Sub-ppt Level Research Articles

Generic method for the detection of short & long chain PFAS extended to the lowest concentration levels of SERS capability

The detection of the highly toxic per- and polyfluoroalkyl substances, PFAS, constitutes a challenging task in terms of developing a generic method that could be rapid and applicable simultaneously to both long and short-chain PFAS at ppt concentration level. In the present study, the method introduced by the USA Environmental Protection Agency, EPA, to detect surfactants, using methylene blue, MB, which is identified an ideal candidate for PFAS-MB ion pairing, is extended at the lowest concentration range by a simple additional step that involves the dissociation of the ion pairs in water. In this work, Surface Enhanced Raman Scattering, SERS, is applied via Ag nanocolloidal suspensions to probe MB and indirectly either/or both short-chain (perfluorobutyric acid, PFBA) and long-chain (perfluoloctanoic acid, PFOA) PFAS downt to 5 ppt. This method, which can be further optimized to sub-ppt level via a custom-made SERS-PFAS dedicated Raman system, offers the possibility to be applied to either specific PFAS (both short and long-chain) in a targeted analysis or to total PFAS in a non-targeted analysis at very low detection limits, following any type of MB detection method in aqueous solutions and obviously with any type of SERS substrate.

Sub-ppt level detection of carbon monoxide and nitrous oxide enabled by mid-infrared doubly resonant photoacoustic spectroscopy.

We report highly sensitive detection of carbon monoxide (CO) and nitrous oxide (N2O) using doubly resonant photoacoustic spectroscopy paired with a quantum cascade laser (QCL) at 4.57 μm. The butterfly-packaged QCL is used to exploit the CO absorption line at 2190.02 cm-1 and the N2O absorption line at 2191.42 cm-1 by scanning the injection current. Leveraging the simultaneous acoustic and optical resonances and adopting a lower photoacoustic detection frequency, we achieve a minimum detection limit of 0.85 part-per-trillion (ppt) for CO over the 500 s averaging time, and 0.7 ppt for N2O over the 200 s averaging time. Our approach demonstrates record sensitivity for CO and N2O detection compared to state-of-the-art optical gas sensors.

PVDF-stimulated surface engineering in ZnO for highly sensitive and water-stable hydrazine sensors

Sensors based on multifunctional n-type metal oxide semiconductors are attracting significant interest in environmental monitoring owing to their distinct characteristics including low production cost, high detection response to different noxious analytes, nontoxic nature, and acceptable biocompatibility. Herein, we present an innovative approach that utilizes surface functionalization on ZnO thin-film transistor (TFT)-type sensors with a fluoropolymer, poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to realize highly sensitive and water-stable liquid-phase sensors. ZnO sensors laminated with PVDF-HFP thin films demonstrate exceptional repeatable stability to DI water and liquid-phase hydrazine, indicating excellent sensitivity in addition to low hydrazine-detection limits approaching 0.01 nM (sub-ppt level) under ambient conditions. This detection limit is five orders of magnitude less than that of the legal limit for an 8 h exposure time-weighted average for hydrazine. Moreover, relatively acceptable repeatability and reproducibility of the sensors were guaranteed over 96% of their initial base current with hydrazine for a month. This outstanding sensing performance is attributed to the enhanced surface interaction between PVDF-HFP with a strong dipole moment and hydrazine, which is completely discriminated from the universal detection mechanism associated with oxygen ion species in ZnO.

A practical approach of high precision U and Th concentration measurement in acrylic

The Jiangmen Underground Neutrino Observatory (JUNO) will build the world’s largest liquid scintillator detector to study neutrinos from various sources. The 20 kilo tonne (kton) liquid scintillator will be stored in a 0.6 kton acrylic sphere with 35.4 m diameter due to the good light transparency, chemical compatibility and low radioactivity of acrylic. The concentration of U/Th in acrylic is required to be less than 1 ppt (10−12 g/g) to achieve a low radioactive background in the fiducial volume of the JUNO detector. The mass production of acrylic has started, and the quality control requires a fast and reliable radioassay on U/Th in acrylic. We have developed a practical method of measuring U/Th in acrylic to sub-ppt level using the Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The U/Th in acrylic can be concentrated by vaporizing acrylic in a class 100 environment, and the residue will be collected and sent to ICP-MS for measuring U/Th. All the other chemical operation is done in a class 100 clean room, and the ICP-MS measurement is done in a class 1000 clean room. The recovery efficiency is studied by adding the natural nonexistent nuclei 229Th and 233U as the tracers. The resulting method detection limit with 99% confidence can reach 0.04/0.09 ppt 238U/232Th in acrylic with (77.5 ± 3.4)% and (73.6 ± 4.1)% recovery efficiency. This equipment and method can not only be used for the quality control of JUNO acrylic, but also be further optimized for the radioassay on other materials with extremely low radioactivity, such as ultra-pure water and liquid scintillator.

Detection of radiocarbon dioxide with double-resonance absorption spectroscopy

Fast and accurate quantitative detection of 14CO2 has important applications in many fields. The optical detection method based on the sensitive cavity ring-down spectroscopy technology has great potential. But currently it has difficulties of insufficient sensitivity and susceptibility to absorption of other isotopes/impurity molecules. We propose a stepped double-resonance spectroscopy method to excite 14CO2 molecules to an intermediate vibrationally excited state, and use cavity ringdown spectroscopy to probe them. The two-photon process significantly improves the selectivity of detection. We derive the quantitative measurement capability of double-resonance absorption spectroscopy. The simulation results show that the double-resonance spectroscopy measurement is Doppler-free, thereby reducing the effect of other molecular absorption. It is expected that this method can achieve high-selectivity detection of 14CO2 at the sub-ppt level.

One-step electrochemical sensor based on an integrated probe toward sub-ppt level Pb2+ detection by fast scan voltammetry

Herein, a one-step electrochemical sensor for selective and sensitive detection of lead ion Pb2+ was developed based on an integrated probe meso-tetra(4-carboxyphenyl) porphine (TCPP)-multi-walled carbon nanotubes (MWCNTs)@Fe3O4, which is TCPP-modified magnetic multi-walled carbon nanotubes. In the integrated probe, TCPP is a porphyrin with a specific cavity structure which could selectively chelate with Pb2+, MWCNTs with good electric conductivity provide a place to load TCPP and form a specific adsorption state of Pb2+ on the electrode surface, and Fe3O4 enables the rapid separation and one-step fabrication of the electrochemical sensor. Based on it, the sample pre-enrichment, separation and determination can be integrated, making the whole process very fast and simple. In addition, fast scan voltammetry (FSV) with a scan rate up to 200 V/s could be used to improve the detection sensitivity greatly, benefitting from the specific adsorption state formed. Under the optimal conditions obtained through orthogonal experiments including adsorption time, integrated probe dosage and solution pH, there was a good linear relationship between the peak current and Pb2+ concentration ranging from 2.0 × 10−4 μg L−1 to 2.0 × 10−3 μg L−1, with the limit of detection (LOD) being 6.7 × 10−5 μg L−1 (S/N = 3) i.e. 0.067 ppt. Analysis of actual water samples was successful. Therefore, being simple, fast, selective and sensitive, the one-step electrochemical sensor proposed has a good potential in practical applications.

Co-precipitation approach to measure amount of 238U in copper to sub-ppt level using ICP-MS

Inductively coupled plasma mass (ICP-MS) spectroscopy is widely used for screening materials of low background detectors in dark matter and double beta decay searches due to its high sensitivity to trace $^{238}$U and $^{232}$Th. This work describes a novel co-precipitation approach to measure the amount of $^{238}$U in high-purity copper to sub-ppt level. Such an approach allows the pre-concentration of U and removal of the matrix, by selecting a proper precipitator to co-precipitate with $^{238}$U and using excess ammonia water to separate the uranium hydroxide from copper by forming water-soluble tetra-amminecopper (II). The isotope dilution method and standard addition method were both used to mitigate the matrix effect and cross-check each other. The latter was also used to measure the recovery efficiency of $^{238}$U by using $^{233}$U as the tracer. The method detection limit (MDL) reached $\sim$0.1 pg $^{238}$U /g Cu for both methods while the recovery efficiency of uranium robustly remains 65\%--85\%. Various sources of interference in the ICP-MS analysis were thoroughly evaluated, and the contamination from reagents were found to be the dominant factor that affected the MDL. Further purification will allow significant improvements in the MDL. This co-precipitate approach can be easily extended to measure $^{232}$Th by using $^{229}$Th as the tracer.

Sub-ppt level voltammetric sensor for Hg2+ detection based on nafion stabilized l-cysteine-capped Au@Ag core-shell nanoparticles

Bimetallic nanoparticles (BMNPs) have received considerable attention due to their distinctive properties when compared to the corresponding monometallic NPs and their bulk counterpart. In this report, the formation of gold@silver core-shell nanoparticles (Au@AgCSNPs) was achieved via a one-pot synthetic approach after mixing 1:1 M solutions of Au and Ag ions. L-cysteine was used as reducing as well as capping agent for preparing Au@AgCSNPs. Ultraviolet-visible (UV-Vis) spectroscopy was employed for surface plasmon study while Fourier-transform infrared (FTIR) spectroscopy gave insights for interaction of NPs with specific functionality of the capping material. Surface morphology of the fabricated Au@AgCSNPs, probed by atomic force microscopy (AFM), indicated an average height of nanoparticles around 43 ± 3 nm and their crystallinity were confirmed via powder X-ray diffraction (PXRD) study. Significantly, the as synthesized Au@AgCSBMNPs were fabricated onto the conductive surface of glassy carbon electrode (GCE), stabilized with nafion, and then utilized as an extremely sensitive/greatly selective sensor for voltammetric detection of Hg2+. The developed sensor responded linearly to Hg2+ between 0.001 and 19 ppb with limit of detection (LOD) as low as 0.0001 ppb (0.1 ppt). Finally, the sensor was effectively applied for Hg2+ detection in different groundwater samples and is workable at concentrations undetectable by several sensing tools.

Determination of cadmium and lead at sub-ppt level in soft drinks: An efficient combination between dispersive liquid-liquid microextraction and graphite furnace atomic absorption spectrometry.

A DLLME method for extraction and preconcentration of Cd and Pb from soft drinks and further determination by GF AAS was developed. Important parameters of DLLME such as the type and volume of dispersive and extraction solvents, concentration of DDTC (complexing agent) and pH were evaluated. Better results were obtained using 500μL of acetone for Cd and 700μL of acetonitrile for Pb as dispersive solvents, 60μL of CCl4 as extraction solvent for both analytes and 500μL of 1.5% DDTC solution. Accuracy was evaluated by recovery assays and ranged from 91 to 113% for Cd and from 95 to 108% for Pb, with RSD below 10 and 7%, respectively. The LODs were 0.006 and 0.072ngL-1 for Cd and Pb, respectively. The optimized method was applied for the determination of Cd and Pb in soft drinks with different brands and flavours.

Ultrasensitive resonance ionization mass spectrometer for evaluating krypton contamination in xenon dark matter detectors

An ultrasensitive resonance ionization mass spectrometer that can be applied to evaluate krypton (Kr) contamination in xenon (Xe) dark matter detectors has been developed for measuring Kr at the parts-per-trillion (ppt) or sub-ppt level in Xe. The gas sample is introduced without any condensation into a time-of-flight mass spectrometer through a pulsed supersonic valve. Using a nanosecond pulsed laser at 212.6nm, 84Kr atoms in the sample are resonantly ionized along with other Kr isotopes. 84Kr ions are then mass separated and detected by the mass spectrometer in order to measure the Kr impurity concentration. With our current setup, approximately 0.4ppt of Kr impurities contained in pure argon (Ar) gas are detectable with a measurement time of 1000s. Although Kr detection sensitivity in Xe is expected to be approximately half of that in Ar, our spectrometer can evaluate Kr contamination in Xe to the sub-ppt level.

Surface-Enhancement Raman Scattering Sensing Strategy for Discriminating Trace Mercuric Ion (II) from Real Water Samples in Sensitive, Specific, Recyclable, and Reproducible Manners

It is of essential importance to precisely probe mercury(II) (Hg(2+)) ions for environment-protection analysis and detection. To date, there still remain major challenges for accurate, specific, and reliable detection of Hg(2+) ions at subppt level. We herein employ gold nanoparticles (AuNPs) decorated silicon nanowire array (SiNWAr) as active surface-enhanced Raman scattering (SERS) substrates to construct a high-performance sensing platform assisted by DNA technology, enabling ultrasensitive detection of trace Hg(2+) in ∼64 min and with low sample consumption (∼30 μL). Typically, strong SERS signals could be detected when the single-stranded DNA structure converts to the hairpin structure in the presence of Hg(2+) ions, due to the formation of thymine (T)-Hg(2+)-T. As a result, Hg(2+) ions with a low concentration of 1 pM (0.2 ppt) can be readily discriminated, much lower than those (∼nM) reported for conventional analytical strategies. Water samples spiked with various Hg(2+) concentrations are further tested, exhibiting a good linear relationship between the normalized Raman intensities and the logarithmic concentrations of Hg(2+) ranging from 1 pM to 100 nM, with a correlation coefficient of R(2) = 0.998. In addition, such SERS sensor features excellent selectivity, facilely distinguishing Hg(2+) ions from various interfering substances. Moreover, this presented SERS sensor possesses good recyclability, preserving adaptable reproducibility during 5-time cyclic detection of Hg(2+). Furthermore, unknown Hg(2+) concentration in river water can be readily determined through our sensing strategy in accurate and reliable manners, with the RSD value of ∼9%.

Ultrafast on-site selective visual detection of TNT at sub-ppt level using fluorescent gold cluster incorporated single nanofiber.

In this communication, a fluorescent gold cluster incorporated electrospun nanofibrous membrane and single nanofiber for selective and sensitive detection of TNT at sub-ppt level are demonstrated.

Methylmercury in water samples at the pg/L level by online preconcentration liquid chromatography cold vapor-atomic fluorescence spectrometry

Ultra-traces of methylmercury at the sub-ppt level can be magnified in the foodweb and is of concern. In environmental monitoring a routine robust analytical method is needed to determine methylmercury in water. The development of an analytical method for ultra-trace speciation analysis of methylmercury (MeHg) in water samples is described. The approach is based on HPLC-CV-AFS with on-line preconcentration of water samples up to 200mL, resulting in a detection limit of 40pg/L (ppq) for MeHg, expressed as Hg. The unit consists of an optimized preconcentration column filled with a sulfur-based sorption material, on which mercury species are preconcentrated and subsequently eluted, separated and detected via HPLC-CV-AFS (high performance liquid chromatography–cold vapor atomic fluorescence spectrometry). During the method development a type of adsorbate material, the pH dependence, the sample load rate and the carry-over were investigated using breakthrough experiments. The method shows broad pH stability in the range of pH0 to 7, without the need for buffer addition and shows limited matrix effects so that MeHg is quantitatively recovered from sewage, river and seawater directly in the acidified samples without sample preparation.

Nanoporous Gold Based Optical Sensor for Sub-ppt Detection of Mercury Ions

Precisely probing heavy metal ions in water is important for molecular biology, environmental protection, and healthy monitoring. Although many methods have been reported in the past decade, developing a quantitative approach capable of detecting sub-ppt level heavy metal ions with high selectivity is still challenging. Here we report an extremely sensitive and highly selective nanoporous gold/aptamer based surface enhanced resonance Raman scattering (SERRS) sensor. The optical sensor has an unprecedented detection sensitivity of 1 pM (0.2 ppt) for Hg(2+) ions, the most sensitive Hg(2+) optical sensor known so far. The sensor also exhibits excellent selectivity. Dilute Hg(2+) ions can be identified in an aqueous solution containing 12 metal ions as well as in river water and underground water. Moreover, the SERRS sensor can be reused without an obvious loss of the sensitivity and selectivity even after 10 cycles.

On-site determination of polynuclear aromatic hydrocarbons in seawater by stir bar sorptive extraction (SBSE) and thermal desorption GC–MS

The identification and quantification of semi-volatile contaminants dissolved in water is currently done in laboratory after a sampling step. This procedure is not satisfactory first because risks of samples contamination and analytes losses remain, in particular when these are present in ultra-trace concentrations, and secondly because procedures are time-consuming. The coupling of the stir bar sorptive extraction (SBSE), a new device of extraction technique, and a new generation of gas chromatography mass spectrometry (GC–MS), the field apparatus EM 640 S from Bruker, could be an answer to the challenge of on-site analysis. This analytical system was used to analyze 24 PAHs, among them 15 EPA priority pollutants. It was shown that this coupling led to encouraging results with LODs around the sub-ppt level for most of the compounds and R.S.D. included between 1 and 48%. The existence of competition phenomena between the various analytes inside the absorbent phase was demonstrated with the release (up to 80%) of light compounds. This result shows the necessity to work on the kinetic domain rather than on the thermodynamic equilibrium that is influenced by nature and concentration of other compounds. The matrix effects were also studied through the comparative analysis of ultrapure water, artificial and natural seawaters spiked with PAHs and the influence of ionic strength and particulate organic matter was investigated.

Stir bar sorptive extraction–thermal desorption–capillary gas chromatography–mass spectrometry applied to the analysis of polychlorinated biphenyls in human sperm

Stir bar sorptive extraction (SBSE) on polydimethylsiloxane (PDMS) was applied to the enrichment of polychlorinated biphenyls (PCBs) from human sperm. The seven Ballschmiter PCBs were used as model compounds. The extracted PCBs were then thermally desorbed from the stir bar and analysed on-line by capillary gas chromatography (CGC) with mass spectrometric detection (MS). Method development started with the analysis of PCBs spiked in water. Methanol had to be added to the samples in order to reduce the influence of glass adsorption on recovery and reproducibility. Recoveries in water for all PCBs varied around 50–60% and were limited for low molecular mass (MM) PCBs by polarity changes in the sample due to methanol addition and for high MM PCBs by non-equilibrium conditions. Matrix suppression by the lipophilic medium lowered the recoveries in the sperm samples proportional with PCB polarity. The method was validated and although limits of detection (LOD) for the individual congeners were in the sub-ppt level (<pg/ml), the limit of quantification (LOQ) was set at 10 ppt (10 pg/ml).

Multielement Profiling Analyses of Biological, Geochemical, and Environmental Samples as Studied by Analytical Atomic Spectrometry

Abstract The progress of modern analytical atomic spectrometry is briefly reviewed with emphasis on ICP-AES and ICP-MS; these provide some excellent analytical features such as high sensitivity, wide linear dynamic range, and multielement detection capability. Thus, the multielement determination of the major-to-ultratrace elements from % to sub-ppt (1 ppt = 10−12 g ml−1) level is possible for the biological, geochemical, and environmental samples. The standard or certified reference materials of plant, human blood serum, and the other biological samples were analyzed to establish the reliable analytical methods for the multielement determination by ICP-AES and ICP-MS. The modern analytical methods were applied to: (i) the correlation analysis of the elements in human blood serum, and (ii) the determination of rare earth elements in human blood serum on a personal basis. The speciation of trace elements in natural water was also carried out by the SEC (size exclusion chromatography)/UV absorption detection/ICP-MS combined system after ultrafiltration of the water samples. The SEC chromatograms indicated that most of trace elements in natural water existed as large organic molecule–metal complexes with the molecular weight of &amp;gt; 300000 and 10000—50000. In addition, trace elements in man and in the sea were discussed from the viewpoints of the geochemical classification of the elements on the earth, the Lewis’s acid-base theory, and the principle of HSAB (hard and soft acids and bases). Such multielement information allows the multielement profiling analyses of various samples to elucidate the elemental distributions, physicochemical characteristics, and behaviors of the major-to-ultratrace elements in relation with the biological functions or with the geochemical and environmental phenomena.

Geographical variations of major and trace elements in East Antarctica

Surface snow samples have been analyzed for a total of 37 elements including Na, Mg, Al, K, Ca, Fe, Ba, Cd, Fe, Cd, Cr, Cu, Ga, Li, Mn, Pb, Se, Sr, V, Zn, As, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Th by inductively coupled plasma mass spectroscopy (ICP-MS). Rare earth elements in surface snow were evaluated after preconcentration of the samples in a class 1000 clean room. These samples were collected between 1991–1993 during the oversnow traverses along a 2200 km route in East Queen Maud Land, Antarctica. They include one at Dome Fuji Station (77°32′S, 24°08′E; 3810 m a.s.l.) built on the top of the second highest dome. In coastal area, fallout flux for Na, Mg, Sr and Cu or more weakly for Ca and Ba shows an apparent decrease according to the distance from the coast. On the other hand, fallout flux for Co, Ni and Cd shows an increase at 2500–3000 m or >3500 m above sea level. For Mn, Se, Zn and As, it shows a combined pattern of these two types. For Al, V and Pb, a constant profile with an intermittent peaks along this route was indicated. These geographical distributions of fallout flux for each element could reflect polar stratospheric precipitation or long-range tropospheric transport from the southern hemisphere. In the present study, concentrations of rare earth elements in Antarctic surface snow at sub-ppt level are first reported. A clear rare earth pattern is noticed in the Antarctic samples and rare earth ratios are also valuable to estimate anthropogenic emissions to the Antarctica.

Polymer-Mediated Extraction of Porphirinato Chelates for a Spectrophotometric Approach to Sub-ppt Level Detection of Metal Ions

Abstract Negatively charged copper(II) and zinc(II) chelates with 5,10,15,20-tetrakis(4-carboxyphenyl)-21H,23H-porphine were incorporated into the precipitates formed from the aqueous solutions of thermo-responsive polymer; poly(N-isopropylacrylamide). In the presence of an appropriate kind and amount of quaternary ammonium ion, the recovery of the chelate was quantitatively. Sub-ppt level zinc(II) in water by the photometric detection in HPLC was successfully achieved, because of the highly efficient concentration.

The optimal amount of isotopic spike solution for ultratrace analysis by isotope dilution mass spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) and high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) are powerful methods of determining metallic impurities in the low- and sub-ppt level in process media such as ultra-pure water used in semiconductor and wafer manufacturing. By using mass spectrometers for analysis, an isotope dilution analysis (IDMS) is possible. The reproducibility of an IDMS is unmatched. For concentration levels near the instrument detection limit a novel method is reported to find the optimal amount of isotopic spike solution. This optimal value can be derived by the law of propagation of uncertainty combined with the Poisson statistics of the measured number of counts. Generally, an excess of isotopic spike solution should be used to provide results of lowest possible uncertainty. The results are presented in a diagram for easy practical use.