ION COMPOSITION ELUCIDATION (ICE)
"ICE is Nice" - A New Approach to High-Resolution Mass Spectrometry for Pollutant Identification
What is ICE?
Ion Composition Elucidation, or ICE, is a powerful analytical technique that determines how many atoms of each element compose the molecular ion and fragment ions observed in a mass spectrum. This information can lead to identification of compounds that are not found in environmental mass spectral libraries. The two components of ICE are data acquisition, using Mass Peak Profiling from Selected Ion Recording Data (MPPSIRD), and automated data interpretation by a Profile Generation Model (PGM).
"ICE is Nice" is an instructional presentation on CD-ROM that describes Ion Composition Elucidation (ICE) to both experienced mass spectroscopists and to University and high school chemistry students. The "ICE is Nice" CD-ROM presents basic definitions, the scientific basis of ICE, and its advantages and limitations. ICE is applied to three analytical problems as illustrative case histories. You can obtain this CD-ROM by phoning or e-mailing Dr. Andrew Grange (702/798-2137, email@example.com).
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What is the Problem?
Matching of low-resolution mass spectra is the pre-eminent identification method for environmental pollutants listed in EPA methods. However, at least three factors limit the utility of mass spectral libraries for tentatively identifying compounds. Mass spectra of poor quality due to low levels of analytes or coelution of interferences can yield errant, multiple library matches; the vast majority of organic compounds that provide gas chromatographic peaks are not found in mass spectral libraries; and most organic compounds are ionic, too polar, too thermolabile, or too high in mass to traverse a gas chromatography column. Unidentified compounds require mass spectral interpretation to hypothesize possible compound identities.
ICE -- The Analytical Approach
Determination of the compositions of the apparent molecular ion and fragment ions in a mass spectrum not found in mass spectral libraries greatly reduces the number of possible compound identities and reduces the number of standards that must be purchased in hope of confirming tentative identifications. Between 5 and 10 mass-to-charge (m/z) ratios are monitored across full or partial mass peak profiles to provide ion chromatograms*. The areas under the chromato-graphic peaks resulting from compounds eluting into the mass spectrometer are integrated and plotted to provide full or partial profiles. A weighted average of the top several points delineating a profile provides its "exact" mass, and the sums of the points used to plot profiles are ratioed to provide relative abundances. In addition to the profile of each ion that arises from the most abundant isotopes of the elements that compose the ion, the profiles heavier by 1 and 2 Da that arise from heavier isotopes such as 13C, 15N, 18O, and 34S are monitored*. The three measured exact masses and two measured relative abundances are then entered into the Profile Generation Model to determine the unique elemental composition that provides similar calculated values.
What are the advantages of using the ICE technique?
Speed, sensitivity, selectivity, and stability are data acquisition advantages of MPPSIRD relative to full scanning or selected ion recording. Up to 31 m/z ratios are monitored sequentially during each 1-s cycle to delineate the chromatographic peaks, whereas measurement of exact masses using abbreviated full scans or peak matching requires several seconds to a minute. The 100-fold gain in sensitivity realized for selected ion recording is retained. Higher sensitivity can be traded for greater selectivity, and analyte peaks can be resolved from interferences by routinely using up to 20,000 mass resolution. Calibration drift is compensated because with each cycle a narrow scan is made across a lock mass, and the voltages used to monitor all m/z ratios are set relative to the lock mass profile's maximum. Two measurement advantages are realized with MPPSIRD: accurate determination of the exact masses (±3 ppm at 20,000 resolution) and relative abundances of the profiles. Quality assurance is provided by delineating all or most of each profile - a Gaussian shape usually indicates that no significant interferences are present.
A survey run is made over a 2000-ppm mass range using 3000 resolution. Three points delineate each profile*. The source of the compound that yields a profile is revealed by the shape of the ion chromatogram that corresponds to the maximum of the profile*. Analyte profiles provide a chromatographic peak. The coarse estimate of the exact mass from an analyte profile plotted from only three points is used as the center mass in a Selected Ion Recording (SIR) descriptor to monitor 10 points across the analyte profile. The exact mass obtained provides the composition of ions containing C, H, N, O, P, or S atoms with masses up to 150 Da*. For heavier ions, a third experiment monitors the 0, +1, and +2 partial profiles. If 20,000 resolution is used, the upper mass limit for determining unique ion compositions for ions containing these six elements is extended to 600 Da*.
ICE Employed in Toms River, NJ samples
Conventional low- and high-resolution mass spectrometry could not identify several isomeric compounds found by the NJ Department of Environmental Protection in a municipal well that serviced 50,000 people near Toms River where an increased incidence of childhood cancer (PDF, 1 pp., 3.7 MB) had been observed ICE, using 20,000 mass resolution to monitor m/z 210, +1, and +2 partial profiles, provided C14H14N2 as the composition of the molecular ion* and was then used to determine the fragment ion and composite neutral loss compositions based on full profiles obtained with 10,000 resolution*. The number of N atoms in each fragment ion and composite neutral loss was determined, greatly reducing the number of possible isomers*. A brief search of the literature located products of an industrial polymerization process (1:2 styrene:acrylonitrile adducts) with similar mass spectra. Comparison with byproducts of the currently used synthesis confirmed the identifications.
In addition to low-resolution mass spectra and retention times, ICE amassed a preponderance of evidence for identification of the NJ well pollutants*. Atomic masses and isotopic abundances are summed for compositions and compared with measured exact masses and relative abundances.* Only two errors are important when full profiles are plotted: the variation of isotopic abundances in nature and instrumental precision. (Two other applications of ICE are illustrated in "ICE is Nice" and in a poster, "Well Pollutants Identified With a New Mass Spectrometric Technique" (PDF, 1 pp., 302 KB) ).