Formation of metastable species

As the gas (M) enters the ion source, an electric potential in the range of +1 to +5 kV is applied to generate a glow discharge. The glow discharge plasma contains and short-lived energetic species including electrons, ions, and excimers. Ion/electron recombination leads to the formation of long-lived excited-state neutral atoms or molecules (metastable species, M*) in the flowing afterglow region. The DART gas can be heated from room temperature (RT) to 550 °C to facilitate desorption of analyte molecules. Heating is optional but may be necessary depending on the surface or chemical being analyzed. The heated stream of gaseous metastable species passes through a porous exit electrode that is biased to a positive or negative potential in the range 0 to 530V. When biased to a positive potential, the exit electrode acts to remove electrons and negative ions formed by Penning ionization from the gas stream to prevent ion/electron recombination and ion loss. If the exit electrode is biased to a negative potential, electrons can be generated directly from the electrode material by surface Penning ionization. An insulator cap at the terminal end of the ion source protects the operator from harm.

${\displaystyle {\ce {{M}+energy->{M^{\ast }}}}}$

DART can be used for the analysis of solid, liquid or gaseous samples. Liquids are typically analyzed by dipping an object (such as a glass rod) into the liquid sample and then presenting it to the DART ion source. Vapors are introduced directly into the DART gas stream.^[7]

Schematic diagram of a DART ion source

Positive ion formation

Once the metastable carrier gas atoms (M*) released from the source, they initiate Penning ionization of nitrogen, atmospheric water and other gaseous species. Although some compounds can be ionized directly by Penning ionization,^[8] the most common positive-ion formation mechanism for DART involves ionization of atmospheric water.

${\displaystyle {\ce {{M^{\ast }}+{N2}->{M}+{N2}^{+\bullet }+e^{-}}}}$

${\displaystyle {\ce {{M^{\ast }}+{H2O}->{M}+{H2O}^{+\bullet }+e^{-}}}}$

Although the exact ion formation mechanism is not clear, water can be ionized directly by Penning ionization. Another proposal is that water is ionized by the same mechanism that has been proposed for atmospheric pressure chemical ionization^[1]

${\displaystyle {\ce {{N2^{+\bullet }}+{2N2}->{N4+}+{N2}}}}$

${\displaystyle {\ce {{N4^{+\bullet }}+{H2O}->{2N2}+{H2O}^{+\bullet }}}}$

Ionized water can undergo further ion-molecule reactions to form protonated water clusters ([(H
₂O)
_nH]⁺
).^[9]

${\displaystyle {\ce {{H2O^{+\bullet }}+{H2O}->{H3O+}+{OH^{\bullet }}}}}$

${\displaystyle {\ce {{H3O^{+}}+{\mathit {n}}H2O->{[{{\mathit {n}}H2O}+H]}^{+}}}}$

The stream of protonated water clusters acts as a secondary ionizing species^[10] and generates analytes ions by chemical ionization mechanisms at atmospheric pressure.^[11] Here protonation, deprotonation, direct charge transfer and adduct ion formation may occur.^[1][7]

${\displaystyle {\ce {{S}+{[{{\mathit {n}}H2O}+H]}^{+}->{[{S}+H]}^{+}+{\mathit {n}}H2O}}}$

${\displaystyle {\ce {{N4}^{+\bullet }+S->{2N2}+S^{+\bullet }}}}$

${\displaystyle {\ce {{O2}^{+\bullet }+S->{O2}+S^{+\bullet }}}}$

${\displaystyle {\ce {{NO^{+}}+S->{NO}+S^{+\bullet }}}}$

${\displaystyle {\ce {{[NH4]^{+}}+S->{[{S}+NH4]}^{+}}}}$

Metastable argon atoms do not have enough internal energy to ionize water, so DART ionization with argon gas requires the use of a dopant.^[12]

Negative ion formation

In negative-ion mode, the potential of the exit grid electrode can be set to negative potentials. Penning electrons undergo electron capture with atmospheric oxygen to produce O₂⁻. The O₂⁻ will produce radical anions. Several reactions are possible, depending on the analyte.^[1]

${\displaystyle {\ce {{O2}+{e}^{-}->{O2}^{-\bullet }}}}$

${\displaystyle {\ce {{O2}^{-\bullet }+{S}->{S}^{-\bullet }+O2}}}$

${\displaystyle {\ce {{S}+{e}^{-}->{S}^{-\bullet }}}}$

${\displaystyle {\ce {{SX}+{e}^{-}->{S}^{-}+{X}^{\bullet }}}}$

${\displaystyle {\ce {{SH}-> {[S-H]}^{-}+ {H}^{+}}}}$

The negative ion sensitivity of DART gases varies with the efficiency in forming electrons by Penning ionization, which means that the negative ion sensitivity increases with the internal energy of the metastable species, for example nitrogenᐸneonᐸhelium.

Source to analyzer interface

A capsule being analyzed is held in the sample chamber between the DART ion source (right) and the spectrometer inlet (cone on left).

Analyte ions are formed at ambient pressure during Penning and chemical ionization. The mass spectrometry analysis, however, takes place at high vacuum condition. Therefore, ions entering the mass spectrometer, first go through a source - to - analyzer interface (vacuum interface), which was designed in order to bridge the atmospheric pressure region to the mass spectrometer vacuum. It also minimizes spectrometer contamination.

In the original JEOL atmospheric pressure interface used for DART, ions are directed to the ion guide through (outer) і and (inner) іі skimmer orifices by applying a slight potential difference between them: orifice і : 20 V and orifice іі : 5 V. The alignment of the two orifices is staggered to trap neutral contamination and protect the high-vacuum region. Charged species (ions) are guided to the second orifice through an intermediate cylindrical electrode ("ring lens"), but neutral molecules travel in a straight pathway and are thus blocked from entering the ion guide. The neutral contamination is then removed by the pump.

The DART source can be operated in surface desorption mode or transmission mode. In the ordinary surface desorption mode, the sample is positioned in a way, which enables the reactive DART reagent ion stream to flow on to the surface while allowing the flow of desorbed analyte ions into interface. Therefore, this mode requires that the gas stream grazes the sample surface and does not block gas flow to the mass spectrometer sampling orifice. In contrast, transmission mode DART (tm-DART) uses a custom-made sample holder and introduces the sample at a fixed geometry.^[10][13]

Schematic diagram of DART-transmission mode

Coupling with separation techniques

DART can be combined with many separation techniques. Thin-layer chromatography (TLC) plates have been analyzed by positioning them directly in the DART gas stream. Gas chromatography has been carried out by coupling gas chromatography columns directly into the DART gas stream through a heated interface. Eluate from a high-pressure liquid chromatograph (HPLC) can be also introduced to the reaction zone of the DART source and analyze. DART can be coupled with capillary electrophoresis (CE) and the eluate of CE is guided to the mass spectrometer through the DART ion source.^[1]