ANALYTICAL APPLICATIONS OF ELECTRON MONOCHROMATOR-MASS SPECTROMETRY Kirk R. Jensen and Kent J. Voorhees* Colorado School of Mines, Department of Chemistry and Geochemistry, 1012 14th Street #204, Golden, Colorado, 80401 Received 26 July 2012; revised 17 June 2013; accepted 17 June 2013 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/mas.21395
An electron monochromator (EM) produces an electron beam with a narrow energy distribution that can be utilized with mass spectrometry (MS). The history and development of the EM from an initial research design to a commercial model are reviewed along with MS research applications. An EM incorporated with a mass spectrometer showed significant improvement in sensitivity over traditional methods for negative-ion generation and selectivity for compounds with electrophilic character. Sensitivity of EM-MS has been shown to be 25 fg for hexachlorobenzene in positive-ion mode and 10 fg for nitrobenzene in negative-ion mode. Reports regarding the analysis of chlorinated compounds, explosives, pesticides, phthalates, polychlorodibenzo-p-dioxins, polycyclic aromatic hydrocarbons (PAHs), nitro-polycyclic aromatic hydrocarbons (NPAHs), antioxidants, and bacterial biomarkers are discussed. Additionally, theoretical methods to predict electron-capture properties are presented. # 2013 Wiley Periodicals, Inc. Mass Spec Rev Keywords: electron monochromator; mass spectrometry; analytical applications; electrophilic compounds
I. INTRODUCTION Negative-ion mass spectrometry (NIMS) affords selectivity over positive-ion mass spectrometry for the analysis of electrophilic compounds at low concentrations in complex mixtures (Hunt & Crow, 1978). Traditionally, negative-ion chemical ionization (NICI) has been used to generate negative ions for NIMS; however, NICI requires a reagent gas and/or a buffer gas, such as a chemical ionization (CI) gas, which must be maintained at a constant temperature and pressure. Ions produced in NICI have multiple chemistries because an electron-energy distribution of 0–12 eV is produced by the CI process (Hunt & Sethi, 1980). The commercial electron monochromator (EM) produces an electron beam with a deviation of 0.3 eV, and eliminates the need for constant pressure and temperature conditions within the ion source. The development of the EM is not a recent effort (Nottingham, 1939; Fox et al., 1955; Stamatovic & Schulz, 1970; Hacaloglu et al., 1989; Illenberger, Scheunemann, & Baumga¨rtel, 1979; Oster, Ku¨hn, & Illenberger, 1989; Hacaloglu, Contract grant sponsor: Department of Energy (National Renewable Energy Laboratory); Contract grant number: KXEA-3-33607-00; Contract grant sponsor: Oakridge National Laboratory; Contract grant number: 74X-SX669C. Correspondence to: Kent J. Voorhees, Colorado School of Mines, Department of Chemistry and Geochemistry, 1012 14th Street #204, Golden, CO 80401. E-mail: [email protected]
Mass Spectrometry Reviews # 2013 by Wiley Periodicals, Inc.
Gokmen, & Suzer, 1990); however, practical application to analytical mass spectrometry (MS) became possible after improvements were reported by Deinzer and co-workers at Oregon State University (OSU) (Larame´e et al., 1996). Under license from OSU, a commercial EM was developed and introduced by JEOL USA, Inc. (Peabody, MA). The following review presents a background on the theory and development of the EM and its application to analytical mass spectrometry. Examples of its use for detection of chlorinated compounds, explosives, pesticides, phthalates, dioxins, polycyclic aromatic hydrocarbons (PAH), nitro-polycyclic aromatic hydrocarbons (NPAH), antioxidants, and bacterial biomarkers are discussed. Theoretical studies to correlate and predict electron capture energies are also included.
II. BACKGROUND A. Ion Formation Traditional mass spectrometry uses a 70 eV electron beam to ionize an analyte [electron ionization (EI)] for positive-ion detection. EI results in an ion in a high-energy state that often fragments into smaller ions. Because of this instability, the molecular ion might not appear in the mass spectrum, a factor that can confound compound identification. Decreasing the electron beam energy to 12 eV often results in the increased intensity of the molecular ion, but at a severe cost of ionization current and sensitivity (Watson & Sparkman, 2007). Analysis of negative ions with an EI source is even more problematic because formation of negative ions is approximately 1,000 times less efficient than positive-ion formation (Hunt et al., 1976). For a low-energy electron beam to be utilized for analytical mass spectrometry, the energy distribution of the beam and the ionization current had to be improved. Formation of negative ions in a mass spectrometer occurs by three types of electron interactions (Craggs & McDowell, 1955):
Resonance electron capture: (Thermal electrons) AB þ e ! AB
Dissociative electron capture: (
An electron monochromator (EM) produces an electron beam with a narrow energy distribution that can be utilized with mass spectrometry (MS). The history and development of the EM from an initial research design to a commercial model are reviewed along with MS research applications. An EM incorporated with a mass spectrometer showed significant improvement in sensitivity over traditional methods for negative-ion generation and selectivity for compounds with electrophilic character. Sensitivity of EM-MS has been shown to be 25 fg for hexachlorobenzene in positive-ion mode and 10 fg for nitrobenzene in negative-ion mode. Reports regarding the analysis of chlorinated compounds, explosives, pesticides, phthalates, polychlorodibenzo-p-dioxins, polycyclic aromatic hydrocarbons (PAHs), nitro-polycyclic aromatic hydrocarbons (NPAHs), antioxidants, and bacterial biomarkers are discussed. Additionally, theoretical methods to predict electron-capture properties are presented. # 2013 Wiley Periodicals, Inc. Mass Spec Rev Keywords: electron monochromator; mass spectrometry; analytical applications; electrophilic compounds
I. INTRODUCTION Negative-ion mass spectrometry (NIMS) affords selectivity over positive-ion mass spectrometry for the analysis of electrophilic compounds at low concentrations in complex mixtures (Hunt & Crow, 1978). Traditionally, negative-ion chemical ionization (NICI) has been used to generate negative ions for NIMS; however, NICI requires a reagent gas and/or a buffer gas, such as a chemical ionization (CI) gas, which must be maintained at a constant temperature and pressure. Ions produced in NICI have multiple chemistries because an electron-energy distribution of 0–12 eV is produced by the CI process (Hunt & Sethi, 1980). The commercial electron monochromator (EM) produces an electron beam with a deviation of 0.3 eV, and eliminates the need for constant pressure and temperature conditions within the ion source. The development of the EM is not a recent effort (Nottingham, 1939; Fox et al., 1955; Stamatovic & Schulz, 1970; Hacaloglu et al., 1989; Illenberger, Scheunemann, & Baumga¨rtel, 1979; Oster, Ku¨hn, & Illenberger, 1989; Hacaloglu, Contract grant sponsor: Department of Energy (National Renewable Energy Laboratory); Contract grant number: KXEA-3-33607-00; Contract grant sponsor: Oakridge National Laboratory; Contract grant number: 74X-SX669C. Correspondence to: Kent J. Voorhees, Colorado School of Mines, Department of Chemistry and Geochemistry, 1012 14th Street #204, Golden, CO 80401. E-mail: [email protected]
Mass Spectrometry Reviews # 2013 by Wiley Periodicals, Inc.
Gokmen, & Suzer, 1990); however, practical application to analytical mass spectrometry (MS) became possible after improvements were reported by Deinzer and co-workers at Oregon State University (OSU) (Larame´e et al., 1996). Under license from OSU, a commercial EM was developed and introduced by JEOL USA, Inc. (Peabody, MA). The following review presents a background on the theory and development of the EM and its application to analytical mass spectrometry. Examples of its use for detection of chlorinated compounds, explosives, pesticides, phthalates, dioxins, polycyclic aromatic hydrocarbons (PAH), nitro-polycyclic aromatic hydrocarbons (NPAH), antioxidants, and bacterial biomarkers are discussed. Theoretical studies to correlate and predict electron capture energies are also included.
II. BACKGROUND A. Ion Formation Traditional mass spectrometry uses a 70 eV electron beam to ionize an analyte [electron ionization (EI)] for positive-ion detection. EI results in an ion in a high-energy state that often fragments into smaller ions. Because of this instability, the molecular ion might not appear in the mass spectrum, a factor that can confound compound identification. Decreasing the electron beam energy to 12 eV often results in the increased intensity of the molecular ion, but at a severe cost of ionization current and sensitivity (Watson & Sparkman, 2007). Analysis of negative ions with an EI source is even more problematic because formation of negative ions is approximately 1,000 times less efficient than positive-ion formation (Hunt et al., 1976). For a low-energy electron beam to be utilized for analytical mass spectrometry, the energy distribution of the beam and the ionization current had to be improved. Formation of negative ions in a mass spectrometer occurs by three types of electron interactions (Craggs & McDowell, 1955):
Resonance electron capture: (Thermal electrons) AB þ e ! AB
Dissociative electron capture: (
