![[logo] US EPA](https://webarchive.library.unt.edu/eot2008/20090508233349im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
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, grange.andrew@epa.gov).
You will need Adobe Acrobat Reader, available as a free download, to view some of the files on this page. See EPA's PDF page to learn more about PDF, and for a link to the free Acrobat Reader. |
ICE is NICE (PDF, 52 pp., 4.9 MB)
Best viewed as a PowerPoint 2000 file (F5 will start presentation)
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.
Successive Approximation
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.
Forensics
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)
).
* Statement is further illustrated on the CD-ROM.