Direct chemical ionization

There are methods for vaporizing solids of low volatility by placing them on a thin wire, which is then raised to a high temperature within a fraction of a second (direct chemical ionization, DCI). This rapid heating allows some vaporization without decomposition, but with the development of later ionization methods, it is now rarely used. 

In order to reveal molecular ions, we moved to desorption/chemical ionization mass spectrometry. This direct chemical ionization (DCI), initially described by Me Lafferty in 1973 68 , was widely used and improved recently with the aim of decreasing sharply thermal degradation of samples under vaporization and to extend mass spectrometry capability both to poorly volatile and to very fragile compounds having high molecular masses 69-74). 

Pyrolysis-direct chemical ionization mass spectrometry ( r-DCI-MS) was recently introduced as a pyrolysis technique for the characterization of complex macromolecular samples and for the analysis of biopolymers. This technique does not require special pyrolysis equipment and can be performed with an instrument which is equipped with a chemical ionization source and a standard DCI probe, which consists of an extended wire used to introduce the sample material directly into the chemical ionization plasma. An important characteristic of this technique is the pyrolysis... 

Derivatized sulfate esters (2,3) and glutathione conjugates (T.B) have been analyzed by these t hniques only rarely. Attempts to analyse underivatized sulfate and glutathione conjugates by electron impact or chemical ionization, (including so-called direct chemical ionization) have resulted in analysis of pyrolysis products. 

Figure 2.13 shows the mass spectrum of a polystyrene sample recorded using an instrument equipped with a Direct Chemical Ionization (DCI) Ion Source. The peaks are due to protonated ions. The authors obtained Mn and Mw using Eqs. (2.1) and (2.2). The result was Mn = 3906 and Mw = 4090, which compares well with the values Mn = 3744 and Mw = 4142 obtained by SEC. DCI can be used for the analysis of narrow-distributed samples in the range below 5 kDa. A series of PEG samples obtained by anionic polymerization was analyzed using an instrument equipped with an Electrospray Ion Source and an FT-ICR cell (ESI-FT), and Mn and Mw... 

One of the most important applications of mass spectrometry has always been the determination of molecular mass. Since this can be difficult with EIMS, the search for soft ionization techniques is almost as old as organic mass spectrometry. The first step was chemical ionization and particularly direct chemical ionization (DCl), which allowed the samples to be desorbed directly from a thin wire into the Cl plasma. It has been successfully u,sed for natural compounds such as glycerides. Later, field ionization (FI) and especially field desorption (FD) opened up new possibilities for the analysis of labile natural compounds such as saccharides, fteptides, and nucleotides. When fast... 

Baldwin M A, McLafferty F W 1973 Direct chemical ionization of relatively involatile samples. Application to underivatized oligopeptides. Org Mass Spec 7 1353-1356... 

Summons, R. E., L. M. Palm, and D. S. Letham Determination of Intact Zeatin Nucleotide by Direct Chemical Ionization Mass Spectrometry. FEBS Lett. 151, 122... 

Modem soft ionization techniques have overcome the sample volatility requirement by combining the first two steps in mass spectrometry sampling and ionization. The soft ionization techniques used for the analysis of surfactants include fast atom bombardment (FAB), field desorption (FD), desorption chemical ionization (DCI, also called direct chemical ionization), secondary-ion mass spectrometry (SIMS), and laser desorption methods. 

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