Basic Mass Spectrometer Vacuum System

Vacuum systems are integral parts of any mass spectrometer, but vacuum technology definitely is a field of its own. [251-255] Thus, the discussion of mass spectrometer vacuum systems will be restricted to the very basics. 

All commercially available SIMS systems have in common some type of computer automation, an ion source, a high-vacuum environment, and some type of mass spectrometer. While the specifics may vary from system to system, the basic requirements are the same. The hardware feature that tends to distii uish the various systems is the type of mass spectrometer used. These fall into three basic catego-... 

It has been the purpose of this paper to provide an overview of the basic differences and similarities of the various types of Instruments which detect Ionized particles emitted from surfaces by energetic particle bombardment. Since the scope of secondary ion mass spectrometry Is so broad, It is not surprising that no one Instrument has been designed to perform optimally for all types of SIMS analyses. Design aspects of the primary beam, extraction optics, mass spectrometer, detection equipment and vacuum system must be considered to construct an Instrument best suited for a particular purpose. 

The mass spectrometer consists of an inlet system, an ionization device, a mass analyzer, and an ion detector, and the system is kept at a vacuum of about 10 4 to 10 7 Torr. (There are various options available for all four basic components which illustrates the versatility of the technique.) The following describes an electron impact-quadruple mass spectrometer. 

The basic elements comprising a SIMS system are an ion source and a mass spectrometer (Fig. 6.4). Maintaining the analysis chamber under ultra-vacuum limits the contamination of the fresh surfaces exposed during analysis. The vacuum system comprises an introduction chamber enabling several samples to be introduced without deteriorating the vacuum. With the impact of the primary ions, secondary electrons are also produced. Addition of a detector... 

Mass spectrometry is a sophisticated instrumental technique that produces, separates, and detects ions in the gas phase. The basic components of a mass spectrometer are shown in Figure 20.9. A sample with a moderately high vapor pressure is introduced in an inlet system, operated under vacuum (10"" to 10 ton) and at high temperarnre (up to 300°C). It vaporizes and is carried to the ionization source. Nonvolatile compounds may be vaporized by means of a spark or other... 

The three basic elements of the RAPyD-400 — the vacuum system, the inlet system, and the quadrupole analyzer — can be seen in Figure 3.5. The quadrupole analyzer, which itself consists of two separate parts, lies inside the main vacuum chamber. The high vacuum is attained using a turbomolecular pump, which is backed by a dual-stage rotary pump mounted externally to the main system. The sample inlet system is connected to the ion source of the mass spectrometer via a heated molecular beam tube. Around the underside of the ion source is a copper cold finger, which is cooled by liquid nitrogen and used as a sample dump to prevent carryover from one sample to another. 

A compact ion attachment mass spectrometer was designed that is simple and small and fulfills all the basic requirements for lAMS the system can be used to obtain only molecular ions by detecting any chemical species in real time [93]. This custom-made apparatus (Fig. 6.11) consists of a Li+ ion attachment ion source into which a stream of gas from a capillary leak inlet is directed, an electrostatic lens system (ELS), and a quadrupole mass spectrometer and detector, all of which are installed in a vacuum-separated envelope. The system employs a single tuibomo-lecular pirmp on the vacuum envelope instead of a differential pumping system. 

Sputtering a sample yields secraidary irais which ccmtain analytical information characteristic of the sample. Several types of SIMS instmments are available to perform the analytical measurements. The basic components are a primary imi source, a sample chamber, a mass spectrometer, a secondary ion detectiOTi system, a vacuum system, and a data acquisition system as illustrated in Fig. 4.3. There are many variations and combinations of the basic components and it is beymid the scope of this chapter to comprehensively cover all instrumentation so only some general and more conmuMi types will be discussed. An in-depth discussion on SIMS instrumentation is provided in the very comprehensive SIMS reference by Benninghoven et al. [10] and in the instrumentation chapter of reference [11]. As SIMS has expanded into more areas of analysis, it has become more difficult to have one instrument to perform all types of analysis so instrument development has trended toward dedication to specific applications [12]. 

Obviously, almost any technique to achieve the goals of ionization, separation and detection of ions in the gas phase can be applied - and actually has been applied -in mass spectrometry. Fortunately, there is a simple basic scheme that all mass spectrometers follow. A mass spectrometer consists of an ion source, a mass analyzer, and a detector which are operated under high vacuum conditions. A closer look at the front end of such a device might separate the steps of sample introduction, evaporation, and successive ionization or desorption/ionization, respectively, but it is not always trivial to identify each of these steps as clearly separated from each other. If the manufacturing date of the instrument is relatively recent, it will have a data system which collects and processes data from the detector. Since the 1990s, mass spectrometers are fully equipped and controlled by data systems (Fig. 1.3). 

---