Mass Spectrometry & Spectroscopy
More than One Way to Hit the Bullseye: Lesser Known Ways to Use Direct Analysis in Real Time (DART) Mass Spectrometry
Robert B. Cody*, Koji Okuda and A. John Dane, JEOL USA, Inc, 11 Dearborn Rd. Peabody, MA 01960 USA, Email:
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 fi rst 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 affi nity 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.
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) orifi ce without additional interface hardware. This is required for the oxygen adduct formation experiment, which cannot be carried out without direct access to the vacuum orifi ce of the API.
Sample Preparation
While many samples can be analysed directly by DART, rapid and inexpensive sample preparation methods greatly enhance the ability to detect trace components, minimise suppression effects, and permit detection of thermally labile compounds.
In-situ derivatisation Figure 1. Schematic diagram of the most common DART ionisation mechanism
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 infl uence ionisation of the target compound.
Hot excited-state atoms in the DART gas stream accelerate derivatising reactions, so derivatisation can often be carried out rapidly in the DART gas stream. This is useful for samples that tend to decompose under DART conditions, such as glycosylated or phosphorylated compounds. The most common in-situ derivatisation reactions in DART are thermal hydrolysis and methylation (THAM) with tetramethylammonium hydroxide [4-7] and silylation with common silylating reagents such as BSTFA (N,O-Bis(trimethylsilyl) trifl uoroacetamide) and MSTFA (N-Methyl-N-trimethylsilyltrifl uoroac etamide) with trichloromethylsilane (TMCS) catalyst. These reagents are typically added onto the sealed tip of a disposable melting point tube together with the sample. The sample and reagent are then suspended in the DART gas stream for analysis.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124
