pluggage problems with a capillary or molecular leak, but these can often exhibit greater memory effects. 

If a pure sample flows into the ion source through a molecular leak, M" " exhibits the same effusion rate as can be determined from the fragment ions. The abundance of M" is proportional to the sample pressure in the ion source. 

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