Vaporization and Ionization Processes

There are a number of vaporization and ionization processes that can be employed, but we will deal only with the most common techniques. In most cases the vaporization process occurs before the ionization takes place however, there is one notable exception to this electrospray ionization. 

The techniques we will discuss are electron impact (El) and chemical 

Historically, electron impact (El) has been the ionization method rriosl frequently employed, but this is a relatively harsh technique and has now been overtaken by electrospray ionization (ESI) in terms ot everyday use. 

The electron lost will be one of the least tightly bound in the molecule, i.e. one of the electrons in the highest occupied molecular orbital (HOMO), and, in general, the order of ease with which electrons are lost upon EI is  

We said earlier that EI is a relatively harsh technique and we will now see why. The amount of energy required to remove an electron from a molecule (which depends upon what type of orbital the electron occupies) is approximately 7 eV (675 kJ mol ), so that the electrons employed in El have ten times the energy required to do the job. Some of this excess energy is imparted to the molecule and results in an excess of vibrational energy and the fragmentation (breaking up) of the molecular ion (see Section 5.3). In some cases, the extent of fragmentation results in the absence of the molecular ion. 

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The APCI source (Table 2.8) has been used for the analysis of various flavonoids, especially flavonols, flavones, flavanones, and chalcones (Table 2.11). APCI is based on gaseous-phase ionization, and is most suitable for compounds that are partially volatile and have a medium polarity. Thus, the application of APCI with respect to analysis of condensed tannins and anthocyanins is more limited. Compared with ESI, APCI produces more fragment ions in the spectrum due to the harsher vaporization and ionization processes. More information about ESI and APCI can be found in Section 1.4.5. 

The analyte (sample being analysed) is dissolved in a low-freezing-point matrix, which not only keeps the analyte in solution in the high vacuum ion source, but also assists in the vaporization and ionization processes. 

Fig. 6.2 Schematic of an Aerodyne aerosol mass spectrometer (AMS). Vaporized aerosol species are ionized and analyzed via mass spectrometry. This figure shows the ion attachment version of ionization methods. Other existing versions of the AMS that utilize several unique ionization methods and developments that are not shown in this schematic are discussed in the text. Extended drawing of flash vaporizer system shows that the particle beam first impacts on a vaporizer, and volatile aerosol components that vaporize are subsequently subjected to cationization. The unique feature of this detection scheme is the fact that a vaporizer is directly coupled into an ion attachment technique to enable a two-step particle vaporization and ionization process. The separation of the vaporization and ionization processes allows for quantitative detection of aerosol mass with the AMS. (Reprinted with permission from Ref [8]. 2007, John Wiley and Sons)...

Laser isotope separation is one area where multistep excitation and ionization has great commercial potential. The research and development efforts in atomic vapor laser enrichment of 235y are a major factor contributing to the current research activities in laser excitation and ionization processes. The first paper on selective multistep photoionization of atoms was published in 1971. (.62) Since then numerous review articles( 15, 16 >L7,63 >54, (i5) ave been written on laser isotope separation and, in each review, there is a section on atomic vapor photoionization processes. The subjects of economics and critical parameters have been well covered in previous reviews and will not be discussed in detail here. We... 

Mass spectrometers operate at high vacuum (Section 2.5), thus they can only analyze samples that are in the vapor state. Equally importantly, the neutral analyte molecules must be converted into ions. The functions of sample introduction systems are to produce vapors from samples (or reduce the pressure of gaseous samples) and to introduce a sufficient quantity of the sample into the ion source in such a way that its composition represents that of the original sample. It is important to note that the concept of sample introduction followed by ionization has changed with the development of recent techniques where the sample introduction and ionization process occur simultaneously. These techniques include atmospheric pressure ionization (API), particularly electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). 

The experimental conditions of these methods differ largely in their intrinsic details, but the main point, common to all of them, is the simultaneous process of vaporization and ionization of the substances. This formation of ions from the probe has, as we think, a fundamental defect. Since the energy for the ion formation is linked directly to the evaporation process, control of the softness in the ionization procedure is not possible. In many cases, this leads to a high amount of fragmentation products and very low yields of molecular ions. Another disadvantage of the simultaneous desorption and ionization process is the formation of molecular adduct ions like M+IT or M+Na" etc. and not the pure molecular ion. In contrast multiphoton ionization (MUPI)/11/ delivers only molecular ions and their fragmentation products, hence multiphoton ionization should be compared with electron ionization. 

An alternative process to PT thin film has been developed using direct surface polymerization. It was proved that PT thin film was formed via surface polymerization by ion-assisted deposition (SPIAD), in which 55-200 eV thiophene ions and R-terthiophene neutrals are codeposited on surfaces [339-342]. This PT film displayed unique optical properties in its photoluminescence (PL) and ultraviolet-visible (UV-vis) absorption compared with the film prepared by direct thiophene ion deposition only. This method was clearly applicable to a wide range of different ions and neutral species. However, vaporized and ionized reagent species could not be applied by SPIAD. 

All qualitative applications of mass spectrometry are based on the determination of the mass-to-charge (m/z) ratio of an analyte. ESI and APCI MS are well known to produce abundant molecular ions and are consequently known as soft ionization techniques. This means that a relatively small amount of energy is transferred to the molecule as a result of vaporization and ionization. The efficiency of ion formation depends, in part, on a molecule s abihty to carry a charge (e.g., its proton affinity). The positive-ion mode ionization process can be defined by the following simple protonation reaction ... 

In solution and in gas phase, the characterization of self-assembled cages/ capsules requires analytical methods operating in timescales that are in accordance with their hfetimes (milliseconds to hours). NMR spectroscopy and mass spectrometry using soft methods (ESI, MALDI) for vaporization and ionization are appropriate. In some cases, the containers structures and their complexes have also been characterized in the solid state by X-ray diffiraction. Generally, the purification of supramolecular containers and their encapsulation complexes using chromatography is not feasible owing to dynamic features of the reversible self-assembly process that led to their formation. 

The process of field ionization presupposes that the substance under investigation has been volatilized by heat, so some molecules of vapor settle onto the tips held at high potential. In such circumstances, thermally labile substances still cannot be examined, even though the ionization process itself is mild. To get around this difficulty, a solution of the substance under investigation can be placed on the wire and the solvent allowed to evaporate. When an electric potential is applied, positive or negative ions are produced, but no heating is necessary to volatilize the substance. This technique is called field desorption (FD) ionization. 

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