40 May / June 2019
More than One Way to Hit the Bullseye: Lesser Known Ways to Use Direct Analysis in Real Time (DART) Mass Spectrometry
by Robert B. Cody*, Koji Okuda and A. John Dane JEOL USA, Inc, 11 Dearborn Rd. Peabody, MA 01960 USA
cody@jeol.com
*Corresponding author
Direct Analysis in Real Time (DART) mass spectrometry was originally introduced [1] as an ambient ionisation method [2] requiring little or no sample preparation. Ionisation with helium DART gas produces primarily protonated or deprotonated molecules directly from solid, liquids or gases. This is still the most common way to use the DART source, but in the past 16 years, new modes of operation and sample handling have evolved that greatly increase the versatility of the technique. In this article, we describe how sample handling methods such in-situ derivatisation, solid-phase microextraction, and thermal desorption and pyrolysis are applied to increase the range of sample types that can be analysed by DART. New modes of operation such as dopant-assisted argon DART and oxygen adduct formation make it possible to detect nonpolar samples and to detect compounds that would not normally be ionised by helium DART. Lastly, the combination of DART with high-pressure liquid chromatography may offer a solution to the problem of ion suppression by nonvolatile buffers.
DART Principle
The DART ion source uses a glow discharge in a gas stream to generate long-lived excited-state neutral atoms or molecules (aka ‘metastables’). Penning ionisation [3] produces a positive ion and an electron when the excited-state atoms in the gas stream interact with a sample or substrate with a lower ionisation energy than the internal energy of the metastable atoms M*. The first substrate encountered by the DART gas is often atmospheric moisture or oxygen (Figure 1). Positive-ion mode with helium DART gas typically results in the formation of protonated water clusters,
which can transfer a proton to any samples S with a higher proton affinity than water. In negative-ion mode, oxygen undergoes electron capture with the Penning electrons. Oxygen anions can remove a proton from acidic compounds, or in some cases, attach to form oxygen anion adducts.
The DART gas is generally heated to desorb low-volatility samples. Although water and oxygen ions are the most common reagent ions, dopants can be added to the DART gas stream to modify the ionisation chemistry. Matrix effects (suppression or enhancement) can occur if interferences in the sample influence ionisation of the target compound.
All of the examples shown in this article were obtained with a JEOL AccuTOF- DART mass spectrometer, which permits the DART source to be positioned within approximately a centimetre of the mass spectrometer atmospheric pressure interface sampling (API) orifice without additional interface hardware. This is required for the oxygen adduct formation experiment, which cannot be carried out without direct access to the vacuum orifice of the API.
Sample Preparation
While many samples can be analysed directly by DART, rapid and inexpensive
Figure 1. Schematic diagram of the most common DART ionisation mechanism
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