Volatile materials, ionization

The arc discharge is commonly used to volatilize and ionize thermally intractable inorganic materials such as bone or pottery so that a mass spectrum of the constituent elements can be obtained. 

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. 

Ionization and fragmentation of materials by a variety of means, principally by electron bombardment, or the softer techniques of chemical ionization, field ionization or fast atom bombardment. Analysis of the range of mass fragments produced. Elemental composition of non-volatile materials by application of an RF spark. 

In the laser ablation technique a portion of a solid sample is volatilized by the laser, and transported by an argon flow to the ICP torch. The volatilized material is atomized and ionized in the ICP for mass spectrometric elemental analysis. 

Caution must be exercised when interpreting mass spectral eliminations in the manner discussed for one of the most common artifacts in mass spectrometry is sample decomposition, particularly by elimination of neutral molecules of those substances listed in Table 2.5 prior to volatilization and ionization. The high temperatures required to vaporize some materials are great enough to promote thermal elimination, which leads in most cases to mixtures of olefins. The occurrence of thermal elimination would certainly produce ambiguous results but is detectable by a number of methods. Low electron energies ( 10 to 15 eV) usually produce less... 

Electron impact (El), the method outlined in Section 9.6.1, is the conventional method of ionizing samples for MS. However, El is not free from disadvantages, such as inability to provide molecular weight information from thermally labile samples, difficulty for study of low-volatility materials, complex spectra arising from interference between molecular and fragment ions, etc. In order to overcome these problems, a large number of alternative ionization methods, collectively known as soft ionization methods have been developed. Reviews of the available techniques have been compiled by Milne and Lacey (1974) and Games (1978). Chemical ionization (Cl), field ionization (FI) and field desorption (FD)... 

GC is regularly used to identify and quantify the presence of aflatoxins in food samples, and many protocols have been developed for these materials. Normally, the system is linked to MS, flame ionization detector (FID), or Fourier transform infrared spectroscopy (FTIR) detection techniques in order to detect the volatile products [81-83]. Most aflatoxins are not volatile and therefore have to be derivatized for analysis using GC [83]. Several techniques have been developed for the derivatiza-tion of aflatoxins. Chemical reactions such as silylation or polyfluoroacylation are employed in order to obtain a volatile material [32]. 

Ionization of Volatile Materials Volatile materials are generally ionized by interaction of their vapors with... 

To analyze any material by MS, the sample must first be vaporized and ionized in the instruments vacuum system. It is generally possible to measure the mass of initial charged species (molecular or parent ion) and those obtained by fragmentation (fragments). In MS, the behavior is studied of the mass-to-charge ratios of volatilized and ionized molecules. The mass spectrum of the fragmentation ions is unique to each molecule as the ion mass pattern is different for each molecule resulting in characteristic mass spectra. 

The molecules that are dissociated and the atoms that are ionized during plasma production can be in any state at the start. Steady-state plasmas are formed most often from gases, although Hquids, such as volatile organics, and soHds are also used. Gases and soHds routinely serve as sources of material in pulsed plasma work. 

Spark source mass spectrometry is used for the examination of non-volatile inorganic samples and residues to determine elemental composition. An RF spark of about 30 kV is passed between two electrodes, one of which may be the sample itself, causing vaporization and ionization. Powdered samples or residues from ashed organic materials can be formed into an electrode after mixing with pure graphite powder. 

However, phosphate salts are not volatile. We must constantly remember that mass spectrometry is a gas-phase experiment. Materials to be examined by mass spectrometry must ultimately be made gaseous. Figure 19.14 shows the atmospheric pressure ionization source chamber of a mass spectrometer after infusion of a 20 mM potassium phosphate-containing mobile phase into the instrument for a few hours. The accumulation of phosphate salts on the striker plate is evident. Visual evidence of salt accumulation is also apparent on the back wall of the source chamber, above the striker plate. The overall haziness of the image is not the result of poor photography, but rather due to the coating of dust on the inner walls of the chamber and all surfaces within. 

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