Mass spectrometers electron impact sources

GC-MS analysis used a Finnigan 4000 quadrupole EI/CI mass spectrometer. Electron Impact spectra were recorded continually using an Incos Nova 4 data system. Ion source temperature was 250°C and the ionisation energy 70 eV. 

Aside from the work of Randall and Wahrhaftig, the only other early attempt to interface supercritical fluid chromatography was the direct introduction work of Gouw et al (34). Although no details were provided, and additional information has not been published, this approach apparently required compound volatility for transfer to a conventional mass spectrometer electron impact ion source. Application appeared limited by compound volatility and poor sensitivity. 

The chemical ionisation (Cl) mass spectrum Fig. 3, was recorded on a Finnigan 4000 Mass Spectrometer with ion source pressure 0.3 Torr, ion source temperature 150°C, emission current 300 yA, electron energy 100 eV using methane as a reagent gas. The electron impact (El) mass spectrum Fig. 4, was recorded on Varian MAT 311 Spectrometer, with an ion source pressure 10 6 Torr, ion source temperature 180, emission current 300 yA and electron energy of 70 eV. 

The analytically important features of Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry (1) have recently been reviewed (2-9) ultrahigh mass resolution (>1,000,000 at m/z. < 200) with accurate mass measurement even 1n gas chromatography/mass spectrometry experiments sensitive detection of low-volatility samples due to 1,000-fold lower source pressure than in other mass spectrometers versatile Ion sources (electron impact (El), self-chemical ionization (self-Cl), laser desorption (LD), secondary ionization (e.g., Cs+-bombardment), fast atom bombardment (FAB), and plasma desorption (e.g., 252cf fission) trapped-ion capability for study of ion-molecule reaction connectivities, kinetics, equilibria, and energetics and mass spectrometry/mass spectrometry (MS/MS) with a single mass analyzer and dual collision chamber. 

Both under electron impact in the mass spectrometer and thermally, the extent of loss of HN02 to give benzyne is small compared to the other processes nevertheless, in the presence of reagents such as hexa-fluorobenzene this reaction becomes significant (Fields and Meyerson, 1967d). Very likely, future work in our laboratories and elsewhere will uncover additional evidence for the ubiquitous role of benzyne in high temperature reactions, as well as exploiting reactions under electron impact in the mass spectrometer as a source of clues to new pyrolysis reactions. 

Much of the work in the early development of the preceding techniques incorporated pulsed electron-impact ionization sources or any of several types of laser ionization techniques. In almost all of these cases the ions were created in a pulsed fashion in vacuum and formed in or sent into the acceleration region of the mass spectrometer, where a static acceleration field present there injected them into the mass spectrometer. Such ion sources use the TOF-MS very efficiently because the repetition rate of the spectrometer is limited by the frequency of the ionization event itself. This arrangement allows the TOF-MS to mass analyze of all of the ions formed completely. However, many of the most popular ionization techniques being used in inorganic analysis today are continuous in nature. 

Until now we have not mentioned in detail the function of the ion source of an accelerator. There are numerous types of ion sources but only a small number is listed in this chapter. Most sources have in common that the element in question enters the source in gaseous form and is extracted from a hole or slit in the source by means of a potential difference. The normal electron impact sources known from mass spectrometers are inconvenient for ion acceleraton because their ionization efficiency is too low (typically 10 %). The most important sources in use are ... 

Figure 16.3. Typical electron-impact source. The source is mounted on a frame and inserted into the flight tube of the mass spectrometer. The voltages in parentheses are typical values for the component parts of a spectrometer operating at 5000 V accelerating potential. Note that the target is at +100 V with respect to the filament, and the filament is at —70 F with respect to the ionization chamber.

Of course, the electron-impact source cannot be used if nonvolatile inorganic samples such as metal alloys or ionic residues are to be analyzed. These substances can be investigated using a different kind of ionization chamber called a spark source, similar to the excitation sources used in emission spectroscopy (Chap. 11). The other parts of the spectrometer can be the same as a general-purpose instrument however, a Mattauch-Herzog double-focusing instrument is preferred (Fig. 16.7 below), because the spark source produces ions with a wide spread of kinetic energies. The entire device is known as a spark-source mass spectrometer (SSMS). 

Time-of-flight (TOF) mass spectrometers are equipped with a modified electron-impact source and a long, straight flight-tube. Different masses are distinguished by their different arrival times at the detector located at the end of the tube. Since the kinetic energy of the ions after acceleration is given by Equation 16.3, we can write... 

Figure 16.16. Schematic diagram of a typical field-ionization source. This source is simply substituted for the conventional electron-impact source. The remainder of the mass spectrometer is the same. Combined El/FI sources have been used.

The 70 eV electron Impact mass spectrum of acetohexamide, presented In Figure 10, was obtained on Varian MAT 311 mass spectrometer using Ion source pressure of 10 Torr, Ion source temperature of 180 C and an emission current of 300 pA. The molecular Ion Is detectable at m/e 324 and the base peak at m/e 56. A proposed fragmentation pattern and the mass/charge ratios of the major fragments are sham in Scheme 1. 

The major components of the mass spectrometer are a gas-handling manifold that enables the gas to be transferred from the ovens to the ion source, the ion source that consists of an electron impact source, magnetic mass analyzer with gap field of 6500 G, four different EM detectors, and an ion pump to ensure low pressure for the EMs. The gas handling system also included a controlled leak valve that could obtain Martian gas and feed the gas directly into the mass spectrometer for analysis. All the tubes in the gas handling system were heated to 35 C to ensure that there is no condensation of water and other volatile vapors on the hardware. 

Experimental. The mass spectra in Figures 1-8 are positive-ion spectra produced by electron impact and were obtained from a single-focusing, magnetic deflection Atlas CH4 Mass Spectrometer. The ionizing potential was 70 e.v. and the ionizing current 18/a a. An enamel reservoir heated to 120°C. was used from which the sample was leaked into the ion source. 

Some of the problems encountered in the mass spectrometric study of ion-molecule reactions are illustrated in a review of the H2-He system (25). If the spectrometer ion source is used as a reaction chamber, a mixture of H2 and He are subjected to electron impact ionization, and both H2+ and He+ are potential reactant ions. The initial problem is iden-... 

The problems of distinguishing H+ produced from H2 by electron impact from the product of dissociative charge transfer reactions between He + and H2 can be studied by determining the kinetic energy distribution in the product H+ (6). The reaction He+ + H2 is exothermic by 6.5 e.v. if the products are atoms or atomic ions. If the reaction is studied with HD substituted for H2, then the maximum kinetic energy that can be deposited in the D + is approximately 2.16 e.v. On the other hand, D + can be produced by electron impact with 5.5 e.v. kinetic energy. If a retarding potential is applied at the repeller in the ion-source of a mass spectrometer, then it is possible to obtain curves related to the kinetic... 

The particles then enter a conventional mass spectrometer source where they are vaporized prior to being ionized using electron impact or chemical ionization. As with other interfaces, this may cause problems during the analysis of thermally labile and highly in volatile compounds. 

Chromatography is typically at atmospheric pressure while source pressures in the mass spectrometer are in the range of 2 to 10 Torr for chemical and electron impact ionization, respectively. The interface must be capable of providing an adequate pressure drop between the two instruments and should also maximize the throughput of seuaple idiile maintaining a gas flow rate compatible with the source operating pressure. Further, the Interface should not introduce excessive dead volume at the column exit and should not degrade or modify the chemical constitution of the sample. 

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