Sample Droplets Research Articles

Simultaneous and Rapid Detection of Glucose and Insulin: Coupling Enzymatic and Aptamer-Based Assays.

Diabetes management demands precise monitoring of key biomarkers, particularly insulin (I) and glucose (G). Herein, we present a bioelectronic chip device that enables the simultaneous detection of I and G in biofluids within 2 min. This dual biosensor chip integrates aptamer-based insulin sensing with enzymatic glucose detection on a single platform, employing a four-electrode sensor chip. The insulin voltammetric sensor employs a G-quadraplex methylene-blue-modified aptamer, while the amperometric biocatalytic glucose sensor utilizes a second-generation mediator-based approach. Simultaneous reagent-less sensing of I and G has been achieved by addressing key challenges. These include combining different surface chemistries, assay formats, and detection principles at closely spaced working electrodes and the substantially different concentration levels of the I and G targets. An attractive analytical performance, with no apparent crosstalk, is demonstrated for the simultaneous detection of millimolar G concentrations and picomolar I concentrations in single microliter serum or saliva sample droplets. This dual biosensor offers rapid, cost-effective, and reliable monitoring, addressing the unmet need for integrated multiplexed diabetes biomarker detection in decentralized settings. Such integration of enzymatic and aptamer-based bioassays could greatly expand the scope of decentralized testing in healthcare beyond diabetes care.

Concurrent Determination of Heat and Capacity Change of a Sessile Droplet Using a Single Measurement

Microcalorimetry, designed for the independent measurement of enthalpy and heat capacity, has been commercially available for a considerable time. However, heat-related states in samples, especially liquids, can introduce complicated phenomena and challenging measurement and data evaluation processes. Such complexity becomes apparent when observing fluctuations in thermal capacity (Cp) while measuring heat consumption (Q) during water evaporation. This paper presents a continuous heat pulse measurement (CHPM) method for concurrently analyzing Q and Cp in a single test using microcalorimetry. The sample droplet of 400 nL was directly dispensed on the microcalorimeter surface, followed by a light-emitting diode (LED) radiation generating heat to perform CHPM. We repetitively heated the microcalorimeter using heat pulses provided by LED irradiation, with their duration set to 100 ms and 10s repetition, while measuring the temperature response of the microcalorimeter sensor. A MATLAB-based simulation model was established to validate the accuracy of our Cp measurements, which show its value of 0.79% of minimum variance. Water evaporation coupled with simultaneous salt crystallization served as our study model, where the Cp values were calculated from real-time responses to heat pulses provided by LED. The experimental outcomes confirm the suitability of CHPM in extracting key thermal properties and emphasize its versatility as a diagnostic tool, providing a significant method for research and applications in the fields of physics, engineering, and beyond.

Continuous monitoring of heart failure biomarker using a sensitive graphene-based electrical sensor

Abstract Heart Failure (HF) is a complex condition that imposes significant burdens on both patients and healthcare systems. The economic impact of HF therapy and management is substantial, underscoring the urgency for proactive measures to mitigate its development. Regular assessments play a pivotal role in identifying individuals at risk and initiating timely interventions to prevent or delay HF progression [1, 2]. NT-proBNP, an amine-terminated form of brain natriuretic peptide, emerges as a promising biomarker in the realm of chronic HF diagnosis, prognosis, and risk assessment [3]. Its utility lies in providing valuable insights into disease progression and prognosis. In recent years, biosensors based on Field Effect Transistors with Graphene (Gr) gate (GFET) have garnered attention for their rapid response, sensitivity, and on-chip integration [4]. Here, we present a sensitive miniaturized GFET sensor for continuous NT-proBNP measurement. The sensor is an array of GFET devices with sensing gates of a monolayer Gr decorated with aptamers as catch probes. Averaging over similar devices in an array approach ensures measurement consistency throughout the experiment. Moreover, the significant challenge posed by the Debye length in FET biosensors is effectively addressed through the functionalization with short folding aptamers [4]. The sensor is built based on a chip comprising an array of 28 individual micro GFETs (Fig. 1a). For the functionalization of Gr gates, we utilized 1-pyrene butyric acid N-hydroxysuccinimide ester as a linker between the aptamer probes and the Gr surface. Subsequently, the Gr surface was incubated with an amine-terminated aptamer solution (Fig. 1c). A schematic diagram of the sensory chip having a droplet of the sample is shown in Fig. 1b. To quantify NT-proBNP levels an 8µL droplet of the buffer solution containing a specific NT-proBNP concentration was deposited onto the sensor. The drain current (IDS) served as the sensor signal for each target concentration (Figs. 2a and 2b). The sensor signal decreased with increasing target concentration, while negligible response was observed when injecting blank buffers. This phenomenon can be attributed to the folding of aptamers upon binding to their target. Also, continuous measurement was conducted at a fixed gate voltage of 0.9V, recording the drain current while the target concentration gradually increased in the sample droplet. This process revealed a decrease in sensor current as we expected (Fig. 2c). The sensor exhibits remarkably low detection limits of 50 pg/mL and a linear range from 100 pg/mL to 10 ng/mL, which are highly conducive for early HF diagnosis and aligning well with reported NT-proBNP levels in patient biofluid [5]. The proposed sensor holds promise as a wearable device for continuous monitoring of high-risk patients. Additionally, this technology shows potential as a detection tool for acute heart failure in hospitalized patients.Sensor viewSensor responses

Research on Deposition Characteristics of a New Air-Assisted Electrostatic Sprayer

This study evaluated the properties of a new double air-assisted electrostatic orchard sprayer, focusing particularly on the impact of its electrostatic system on droplet deposition. The sprayer featured 10 nozzles, a centrifugal fan, and an axial fan, with an overall flow rate of 3.5 L/min and nozzle air velocities of 25 m/s (interior) and 12.5 m/s (exterior). Droplet sampling was conducted using paper cards and ponceau solution as a tracer. Field measurements in a Y-tree trained pear orchard assessed the effects of tractor speeds (0.34, 0.52, 0.84, and 1.05 m/s) and sampling positions (exposed and hidden faces). Tractor speed did not significantly affect overall droplet density, but the normalized droplet density on the exposed face increased with speed, while it decreased on the hidden face. The average normalized droplet density at all speeds was approximately 114.6 dot/cm2. The electrostatic system improved droplet deposition on the hidden face of leaves, particularly on the side of the tree near the sprayer. The coefficient of variation (CV) of the exposed face increased with tractor speed, while tractor speed had no impact on the CV of the hidden face. Overall, the electrostatic system reduced the CV. This research offers valuable insights into the use of electrostatic sprayers.

High-resolution acoustic ejection mass spectrometry for high-throughput library screening

An approach is described for high-throughput quality assessment of drug candidate libraries using high-resolution acoustic ejection mass spectrometry (AEMS). Sample introduction from 1536-well plates is demonstrated for this application using 2.5 nL acoustically dispensed sample droplets into an Open Port Interface (OPI) with pneumatically assisted electrospray ionization at a rate of one second per sample. Both positive and negative ionization are shown to be essential to extend the compound coverage of this protease inhibitor-focused library. Specialized software for efficiently interpreting this data in 1536-well format is presented. A new high-throughput method for quantifying the concentration of the components (HTQuant) is proposed that neither requires adding an internal standard to each well nor further encumbers the high-throughput workflow. This approach for quantitation requires highly reproducible peak areas, which is shown to be consistent within 4.4 % CV for a 1536-well plate analysis. An approach for troubleshooting the workflow based on the background ion current signal is also presented. The AEMS data is compared to the industry standard LC/PDA/ELSD/MS approach and shows similar coverage but at 180-fold greater throughput. Despite the same ionization process, both methods confirmed the presence of a small percentage of compounds in wells that the other did not. The data for this relatively small, focused library is compared to a larger, more chemically diverse library to indicate that this approach can be more generally applied beyond this single case study. This capability is particularly timely considering the growing implementation of artificial intelligence strategies that require the input of large amounts of high-quality data to formulate predictions relevant to the drug discovery process. The molecular structures of the 872-compound library analyzed here are included to begin the process of correlating molecular structures with ionization efficiency and other parameters as an initial step in this direction.

On-Chip Droplet Splitting with High Volume Ratios Using a 3D Conical Microstructure-Based Microfluidic Device.

This work reports a simple microfluidic method for splitting a mother droplet into two daughter droplets with high and precise volume ratios. To achieve this, a droplet-splitting microfluidic device embedded with a three-dimensional (3D) conical microstructure is fabricated, in which the high splitting ratios of monodisperse mother droplets are achieved. The volume ratio of the split daughter droplets can reach up to 265. In addition, we examined factors that affect the splitting ratio of the daughter droplets and found that the ratio is affected by the flow rates of the two individual outlet channels, the injection length of the conical microstructure, and the diameter of the original mother droplets. Numerical simulations of these parameters were conducted to gain a clearer understanding of the splitting behavior. The proposed droplet splitting device with a conical microstructure enables on-chip sample extraction and droplet volume control, which can be a powerful tool for various droplet-based applications in microfluidic devices such as viral infectivity assays and sequencing heterogeneous populations.

Artificial intelligence-enabled multipurpose smart detection in active-matrix electrowetting-on-dielectric digital microfluidics

An active-matrix electrowetting-on-dielectric (AM-EWOD) system integrates hundreds of thousands of active electrodes for sample droplet manipulation, which can enable simultaneous, automatic, and parallel on-chip biochemical reactions. A smart detection system is essential for ensuring a fully automatic workflow and online programming for the subsequent experimental steps. In this work, we demonstrated an artificial intelligence (AI)-enabled multipurpose smart detection method in an AM-EWOD system for different tasks. We employed the U-Net model to quantitatively evaluate the uniformity of the applied droplet-splitting methods. We used the YOLOv8 model to monitor the droplet-splitting process online. A 97.76% splitting success rate was observed with 18 different AM-EWOD chips. A 99.982% model precision rate and a 99.980% model recall rate were manually verified. We employed an improved YOLOv8 model to detect single-cell samples in nanolitre droplets. Compared with manual verification, the model achieved 99.260% and 99.193% precision and recall rates, respectively. In addition, single-cell droplet sorting and routing experiments were demonstrated. With an AI-based smart detection system, AM-EWOD has shown great potential for use as a ubiquitous platform for implementing true lab-on-a-chip applications.

An analytical study of TEG loss in a natural gas purification plant based on high-pressure droplet angular scattering

In the process of purifying natural gas, natural gas dehydration plants use triethylene glycol (TEG) as a solvent for absorption. However, the loss of TEGs in the form of droplets can pollute pipelines and create a technological challenge. Using the geometric optical approximation (GOA) method, a model was developed to predict the scattering of droplets at high pressure. Based on this model, a device was designed and constructed to inspect droplets in external pipelines and distillation column tailgate lines online, which are the outlets of the natural gas dehydration unit (TEG absorber tower) of four columns in stable production. This device analyzes the components of the sampled droplets and performs online inspection to ensure smooth and safe operation. The results of the experiment indicate that the TEG content in the droplet samples exceeded 80%, which helped identify the source of the loss. The amount of treatment directly affected the concentration and distribution features of the entrained droplets, whereas the impact of the gas-phase pressure change was negligible. This study investigated three different forms of filtration separation, which provided industrial data for the separation and recovery process of entrained droplets. This study provides a new testing tool and method to examine plate towers under high pressure and provides a theoretical reference for dewatering procedures in natural gas purification plants.

Effect of an extraoral scavenging device on eliminating droplets and aerosols generated in ultrasonic supragingival scaling: A randomised clinical trial.

Ultrasonic scaling is extensively applied as part of the initial therapy for periodontal diseases, which has been restricted since the outbreak of the COVID-19 pandemic due to droplets and aerosols generated by ultrasonic devices. An extraoral scavenging device (EOS) was designed for diminishing droplets and aerosols in dental clinics. The objective of this study is to evaluate the effect of EOS on eliminating droplets and aerosols during ultrasonic supragingival scaling. This single-blinded, randomised controlled clinical trial enrolled 45 patients with generalised periodontitis (stage I or II, grade A or B) or plaque-induced gingivitis. The patients were randomly allocated and received ultrasonic supragingival scaling under three different intervention measures: only saliva ejector (SE), SE plus EOS and SE plus high-volume evacuation (HVE). The natural sedimentation method was applied to sample droplets and aerosols before or during supragingival scaling. After aerobic culturing, colony-forming units (CFUs) were counted and analysed. Compared with the level before treatment, more CFUs of samples throughout treatment could be obtained at the operator's chest and the patient's chest and the table surface when using SE alone (p < 0.05). Compared with the SE group, the SE + EOS group and the SE + HVE group obtained decreasing CFUs at the operator's chest and the patient's chest (p < 0.05), while no significant difference was determined between these two groups. The EOS effectively eliminated splatter contamination from ultrasonic supragingival scaling, which was an alternative precaution for nosocomial contamination in dental clinics.

Thermoset droplet curing performance in the microbond test

ABSTRACT Users of the microbond test assume that a microbond resin droplet’s properties are equivalent to a macroscale specimen. However, there is currently no standardised methodology for determining the cure state of droplet specimens used in the microbond test. In this paper, we present a technique for microbond test users to better understand the properties of thermoset droplet specimens. Utilising a conventional benchtop spectrometer, a novel sample preparation technique involving curing epoxy droplets on thin-steel filaments allowed for high-throughput determination of the microbond droplet cure state. The parity between steel filament and glass fibre microbond samples was confirmed by infrared microspectroscopy. It is shown that cure schedules used in manufacturing composite parts produced microbond droplets with degrees of cure lower than that of bulk matrix specimens subjected to an identical thermal history. Testable microbond droplets could only be prepared for commercial resin systems when introducing a room temperature pre-curing time of at least 2 h. It is concluded that microbond testing should be supported by some droplet cure state characterisation methods to ensure that interfacial effects are not artefacts of droplet sample preparation.

Observation and analysis of the ultra-rapid cooling effect of cryoprotectant droplets on a sapphire surface

Cryopreservation has emerged as a promising technology to realize semipermanent storage of biomaterials widely used in assisted reproduction and cell therapy. A large portion of successful cryopreservation requires the combination of ultra-rapid cooling and appropriate low-toxicity cryoprotectant agents, which inhibits the formation of ice crystals that damages the cells lethally. Conventional approaches are not competent in accurately capturing high enough cooling rate and more importantly, not able to simultaneously track the instant appearance of the sample at a microscale level during the momentary cooling process. A rapid freezing device for the droplet sample study purpose was designed and developed, which is capable of visualization of the sample on a sapphire surface at a vitrification cooling rate over 104 K/min. The instantaneous temperature changes and the appearance of the cooling samples were recorded at the speed of 10 kHz and 2000 frames. Typical vitrification and crystallization processes were observed and analysed. Low concentrations of the CPA solution inhibited the formation and growth of the crystals, and resulted in the formation of layered ice, punctate ice and scattered small ice. The experimental results could contribute to a better understanding of the ultra-rapid cooling mechanism on cryoprotectant droplets.

Deep-learning based 3D birefringence image generation using 2D multi-view holographic images

Refractive index stands as an inherent characteristic of a material, allowing non-invasive exploration of the three-dimensional (3D) interior of the material. Certain materials with different refractive indices produce a birefringence phenomenon in which incident light is split into two polarization components when it passes through the materials. Representative birefringent materials appear in calcite crystals, liquid crystals (LCs), biological tissues, silk fibers, polymer films, etc. If the internal 3D shape of these materials can be visually expressed through a non-invasive method, it can greatly contribute to the semiconductor, display industry, optical components and devices, and biomedical diagnosis. This paper introduces a novel approach employing deep learning to generate 3D birefringence images using multi-viewed holographic interference images. First, we acquired a set of multi-viewed holographic interference pattern images and a 3D volume image of birefringence directly from a polarizing DTT (dielectric tensor tomography)-based microscope system about each LC droplet sample. The proposed model was trained to generate the 3D volume images of birefringence using the two-dimensional (2D) interference pattern image set. Performance evaluations were conducted against the ground truth images obtained directly from the DTT microscopy. Visualization techniques were applied to describe the refractive index distribution in the generated 3D images of birefringence. The results show the proposed method’s efficiency in generating the 3D refractive index distribution from multi-viewed holographic interference images, presenting a novel data-driven alternative to traditional methods from the DTT devices.

Selective Sparse Sampling of Water Droplets in Oil with Acoustic Tweezers.

In microfluidics, water droplets are often used as independent biochemical microreactor units, enabling the implementation of massively parallel screening assays where only a few of the reacting water droplets yield a positive result. However, sampling the product of these few successful reactions is an unsolved challenge. One possible solution is to use acoustic tweezers, which are lab-free, easily miniaturized, and biocompatible manipulation tools, and existing acoustic tweezers manipulating particles or cells, and water droplet manipulation in oil with an acoustic tweezer is absent. The first challenge in attempting to recover a few water droplets from a large batch is the selective manipulation of water droplets in an oil system. In this paper, we trap and manipulate single water droplets in oil using integrated single-beam (focused beam/vortex beam) acoustic tweezers for the first time. We find that water droplets with a diameter smaller than half a wavelength are trapped by acoustic vortices, while larger ones are better captured by focused acoustic beams. It is the first step to extract the target water droplet microreactors (positive ones) in an oil system and analyze their content. Compared to previous techniques, such as fluorescence-activated cell sorting (FACS), our technique is sparse, meaning that the sampling time is proportional to the number of droplets required and very insensitive to the total number of microreactors, making it well suited for large-scale screening assays.

Solvent-free sample preparation for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of polymer blends.

Solvent-free sample preparation offers some advantages over solvent-based techniques, such as improved accuracy, reproducibility and sensitivity, for matrix-assisted laser desorption/ionization (MALDI) analysis. However, little or no information is available on the application of solvent-free techniques for the MALDI analysis of polymer blends. Solvent-free sample preparation by ball milling was applied with varying sample-to-matrix ratios for MALDI time-of-flight mass spectrometry analysis of various polymers, including polystyrenes, poly(methyl methacrylate)s and poly(ethylene glycol)s. The peak intensity ratios were compared with those obtained after using the conventional dried droplet sample preparation method. In addition, solvent-assisted milling was also applied to improve sample homogeneities. Depending on the sample preparation method used, different peak intensity ratios were found, showing varying degrees of suppression of the signal intensities of higher mass polymers. Ball milling for up to 30 min was required to achieve constant intensity ratios indicating homogeneous mixtures. The use of wet-assisted grinding to improve the homogeneity of the blends was found to be disadvantageous as it caused partial degradation and mass-dependent segregation of the polymers in the vials. The results clearly show that solvent-free sample preparation must be carefully considered when applied to synthetic polymer blends, as it may cause additional problems with regard to homogeneity and stability of the blends.

High-Throughput Capillary Liquid Chromatography Using a Droplet Injection and Application to Reaction Screening.

The cycle time of a standard liquid chromatography (LC) system is the sum of the time for the chromatographic run and the autosampler injection sequence. Although LC separation times in the 1-10 s range have been demonstrated, injection sequences are commonly >15 s, limiting throughput possible with LC separations. Further, such separations are performed on relatively large bore columns requiring flow rates of ≥5 mL/min, thus generating large volumes of mobile phase waste when used for large scale screening and increasing the difficulty in interfacing to mass spectrometry. Here, a droplet injector system was established that replaces the autosampler with a four-port, two-position valve equipped with a 20 nL internal loop interfaced to a syringe pump and a three-axis positioner to withdraw sample droplets from a well plate. In the system, sample and immiscible fluid are pulled alternately from a well plate into a capillary and then through the injection valve. The valve is actuated when sample fills the loop to allow sequential injection of samples at high throughput. Capillary LC columns with 300 μm inner diameter were used to reduce the consumption of mobile phase and sample. The system achieved 96 separations of 20 nL droplet samples containing 3 components in as little as 8.1 min with 5-s cycle time. This system was coupled to a mass spectrometer through an electrospray ionization source for high-throughput chemical reaction screening.

Development of a multi-stage fog droplet screening system based on the virtual impact principle.

Accurately measuring fog droplet spectra is essential for understanding fog's formation, dissipation, and composition, which makes a challenge to the performance of droplet sampling and measurement systems. Standard particles such as glass beads are widely used to characterize their performance. However, the disparities between glass beads and fog droplets, including refractivity, size distribution, and composition, may lead to calibration errors. In this context, we developed a three-stage fog droplet screening system based on the virtual impact principle. We determined the Stokes number and the diameter of the acceleration nozzle through theoretical analysis. Subsequently, we utilized the computational fluid dynamics software Fluent to explore the influence of key system parameters on screening efficiency, including the diameter of the collection nozzle (D1) and the distance between the acceleration nozzle and the collection nozzle (S). The simulation results indicated that the screening efficiency improved with S. The best performance was achieved when D1 = 1.35 D0 and S = 1.90 D0 (where D0 represents the diameter of the acceleration nozzle), resulting in an average screening efficiency of 75.4%. Finally, we conducted experiments to validate the effectiveness of the screening system. The screening efficiency of each outlet was estimated at 42.2%, 66.1%, 84.0%, and 95.3%, with differences of 2.0%, 3.3%, 4.1%, and 4.7% compared to the simulations. The average screening efficiency was 71.9%, with a deviation of 3.5% from the simulation. These findings demonstrated that the screening system could provide an alternative technical apparatus for characterizing droplet sampling and measurement systems.

Rapid and ultra-sensitive trace metals detection of water by partial Leidenfrost superhydrophobic array surface enhanced laser-induced breakdown spectroscopy

The rapid and ultra-sensitive detection of trace elements in liquid is a primary concern for researchers. In this study, a partial Leidenfrost effect superhydrophobic (PLSHB) array surface was used for rapid in situ evaporation enrichment of sample droplets. Within 4 min, a 50 μL droplet sample was completely evaporated, resulting in all solutes in it being concentrated within a circular range measuring approximately 350 μm in diameter, without the formation of a coffee ring structure. The limits of detection for six metals (Pb, Ba, Be, Mn, Cr, Cu) in water were determined to be as follows: 0.82 μgL−1, 0.27 μgL−1, 0.033 μgL−1, 0.136 μgL−1, 0.241 μgL−1, and 0.083 μgL−1. Furthermore, laser-induced breakdown spectroscopy (LIBS) was employed to detect the enriched solutes from ten liquid samples with identical concentrations on the PLSHB array surface; these measurements exhibited a relative standard deviation (RSD) of only 3.7%. Spike experiments involving the addition of the aforementioned six metals into drinking water demonstrated recovery rates ranging from 85.7% to 117.7%. Therefore, the application potential of PLSHB array surface enhanced LIBS for rapid, stable, and ultra-sensitive detection and analysis of trace metal elements across various fields such as industry, environmental science, and biomedicine might be highly promising.

Thermophysical Properties of Vanadium Melts and Discussion of Thermal Diffusivity in Mott’s Theory

The temperature dependence of density, normal spectral emissivity, heat capacity at constant pressure, and thermal conductivity of the V melt were measured with high accuracy using electromagnetic levitation in a static magnetic field. Surface vibration, translational motion, and convection of the electromagnetically levitated droplet sample were suppressed by the magnetic field. In the measurement of thermal conductivity, convection in the V-melt was sufficiently suppressed by the application of a field of 7 T or higher. In this study, the measured emissivity and thermal conductivity are compared with those evaluated using the free-electron models (Drude model and Wiedemann–Franz rule). Correlations between the density of states and thermal diffusivity at the Fermi energy of transition metals in the liquid state are investigated and the applicability of Mott's s–d scattering model is discussed.

Exploring drying-induced separation of main constituents in melamine-blended milk infant formula using DCDR spectroscopy

Drop coating deposition Raman (DCDR) spectroscopy was employed to investigate the melamine compound susceptibility to milk infant formula main constituents as lactose and lipids (specifically 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE)). DCDR approach is based on the deposition and drying of a small droplet (a few µl) of liquid sample on a hydrophobic substrate. After complete evaporation, the analyte is preconcentrated into the dried pattern from which the classical Raman spectra are acquired. A simple melamine-blended model reactions were tested (melamine-blended lactose, DOPC, and DOPE). From these, lactose was shown to be the most potent reaction partner for melamine, where a significant difference in dried pattern between pure and blended lactose was noticed, and the relevant Raman spectral changes were observed. Pure and melamine-blended infant formula solutions from two kinds of powdered infant formula purchased in the local market were then studied. For pure milk solutions, the drying process, as an integral part of the DCDR approach, led to the spatial separation of lipids and carbohydrates in the resulting pattern. Acquired Raman spectra revealed that the ring edge of the dried pattern was composed mainly of lipids, while the thin film in the central part contained mainly carbohydrates, especially lactose. For the melamine-blended formula, the DCDR approach identified that melamine was present only in the central part of the dried pattern together with carbohydrates. As a result, it was assumed that melamine has a higher susceptibility for carbohydrates than for lipid molecules, even in milk infant formula.

Utilizing a Metal Inlet Coiled with Copper Wire as the Ion Source for Ambient Ionization Mass Spectrometry.

In ambient ionization mass spectrometry (MS), a customized metal inlet is typically adapted to the orifice of the mass spectrometer for ease of introduction of the sample. We herein explore that the metal inlet coiled with a copper wire (∼50 μm) can be directly used as an ion source to induce corona discharge-like processes for ionization of analytes in the gas phase. When the metal inlet is subjected to a high voltage in the mass spectrometer, the electric field provided by the mass spectrometer enables the generation of corona discharge to ionize volatile/semivolatile analytes derived from the sample in the condensed phase. The limit of detection for azulene derived from the aqueous sample was as low as ∼1 pM. Moreover, we also demonstrated the feasibility of coupling ultraviolet-visible absorption spectroscopy with MS by using the metal inlet coiled with a thin copper wire as the interface. Integration of these two techniques enables the simultaneous acquisition of spectra from both instruments for quantitative and qualitative analysis of the sample. Furthermore, we showed that polar and nonpolar analytes in a mixture can be acquired in the same mass spectrum by simply depositing a sample droplet (∼20 μL) on a dielectric substrate near the copper wire-coiled metal inlet of the mass spectrometer. The ionization processes involved both electrospray ionization and corona discharge. To demonstrate the applicability of our method for detecting nonpolar and polar analytes in complex samples, we spiked a nonpolar analyte, benzo[a]pyrene, to a spice sample and successfully detected analytes with different polarities using our approach.