Mass spectrometric technique

1986 John Wiley Sons, Inc. Reprinted by permission of the publisher.) 

All ions leaving the ion source have the same kinetic energy E, but the velocity v of a particular ion will depend on its mass. The velocity of an ion is given by rearranging the above  

The equation of motion of a charged particle in a magnetic field of strength B is given by the following, where r is the radius of the circular track taken by the ion  

Combining these last two equations to eliminate v gives the following  

Laser ablation is another important development, allowing solid samples to be directly analysed by ICP, since it offers the possibility of spatially-resolved microanalysis of solid samples, in a manner similar to electron microscopy, but with greater sensitivity and the potential for isotopic analysis. In this technique, a high-energy pulsed laser is directed onto a solid sample, with a beam diameter of less than 25 pm. The pulse vaporizes about 1 pg of material, to leave a crater 

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Benzene profile in a fuel-rich propene flame (C/O = 0.77), measured independently with three different mass spectrometric techniques X(. mole fraction, h height above burner. 

Sim PG, Boyd RK, Gershey RM, Gueveemont R, Jamieson WD, Qdilliam MA, and Geegely RJ (1987) A comparison of chromatographic and chromatographic/mass spectrometric techniques for the determination of polycyclic aromatic hydrocarbons in marine sediments. Biomed Environ Mass Spectrosc 14 375-381. 

TABLE 5.2. Representative BDEs for closed-shell organic molecules measured by using mass spectrometric techniques. ... 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

In this chapter we discuss improvements documented in the literature over the past decade in these areas and others. Chemical procedures, decay-counting spectroscopy, and mass spectrometric techniques published prior to 1992 were previously discussed by Lally (1992), Ivanovich and Murray (1992), and Chen et al. (1992). Because ICPMS methods were not discussed in preceding reviews and have become more commonly used in the past decade, we also include some theoretical discussion of ICPMS techniques and their variants. We also primarily focus our discussion of analytical developments on the longer-lived isotopes of uranium, thorium, protactinium, and radium in the uranium and thorium decay series, as these have been more widely applied in geochemistry and geochronology. 

In addition to instrumental improvements, various approaches have been used to improve the purity or geometry of sources of natural samples for gamma spectrometric measurement. For example, improvements in source preparation for " Th measurement in water and sediment samples by gamma spectrometry are discussed in Cochran and Masque (2003). It should be emphasized that one of the main advantages of gamma spectrometry is ease of use, since in many cases samples may be analyzed directly or with significantly reduced sample preparation compared to alpha, beta, or mass spectrometric techniques. 

Measurement of U-series disequilibria in MORB presents a considerable analytical challenge. Typical concentrations of normal MORB (NMORB) are variable but are generally in the 50-150 ppb U range and 100-400 ppb Th range. Some depleted MORB have concentrations as low as 8-20 ppb U and Th. The concentrations of °Th, Pa, and Ra in secular equilibrium with these U contents are exceedingly low. For instance, the atomic ratio of U to Ra in secular equilibrium is 2.5 x 10 with a quick rule of thumb being that 50 ng of U corresponds to 20 fg of Ra and 15 fg of Pa. Thus, dissolution of a gram of MORB still requires measurement of fg quantities of these nuclides by any mass spectrometric techniques. 

Mass spectrometric techniques for analysis of Th- U disequilibria were first developed to date corals for paleoclimate research (Edwards et al. 1987). Soon thereafter, workers at Los Alamos National Laboratory (LANE) developed methods for silicate analysis by TIMS (Goldstein et al. 1989). Typical TIMS analysis of MORE requires 0.5 to 1 gram of material in order have an analyzable load of 100 ng of Th. TIMS analyses of U and Th last 2-3 hrs and produce a precision of 0.5-2% (2a). SIMS techniques for measuring Th isotopes have also been developed (England et al. 1992 Layne and Sims 2000). Analysis of Ra and Pa isotopes by TIMS was developed in the early 1990 s significantly increasing the sensitivity over decay counting analysis (Volpe et al. 1993 Cohen and Onions 1993 Pickett et al. 1994 Chabaux et al. 1994). 

Of note are the values for °Th and as the revised values postdate the development of mass spectrometric techniques for measurement of U and Th in natural materials. Data published prior to Cheng et al. (2000b) does use not use the revised values whereas data published subsequently may or may not use the new values. The revised half-lives do have a small, but significant effect on calculated °Th ages, particularly ages older than about 100 ka. Furthermore, the new value for the half-life changes values as these are calculated from measured atomic ratios using... 

L234 are about 3 per mil lower than those calculated with commonly used X234 values, hence the revised modem sea water of 145.8 1.7 per mil (Cheng et al. 2000b), compared to earlier values about 3 per mil higher. In general half-lives are now known precisely enough so that their contribution to error in age is comparable to or smaller than typical errors in isotope ratios (determined with mass spectrometric techniques). 

As discussed above, mass spectrometric techniques are the methods of choice for measurement of nuclides pertinent to this study. They supercede earlier decay-counting techniques because of their ability to detect a much larger fraction of the nuclides of... 

Figure 15. Error in age vs. age, both on log scales (after Edwards et al. 1997). Each data point represents data from a particular sample analyzed by thermal ionization mass spectrometric techniques. Solid circles represent Pa ages. Contours of aniytical error in Pa/ U pertain to the Pa data points. Shaded squares represent °Th ages. See text for discussion.

Speleothem frequency distributions have provided a useful tool for broad comparisons, but they suffer from the problem of biased sampling strategies and low resolution at times of known abrupt change. The increased precision afforded by mass-spectrometric techniques will result in fewer studies using this approach to assess of growth frequency and, more often, records of continuous deposition and growth rate studies will be graphically illustrated. 

The situation is somewhat better for the gas-phase chemistry of isolated transition-metal ions or complexes, and this area of research has received a lot of attention in the past. On the experimental side, comprehensive mass-spectrometric techniques allow for an explicit measurement of thermochemical and kinetic parameters of reactants, intermediates, and products occurring along the reaction pathways. These data can be obtained without the influence of ligands, counter ions, solvents etc. which would be a highly complicated enter-... 

Section 6.4 deals with other EI-MS analyses of samples, i.e. analyses using direct introduction methods (reservoir or reference inlet system and direct insertion probe). Applications of hyphenated electron impact mass-spectrometric techniques for poly-mer/additive analysis are described elsewhere GC-MS (Section 7.3.1.2), LC-PB-MS (Section 7.3.3.2), SFC-MS (Section 13.2.2) and TLC-MS (Section 7.3.5.4). 

Applications Various surfactant types (ABS, AES, secondary alkane sulfonates, and alkylphenol ethoxy-sulfates) have been analysed by means of a QQQ using a thermospray source [89]. Other applications of hyphenated thermospray ionisation mass-spectrometric techniques (LC-TSP-MS) are described elsewhere (Section 73.3.2). 

Applications of hyphenated ESI mass-spectrometric techniques are described elsewhere LC-ESI-MS (Section 73.3.2), SFC-ESI-MS (Section 73.2.2) andCE-ESI-MS (Section 73.6.1) for polar nonvolatile organics. 

Applications APCI-MS is often more widely applicable than ESI-MS to the analysis of classes of compounds with a low molecular weight, such as basic drugs and their metabolites, antibiotics, steroids, oestrogens, benzodiazepines, pesticides, surfactants, and most other organic compounds amenable to El. LC-APCI-MS has been used to analyse PET extracts obtained by a disso-lution/precipitation procedure [147]. Other applications of hyphenated APCI mass spectrometric techniques are described elsewhere LC-APCI-MS (Section 7.33.2) and packed column SFC-APCI-MS (Section 73.2.2) for polar nonvolatile organics. 

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