Analytica Chimica Acta 532 (2005) 37–45
Separation efficiency of a chemical warfare agent simulant in an
atmospheric pressure ion mobility time-of-flight mass
spectrometer (IM(tof)MS)
Wes E. Steiner, William A. English, Herbert H. Hill Jr.∗
Department of Chemistry, Washington State University, Pullman, WA 99164-4630, USA
Received 16 June 2004; received in revised form 18 October 2004; accepted 18 October 2004
Available online 28 January 2005
Abstract
An electrospray ionization atmospheric pressure ion mobility orthogonal reflector time-of-flight mass spectrometer (IM(tof)MS) that
routinely achieves mobility and mass separation efficiencies in line with theoretical limits is reported. The maximum IM(tof)MS efficiency
for a given instrumental design depends widely upon the various key parameters such as voltage, temperature, initial pulse width, interface
and reflectron energies. Optimization of the current IM(tof)MS instrument, resulted in an IMS separation efficiency over 133,000 theoretical
plates (a resolving power of 155) and a resolving power of 1200 for the TOFMS using a singly charged G/V-type chemical warfare agent
(CWA) nerve simulant (dimethyl methylphosphonate (DMMP)) in less than 12 ms.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Ion mobility spectrometry; Time-of-flight mass spectrometry; Chemical warfare agent simulant; Resolving power
1. Introduction
Coupling of ion mobility spectrometry (IMS) with mass
spectrometry (MS) [1] to produce an ion mobility mass spectrometer (IMMS) for chemical analysis has proven to be
a powerful means for characterizing mixtures; via a twodimensional matrix of gas phase size-to-charge drift and
mass-to-charge flight times. Traditionally the first IMMS instruments employed quadrupole mass (QMS) spectrometers
[1–4]. This arrangement was found to be relatively slow
because of the need to scan m/z values sequentially in the
QMS filter. The ability of a time-of-flight mass spectrometry
(TOFMS) to acquire all ions without having to scan through
the m/z made the TOFMS analyzer a more rapid method for
an IMMS hybrid instrument. Young et al. first coupled a low
pressure (2–10 Torr) low temperature (∼298 K) IMS tube to
an orthogonal TOFMS analyzer in order to measure the formation and decomposition rates of hydrates of the hydronium
∗
Corresponding author. Tel.: +1 509 335 5648; fax: +1 509 335 8867.
E-mail address: [email protected] (H.H. Hill Jr.).
0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.aca.2004.10.073
ion [5]. More recently, this approach has been used by several
research groups in combination with electrospray ionization
(ESI) for the analytical separation and determination of various biochemical compounds [3,6–8]. Additionally, Clemmer
et al. have coupled both an ion trap and a collision-induced
dissociation (CID) cell to their low pressure IMS–TOFMS
instrument to improve sensitivity and analyze fragmentation
products [9,10].
For the most part, IMS–TOFMS instruments have used
low-pressure IMS drift tubes for the mobility separation step.
However, because ESI was suited for operation at elevated
pressures and higher IMS resolving powers may be obtained
under these same conditions, it was desirable to assess the
IMS–TOFMS hybrid with an ESI atmospheric pressure ion
mobility spectrometer (APIMS). While Guevremont et al.,
did use APIMS separation prior to TOFMS analysis, the interface between the APIMS and TOFMS was a capillary tube,
which created band broadening of the mobility separated ions
leading to a decrease in APIMS resolving power. Also, the
configuration of the TOFMS instrument was linear causing
the initial conditions (temporal, spatial, and kinetic energy
38
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
Fig. 1. Schematic diagram of the electrospray ionization high-resolution atmospheric pressure ion mobility orthogonal time-of-flight mass spectrometer
(IM(tof)MS).
distributions of ion packets) to be rather large; which gave
raise to low mass resolving powers and an overall drop in
sensitivity [6,11]. These issues were addressed by interfacing a novel high-resolution ESI–APIMS to an orthogonal
TOFMS through a series of static lenses [12]. This helped to
not only minimize any loss of APIMS resolving power due
to ion transport inefficiency, but had the added effect of diminishing initial ion distribution conditions in the TOFMS
by extracting ions in an orthogonal fashion. While initial results have demonstrated the potential of IM(tof)MS for the
detection of chemical warfare agents the effect of various
parameters on these agents were unknown.
In this study, investigations of the overall operating efficiency of the atmospheric pressure ion mobility orthogonal
reflector time-of-flight mass spectrometer (IM(tof)MS) that
was developed in our lab were conducted. Traditionally, the
G/V-type chemical warfare agent (CWA) nerve simulant standard DMMP has been employed to represent the detection of
a class of schedule 1, 2, or 3 toxic chemicals or their precursors as stated in the CWC verification and related analysis annex [13]. Parametric investigations using this simulant were
conducted for the following critical instrumental parameters:
(1) voltage, (2) temperature, (3) APIMS gate pulse widths,
(4) interface transport energies, and (5) TOFMS reflectron
energies.
2. Experimental
2.1. Chemicals and solvents
The CWA simulant (97% dimethyl methylphosphonate
(DMMP)) that was used in this study was obtained from
Sigma–Aldrich Chemical Company (St. Louis, MO) and
was used without further purification. A stock solution for
this CWA simulant was prepared in ESI solvent (47.5% water, 47.5% methanol, 5% acetic acid) at a concentration of
1000 ppm (1000 ␮g/mL). Further dilutions of this stock solution ranged from 1 to 500 ppm (1–500 ␮g/mL), depending upon the experiment. The HPLC grade solvents (water, methanol, acetic acid) were purchased from J.T. Baker
(Phillipsburgh, NJ).
2.2. Instrumentation
The IM(tof)MS instrument used in this study was constructed at Washington State University where the fundamental components (ESI source; APIMS drift tube; pressure interface; TOFMS analyzer; data acquisition system) and modes
of operation have been previously described in considerable
detail [12,14] and are shown in Fig. 1. Thus, only a brief
outline of a typical experimental sequence is provided. A
continuous flow (3.0 ␮L/min) of solvent was electrosprayed
in the positive ion mode with a needle voltage of +5.0 kV
with respect to the target screen of the APIMS. The APIMS
was divided into two regions – the desolvation (7.5 cm in
length) and the drift (18.0 cm in length) regions – that were
separated by a Bradbury–Nielsen style ion gate [15]. Fig. 2
shows the relative ion cut off potential voltage (∼35.0 V) for
this particular gate. Desolvated ions typically drifted through
the 473 K APIMS tube under a weak uniform electric field
(408 V/cm), which facilitated separation based upon differing analyte ion mobility constants. A counter current flow of
preheated nitrogen drift gas was introduced at the end of the
drift region at a rate of 1.0 L/min. Ions exiting the APIMS
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
drift tube (690 Torr) traversed a pressure interface (1.5 Torr)
where parent and daughter ions could be transported through
a series of lenses into the TOFMS (4.0 × 10−6 Torr) for analysis [12].
Data acquisition for this instrumental setup consisted of
a timing sequence that was comprised of a real-time twodimensional matrix of simultaneous mobility drift and mass
flight times. In the ion mobility portion of the spectrometer,
ions were typically “gated” for 0.2 ms into the drift region at
a frequency of 50 Hz. This allowed for a maximum of 20 ms
for the APIMS mobility data to be acquired. The TOFMS
extraction frequency was set to 50 KHz, which provided a
mass spectrum that consisted of ions with flight times up to
20 ms. Therefore, within each 20 ms mobility time window
there were effectively 1000 TOF extractions. The APIMS
ion gate, TOFMS extractor, and TOFMS time-to-digital converter were all triggered by a personal computer (PC) based
timing controller. Synchronization of this electronic hardware was facilitated by the use of a dual Pentium III workstation running Ionwerks® two-dimensional acquisition software [16]. Experimental data acquisitions were acquired in
triplicate for a typical run time of 1 min. These 1 min spectral
compilations of data were exported into both 2D transform
[17] and 3D NoeSYS [18] software for processing.
In the case of APIMS, a practical measure, and often the
most useful definition of resolving power [19] RAPIMS , is
given by:
RAPIMS =
td
tFWHM
drift cell space, Ld , the voltage drop across this drift space,
Vd , and the mobility constant of the ion K is shown by:
td =
Ld 2
KVd
(1)
where td is the ion drift time and tFWHM the peak width
measured at half of the maximal intensity. The drift time of
an ion in the APIMS drift tube is related to the length of the
(2)
To correct for varying environmental and experimental conditions [2], it is often more practical to report ion drift times
in terms of reduced mobility constants (K0 ) which is defined
by:
2 273.15
Ld
P
K0 =
(3)
V d td
T
760
where T is the effective temperature in the drift region, and
P the pressure. The measure of separation efficiency is similar to that normally used in chromatography given by the
relation:
2
td
N = 5.55
(4)
tFWHM
where N is the theoretical number of plates. Thus, the measure
of separation efficiency of an APIMS can be directly related
to the traditional chromatographic term by combining Eqs.
(1) and (4) to give:
N = 5.55(RAPIMS )2
2.3. APIMS–TOFMS calculations
39
(5)
The major factors determining APIMS resolving power are
the initial widths of the ion packets admitted into the drift
region and the diffusional broadening of that ion packet as
it travels toward the TOFMS interface region. To this end,
Revercomb and Mason [20] developed an expression for measured peak width where only the initial pulse width and the
broadening due to normal diffusion are given by:
16kT ln 2
tFWHM 2 = tg 2 +
(6)
td 2
Vd ez
Fig. 2. The relative APIMS ion cut off potential voltage (∼5.0 V) for this particular Bradbury–Nielsen gate.
40
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
where tg is the initial ion pulse width, k is Boltzman’s constant, e the elementary charge, and z the number of charges
on the ion. By simply combing Eqs. (1), (2) and (6) to
give:
RAPIMS = (Ld 2 /KVd )
2
(7)
tg 2 + (16kT ln 2/Vd ez)(Ld 2 /KVd )
or Eqs. (1), (2), (4) and (6) to yield:
2
N=
(Ld 2 /KVd )
2
[tg 2 + (16kT ln2/Vd ez)(Ld 2 /KVd ) ]
5.55
(8)
To optimize the resolving power (Eq. (7)) or the number of
theoretical plates (Eq. (8)) in APIMS voltage, drift length,
temperature, and gate width are easy instrumental parameters
to adjust. For APIMS (with a gate width of 0.2 ms, a drift time
of 17 ms for a + 1 charged ion, traversing a voltage drop of
7500 V at 473.15 K), the predicted resolving power would be
around 100, while the number of theoretical plates would be
approximately 56,000.
In the case of TOFMS, where ions are accelerated to a
constant energy, the mass resolving power [11], RTOFMS, is
defined by:
RTOFMS =
τf
2τFWHM
(9)
where, πf denotes the flight time of an ion species in the
TOFMS and τ FWHM denotes the width of the distribution
of the flight times at half maximum. The flight time of an ion
in the simplest sense of terms refers to the time required for
ions to traverse the length, Lf , of the TOFMS flight chamber.
The time it takes from the ion extraction chamber through
the reflectron back to the micro-channel detection plates as
shown by:
1/2
m
τf =
(10)
Lf
2Vf ez
Fig. 3. Voltage effect on the number of theoretical plates (a) and resolving power (b) for DMMP at differing APIMS gate pulse widths (z = 1, L = 18.0 cm,
T = 473 K, P = 690 Torr, K = 1.33). Experimentaly obtained values for gate pulse widths of 0.1, 0.15 and 0.2 ms are shown as boxes, diamonds, and triangles,
respectively. These points had uncertainties of less than 10% (error bars not shown) and were on average within 5% of theory.
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
here, m is the ion mass and Vf the acceleration voltage. For a
given TOFMS instrumental design of a fixed length, there are
a couple of ways to increase the resolving power efficiency:
one is by focusing the ion packet along its flight axis by
voltage adjustments of the primary entrance beam and the
other is by increasing the distance between two differing ion
mass packets at the detector by increased extraction voltages
[21].
3. Results and discussion
3.1. The effect of drift voltage on APIMS efficiency
According to Eq. (7) and (8), the APIMS resolving power
and separation efficiency should increase as a function of the
41
square root of the applied voltage over the ion drift region.
The effects of voltage and gate pulse widths on the number
of theoretical plates and resolving power for APIMS can be
seen in Fig. 3 a and b, respectively. Experimentaly obtained
values for DMMP at varying gate pulse widths of 0.10, 0.15,
and 0.20 ms are shown as boxes, diamonds, and triangles,
respectively. These 18 points had uncertainties of less than
10% and were on average within 5% of theory. In the narrow range where the theory was tested (6000–9000 V due to
instrumental limitations) the measured number of theoretical plates and resolving power agreed resonable well with
the theoretical vaules predicted by theory. For the narowest
pulse width tested, 0.10 ms, the number of theoretical plates
and resolving power increased with voltage, but for the other
two pulse widths tested (0.15 and 0.2 ms) the number of theoretical plates and resolving power passed through a local
Fig. 4. Temperature distrubutions of the number of theoretical plates (a) and resolving power and (b) for DMMP at differing APIMS gate pulse widths (z = 1,
L = 18.0 cm, V = 8500, P = 690 Torr, K = 1.33). Experimentaly obtained values for gate pulse widths of 0.1, 0.15 and 0.2 ms are shown as boxes, diamonds, and
triangles, respectively. These points had uncertainties of less than 10% (error bars not shown) and were on average within 5% of theory.
42
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
maximum as the voltage was increased. According to Eqs.
(7) and (8), the local maximums occured at the point at which
the contribution to band broadening due to difussion and that
of the original gate pulse became equal. At voltages higher
than the local maximum, the gate pulse width became the
dominent contribution to the overall ion band width. Thus,
for example, compounds with drift times less than protonated
DMMP would reach a local maximum number of theoretical
plates and resolving power at a lower voltage than compounds
with drift times that were larger.
3.2. The effect of temperature on APIMS efficiency
Temperature is another APIMS parameter, which was critical to the optimization of the maximum number of theoretical
plates and resolving power for separation efficiency. The diffusion limited effect of temperature on the number of theoretical plates and resolving power for DMMP are shown in Fig. 4.
Again experimentaly differing APIMS gate pulse widths of
0.10, 0.15, and 0.20 ms for (a) number of theoretical plates
and (b) resolving power are shown as boxes, diamonds, and
triangles, respectively. These 15 points also had uncertainties
of less than 10% and were on average within 5% of theory.
According to theory, the number of theoretical plates and resolving power increased as temperature decreased. This was
because as the temperature decreased the ion mobility also
decreased and the time of drift of an ion increased. Thus as the
temperature decreased there was an increase in the number
of theoretical plates and resolving power due to reduced ion
diffusion. Thus, APIMS should achieve better efficiencies as
the temperature was decreased. In theory this would make
decreasing the temperature of an ion mobility spectrometer
an attractive way to increase the number of theoretical plates
and resolving power. According to theory, if the operational
temperature of the APIMS were around 77.0 K (liquid nitrogen temperature) it would be possible to achieve separation
Fig. 5. Effect of varying IM(tof)MS interface voltage conditions on arbritrary signal intensity for protonated DMMP (drift gas = N2 , P = 1.5 Torr, T = 296 K). (a)
The signal intensity of DMMP as a function of four different nozzle voltages (Vn ) arcross the focus voltage spectrum. (b) Represents the optimal nozzle-to-focus
ratios for a given nozzle voltage.
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
efficiencies as high as 500,000 theoretical plates with a resolving power of approximately 300. In practice, however,
the number of theoretical plates and resolving power usually decreased as temperature decreased due to increased ion
clustering with water [2] and other polar containments in the
drift gas. For the limited temperature range investigated in
this study (375–575 K) the number of theoretical plates and
resolving power did increase with decreasing temperature as
predicted by theory.
3.3. The effect of voltage on IM(tof)MS interface
efficiency
Both the nozzle and focus voltages effected ion transmission. The APIMS was interfaced to the TOFMS through
an interface where ions exiting the APIMS drift region at
a voltage of roughly 160 V entered the mass spectrometer
through a 300 ␮m pinhole nozzle (+150 V) that served as the
first stage in a pressure drop from 690–705 Torr to roughly
1.5 Torr. Immediately proceeding this nozzle a focusing lens
(+147 V) served to control the collisional energy of the ions
with residual gas particles as they traversed interface to the
second 300 ␮m skimmer cone (+92.0 V) lens. Fig. 5 shows
the effect of varying IM(tof)MS interface voltage conditions
on arbritrary signal intensity for protonated DMMP. Where
(a) shows the signal intensity of DMMP as a function of four
different nozzle voltages (Vn ) arcross the focus voltage spectrum and (b) represents the optimal nozzle-to-focus ratios for
a given nozzle voltage. These data show that a nozzle voltage from 150 to 160 V with a focus voltage about 5 V lower
gave the highest level of ion transmission. Moreover, even
with nozzle voltages as low as 130 V it was still clear that a
nozzle-to-focus voltage ratio close to 1.0 gave the best signal
intensity.
43
3.4. The effect of reflectron voltage on TOFMS efficiency
Adjustment of both the reflector backplane and grid voltage with respect to each other it was possible to obtain an
optimal setting for maximal ion resolving power. The primary beam of ions, upon exiting the interface that adjoins
the APIMS to the TOFMS, was focused via a series of lenses
(labeled L1–L3, DU, DD in Fig. 1) into the TOFMS extraction chamber. This served to help to minimize initial starting
conditions. Segments of the primary beam of ions were orthogonally extracted to an energy of −2000 V by a bipolar
extractor into the reflection region; were they were refocused
on their way to the detector. The overall focusing effects of
the reflection region can vary in magnitude by the adjustment of the voltage placed on the reflector grid with respect
to the backplane. Fig. 6 shows the TOFMS resolving power
for protonated DMMP as a function of five different reflector backplane voltages (Vrb ) arcross a moderate reflector grid
voltage spectrum. Mass spectral resolving powers of around
∼600 were typically obtained for most voltage ratios explored. Once the reflectron was tuned to match the extraction
energies of the ion packets, however, a marked improvement
in resolving power (∼1200) was realized. The optimal setting for the reflector backplane and grid voltage was found to
be 656 and −245 V, respectively. Moreover, given the overal
physical flight length (∼0 cm) of the TOFMS, the resolving
power achieved is in line with current theoretical calculations
[11,15].
3.5. High-resolution IM(TOFMS)
An early paper describing high-resolution ion mobility
spectrometry [2] showed the highest number of theoretical
plates and resolving power achieved for a singly charged
Fig. 6. TOFMS resolving power for protonated DMMP (P = 4.0 × 10−6 Torr, T = 296 K) as a function of five different reflector backplane (Vrb ) voltages arcross
the reflector grid voltage spectrum.
44
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
Fig. 7. (a) The APIMS mobility spectrum (Ld = 18.0 cm, Vd = 13.0 kV or 722 V/cm, T = 423 K, tg = 0.05 ms) of protonated DMMP demonstrating a resolving
power of 155 and (b) the corresponding TOFMS mass distribution resolving power of 1200.
ion was about 50,000 and 90, respectively. Modifications to
that design, APIMS drift tube length of 22.5 cm operating at
12.5 kV, Asbury and Hill were able to achieve around 130,000
theoretical plates with a resolving power of 150 for singly
charged arginine ions (174 m/z) [22]. Here the relationship
of IMS resolving power with drift gas temperature and drift
voltage was explored. In these studies, a more compact version of the APIMS was able to maintain remarkable efficiency
while still obtaining a high level of TOFMS resolving power.
These data can be seen in Fig. 7, where (a) shows the APIMS
mobility spectrum and (b) the corresponding TOFMS mass
distribution. An observed experimental number of theoretical plates around 133,000 with a resolving power of approximately 155 for the protonated DMMP ions (125 m/z) were
found for APIMS; a resolving power of approximately 1200
was found for TOFMS. When compared to the APIMS theoretical maximum (N = 140,000, RAPIMS = 160) calculated
from Eqs. (7)–(9) for these experimental conditions it was observed that the experimental values were within 5% of those
predicted by theory. This excellent agreement with theory indicates that the IMS is operating under optimal conditions.
There is still room for further improvements in IMS resolving
power by adjusting parametric conditions. In fact, as Asbury
et al. have pointed out, with a commonly available 30 kV
power supply a calculated number of theoretical plates over
500,000 should be possible [22].
4. Conclusion
Parametric investigation of both drift voltage and temperature showed that for a given APIMS (e.g. IMS technology in
general) gate pulse width there is an experimentally achievable maximum separation efficiency. Although diffusion lim-
W.E. Steiner et al. / Analytica Chimica Acta 532 (2005) 37–45
ited calculations predict that resolving power should increase
as a function of the square root of the voltage, in practice
the initial ion pulse width limits the maximum optimal voltage. The experimental APIMS separation efficiencies were
found to be within 5% of that predicted by theory; which
indicated that this instrumental design does not suffer significantly from heterogeneity of the applied electric field or
columbic repulsions. The resolving powers reported in this
work are the maximum obtained to date for APIMS, corresponding to 133,000 theoretical plates for a singly charged
ion. This separation efficiency greatly exceeds that normally
seen in HPLC and is on par with that of high-resolution GC.
Additionally, resolving powers of around 1200 for
TOFMS were routinely achievable regardless of what APIMS
or interface operating voltages were used as long as the
nozzle-to-focus voltage ratio was maintain at unity. Thus, the
APIMS and interface can be fine tuned for a given separation
and transport environment, respectively, without sacrificing
TOFMS efficiency.
Acknowledgement
This work was supported in part the Geo-Centers Incorporated (Grant No. 40853CMGC3173).
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