Spectra soft-source

Both hard- and soft-source spectra arc uselTil for analysis. The many peaks in a hard-source spectrum provide useful information about the kinds of functional groups and thus structural information about analytes. Soft-source spectra are useful because they supply accurate information about the molecular mass of the analyte molecule or molecules. 

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. 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

In soft ionization methods the excess energy deposited onto the ionized molecule is very small and stable even-electron ions are formed. This leads to easy determination of the molecular weight of the analyte, but as fragmentation is absent or it occurs to a very low extent, structural information is missing in the mass spectrum. However, one can obtain structural information by causing ion fragmentation out of the source by means of tandem mass spectrometry experiments (see below). 

There are several methods of producing gas-phase inorganic ions, the starting materials in mass spectrometric studies. The properties of the source of the ions required for study are important in the choice of ionization method. The production of bare metal ions from an involatile nonmolecular source requires a large amount of energy deposited on the surface of the material. The processes that occur after the initial ionization process may also affect the ions finally observed (e.g., clustering). At the other end of the ionization energy spectrum, gas-phase ions of a complexity similar to those observed in the condensed phases require a soft ionization process. A brief description of some of the ionization methods follows. 

The XPS mechanism, which can be used for quantitative and qualitative chemical analysis of surfaces, is based on the photoelectric effect. A monochromatic soft Mg or Al anode X-ray source is used to irradiate the surface. The absorbed X-rays ionize die core shell, and in response, the atom creates a photoelectron that is transported to the surface and escapes. The ionization potential of a photoelectron that must be overcome to escape into vacuum is the binding energy (BE) plus the work function of the material. The emitted photoelectrons have a remaining kinetic energy (KE), which is measured by using an electron analyzer. Individual elements can be identified on the basis of their BE. The resulting XP spectrum is a characteristic set of peaks for a specific element, with BE as the abscissa and counts per unit time as... 

It may be mentioned here that this decade probably will see the soft lunar landing of an atomic absorption spectroscope (M8) intended to analyze surface material of the moon and relay signals back to earth. The system makes use of a solar furnace for sample vaporization and depends on the sun s spectrum as a light source. 

In such experiments, one should also note that the clusters were not free, but were deposited or frozen into a solid rare-gas matrix or host, so as to achieve a high enough density of clusters to observe the soft X-ray spectrum using a synchrotron radiation source. This technique, called matrix isolation spectroscopy, is often used to probe the interaction... 

Electrospray ionisation (ESI) is a technique that takes place at atmospheric pressure and is considered to be a soft ionisation process. It is very useful for liquids. Unlike hard processes, the molecule is not normally fragmented and so the resulting mass spectrum is much simpler, the principal peak of which will be the pseudo-molecular ion, i.e. a pro-tonated or sodiated peak. It is therefore much easier to decipher the molecular weight of a compound from an ESI source but there is less structural information given about the molecule, if any. 

---