Simultaneous Preconcentration and Desalting of Organic Solutes in Aqueous Solutions by Bubble Bursting

Sensitive quantitation of ultra-trace toxic aconitines in complex matrices by perfusion nano-electrospray ionization mass spectrometry combined with gas-liquid microextraction

The accurate analysis of ultra-trace (e.g. <10-4 ng/mL) substances in complex matrices is a burdensome but vital problem in pharmaceutical analysis, with important implications for precise quality control of drugs, discovery of innovative medicines and elucidation of pharmacological mechanisms. Herein, an innovative constant-flow perfusion nano-electrospray ionization (PnESI) technique was developed firstly features significant quantitative advantages in high-sensitivity ambient MS analysis of complex matrix sample. More importantly, double-labeled addition enrichment quantitation strategies of gas-liquid microextraction (GLME) were proposed for the first time, allowing highly selective extraction and enrichment of specific target analytes in a green and ultra-efficient (>1000-fold) manner. Using complex processed Aconitum herbs as example, PnESI-MS directly enabled the qualitative and absolute quantitative analysis of the processed Aconitum extracts and characterized the target toxic diester alkaloids with high sensitivity, high stability, wide linearity range, and strong resistance to matrix interference. Further, GLME device was applied to obtain the highly specific enrichment of the target diester alkaloids more than 1000-fold, and accurate absolute quantitation of trace aconitine, mesaconitine, and hypaconitine in the extracts of Heishunpian, Zhichuanwu and Zhicaowu was accomplished (e.g., 0.098 pg/mL and 0.143 pg/mL), with the quantitation results well below the LODs of aconitines from any analytical instruments available. This study built a systematic strategy for accurate quantitation of ultra-trace substances in complex matrix sample and expected to provide a technological revolution in many fields of pharmaceutical research.

Ultrasensitive and Green Bubbling Extraction Strategies: An Extensible Solvent-Free Re-Enrichment Approach for Ultratrace Pollutants in Aqueous Samples

Ultratrace organic pollutants in the environment pose severe threats to human health; hence, their accurate detection is essential. In this study, we develop a secondary solvent-free enrichment strategy based on bubbling extraction (BE). Especially, we used BE solid-phase microextraction and BE carbon nanotube paper absorption to capture aerosols from a liquid water surface, desorb analytes, and analyze the analytes using mass spectrometry. The application of a solvent-free enrichment strategy helps overcome technical challenges in implementing BE technology, including reproducibility, quantification, and sensitivity. This approach objectively demonstrates the enrichment efficiency of BE, resulting in improved mass spectrometry response and quantification. It effectively tackles the difficulties in detecting and quantifying ultratrace environmental pollutants in mass spectrometric analysis. The present study successfully conducted a quantitative analysis of 16 polycyclic aromatic hydrocarbons and 7 antibiotics in 48 environmental water samples. This strategy proved effective in detecting the presence and distribution of polar and nonpolar environmental pollutants in rivers and lakes. Moreover, this strategy has several advantages, such as ultrahigh sensitivity at the femtograms per liter level, good greenness, multiplexed quantitation, low sample consumption, and ease of operation. Overall, the utilization of the ultrasensitive and environmentally friendly BE approach presents a reliable and adaptable method for the identification of ultratrace environmental pollutants in water specimens, thereby enabling early monitoring of pollutant levels.

Carbon Dioxide Microbubble Bursting Ionization Mass Spectrometry

Aerosols generated by bubble bursting have been proved to promote the extraction of analytes and have ultrahigh electric fields at their water-air interfaces. This study presented a simple and efficient ionization method, carbon dioxide microbubble bursting ionization (CDMBI), without the presence of an exogenous electric field (namely, zero voltage), by simulating the interfacial chemistries of sea spray aerosols. In CDMBI, microbubbles are generated in situ by continuous input of carbon dioxide into an aqueous solution containing low-concentration analytes. The microbubbles extract low- and high-polarity analytes as they pass through the aqueous solution. Upon reaching the water-air interface, these microbubbles burst to produce charged aerosol microdroplets with an average diameter of 260 μm (8.1-10.4 nL in volume), which are immediately transferred to a mass spectrometer for the detection and identification of extracted analytes. The above analytical process occurs every 4.2 s with a stable total ion chromatogram (relative standard deviation: 9.4%) recorded. CDMBI mass spectrometry (CDMBI-MS) can detect surface-active organic compounds in aerosol microdroplets, such as perfluorooctanoic acid, free fatty acids epoxidized by bubble bursting, sterols, and lecithins in soybean and egg, with the limit of detection reaching the level of fg/mL. In addition, coupling CDMBI-MS with an exogenous voltage yields relatively weak gains in ionization efficiency and sensitivity of analysis. The results suggested that CDMBI can simultaneously accomplish both bubbling extraction and microbubble bursting ionization. The mechanism of CDMBI involves bubbling extraction, proton transfer, inlet ionization, and electrospray-like ionization. Overall, CDMBI-MS can work in both positive and negative ion modes without necessarily needing an exogenous high electric field for ionization and quickly detect trace surface-active analytes in aqueous solutions.

Highly efficient preconcentration using anodically generated shrinking gas bubbles for per- and polyfluoroalkyl substances (PFAS) detection

Here we report a highly efficient PFAS preconcentration method that uses anodically generated shrinking gas bubbles to preconcentrate PFAS via aerosol formation, achieving ~ 1400-fold enrichment of PFOS and PFOA-the two most common PFAS-in 20 min. This new method improves the enrichment factor by 15 to 105% relative to the previous method that uses cathodically generated H₂ bubbles. The shrinking gas bubbles are in situ electrogenerated by oxidizing water in an NH₄HCO₃ solution. H⁺ produced by water oxidation reacts with HCO₃^- to generate CO₂ gas, forming gas bubbles containing a mixture of O₂ and CO₂. Due to the high solubility of CO₂ in aqueous solutions, the CO₂/O₂ bubbles start shrinking when they leave the electrode surface region. A mechanistic study reveals two reasons for the improvement: (1) shrinking bubbles increase the enrichment rate, and (2) the attractive interactions between the positively charged anode and negatively charged PFAS provide high enrichment at zero bubble path length. Based on this preconcentration method, we demonstrate the detection of ≥ 70 ng/L PFOA and PFOS in water in ~ 20 min by coupling it with our bubble-nucleation-based detection method, fulfilling the need of the US Environmental Protection Agency.

Catalytic Oxygenation-Mediated Extraction as a Facile and Green Way to Analyze Volatile Solutes

Sparging-based methods have long been used to liberate volatile organic compounds (VOCs) from liquid sample matrices prior to analysis. In these methods, a carrier gas is delivered from an external source. Here, we demonstrate "catalytic oxygenation-mediated extraction" (COME), which relies on biocatalytic production of oxygen occurring directly in the sample matrix. The newly formed oxygen (micro)bubbles extract the dissolved VOCs. The gaseous extract is immediately transferred to a separation or detection system for analysis. To start COME, dilute hydrogen peroxide is injected into the sample supplemented with catalase enzyme. The entire procedure is performed automatically-after pressing a "start" button, making a clapping sound, or triggering from a smartphone. The pump, valves, and detection system are controlled by a microcontroller board. For quality control and safety purposes, the reaction chamber is monitored by a camera linked to a single-board computer, which follows the enzymatic reaction progress by analyzing images of foam in real time. The data are instantly uploaded to the internet cloud for retrieval. The COME apparatus has been coupled on-line with the gas chromatography electron ionization mass spectrometry (MS) system, atmospheric pressure chemical ionization (APCI) MS system, and APCI ion-mobility spectrometry system. The three hyphenated variants have been tested in analyses of complex matrices (e.g., fruit-based drinks, whiskey, urine, and stored wastewater). In addition to the use of catalase, COME variants using crude potato pulp or manganese(IV) dioxide have been demonstrated. The technique is inexpensive, fast, reliable, and green: it uses low-toxicity chemicals and emits oxygen.

Bursting-bubble flow microextraction combined with gas chromatography to analyze organophosphorus pesticides in aqueous samples

Bubble-bursting flow microextraction combined with gas chromatography as a green and sustainable microextraction method is used to determine some organophosphorus pesticide residues in water samples. The extraction process occurs at the surface of liquid-gas contact, where the analytes interact with the gas molecules in the bubble. The analytes are transferred to the surface of the sample solution by moving the gas bubbles upwards. The bursting of gas bubbles causes the analytes to disperse in the headspace. Eventually, they are collected for injection into the chromatography system. A one-factor-at-one-time approach was applied to optimize the independent variables in the proposed method. Validation studies were performed according to reliable guidelines. Under optimal conditions, the method indicated a dynamic linear range from 1.0 to 100.0 μg/L. The limit of detection and quantification of the method was 0.29-0.38 and 1.21-1.70 μg/L, respectively. The proposed method was successfully utilized to determine malathion, diazinon, profenofos, and ethion as the target analytes in various water samples with satisfactory relative recoveries ranged from 90.1 to 102.2%.

Harnessing bubble behaviors for developing new analytical strategies

Gas bubbles are easily accessible and offer many unique characteristic properties of a gas/liquid two-phase system for developing new analytical methods. In this minireview, we discuss the newly developed analytical strategies that harness the behaviors of bubbles. Recent advancements include the utilization of the gas/liquid interfacial activity of bubbles for detection and preconcentration of surface-active compounds; the employment of the gas phase properties of bubbles for acoustic imaging and detection, microfluidic analysis, electrochemical sensing, and emission spectroscopy; and the application of the mass transport behaviors at the gas/liquid interface in gas sensing, biosensing, and nanofluidics. These studies have demonstrated the versatility of gas bubbles as a platform for developing new analytical strategies.

Rapid detection of perfluorinated sulfonic acids through preconcentration by bubble bursting and surface-assisted laser desorption/ionization

We developed a preconcentration method in which aerosol droplets containing enriched perfluorinated sulfonic acids (PFSs) are generated through bubble bursting and collected. The droplets were subjected to PFS analysis of perfluorohexane sulfonic acid (PFHxS) and perfluorooctanesulfonic acid (PFOS) through surface-assisted laser desorption/ionization-time-of-flight mass spectrometry; silver nanoplates (AgNPts) were assisting materials. The method was highly efficient, with an approximately three-order magnitude enhancement (5 × 10-13 to 1 × 10-11 M). Ultralow PFS concentrations (0.5 ng/L of PFOS; 0.4 ng/L of PFHxS) were detected in preconcentrated tap water containing PFSs. Our method has potential for rapid real-world PFS detection in water.

Online Scavenging of Trace Analytes in Complex Matrices for Fast Analysis by Carbon Dioxide Bubbling Extraction Coupled with Gas Chromatography-Mass Spectrometry

Carbon dioxide (CO₂) microbubbles can selectively enrich organic solutes from sea spray aerosols. Common bubbling extractions are normally followed by off-line separation/detection through methods such as mass spectrometry, chromatography, and spectroscopy. However, it is necessary to establish extractions with online separation and identification systems to improve efficiency and minimize sample loss. In this study, CO₂ is used to form microbubbles in the sample solution, and trace analytes in the solution are transported to the gas phase by bubble bursting. Analytes at the liquid-gas interface are directly released into the trapping device, followed by thermal desorption for gas chromatography-mass spectrometry. For polycyclic aromatic hydrocarbons, the dependence of the extraction efficiency on various parameters has been analyzed. The method reported here provides high efficiency and minimizes the loss of trace volatiles with a better signal strength and signal-to-noise ratio than other gases. These features make the proposed method a rapid method to detect and quantify volatile/semivolatile analytes in complex liquid matrices. In addition to the preconcentration of organics, metal ions, and inorganic anions, a noticeable decrease of metal-organic compounds in the aqueous solution was shown for the first time. We finally propose a simple model of chemical partitioning in CO₂ bubbling extraction of liquid samples for guiding online monitoring of trace analytes in real-world samples.

Rapid Extraction and Analysis of Volatile Solutes with an Effervescent Tablet

Extraction of volatile compounds from complex liquid matrices is a critical step in volatile compound analysis workflows. Recently, green chemistry principles are increasingly implemented in extraction processes. Some of the available approaches are solvent-free but still require concentration or trapping of analytes. Here, we propose effervescent tablet-induced extraction (ETIE) as a method of transferring volatile/semivolatile compounds from liquid matrices to the gas phase for analysis. This technique relies on the release of carbon dioxide produced in situ during a neutralization reaction, which occurs when a tablet is inserted into an aqueous sample matrix. In this process, many bubbles of carbon dioxide are instantly formed in the sample matrix. The bubbles rapidly extract and liberate volatile compounds from the sample. The gaseous effluent is then immediately transferred to a detector (atmospheric pressure chemical ionization mass spectrometry (MS) or gas chromatography (GC) hyphenated with MS). ETIE-GC-MS can be used for analysis of volatile compounds present in real samples. The method was validated for analysis of selected ethyl esters present in a yogurt drink. The calibration data set was linear over a range from 5 × 10^-7 to 1 × 10^-5 M. The limits of detection ranged from 1.51 × 10^-7 to 6.82 × 10^-7 M, while the recoveries ranged from 71 to 118%. Inter- and intraday precision of selected ethyl esters in aqueous solution was satisfactory (relative standard deviation, 3.6-18.3%). Furthermore, it is shown that ETIE improves the performance of headspace solid-phase microextraction while eliminating the need for heating and shaking samples.

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.

Real-time monitoring of the reaction between aniline and acetonylacetone using extractive electorspray ionization tandem mass spectrometry

In this work an on-line monitoring method was developed to study the mechanism of acetic acid catalyzed reaction between aniline and acetonylacetone using extractive electorspray ionization-tandem mass spectrometry (EESI-MS). The signals of reactants, intermediates and various byproducts were continuously detected as a function of reaction time. The chemical assignment of each signal was done via multi-stage collision induced dissociation (CID) analysis, and the reaction mechanism between aniline and acetonylacetone was deduced based on the generated molecular ions and fragment ions. The results indicate that on-line EESI-MS is an effective technique for the real time analysis of chemical reactions. EESI avoids off-line sample pretreatment and provides "soft" ionization, which allows direct analysis of various analytes at molecular level.

Nanosecond photochemically promoted click chemistry for enhanced neuropeptide visualization and rapid protein labeling

Comprehensive protein identification and concomitant structural probing of proteins are of great biological significance. However, this is challenging to accomplish simultaneously in one confined space. Here, we develop a nanosecond photochemical reaction (nsPCR)-based click chemistry, capable of structural probing of proteins and enhancing their identifications through on-demand removal of surrounding matrices within nanoseconds. The nsPCR is initiated using a photoactive compound, 2-nitrobenzaldehyde (NBA), and is examined by matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). Benefiting from the on-demand matrix-removal effect, this nsPCR strategy enables enhanced neuropeptide identification and visualization from complex tissue samples such as mouse brain tissue. The design shows great promise for structural probing of proteins up to 155 kDa due to the exclusive accessibility of nsPCR to primary amine groups, as demonstrated by its general applicability using a series of proteins with various lysine residues from multiple sample sources, with accumulated labeling efficiencies greater than 90%.

Mass spectrometry-based quantitative proteomics aim to identify and quantify proteins from complex biological samples. Here, the authors developed a method for simultaneous high-throughput protein labelling and on-demand matrix removal within nanoseconds.

On-line coupling of fizzy extraction with gas chromatography

Fizzy extraction (FE) is carried out by first dissolving a carrier gas (typically, carbon dioxide) in a liquid sample at a moderate pressure (typically, 150 kPa) and then rapidly depressurizing the sample. The depressurization leads to instant release of numerous microbubbles in the liquid matrix. The abruptly released gas extracts the volatile solutes and elutes them toward a detector in a short period of time. Here, we describe on-line coupling of FE with gas chromatography (GC). The two platforms are highly compatible and could be combined following several modifications of the interface and adjustments of the extraction sequence. The analytes are released within a short period of time (1.5 s). Thus, the chromatographic peaks are satisfactorily narrow. There is no need to trap the extracted analytes in a loop or on a sorbent, as it is done in standard headspace and microextraction methods. The approach requires only minor sample pretreatment. The main parameters of the FE-GC-mass spectrometry (MS) method were optimized. The results of FE were compared with those of headspace flushing (scavenging headspace vapors), and the enhancement factors were in the order of ~ 2 to 13 (for various analytes). The limits of detection for some of the tested analytes were lower in the proposed FE-GC-MS method than in FE combined with atmospheric pressure chemical ionization MS. The method was further tested in analyses of selected real samples (apple flavor milk, mixed fruit and vegetable juice drink, mango flavored drink, pineapple green tea, toothpaste, and yogurt). Graphical abstract.

Enrichment of Surface-Active Compounds in Bursting Bubble Aerosols

The pronounced enrichment of surface-active compounds in the aerosols produced by bubble bursting plays a central role in the chemical transfer from the ocean into the atmosphere and has an important impact on the global Earth's climate. However, the mechanism of chemical enrichment in bursting bubble aerosols remains poorly understood and controversial due to the high complexity and diversity of experimental behaviors. Contrary to the common belief, here we show that the major share of surfactants in the jet droplets produced by individually bursting bubbles at a calm solution surface is released directly from the bubble surface rather than from the solution surface or subsurface microlayer. We reveal that surfactants are accumulated at the surface of a rising bubble in solution following three successive stages with strongly distinct adsorption profiles: linear kinetic, mixed kinetic, and equilibrium. The magnitude of surfactant enrichment in the aerosol is directly determined by which adsorption mode is in control by the moment of the bubble bursting at solution surface. Our mechanistic description explains the diversity of experimental observations regarding the surfactant enrichment in aerosol droplets and lays the ground for understanding the more complex behaviors associated with collective effects at the solution surface (e.g., breaking waves).

Experimental verification of simultaneous desalting and molecular preconcentration by ion concentration polarization

While the ion concentration polarization (ICP) phenomenon has been intensively researched for the last decade, a complete picture of ion and analyte distributions near nanoporous membranes is strongly desired, not only for fundamental nano-electrokinetic studies but also for the development of lab-on-a-chip applications. Since direct concentration measurements, using either time-consuming collection or microelectrodes, are limited due to low throughput (<nL min^-1 in typical micro/nanofluidic device) and Faradaic reactions, respectively, we measured the concentration changes of prefilled solutions in individual reservoirs in this work. As a result, analytes larger than the size of nanopores were completely repelled by the ICP layer, 65% of cations were transported through the nanoporous membrane to sustain the ICP phenomenon, and the remaining anions were consumed by electrode reactions for electro-neutrality requirements. These combined effects would enable the perfect recovery of a target analyte and the removal of unnecessary salts simultaneously. Using this scenario, the novel concept of an ink recycler was also demonstrated in this work. We showed that 40% of unnecessary salt, which causes serious deterioration of inkjet heads, was removed, while the concentration of ink molecules was doubled in a single-step operation. This simultaneous desalting and molecular preconcentration mechanism would be a key operational strategy of various refinery/purification applications for drug discovery and the chemical industry, etc.

Fizzy Extraction of Volatile and Semivolatile Compounds into the Gas Phase

Extraction of volatile and semivolatile compounds from liquid matrixes with high yields, and transferring the extracts to detectors in real time, is challenging. Common extraction procedures involve heating the samples to release the analytes to the gas phase and, in some cases, trapping the gas-phase analytes into sorbents or containers. Here, we propose a new method for fast extraction of volatile and semivolatile compounds from liquid matrixes. This method involves dissolution of a carrier gas in the liquid sample by applying a moderate overpressure (∼150 kPa) and stirring the sample. An abrupt decompression of the extraction chamber leads to effervescence. In this step, many bubbles are instantly formed in the sample matrix. The dissolved carrier gas as well as dissolved volatiles are liberated into the headspace of the extraction chamber within a short period of time (few seconds). The gaseous effluent of the extraction chamber is immediately transferred to the online detector; in this case, an atmospheric pressure chemical ionization interface of a triple quadrupole mass spectrometer. The fast release of the gas-phase extract gives rise to a high signal recorded by the detector; several times higher than the signal recorded during direct infusion of headspace vapors without fizzy extraction. This feature provides the means to detect and quantify analytes present in solutions in a short period of time. Here we show that fizzy extraction is suitable for analysis of volatile/semivolatile compounds present in various samples, including those containing complex matrixes.