Nitrogen Buffer Gas Research Articles

Large-Scale Prediction of Collision Cross-Section Values for Metabolites in Ion Mobility-Mass Spectrometry.

The rapid development of metabolomics has significantly advanced health and disease related research. However, metabolite identification remains a major analytical challenge for untargeted metabolomics. While the use of collision cross-section (CCS) values obtained in ion mobility-mass spectrometry (IM-MS) effectively increases identification confidence of metabolites, it is restricted by the limited number of available CCS values for metabolites. Here, we demonstrated the use of a machine-learning algorithm called support vector regression (SVR) to develop a prediction method that utilized 14 common molecular descriptors to predict CCS values for metabolites. In this work, we first experimentally measured CCS values (ΩN2) of ∼400 metabolites in nitrogen buffer gas and used these values as training data to optimize the prediction method. The high prediction precision of this method was externally validated using an independent set of metabolites with a median relative error (MRE) of ∼3%, better than conventional theoretical calculation. Using the SVR based prediction method, a large-scale predicted CCS database was generated for 35 203 metabolites in the Human Metabolome Database (HMDB). For each metabolite, five different ion adducts in positive and negative modes were predicted, accounting for 176 015 CCS values in total. Finally, improved metabolite identification accuracy was demonstrated using real biological samples. Conclusively, our results proved that the SVR based prediction method can accurately predict nitrogen CCS values (ΩN2) of metabolites from molecular descriptors and effectively improve identification accuracy and efficiency in untargeted metabolomics. The predicted CCS database, namely, MetCCS, is freely available on the Internet.

Observation of the C2H radical using (1 + 2) REMPI via the [formula omitted] transition

We report the detection of the ethynyl (C2H) radical by (1+2) REMPI on the B̃2A′(T,Ka=0,1,2)←X̃2Σ+(0,31,0) transition. One parallel, B̃2A′(T,Ka=1)←X̃2Σ+(0,31,0) with EeV=37,946cm-1, and two perpendicular transitions, B̃2A′(T,Ka=0)←X̃2Σ+(0,31,0) with EeV=37,927cm-1 and B̃2A′(T,Ka=2)←X̃2Σ+(0,31,0) with EeV=38,002cm-1, are observed. The C2H radical was generated by the photodissociation of C2H2 at 193nm. By employing REMPI spectroscopy, we also studied the collisional induced rotational relaxation of C2H and measured rotational relaxation rate constants for this radical. The measured rotational relaxation rate constants for several values of J″ are (0.08–0.26)×10-9cm3molec-1s-1 and (0.75–1.92)×10-9cm3molec-1s-1 for nitrogen buffer gas and acetylene, respectively.

Observing random walks of atoms in buffer gas through resonant light absorption

Using resonant light absorption, random walk motions of rubidium atoms in nitrogen buffer gas are observed directly. The transmitted light intensity through atomic vapor is measured and its spectrum is obtained, down to orders of magnitude below the shot noise level to detect fluctuations caused by atomic motions. To understand the measured spectra, the spectrum for atoms performing random walks in a gaussian light beam is computed and its analytical form is obtained. The spectrum has $1/f^2$ ($f$: frequency) behavior at higher frequencies, crossing over to a different, but well defined behavior at lower frequencies. The properties of this theoretical spectrum agree excellently with the measured spectrum. This understanding also enables us to obtain the diffusion constant, the photon cross section of atoms in buffer gas and the atomic number density, from a single spectral measurement. We further discuss other possible applications of our experimental method and analysis.

Molecular Structures and Ion Mobility Cross Sections: Analysis of the Effects of He and N2 Buffer Gas.

An empirically observed correlation between ion mobility cross sections in helium and nitrogen buffer gases was examined as a function of temperature, molecular size, and shape. Experimental cross sections were determined for tetraglycine, bradykinin, angiotensin 2, melittin, and ubiquitin at 300 K and in the range from 80 to 550 K on home-built instruments and calculated by the projection superposition approximation (PSA) method. The PSA was also used to predict cross sections for larger systems such as human pancreatic alpha-amylase, concanavalin, Pichia pastoris lysyl oxidase, and Klebsiella pneumoniae acetolactate synthase. The data show that the ratio of cross sections in helium and nitrogen depends significantly on the temperature of the buffer gas as well as the size and shape of the analyte ion. Therefore, the analysis of the data indicates that a simple formula that seeks to quantitatively relate the momentum transfer cross sections observed in two distinct buffer gases lacks a sound physical basis.

Study of electrospray ion mobility dependence on the ion bunch drift velocity in the radio frequency quadrupole

The objective of this work is the study of the ion mobility dependence of electrospray Gramicidin S ions [M + 2H]2+ on the ion bunch drift velocity under variation of the specific electrical field strength at certain gas number density in the range from 0 to 300 Td. The ion mobility measurements were carried out under rotational excitation of ions using custom built gas filled segmented radio frequency quadrupole with the nitrogen buffer gas pressure of 1.9 Pa. The resulting ion mobility dependence of [M + 2H]2+ Gramicidin S ions upon the bunch drift velocity is essentially weaker, than it could be expected from the hard sphere collision model. By comparing the acquired ion mobility dependencies with the theoretical ones derived from the hard sphere collision model we suppose that the attractive polarization potential takes an essential part into the ion-molecular interaction even for such big organic ions as Gramicidin S.

Nanoclusters formation in ion mobility spectrometry and change separation selectivity of picoline isomers

Ion mobility spectrometry (IMS) was applied to determine the influence of structural features of nanocluster formation of picoline isomers in ion mobility spectrometry. Since the results of our studies show that different isomers have the same mobilities in pure nitrogen buffer gas and their corresponding peaks are totally overlapped, 2-butanol vapor was introduced into buffer gas by means of an online system from 0 to 300 mL min−1. We found different structural features of these isomeric compounds which cause distinct differences in ion mobility spectra. These differences result from the formation of different nanocluster product ions (~1 nm3) with different cross section areas formed depending on the occurrence of certain structural features (position of the methyl group on the pyridine ring). The size of cluster product ions formed was determined using cross section area measurements. The effects of temperature in the range from 80 to 200 °C and electric field strength have also been investigated. At 140–160 °C and 636 V cm−1, optimum peak-to-peak resolution can be obtained.

Effect of buffer gases on the performance of SO2 trace measurement based on photoacoustic spectroscopy

In this experimental work a laser photoacoustic spectrometer designed and fabricated. System sensitivity for detection of SO2 and NO2 was measured. Resonance frequency variation versus pressure increase of Nitrogen, Argon, Helium and Air buffer gases was studied. Results show that, sensitivity of system for SO2 and NO2 are 353ppb and 963ppb respectively. It was shown that resonance frequency for Nitrogen, Argon, and Air buffer gases was not noticeably varied by buffer gas pressure increasing, but for Helium, resonance frequency not only is not in range of three other gases, but also grows by pressure increasing. The system noises were damped preparing two buffer chambers.

Intrinsic relaxation rates of polarized Cs vapor in miniaturized cells

The intrinsic relaxation rates of the vector magnetization of cesium vapor enclosed in microfabricated atomic magnetometer cells are investigated. Two methods—the optically detected magnetic resonance and the ground-state Hanle effect—are used to carry out automated measurements in dependence on cell temperature and nitrogen buffer gas pressure. The experimental results are compared with expected contributions of the different relaxation processes and in this way allow the discrimination between them to help further optimization of cell design. The methods are compared in terms of basic features, data quality, and practical applicability.

Electron attachment to oxygen in nitrogen buffer gas at atmospheric pressure

We have carried out experimental and theoretical studies of three body electron attachment (TBEA) to O2 in N2/O2 mixtures. We have applied three different experimental methods to determine the apparent rate constant k for TBEA to O2 for reduced electric fields E/ n from 0.5 Td up to 4.5 Td and O2 concentrations from 0.02% up to 3%. From the apparent rate constant k we have evaluated three body rate constant for electron attachment to O2 in pure O2 $$\left( {k_{O_2 } } \right)$$ and in pure N2 $$\left( {k_{N_2 } } \right)$$ . The comparison of present data with former studies shows that the former values of $$k_{N_2 }$$ overestimated the efficiency of this reaction, while in case of $$k_{O_2 }$$ we have found agreement with earlier studies. We have solved numerically the Boltzmann equation of the electrons and calculated the values of k, $$k_{N_2 }$$ and $$k_{O_2 }$$ using well established cross sections. Using the known collision cross section set for TBEA to O2, very good agreement between calculated and measured results for $$k_{O_2 }$$ was found, while in the case of k and $$k_{N_2 }$$ we had to introduce a scaling function, which describes the decrease of the efficiency of TBEA to O2 in presence of N2 and the dependence of the scaling function on E/n was determined.

Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping

Spin-exchange optical pumping (SEOP) has been widely used to produce enhancements in nuclear spin polarisation for hyperpolarised noble gases. However, some key fundamental physical processes underlying SEOP remain poorly understood, particularly in regards to how pump laser energy absorbed during SEOP is thermalised, distributed and dissipated. This study uses in situ ultra-low frequency Raman spectroscopy to probe rotational temperatures of nitrogen buffer gas during optical pumping under conditions of high resonant laser flux and binary Xe/N2 gas mixtures. We compare two methods of collecting the Raman scattering signal from the SEOP cell: a conventional orthogonal arrangement combining intrinsic spatial filtering with the utilisation of the internal baffles of the Raman spectrometer, eliminating probe laser light and Rayleigh scattering, versus a new in-line modular design that uses ultra-narrowband notch filters to remove such unwanted contributions. We report a ~23-fold improvement in detection sensitivity using the in-line module, which leads to faster data acquisition and more accurate real-time monitoring of energy transport processes during optical pumping. The utility of this approach is demonstrated via measurements of the local internal gas temperature (which can greatly exceed the externally measured temperature) as a function of incident laser power and position within the cell.

芯片级原子器件MEMS碱金属蒸气腔室制作

This paper reported on the microfabrication of alkali metal vapor cells based on the two-step low temperature anodic bonding for the chip-scale integration of atomic clock,atomic magnetometer,atomic gyroscope and other atomic devices.Cell structures were fabricated by Micro-electromechanical System(MEMS) bulk silicon process,and the etched silicon with cells was firstly bonded to Pyrex glass to fabricate preformed chambers by the standard anodic bonding process.Then,nitrogen buffer gas and micro-scale alkali metal(rubidium or cesium) were introduced into the preformed cells.The two-step anodic bonding process was used to seal the cells at a temperature lower than the paraffin flash point(198 ℃).In the first step,bonding voltage was lower than the breakdown voltage of nitrogen buffer gas to pre-seal the cells.In the second step,the bonding was in air atmosphere,and the bonding voltage increased up to 1 200 V to strengthen packaging quality.A high power laser system locally heated the micro-scale alkali metal packets to release alkali metal,and a uniform coating of paraffin was formed on cell walls to prolong the life of the polarized atoms.With proposed method,95% bonding is achieved by the two-step low temperature anodic bonding,and the alkali rubidium still has a metallic luster after anodic packaging.The achieved minimum volume of double-cells is about 6.5 mm×4.5 mm×2mm.Rubidium absorption spectrum shows that alkali rubidium is effectively encapsulated in the cells.It is feasible to fabricate MEMS alkali metal vapor cells by the two-step low temperature anodic bonding process.

Pressure and temperature dependent photolysis of glyoxal in the 355–414 nm region: evidence for dissociation from multiple states

The photolysis of glyoxal has been investigated in the 355-414 nm region by dye laser photolysis coupled with cavity ring-down spectroscopy. Absolute quantum yields of HCO, ΦHCO, were determined using the reaction of chlorine atoms with formaldehyde as an actinometer. The dependence of the quantum yield on pressure was investigated in 3-400 Torr of nitrogen buffer gas and at four temperatures: 233 K, 268 K, 298 K and 323 K. For 355 nm ≤ λ < 395 nm the HCO quantum yield is pressure dependent with linear Stern-Volmer (SV) plots (1/ΦHCO vs. pressure). The zero pressure quantum yield, obtained by extrapolation of the SV plots, rises from 1.6 to 2 between 355 and 382 nm and remains at 2 up to 395 nm. For λ ≥ 395 nm ΦHCO shows a stronger pressure dependence and non-linear SV plots, compatible with formation of HCO by dissociation from two electronic states of glyoxal with significantly different lifetimes. These observations are used to develop a mechanism for the photolysis of glyoxal over the wavelength range studied.

Real time pharmaceutical reaction monitoring by electrospray ion mobility-mass spectrometry

Reaction monitoring is a critical step in the synthesis of new drug entities, and many pharmaceutical companies spend significant resources on equipment and new methods to perform this task. The potential of electrospray ion mobility-mass spectrometry (ESI-IMMS) as a method for monitoring pharmaceutical reactions in real time is demonstrated with a reductive amination reaction that is commonly used in the synthesis of amines for drug discovery but poses problems for detection with current analytical instrumentation. Data were collected on the timescale of 300s and reaction completion time was determined by monitoring the starting, intermediate and product materials during the reaction. The detection limits for nicotinaldehyde (starting material 1), 4-picolylamine (starting material 2), and di-(2-picolyl)amine (the product) were 0.32, 0.10 and 0.05μM, respectively, with a linear dynamic range of more than 2 orders of magnitude for each analyte. Starting materials were not separated by their mobility in a nitrogen buffer gas, but could be separated in a buffer gas of carbon dioxide. In the nitrogen buffer gas, the reduced mobility (Ko) values for the reaction starting materials (nicotinaldehyde, 4-picolylamine, 4-pyridinementhanol) were 1.86cm2/Vs and the reduced mobility value for the product (di-(2-picolyl)amine) was 1.69cm2/Vs. In carbon dioxide the Ko values for the reaction starting materials were 1.43cm2/Vs and 1.38cm2/Vs for nicotinaldehyde and 4-picolylamine, respectively. The coupling of IMS to MS enabled the discovery of a hemiacetal complex forming between nicotinaldehyde and methanol, limiting the availability of nicotinaldehyde for the reaction. Thus use of IMS in conjunction with MS provided information about the reaction mechanism that would not have been possible with MS alone. The complexation prevented the formation of the intermediate species, presumably due to the formation of the hemiacetal from SM1; however, the intermediate species could be formed when the nicotinaldehyde concentration was increased by 10-fold. While either IMS or MS alone could be used to monitor this and similar reactions, the combined ESI-IMMS approach offered more complete information about the reaction, demonstrating potential as a rapid and selective technique to aid pharmaceutical process control.

On the Road from Formamide Ices to Nucleobases: IR-Spectroscopic Observation of a Direct Reaction between Cyano Radicals and Formamide in a High-Energy Impact Event

The formamide-based synthesis of nucleic acids is considered as a nonaqueous scenario for the emergence of biomolecules from inorganic matter. In the current study, we scrutinized the chemical composition of formamide ices mixed with an FeNi meteorite material treated with laser-induced dielectric breakdown plasma created in nitrogen buffer gas. These experiments aimed to capture the first steps of those chemical transformations that may lead to the formation of nucleobases during the impact of an extraterrestrial icy body containing formamide on an early Earth atmosphere. High-resolution FT-IR spectroscopy combined with quantum chemical calculations was used to analyze the volatile fraction of the products formed during such an event. We have found that the spectrum of the evaporated formamide ices is dominated by the spectral signatures of the dimeric form of formamide. Upon exposure to laser sparks, new well-defined bands appear in the spectrum centered at ~820, ~995, and ~1650 cm(-1). On the basis of quantum chemical calculations, these bands can be assigned to the absorptions of 2-amino-2-hydroxy-acetonitrile and to 2-amino-2-hydroxy-malononitrile, which are formed in a direct reaction between formamide and CN(•) radicals upon the high-energy impact event. We also show that there is an exergonic reaction route via these intermediates leading to diaminomaleonitrile, which is generally considered to play a key role in the synthesis of nucleobases.

Two-photon laser excitation of trapped232Th+ions via the 402-nm resonance line

Experiments on one- and two-photon laser excitation of ${}^{232}$Th${}^{+}$ ions in a radio-frequency ion trap are reported. As the first excitation step, the strongest resonance line at 402 nm from the $(6{d}^{2}7s)J=3/2$ ground state to the $(6d7s7p)J=5/2$ state at 24874 cm${}^{\ensuremath{-}1}$ is driven by radiation from an extended-cavity diode laser. Spontaneous decay of the intermediate state populates a number of low-lying metastable states, thus limiting the excited state population and fluorescence signal obtainable with continuous laser excitation. We study the collisional quenching efficiency of helium, argon, and nitrogen buffer gases, and the effect of repumping laser excitation from the three lowest-lying metastable levels. The experimental results are compared with a four-level rate equation model, which allows us to deduce quenching rates for these buffer gases. Using laser radiation at 399 nm for the second step, we demonstrate two-photon excitation to the state at 49 960 cm${}^{\ensuremath{-}1}$, among the highest-lying classified levels of Th${}^{+}$. This is of interest as a test case for the search for higher-lying levels in the range above 55 000 cm${}^{\ensuremath{-}1}$ which can resonantly enhance the excitation of the ${}^{229}$Th${}^{+}$ nuclear resonance through an inverse two-photon electronic bridge process.

Considerations in experimental and theoretical collision cross-section measurements of small molecules using travelling wave ion mobility spectrometry-mass spectrometry

Travelling wave ion mobility spectrometry-mass spectrometry (TWIMS-MS) has the capability to separate ions based on their mobility through a gas-filled travelling wave (T-wave) device in the presence of a train of transient voltage pulses. By calibration of this device using analytes of previously determined cross-sectional area (from conventional IMS experiments), collision cross-sections of ions can be determined based on their drift time through the T-wave device. Comparison of experimentally determined cross-sections with theoretical calculations from structural models has the potential to provide methodology which can be applied to analytes of previously uncharacterised structure; however, this comparison relies on a high degree of confidence in both experimental and theoretical methods. Focussing on small (≤200 Da) molecules, collision cross-sections have been measured by TWIMS-MS employing a calibration procedure that uses both oligo-glycine peptides and human haemoglobin-derived tryptic peptides in order to extend the calibration range for the measurement of high mobility ions. The effect of TWIMS wave height parameters on the calibration is addressed. Theoretical TWIMS cross-section calculations have been performed using a rapid, Windows-based, in-house developed projection approximation algorithm. These estimates were optimised by comparison with experimental values for a series of small molecules with rigid core structures by systematic variation of the interaction radii of the atoms comprising these species until theoretical measurements were in agreement with experimentally derived TWIMS cross-sections. The effect of varying the interaction radius for the buffer gas was subsequently studied by comparison of theoretical collision cross-sections calculated using helium and nitrogen radii with TWIMS-MS cross-section measurements determined in either helium or nitrogen buffer gases. It was found that the buffer gas used in theoretical calculations should ideally match the buffer gas in which the cross-sections of the calibrants were originally determined by conventional IMS. The resolving potential of the experimental methodology was demonstrated by separation of two isomeric amino acids, leucine and isoleucine, which showed <3% difference in cross-sectional areas. Furthermore, the application of both experimental and theoretical methods to compare the gas phase and solution conformations of seven amino acids was demonstrated. This study describes important considerations for the comparison of TWIMS-MS collision cross-sections with those obtained by theoretical methods, and also demonstrates that TWIMS-MS could be applied specifically to study a range of small molecules including those of pharmaceutical interest, in particular structural isomers and diastereoisomers that may not be distinguishable by mass spectrometry alone.

Standoff detection of nitrotoluenes using 213-nm amplified spontaneous emission from nitric oxide

A method of standoff detection based on the observation of laser-induced fluorescence-amplified spontaneous emission (LIF-ASE) is described. LIF-ASE generates uniaxial intensity distributions of the observed fluorescence with the majority of intensity propagating along the excitation axis in both the forward and backward directions. The detection of bulk vapor at significant standoff distances is readily achieved. This method was used to detect NO directly and as a photoproduct after 213-nm excitation of 2-, 3-, and 4-nitrotoluene. The NO LIF-ASE spectra were studied as a function of buffer gas. These studies showed that the emission from different vibrational states was dependent upon the buffer gas used, suggesting that the populations of vibrational states were influenced by the environment. A similar sensitivity of the vibrational populations was observed when the different nitroaromatic precursors were used in nitrogen buffer gas. Such sensitivity to environmental influences can be used to distinguish among the different nitroaromatic precursors and facilitate the identification of the bulk vapor of these analytes.

Novel resonance profiling and tracking method for photoacoustic measurements

Resonant photoacoustic (PA) detection is widely used in several atmospheric and industrial monitoring applications due to its high sensitivity and short response time. However, unexpected changes in the acoustic resonance frequency of a PA cell caused by sudden changes either in the composition or the temperature of the sample gas can largely diminish the precision of the PA measurement. This paper describes a novel method for tracking such changes in resonance frequency. Besides improving the measurement precision, the introduced CHIrped modulation for Resonance Profiling (CHIRP) method has the additional advantage of maintaining the fast response time of the PA system without using any additional hardware components. The minimum detectable water vapor concentration, depending on the modulation bandwidth of the CHIRP, was found to be 0.3–0.5 ppm in nitrogen buffer gas. The applicability of the CHIRP method was demonstrated in PA measurements in a buffer gas with varying composition, which are typical in, e.g., industrial monitoring applications.

High-resolution infrared polarization spectroscopy and degenerate four wave mixing spectroscopy of methane

High-resolution infrared polarization spectroscopy (IR-PS) and degenerate four wave mixing (IR-DFWM) spectroscopy of methane using a diode-seeded modeless laser (DSML) system are reported. Mid-infrared radiation around 3.3 μm is generated by difference frequency mixing of the single-mode output of the DSML around 0.634 μm with the frequency-doubled output of a single-mode Nd:YAG pump laser at 0.532 μm. Polarization spectroscopy signals in the forward geometry were generated in methane at around 5 Torr pressure. IR-PS spectra were recorded with a typical signal-to-noise ratio of 150:1 with methane pressures of at least 1 Torr. The line shape of the IR-PS signals was analysed to measure pressure broadening induced by nitrogen buffer gas yielding a value of 6.3±1.5 MHz Torr−1. IR-DFWM spectra of methane were generated in the counter-propagating pump geometry yielding Doppler-free signals with signal-to-noise ratios of typically 650:1. Signals were obtained at methane pressures down to less than 10 mTorr. A comparison of IR-PS and IR-DFWM is made indicating that IR-DFWM has some advantages over IR-PS in this spectral region in terms of sensitivity, signal-to-noise ratio and ease of use. The results illustrate the utility of the DSML for high-resolution nonlinear spectroscopy in the mid infrared.

Wafer-level filling of microfabricated atomic vapor cells based on thin-film deposition and photolysis of cesium azide

The thin-film deposition and photodecomposition of cesium azide are demonstrated and used to fill arrays of miniaturized atomic resonance cells with cesium and nitrogen buffer gas for chip-scale atomic-based instruments. Arrays of silicon cells are batch fabricated on wafers into which cesium azide is deposited by vacuum thermal evaporation. After vacuum sealing, the cells are irradiated with ultraviolet radiation, causing the azide to photodissociate into pure cesium and nitrogen in situ. This technology integrates the vapor-cell fabrication and filling procedures into one continuous and wafer-level parallel process, and results in cells that are optically transparent and chemically pure.