The isotope mass spectrometer

If an ion of mass m and charge e is propelled by an accelerating voltage V through a magnetic field of strength B, it will follow a curved path of radius R. The relationship of these variables is given by  

Isotope mass spectrometers typically cover the range m e 2—100. A high vacuum pumping system is required to maintain a sample inlet pressure of 10 torr and an analyser pressure of 10 torr. Both inlet and analyser components can be baked to 300°C to reduce the background spectrum. 

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Mass spectrometry is the most versatile and widely used technique and is capable of great sensitivity. It can be employed for a whole range of isotope analyses. The analysis of simple gases using the isotope mass spectrometer is of particular importance in clinical applications and will be treated in some detail. The pharmacologist is frequently concerned with complex mixtures and it is in this area that the organic mass spectrometer linked to the gas chromatograph has assumed an important role. 

Isotope mass spectrometers are designed to measure the abundance of an isotope in an unknown gas sample compared with that in a known reference gas. The layout of a typical instrument analyser is shown in Figure 1.3. There are important differences between the isotope mass spectrometer and the high resolution analytical mass spectrometer. In both types of instnunent the sample is ionised... 

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

Historical That positive rays could be deflected in electric and magnetic fields was shown as early as 1898 by Wien, but it was not until 1912 that what was to become the forerunner of the modem mass spectrometers was built by JJ. Thompson, who became known as the father of mass spectrometry. The existence of two isotopes of neon (m/e 20 and 22) was demonstrated by Thompson with this instrument. The discovery of stable isotopes of elements has been generally considered the... 

The collected sample at -196°C was isolated from the flow of the GC s helium gas stream and then the loop was warmed to ambient temperature for GC-mass spectroscopic analyses. The gas cell, which contained the isotopic CO2 and the C2Hg standard in helium at one atmosphere, was placed in the injection helium flow of the GC-mass spectrometer for ten minutes, before the mini-switching valve was turned to inject the vapor contents into the instrument. After three minutes, the CO2 peak eluted. The superimposed peaks were sampled ten times during their elution and the relative isotopic quantities of - C02 C02 and C02 were determined. 

Figure 9. The quadrupole mass spectrometer signal for volatile species released from 0.90 nm palladium acetate film as a function of 2 MeV He+ ion dose. Mass 15 is shown for both CH3 and CH4 because of overlap at m/e 16 with oxygen. Mass 31 is shown for C2H6 (13C isotope) because of overlap at m/e 30 with major fragments of other parent ions.

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... 

Shortly after the development of the early mass spectrometers, Li isotopes were identified by Francis Aston (1932). Although mass spectrometric techniques are those most commonly applied to the measurement of isotope ratios in geochemistry, attempts to quantify Li isotopes have been made using non-mass based emission methods (e.g., atomic absorption Zaidel and Korennoi 1961 various nuclear methods Kaplan and Wilzbach 1954 Brown et al. 1978 ... 

In order to successfully interpret a mass spectrum, we have to know about the isotopic masses and their relation to the atomic weights of the elements, about isotopic abundances and the isotopic patterns resulting therefrom and finally, about high-resolution and accurate mass measurements. These issues are closely related to each other, offer a wealth of analytical information, and are valid for any type of mass spectrometer and any ionization method employed. (The kinetic aspect of isotopic substitution are discussed in Chap. 2.9.)... 

The foundations of stable isotope geochemistry were laid in 1947 by Urey s classic paper on the thermodynamic properties of isotopic substances and by Nier s development of the ratio mass spectrometer. Before discussing details of the naturally occurring variations in stable isotope ratios, it is useful to describe some generalities that are pertinent to the field of non-radiogenic isotope geochemistry as a whole. 

The instrument can be run in various combinations of fixed or scanning modes for Ql and Q2 (e.g., see Johnson and Yost, 1985). Particularly useful is the continuous mode, where particular peaks in the Ql scan and certain fragments in the Q2 mass spectrometer are followed, rather than scanning one (or both) of the quadruples. Indeed, this method has been used to measure Cl2 specifically in the marine boundary layer (Spicer et al., 1998). In these studies, Cl2 was generated as described earlier and the mass combinations (Q1/Q2) for 70/35, 72/35, 72/37, and 74/37 were followed. The combination of MS-MS and the isotope ratios provided unique confirmation that the species being measured was indeed Cl2. Concentrations down to 15 ppt Cl2 could be measured using this approach, with slightly better sensitivity for Br2. 

As described in 2.2.3.1, Principles of Assay , tHcy must be produced by chemical reduction, which is achieved in the method described here by dithiothreitol. tHcy is analysed by HPLC separation followed by electrospray ionisation and then separation of the ionised molecule in the first mass spectrometer, then fragmentation into a specific ion fragment in the second. Quantification is based on comparison of the signal from natural Hey (transition m/z 135.9 —< m/z 89.9) with that of the stable isotope internal standard (transition m/z 139.9 —< m/z 93.9). 

The isotopic dilution method can be extended to non-radioactive tracers by using mass spectrometry or NMR to determine the variation in isotopic ratios. This method can be used for the measurement of molecules or elemental species (about 60 elements have stable isotopes). This approach allows ultra-trace analysis because, contrary to radioactive labelling where the measurement relies on detecting atoms that decompose during the period of measurement, all labelled atoms are measured. Isotopic mass spectrometers are well suited for these measurements. 

Tests on copper SRMs show that a stable signal is obtained when each isotope is measured using the peak jumping mode and one point per peak, with a dwell time of 25,000 ps. The quadrupole mass spectrometer scans three times the mass range per replicate and accumulates 10 replicates for a total acquisition time of about 1 minute. For this application, 22 isotopes were selected (Table I). 

How do we know isotopes exist They were first discovered by scientists using apparatus called a mass spectrometer (Figure 3.6). The first mass spectrometer was built by the British scientist Francis Aston in 1919 and enabled scientists to compare the relative masses of atoms accurately for the first time. 

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