18
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+
• instead of protonated molecules [M + H]+ for compounds
with relatively low 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 b-carotene in the hexane extract from a carrot. Protonated b-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.
Figure 5 shows the oxygen adduct/DART mass spectrum of a hexane/ ethanol extract from the inside surface of cardboard packaging for a commercial pancake batter mix. The base peak is octadecanol with [M + O2]—
the deprotonated molecules [M – H]—
at m/z 302.281. Palmitic and stearic acids are detected as at m/z 255.232 and 283.263,
with 23 to 40 carbons. The relative abundance for the mineral oil peaks is low because oxygen adduct formation for hydrocarbons is not as sensitive as for alcohols. These assignments were confirmed by an independent study in our laboratory with comprehensive gas chromatography and high-resolution time-of-flight mass spectrometry (GCxGC/HRTOFMS).
respectively. Mineral oil is detected as a series of peaks corresponding to the oxygen adducts [CnH2n+2 + O2]—
for saturated hydrocarbons
Figure 5. Negative-ion oxygen adduct/DART mass spectrum of mineral oil extracted from cardboard packaging for a commercial pancake batter mixture.
Thermal Desorption and Pyrolysis with DART
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 peaks corresponding to C15H24 represent sesquiterpenes, and chlorophenol is a trace impurity in the chlorobenzene dopant.
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-. Because large alkanes are highly polarisable, O2 - 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].
By using high gas temperatures (>350°C), DART can be operated in a pyrolysis mode to characterise large molecules such as industrial polymers that cannot be ionised directly by DART. A better approach is to use a thermal desorption and pyrolysis attachment (‘ionRocket’ from Biochromato LLC, Fujisawa, Kanagawa-ken, Japan) for controlled heating of samples. Samples are deposited onto disposable copper sample stubs, mounted on a heating element, and heated gradually, typically from ambient to 600°C at a rate of 100°C min-1
. This results in thermal separation
of polymer additives followed by pyrolysis of the base polymer. Thermal desorption and pyrolysis DART (TDP/DART) has been used to characterise polylactic acid in 3D-printer feedstock [28], for forensic identification of automotive paint chips [29], forensic identification of sexual lubricants [30] and ignitable liquids in complex matrices [31]. The TDP accessory also facilitates the analysis of fibres and powders that are otherwise difficult to contain in the DART gas stream.
Figure 6 shows the reconstructed ion current chromatograms corresponding to thermal desorption profiles for different components in a 0.5 mm-diameter particle of duct tape and their corresponding DART mass spectra. At temperatures less than about 200°C, the plasticiser diethyl phthalate is observed. Between 200°C and 400°C, we observe a complex pattern of peaks including a peak with an elemental composition consistent with abietic acid. Around 400°C, we observe pyrolysis fragments from polyisoprene (the rubber-based adhesive) and at the highest temperature, we detect the pyrolysis fragments from the low-density polyethylene backing. Peaks associated with pyrolysis of the cotton mesh fibres appear between 500°C and 540°C (not shown).
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