Vacuum lock

Liquids examined by FAB are introduced into the mass spectrometer on the end of a probe inserted through a vacuum lock in such a way that the liquid lies in the target area of the fast atom or ion beam. There is a high vacuum in this region, and there would be little point in attempting to examine a solution of a sample in one of the commoner volatile solvents such as water or dichloromethane because it would evaporate extremely quickly, probably as a burst of vapor when introduced into the vacuum. Therefore it is necessary to use a high-boiling solvent as the matrix material, such as one of those listed in Table 13.1. 

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

Vacuum-lock inlet. An inlet through which a sample is first placed in a chamber the chamber is then pumped out, and a valve is opened so that the sample can be introduced to the mass spectrometer ion source. A vacuum-lock inlet commonly uses a direct-inlet probe, which passes through one or more sliding seals, although other kinds of vacuum-lock inlets are also used. 

A versatile Laser-SNMS instrument consists of a versatile microfocus ion gun, a sputtering ion gun, a liquid metal ion gun, a pulsed flood electron gun, a resonant laser system consisting of a pulsed Nd YAG laser pumping two dye lasers, a non-resonant laser system consisting of a high-power excimer or Nd YAG laser, a computer-controlled high-resolution sample manipulator on which samples can be cooled or heated, a video and electron imaging system, a vacuum lock for sample introduction, and a TOF mass spectrometer. 

The direct insertion probe consists of a metal sample holder drilled to accept standard melting point capillaries up to 1 inch in length. This is inserted into the ion source through a vacuum lock and may be heated to 250°C at varying rates. 

Direct insertion probe pyrolysis mass spectrometry (DPMS) utilises a device for introducing a single sample of a solid or liquid, usually contained in a quartz or other non-reactive sample holder, into a mass spectrometer ion source. A direct insertion probe consists of a shaft having a sample holder at one end [70] the probe is inserted through a vacuum lock to place the sample holder near to the ion source of the mass spectrometer. The sample is vaporized by heat from the ion source or by heat from a separate heater that surrounds the sample holder. Sample molecules are evaporated into the ion source where they are then ionized as gas-phase molecules. In a recent study, Uyar et al. [74] used such a device for studying the thermal stability of coalesced polymers of polycarbonate, PMMA and polylvinyl acetate) (PVAc) [75] and their binary and ternary blends [74] obtained from their preparation as inclusion compounds in cyclodextrins. 

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. 

Field desorption (FD) is similar in principle to FI. It enables ions to be produced directly from solid samples which are deposited from solution onto an anode fitted to a probe that can be inserted into the instrument via a vacuum lock. It is even more gentle than Cl and FI, producing molecular ions and virtually no fragmentation. However, the ionization process decays very rapidly so spectra must be scanned quickly and cannot be re-recorded without introducing more sample. 

Both systems A and B can be used for continuous operation with vacuum locks. In system A (Fig. 2.53), one or two carriers are be moved into a lock (4) in front of several connected tunnels (8). The lock is evacuated and the carrier(s) are moved into the tunnels. At the same time, the equal number of carriers is moved from the tunnels into an exit lock (10). Both locks can be separated from the tunnels by two large slide valves (7). In system B (Fig. 2.54), each tray (1) passes through an entrance lock (2) into a paternoster lift (3), which moves the tray to a certain level and pushes it into the drying zone. The last tray is pushed by the entering tray into the exit paternoster lift (6), which moves the tray into the exit lock (7). These plants are illustrated in Fig. 2.55 and 2.56. 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

System resolution in excess of 50 000 is achieved and excellent performance is obtained at high masses. The instrument features a novel ion source which can be exchanged in a few seconds via vacuum lock. Optimized El and Cl systems are supplied. 

Fig. 1.1. General scheme of a mass spectrometer. Often, several types of sample inlets are attached to the ion source housing. Transfer of the sample from atmospheric pressure to the high vacuum of the ion source and mass analyzer is accomplished by use of a vacuum lock (Chap. 5.3).

Junk, G.A. Svec, H.J. A Vacuum Lock for the Direct Insertion of Samples into a Mass Spectrometer. Anal. Chem. 1965, S7, 1629-1630. 

Fig. 8.8. FD probe inserted into the vacuum lock. FD probes are generally inserted in axial position to leave the vacuum lock of the DIP free for FI use. The emitter wire is now oriented vertically to comply with the beam geometry of the magnetic sector analyzer.

Numerous analytes could be good candidates for FD-MS, but undergo immediate decomposition by reacting with ambient air and/or water under the conditions of conventional emitter loading. Inert conditions such as emitter loading in a glove box does not really avoid the problem, because the emitter still needs to be mounted to the probe before insertion into the vacuum lock. Furthermore, the tuning of an FD ion source usually has to be optimized for the emitter actually in use which is not practicable with the sample already loaded onto its surface. 

Many manufacturers now offer other sample injection systems compatible with the vacuum lock used for the solids probe. These include small (e.g., 75-ml) heatable batch inlet systems, usually accessible via syringe (gas syringe or GC microliter syringe for liquids), which can be particularly useful as inlets for mass reference compounds. Other probes are designed as flexible, easily removed connections to a gas chromatograph via some form of interface. 

Fig. 1. Double glove-box system for the preparation and refining of actinide metals 1. RF heating system 2. Water cooled quartz vacuum furnace 3. Box filled with circulating nitrogen 4. -I- 5. Argon in and out filters filter 6. Stainless steel box filled with circulating argon 7. Vacuum lock chamber 8. Pump...

Fig. 3.4 (c) Penn State design tip exchange setup ffor an FIM or a pulsed-laser atom-probe with a vacuum lock. The tip can be heated by passing a current through the wire loop. The tip is mounted on an internal gimbal and is cooled through a copper braid by a refrigerator. 

It is often desirable to have a separate chamber with a vacuum lock for the purpose of tip exchange and tip manipulation without the need of exposing the FIM chamber to atmosphere. There are many different designs used by different investigators. One employed in the author s laboratory, designed by G. L. Xiao, is shown in Fig. 3.4(c). A tip is mounted on a cone-shaped sapphire piece with a cross-shaped stainless steel thin rod attached at the back end. The tip mounting sapphire piece... 

Solid samples that have a sufficient vapour pressure at 300 °C are deposited on the tip of a heated metal probe which is then inserted into the instrument through a vacuum lock. With some ionisation methods, the solid sample is mixed with a liquid matrix (e.g. glycerol or benzoic acid). 

In direct introduction the sample can be introduced via a sample probe or plate through a vacuum lock, and can subsequently be ionized via El, Cl or matrix-assisted laser desorption ionization (MALDI see Section 2.4). Alternatively, the sample can be introduced as a liquid stream into an ion source at atmospheric pressure, after which it is subjected to electrospray ionization (ESI see Section 2.3). Direct injection does not offer any form of sample separation. 

Special Gun Mounts The guns described above are mounted on flanges or or introduced through the vacuum lock. A few specialists have mounted the FABMS gun on the mass spectrometer ionization source structure with good results. These mounting procedures require careful planning and alterations of the source structure to assure alignment and minimal operational interference. 

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