Multigenerational Collision-Induced Dissociation for Characterization

Sep 13, 2016 - The advent of tandem mass spectrometry (MS2, or MS/MS), specifically collision-induced dissociation (CID), was a critical step in the d...
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Multigenerational Collision-Induced Dissociation for Characterization of Organic Compounds Dalton T. Snyder, Patrick W. Fedick, and R. Graham Cooks* Department of Chemistry and Center for Analytical Instrumentation Development, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: There are many cases in which limited information is obtained from a single stage of tandem mass spectrometry (MS2 or MS/MS). For example, isomeric cathinones give similar product ion MS2 spectra, but they can be differentiated by their unique MS3 fragments. Other drugs such as oxycodone and noroxycodone lose water in a single-stage tandem mass spectrometry experiment but give rich structural information in subsequent stages, as do many peptides. Here we utilize multigenerational collision-induced dissociation (CID) on a miniature mass spectrometer and emphasize useful applications. Unique fragmentation patterns consisting of several generations of fragment ions are obtained in these multigenerational spectra, allowing discrimination of cathinone isomers and structural characterization of small molecules, including drugs and peptides, all using a single sequence of ion injection, isolation, and mass scanning.

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he advent of tandem mass spectrometry (MS2, or MS/ MS), specifically collision-induced dissociation (CID), was a critical step in the development of mass spectrometry.1,2 With MS2 capabilities, a precursor ion can be separated, isolated, and fragmented, generating product ions from which the precursor ion structure can be deduced.3,4 Tandem mass spectrometry is useful for ion structure elucidation, but it is also important in compound identification, quantitation, and reaction monitoring.5 Not only is MS2 a workhorse for organic chemists, it is a critical tool for the fields of “omics,”6,7 forensics,8,9 and environmental monitoring.10,11 While MS2 is valuable for many analyses, there are instances when further fragmentation of product ions is required (MS3 or MSn) in order to obtain desired results, usually because the major product from an MS2 experiment is a loss of H2O, or when structurally similar or isomeric compounds are being examined. Natural and designer drugs of abuse can be detected and monitored by tandem mass spectrometry, but often synthetic cathinones or cannabinoids do not have information-rich MS2 spectra, and structural and compositional identification requires higher order tandem experiments.12,13 Similarly, much of biological mass spectrometry relies on MS3 or MSn for structural studies.14,15 Instruments that can perform multiple stages of tandem mass spectrometry can be broadly divided into two categories: (1) multianalyzer instruments and (2) ion traps. The former category encompasses pentaquadrupole instruments,16−20 which may provide MS3 information using two rf-only CID cells and three mass-selective quadrupole filters, and multistage quadrupole (or ion trap) time-of-flight instruments. Another system is the triple quadrupole ion trap.21 Multianalyzer © XXXX American Chemical Society

instruments offer high performance and access to precursor and neutral loss scans22−24 but have more stringent pressure and size requirements than ion trap instruments. Ion traps have the inherent capability to perform MSn scans with high sensitivity. The Fourier transform ion cyclotron resonance (FTICR) instrument25−27 provides high resolution in MS/MS product ion scans, but requires substantial maintenance and is, in general, not amenable to miniaturization, which is an attractive feature of quadrupole ion traps.28−30 Quadrupole ion traps are less expensive mass analyzers that can perform MSn scans in a similar manner to the FTICR. Ions are introduced to the trap, and the analyte of interest is isolated using one of several available isolation techniques. A small alternating current (ac) signal is then applied in a dipolar manner to opposite electrodes, and the frequency is chosen to match a characteristic frequency of the analyte ion. The ion then increases its amplitude in the trap, gains kinetic energy, collides with bath gas molecules, and fragments, giving structural information corresponding to the structure of the precursor ion.31−33 Nonresonance activation techniques may also be used in conjunction with traps. These techniques include surfaceinduced dissociation,34−36 which shows particular promise for analysis of peptides,37 proteins,38 and protein complexes,39 and photodissociation (including infrared multiphoton dissociation),40,41 which shows similar promise.42,43 Here we will purposely limit our comparison to ac resonance methods. Received: June 6, 2016 Accepted: September 13, 2016

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DOI: 10.1021/acs.analchem.6b02209 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry Single-resonance techniques include high-amplitude short time excitation,44 dynamic CID,45,46 “fast excitation” CID,47 red-shifted off-resonance excitation,48 and other off-resonance excitation techniques.49 Broad-band techniques, which fragment ions over a range of mass-to-charge ratios (m/z), are also available. Here we will use a wide definition of “broad-band fragmentation.” We include methods that fragment ions over a range of values simultaneously as well as those that fragment ions successively with a single scan of a given parameter (e.g., rf amplitude or ac frequency). The most popular broad-band CID method is the stored waveform inverse Fourier transform (SWIFT), in which multiple frequencies with varying amplitudes are integrated into a single activation waveform (a broad-band waveform).50−53 WideBand Activation, trademarked by Thermo Finnigan, is a method of fragmenting ions over a range of 20 Th by fixing the excitation frequency and scanning the driving rf amplitude slowly so that water loss products (or others falling in the excitation range) may also be fragmented.54 In principle, this technique can be extended to any arbitrary loss, but as commercially implemented in Thermo ion traps (e.g., the LTQ XL) only a range of 20 Th can be fragmented using this method. The dipolar dc method, in which dc potentials of opposite polarities are applied to opposing electrodes, has also been shown to effect broad-band ion dissociation, but the mass selectivity in this method is poor since the entire ion cloud is displaced from the center of the trap.55,56 The secular frequency scan, in which the frequency of the resonance ejection signal is swept linearly with time, can also be used for mass selective fragmentation;57−59 it thus allows precursor and neutral loss scans to be performed in ion traps,58,60 but the differing precursor ion Mathieu q values and nonlinear mass scan with time are deterrents to using this method. In the cases in which a parameter is scanned to give broad-band CID spectra, fragmentation is successive rather than simultaneous. Nonetheless, this fits into our definition of “broad-band fragmentation.” We recently demonstrated the implementation of an extended wideband activation method for broad-band collision-induced dissociation of precursor ions using a miniature mass spectrometer. In this technique ions are sequentially fragmented from high to low mass by ramping the rf amplitude in the reverse direction while a constant frequency ac signal is used for resonance excitation at an optimal Mathieu q value (0.2−0.35).61 The method is an extension of Thermo Finnigan’s trademarked WideBand Activation method, but the mass range is not limited to 20 Th and it was implemented for the first time on a miniature mass spectrometer. Here we emphasize the multigenerational fragmentation that is observed in this extended wideband activation method, which we will refer to as “multigenerational CID” (MCID). We demonstrate its utility for discrimination of isobaric cathinones and structural elucidation of small molecules which undergo a single primary loss of arbitrary mass in CID. All multigenerational spectra are obtained with a single sequence of ion injection, isolation, excitation, and mass scanning, unlike conventional MSn which requires several sequences to obtain information beyond MS2.

Figure 1. Scan table illustrating conventional CID and multigenerational CID (this work). (a) In conventional CID, the rf amplitude is constant while a fixed frequency ac signal is set on the precursor ion m/z (P), giving a single generation of fragments (F1). In this work, the rf amplitude is scanned from high to low, thereby generating multiple generations of fragment ions (F2, second generation; F3, third generation; etc.) since ions will be fragmented successively from high to low mass. Panel b shows that a constant frequency ac signal is used to fragment ions at a fixed Mathieu q parameter (100 kHz, q = 0.279), and panel c shows that the ac amplitude was decreased with time in order to scale the excitation energy to the m/z of each ion.

U.S.A.). Sulfated leucine enkephalin was purchased from Peptide Institute Inc. (Saitoasagi Ibarakishi, Osaka, JPN) and leucine enkephalin (YGGFL) was purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.). Cathinone standards were prepared in 50:50 MeOH/H2O with 0.1% formic acid. Oxycodone and noroxycodone standards were prepared in MeOH, and the leucine enkephalin standards were prepared in H2O. All samples had final concentrations of ∼10 ppm and were stored at −20 °C prior to analysis. Ionization. Nanoelectrospray ionization (nESI) at ∼2000 V generated ions in all experiments. Borosilicate glass capillaries (1.5 mm o.d., 0.86 mm i.d., Sutter Instrument Co.) pulled to a 5 μm tip using a Flaming/Brown micropipette puller (Sutter Instrument Co. model P-97, Novato, CA, U.S.A.) were used. Instrumentation. All multigenerational CID scans were performed using the Mini 12 miniature mass spectrometer, developed in-house at Purdue University.62 The rf frequency was tuned to 0.999 MHz. Ions were introduced through a discontinuous atmospheric pressure interface (DAPI)63 for 13 ms, after which they were allowed to cool for ∼700 ms. The ion of interest was then isolated by first applying a highamplitude (∼5.5 Vpp) ac waveform at a frequency below the analyte’s secular frequency for 30 ms, thereby removing ions higher in mass than the analyte. Then, the rf amplitude was increased to bring the analyte to the edge of the Mathieu stability diagram (q ∼ 0.9), ejecting all ions lower in mass. The rf amplitude was then decreased linearly during the 400 ms CID scan, during which a low-amplitude (20 Th) of unknown composition is observed is in the MS2 spectrum of leucine enkephalin, a peptide (YGGFL). Because WideBand Activation has a limited mass range (as commercially implemented) and because the large neutral loss (m/z 578 → 465) is not common (like H2O loss), neither the wideband nor the dual-frequency technique would suffice for further fragmentation in a single scan without prior knowledge of the neutral loss. Figure 5 compares the mass spectra obtained from multigenerational CID on the Mini 12 with that obtained from a simple MS2 experiment using the benchtop LTQ. Few fragments are observed in MS2 because a prominent loss of 113 Th dominates the spectrum, whereas the multigenerational method yields significantly more information because the major fragment (m/z 465) becomes a precursor ion later in the multigenerational scan and thus dissociates further, giving more structurally diagnostic fragments. Note that we restrict our discussion to MS3 fragmentation as the highest level of tandem mass spectrometry because generally higher order experiments are not needed. However, certainly higher order fragments are produced with multigenerational CID. So far we have not shown that multigenerational CID is mass selective, that is, that the excitation range can be tuned. This can be accomplished in two ways: (1) changing the amplitude range over which the rf is ramped (Figure 1a) or (2) changing the ac frequency (Figure 1b) or the amplitude over which the ac is ramped (Figure 1c). Figure 6 illustrates the second method with sulfated leucine enkephalin (m/z 636, [M + H]+). Figure 6a shows a typical MS2 spectrum of sulfated leucine enkephalin obtained on the benchtop LTQ. One major fragment, which is due to sulfite loss, is observed. A similar spectrum can be obtained using the multigenerational CID method if the ac amplitude at the time m/z 556 comes into resonance with the excitation waveform is kept too low for the product ion to fragment (Figure 6b). In order to obtain the spectrum in Figure 6b, the rate of change of ac amplitude with respect to time was much greater than, say, Figure 6d. That is, the slope of the curve in Figure 1c was fairly high, meaning that higher m/z ions were given sufficient collision energy to fragment, but lower m/z ions were not, thereby limiting fragmentation to a single stage. Hence, only SO3 loss is observed (m/z 636 → 556). However, if the ac amplitude is ramped more slowly (small slope in Figure 1c), the collision energy will be relatively high for both the precursor ion as well as its product ions. Thus, m/z 556 is also allowed to fragment, giving rise to many more product ions (Figure 6d). These product ions can then become precursor ions for further fragmentation, giving rise to a “fingerprint” mass spectrum unlike those obtained in standard MSn excitation. For reference the MS3 spectrum for the transition m/z 636 → 556 → products is given in Figure 6c. In comparing Figure 6, parts c and d, we notice that most of the additional peaks are the product ions of

Figure 6. Mass-selective multigenerational CID of sulfated leucine enkephalin: (a) MS2 spectrum of singly protonated ([M + H]+) sulfated leucine enkephalin obtained using standard single-resonance excitation on a benchtop LTQ at q = 0.25 compared to (b) multigenerational CID of sulfated leucine enkephalin using the Mini 12 (ac frequency of 100 kHz, q = 0.279) where the ac amplitude was ramped with a high rate of change with respect to time such that m/z 556 was not fragmented further. (c) MS3 spectrum m/z 636 → 556 → products on the LTQ compared with (d) multigenerational CID of sulfated leucine enkephalin using the Mini 12 (ac frequency of 100 kHz, q = 0.279) where the ac amplitude was ramped with a low rate of change with time such that m/z 556 and other lower mass fragments were further dissociated to give additional product ions. Panels a and c were recorded on an LTQ. Panels b and d were recorded on the Mini 12. “Steep” (high) and “shallow” (low) ac amplitude ramp refer to the slope of the line in Figure 1c.

fragmentation of m/z 556. However, there are some that cannot be accounted for: m/z 118, 130, 134, 146, and 294. Presumably these are the result of MS4-like (or higher order) fragmentation. H

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CONCLUSION The utility of multigenerational CID was demonstrated using a miniature mass spectrometer. Applications to forensics, including differentiation of isomeric cathinones and structural characterization of oxycodone and noroxycodone, were shown. Peptide analysis may also benefit from the successive fragmentation steps that are observed with this method, allowing for the rapid acquisition of data in MSn space. The fingerprint mass spectra that are acquired are more informative than simple MS2 or MS3 spectra and enhance isomer discrimination in the cases tested.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b02209. Description of Movie S1 (PDF) Movie S1, a visual depiction of the multigenerational CID scan on the well-known Mathieu stability diagram (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Steve Ayrton (Purdue University) for creating the TOC graphic and Valentina Pirro and Karen Yannell (Purdue University) for helpful discussions. The authors acknowledge discussions with Professor Scott McLuckey (Purdue University) and thank Mack Shih (Purdue University) for providing leucine enkephalin. The authors acknowledge with particular gratitude the engaged, high-quality efforts of the reviewers of this manuscript. This research was funded by NSF (CHE 1307264) and NASA (NNX16AJ25G).

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DOI: 10.1021/acs.analchem.6b02209 Anal. Chem. XXXX, XXX, XXX−XXX