Particle Population Research Articles

Water desalination system via ion immobilization on iron corrosion-based colloids and filtration by kevlar support

An efficient, scalable water desalination line has been introduced based on ion immobilization on the corrosion-based colloids (Diameter: from 10 nm to 20 µm) and filtration by a kevlar support. These particles are generated based on the galvanic cell behavior of some semi-galvanized wiring bundles inside a water medium according to the electrochemical cell schematic: Fe|Fe2+, OH-, H2|H2O with ΔG° of −73.0 KJ mol−1. To have an efficient desalination process, the contributions of different mechanisms such as mixed salt crystals, co-precipitation, and /or formation of various ionic charged species are present. These interactions resulted in the formation of micro/nano colloids (suspensions) with Fe2+ with other cations and anions, randomly, inside the bulk water medium. These particles are sandwiched among the arrays of the kevlar slices (5.0 × 5.0 cm) and play the role of the filter medium. In the introduced water desalination process, the population of the iron-based suspensions is controlled via a descending setting of the mass percentage of the semi-galvanic iron bundling wires along the water desalination line to control the number (population) of iron-based particles. This phenomenon consequently approves the condition for the efficient water desalination process at a laminar mass transfer condition with Reynolds number < 2000. The important factors of the removal efficiency are optimized using a multivariate optimization method based on a controllable waste purification efficiency threshold. This system possesses adequate advantages such as high water desalination efficiency, an acceptable water purification rate (3.0 L h−1, for only one water desalination line), low cost, enough simplicity, no residual water, and no need for any external driving source(s).

Modeling and Simulation of Hydroxyapatite Recovery in the Desliming Circuit of the Tapira Industrial Plant, Brazil

The modeling and simulation of industrial mineral processing operations are traditionally used for cyclone sizing and optimizations of industrial operations. However, the main models used are based on the total population of particles in the pulp, thus not distinguishing the individual minerals. This article presents the results of an innovative method that investigated the optimization of the metallurgical recovery of P2O5 in the desliming circuit of a phosphate ore processing plant in Brazil. A survey campaign was carried out in the existing industrial circuit, followed by determining the partition curves for the overall particles and specifically for the hydroxyapatite particles. The results were used to calibrate the Narasimha–Mainza cyclone model. From a Base Case determined with reference to the industrial survey, three optimization scenarios were simulated through cyclone geometries and respective operating conditions changes. Simulated scenarios indicated the possibility of P2O5 metallurgical recovery increasing from 9.4% to 12.7% compared to the Base Case.

Evolving Phase Propagation in an Intermediate‐m ULF Wave Driven by Substorm‐Injected Particles

AbstractAn ultralow frequency (ULF) wave was simultaneously observed in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) radar at Hankasalmi, Finland and on the ground by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers with close proximity to the radar. The onset time of the wave event was around 03:00 magnetic local time. Fourier wave analysis of the event suggests a wave period of about 1,340 s with an equatorward latitudinal and eastward longitudinal wave phase propagation, and an effective azimuthal wave number of 17 ± 1, in the intermediate range of those observed in ULF waves. This wave has been interpreted as resulting from drifting electrons of energies of 13 ± 5 keV in a drift resonance condition linked to energetic particle populations during a magnetospheric substorm. The latitudinal phase characteristics of this wave experienced temporal evolution, believed to be caused by additional injected particle populations associated with the same substorm driving the wave, which resulted in an observed loss of HF backscatter. This observation of a unique type of temporal evolution in the phase propagation characteristics of ULF waves enhances current understanding about the structure, dynamics and source of these types of ULF waves.

Exact numerical analysis of EMEC mode instability in more realistic Cairns distributed non-thermal plasmas

Non-thermal plasma systems beyond the state of thermal equilibrium must have non-thermality dependent effective temperatures. These particle populations cannot have Maxwellian temperatures T∥,⊥e(M) which are typically considered at thermal equilibrium in the context of Maxwellian plasmas. Previously, in such dilute environments, numerous non-thermal distributions incorporating the concept of Maxwellian temperature T∥,⊥e(M) have been utilized, one of them is Cairns distribution (here termed as type-A). To ameliorate this inconsistency, we propose a more realistic form of Cairns distribution (type-B) with the vigorous definition of effective temperature and effective thermal velocity. Both Cairns distributions (A and B) are utilized to calculate the transverse dielectric response function (TDERF) of electromagnetic electron cyclotron (EMEC) mode. The exact numerical solution of TDERFs of EMEC instability reveals interesting and more promising repercussions in the case of Cairns-B i.e. an augmentation in the behavior of oscillatory and imaginary frequencies of the instability.

Design of polymeric carriers to enhance antimicrobial photodynamic inactivation

Mixed polymeric micelles are a promising tool for the design of nanocarriers with pharmaceutical properties. These micellar systems allow reversing numerous properties of lipophilic drugs. Particularly, when applied to transport photosensitizers (PSs), they give rise to a wide variety of nanophotosensitizers (nPSs). These compounds are used to inactivate numerous bacteria resistant to conventional antibiotics. This research proposes the development of new mixed micelles, with better properties than simple micelles, for the trapping of Neutral Red (NR) and its monobrominated derivative (NRBr). Based on the combination of Pluronic P-123 with Pluronic F-108, Pluronic L-61 and Tween 80, 15 binary micellar systems were obtained. These mixed micelles presented a single population of homogeneous particles, with a diameter less than 70 nm. Moreover, several mixed micelles show smaller CMC than the simple system. These properties are predictors of high micellar stability and good anti-dilution properties, favoring their drug delivery applications. For these reasons, were loaded with NR and NRBr, showing good encapsulation efficiency. The 30 new nPSs presented an adequate size for pharmaceutical use and markedly enhanced the photochemical reactivity of the free PSs. Many of the mixed micellar systems showed greater production of singlet oxygen than the simple micelles of Pluronic P-123. Furthermore, all micellar systems increased the chemical stability of NRBr, which degrades in pH 7.4 buffer solution. Finally, all nPSs presented greater phototoxic activity against methicillin-resistant Staphylococcus aureus than free PSs. The nPSs formed by P-123: F-108 (90:10) + NR, P-123: F-108 (90:10) + NRBr, P-123: L-61 (80:20) + NRBr and P −123: T80 (80:20) + NRBr caused greater photodynamic inactivation of the microorganism than simple P-123 micellar systems. These nPSs could be considered excellent candidates for application in the treatment of infections caused by bacteria resistant to antibiotics.

Self-weighting of the overlying soil horizon catalyzed by freeze–thaw cycles leads to silt particle enrichment in the soil profile

Freeze–thaw cycles, as a strong weathering effect, can cause changes in the soil particle size and content, which can affect changes in the geological environment of the soil. A zone of silt enrichment was found at certain depths in the soil profile of the study regions. To understand and analyze the causes of this substratum phenomenon, a unidirectional freeze–thaw test on soil under static loading was designed and simulated in the laboratory in accordance with actual field conditions to investigate the process of changes in the physical index parameters after freeze–thaw cycles when the soil is under pressure. The changes in the parameters in the upper, middle and lower parts of the test soil column after 6, 10, 20, 40 and 50 freeze–thaw cycles were measured for the three regions. The test results show that the mechanical composition of the soil changes significantly after freeze–thaw cycles, the sand content decreases significantly, the clay and silt contents increase, and the particle frequency curve moves in the direction of particle size reduction. The freeze–thaw process is always accompanied by the fragmentation and aggregation of soil particles, and the fractal dimension is used to describe the distribution width of the soil particle population and to compare the clay content. The larger the fractal dimension is, the higher the clay content in the sample. Kvar(Coefficient of granulometric variation) decreases as the number of freeze–thaw cycles increases, the soil particles change from high-intensity to low-intensity variation, and the soil particles gradually transition to silt particles. The occurrence of silt-rich zones is the result of a combination of freeze–thaw action and static loading. This study can provide a new reference and vision for the freeze–thaw evolution process and tectonic development of soil in middle- and high-latitude cold regions.

Kinematics inverse solution of assembly robot based on improved particle swarm optimization

AbstractInverse kinematics of robot is the basis of robot assembly, which directly determines the pose of robot. Because the traditional inverse solution algorithm is limited by the robot topology structure, singular pose and inverse solution accuracy, it affects the use of robots. In order to solve the above problems, an improved particle swarm optimization (PSO) algorithm is proposed to solve the inverse problem of robot. This algorithm initializes the particle population based on joint angle limitations, accelerating the convergence speed of the algorithm. In order to avoid falling into local optima and premature convergence, we have proposed a nonlinear weight strategy to update the speed and position of particles, enhancing the algorithm’s search ability, in addition introducing a penalty function to eliminate particles exceeding joint limits. Finally, the positions of common points and singular points are selected on PUMA 560 robot and redundant robot for inverse kinematics simulation verification. The results show that, compared with other algorithms, the improved PSO algorithm has higher convergence accuracy and better convergence speed in solving the inverse solution, and the algorithm has certain universality, which provides a new solution for the inverse kinematics solution of the assembly robot.

TRU utilization and MA transmutation in thorium-based fluorinated molten salt fast reactor

In this paper, a thorium-based fluorinated molten salt fast neutron reactor is taken as the research object, and the utilization of thorium resources, reuse of transuranic waste (TRU), and transmutation of minor actinide (MA) in fast reactor are the research purpose. The fast reactor adopts a multi-layer core structure, in which the fuel is divided into an inner fission fuel zone and an outer breeding fuel layer, respectively loaded with fluorinated molten salts LiF+ThF4+XF4 (where X is 233U, TRU, or 233U+MA) and LiF+ThF4. In addition to serving as nuclear fuel, these two molten salts also act as coolants in the primary circuits.Through the cross sections of nuclear reactions, transmutation chain of actinide nuclides, and the particle population density formula of actinide nuclides in nuclear reaction equilibrium, the possible nuclear reactions of actinide nuclides in thorium-based fast reactors are analyzed. The calculated results show that the thorium-based fast reactor has the following characteristics: fast neutron energy spectrum, negative temperature reactivity coefficient, effective utilization of thorium resources, the ability to breed fissle fuel such as 233U, production of high-purity radioisotope 238Pu material, and the ability to incinerate Pu and MAs in the TRU spent fuel.In the 233U fuel scheme, the 233U fuel is breeded, breeding ratio of fissile fuels is 1.21, the reaction products contain very little Np, and almost no Pu, Am, and Cm long-lived radionuclides. In the TRU fuel scheme, the annual capacity to incinerate MAs and Pu is 1072.6 kg, while producing a large amount of 233U and 233Pa fuel (together 1162.5 kg/year).In the 233U+2.0 wt% TRU-MAs fuel scheme, 127.35 kg of MA waste can be incinerated annually, and their transmutation rate is 15.95%, which is equivalent to the annual production of MA nuclear waste from 4.9 conventional 1000 MWe light water reactors. Here, TRU-MAs is the mixtured MA fuels after separation of Pu from the TRU fuel. At the same time, radioactive fuel 238Pu, which can be used for space nuclear power, is produced with an annual yield of 87.89 kg and an isotopic abundance of 87.57%. If the added TRU-MAs fuel is replaced with 2.0 wt% 237Np fluorinated salt, the annual yield and isotope abundance of 238Pu are further increased to 130.24 kg and 95.4%.

Intranight Variability of Ultraviolet Emission from High-z Blazars

Rapid intranight variability of continuum and polarization in blazars is a valuable tool for probing the beaming of a relativistic jet and the associated population of the relativistic particles. Although such intranight variability in the rest-frame optical continuum has been extensively studied, there is a scarcity in the rest-frame ultraviolet (UV), where the variability might be caused by a secondary population of relativistic particles. To address this gap, we recently reported, for the first time, an intranight variability study of a sample of high-z blazars (Chand et al., 2022), with the monitored optical radiation is being their rest-frame UV radiation. Here, we discuss in detail the implications of this investigation, including a proper comparison of high-z blazar samples with fractional optical polarization (popt) lower and greater than 3%. In this context, we also report an intranight variability study of an additional high-z blazar at z = 2.347, namely J161942.38+525613.41, monitored over three sessions, each lasting approximately five hours. Our investigation does not reveal any compelling evidence for a stronger intranight variability of the UV emission in high polarization blazars, in contrast to the blazars monitored in the rest-frame blue-optical. We also discuss this trend in the light of the suggestion that the synchrotron radiation of blazar jets in the UV/X-ray regime may arise from a relativistic particle population different from that radiating up to near-infrared/optical frequencies.

A live cell imaging-based assay for tracking particle uptake by clathrin-mediated endocytosis.

A popular strategy for therapeutic delivery to cells and tissues is to encapsulate therapeutics inside particles that cells internalize via endocytosis. The efficacy of particle uptake by endocytosis is often studied in bulk using flow cytometry and Western blot analysis and confirmed using confocal microscopy. However, these techniques do not reveal the detailed dynamics of particle internalization and how the inherent heterogeneity of many types of particles may impact their endocytic uptake. Toward addressing these gaps, here we present a live-cell imaging-based method that utilizes total internal reflection fluorescence microscopy to track the uptake of a large ensemble of individual particles in parallel, as they interact with the cellular endocytic machinery. To analyze the resulting data, we employ an open-source tracking algorithm in combination with custom data filters. This analysis reveals the dynamic interactions between particles and endocytic structures, which determine the probability of particle uptake. In particular, our approach can be used to examine how variations in the physical properties of particles (size, targeting, rigidity), as well as heterogeneity within the particle population, impact endocytic uptake. These data impact the design of particles toward more selective and efficient delivery of therapeutics to cells.

Finite size effects on the metamagnetic phase transition in a thick B2 FeRh nanocluster film.

FeRh alloys in the CsCl-type (B2) chemically ordered phase present an antiferromagnetic to ferromagnetic order transition around 370 K observed in bulk and continuous films but absent in nanoclusters. In this study, we investigate the thermal magnetic behavior of a thick film composed of assembled FeRh nanoclusters preformed in the gas phase. This work reveals a broad and asymmetric metamagnetic transition with a consequent residual magnetization at low temperature. Due to the coexistence of different grain sizes in the sample, we confront the results with a description that involves two populations of B2-FeRh particles, and the existence of a discriminating size below which the magnetic order transition does not take place.

Room temperature spin crossover properties in a series of mixed-anion Fe(NH2trz)3(BF4)2-x(SiF6)x/2 complexes.

A series of mixed-anion Fe(NH2trz)3(BF4)2-x(SiF6)x/2 spin crossover complexes is obtained modifying the reaction time but also using an increase amount of tetraethyl orthosilicate as the source for the production and the incorporation of SiF62- competing with the BF42- anions present in the mother solution. The increase of the SiF62- anion inclusion to the detriment of the BF4- counterpart induces a shift of the temperature transition toward high temperatures leading to interesting bistability properties around room temperature with T1/2 spanning from 300 K to 325 K. Moreover, the implementation of a solid-liquid post synthetic modification approach from the Fe(NH2trz)3(BF4)2 parent complex with identical TEOS proportions and under certain experimental conditions lead systematically to the same Fe(NH2trz)3(BF4)1.2(SiF6)0.4 composition. This compound presents an abrupt spin crossover behaviour with a narrow hysteresis loop just above room temperature (320 K), which is stable under thermal cycling and along time with no specific storage conditions. Such crystalline powder sample incorporates homogeneous rod-shaped particles whose formation and physical properties can be followed simultaneously using infra-red spectroscopy, dynamic light scattering (DLS), transmission electronic microscopy (TEM) and optical reflectance. The observation of a stabilized single ca. 800 nm population of mixed-anion particles starting from insoluble various sizes (from nano- to microscale) Fe(NH2trz)3(BF4)2 particles supports the key role of the solvent (water molecules) on the separation, the reactivity and the reorganization of the 1D iron-triazole chains forming the packing of the structure.

Microfluidic device for the high-throughput and selective encapsulation of single target cells.

Droplet-based microfluidic technologies for encapsulating single cells have rapidly evolved into powerful tools for single-cell analysis. In conventional passive single-cell encapsulation techniques, because cells arrive randomly at the droplet generation section, to encapsulate only a single cell with high precision, the average number of cells per droplet has to be decreased by reducing the average frequency at which cells arrive relative to the droplet generation rate. Therefore, the encapsulation efficiency for a given droplet generation rate is very low. Additionally, cell sorting operations are required prior to the encapsulation of target cells for specific cell type analysis. To address these challenges, we developed a cell encapsulation technology with a cell sorting function using a microfluidic chip. The microfluidic chip is equipped with an optical detection section to detect the optical information of cells and a sorting section to encapsulate cells into droplets by controlling a piezo element, enabling active encapsulation of only the single target cells. For a particle population including both targeted and non-targeted particles arriving at an average frequency of up to 6000 particles per s, with an average number of particles per droplet of 0.45, our device maintained a high purity above 97.9% for the single-target-particle droplets and achieved an outstanding throughput, encapsulating up to 2900 single target particles per s. The proposed encapsulation technology surpasses the encapsulation efficiency of conventional techniques, provides high efficiency and flexibility for single-cell research, and shows excellent potential for various applications in single-cell analysis.

Enhancing K-Nearest Neighbors Algorithm in Wireless Sensor Networks through Stochastic Fractal Search and Particle Swarm Optimization

The utilization of wireless sensor networks (WSNs) holds significant importance in diverse data collection applications. Efficient operation of computers, especially in predictive tasks, is imperative for obtaining accurate results within WSNs. This research introduces an innovative approach employing Stochastic Fractal Search-Particle Swarm Optimization (SFS-PSO) to enhance the performance of the K-Nearest Neighbors (KNN) algorithm. The proposed methodology initiates with the establishment of a particle population, dynamically adjusting their positions and velocities and integrating a diffusion process. Through an iterative process of incremental adjustments and evaluations, the algorithm fine-tunes its parameters, resulting in a refined KNN regression model. The enhanced model exhibits substantial improvements, as indicated by the notable reduction in root mean square error (RMSE) and mean absolute error (MAE), accompanied by a strengthened correlation between variables. The favorable outcomes underscore the efficacy of the SFS-PSO optimization technique in augmenting the KNN algorithm's performance within wireless sensor networks. In simpler terms, the application of SFS-PSO in conjunction with KNN leads to a significant decrease in RMSE, reaching a value as low as 0.00894, demonstrating the notable effectiveness of this optimization approach in refining the predictive capabilities of the KNN algorithm in the context of WSNs.

Measuring Size and Number Concentration of Metallic Nanoparticles Using spICP-MS.

This protocol describes how to measure the size and concentration of individual metallic nanoparticles using inductively coupled plasma-mass spectrometry (ICP-MS) in single-particle (sp) mode. Accurately determining the size of individual nanoparticles on a per-particle basis, both quickly and accurately, is an ever-increasing need within nanoparticle characterization. ICP-MS is capable of measuring a broad range of metallic nanoparticle sizes with high resolution, thus allowing the measurement of multiple particle populations for the quality assessment of nanoformulations. Additionally, spICP-MS can accurately determine particle concentrations without the need for concentration standards.

Optofluidic flow cytometer with in-plane spherical mirror for signals enhancement

Statistical analysis of properties of single microparticles, such as cells, bacteria or plastic slivers, has attracted increasing interest in recent years. In this field flow cytometry is considered the gold standard technique, but commercially available instruments are bulky, expensive, and not suitable for use in Point-of-Care (PoC) testing. Microfluidic flow cytometers, on the other hand, are small, cheap and can be used for on-site analysis. However, in order to detect small particles, they require complex geometries and the aid of external optical components. To overcome these limitations here we present an opto-fluidic flow cytometer with an integrated 3D in-plane spherical mirror for enhanced optical signal collection. As result the signal-to-noise ratio is increased by a factor of 6, enabling the detection of particle sizes down to 1.5μm. The proposed optofluidic detection scheme allows the simultaneous collection of particles fluorescence and scattering - using a single optical fiber - which is crucial to easily distinguish particle populations with different optical properties. The devices have been fully characterized using fluorescent polystyrene beads of different sizes. As a proof of concept for potential real-world applications, signals from fluorescent HEK cells and Escherichia coli bacteria were analyzed.

Introducing a Novel Light Scattering Technique for Visualising Operando State-of-Charge Changes Within Individual Active Particles and Across the Electrode

In our presentation we will demonstrate the application of a recently developed operando optical microscopy technique to mechanistic studies of lithium-ion battery electrodes and active material characterisation. Our technique probes state-of-charge (SoC) changes in the active particles within the electrode during battery operation, based on the intensity variation of scattered light across each active particle. This enables detailed spatially resolved data on an individual particle level to be utilised for furthering mechanistic understanding of battery capability and degradation, and it also allows global statistics to be established for the optimisation and screening of new electrode materials. The technique is agnostic to the underlying battery chemistry, so in principle can be applied to study any battery electrode.In this talk, we will demonstrate how our technique can be used in the study of two key dynamic processes, solid-state ion transport and mechanical degradation. The ability to quantify ion transport during battery operation helps to provide insights into how lithium-ion dynamics affect the (de)lithiation mechanisms and degradation mechanisms of battery electrodes, which is crucial to improving their electrochemical performance. Using lithium cobalt oxide (LCO) as a case study, we will show how ion-transport mechanisms can be deduced by observing moving phase boundaries within particles during cycling.We will then demonstrate how local SoC changes within Ni-rich nickel manganese cobalt oxide (NMC) active particles can be visualised during operation, and how the intra-particle ion gradients identified can be used to determine the mechanistic origins of first-cycle capacity losses. Building on this, we will highlight how variation in the rates of (dis)charge between a population of active particles could be used to characterise the performance of new electrode compositions.In addition, we will highlight how lithium-ion concentration gradients can also be identified in high-rate niobium tungsten oxide (NWO) anode materials during initial lithiation and under initial rapid delithiation conditions and demonstrate how this can lead to particle cracking. Expanding on this, we illustrate how the proportion of cracked particles within an electrode can be quantified and then monitored over successive cycles as a potential metric for screening the stability of new electrode materials.

Uniformity of neutron absorber distribution in Gd-containing neutron absorber materials

For neutron absorber materials (NAMs), it is important to evaluate their neutron-absorption capability and quantitatively evaluate the uniformity of the distribution of neutron-absorbing elements within the materials. In this study, neutron attenuation testing with cold neutrons was conducted on Ti-based model alloys with Gd concentrations ranging from 1 to 10 wt%. The neutron attenuation coefficients (μ) of the Ti-Gd alloys were determined and compared with the total macroscopic cross sections (Σt), which were computed using the latest ENDF/B library. Although the measured μ increases with the Gd-content, the coefficients were not linearly proportional to the absorber amount, and the ratio of μ to Σt gradually decreased. This was caused by beam hardening, which is more significant for higher-absorbing and thicker NAMs. To examine the uniformity of the neutron absorbers (NAs) in plates, we propose a method for characterizing the degree of homogeneity by introducing a radial distribution function (RDF). After the optical microscopic images of the Ti-Gd alloys were converted to grayscale, the RDF of the Gd particles in the alloys was calculated under the assumption that a large particle is composed of many adjacent unit particles. A unit particle was regarded as one pixel in the digital image. For all Ti-Gd alloys, the RDF had a value of > 1 at short interparticle distances, which indicated a dense population of particles in a small area due to cluster formation. The method of using the RDF as a uniformity parameter was validated through a case study involving a fixed NA content with different distributions.

Impact of Non-Thermal Particle Acceleration on Radiative Signatures of AGN Jet-Cloud Interactions

This study investigates the complex dynamics of AGN (Active Galactic Nucleus) jet-cloud interactions, particularly focusing on the impact of non-thermal particle acceleration on the resulting radiative signatures. We utilize advanced computational simulations, tracking changes in jet properties and emissions over a span of 0.2 Myr (millions of years). The research design incorporates the modeling of jet-cloud interactions, with a key focus on variations in the jet's density, velocity, and magnetic field. Findings reveal a two-fold increase in the magnetic field strength up to ~5 μG due to cloud incorporation, which, coupled with an elevated non-thermal particle population, enhances synchrotron emissions, shifting the spectral index from 2.2 to 2.4. Inverse Compton scattering saw a 30% increase within the first 0.125 Myr, reflecting in an abrupt X-ray and gamma-ray emissions spike. Furthermore, the jet's light curve flux variability in the X-ray band showcased an initial peak increase of about 28% by 0.175 Myr, settling to a 20% increase by 0.2 Myr, attributable to cloud disruption and absorption. Conclusions drawn from these findings confirm our hypothesis that non-thermal particle acceleration dramatically influences the radiative signatures of AGN jet-cloud interactions .It underscores the necessity of considering such acceleration processes in modeling AGN jet-cloud interactions and posits that these changes could be instrumental as observational indicators, thereby contributing to more accurate interpretations of AGN activity and evolution.&#x0D;

Understanding HDL Metabolism and Biology Through In Vivo Tracer Kinetics.

HDL (high-density lipoprotein), owing to its high protein content and small size, is the densest circulating lipoprotein. In contrast to lipid-laden VLDL (very-low-density lipoprotein) and LDL (low-density lipoprotein) that promote atherosclerosis, HDL is hypothesized to mitigate atherosclerosis via reverse cholesterol transport, a process that entails the uptake and clearance of excess cholesterol from peripheral tissues. This process is mediated by APOA1 (apolipoprotein A-I), the primary structural protein of HDL, as well as by the activities of additional HDL proteins. Tracer-dependent kinetic studies are an invaluable tool to study HDL-mediated reverse cholesterol transport and overall HDL metabolism in humans when a cardiovascular disease therapy is investigated. Unfortunately, HDL cholesterol-raising therapies have not been successful at reducing cardiovascular events suggesting an incomplete picture of HDL biology. However, as HDL tracer studies have evolved from radioactive isotope- to stable isotope-based strategies that in turn are reliant on mass spectrometry technologies, the complexity of the HDL proteome and its metabolism can be more readily addressed. In this review, we outline the motivations, timelines, advantages, and disadvantages of the various tracer kinetics strategies. We also feature the metabolic properties of select HDL proteins known to regulate reverse cholesterol transport, which in turn underscore that HDL lipoproteins comprise a heterogeneous particle population whose distinct protein constituents and kinetics likely determine its function and potential contribution to cholesterol clearance.