41
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 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).
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) trifluoroacetamide) and MSTFA (N-Methyl- N-trimethylsilyltrifluoroacetamide) 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.
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 (C48
H80 O40 )
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 [C48
H80O40 + 24CH2 + (CH3 NH]+ )3
The plant hallucinogen psilocybin (C12
H17 N2 O4 H16 N2 . P) undergoes dephosphorylation
with DART, making it indistinguishable from psilocin (C12
O). By adding a drop of
MSTFA (1% TMCS) onto the melting point tube with the sample, silylated psilocybin is readily detected (Figure 2b).
Solid-phase microextraction
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.
Solid-phase microextraction (SPME) is easily combined with DART to screen for drugs and environmental contaminants present at
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