Factors determining relative sensitivity of analytes in positive mode atmospheric pressure ionization mass spectrometry

The behavior in atmospheric pressure chemical ionization of selected model polycyclic aromatic compounds, pyrene, dibenzothiophene, carbazole, and fluorenone, was studied in the solvents acetonitrile, methanol, and toluene. Relative ionization efficiency and sensitivity were highest in toluene and lowest in methanol, a mixture of molecular ions and protonated molecules was observed in most instances, and interferences between analytes were detected at higher concentrations. Such interferences were assumed to be caused by a competition among analyte molecules for a limited number of reagent ions in the plasma. The presence of both molecular ions and protonated analyte molecules can be attributed to charge-transfer from solvent radical cations and proton transfer from protonated solvent molecules, respectively. The order of ionization efficiency could be explained by incorporating the effect of solvation in the ionization reactions. Thermodynamic data, both experimental and calculated theoretically, are presented to support the proposed ionization mechanisms. The analytical implications of the results are that using acetonitrile (compared with methanol) as solvent will provide better sensitivity with fewer interferences (at low concentrations), except for analytes having high gas-phase basicities. (J Am Soc Mass Spectrom 2008, 19, 1926 -1941) © 2008 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry T he analysis of polycyclic aromatic compounds (PACs) in petroleum and environmental samples has been the subject of many investigations [1-3], due to their impact on the environment, refining processes, and product quality. However, success is often hindered by the number and low concentration of these compounds in such samples, as well as by the lack of standards. Gas chromatography (GC) and GC coupled with electron ionization (EI) mass spectrometry (GC/ MS) are normally employed for samples containing robust, thermally stable analytes [4,5]. Atmospheric pressure ionization methods [6]-electrospray ionization (ESI) [7], atmospheric pressure chemical ionization (APCI) [8], and atmospheric pressure photoionization (APPI) [9]-in conjunction with liquid chromatography (LC) have been used for samples of more problematic analytes.It is generally believed that ESI suffers more from nonlinear response and matrix effects than APCI. Linear response over a wide concentration range is an attractive feature of APCI and has been reported by several authors [10,11]. Recently, Roussis and Fedora compared the ability of APCI and ESI to quantify polar and ionic compounds in petroleum products [12]. They obtained linear ranges of three orders of magnitude for both techniques, and higher sensitivity for ESI. However, the ESI response was nonlinear over the concentration range of interest, and they recommended the use of APCI for quantitative LC/MS applications.In the course of a study of the application of APCI to the analysis of nitrogen-and particularly sulfurcontaining PACs in petroleum samples, we observe...

Section: Formation Of Reagent Ionsmentioning

confidence: 84%

Section: Effects Of Solvation On the Ionization Processmentioning

confidence: 99%

Quantitative aspects of and ionization mechanisms in positive-ion atmospheric pressure chemical ionization mass spectrometry

Herrera

Grossert

Ramaley

2008

“…In an APCI source, corona discharge is used to ionize gases, such as He, N 2 , or CO 2 , to form radical cations in the positive ion mode [23][24][25]. These ions collide with vaporized solvent molecules to form secondary reagent ions, usually [M + H] + , M +• , and/or [M -H] + , and their solvent molecule clusters [25].…”

Section: Introductionmentioning

confidence: 99%

HPLC/APCI Mass Spectrometry of Saturated and Unsaturated Hydrocarbons by Using Hydrocarbon Solvents as the APCI Reagent and HPLC Mobile Phase

Gao

Owen

Borton

et al. 2012

Saturated and unsaturated, linear, branched, and cyclic hydrocarbons, as well as polyaromatic and heteroaromatic hydrocarbons, were successfully ionized by atmospheric pressure chemical ionization (APCI) using small hydrocarbons as reagents in a linear quadrupole ion trap (LQIT) mass spectrometer. Pentane was proved to be the best reagent among the hydrocarbon reagents studied. This ionization method generated different types of abundant ions (i.e., [M + H] , with little or no fragmentation. The radical cations can be differentiated from the even-electron ions by using dimethyl disulfide, thus facilitating molecular weight (MW) determination. While some steroids and lignin monomer model compounds, such as androsterone and 4-hydroxy-3-methoxybenzaldehyde, also formed abundant M +• and [M+H] + ions, this was not true for all of them. Analysis of two known mixtures as well as a base oil sample demonstrated that each component of the known mixtures could be observed and that a correct MW distribution was obtained for the base oil. The feasibility of using this ionization method on the chromatographic time scale was demonstrated by using high-performance liquid chromatography (HPLC) with hexane as the mobile phase (and APCI reagent) to separate an artificial mixture prior to mass spectrometric analysis.

“…The rate coefficients for fast ion/molecule reactions are some two orders of magnitude lower [37], therefore k iM was assumed to be 10 Ϫ9 cm 3 -s Ϫ1 . It is appealing to describe the ASGDI ionization dynamics by methods similar to those used to explain the relative sensitivities in corona atmospheric pressure ionization mass spectrometry (APCI-MS); the rate expression for analyte ion intensities (under kinetic control) in the corona APCI source is [38]:…”

Section: Functional Dynamicsmentioning

confidence: 99%

Factors influencing the analytical performance of an atmospheric sampling glow discharge ionization source as revealed via ionization dynamics modeling

Goeringer

2003

A kinetic model is developed for the dynamic events occurring within an atmospheric sampling glow discharge that affect its performance as an ion source for analytical mass spectrometry. The differential equations incorporate secondary electron generation and thermalization, reagent and analyte ion formation via electron capture and ion-molecule reactions, ion loss via recombination processes, diffusion, and ion-molecule reactions with matrix components, and the sampling and pumping parameters of the source. Because the ion source has a flow-through configuration, the number densities of selected species can be estimated by applying the steady-state assumption. However, understanding of its operation is aided by knowledge of the dynamic behavior, so numerical methods are applied to examine the time dependence of those species as well. As in other plasma ionization sources, the ionization efficiency is essentially determined by the ratio of the relevant ion formation and recombination rates. Although thermal electron and positive reagent ion number densities are comparable, the electron capture/ion-molecule reaction rate coefficient ratio is normally quite large and the ion-electron recombination rate coefficient is about an order of magnitude greater than that for ion-ion recombination. Consequently, the efficiency for negative analyte ion formation via electron capture is generally superior to that for positive analyte ion generation via ion-molecule reaction. However, the efficiency for positive analyte ion formation should be equal to or better than that for negative analyte ions when both ionization processes occur via ion-molecule reaction processes (with comparable rate coefficients), since the negative reagent ion density is considerably less than that for positive reagent ions. Furthermore, the particularly high number densities of thermal electrons and reagent ions leads to a large dynamic range of linear response for the source. Simulation results also suggest that analyte ion number densities might be enhanced by modification of the standard physical and operating parameters of the source. . In these and other applications, a number of methods have been utilized for generation of ions from compounds of interest. Electron ionization (EI) has been employed for formation of positive ions in situations for which air is leaked directly into the mass spectrometer [5,6]. To enhance ionization selectivity over that exhibited by EI, single-and multi-photon ionization techniques have been used for determination of targeted aromatic compounds in automobile exhaust [7][8][9][10]. More widely used approaches for direct sampling are based on atmospheric pressure chemical ionization (APCI) [11] which is capable of making positive or negative ions. APCI is effected by a sequence of electron-and ion-molecule reactions initiated by electrons that are generally produced via either a corona discharge [12] or a ␤-emitter such as 63 Ni [1,13]. Because the equilibrium distribution in such reactions under normal APCI operati...