Primary ion columns

The primary ion column is the section in which the primary ions used to generate the secondary ions are formed, filtered, focused, aud directed onto the sample. Cross sections of primary ion columns found on commercially available instruments are shown in Figure 4.3(a-c). The primary difference lies in the primary ion filtering... 

Figure 4.3 Simplified layouts of primary ion columns employing (a) Magnetic Sector filters, (b) Wien filters, and (c) Time Of Flight (TOF) mass filters. Operational aspects are discussed in the text. Additional lenses/deflectors/apertures are omitted for sake of clarity.

The remainder of the primary ion column consists of a series of electrostatic lenses/deflectors and apertures. Additional sources, discussed further in Section 4.2.2.1, may also be present. Electrostatic lenses and deflectors are used to shape and transfer the ion beam to the region of interest. Apertures, along with focus-ing/defocusing of the beam, are used to control the current. As many analytical conditions are available (primary ion beam type, polarity, energy, and, in some cases, the angle of incidence), the primary ion column must be prealigned for the specific analysis on a daily basis. 

Electron sources are also commonly found on primary ion columns as these allow for the analysis of insulating samples (issues accompanying the analysis of insulating samples are covered in Section 5.2.1, with methods used to circumvent these issues covered in Section 5.2.1.1.1). These sources are focused/tuned to suite the analysis need. 

TABLE 4.4 Typical Properties of Ion Sources Used in SIMS within Optimized Primary Ion Columns along with the More Commonly Generated Primary Ions. 

In addition, the primary ion analysis beam focusing/pulse sequencing can be tailored to optimize the specific information required. As an example, the pulse width, which defines the mass resolution, can be adjusted by pulsing the electrodynamic fields within the primary ion column such as to accelerate slower ions/decelerate faster ions within a specific pulse, such that they all arrive at the sample surface at the same time. This is referred to as beam bunching. The downside is that spatial resolution is lost when operating in the micro-probe mode. 

In general the instrumentation for SSIMS and SNMS can be divided into two parts, that of the primary-ion column in which the primary ions are generated, focused and transported toward the target and that in which desorbed secondary species are extracted, postionized (where appropriate), mass. separated and detected (see Fig. 2). 

The confirmation of pesticides by GC/MS should be more reliable than that on the GC-ECD using an alternate column. Presence of stray interference peaks, even after sample cleanup, and the retention time shift and coelution problem, often necessitate the use of GC/MS in compounds identification If a quantitative estimation is to be performed, select the primary ion or one of the major characteristic ions of the compounds and compare the area response of this ion to that in the calibration standard. Quantitation, however, is generally done from the GC-ECD analysis, because ECD exhibits a much greater sensitivity than the mass selective detector (MSD). For example, while ECD is sensitive to 0.01 ng dieldrin, the lowest MSD detection for the same compound is in the range of 1 ng. The primary and secondary characteristic ions for qualitative identification and quantitation are presented in Table 2.20.3. The data presented are obtained under MS conditions utilizing 70 V (nominal) electron energy under electron impact ionization mode. 

In 1967 Liebl reported the development of the first imaging SIMS instrument based on the principle of focused ion beam scanning [24]. This instrument, the ion microprobe mass analyzer, was produced by Applied Research Laboratories (Fig. 4.5). It used an improved hollow cathode duoplasmatron [25] ion source that eliminated filaments used in earlier sources and allowed stable operation with reactive gases. The primary ion beam was mass analyzed for beam purity and focused in a two-lens column to a spot as small as 2 pm. The secondary ions were accelerated from the sample surface into a double focusing mass spectrometer of Mattauch-Herzog geometry. Both positive and negative secondary ions were de-... 

In 1965 Long published a proposed ion microprobe analyzer [27]. Long s student, Drummond, began construction and in 1967 published secondary electron micrographs showing 0.5-pm resolution [28] using a primary beam column. This became the basis for the Associated Electrical Industries (AEI) Ltd. SIMS instrument [29]. This instrument utilized AEI s MS702R spark source mass spectrometer for secondary ion analysis and had a mass resolution of -5000. 

Propyn-l-ol is analyzed by GC-FID using a suitable column for alcohol. It can be analyzed in all types of samples, such as ground-water or soils, sediments, and sludges by GC/MS using a purge and trap technique or by direct injection (U.S. ERA 1986, Method 8240, SW-846). A column containing 1% SP-1000 on Carbopack-B is suitable. The primary ion is 55 and the secondary ions are 39, 38, and 53 (electron-impact ionization). 

A typical SIMS configuration includes one or two ion sources (ion gun) used to produce the primary ion beam, an ion column that accelerates and focuses the... 

A series of lenses and apertures in a primary beam column allow the accelerated primary beam to be focused on the sample surface. Generally, the ion source and primary column are tuned by the analyst to get the maximum amount of primary beam current into the smallest spot size. After focusing, the beam may be moved on the sample surface in a raster pattern. The current density of the primary beam is often expressed as mA/cm. Eor example, a 100 nA primary beam rastered over a 250 pm square has a current density of ... 

S.3.2.3 The O2 Leak Methodology As covered in Section 4.2.1.1, SIMS requires a vacuum of better than 1 X 10 " Torr (that at which the mean free path of an ion equates to 1 m, which equates to the typical path length of a primary or secondary ion column). More typically, a vacuum of better than 1 x 10 Torr is used to control the adsorption of gas phase molecules. As an example, a monolayer of Oxygen will form on Silicon in Is when in the 10 Torr range. 

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