Article pubs.acs.org/JPCA
Dissociation of Proton Bound Ketone Dimers in Asymmetric Electric Fields with Differential Mobility Spectrometry and in Uniform Electric Fields with Linear Ion Mobility Spectrometry Xinxia An,† Gary A. Eiceman,*,† Riikka-Marjaana Ras̈ an̈ en,†,§ Jaime E. Rodriguez,† and John A. Stone‡ †
Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, United States Department of Chemistry, Queens University, Kingston, Ontario Canada
‡
ABSTRACT: The kinetics for the decomposition of the symmetrical protonbound dimers of a series of 2-ketones (M) from acetone to 2-nonanone have been determined at ambient pressure by linear ion mobility spectrometry (IMS) and by differential mobility spectrometry (DMS). Decomposition, M2H+ →MH+ + M, in the IMS instrument, observed under thermal conditions over the temperature range 147 to 172 °C, yielded almost identical Arrhenius parameters Ea = 122 kJ mol−1 and ln A = 38.8 for the dimers of 2-pentanone, 2-heptanone, and 2-nonanone. Ion decomposition in the DMS instrument was due to a combination of thermal and electric field energies at an effective ion internal temperature whose value was estimated by reference to the IMS kinetic parameters. Decomposition was observed with radio frequency (RF) fields with maximum intensities in the range 10 kV cm−1 to 30 kV cm−1 and gas temperatures from 30 to 110 °C, which yielded effective temperatures that were higher than the gas temperature by 260° at 30 °C and 100° at 110 °C. There was a mass dependence of the field for the onset of decomposition: the higher the ion mass, the higher the required field at a given gas temperature, which is ascribed to the associated increasing heat capacity with the increasing carbon number, but similar, internal vibrations and rotations.
■
surrounding gas. The effective kinetic temperature Tkeff of a monatomic ion moving under the influence of a uniform electric field E through a monatomic gas is given by eq 1.1
INTRODUCTION Ion excitation followed by a monitored dissociation reaction is an important method in the investigation of the properties of gas phase ions. The internal energy of an ion may be readily increased in near-vacuum by several methods, including collision with one or more molecules or with a surface after prior acceleration, by single and multiphoton absorption, and by blackbody radiation. The energy content of the hyperenergetic ion prior to dissociation may be associated with an effective temperature Teff, a value that is problematical both to define and to determine for polyatomic systems.1−10 For example, values of Teff are derived and used, but not without some controversy, in determining the entropy factor in the kinetic method for proton and other ion affinity determinations.11−15 The estimation of Teff for collisionally activated decomposing ions has sometimes been made by assuming that only a few collisions are required for dissociation to proceed statistically from a “Boltzmann-like” distribution of internal energies.16−18 However, the only accurate measure of Teff that can be made for collisionally activated polyatomic ions is the trivial one when Teff is equal to T, the ambient temperature of a bath gas in which a Boltzmann energy distribution holds at all times. The concept and theory of an effective temperature is most fully developed for the drifting of ions through gases under the influence of electric fields. Ions at very low concentration in a drift tube have a temperature that is higher than that of the © 2013 American Chemical Society
3 3 1 k kBTeff = kBT + Mνd2(1 + β) 2 2 2
(1)
T is the drift gas temperature, M is the mass of the drift gas molecule, νd is the ion drift velocity, kB is the Boltzmann constant, and β is a small correction factor. This equation successfully establishes Tkeff for use in eq 2 that describes the ion mobility coefficient K = νd/E, which is measured by ion mobility spectrometry (IMS) with small electric fields E, usually in linear drift tubes operating at atmospheric pressure19 or at much lower pressure.20 K is a function of a temperaturedependent cross section Ω1.1(Tkeff), the reduced mass of ion and neutral μ, the charge on the ion q, and the neutral number density N. The small correction factor α, and also β of eq 1, may be calculated from ion-neutral interaction potentials. 3q ⎛ π ⎞ ν ⎜⎜ ⎟ K= d = k ⎟ E 8N ⎝ 2μkBTeff ⎠
1/2
(1 + α) k Ω1.1(Teff )
(2)
Received: February 15, 2013 Revised: May 30, 2013 Published: June 17, 2013 6389
dx.doi.org/10.1021/jp401640t | J. Phys. Chem. A 2013, 117, 6389−6401
The Journal of Physical Chemistry A
Article
An ion that gains sufficient field energy may decompose, and the rate of decomposition will be a function of its internal temperature. A comparison of the rate constants for a reaction in a differential mobility spectrometer with those obtained for the same reaction under strictly thermal conditions should i provide a method for obtaining Teff .The combination of Arrhenius parameters obtained for the reaction of ions in a bath gas at a well-defined temperature with dissociation data obtained under non-Boltzmann conditions is a method that has been utilized to determine Tieff. This method, sometimes called the thermal extrapolation method, has yielded effective temperatures and related internal energies of ions activated by ion−molecule reactions, gas phase ion−molecule collisions, and desorption techniques.4,9,30,31 The method has also been used to estimate the effective temperature for ion isomerization by comparing the extent of unfolding of gas-phase multiprotonated ubiquitin ions heated in a bath gas with that caused by excitation in a high asymmetric electric field differential mobility spectrometer.32 Comparison of the effects of FAIMS and thermal heating (IMS) suggested that passage through the high field produced an internal ion Tieff that was ca. 50 °C above the ambient temperature. Kinetic parameters, activation energy, and pre-exponential factor for the decomposition of weakly bound ions may be obtained under thermal conditions by observing their decomposition at low fields in an ion mobility spectrometer, as illustrated for the dissociation of the proton bound dimers of methyl methylphosphonate (DMMP) and dimethylpyridine.33 These parameters were then used to estimate Tieff for the decomposition of (DMMP)2H+ occurring in a planar differential mobility spectrometer under nonthermal conditions as a function of temperature and high electric field.34 Tieff was found to increase by ∼0.01 °C cm V−1, in agreement with the value estimated for the decomposition of protonated diaryl molecules in a similar differential mobility spectrometer.35 Protonated alkyl esters of carboxylic acids mainly decompose in the high field of a differential mobility spectrometer by loss of alkene to give the protonated carboxylic acid.21 A qualitative observation from the study was the mass dependence of the threshold electric field required for decomposition, the higher the ion mass the higher the required amplitude of the radio frequency (RF) field at the same gas temperature. As expected, all thresholds decreased with increasing gas temperature as the energy required from the field to attain the same Tieff decreased. In this paper, we report on a study similar to that for the decomposition of (DMMP)2H+, involving a combination of the Arrhenius parameters obtained from ion mobility experiments at low field with those obtained from differential mobility experiments at high electric field. The objective was to confirm a mass dependence on the high field threshold for ion decomposition and to obtain estimates for Tieff as a function of ambient temperature and electric field amplitude. The working temperature range for the ion mobility spectrometer was ambient to ∼450 K, and within this range, the decomposition of ions with dissociation energies up to ca.130 kJ/mol can be observed over their approximately 10−2 s drift times. The rate constants for observation of the first order decompositions are therefore on the order of 102 s−1. We describe experiments with a homologous series of 2ketones, from acetone to 2-nonanone. Ketones form symmetrical proton bound dimers in which the proton bridges two oxygen atoms. Such symmetrical dimers have almost identical bond strengths for dissociation of 130 ± 6 kJ mol−1,36 and their
This two-temperature theory of mobility does not hold when the ion and the neutral molecules are not monatomic.1,6,7 In collisions involving polyatomic ions and neutrals, inelastic collisions result in energy transfer to the internal modes of the participants, and the ion drift velocity increases with E at less than a linear rate. The neutral molecules, present in extremely high concentration compared with the ions, eventually through intermolecular collisions transfer excess energy to the walls so that the gas temperature T remains constant, described by a Boltzmann distribution of both internal (vibrational and rotational) and external (translational) energies. A constant ion velocity after a short time in a uniform electric field must lead to a situation where the average internal energy gained in a collision is the same as the average energy lost. The distributions of the energies of the internal and external motions of the ion are then accurately described by the same effective temperature. The first term on the right-hand side of eq 1 is the thermal energy of the ion at temperature T, and the second term is the energy gained from the field. For ions in low fields with atmospheric gas pressure, that is fields less than ca. 1000 V cm−1, and neglecting the small correction factor β, the energy gained from the field is far less than the thermal energy. The kinetic temperature of the ion Tkeff is essentially the temperature of the gas. For example, an ion of mass 100 Da with a typical measured mobility K = 2.0 cm2 V−1 s−1 in an electric field of 1000 V cm−1 has an average field energy of 3.3 × 10−25 J and a thermal energy of 6.2 × 10−21 J. Tkeff exceeds T by only 0.016 °C. The technique known as differential mobility spectrometry (DMS) or field asymmetric ion mobility spectrometry (FAIMS) operates with asymmetric electric fields with maximum amplitudes up to 75 000 V cm−1 and even higher.21,22 Ions oscillate in the high and low fields at frequencies of 0.3 to 25 MHz.23−29 In these high fields, the second term in eq 1 becomes significant, and the analysis becomes complex even for atomic systems and becomes even more complex when polyatomic species are involved.10 Krylov and co-workers modified eq 1 with a factor ζ (≤1) to give an expression for Tkeff, eq 3, which acknowledges that not all the field energy is converted to the energy of ion motion through a bath gas.5 From an examination of temperature effects on the differential mobility of a variety of ions, ζ was found to be different for different ions and also to vary with transport gas temperature. 3 3 1 k kBTeff = kBT + ζMνd2 2 2 2
(3)
There is no theoretical basis for this simple modification, but experimental data for the mobilities of a variety of ions at high field obtained with a differential mobility spectrometer could be modeled for different gas temperatures. The trend for all ions was that ζ decreased with increasing gas temperature. The field energy not converted to kinetic energy of motion is distributed between the internal energy of the ion and the translational and internal energies of the surrounding gas molecules. By analogy with eqs 1 and 3, the internal effective temperature Tieff of an ion heated both thermally by a bath gas and by field energy, may be written as the sum of two temperatures, that of the gas T, and the increase due to the field, ΔTfield: i Teff = T + ΔTfield
(4) 6390
dx.doi.org/10.1021/jp401640t | J. Phys. Chem. A 2013, 117, 6389−6401
The Journal of Physical Chemistry A
Article
be synonymous with the maximum voltage of the waveform. A suitable small applied DC voltage, the compensation voltage (CV), can counteract the SV-induced drift and allow ions to pass to the detector plate. The required CV is, in general, different for different ions and a scan of CV at fixed SV generates a DMS spectrum of ion intensity as a function of CV. The experimental variables pertinent to the present work are the CV and SV for an ion and the flow rate and temperature of the transport gas. A vapor generator supplied suitable concentrations of ketones to the ion sources for both IMS and DMS experiments. Ion identities were determined by IMS/MS/MS and DMS/ MS/MS as has been previously described.34,37 Chemicals and Reagents. Seven homologous ketones acetone, 2-butanone, 2-pentanone, 2-hexanone, 2-heptanone, 2octanone, and 2-nonanonewere obtained from SigmaAldrich (Milwaukee, WI). The individual neat ketones were used, as received, in the vapor generator for both the DMS and the IMS experiments because none was found by gas chromatography−mass spectrometry to contain significant levels of impurities. Procedures. Ion Mobility Spectrometry. The temperature range over which dimer formation in the ion source followed by dissociation in the drift region was observable was 146 to 174 °C. Experiments were carried out in this range at ambient pressure (660 Torr) and different set temperatures that were approximately 4 °C apart. In the absence of ketone, the single peak in the mobility spectrum was that of the reactant ion, (H2O)nH+. Ketone introduced to the source region produced two new peaks” the protonated molecule MH+ and the proton bound dimer M2H+. The concentration of ketone was adjusted until the (H2O)nH+ peak was reduced to approximately 10% of its initial value, and before any measurement of M2H+ dissociation was commenced the instrument was left overnight to equilibrate at a set temperature. At a fixed temperature, mobility spectra were obtained at 25 V cm−1 increments as the field in the 7.0 cm drift region was changed over the range 175 V cm−1 to 400 V cm−1. The dual shutter capability of the instrument was utilized in order to ensure that only M2H+ entered the drift region. The timing of the two shutters at each temperature was set as follows: (1) with shutter 1 operating and shutter 2 open, the drift time t1 of M2H+ over the 14.0 cm drift length was determined; (2) with shutter 1 open and shutter 2 operating, the drift time t2 of M2H+ over the final 7.0 cm drift length was determined; (3) spectra were obtained with shutter 1 operating and shutter 2 opening after a delay of t1 − t2. The balance between resolution and ion intensity was achieved with a shutter 1 pulse width of 1.45 ms and a shutter 2 pulse width of 0.500 ms. Mobility spectra were obtained by monitoring ion intensities as a function of drift time over the final 7.0 cm drift length. Each recorded spectrum was the average of ∼2000 individual spectra, and six to eight of these averaged spectra were recorded and their average constituted the spectrum for each electric field strength. Other parameters for data collection included a 40 000 Hz sampling rate, and 1500 points/per spectrum. Differential Mobility Spectrometry. The effect of the combination of instrument temperature and separation field on the dissociation of M2H+ for each ketone was examined with a constant concentration of a single ketone in the transport gas. The DMS analyzer temperature was fixed, and SV was scanned in 100 s over the SV range of 500 V to 1500 V in 10 V steps. At each SV, a DMS spectrum was obtained by scanning CV from
expected activation energies and pre-exponential factors for dissociation should therefore be almost identical since the reaction path is the simple stretching of an O···H+−O hydrogen bond. This provides an opportunity to investigate the effect of ion mass on Tieff for dimer decomposition as a function of ambient temperature and electric field strength.
■
EXPERIMENTAL SECTION Instrumentation. The ion mobility spectrometer, built from a series of steel rings separated by Teflon spacers, is of a design previously described in detail.34 The total length of the instrument is 21.0 cm from the ion repeller to the ion detector, comprising 1.8 cm for the ionization region, 5.2 cm for the reaction region, and 14.0 cm for the drift region. The ionization region has an internal diameter of 1.5 cm and contains a 10 μCi 63 Ni ionization source, and all the other regions have the same internal diameter, 1.99 cm. The drift region has two Tyndall ion shutters, shutter 1 is 14.0 cm and shutter 2 is 7.0 cm from the Faraday detector plate, providing drift lengths of 14.0 and 7.0 cm. A 1.7 cm diameter aperture grid is situated 1.0 mm from the 1.0 cm diameter detector plate. The whole instrument is covered by a stainless steel housing wrapped with stainless steel tubes that carry and preheat sample and drift gases. It is housed in a gas chromatograph oven that provides a uniform temperature from ambient to 523 K. The electric fields along the mobility spectrometer are provided by a voltage divider chain of resistances with a voltage, variable from +4980 V to +7005 V, applied to the source end of the instrument relative to the grounded detector end. The electric field from the source end to the second shutter was held constant in all experiments at 300 V cm−1 and the field over the final 7.0 cm drift region was variable between 175 V cm−1 and 400 V cm−1. Variation of the residence time of ions in this drift region with change of drift field did not alter ion formation in, or transfer of ions from, the source region. Two shutter operation allowed mobility-based selection of ions entering the 7.0 cm drift region. The drift gas, 320 cm3 min−1 of purified air, was introduced through the detector-end flange and vented at the source end to create a unidirectional flow, counter to ion drift. The water concentration of the drift gas was always less than 0.1 ppmv as measured with a Moisture Image Series 2 (Panametrics, Inc., Waltham, MA). A controlled flow of sample from a vapor generator entered the ionization region in a separate, smaller air stream and the combined gas temperature was monitored at the exhaust of the instrument by a thermocouple. The positive ion spectra were digitized using software derived from Labview (National Instruments Corporation, Austin, TX, USA). The structure and operation of planar differential mobility spectrometers have been described in detail and only a bare outline will be given here.5,34,35 The instrument was an SVAC (Sionex Corporation, Bedford, MA, USA) that contained a 5 mCi 63Ni source providing ions that are carried through the instrument in a flowing transport gas stream. The ions travel to a Faraday plate detector between two parallel metal plates, each of length 15 mm, width 1.5 mm, set 0.5 mm apart. A mass flow controller supplies 500 cm3 min−1 at STP of transport gas (dry air moisture level
