Sensitive Mass Detection Research Articles

Nonlinearity and temperature dependence of CVD graphene nanoelectromechanical resonator

As a two-dimensional material, graphene has outstanding properties, with high electron mobility and mechanical resistance. Therefore, it has good prospects to be used in nanoelectromechanical resonators for sensitive mass detection. Previously, a graphene resonator with graphene/metal contact able to zeptogram level sensing and a with quality factor of 42 at RT was presented. However, over-etching under the metal contacts degrades the quality factor, and resonators with metal/graphene/metal contacts were fabricated to overcome this issue. A quality factor of 350 at RT was obtained for a device with width = 0.8 μm and length = 1 μm. Tunning of the resonance frequency from 9.6 to 13.5 MHz with dc gate bias is demonstrated. Temperature reduction from 300 K to 30 K increased the resonance frequency by about 4–6 MHz. The temperature influence on nonlinearity is also discussed, with a resonator demonstrating softening and hardening nonlinearities at cryogenic temperatures.

Supercritical Fluid Nanospray Mass Spectrometry: II. Effects on Ionization.

Nanospraying supercritical fluids coupled to a mass spectrometer (nSF-MS) using a 90% supercritical fluid CO2 carrier (sCO2) has shown an enhanced desolvation compared to traditional liquid eluents. Capillaries of 25, 50, and 75 μm internal diameter (i.d.) with pulled emitter tips provided high MS detection sensitivity. Presented here is an evaluation of the effect of proton affinity, hydrophobicity, and nanoemitter tip size on the nSF-MS signal. This was done using a set of primary, secondary, tertiary, and quaternary amines with butyl, hexyl, octyl, and decyl chains as analytes. Each amine class was analyzed individually to evaluate hydrophobicity and proton affinity effects on signal intensity. The system has shown a mass sensitive detection on a linear dynamic range of 0.1-100 μM. Results indicate that hydrophobicity has a larger effect on the signal response than proton affinity. Nanospraying a mixture of all amine classes using the 75 μm emitter has shown a quaternary amine signal not suppressed by competing analytes. Competing ionization was observed for primary, secondary, and tertiary amines. The 75 and 50 μm emitters demonstrated increased signal with increasing hydrophobicity. Surprisingly, the 25 μm i.d. emitter yielded a signal decrease as the alkyl chain length increased, contrary to conventional understanding. Nanospraying the evaporative fluid in a sub-500 nm emitter likely resulted in differences in the ionization mechanism. Results suggest that 90% sCO2 with 9.99% methanol and 0.01% formic acid yielded fast desolvation, high ionization efficiency, and low matrix effect, which could benefit complex biological matrix analysis.

Supercritical Fluid Nanospray Mass Spectrometry.

Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO2 with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5-1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15-25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO2, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.

Implementing 1.5 mm internal diameter columns into analytical workflows

The use of smaller column diameters in liquid chromatography (LC) is often associated with capillary LC. Although there are many analytical benefits gained by adapting this format, routine use continues to be challenging due to column fragility and extra column dispersion. Bridging the gap between routinely used 2.1 mm columns and capillary bore columns allows for a sequential but far from insignificant increase in performance without the need for specialized equipment associated with very low dispersion LC systems. Moreover, an incremental decrease in column internal diameter (i.d.) allows for similar mass load (avoiding column overload that may be observed in much larger decreases in i.d. without trapping) and thus an increase in measured signal. As such, 1.5 mm i.d. columns provide an alternative intermediate dimension between the more regularly used 2.1 mm i.d. columns and 1 mm i.d. columns. These columns balance an increase in sensitivity compared to 2.1 mm i.d. columns (theoretically doubling the time-domain peak area in mass sensitive detectors for the same mass load), while mitigating the efficiency losses due to extra-column dispersion effects that are commonly observed with 1.0 mm i.d. columns. Here, the use of 1.5 mm i.d. columns was applied to LC/UV analysis of small molecules and LC/MS methods for the analysis of monoclonal antibodies. With equivalent mass load on column, the 1.5 mm i.d. columns provide two-to-threefold improvement in analyte peak area signal for small molecules as well as intact, subunit, and peptide levels of antibody analysis. Peak height was also increased using the 1.5 mm i.d. column, although the scale of increase varies between isocratic and gradient modes, likely due to differences in system dispersion effects and variation in electrospray ionization efficiency at different flow rates.

Evaluation of nanospray capillary LC-MS performance for metabolomic analysis in complex biological matrices

LC-MS metabolomic analysis in complex biological matrices may be complicated by degeneracy when using large-bore columns. Degeneracy is the detection of multiple mass spectral peaks from the same analyte due to adduction of salts to the metabolite, dimerization, or loss of neutrals. This introduces interferences to the MS spectra, diminishes quantification, and increases the rate of false identifications. Analysis using 2.1 mm inner diameter (i.d.) columns typically leads to degenerate peaks whereas nanospray using capillary columns (25, 50, and 75 µm i.d.) reduces degeneracy. Optimization of chromatographic parameters of capillary LC for amino acid standards showed the lowest HETP at 1.25 mm/sec across all capillary i.d. columns. Results suggest mass-sensitive detection below the optimum velocity. At faster velocities, concentration-dependent detection occurred across all capillaries. The 2.1 mm i.d. analytical scale column showed the greatest level of degeneracy, particularly in the low signal intensity range. 25 µm i.d. columns showed higher levels of metabolite annotation for the same signal intensity range. It also provided the lowest level of degeneracy, making it best suited for untargeted analysis. The 25 µm i.d. column achieved a peak capacity (nc) of 144 in a 30-minute gradient method with nc decreasing as the column i.d. increased. 75 µm i.d. capillary columns showed the highest signal intensity, which is beneficial for targeted analysis. These effects of chromatographic performance, resolution, and degeneracy profile of capillary and analytical scale columns were compared for metabolomic analyses in complex serum and cell lysate matrices.

Rapid and sensitive detection of Staphylococcus aureus and Klebsiella pneumonia based on bacitracin-modified Fe3O4@PDA magnetic beads combined with matrix-assisted laser desorption ionization-time of flight mass spectrometry.

The highly effective detection of pathogens in complex biological samples is an attractive and critical topic of study. Bacitracin is a novel broad-spectrum antimicrobial peptide to enrich bacteria via interactions with the membrane surface of the different bacterial cells. In this study, for the first time, bacitracin was immobilized on the surface of Fe3O4@PDA magnetic nanoparticles for the enrichment of Staphylococcus aureus (G+) and Klebsiella pneumoniae (G-). Combined with matrix-assisted laser desorption ionization time-of-flight mass spectrometry, a rapid and sensitive detection method for these two bacteria was developed. In this method, the detectable concentration of bacteria was decreased by 2-3 orders of magnitude in unenriched samples. The enrichment and identification can be completed in one hour. All these results demonstrated that the bacitracin-functionalized magnetic composite has potential for use in the large-scale enrichment and isolation of target pathogens from complex biological samples, opening a new avenue for the rapid and sensitive detection of pathogens.

New Methods Used in Pharmacokinetics and Therapeutic Monitoring of the First and Newer Generations of Antiepileptic Drugs (AEDs).

The review presents data from the last few years on bioanalytical methods used in therapeutic drug monitoring (TDM) of the 1st–3rd generation and the newest antiepileptic drug (AEDs) cenobamate in patients with various forms of seizures. Chemical classification, structure, mechanism of action, pharmacokinetic data and therapeutic ranges for total and free fractions and interactions were collected. The primary data on bioanalytical methods for AEDs determination included biological matrices, sample preparation, dried blood spot (DBS) analysis, column resolution, detection method, validation parameters, and clinical utility. In conclusion, the most frequently described method used in AED analysis is the LC-based technique (HPLC, UHPLC, USLC) combined with highly sensitive mass detection or fluorescence detection. However, less sensitive UV is also used. Capillary electrophoresis and gas chromatography have been rarely applied. Besides the precipitation of proteins or LLE, an automatic SPE is often a sample preparation method. Derivatization was also indicated to improve sensitivity and automate the analysis. The usefulness of the methods for TDM was also highlighted.

Towards Acoustic Radiation Free Lamb Wave Resonators for High-Resolution Gravimetric Biosensing

A novel Lamb wave resonator with fully suppressed acoustic radiation in water is proposed for high-resolution mass-sensitive detection of biomolecules. The elimination of acoustic radiation is achieved by slowing down the Lamb wave to a velocity lower than the sound speed in water. This enables high-quality-factor resonance in water and reduces the sensing frequency noise. High aspect ratio electrodes (HAREs) are used to slow down the Lamb wave. The elastic resonance and large surface area of the HAREs can also enhance the mass sensitivity of the device. The improved mass sensitivity together with the low frequency noise substantially improves the overall sensing resolution. Although reducing the plate thickness can also slow down the Lamb wave, it makes the device very fragile and not practical to use. In contrast, slowing down the Lamb wave by increasing electrode height allows the use of thick plates which is robust. In this article, the behavior and performance of the proposed high aspect ratio electrode Lamb wave resonator (HARE-LWR) are theoretically analyzed using finite element method simulations. Optimum design parameters were found through the simulations. Reported results show that a significant figure of merit improvement was achieved by the proposed HARE-LWR design.

Concentration Sensitive vs Mass Sensitive Detectors

The concept of mass or concentration sensitive detection in LC can be a tricky concept to understand. We therefore thought it would be useful in this article to clarify the operating principles and differences behind these two modes of detection

Design of heterostructured hybrids comprising ultrathin 2D bismuth tungstate nanosheets reinforced by chloramphenicol imprinted polymers used as biomimetic interfaces for mass-sensitive detection

Combining nanomaterials in varying morphology and functionalities gives rise to a new class of composite materials leading to innovative applications. In this study, we designed a heterostructured hybrid material consisting of two-dimensional bismuth nanosheets augmented by molecularly imprinted networks. Antibiotic overuse is now one of the main concerns in health management, and their monitoring is highly desirable but challenging. So, for this purpose, the resulting composite interface was used as a transducer for quartz crystal microbalances. The main objective was to develop highly selective mass-sensitive sensor for chloramphenicol. Morphological investigation revealed the presence of ultrathin, square shaped nanosheets, 2−3 nm in height and further supplemented by imprinted polymers. Sensor responses are described as the decrease in the frequency of microbalances owing to chloramphenicol re-binding in the templated cavities, yielding a detection limit down to 0.74 μM. This sensor demonstrated a 100 % specific detection of chloramphenicol over its interfering and structural analogs (clindamycin, thiamphenicol, and florfenicol). This composite interface offers the advantage of selective binding and excellent sensitivity due to special heterostructured morphology, in addition to benefits of robustness and online monitoring. The results suggest that such composite-based sensors can be favorable platforms, especially for commercial prospects, to obtain selective detection of other biomolecules of clinical importance.

On the mechanism of automated fizzy extraction

Fizzy extraction (FE) facilitates analysis of volatile solutes by promoting their transfer from the liquid to the gas phase. A carrier gas is dissolved in the sample under moderate pressure (Δp ≈ 150 kPa), followed by an abrupt decompression, what leads to effervescence. The released gaseous analytes are directed to an on-line detector due to a small pressure difference. FE is advantageous in chemical analysis because the volatile species are released in a short time interval, allowing for pulsed injection, and leading to high signal-to-noise ratios. To shed light on the mechanism of FE, we have investigated various factors that could potentially contribute to the extraction efficiency, including: instrument-related factors, method-related factors, sample-related factors, and analyte-related factors. In particular, we have evaluated the properties of volatile solutes, which make them amenable to FE. The results suggest that the organic solutes may diffuse to the bubble lumen, especially in the presence of salt. The high signal intensities in FE coupled with mass spectrometry are partly due to the high sample introduction rate (upon decompression) to a mass-sensitive detector. However, the analytes with different properties (molecular weight, polarity) reveal distinct temporal profiles, pointing to the effect of bubble exposure to the sample matrix. A sufficient extraction time (~12 s) is required to extract less volatile solutes. The results presented in this report can help analysts to predict the occurrence of matrix effects when analyzing real samples. They also provide a basis for increasing extraction efficiency to detect low-abundance analytes.

Early Detection of Growth Hormone Secretagogue Receptor Antagonists Exploiting Their Atypical Behavior in Competitive Assays.

We report the proof-of-concept of a bioaffinity format designed for the early detection of growth hormone secretagogue receptor (GHS-R1a) antagonists in urine samples. We exploit here their atypical behavior in competitive experiments with labeled ghrelin (GHR), namely, the strong promoting effect on the GHR/GHS-R1a interaction at low molar ratios GHR/antagonist. The antagonists potentiate the GHR/GHS-R1a interaction, and they display the same effect on the interaction of GHS-R1a with other agonists listed as doping agents. The developed assay allows the estimation of affinity constants of ligand/receptor and antagonist/receptor binding and is amenable to optical, electrochemical, and mass-sensitive detection. The estimated affinity constants for GHR/GHS-R1a and antagonist/GHS-R1a in the absence of G proteins are in good agreement with recently reported data.

Highly Sensitive Mass Detection Using Optically Levitated Microdisks

We propose a mass sensor using optically trapped and cooled dielectric microdisks. The center-of-mass motion of a trapped particle in vacuum can experience extremely low dissipation resulting in robust decoupling from the heat bath. It leads to a long heating time thus the measurement can be accomplished before a jump in the phonon number without the optomechanical cooling. This method can lead to milli-dalton mass sensitivity which approximates to the mass of one electron.

Highly sensitive mass detection based on nonlinear sum-sideband in a dispersive optomechanical system.

We present a theoretical scheme to realize high-sensitive mass detection in a dispersive optomechanical system (DOMS) via nonlinear sum-sideband. In this scheme, DOMS assisted by a degenerate parametric amplifier (DPA) provides a well-established optomechanical circumstance, where nonlinear optomechanical interaction between cavity mode and mechanical mode of dielectric membrane is expected for creating the frequency components at optical sum-sideband. Such a scheme for mass detection mainly relies on monitoring the conversion efficiency of generated sum-sideband after the added mass is absorbed on the dielectric membrane. Using experimentally achievable parameters, we find that the conversion efficiency of sum-sideband and the sensitivity of mass detection can be simultaneously improved when the nonlinear gain of DPA increases. Furthermore, our results also demonstrate that this mass detection of DOMS can reach femtogram (fg) level resolution, when the method of mass detection relies on a direct relationship between maximum efficiency of sum-sideband and mass-change of membrane.

Online coupling of thermal field-flow fractionation and Fourier transform infrared spectroscopy as a powerful tool for polymer characterization

The rapid growth and increasing importance of complex polymers has accelerated the need for advancement in characterization techniques. The analysis of such materials is challenging and the determination of different molecular parameters requires the combination of different advanced analytical techniques. Preferably, coupled and hyphenated techniques are used that provide different molecular parameters simultaneously in one experiment.Complementary to column-based fractionation techniques, channel-based methods such as thermal field-flow fractionation (ThFFF) are attractive as they can address different molecular parameters for a variety of analytes ranging from single macromolecules to macromolecular assemblies and nanoparticles. In the past, ThFFF has been coupled to a variety of size (molar mass) sensitive detectors such as multiangle laser light scattering and dynamic light scattering. Now, for the first time, the online coupling of ThFFF to Fourier transform infrared (FTIR) spectroscopy is presented. Different from standard single- or dual-wavelength IR detectors, this detector provides full spectra over the complete mid infrared range that can be used for concentration detection as well as chemical composition information. The applicability of online multidetector ThFFF-FTIR is demonstrated for selected polymer blends and copolymers comprising styrene and methyl methacrylate.

Selection of aptamers against pathogenic bacteria and their diagnostics application.

Aptamers are short nucleotide sequences which can specifically bind to a variety of targets with high affinity. They are identified and selected via systematic evolution of ligands by exponential enrichment (SELEX). Compared to antibodies, aptamers offer several advantages including easy labeling, high stability and lower cost. Those advantages make it possible to be a potential for use as a recognition probe to replace antibody in the diagnostic field. This article is intended to provide a comprehensive review, which is focused on systemizing recent advancements concerning SELEX procedures, with special emphasis on the key steps in SELEX procedures. The principles of various aptamer-based detections of pathogenic bacteria and their application are discussed in detail, including colorimetric detection, fluorescence detection, electrochemical detection, lateral flow strip test, mass sensitive detection and PCR-based aptasensor. By discussing recent research and future trends based on many excellent publications and reviews, we attempt to give the readers a comprehensive view in the field of aptamer selection against pathogenic bacteria and their diagnostics application. Authors hope that this review will promote lively and valuable discussions in order to generate new ideas and approaches towards the development of aptamer-based methods for application in pathogenic bacteria diagnosis.

Separation and characterization of cellulose nanocrystals by multi-detector asymmetrical-flow field-flow fractionation.

Cellulose nanocrystals (CNCs) are renewable, naturally derived polymeric nanomaterials receiving substantial attention for a wide range of potential applications. The recent availability of high quality reference materials will facilitate the development and validation of measurement methods needed to advance the scientific and commercial use of CNCs. In the present study, we demonstrate an optimized method to fractionate CNCs with narrow size dispersion based on asymmetrical-flow field-flow fractionation (AF4) coupled with on-line multi-angle light scattering (MALS), dynamic light scattering (DLS), and differential refractometry (dRI). A stable suspension of CNC (Certified Reference Material CNCD-1, National Research Council-Canada) in deionized water was prepared using a dispersion method provided by NRC and adopted from a protocol originally developed at the National Institute of Standards and Technology. The as-prepared material was initially characterized in batch mode to validate the NRC dispersion method. AF4 was then optimized for channel and cross flow, mobile phase composition, and injection volume, among other parameters. Additionally, suspensions containing (1.25-10) mg mL-1 CNC were injected directly into the dRI detector (off-line), yielding a dn/dc value of 0.148 ± 0.003 mL g-1. dRI was then used as an on-line mass sensitive detector to quantify recovery. Results show that maximum recovery (≈ 99%) was achieved under optimized conditions. The weight-averaged molar mass (Mw) was estimated at roughly 107 Da from a partial Zimm analysis. The optical radius of gyration, Rg, and the hydrodynamic radius, Rh, were measured during elution. The shape factor (Rg/Rh) ranged from 1.5 to 1.9 for the fractionated material, supporting an elongated or rod-like structure. To our knowledge, this is the first time that both the morphology and molar mass of CNCs have been directly measured for the full distribution of species. Finally, we developed and demonstrated a semi-preparatory fractionation method to separate CNCs at the milligram scale for off-line research and analysis.

Profiling of a wide range of neurochemicals in human urine by very-high-performance liquid chromatography-tandem mass spectrometry combined with in situ selective derivatization

Development of a reliable analytical method of neurochemicals in biological fluids is important to discover potential biomarkers for the diagnosis, treatment and prognosis of neurological disorders. However, neurochemical profiling of biological samples is challenging because of highly different polarities between basic and acidic neurochemicals, low physiological levels, and high matrix interference in biological samples. In this study, an ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method combined with in situ selective derivatization for comprehensive profiling of 20 neurochemicals in urine was developed for a wide range of neurochemicals. In situ selective derivatization greatly improved the peak capacity on a reversed-phase C18 column and sensitive mass detection in LC-ESI-MS/MS-positive ion mode due to reduction of the distinct physicochemical properties between acidic and basic neurochemicals. The MS/MS spectra of neurochemicals exhibited specific ions, such as losses of amine, methanol, or methyl formate molecules from protonated molecules, enabling selection of appropriate multiple reaction monitoring (MRM) ions for selective and sensitive detection. The developed method was validated in terms of linearity, limit of detection (LOD) and limit of quantification (LOQ), precision, accuracy, and recovery. The correlation coefficients (R2) of calibration curves were above 0.9961. The ranges of LODs and LOQs were 0.1–3.6ng/mL and 0.3–12.0ng/mL, respectively. The overall precision and accuracy were 0.52–16.74% and 82.26–118.17%, respectively. The method was successfully applied to simultaneously profile the metabolic pathways of tyrosine, tryptophan, and glutamate in Parkinson’s disease patient urine (PD, n=21) and control urine (n=10). Significant differences (P≤0.01) between two groups in the activity of phenylethanolamine N-methyltransferase (PNMT) and alcohol dehydrogenase (ADH) were observed. In conclusion, this method provides reliable quantification of a wide range of neurochemicals in human urine and would be helpful for finding biomarkers related to specific neuronal diseases.

Metabolic profiling of tyrosine, tryptophan, and glutamate in human urine using gas chromatography-tandem mass spectrometry combined with single SPE cleanup.

The tyrosine, tryptophan, and glutamate metabolic pathways play key roles on pathological state of neuronal functions and the change of their levels in biological systems reflects the progress degree of neuronal diseases. Comprehensive profiling of these metabolites is important to find new biomarkers for diagnosis or prognosis of various neuronal diseases. However, the overall profiling analysis of various neurochemicals in biological sample is confronted with several limitations due to their low concentration and physicochemical properties and the coexistence of matrices. We developed an efficient and feasible method using gas chromatography-tandem mass spectrometry (GC-MS/MS). Wide-bore mixed cation exchange (MCX) SPE process enables a rapid and effective cleanup of 20 neurochemicals even including acidic and basic neurochemicals in a single SPE cartridge by using different composition of eluents. Selective derivatization of various types of metabolites was applied to achieve highly chromatographic separation and sensitive mass detection. Appropriate selection of precursor and product transition ions used in multiple reaction-monitoring (MRM) mode based on the MS/MS fragmentations of the derivatized neurochemicals could be significantly minimized the matrix effects and enhanced the reliability of quantification results. The developed method was validated in terms of linearity, limits of detection, precision, accuracy, and matrix effects. The intra- and inter-assay analytical variations were less than 10%. The overall linearity for all of the targets was excellent (R2≥0.996). The detection limits ranged between 0.38 and 8.13ng/mL for the acidic neurochemicals and between 0.02 and 11.1ng/mL for the basic neurochemicals. The developed protocol will be expected to be a promising tool for the understanding of the pathological state and diagnosis of various neuronal diseases.

Characterization of poly(methyl methacrylate)-graft-poly(styrene)s using various chromatographic techniques

Two graft copolymer samples of identical average composition were synthesized by grafting living polystyrene anions onto a broadly distributed PMMA backbone. Size exclusion chromatography (SEC) with only RI-detection, SEC with viscometry and light scattering detection, SEC with UV and RI dual detection, gradient chromatography and 2-dimensional chromatography were applied to compare the information that can be obtained by the different techniques. While only limited information was retrieved by conventional SEC or SEC with molar mass sensitive detection, SEC with UV and RI revealed different chemical heterogeneity of the samples. Using gradient chromatography and 2-dimensional chromatography it was possible to identify non-grafted side chains and unreacted parent PMMA besides the actual graft copolymer molecules. While in one sample a heavily grafted product was formed besides non-grafted PMMA, the second sample did not contain ungrafted PMMA but a graft product of lower grafting density. The different product distributions were explained by the different synthetic procedures.