Effects of the LC mobile phase in vacuum differential mobility spectrometry-mass spectrometry for the selective analysis of antidepressant drugs in human plasma.

The effect of LC mobile phase composition and flow rate (2-50 µL/min) on mobility behavior in vacuum differential mobility spectrometry (vDMS) was investigated for electrosprayed isobaric antidepressant drugs (AD); amitriptyline, maprotiline, venlafaxine; and structurally related antidepressants nortriptyline, imipramine, and desipramine. While at 2 µL/min, no difference in compensation voltage was observed with methanol and acetonitrile, at 50 µL/min, acetonitrile used for LC elution of analytes enabled the selectivity of the mobility separation to be improved. An accurate and sensitive method could be developed for the quantification of six AD drugs in human plasma using trap/elute micro-LC setup hyphenated to vDMS with mass spectrometric detection in the selected ion monitoring mode. The assay was found to be linear over three orders of magnitude, and the limit of quantification was of 25 ng/mL for all analytes. The LC-vDMS-SIM/MS method was compared to a LC-MRM/MS method, and in both cases, inter-assay precisions were lower than 12.5 and accuracies were in the range 91.5-110%, but with a four times reduced analysis time (2 min) for the LC-vDMS-SIM/MS method. This work illustrates that with vDMS, the LC mobile phase composition can be used to tune the ion mobility separation and to improve assay selectivity without additional hardware.

Ion Mobility Spectrometry of Superheated Macromolecules at Electric Fields up to 500 Td.

Since its inception in 1980s, differential or field asymmetric waveform ion mobility spectrometry (FAIMS) has been implemented at or near ambient gas pressure. We recently developed FAIMS at 15-30 Torr with mass spectrometry and utilized it to analyze amino acids, isomeric peptides, and protein conformers. The separations broadly mirrored those at atmospheric pressure, save for larger proteins that (as predicted) exhibited dipole alignment at ambient but not low pressure. Here we reduce the pressure down to 4.7 Torr, allowing normalized electric fields up to 543 Td-double the maximum in prior FAIMS or IMS studies of polyatomic ions. Despite the collisional heating to ∼1000 °C at the waveform peaks, the proteins of size from ubiquitin to albumin survived intact. The dissociation of macromolecules in FAIMS appears governed by the average ion temperature over the waveform cycle, unlike the isomerization controlled by the peak temperature. The global separation trends in this "superhot" regime extend those at moderately low pressures, with distinct conformers and no alignment as theorized. Although the scaling of the compensation voltage with the field fell below cubic at lower fields, the resolving power increased and the resolution of different proteins or charge states substantially improved.

Ion Mobility Spectrometry of Macromolecules with Dipole Alignment Switchable by Varying the Gas Pressure.

Since inception in the 1980s, differential or field asymmetric waveform ion mobility spectrometry (FAIMS) was implemented at or near the ambient gas pressure (AP). Recently, we developed FAIMS at 15-30 Torr within a mass spectrometer and demonstrated it for small and medium sized ions, including peptides. The overall separation properties mirrored those at AP, reflecting the shared underlying physics. Here we extend these analyses to macromolecules, namely, multiply charged proteins generated by electrospray ionization. The spectra for smaller proteins (ubiquitin, cytochrome c, myoglobin) again resemble those at AP, producing features for one or a few adjacent well-defined conformers with type C behavior. Large proteins (single aldolase domain and albumin) now follow, with no broad bands for type A or B species that dominated at 1 atm. Those unique behaviors were ascribed to pendular ions with electric dipoles reversibly locked by the strong field in FAIMS. Disappearance of those bands shows loss of alignment predicted by first-principles theory, further supporting dipole locking at AP. The capability to modulate dipole alignment by varying gas pressure at constant normalized field provides the basis for determining the ion dipole moment and direction within the molecular frame from the pressure of onset and characteristics of spectral drift. This new approach to alter FAIMS separations of proteins could make a powerful tool for structural biology and be useful for proteomics and imaging.

Differential Ion Mobility Separations in the Low-Pressure Regime.

Ion mobility spectrometry (IMS) in conjunction with mass spectrometry (MS) has emerged as a powerful platform for biological and environmental analyses. An inherent advantage of differential or field asymmetric waveform IMS (FAIMS) based on the derivative of mobility vs electric field over linear IMS based on absolute mobility is much greater orthogonality to MS. Effective coupling of linear IMS to MS and diverse IMS/MS arrangements and modalities impossible at ambient buffer gas pressure were enabled at much reduced pressures. In contrast, FAIMS devices operate at or near atmospheric pressure, which complicated integration with MS. Here, we show FAIMS at ∼15-30 Torr using a planar-gap stage within the MS instrument envelope. Fields up to ∼300 Td permitted by the Paschen law at these pressures greatly raise the separation speed, providing fair resolution in ∼10 ms and FAIMS scans in under 5 s. Rapid separation and efficient ion collection at low pressure minimize losses in the FAIMS step. Separations for key analyte classes and their dependences on electric field mirror those at ambient pressure. The potential for proteomics is demonstrated by separations of isomeric peptides with variant localization of post-translational modifications.

A case of recurrent spontaneous pneumothorax during pregnancy treated with video assisted thoracoscopic surgery.

Pneumothorax during pregnancy is uncommon. Recently ambulatory chest drainage has been advised to treat the pneumothorax and to cover the delivery period. This imposes restrictions on the mother with associated co-morbidity. The authors present a case of recurrent chest-tube resistant pneumothorax during pregnancy which had persisted for 4-weeks. To guide management of a patient referred in the third trimester of pregnancy the authors undertook a systematic review. This led to definitive video assisted thoracoscopic surgery (VATS) for bullectomy and pleurodesis which was successful without either peri-operative or peri-partum complications or recurrence of pneumothorax. Our review suggests that a VATS approach during pregnancy is both safe and effective.

A digital ion trap mass spectrometer coupled with atmospheric pressure ion sources.

In a digital ion trap (DIT), the quadrupole trapping and excitation waveforms are generated by the rapid switching between discrete d.c. voltage levels. As the timing of the switch can be controlled precisely by digital circuitry, the approach provides an opportunity to generate mass spectra by means of a frequency scan in contrast to the conventional voltage scan, thus providing a wider mass range of analysis. An instrument has been constructed which employs a 'non-stretched' ion trap and the field fault around the aperture of the end-cap electrode can be corrected electronically using a field-adjusting electrode. The ion trap was coupled with electrospray ionization (ESI) and atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) sources to demonstrate the capability of the digital method. AP-MALDI mass spectra of singly charged ions with mass-to-charge ratios upto 17 000 Th were generated using a trapping voltage of only 1000 V. Forward and reverse mass scans at resolutions up to 19 000 and precursor ion isolation at resolutions up to 3500 with subsequent tandem mass spectrometric analysis were demonstrated. The method of generating the digital waveforms and period scan is described. Discussion of the issues of mass range, scan speed, ion trapping efficiency and collision-induced dissociation efficiency are also provided.