Isotopes common, masses

A common mistake for beginners in mass spectrometry is to confuse average atomic mass and isotopic mass. For example, the average atomic mass for chlorine is close to 35.45, but this average is of the numbers and masses of Cl and Cl isotopes. This average must be used for instruments that cannot differentiate isotopes (for example, gravimetric balances). Mass spectrometers do differentiate isotopes by mass, so it is important in mass spectrometry that isotopic masses be used... 

Tritium [15086-10-9] the name given to the hydrogen isotope of mass 3, has symbol or more commonly T. Its isotopic mass is 3.0160497 (1). Moletecular tritium [10028-17-8], is analogous to the other hydrogen isotopes. The tritium nucleus is energetically unstable and decays radioactively by the emission of a low-energy P particle. The half-life is relatively short (- 12 yr), and therefore tritium occurs in nature only in equiUbrium with amounts produced by cosmic rays or man-made nuclear devices. 

Separation and detection methods The common methods used to separate the Cr(III)/(VI) species are solvent extraction, chromatography and coprecipitation. In case of Cr(VI) from welding fumes trapped on a filter, a suitable leaching of the Cr(VI) from the sample matrix is needed, without reducing the Cr(VI) species. The most used detection methods for chromium are graphite furnace AAS, chemiluminescence, electrochemical methods, ICP-MS, thermal ionization isotope dilution mass spectrometry and spectrophotometry (Vercoutere and Cornelis 1995)- The separation of the two species is the most delicate part of the procedure. 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

While competitive methods to determine KIE s are free from errors due to differences in reaction conditions (impurities, temperature, pH, etc.) they do require access to equipment that allows high precision measurements of isotope ratios. The selection of an appropriate analytical technique depends on the type of the isotope and its location in the molecule. For studies with stable isotopes the most commonly used technique (and usually the most appropriate) is isotope ratio mass spectrometry (IRMS). 

The most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by ... 

Accurate Masses of Selected Isotopes Common Fragments and their Masses... 

PROBLEM 2.5 Chlorine, one of the elements in common table salt (sodium chloride), has two main isotopes, with mass numbers 35 and 37. Look up the atomic number of chlorine, tell how many neutrons each isotope contains, and give the standard symbol for each. 

There are two main types of internal standards. The first ones are stable isotope labeled (SIL) internal standards. They are compounds in which several atoms in the analytes are replaced by their respective stable isotopes, such as deuterium (2H, D or d), 13C, 15N, or 170. Labeling with the first three isotopes are most common, particularly labeling with deuterium (due to less difficulty in synthesis and therefore less expensive). For examples, raloxifene-d4-6-glucuronide was used as the internal standard for the determination of raloxifene-6-glucuronide [5] and 1, 2, 3, 4-13C4 estrone (PCJEl) was used as the internal standard for estrone (El) [6], The usage of stable isotope labeled internal standards in quantitative LC-MS or GC-MS analysis is often termed as isotope dilution mass spectrometry (IDMS) [7],... 

Targeted analysis refers to metabolome analysis that targets one, or a few metabolites, and typically uses an internal standard for quantitation. The most common method is isotope dilution mass spectrometry (IDMS) [34], which relies on the use of stable isotope internal standards to enable the absolute quantitation of metabolites. This method has proven highly effective and has been successfully used in numerous studies. 

Ordinary hydrogen consists of a mixture of three isotopes. The isotope with mass number 1, H, is about 6400 times as abundant as deuterium, D, the isotope with mass number 2. Tritium, T, the isotope with mass number 3, is many orders of magnitude less abundant than deuterium. Although six different diatomic molecules are possible from these three isotopes, H2, D2, HD, and T2 have been more thoroughly studied. Of these, the two most common forms are H2 and D2. Some of the properties of these two forms of hydrogen are listed in Table 6.1. 

The atomic masses used in common chemical calculations are based on averages resulting from mixtures of isotopes. In mass spectrometry, the calculation is based on the mass of the predominant isotope of each element. As the isotopes are separated in the spectrometer, we always face several peaks with different masses, and with intensity ratios defined as described earlier. Thus, for example, dichloromethane has a classical molecular mass equal to 12.01 + 2 x 1.00 + 2 x 35.45 = 84.91 Da. The molecular mass in mass spectrometry is (if mass defects are neglected) 12 + 2 + 2 x 35 = 84 u. Several isotopic peaks are observed in the spectrum, the second most important being observed at m/z 86 with an intensity equal to 64.8 % of that of the m/z 84 peak. 

The problem of excess substrate addition in tracer experiments has been largely overcome by the advent of more sensitive mass spectrometers, however, and estimates obtained under conditions approaching in situ are possible. Owing to the great sensitivity of isotope ratio mass spectrometry, much shorter incubations (compared to the inhibitor and inventory methods) are possible (a few hours to 24 h are commonly used). Although they have not yet been widely applied for this purpose, the recently introduced sensitive isotope methods for determination of content ofN02 and N03 (Mcllvin and Altabet, 2005 Sigman et al., 2001) should make true tracer level incubations possible. Details on the most commonly used isotope tracer methods for... 

The most common isotope of oxygen has a mass number of 16, but two other isotopes having mass numbers of 17 and 18 are also known. For each isotope, give the following information (a) the number of protons (b) the number of neutrons (c) the number of electrons in the neutral atom and (d) the group number. 

Stable carbon isotope compositions of organic substances are measured via isotope ratio mass spectrometry. Samples are combusted to convert organic matter into gases (e.g. CO2, N2) that are then ionized and admitted into the isotope ratio-mass spectrometer (IR-MS) where the ratios of the isotopes are quantified (see Chapter 5 for further information). Owing to the low abrmdances of some of the isotopes, the precision and accuracy of the measurements is important (Merritt and Hayes, 1994). One of the most common and informative isotopes is because it is not readily altered by equilibration or degradation processes. Hence, values are most commonly employed as indicators of ultimate plant or geographic sources. In... 

Very low impurity contents have been detected by the measurements of impurity sensitive properties like residual resistivity. Examples of impurity contents in less-common actinide metals are published for Pa (7), Am (8) and Cm (9). Isotope dilution mass spectrometry is expected to be increasingly applied to the accurate determination of selected elements, or to standardisation of routine methods or reference samples. 

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