APCI(+)FT-ICR MS Analysis of Hydrocarbons Using Isooctane as Ionizing Reagent - A Comparison with HTGC-FID, GC×GC-MS and NMR

Abstract

Hydrocarbons present in saturated fractions of crude oils can be assessed by atmospheric pressure chemical ionization (APCI) using small hydrocarbons as ionizing reagents in a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). In this work, five paraffin standards of different average molar mass distributions (M w) were easily ionized by APCI(+)FT-ICR MS using isooctane as the reagent gas. Data of M w , carbon number and double bond equivalent (DBE) distributions corresponding to linear and cyclic hydrocarbons (HCs) were compared to results obtained from analysis of high temperature gas chromatography with a flame ionization detector (HTGC-FID), comprehensive two-dimensional gas chromatography coupled to mass spectrometry (GC×GC-MS) and 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy. APCI(+)FT-ICR MS data showed good agreement with those of analytical techniques. Furthermore, the ability of APCI(+) to assess n-paraffin, even in blends with polyaromatic hydrocarbon molecules such as coronene (M w = 301 Da) at concentrations from 2.5 to 25 µg mL-1 , was demonstrated. The typical MS paraffin profile (containing repeating mass units of 14 Da) was clearly confirmed, being totally suppressed when a concentration of 25 µg mL-1 of coronene was used. This phenomenon was also evidenced in one of two saturated fractions produced using saturates, aromatics and polar (SAP) compound fractionation methodology.

Fragmentation of Saturated Hydrocarbons upon Atmospheric Pressure Chemical Ionization Is Caused by Proton-Transfer Reactions

“…The ionization is thought to involve multiple steps. 5,16,26,27 For example, Scheme 1 shows the APCI mechanism proposed 5,6,8,16,22 for the ionization of methylcyclohexane dissolved in isooctane when using nitrogen sheath gas. A high voltage was applied to the tip of a corona discharge needle, generating an electric field.…”

“…While accurate characterization of the chemical compositions of complex mixtures of large saturated hydrocarbons is desirable in fields as diverse as forensics, evolutionary biology, food science, petroleomics, − and renewable transportation fuels, modern analytical techniques have had limited success in characterizing such mixtures . An improved knowledge on the chemical compositions of these mixtures would enable a rational optimization of processes such as crude oil refining and conversion of renewable resources into transportation fuels, ultimately saving energy and resources while mitigating pollution. , A plethora of techniques can be used to provide bulk information for complex mixtures of large saturated hydrocarbons, ,, but of these, only tandem mass spectrometry can provide structural and quantitative information for individual compounds without extensive separation and purification …”

“…However, all contemporary ionization methods have significant drawbacks when employed to analyze complex mixtures of large saturated hydrocarbons. GC/MS based on electron ionization (EI) can be used to characterize only the most volatile components in these mixtures. ,,, Furthermore, EI fragments saturated hydrocarbons extensively, , which complicates mass spectral interpretation and sometimes prevents the identification of the structures of the saturated hydrocarbons. Larger saturated hydrocarbons do not generate stable molecular ions upon EI and often generate identical fragmentation patterns, preventing their identification.…”

“…In contrast, atmospheric pressure chemical ionization (APCI) has been demonstrated to be a quick and reproducible technique for evaporation and ionization of saturated hydrocarbon mixtures . Furthermore, APCI performed with a hydrocarbon solvent/reagent has been demonstrated to generate primarily [M–H] + ions. ,,,,, For instance, a method has been published that used laser-induced acoustic desorption (LIAD) to evaporate saturated hydrocarbons in lubricant base oils (complex mixtures containing mostly saturated hydrocarbons) dissolved in toluene followed by ionization via APCI with O 2 as the sheath gas to generate [M–H] + ions . Another method involved direct infusion of base oils dissolved in hexane or isooctane into an APCI source with O 2 as the sheath gas .…”

“…Ionization of polar analytes upon APCI is thought to occur primarily via gas-phase proton-transfer reactions of protonated water or water clusters of protonated water; conversely, the APCI mechanism for generating [M–H] + ions and fragment ions from saturated hydrocarbons is less well understood. The ionization is thought to involve multiple steps. ,,, For example, Scheme shows the APCI mechanism proposed ,,,, for the ionization of methylcyclohexane dissolved in isooctane when using nitrogen sheath gas. A high voltage was applied to the tip of a corona discharge needle, generating an electric field.…”