Computer systems for analytical chemistry

Microcomputers are becoming integral parts of all analytical instrumentation and 'personal computers' are invading the laboratory. The analytical chemist is confronted with the question of how to computerize his work and what to request from a computer system. This article attempts to provide some guidelines.     

Laser-induced low-temperature fluorescence spectroscopy

The absorption and fluorescence spectra of molecules are often very sharp under cryogenic sampling conditions. When a tunable laser is used to excite fluorescence, selective determinations of individual constituents of very complex samples are possible, and the highly resolved fluorescence spectra can serve as 'molecular fingerprints'     

An investigation of the applicability of 2,2'-dianthrimide in spectrophotometry: The determination of selenium(IV)

2,2'-Dianthrimide was studied as an analytical reagent and compared with the properties of 1,1'-dianthrimide. While 1,1'-dianthrimide reacts with B, Ge, Se and Te, 2,2'-dianthrimide was found to react only with selenium(IV).A straight line calibration curve is obtained up to 50 μg of selenium under the optimum conditions but the value of 2,2'-dianthrimide as a reagent for selenium(IV) is reduced by the high absorption of the reagent itself.     

Environmental sampling: waters and effluents

Has sampling for environmental purposes lagged behind the sampling procedures of industry? In this environmentally conscious age there may be a danger of statistical control of sampling, overlooking the fact that conditions may change rapidly in an uncontrollable manner. Samples should always be 'representative' and 'valid'.     

Electrophoric release tags: Ultrasensitive molecular labels providing multiplicity

Electrophoric release tags are a new class of analytical reagents which function as molecular labels. These tags can be more sensitive than radioisotopes and their potential for multiplicity allows release-tagged reagents to be used as 'teams' in chemical analysis. Monoclonal antibodies, nucleic acid probes, receptor ligands and enzyme substrates may all benefit from this approach.     

Conformational analysis of 2', 3' and 4'-azachalcones by molecular orbital calculations

The semi-empirical quantum chemical PCILO method is used to perform a detailed conformational analysis of 2'-, 3'-and 4'-azachalcones. Several energy minima are obtained for each compound. Global energy minima (most stable conformations) are found for antiperiplanar-s-cis 2'-azachalcone, synperiplanar-s-cis 3'-azachalcone and s-cis 4'-azachalcone. The calculated results are compared with available experimental data.     

Polybrominated oxydiphenol derivatives from the sponge dysidea herbacea: Structure determination by analysis of 13C spin-lattice relaxation data for quaternary carbons and 13C-1H coupling constants

Five polybrominated oxydiphenol derivatives have been isolated from various Great Barrier Reef collections and one Fijian collection of the sponge Dysidea herbacea: 3,4',5,6,6'-hexabromo-2, 2'-oxydiphenol (11), 3,4',5,6,6'-pentabromo-2,2'-oxydiphenol (12), 3.4',5,6,6'-pentabromo-2,2'-oxydiphenol 1-methyl ether (13), 3,4,4',5,6'-pentabromo-2,2'-oxydiphenol 1,1'-dimethyl ether (14) and 3,4',5,6'-tetrabromo-2,2'-oxydiphenol 1'-methyl ether (15).The structure of the first member of this series is determined by a new method involving ¹³C spin-lattice relaxation data. The contributions of nearby hydrogens to quaternary carbon spin-lattice relaxation times are calculated for various possible structures and compared with the experimental data, leading to an unequivocal proof of structure. The structures of the remaining compounds in the series are established principally by analysis of ¹³C chemical shifts and ¹³C-¹H coupling constants.     

Sample preparation for the analysis of heavy metals in foods

The analysis of foodstuffs for heavy metals continues to be an area of intense activity for analytical chemists. Methods of sample preparation are changing to allow a growing number of samples to be handled and to facilitate 'speciation' studies.     

Meeting reports: International conference on chemometrics in analytical chemistry

In the field of analytical chemistry, chemometrics has been defined as 'the chemical discipline which uses mathematical and statistical methods to achieve the aim of analytical chemistry, namely the obtention, in the optimal way, of relevant information about material systems'. There has been a tremendous growth of interest in the role of chemometrics in analytical chemistry in recent years and this has been reflected in the number of symposia devoted to the subject. Two meetings, held in Europe in September 1982, covered different aspects of chemometrics and some of the more significant points discussed are summarized in the reports below.     

Measurement of radiocesium in Irish peatland soils

Results are presented for soils taken from twelve peatland sites in Ireland. Two depth horizons, 0–5 and 5–15 cm, were sampled on each site and sampling was carried out in summer and autumn. Deposition of Chernobyl¹³⁷Cs varied between 0.4 and 4.6 kBq m^–2, which is at the lower end of values reported by other authors for Irish agricultural soils. The data showed that the relative percentages of activity and deposition remained internally consistent in each soil horizon for Chernobyl and weapons fallout sources on both sampling occasions: this provided evidence that sampling and analytical procedures were consistent. An apparent reversal in the relative importance of weapons fallout in soils was found when deposition data were compared to the same data presented in activity format, and this same reversal was found for both soil horizons. Different conclusions may erroneously be drawn from data depending on the parameter by which we make measurements. Presentation of results should be carefully considered and depends on the purpose for which a study is intended. A statistically significant change in bulk density of the deeper soil horizon was found between summer and autumn. Changes in bulk density important to bear in mind that in addition to analytical errors, sampling methods can only provide estimations which themselves have inherent sources of error.     

Using an 'electronic spreadsheet' for Plackett-Burman calculations

The trend in analytical chemistry toward statistically designed experiments is reinforced by the trend toward using personal computers. The tedious calculations required to determine which factors significantly affect the performance or yield of an experiment are easily done with one of the 'electronic spreadsheet' programs now available on personal computers.     

Sampling and metrology

The result of a measurement refers in principle only to the amount of substance actually contributing to the analytical signal. However, an appropriate definition of the measurand must include a specification of the system for which the result of the measurement should apply. All systems being inherently heterogeneous, representativity assumes importance for the metrological quality of a measurement, and the process needed to ascertain representativity is sampling. The contribution from this characteristic must be included when expressing the uncertainty of the reported value of the measurand. Representative sampling of systems that are infinite or non-uniform was developed by Pierre Gy in his Theory of Sampling. Finite systems can achieve uniformity by mechanical treatment and mixing; the heterogeneity of these systems can be characterized by a sampling constant, expressed in units of weight, for each particular species being determined. Examples of the contribution of sampling to the uncertainty of analytical results are discussed for some biological materials. Presented at the 2nd International Conference on Metrology – Trends and Applications in Calibration and Testing Laboratories, November 4–6, 2003, Eilat, Israel.     

Relationship between the general laws of vapor-liquid equilibria and the analytical curve of emission spectrometry

The relationship between the vapor-liquid equilibrium and the analytical curve of emission spectrometry is discussed on the basis of Hirata's equation, i.e., $\text{y}{}_1\text{y}{}_2$ = α($\text{x}{}_1\text{x}{}_2$)^β, where y is the mole fraction in the vapor phase, x in the liquid phase, α and β are constants for binary systems in a certain concentration range, and the indices $\text{1}$ and $\text{2}$ refer to the two components. Evaporation produced by different atomization-excitation systems can be characterized by apparent evaporation constants, α' and β', the values of which fall between the true values, α and β, of the normal distillation, and unity. Two powder techniques were selected representing the limiting cases: injection spark excitation (α' ≈ α, β' ≈ β) and controlled arc excitation (α' ≈ 1, β' ≈ 1). Using these excitation methods, the analytical curves of the impurities in industrial alumina and electrocorundum were studied using cup-electrode techniques with arc excitation. As a result, one of Hirata's rules could be selected for a given pair of elements. An unambiguous correlation between the properties of the analytical curves and Hirata's rules could be established.     

Reliability of measurements of pesticide residues in food

This paper accounts for the major sources of errors associated with pesticide residue analysis and illustrates their magnitude based on the currently available information. The sampling, sample processing and analysis may significantly influence the uncertainty and accuracy of analytical data. Their combined effects should be considered in deciding on the reliability of the results. In the case of plant material, the average random sampling (coefficient of variation, CV=28–40%) and sample processing (CV up to 100%) errors are significant components of the combined uncertainty of the results. The average relative uncertainty of the analytical phase alone is about 17–25% in the usual 0.01–10 mg/kg concentration range. The major contributor to this error can be the gas-liquid chromatography (GLC) or high-performance liquid chromatography (HPLC) analysis especially close to the lowest calibrated level. The expectable minimum of the combined relative standard uncertainty of the pesticide residue analytical results is in the range of 33–49% depending on the sample size.The gross and systematic errors may be much larger than the random error. Special attention is required to obtain representative random samples and to eliminate the loss of residues during sample preparation and processing.     

Characterization (certification) of three wheat flours and a wheat gluten reference material (NIST RM 8436, 8437, 8438 and 8418) for essential and toxic major, minor and trace element constituents

Summary Three wheat flours, Durum Wheat Flour (NIST RM 8436), Hard Red Spring Wheat Flour (NIST RM 8437), Soft Winter Wheat Flour (NIST RM 8438) and Wheat Gluten (NIST RM 8418) Reference Materials were characterized for essential and toxic major, minor and trace elemental composition by analysts in an interlaboratory cooperative characterization campaign. Extensive application of widely varied analytical methods yielded 16–27 best estimate and 3–8 informational concentration values for each of these materials. These reference materials are intended for analytical quality control of element determinations in flour and flour products as well as other agricultural/food materials with related matrices.Contribution No. 92–145 from Centre for Land and Biological Resources Research     

Flow-Injection Chemiluminescence as a Means of Studying the Metabolism of Unbound Chromium(III) in Rabbit Blood with On-Line Microdialysis Sampling

In this paper, we present an in-vivo, on-line, real-time analytical system for monitoring the metabolism of free chromium(III) in rabbit blood. This system includes microdialysis sampling, ion-exchange on-line separation and chemiluminescence detection. The results show that none of the co-existing substances in the blood, including protein and other small molecules, interfere with the determination. CrCl₃·6H₂O was administrated orally (0.5 g), and the microdialysis probe was utilized to sample rabbit blood with a perfusion rate of 10 µL min⁻¹. The dialytic efficiency of chromium(III) under the experimental conditions was 18.1 ± 5.1% (n=3). The concentration-time curve of chromium(III) is in accordance with the one-compartmental open model, the T_1/2 is 16.62 min.     

Estimation of C2−C5 hydrocarbons in air by pre-concentration on silica gel at dry ice temperature

Summary A short stainless steel column packed with silica gel is cooled to –78°C and atmospheric hydrocarbons are preconcentrated from 3 I of air on it. Desorption to the analytical column is by heating for 10 min to 200°C. Analysis is by GC on OPN-Poracil C at 27°C. The sorption efficiencies were nearly 100% for most of the light hydrocarbons.     

Evaluation of the fundamental sampling error in the sampling of particulate solids

Chemical analysis is a multi-stage process, which starts with primary sampling and ends with evaluation of the resuts. Especially in trace analysis and microanalysis of solid materials, sampling can far outweigh all other sources of error. For estimating the reliability of complete analytical procedures, a method is needed which can be used to estimate the errors made in the primary and the secondary sampling and sample preparation steps. Based on Gy's theory of sampling, a computer program (SAMPEX) was written for the solution of practical sampling problems. The method involves the estimation of the sampling constant, C. For well-characterized materials, C can be estimated from the material properties. If the necessary material properties are difficult to estimate, C can be evaluated experimentally. The program can be used to solve the following problems: minimum sample size for a tolerated relative standard deviation of the fundamental sampling error; relative size for a tolerated for a given sample size; maximum particle size of the material for a specified standard deviation and sample size; balanced design of a multi-stage sampling and sample-reduction process; and sampling for particle size determination.     

Application of Covalent Organic Porous Polymers-Functionalized Basalt Fibers for in-Tube Solid-Phase Microextraction

To establish an online analytical method towards estrogenic pollutants, a covalent organic porous polymer (COP) was in-situ synthesized on the surface of basalt fibers (BFs) for in-tube solid-phase microextraction (IT-SPME). The extraction tube, obtained via placing the modified BFs into a polyetheretherketone tube, was combined with high-performance liquid chromatography (HPLC) to achieve online IT-SPME-HPLC analysis. The important parameters, including sampling volume, sampling rate, organic solvent content and desorption time, were carefully investigated. Under the optimized conditions, the online analytical method was established for five estrogenic targets, with low limits of detection (0.001–0.005 μg/L), high enrichment factors (1800–2493), wide linear ranges (0.003–20, 0.015–20 μg/L) and satisfactory repeatability. It was successfully applied to detect five estrogens in a wastewater sample and a water sample in a polycarbonate cup. The BFs functionalized with COPs displayed excellent extraction effect for estrogenic pollutants, furthermore it has great potential in sample preparation or other fields.