DART IONISATION

DART ionisation

DART Ionisation

Introduction

Direct analysis in real time (DART) is a new ion source technique, which is conducted in the open air under ambient conditions, applied to the rapid and direct analysis of any material (gases, liquids, and solids) with minimal or no sample preparation. In order to take advantage of the capacity of DART mass spectrometry for the real-time analysis of hazardous ingredients in commercial agrochemicals, a pilot study of rapid qualitative determination of hazardous pesticides was performed (Musah & Cody, 2012: 1039). Interestingly, can form adduct with nucleobases in different molar ratios in the DART ion source. In contrast to nucleotides, the ions representing the intact molecules of nucleosides are detected in both positive-ion and negative-ion mode using DART mass spectrometry (Williams & Patel, 2006: 1447). Surprisingly, the fragmentation pattern of nucleosides is different from that of nucleotides in the DART ion source. In the cases of nucleosides (under positive-ion conditions), the production of 1 is not observed, indicating that the phosphate group plays an important role for the multiple eliminations observed in the spectra of nucleotides.

Basis of Mass Spectrometry

There are many different types of MS instruments, but they all have the same three essential components.  First, there is an ionization source, where the molecule is given a positive electrical charge, either by removing an electron or by adding a proton (Musah & Domin, 2012: 1109). Depending on the ionization method used, the ionized molecule may or may not break apart into a population of smaller fragments. In the figure below, some of the sample molecules remain whole, while others fragment into smaller pieces. Next in line there is a mass analyzer, where the cationic fragments are separated according to their mass (Song & Dykstra, 2009: 42). 

Finally, there is a detector, which detects and quantifies the separated ions. One of the more common types of MS techniques used in the organic laboratory is electron ionization.  In the ionization source, the sample molecule is bombarded by a high-energy electron beam, which has the effect of knocking a valence electron off of the molecule to form a radical cation.  Because a great deal of energy is transferred by this bombardment process, the radical cation quickly begins to break up into smaller fragments, some of which are positively charged and some of which are neutral (McEwen & Larsen, 2009: 1518).  The neutral fragments are either adsorbed onto the walls of the chamber or are removed by a vacuum source.  In the mass analyzer component, the positively charged fragments and any remaining un-fragmented molecular ions are accelerated down a tube by an electric field. 

This tube is curved, and the ions are deflected by a strong magnetic field.  Ions of different mass to charge (m/z) ratios are deflected to a different extent, resulting in a 'sorting' of ions by mass (virtually all ions have charges of z =  +1, so sorting by the mass to charge ratio is the same thing as sorting by mass).  A detector at the end of the curved flight tube records  ...