Dynamic range, SIMS

The LIMS technique is rarely used for quantitative elemental analysis, since other techniques such as EPMA, AES or SIMS are usually more accurate. The limitations of LIMS in this respect can be ascribed to the lack of a generally valid model to describe ion production from solids under very brief laser irradiation. Dynamic range limitations in the LIMS detection systems are also present, and will be discussed below. 

Figure 3.17 depicts an ultra-shallow TOF SIMS depth profile of a 100-eV B-implant in Si, capped with 17.3 nm Si. The measurement was performed with 600-eV SF5-sputtering and with 02-flooding. The original wafer surface, into which the B was implanted, is indicated by the maxima of the alkali- and C-signals. Because of these contaminants, a minimum is observed in the °Si-signal. The dynamic range of the B-profile is more than 3.5 decades and the depth resolution is <0.5 nm. 

In another review, Magee and Honig [24] discuss three important aspects of depth profiling by SIMS depth resolution, dynamic range and sensitivity. First, the depth resolution is a measure of the profile quality. They point out that the depth resolution is limited by atomic mixing effects and the flatness of the sputtered crater within the analyzed area. Second, the dynamic range of depth profiles is limited by crater edge... 

Other attributes of SIMS Include a wide dynamic range good depth resolution the ability to detect all elements the ability to differentiate between different Isotopes of the same element good elemental specificity and exceptionally good sensitivity for the low atomic number elements. An excellent discussion of the strengths and weaknesses of SIMS has been provided by Magee (Ji). Comparisons between SIMS and other surface analysis techniques have been published (Ifi). 

The sensitivity and the dynamic range of this type of sensor are related to the modal stmcture of the fiber, and to the behavior of the materials surrounding it. Therefore, the developed SIM technique can provide a compatible methodology suitable for sensors applications. Experimental and theoretical feasibility studies show that the developed sensing technique is sensitive, inexpensive, and can be manufactured in microstmcture components. A simple example on using the SIM technique for chemical sensors application is presented in the next section. 

Difficulties of SIMS are the complexity and large dynamic range of the ion beams produced. This may complicate the identification of the positive ion spectrum and cause sometimes an insufficient reproducibility of the results. Important advantages. 

High mass resolution is necessary to avoid peak interferences in SIMS. However, relative intensity is strongly reduced with the increasing mass resolution due to narrower entrance slits in the mass spectrometer. Lateral resolution of 0.3 pm or less is possible but the sensitivity is reduced in proportion to the resolution. The excellent mass resolution of less than 1 ppm and wide dynamic range is available for many elements. 

Secondary ion mass spectrometry (SIMS) is to measure the secondary ions, ionized clusters, atoms and atomic clusters, which are emitted from the surface of particles, when it is bombarded with a primary beam of ions, such as He", Ne", or Ar", with energies in the range of hundreds of eV to keV scale. The emitted ions and ionized clusters are analyzed directly by using a mass spectrometer. Therefore, chemical composition of the surface can be analyzed with the obtained accordingly. SIMS has two modes of analysis (i) static and (ii) dynamic. Static SIMS uses an ion beam with low current density, so as to confine the analysis to the outermost layers. Dynamic SIMS uses beams of high current density, so that successive atomic layers can be eroded at a relatively high rate. Comparatively, the analytical conditions of dynamic SIMS are less suitable for surface analysis. 

The amount of internal standard to add to a biological sample has been a matter of some debate. Many workers have added a large excess in the hopes that this would act as a carrier. However, isotopic impurities can in such cases cause considerable "crosstalk" into the SIM channel of the biological sample. A more ideal approach is to add the identical amount of internal standard as is present in the biological sample under normal conditions. With this procedure, decreases and increases in the endogenous ammo acid level can be accurately monitored. An additional feature of this approach is that a full standard curve need not always be constructed since with deuterated internal standards a wide dynamic range in such curves is observed. Indeed, mole ratios (endogenous/internal standard) are linear up to 100 and down to 0.1 (Colby and McCamen, 1979). 

ITMS can also be used for pantothenic acid analysis in multivitamin dietary supplements. Although the quality of the quantitative analysis using ITMS is highly dependent on the analytes, pantothenic acid displays good linearity and dynamic range when the LCQ series (Thermo Fisher Scientific Inc. Waltham, MA, USA) ITMS is used (unpublished data). Both the SIM mode and SRM mode (SRM transition 220- 90) can be used. 

A major application of the SIMS technique is depth profiling. As illustrated with the ion implant standards, secondary ion counts may be monitored as a primary beam erodes the sample resulting in a depth profile of one or several ions. This section will discuss some of issues associated with depth profiles and the analytical choices available. Three terms of interest that will be discussed are dynamic range, depth resolution, and detection limit. The dynamic range is the ratio of the maximum secondary ion counts to the minimum secondary ion counts and is often measured on an ion implantation profile. The dynamic range can be several orders of magnitude. The depth resolution is the depth across which a secondary ion signal drops from a defined upper limit to a defined lower limit or rises from a defined lower limit to a defined upper limit. The detection limit is the minimum concentration that can be detected for the element of interest and will vary with the element and the matrix. 

Generally, in LC-MS analysis, a particular pair or pairs of precursor-/product-ion transitions are monitored at a specified elution time. Of course, these transitions at such an elution time should be predetermined utilizing authentic compounds or close analogs. Alternatively, a data-dependent acquisition approach could be set up with a certain type of instruments [60]. In either case, some degree of preknowledge about the individual lipid species present in the samples is required since currently available instruments are still unable to perform an infinite number of transitions at an elusion time due to the limitation of instmmental duty cycle and/or sensitivity. Moreover, similar to the SIM method, the linear dynamic range, limit of detection, and calibration curves of the species of interest should generally be predetermined for quantitative analysis of lipid species. Thus, the constmcted ion peak area of each species can be compared to a standard curve of the species under identical experimental conditions. 

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