SIM quantification

D. Hainzl, J. Burhenne and H. Pariar, HRGC-ECD and HRGC-NICI SIM quantification of toxaphene residues in selected marine organism by envir onmentally relevant chloroboT nanes as standard , Chemosphere 2S 237-243 (1994). 

Vanden Eynde X, Bertrand P (1997) ToF-SIMS quantification of polystyrene spectra based on principal component analysis (PCA). Surf Interface Anal 25 878... 

The most satisfactory results for SIMS quantification have been achieved through the use of relative sensitivity factors (RSFs). In its simplest form, RSFs are determined by measuring ion collection efficiencies from known matrices and recording them as a ratio to a reference element that is usually the most abundant metallic atom in the matrix. A matrix that matches the unknown is necessary. If a good matrix match is available, excellent results can be achieved—especially in the dilute concentration range. A relative sensitivity factor (RSF) may be defined as... 

H. Zhang, M. Balban, K. Portier, C.A. Sims, Quantification of spice mixture compositions by electronic nose, part II comparison with GC and sensory methods. J. Food Sci. 70(4), E259-E264 (2006)... 

Wilson, R.G. (1995) SIMS quantification in Si, GaAs, and diamond—an update. International Journal of Mass Spectrometry and Ion Processes, 143, 43 9. 

Fujiyama,N., Karen,A.,Sams,D.B., Hockett,R.S.,Shingu, K., Inoue, N. (2003) SIMS quantification of low concentration of nitrogen doped in silicon crystals. Applied Surface Science, 203-204,457-460. 

Torrisi, A., Scandurra, A., Licciardello, A. (1994) Evaluation of matrix effects in SIMS quantification of AljGai.xAs/GaAs heterostructures a SNMS approach. Applied Surface Science, 81,259-264. 

Ferrari, S., Ratner, B.D. (2000) ToF-SIMS quantification of albumin adsorbed on plasmadeposited fluoropolymers by partial least-squares regression. Surf Interface Anal., 29,837-844. 

Secondary ion emissions are collected as a function of sputter time, or more precisely, the primary ion dose. This is a result of the fact that each primary ion impact has a statistical likelihood of removing atoms and/or molecules from the surface of the substrate of interest. If more than one atomic layer is removed per analytical cycle, as is carried out in Dynamic SIMS, quantification of the depth scale may be required. For obvious reasons, this requirement does not extend to the Static SIMS regime. 

As for molecular distributions noted in Dynamic SIMS, quantification is eased by the weaker variability of matrix effects noted, which is particularly evident for organic matrices. Indeed, the intensities of specific molecular emissions, whether fragment or otherwise, from organic substrates can be seen to follow the concentrations of the respective constituents. The useful emissions will be specific to the substrate being analyzed and exceptions will be noted when there exist variations in the concentration of chemically active species such as Oxygen, Fluorine, and Cesium. 

In the case of Static SIMS, quantification can be eased by the reduced matrix variations noted when the element or molecule of interest exists in submonolayer coverage on some well-defined support, i.e. Gold, Silver, or Silicon. The reduced matrix effect variations stem from the fact that all of the secondary ions essentially see the same matrix during their departure, i.e. that of the support. 

The requirement for matrix-matched reference materials highlights the limitations of the RSF method. Without these, quantification via the RSF method is not possible. As a result, other SIMS quantification methodologies that do not require reference samples, or some form of referencing procedure, have been suggested. These are, however, not covered within this section as these have not found extensive use, a fact arising from the realization that these alternative procedures are not capable of providing the same level of precision, sensitivity, or detection limits as the RSF method. Some examples of alternative methods are covered in Appendices A.9.1-A.9.3. 

S.4.3.2 Fabrication of Reference Materials The effectiveness of the RSF method for SIMS quantification hinges critically on the quality of the matrix-matched reference material (internal standard) used. 

Infinite velocity method A suggested SIMS quantification method... 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

Ion implantation is often used to produce reliable standards for quantification of SIMS analyses. Ion implantation allows the introducdon of a known amount of an element into a solid sample. A sample with a major component composition similar to that of the unknown sample may be implanted to produce an accurate standard. The accuracy of quandfication using this implantation method can be as good as 2%. 

SALI compares fiivorably with other major surface analytical techniques in terms of sensitivity and spatial resolution. Its major advantj e is the combination of analytical versatility, ease of quantification, and sensitivity. Table 1 compares the analytical characteristics of SALI to four major surfiice spectroscopic techniques.These techniques can also be categorized by the chemical information they provide. Both SALI and SIMS (static mode only) can provide molecular fingerprint information via mass spectra that give mass peaks corresponding to structural units of the molecule, while XPS provides only short-range chemical information. XPS and static SIMS are often used to complement each other since XPS chemical speciation information is semiquantitative however, SALI molecular information can potentially be quantified direedy without correlation with another surface spectroscopic technique. AES and Rutherford Backscattering (RBS) provide primarily elemental information, and therefore yield litde structural informadon. The common detection limit refers to the sensitivity for nearly all elements that these techniques enjoy. 

The limitations of SIMS - some inherent in secondary ion formation, some because of the physics of ion beams, and some because of the nature of sputtering - have been mentioned in Sect. 3.1. Sputtering produces predominantly neutral atoms for most of the elements in the periodic table the typical secondary ion yield is between 10 and 10 . This leads to a serious sensitivity limitation when extremely small volumes must be probed, or when high lateral and depth resolution analyses are needed. Another problem arises because the secondary ion yield can vary by many orders of magnitude as a function of surface contamination and matrix composition this hampers quantification. Quantification can also be hampered by interferences from molecules, molecular fragments, and isotopes of other elements with the same mass as the analyte. Very high mass-resolution can reject such interferences but only at the expense of detection sensitivity. 

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

Resonant (R-) laser-SNMS [3.107-3.112] has almost all the advantages of SIMS, e-SNMS, and NR-laser-SNMS, with the additional advantage of using a resonance laser ionization process which selectively and efficiently ionizes the desired elemental species over a relatively large volume (Eig. 3.40 C). Eor over 80% of the elements in the periodic table, R-laser-SNMS has almost unity ionization efficiency over a large volume, so the overall efficiency is greater than that of NR-laser-SNMS. Quantification is also simpler because the unsaturated volume (where ionization is incom-... 

The advantages of SIMS are its high sensitivity (detection limit of ppms for certain elements), its ability to detect hydrogen and the emission of molecular fragments that often bear tractable relationships with the parent structure on the surface. Disadvantages are that secondary ion formation is a poorly understood phenomenon and that quantification is often difficult. A major drawback is the matrix effect secondary ion yields of one element can vary tremendously with chemical environment. This matrix effect and the elemental sensitivity variation of five orders of magmtude across the periodic table make quantitative interpretation of SIMS spectra oftechmcal catalysts extremely difficult. 

CB. Bertrand and Weng [47] have reported CB surface characterisation by ToF-SIMS and XPS. The sensitivity of the former (ToF-SIMS) is much higher than that of the latter (XPS) but quantification is much better with XPS. 

GC-MS operated in electron impact (El) mode was only sporadically used for the determination of some UV filters such as 4-MBC, EHMC, and OC. Separation was achieved on a 60 m x 0.25 mm i.d. DB-5 column, with 0.25-pm film thickness. For quantification of the compounds, data acquisition was performed in selected ion monitoring (SIM) mode recording three characteristic ions per compound. GC-MS allowed the differentiation between the two isomers (cis/trans) for 4-MBC and EHMC. 

GC/MS separation of mixtures of the compounds are usually performed on capillary columns with low and mid polarity and a length in the range of 30 50 m, with a total separation time of20 40 min, and temperature ramping from 40 to 300 °C. Total ion current (TIC) profiles are often obtained using ion trap or quadrupole analysers. Quantification is performed by selected-ion monitoring (SIM) detection using calibration curves. 

Table 7.1 reports the ions commonly used in SIM acquisition for the quantification of trimethylsilyl (TMS) and f-butyldimethylsilyl (TBDMS) derivatives. 

Total or partial ion suppression is a well-known LC-MS effect, which is induced by coeluting matrix components that can have a dramatic effect on the intensity of the analyte signal. As can be observed in Fig. 1, analyte suppression occurs as a consequence of the different matrix interferences present in waste-water samples, making the identification and/or quantification process difficult or unfeasible. Even when working under selection ion monitoring (SIM) conditions, these matrix effects can cause ion suppression in the detection of some analytes that are present at low levels of concentration, as seen in this figure. Several papers have reported this effect [30-32] and different alternatives to overcome these problems, such as the inclusion of a size-exclusion step [33] or sequential SPE [28], have been applied for the determination of pesticides in... 

The quantification capability is normally limited by the detector and/or the ion source. The MCP that is often utilized in TOF instruments cannot fully handle the ion currents that are produced in MALDI and are often saturated to some extent. With other ion sources, such as SIMS, the detection system is less strained so the detector is less limiting. Instead the ion source will limit the quality in quantification. Magnetic sectors and also qudmpoles are more often utilized when quantification is important. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

Although SIMS can provide quite valuable information on the molecular (rather than atomic) composition of the surface, this is a difficult technique to use. Moreover, the resulting spectra are complex, and quantification of the data is almost impossible. To date, SIMS remains a special and seldom-used technique for catalyst characterization. 

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