Positive Ion Electrospray Ionization Suppression in Petroleum and Complex Mixtures

Abstract

Since the emergence of high resolving power crude oil mass spectrometry two decades ago, hundreds of publications and presentations have detailed petroleum complex mixtures by electrospray ionization (ESI) mass spectrometry (MS). None of these works have reported or detailed ion suppression (also referred to as ionization biasing or matrix effects) which is a well-known feature of ESI. Here, we show the extreme consequences of ionization biasing within a narrow, 1 order of magnitude concentration range for crude oil mixture direct infusion experiments in positive ion (+) ESI. An oil spill contaminant, a crude oil, and an equimolar model compound mixture were electrosprayed at various analyte and acid modifier concentrations for Fourier transform ion cyclotron resonance (FT-ICR) and time-of-flight (TOF) MS analysis. A 3-fold increase in the number of elemental compositions is achieved by optimization of analyte and acid concentration. At high analyte concentration, oxygen heteroatom class (i.e., CcHhO x species, denoted henceforth simply as O x ) abundance is attenuated and practically nullified. The suppression can be understood from (+) ESI TOF mass analysis of a prepared equimolar model compound mixture, particularly those with ketone functional groups. At sufficiently low concentration of analyte, the relative abundances of nitrogen- and oxygen-containing model compounds no longer vary. For (+) ESI at the flow rates and voltages described in this study, we recommend operating at mass/volume petroleum residue concentration below 0.1 mg/mL in 1:1 (v:v) toluene/methanol with formic acid at 2.5% (v:v).

Investigation of space charge effects and ion trapping capacity on direct introduction ultra‐high‐resolution mass spectrometry workflows for metabolomics

“…For this reason, we recommend not surpassing total metabolomics product concentrations of 100 μg/ml, which is similar to the limitations endorsed for petroleomics studies. 23 The Orbitrap Fusion showed a major advantage in speed of analysis at very high resolution, enabling detection of 218 of 440 unique masses present in a complex mixture in 5 min. Of course, a rapid analysis time is of primary importance to high-throughput complex mixture analyses that seek to profit from high resolution mass spectrometry in the absence of chromatography.…”

“…19,20 The modeling of the signal suppression phenomenon was further advanced by Enke 21,22 who, using an equilibrium partition model, emphasized the surface-activity aspect of desorption, arguing that lower response factors could be expected from less surface-active ionic species. Signal suppression during ESI-MS has more recently been studied in the context of "petroleomics" investigations 23,24 that employ "dilute and shoot" direct infusion for introduction of highly complex petreoleum crude oil samples. To date, FT-ICR 25 has been used in the vast majority of ultra-high-resolution MS analyses for petroleomics, but the use of Orbitrap analyzers has also been making strides.…”

“…To date, FT-ICR 25 has been used in the vast majority of ultra-high-resolution MS analyses for petroleomics, but the use of Orbitrap analyzers has also been making strides. 26 Marshall and coworkers cite competition for available charge (protons) as a major contributor to ionization suppression in +ESI of petroleum products 23 and recommend chromatographic fractionation as an approach to reduce discrimination in ionization of complex crude oil samples run by direct infusion ESI. 24 Mass accuracy refers to the ability of a mass spectrometer to measure the correct m/z value of an ionic species, whereas precision indicates the reproducibility of such a measurement.…”

Analysis of non-conjugated steroids in water using paper spray mass spectrometry

“…The most likely point of protonation on the steroid molecule is at the hydroxyl (OH) group 58 . Many biologically important molecules, such as the steroids studied herein, cannot be easily ionized using the typical paper spray solvent (methanol/ water (1:1, v/v)) due to ion suppression 59 . This is because many biological samples contain salts, and many ions compete for charge 60 .…”

Towards a New Pseudo-quantitative Approach to Evaluate the Ionization Response of Nitrogen Compounds in Complex Matrices

“…On the other hand, ultra-high resolution mass spectrometry (FT-ICR MS) provides an unsurpassed level of detail and usually more than 10 times faster than GC×GC-NCD analysis hence improving analysis efficiency. FT-ICR MS is often coupled to electrospray ionization (ESI) to characterize polar compounds within complex mixtures but ESI undergoes important and complex ionization competition phenomena between the compounds of interest as well as possible different ionization responses depending on the samples (matrix effect) [7][8][9]. On the other hand, multiple linear regression (MLR) can be used to quantify some complex analytes while considering the matrix effect related to the analytical method used that could prevent the quantification [10,11].…”