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Anal. Chem. 1985, 57,567-569
Ion Mobility Spectrometry/Mass Spectrometry of Two Structurally Different Ions Having Identical Ion Mass Sir: Ion mobility spectrometry (IMS) usually provides quasi-molecular ions which can be used for identification purposes ( I , 2). These ions are separated in the drift region according to intrinsic mobility through a counterflow of drift gas a t atmospheric pressure. Ion mobility is dependent on ion mass and average collision cross section (ionic radii) as the ions undergo binary collisions with neutral molecules. Ion mobility KO(cm2V-' ) r educed to standard temperature and pressure is given by
where I,, is the drift length between the shutter grid and aperture grid (cm), I,, is the drift length between the aperture grid and collector (cm), V, is the potential applied between the shutter grid and aperture grid (V),V,, is the potential applied between the aperture grid and collector (V),t d is the total drift time for the ions (s), t , is the width of the shutter grid pulse (s), T is the drift temperature (K), and P is the pressure (torr). Theoretically ion mobility K (cm2 V ' s - l ) is given by Figure 1. Ion mobility spectrum (A) and corresponding mass spectrum
K=
(-)(-m+ 16N 3e
?*');
(k T
2 r y 2
( ) -
(5)for the positive reactant ions at an IMS cell temperature of 50 "C.
(2)
Peaks labeled with a divide by two notation are actually twice the indicated amplitude.
OOT
where e is the electronic charge (esu), N is the gas density (molecules/cm3), m is the ion mass (g), A4 is the mass of drift gas (g), k is the Boltzmann constant (erg/K), and R O T is the average ionic collision cross section (cm2)a t temperature T ("C). Equation 2 predicts that if m is identical for two compounds having different ion mobilities, OoT provides an additional means for separating ion mobility peaks in an ion mobility spectrum. On the basis of this inherent feature of IMS, identification of isomeric phthalic acids ( 3 ) ,isomeric dihalogenated benzenes ( 4 ) were previously reported. Average collision cross sections for some alkyl or group substituted ring compounds were also reported using observed KOvalues ( 5 ) .
A
3
Table I. Experimental Conditions
'1
83
:
Ion Mobility Spectrometer reactor length reactor voltage drift length drift voltage carrier gas (purified air) drift gas (purified air) sample gas (prepurified nitrogen) drift temperature reaction temperature membrane temperature pressure
6.9 cm 1500 V 10.92 cm 2150 V 200 cm3/min 800 cm3/min 200 cm3/min 50 "C 50 O C 45 "C atmosphere
Mass Spectrometer inlet pressure chamber pressure resolution
1.5 X torr 4 x 10" torr Am = 1 (low mass) Am = 3-4 (hieh mass)
01
C
Figure 2. Positive ion mobility spectrum (A) and corresponding mass spectra (B and C) for GB (2 X IO-' g/L) at an I M S cell temperature 50 "C.
Under atmospheric pressure conditions, ions undergo clustering reactions with neutral molecules such as N2 and
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985 K.:
2 I2
25
! 1.25 (GB), H +
I REACTANT IONS
K,
',
:I
41
m/z 8 3
m/z
F
TB4
IO1 1
0
G
A
m/z 111
n
A
m/z 129
i
I0
20
~_
_ ~
30 DRIFT TIME (msi
I
40
50
60
Figure 4. Mass identified ion mobility spectral data for TBA: trace A is the total ion mobility spectrum for TBA and traces B-D are MIMS data for the K O = 1.43 cm2 V-' s-l I MS peak.
m / z 139 ;K
41
m/z IS7
m/r
P
307-311
Figure 3. Mass identified ion mobility spectral (MIMS) data collected at 50 OC for GB: trace A is the total ion mobility spectrum of GB, traces B-J are MIMS data for the positive reactant ions, traces K-N are MIMS data for the K O= 1.69 cm2 V-' s-l IMS peak, and traces 0-P are MIMS data for the K O = 1.43 cm2 V-' s-' IMS peak.
C
A
m/i
215
H20. For instance, the most prominent reactant ion species exist as an equilibrium mixture expressed as (HzO)pi(Nz)m-IH+ + H2O
k".b
+ Nz Z Y (HzO),(Nz),H+ (3)
where n and m are integers (Le., 2, 3, or 4 for H 2 0 and 0, 1, and 2 for N2)(6). The values of n and m are dependent upon temperature and pressure. Each of these ion species interchange rapidly ( lo5 times/s) to maintain a dynamic equilibrium characterized by a forward reaction rate constant 2.3 X cm6 molecules-2 s-l for k2,3 and a reverse rate constant cm6 molecules-2 s-l for k3,2(7). The reduced of 9.3 X mobility (KO)value observed for these ionic species is single valued corresponding to an ensemble average with each ion contributing proportionally to its abundance within the rapidly exchanging ion cloud (8). Consequently one can treat the ion cloud as a specific ion characterized by an effective collision cross section. This report describes an example where IMS data could separate otherwise ambiguous ions of identical mass, one ion being a member of a cluster series and the other being a quasi-molecular ion. This is unlike atmospheric N
Flgure 5. Ion mobility spectra collected from GB (A) and from TBA (B).
pressure ionization mass spectrometry (APIMS) where collisional stripping of the ion clusters is needed to derive information on the quasi-molecular ion (9, I O ) .
EXPERIMENTAL SECTION Ion Mobility Spectrometer/Mass Spectrometer (IMS/ MS). The IMS/MS system used to collect the data in this report is described elsewhere (11). The experimental parameters used to operate the IMS/MS are tabulated in Table I. The sample was introduced into the IMS through a 25-pm dimethylsilicone membrane of a membrane inlet. Gases. Purified air used for the drift and carrier gases of the IMS was generated by an AADCO 737 pure air generator and scrubbed by an activated 13X molecular sieve trap. The moisture level in this air was approximately 9 ppm as measured with a Du Pont phosphorus pentoxide moisture monitor (Model 303). Prepurified nitrogen (99.998% minimum purity) was used for
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Anal. Chem. 1985, 57, 569-571
the chamber and dilution gases in the diffusion tube standards generators which delivered sample vapor to the IMS. Chemicals. Isopropylmethylphosphonofluoridate(GB) was obtained from the Chemical Research and Development Center, Aberdeen Proving Ground, MD. It was of high purity (295%) and contained a small amount (2%) of tributylamine (a stabilizer) (12). Head space vapors of this sample were introduced into the sample gas of the IMS/MS by means of a Tracor 432 Tri-Perm or an Analytical Instrument Development, Inc., Model 350 diffusion tube standards generator. All samples were generated at 30 "C and vapor concentrations were determined by a well-established fluorometric method (13). The tributylamine was purchased from Polyscience Co., Evanston, IL.
RESULTS AND DISCUSSION In the positive ion mode the reactant ions (H20),(N2),NH4+, (H20),(N2),NO+, and (H2O).(NJ,H+, where n and m = 0, 1, 2, 3, or 4, are formed in both IMS/MS ( 7 )and APIMS (10). These ions undergo ion/molecule reactions with sample vapors to produce product ions. Figure 1 shows an ion mobility spectrum (trace A) and corresponding mass spectrum (trace B) for the positive reactant ions with an IMS cell temperature of 50 "C. In Figure 2, a positive ion mobility spectrum (A) and corresponding mass spectra (B and C) are shown for GB (2 X g/L). These data were collected under the same conditions of Figure 1. Spectrum C was obtained with a lower resolution for the mass spectrometer than spectrum B. Mass identified mobility spectral data are shown in Figure 3. From these data, it is observed that GB produces two major product ion mobility peaks with reduced mobilities of 1.65 and 1.25 cm2 V-' sd. According to the mass identified mobility data (MIMS) shown in Figure 3, the major ions contributing to the IMS peak a t KO= 1.65 cm2 V-' s-l a re m / z 141, 159, 187, and 215 corresponding to (H,O),(N,),(GB)H+ where n = 1 and m = 0, 1, or 2. The ions with m / z 280-281 contribute to the k , = 1.25 cm2 V-' s-l IMS peak corresponding to the protonated GB dimer ion (GB)2H+. These dimer ions clustered with nitrogen also account for the broad mass spectral peak having m / z 307-311. Since tributylamine (TBA) with molecular weight 186 is added to GB as a stabilizer (12),the particular ion with m / z 187 may be either (H20)(N2)(GB)H+ or (TBA)H+. The mass identified mobility spectra for TBA in Figure 4 shows a single ion mobility peak with K , = 1.41 cm2 V-' s-'. The ions with masses m / z 187, 215, and 243 contribute to the ion mobility peak. Thus the ion with mass m/z 187 has a reduced mobility
of KO= 1.41 cm2 V-l s-l for TBA whereas the ion with same mass for GB has a reduced mobility of KO= 1.65 cm2 V-ls-l. If one were to rely on mass spectral information only, one would be unable to determine the origin of the m / z 187 ions. One would need additional data from either GC/MS ( 1 4 ) , triple quadrupole mass spectrometry (15),or collisional ion stripping (10)to make an assignment to this controversial ion. Clear-cut evidence, however, is provided by IMS for the ion (Figure 5). The calculated average collision cross sections (no,) under the conditions of temperature and pressure used in this work are 166 A2 and 195 A2 for (H20),(N2),(GB)H+ and (TBA)H+,respectively.
Registry No. GB, 107-44-8;tributylamine, 102-82-9. LITERATURE CITED (1) Karasek, F. W. Anal. Chem. 1974, 4 5 , 710A. (2) Spangler, G. E.; Cohen, M. J. "Plasma Chromatography"; Carr, T. W.. Ed.; Plenum Press: New York, 1984; Chapter 1. (3) Karasek, F. W.; Kim, S.H. Anal. Chem. 1975, 4 7 , 1166. (4) Carr, T. W. J. Chromatogr. Sd.1977, 15, 85. (5) Hagen, D. F. Anal. Chem. 1979, 5 1 , 870. (6) Kim, S. H.;Betty, K. R.;Karasek, F. W. Anal. Chem. 1978, 50, 2006. (7) Good, A.; Kebarle, P. J. Chem. Phys. 1970, 5 0 , 212. (8) Mason, E. A. "Plasma Chromatography"; Carr, T. W., Ed.; Plenum Press: New York, 1984; Chapter 2. (9) Carroll, D. 1.; Dzidic. I.; Homing. E. C.; Stillwell, R. N. Appl. Spectros. Rev. 1981, 17. 337. (10) Kambara, H.; Mitsui, Y.; Kanomata, I . Anal. Chem. 1979, 5 1 , 1447. (11) Spangler, G. E.; Carrico, J. P. Int. J. Mass Spectrom. Ion Phys. 1983, 52, 287. (12) Franke, S. "Manual of Military Chemistry-Volume I , Chemistry of Chemical Warfare Agents"; Headquarters, Department of the Army: Washington, DC, April 1968, AD849-7866. (13) "Procedure for Chemical Analysis of Chemical Agents Tabun (GA) and Sarin (GB)"; Chemical System Laboratory, Aberdeen, MD, Report 136-350-1, 15 June 1964. (14) McFadden, W. "Technique of Combined GC/MS"; Wiley: New York, 1973. (15) Yost. R. A,; Enke, C. G. Anal. Chem. 1979, 5 1 , 1251A.
S. H. Kim G. E. Spangler* Allied/Bendix Aerospace Environmental Systems Division 1400 Taylor Avenue Baltimore, Maryland 21204 RECEIVED for review September 178 1984. Accepted November 19,1984. This effort was partially funded by the Chemical Research and Development Center, Aberdeen Proving Ground, MD. Official DOD position and use of brand name equipment do not constitute endorsement of same.
AIDS FOR ANALYTICAL CHEMISTS Sample Preparation Considerations for Measurement of Fluorescence Enhancement Using Cyclodextrins and Micelles Gabor Patonay, Mae E. Rollie, and Isiah M. Warner* Department of Chemistry, Emory University, Atlanta, Georgia 30322 In the past few years, there has been an increasing interest in the study of the fluorescence of many fluorophores in the presence of micelles and cyclodextrins. The consensus is that micelles and cyclodextrins enhance fluorescence by compartmentalizing and shielding the excited singlet species from quenching and nonradiative decay processes that occur in bulk
solution (1-6). However, in some studies, an equally plausible explanation is the ability of micelles and cyclodextrins to solubilize fluorophores that are otherwise sparingly soluble in the bulk aqueous phase. Differences in the degree of solubilization of the fluorescent species by the micellar solution relative to an aqueous solution of the same fluorophore may
0003-2700/85/0357-0569$01.50/0 1985 American Chemical Society
