Anal. Chem. 1993, 65,3010-3014
3010
Theory and Technique for Measuring Mobility Using Ion Mobility Spectrometry Glenn E. Spangler Environmental Technologies Group, Inc., 1400 Taylor Avenue, Baltimore, Maryland 21284-9840
Theory and techniques to measure ion mobilities using ion mobility spectrometry are presented. With two IMS cells of very different designs, reduced mobility data for the protonated water reactant ion were collected at 50-55 "C. Sources of error for the final values are reported.
for the ion, and the mobility K is given by
K = VdIE (2) E is the constant electric field applied across the drift tube. To remove variations due to changing pressure and temperature, reduced mobility K O (3)
INTRODUCTION Ion mobility spectrometry (IMS)is an atmospheric pressure ionization technique in which organic sample molecules are submitted to ionizing reactions to produce product ions. The ionization is accomplished in a reactor by using either a radioactive 'j3Ni,photoionization, or alkali cation thermionic emitting source.14 The radioactive and alkali cation sources generate reactant ions which react with the sample. The photoionization source emits vacuum ultraviolet photons which photoeject valence electrons from the sample. The chemistry of the reactions can be altered by adding reagent gases to the carrier gas. After the product ions are formed, they are extracted from the reactor by an electric field and introduced into a linear drift tube.5 A shutter grid is opened approximately 100-700 ps to inject a narrow band of the ions into the drift tube. Under the influence of an electric field applied across the drift tube, this band of ions travels toward an ion collector and breaks up as each ion species of the original ion mixture arrives at the collector with different drift times. As they arrive at the collector, the ions generate an ion current which is amplified by an electrometer circuit for data processing. For a more detailed description of IMS, the reader is referred to several review articles and books.Sl0 A parameter of interest to IMS is mobility. Experimentally, it is derived from the time, td, it takes the ions to travel the length of the drift tube. It is obtained from the peak maxima of the ion mobility peaks in the ion mobility spectrum measured relative to the time when the ions were introduced into the drift tube. Theoretically, the drift time is given by td = ldlvd (1) where l d is the length of the drift tube, vd is the drift velocity (1)Cohen, M. J.; Karasek, F. W. J.Chromatogr. Sci. 1970,8,330-337. (2) Leasure, C. S.; Fleischer, M. E.; Anderson, G. K.; Eiceman, G. A. Anal. Chem. 1986,58,2142-2147. (3) Baim, M. A.; Eatherton, R. L.; Hill, H. H., Jr. Anal. Chem. 1983, 55,1761-1766. (4) Roehl, J.;Spangler, G. E.; Donovan, W. H.; Nowak, D. Proceedings 1992 Workshop on Ion Mobility Spectrometry; Mescalero, NM, June 1992;Eiceman, G. A., Ed.; Final Report on Contract DAAL03-91-C-0034; U.S.Army Chemical Research, Development and Engineering Center: Aberdeen Proving Ground, MD 21010, January 27, 1993. (5) Mason, E. A.; McDaniel, E. W. Transport Properties of Ions in Gases; Wiley: New York, 1988. (6) Hill, H. H., Jr.; Siems, W. F.;St. Louis, R. H.;McMinn, D. G. Anal. Chem. 1990,62, 1201A-1209A. (7) St. Louis, R. H.; Hill, H. H., Jr. Crit. Rev. Anal. Chem. 1990, 21, 321-355. (8) Eiceman, G. A. Crit. Rev. Anal. Chem. 1991, 22 (1&2), 17-36. (9) Roehl, J. E. Appl. Spectrosc. Rev. 1991, 26 (1&2), 1-57. (10) Carr, T. W. Plasma Chromatography; Plenum: New York, 1984.
0003-2700/93/0365-30 10$04.00/0
is also defined.5 Because of the fundamental nature of reduced mobility to IMS theory, it is used to compare data generated in different laboratories. One compilation of reduced mobilities has already been assembled for this purpose.ll On the other hand, there is nothing in the literature to assist a researcher desiring to make careful mobility measurements. If the IMS is bought from a reputable manufacturer, a relationship between the measured drift time and mobility may be provided. However, the details supporting this relationship will most likely be lacking. Presently, IMS technology is rapidly expanding and hardware can be purchased from a variety of vendors. More often than not, aminiaturized IMS is used which was originally developed and designed as a detector in an environmental sensor. As such, no information is available to support a researcher desiring to measure mobilities. Furthermore, it is easy for anyone to build an IMS for use in his own laboratory. To reliably measure mobility using these instruments, it is necessary to deduce mobility values from measured drift times and instrument parameters. A systematic approach to measuring mobilities is preferred over making qualitative comparisons with previously published mobility values in the open literature. This paper concerns the reporting of reduced mobilities from drift time data. Drift time data were collected on the protonated water reactant ion12 by using two IMS cells of very different constructions. Theory and measurement techniques were developed to deduce reduced mobility values from both sets of data. Sources of error for the measurement are evaluated.
EXPERIMENT The first IMS cell was the IMS cell of an ion mobility spectrometer/mass spectrometer (IMS/MS) previouslydescribed in the 1iterat~re.l~ The shutter grid of this cell was two coplanar grids of a Bradbury-Nielsen design,14which was electronically pulsed by momentarilyequalizingthe normally unequal potential applied between the two halves of the grid. The drift region was a stacked ring drift tube with the drift length between the shutter grid and aperture grid being 10.92 cm and between the aperture grid and collector being 0.09 cm. All data were collected using an IMS cell temperature of 50 OC (as measured with two (11)Shumate, C.; St. Louis, R. H.; Hill, H. H., Jr. J.Chromatogr. 1986, 373, 141-173. (12) Kim, S. H.; Betty, K. R.; Karasek, F. W. Anal. Chem. 1978, 50,
2006-2012. (13) Spangler, G. E.; Carrico, J. P. Int. J. Mass Spectrom. Ion Phys. 1983, 52, 267-287. (14) Bradbury, N. E.; Nielsen, R. A. Phys. Reu. 1936, 49, 388-393. 0 1993 American Chemical Society
CHEMISTRY,VOL. 65, NO. 21, NOVEMBER 1, 1993 3011
ANALYTICAL
thermocouples located between the housing and the electrode structure for the IMS cell) and an IMS cell pressure of atmospheric. The second IMS cell was a parametric cell assembled to study the effecta of geometric design on the performance of IMS. This cell was a series of ceramic rings stacked on two glass insulated rods inside a rectangular metal box. The ionization source was contained in the first ceramic ring, a reactor ring positioned the ionization source 1 cm from the shutter grid, a parallel plane shutter grid interrupted ion flow from the reactor to the drift tube, a ceramic tube coated internally with thick film resistor ink served as the drift tube,lS and an aperture grid/collector assembly collected the ions. The parallel plane shutter grid consisted of two Buckbee MearsIs photoetched grids separated by 0.14 cm along the axis of the IMS cell. The shutter grid was activated by forward biasing the normally reversed electrostatic field between the two grids. The drift length was 8.06 cm from the last grid of the shutter grid to the aperture grid and 0.71cm from the aperture grid to the collector. The reactor and drift voltages were independentlyappliedto the reactor and drift tube using two Bertan high-voltage power supplies. All data were collected using an IMS cell temperature of 55 OC (as measured witha thermocouplelocated between the housing and the ceramic electrode structure for the IMS cell) and an IMS pressure of atmospheric. Both IMS cella contained a 10-mCi"Ni radioactive source for ionization and air purified to approximately 3 ppm water for the carrier and drift gases. The purified air was laboratory air processed by molecular sieve towers in a pressure/purgecycle of an AADCO 737-R pure air generator." The output of this generator was passed through a thermal converter to oxidize hydrocarbons and through an activated 13X molecular sieve trap to remove COz. The water content of the purified air was 2-4 ppm as measured with a DuPont Instrumenta (now Ametek)'s 303 moisture monitor. The IMS data are collected with the aid of a Tracor TN-1500 signal averager.le The signal averager was triggered using the positive slope (or trailing edge) of the negative gate pulse applied to the shutter grid. The drift time of ion mobility peaks was determined by recording the channel containing the maximum number of counta in the spectrum.
THEORY Figure 1shows schematically the drift regions for the two IMS cells. When an ion is introduced into the drift region, it must travel a distance d d = d,
+ d,,
+ d, + d,,
(5) for the parametric cell. d, is the separation between the parallel plane shutter grid, d, is the drift length from the shutter grid to the aperture grid, and d, is the separation between the aperture grid and collector. Corresponding to these distances are the times (i.e., t,, t,, and t,, respectively) the ions spend between each electrode structure. The total drift time is
td = t,
0
0
0 COLLECTOR
0 0
0 0
0 0
0
0
0
0
0
L-)
ess
h a
(6)
+ t, + t ,
(7)
for the parametric cell. The actual measured drift time, td", (15) Browning, D. R.;Sima,G. R.; Schmidt,J. C.;Sickenberger,D. W. U.S.Patent 4,390,784,June 28, 1983.
(16) Buckbee-Meara Div. of BMC Ind., 245 E. 6 St., St. Paul, MN 55101. (17)-0, Box 1791, Rockville, MD 20854. (18) AmetekInc.,Proceee& AnalyticalInstnunentDiv., 455 Corporate Blvd.. Newark. DE 19702. (19) Tracor 'Northern Scientific Inc., 2651 West Beltline Highway, Middleton, WI 53562.
dac
PARAMETRIC CELL
SHUTTER GRID
APERTURE GRID
0
0
X
0
COLLECTOR
X
0 X
0
0
X
0
0 X
0
0
dsa IMS/MS CELL
dac
Flgurr 1. Schematic representation for the drift reglons of the parametric (top) and IMSIMS (bottom) lon mobility spectrometers. The distances used in this paper are labeled. differs from these times by a fraction, CY-', of the gate width, t, (i.e., &tB/a,where the value of a depends on the construction of the shutter grid and the method of triggering the signal averager). For the IMS/MS cell, half of the gate width applied to the shutter must be either added or subtracted from the measured drift time. For the parametric cell, a must be corrected for the portion of the ion cloud lost between the two half-grids of the shutter grid when the grid is reverse biased. a is determined by collecting drift time, tda-, data versus gate width, t,, and applying a-1
f-
I
at,
From eqs 1and 2 and applying the definition for electric field as a potential gradient over distance, eqs 6 and 7 can be written as -12
- 1 2
for the IMS/MS cell and d,'
e-+-+-
td
+ t,
td = t , for the IMS/MS cell and
GRID
0
(4)
for the IMS/MS cell and d = d,
APERTURE
SHUTTER GRID
KV,
dm2
KV,
d,:
KVac
(10)
for the parametric cell. The 6 term is included in eq 9 to acknowledge possible distortions in the drift field due to the voltage V, applied to a closed Bradbury-Nielsen grid. Since 6Vg
