Mass spectrometers isotope abundances

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

Before measurement it must be decided exactly which isotopes are to be compared. For oxygen, it is usually the ratio of 0 to 0, and for hydrogen it is H to H. Such isotope ratios are measured by the mass spectrometer. For example, examination of a sample of a carbonaceous compound provides abundances of ions at two m/z values, one related to C and one to C (it could be at m/z 45 and COj at m/z 44). By convention, the heavier isotope is always compared with the lighter isotope. The ratio of isotopes is given the symbol R (Figure 48.1). 

For example, if a carbonaceous sample (S) is examined mass spectrometrically, the ratio of abundances for the carbon isotopes C, in the sample is Rg. This ratio by itself is of little significance and needs to be related to a reference standard of some sort. The same isotope ratio measured for a reference sample is then R. The reference ratio also serves to check the performance of the mass spectrometer. If two ratios are measured, it is natural to assess them against each other as, for example, the sample versus the reference material. This assessment is defined by another ratio, a (the fractionation factor Figure 48.2). 

Atoms of elements are composed of isotopes. The ratio of natural abundance of the isotopes is characteristic of an element and is important in analysis. A mass spectrometer is normally the best general instrument for measuring isotope ratios. 

Special instruments (isotope ratio mass spectrometers) are needed to measure the required very accurate, precise ratios of abundances. 

In its simplest form, a mass spectrometer is an instmment that measures the mass-to-charge ratios ml of ions formed when a sample is ionized by one of a number of different ionization methods (1). If some of the sample molecules are singly ionized and reach the ion detector without fragmenting, then the ml ratio of these ions gives a direct measurement of the molecular weight. The first instmment for positive ray analysis was built by Thompson (2) in 1913 to show the existence of isotopic forms of the stable elements. Later, mass spectrometers were used for precision measurements of ionic mass and abundances (3,4). 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

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. 

Fortunately, isotopic abundances as well as isotopic masses can be determined by mass spectrometry. The situation with chlorine, which has two stable isotopes, 0-35 and 0-37, is shown in Figure 3.2. The atomic masses of the two isotopes are determined in the usual way. The relative abundances of these isotopes are proportional to the heights of the recorder peaks or, more accurately, to the areas under these peaks. For chlorine, the data obtained from the mass spectrometer are... 

Atomic masses calculated in this manner, using data obtained with a mass spectrometer can in principle be precise to seven or eight significant figures. The accuracy of tabulated atomic masses is limited mostly by variations in natural abundances. Sulfur is an interesting case in point. It consists largely of two isotopes, fiS and fgS. The abundance of sulfur-34 varies from about 4.18% in sulfur deposits in Texas and Louisiana to 4.34% in volcanic sulfur from Italy. This leads to an uncertainty of 0.006 amu in the atomic mass of sulfur. 

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

Neder H, Heusser G, Laubenstein M (2000) Low-level y-ray germanium-spectrometer to measure veiy low primordial radionuclide concentrations. ApplRadiat Isot 53 191-195 Palacz ZA, Freedman PA, Walder AJ (1992) Thorium isotope ratio measurements at high abundance sensitivity using a VG 54-30, an energy-filtered thermal ionization mass spectrometer. Chem Geol 101 157-165... 

Table I also shows that the main component of the total CO2 evolved, which is determined by GC, is the C02, and the total of the other CO2 isotopes (e.g., 6C02 and 8C02) is about 10 of C02. The low concentrations of the isotopic 8C02 and C02 are still, however, an easily measurable quantity of GC-mass spectrometer. The C02 is used as the standard, because the measurement of relates directly to the quantity of the abundant C02. Its low concentration levels help to improve the measuring accuracy of the other CO2 isotopes, chich are also in low concentration. The moles...

For many years, meteorites have provided the only means to determine the abundance of 3He in protosolar material. The values obtained by mass spectroscopy techniques in the so-called planetary component of gas-rich meteorites have been critically examined by Geiss (1993) and Galli et al. (1995). The latter recommend the value 3He/4He= (1.5 0.1) x 10-4. The meteoritic value has been confirmed by in situ measurement of the He isotopic ratio in the atmosphere of Jupiter by the Galileo Probe Mass Spectrometer. The isotopic ratio obtained in this way, 3He/4He= (1.66 0.04) x 10 4 (Mahaffy et al. 1998), is slightly larger than, but consistent with, the ratio measured in meteorites, reflecting possible fractionation in the protosolar gas in favor of the the heavier isotope, or differential depletion in Jupiter s atmosphere. 

In this method, chromium is extracted and preconcentrated from seawater with trifluoroacetylacetone [H(tfa)] which complexes with trivalent but not hexavalent chromium. Chromium reacts with trifluoroacetylacetone in a 1 3 ratio to form an octahedral complex, Cr(tfa)3. The isotopic abundance of its most abundant mass fragment, Cr(tfa)2 was monitored by a quadrupole mass spectrometer. 

The earlier stable isotope dilution mass spectrographic work was accomplished with a thermal ion mass spectrometer which had been specifically designed for isotope abundance measurements. However, Leipziger [829] demonstrated that the spark source mass spectrometer could also be used satisfactorily for this purpose. Although it did not possess the excellent precision of the thermal unit, Paulsen and coworkers [830] pointed out that it did have a number of important advantages. 

The thermal ion mass spectrometer was specifically developed for the measurement of isotope abundances and is capable of excellent precision. Although the spark source mass spectrometer used in this work lacks some of this precision, it has proved very useful in stable isotope dilution work. It has a number of advantages, including greater versatility, relatively uniform sensitivity, and better applicability to a wide range of elements. 

A. O. Nier. A Mass Spectrometer for Routine Isotope Abundance Measurements. Rev. Sci. Instrum., 11(1940) 212-216. 

Mass spectrometry is one of the oldest instrumental analytical methods. Positive rays were discovered by Goldstein in 1886 (after Barrie Prosser, 2000). The first mass spectrometer for routine measurements of stable isotope abundances was reported in 1940 and improved upon over the following ten years Nier, 1940, Nier, 1947, Murphey, 1947, McKinney et al, 1950, after Prosser, 1993. It is remarkable that the vast majority of active gas spectrometers in use today are little changed from those described around 50 years ago. For most people, mass spectrometry now means organic molecular structure determination. However, within the last 15... 

The elements whose isotopes are routinely measured with gas inlet mass spectrometers are carbon (12C and 13C, but not 14C), oxygen (160, 170, l80), hydrogen ( H, 2H, but not 3H), nitrogen (14N and 1SN) and sulphur (32S, 33S, 34). Stable isotopes of H, C, N, O, and S occur naturally throughout atmosphere, hydrosphere, lithosphere, and biosphere. They are atoms of the same elements with a different mass. Each element has a dominant light isotope with the nominal atomic weight (I2C, 160,14N, 32S, and H) and one or two heavy isotopes (l3C, nO, 180, 15N, 33S, 34S, and, 2H) with a natural abundance of a few percent or less Table 1). 

McKinney, C.R., McCrea, J.M., Epstein, S., Allen, H.A., Urey, H.C. Improvements in mass spectrometers for the measurement of small differences in isotope abundance ratios. Rev. Sci. Instrum. 21,1950,724. 

The isotope dilution gas chromatography-mass spectrometry method described by Lopez-Avila et al. [16] and discussed in section 5.3.1.3 has been applied to the determination of Atrazine in soil. In this method known amounts of labelled Atrazine were specked into soil samples before extraction with acetone-hexane. The ratio of the naturally abundant compound and the stable-labelled isotope was determined by high-resolution gas chromatography-mass spectrometry with the mass spectrometer in the selected ion monitoring mode. Detection limits of 0.1-l.Oppb were achieved. Accuracy was >86% and precision better than 8%. 

Different isotopes are detected because the differing count of neutrons that characterizes atoms of different isotopes produces ions of different mass-to-charge ratios in the mass spectrometer. Their relative abundances are related to the intensity of the respective signals in the mass spectrum. 

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