On The Time Scale of Internal Energy Relaxation of AP-MALDI and nano-ESI Ions in a Quadrupole Ion Trap

Recently reported results (Konn et al. [14]) on the collisional cooling of atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI) and nano-electrospray ionization (nano-ESI) generated ions in a quadrupole ion trap mass spectrometer (QITMS) are inconsistent with measured collisional cooling rates. The work reported here presents a re-examination of those previous results. Collision induced dissociation (CID) has been used to probe various properties of ions contained in a QITMS. It is shown experimentally that when trapping large numbers of ions, an effective dc trapping voltage is induced that varies with changes in the size of the ion cloud. A decrease in the resonant frequency for maximum CID efficiency is observed as the cool time between parent ion isolation and CID is increased. Ion trajectories in a QITMS are simulated to demonstrate how ion density changes over the course of parent ion isolation. The effect of space charge on ion motion is simulated, and Fourier transformations of ion axial motion plus simple calculations corroborate the experimentally observed transient frequency shifts. The relative stability of ions formed by AP-MALDI and nano-ESI is compared under low charge density conditions. These data show that the ions have reached equilibrium internal energy and, thus, that differences in dissociation onsets and “50% fragmentation efficiency points” between the ionization mechanisms are due to the formation of distinct ion conformations as previously shown in reference [28]. The conclusions of Konn et al. [14] are based on invalid experimental procedures as well as inappropriate comparisons of QITMS data to low-pressure FT-ICR data

High Amplitude Short Time Excitation: A Method to Form and Detect Low Mass Product Ions in a Quadrupole Ion Trap Mass Spectrometer

Collision induced dissociation (CID) in a quadrupole ion trap mass spectrometer using the conventional 30 ms activation time is compared with high amplitude short time excitation (HASTE) CID using 2 ms and 1 ms activation times. As a result of the shorter activation times, dissociation of the parent ions using the HASTE CID technique requires resonance excitation voltages greater than conventional CID. After activation, the rf trapping voltage is lowered to allow product ions below the low mass cut-off to be trapped. The HASTE CID spectra are notably different from those obtained using conventional CID and can include product ions below the low mass cut-off for the parent ions of interest. The MS/MS efficiencies of HASTE CID are not significantly different when compared with the conventional 30 ms CID. Similar results were obtained with a two-dimensional (linear) ion trap and a three-dimensional ion trap

Selective Ion Isolation/Rejection Over a Broad Mass Range in the Quadrupole Ion Trap

AbstractTechniques are presented for mass-selective ion manipulation over a wide mass range in a three-dimensional quadrupole. The methods use an auxiliary, low-amplitude radio-frequency signal applied to the endcap electrodes. This signal is either held at a single frequency as the fundamental radio-frequency trapping amplitude is ramped or swept over a frequency range while the fundamental radio-frequency trapping amplitude is held at a fixed level. Ion isolation and ejection are demonstrated for ions formed within the ion trap using electron ionization and for ions injected into the ion trap formed either by an air-sustained glow discharge or by electrospray. Mass-selective ion ejection is used to reduce matrix-ion-induced space charge during ion injection, thereby producing signal enhancement for the detection of 2,4,6-trinitrotoluene in air. Mass-selective isolation of ions with mass-to-charge ratios above the normal operating range (m/z 650) for the ion trap is also demonstrated after injection of myoglobin ions formed via electrospray

Optimization of Peptide Separations by Differential Ion Mobility Spectrometry

Differential ion mobility spectrometry (DIMS) has the ability to separate gas phase ions based on their difference in ion mobility in low and high electric fields. DIMS can be used to separate mixtures of isobaric and isomeric species indistinguishable by mass spectrometry (MS). DIMS can also be used as a filter to improve the signal-to-background of analytes in complex samples. The resolving power of DIMS separations can be improved several ways, including increasing the dispersion field and increasing the amount of helium in the nitrogen carrier gas. It has been previously demonstrated that the addition of helium to the DIMS carrier gas provides improves separations when the dispersion field is the kept constant as helium content is varied. However, helium has a lower breakdown voltage than nitrogen. Therefore, as the percent helium content in the nitrogen carrier gas is increased, the highest dispersion field accessible decreases. This work presents the trade-offs between increasing dispersion fields and using helium in the carrier gas by comparing the separation of a mixture of isobaric peptides. The maximum resolution for a separation of a mixture of three peptides with the same nominal molar mass was achieved by using a high dispersion field (~72 kV/cm) with pure nitrogen as the carrier gas within the DIMS assembly. The conditions used to achieve the maximum resolution also exhibit the lowest ion transmission through the assembly, suggesting that it is necessary to consider the trade-off between sensitivity and resolution when optimizing DIMS conditions for a given application

Simultaneous Collision Induced Dissociation of the Charge Reduced Parent Ion during Electron Capture Dissociation

A method of performing collision induced dissociation (CID) on the charge-reduced parent ion as it is formed during electron capture dissociation (ECD), called ECD+CID, is described. In ECD+CID, the charge-reduced parent ion is selectively activated using resonant excitation and collisions with the helium bath gas inside a linear quadrupole ion trap ECD device (ECDLIT). It has been observed that ECD+CID can improve the sequence coverage for melittin over performing ECD alone (i.e., from 76 % to 88 %). Perhaps just as important, ECD+CID can be used to reduce the extent of multiple electron capture events observed when performing ECD in the ECDLIT. Consequently, the abundance of mass-to-charge ratios corresponding to ECD product ions that contain neutralized protons is decreased, simplifying the interpretation of the product ion spectrum

Iterative Accumulation Multiplexing Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

A multiplexed tandem mass spectrometry (MS/MS) technique known as iterative accumulation multiplexing (IAM) has been implemented on a hybrid quadrupole Fourier transform ion cyclotron resonance mass spectrometer (Q-FTICR-MS). The IAM experiment resulted in obtaining MS/MS spectra for six analytes in two MS/MS experiments while characteristic resolving power and mass measurement accuracies were maintained. Parent-product ion correlations were graphically represented in a “ratiogram” where each product ion is encoded with a ratio unique to the parent ion from which it was formed. This is the first example of multiplexed MS on a FTICR instrument where the ions are encoded externally to the ICR cell. By performing the encoding external to the ICR cell, one set of ions can be encoded while the previous set of ions is being analyzed in the cell, maximizing the use of the continuous ion current emanating from the electrospray ionization source

Pulsed Nano-Electrospray Ionization: Characterization of Temporal Response and Implementation with a Flared Inlet Capillary

The temporal response of pulsed nano-electrospray ionization mass spectrometry (nano-ESI-MS) was studied and its influence on ion formation and detection was characterized. Rise and decay times for the mass resolved ion current were determined to be 20 ± 3 msec and 61 ± 5 msec, respectively, which led to a maximum pulse rate of 12 Hz. Pulsed nano-ESI operation was demonstrated from a multi-sprayer source controlled by a high voltage pulsing circuit constructed in-house. The desired source mode of operation (e.g. pulsing or continuous) can be realized solely by controlling the voltage applied to each sprayer

Cation Recombination Energy/Coulomb Repulsion Effects in ETD/ECD as Revealed by Variation of Charge per Residue at Fixed Total Charge

Electron capture dissociation (ECD) and electron transfer dissociation (ETD) experiments in electrodynamic ion traps operated in the presence of a bath gas in the 1-10 mTorr range have been conducted on a common set of doubly protonated model peptides of the form X(AG)nX (X = lysine, arginine, or histidine, n = 1, 2, or 4). The partitioning of reaction products was measured using thermal electrons, anions of azobenzene, and anions of 1,3-dinitrobenzene as reagents. Variation of n alters the charge per residue of the peptide cation, which affects recombination energy. The ECD experiments showed that H-atom loss is greatest for the n = 1 peptides and decreases as n increases. Proton transfer in ETD, on the other hand, is expected to increase as charge per residue decreases (i.e., as n increases). These opposing tendencies were apparent in the data for the K(AG)nK peptides. H-atom loss appeared to be more prevalent in ECD than in ETD and is rationalized on the basis of either internal energy differences, differences in angular momentum transfer associated with the electron capture versus electron transfer processes, or a combination of the two. The histidine peptides showed the greatest extent of charge reduction without dissociation, the arginine peptides showed the greatest extent of side-chain cleavages, and the lysine peptides generally showed the greatest extent of partitioning into the c/z•-product ion channels. The fragmentation patterns for the complementary c- and z•-ions for ETD and ECD were found to be remarkably similar, particularly for the peptides with X = lysine

Little Cigars are More Toxic than Cigarettes and Uniquely Change the Airway Gene and Protein Expression

Little cigars (LCs) are regulated differently than cigarettes, allowing them to be potentially targeted at youth/young adults. We exposed human bronchial epithelial cultures (HBECs) to air or whole tobacco smoke from cigarettes vs. LCs. Chronic smoke exposure increased the number of dead cells, lactate dehydrogenase release, and interleukin-8 (IL-8) secretion and decreased apical cilia, cystic fibrosis transmembrane conductance regulator (CFTR) protein levels, and transepithelial resistance. These adverse effects were significantly greater in LC-exposed HBECs than cigarette exposed cultures. LC-exposure also elicited unique gene expression changes and altered the proteomic profiles of airway apical secretions compared to cigarette-exposed HBECs. Gas chromatography-mass spectrometry (GC-MS) analysis indicated that LCs produced more chemicals than cigarettes, suggesting that the increased chemical load of LCs may be the cause of the greater toxicity. This is the first study of the biological effects of LCs on pulmonary epithelia and our observations strongly suggest that LCs pose a more severe danger to human health than cigarettes

Quantification of Human Uridine-Diphosphate Glucuronosyl Transferase (UGT) 1A Isoforms in Liver, Intestine and Kidney using nanoLC-MS/MS

Uridine-disphosphate glucuronosyl transferase (UGT) enzymes catalyze the formation of glucuronide conjugates of Phase II metabolism. Methods for absolute quantification of UGT1A1 and UGT1A6 were previously established utilizing stable isotope peptide internal standards with LC-MS/MS. The current method expands upon this by quantifying eight UGT1A isoforms by nanobore HPLC coupled with a linear ion trap-time of flight mass spectrometer platform. Recombinant enzyme digests of each of the isoforms were used to determine assay linearity and detection limits. Enzyme expression level in human liver, kidney and intestinal microsomal protein was determined by extrapolation from spiked stable isotope standards. Intraday and Interday variability wa