Direct analysis in real time

Direct analysis in real time (DART) is an ion source used in mass spectrometry. It is applied for rapid analysis of a wide variety of samples at atmospheric pressure and in the open laboratory environment. It does not need a specific sample preparation, so it can be used for the analysis of solid, liquid and gaseous samples in their native state.

DART produces excited state species from the carrier gases and let them to react with reagent molecules in atmosphere and eventually the analytes. DART ionization process can produce positive or negative ions depending on the potential applied.This ionization can take place directly on the sample surface, such as currency bills, tablets, bodily fluids (blood, saliva and urine), glass, plant leaves, fruits & vegetables, clothing and living organisms.

With the aid of DART, exact mass measurements can be done rapidly with higher resolution.The use of DART in support of mass spectrometry is well known in pharmaceutical applications, forensic studies, quality control and environmental studies etc.

DART

DART ion source

History

DART was developed in 2005 by J.A Laramee and R.B Cody as a new atmospheric pressure desorption ionization process. DART was introduced as a result of attempts with the goal of looking into methods, to provide a thermal electron source that can be used at atmospheric pressure.The introduction of DART took place after the desorption electrospray ionization (DESI). DESI and DART are considered as pioneer techniques in the ambient desorption ionization, since they operate in the open laboratory environment and do not require sample pretreatment. In contrast to the DESI, the heated plasma escaped from the DART ion source contains a dry stream containing excited state carrier gas species.

Principle of operation

Ionization process

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. This higher potential ionizes the carrier gas and generate electrons, ions and excited state atoms/molecules (metastable species-M*). Then a potential of 100 V applied to the series of perforated disk electrodes and they remove charged particles from the gas stream and only excited state species flow to the third chamber. In the third chamber the cold plasma can be heated from room temperature (RT) to 250 °C. Heating is optional but may be necessary depending on the surface or chemical being analyzed.The heated stream of gaseous metastable species comes out from the DART ion source. An insulator cap at the terminal end of the ion source protects the operator from harm.

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

DART can be used for the analysis of solid, liquid or gaseous samples. Liquids are 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.

Schematic diagram of 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.

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

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

The ionization of water can be happened via a multi-step pathway involving nitrogen.

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

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

These reagent ions undergo ion-molecule reactions and form protonated water clusters([(H₂O)_nH]⁺).

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

${\ce {{H3O^{+}}+{nH2O}->{[{nH2O}-{H}]}^{+}}}$

The stream of protonated water clusters act as secondary ionizing species and generate analyte(s) ions by chemical ionization mechanisms at atmospheric pressure. Here protonation, deprotonation, direct charge transfer and adduct ion formation may occur.

${\ce {{S}+{[{nH2O}-{H}]}^{+}->{[S-H]}^{+}+nH2O}}$

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

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

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

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

Negative ion formation

The potential of the second perforated electrode and the exit grid electrode can be set to negative potentials for the negative ion mode.In this mode the most common mechanism is , Penning electrons undergo electron capture with atmospheric oxygen to produce O₂⁻. The O₂⁻ will produce radical anions.Also depending on the analyte several reactions are feasible.

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

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

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

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

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

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

Instrumentation

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 this interface, ions are directed to the ion guide through orifice (outer) і and (inner) іі by applying a slight potential difference between them: orifice і : 30 V and orifice іі : 5 V. Species containing charge (ions) are attracted to the second orifice, but neutral molecules travel in a straight pathway and thus get trapped in that region. The 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 to optimize the sample position. In contrast, the transmission mode DART (tm-DART) uses a custom made sample holder and introduces the sample at a fixed geometry.

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 to the DART gas stream.The gas chromatographic (GC) columns directly coupled with mass spectrometer through DART as its ability to directly analyze gases.The eluate from the 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 DART ion source.

Mass spectra

In positive ion mode, DART produces predominantly protonated molecules [M+H]⁺ ] and in negative-ion mode deprotonated molecules [M-H]⁻. Both negative and positive modes of DART provides relatively simple mass spectra.Depending on the type of analyte, other species may be formed, such as multiple charged adducts .DART categorized under soft ionization technique.Fragmentation can be rarely observed for some molecules.

DART source red ink spectrum

Use of DART compared to traditional methods minimizes sample amount, sample preparation, eliminates extraction steps, decreases limit of detection and analysis time. Also it provides a broad range sensitivity, simultaneous determination of multi-drug analytes and sufficient mass accuracy for formulation determination.

But DART ion source is a kind of gas phase ionization and it requires some sort of volatility of the analyte to support thermally assisted desorption of analyte ions. This limits the size range of the molecules that can be analyzed by DART i.e. m/z 50 to 1200. DART-MS is capable of semi-quantitative and quantitative analysis. To accelerate sample release from the surface, the DART gas stream is usually heated to temperature in the range 100-400 °C. High temperature evaporates less volatiles such as higher molecular weight and/or highly polar compounds in the sample.

Applications

DART is being applied in many fields, including the fragrance industry, pharmaceutical industry, foods and spices, forensic science and health etc.

In forensic science, DART has been used for analysis of explosives, warfare agents, drugs, inks and sexual assault evidence. In clinical and pharmaceutical sector, DART is utilized for body fluid analysis such as blood, plasma, urine etc. and study traditional medicines. Also DART can detect composition in medicine in a tablet form as per there is no need for sample preparation such as crushing or extracting.

In food industry the quality and authenticity assessment of food is assured with DART.Also it's used in mycotoxins analysis of beverages, semi-quantitative analysis of caffeine, monitoring heat accelerated decomposition of vegetable oils and many other food safety analysis. In the manufacturing industry, to determine the deposition and release of a fragrance on surfaces such as fabric and hair and dyes in textiles, DART is often utilized.

DART is used in environmental analysis.For example analysis of organic UV filters in water, contaminants in soil,petroleum products and aerosols etc.DART plays an important role in biological studies as well.It enables studying chemical profile of plants and organisms.

See also

• Ambient ionization
• Desorption electrospray ionization
• Electric glow discharge
• Atmospheric pressure chemical ionization
• Desorption atmospheric pressure photoionization

Patents

• Robert B. Cody and James A. Laramee, “Method for atmospheric pressure ionization” U.S. patent 6,949,741 issued September 27, 2005. (Priority date: April 2003).
• James A. Laramee and Robert B. Cody “Method for Atmospheric Pressure Analyte Ionization” U.S. patent 7,112,785 issued September 26, 2006.