Quantitative analysis, SIMS techniques

Charge exchange processes as discussed above are important for a good understanding of LEIS, and of SIMS as well. Unfortunately, the subject is still not yet completely understood, which forms an impediment to quantitative analysis by both techniques. Quantitative interpretation of LEIS spectra is nevertheless perfectly possible if one uses appropriate calibration standards. 

This paper discusses SIMS as a multi-dimensional technique for the analysis of inorganic and organic materials. The paper is divided into two parts inorganic and organic (or molecular) SIMS. The inorganic SIMS part focuses on the methods of quantitative analysis and depth profiling applications. In particular, SIMS matrix effects are defined and the physical models and empirical methods used to quantify SIMS results are reviewed. 

By itself, SIMS has been shown to be a powerful tool for elemental surface characterization by Benninghoven21 and Schubert and Tracy 22 however, uncertain or rapidly changing secondary ion yield due to changes in chemical bonding makes quantitative analysis virtually impossible using SIMS alone.23,24 SIMS is most helpful when combined with other techniques, such as ISS and AES, which use a beam of ions of correct energy for combined use with SIMS. 

For purposes of quantitative analysis, selected ion monitoring (SIM) and selected reaction monitoring (SRM) are two commonly utilized approaches. The latter is also referred to as multiple reaction monitoring (MRM). In both modes, considerable structural information is lost nonetheless, these techniques are extremely powerful for target compound quantihcation in biological matrices, if the compound of interest is known. 

When only a few analytes are of interest for quantitative analysis and their mass spectrum is known, the mass spectrometer can be programmed to monitor only those ions of interest. This selective detection technique is known as selected ion monitoring (SIM). Because SIM focuses on a limited number of ions, more signal can be collected for each selected mass. This increases the signal-to-noise ratio of the analyte and improves the lower limit of detection. In general, however, an unknown is considered identified if the relative abundances of three or four ions agree within 20% of those from a reference compound. 

Such features make SIMS a powerful technique for surface analysis. However, SIMS as a surface analysis technique has not yet reached a mature stage because it is still under development in both theoretical and experimental aspects. This lack of maturity is attributed to the complicated nature of secondary ion yield from a solid surface. Complexity of ion yield means that SIMS is less likely to be used for quantitative analysis because the intensity of secondary ions is not a simple function of chemical concentrations at the solid surface. SIMS can be either destructive or non-destructive to the surface being analyzed. The destructive type is called dynamic SIMS it is particularly useful for depth profiling of chemistry. The nondestructive type is called static SIMS. Both types of SIMS instruments are widely used for surface chemical examination. 

Secondary ion mass spectroscopy (SIMS) is a valuable technique for identifying the structure and composition of polymer surfaces and complements ESCA. Similar spectra are difficult to resolve by ESCA, while SIMS can differentiate among different polymers. This is partly due to the smaller sampling depth required by SIMS. In atypical analysis, the surface of the polymer sample is bombarded by a primary ion at low current density, mainly intended to minimize alteration of the sample surface. A polymer surface generates positive and negative ions that are analyzed using a mass analyzer. The results of detailed analysis provide chemical structure and composition information about the surface. A traditional shortcoming of SIMS is its inability to perform quantitative analysis. 

As mentioned in the Section 1, physico-chemical methodology for quantitative analysis of plant hormone focuses primarily on GC-SIM, although HPLC with selective fluorescence detection continues to be used for lAA analysis in some laboratories. Procedures, such as the 2-methylindolo-a-pyrone assay for lAA analysis [82], are now rarely utilised. With the exception of ethylene quantification [2] there is little use of non-MS-based GC detection techniques, despite the fact that selective analysis at the picogram level is achieved for ABA with an electron capture detector [83], and lAA and cytokinins with a nitrogen phosphorus detector [84,85]. The reason for the demise of these GC procedures is that the detectors are destructive and this precludes the reliable recovery of labelled internal standards for radioassay and isotopic dilution analysis. The usual compromise was to take two aliquots of the purified samples, one for GC analysis and the other for the determination of radioactivity. The accuracy of this approach is dependent upon the questionable assumption that the radioactivity in the purified sample is associated exclusively with the compound under study. In an attempt to circumvent this problem, a double standard isotope dilution procedure was devised for the quantitative analysis of lAA in which one internal standard was used to correct for losses during sample preparation and a second for GC quantification [86]. This procedure was used in several... 

MS detection may well prove to be the most informative of all detectors for HPLC as not only does it afford quantitative analysis, but SIM procedures offer high sensitivity and selectivity and in principle it is a universal detector and can be used for all analytes. The power and utility of this approach has been extended further by the development of LC-MS-MS systems and these and other of the hyphenated techniques are discussed in detail in Chapter 7. 

Most LC-MS sample introduction techniques, such as thermospray, electrospray, and APCI, do not employ an electron beam to induce ionization, and lead to the formation of predominantly molecular ion species. This leads to the formation of higher mass fragments that may be monitored by SIM or SRM. Such techniques are frequently used in biomedical applications, and find particular application in the quantitative analysis of labile or polar compounds such as biomolecules from complex biological matrices. 

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