Analytical Methods (v.8, #37)

Front cover (6659-6660).

Contents list (6661-6667).

Identification and quantification of foodborne pathogens are becoming increasingly important to public health and food safety since the majority of foodborne illnesses and deaths are caused by pathogenic bacteria. Conventional methods for foodborne pathogen detection are time-consuming and laborious due to the requirement of a series of processes including cell enrichment, isolation and morphological identification. Therefore the demand for rapid, sensitive, inexpensive and convenient approaches to detect foodborne pathogenic bacteria has emerged in recent years. Among the new approaches, microfluidic chip-based detection has generated growing interest because of the miniaturized size, improved sensitivity and reduced detection time of the devices. In addition, the applications of nanomaterials and magnetic microbeads have further facilitated target recognition and signal transduction processes in microfluidic pathogen detection. The lab-on-a-chip technique has developed into an alternative to conventional methods to detect foodborne pathogens owing to its potentials to offer desired sensitivity and to respond in a short test time. Most recently, smartphones and 3-D printing technologies are attracting growing attention for enhanced detection performances. This paper reviews the most recent developments and trends in miniaturized microfluidic devices based on different recognition principles and signal amplification methods to detect foodborne pathogens. In particular, emphasis will be put on those that offer both rapid detection result turn-around and ultra-low detection limit of 102 to 103 cells per mL or even single-cell detection.

A fully disposable paper-based electrophoresis microchip with integrated pencil-drawn electrodes for contactless conductivity detection by Cyro L. S. Chagas; Fabrício R. de Souza; Thiago M. G. Cardoso; Roger C. Moreira; José A. F. da Silva; Dosil P. de Jesus; Wendell K. T. Coltro (6682-6686).
We describe for the first time the fabrication of a paper-based microchip electrophoresis (pME) device with integrated hand-drawn pencil electrodes to perform capacitively coupled contactless conductivity detection (C4D). This low-cost device (less than $0.10) revealed great capability to dissipate heat, good injection-to-injection repeatability, and ease of attaching pencil-drawn electrodes on the separation channel. The feasibility of the proposed pME-C4D device was successfully demonstrated with the separation of bovine serum albumin and creatinine within 150 s with baseline resolution. The limits of detection values for albumin and creatinine were 20 and 35 μmol L−1, respectively. These biomolecules present clinical relevance as evidence of kidney failure. The proposed pME-C4D offers great potential to be explored in the diagnosis of diabetes mellitus and heart disease.

This paper describes the optical properties of micrometer-long gold nanowires studied by differential interference contrast (DIC) microscopy. A far-field DIC microscope enables us to visualize the modulated field distribution along the nanowires resulting from the interference of the longitudinal surface plasmon resonance (SPR) modes and the linearly polarized incident light. Therefore, we demonstrate a new way of using far-field DIC microscopy to elucidate the relationship between incident light and the induced plasmon modes on single nanowires.

We attempted to investigate the colorimetric detection of H2O2 using silver nanoparticles (AgNPs) with three different morphologies (triangular, spherical, and cubic), and the activity of AgNPs toward H2O2 increased in the order of cubic < spherical < triangular AgNPs.

Multiplexed detection of pathogen-specific DNA sequences in a simple and reliable way is in great demand for clinical and biomedical applications. However, there is still a lack of available DNA detection methods that are simple and pathogen-selective for point-of-care (POC) testing. Here, we report a novel zinc finger protein (ZFP)-based chemiluminescence method for direct detection of pathogenic double-stranded DNA (dsDNA) in a multiplexed platform. ZFPs are custom-designed to identify unique pathogenic DNA sequences. ZFP-based chemiluminescence detection of dsDNA provides sufficient sensitivity (≤50 fmol) and high specificity without target DNA amplification. Our study addresses the potential of developing a simple and selective pathogen detection method in a multiplexed fashion needed for POC applications.

Binding-induced DNA strand-displacement reactions diversify the applications of DNA strand-displacement reactions beyond nucleic acids and small molecules. To gain an in-depth understanding of the reaction induced by the biotin-streptavidin interaction, we studied the kinetics of the reaction and established a protein detection method based on the binding-induced strand-displacement reaction rate.

The measurement of nutritional elements in internal tissues is important in modern breeding; calcium has an important role in keeping the cell structure and cell metabolism. However, existing measurement methods require a large number of seed samples and a pretreatment procedure and cannot quickly provide the element content in a single grain seed. In this paper, with a single grain of corn, we studied the spectral data of its endosperm as well as its embryo through the seed epidermis according to the erosion characteristics of laser pulses and implemented the direct rapid quantitative detection of calcium content in different tissues in the seed. We studied the main emission characteristic spectral lines of the calcium element and established this quantitative regression model with the characteristic spectral line at 396.8 nm for the analysis of calcium. The detection limit reached 1.05 PPM (parts per million) and spectral acquisition took less than 1 second. The measurement results indicated that the LIBS method was consistent with the traditional AAS method. This study confirmed that the LIBS method could realize the measurements of element contents in different tissues of a single grain seed and might be applied in modern breeding due to its fast measurement, slight damage, and simple procedure without pretreatment.

Measurement uncertainty associated with shipboard sample collection and filtration for the determination of the concentration of iron in seawater by Robert Clough; Geerke H. Floor; Christophe R. Quétel; Angela Milne; Maeve C. Lohan; Paul J. Worsfold (6711-6719).
A flow injection with chemiluminescence detection (FI-CL) method was used to determine the concentration of dissolved iron in seawater samples collected in the South Atlantic during the GEOTRACES GA10 cruise that took place from 24th December 2011–27th January 2012 on board the R.R.S. James Cook (cruise JC068). Six different sample collection and filtration strategies were used. Open ocean (shallow and deep) and coastal (shallow and deep) samples were collected and five sub-samples from each collection were filtered through a cartridge filter. For the deep open ocean sample, separate sub-samples were also filtered through a membrane disc filter. In addition, deep open ocean sub-samples were also taken from five separate sampling bottles. Each sub-sample (29 in total) was analysed six times (giving 174 discrete measurements in total) and the within sub-sample precision was in the range 1.4–12.2%. There was no statistically significant difference for the deep, open ocean sample between the mean results obtained with the two different filter types or the single sample bottle versus separate sample bottle sub-samples. Application of classical ANOVA showed that the relative combined standard uncertainty for each of the six sampling strategies ranged from 2.3–3.8%. This approach did not include an estimation of sampling bias. Application of robust ANOVA to the deep open ocean samples showed that contributions to the total variance were 0% from the different sample collection and filtration strategies, 42% from the sub-sample precision and 58% from between sub-sample measurements.

An analytical method for simultaneous determination of seven commonly used antibiotics and three antiretrovirals in surface water and urban wastewater at the ng L−1 level has been developed. The method involves concentration and clean-up by solid phase extraction (SPE) followed by identification and quantification by liquid chromatography tandem mass spectrometry (LC-ESI-MS/MS). The use of matrix-matched calibration curves constructed by spiking surface water was evaluated for quantification and compared with the internal standard method using isotopically labelled compounds. The method gave absolute recoveries of 41–116% with most of the compounds having recoveries >50%. The LOQs ranged from 5 to 63 ng L−1 allowing for the determination of the analytes at trace levels in the environmental samples considered. The difference in the quantification results obtained using surface water matrix-matched standards and internal standards was within a ±20% margin. The method provides an affordable and relatively fast alternative with acceptable accuracy and precision. The method was applied to study the occurrence of the target analytes in the surface water of Lake Päijänne and wastewater from the Jyväskylä wastewater treatment plant (WWTP) in Central Finland. All target compounds were detected in the WWTP streams with concentrations ranging between 10 and 570 ng L−1 while low ng L−1 levels were measured for some of the analytes in surface waters. The results institute the need for further monitoring in other WWTP streams and receiving waters as well as improvements of the wastewater treatment process with the aim of minimizing the pharmaceutical load in the effluents.

Micro-laser Raman spectrometry is a potential method for quantitatively analysing stable isotopes in individual fluid inclusions. In this study, we demonstrate that micro-laser Raman spectroscopy is a suitable and non-destructive technique to quantitatively determine the carbon isotopic compositions (δ13C value) of CO2 in individual fluid inclusions. A series of 12CO2/N2 and 13CO2/N2 binary mixtures with various molar fraction ratios were synthesized with the aim of obtaining Raman quantification factors. It was found that the Raman peak area ratios were proportional to the molar fraction ratios. The linearity of the working curves was very good (higher than 0.999). Moreover, the slopes of working curves were regarded as Raman quantification factors (F12CO2 was 1.16259 and F13CO2 was 1.61089), which offered a theoretical foundation for the research on computing the value of carbon isotopic composition. The artificial inclusions with known molar fraction ratios were presented to testify the validity and precision of the method. In addition, the individual fluid inclusions collected from the Yanchang formation sandstone reservoir, Ordos Basin, have been applied to estimate their carbon isotopic compositions. The possibility of quantifying the carbon isotopic compositions of CO2 by micro-laser Raman spectroscopy was demonstrated.

Driven by the need to detect narcotics, a new and simple colorimetric assay is presented for the sensitive and visual detection of morphine (MOR) and codeine (COD) using melamine modified gold nanoparticles (MA–AuNPs). Hydrogen-bonding interactions between melamine and the drugs induce aggregation of the AuNPs with a consequent color change from wine red to blue, which can be monitored by a UV-vis spectrophotometer or even the naked eye. Furthermore, we demonstrated the practicality of using mean centering of ratio spectra (MCRS) to remove the contribution of the reagent blank and obtain the net analyte signal in nanoparticle-based colorimetric assays, an aspect which appears to have received no attention to date. This procedure gives more accurate results than the conventional approach using the absorption ratio at maximum wavelength of the aggregated state to the dispersion state (Aagg/Adis). Comparison of the proposed method with the usual method reveals that MCRS slightly improves the figures of merit of the colorimetric assays. The systems exhibited a linear range from 0.07 to 3 μM for MOR and from 0.03 to 0.8 μM for COD using MCRS. Detection limits (3σ/m) of 17 and 9 nM were achieved for MOR and COD, respectively, which are much lower than the therapeutic plasma/serum concentrations and cutoff levels for opiates in urine samples set by the National Institute on Drug Abuse (NIDA). Furthermore, we demonstrate the application of the approach in biological fluids and pharmaceuticals, which suggests that the present assay could satisfy the requirements of clinical toxicology and forensic cases.

A long wavelength fluorescent probe for biothiols and its application in cell imaging by Jiawei Li; Caixia Yin; Yongbin Zhang; Jianbin Chao; Fangjun Huo (6748-6753).
A long wavelength and turn-on fluorescent probe (1) containing the maleimide group as a reactive site was designed and synthesized. A series of sensing studies showed that probe 1 was highly selective and sensitive with fast responses towards bio-thiols over non-thiolated amino acids in DMSO : HEPES = 1 : 1 (v/v, pH 7.4) solution. Furthermore, the detection limit was as low as 0.015 μM. More importantly, the potential application of probe 1 as a biosensor for thiols was demonstrated by employing it in fluorescence imaging of Cys in HepG2 cells.

An inductively coupled plasma mass spectrometry (ICP-MS) method was developed for the simultaneous determination of the following thirteen trace elements in human urine: Cr, Mn, Co, As, Se, Sr, Mo, Cd, Sb, Hg, Tl, Pb, and U. An Agilent 7700x ICP-MS fitted with a helium mode collision cell was used, along with an integrated sample introduction system for flow injection analysis. An optimal helium flow rate of 5.5 mL min−1 through the collision cell was determined based on background levels, analyte-to-background signal ratios, and analytical precisions. The sample diluent was added with n-butanol as a carbon source to enhance analytical sensitivity, which yielded higher signal intensities than methanol or ethanol. The basic diluent prepared with 4% (v/v) n-butanol, 1% (w/v) ammonium hydroxide, 0.1% (w/v) ethylenediaminetetraacetic acid (H4EDTA), and 0.05% Triton X-100, also demonstrated excellent washout efficiency, requiring only <60 s of rinse time to minimize the Hg memory effect. Sodium chloride (0.95%, w/v) was added to intermediate calibration standards as a matrix-matching component in order to equalize analyte signal suppression caused by inorganic urine components. When both calibration standards and urine specimens were diluted 1 : 10 with the diluent, no considerable variability was observed in internal standard signals throughout the analyses, in spite of a wide range of Na content in urine specimens. Potential Mo oxide interferences on Cd signals were taken into account and minimized with empirically estimated correction factors. Analysis of standard reference materials as well as proficiency testing specimens yielded accuracies of 100 ± 10% for most certified/reference values. Long-term precision of the method was also routinely monitored with internal quality control urine specimens, and after a period of 60 days, the relative standard deviations were <8% for all the analytes.

Specificity of high resolution analysis of naphthenic acids in aqueous environmental matrices by Pamela Brunswick; L. Mark Hewitt; Richard A. Frank; Graham van Aggelen; Marcus Kim; Dayue Shang (6764-6773).
The determination of naphthenic acids to ultra-trace level is reported using liquid chromatography with quadrupole time-of-flight mass spectrometry detection (LC/QToF). The application of this method is in the rapid screening for low level oxygenated naphthenic acid species from potential oil sands process water (OSPW) spills and leakage from storage facilities into aquafers and waterways. During LC/QToF method optimization it was found to be essential to adjust sample pH to >10 prior to any sample container transfer or sub-sampling in order to avoid naphthenic acid losses. Under acidic conditions the loss of O4 species from AEOs was shown to range from 17% for earlier eluting C17 species, to 86% for a later C21 species. Despite application of high resolution mass spectrometry with a mass accuracy of 5 ppm, the potential for interference from environmental contaminants was demonstrated for recognized estrogenic contaminants, resin acids, and fatty acids. These contaminants occur in the environment from natural and human activity, for example fatty acids derived from the pulp and paper mill industry upstream of the Athabasca oil sands. In an effort to avoid potential false positive identifications at low NA concentration levels, formulae for Cn z-0 O2 fatty acids were excluded from total NA screening. It was further noted that OSPW-derived acid extractable organics (AEOs) and Merichem technical mixtures contained ∼2.5% and ∼1.8% respectively of compounds identified with formulae equivalent to both naphthenic and resin acids. The C20 resin acids, anticipated at higher levels in dystrophic water samples, were recommended for future exclusion. Similarly, estrogens, namely estrone, 17α-ethinylestradiol, and 17β-estradiol estrogens, were also identified as NA during screening but eluted at different retention times to the naphthenic acid homologs observed in NA technical mixtures. The potential for their erroneous inclusion in total naphthenic acid results is also discussed.

Speciation of a lanthanide (Sm) using an ion exchange resin by Ildephonse Nduwayezu; Fatemeh Mostafavirad; Madjid Hadioui; Kevin J. Wilkinson (6774-6781).
Rare earth elements (REE) are economically important metals that are found in numerous high technology applications. Their release into the environment is expected to greatly increase in the near future. As for most metals, their bioavailability is expected to be strongly related to their chemical speciation. In this study, an economical and simple means, using ion exchange on a strong acid resin, was developed to evaluate the speciation of a REE (samarium) in environmental matrices. In flow experiments, resin adsorption kinetics were first determined for buffered solutions (pH = 6.0) containing Sm and for which sodium nitrate was added to control ionic strength. Those experiments showed that resin equilibration was very slow, except when using very small quantities of the resin, relatively high ionic strengths (≥0.5 M NaNO3) or high flow rates. Sm adsorption dynamics were evaluated in the presence of five ligands: NTA, EDTA, citric acid, malic acid and the Suwannee River standard fulvic acid. Both in the absence of ligand and in the presence of NTA and EDTA, a strong correlation (r2 = 0.99, slope = 0.67 L−1 g−1 min−1) was found between the rate of Sm accumulation on the resin and free samarium concentrations in solution. On the other hand, in the presence of citric and malic acid, accumulation rates were much higher than those predicted on the basis of free ion concentrations alone, suggesting that these complexes were also being measured by the resin. In the presence of the fulvic acid, the Sm accumulation rate on the resin was much higher than that predicted using thermodynamic calculations, based upon literature values of the Sm–humic acid stability constant. When accumulation rates were compared with free ion concentrations determined by fluorescence quenching titration (FQT), an excellent correlation was found (r2 = 0.98). Overall, the results suggest that the dynamic mode of the IET may be a useful tool for determinations of bioavailable Sm in natural media where humic and fulvic acids are often the main organic ligands binding the REE.

The fabrication of a label-free electrochemical immunosensor using an aldehyde-functionalized pyridinium salt for antibody immobilization by Chunxiang Zhang; Yibing Pi; Youming Shen; Guangyu Shen; Youyu Zhang; Shouzhuo Yao (6782-6786).
This paper reported a strategy based on the use of an aldehyde-functionalized pyridinium salt for the fabrication of a label-free electrochemical immunosensor. The aldehyde-functionalized pyridinium salt was incorporated into Nafion to form a composite film modified on the surface of an Au electrode. The aldehyde-functionalized pyridinium salt was used to capture an antibody directly and improve the conductivity of the sensing interface, which ensured the convenient fabrication of a sensitive label-free electrochemical immunosensor for α-fetoprotein detection. Under optimal conditions, α-fetoprotein was detected in the range from 0.005 to 25 ng mL−1 with a detection limit of 0.003 ng mL−1 based on a signal-to-noise ratio of 3. This strategy would offer potential promise for the convenient fabrication of immunosensors and for clinical applications.

Determination of fifteen water and fat-soluble UV filters in cosmetics by pressurized liquid extraction followed by liquid chromatography tandem mass spectrometry by Marlene Vila; Rocio Facorro; J. Pablo Lamas; Carmen Garcia-Jares; Thierry Dagnac; Maria Llompart (6787-6794).
An analytical method based on pressurized liquid extraction (PLE) followed by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was developed for the simultaneous analysis of fifteen different organic UV filters, both water and fat soluble, in cosmetic products. The extractions were carried out in 10 mL extraction cells and the amount of sample employed was only 100 mg. Several factors affecting the PLE procedure such as the extraction solvent and the dispersive agent used to fill the PLE cell were studied. The use of smaller cells (1 mL) and with Teflon as an inner cell coating was also evaluated. Good linearity (R2 > 0.991), quantitative recoveries (82–101%), and satisfactory precision (RSD < 12%) were obtained under the optimal conditions. LOQ values <0.1 μg g−1 were achieved. The proposed methodology was applied to the analysis of daily personal care products, specific sunscreen products, and cosmetics, such as moisturizing creams, facial creams, anti-wrinkle creams, body milks, hair care products, nail polish, make-up, color lipsticks, lip glosses, protector lipsticks, sun milks, sun sticks, facial sun creams, and sun lotions. For quantification purposes, an external calibration method was implemented since significant matrix effects were not observed. Nine out of the target compounds were found, frequently in combinations of 2–3 compounds, at concentrations ranging from 0.044 to 9.6%. In all cases, these compounds were correctly included in the product label.

Eight years of intensive research have demonstrated that Matrix Assisted Laser Desorption Ionisation-Mass Spectrometry Profiling and Imaging (MALDI-MSP and MSI) are powerful tools to gather intelligence around a suspect lifestyle, directly from the identifying ridges of a latent fingermark. In the past three years, many efforts have been invested into translating laboratory methodologies to the field; this was undertaken by devising protocols either for (a) enabling initial fingermark visualisation, such as through the Dry-Wet method, recovery and subsequent MALDI MS based analysis, or for (b) rendering the MS methodologies compatible with the prior application of commonly employed fingermark enhancement techniques (FET). In the present work a major point of interest concerned the sample treatment of FET visualised-lifted fingermarks and the subsequent MS performance of primary tape lifted fingermarks (“primary lifts”) versus secondary tape lifted fingermarks (recovery from the surface a second time following the initial primary lift). This was necessary since it may not always be possible to obtain primary lifts of marks visualised at crime scenes for remote MALDI-MSP and MSI. The work illustrated here has provided methodological insights into establishing how to best treat a few types of developed marks in preparation for MALDI-MSI when presented as both secondary and primary lifts; it was demonstrated, as expected, that primary lifts generally yield much higher quality chemical/physical information and are therefore crucial to maximise chances of suspect identification and of retrieval of chemical intelligence. When analysing secondary lifted marks that have been initially developed using aluminium or carbon powders, any of the trialled sample preparation methodologies can be employed except the Dry-Wet method. In the case of TiO2 powder developed marks, the best ridge coverage was achieved by re-enhancing the mark using the initial powder and spray-coating with MALDI matrix. Primary lifts of fingermarks contaminated with an exogenous substance (used as a reference model) yielded the best ridge detail quality whilst for secondary lifts of natural marks pre-enhanced with aluminium powder, significantly greater intensity of the ion image was observed for the sections subjected to either no further enhancement or re-enhancement using aluminium powder.

A method based on flow injection-hydride generation-inductively coupled plasma mass spectrometry (FI-HG-ICP-MS) for the determination of trace amounts of As, Bi, Sb, Se, Sn and Te in lead alloys was developed. The analytical parameters for the analyte determination were 0.60% m/v sodium tetrahydroborate (2.6 mL min−1); 90 μL sample volume and 1.25 L min−1 carrier gas (Ar) flow rate. Hydrochloric acid (6.0 mol L−1 for As, Bi, Se and Te and 0.1 mol L−1 for Sb and Sn) was used as sample carrier at 7.4 mL min−1. Three FI systems were developed and consist of a single channel and dual channel, with one and two gas–liquid separators, respectively. The systems with a dual channel permit the simultaneous determination of all elements, by using two reaction conditions. The effect of Ag, Bi, Cu, Pb, Sb and Te, which were the main elements present in the sample, on the analyte determination was evaluated. With the use of optimized experimental conditions, all elements could be determined by external calibration. The LOQ were 0.14, 0.02, 0.03, 0.02, 0.02 and 0.02 μg g−1 for As, Bi, Sb, Se, Sn and Te, respectively, and the RSD was 8% or lower for five consecutive measurements of a sample solution. Analyte recoveries were in the range of 94 to 101% and the results were in good agreement with those obtained by ICP OES. The method was applied to determine the analytes in lead alloy samples used in automotive battery manufacture, with a sample throughput of 10 determinations h−1.

Evaluation of a high-resolution micro-sized magic angle spinning (HRμMAS) probe for NMR-based metabolomic studies of nanoliter samples by Nghia Tuan Duong; Yuki Endo; Takahiro Nemoto; Hiroshi Kato; Anne-Karine Bouzier-Sore; Yusuke Nishiyama; Alan Wong (6815-6820).
This study evaluates the performances of a recently introduced 1 mm HRμMAS NMR probe for metabolic profiling of nanoliter samples. It examines essential NMR criteria such as spectral data qualities and repeatability, and experimental practicality. The report also discusses the difficulties related to the sample preparation in HRμMAS experiments and considers possible solutions and improvements.

Back cover (6821-6822).