Mass spectrometer ionization methods

There is a recent hybrid between AP-MALDI and ESI, matrix-assisted laser desorption electrospray ionization (MALDESI) [202], where species desorbed from a MALDI target are subjected to an electrospray before entering the mass spectrometer. The method is similar to ELDI except that the analyte is embedded in a matrix as in MALDI. 

Many interfaces have been developed to meet these demanding challenges. Some of these coupling methods, such as the moving belt or the particle beam interface, are based on the concomitant elimination of the solvent before it enters the mass spectrometer. Other methods such as direct liquid introduction (DLI) or continuous flow FAB rely on splitting the flow of the liquid that is introduced into the interface in order to obtain a flow that can be directly infused into the ionization source. However, these types of interfaces can only handle a fraction of the liquid flow from the LC. 

All mass spectrometers share common features. (See Figure 1.2) Some sort of chromatography usually accomplishes introduction of the sample into the mass spectrometer, although many instruments also allow for direct insertion of the sample into the ionization chamber. All mass spectrometers have methods for ionizing the sample and for separating the ions on the basis of mlz. These methods are discussed in detail below. Once separated, the ions must be detected and quantified. A typical ion collector consists of collimating slits that direct only one set of ions at a time into the collector, where they are detected and amplified by an electron multiplier. The method of ion detection is dependent to some extent on the method of ion separation. 

A mass spectrometer ionizes molecules in a high vacuum, sorts the ions according to their masses, and records the abundance of ions of each mass. A mass spectrum is the graph plotted by the mass spectrometer, with the masses plotted as the x axis and the relative number of ions of each mass on the y axis. Several methods are used to ionize samples and then to separate ions according to their masses. We will emphasize the most common techniques, electron impact ionization for forming the ions, and magnetic deflection for separating the ions. 

In thermal ionization mass spectrometry (TI-MS), solid, inorganic compounds may be volatilized from a heated surface. TI-MS is the most precise method for the measurement of isotopic ratios of minerals and has been used to analyze 58pe in fecal samples collected from a human study (H). The major drawbacks of this technique are the costly instrument and the slow sample through-put. Conventional mass spectrometry produces ions by electron bombardment of the vapor of volatile compoimds. This is called electron-impact ionization mass spectrometry (EI-MS). Analysis of iron by EI-MS requires derivitization to volatile forms before introduction into the mass spectrometer. A method has been developed for the synthesis of volatile iron-acetylacetone chelates from iron in blood serxm (1 ). A tetraphenylporphyrin chelate has also been synthesized and used in an absorption study in which 54pe and 57pe were given orally (16). 

The on-line coupling between TLC and mass spectrometry provides a powerful combination for the detection and identification of substances separated by a planar chromatographic method. The on-line coupling between these two methods has to overcome the problem of vaporizing and introducing the sample into the mass spectrometer. Different methods are reported in the literature, but the analytical principle is the same the sample is ionized from the layer surface by means of a laser beam, under vacuum, and in the presence of an energy-buffering matrix. Once the ions are transferred into the mass spectrometer, more sophisticated methods can be applied for data analysis and interpretation, e.g., MS-MS. 

Atmospheric pressure chemical ionization, like electrospray ionization, is a mass spectrometer ionization source in which ionization occurs not in a vacuum but at atmospheric pressure. In contrast to electrospray ionization, in which the ionization process occurs in solution phase, atmospheric pressure chemical ionization is a gas-phase ionization process whereby gas-phase molecules are isolated from the carrier solvent before ionization [6]. Because the ionization mechanisms of APCI and electrospray are fundamentally different (gas-phase and liquid-phase ionization, respectively) the two methods have the potential to provide complimentary analyte characterization. To generalize, electrospray ionization is more... 

ABSTRACT. Details of the novel method of laser evaporation of intact neutral molecules (LEIM) with a low powered IR-laser and the multiphoton ionization (MUPI) combined with a high-resolution Reflectron-Time-of-Flight (RETOF) mass spectrometer are explained. Some features of the method are discussed. Mass Spectra of biomolecules obtained with this method are displayed and their differences to other mass spectrometric techniques are discussed. It is shown, that Multiphoton Ionization is a general activation method for forming ions in a mass spectrometer, with additional features for an easy deducing of structures and intrinsic properties of biomolecules in contrast to other mass spectrometric ionization methods. 

MALDI is principally an off-line technique. The most common method consists in spotting the analytes, dissolved in a matrix solution, on a plate, where the matrix co-crystallizes with the analyte contained in its lattice. The function of the matrix is to adsorb the laser energy and transfer the resulting charge to the analyte, so that the analyte evaporates and can be transported by the electric field that is set up between the plate and the inlet of the mass spectrometer. Ionization usually takes place in a vacuum chamber, although atmospheric pressure (AP) MALDI exists however, this method is not (yet) very well developed and less preferred because of the limited sensitivity and mass range. 

The following protocol describes a mass spectromet-ric method for characterization of a block copolymer consisting of methoxy poly(ethylene oxide) (mPEO), an -e-caprolactone (CL) segment, and linoleic acid (LA), used as surfactant in water-based latex paints by liquid chromatography electrospray ionization (LC-ESI) or API-MS [10]. 

Inductively coupled plasma-mass spectrometry is a very rapid technique for the determination of long-lived radionuclides. This technique is based on the ionization of elements in the plasma source. Typically, radiofrequency and argon are used to reach plasma excitation temperatures ranging from 4900 to 7000 K [18,19]. The ions produced are introduced through an interface into a vacuum chamber and are analyzed by a quadru-pole mass spectrometer. Other attempts are being made to use faster mass-spectrometer detectors, such as time-of-flight mass spectrometers, but methods are still not available. 

Recendy several studies have reported the use of analytical columns packed with sub-2 pm size particles (11, 12, 14, 15). These columns are especially designed for use with ultrahigh-performance liquid chromatography (UHPLC). The smaller particles are more efficient because they can be used at higher linear velocities, which provides both better resolution and shorter analysis times. UHPLC methods have been reported using isocratic and gradient elutions, a variety of mobile phases, and aqueous buffers with pH ranging from 2.6 to 8.6 to ensure optimum ionization and intensities for specific mass spectrometer ionization sources (14). 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

A third method for generating ions in mass spectrometers that has been used extensively in physical chemistry is chemical ionization (Cl) [2]. Chemical ionization can involve the transfer of an electron (charge transfer), proton (or otlier positively charged ion) or hydride anion (or other anion). 

For mixture.s the picture is different. Unless the mixture is to be examined by MS/MS methods, usually it will be necessary to separate it into its individual components. This separation is most often done by gas or liquid chromatography. In the latter, small quantities of emerging mixture components dissolved in elution solvent would be laborious to deal with if each component had to be first isolated by evaporation of solvent before its introduction into the mass spectrometer. In such circumstances, the direct introduction, removal of solvent, and ionization provided by electrospray is a boon and puts LC/MS on a level with GC/MS for mixture analysis. Further, GC is normally concerned with volatile, relatively low-molecular-weight compounds and is of little or no use for the many polar, water soluble, high-molecular-mass substances such as the peptides, proteins, carbohydrates, nucleotides, and similar substances found in biological systems. LC/MS with an electrospray interface is frequently used in biochemical research and medical analysis. 

The advent of atmospheric-pressure ionization (API) provided a method of ionizing labile and nonvolatile substances so that they could be examined by mass spectrometry. API has become strongly linked to HPLC as a basis for ionizing the eluant on its way into the mass spectrometer, although it is also used as a stand-alone inlet for introduction of samples. API is important in thermospray, plasmaspray, and electrospray ionization (see Chapters 8 and 11). 

If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

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