Kinetic energy spread

An electron impact (El) ion source uses an electron beam, usually generated from a rhenium filament, to ionize gas-phase atoms or molecules. Electrons from the beam (usually 70 eV) knock an electron from a bond of the atoms or molecules creating fragments and molecular ions [366,534,535]. Several factors contribute to the popularity of El ionization in environmental analyses such as stability, ease of operation, simple construction, precise beam intensity control, relatively high efficiency of ionization, and narrow kinetic energy spread of the ions formed. 

Traditionally thermal ionization mass spectrometry was the instrument of choice for the isotopic analysis of metals because thermal ionization produced an ion beam with a very small kinetic energy spread ( 0.5 eV). Therefore only a magnetic mass analyzer is needed to resolve one isotope from another. Moreover, ionization of unwanted material, such as atmospheric contaminates, hydrocarbons from pump oil, or production of doubly ionized particles is almost non existent, thus background counts are minimized and signal-to-noise ratio is maximized. 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

With soft ionization techniques such as MALDI, ions of m/z 200000 can be routinely detected. The mass range is mainly limited by the fact that with the detector the response decreases with increasing m/z of the ions. The mass resolution of a TOF mass analyzer is relatively poor (unit mass resolution and less) and is affected by factors that create a distribution in the flight time of ions with the same m/z. The simplest way to increase the mass resolution is to increase the length of flight tube or to reduce the kinetic energy spread of the ions leaving the source. 

One way to reduce the kinetic energy spread is to introduce a time delay between ion formation and acceleration, referred to as delayed pulsed extraction. After a certain time delay ranging from nanoseconds to microseconds a voltage pulse is applied to accelerate the ions out of the source. 

Turner et al. [114] described an ICP-MS with a hexapole transfer lens (Fig. 3.12a) in a tube that allows the pressure to be maintained. The cell was initially reported to contain He, although it is likely that H2 or H20 vapor was also in the cell and is now purposely added to the cell [115]. Ar2+, ArCl+, ArO+, and Ar+ signals were reduced relative to Se+, As+, Fe+, and Ca+, respectively. Detection limits for Fe, Ca, K, Se, and As near 10 parts per trillion (ppt) have been reported [115]. Recently, Beaty and Liezers [116] also described a collision-reaction cell at a pressure of 30 mtorr that reduced the ion kinetic energy spread as well as continuum ICP-MS background to less than 1 count/sec. Previously, Douglas and French had described the use of an rf-only quadrupole for collisional focusing and reduction of the ion kinetic energy spread [117]. 

To reduce the kinetic energy spread among ions with the same m/z ratio leaving the source, a time lag or delay between ion formation and extraction can be introduced. The ions are first allowed to expand into a field-free region in the source and after a certain delay (hundreds of nanoseconds to several microseconds) a voltage pulse is applied to extract the ions outside the source. This mode of operation is referred to as delayed pulsed extraction to differentiate it from continuous extraction used in conventional instruments. Delayed pulsed extraction, also known as pulsed ion extraction, pulsed extraction or dynamic extraction, is a revival of time-lag focusing, which was initially developed by Wiley and McLaren in the 1950s, shortly after the appearance of the first commercial TOF instrument. 

Advances in TOF/MS technology, such as reflectron ion optics, delayed ion extraction, and orthogonal ion acceleration, have given rise to bench-top instrumentation with resolution capabilities in excess of that obtained by quadra-pole MS systems. Each of these devices limits or corrects the kinetic energy spread of ions with identical m/ ratios. 

The first experiment combining ion beams with laser excitation was performed by Andra et al. [22] in 1973 in lifetime studies. Crossed-beam spectroscopy on ion-beams suffers from the short interaction time. This is why a collinear superposition of laser and ion beams is favourable. Another important advantage of a collinear geometry was proposed in 1976 by Kaufmann [23] and, independently, demonstrated by Wing et at. [24]. Since the kinetic energy spread(6E) of the ions remains constant under acceleration ... 

In magnetic sector instruments, the result of x-axial (i.e. along the beam path) KER is the observation of fragment ion peaks in the MIKE or CID spectra which have a broader kinetic energy spread than the precursor ion peak. In a CID, however, this spread is complicated by collisional scattering and so KER is most often discussed for peaks in MIKE spectra. If the mass... 

Electron impact ion source. The electron impact ion source is efficient, simple to construct, stable and produces ions with a narrow kinetic energy spread (Figures 7.4 and 7.5). The spectra obtained are specific and characteristic of the molecular structure of the analyte since the fragmentation processes produce ions in a relative abundance ratio which is characteristic of the molecular structure of the compound. The ions and their relative intensities provide a fingerprint for the compound. 

Despite the simplicity of the equation, an ion s initial spatial and angular kinetic energy spread complicate the precise determination of m/z. These issues are addressed to improve m/z accuracies in high performance time-of-flight mass spectrometers (Guilhaus et ah, 2003). 

El ionization is popular in environmental analyses because of its stability, ease of operation, simple construction, precise beam intensity control, relatively high efficiency of ionization and narrow kinetic energy spread of the ions formed... 

Time-of-flight (TOP) mass spectrometers are ideally suited for use with the MALDI technique because of their theoretically unlimited mass range, high ion transmission, and for the pulsed nature of the laser used in this method. Mass resolution is the ability of an instrument to separate the signals from ions of similar mass, expressed as the mass of a given ion divided by the full width at half maximum of the peak (fwhm). Resolution in MALDl-TOP MS is mainly restricted by the ionization process, rather than by instrumental limits, because the ions have a certain time-span of formation, a spatial distribution, and a kinetic energy spread. ... 

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