Molecular ions ionization

The detection efficiency of C6H5X (X = F, Cl, Br and I) was investigated with the laser multiphoton ionization method152. The laser-induced ion yield depends mainly on the cross sections of the transitions available to the molecule ground state and on the lifetime of the intermediate electronic state that is initially excited. If a species has a radiative lifetime that is very short compared to the pulse duration, it may relax after excitation and will not be ionized. Molecular ions will therefore be obtained when laser pulses that are at least as short as the excited-state lifetimes are employed. The S excited states of halobenzenes are estimated to have subnanosecond lifetimes, with the exception of fluorobenzene for which a lifetime of the order of 9-10 ns has been calculated at 2ex = 266 nm. Picosecond laser pulses are therefore found effective in producing ionization of halobenzenes with short lifetimes, whereas nanosecond pulses are not152. 

A.26.15 (a) Mass spectrometry uses the mass-to-charge ratio (m/z) of the gaseous ionized molecular ions. Its separation results in a plot that shows the abundance of a particular molecular fragment as a function of its m/z ratio, (b) The compounds could have identical masses and charges. One compound could be a multiple of the mass of the other compound, e.g. 2 1, and have die same ratio in charge. The researcher could have mistakenly used the same compound for both tests. Because of the last possibility, the researcher should repeat her experiment to make certain the result is reproducible. 

In soft ionization, e.g., chonical and electrospray ionization, the energy associated with the added charge is usually only sufficient to generate adducted ions, such as [M + HJ, i.e., M + proton (Section 2.2.2), but not to cause fragmentation. Note that the ions formed are protonated molecules and not protonated molecular ions, as the latter would have two charges, i.e., the already ionized molecular ion and the proton that is itself an ion. 

Keywords chemical analysis identification chemical ionization molecular ion MS/MS ion trap amines Mass Frontier fragmentation pathways... 

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

The mass spectra of phenylthiazoles are characterized by the presence of intense molecular ion peaks, due to the aromatic nature of the molecules, which represent 35, 41, and 44% of the total ionization for 2-, 4-, and 5-phenylthiazoles, respectively. 

We say the molecule AB has been ionized by electron impact The species that results called the molecular ion, is positively charged and has an odd number of electrons—it IS a cation radical The molecular ion has the same mass (less the negligible mass of a single electron) as the molecule from which it is formed... 

Unlike the case of benzene in which ionization involves loss of a tt electron from the ring electron impact induced ionization of chlorobenzene involves loss of an elec tron from an unshared pair of chlorine The molecular ion then fragments by carbon-chlorine bond cleavage... 

Molecular Identification. In the identification of a compound, the most important information is the molecular weight. The mass spectrometer is able to provide this information, often to four decimal places. One assumes that no ions heavier than the molecular ion form when using electron-impact ionization. The chemical ionization spectrum will often show a cluster around the nominal molecular weight. 

This chapter should be read in conjunction with Chapter 3, Electron Ionization. In electron ionization (El), a high vacuum (low pressure), typically 10 mbar, is maintained in the ion source so that any molecular ions (M +) formed initially from the interaction of an electron beam and molecules (M) do not collide with any other molecules before being expelled from the ion source into the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics). 

Decomposition (fragmentation) of a proportion of the molecular ions (M +) to form fragment ions (A B+, etc.) occurs mostly in the ion source, and the assembly of ions (M +, A+, etc.) is injected into the mass analyzer. For chemical ionization (Cl), the Initial ionization step is the same as in El, but the subsequent steps are different (Figure 1.1). For Cl, the gas pressure in the ion source is typically increased to 10 mbar (and sometimes even up to atmospheric pressure) by injecting a reagent gas (R in Figure 1.1). 

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

This is entirely analogous to the problem with simple chemical ionization, and the solution to it is similar. To give the quasi-molecular ions the extra energy needed for them to fragment, they can be passed through a collision gas and the resulting spectra analyzed for metastable ions or by MS/MS methods (see Chapters 20 through 23). 

The FAB source operates near room temperature, and ions of the substance of interest are lifted out from the matrix by a momentum-transfer process that deposits little excess of vibrational and rotational energy in the resulting quasi-molecular ion. Thus, a further advantage of FAB/LSIMS over many other methods of ionization lies in its gentle or mild treatment of thermally labile substances such as peptides, proteins, nucleosides, sugars, and so on, which can be ionized without degrading their. structures. 

Some mild methods of ionization (e.g., chemical ionization. Cl fast-atom bombardment, FAB electrospray, ES) provide molecular or quasi-molecular ions with so little excess of energy that little or no fragmentation takes place. Thus, there are few, if any, normal fragment ions, and metastable ions are virtually nonexistent. Although these mild ionization techniques are ideal for yielding molecular mass information, they are almost useless for providing details of molecular structure, a decided disadvantage. 

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

The study of metastable ions concerns substances that have been ionized by electrons and have undergone fragmentation. The stable molecular ions that are formed by soft ionization methods (chemical ionization. Cl field ionization, FI) need a boost of extra energy to make them fragment, but in such cases other methods of investigation than linked scanning are generally used. 

There are ill-defined limits on EI/CI usage, based mostly on these issues of volatility and thermal stability. Sometimes these limits can be extended by preparation of a suitable chemical derivative. For example, polar carboxylic acids generally give either no or only a poor yield of molecular ions, but their conversion into methyl esters affords less polar, more volatile materials that can be examined easily by EL In the absence of an alternative method of ionization, EI/CI can still be used with clever manipulation of chemical derivatization techniques. 

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