Direct comparison of the hygroscopic properties of ammonium sulfate and sodium chloride aerosol at relative humidities approaching saturation

Holographic optical tweezers are used to make comparative measurements of the hygroscopic properties of single component aqueous aerosol containing sodium chloride and ammonium sulfate over a range of relative humidity from 84% to 96%. The change in RH over the course of the experiment is monitored precisely using a sodium chloride probe droplet with accuracy better than ±0.09%. The measurements are used to assess the accuracy of thermodynamic treatments of the relationship between water activity and solute mass fraction with particular attention focused on the dilute solute limit approaching saturation vapor pressure. The consistency of the frequently used Clegg-Brimblecombe-Wexler (CBW) treatment for predicting the hygroscopic properties of sodium chloride and ammonium sulfate aerosol is confirmed. Measurements of the equilibrium size of ammonium sulfate aerosol are found to agree with predictions to within an uncertainty of ±0.2%. Given the accuracy of treating equilibrium composition, the inconsistencies highlighted in recent calibration measurements of critical supersaturations of sodium chloride and ammonium sulfate aerosol cannot be attributed to uncertainties associated with the thermodynamic predictions and must have an alternative origin. It is concluded that the CBW treatment can allow the critical supersaturation to be estimated for sodium chloride and ammonium sulfate aerosol with an accuracy of better than ±0.002% in RH. This corresponds to an uncertainty of ≤1% in the critical supersaturation for typical supersaturations of 0.2% and above. This supports the view that these systems can be used to accurately calibrate instruments that measure cloud condensation nuclei concentrations at selected supersaturations. These measurements represent the first study in which the equilibrium properties of two particles of chemically distinct composition have been compared simultaneously and directly alongside each other in the same environment.     

Comparative thermodynamic studies of aqueous glutaric acid, ammonium sulfate and sodium chloride aerosol at high humidity

Aerosol optical tweezers are used to simultaneously characterize and compare the hygroscopic properties of two aerosol droplets, one containing inorganic and organic solutes and the second, referred to as the control droplet, containing a single inorganic salt. The inorganic solute is either sodium chloride or ammonium sulfate and the organic component is glutaric acid. The time variation in the size of each droplet (3-7 microm in radius) is recorded with 1 s time resolution and with nanometre accuracy. The size of the control droplet is used to estimate the relative humidity with an accuracy of better than +/-0.09%. Thus, the Kohler curve of the multicomponent inorganic/organic droplet, which characterizes the variation in equilibrium droplet size with relative humidity, can be determined directly. The measurements presented here focus on high relative humidities, above 97%, in the limit of dilute solutes. The experimental data are compared with theoretical treatments that, while ignoring the interactions between the inorganic and organic components, are based upon accurate representations of the activity-concentration relationships of aqueous solutions of the individual salts. The organic component is treated by a parametrized fit to experimental data or by the UNIFAC model and the water activity of the equilibrium solution droplet is calculated using the approach suggested by Clegg, Seinfeld and Brimblecombe or the Zdanovskii-Stokes-Robinson approximation. It is shown that such an experimental strategy, comparing directly droplets of different composition, enables highly accurate measurements of the hygroscopic properties, allowing the theoretical treatments to be rigorously tested. Typical deviations of the experimental measurements from theoretical predictions are shown to be around 1% in equilibrium size, comparable to the variation between the theoretical frameworks considered.     

Crystal structures and physicochemical properties of mixed salts of ammonium nitrate and sulfate

Mixed salts (NH₄)₂SO₄·2NH₄NO₃ (1) and (NH₄)₂SO₄·3NH₄NO₃ (2) were synthesized and studied by X-ray diffraction analysis. The unit cell parameters of these salts were determined and their crystal structures were solved. The thermal stability of the salts was studied by differential scanning calorimetry and thermogravimetric analysis. The temperatures and enthalpies of incongruent melting of compounds 1 and 2 were determined. The enthalpies of formation from the constituent salts were estimated.     

Tumour cell identification by means of Raman spectroscopy in combination with optical traps and microfluidic environments

Raman spectroscopy has been recognized to be a powerful tool for label-free discrimination of cells. Sampling methods are under development to utilize the unique capabilities to identify cells in body fluids such as saliva, urine or blood. The current study applied optical traps in combination with Raman spectroscopy to acquire spectra of single cells in microfluidic glass channels. Optical traps were realized by two 1070 nm single mode fibre lasers. Microflows were controlled by a syringe pump system. A novel microfluidic glass chip was designed to inject single cells, modify the flow speed, accommodate the laser fibres and sort cells after Raman based identification. Whereas the integrated microchip setup used 514 nm for excitation of Raman spectra, a quartz capillary setup excited spectra with 785 nm laser wavelength. Classification models were trained using linear discriminant analysis to differentiate erythrocytes, leukocytes, acute myeloid leukaemia cells (OCI-AML3), and breast tumour cells BT-20 and MCF-7 with accuracies that are comparable with previous Raman experiments of dried cells and fixed cells in a Petri dish. Implementation into microfluidic environments enables a high degree of automation that is required to improve the throughput of the approach for Raman activated cell sorting.     

Experimental investigation of the homogeneous freezing of aqueous ammonium sulfate droplets

We have measured the light scattering intensity and homogeneous ice nucleation temperatures from water droplets containing 0-33 wt % ammonium sulfate. In these laboratory experiments, we used a free-fall freezing tube technique to determine the fraction of frozen droplets at a particular droplet temperature by measuring the depolarized light scattering intensity from the droplets in free-fall. Previously reported freezing temperatures for solution concentrations greater than 5 wt % display a larger spread than can be accounted for by the reported experimental errors. We find freezing temperatures in good agreement with the lowest temperature freezing results reported by previous experiments. Our ammonium sulfate freezing temperature data set with water activity less than 0.98 is consistent with a curve that deviates in activity shift by about 5% from the best-fit ice nucleation temperature versus water activity curve found by Koop et al. in 2000, but the significance of this deviation will only be known with further high-precision ice nucleation temperature measurements for other aqueous solutions.     

Observation of the crystallization and supersaturation of mixed component NaNO3-Na2SO4 droplets by FTIR-ATR and Raman spectroscopy

We present here a study of the phase behavior of mixed component NaNO(3)-Na(2)SO(4) (SNS) droplets with NaNO(3) to Na(2)SO(4) molar ratios of 1:1, 3:1, and 10:1, comparing observations with thermodynamic predictions. Measurements are made by Fourier transform infrared attenuated total reflection and micro-Raman spectroscopy for SNS droplets deposited on ZnSe and quartz substrates, respectively. The conventional deliquescence/efflorescence hysteresis in phase behavior is observed. On drying, heterogeneous crystallization leads to phase behavior that is consistent with bulk solution thermodynamics, with the formation of the mixed salt NaNO(3)·Na(2)SO(4)·H(2)O, Na(2)SO(4) (s), and NaNO(3) (s) all observed to form at relative humidities that coincide with predictions by the aerosol inorganics model. However, conditioning of the droplet at high relative humidity prior to drying is observed to lead to quantitative differences between the fractions of different salts formed. When substrate effects do not influence the crystallization process, supersaturated solutions are formed, and this leads to the observation of contact ion pairs. Such measurements of the phase behavior of mixed component droplets are important for testing the reliability of thermodynamic models.     

Magnesium sulfate aerosols studied by FTIR spectroscopy: hygroscopic properties, supersaturated structures, and implications for seawater aerosols

Supersaturated MgSO4 aerosols and dilute MgSO4 solutions were studied by FTIR spectroscopic techniques (i.e., aerosol flow tube (AFT) and attenuated total reflection (ATR)). The hygroscopic properties of MgSO4 aerosols were investigated with results in good agreement with previous measurements by a scanning electrodynamic balance (SEDB). Well-defined spectral evolutions with changing relative humidity (RH) for the v3 band of SO4(2-) and the water O-H stretching envelope could be directly related to the observed hygroscopic properties of MgSO4 aerosols. When the RH decreased from approximately 55 to approximately 40%, the v1 band of SO4(2-) in supersaturated MgSO4 aerosols was observed to transform from a sharp peak at approximately 983 cm(-1) into a wide band at approximately 1005 cm(-1). The sharp peak at approximately 983 cm(-1) was mainly assigned to such associated complexes of Mg2+ and SO4(2-) as double solvent-separated ion pairs (2SIPs), solvent-shared ion pairs (SIPs), and simple contact ion pairs (CIPs) in supersaturated MgSO4 aerosols, while the wide band at approximately 1005 cm(-1) was due to polymeric CIPs chains, probably the main component of gels formed in MgSO4 aerosols at low RHs. Relating to this v1 band transformation, the peak position of the v(3) band was first shown to be a sensitive indicator of CIPs formation, spanning across approximately 40 cm(-1) on the formation of polymeric CIPs chains, which could also be supported by aerosol composition analysis in the form of water-to-solute molar ratios (WSR). In the water O-H stretching envelope, the absorbance intensities at 3371 and 3251 cm(-1) were selected to represent contributions from weak and strong hydrogen bonds, respectively. The absorbance intensity ratio changing with RH of 3371 to 3251 cm(-1) could be related to the previous observations with the v1 and v3 bands of SO4(2-). As a result, the formation of CIPs with various structures in large amounts was supposed to significantly weaken hydrogen bonds in supersaturated MgSO4 aerosols, while 2SIPs and SIPs were not expected to have similar effects even when occurring in abundance. In comparison with MgSO4 aerosols, the peak positions of the v3 band of SO4(2-) in artificial seawater aerosols implied that the MgSO4 component should be contained as gels or concentrated solutions in the fissures of microcrystals of sea salts for freshly formed seawater aerosols at low RHs.     

TG,DTA, FTIR and Raman spectral analysis of Zna/Mgb ammonium sulfate mixed crystals

To understand the structural and thermal properties of the mixed crystals, thermogravimetric (TG) and differential thermal analysis (DTA), and FTIR and Raman spectral studies were carried out for the mixed crystals of Zn_a/Mg_b ammonium sulfate of composition namely 'a' (fraction by mass of salt Zn[NH₄]₂[SO₄]₂·6H₂O to the total salt (both Zn[NH₄]₂[SO₄]₂·6H₂O, Mg[NH₄]₂[SO₄]₂·6H₂O or it can be explained as Zn_aMg_b[NH₄]₂[SO₄]₂·6H₂O, a + b =1), and a = 0.1, 0.25, 0.333, 0.5, 0.666, 0.75 and 0.9 grown by a solution technique. From the correlation and analysis of the results obtained for the various crystals, the desolvation, decomposition, crystalline transition phenomena were identified. By close comparison of the endotherms, obtained for the various crystals, it was found that isomorphous substitution takes place in the crystals. Up to 0.5, Zn²⁺ ion replaces isomorphous Mg²⁺ ions in the lattice sites of Mg[NH₄]₂[SO₄]₂·6H₂O and above 0.5, Mg²⁺ ions occupies the Zn²⁺ ion in the lattice sites of Zn[NH₄]₂[SO₄]₂·6H₂O. Both crystals belong to monoclinic system with P 2(1)/a symmetry. The vibrations of NH₄ ⁺ ion, SO₄ ^2- ion, the complex [Mg(OH₂)₆]²⁺ the complex [Zn(OH₂)₆]²⁺ and the three different water molecules are identified. The linear distortion of SO₄ ^2- ion is found to be greater than its angular distortion, while the NH₄ ⁺ ion has suffered more angular distortion. The possibility of free rotation of the NH₄ ⁺ ion is ruled out.     

Raman acoustic levitation spectroscopy of red blood cells and Plasmodium falciparum trophozoites

Methods to probe the molecular structure of living cells are of paramount importance in understanding drug interactions and environmental influences in these complex dynamical systems. The coupling of an acoustic levitation device with a micro-Raman spectrometer provides a direct molecular probe of cellular chemistry in a containerless environment minimizing signal attenuation and eliminating the affects of adhesion to walls and interfaces. We show that the Raman acoustic levitation spectroscopic (RALS) approach can be used to monitor the heme dynamics of a levitated 5 microL suspension of red blood cells and to detect hemozoin in malaria infected cells. The spectra obtained have an excellent signal-to-noise ratio and demonstrate for the first time the utility of the technique as a diagnostic and monitoring tool for minute sample volumes of living animal cells.     

Investigations of kinetics and mechanism of chloropinnoite in boric acid aqueous solution at 303 K by Raman spectroscopy

Raman spectroscopy of dissolution and transformation of chloropinnoite in 4.5% (wt.%) boric acid aqueous solution at 303 K has been recorded. The Raman spectra of kinetics process have been obtained. The phase transformation product is 2MgO.3B2O3.15H2O (kurnakovite). The main polyborate anions and their interaction in aqueous solution have been proposed according to the Raman spectrum. Some assignments were tentatively given and the relations between the existing forms of polyborate anions and the crystallizing solid phases have been gained. A mechanism of dissolution and crystallization reactions and the formation condition of kurnakovite in Qinghai-Tibet plateau were proposed and discussed.     

Quantitative analysis of serum and serum ultrafiltrate by means of Raman spectroscopy

The fast and reliable determination of concentrations of blood, plasma or serum constituents is a major requirement in clinical chemistry. We explored Raman spectroscopy as a reagent-free tool for predicting the concentrations of different parameters in serum and serum ultrafiltrate. In an investigation using samples from 247 blood donors (which we believe to be the largest study on Raman spectroscopy of serum so far) the concentrations of glucose, triglycerides, urea, total protein, cholesterol, high density lipoprotein, low density lipoprotein and uric acid were determined with an accuracy within the clinically interesting range. After training a multivariate algorithm for data analysis, using 148 samples, concentrations were predicted blindly for the remaining 99 serum samples based solely on the Raman spectra. Relative errors of prediction around 12% were obtained. Moreover, to the best of our knowledge, differentiation between HDL and LDL cholesterol as well as the quantification of uric acid was for the first time successfully accomplished for serum-based Raman spectroscopy. Finally, we showed that ultrafiltration can efficiently reduce fluorescent light background to improve prediction accuracy such that the relative coefficient of variation was reduced for glucose and urea in ultrafiltrate by more than a factor of 2 when compared to serum.     

Screening the optical properties of Ag-Au alloy gradients formed by bipolar electrodeposition using surface enhanced Raman spectroscopy

We report the synthesis of Ag-Au alloy gradients on stainless steel substrates using bipolar electrodeposition (BP-ED), a technique based on the existence of a potential gradient at the interface of a bipolar electrode (BPE) and an electrolytic solution. The interfacial potential gradient causes the rates of electrodeposition of Ag and Au to vary along the length of the BPE, leading to the electrodeposition of a chemical concentration gradient. The surface morphology of the electrodeposits was characterized using scanning electron microscopy (SEM), and their chemical composition was determined using energy dispersive X-ray spectroscopy (EDX). Self-assembled monolayers of a Raman-active probe molecule (benzene thiol) were allowed to form on the surface of the alloy gradients, and confocal Raman microscopy was employed to determine the alloy composition that resulted in the maximum surface enhanced Raman scattering (SERS) intensity. An alloy composition of ca. 70% Ag/30% Au was found to be optimum for SERS excited using 514.5 nm radiation, and it is explained on the basis of composition-dependent changes in the local surface plasmon resonance (LSPR) of the electrodeposited Ag-Au alloy.     

Structure and conformation analysis on perfluorodecanoic acid and its lithium,sodium and ammonium salts as studied by Raman spectroscopy

Raman spectroscopy is shown to be a useful method for structure and conformation analysis in fluorinated surfactants (perfluoro decanoic acid and its lithium, sodium and ammonium salts). The most sensitive part of the spectra in this respect is formed by the frequency changes occurring in the 1400–1700 cm^?1 region and the changes in the relative intensity of some bands associated either with C-F or C-C stretching vibrations. The successive increase in the degree of disorder at transitions into liquid crystalline and micellar phases can be seen from a decrease in the 1370/1300 cm^?1 intensity ratio.     

Biomedical tissue phantoms with controlled geometric and optical properties for Raman spectroscopy and tomography

To support the translation of Raman spectroscopy into clinical applications, synthetic models are needed to accurately test, optimize and validate prototype fiber optic instrumentation. Synthetic models (also called tissue phantoms) are widely used for developing and testing optical instrumentation for diffuse reflectance, fluorescence, and Raman spectroscopies. While existing tissue phantoms accurately model tissue optical scattering and absorption, they do not typically model the anatomic shapes and chemical composition of tissue. Because Raman spectroscopy is sensitive to molecular composition, Raman tissue phantoms should also approximate the bulk tissue composition. We describe the fabrication and characterization of tissue phantoms for Raman tomography and spectroscopy. These phantoms have controlled chemical and optical properties, and also multilayer morphologies which approximate the appropriate anatomic shapes. Tissue phantoms were fabricated to support on-going Raman studies by simulating the human wrist and rat leg. Surface meshes (triangle patch models) were generated from computed tomography (CT) images of a human arm and rat leg. Rapid prototyping was used to print mold templates with complex geometric patterns. Plastic casting techniques used for movie special effects were adapted to fabricate molds from the rapid prototypes, and finally to cast multilayer gelatin tissue phantoms. The gelatin base was enriched with additives to model the approximate chemistry and optical properties of individual tissue layers. Additional studies were performed to determine optimal casting conditions, phantom stability, layer delamination and chemical diffusion between layers. Recovery of diffuse reflectance and Raman spectra in tissue phantoms varied with probe placement. These phantoms enable optimization of probe placement for human or rat studies. These multilayer tissue phantoms with complex geometries are shown to be stable, with minimal layer delamination and chemical diffusion.     

Investigations on the process of dissolution and transformation of chloropinnoite in boric acid aqueous solution at 303 K by kinetics and Raman spectroscopy

Kinetics of dissolution and transformation of chloropinnoite (2MgO·2B₂O₃· MgCl₂·14H₂O) in 4.5% (wt%) boric acid aqueous solution at 303 K had been studied, and the kinetic process and mechanism were investigated by the Raman spectroscopy. According to the kinetic curve, the process of dissolution and phase transformation can be divided into three stages: dissolution stage, phase transformation induction period, and crystallization stage. The experimental results showed that when chloropinnoite dissolved in boric acid aqueous solution, MgCl₂ was removed and midproduct (MgB₂O(OH)₆·H₂O) dissolved at the same time, which was different from that of in water. Kurnakovite was a final product. The experimental data of the crystallization process have been fitted with the following mathematical modification with the aid of simplex optimum method and Runge–Kutta digital solution of differential equation system: ?dc/dt = 6.31(c?0.3906)² + 0.975(c?0.3906). Based on our experimental results and the Raman spectroscopy, kinetics and mechanism of dissolution, phase transformation, and crystallization of borate in chloropinnoite–boric acid water system are discussed. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 136–143, 2006     

Determination of the absolute stereochemistry of limonene and alpha-santalol by Raman optical activity spectroscopy

Determining the absolute stereochemistry of organic compounds in solution remains a challenge. We investigated the use of Raman optical activity (ROA) spectroscopy to address this problem. The absolute configurations of (+)-(R)- and (-)-(S)-limonene were determined by ROA spectroscopy, which can be applied to smaller amounts of sample as compared with vibrational circular dichroism (VCD) spectroscopy. This ROA method was also applied to (+)-(E)-alpha-santalol and shown to be successful in the determination of the absolute configuration of this compound. ROA spectroscopy shows promise as a useful tool for determining the absolute stereochemistry of many natural compounds.     

The determination of the Fe content in natural sphalerites by means of Raman spectroscopy

Natural sphalerite samples collected from the Baia Sprie ore deposit (Romania) were analyzed through Raman spectroscopy, SEM-EDX and XRD. The most intense Raman lines at 300, 331 and 350 cm⁻¹ were used to improve iron determination method from sphalerites by Raman spectroscopy. It is well known that the iron content of synthetic sphalerite can be quantified by measuring the height of Raman lines (h₁, h₃). By using the new h₂/h₃ and (h₁ + h₂)/h₃ ratios and two additional linear equations, this method is improved and becomes suitable to natural sphalerites. The results are in good agreement with the SEM-EDX data.     

Role of interfacial water in the heterogeneous uptake of glyoxal by mixed glycine and ammonium sulfate aerosols

This study focuses on the heterogeneous reactions of gas phase glyoxal with aerosols of glycine, the most abundant amino acid in atmospheric aerosols, as well as with a mixture of glycine and ammonium sulfate (AS) at a molar ratio of 1:100 (glycine-AS 1:100). Aerosols were exposed to varying relative humidity (RH) conditions in the presence of gas phase glyoxal for ～1 h, followed by drying and efflorescence. The changes in size, chemical composition, and optical properties were consequently measured. The reactions occur over a wide range of relative humidities, from ～30% up to 90% RH, covering values that are substantially lower as well as above the deliquescence point of the investigated aerosols. The product aerosols exhibit a trend of increasing growth in size, in optical extinction cross sections, and in extinction efficiencies (at λ = 355 nm) with decreasing seed aerosol size, and with decreasing RH values from 90% to ～50%. For glycine-AS 1:100 particles, the ratio of the geometric cross section of the product aerosol to the original seed aerosol reached a value of ～3, the optical extinction cross section ratio was up to ～25, and the Q(ext) ratio was up to ～8, exceeding those of both AS and glycine separately, suggesting a synergistic effect. Aerosol mass spectrometer analyses show that the main products of all the studied reactions are glyoxal oligomers (light scattering compounds), with a minor contribution from imidazoles (absorbing compounds at λ = 355 nm). These findings imply that the changes in the optical properties are likely due to enhanced scattering by the reaction products. The fraction of absorbing substances in the reacted aerosol increases with increasing RH, suggesting that the absorption component may become more substantial after longer reaction times, possibly in cloud or fog droplets. The results suggest that these reactions are possibly important in low RH regions, plausibly due to the reaction occurring in a few interfacial monolayers of water well before deliquescence.     

Internal structure, hygroscopic and reactive properties of mixed sodium methanesulfonate-sodium chloride particles

Internal structures, hygroscopic properties and heterogeneous reactivity of mixed CH(3)SO(3)Na/NaCl particles were investigated using a combination of computer modeling and experimental approaches. Surfactant properties of CH(3)SO(3)(-) ions and their surface accumulation in wet, deliquesced particles were assessed using molecular dynamics (MD) simulations and surface tension measurements. Internal structures of dry CH(3)SO(3)Na/NaCl particles were investigated using scanning electron microscopy (SEM) assisted with X-ray microanalysis mapping, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The combination of these techniques shows that dry CH(3)SO(3)Na/NaCl particles are composed of a NaCl core surrounded by a CH(3)SO(3)Na shell. Hygroscopic growth, deliquescence and efflorescence phase transitions of mixed CH(3)SO(3)Na/NaCl particles were determined and compared to those of pure NaCl particles. These results indicate that particles undergo a two step deliquescence transition: first at ～69% relative humidity (RH) the CH(3)SO(3)Na shell takes up water, and then at ～75% RH the NaCl core deliquesces. Reactive uptake coefficients for the particle-HNO(3) heterogeneous reaction were determined at different CH(3)SO(3)Na/NaCl mixing ratios and RH. The net reaction probability decreased notably with increasing CH(3)SO(3)Na and at lower RH.     

Structural characterization of thin films formed or changed on materials by micro Raman spectroscopy

Micro Raman spectroscopy is shown as a useful method of high spatial resolution in characterization of thin films on or in materials formed or changed as a result of external influences. Identification of black films in the glaze of porcelain, of impairing deposits on components of an ICP mass spectrometer and of tribologically stressed contacts of ADLC films or (Ti,Mo)(C,N) ceramics, respectively, are examples for application.