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Ion Composition Elucidation (ICE)  

Plotting Mass Peak Profiles from Selected-Ion Recording Data

Andrew H. Grange1 and William C. Brumley2

1Lockheed Environmental Systems & Technologies Co., 980 Kelly Johnson Drive, Las Vegas, Nevada 89119.
2U.S. Environmental Protection Agency, National Exposure Research Laboratory,
Environmental Monitoring Systems Laboratory, P.O. Box 93478, Las Vegas, Nevada 89193.

SPONSOR REFEREE: Dr Ron Hass, Triangle Labs Inc., Durham, NC, USA

ABSTRACT

Plotting of mass peak profiles at high mass resolution for chromatographic peaks with low ion abundance is possible from single-ion monitoring data due to the high sensitivity and low cycle time for single-ion monitoring.  The mass profiles provide accurate masses, document mass resolution at a mass as a chromatographic peak elutes and provide relative abundances for two ions at the same nominal mass.  A personal computer is used to plot the mass peak profiles.

INTRODUCTION

As pointed out previously by Harvan et al.,1 it is generally impossible to demonstrate the absence of co-eluting interferences when validating quantitative analyses using gas chromatography with mass spectrometry (GC/MS).  Increasing the resolving power of the mass spectrometer to 10,000 or more enhances selectivity, but requires the use of selected-ion recording (SIR) to provide adequate sensitivity and a cycle time small enough to profile chromatographic peaks with widths of only several seconds as they elute from capillary columns.  As usually practiced, SIR techniques monitor a mass-window centered on the exact mass of the analyte ion, whose width is fixed by the resolving power.  This procedure is limited by the possibility that a co-eluting ion can contribute significant intensity within the mass-window being monitored, even though the accurate mass of the interfering ion is outside the monitored window.

Additional confirmatory evidence is required if a chromatographic peak, monitored by conventional SIR methods, is to be confidently assigned to an analyte and subsequently quantified.  Harvan et al. suggested two confirmatory mass spectrometric methods.  One of these is tandem mass spectrometry (MS/MS), which has recently been assessed2 in comparison to high resolution SIR in the context of trace analysis for chlorinated dioxins.  The second confirmatory technique1 is a modification of the SIR approach, in which one monitors the integrated ion intensity in the SIR mass-window and, in addition, scans and records the mass-peak profile within this window.  The profile allows one to assess whether or not the integrated intensity observed with conventional SIR is entirely due to the analyte ion.

Harvan et al.1 acquired their mass-peak profile data using what amounts to a continuous scan over the specified mass-window, and stored the data in profile mode using a Finnigan-Incos data system (Finnigan-MAT, San Jose, CA, USA).  We wish to report a modification of their technique, whereby the mass-peak profiles in a GC/MS experiment are acquired using a series of overlapping SIR channels covering the mass peak monitored in the conventional SIR mode.  The mass increment between SIR channels is usually chosen to provide 10 samplings across the mass profile.  The width of the mass-window is the same as that used to sample the maximum in the profile for conventional SIR when the same resolving power is used.  There is no apparent difference in shape between mass peak profiles obtained with a full scan over a range of 1 u and that obtained by plotting SIR data acquired at the same mass resolution.

EXPERIMENTAL

All experiments reported herein employed a type 70SE double-focusing mass spectrometer (VG Analytical, Manchester, UK) equipped with a VG 11-250 data system.  A 30 m, 0.25 mm ID, 0.25 mm film, DB-5 capillary column was used within a type 5890 gas chromatograph (Hewlett-Packard, Avondale, CA, USA) that was interfaced to the mass spectrometer source through a deactivated transfer line.  Certain capabilities of the instrument can be expanded by plotting mass-peak profiles from data acquired with SIR at high resolving power.  Compared to the scan modes normally used to obtain mass-peak profiles,1 the SIR mode provides increased sensitivity and decreased cycle times when high mass resolution is used.  Disadvantages of this approach are that only a narrow mass range can be monitored and that an ancillary computer is necessary to plot the profiles off-line.

The enhanced sensitivity permits accurate masses to be determined for fragment and molecular ions with low ion abundances.  It also allows higher mass resolution to be used.  Because the cycle time of data acquisition is decreased, chromatographic peak profiles can be monitored at high mass resolution when capillary columns are used in GC/MS analyses.  From these chromatographic profiles, multiple ions found at the same nominal mass, but with different accurate masses, can be correlated with their parent compounds.

To obtain a mass-peak profile for a chromatographic peak, the areas under 21 SIR traces are plotted as a function of the m/z ratio as illustrated in Figure 1.

Figure 1 - For further information contact grange.andrew@epa.gov

Figure 1.  The areas under the 7 ion chromatograms were used to plot the ion profile.

To determine an accurate mass for a single ion, the center mass in the SIR descriptor is set to the anticipated mass of the ion and the other m/z ratios are incremented to lower and higher values to provide 10 samplings of the mass-peak profile at the resolving power to be used.  The GC/MS software provided by the manufacturer3 cannot plot the mass-peak profiles for a GC peak, but they can be plotted using LOTUS 1234 (or any other spreadsheet software with graphics capability) after a report file containing the m/z ratios listed in the SIR descriptor and the corresponding chromatographic peak areas are transferred using KERMIT5 to a personal computer (PC).  The areas rather than the maximum intensities of the chromatographic peaks are used, because more samplings across the peak provide a better signal-to-noise ratio for the mass peak profile.  The mass peak profiles can be printed using PRINTGRAPH which is a part of the LOTUS 123 software package, or imported into a WordPerfect6 document.  In summary, less than 10 min are required to enter the m/z ratios into the SIR descriptor, determine the areas of the chromatographic peaks, transfer the area report between computers, and plot and print the mass-peak profiles.  The simple VG, LOTUS and WordPerfect programs needed for these tasks, a detailed description of these programs and example mass-peak profiles are available from the corresponding author upon request.  (The VG, LOTUS 123 v2.2 and WordPerfect 5.1 software packages must be purchased separately.)

Mass-peak profiles plotted from SIR mode data can be used to:  (1) determine the accurate masses of ions; (2) document the mass resolution for a mass at the time a compound elutes; (3) determine the relative abundances of two ions of similar mass; and (4) document the resolution necessary to resolve target ions from interfering ions.  The mass range, mass increment and mass resolution used in the SIR descriptor must be determined from the masses of the target ion and the elemental compositions of the possible interfering ions that must be resolved.

In practice, when only the nominal mass of an ion is known, a relatively wide mass range (e.g. 2 u) is monitored to determine if more than one ion contributed to the ion abundance.  Estimates are thus provided of the relative contributions and accurate masses of the ions, if more than one is present.  The mass range monitored should include the mass limits corresponding to a list of ion compositions that are arithmetically possible for the mass-window used to determine the nominal mass, together with the maximum mass assignment error for the nominal mass determination.

When 21 m/z ratios are used in the SIR descriptor, the mass increment is the (mass range)/20.  The mass resolution must provide a peak width no smaller than the mass increment to ensure that all of the signal within the mass range is monitored.

After an accurate mass has been estimated, resolutions of up to 20,000 to 25,000 can be conveniently used with a VG 70SE mass spectrometer to obtain a better mass assignment.  To monitor 10 points across a mass-peak profile at a resolution of 20,000 requires a mass increment of 5 ppm (M/(resolution x 10).  Smaller mass increments can result in repeated points across the profile when the accelerating potential is digitized to the same value for pairs of m/z ratios.  Figure 2 is a mass-peak profile with a maximum at m/z 123.1181 that was obtained after tuning the instrument to a resolution of 20,000.

Figure 2 - For further information contact grange.andrew@epa.gov

Figure 2.  An ion profile at m/z 123 plotted from the peak areas from the 21 ion chromatograms specified by the SIR descriptor.  The resolution determined from the points at 5% of the maximum was 21,000 and the mass was in error by 0.0008 u.

Once the accurate masses of two ions are known, a split SIR group employing one lock-mass can be used to determine their relative abundances more accurately.  Approximately 8 points are monitored across each profile to ensure that each plot starts and ends near the baseline.

Interference with a target ion used for quantification in conventional SIR is usually illustrated by displaying an ion chromatogram with the distorted analyte peak.2  The mass-peak profile of an ion in the SIR descriptor provides more information about the interference; its accurate mass and the resolution necessary to remove it can be determined.1  In addition, if the interference coelutes exactly with the analyte, only mass-peak profiles can reveal the interference.  A mass resolution of about 22,000 is required to separate the ions shown in Figure 3.


Figure 3 - For further information contact grange.andrew@epa.gov

Figure 3.  The left ion profile is for a quantification ion, while the right profile is for a coeluting interference.

These profiles document that the interference has been adequately resolved.  In this example.  monitoring the centroid mass of the quantification mass-peak profile in the conventional SIR mode at this resolution will provide good quantitative results.

REFERENCES

  1. D. J. Harvan, J. R. Hass, J. L. Schroeder and B. J. Corbett, Anal. Chem. 53, 1755 (1981).

  2. M. J. Charles and Y. Tondeur, Environ. Sci. Technol. 24, 1856 {1990).

  3. VGII-250 Instruction Manual, Vol. 1-6, Release Bl.0 (1987).

  4. Lotus 1-2-3 Release 2.2, Reference, Lotus Development Corp., 55 Cambridge Parkway, Cambridge, MA, USA.

  5. C. Gianone, Editor, Kermit User Guide, Columbia Univ. Center for Computing Activities, New York, NY, USA (1988).

  6. WordPerfect for IBM Personal Computers and PC Networks, Version 5.1, WordPerfect Corp., 1555 N. Technology Way, Orem, UT, USA (1989).

Analytical Environmental Chemistry
ICE Home Page

Environmental Sciences | Office of Research & Development
 National Exposure Research Laboratory
Author: Andrew Grange
Email: grange.andrew@epa.gov


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