Information in a Mass Spectrum

The mass spectrum of a molecule provides information about its structure. At the frontier of mass spectrometry today, scientists are elucidating the sequences of amino acids in proteins and the structures of complex carbohydrates by their fragmentation patterns. In this section, we touch on some of the simplest information available from mass spectrometry. 

The unit of atomic mass is the dalton, Da, defined as 1/12 of the mass of C. Atomic mass is the weighted average of the masses of the isotopes of an element. Table 21-1 tells us that bromine consists of 50.69% Br with a mass of 78.918 34 Da and 49.31% Br with a mass of 80.916 29 Da. In the weighted average, each mass is multiplied by its abundance. Therefore the atomic mass of Br is (0.506 9)(78.918 34) + (0.493 1)(80.916 29) = 79.904 Da. 

The nominal mass of a molecule or ion is the integer mass of the species with the most abundant isotope of each of the constituent atoms. For carbon, hydrogen, and bromine, the most abundant isotopes are H, and Br. Therefore the nominal mass of C4H9Br is (4 X 12) + (9 X 1) + (1 X 79) = 136. 

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In contrast to IR and NMR spectroscopy, the principle of mass spectrometry (MS) is based on decomposition and reactions of organic molecules on theii way from the ion source to the detector. Consequently, structure-MS correlation is basically a matter of relating reactions to the signals in a mass spectrum. The chemical structure information contained in mass spectra is difficult to extract because of the complicated relationships between MS data and chemical structures. The aim of spectra evaluation can be either the identification of a compound or the interpretation of spectral data in order to elucidate the chemical structure [78-80],... 

As we have just seen, interpreting the fragmentation patterns in a mass spectrum in terms of a molecule s structural units makes mass spectrometry much more than just a tool for-determining molecular- weights. Nevertheless, even the molecular- weight can provide more information than you might think. 

MS-MS is a term that covers a number of techniques in which two stages of mass spectrometry are used to investigate the relationship between ions found in a mass spectrum. In particular, the product-ion scan is used to derive structural information from a molecular ion generated by a soft ionization technique such as electrospray and, as such, is an alternative to CVF. The advantage of the product-ion scan over CVF is that it allows a specific ion to be selected and its fragmentation to be studied in isolation, while CVF bring about the fragmentation of all species in the ion source and this may hinder interpretation of the data obtained. 

One can get an enormous amount of information from studying the region of the molecular ion in a mass spectrum. The mass of M+ is the molecular mass of the analyte. The ratio of the isotopic peaks (see below) allows one to roughly establish the elemental composition, while accurate mass measurements using high resolution mass spectrometry give exact elemental composition. The relative intensity of the M+ peak... 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

All analytical techniques are designed to provide the answer to one or both of the two important questions what is it and how much is there Mass spectrometry possesses attributes that allow it to contribute answers to both of these questions. The nominal (integral) m/z ratio of the molecular ion can sometimes be sufficient to identify a chemical compound, particularly if there is additional information available (either from the mass spectrum itself or another analytical technique). The presence of other signals in a mass spectrum attributable to... 

The molecular ion peak directly provides valuable information on the analyte. Provided the peak being of sufficient intensity, in addition to mere molecular mass, the accurate mass can reveal the molecular formula of the analyte, and the isotopic pattern may be used to derive limits of elemental composition (Chaps. 3.2 and 3.3). Unfortunately, the peak of highest m/z in a mass spectrum must not necessarily represent the molecular ion of the analyte. This is often the case with El spectra either as a result of rapidly fragmenting molecular ions or due to thermal decomposition of the sample (Chaps. 6.9 and 6.10.3)... 

In the past, PTRC screening was mainly based on gas chromatography-mass spectrometry (GC-MS) [116]. The choice of GC-MS was based on a number of good reasons (separation power of GC, selectivity of detection offered by MS, inherent simplicity of information contained in a mass spectrum, availability of a well established and standardized ionization technique, electron ionization, which allowed the construction of large databases of reference mass spectra, fast and reliable computer aided identification based on library search) that largely counterbalanced the pitfalls of GC separation, i.e., the need to isolate analytes from the aqueous substrate and to derivatize polar compounds [117]. 

The major limitation of both UV and MS detectors is that neither can provide quantitative or even semiquantitative information without reference standards. Ultraviolet response depends on the presence of a chromophore in a molecule and evidently might vary from one molecular species to another in a library. Although successful application of electrospray mass spectrometry for quantitative analysis of peptides has been reported [35], one should always keep in mind that signal intensity in a mass spectrum depends on the ability of a molecule to ionize. The ability to produce ions, especially with soft ionization techniques, might be very different for different molecules within one library, and the difference might be even bigger from one library to another. 

An added piece of information is available in the isotopic fingerprints in a mass spectrum. The isotopic ratios can assist in Identifying the species and fragmentation patterns of the substance being analyzed. The ability of SIMS to detect different Isotopes of an element adds richness to the data obtained and flexibility to the analysis. 

In gas chromatography coupled with mass spectrometry a procedure has been given, how to separate the inadequate resolved peaks [7,8]. It is remarkable that the mass spectra of the individual peaks that are overlapping need not to be known. Such a procedure is only possible, because in a mass spectrum of an individual compound there is so much information inside. The mass spectrum in a gas chromatographic peak is, for each mass number, the sum of the concentration of each individual compound, weighted by the relative intensity of the respective mass number and the relative amount of the respective compound. In the procedure, a normalized mass spectrum is constructed as a vector with the relative intensities of the mass numbers. Here it is necessary that the number of the mass numbers constituting a compound is equal for each compound, i.e., all the vectors should have the same dimension. Because of the formalism of the matrix multiplication, the overlapped spectrum is the product of the matrix of the spectra and the concentration vector. The overlapped spectrum is also addressed as matrix of the observable intensities... 

Mass spectrometry has been applied in electrochemical investigations predominantly as an ex situ method because of the obvious incompatibility of the high vacuum needed for all types of mass spectrometry and the presence of a liquid electrolyte solution. Because of the amount of information provided in a mass spectrum, there have been various attempts to couple mass spectrometers with electrochemical cells as described below. 

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