17
Oligosaccharides such as cyclodextrins undergo thermal decomposition by DART and cannot be analysed directly without derivatisation. Figure 2a shows a mass spectrum of γ-cyclodextrin
(C48H80O40) measured by touching the sealed end of a melting point tube to the powdered sample. Three microlitres of the methylating agent 2.5% tetramethylammonium hydroxide (TMAH) in methanol were applied onto the tip of the melting point tube with the sample. The sample and methylating agent were suspended in the DART gas stream (helium with a gas heater setting of 350°C), and complete methylation occurred within seconds. The permethylated cyclodextrin was observed as the trimethylammonium adduct [C48H80O40 + 24CH2 + (CH3)3NH]+
.
The plant hallucinogen psilocybin (C12H17N2O4P) undergoes dephosphorylation with DART, making it indistinguishable from
psilocin (C12H16N2O). By adding a drop of MSTFA (1% TMCS) onto the melting point tube with the sample, silylated psilocybin is
readily detected (Figure 2b).
application of SPME to toxicological screening for drugs in urine, 17 drugs (including oxazepam) were spiked into urine at concentrations ranging from 0.3 ppb to 300 ppb. Without SPME, only methoxyamphetamine and codeine could be detected at the 300 ppb level. With SPME all of the drugs are detected at 300 ppb and 30 ppb, and some are detected at 3 ppb. SPME/DART has also been applied to the trace detection of synthetic cathinones in human saliva [18].
Figure 2. Positive-ion DART mass spectra. (A) γ-Cyclodextrin analysed by DART with added tetramethylammonium hydroxide, (B) psilocybin analysed by DART with added MSTFA (1% TMCS).
Solid-phase microextraction
Solid-phase microextraction (SPME) is easily combined with DART to screen for drugs and environmental contaminants present at low concentrations. SPME is a convenient way to analyse headspace vapours by DART. An early application of SPME to DART in our lab was to detect volatile esters from ripening bananas [8]. SPME/ DART was applied to beer brand profi ling using headspace vapour [9] and to the detection of cocaine and methadone [10]. Recently, SPME/DART was used to detect transient and reactive volatiles emitted when the root of the Mimosa pudica plant is disturbed [11]. Other forms of rapid sample cleanup have been reported with DART. Microextraction with a packed sorbent (MEPS) was combined with DART to detect cocaine metabolites in urine [12]. Two independent investigations used stir-bar sorptive extraction (SBSE) to detect part-per-trillion contaminants in water [13-15]. Disposable pipette extraction was also applied to the detection of drugs in urine [16].
For analysing trace compounds, SPME has two advantages for DART analysis: it concentrates the sample and removes suppressing interferences. The classic example of sample suppression in DART is the loss of signal for oxazepam in urine due to the presence of excess creatinine. Stout and Ropero-Miller showed that with suffi ciently high concentrations of creatinine, oxazepam could not be reliably detected at levels of 100 ppm [17]. To test the
Figure 3. (a) Positive-ion DART mass spectrum of 17 drugs spiked into urine at a concentration of 300 ppb without SPME sample cleanup, (b) The same sample measured with SPME cleanup, (c) Reconstructed ion chromatograms for oxazepam in urine showing detectable signal at concentrations of 3 ppb, 30 ppb, and 300 ppb. Note that for clarity, not all of the drug labels are shown in Figure 3b.
Other modes of DART operation Argon DART
Helium is the most commonly used DART gas; although other gases can be used (Chapter 2 in reference [2]). Nitrogen can be useful for target compound identifi cation, but it does not heat the sample as effectively as helium and its higher reactivity relative to helium makes it problematic for identifi cation of unknowns [19]. Neon works by the same mechanism as helium, but its higher cost limits its utility. Argon is a particularly interesting DART gas because the internal energy of metastable argon (Ar*) is 11.55 eV. This is not enough energy to ionise water or oxygen, so as an atmospheric pressure ion source, argon DART is energetically equivalent to atmospheric pressure photoionisation (APPI). Therefore, it requires the use of a dopant with an ionisation energy lower than 11.55 eV. Argon DART was fi rst used with a mixed dopant consisting of acetyl acetone and pyridine for the selective ionisation of melamine in powdered milk [20]. Yang and coworkers showed that argon DART with certain dopants (absolute ethyl
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
