Fundamentals of tandem mass spectrometry: a dynamics study of simple C-C bond cleavage in collision-activated dissociation of polyatomic ions at low energy

Quantitative LC-MS/MS. 1. Impact of Points across a Peak on the Accuracy and Precision of Peak Area Measurements

The number of points across a chromatographic peak has long been recognized as a key determinant of the accuracy and precision of the measured peak area. In LC-MS-based quantitation experiments in drug discovery and development, the "rule-of-thumb" has been to use 15 or more points. This "rule" is based on the literature describing chromatographic methods where the goal was to achieve the lowest possible imprecision in the measurements, especially when unknown analytes are being detected. Restricting methods to the requirement of at least 15 points across a peak can be detrimental to the development methods that fully optimize the signal-to-noise ratio for the assay using longer dwell times and/or transition summing. This study aims to show that 7 points across the peak for peaks that are 9 s or less wide provide more than sufficient accuracy and precision for drug quantitation studies. Data from simulated Gaussian curves using a sampling interval of 7 points across the peak gave peak area calculations within 1% of the expected total peak area using the Trapezoidal and Riemann rules and 0.6% for the Simpson rule. Low and high concentration samples (n = 5) were assayed using three different LC methods on three different days on two different instruments (API5000 and API5500). The difference in peak area (%ΔPA) and relative standard deviation of the peak areas (%RSD) was less than ∼5%. No significant difference was observed from the data that were obtained from different sampling intervals, different peak widths, different days, different peak sizes, and different instruments. Three core analytical runs were performed on three different days. In each core run, the lower limit of quantitation (LLOQ, n = 5), low quality control (LQC, n = 5), middle quality control (MQC, n = 5), and high-quality control samples (HQC, n = 5) were processed and run simultaneously with a standard curve. The range of the intra- and interday accuracy and precision for 3 core runs was 98.0-105% and 0.9-3.0% for 7 data points and 97.5-105% and 0.8-4.3% for 17 data points, respectively. No significant difference was observed for the different sampling intervals. The results show a sampling interval of 7 points for peaks up to 9 s wide is sufficient to define a peak accurately and precisely for drug quantitation studies in drug discovery and development.

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.

Ion-molecule reaction dynamics: Velocity map imaging studies of N(+) and O(+) with CD3OD

We present a study of the charge transfer reactions of the atomic ions N(+)and O(+) with methanol in the collision energy range from ∼2 to 4 eV. Charge transfer is driven primarily by energy resonance, although the widths of the product kinetic energy distributions suggest that significant interchange between relative translation and product vibration occurs. Charge transfer with CD3OD is more exoergic for N(+), and the nascent parent ion products appear to be formed in excited B̃ and C̃ electronic states, and fragment to CD2OD(+) by internal conversion and vibrational relaxation to the ground electronic state. The internal excitation imparted to the parent ion is sufficient to result in loss of one or two D atoms from the carbon atom. The less exoergic charge transfer reaction of O(+) forms nascent parent ions in the excited à state, and internal conversion to the ground state only results in ejection of single D atom. Selected isotopomers of methanol were employed to identify reaction products, demonstrating that deuterium atom loss from nascent parent ions occurs by C-D bond cleavage. Comparison of the kinetic energy distributions for charge transfer to form CD3OD(+) and CD2OD(+) by D atom loss with the known dynamics for hydride abstraction from a carbon atom provides strong evidence that the D loss products are formed by dissociative charge transfer rather than hydride (deuteride) transfer. Isotopic labeling also demonstrates that chemical reaction in the N(+) + CD3OD system to form NO(+) + CD4 does not occur in the energy range of these experiments, contrary to earlier speculation in the literature.

Velocity Map Imaging Study of Charge-Transfer and Proton-Transfer Reactions of CH3 Radicals with H3(.)

The velocity map imaging method has been applied to crossed beam studies of charge transfer and proton transfer between methyl (CH3) radicals formed by pyrolysis and H3(+) cations over the collision energy range from 1.2 to 3.4 eV. Vibrational excitation in the H3(+) reactants plays an important role both in promoting endoergic charge transfer and in supplying energy to the products of the proton-transfer reaction. Excited H3(+) reactants with vibrational energy in excess of the barrier lead to energy-resonant charge transfer via long-range collisions. A small fraction of collisions that take place at low impact parameters appear to form charge-transfer products with higher levels of internal excitation. The proton-transfer reaction exhibits direct, stripping-like dynamics. Consistent with the kinematics of proton transfer, incremental kinetic energy supplied to the reactants is strongly directed into product relative kinetic energy, as predicted by the concept of "induced repulsive energy release".

Collision-induced dissociation (CID) of peptides and proteins

The most commonly used activation method in the tandem mass spectrometry (MS) of peptides and proteins is energetic collisions with a neutral target gas. The overall process of collisional activation followed by fragmentation of the ion is commonly referred to as collision-induced dissociation (CID). The structural information that results from CID of a peptide or protein ion is highly dependent on the conditions used to effect CID. These include, for example, the relative translational energy of the ion and target, the nature of the target, the number of collisions that is likely to take place, and the observation window of the apparatus. This chapter summarizes the key experimental parameters in the CID of peptide and protein ions, as well as the conditions that tend to prevail in the most commonly employed tandem mass spectrometers.

Contributions of C3H6O+. ions with the oxygen on the middle carbon to gas phase ion chemistry

The numerous ways in which studies of C3H6O+. ions with the oxygen on the second carbon have added to our knowledge of gas phase ion chemistry are reviewed. The enol form of this ion (1) first attracted interest during early investigations of the mechanism of the McLafferty Rearrangement and later in characterizing the double McLafferty Rearrangement. Next, it was found that 1 isomerizes to the higher energy acetone ion (2). This discovery sparked studies of the relative stabilities of ionized and neutral enol and ketone species. It also led to the discovery that 1-->2 surmounts a substantial barrier, and that 2 dissociates faster than the excess energy deposited in it by the isomerization can become randomly distributed; i.e., dissociation following 1-->2 is nonergodic. This fact is manifested by a more abundant loss of the methyl formed by isomerization and a greater associated translational energy release. Propene oxide and methyl vinyl ether ions also appear to ionize to 2 and to dissociate nonergodically. Methane is eliminated from 2 through a methyl-acetyl ion complex. Characterization of this reaction by photoionization mass spectrometry, ab initio theory, and RRKM calculations helped to establish that complex-mediated alkane eliminations are generally confined to a region just above threshold. At higher energies, because attractions between ions and nonpolar neutrals are weak, simple dissociation is too fast for complex-mediated H-transfer to compete with it. Studies of the collision-induced dissociations of 2 demonstrate that the first electronically excited A state of 2 is very long-lived and efficiently releases its energy into translational energy when the ion collides with a neutral at low impact energies. Finally, ion-molecule reactions of 2 and acetone-containing ion clusters in the gas phase are described.

High- and low-energy collisionally activated decompositions of octaethylporphyrin and its metal complexes

High-energy (HE) and low-energy (LE) collisionally activated decompositions of octaethylporphyrin (OEP) and its metal complexes (ZnOEP and CuOEP) depend on whether the precursor is produced by electrospray ionization as protonated molecules or by fast atom bombardment as radical cations or protonated molecules. LE activation leads to such simple product-ion spectra that a complete picture of fragmentation emerges only after nine stages of tandem mass spectrometry (MS). HE activation, on the other hand, gives product-ion spectra that afford an integrated view of all the decomposition channels in a single MS/MS experiment. These results are the basis of a recommendation that OEP is an appropriate model compound for investigating energy effects in the collisional activation of organic and bioorganic molecule ions.

Mass spectrometric analyses of beta-ketolactone oligomers, macrocyclic or catenane structures?

Mass spectrometry is used to develop an analytical method for a new class of organic oligomeric material, beta-ketolactones. Three different series with varying oligomeric sizes are examined. The oligomers may form at least two different structures, a macrocyclic and a catenane ring. Fast atom bombardment coupled with Fourier transform mass spectrometry provides a rapid and convenient method that provides both molecular weight information and abundant fragment ions that are structurally relevant. All the compounds are found to work well with the mass spectrometric analysis. Electrospray ionization coupled with triple-quadrupole mass spectrometry was also evaluated and found to yield results that are similar to FAB. On the basis of the FAB and the low-energy collisionally activated dissociation spectra, we conclude that these compounds are macrocyclic rings and not catenanes.

High-energy collisional activation studied via angle-resolved translational energy spectra of survivor ions

Angle-resolved translational energy spectroscopy has been applied to Cs4I + (3) ions that survived 8 keV collisions with a range of collision gas targets, including inert gases and deuterium. The experimental data comprise values of the translational energy loss ΔTR as a function of the (laboratory-frame) scattering angle θ R for each collision gas under conditions such that single-collision events dominated the scattering. The values of ΔTR increase with θ R, in accordance with very general expectations. However for any value of θ R, the values of ΔTR for helium and deuterium as targets were almost indistinguishable from one another but were at least five to six times larger than those for neon and all other collision gases. These data have been shown to be consistent with theoretical considerations based upon conservation of energy and linear momentum. Theoretical approaches include the simple "elasticlimit" model, which makes no mechanistic assumptions, and a particular "binary-model" theory, which excludes electronic excitation as a possibility. Both theories are consistent with the experimental data and interpret the surprisingly large values of ΔTR for low-mass targets in terms of large recoil energies of the target required to ensure conservation of momentum. The most likely alternative candidate as sink for ΔTR is internal excitation of the target, but this possibility was excluded in the present work by choosing ΔTR values less than the lowest excitation energies of the inert gas targets. Moreover, such an interpretation cannot explain the similar results obtained using helium and deuterium, which were markedly different from those obtained for all other collision gases.

Principles of collisional activation in analytical mass spectrometry

Collisional activation has played an essential role in the development of mass spectrometry/mass spectrometry (MS/MS). It was the first activation method to be employed and continues to be by far the most widely used. As instrumentation for MS/MS has evolved it has been found that collisional activation can be effected under a remarkably wide range of conditions for a wide range of ions. It is fair to conclude from the growth of MS/MS over the past fifteen years that collisional activation has been spectacularly successful. However, it has limitations. As a community, we have learned much over the years regarding these limitations both from empirical and fundamental points of view. This overview provides background on the development of collisional activation and discusses the importance of the interaction potential and timing on mechanisms for energy transfer. Parts of the discussion is devoted to changing reference frames from the laboratory to the center of mass to simplify visualizing what is possible and what is probable in collisional activation.

Kinetics and mechanism of the collision-activated dissociation of the acetone cation

For center-of-mass collision energies Ecm = 1-60 eV, the major fragment ions for the collision-activated dissociation (CAD) of the acetone cation are the acetyl cation (m / z 43; absolute branching ratios of 0.96-0.60) and the methyl cation (m/ z 15; absolute branching ratios of 0.02-0.26); the absolute total cross-sections were 24-35) Å(2). The breakdown curves (viz, plots of the absolute branching ratios versus Ecm) show complex, complementary energy dependences for production of MeCO(+) and Me(+), indicating apparent closure of the Me(+) channel for Ecm > 30 eV. Our observations are consistent with a competition between three fast, primary (direct) reactions, each of which opens sequentially at its respective threshold energy (viz, reactions 8, 10, and 8'). [Formula: see text] [Formula: see text] [Formula: see text] That is, the breakdown curves for MeCO(+) and Me(+) (and other CAD fragments) are consistent with the interpretation by other authors that the collisional activation of the acetone cation involves electronic transitions, so that CAD occurs primarily from isolated electronic states (i.e., non-quasi-equilibrium theory (QET) behavior). For acetone we found a correspondence between the photoelectron-photoion-coincidence and CAD breakdown curves. This may indicate that collisional activation in non-QET systems corresponds to scattering angles that emphasize optically allowed transitions accessed by photoionization.