Continuous ionization techniques

TOF analysers are directly compatible with pulsed ionization techniques such as plasma or laser desorption because they provide short, precisely defined ionization times and a small ionization region. However, to take advantage of TOF analysers, it is interesting to combine such powerful analysers with continuous ionization techniques. These ionization techniques can be compatible with TOF analysers but require some adaptations to pulse the source or to transform a continuous ion beam into a pulsed process. For instance, the coupling of an ESI (or any other API) source with a TOF mass spectrometer is difficult, because ESI yields a continuous ion beam, whereas the TOF system works on a pulsed process. 

TOF-TOF Instruments. A simple design for a tandem (TOF-TOF) instrument that incorporates two drift regions separated by a collision chamber has been reported by Jardine et al. As shown in Figure 9.5, the instrument uses fast-atom bombardment ionization. Because this is a continuous ionization technique, a deflection gating system is used to define the initial ion packet, while a second set of deflectors (located close to the collison chamber) is used for mass selection. In addition, the collision cell is raised to a voltage that is intermediate between ground and the source potential, so that product ions will be reaccelerated and will have velocities different from their precursors. 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

Thermospray interface. Provides liquid chromatographic effluent continuously through a heated capillary vaporizer tube to the mass spectrometer. Solvent molecules evaporate away from the partially vaporized liquid, and analyte ions are transmitted to the mass spectrometer s ion optics. The ionization technique must be specified, e.g., preexisting ions, salt buffer, filament, or electrical discharge. 

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. 

ESI has become the most commonly used interface for LC/MS. It was recognized by John Fenn and co-workers as an important interface for LC/MS immediately after they developed it as an ionization technique for MS. ESI transforms ions in solution to ions in the gas phase and may be used to analyze any polar molecule that makes a preformed ion in solution. The technique has facilitated the ionization of heat-labile compounds and high-molecular-weight molecules such as proteins and peptides. ESI is a continuous ionization method that is particularly suitable for use as an interface with FiPLC. It is the most widely accepted soft-ionization technique for the determination of molecular weights of a wide variety of analytes and, has made a significant impact on drug discovery and development since the late 1980s. 

In summary, the use of mass spectrometric methods, combined with various approaches to vaporizing and ionizing the particles, is gaining increasing popularity and interest for the analysis of continuous sources of particles or single particles. The problem of quantification of the components seen by single-particle laser ionization techniques remains to be solved. On the other hand, the vaporization approaches can provide quantitative data on some volatile and semivolatile components but cannot measure the nonvolatile species and, at present, do not provide a full mass spectrum for a single particle. 

The sample is usually dissolved in a mixture of water and organic solvent, commonly methanol, isopropanol, or acetonitrile. It can be directly infused, or injected into a continuous flow of this mixture, or be contained in the effluent of an HPLC column or CE capillary. First introduced in late 1980s, MALDI is a soft ionization technique that allows the analysis of intact molecules of high masses. It allows determination of the molecular mass of macromolecules such as peptides and proteins more than 300 kDa in size. 

Most mass spectrometers equipped for electrospray ionization can be converted to APCI, and many commercial LC-APCI-MS instruments are equipped with both ionization techniques. During APCI, ionization takes place in an atmospheric pressure chamber when the sample molecules collide with solvent ions formed in a continuous corona discharge. Unlike electrospray, the needle used to spray the HPLC effluent is not at high voltage. 

FAB and LSIMS are matrix-mediated desorption techniques that use energetic particle bombardment to simultaneously ionize samples like carotenoids and transfer them to the gas phase for mass spectrometric analysis. Molecular ions and/or protonated molecules are usually abundant and fragmentation is minimal. Tandem mass spectrometry with collision-induced dissociation (CID) may be used to produce abundant structurally significant fragment ions from molecular ion precursors (formed using FAB or any suitable ionization technique) for additional characterization and identification of chlorophylls and their derivatives. Continuous-flow FAB/LSIMS may be interfaced to an HPLC system for high-throughput flow-injection analysis or on-line LC/MS. 

Since FAB (or LSIMS) requires that the analyte be dissolved in a liquid matrix, this ionization technique was easily adapted for infusion of solution-phase samples into the FAB ionization source, in an approach known as continuous-flow FAB. Continuous-flow FAB was connected to microbore HPLC columns for LC/MS applications (Ito et al., 1985). Since this method is limited to microbore HPLC applications at flow rates of <10 pl/min... 

MALDI is considered the most sensitive of these techniques, with detection limits in the femtomoles/microliter range being relatively behind these other two, and this probably explains the general lack of continued interest in them in recent years. Of course statements concerning the relative sensitivities of the four ionization techniques have to take into account the chemical differences between compound classes, which plays a major part in the ionization of a given compound by a given ionization technique. 

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. 

Contrary to most other ionization sources that yield a continuous ion beam, MALDI is a pulsed ionization technique that produces ions in bundles by an intermittent process. The pulsed nature of the MALDI source is well suited for the time-of-flight (TOF) analyser. In addition, the TOF analyser has the ability to analyse ions over a wide mass range and thus... 

An ideal interface should not cause extra-column peak broadening. Historical interfaces include the moving belt and the thermospray. Common interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Several special interfaces include the particle beam—a pioneering technique that is still used because it is the only one that can provide electron ionization mass spectra. Others are continuous fiow fast atom bombardment (CF-FAB), atmospheric pressure photon ionization (APPI), and matrix-assisted laser desorption ionization (M ALDl). The two most common interfaces, ESI and APCI, were discovered in the late 1980s and involve an atmospheric pressure ionization (API) step. Both are soft ionization techniques that cause little or no fragmentation hence a fingerprint for qualitative identification is usually not apparent. 

Earlier methods of ionization applied to carotenoids, including electron impact (El), chemical ionization (Cl), a particle beam interface with El or Cl, and continuous-flow fast atom bombardment (CF-FAB), have been comprehensively reviewed elsewhere (van Breemen, 1996, 1997 Pajkovic and van Breemen, 2005). These techniques have generally been replaced by softer ionization techniques like electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), and more recently atmospheric pressure photoionization (APPI). It should be noted that ESI, APCI, and APPI can be used as ionization methods with a direct infusion of an analyte in solution (i.e. not interfaced with an HPLC system), or as the interface between the HPEC and the MS. In contrast, matrix-assisted laser desorption ionization (MALDI) cannot be used directly with HPEC. 

In addition to ESI and APCI, ionization techniques such as electron ionization (El) and chemical ionization (Cl) have also been used in the analysis of flavanones (Weintraub et al., 1995). As the advancement and refinement in analytical techniques continues, it is expected that these techniques will find an application in flavanone analysis. 

Continuous-flow FAB (Ch. 4.6, [10]). This was the most successful approach to an on-line combination of LC-MS via a desorption ionization technique. 

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