Molecular diagnostics mass spectrometry

A.G. Lowe, A.T. Hartlier, J. Brand, B. Atakan, and K. Kohse-Hoinghaus, In situ Temperature and Species Concentration Measurements by Laser Diagnostics and Molecular Beam Mass Spectrometry, Combustion Flame, Vol.118, 1999, pp.37-50. 

Fig. 51. A RE capacitively-coupled diode with a variety of plasma diagnostics, including Langmuir probe, optical emission spectroscopy, molecular beam mass spectrometry, and ion flux/energy analysis systems. After [35).

The hyphenation of capillary electromigration techniques to spectroscopic techniques which, besides the identification, allow the elucidation of the chemical structure of the separated analytes, such as mass spectrometry (MS) and nuclear magnetic resonance spectroscopy (NMR) has been widely pursued in recent years. Such approaches, combining the separation efficiency of capillary electromigration techniques and the information-rich detection capability of either MS or NMR, are emerging as essential diagnostic tools for the analysis of both low molecular weight and macromolecular compounds. 

In the past decade, eight inherited disorders have been linked to specific enzyme defects in the isoprenoid/cholesterol biosynthetic pathway after the finding of abnormally increased levels of intermediate metabolites in tissues and/or body fluids of patients (Table 5.1.1) [7, 9, 10]. Two of these disorders are due to a defect of the enzyme mevalonate kinase, and in principle affect the synthesis of all isoprenoids (Fig. 5.1.1) [5]. The hallmark of these two disorders is the accumulation of mevalonic acid in body fluids and tissues, which can be detected by organic acid analysis, or preferably, by stable-isotope dilution gas chromatography (GC)-mass spectrometry (GC-MS) [2]. Confirmative diagnostic possibilities include direct measurement of mevalonate kinase activities in white blood cells or primary skin fibroblasts [3] from patients, and/or molecular analysis of the MVK gene [8]. 

Detection of additional but non-diagnostic ion High-resolution mass spectrometry Peak match of molecular ion... 

Interpretation of mass spectra depends on the type of mass spectrometer and ionisation technique used. Hard ionisation methods such as El produce molecular ion fragmentation, which can be used to identify diagnostic fragmentation patterns and functional groups. Softer ionisation techniques such as ESI and MALDI provide pseudomolecular ion formation, and rules in accordance with spectral information can be used to identify corresponding molecular structure and elemental composition. Table 13.3 lists some of the types of information that can be provided by mass spectrometry, and Table 13.4 gives dehnitions of molecular masses that are highly relevant in mass spectrometry. 

Photoionization is not a common technique in mass spectrometry, but it was reported to be utilized for both Py-GC/MS [61, 62] and for Py-MS [61]. Compared to El spectra, for which extended mass spectral libraries are available, the photoionization (PI) spectra are not standard and therefore not library searchable. However, the ionization being softer, the molecular ion is commonly more intense providing additional information in certain cases when the El mass spectrum is not diagnostic. 

Electron ionization (El) is surely the ionization method most widely employed (Mark and Dunn, 1985). This method was proposed and used from the early days of mass spectrometry (MS) applications in the chemical world and is still of wide interest. This interest is due to the presence of libraries of El mass spectra, which allows easy identification of unknown previously studied analytes. The El method suffers from two limitations It is based on the gas-phase interactions between the neutral molecules of the analyte and an electron beam of mean energy 70 eV. This interaction leads to the deposition of internal energy in the molecules of the analyte, which is reflected in the production of odd-electron molecular ([M]+ ) and fragment ions. These ions are highly diagnostic from a structural point of view. 

Moreover, a rapid diagnostic method for analysing the three new cytokinins (1 MeZ, its 9-P-D-riboside and 2 deOZR) was developed to detect them in complex samples. The natural cytokinins and some of their above-mentioned derivatives and analogues, were characterised by fast atom bombardment tandem mass spectrometry (FAB MS MS). The protonated molecular ions of the examined cytokinins could be fingerprinted from breakdown patterns of their gaseous unimolecular... 

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