Ion Composition Elucidation (ICE)
Powerful New Tools for Analyzing Environmental Contaminants: Mass Peak Profiling from Selected-Ion-Recording Data (MPPSIRD) and a Profile Generation Model
Andrew H. Grange, William C. Brumley, and G. Wayne Sovocool
Environmental Sciences Division, NERL, U.S. EPA, PO Box 93478, Las Vegas, NV 89193-3478
Phone: 702-798-2137; Fax: 702-798-2142
There are numerous applications for MPPSIRD and the PGM. Five that have been demonstrated are briefly discussed below.
Capillary gas chromatography with mass spectrometric detection is the most commonly used technique for analyzing samples from Superfund sites. While the U.S. EPA has developed target lists of compounds for which library mass spectra are available on most mass spectrometer data systems, only a small fraction of compounds generated in industrial processes are included in the libraries. Further, the low accuracy of the mass determined for ions can correspond to multiple elemental compositions. Finally, coelution of components can yield poor, multiple matches with library mass spectra. Consequently, comparison of mass spectra with those in libraries cannot identify most components in these complex samples.(1) This applications note discusses a new mass spectrometric approach that overcomes these deficiencies by determining unique elemental compositions for ions, which can lead to identification of otherwise analytically problematic pollutants. In principle, high-resolution mass spectrometry could be used to determine the exact mass of the molecular ion (M+) from each component, which, if known within narrow error limits, would provide the elemental composition of each compound. The elemental composition of M+ excludes library matches having other compositions and limits a compound's identity to a finite number of isomers. The compositions of fragment ions, and neutral losses determined from their exact masses, can greatly reduce the number of possible isomers, especially when fragment ions characteristic of specific moieties are observed. Historically, individual mass peak profiles have been examined using high mass resolution for quality assurance.(2-6) A non-Gaussian peak shape indicates the presence of an interfering ion. The exact mass determined from a profile aids in identifying the compound responsible for the interference.2 However, until now, no analytical method had been developed to systematically identify ions based on information obtained from one or more profiles.
RESULTS AND DISCUSSION
Mass peak profiling from selected ion recording data (MPPSIRD)
Full scans sample m/z ratios across a broad mass range. Unfortunately, a slow scanning cycle prevents delineation of chromatographic peaks when high mass resolution is used. For example, 5.4 sec/scan is required to view a range of 14 Da at 20,000 resolution. Mass resolution is defined as R = M/deltaM for a 10% valley between overlapping profiles of equal height, where M is the average center mass of the two profiles and deltaM is the mass difference between the maxima of the two profiles.(7) A new data acquisition strategy (MPPSIRD) developed at the U.S. EPA's Environmental Sciences Division (Las Vegas, NV) provides more than a sixfold faster sampling time (0.8-sec scans) for the above example. Equally important, MPPSIRD provides more than 100 times greater sensitivity than full-scan modes.(8) Using high mass resolution (20,000 + to - 10%), ion chromatograms are plotted (Figure 1a) for m/z ratios across a single mass peak profile.
Figure 1. Ion chromatograms (a) for m/z ratios corresponding to the five shaded points in (b), a full mass peak profile, and (c) three partial profiles.
The areas in Figure 1a under the chromatographic peaks, which were observed as a targeted component entered the mass spectrometer, were plotted to provide the profile in Figure 1b. The exact mass was determined as the weighted average of several points near its maximum. At 10,000 resolution, experimental exact masses of less than 150 Da for ions that contain only C, H, N, O, P, or S atoms usually correspond to a single elemental composition within the maximum mass error of 6 ppm for a single determination.(8-9) However, many components in Superfund site samples have larger molecular weights, and the number of possible compositions increases rapidly with mass. The abundance of the (M+1)+ peak relative to the M+ peak in low-resolution mass spectra has long been used to estimate the number of C atoms in an ion.(10) At high mass resolution, the exact masses of the (M+1)+ and (M+2)+ profiles and their abundances relative to the M+ profile provide four additional physical properties for distinguishing among compositions. These values are determined from partial profiles obtained by monitoring six points each across the top of the M+, (M+1)+, and (M+2)+ profiles.(11) The relative abundances in Figure 1c are the ratios of the sums of the six points for each partial profile normalized to 100%. At 20,000 resolution, interferences from the calibrant, column bleed, and coeluting analytes are usually eliminated. Compositions can generally be determined for M+ up to 600 Da, when up to 15 determinations of the exact masses and relative abundances are made to reduce the error limits.
MPPSIRD requires a personal computer and an RS232 cable, but no modifications to the VG70-250SE mass spectrometer (Micromass, Danvers, MA). Simple programs in the data system macro language (version B2.2), DOSTM (version 3.1) (Microsoft Corp., Bellevue, WA), Lotus 1-2-3TM(version 2.1) (Lotus Development Corp., Cambridge, MA), ReflectionTM (version 4.2) (Walker, Richer, and Quinn, Inc., Seattle, WA), and WordPerfectTM (version 5.1) (WordPerfect Corp., Orem, UT) automatically prepare SIR descriptors for data acquisition and plot the profiles. To fully utilize the data obtained, however, a profile generation model (PGM) is necessary.
The PGM(9) written in QuickBasicTM (Microsoft Corp.) is used to plan and interpret experiments. The model lists all possible compositions with zero or more rings and double bonds for a user-specified mass and possible elements. It calculates the exact masses for the M+, (M+1)+, and (M+2)+ partial profiles and ranges of relative abundances for each possible composition based on experimental errors determined from experiments with several standards.(11) The model compares these values for each composition with that of a hypothetical composition selected from the list by the user, and places an "X" next to each value outside the error limits for the hypothetical composition. A hypothetical composition is needed to calculate the m/z ratios to be monitored for the partial profiles. One then predicts the mass resolution and number of determinations needed to reject all but a single composition for an ion. At 20,000 resolution, the error limits for the exact masses decrease from + to - 3 ppm to + to - 1 ppm as the number of determinations increases from 1 to 15. After data acquisition, the experimental values are entered, and the compositions are tested as illustrated in Table 1.
Table 1. Partial list of elemental compositions and quantities useful for distinguishing among them
Application 1: Characterization of a complex sample
MPPSIRD and the PGM were used to characterize a tar-like sample from a Superfund site.(1) Sample diluted with methylene chloride was injected onto an Rtx-5 GC column (Restek, Bellefonte, PA), and 47 chromatographic peaks in the total ion chromatogram (TIC) were investigated. Compositions for several presumed molecular ions with masses less than 150 Da were determined from full mass peak profiles acquired at 10,000 resolution. The compositions of the remaining ions were determined from full and partial profile data acquired at 20,000 resolution by applying all five criteria. A portion of the TIC labeled with the unique composition identified for each peak is shown in Figure 2.
Figure 2. A portion of the total ion chromatogram for a Superfund site sample dissolved in methylene chloride. The elemental compositions were determined for the largest mass ion in the low-resolution mass spectra that did not contain higher isotopes.
In addition, compositions were determined for ions from four compounds that did not provide chromatographic peaks by using a heated insertion probe to introduce the extract into the ion source. At least 25 compounds provided the C8H7NS+ ion (m/z 149.0299) characteristic of alkylbenzothiazoles, which was monitored at 20,000 resolution to discriminate against potential interferences such as the C8H5O3+ ion (m/z 149.0239) formed from phthalates. The finding that a majority of the compounds contained the benzothiazole moiety suggested that dye or rubber plants produced the waste. These results were consistent with the known source of this contamination. Single, good library matches for low-resolution mass spectra were found for several low-mass compounds, most of which were not benzothiazoles. Characterization of the complex mixture with MPPSIRD and the PGM provided elemental compositions for ions from components that accounted for most of the mass of the sample compounds that provided chromatographic peaks and provided the chemical information needed to identify the source of the waste.
Application 2: Rapid screening for a target analyte
A library match for strychnine (C21H22N2O2) was found for a very minor component in Application 1. However, only the composition, C24H18N2, passed all five criteria. The absence of strychnine was confirmed by monitoring for its partial profiles with probe introduction of the sample and none was found.(1) Probe introduction using 20,000 resolution is a rapid, selective, and sensitive way to screen for target analytes within complex mixtures.
Application 3: Identification and quantification of Aroclors
To rapidly screen for and identify Aroclors (commercial mixtures of polychlorobiphenyl congeners), partial profiles were used to monitor the most abundant ion of the molecular ion group for three congeners at a time with probe introduction.(12) The binary mixture of Aroclors was prepared that provided the same ratio of the partial profile areas for the two most abundant congeners. Then partial profiles were used to monitor the two most abundant congeners and a 13C-labeled isomer of one of the congeners. A useful calibration curve was obtained over a concentration range of 0.01 ng/ml to 1000 ng/ml.(12) Quantitation was peformed rapidly since probe introduction was used.
Application 4: Identification of well contaminants
MPPSIRD and the PGM were used to determine the elemental compositions of several isomers that were environmental contaminants in a municipal well that serviced 50,000 people in an area where an increased incidence of childhood cancer was observed. The number of candidate isomers was greatly reduced by acquiring full profiles at 10,000 resolution for 10 fragment ions, many of which were present in very low abundance. Unique compositions constrained by the number of atoms of each element in the molecular ion corresponded to the exact masses of the fragment ions and neutral loss fragments. Mass spectral interpretation was simplified, and searching of mass spectral databases and the chemical literature was more rapid when fewer isomers were considered. Identification of these isomers and confirmation by comparison to a standard were described in Reference 13.
Application 5: Confirmation of products from syntheses
High-resolution mass spectrometry is used to confirm the identity of synthesis products based on their exact masses. Use of MPPSIRD and the PGM to make such confirmations based on five criteria and to eliminate all other compositions based on the elements in the reagents, solvents, and catalysts provides much stronger confirmatory evidence. In addition, the structure of unintended synthesis products can be determined from fragment ion and neutral loss compositions.(14) This information can guide modification of the synthesis.
The Future: Liquid sample introduction
MPPSIRD has provided the exact mass of acid orange 8 introduced by coaxial flow and ionized by Cs+ atoms (secondary ionization mass spectrometry [SIMS]). Studies are now directed toward using the technique with electrospray ionization and atmospheric pressure chemical ionization sources. This will expand application of MPPSIRD and the PGM to identification of environmental contaminants that cannot be studied using gas chromatography or probe introduction.
The sampling speed, selectivity, and sensitivity of MPPSIRD enabled use of a high-resolution, double focusing mass spectrometer to determine elemental compositions of ions formed from minor and major components in complex mixtures, to rapidly and selectively screen for a target analyte with high sensitivity, and to identify and quantify Aroclors. Separation in the mass domain removed interferences and provided a separation orthogonal to chromatographic separations. Two manufacturers of high-resolution mass spectrometers have expressed interest in this new analytical technique. It is hoped that MPPSIRD and the PGM will be incorporated into future versions of their software to greatly enhance the utility of their instruments for environmental and other applications. In the meantime, the programs needed to perform MPPSIRD with a VG70-250S mass spectrometer and the PGM are available from the authors.
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