Isotopes elements

Though individual atoms always have an integer number of amus, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. Isotopes can have a weight either more or less than the average. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass. 

Elemental isotopic compositions (isotope ratios) can be used mass spectrometrically in a routine sense to monitor a substance for the presence of different kinds of elements, as with chlorine or platinum. It can also be used in a precise sense to examine tiny variations in these ratios, from which important deductions can be made in a wide variety of disciplines. 

Inductively coupled plasmas are used to obtain the ions needed to measure either relative concentrations (amounts) of the various elements in a sample or to obtain accurate elemental isotope ratios. 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

Qualitatively, the spark source mass spectrum is relatively simple and easy to interpret. Most instrumentation has been designed to operate with a mass resolution Al/dM of about 1500. For example, at mass M= 60 a difference of 0.04 amu can be resolved. This is sufficient for the separation of most hydrocarbons from metals of the same nominal mass and for precise mass determinations to identify most species. Each exposure, as described earlier and shown in Figure 2, covers the mass range from Be to U, with the elemental isotopic patterns clearly resolved for positive identification. 

Detection limits in ICPMS depend on several factors. Dilution of the sample has a lai e effect. The amount of sample that may be in solution is governed by suppression effects and tolerable levels of dissolved solids. The response curve of the mass spectrometer has a large effect. A typical response curve for an ICPMS instrument shows much greater sensitivity for elements in the middle of the mass range (around 120 amu). Isotopic distribution is an important factor. Elements with more abundant isotopes at useful masses for analysis show lower detection limits. Other factors that affect detection limits include interference (i.e., ambiguity in identification that arises because an elemental isotope has the same mass as a compound molecules that may be present in the system) and ionization potentials. Elements that are not efficiently ionized, such as arsenic, suffer from poorer detection limits. 

State-of-the-art TOF-SIMS instruments feature surface sensitivities well below one ppm of a mono layer, mass resolutions well above 10,000, mass accuracies in the ppm range, and lateral and depth resolutions below 100 nm and 1 nm, respectively. They can be applied to a wide variety of materials, all kinds of sample geometries, and to both conductors and insulators without requiring any sample preparation or pretreatment. TOF-SIMS combines high lateral and depth resolution with the extreme sensitivity and variety of information supplied by mass spectrometry (all elements, isotopes, molecules). This combination makes TOF-SIMS a unique technique for surface and thin film analysis, supplying information which is inaccessible by any other surface analytical technique, for example EDX, AES, or XPS. 

SIMS Ions 20 nm-1 jim depending on ion source Ppm for most elements Ppb for most favourable All elements isotopes distinguishable Yes Not in general, but internal standards usable in some cases. 

Q Categorial variable characterizing chemical entities (species) being investigated, e.g., elements, isotopes, ions, compounds ... 

When we look at a mass spectrum we can observe that a single peak is never isolated but there is one or more small peaks accompanying it to its right (Figure 2.16). These peaks are due to the isotopes of the elements. Isotopes are atoms of the same element that have the same number of protons and electrons but a different number of neutrons. The total number of protons and neutrons constitutes the mass number and it is indicated as a superscript preceding the atomic symbol, i.e. 1H, 12C, 23Na. 

Element Isotope Fractional abundance 1 %o change International standard... 

Light element (isotope ratio) mass spectrometry 14, c — 14, 19 24... 

Element Isotope Isotope Type Abundance to All the Isotopes (%) Abundance to the Main Isotope (%) Element Type... 

Element Isotope Relative Abundance (%) Mass (Da) Isotope Relative Abundance (%) Mass (Da)... 

Elemental uranium, crystal structures of, 25 409. See also Uranium (U) Elementary reaction, 21 336 Element/isotope ratios, in fine art examination/conservation, 11 419 Element mapping, in fine art examination/ conservation, 11 406 Element names/symbols, 17 386-387 transfermium, 17 387t Elements, in earth s crust, 26 23 a-Eleostearic acid, physical properties, 5 33t... 

Oebelmann, J., Juchelka, D., Hilkert, A., Avak, H., Douthitt Ch. Authenticity control by multiple element isotope ratio determination. Unpublished Materials. Thermo Finnigan, 2001. 

Element Isotope ratio1 Natural isotopic abundance (%) Substances studied Application... 

Various analytical applications of different element isotopes are discussed in the following. 

Element Isotope Natural abundance (%) or half-life Standard isotope ratio or energy... 

Deines P (2003) A note on intra-elemental isotope effects and the interpretation of non-mass-dependent isotope variations. Chem Geol 199 179-182... 

Element Isotope Available quantity Price ( /g) Limited handling quantity Hazardous radiation... 

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