Mass analysis

After the analytes are ionized, they enter the mass spectrometer through a series of lenses and skimmers. These lenses and skimmers cause focusing of the ion beam. 

In addition to this, they also divide the mass spectrometer into several compartments. For each compartment closer to the analyzer, the pressure drops. The vacuum is created gradually throughout the system. Vacuum is essential for mass spectrometry the presence of air (gas molecules) hampers the ions to fly toward the analyzer. Performance of the mass analyzers can be summarized with the following parameters  

The higher the value for R, the better the separation of closely related m/z values. 

There are several mass analyzers. In the following sections, the most common ones are being described with their functioning principles. 

Developed around the same time as the DART technique (Section 3.3B), DESI interfaces with a mass analyzer using a heated ion transfer tube that is in some cases flexible and can be held in the researcher s hand directly above the sample surface. 

Once the sample has been ionized, the beam of ions is accelerated by an electric field and then passes into the mass analyzer, the region of the mass spectrometer where the ions are separated according to their mass-to-charge (m/z) ratios. Just like there are many different ionization methods for different applications, there are also several types of mass analyzers. While some mass analyzers are more versatile than others, none of the options are one-size-fits-all. 

Ions generated in the source of the spectrometer have to be discriminated from each other on the basis of their mass-to-charge ratio. Conventional mass spectrometers usually use one of the following three methods to achieve this. 

Using appropriate electric fields the radical cation IVT can now be sent to the mass analyzer for subsequent mass analysis. This radical cation can also undergo fragmentation. 

The fragment ions (F+) formed can be used to provide additional structural information about the original compound. 

Another conventional ionization technique termed chemical ionization (Cl), utilizes a reagent gas (such as isobutane, methane, ammonia) to form reagent ions (Rif) which can undergo ion-molecule reactions with the compound of interest to form protonated molecules. 

The protonated molecules are much less energetic than the related radical cations formed during electron impact and are less prone to fragmentation. Both El and Cl require volatile samples. This puts a tremendous limitation on the types of molecules that can be ionized, in particular, compounds found in biological systems. 

An ion trap mass analyzer has a variety of differing physical arrangements of its electrodes, but the primary objective remains the same to allow the ions to enter and then to trap them in space between the electrodes. Unlike the fly-through mass analysis scheme of a quadrupole, the ion trap mass analyzer stores the ions. They are then ejected to the detector as a function of the mass-to-charge ratio, typically by scanning the rf voltage. 

The majority of H/D studies that have been reported employ quadrupole ion trap (QIT) instruments due to their ease of use, excellent sensitivity, ability to perform MS/MS experiments, compact size, and low cost. Other reports discuss the use of instruments with higher mass-resolving power such as the hybrid QqTOF instruments [47]. A few groups have utilized FT-ICR mass spectrometry, which offers ultra-high mass-resolving power and improved mass accuracy [48, 49]. 

A fully automated system for performing detailed studies has been developed to improve the reproducibility and throughput (Fig. 12.2) [8]. It consists of two functional components a sample-deuteration device and a protein processing unit. The preparation operations (shown at the top of Fig. 12.2) are performed by two robotic arms equipped with low volume syringes and two temperature-controlled chambers, one held at 25 °C and the other held at 1 °C. To initiate the exchange experiment, a small amount of protein solution is mixed with a deuter-ated buffer and the mixture is then incubated for a programmed period of time in the temperature-controlled chamber. This on-exchanged sample is immediately transferred to the cold chamber where a quench solution is added to the mixture. 

The exchange-quenched solution is then injected onto the protein processing system which includes injection loops, protease column(s), a trap column, an analytical column, electronically controlled valves, and isocratic and gradient pumps. 

The injector, columns and valves reside in a low temperature chamber to minimize the loss of deuterium by back exchange (Fig. 12.2). The quenched protein solution is pumped in series through a column containing an immobilized protease and a trap column to capture the peptide fragments. The gradient pump is activated following digestion and the peptides captured on the trap column are eluted and separated over an analytical reverse-phase HPLC column directly into the mass spectrometer. 

ESI-MS is not limited to the study of large biomolecules, however. Many small molecules with molecular weight in the 100-1500 range can be studied by ESI-MS. Compounds that are too nonvolatile to be introduced by direct probe methods or are too polar or thermally labile to be introduced by GC-MS methods are ideal for study by LC-MS using ESI techniques. 

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Arnold F and Henschen G 1978 First mass analysis of stratospheric negative ions Nature 257 521-2 Eisele F L 1989 Natural and anthropogenic negative ions in the troposphere J. Geophys. Res. 94 2183-96 Oka T 1997 Water on the sun—molecules everywhere Science 277 328-9... 

Another approach to mass analysis is based on stable ion trajectories in quadnipole fields. The two most prominent members of this family of mass spectrometers are the quadnipole mass filter and the quadnipole ion trap. Quadnipole mass filters are one of the most connnon mass spectrometers, being extensively used as detectors in analytical instnunents, especially gas clnomatographs. The quadnipole ion trap (which also goes by the name quadnipole ion store, QUISTOR , Paul trap, or just ion trap) is fairly new to the physical chemistry laboratory. Its early development was due to its use as an inexpensive alternative to tandem magnetic sector and quadnipole filter instnunents for analytical analysis. It has, however, staned to be used more in die chemical physics and physical chemistry domains, and so it will be described in some detail in this section. 

The mix of ions, formed essentially at or near ambient temperatures, is passed through a nozzle (or skimmer) into the mass spectrometer for mass analysis. Since the ions are formed in the vapor phase without having undergone significant heating, many thermally labile and normally nonvolatile substances can be examined in this way. 

The beam entering the ion chamber is suitable for both electron (El) and chemical (Cl) ionization, and either mode can be used (Figure 12.3). Mass analysis follows in the usual way, typically using quadruple or magnetic-sector instruments. 

After the skimmer, the ions must be prepared for mass analysis, and electronic lenses in front of the analyzer are used to adjust ion velocities and flight paths. The skimmer can be considered to be the end of the interface region stretching from the end of the plasma flame. Some sort of light stop must be used to prevent emitted light from the plasma reaching the ion collector in the mass analyzer (Figure 14.2). 

Evaporation of solvent from a spray of electrically charged droplets at atmospheric pressure eventually yields ions that can collide with neutral solvent molecules. The assemblage of ions formed by evaporation and collision is injected into the mass spectrometer for mass analysis. 

Electrostatic analyzer. A velocity-focusing device for producing an electrostatic field perpendicular to the direction of ion travel (usually used in combination with a magnetic analyzer for mass analysis). The effect is to bring to a common focus all ions of a given kinetic energy. 

Mass analysis. A process by which a mixture of ionic (or neutral) species is separated according to the mass-to-charge (m/z) ratios (for ions) or their aggregate atomic masses (for neutrals). The analysis can be qualitative or quantitative. 

Total ion current (TIC), (a) After mass analysis the sum of all the separate ion currents carried by the different ions contributing to the spectrum, (b) Before mass analysis the sum of all the separate ion currents for ions of the same sign. 

The simplest example of this type of instmment is the triple quadmpole ms (27), in which and 2 used for mass analysis of the precursor and product ions, respectively, and 2 is a rf-only quadmpole coUision cell. The maximum possible energy uptake during coUisional activation, ... 

The methods just noted tell something about the physical characteristics of atmospheric particulate matter but nothing about its chemical composition. One can seek this kind of information for either individual particles or all particles en masse. Analysis of particles en masse involves analysis of a mixture of particles of many different compounds. How much of... 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

Although a number of studies have been made concerning the basic properties of the RF vacuum spark used for excitation, the discharge is typically erratic, producing a widely fluctuating signal for mass analysis. For this reason, the most widely used form of this instrumentation consists of a mass spectrometer of the... 

Laser Ionization Mass Spectrometry Laser Microprobe Mass Analysis Laser Microprobe Mass Spectrometry Laser Ionization Mass Analysis Nonresonant Multi-Photon Ionization... 

Fig. 3.3. Experimental arrangement used by Krauss and Gruen for SSIMS [3.8] a qua-drupole mass spectrometer was used for mass analysis and a retarding-field analyzer for prior energy selection (a) ion gun (b)-(d) lenses 1-3 (e) quadrupole mass spec-...

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. 

The surface is bombarded with a stream of inert gas ions of energy, Eo, and the sputtered target secondary ions of energy, E, are monitored, rather than the backscattered primary beam ions. Mass analysis of the secondary ions is carried out. The intensity and the energy are also determined, Each element has a characteristic value of E/ Eq. This allows the elemental analysis of the surface. 

The low energy ions leaving the reaction chamber are re-accelerated for conventional mass analysis. Many of these instruments use a pair of quadrupole lenses (13) following re-acceleration to increase the intensity of secondary ions. Such a lens system is particularly well adapted to this application because of its large physical size and strong focussing properties. 

More direct observations of the kinetic energy dependence of cross-sections should be possible using external ionization techniques where the reactant ion can be chosen by initial mass analysis and, in principle, its energy more readily controlled. Several studies using external ionization techniques, both with (2, 10, 45) and without (20, 21, 27, 41) preliminary mass selection of the reactant ion, have been reported. However, apparently with these techniques it is not possible to obtain well-defined primary ion beams at energies below 0.5-1 e.v. a region of critical importance both experimentally and theoretically. 

In 1960 Tal roze and Frankevich (39) first described a pulsed mode of operation of an internal ionization source which permits the study of ion-molecule reactions at energies approaching thermal energies. In this technique a short pulse of electrons is admitted to a field-free ion source to produce the reactant ions by electron impact. A known and variable time later, a second voltage pulse is applied to withdraw the ions from the ion source for mass analysis. In the interval between the two pulses the ions react under essentially thermal conditions, and from variation of the relevant ion currents with the reaction time the thermal rate constants can be estimated. 

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