Matrix effect in SIMS

With the use of the MCs+ technique to reduce matrix effects in SIMS, Gao et al measured the composition of InxGai.xN alloys and confirmed the validity of this technique with the RBS method [12], It was shown that the SIMS-MCs+ method can provide very accurate measurements of composition on complex structures and devices [13],... 

SIMS is an inherently nonquantitative technique -there is no direct relationship between peak intensity and the concentration of the species it represents. This is because secondary ion yields are strongly dependent on the chemical environment (or matrix), examples of which were noted in the previous section. Matrix effects in SIMS are most notable for yields of elemental species from inorganic matrices, but the situation is less severe for polymer matrices. Thus, relative intensities such as 0 /CH or F /CH correlate with X-ray photoelectron spectroscopy (XPS) atomic ratios (O/C, F/C) when trends in composition are restricted to similar materials however, the C1 CH ratio for a variety of chlorine-containing polymers does not correlate with the XPS Cl/C atomic ratio. 

Sdnchez-Almazdn, R, Napolitani, E., Camera, A., Drigo, A.V., Isella, G, von Kfinel, H., Berti, M. (2004) Matrix effects in SIMS depth profiles of SiGe relaxed buffer layers. Applied Surface Science, 231-232,704-707. 

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. 

Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

As a surface analytical tool, SIMS has several advantages over X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). SIMS is sensitive to all elements and isotopes in the periodic table, whereas XPS and AES cannot detect H and He. SIMS also has a lower detection limit of 10 5 atomic percent (at.S) compared to 0.1 at.S and 1.0 at.% for AES and XPS, respectively. However, SIMS has several disadvantages. Its elemental sensitivity varies over five orders of magnitude and differs for a given element in different sample matrices, i.e., SIMS shows a strong matrix effect. This matrix effect makes SIMS measurements difficult to quantify. Recent progress, however, has been made especially in the development of quantitative models for the analysis of semiconductors [3-5]. 

Another area of SIMS that has received a great deal of attention is the study of simple inorganic salts. Marien and De Pauw (92) have looked at the ion-beam-induced effects in SIMS spectra. It is clear that under high ion fluxes significant damage to the surface occurs, with the resulting complication of the spectrum. FAB, or liquid-matrix SIMS, is free of this effect but has the equivalent complication of reactions with the matrix, as discussed previously. 

Other techniques are based on sample sputtering followed by mass spectrometry of the vaporized products, including secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Beam sizes are in the few to tens of pm range. Elemental sensitivities for SIMS are highly variable depending on ion yield, and quantification can be difficult because of matrix effects in the ion production process. SIMS and LA-ICP-MS have very high sensitivities for some elements and low sensitivity for others. These and other microanalytical techniques used in earth science research are described in Potts et al. (1995). 

Eiler JM, Graham CM, Valley JW (1997) SIMS analysis of oxygen isotopes matrix effects in complex minerals and glasses. Chem Geol 138 221-244... 

The relative insensitivity of SALI to matrix effects in comparison to SIMS makes the method particularly suited to the characterization of surface coatings, e.g., inorganic overlayers of nitrides and oxides, intentionally or accidentally deposited organic layers on an inorganic surface. Continuing bombardment of the sample with an ion dose above... 

Ottolini, L., Cdmara, E, Hawthorne, FC., Stirling, J. (2002) SIMS matrix effects in the analysis of light elements in silicate minerals comparison with SREF and EMPA data. Am. Mineral, 87,1477-1485. 

As for their validation, such reference materials can contain species introduced either at the low concentrations amenable to the detection limits afforded by SIMS or at higher concentrations where they themselves can induce significant matrix effects. In addition, the use of thin films places further limits on the array of analytical techniques to those displaying high surface specificity. 

Detection limits of analytes measured in ESI and MALDI using the mass analyzers covered in this chapter are quite noteworthy. Table 9.3 lists detection limits for various molecules analyzed by LC, CE, and EIA coupled to ESI QIT, LQIT, and the orbitrap or with the use of MALDI. The lowest limits of detection are measured when an isolation step is used to accumulate and/or isolate only the ion(s) of interest. For example, in Table 9.3, the detection limits for reserpine in SIM and SRM scan modes are better than in the full-scan mode. Although ion trap full MS scans have a higher duty cycle over QMF ion beam instruments, detection limits can be reduced in the full-scan MS mode due to matrix effects. In this latter case, at low analyte amounts, the QIT is filled up primarily with unwanted matrix ions, leaving little space to store the ions of interest. The advantage of the high-charge-capacity... 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

The ionization probabilities It vary over some five decades across the elements in the periodic table. In addition, they vary also with the chemical environment of the element. This effect, usually referred to as the matrix effect, makes quantitation of SIMS spectra extremely difficult. As illustrated in Table 4.1, positive secondary ion yields from metal oxides are typically two orders of magnitude higher than those of the corresponding metals. A similar increase in yields from metals is observed after adsorption of gases such as oxygen or carbon monoxide. 

Matrix effects and inhomogeneous sample charging seriously hinder quantitative analysis of SIMS on technical catalysts. Although full quantitation is almost impossible in this area, the interpretation of SIMS data on a more qualitative base nevertheless offers unique possibilities. Molecular cluster ions may be particularly informative about compounds present in a catalyst. 

In SNMS, sputtered neutrals are post-ionized before they enter the mass spectrometer. In contrast to SIMS, SNMS does not suffer from the matrix effects associated with the ionization probability of sputtered particles. Here, the sensitivity for a cer-... 

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... 

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