5 Results and Discussion
Figure 1 shows the contour plots obtained from EI and PI for a pyrolysis oil sample. In the reverse phase GC×GC condition developed in this study, first dimension separation was based on polarity and second dimension separation on volatility. It allowed a nice separation for the majority of the heteroatoms by stripping off those polar compounds from the major apolar hydrocarbons. EI and PI contour plots revealed that although the majority of the chemical classes were ionised by both EI and PI, relative intensity indicated more selectivity of PI towards aromatic and heteronate compounds than paraffinic and naphthenic ones. Noteworthy to mention is that commonly two types of PI are used - single photon ionisation (SPI) and resonance enhanced multi-photon ionisation (REMPI). Among those, SPI is considered as relatively universal and REMPI is selective mainly towards aromatics. Current PI configuration is based on SPI, thus a universal selectivity was observed covering both paraffinic as well as poly-aromatic compounds.
Figure 2: Mass spectra revealing ionisation impact on alkane and aromatic classes specified in Figure 1, as well as for compound specific ionisation impact of EI and PI.
data on both molecular ions and structurally significant fragments in low-energy (10.8 eV) PI mass spectra.
Experimental
Reverse and normal phase GC×GC column setups comprised of an Agilent 7890B gas chromatograph equipped with a split/ split-less injector at 300ºC were used to analyse pyrolysis oil (pyoil) from mixed plastic waste. Helium was used as the carrier gas in constant flow mode with a flow rate of 1.3 mL min-1. The order of columns in reverse phase GC×GC was- 1st column: 30 m polar, Zebron ZB-FFAP; and 2nd column: 3.7 m polar, Agilent VF-17ms. The chromatographic analysis commenced with an oven temperature at 60 ºC for 1 min; then programmed at 2ºC min-1 up to 260ºC; and held there for 5 min. Normal phase configuration was as follows: 1st column: 30 m apolar, Rxi® 1Sil MS; and 2nd column: 2 m polar, Rxi® 17Sil MS. Analysis commenced with an oven temperature at 60ºC for 0.2 min; then programmed at 2ºC min-1 up to 280ºC; and held there for 5 min. Modulation was performed with a cryogenic thermal loop modulator ZX-1 (ZOEX Corporation, USA) with a modulation time of 16 s and 7 s for reverse and normal phase, respectively.
An EI/PI combination ion-source (JEOL, Japan) was used to ionise chromatographically separated compounds eluted in ion-block. A deuterium lamp (L7293; Hamamatsu, Japan) was used for photon emission in PI with a maximum energy output of 10.78 eV at 118 nm. A time- of-flight mass spectrometer (AccuTOF GCv 4G, JEOL, Japan) was used to acquire data at a mass resolution of >10,000 FWHM. Data processing and visualisation was done based on GCImage (v-2.6, ZOEX Corporation, Houston, U.S.A.) and mMass software (V.3, open source).
The comparison of cumulative global mass spectra of a pyrolysis oil for EI and PI (Figure 2) revealed exhaustive fragmentation in EI leading to the smaller fragments of homologous series of butyl, pentyl, hexyl (m/z 57, 71, 85, respectively) etc, together with corresponding alkenyl and alkynyl carbocations (loss of 2H or 4H, respectively) with limited or no presence of molecular ions for paraffins. For aromatic zone, a series of fragments at m/z 91, 105, 119, and 133 and so on, signified cleavage of C−C bond next to methyl substituted alkylbenzene ring (mono-, di-, tri-, tetra-methyl, respectively). For both paraffinic and aromatic zones, prominent ions in EI spectrum were odd- mass fragment ions. On the other hand, PI not only retained the molecular ions but also
Figure 3: GC×GC‐TOF/MS contour plots of pyrolysis oil sample for PI revealing elution patterns of hydrocarbon group types as visualised by 2D plots using normal phase column configuration (1 dimension retention time; 2
tR tR : second dimension retention time).
: first
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