IT-SIMS

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

March. J. Advanced Oiyiinn Cheinism. 4ih cJ, John Wiley it Sims. Inc.. New York. NY. 1992. 

IMPA IT-SIMS isopropyl methylphosphonic acid ion-trap secondary ion mass spectometry... 

For example the Identification of carbon contamination on an IC surface as a hydrocarbon was based on Its SIMS mass spectrum showing the C H cracking pattern typical of hydrocarbons (52). Both tne"type of Instrument used and the substrate on which the sample Is adsorbed can affect the cracking pattern obtained (12)< thus good standards are necessary for accurate compound Identification. 

The END theory answers this question by treating the simultaneous dynamics of electrons and nuclei and thereby avoiding the use of potential energy surfaces and stationary electronic states. The END formalism, even in its sim-... 

Usually molecules that have significant functional groups are most aggressively adsorbed, and in many instances, these molecules are of high analytical interest. The functionalized molecules can be bound strongly, which can defeat attempts to remove them by solvent extraction. This difficulty and the need for direct analytical strategies motivated the development of direct surface analysis, one of which is the ion trap secondary ion mass spectrometer (IT-SIMS). 

In this protocol, direct surface analyses using IT-SIMS will be described from the perspective of analyses that were conducted to measure chemical warfare agents and related compounds (precursors, degradation products, and surrogates) that were adsorbed on samples from exposed environments. During the course of the research that is described in this chapter, the basic instrumental design of the IT-SIMS was modified for the analysis of absorbed compounds, and so the utilization of the instrument for these purposes is also described. 

Another technological advance that was a key to the development of the IT-SIMS, particularly for analysis of electrically insulating surfaces, is the control of surface charging. As noted above, when primary ions impact... 

FIGURE 22.2 Photographs of the IT-SIMS instrument based on the Teledyne ion trap. (A) Vacuum unit situated on top of a cart, next to a monitor. Pumping, electronics, and computer are shown below the cart platform. (B) Top view (glass cover removed). The perrhenate ion gun is the lens stack on the left side, the curved channeltron is in the center, and the end caps and ring electrode are on the far right side. The small ruler on the edge of the vacuum chamber is 15 cm. 

The IT-SIMS instruments were custom designed and fabricated in-house at the Idaho National Laboratory. All instruments were based on modifications of commercial gas chromatography/mass spectrometry (GC/ MS) instruments that were altered by removing the gas chromatography (GC) and inlet, and replacing it with a direct insertion probe, a modified conversion dynode and a Venetian blind electron multiplier, and a perrhenate primary ion gun. The primary ion gun and the direct insertion probe are located coaxial with the IT,... 

The analysis involved attaching the solid sample to the direct insertion probe, which was generally accomplished using double-sided tape. A small piece of tape was attached to the end of a small finishing nail (Figure 22.3) that functioned as a sample holder. The end of the direct insertion probe contained a cylindrical sleeve with a set screw that was used to attach the loaded sample holder. Prior to attachment, the sample holder was tapped to remove any loosely held material. The direct insertion probe was then placed into the insertion lock, which was evacuated. This evacuation procedure normally required several minutes however, for samples such as damp soils, up to 5 min was sometimes allowed. Pressure in the forelock dropped into the millitorr regime, after which a ball valve was opened and the probe was inserted into the vacuum chamber of the IT-SIMS. This caused a momentary rise in the pressure inside the IT-SIMS. 

One of the first direct surface analyses conducted using the IT-SIMS was the measurement of AMPAs on soil and vegetation. The AMPAs are the hydrolysis products of nerve agents (Scheme 22.1) and have proven difficult to detect on account of their tendency to strongly adsorb... 

Samples were generated to evaluate the efficacy of the IT-SIMS for the detection of the AMPAs [9,10], which involved exposing soil samples to solutions at concentrations of 100 ppm and allowed to dry. 

After trapping sputtered ions, MS-1 spectra were acquired by scanning ions out of the IT [8], or MS-2 spectra were produced by first isolating the ions of interest, energizing them to induce dissociation and then scanning the fragment ions out of the IT-SIMS. 

Ion dissociation was achieved by subsequent application of a voltage having a frequency equivalent to the secular frequency of the parent ion in the IT-SIMS. A typical voltage value was a few hundred millivolts. 

Results of IT-SIMS Analyses of Soil Samples Dosed with AMPAs... 

The IT-SIMS was used to evaluate conjugate sulfonium ion detectability by application of a hydrolyzed CEES solution to soil. Hydrolysis was initiated by using methanol and H2O as a solvent. Intact sulfonium samples were detected on the surface using the IT-SIMS that corresponded to structures similar to ion B but having OH and OCH3 substituents in addition to Cl. The sulfonium ions are readily detected because they contain a fixed charge and are fairly surfactant. 

SCHEME 22.4 Ion fragmentation observed in the analyses of [H-Cl] formed in the IT-SIMS from H adsorbed to soil particles. 

The IT-SIMS was also applied for the characterization of soil samples contaminated with nitrogen mustard compounds [16]. These are much less common than sulfur mustards and are comprised by three compounds tris(2-chloroethyl)amine (HN-3), bis(2-chloroethyl)methyl-amine (HN-2), and bis(2-chloroethyl)ethylamine (HN-1). Of these, only HN-3 received serious consideration... 

IT-SIMS that have enabled application to the analysis of nerve agent-contaminated surfaces. Specifically, the bread box-sized IT-SIMS shown in Figure 22.2 is small enough to be situated in a surety laboratory, which is exactly what was done to enable the characterization of VX on soil surfaces. 

VX is very readily detected using the IT-SIMS and is in sharp contrast to GB and GD, which are not easily detected. This is principally because the G-agents were too volatile to effectively adsorb to the surfaces of sample materials, and hence, were lost in the evacuation of the sample in the direct insertion probe forelock. VX, on the other hand, aggressively adsorbed to sample surfaces this, together with its high proton affinity, resulted in extremely sensitive detection. 

IT-SIMS Analysis The loaded sample probe was evacuated in the insertion probe forelock to about 100 mTorr,then inserted into the IT-SIMS vacuum chamber. This procedure caused a temporary increase in pressure to about 10" Torr, which is followed by a decrease to 10 Torr, which is where the instrument is operated during analysis. Background pressure for the IT-SIMS used in these experiments was on the order of... 

Data Analysis As compounds are depleted from the surface, compounds in the near-surface bulk permeate back to the surface, and evaporate into the gas phase of the IT-SIMS, where they are ionized and detected. Mass spectra are recorded as a function of increasing time in vacuum, and permeation was deduced from the time-dependent behavior of the analyte ions. 

It may be that in light of advances made using both desorption electrospray ionization (DESI) [20,21] and direct analysis in real time (DART) [22,23] that the use of IT-SIMS protocols may be supplemented in some applications. These techniques differ in that they sample surface contaminants at atmospheric pressure and do not necessarily require collection of a sample, or transporting the sample into the vacuum environment of the mass spectrometer. However, preliminary experiments have shown that these techniques are tremendously matrix dependent, even more so than SIMS, and thus the generation of even semiquantitative data for environmental surfaces like soil samples remains challenging. Comparisons of ultimate detection limits between the techniques have not yet been accomplished. 

Sorption of toxins that are absorbed into the nearsurface bulk of permeable polymers represents one area where substantial improvements in the understanding of contaminant behavior are needed. Applications of the IT-SIMS in the DSIMS mode provide a relatively easy approach for measuring absorbed contamination. 

To overcome matrix-related problems, the simplest quantification can be performed with a standard adcfition method and spiking the analyzed wine with a known concentration of TCA. The linearity range of the method should be satisfactory between 5 and 5000 ng/L. Typical scan/SIM chromatogram of red wine spiked with TCA together with its SIM spectrum is shown at Figure 24.5. Alternatively, TCA may be analyzed using GC x GC-TOF-MS system in a configuration described in the section devoted to the analysis of trichothecenes. 

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