6 May / June 2019
retaining lower amount of dehydrated molecular ion, mainly exhibited series of ions corresponding to alkyl chain breaking, making the identification indecisive. Similarly, by retention of molecular ions and other structurally important ions by using PI, identification of several other classes of heteronates including, alcohols, acids, ketones, nitriles, phenols, thiols etc were confirmed.
Figure 4: Mass spectra of 3-methyl isoparaffins (a-C12, b-C14, and c-C16 pointed in Figure 3) confirming the fragmentation hypothesis presented. Pattern recognition of major fragments enables scripting to identify/extract the information on isomers.
exhibited significant even-mass fragment ions. These characteristic even-mass fragment ions could be generated either from the molecular ions or corresponding to naphthenic and aromatic rings, or from the secondary carbocation at C−C bond adjacent to tertiary carbon atom followed by a hydrogen rearrangement. In PI, the absorption of photon by a gaseous molecule resulted in molecular radical cation as the primary product. Fragment ions might be generated as a result of absorption of a photon by a primary photo-dissociation product, neutral loss from the radical ions, and ionic fragmentation during photoionisation step.
Mass spectral patterns of selected heteroatoms (2-Ethylhexanol, 2-EH; and 13-Octadecenoicacid, 13-OA) for EI and PI are presented in Figure 2. 2-EH, a primary alcohol, exhibited small or nonexistent molecular ion by EI. Moreover, cleavage of the C−C bond resulted in series of ions corresponding to alkyl chain breaking. However, PI fragmentation resulted dehydrated molecular ion (M−H2
O) as
a base peak. This agrees with previous literature referring to dehydration of alcohols by the hot surface of the ion- source system [2]. PI also exhibited loss of ethylene, propylene, butylene, etc. as a result of McLafferty rearrangement. For 13-OA, PI exhibited both molecular (M) as well as dehydrated molecular ion (M− H2
O) as base peak. However, EI, though
For conventional group type analysis, classic normal phase column configuration is preferable as typical structured pattern of separation for several groups of hydrocarbons including n‐paraffins, iso‐ paraffins, monocyclic, dicyclic, polycyclic saturated, and aromatic compounds etc, could be easily achieved (Figure 3). GC×GC being widely orthogonal compared to 1D GC separation, compounds were separated in the first dimension based on volatility and in the second dimension based on polarity. Therefore, the molecular mass, one of the most selective information, is highly related to the elution order of the corresponding compound. On the other hand, along the second axis relatively apolar saturated alkanes were eluted first followed by saturated and aromatic cyclics with increased polarity.
The fragmentation pattern for n-paraffins and isoparaffins at EI generally reveals clusters of peaks 14 mass unit apart representing loss of (CH2
)n CH3
be used to design scripting for isomeric species identification and extraction of other relevant information from PI.
PI also resulted in dominant molecular ion (M+) and the ions at m/z 82 and 68 for alkylcyclohexanes and akylcyclopentanes respectively, related to the corresponding ring. Methyl-substituted isomers were tentatively identified by the presence of molecular ions and predominant ions at m/z 96, 110, 124 and 138 for alkylcyclohexane and at m/z 82, 96, 110 and 124 for alkylcyclopentane corresponding to the C-C cleavage of the alkyl chain from methyl substituted ring. For bi-cyclic and polycyclic naphthenics as well as aromatic compounds, PI resulted exclusively molecular ions; thus EI remains advantageous to elucidate isomeric species by providing molecular ions as well as structurally important fragment ions.
Chromatographic resolution achieved by GC×GC can be synergised by soft- ionisation, and can further be enhanced by coupling high resolution (HR) MS to extract useful information from molecular ions. Depending on the mass resolving power used, HRMS has the ability to resolve
with limited
or no presence of molecular ion. In contrast, the superficial fragmentation achieved by PI greatly simplified mass spectra with reduced background noise as well as enhanced molecular ions for hydrocarbons. For paraffinic compounds, PI retained molecular ions and predominant ions signified the probability of cleavage at lower state of excitement, revealing the branching position. The structured elution pattern of branched alkanes in GC×GC revealed several mono-, di-, and heavily branched alkanes for pyrolysis oil.
Elution of 3-methyl alkanes belonging to C12, C14, and C16 is shown in Figure 3. Corresponding PI mass spectra agrees with the postulated structures as shown in Figure 4. Further, the fragmentation pattern of 3-methyl alkanes can be generalised as follows for the 3 major fragments: 1. molecular ions- C(n) C(n-2)
H2(n)+2 H2(n-2)+1 .; 3. daughter ions-C(n-4)
.; 2. base peaks- H2(n-4)+1
. as
shown in Figure 4. Such pattern recognition of the classes or even sub-classes of hydrocarbons followed by weighing out the patterns (relative intensity 30%, 100% and 15%, respectively) can potentially
Figure 5: Cumulative global mass spectra obtained from EI and PI for pyrolysis oil sample (a); Kendrick Mass Defect (KMD) plot for zoomed area of PI revealing alkylation and double bond equivalent (DBE) series (b); and carbon number vs DBE plot (c).
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