Qualitative Analysis Revisited

Gas Identification by Simultaneous Permeation through Parallel Membranes: Proof of Concept

This paper describes an experimental system for simultaneous permeation of a pressurized test gas through different gas permeable membranes and provides a proof of concept for a novel approach for gas identification/fingerprinting for potential construction of electronic noses. The design, construction, and use of a six-channel system which allows simultaneous gas permeation from a single pressurized gas compartment through six different parallel membranes are presented. The permeated gas is accumulated in confined spaces behind the respective membranes. The rate of gas pressure accumulation behind each membrane is recorded and used as a measure of the gas permeation rate through the membrane. The utilized gas permeable membranes include Teflon AF, silicone rubber, track-etch hydrophilic polycarbonate, track-etch hydrophobic polycarbonate, track-etch polyimide, nanoporous anodic aluminum oxide, zeolite ZSM-5, and zeolite NaY. An analogy between the rate of pressure accumulation of the permeating gas behind the membrane and the charging of an electric capacitor in a single series RC circuit is proposed and thoroughly validated. The simultaneous permeation rates through different membranes demonstrated a very promising potential as characteristic fingerprints for 10 test gases, that is, helium, neon, argon, hydrogen, nitrogen, carbon dioxide, methane, ethane, propane, and ethylene, which are selected as representative examples of mono-, di-, tri-, and polyatomic gases and to include some homologous series as well as to allow testing the potential of the proposed system to discriminate between closely related gases such as ethane and ethylene or carbon dioxide and propane which have almost identical molecular masses. Finally, a preliminary investigation of the possibility of applying the developed gas permeation system for semiquantitative analysis of the CO₂-N₂ binary mixture is also presented.

Passive Sampling with Active Carbon Fibres in the Determination of Organic Pollutants in Groundwater

Legislation addressing the quality of groundwater and increasing concerns over public health calls for the development of analytical methods that can produce accurate and precise results at the ppt level. Passive sampling has been recognised as a helpful tool in identifying various organic pollutants in groundwater, even when their presence had not yet been identified through conventional groundwater quality monitoring. The article presents an analytical method involving a simple and cost-effective passive sampling device using Zorflex® activated carbon fibres (ACFs) for the qualitative monitoring of a broad range of organic pollutants in water in a single run. The applicability of the method developed was tested in three hydrogeological studies. In the first case, we present a non-targeted qualitative screening and a list of 892 different contaminants detected in the groundwater in Slovenia. In the second case, we discuss the presence and origin of organic compounds in the groundwater from a pilot area of the urban aquifer, Ljubljansko polje. The third case presents a comparison of results between passive and grab sampling. Passive sampling with ACFs confirmed the presence of a pollutant, even when it had not been previously detected through a quantitative method.

Maximizing the Accuracy of Continuous Quantification Measures Using Discrete PackTest Products with Deep Learning and Pseudocolor Imaging

Using the standard colors provided in the instructions, PackTest products can approximate and quickly estimate the chemical characteristics of liquid samples. The combination of PackTest products and deep learning was examined for its accuracy and precision in quantifying chemical oxygen demand, ammonium ion, and phosphate ion using a pseudocolor imaging method. Each PackTest product underwent reactions with standard solutions. The generated color was scanner-read. From the color image, ten grayscale images representing the intensity values of red, green, blue, cyan, magenta, yellow, key black, and L ^∗ , and the values of a ^∗ and b ^∗ were generated. Using the grayscale images representing the red, green, and blue intensity values, 73 other grayscale images were generated. The grayscale intensity values were used to prepare datasets for the ten and 83 (=10 + 73) images. For both datasets, chemical oxygen demand quantification was successful, resulting in values of normalized mean absolute error of less than 0.4% and coefficients of determination that were greater than 0.9996. However, the quantification of ammonium and phosphate ions commonly provided false positive results for the standard solution that contained no ammonium ion/phosphate ion. For ammonium ion, multiple regression markedly improved the accuracy using the pseudocolor method. Phosphate ion quantification was also improved by avoiding the use of an estimated value for the reference solution that contained no phosphate ion. Real details of the measurements and the perspectives were discussed.

A New Bayesian Approach for Estimating the Presence of a Suspected Compound in Routine Screening Analysis

A novel method for compound identification in liquid chromatography-high resolution mass spectrometry (LC-HRMS) is proposed. The method, based on Bayesian statistics, accommodates all possible uncertainties involved, from instrumentation up to data analysis into a single model yielding the probability of the compound of interest being present/absent in the sample. This approach differs from the classical methods in two ways. First, it is probabilistic (instead of deterministic); hence, it computes the probability that the compound is (or is not) present in a sample. Second, it answers the hypothesis "the compound is present", opposed to answering the question "the compound feature is present". This second difference implies a shift in the way data analysis is tackled, since the probability of interfering compounds (i.e., isomers and isobaric compounds) is also taken into account.

Screening of N-nitrosamines in tap and swimming pool waters using fast gas chromatography

N-Nitrosamines (NAms) are suspected human carcinogens that have been identified as drinking water and wastewater pollutants. In this work, a sensitive screening/confirmation method was proposed for the determination of the most toxic NAms that can be found in water samples (N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodibutylamine, N-nitrosomorpholine, N-nitrosopiperidine, and N-nitrosopyrrolidine). A sample volume of 250 mL was first preconcentrated in an automatic SPE unit and then the extract was concentrated to a final volume of 10 microL (providing a preconcentration factor of 25,000). Aliquots of the extract were subjected to a rapid screening process (1.6 min) by using a short capillary polar column (1.5 m length) and GC with nitrogen-phosphorous detection. In this way, the high number of samples to be tested routinely in a water laboratory is simplified due to a reduction in the analysis time. Thus, the screening method acts as a filter that indicates whether target analytes are present, above or below the cut-off level (3.8 or 10.4 ng/L), giving no false negatives at concentrations below the guide values for NAms in drinking water established by different countries. Positive samples (tap and swimming pool waters) were then confirmed by GC-MS detection.

Principles of qualitative analysis in the chromatographic context

This article presents the state of the art of qualitative analysis in the framework of the chromatographic analysis. After establishing the differences between two main classes of qualitative analysis (analyte identification and sample classification/qualification) the particularities of instrumental qualitative analysis are commented on. Qualitative chromatographic analysis for sample classification/qualification through the so-called chromatographic fingerprint (for complex samples) or the volatiles profile (through the direct coupling headspace-mass spectrometry using the chromatograph as interface) is discussed. Next, more technical exposition of the qualitative chromatographic information is presented supported by a variety of representative examples.

Evaporative light scattering detection: trends in its analytical uses

Evaporative light scattering detection (ELSD) is widely recognized as a universal tool for liquid and supercritical chromatographies. In addition, this detection technique is fully compatible with continuous-flow systems. In fact, the combination of continuous non-chromatographic techniques and ELSD affords the design of simple, reliable systems for extracting qualitative information. This paper reviews instrumental innovations regarding the miniaturization of evaporative light scattering detectors and their uses in micro and capillary liquid chromatography; also, it discusses their increasingly important role in the development of vanguard configurations for sample screening and the determination of total indices without the need for chromatographic separation. Moreover, it compares them with other types of chromatographic detectors in terms of performance. Finally, the potential of ELSD for solving real-life analytical problems arising from the need to meet (bio)chemical information needs is illustrated with various selected applications.

Coupling micelle-mediated extraction using mixtures of surfactants and fluorescence measurements with a fiber-optic for the screening of PAHs in seawater

The screening of PAHs from seawater samples using cloud-point extraction (CPE) as a step prior to their determination by fluorescence measurements with a fiber-optic is proposed. The CPE is carried out with the nonionic surfactants mixture POLE and Brij 30. The fluorescence measurement parameters were optimized, allowing selection of benzo(a)pyrene (B(a)Py) and benzo(k)fluoranthene (B(k)Ft) as the target analytes for the screening. The reproducibility of the whole screening system, expressed as relative standard deviation, was 9.0% for B(a)Py and 12.1% for B(k)Ft (both for n = 7). The reliability of the method was established at five concentrations for B(a)Py (between 0.5 and 3.3 times the detection limit: 0.31 ng ml(-1)) and at three concentrations for B(k)Ft (between 0.6 and 2.5 times the detection limit: 0.56 ng ml(-1)). The resolution of binary mixtures of these PAHs at different levels of concentration, and a study of the interferences with the rest of the PAHs were also carried out.

Determination of natural and synthetic colorants in prescreened dairy samples using liquid chromatography-diode array detection

A simple and novel screening method for food colorants was proposed. Synthetic or natural colorants were discriminated as they were selectively adsorbed on cotton or RP-C18 sorbent columns, respectively. After elution, each fraction was monitored at 400, 530, and 610 nm for yellow, red, and green-blue-brown additives, respectively, with a DAD spectrophotometer. The screening method serves as a filter to indicate whether the target colorants are present above or below the detection limit of the method (6-15 or 25-10,000 n/mL for synthetic or natural colorants, respectively). Positive samples were discriminated by LC-DAD, using a flow system similar to that of the screening method. The LC-DAD discrimination/confirmation method is very sensitive; it exhibits a linear range of 0.01-50 microg/mL (excluding curcumin and caramel, which are linear up to 200 and 1,500 microg/mL, respectively). The method was applied to the determination of natural and synthetic colorants in dairy samples with good precision.