Jawahar Lal Sharma

Recent applications of sample preparation techniques in food analysis

Even with the emergence of advanced techniques of separation and identification, it is rarely possible to analyse food without manipulation. The traditional techniques for sample preparation are time consuming and require large amount of reagents, which are expensive, generate considerable waste, contaminate the sample and can enrich it for analytes. The more analytical techniques have become highly developed, the more has sample clean-up become important in order to fully take advantage of them. Due to the multiplicity of food matrices, it is not possible to use one sample preparation technique, so many methods have been proposed for meeting all the requirements. The newest variations of wet digestion, solvent and sorbent extraction and membrane separation are summarised and their most recent applications to food analysis are provided.

Ion chromatographic and voltammetric determination of heavy and transition metals in honey

Heavy metals in honey are of interest not only for quality control, but can be used also as an environmental indicator. In the present work, in order to minimize sample pre-treatment, the interference by organic constituents of the matrix is overcome by using oxidative UV photolysis. The matrix degrades in less than 1 h, while most common metallic impurities, like iron, copper, nickel, zinc, lead, cadmium and cobalt, remain unaffected by UV radiation, with the exception of manganese. After UV photolysis, the resulting solution is directly analyzed by ion chromatography and differential pulse anodic or cathodic stripping voltammetry. In absence of official standards, the results obtained by these techniques on spiked matrix-matched blank solutions and original and spiked real samples are compared with those of the well established electrothermal atomic absorption spectrometry and they are found in good agreement. The proposed techniques show satisfactory sensitivity, detection limits and standard deviation for heavy and transition metals determination in honey. In addition, both ion chromatography and pulsed voltammetries permit multielement analyses which can be fully automated.

Determination of some inorganic species in edible vegetable oils and fats by ion chromatography

Abstract A procedure has been developed for the ion chromatographic determination of total chlorine, phosphorus and sulphur and of iron, copper, nickel, zinc, cobalt, lead and cadmium in edible vegetable oils and fats. The organic matrix which strongly interferes in analytical procedures, is completely removed by saponification followed by oxidative UV photolysis. This method is simpler and requires less reagents compared with other sample pretreatment procedures. To oil and fats, ethanol and potassium hydroxide were added, and they were saponified for half an hour. Hydrogen peroxide was added and the sample was subjected to UV photolysis at 85±5°C. In less than 1 h organic constituents were completely degraded, while inorganic constituents, except nitrates, iodides and manganese, remained unaffected by UV radiation. Chlorine, phosphorus and sulphur, present in different organic compounds, can be quantified as total amounts only, without speciation. The clear sample solutions were directly injected on to an ion chromatograph equipped both with a conductivity detector, for the determination of total chloride, phosphate and sulphate ions using a carbonate–hydrogencarbonate eluent, and a post-column reactor and a variable-wavelength UV–Vis detection system, for the determination of lead and cadmium using an oxalate eluent and iron, copper, nickel, zinc and cobalt using a pyridine-2,6-dicarboxylic acid eluent. The method has been tested on spiked and unspiked samples of vegetable oils and fats and was found to be satisfactory for the determination of the previously cited elements. Ion chromatographic responses were compared to voltammetric ones and were found to be in good agreement (±3%).

Matrix removal for the ion chromatographic determination of some trace elements in milk

Abstract An ion chromatographic method has been developed for the determination of some microelements in different types of milk. It involves oxidative photodegradation of the organic matrix with H2O2 in a UV digester, equipped with a high-pressure mercury lamp. The temperature of the sample is maintained at 85±5 °C by a combined air/water cooling system. This procedure provides an efficient alternative to traditional dry ashing and wet digestion methods. Milk degrades in less than 2 h, while inorganic constituents, except for iodide, nitrate, nitrite, sulfite and manganese (II), are unaffected by UV radiation. Depending upon the type of milk (whole, skimmed, powdered, evaporated, etc.) to be analysed, the amount of sample and the UV photolysis time can be adjusted as per requirements. The clear solution resulting from the UV digestion is diluted, filtered and injected onto an ion chromatograph equipped with both conductivity and variable-wavelength UV-Vis detectors. The method has been tested with standards and real milk samples and has been found to be satisfactory for the determination of total chloride, bromide, phosphorus (as phosphate) and sulfur (as sulfate), and of copper, nickel, zinc, cobalt, iron and lead.

Determination of total phosphorus in soaps/detergents by ion chromatography

Abstract The quantitative determination of phosphorus in soaps and detergents is a classical analytical problem owing to the complexity of the matrix containing a variety of chemical species (i.e., surfactants, complexones and/or zeolites, optical whiteners, perfumes, etc.). A new method has been developed for the analysis of total phosphorus in soaps and detergents which employs UV photolysis of the analyte. It has the advantages that it is a simple procedure and has very low blank values because of the small amount of reagent required for the sample pretreatment. Different types of soaps and detergents were subjected to oxidative UV photolysis. It was found that the organic matrix was degraded in about 60 min, thereby permitting the quantitative analysis of various inorganic species, especially the phosphate ion. Soaps and detergents of different “types and brands” were found to contain phosphate from a few mg/l or mg/kg to the 1% level. The results were compared with those obtained by conventional alkaline fusion followed by reduction to molybdenum blue, and were found to be in good agreement.

LA-ICP-MS, IC and DPASV-DPCSV determination of metallic impurities in solar-grade silicon

Stringent specifications are laid down for the silicon used for solar cells. The present work deals with the application of different techniques to the simultaneous determination of most common metallic impurities like iron, copper, nickel, zinc, lead and cadmium, in industrial process control. Laser ablation inductively coupled plasma mass spectrometry is quite expensive in apparatus, but it directly performs the analysis of solid silicon with very good sensitivity, even if coupled to considerable standard deviation, probably due to the material defects. Both ion chromatography and voltammetry need sample pre-treatment, but they are characterized by cheap and simple apparatus, suitable detection limits, good sensitivity and small standard deviation.

Determination of trace amounts of anionic impurities in hydrochloric acid by ion chromatography

The purity of the hydrochloric acid used in semiconductor processing has a direct impact on device reliability because ionic impurities adhere to the wafers surface during processing, and the impurities must be monitored in order to avoid both unnecessary wastes and exceeding of the permitted levels. Ion-chromatographic procedures for the determination of bromide, nitrate, phosphate, selenate and arsenate in hydrochloric acid are described. The high chloride concentration and the pH of the sample cause major problems that, for monovalent anions, can be overcome by simply diluting the sample, while matrix elimination in the presence of potassium permanganate is effective in the determination of polyvalent anions. The optimum pH range, concentration of the matrix and of the eluent, and working conditions have been investigated. The use of different eluents has been suggested for the determination of bromide and nitrate after matrix dilution, as these ions elute very near to the large chloride peak. The procedures described have been successfully used for the routine analysis of hydrochloric acid batches used for cleaning semiconductors.

Ion-chromatographic determination of inorganic anions and cations in some reagents used in the electronics industry

Abstract In the electronics industry, wafer cleaning is usually performed by means of hot ammoniacal or acidic hydrogen peroxide solutions, which remove organic surface films by oxidative breakdown and dissolution to expose the substrate surface for concurrent and/or subsequent decontamination reactions. It is difficult to establish the optimum renewal cycle of the cleaning solutions to avoid economic as well as pollution problems. The present work deals with the application of ion chromatography to the determination of anionic as well as cationic impurities in hydrogen peroxide and ammonia, which are extensively used in wafer cleaning technology. As these two reagents cannot be injected directly into the separator column, methods are suggested for the pretreatment of the samples. Hydrogen peroxide was subjected to UV photolysis for about 30 min in an UV digestor at 85 ± 5°C. Though ammonia could also be treated in the same manner, simply heating it at 85 ± 5°C for 45 min in a dust-free cell, to expel most of the ammonia as gas, was found to be satisfactory. The samples were then analysed by ion chromatography and were found to contain chloride, phosphate, sulphate, copper, zinc, iron and manganese as impurities in variable amounts, when analysed at different stages of the wafer cleaning operation (20–1000 μg/l).

Application of ion chromatography to the analysis of lead–acid battery electrolyte

Stringent specifications are laid down for the sulfuric acid used as battery electrolyte. In the present work ion chromatography (IC) was applied to the determination of several species in sulfuric acid and it proved possible to separate impurities in the presence of large amounts of sulfate ion without the need for a preconcentration or pre-separation step. Fluoride, chloride, bromide, nitrate, phosphate, sodium, potassium, magnesium, calcium, iron, copper, nickel, zinc and cadmium can easily be determined by diluting the lead–acid battery electrolyte (usually 38% H2SO4) 40-fold. Most of the species to be determined were found to be adversely affected by sample pre-treatments, such as sulfate precipitation or ion-exchange resin separation. The pH of the analyte strongly affects the analytical response and hence the optimum pH range was established for anions, alkali and alkaline earth metals, and transition metals. In the presence of organic compounds, oxidative UV photolysis of the sample permits determinations to be performed without any interference. The proposed procedure involves the IC determination of fluoride, chloride, bromide, nitrate, and phosphate using a carbonate–hydrogencarbonate eluent with suppressed conductivity detection, that of sodium, potassium, magnesium and calcium using a hydrochloric acid–2,3-diaminopropionic acid hydrochloride eluent with suppressed conductivity detection and that of iron, copper, nickel, zinc and cadmium using pyridine-2,6-dicarboxylic acid as eluent with visible absorbance detection. Detection limits ranged between 0.2 and 1.3 ppm and the linearity of the calibration graphs was found to be extended by up to two orders of magnitude.

Determination of inorganic ions in carboxylic acids by ion chromatography

Abstract A method has been developed for the quantitative determination of various inorganic impurities present in carboxylic acids. The spiked as well as unspiked analytes were subjected to oxidative UV photolysis prior to ion chromatographic analysis. This procedure has definite advantages compared with other sample pretreatment methods: it is a simple procedure and the reagent requirement is minimal. Inorganic cations and anions, except nitrite, nitrate, iodine, sulphite and manganese(II), are unaffected by UV radiation. Depending upon the nature of impurity to be analysed and the amount of carboxylic acid, the UV photolysis time can be adjusted as required. The method was tested on several aliphatic as well as aromatic acids and found to be satisfactory for the determination of chloride, bromide, phosphate, sulphate, iron(III), lead(II), copper(II), zinc(II), nickel(II) and cobalt(II). Detection limits of the proposed method are between 5 and 25 ng/ml and calibration curves were found to be linear up to 1–2 μg/ml.