Pulse mass analysis

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

ToF analysers are able to provide simultaneous detection of all masses of the same polarity. In principle, the mass range is not limited. Time-of-flight mass analysis is more than an alternative method of mass dispersion it has several special qualities which makes it particularly well suited for applications in a number of important areas of mass spectrometry. These qualities are fast response time, compatibility with pulsed ionisation events (producing a complete spectrum for each event) ability to produce a snapshot of the contents of the source volume on the millisecond time-scale ability to produce thousands of spectra per second and the high fraction of the mass analysis cycle during which sample ions can be generated or collected. 

Fig. 9. Pulse microreactor system for use with 13C-labeled hydrocarbons. D, E, and J are microreactors J contains the catalyst to be used for hydrocarbon skeletal reaction D and E are used, when necessary, to generate the required reactant hydrocarbon from a non-hydrocarbon precursor (e.g., alcohol dehydration in D and olefin hydrogenation in E) reactant injected at C. F is a trap which allows the accumulation of products from several reaction pulses before analysis G is a G.P.C. column, K a katharometer. Traps H collect fractions separated on G for subsequent mass spectrometric study. When generating reactant hydrocarbon in D and E, a two-step process is preferable in which, with J below reaction temperature, the purified reactant hydrocarbon is collected in H, and this is recycled as reactant with D and E below reaction temperature but with J at reaction temperature. After C. Corolleur, S. Corolleur, and F. G. Gault, J. Catal. 24, 385 (1972).

Rate constants are usually determined in the pulsed mode by switching on the electron gun for several microseconds followed by reaction of the ions in a field-free region. Magnetic mass analysis coupled with an electron multiplier in which the pulses are collected in a multiscaler provide a time development of the particular ions under analysis. There have been few reactions relevant to this review which have been studied by HPMS. 

This desorption ionisation technique leads to weak fragmentation. The analyte is incorporated into a solid organic matrix (such as hydroxybenzoic acid) and the mixture is placed on a sample holder that is irradiated with UV laser pulses (e.g. N2 laser, A = 337 nm, pulse width = 5 ns). The laser energy is absorbed by the matrix and transferred to the analyte, which becomes desorbed and ionised (Fig. 16.18c). Although MALDI is considered to be a soft ionisation technique, a substantial amount of energy is involved. Because the technique involves pulsed ionisation, it is well suited for time-of-flight mass analysis of biomolecules. The analysis of small molecules (M < 500 Da) is limited because the matrix decomposes upon absorption of the laser radiation. However, solid supports such as silicone can be used as the matrix to overcome this disadvantage. 

In 1988, Karas and HiUenkamp embedded proteins in a large molar excess of a UV-absorbing crystal matrix and irradiated the sample with a laser beam at suitable wavelengths (Karas and HiUenkamp, 1988). In this process of matrix-assisted laser desorphon/ionization (MALDI), large proteins can be transferred into the gas phase as intact molecules and become, for the most part, singly protonated. Since MALDI rehes on the use ofa pulsed laser, mass analysis is usuaUy performed... 

The speed of MALDI analysis depends on the laser pulse rate. With the recent introduction of 200-Hz lasers, samples can be analyzed ten times faster than before. This development is especially advantageous for offline liquid chromatography (LC)/MALDI applications (Ericson et al., 2003). MALDI mass analysis is performed considerably faster than LC separation, allowing for chromatographic... 

The equipment consists of a feed system, the reactor section, a GC, and an MS for product analysis. A Rupprecht and Patashnick TEOM 1500 PMA (Pulse Mass Analyzer) was used in the experimental design shown in Fig. 2. The tapered element with the catalyst bed on its end oscillates in a clamped-free mode. This is accomplished through a sensitive feedback amplifier control circuit connected to a mechanical drive to supply the necessary energy. The reactor tube is constructed of proprietary glass ( engineered glass ) (5). The reactor material has proved to be sufficiently inert for a number of applications. The catalyst bed is held in place by quartz, a-alumina, or carbon wool, depending on the conditions, and a metal cap. 

Mass spectrometry is one physical technique that does not (at least directly) involve electromagnetic radiation. However, some sample desorption and ionization processes do use high intensity pulses of laser light in techniques such as MALDI (Matrix-Assisted Laser Desorption Ionization) that have proved very useful in mass analysis of proteins and other biologic macromolecules. High resolution mass spectrometry derives from atomic/molecular beam studies in which the trajectories of ionized particles in a vacuum can be manipulated by static... 

Fig. 6 Schematic of a FTICR MS instrument. This type of MS consists of an ion cyclotron resonance (ICR) analyzer cell that is situated in the homogeneous region of a large magnet. The ions introduced into the ICR analyzer are constrained (trapped) by the magnetic field to move in circular orbits with a specific frequency that corresponds to a specific mass-to-charge ratio (m/z). Mass analysis occurs when radiofrequency (rf) potential is applied (pulsed) to the ICR analyzer so that all ions are accelerated to a larger orbit radius. After the pulse is turned off, the transient image current is acquired and a Fourier transform separates the individual cyclotron frequencies. Repeating this pulsing process to accumulate several transients is used to improve the signal-to-noise ratio. (Courtesy of Bruker Daltonics, Billerica, MA.)...

This value is in agreement with the one derived from band profiles calculated with the equilibrium-dispersive model [9]. The time given by Eq. 16.20 provides useful information regarding the specifications for the experimental conditions under which staircase binary frontal analysis must be carried out to give correct results in the determination of competitive isotherms. The concentration of the intermediate plateau is needed to calculate the integral mass balances of the two components, a critical step in the application of the method (Chapter 4). This does not apply to single-pulse frontal analysis in which series of wide rectangular pulses are injected into the column which is washed of solute between successive pulses. 

---