Mono-isotopic elements

Figure 1. Number of stable isotopes relative to atomic number (Z) for the elements. Mono-isotopic elements shown in gray diamonds. Elements discussed in this volume are shown as large gray circles. Other elements that have been the major focus of prior isotopic studies are shown in small white circles, and include H, C, O, and S. Nuclides that are radioactive but have very long half-lives are also shown in the diagram.

Isotopes of a chemical element are nuclides with the same number of protons (Z) but a different number of neutrons N) in the atomic nucleus. Isotopes of a chemical element (e.g., H and of hydrogen Cl and Cl of chlorine or Fe, Fe, Fe and Fe of iron, respectively) have the same number of protons (Z) and possess the same chemical properties, but differ in the number of neutrons (N) and thus in the mass number (A). With increasing Z, the number of neutrons in a stable atomic nucleus is higher than the number of protons. For mono-isotopic elements. 

Confidence in elemental assignment can be assured by comparison of the intensities of signals that can be ascribed to other isotopes of the element of interest, i.e. these intensity ratios should match the natural abundances isotopes listed in Appendix A.2, unless, of course, isotopic enrichment of one form or another has occurred (as is the case in nuclear reactions, extra-terrestrial samples, and so on) or has been applied (isotopic implantation). In the case of mono-isotopic elements such as Fluorine and Aluminum, molecular patterns, or even multiply charged ions, can be used to confirm assigmnents. 

This application has already been described in the previous section on ICP-AES. Relative to ICP-AES, ICP-MS yields simpler spectra and better detection limits for rare earths. Representative detection limits typical of the performance of most devices under multi-element operating conditions are given in table 4. The detection limits follow the trend of isotopic abundances, i.e., detection limits are best for the mono-isotopic elements and poorest for Gd and Nd where less abundant isotopes are monitored. 

Multi-element IDMS is available when using ICP-MS. For transient signals, however, the multi-element capability is reduced as two isotopes have to be measured for each element. In principle all elements of the periodic system having two natural isotopes can be analysed by IDMS as long as mass spectrometric methods for isotope ratio determinations are available. Moreover even mono-isotopic elements can be determined, if they also have a long-lived radionuclide isotope such as iodine-129. 

Because the beam monitor allows accurate measurement of the total number of ions that are analyzed, a graded series of exposures (i.e., with varying numbers of ions impinging on the plate) is collected, resulting in the detection of a wide range of concentrations, from matrix elements to trace levels of impurities. In Figure 2, the values of the individual exposures have been replaced with the concentration range that can be expected for a mono-isotopic species just visible on that exposure. In this example, exposures from a known Pt sample have been added to determine the response curve of the emulsion. 

Of more relevance, in the present context, HW reasoned that large deviations from the whole number rule among the light elements indicates the existence of isotopes. They, thus, predicted that Mg, Si, and Cl had isotopes (were not mono-isotopic). Harkins decided to show that the element chlorine which had the atomic weight deviating the most from integer value among the 27 low atomic number... 

If the isotopic distribution is very broad and/or there are elements encountered that have a lowest mass isotope of very low abundance, recognition of the mono-isotopic peak would become rather uncertain. However, there are ways to cope with that situation. 

Since stable isotope dilution depends on the mass spectrometric determination of isotope ratios, the method is generally restricted to elements which have at least two stable isotopes. However, in the case of mono isotopic cesium, the long-lived fission product Cs136 or Cs137 can be used as a tracer for the analysis of this element. In all, some 68 elements can be... 

What are the differences between nominal mass, exact mass, accurate mass, mono-isotopic mass, molecular mass, and isobaric mass The nominal mass of a compound, ion, or fragment is calculated using the masses of the elements rounded to the nearest whole number (e.g., C = 12, H = 1, O = 16). For example, cholesterol with an empirical formula C27H4gO has a nominal molecular mass of 386 Da. It is common to see molecular mass referred to as molecular weight (MW). 

Mono-isotopic mass is the calculated mass of a compound based on the accurate masses of the most common stable isotopes of the elanents present. For the elements commonly found in organic molecules the mono-isotopic mass is composed of the isotopes of lowest mass, e.g., 12.0000 for carbon and 15.9949 for oxygen. The mono-isotopic mass is usually the most relevant measure of mass, because of the ability of instruments to observe the masses of the individual isotopes. 

Molecular mass is the average mass of a compound obtained when an accounting is made for all isotopes of the elements present based on their relative abundances, e.g., C = 12.0108 Da, O = 15.9994 Da. If the instrument used cannot resolve the individual isotopes, the observed peaks include all isotopes present. Molecular mass is sometimes referred to as average mass. For cholesterol the molecular (average) mass is 386.6616 Da. (Note the difference between this number and the mono-isotopic exact mass value, 386.3549 Da, as described above.) Molecular mass does have a dimension as it is an absolute value unit, based on 1/12 of the mass of the isotope (in lUPAC units), i.e., 1.6605 x 10- kg. If, however, the mass of an analyte is considered as a ratio with respect to the mass of C, then the dimensions cancel out and the resulting dimensionless number is the relative molecular mass. These two terms are equivalent in everyday usage. 

The possibility that a particular element with a value of Z may have varying values of A - Z, the number of neutrons, gives rise to the phenomenon of isotopy. Atoms having the same value of Z but different values of A-Z (i.e. the number of neutrons) are isotopes of that atom. Isotopes of any particular element have exactly the same chemical properties, but their physical properties vary slightly because they are dependent upon the atomic mass. A minority of elements, such as the fluorine atom, are mono-isotopic in that their nuclei are unique. Examples of isotopy are shown in Table 1.2. 

The exact mass of the most abundant isotope of an element is termed monoisotopic mass [6]. The monoisotopic mass of a molecule is the sum of the monoisotopic masses of the elements in its empirical formula. As mentioned before, the mono- isotopic mass is not necessarily the naturally occurring isotope of lowest mass. However, for the common elements in organic mass spectrometry the monoisotopic mass is obtained using the mass of the lowest mass isotope of that element, because this is also the most abundant isotope of the respective element (Chap. 3.1.5.1). 

Let us briefly repeat i) the isotopic mass is also the exact mass of an isotope it) the isotopic mass is very close but not equal to the nominal mass of that isotope Hi) accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass iv) due to the definition of our mass scale, the isotope represents the only exception from non-integer isotopic mass. As a consequence of these individual non-integer isotopic masses, almost no combination of elements in a molecular or ionic formula has the same calculated exact mass, or simply exact mass as it is often referred to, as any other one [36]. 

It has been pointed out that routine accurate mass measurements are conducted at resolutions which are normally too low to separate isobaric isotopologs. Unfortunately, multiple isotopic compositions under the same signal tend to distort the peak shape (Fig. 3.31) [64], This effect causes problems when elemental compositions have to be determined from such multi-isotopolog peaks, e.g., if the mono-isotopic peak is too weak as in case of many transition metals. Generally, the observed decrease in mass accuracy is not dramatic and is somewhat counterbalanced by the information derived from the isotopic pattern. However, it can be observed that mass accuracy decreases by about 50% on such unresolved signals. 

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. 

In spite of the excellent capability and advantages (high selectivity and sensitivity) of RIMS for the ultratrace analysis of isotopes with naturally rare abundance in environmental, geological, medical and nuclear samples, no commercial instrumentation is available to date. In contrast to AMS and RIMS as mono-elemental (element-specific) analytical techniques, ICP-MS and LA-ICP-MS possess, in analogy to GDMS and SIMS, have the ability for multi-element analysis and thus could have the widest fields of application. 

S. Gas source mass spectrometry (GSMS) with electron impact (El) ion source produces nearly mono-energetic ions (similar to TIMS) and is an excellent tool for the high precision isotope analysis of light elements such as H, C, N and O, but also for S or Si.7,100,101 Precise and accurate measurements of isotope ratios have been carried out by gas source mass spectrometers with multiple ion collectors by a sample/standard comparison and the 8 values of isotope ratios were determined (see Equation 8.4). Electron impact ionization combined with mass spectrometry has been applied for elements which readily form gaseous compounds (e.g., C02 or S02) for the isotope analysis of carbon and sulfur, respectively). 

The differences in sorptive behavior of Th, Pu, U, and Np are evident by examining Table II. Plutonium and thorium isotopes at tracer concentrations (parts per billion, element mass/clay mass) were equilibrated for 24 hours with the < 2-pm fraction (clay) of a silt loam soil. The pH of the equilibration solutions was 6.5 and the aqueous phase contained Ca at a concentration of 5 mM. Both tetravalent actinides failed to remain 1n solution. Whether this is a direct function of sorption mechanisms or simply related to the solubility of the ions in solution is not distinguished by the results. Uranyl ion was not removed to the same extent as the tetravalent species. Neptunium(V) sorbed very poorly. It should be noted that while Np(V) is a mono-charged cation, Np02+ does not sorb like Na+. 

Figure 16.2 A 180 ° magnetic deflection spectrograph with a velocity filter. The filter eliminates the problem due to the quasi impossibility of having homogeneous and mono-kinetic beams. The diagram represents a sort of photographic recording of the spectrum of neon. The two series of broken lines represent the removal of one or two of the electrons from the different isotopes of elemental neon. The symbol - - signifies that the ion is both a radical (i.e. has an unpaired number of electrons) and a cation. J.J. Thomson, who around 1913 proved the existence of the isotopes of neon, is considered as the father of this kind of assembly. Later, work performed by F.W. Aston showed the existence of the isotopes of sulphur and chlorine.

There are two stable isotopes 203-TI (29.52% nat. occurrence) and 205-TI (70.48% nat. occurrence). Thallium can occur in the zero-, mono- and trivalent state. The element is easily oxidized and of no pronounced technical use. 

Speciation Analysis Simultaneous Determination of Multiple Species of the Same Element At the beginning of the development of species-specific isotope dilution, only a single spike compound was used [77]. Later, species-spedfic IGP-IDMS has also been used for the determination of more than one spedes of the same element, using different spike spedes labeled with the same isotope. Examples include the determination of mono- and dimethylmercury (MMM and DMM) and of the three butyltin compounds TBT, DBT, and MBT [80-83]. Various spike isotopes, usually of medium natural abundance, such as Hg or ° Hg and Sn, Sn, or Sn, have been applied. The reason for selecting this type of spike... 

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