Mass spectral techniques field ionization

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

A different perspective on mass spectral fragmentation can be obtained considering the energy of the molecular ion formed during the El process. The ions with low energy states can be related to thermal processes [21.22], and this may provide information on the thermal decompositions. In addition to standard ionization procedures, special techniques that provide milder ionization conditions such as field ionization (see Section 5.4) offer an even closer similarity to the pyrolytic process. 

A possible solution to the above problems would be the triple-dimensional analysis by using GC x GC coupled to TOFMS. Mass spectrometric techniques improve component identification and sensitivity, especially for the limited spectral fragmentation produced by soft ionization methods, such as chemical ionization (Cl) and field ionization (FI). The use of MS to provide a unique identity for overlapping components in the chromatogram makes identification much easier. Thus MS is the most recognized spectroscopic tool for identification of GC X GC-separated components. However, quadru-pole conventional mass spectrometers are unable to reach the resolution levels required for such separations. Only TOFMS possess the necessary speed of spectral acquisition to give more than 50 spectra/sec. This area of recent development is one of the most important and promising methods to improve the analysis of essential oil components. 

Field desorption (FD) and fast atom bombardment (FAB) mass spectrometry provides mass spectral information about compotmds that are not very volatile but these two techniques are not used often in polymer science since they have several disadvantages. Electrospray ionization (ESI) mass spectrometry can also be used to obtain the above information about polymers, but ESI spectra are generally complicated due to differences in charge state distributions. Static secondary ion mass spectrometry (static SIMS) is a surface-sensitive MS technique, which is suitable for studying the interfaces of polymers with respect to chemical structure and molecular weight as well as end groups and surface contaminants. Laser desorption... 

Inductively coupled plasma (ICP) ionization has currently assumed a more prominent role in the field of elemental and isotopic analysis [1,2,14]. It is apphcable to solid-state as well as to solution-phase samples. A plasma is defined as a form of matter that contains a significant concentration of ions and electrons. The heart of this technique is a plasma torch, first developed as an efficient source for optical emission spectroscopy (OES) [15,16]. Multielement analysis with OES has, however, some serious shortcomings, such as complicated spectra, spectral interferences, high background levels, and inadequate detection of some rare-earth and heavy elements. The high ionization efficiency (>90%) of ICP for most elements is an attractive feature for its coupling to mass spectrometry. 

To compensate for the energy spread, the first stage of the two-stage acceleration field can be activated shortly after ion production or ejection from the sample. This event typically occurs in the nanosecond to low microsecond range. The method is known as time-lag focusing or delayed extraction technique. In TOF-MS equipped with matrix-assisted laser desorption/ionization (MALDI) ion source, delayed extraction may improve the mass resolving power by a factor of two- to fivefold [9, 10]. Mass resolving power defines the sharpness of spectral features, which is explained in Chapter 5. 

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