43
Figure 5. Negative-ion oxygen adduct/DART mass spectrum of mineral oil extracted from cardboard packaging for a commercial pancake batter mixture.
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).
Chromatography with DART
Figure 6. Thermal desorption/pyrolysis DART analysis of a particle of duct tape showing the reconstructed ion chromatograms for selected components (centre) and their corresponding mass spectra.
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
]— at m/z 302.281. Palmitic
and stearic acids are detected as the deprotonated molecules [M – H]—
at
m/z 255.232 and 283.263, respectively. Mineral oil is detected as a series of peaks corresponding to the oxygen adducts [Cn
H2n+2 + O2]— for saturated hydrocarbons
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).
Thermal Desorption and Pyrolysis with DART
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
In the absence of chromatography, DART analysis relies on the specificity of the mass spectrometer - that is, resolving power, mass accuracy and tandem mass spectrometry (MS/MS) - to achieve specificity. However, DART has been combined with the most common forms of chromatography including thin-layer chromatography [32-47], gas chromatography [23], liquid chromatography [48,49] and capillary electrophoresis [50]. Of these, the coupling with thin-layer chromatography has been the most common. Figure 7 shows the extracted ion chromatograms for 9 drugs separated on a silica TLC plate with ethyl acetate/ methanol/ammonium hydroxide (85:10:5). The TLC plate was cut to a 1 cm width prior to analysis. After separation, the plate was sprayed with 4% glycerol in methanol to facilitate desorption of the drugs from the silica substrate and then scanned horizontally through the DART gas stream at a rate of 3 mm s-1
using the DART-SVP
linear rail (IonSense LLC, Saugus MA USA). The DART gas heater was set to 450°C. The chromatograms are normalised to show all compounds on the same scale because the response of the weakest signal is only 1/100 relative to the strongest signals. In contrast to fluorescence (inset), DART analysis reveals all 9 drugs.
The two publications from Johannes Kepler University [48,49] demonstrated the surprising observation that HPLC/DART does not exhibit sample suppression from nonvolatile buffers (such as phosphates), a problem that plagues common HPLC/MS methods such as APCI and ESI. The lack of a commercially available HPLC/DART interface may explain why HPLC/DART has not been more widely used. Given
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