EPA/600/A-96/082
A FIELD DEPLOYABLE ION TRAP MASS SPECTROMETER FOR THE
MEASUREMENT OF TRACE ORGANIC COMPOUNDS IN AIR
Sydney M. Gordon,1* Patrick J. Callahan,1 Donald V. Kenny,1 and Joachim D. Pleil2
1 Battelle, 505 King Ave., Columbus, OH 43201; 2 National Exposure Research Laboratory
(MD-44), US Environmental Protection Agency, Research Triangle Park, NC 27711
Keywords: Direct air sampling interface, VOCs, benchtop ion trap MS, MS/MS, FNF technique
INTRODUCTION
A number of new monitoring techniques for characterizing hazardous air pollutants have arisen
in response to the limitations of standard batch methods. Based on mass spectrometry and direct
air sampling interfaces, the techniques measure all of the target analytes in a sample simul-
taneously without prior sample preconcentration or chromatographic separation [1). Those based
on the ion trap mass spectrometer (ITMS) offer a powerful means of developing compact, field-
deployable systems with high sensitivity, specificity, and speed for continuous real-time
monitoring of volatile organic compounds (VOCs) of environmental interest in air [1-5]. The
necessary specificity is provided by tandem mass spectrometry (MS/MS) in which parent ions
are isolated selectively before undergoing dissociation to form characteristic product ions.
Although the ion trap is uniquely suited to take advantage of the inherent capabilities of MS/MS
[6], its storage capacity is limited by space charge effects. A broadband resonance excitation
technique that makes use of filtered noise fields (FNF) has been developed to enhance the trap's
storage capacity for analyte ions [7-8J. A commercially-available instrument with full FNF and
MS/MS capability is the compact field-deployable Teledyne 3DQ Discovery ion trap.
We have attached a direct air sampling (DAS) interface, an air sampling glow discharge ioniza-
tion (ASGDI) source [2,9], to the 3DQ ion trap. This allows trace organics in air to be ionized
externally before they are injected into the trap where they are selectively stored and dissociated
using the FNF technique [3,8]. We have explored the potential of the ASGDI/3DQ combination
and filtered noise fields for rapid field MS/MS analysis of VOCs at trace levels in air.
EXPERIMENTAL
All experiments were carried out on a Teledyne (Mountain View, CA) 3DQ Discovery ITMS.
The 3DQ is a compact, field-deployable ion trap with full FNF capability, and has been
described in detail previously [3]. With the FNF method, a flat broadband waveform with a large
number of evenly spaced frequency components is applied to the end caps of the trap [7], The
frequency components span the entire trappable mass/charge ratio (m/z) range of the trap, and
their amplitudes are set sufficiently large so that initially all ions are resonantly ejected. Then,
"notches" that correspond to specific masses are inserted at the appropriate resonance frequencies
in the basic FNF waveform, and the notched FNF signal is applied to the end caps. In this way,
unwanted ions are ejected and the full storage capacity of the trap is used to accumulate only
selected target ions. The isolated ions may be collisionally dissociated (MS/MS mode) by
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applying resonance excitation to the end caps using either a single frequency or a frequency
bandwidth.
Filtered noise fields are implemented through scan functions, which are generated by
constructing a contiguous series of scan descriptor "tables." Each table contains a description of
the procedures needed to perform a specific task within the scan function. By allowing a single
table to perform several functions, a standard ion isolation step can be defined with only two
tables, and an MS/MS experiment with just three tables. Tables can be viewed and edited
through an interactive Windows™ edit program. Additional tables may be quickly added to
extend the functionality of each scan function.
The ASGDI source has been described in detail elsewhere [2,9]. Briefly, the ionization region is
defined by two circular plates mounted parallel to one another on the outside faces of a
hollowed-out vacuum flange. Air enters the source, at a pressure of-0.6-1.0 torr, through a
narrow aperture in the outer plate and sustains the discharge which, in the positive ion mode, is
established by applying a potential of about 400 V between two half plates mounted in the source
region perpendicular to the two outside circular plates [2,9]. Ions formed in the discharge are
focused into the trap via a three-element einzel lens. The central element of the lens is
electronically pulsed using the standard ITMS circuitry to gate ions into the trap. Helium flows
into the vacuum chamber at an uncorrected pressure of 4 x 10"4 torr and serves as the buffer gas
as well as the collision gas in the MS/MS dissociation experiments.
RESULTS
Figure 1. ASGDI/3DQ ion trap mass spectra
of toluene: (a) molecular ion region without
FNFion isolation; (b) spectrum with FNF
single-notch isolation ofm/z 92; (c) MS/MS
spectrum resulting from collisional dissociation
ofm/z 92 to m/z 65.
Ion Isolation with Unit Mass Resolution.
Figure la presents the mass spectrum of the
molecular ion region from toluene obtained with
the ASGDI/3DQ combination in the absence of a
filtered noise field. Application of the single
notch FNF broadband signal has a filtering effect
on the signal since it removes all of the unwanted
ions from the spectrum except those of the
selected mass, m/z 92, as shown in Figure Ib.
As a result, the ion gate can be opened for
extended periods, significantly increasing the
intensity of the mass of interest. The specificity
and sensitivity required to analyze complex
mixtures is obtained by using resonance excita-
tion to bring about MS/MS dissociation of the
isolated ion, which produces only those fragment
ions that are formed directly from the parent.
Figure Ic shows the collisional dissociation of
the isolated molecular ion of toluene to form the
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Table 1. Product Ion (MS/MS-Mode) Detection Limits (DL) for Selected Nonpolar
and Polar VOCs in Humidified Air Using the ASGDI/3DQ Ion Trap MS System.
Compound
Benzene
1,1,1 -Trich loroethane
o-Dichlorobenzene
Acetone
Methyl ethyl ketone
Parent
Ion -*•
78
97
146
59
73
Product
Ion
5lb
61
lllb
3lb
55
DL
(ppbv)
0.3
0.8
13
1.0
0.1
Conversion
Efficiency (%)a
36
77
24
34
81
a Ratio of product ion signal intensity to isolated (parent) ion signal intensity.
b More than one product ion observed. Sum of product ions yields conversion efficiency g 75%.
product ion at m/z 65, which takes place with a conversion efficiency (i.e., the ratio of the
fragment ion signal intensity to the initial parent ion signal intensity) of about 90%. This high
conversion efficiency is typical of many positive ion MS/MS dissociations in the ion trap.
Wide Linear Dynamic Range and Low-ppb Detection. MS/MS detection limits obtained for
product ions from a representative set of VOCs in humidified air are listed in Table 1. Two of
the compounds, 1,1,1-trichloroethane and methyl ethyl ketone, form only one fragment ion each,
and the resulting detection limits are in the sub-ppbv range with dissociation efficiencies of
-80%. However, the parent m/z 146 ion from o-dichlorobenzene dissociates to form several
fragment ions. Although the conversion efficiency is high for the sum of these fragment ions, it
is no more than 24% for the m/z 111 fragment and the associated detection limit is only 13 ppbv.
Benzene, on the other hand, is detected with high sensitivity, despite the fact that it also yields
several product ions upon MS/MS dissociation.
Continuous Analyte Monitoring. Figure 2
shows the "step change" in p-dicbJorobenzene
concentration that results when two "moth
cakes" are unwrapped and placed on shelves in
a small storage room. The compound was
measured in the MS/MS mode by monitoring
the intensity of the m/z 111 fragment ion.
Although only a single compound was
monitored in this test and the increase in
concentration occurred over a relatively long
period of time, the system is capable of
measuring several selected VOCs simul-
taneously in the MS/MS mode. Furthermore,
the fact that measurements were made at 5-
second intervals indicates that the technique
can track changes in concentration that occur
rapidly and at trace levels.
omit it U U U*ttttM«KetU3U2UX«X;0)UX«BIXtXO
m/z
Figure 2. Increase in air concentration ofp-
dichlorobenzene, measured continuously in
real time in the MS/MS mode, as a result of
exposure of two "moth cakes " in a small
storage room.
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CONCLUSIONS
Monitoring trace levels of specific VOCs in air in the field can be effectively accomplished using
the 3DQ Discovery ion trap mass spectrometer with filtered noise fields and an air sampling
glow discharge ionization source as a DAS interface. The instrument provides continuous real-
time measurement capability with detection limits for selected nonpolar and polar VOCs in the
low- to sub-ppbv range. Although the system described here is still in the developmental stage,
it has the potential for direct application in a variety of important real-world applications.
REFERENCES
1. MB. Wise, C.V. Thompson, M.V. Buchanan, R. Merriweather, and M.R. Guerin,
Spectroscopy 8,14-22 (1993).
2. S.A. McLuckey, G.L. Glish, and K.G, Asano, Anal. Chim. Acta 225,25-35 (1989).
3. S.M. Gordon, PJ. Callahan, D.V. Kenny, and J.D. Pleil, Proc. 1995 EPA/A&WMA
International Symposium on Field Screening Methods for Hazardous Wastes and Toxic
Chemicals, Vol. 1, VIP-47, Air & Waste Management Association, Pittsburgh, PA, 1995,
pp. 670-679.
4. M.E. Cisper, C.G. Gill, L.E. Townsend, and P.M. Hemberger, Anal. Chem. 67,1413-1417
(1995).
5. S.M. Gordon, PJ. Callahan, D.V. Kenny, and J.D. Pleil, Rapid Commun. Mass Spectrom.
in press (1996).
6. J.V. Johnson, R.S. Yost, P.E. Kelley, and D.C. Bradford, Anal. Chem. 62,2162-2172 (1990).
7. P.E. Kelley, US Patent No. 5,134,286, (1992).
8. D.V. Kenny, PJ. Callahan, S.M. Gordon, and S.W. Stiller, Rapid Commun. Mass
Spectrom. 7,1086-1089 (1993).
9. S.A. McLuckey, G.L. Glish, K.G. Asano, and B.C. Grant, Anal. Chem. 60,2220-2227 (1988).
ACKNOWLEDGMENTS
The information in this document has been funded wholly or in part by the U.S. Environmental
Protection Agency under Cooperative Agreement CR 822062-01-0 to Battelle Memorial Institute.
It has been subjected to Agency review and approved for publication. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use. We are
grateful to Teledyne Electronic Technologies for the loan of the 3DQ Discovery ion trap mass
spectrometer.
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TECHNICAL REPORT DATA
1. REPORT NO. 2.
EPA/600/A-96/082
4. TITLE AND SUBTITLE
A Field Deployable Ion Trap Mass Spectrometer for the
Measurement of Trace Organic Compounds in Air
7. AUTHOR (S)
S. M. Gordon, P. J. Callahan, D. V. Kenny; Battelle
J. D. Pleil, US EPA/NERL
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Battelle Columbus
505 King Avenue
Columbus, OH 43201
12. SPONSORING AGENCY NAME AND ADDRESS
US Environmental Protection Agency
National Exposure Research Laboratory
Research Triangle Park, NC 27711
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
11. CONTRACT /GRANT NO.
822062
13. TYPE OF REPORT AND PERIOD COVERED
Symposium paper
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16 . ABSTRACT
A number of new monitoring techniques for characterizing hazardous air pollutants have
arisen in response to the limitation of standard batch methods. Based on mass
spectrometry and direct air sampling interfaces, the techniques measure all of the
target analytes in a sample simultaneously without prior sample preconcentat ion or
chromatographic separation. Those based on the ion trap mass spectrometer (ITMS) offer
a powerful means of developing compact, f ield-deployable systems with high sensitivity,
specificity, and speed for continuous real-time monitoring of volatile organic
compounds (VOCs) of environmental interest in air. The necessary specificity is
provided by tandem mass spectrometry (MS/MS) in which parent ions are isolated
selectively before undergoing dissociation to form characteristic product ions.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
18 . DISTRIBUTION STATEMENT
RELEASE TQ PUBLIC
b. IDENTIFIERS/ OPEN ENDED c.COSATI
TERMS
19. SECURITY CLASS (This 21. NO. OF PAGES
Report )
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20. SECURITY CLASS (This Page) 22. PRICE
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