42 May / June 2019
pressure photoionisation (APPI). Therefore, it requires the use of a dopant with an ionisation energy lower than 11.55 eV. Argon DART was first 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 alcohol, methanol, fluorobenzene, or acetone) showed reduced fragmentation for labile compounds [21]. In our laboratory, we found that argon with chlorobenzene dopant produces molecular ions M+• [M + H]+
instead of protonated molecules for compounds with relatively low
Figure 4. (a) Positive-ion helium DART mass spectrum of a hexane extract of a carrot. (b) Positive-ion argon DART mass spectrum with chlorobenzene dopant for the same extract. The peak corresponding to C15
H24 represents sesquiterpenes, and chlorophenol is a trace impurity in the chlorobenzene dopant.
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 profiling 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 sufficiently high concentrations of creatinine, oxazepam could not be reliably detected at levels of 100 ppm [17]. To test the 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].
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 identification, but it does not heat the sample as effectively as helium and its higher reactivity relative to helium makes it problematic for identification 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
O2
- attachment for nonpolar compounds and alcohols
Normal DART methods are not suitable for the analysis of saturated hydrocarbons and alcohols. Saturated hydrocarbons do not form protonated molecules (although molecular ions can rarely be observed under very dry conditions [23]). Unless other functional groups are present, alcohols tend to lose water upon protonation. One solution is to derivatise the alcohols [24.25] but this is difficult to do for trace alcohols in the presence of other compounds.
An alternative approach is oxygen anion adduct formation with DART. Samples dissolved in a volatile solvent such as hexane are aspirated directly into vacuum through the mass spectrometer sampling orifice while the DART is operated in negative-ion mode as a source of O2
alkanes are highly polarisable, O2 ]-
-. Because large -
can
attach to the positive end of the induced dipole to form adducts [M + O2
without
fragmentation [26]. Rapid expansion into vacuum cools the weakly bound adducts so that they can be detected. Alcohols exhibit the same behaviour, making it easy to detect them without derivatisation. Oxygen anion adduct formation was used to identify blowfly species from puparial casings based upon differences in their hydrocarbon profiles [27].
ionisation energies [22]. This enables us to selectively detect compounds that might otherwise be suppressed in a complex matrix. An example is shown in Figure 4 for the detection of β-carotene in the hexane extract from a carrot. Protonated β-carotene (m/z 537.446) is not visible in the helium DART mass spectrum of the extract (Figure 4a) because it is either suppressed or masked by diglycerides present in the same mass range.
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