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Determination of Collision Cross Sections Using a Fourier Transform Electrostatic Linear Ion Trap Mass Spectrometer

  • Eric T. Dziekonski
  • Joshua T. Johnson
  • Kenneth W. Lee
  • Scott A. McLuckey
Focus: 32nd Asilomar Conference, Novel Instrumentation in MS and Ion Mobility: Research Article

Abstract

Collision cross sections (CCSs) were determined from the frequency-domain linewidths in a Fourier transform electrostatic linear ion trap. With use of an ultrahigh-vacuum precision leak valve and nitrogen gas, transients were recorded as the background pressure in the mass analyzer chamber was varied between 4× 10-8 and 7 × 10-7 Torr. The energetic hard-sphere ion–neutral collision model, described by Xu and coworkers, was used to relate the recorded image charge to the CCS of the molecule. In lieu of our monoisotopically isolating the mass of interest, the known relative isotopic abundances were programmed into the Lorentzian fitting algorithm such that the linewidth was extracted from a sum of Lorentzians. Although this works only if the isotopic distribution is known a priori, it prevents ion loss, preserves the high signal-to-noise ratio, and minimizes the experimental error on our homebuilt instrument. Six tetraalkylammonium cations were used to correlate the CCS measured in the electrostatic linear ion trap with that measured by drift-tube ion mobility spectrometry, for which there was an excellent correlation (R 2 ≈ 0.9999). Although the absolute CCSs derived with our method differ from those reported, the extracted linear correlation can be used to correct the raw CCS. With use of [angiotensin II]2+ and reserpine, the corrected CCSs (334.9 ± 2.1 and 250.1 ± 0.5, respectively) were in good agreement with the reported ion mobility spectrometry CCSs (335 and 254.3, respectively). With sufficient signal-to-noise ratio, the CCSs determined are reproducible to within a fraction of a percent, comparable to the uncertainties reported on dedicated ion mobility instruments.

Graphical Abstract

Keywords

Collision cross section Electrostatic linear ion trap Fourier transform Electrospray ionization 

Introduction

Mass spectrometry is widely used for the identification and quantitation of species of interest on the basis of the mass-to-charge (m/z) measurement of their relevant ions. Important information regarding ion structure, such as bond connectivity, can be derived from ion fragmentation. However, the measurement of m/z alone cannot provide information about the three-dimensional gas-phase structure, which is often critical to biological functionality [1, 2]. To obtain such information, instruments containing a dedicated ion mobility spectrometry (IMS) platform may be coupled to a tandem mass spectrometer [3, 4], thereby enabling the user to identify the molecule and elucidate the rotationally averaged collision cross section (CCS) of the ion–neutral pair. It has, however, long been recognized that the spectral linewidth in Fourier transform (FT) mass spectrometry is related to the CCS of the ion–neutral pair [5, 6, 7, 8, 9]. Recently, several groups have used FT ion cyclotron resonance (ICR) mass analyzers and linewidth analysis to determine the CCSs of ions derived from amino acids [10], peptides [11], and proteins [12], among others [13]. The process of determining CCSs from the frequency linewidth with an FT-ICR mass analyzer has been given the acronym “CRAFTI,” denoting “cross-sectional areas by Fourier transform ion cyclotron resonance” (FT-ICR) [13]. The relative CCSs of very large ions (e.g., those derived from myoglobin, carbonic anhydrase, enolase, bovine serum albumin, and transferrin) have also been reported with use of an Orbitrap mass analyzer. However, the authors were unable to directly measure the pressure in the Orbitrap chamber and therefore were unable to determine the absolute CCSs [14]. It has been proposed, on the basis of theory, that the CCS can be derived in a quadrupole ion trap through a time–frequency analysis method [15].

In an FT electrostatic linear ion trap (ELIT) [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27], ions are confined between two axially opposing reflectrons. The image charge induced on a central pickup electrode is digitized, subjected to Fourier transformation, and calibrated to generate a mass spectrum. The rate at which the detected signal intensity decays depends on many factors, including the divergence angle of the injected ion beam, the focal point of the einzel lenses [28], the kinetic energy (KE) focusing characteristics of the reflectrons and einzel lens [29, 30], the CCS of the ion–neutral pair, the background gas density [31, 32], the energy exchanged owing to a collision, space charge [33, 34, 35, 36], and any geometric/field imperfections in the trap. Therefore, the source(s) of signal decay on our homebuilt ELIT could be much more complicated than in an FT-ICR mass analyzer, where ions start at the center of the mass analyzer and are excited to radii where field imperfections are minimized by shimming of the electrode potentials. However, all experimental observations point to the fact that, even at the lowest pressures that we have achieved (i.e., approximately 3 × 10-9 Torr), we are operating under a pressure-limited circumstance [24]. Therefore, the collision rate should be the dominant ion loss mechanism such that a CCS may be derived.

Herein we report measurement of the CCSs of six quaternary ammonium cations, [angiotensin II + 2H]2+, and [reserpine + H]+ in an ELIT using the energetic hard-sphere collision model proposed by Xu and coworkers [12, 37]. Neither monoisotopic isolation nor high resolution was needed as the frequency-domain spectrum was fit to a sum of Lorentzians, from which the linewidth was extracted. We found there to be good correlation (R 2 ≈ 0.9999) between the CCS measured in the FT-ELIT and that measured by drift-tube IMS. However, the absolute CCSs differ, which has also been observed in the measurement of the CCSs of ions with an FT-ICR mass analyzer by several different methods [10, 12, 13].

Experimental

Materials

The following chemicals were purchased from Sigma-Aldrich (St Louis, MO, USA): reserpine, angiotensin II (human), tetraethylammonium bromide, tetrapropylammonium bromide, tetrabutylammonium bromide, tetrahexylammonium bromide, tetraoctylammonium bromide, and tetradodecylammonium bromide. Methanol (MeOH) was purchased from Thermo Fisher Scientific (Waltham, MA, USA), glacial acetic acid (AcOH) was purchased from Mallinckrodt (Phillipsburg, NJ, USA), isopropyl alcohol (IPA) was purchased from Macron Fine Chemicals (Center Valley, PA, USA), and high-performance liquid chromatography grade water was purchased from Fisher Scientific (Pittsburgh, PA, USA). Reserpine and angiotensin II were prepared to final concentrations of 100 μM in 49.5:49.5:1 v/v/v MeOH–H2O–AcOH. All tetraalkylammonium salts were prepared to final concentrations of 100 μM in 50:50 v/v MeOH–H2O, except for tetradodecylammonium bromide, which was prepared to a final concentration of 100 μM in 45:45:10 v/v/v MeOH–H2O–IPA.

Mass Spectrometry

The nanoelectrospray ionization (nESI) source and the method by which ions are concentrated and injected into the ELIT have been described previously [18, 21]. In brief, the sample is loaded into a pulled-glass capillary and placed in front of the sampling orifice. High voltage is applied to a platinum wire in contact with the solution to generate an electrospray [38]. Ions generated via nESI are transported to a trapping quadrupole equipped with LINAC II electrodes [39], where their accumulation and collisional cooling is facilitated by the continuous injection of nitrogen gas. Once the ions have cooled, the voltage to all quadrupole elements is ramped to their injection potentials, where the KE of the ions is set by the rod offset. An RF phase-locked circuit was used to trigger both the injection of the bunched ion packet and the start of data collection at a zero crossing of the trapping RF such that a consistent ion energy was sampled. The pressure within the ELIT analysis chamber is continuously variable (manual) between 5 × 10-9 and 5 × 10-6 Torr (N2) via a VZLVM263R LVM series ultrahigh-vacuum leak valve that is attached directly to the chamber. The pressure is measured with a Granville Phillips 355001-YF ion gauge and displayed on a Granville Phillips 358 Micro-Ion controller. If the controller is out of calibration, in that the response is not 1 V per pressure decade, as it is configured to be, all CCS measurements will suffer from a systematic error and be off by a constant factor from those derived by IMS.

The ELIT used in this work has been described previously [23]. Briefly, the ELIT captures ions via mirror-switching and detects the resulting image charge using a central electrode. The trap itself is made up of ten parallel stainless steel plates (5.08 cm × 5.08 cm × 0.635 mm thick, Kimball Physics, Wilton, NH, USA) with holes 6.48 mm in diameter drilled through the center; eight of the plates control ion acceleration, deceleration, and radial focusing (plates 1–8), and the other two plates are used to enclose the grounded housing for the detector. The spacing between elements (plates 1–3, 7.62 mm; plate 3 to central housing, 11.43 mm) is maintained by alumina spacers (Kimball Physics). The copper pickup tube used for image charge detection [8.26-mm inner diameter (i.d.), 9.53-mm outer diameter (o.d.), 25.4 mm long] is centered within its grounded housing (50.8 mm long, 33.02-mm i.d.) by a polyether ether ketone spacer. Stainless steel tubes (6.35-mm i.d., 10.16-mm o.d., 19.05 mm long) were welded to plates 1 and 8 to make the electric fields in both ion mirrors identical. To trap positive ions, plate 1 is pulsed from ground to 2356 V (ORTEC model 556, Advanced Measurement Technology, Oak Ridge, TN, USA) by a fast, high-voltage switch (HTS 31-03-GSM, Behlke Power Electronics, Billerica, MA) at a time defined relative to the ejection of ions from the trapping quadrupole. All other plate potentials are generated by additional ORTEC 556 power supplies, with typical trapping potentials being 2364 V (plate 8), 1646 V (plates 2 and 7), 1047 V (plates 3 and 6), and -2106 V (plates 4 and 5) as measured with a calibrated 1000x probe and an HP 34401A multimeter. Frequency shifts arising from the transient voltage recovery of the pulsed power supply are minimized by use of the ORTEC 556 power supply as the power supply from plate 1 [23]. Considering the linewidths (frequency resolution) at elevated background pressures, the frequency correction circuit was not included. Daily variations in the output voltages of the power supplies were minimized by or leaving the ORTEC power supplies on. With the detailed trap dimensions, ion energy, and trapping voltages, typical ion frequencies are between 400 kHz and 100 kHz for m/z 100 and 1500, respectively.

Signal Processing

The charge-sensitive detection electronics have been described previously [19, 23]. The output of the charge-sensitive preamplifier (A250, Amptek) was filtered (band pass, Krohn-Hite model 3940, Brockton, MA, USA) and amplified (gain of 5) before digitization by a PCI-based digitizer (CS1621, 16 bit, Gage Applied Technologies, Lanchine, QC, Canada) at a rate of 10 MS/s (AC coupled, 1-MΩ input impedance, 25-MHz low-pass filter enabled). A program written in LabVIEW 13.0 (National Instruments, Austin, TX, USA) was used to acquire each transient. A custom program, written in MATLAB 2015, was used to process the transients (100 averages, 75 ms, 10 MSa/s, 1 zero fill, rectangular window) and determine the CCS of the ion.

Theory

Signal Decay in an Electrostatic Linear Ion Trap

To measure the CCS of an ion in an FT-ELIT, the relationship between the frequency-domain linewidth and the CCS must be established, which requires the use of an appropriate ion–neutral collision model. Three collision models (i.e., Langevin, hard sphere, and energetic hard sphere) were described by Guo et al. [37] and implemented by Jiang et al. [11] to determine biomolecular CCSs with an FT-ICR mass analyzer. Typically, the appropriate collision model is chosen on the basis of the size of the ion and/or its trapped KE. In a modern Orbitrap or FT-ICR mass analyzer, ions rotate about the trap axis at a constant angular speed and a high ion energy, and it is therefore expected that the energetic hard-sphere collision model is a good description of the ion–neutral collisions so long as the energy exchanged by a single collision is sufficient to dephase or dissociate the ion. Unlike in Orbitrap and FT-ICR mass analyzers, ions trapped within an ELIT have a time-dependent axial KE (KE z ) that can range from 0 to approximately 3.7 keV 7per charge throughout one oscillation of the trap. As such, it is reasonable to presume that the appropriate collision model depends on the location (potential, time) at which the collision occurs. However, for an ion to continue to contribute to the observed signal after a collision has occurred in the ELIT, several criteria must be met:
  1. 1.

    The collision must not lead to fragmentation. The likelihood for this scenario depends on the KEs of the ions, the relative masses of the ion and background gas, and the stability of the ion with respect to fragmentation. The first two factors determine the collision energy in the center-of-mass frame of reference, whereas the latter factor relates to the energies and entropies associated with ion fragmentation as well as the number of degrees of freedom in the ion.

     
  2. 2.

    The collision must not lead to lateral scattering sufficient for ion loss. In other words, all the KE needs to be retained within the axial motion of the ion (KE z >> KE x,y ).

     
  3. 3.

    The postcollision KE (laboratory frame of reference) must be within the focusing capabilities of the reflectrons and einzel lenses to avoid loss of ions in the z-dimension.

     
  4. 4.

    An extension to criterion 3 is the requirement that the ions maintain the same frequency of oscillation. To achieve this condition, the reflectron potentials must be tuned such that a broad range of KEs exhibit isochronous axial motion. Otherwise, dephasing of the ions leads to loss of signal even when the ions remain trapped.

     

By definition, if every collision were to lead to fragmentation (i.e., criterion 1 is never met), the energetic hard-sphere collision model can be used to describe the signal decay. If the second or third criterion is not met, the ion will follow an unstable trajectory and be lost from the trap within several oscillations. Smaller ions are especially susceptible to ejection from the trap via criterion 2, where anything but a head-on collision is likely to scatter the ion and partition axial KE into the radial dimensions. As the ion mass increases, the maximum exchange energy [37] is decreased and the collision is less likely to scatter or fragment the ion, as is the case for megadalton-sized complexes [40]. Nevertheless, an ion that does not fragment, is not scattered, and remains within the focusing conditions of the reflectron can still dephase from the packet owing to a change in its total KE if the fourth criterion is not satisfied. As the four plate reflectrons used in the current ELIT do not generate an ideal quadratic electric field, only a small range of KEs will exhibit isochronous motion. Per ion optical simulations performed in SIMION version 8.1 (Scientific Instrument Services, Ringoes, NJ, USA), simply changing the trapped KE of cesium (m/z 132.906, approximately 1960 eV) by 10 eV will change the detected frequency by approximately 80 Hz. If cesium were to undergo a collision with helium in the field-free region, the maximum exchange energy would be approximately 120 eV, causing it to rapidly dephase from the coherent ion packet and/or be ejected from the trap.

From this discussion, even though ions trapped within the ELIT exhibit a time-dependent KE z , it is expected that a single collision will remove smaller m/z ions from the spatially coherent ion packet by a dephasing mechanism, dissociation, or ejection from the trap. As such, the energetic hard-sphere collision model is used to relate the transient signal decay to the CCS of the ions in this study. Briefly, the number of ions remaining in the coherent packet, N(t), can be described by an exponential decay:
$$ N(t)={N}_0{e}^{-\frac{t}{\tau}}, $$
(1)
where N 0 is the initial number of ions in the packet, t is the time, and τ is an exponential damping time constant defined as
$$ \tau =\frac{1}{\sigma nv}, $$
(2)
in which σ is the CCS of the ion (m2), n is the neutral gas density (molecules per cubic meter), and v is the ion speed (m/s). The Fourier transformation of the time-domain exponential-damped sinusoid (no apodization) generates a frequency-domain line shape corresponding to a Lorentzian function:
$$ A(f)=\frac{2\alpha}{\pi}\;\frac{\varGamma}{4\cdot {\left( f-{f}_0\right)}^2+{\varGamma}^2}+ C, $$
(3)
in which A ( f) is the signal magnitude at the specified frequency (f), α is the peak area, Γ is the full width at half maximum (FWHM) in hertz, f 0 is the center frequency of the peak in hertz, and C is the offset. The FWHM of a Lorentzian function is related to the exponential decay time constant by
$$ \varGamma =\frac{1}{\pi \tau}. $$
(4)
Therefore, rearrangement of Eqs. 3 and 5 allows the ion CCS to be calculated:
$$ \sigma =\frac{\pi}{v}\cdot \frac{\varGamma}{n}. $$
(5)
Given that the ion speed is not constant in the ELIT, the time-averaged ion speed is used instead:
$$ \overline{v}= path\_ length\;(KE)\cdot {f}_0= path\_ length\;(KE)\cdot \left(\frac{k}{\sqrt{m/ z}}+ b\right), $$
(6)
where path length(KE) is the distance between the turning points (Figs. 1 and 2), which varies with the trapped KE, and f 0 is the center frequency of the ion extracted from a Lorentzian fit of the experimental line shape (Eq. 4). The frequency of an ion within the ELIT is inversely proportional to the square root of the mass-to-charge (m/z) ratio. With use of the ideal trap dimensions, measured plate potentials, injection conditions, and SIMION version 8.1, the potential energy surface along the central axis of the ELIT was extracted (Fig. 2) and the KE of the ion as it passed through the central plane of the ELIT was determined to be approximately 1960 eV per charge. The path length was estimated to be 123.68 mm by linear interpolation of the distance between the turning points (potential energy 1960 V). In part, the experimental error in the absolute CCSs determined with an FT-ELIT can be related to physical imperfections of the ELIT, which could cause accelerated dephasing, or to error in the estimated path length. Both cases lead to systematic errors that are difficult to quantify independently.
Figure 1

The instrument (not to scale). BP band pass, ESI electrospray ionization, UHV ultrahigh vacuum

Figure 2

Top: AutoCAD assembly of the electrostatic linear ion trap (ELIT). Bottom: Potential (in volts) along the central axis of the trap. The effective path length is equal to the distance between the two turning points for a known trapped kinetic energy (KE)

Use of Isotopic Information to Improve Performance

Apex ion isolation is relatively inefficient on our homebuilt instrument and causes the intensity of the isotope of interest to be heavily depleted while not providing a monoisotopic isolation. If it is used, the signal-to-noise ratio of the frequency-domain spectrum is drastically reduced, leading to a systematically high standard deviation in the extracted FWHM at each pressure and, consequently, a high uncertainty in the calculated CCS. In lieu of monoisotopic isolation, two methods can be adopted: (1) with high-frequency resolution, as is found on an FT-ICR mass analyzer, the FWHM of each individual isotope can be recorded at several pressures before the increasing isotopic linewidths begin to interfere with each other [11, 12]; and (2) if the isotopic distribution of the ions is known, the frequency-domain spectrum can be fit to a sum of Lorentzians (Eq. 7):
$$ A(f)=\frac{2{\alpha}_1}{\pi}\;\frac{\varGamma}{4\cdot {\left( f-{f}_{0,1}\right)}^2+{\varGamma}^2}+\frac{2{\alpha}_2}{\pi}\;\frac{\varGamma}{4\cdot {\left( f-{f}_{0,2}\right)}^2+{\varGamma}^2}+\cdots + C, $$
(7)
in which the subscripts indicate the isotope to which the variables refer. It is assumed that the FWHM of all isotopes are the same and that the areas are proportional to the relative intensities; that is, \( {\alpha}_2=\frac{R_2}{100}{\alpha}_1 \), where R 2 is the relative intensity of the second isotope.

The second method was used throughout this work, and only those isotopes with a relative abundance greater than 3% were included in the fit.

Results and Discussion

Figure 3 shows eight transients (100 averages each) of tetraoctylammonium (TOA), collected over a pressure range from 4.2 × 10-8 to 7.0 × 10-7 Torr with nitrogen as a buffer gas. Considering the high operating pressures and the CCS of the ion–neutral pair, the observed signal decayed very rapidly, and therefore only the first 25 ms of the full 75-ms transients are plotted to aid the reader. At low pressures, a beat pattern was observed in the transient that resulted from isotopic interferences.
Figure 3

Time-domain signal of tetraoctylammonium (TOA) at different buffer gas pressures. To aid with visualization, only the first 25 ms of the 75-ms transient was plotted. Each trace represents the average of 100 transients

The corresponding magnitude-mode fast FTs (power spectrum) of the transients shown in Fig. 3 are plotted in Fig. 4 and expanded about the fundamental distribution of TOA. At low buffer gas pressures, the three major isotopes (relative abundance greater than 3%) were clearly resolved. However, as the pressure was increased, the linewidth of the isotopes increased concurrently to the point that they were no longer resolved (although a shoulder is visible). The frequency-domain spectra were fit to a sum of Lorentzians with use of Eq. 7 and the nonlinear least squares solver in MATLAB (lsqcurvefit). It was assumed that the area of each Lorentzian was proportional to the isotopes’ respective relative abundance and that the FWHM of all isotopes was equal. Selected spectra (black trace) and their respective fits (dashed red trace) are shown in Fig. 5. The experimentally observed Lorentzians were asymmetric, an observation made apparent by the seemingly poor fit at 42 nTorr. However, even when the isotopes were unresolved (and uncoalesced), the algorithm could fit the experimental spectrum very well, as is observed at 600 nTorr.
Figure 4

Magnitude-mode frequency-domain spectra of TOA at different buffer gas pressures (power spectra). The corresponding transients (100 averages) are shown in Fig. 3

Figure 5

Frequency-domain spectrum (black) and Lorentzian fit (red) for TOA at selected pressures. When the isotopic information is known, it can be used to fit the spectrum to multiple Lorentzians, thus eliminating the need for isotopic isolation

The linewidth at each pressure was extracted from the solver and is plotted against the neutral gas density in Fig. 6 (n = P/k B T, where P is the pressure, k B is the Boltzmann constant, and T is the temperature). The slope of the linear regression represents the quantity Γ/n in Eq. 5, whereas no physical meaning is attributed to the intercept. As the signal was completely absent by the end of the collected 75-ms transient, no frequency-domain peak broadening (windowing effect) was expected, and therefore no linewidth correction was applied [11]. This procedure was repeated for a total of six different quaternary ammonium cations (five replicate measurements at each pressure) ranging from tetraethylammonium (m/z 130.2) to tetradodecylammonium (m/z 690.8). In all cases, the coefficient of determination (R 2) is near unity. Standard deviations in the measured linewidth were typically within 0.2–2 Hz, and are not represented on the plot as they are unable to be visualized on its scale.
Figure 6

Linewidth (full width at half maximum, FWHM) of several tetraalkylammonium cations at different buffer gas pressures. With high signal-to-noise ratio, the coefficient of determination (R 2) is near unity, demonstrating excellent linearity. Error bars have been omitted as they are unable to be visualized on this scale (typically 0.2–2 Hz). TBA tetrabutylammonium, TDDA tetradodecylammonium, TEA tetraethylammonium, THA tetrahexylammonium, TPrA tetrapropylammonium

From the simulated path length and center frequency of the most abundant isotope, the time-averaged velocity of the ion could be determined (Eq. 6) and used to calculate the CCS via Eq. 5. The CCS of each quaternary ammonium cation, calculated from the FT-ELIT data, is plotted against the reported drift-tube IMS CCS [41, 42] in Fig. 7 (left, dashed line). The solid line represents a 1:1, and therefore perfect, correlation. The FT-ELIT-derived CCS trend correlates very well with that derived from drift-tube IMS measurements (R 2 ≈ 0.9999). However, the absolute CCSs differ. This observation is not unique to measurements made with an ELIT, and was reported by others when calculating CCSs on an FT-ICR mass analyzer by different methods [10, 12, 13]. Figure 7, right, shows the absolute CCS as a function of the number of carbons in the alkyl substituent. As one could expect from the excellent correlation between the IMS and FT-ELIT CCSs, the trend is observed to be the same in both cases, with only the absolute values differing. Again, the standard deviations of the calculated CCSs in Fig. 7 are smaller than the symbols used for the data points (see Table 1).
Figure 7

Left: Correlation between the collision cross section (CCS) measured via Fourier transform (FT) ELIT mass spectrometry (MS) and that reported from drift-tube ion mobility spectrometry (IMS). The solid line represents a 1:1 correlation. Right: Collision cross section of tetraalkylammonium cations with dissimilar chain lengths for both IMS and FT-ELIT MS. Error bars for both plots have been omitted as they are unable to be visualized with the corresponding scale (see Table 1)

Table 1

Comparison of collision cross sections derived from drift-tube ion mobility spectrometry (IMS) and Fourier transform electrostatic linear ion trap mass spectrometry (FT-ELIT MS)

Species

m/z

Collision cross section (Å2)

Relative error (%)

Drift tube IMS

FT-ELIT MS, raw

FT-ELIT MS, corrected

TEA

130.2

122.2a

85.7 ± 0.2

121.1 ± 0.2

0.9

TPrA

186.2

143.8a

116.2 ± 0.5

144.1 ± 0.4

0.2

TBA

242.3

166.0a

145.5 ± 0.2

166.2 ± 0.2

0.1

THA

354.4

214.0a

210.5 ± 0.2

215.2 ± 0.2

0.6

TOA

466.5

258.3a

267.8 ± 0.4

258.4 ± 0.3

<0.1

TDDA

690.8

319.0b

347.3 ± 2.6

318.3 ± 2.0

0.2

[Angiotensin II + 2H]2+

523.3

335c

369.3 ± 2.7

334.9 ± 2.1

<0.1

[Reserpine + H]+

609.4

254.3a

256.8 ± 0.7

250.1 ± 0.5

1.7

TBA tetrabutylammonium, TDDA tetradodecylammonium, TEA tetraethylammonium, THA tetrahexylammonium, TOA tetraoctylammonium, TPrA tetrapropylammonium

aFrom [43]

bFrom [42]

cFrom [41]

The CCSs of [angiotensin II + 2H]2+ and [reserpine + H]+ were measured with the ELIT and corrected with use of the relationship derived between the FT-ELIT and IMS CCSs in Fig. 7, left. The experimental (raw) CCS for [angiotensin II + 2H]2+ was 369.3 ± 2.7 Å2, whereas the corrected CCS was 334.9 ± 2.1 Å2, representing less than 0.1% relative error over the reported IMS CCS of 335 Å2 [41]. For [reserpine + H]+, the experimental (raw) CCS was 256.8 ± 0.7 Å2, whereas the corrected CCS was 250.1 ± 0.5 Å2, representing a 1.7% relative error when compared with the reported IMS CCS of 254.3 Å2 [43]. It should be noted, however, that Giles et al. [44] reported the CCS (N2) of reserpine to be between 251.0 and 253.2 Å2 depending on the instrument used (traveling-wave versus drift-tube IMS), thereby lowering the relative error to within the range of 0.4–1.2%.

The results of all experiments are summarized in Table 1. Generally, the corrected CCSs derived from FT-ELIT mass spectrometry demonstrate relative errors of less than 1% from the reported drift-tube IMS CCSs. As expected, even though the mass of reserpine is approximately 143 Da greater than that of TOA, its measured CCS is smaller. Transients exhibiting a lower signal-to-noise ratio generally have a higher uncertainty in the calculated CCS, as exemplified by the fact that tetradodecylammonium and [angiotensin II + 2H]2+ had lower signal intensities than the other compounds. Overall, the uncertainties in the measured CCSs and relative errors are comparable to those derived with dedicated IMS instruments. With higher-sensitivity detection electronics, a voltage-controlled leak valve, and lower pressure, fewer averages would need to be recorded to provide the same measurement precision, and therefore the CCS of an ion could be derived in less than 1 min. Currently, no method has been generated or reported that allows one to determine the CCS of an ion in an FT-based mass analyzer if it adopts multiple conformational states; thus, this remains an area of future research.

Conclusions

The FT-ELIT was used to determine the CCS of several molecules in a nitrogen buffer gas. The potentials applied to the reflectron plates are no different from those used during high-resolution analysis, and thus it is trivial to implement this technique during a normal experimental workflow. Owing to the trajectory stability criteria in an ELIT, even though an ion exhibits a KE between 0 and approximately 3.7 keV per charge throughout one cycle of the trap, a single collision can remove an ion from the packet, and therefore the energetic hard-sphere collision model well approximates the transient decay profile. It is not necessary to perform monoisotopic isolation before analysis and, in fact, preservation of the high signal-to-noise ratio provides higher CCS precision even when the isotopic peaks overlap in the frequency domain. Although the absolute CCSs determined via FT-ELIT mass spectrometry deviate from the values reported in the IMS literature, there is a very strong correlation between the two. After correction, this method provides CCSs and relative errors comparable to those generated with dedicated IMS instruments, but cannot determine if multiple conformational states exist for a single m/z. The implementation of this measurement, from a hardware perspective, requires only the addition of a precision leak valve compatible with an ultrahigh vacuum. The ability to determine CCSs adds to the capabilities of the low-cost, high-performance, homebuilt, FT tandem mass spectrometer.

Notes

Acknowledgements

This work was supported by the Purdue Research Foundation and SCIEX. We thank Mark Carlsen, Randy Replogle, Phil Wyss, and Tim Selby of the Jonathan Amy Facility for Chemical Instrumentation for helpful discussions and their help with construction of the mass spectrometer. We also acknowledge Mircea Guna, and James W. Hager of SCIEX for helpful discussions and for providing the collision cell with LINAC II electrodes.

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Copyright information

© American Society for Mass Spectrometry 2017

Authors and Affiliations

  1. 1.Department of ChemistryPurdue UniversityWest LafayetteUSA

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