Secondary mass spectrometer

The cross section measurements were made with an apparatus described previously (1). The instrument consists of a primary mass spectrometer (PMST in tandem with a secondary mass spectrometer (SMS). The PMS is a 1 cm radius of curvature 2500 G permanent magnet mass analyzer. The SMS is a 60° magnetic sector instrument with an 8 in. radius of curvature. The detection of product ions was made by counting ions with a 17 stage electron multiplier. The gas pressure in the reaction cell was measured with an MKS Baratron differential pressure gauge. 

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

Ar, Cs, Ga or other elements with energies between 0.5 and 10 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection of atoms, ions and multi-atomic clusters (figure Bl.25.8). In SIMS, secondary ions (positive or negative) are detected directly with a mass spectrometer. 

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

A gun is used to direct a beam of fast-moving atoms or ions onto the liquid target (matrix). Figure 4.1 shows details of the operation of an atom gun. An inert gas is normally used for bombardment because it does not produce unwanted secondary species in the primary beam and avoids contaminating the gun and mass spectrometer. Helium, argon, and xenon have been used commonly, but the higher mass atoms are preferred for maximum yield of secondary ions. 

The basic principles of fast-atom bombardment (FAB) and liquid-phase secondary ion mass spectrometry (LSIMS) are discussed only briefly here because a fuller description appears in Chapter 4. This chapter focuses on the use of FAB/LSIMS as part of an interface between a liquid chromatograph (LC) and a mass spectrometer (MS), although some theory is presented. 

By passing a continuous flow of solvent (admixed with a matrix material) from an LC column to a target area on the end of a probe tip and then bombarding the target with fast atoms or ions, secondary positive or negative ions are ejected from the surface of the liquid. These ions are then extracted into the analyzer of a mass spectrometer for measurement of a mass spectrum. As mixture components emerge from the LC column, their mass spectra are obtained. 

An ion beam causes secondary electrons to be ejected from a metal surface. These secondaries can be measured as an electric current directly through a Faraday cup or indirectly after amplification, as with an electron multiplier or a scintillation device. These ion collectors are located at a fixed point in a mass spectrometer, and all ions are focused on that point — hence the name, point ion collector. In all cases, the resultant flow of an electric current is used to drive some form of recorder or is passed to an information storage device (data system). 

Once basic requirements and secondary objectives have been established, the prospective purchaser will find it easier to discuss details with sales representatives. From the latter s viewpoint, it is easier to talk to a potential customer who knows what he needs from a mass spectrometer system rather than to a customer who has only a vague idea of what is required. In fact, an uninformed customer can end up purchasing an expensive instrument that is far too good for the analyses required or, at the other extreme, a cheap instrument that is inadequate for immediate needs, let alone ones that might arise in the near future. 

Lasers are used to deliver a focused, high density of monochromatic radiation to a sample target, which is vaporized and ionized. The ions are detected in the usual way by any suitable mass spectrometer to produce a mass spectrum. The yield of ions is often increased by using a secondary ion source or a matrix. 

In Secondary Ion Mass Spectrometry (SIMS), a solid specimen, placed in a vacuum, is bombarded with a narrow beam of ions, called primary ions, that are suffi-ciendy energedc to cause ejection (sputtering) of atoms and small clusters of atoms from the bombarded region. Some of the atoms and atomic clusters are ejected as ions, called secondary ions. The secondary ions are subsequently accelerated into a mass spectrometer, where they are separated according to their mass-to-charge ratio and counted. The relative quantities of the measured secondary ions are converted to concentrations, by comparison with standards, to reveal the composition and trace impurity content of the specimen as a function of sputtering dme (depth). 

In Dynamic Secondary Ion Ma s Spectrometry (SIMS), a focused ion beam is used to sputter material from a specific location on a solid surface in the form of neutral and ionized atoms and molecules. The ions are then accelerated into a mass spectrometer and separated according to their mass-to-charge ratios. Several kinds of mass spectrometers and instrument configurations are used, depending upon the type of materials analyzed and the desired results. 

Static SIMS entails the bombardment of a sample surface with an energetic beam of particles, resulting in the emission of surface atoms and clusters. These ejected species subsequendy become either positively or negatively charged and are referred to as secondary ions. The secondary ions are the actual analytical signal in SIMS. A mass spectrometer is used to separate the secondary ions with respect to their charge-to-mass ratios. The atomic ions give an elemental identification (see... 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Secondary Ion Mass Spectrometry Dynamic Secondary Ion Mass Spectrometry Static Secondary Ion Mass Spectrometry SIMS using a Quadruple Mass Spectrometer SIMS usii a Magnetic Sector Mass Spectrometer See Magnetic SIMS... 

To minimize surface damage, static SIMS mass spectrometers should be as efficient as possible for detecting the total yield of secondary ions from a surface. Also, to be able to separate elemental from molecular species, and molecular species from each other, the mass resolution usually given as the mass m divided by the separable mass Am, should be very high. With this in mind, two types of mass spectrometer have been used - in early work mainly quadrupole mass filters and, more recently, time-of-flight mass spectrometers. 

For IBSCA analysis, standard HV or, better, UHV-equipment with turbomolecular pump and a residual gas pressure of less than 10 Pa is necessary. As is apparent from Fig. 4.46, the optical detection system, which consists of transfer optics, a spectrometer, and a lateral-sensitive detector, is often combined with a quadrupole mass spectrometer for analysis of secondary sputtered particles (ions or post-ionized neutrals). 

In secondary ion mass spectrometry (SIMS), a beam of energetic primary ions is focused onto the surface of a solid. Some of the ions are reflected but most of the energy of the primary ions is dissipated in the surface by binary collisions that cause neutrals, excited neutrals, and ions (positive and negative) to be ejected or sputtered from the surface. The secondary ions can be analyzed by a mass spectrometer to provide information about the surface composition of the solid. 

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