Development and characterization of discontinuous atmospheric pressure interface - Dual pressure chamber miniature mass spectrometer

Miniaturized mass spectrometers have become increasingly prevalent for real-time detection and analysis, owing to their compact size and portability. The pursuit of performance enhancement in these instruments is a pivotal objective within the domain of mass spectrometry miniaturization. This study introduces a novel miniature mass spectrometer featuring a discontinuous atmospheric pressure interface and a dual pressure chamber. Compared to conventional single-chamber, discontinuous sampling interface mass spectrometers, the newly developed instrument demonstrates a more than tenfold improvement in detection efficiency. This significant enhancement is achieved without the need for complex control of switch coupling time series, thereby streamlining the circuit design and improving the instrument's fault tolerance. Furthermore, by capitalizing on the benefits of discontinuous sampling, the instrument reduces the operational pressure relative to traditional continuous sampling in differential pressure vacuum chambers. It accommodates larger inlet capillary (0.38 mm) and skimmer (0.5 mm) diameters, leading to a ninefold increase in response strength for risperidone and lowering the detection limit to 0.5 ppb. The instrument's capacity for rapid drug detection, along with enhanced resolution and detection limits, underscores its potential utility. Additionally, it facilitates the use of smaller mechanical pumps, significantly diminishing both the instrument's volume and power consumption. This presents a promising avenue for further miniaturization of mass spectrometers.

Differentiation of free d-amino acids and amino acid isomers in solution using tandem mass spectrometry of hydrogen-bonded clusters

Free d-amino acids and amino acid isomers were differentiated using tandem mass spectrometry without chromatographic separation. Ultraviolet photodissociation and water adsorption of leucine (Leu) and isoleucine (Ile) enantiomers hydrogen-bonded with tryptophan (Trp) were investigated at 8 K in the gas phase. The enantiomer-selective Cα-Cβ bond cleavage of Trp was observed in the product ion spectra obtained by 285 nm photoexcitation, where the abundance of NH2CHCOOH-eliminated ion of heterochiral H+(d-Trp)(l-Leu) was higher than that of homochiral H+(l-Trp)(l-Leu). When comparing water adsorption on the surfaces of the heterochiral and homochiral clusters in a cold ion trap, the number of water molecules adsorbed on the heterochiral cluster was greater than that adsorbed on the homochiral cluster. These results indicate that the stronger intermolecular interactions within the homochiral H+(l-Trp)(l-Leu) compared to the heterochiral cluster inhibit enantiomer-selective photodissociation. Leu and Ile were differentiated by the isomer-selective Cα-Cβ bond cleavage of Trp in the clusters. Calibration curves for the differentiation of isomeric amino acids and their enantiomers were developed using monitoring isomer- and enantiomer-selective photodissociation, indicating that the molar fractions in solution could be determined from a single product ion spectrum.

Quantification of monosaccharide enantiomers using optical properties of hydrogen-bonded tryptophan

Chiral recognition between amino acids and monosaccharides in the gas phase was investigated as a model for chemical evolution in interstellar molecular clouds. Ultraviolet (UV) photodissociation spectra and product ion spectra of cold gas-phase hydrogen-bonded clusters of protonated tryptophan (Trp) and a pentose, including ribose and arabinose, were obtained using a tandem mass spectrometer equipped with an electrospray ionization source and a temperature-controlled ion trap. The relative intensity of the signal arising from the S₁-S₀ transition of protonated Trp observed at approximately 285 nm in the UV photodissociation spectrum of homochiral H⁺(d-Trp)(d-ribose) was significantly higher than that of heterochiral H⁺(l-Trp)(d-ribose), corresponding to the ππ* state of the Trp indole ring. Optical properties of Trp in the clusters induced by 285-nm photoexcitation were applied to the identification and quantification of pentose enantiomers in solution. Pentose enantiomeric excess in solution was determined from relative abundances observed in a single product ion spectrum of 285-nm photoexcited hydrogen-bonded clusters of H⁺(l-Trp) and pentose. A mixture of two pentoses could also be quantified by this method. The geometric and electronic structures of Trp enable recognition of biological molecules through hydrogen bonding.

Molecular Adsorption on Cold Gas-Phase Hydrogen-Bonded Clusters of Chiral Molecules

Gas-phase molecular adsorption was investigated as a model for molecular cloud formation. Molecular adsorption on cold gas-phase hydrogen-bonded clusters containing protonated tryptophan (Trp) enantiomers and monosaccharides such as methyl-α-D-glucoside, D-ribose, and D-arabinose was detected using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The adsorption sites on the surface of cold gas-phase hydrogen-bonded cluster ions were quantified using gas-phase N₂ adsorption-mass spectrometry. The gas-phase N₂ adsorption experiments indicated that the number of adsorption sites on the surface of the hydrogen-bonded heterochiral clusters containing L-Trp and D-monosaccharides exceeded the number of adsorption sites on the homochiral clusters containing D-Trp and D-monosaccharides. H₂O molecules were preferentially adsorbed on the heterochiral clusters, and larger water clusters were formed in the gas phase. Physical and chemical properties of cold gas-phase hydrogen-bonded clusters containing biological molecules were useful for investigating enantiomer selectivity and chemical evolution in interstellar molecular clouds.

Experimental demonstration of suppressing residual geometric dephasing

The geometric phase is regarded as a promising strategy in fault tolerance quantum information processing (QIP) domain due to its phase only depending on the geometry of the path executed. However, decoherence caused by environmental noise will destroy the geometric phase. Traditional dynamic decoupling sequences can eliminate dynamic dephasing but can not reduce residual geometric dephasing, which is still vital for high-precision quantum manipulation. In this work, we experimentally demonstrate effective suppression of residual geometric dephasing with modified dynamic decoupling schemes, using a single trapped ¹⁷¹Yb⁺ ion. The experimental results show that the modified schemes can reduce dephasing rate up to more than one order of magnitude compared with traditional dynamic decoupling schemes, where residual geometric dephasing dominates. Besides, we also investigate the impact of intensity and correlation time of the low-frequency noise on coherence of the quantum system. And we confirm these methods can be used in many cases.

Ion-Neutral Collision Effects on Ion Trapping and Pseudopotential Depth in Ion Trap Mass Spectrometry

Ion trapping using radio-frequency (RF) devices has been widely used in mass spectrometry (MS). The pseudopotential well (PW) model enables the use of a pseudopotential depth, D, to evaluate the ion trapping capability of the RF devices in the pure electric field. It remains unclear how gas pressures regulate the ion trapping and D. Here, we calculated the D of a linear ion trap (LIT) from 1 mTorr to 2 Torr, a pressure range critical for the operation of the RF devices, through ion cloud simulations. Compared with the case of pure electric field, ion-neutral collision effects at pressures of 1 to 100 mTorr were beneficial for the ion trapping and revealed an optimal trapping depth, D, at around 10 mTorr. We explained the mechanism and validated the observation via ion trapping experiments performed in a home-made dual LIT mass spectrometer. We also showed that near the stability boundary, the RF heating became comparable with the D, which led to the decrement of ion trapping capability characterized by the available D.

Optimized Electrostatic Linear Ion Trap for Charge Detection Mass Spectrometry

In charge detection mass spectrometry (CDMS), ions are passed through a detection tube and the m/z ratio and charge are determined for each ion. The uncertainty in the charge and m/z determinations can be dramatically reduced by embedding the detection tube in an electrostatic linear ion trap (ELIT) so that ions oscillate back and forth through the detection tube. The resulting time domain signal can be analyzed by fast Fourier transforms (FFTs). The ion's m/z is proportional to the square of the oscillation frequency, and its charge is derived from the FFT magnitude. The ion oscillation frequency is dependent on the physical dimensions of the trap as well as the ion energy. A new ELIT has been designed for CDMS using the central particle method. In the new design, the kinetic energy dependence of the ion oscillation frequency is reduced by an order of magnitude. An order of magnitude reduction in energy dependence should have led to an order of magnitude reduction in the uncertainty of the m/z determination. In practice, a factor of four improvements was achieved. This discrepancy is probably mainly due to the trajectory dependence of the ion oscillation frequency. The new ELIT design uses a duty cycle of 50%. We show that a 50% duty cycle produces the lowest uncertainty in the charge determination. This is due to the absence of even-numbered harmonics in the FFT, which in turn leads to an increase in the magnitude of the peak at the fundamental frequency. Graphical Abstract ᅟ.

Stimulated Motion Suppression (STMS): a New Approach to Break the Resolution Barrier for Ion Trap Mass Spectrometry

Ion trap is an excellent platform to perform tandem mass spectrometry (MS/MS), but has an intrinsic drawback in resolving power. Using ion resonant ejection as an example, the resolution degradation can be largely attributed to the broadening of the resonant frequency band (RFB) between ion motion and driving alternative-current (AC). To solve this problem, stimulated motion suppression (STMS) was developed. The key idea of STMS is the use of two suppression alternative-current (SAC) signals, which both have reversed initial phases to the main AC. The SACs can block the unexpected sideband ion resonances (or ejections), therefore playing a key role in sharpening the RFB. The proof-of-concept has been demonstrated through ion trajectory simulations and validated experimentally. STMS provides a new and versatile means for the improvement of the ion trap resolution, which for a long time has reached the bottleneck through conventional methods, e.g., increasing the radio-frequency (RF) voltage and decreasing the mass scan rate. At the end, it is worth noting that the idea of STMS is very general and principally can be applied in any RF device for the purposes of high-resolution mass analysis and ion isolation. Graphical Abstract ᅟ.

Operational effects of the UNOT gate on classical and quantum correlations

The NOT gate that flips a classical bit is ubiquitous in classical information processing. However its quantum analogue, the universal NOT (UNOT) gate that flips a quantum spin in any alignment into its antipodal counterpart is strictly forbidden. Here we explore the connection between this discrepancy and how UNOT gates affect classical and quantum correlations. We show that while a UNOT gate always preserves classical correlations between two spins, it can non-locally increase or decrease their shared discord in ways that allow violation of the data processing inequality. We experimentally illustrate this using a multi-level trapped ¹⁷¹Yb⁺ ion that allows simulation of anti-unitary operations.

A Linear Ion Trap with an Expanded Inscribed Diameter to Improve Optical Access for Fluorescence Spectroscopy

We report a custom-geometry linear ion trap designed for fluorescence spectroscopy of gas-phase ions at ambient to cryogenic temperatures. Laser-induced fluorescence from trapped ions is collected from between the trapping rods, orthogonal to the excitation laser that runs along the axis of the linear ion trap. To increase optical access to the ion cloud, the diameter of the round trapping rods is 80% of the inscribed diameter, rather than the roughly 110% used to approximate purely quadrupolar electric fields. To encompass as much of the ion cloud as possible, the first collection optic has a 25.4 mm diameter and a numerical aperture of 0.6. The choice of geometry and collection optics yields 10⁷ detected photons/s from trapped rhodamine 6G ions. The trap is coupled to a closed-cycle helium refrigerator, which in combination with two 50 Ohm heaters enables temperature control to below 25 K on the rod electrodes. The purpose of the instrument is to broaden the applicability of fluorescence spectroscopy of gas-phase ions to cases where photon emission is a minority relaxation pathway. Such studies are important to understand how the microenvironment of a chromophore influences excited state charge transfer processes. Graphical Abstract ᅟ.

Design and Application of a High-Temperature Linear Ion Trap Reactor

A high-temperature linear ion trap reactor with hexapole design was homemade to study ion-molecule reactions at variable temperatures. The highest temperature for the trapped ions is up to 773 K, which is much higher than those in available reports. The reaction between V₂O₆^- cluster anions and CO at different temperatures was investigated to evaluate the performance of this reactor. The apparent activation energy was determined to be 0.10 ± 0.02 eV, which is consistent with the barrier of 0.12 eV calculated by density functional theory. This indicates that the current experimental apparatus is prospective to study ion-molecule reactions at variable temperatures, and more kinetic details can be obtained to have a better understanding of chemical reactions that have overall barriers. Graphical Abstract.

Influence of NanoLC Column and Gradient Length as well as MS/MS Frequency and Sample Complexity on Shotgun Protein Identification of Marine Bacteria

Protein identification by shotgun proteomics, i.e., nano-liquid chromatography (nanoLC) peptide separation online coupled to electrospray ionization (ESI) mass spectrometry (MS)/MS, is the most widely used gel-free approach in proteome research. While the mass spectrometer accounts for mass accuracy and MS/MS frequency, the nanoLC setup and gradient time influence the number of peptides available for MS analysis, which ultimately determine the number of proteins identifiable. Here, we report on the influence of (i) analytical column length (15, 25, or 50 cm) coupled to (ii) the applied gradient length (120, 240, 360, 480, or 600 min), as well as (iii) MS/MS frequency on peptide/protein identification by shotgun proteomics of (iv) 2 marine bacteria. Longer gradients increased the number of peptides/proteins identified as well as the reproducibility of identification. Furthermore, longer analytical columns strictly enlarge the covered proteome complement. Notably, the proteome complement identified with a short column and applying a long gradient is also covered when using longer columns with shorter gradients. Coverage of the proteome complement further increases with higher MS/MS frequency. Compilation of peptide lists of replicate analyses (same gradient length) improves protein identification, while compilation of analyses with different gradient lengths yields a similar or even higher number of proteins using comparable or even less total analysis time.

Direct Analysis of Organic Compounds in Liquid Using a Miniature Photoionization Ion Trap Mass Spectrometer with Pulsed Carrier-Gas Capillary Inlet

A miniature ion trap mass spectrometer with capillary direct sampling and vacuum ultraviolet photoionization source was developed to conduct trace analysis of organic compounds in liquids. Self-aspiration sampling is available where the samples are drawn into the vacuum chamber through a capillary with an extremely low flow rate (less than 1 μL/min), which minimizes sample consumption in each analysis to tens of micrograms. A pulsed gas-assisted inlet was designed and optimized to promote sample transmission in the tube and facilitate the cooling of ions, thereby improving instrument sensitivity. A limit of detection of 2 ppb could be achieved for 2,4-dimethylaniline in a methanol solution. The sampling system described in the present study is specifically suitable for a miniature photoionization ion trap mass spectrometer that can perform rapid and online analysis for liquid samples. Graphical Abstract ᅟ.

Analysis of the Proteome of Hair-Cell Stereocilia by Mass Spectrometry

Characterization of proteins that mediate mechanotransduction by hair cells, the sensory cells of the inner ear, is hampered by the scarcity of these cells and their sensory organelle, the hair bundle. Mass spectrometry, with its high sensitivity and identification precision, is the ideal method for determining which proteins are present in bundles and what proteins they interact with. We describe here the isolation of mouse hair bundles, as well as preparation of bundle protein samples for mass spectrometry. We also describe protocols for data-dependent (shotgun) and parallel reaction monitoring (targeted) mass spectrometry that allow us to identify and quantify proteins of the hair bundle. These sensitive methods are particularly useful for comparing proteomes of wild-type mice and mice with deafness mutations affecting hair-bundle proteins.

Making Mass Spectrometry See the Light: The Promises and Challenges of Cryogenic Infrared Ion Spectroscopy as a Bioanalytical Technique

The detailed chemical information contained in the vibrational spectrum of a cryogenically cooled analyte ion would, in principle, make infrared (IR) ion spectroscopy a gold standard technique for molecular identification in mass spectrometry. Despite this immense potential, there are considerable challenges in both instrumentation and methodology to overcome before the technique is analytically useful. Here, we discuss the promise of IR ion spectroscopy for small molecule analysis in the context of metabolite identification. Experimental strategies to address sensitivity constraints, poor overall duty cycle, and speed of the experiment are intimately tied to the development of a mass-selective cryogenic trap. Therefore, the most likely avenues for success, in the authors' opinion, are presented here, alongside alternative approaches and some thoughts on data interpretation.

Ion-molecule adduct formation in tandem mass spectrometry

Nowadays most LC-MS methods rely on tandem mass spectrometry not only for quantitation and confirmation of compounds by multiple reaction monitoring (MRM), but also for the identification of unknowns from their product ion spectra. However, gas-phase reactions between charged and neutral species inside the mass analyzer can occur, yielding product ions at m/z values higher than that of the precursor ion, or at m/z values difficult to explain by logical losses, which complicate mass spectral interpretation. In this work, the formation of adduct ions in the mass analyzer was studied using several mass spectrometers with different mass analyzers (ion trap, triple quadrupole, and quadrupole-Orbitrap). Heterocyclic amines (AαC, MeAαC, Trp-P-1, and Trp-P-2), photo-initiators (BP and THBP), and pharmaceuticals (phenacetin and levamisole) were selected as model compounds and infused in LCQ Classic, TSQ Quantum Ultra AM, and Q-Exactive Orbitrap (ThermoFisher Scientific) mass spectrometers using electrospray as ionization method. The generation of ion-molecule adducts depended on the compound and also on the instrument employed. Adducts with neutral organic solvents (methanol and acetonitrile) were only observed in the ion trap instrument (LCQ Classic), because of the ionization source on-axis configuration and the lack of gas-phase barriers, which allowed inertial entrance of the neutrals into the analyzer. Adduct formation (only with water) in the triple quadrupole instruments was less abundant than in the ion trap and quadrupole-Orbitrap mass spectrometers, because of the lower residence time of the reactive product ions in the mass analyzer. The moisture level of the CID and/or damper gas had a great effect in beam-like mass analyzers such as triple quadrupole, but not in trap-like mass analyzers, probably because of the long residence time that allowed adduct formation even with very low concentrations of water inside the mass spectrometer.

Structural analysis of quazepam metabolites in bile by ion trap time-of-flight mass spectrometry

Quazepam (QZP) is a long-acting benzodiazepine-type hypnotic. We searched for novel QZP metabolites in bile and determined their structures by liquid chromatography-ion trap time-of-flight mass spectrometry (LC-IT-TOF MS). The metabolites were extracted with ethyl acetate after β-glucuronidase treatment. First, a single MS spectrum was acquired. Second, MS(n) spectra were acquired for peaks that consisted of ions with the isotope pattern corresponding to molecules bearing one chlorine atom. The novel QZP metabolites found in this study were hydroxyquazepam, hydroxy-methoxyquazepam, hydroxy-oxoquazepam, and hydroxy-methoxy-oxoquazepam, which have the hydroxy and methoxy groups on the fluorophenyl group, and dihydroxy-oxoquazepam and dihydroxy-methoxy-oxoquazepam, which have one hydroxy group at the 3-position of the seven-membered ring and the other hydroxy group and the methoxy group on the fluorophenyl group. We demonstrated that LC-IT-TOF MS was a useful tool for determining the structure of the metabolites. However, the exact locations of the hydroxy and methoxy groups on the fluorophenyl group could not be identified.

Development of a capillary high performance liquid chromatography-ion trap-mass spectrometry method for the determination of VLIVP antihypertensive peptide in soybean crops

Soybean peptide VLIVP presents a very high antihypertensive activity (IC50 value 1.69μM), even higher than extensively studied IPP and VPP peptides from milk. Nevertheless, no much attention has been paid to this peptide and there is no method enabling its determination in soybeans. The aim of this work was the development of an analytical methodology for this purpose. A methodology consisting of the extraction of soybean proteins, their digestion with Protease P enzyme, their chromatographic separation using capillary-HPLC, and IT-MS detection was optimized. Protein extraction was performed by the use of high intensity focused ultrasounds to obtain a reduced extraction time. Optimization of chromatographic and mass spectrometry parameters enabled the separation of VLIVP peptide within just 7min and its sensitive detection. The analytical characteristics of the capillary-HPLC-IT-MS method were evaluated through the study of linearity, LOD, LOQ, study of the presence of matrix interferences, precision, and recovery. The method enabled to detect as low as 3.6ng of peptide and to determine as low as 12ng of peptide in 1g of soybean (as dry basis). Finally, the developed method was applied to the determination of the antihypertensive peptide VLIVP in different soybean varieties. The results showed the highest yield of VLIVP peptide in variety Mazowiecka II from Poland.

A serially coupled stationary phase method for the determination of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine by liquid chromatography ion trap tandem mass spectrometry

Oxidative attack to DNA is of particular interest since DNA modifications can lead to heritable mutations. The most studied product of DNA oxidation is 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG). While 8-oxodG determination in blood and tissue cells is prone to artifacts, its measurement in urine employing liquid chromatography tandem mass spectrometry (LC-MS/MS) has gained more and more interest for increased reliability. LC-MS/MS can be affected by matrix effects and this is particularly true when ion trap is used as MS analyzer, due to ion accumulation in the trap and related space charge effect. In the present work, we have developed a LC-MS/MS method where the combination of cation exchange and reverse phase solid phases resulted in LC separation optimization. This together with the employment of an isotopically labeled internal standard, allowed the usage of ion trap LC-MS/MS, typically not employed for quantitative measurement in biological samples, for the measurement of 8-oxodG in urine samples from control populations.

Four different urine matrices were employed for method validation. Limit of quantitation was set at least at 0.5 ng/ml. While analyzing urine samples from healthy volunteers, 8-oxodG levels reported as ng/ml were statistically different comparing males with females (p<0.05, Mann Whitney test); while comparing results normalized for creatinine no statistical significant difference was found. Mean urinary 8-oxodG level found in healthy volunteers was 1.16±0.46 nmol/mmol creatinine.

The present method by enhancing at best the chromatographic performances allows the usage of ion trap LC-MS/MS for the measurement of 8-oxodG in urine samples from control populations.