Monoisotopic elements

The chemistry of rare earth elements makes them particularly useful in studies of marine geochemistry [637]. But the determination of rare earths in seawater at ultratrace levels has always been a difficult task. Of the various methods applied, instrumental neutron activation analysis and isotope dilution mass spectrometry were the main techniques used for the determination of rare earths in seawater. However, sample preparation is tedious and large amounts of water are required in neutron activation analysis. In addition, the method can only offer relatively low sample throughputs and some rare earths cannot be determined. The main drawbacks of isotopic dilution mass spectrometry are that it is time-consuming and expensive, and monoisotopic elements cannot be determined as well. 

The exact mass of an ion (4 to 6 decimal points) reliably defines its elemental and isotopic composition, while the method is called high resolution mass spectrometry. The measurements are conducted manually or automatically (computerized). Manual measurements are based on the parallel acquisition of the peak of interest with the closest peak of an ion with the known composition. Any compound with an intense ion peak with m/z value in the region +10% may serve as a marker. The most widespread markers are perfluorokerosene, perfluorotributylamine, and other polyfluorinated compounds. The use of these compounds is based on their volatility, as well as on the fact that fluorine is a monoisotopic element. In the spectra of these compounds intense ion peaks randomly cover all the range between m/z 19 and M+. ... 

When the nature and number of A + 2 and A + 1 elements in the molecule is known it is easy to derive conclusions about monoisotopic elements. As the masses of these elements are rather unique, it is not a difficult task. 

The primary and immediate need is for a trace metal reference material, but a certified reference material would provide even greater benefits. A technique based on isotope dilution with detection by inductively-coupled plasma mass spectrometry (ICP-MS) (Wu and Boyle, 1998) most clearly meets the traceability criteria required for a certified reference material. Although useful for iron and several other metals, isotope dilution is not possible for monoisotopic elements like cobalt, so other techniques must also be used. Indeed, it is advisable that several techniques be used to certify a trace metal reference material. 

Some elements do exist in only one naturally occurring stable isotope, and therefore, they are termed monoisotopic elements. Among these, fluorine ( ), sodium ( Na), phosphorus ( P) and iodine belong to the more prominent examples in organic mass spectrometry, but there are several more such as beryllium ( Be), aluminum ( Al), scandium (" Sc), manganese ( Mn), cobalt ( Co), arsenic... 

Au) and some heavier transition metals, too. The monoisotopic elements are also referred to as A or X elements (see below). [2,3]... 

Even if the analyte is chemically perfectly pure it represents a mixture of different isotopic compositions, provided it is not composed of monoisotopic elements only. Therefore, a mass spectrum is normally composed of superimpositions of the mass spectra of all isotopic species involved. [11] The isotopic distribution or isotopic pattern of molecules containing one chlorine or bromine atom is listed in Table 3.1. But what about molecules containing two or more di-isotopic or even polyisotopic elements While it may seem, at the first glance, to complicate the interpretation of mass spectra, isotopic patterns are in fact an ideal source of analytical information. 

The polynomial approach is the logical expansion of the binomial approach. It is useful for the calculation of isotopic distributions of polyisotopic elements or for formulas composed of several non-monoisotopic elements. [2,14] In general, the isotopic distribution of a molecule can be described by a product of polynominals... 

Precise and accurate measurements of ultralow Nb, , Zr and Hf concentrations and the chondritic Zr/Hf and Nb/Ta ratios by MC-ICP-MS (Isoprobe, Micromass) were examined by Weyer et al.64 For the development of the isotope dilution technique, enriched 180Ta, 94Zr and 180Hf isotopes were applied for quantitative Zr, and Hf determination, and the monoisotopic element Nb was measured relative to Zr after quantitative separation from the matrix by ion exchange. 

Once the numbers of these two groups of elements have been estimated, the remainder of the mass of the ion must be due to the monoisotopic elements, the numbers of which can then usually be deduced. 

Different approaches have been used to assign the signals in the measured mass spectrum to particular elements and polyatomic ions. The TotalQuant approach by PE-Sciex uses a combination of equations and heuristics ( rules of thumb ) [207]. For example, constraints are included on the relative detected oxide ion (such as LaO+) to elemental ion (such as La+) signals. The LaO+ signal is assumed to be no more than 2% of the La+ signal. Instrument response values for each isotope of each elemental ion are stored in the computer, as are spectra of potential interferents. The steps for the semiquantitative determination are shown in Table 3.5. The approach takes advantage of the isotopic pattern of masses for elemental, polyatomic, and doubly charged ions. Monoisotopic element... 

Assignments are made for monoisotopic elements and associated polyatomic and doubly charged ions. 

There are three potential limitations of the MCA approach. If the signals at some masses (such as mfz = 56) are over range because of large signals (ArO+, Fe+, and/or CaO+), it is more difficult to assign their element intensities at other masses. Monoisotopic elements have more uncertainty because there is no isotopic pattern to assess the model fit. If there are more species than masses over a region of the spectrum, there is not a unique solution. For example, Cr+ may require correction for Ti+, V+, Fe+, and ArO+. 

The success of ratioing techniques is entirely dependent upon the proper choice of an internal standard or ratio reference. Certainly, another isotope of the same element under study is the best choice indeed, it has been shown that lead signals temporally varying with 17% RSD can be radioed to yield 1.9% RSD, with this limitation imposed by the data acquisition system. In some cases, such as the monoisotopic elements or expense considerations, isotopic dilution may not be attractive and thus alternative elements must be utilized for ratioing. Here, it was found that improvements in precision were greatly dependent upon the choice of internal standard, with best results obtained for elements of close mass and ionization energy. 

Figure 3. Schematics of the depth profiling approach for a sample of a homogeneous mixture of two monoisotopic elements A and B.

From Condition 2, iodine is a monoisotopic element giving a very sharp line at 2062.4 A. (15). (Using the 1849 A. line of normal Hg in Hg sensitized CO produced C3O2 but no enrichment of the C isotope.)... 

The investigation of isotopic separations in systems with cyclic polyethers has been carried out up to now for the elements lithium, calcium and sodium, in particular. Among these elements, the enrichment of Li is of essential importance for the production of tritium and that of the heavy calcium isotopes for medical labeling experiments. An enrichment aspect does not exist for the monoisotopic element sodium. Investigations with the radioactive nuclides Na and Na are obviously of interest for fundamental investigations because these isotopes can be easily and precisely measured by their y-activity. Except for uranium, most of the investigations on other chemical exchange systems with metal ions are also based on measurements with lithium and calcium, respectively. 

Phosphorus ( P,/= 1/2) another common monoisotopic element, has been studied since the beginnings of NMR. It has a great sensitivity and it is an important element in the composition of numerous biological compounds. 

IDMS is best illustrated by its most simple case the determination of an unknown number N% of atoms of a monoisotopic element, say 1. When approximately a same and known number Ny of atoms of a sufficiently stable isotope 1 of that element is added to this sample, a homogeneous mixture or blend" of both isotopes can be made and the isotope ratio R = 27 129 measured in that blend as Rb (see Fig. la). Then ... 

Further extension of the possibilities of the method is the introduction of isotope dilution to MS. It is applicable to elements with at least two stable isotopes or to monoisotopic elements having a long lived radioactive isotope (e.g. Th). IDA-ICP-MS has been applied to analysis of various standard reference materials sediments, waters etc. (Barnes, 1996). Analytical precision of 2% has been reached and even down to 1 % in the case of flow injection IDA-ICP-MS and determination of Cu, Cd and Pb in river water. 

Quantitative determination of the monoisotopic element cesium can only be performed by using a radioactive isotope or a different alkali element as an internal standard or by establishing a calibration curve as an external standard. A plot of such a curve is given in Fig. 13 for [Cs]" . For measurements, a number of standard solutions of CsCl in CCI4 are prepared and analyzed. The resultant values are displayed with an error of 22% corresponding to 2a (a = standard deviation). 

There is no contribution to isotope peak intensities by these elements. Therefore, weak isotope peak intensities for relatively high-mass ions are evidence for the presence of monoisotopic elements. 

In simple molecules containing only carbon, hydrogen, nitrogen, oxygen, and the monoisotopic elements, elemental compositions of ions may be calculated using isotope peak intensities. As a hrst approximation, one atom in an ion contributes an amount to the intensity of the isotope peaks that is equal to the relative abundances of the isotopes of that atom. When more than one atom is present, the intensity contribution is multiplied by the number of each atom present. 

Isotope dilution mass spectrometry (ID-MS) is widely accepted as a quantification procedure of proven accuracy in elemental analysis and isotope ratio measurements [4]. Several areas of research in nuclear science, geochronology, medicinal chemistry, environmental science, and agricultural science have benefited from this technique. ID-MS is applicable to all elements that have at least two stable isotopes. Monoisotopic elements can be analyzed only if they have a long-lived natural or artificial radioisotope. For example, iodine and thorium have been determined with spikes of the long-lived isotopes 29i and 25 Th, respectively [44]. TI-MS and ICP-MS are the methods of choice for accurate ID-MS analysis. ICP-MS has the advantage that several elements can be analyzed simnltaneously under the same experimental conditions. Other ionization techniqnes discussed in this chapter have also been coupled with ID-MS. 

Detailed inspection of ranking lists reveals that the top positions are usually occupied by molecular formulas containing several elements. This is because the number of possible fragment formulas are higher for these candidates than for those made of few elements. In particular, molecular formulas made of several monoisotopic elements... 

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