Review: The approaches for estimation of limit of detection for ICP-MS trace analysis of arsenic

4D-Printed Elution-Peak-Guided Dual-Responsive Monolithic Packing for the Solid-Phase Extraction of Metal Ions

Four-dimensional printing (4DP) technologies are revolutionizing the fabrication of stimuli-responsive devices. To advance the analytical performance of conventional solid-phase extraction (SPE) devices using 4DP technology, in this study, we employed N-isopropylacrylamide (NIPAM)-incorporated photocurable resins and digital light processing three-dimensional printing to fabricate an SPE column with a [H+]/temperature dual-responsive monolithic packing stacked as interlacing cuboids to extract Mn, Co, Ni, Cu, Zn, Cd, and Pb ions. When these metal ions were eluted using 0.5% HNO3 solution as the eluent at a temperature below the lower critical solution temperature of polyNIPAM, the monolithic packing swelled owing to its hydrophilic/hydrophobic transition and electrostatic repulsion among the protonated units of polyNIPAM. These effects resulted in smaller interstitial volumes among these interlacing cuboids and improvements in the elution peak profiles of the metal ions, which, in turn, demonstrated the reduced method detection limits (MDLs; range, 0.2-7.2 ng L-1) during analysis using inductively coupled plasma mass spectrometry. We studied the effects of optimizing the elution peak profiles of the metal ions on the analytical performance of this method and validated its reliability and applicability by analyzing the metal ions in reference materials (CASS-4, SLRS-5, 1643f, and Seronorm Trace Elements Urine L-2) and performing spike analyses of seawater, groundwater, river water, and human urine samples. Our results suggest that this 4D-printed elution-peak-guided dual-responsive monolithic packing enables lower MDLs when packed in an SPE column to facilitate the analyses of the metal ions in complex real samples.

Recent advances in fluorescence-based chemosensing of organoarsenic feed additives using luminescence MOFs, COFs, HOFs, and QDs

Organoarsenics are low-toxicity compounds that are used widely as feed additives to promote livestock growth, enhance meat pigmentation, and fight against intestinal parasites. The organoarsenic compounds are commonly found in poultry waste and the degradation of organoarsenic produces the toxic carcinogen inorganic arsenic such as As(V) and As(III), which results in severe arsenic pollution of soil and groundwater. As a consequence, there exists a high necessity to develop suitable sensing methods for the trace detection and quantification of organoarsenic feed additives in wastewater. Among various detection methods, in particular, fluorescence-based sensing has become a popular and efficient method used extensively for sensing water contaminants and environmental contaminants. In the recent past, a wide variety of fluorescence chemosensors have been designed and employed for the efficient sensing and quantification of the concentration of organoarsenic feed additives in different environmental samples. This review article systematically highlights various fluorescence chemosensors reported to date for fluorescence-based sensing of organoarsenic feed additives. The fluorescence sensors discussed in this review are classified and grouped according to their structures and functions, and in each section, we provide a detailed report on the structure, photophysics, and fluorescence sensing properties of different chemosensors. Lastly, the future perspectives on the design and development of practically useful sensor systems for selective and discriminative sensing of organoarsenic compounds have been stated.

Cationic Copolymer-Augmented DNA Hybridization Chain Reaction

Various DNA assembly techniques and structures have emerged with the continuous progress of DNA nanotechnology. DNA hybridization chain reaction (HCR) is a representative example owing to isothermal and enzyme-free features. However, HCR is time consuming and is inhibited by nucleases present in biological samples. Herein, we demonstrated that a cationic copolymer, poly(l-lysine)-graft-dextran (PLL-g-Dex), significantly facilitated HCR and increased its initiator sensitivity by 40-fold. PLL-g-Dex promoted the generation of HCR products with high molecular weight by accelerating the initiation and the subsequent growth steps of HCR. Moreover, PLL-g-Dex protected the HCR system from nucleases, permitting HCR in the presence of serum components. Addition of PLL-g-Dex is a universal and efficient strategy that does not require optimization of the reactor setup or DNA sequences, thus laying a solid foundation for the wider application of HCR.

A simple method for daily inspections of gas chromatography-mass spectrometry systems with an instrumental detection limit as an indicator

The purpose of this paper is to propose a simple method for daily inspections of a gas chromatography-mass spectrometry (GC-MS) system with an instrumental detection limit (IDL) as an indicator. A definition of DLs by ISO is 3.3σ where σ denotes the standard deviation (SD) of blank measurements. Estimation of σ is carried out according to the function of mutual information (FUMI) theory and actually with commercial software (TOCO19). An IDL which is a combination of a signal area, width and noise level is concluded to be a good indicator for daily inspections compared with each of its constituents. Methyl stearate is used as a standard material for the daily inspection of a programmed-temperature GC-MS system. A short chromatogram of 1800 data points (1.5 min) containing a target signal and background noise is fit for the IDL prediction by TOCO19. The relative SDs (RSDs) based on the theoretically estimated σ are shown to coincide with statistical results from repeated measurements within 95% confidence intervals. Column temperature is observed to affect IDLs through background fluctuation and then temperature-IDL relationship is examined in a range from 170 to 270 °C. Actual daily inspections over a month are demonstrated.

Flexible Enzymatic Glucose Electrochemical Sensor Based on Polystyrene-Gold Electrodes

Metabolic disorders such as the highly prevalent disease diabetes require constant monitoring. The health status of patients is linked to glucose levels in blood, which are typically measured invasively, but can also be correlated to other body fluids such as sweat. Aiming at a reliable glucose biosensor, an enzymatic sensing layer was fabricated on flexible polystyrene foil, for which a versatile nanoimprinting process for microfluidics was presented. For the sensing layer, a gold electrode was modified with a cysteine layer and glutaraldehyde cross-linker for enzyme conformal immobilization. Chronoamperometric measurements were conducted in PBS buffered glucose solution at two potentials (0.65 V and 0.7 V) and demonstrated a linear range between 0.025 mM to 2mM and an operational range of 0.025 mM to 25 mM. The sensitivity was calculated as 1.76µA/mM/cm² and the limit of detection (LOD) was calculated as 0.055 mM at 0.7 V. An apparent Michaelis–Menten constant of 3.34 mM (0.7 V) and 0.445 mM (0.65 V) was computed. The wide operational range allows the application for point-of-care testing for a variety of body fluids. Yet, the linear range and low LOD make this biosensor especially suitable for non-invasive sweat sensing wearables.

Censoring Trace-Level Environmental Data: Statistical Analysis Considerations to Limit Bias

Trace-level environmental data typically include values near or below detection and quantitation thresholds where health effects may result from low-concentration exposures to one chemical over time or to multiple chemicals. In a cook stove case study, bias in dibenzo[a,h]anthracene concentration means and standard deviations (SDs) was assessed following censoring at thresholds for selected analysis approaches: substituting threshold/2, maximum likelihood estimation, robust regression on order statistics, Kaplan-Meier, and omitting censored observations. Means and SDs for gas chromatography-mass spectrometry-determined concentrations were calculated after censoring at detection and calibration thresholds, 17% and 55% of the data, respectively. Threshold/2 substitution was the least biased. Measurement values were subsequently simulated from two log-normal distributions at two sample sizes. Means and SDs were calculated for 30%, 50%, and 80% censoring levels and compared to known distribution counterparts. Simulation results illustrated (1) threshold/2 substitution to be inferior to modern after-censoring statistical approaches and (2) all after-censoring approaches to be inferior to including all measurement data in analysis. Additionally, differences in stove-specific group means were tested for uncensored samples and after censoring. Group differences of means tests varied depending on censoring and distributional decisions. Investigators should guard against censoring-related bias from (explicit or implicit) distributional and analysis approach decisions.

One-step isothermal RNA detection with LNA-modified MNAzymes chaperoned by cationic copolymer

RNA detection permits early diagnosis of several infectious diseases and cancers, which prevent propagation of diseases and improve treatment efficacy. However, standard technique for RNA detection such as reverse transcription-quantitative polymerase chain reaction has complicated procedure and requires well-trained personnel and specialized lab equipment. These shortcomings limit the application for point-of-care analysis which is critical for rapid and effective disease management. The multicomponent nucleic acid enzymes (MNAzymes) are one of the promising biosensors for simple, isothermal and enzyme-free RNA detection. Herein, we demonstrate simple yet effective strategies that significantly enhance analytical performance of MNAzymes. The addition of the cationic copolymer and structural modification of MNAzyme significantly enhanced selectivity and activity of MNAzymes by 250 fold and 2,700 fold, respectively. The highly simplified RNA detection system achieved a detection limit of 73 fM target concentration without additional amplification. The robustness of MNAzyme in the presence of non-target RNA was also improved. Our finding opens up a route toward the development of an alternative rapid, sensitive, isothermal, and protein-free RNA diagnostic tool, which expected to be of great clinical significance.

Anti-p53 Autoantibody Detection in Automatic Glass Capillary Immunoassay Platform for Screening of Oral Cavity Squamous Cell Carcinoma

The incidence of oral squamous cell carcinoma (OSCC), which is one of the most common cancers worldwide, has been increasing. Serum anti-p53 autoantibody is one of the most sensitive biomarkers for OSCC. Currently, the most commonly used method on clinical screening platforms is the enzyme-linked immunosorbent assay, owing to its high specificity and repeatability. However, conducting immunoassays on 96-well plates is typically time consuming, thereby limiting its clinical applications for fast diagnosis and immediate prognosis of rapidly progressive diseases. The present study performed immunoassays in glass capillaries of 1-mm internal diameter, which increases the surface to volume ratio of the reaction, to shorten the time needed for immunoassay. The immunoassay was automated while using linear motorized stages and a syringe pump. The results indicated that, when compared with the 96-well plate immunoassay, the glass capillary immunoassay decreased the reaction time from typical 120 min to 45 min, reduced the amount of reagent from typical 50 µL to 15 µL, and required only simple equipment setup. Moreover, the limit of detection for glass capillary anti-p53 autoantibody immunoassay was 0.46 ng mL⁻¹, which is close to the 0.19 ng mL⁻¹ value of the conventional 96-well plate assay, and the glass capillary method had a broader detection range. The apparatus was used to detect the serum anti-p53 autoantibody concentration in clinical patients and compare its results with the conventional 96-well plate method results, which suggested that both of the methods detect the same trend in the relative concentration of serum anti-p53 autoantibody in healthy individuals or patients with OSCC.

A Facile Fabrication of a Potentiometric Arrayed Glucose Biosensor Based on Nafion-GOx/GO/AZO

In this study, the potentiometric arrayed glucose biosensors, which were based on zinc oxide (ZnO) or aluminum-doped zinc oxide (AZO) sensing membranes, were fabricated by using screen-printing technology and a sputtering system, and graphene oxide (GO) and Nafion-glucose oxidase (GOx) were used to modify sensing membranes by using the drop-coating method. Next, the material properties were characterized by using a Raman spectrometer, a field-emission scanning electron microscope (FE-SEM), and a scanning probe microscope (SPM). The sensing characteristics of the glucose biosensors were measured by using the voltage–time (V-T) measurement system. Finally, electrochemical impedance spectroscopy (EIS) was conducted to analyze their charge transfer abilities. The results indicated that the average sensitivity of the glucose biosensor based on Nafion-GOx/GO/AZO was apparently higher than that of the glucose biosensor based on Nafion-GOx/GO/ZnO. In addition, the glucose biosensor based on Nafion-GOx/GO/AZO exhibited an excellent average sensitivity of 15.44 mV/mM and linearity of 0.997 over a narrow range of glucose concentration range, a response time of 26 s, a limit of detection (LOD) of 1.89 mM, and good reproducibility. In terms of the reversibility and stability, the hysteresis voltages (V_H) were 3.96 mV and 2.42 mV. Additionally, the glucose biosensor also showed good anti-inference ability and reproducibility. According to these results, it is demonstrated that AZO is a promising material, which could be used to develop a reliable, simple, and low-cost potentiometric glucose biosensor.

An automated system for predicting detection limit and precision profile from a chromatogram

This paper presents a basic model of an automated system for predicting the detection limit and precision profile (plot of relative standard deviation (RSD) of measurements against concentration) in chromatography. The fundamental assumption is that the major source of response errors at low sample concentrations is background noise and at high concentrations, it is the volumes injected into an HPLC system by a sample injector. The noise is approximated by the mixed random processes of the first order autoregressive process AR(1) and white noise. The research procedures are: (1) the description of the standard deviation (SD) of measurements in terms of the parameters of the mixed random processes; (2) the algorithm for the parameter estimation of the mixed processes from actual background noise; (3) the mathematical distinction between noise and signal in a chromatogram. When compounds are chromatographically separated, each obtained signal is given the detection limit and precision profile on laboratory-made software. A file of a chromatogram is the only requirement for the theoretical prediction of measurement uncertainty and therefore the repeated measurements of real samples can be dispensed with. The theoretically predicted RSDs are verified by comparing them with the statistical RSDs obtained by repeated measurements. Signal shapes on noise are illustrated at the detection limit and quantitation limit, the signal-to-noise ratios of which are close to the widely adopted values, 3 and 10, respectively.

ICH Q3D based elemental impurities study in liquid pharmaceutical dosage form with high daily intake - comparative analysis by ICP-OES and ICP-MS

Guideline for Elemental Impurities-Q3D of the International Conference on Harmonisation represents a new paradigm in the control of elemental impurities (EIs) in pharmaceuticals. It changes the approach toward control of EIs from the historical 'heavy metals test', to a scientific-based risk assessment and testing by modern analytical instrumentation such as inductively coupled plasma-optical emission spectroscopy (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS). Management of EIs related to all finished drug products must be implemented in strict compliance with the regulatory requirements of pharmaceutical industry due to their quality and safety concerns. Testing for presence of EIs from Class 1 and Class 2a in methadone hydrochloride 1 mg/ml oral solution with recommended daily intake of 150 mg methadone hydrochloride was initially performed on ICP-OES using in-house validated method according to the requirements of pharmacopoeias, in line with Q3D. During the procedure, it became apparent that ICP-OES has its own limitations, especially when it comes to testing arsenic and lead in low concentrations. ICP-MS in-house validated method was developed and employed for determination of trace concentrations of arsenic and lead, providing resourceful information that were compared and correlated to the data obtained by ICP-OES analysis. Sample preparation using microwave digestion technique was applied for the analyses by both techniques. Although the applied ICP-OES in-house method is suitable for determination of Hg, Cd, Co, V, and Ni, more sensitive technique such as ICP-MS is required for accurate determination of As and Pb concerning pharmaceuticals with high daily intakes.

A New Calibration Circuit Design to Reduce Drift Effect of RuO2 Urea Biosensors

The goal of this study was to reduce the drift effect of RuO₂ urea biosensors. A new calibration circuit (NCC) based on the voltage regulation technique with the advantage of having a simple structure was presented. To keep its simplicity, the proposed NCC was composed of a non-inverting amplifier and a voltage calibrating circuit. A ruthenium oxide (RuO₂) urea biosensor was fabricated to test the calibrating characteristics of the drift rate of the proposed NCC. The experiment performed in this study was divided into two main stages. For the first stage, a sound RuO₂ urea biosensor testing environment was set-up. The RuO₂ urea sensing film was immersed in the urea solution for 12 h and the response voltage was measured using the voltage-time (V-T) measurement system and the proposed NCC. The results of the first stage showed that the RuO₂ urea biosensor has an average sensitivity of 1.860 mV/(mg/dL) and has a linearity of 0.999 which means that the RuO₂ urea biosensor had been well fabricated. The second stage of the experiment verified the proposed NCC's functions, and the results indicated that the proposed NCC reduced the drift rate of RuO₂ urea biosensor to 0.02 mV/hr (98.77% reduction).

Achieving confidence in trace element analysis for nuclear forensic purposes: ICP-MS measurements using external calibration

In this work, problems arising from performing trace element analysis using inductively coupled plasma—mass spectrometry with low measurement uncertainties are addressed. It is shown that some reference materials certified for massic concentration of lanthanides may have either deviating concentrations or underestimated measurement uncertainties. It is also shown that the choice of methods for sample preparation and linear regression to perform external calibration is affecting the outcome of the measurement results and their uncertainties. The results show that, from the selection of methods investigated in this work, the lowest measurement uncertainties can be achieved by using weighted linear regression to evaluate the calibration function and gravimetric dilutions of samples.

Trace elements in atmospheric wet precipitation in Detroit metropolitan area: Levels and possible sources

Rain and snow samples were collected in the Detroit metropolitan area and analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Twenty-two elements were detected in a concentration range from ∼0.03 to ∼1.8 × 10³ μg/kg. An enrichment factor (EF), defined as, EF=(X/Al)sample(X/Al)crust , was estimated for each element (X) detected, and used to determine possible origins of the element. Based on the hypothesis that crustal material is the only source of aluminum (Al) found in the atmospheric environment, an EF value near unity for an element suggests that crustal material is a major source of this element. If EF>10, an element is enriched in the atmosphere relative to its concentration in the earth's crust, implying a source other than the crust. Alkali, alkaline earth and lanthanide metals detected in the samples exhibit low EF values, indicating mainly a crustal source. Six elements (As, Cd, Cu, P, Pb and Zn) were significantly enriched in the samples as their EF values were greater than 10, thus originated likely from anthropogenic emissions. The relative order of the significantly enriched elements is estimated as follows: Cd > Pb > Zn > As > Cu > P. A high enrichment (EF∼100) for phosphorus was observed and caused plausibly by extensive usage of phosphorus-containing fertilizers and pesticides.

Polystyrenesulfonate-coated nanoparticles with low cytotoxicity for determination of copper(II) via the luminescence of Tb(III) complexes with new calix[4]arene derivatives

The authors describe new ligands with two 1,3-diketone groups and two heteroaromatic (pyridyl or quinolyl) moieties embedded to the upper and lower rims of dibromo-substituted calix[4]arene scaffold. The ligands bind Tb(III) ions in alkaline DMF solutions to form 1:1 complexes. The strong Tb(III)-centered luminescence (with excitation/emission peaks at 330/545 nm) of the complexes results from efficient ligand-to-metal energy transfer. The complexes were incorporated into polystyrenesulfonate (PSS) colloids by diluting a DMF solution of the complex with aqueous solution of PSS. The luminescence of the colloids is quenched by copper(II), and this was used to develop a method for its fluorometric determination in nanomolar concentrations. The lower limit of detection is 0.88 nM. Quenching is a result of (a) ion exchange which converts the terbium complexes into their copper counterparts, and (b) energy transfer from Tb(III) to Cu(II) complexes. The low cytotoxicity of the colloidal nanoprobe conceivably makes it a promising tool for use in cellular imaging. Graphical abstract New calix[4]arene derivative provide efficient binding sites for Tb(III) and Cu(II) ions. The Tb(III) complexes were embedded to core-shell nanoparticles by solvent-mediated aggregation followed by polystryrenesulfonate deposition. The nanoparticles exhibit luminescence response on copper ions in nanomolar concentration range.

Inorganic Arsenic Determination in Food: A Review of Analytical Proposals and Quality Assessment Over the Last Six Years

Here we review recent developments in analytical proposals for the assessment of inorganic arsenic (iAs) content in food products. Interest in the determination of iAs in products for human consumption such as food commodities, wine, and seaweed among others is fueled by the wide recognition of its toxic effects on humans, even at low concentrations. Currently, the need for robust and reliable analytical methods is recognized by various international safety and health agencies, and by organizations in charge of establishing acceptable tolerance levels of iAs in food. This review summarizes the state of the art of analytical methods while highlighting tools for the assessment of quality assessment of the results, such as the production and evaluation of certified reference materials (CRMs) and the availability of specific proficiency testing (PT) programmes. Because the number of studies dedicated to the subject of this review has increased considerably over recent years, the sources consulted and cited here are limited to those from 2010 to the end of 2015.

Tutorial on estimating the limit of detection using LC-MS analysis, part I: Theoretical review

A large body of literature exists on the limit of detection (LOD), but there is still a lot of confusion about this important validation parameter. This confusion mainly stems from its statistically complex background. The goal of this two-part tutorial is to discuss and clarify the topic of LOD for practitioners. The two main conclusions of this tutorial are: (1) the choice of how to estimate LOD should be based on the purpose of the analytical method that is being validated (e.g. considerable effort should not be made to estimate LOD for a method that is not used for detecting traces in the vicinity of LOD), and (2) LOD estimates are strongly dependent on different assumptions and the approach used, and therefore caution must be exercised when using the estimate or when comparing different estimates. Part I of the tutorial contains a theoretical discussion (without excessively sophisticated statistics) and part II contains examples based on experimental data. In Part I, LOD and other definitions related to it are reviewed, and their estimation and use are discussed. The assumptions and practicality of different approaches to estimate LOD are compared. Different aspects of the analytical method that strongly influence LOD estimates (e.g. linearity, scedasticity and day-to-day variability of LOD) together with experimental design considerations when estimating LOD are discussed. In part II, LOD is estimated for the LC-MS/MS analysis method to detect pesticides on separate days. The performance of different tests on the data necessary for LOD estimation are discussed and the results of different approaches under review in this tutorial are compared. In conclusion, a decision tree is proposed for estimating and monitoring LOD. A detailed working procedure for estimating LOD is presented. Several topics are pointed out in which further research and discussion is needed.

Arsenic(III) detection using electrochemical-chemical-chemical redox cycling at bare indium-tin oxide electrodes

Sensitive As(III) detection in ground water is of great importance for evaluating the quality of drinking water. We report a sensitive electrochemical method for As(III) detection based on electrochemical-chemical-chemical (ECC) redox cycling involving Ru(IV) [an oxidized species of Ru(III)(NH3)5NH2(2+)], As(III), and tris(3-carboxyethyl)phosphine (TCEP). Electrochemical oxidation of Ru(III)(NH3)5NH2(2+) formed from Ru(III)(NH3)6(3+) generates Ru(IV), which quickly oxidizes As(III). This electro-mediated oxidation of As(III) produces As(V), which is reduced back to As(III) by TCEP. Electrochemically generated Ru(IV) then reoxidizes As(III), allowing ECC redox cycling to occur at a high rate on bare indium-tin oxide (ITO) electrodes without modifying the surfaces with electrocatalytic materials. Because most interfering metal ions precipitate in a carbonate buffer, water samples are mixed with carbonate buffers prior to electrochemical measurements, rendering the effects of Cu(+), Cu(2+), Fe(2+), Fe(3+), and Pb(2+) insignificant. The detection limit calculated by ECC redox cycling using a chronocoulogram is 1.2 μM, much lower than that obtained using only the electro-mediated oxidation of As(III) (90 μM).

Bubble-driven mixer integrated with a microfluidic bead-based ELISA for rapid bladder cancer biomarker detection

In this study, fine bubbles were successfully generated and used as a simple, low-cost driving force for mixing fluids in an integrated microfluidic bead-based enzyme-linked immunosorbent assay (ELISA) to rapidly and quantitatively detect apolipoprotein A1 (APOA1), a biomarker highly correlated with bladder cancer. A wooden gas diffuser was embedded underneath a microfluidic chip to refine injected air and generate bubbles of less than 0.3 mm. The rising bubbles caused disturbances and convection in the fluid, increasing the probability of analyte interaction. This setup not only simplifies the micromixer design but also achieves rapid mixing with a small airflow as a force. We used this bubble-driven micromixer in a bead-based ELISA that targeted APOA1. The results indicate that this micromixer reduced the time for each incubation from 60 min in the conventional assay to 8 min with the chip, resulting in a reduction of total ELISA reaction time from 3-4 h to 30-40 min. Furthermore, the concentration detection limit was 9.16 ng/mL, which was lower than the detection cut-off value (11.16 ng/mL) for bladder cancer diagnosis reported in the literature. Therefore, this chip can be used to achieve rapid low-cost bladder cancer detection and may be used in point-of-care cancer monitoring.

Over a century of detection and quantification capabilities in analytical chemistry--historical overview and trends

The detection limit (LD) and the quantification limit (LQ) are important parameters in the validation process. Estimation of these parameters is especially important when trace and ultra-trace quantities of analyte are to be detected. When the apparatus response from the analyte is below the detection limit, it does not necessarily mean that the analyte is not present in the sample. It may be a message that the analyte concentration could be below the detection capabilities of the instrument or analytical method. By using a more sensitive detector or a different analytical method it is possible to quantitatively determine the analyte in a given sample. The terms associated with detection capabilities have been present in the scientific literature for at least the past 100 years. Numerous terms, definitions and approaches to calculations have been presented during that time period. This paper is an attempt to collect and summarize the principal approaches to the definition and calculation of detection and quantification abilities published from the beginning of 20th century up until the present. Some of the most important methods are described in detail. Furthermore, the authors would like to popularize the knowledge of metrology in chemistry, particularly that part of it which concerns validation of the analytical procedure.

A new approach in regression analysis for modeling adsorption isotherms

Numerous regression approaches to isotherm parameters estimation appear in the literature. The real insight into the proper modeling pattern can be achieved only by testing methods on a very big number of cases. Experimentally, it cannot be done in a reasonable time, so the Monte Carlo simulation method was applied. The objective of this paper is to introduce and compare numerical approaches that involve different levels of knowledge about the noise structure of the analytical method used for initial and equilibrium concentration determination. Six levels of homoscedastic noise and five types of heteroscedastic noise precision models were considered. Performance of the methods was statistically evaluated based on median percentage error and mean absolute relative error in parameter estimates. The present study showed a clear distinction between two cases. When equilibrium experiments are performed only once, for the homoscedastic case, the winning error function is ordinary least squares, while for the case of heteroscedastic noise the use of orthogonal distance regression or Margart's percent standard deviation is suggested. It was found that in case when experiments are repeated three times the simple method of weighted least squares performed as well as more complicated orthogonal distance regression method.

Limit of detection and limit of quantification development procedures for organochlorine pesticides analysis in water and sediment matrices

Background

Reliable values for method validity of organochlorine pesticides determination were investigated, in water by solid phase extraction and in sediment by Soxhlet extraction, followed by gas chromatography equipped with an electron capture detector. Organochlorine pesticides are categorized as Persistent Organic Pollutants. Hence, critical decisions to control exposure to these chemicals in the environment are based on their levels in different media; it is important to find valid qualitative and quantitative results for these components. In analytical chemistry, internal quality procedures are applied to produce valid logical results.

Result

In this study, 18 organochlorine pesticides were targeted for analysis and determination in water and river sediment. Experiments based on signal-to-noise ratio, calibration curve slope and laboratory fortified blank methods were conducted to determine the limits of qualification and quantification. The data were compared with each other. The limitation values, following Laboratory Fortified Blank, showed significant differences in the signal-to-noise ratio and calibration curve slope methods, which are assumed in the results for the sample concentration factor to be 1,000 times in water and 10 times in sediment matrices. The method detection limit values were found to be between 0.001 and 0.005 μg/L (mean of 0.002 ± 0.001) and 0.001 and 0.005 μg/g (mean of 0.001 ± 0.001). The quantification limits were found to be between 0.002 and 0.016 μg/L (mean of 0.006 ± 0.004) and 0.003 and 0.017 μg/g (mean of 0.005 ± 0.003 μg/L) for water and sediment, respectively, based on the laboratory fortified blank method. Because of different slopes in the calibration methods, it was also found that the limitation values for some components from the internal standard were higher than from external standard calibration, because in the latter a factor for injection efficiency is applied for calibration.

Conclusion

Technically, there are differentiations between detection limits for quality and quantity from component to component, resulting from noise, response factors of instruments and matrix interference. However, the calculation method is the cause of differentiation for each component of the different methods. The results show that for no matter what component, the relationship between these levels in different methods is approximately: Signal to Noise : Calibration Slope = 1:10. Therefore, due to different methods to determine LOD and LOQ, the values will be different. In the current study, laboratory fortified blank is the best method, with lower limitation values for Soxhlet and solid phase extraction of OCPs from sediment and water, respectively.