Molecular-ion samples

After cleaning of BeH" " ions generated in addition to Equation 18.12 using the procedure mentioned in Section 18.5.4, only and Be" " ions remain Figure 18.27a. Thus, this is an example of the production of a sympathetically cooled molecular ion sample that contains only one molecular species. 

Molecular beam sample introduction (described in section (Bl.7.2)). followed by the orthogonal extraction of ions, results in improved resolution in TOP instruments over eflfrisive sources. The particles in the molecular beam typically have translational temperatures orthogonal to the beam path of only a few Kelvin. Thus, there is less concern with both the initial velocity of the ions once they are generated and with where in the ion source they are fonned (since the particles are originally confined to the beam path). 

Formation of reactive ions (CH," ) from methane (CH4) reagent gas and their reaction with sample molecules (M) to form protonated molecular ions [M + Hp. 

Typical Cl processes in which neutral sample molecules (M) react with NH to give either (a) a protonated ion [M + HJ or (b) an adduct ion [M + NHJ+ the quasi-molecular ions are respectively 1 and 18 mass units greater than the true mass (M). In process (c), reagent ions (CjHf) abstract hydrogen, giving a quasi-molecular ion that is 1 mass unit less than M. 

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. 

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

At the target, clusters are broken up and sample molecular ions, accompanied by some remaining solvent ions, are extracted by an electrical potential through a small hole into the mass spectrometer analyzer (Figure 11.1), where their mass-to-charge (m/z) ratios are measured in the usual way. The mass spectrometer may be of any type. 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

For this FAB experiment, a sample of the pentapeptidic enkephalin, Tyr.Gly.Gly.Phe.Leu., dissolved in glycerol was bombarded by xenon atoms. The resulting mass spectrum shows abundant protonated molecular ions at m/z 556. 

This last m/z value is easy to measure accurately, and, if its relationship to the true mass is known (n = 10), then the true mass can be measured very accurately. The multicharged ions have typical m/z values of <3000 Da, which means that conventional quadrupole or magnetic-sector analyzers can be used for mass measurement. Actually, the spectrum consists of a series of multicharged protonated molecular ions [M + nWY for each component present in the sample. Each ion in the series differs by plus and minus one charge from adjacent ions ([M + uH] + n -an integer series for example, 1, 2, 3,. .., etc.). Mathematical transformation of the spectrum produces a true molecular mass profile of the sample (Figure 40.5). 

A sample of the protein, horse heart myoglobin, was dissolved in acidified aqueous acetonitrile (1% formic acid in HjO/CHjCN, 1 1 v/v) at a concentration of 20 pmol/1. This sample was injected into a flow of the same solvent passing at 5 pl/min into the electrospray source to give the mass spectrum of protonated molecular ions [M + nH] shown in (a). The measured ra/z values are given in the table (b), along with the number of protons (charges n) associated with each. The mean relative molecular mass (RMM) is 16,951,09 0.3 Da. Finally, the transformed spectrum, corresponding to the true relative molecular mass, is shown in (c) the observed value is close to that calculated (16,951.4), an error of only 0.002%. 

This example can be used in reverse to show the usefulness of looking for such isotopes. Suppose there were an unknown sample that had two molecular ion peaks in the ratio of 3 1 that were two mass units apart then it could reasonably be deduced that it was highly likely the unknown contained chlorine. In this case, the isotope ratio has been used to identify a chlorine-containing compound. This use of mass spectrometry is widespread in general analysis of materials, and it... 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

Fast-atom bombardment (FAB) is an ionization technique that produces a protonated or deprotonated molecular ion, hence a molecular mass for the sample. It can be used for analysis of peptides up to m/z about 5000. 

Ideally, a mass spectmm contains a molecular ion, corresponding to the molecular mass of the analyte, as well as stmcturaHy significant fragment ions which allow either the direct deterrnination of stmcture or a comparison to Hbraries of spectra of known compounds. Mass spectrometry (ms) is unique in its abiUty to determine direcdy the molecular mass of a sample. Other techniques such as nuclear magnetic resonance (nmr) and infrared spectroscopy give stmctural information from which the molecular mass may be inferred (see Infrared technology and raman spectroscopy Magnetic spin resonance). 

In plasma chromatography, molecular ions of the heavy organic material to be analy2ed are produced in an ionizer and pass by means of a shutter electrode into a drift region. The velocity of drift through an inert gas at approximately 101 kPa (1 atm) under the influence of an appHed electric field depends on the molecular weight of the sample. The various sonic species are separated and collected every few milliseconds on an electrode. The technique has been employed for studying upper atmosphere ion molecule reactions and for chemical analysis (100). 

Tandem mass spectrometry or ms/ms was first introduced in the 1970s and gained rapid acceptance in the analytical community. The technique has been used for stmcture elucidation of unknowns (26) and has the abiUty to provide sensitive and selective analysis of complex mixtures with minimal sample clean-up (27). Developments in the mid-1980s advancing the popularity of ms/ms included the availabiUty of powerhil data systems capable of controlling the ms/ms experiment and the viabiUty of soft ionisation techniques which essentially yield only molecular ion species. 

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