Mass spectrometer ion-source techniques

The careful establishment of the merging-beams technique is an excellent example of what is needed. McDaniel s recent formulation of necessary criteria to be met for a reliable drift-tube measurement is another example. In contrast, one notes the lack of the necessary control experiments during the past development of the mass-spectrometer ion-source technique (Section 3.4.1) and during the recent development of the ion cyclotron resonance technique (Section 3.4.7). The necessary techniques may not always be available, but the effort must be directed to that end and not to the premature acquisition of data of unproven reliability. 

NH3. The rate constant, determined using a mass spectrometer ion source technique, decreased slightly with increasing temperature (k(320 K) = 2.0x10" , k(640 K) = 1.7xi0 cm molecule" -s" ). This effect is attributed to an ion-dipole interaction [11]. AH = -401 kJ/mol was reported [1]. 

Zitomer (67) was the first to describe the coupling of a thermobalance to a time-of-flight mass spectrometer and a magnetic sector mass spectrometer. This technique eliminated the practice of collecting or trapping fractions for subsequent analysis and also permitted careful control of the furnace atmosphere. One of the important features of the TG-MS system is its relatively short dead time, that is, the time between product evolution and introduction into the mass spectrometer ion source. Under proper flow conditions, this time is of the order of seconds. There is also less probability of the formation of secondary reaction that can lead to products other than those initially evolved. 

Although many reactions have been reported to have no activation energy, they need careful re-examination since most of the earlier experiments were carried out in a mass spectrometer ion-source where ions acquire some translational energy from the repeller field. In fact, recent experimental- techniques have revealed the existence of activation energies for some ion—molecule reactions. 

This chapter is concerned principally with two-body reactions between negative ions and neutral molecules at low energies in the gas phase. Introductory material on the formation and disappearance of negative ions is also included. Emphasis is placed on results arising from the use of mass spectrometer ion sources or double mass spectrometers as the experimental technique. Additional information on negative-ion-neutral reactions may be found in Chapters 7, 8, 11, 15, and 16 of this book. 

This chapter is largely restricted to the study of negative-ion-neutral reactions using the mass spectrometer ion source and double mass spectrometer techniques, and only these methods are discussed in detail in this section. 

The Cerm k-Herman technique, which ingeniously reproduces the capabilities of a transverse tandem machine within the compass of a traditional mass-spectrometer ion source, is not suited for the measurement of excitation functions. This is a consequence of the large energy dispersion of the reactant ions, determined by the voltage between the ionization chamber and the electron trap. 

Mass-Spectrometer Ion Source—Low-Pressure Experiments The early history of ion-molecule studies is largely a chronicle of mass-spectrometer ion-source experiments. Data were easy to obtain, but data which are only dependent on the kinetics of the collision processes were and still are exceedingly difficult to obtain. Moreover, as will be seen, reliable data so obtained have only an indirect relevance to present needs (Section 1.2). Nowadays the technique is not widely used and it might therefore seem appropriate to confine discussion of it to Chapter 2. It will, however, be discussed here for two reasons. The technique is still used and it will be a major purpose of the following remarks to indicate... 

Fig. 7. Energy analysis by the retarding potential technique of argon primary ions produced in a standard mass-spectrometer ion source operated with a dc repeller field. The shape of the curve should be independent of the ion exit energy E data shown for three different values of E demonstrate this in the appropriately normalized plot. The distribution of experimental ion exit energies results from the finite thickness of the electron beam, and the figure shows theoretical distributions predicted for various assumed thicknesses of the electron beam. Fair agreement is found for an assumed thickness of 0.5 mm, the actual dimension of the slit through which the electron beam enters the source.

Mass-Spectrometer Ion Source—Photoionization Technique Reference is made here to the discussion in Chapter 3 of the photoionization technique, which is clearly the most powerful ion-source method. Not only is state selection of the reactant ion often possible, but operation of the source at 77°K minimizes the initial thermal energy spread to such an extent that unfolding of the excitation function is possible from the measured phenomenological cross section. Moreover, two types of ion source are described which allow direct measurement of the excitation... 

The second and third examples have been selected because these reactions are in principle among the simplest reaction rates to measure. Problems of product ion detection efficiency are eliminated by the collision dynamics, whereby both the reactant and the product ions exhibit very similar velocity and angular distributions, irrespective of the collision energy. However, the reactant ion is formed in two states by electron impact and the cross sections for these states are significantly different. Rate parameters will therefore depend on the relative populations of these two states. In this case, both the chemists techniques (mass-spectrometer ion source and ion cyclotron resonance) and some of the physicists ... 

MSP Mass-spectrometer ion source—pulse technique. TOP Time-of-flight mass analysis. LP Low-pressure experiment < 10 /xm. MP Medium-pressure experiment < 100 /im. HP High-pressure experiment > 100 /im. ICR Ion cyclotron resonance. CR Constant repeller field—variable reaction time. IMP Impulse technique. PPE Pulsed product ion ejection. MS Mass-spectrometer ion source. 

Personnel working in some programs at the Los Alamos National Laboratory (LANL) may handle radioactive materials that, under certain circumstances, could be taken into the body. Employees are monitored for such intakes through a series of routine and special bioassay measurements. One such measurement involves a thermal ionization mass spectrometer. In this technique, the metals in a sample are electroplated onto a rhenium filament. This filament is inserted into the ion source of the mass spectrometer and a current is passed through it. The ions of the plutonium isotopes are thus formed and then accelerated through the magnetic held. The number of ions of each isotope are counted and the amount of Pu-239 in the original sample calculated by comparison to a standard. 

In atmospheric pressure ionization sources (API) the ions are first formed at atmospheric pressure and then transferred into the vacuum. In addition, some API sources are capable of ionizing neutral molecules in solution or in the gas phase prior to ion transfer to the mass spectrometer. Because no liquid is introduced into the mass spectrometer these sources are particularly attractive for the coupling of liquid chromatography with mass spectrometry. Pneumatically assisted electrospray (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) are the most widely used techniques. 

Table 1 lists a number of ionization sources which produce ions at either atmospheric pressure or under vacuum conditions. For atmospheric pressure ionization sources a suitable interface is required which allows a controlled leak of ions into the vacuum region of the mass spectrometer. Vacuum ionization techniques likewise require a controlled leak, or mechanical introduction, of neutral molecules into the vacuum chamber, followed by ionization. 

Most of the studies in this decade were carried out with conventional single source mass spectrometers, which limited the kind and accuracy of the information. During the next decade, however, various sophisticated techniques for the study of ion—molecule reactions, such as tandem mass spectrometers, photoionization sources, pulsed sources, flowing afterglow and drift tube methods, crossed and merging beams and ion cyclotron resonance, have been developed. Much detailed information on various aspects of ion—molecule reactions has accumulated, and this has consequently stimulated the theoretical studies as well. This decade was, so to speak, the second epoch in the history of ion—molecule studies. 

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