Abstract
In recent years nutrition has become a contemporary subject with increased apprehension over the adequate intake of minerals countered by the concern over the inadvertent consumption of contaminants such as heavy metals (lead, cadmium and mercury) and pesticides (organochlorine and organophosphorus). The advent of neutron activation analysis (NAA), has led to the widespread acceptance of multielemental techniques for the analysis of biological matrices. Inductively coupled plasma mass spectrometry (ICP-MS) is a relatively new technique which in the last decade has gained recognition for its multielemental capabilities. This thesis deals with the application of ICP-MS for the determination of essential, non-essential and toxic elements namely, sodium, magnesium, calcium, manganese, iron, vanadium, chromium, nickel, cobalt, copper, zinc, rubidium, strontium, caesium, molybdenum, cadmium and lead in food matrices; breakfast cereal (wholewheat, wheatbran, rice, oats, corn), flour (white, brown, wholemeal, organic) and fresh milk (full fat, semi-skimmed, skimmed, enriched, organic) samples. Wet and dry mineralization procedures have been examined throughout the course of this study, but losses were incurred in utilizing the dry ashing procedures for both cereal and milk matrices. Consequently, wet digestion techniques using HNO3 for the cereals and a mixture of HNO3, HCIO4 and H2O2 for the milks were employed. Elemental recoveries from both cereal and milk matrices were found to excellent and generally ranged from 80.7% for manganese to 115.4% for sodium. Matrix interference effects have been examined through standard addition studies and observed to be insignificant. Quality control has been maintained throughout, and accuracy of results ensured through the use of the international standard reference materials, NIST:SRM 1567a Wheat Flour (for zinc typically, 11.6±0.4 certified compared with 11.1±0.1 μg g-1 calculated) and NIST 1549 Non-Fat Milk Powder (for iron typically, 1.78±0.1 certified compared with 1.75±0.4 μg g-1 calculated). The results obtained from the elemental study indicate that the wheat-based breakfast cereals were richer in mineral content which is in agreement with previously published data. However, no significant differences were found between the individual varieties of flour and similarly no conclusive differences in the elemental composition of the milks was observed. The Ministry of Agriculture Fisheries and Food publishes figures for the estimated weight of food eaten per person per day, and the Department of Health issues recommended nutrient intake values (RNI), which include data for essential minerals. Employing these figures and the data obtained from the elemental studies of the milk and cereal food groups, estimates of the individual intake of essential elements have been obtained. Results indicate that wheat based cereals are the most nutritious and based on this matrix, the consumption of ready-to-eat breakfast cereals on average yield 1.9% (calcium) to 57.0% (magnesium) of the recommended intakes; flours 0.1% (sodium) to 43.2% (zinc); and milk 0.6% (iron) to 72.4% (calcium). The intake of the elemental contaminants was in line with documented reports; cadmium <8 μg day-1 and lead 13 μg day-1. Investigations into the contamination of both cereals and milks by organophosphorus pesticide residues have been carried out, using gas chromatography with flame photometric detection. Documented analysis procedures were observed to employ vast quantities of solvent and were found to be extremely time consuming. In order to minimize analysis times and reduce the quantity of chemicals used, analytical procedures have been developed employing solid phase extraction. Cereal matrices are extracted directly with hexane, concentrated by rotary evaporation and then passed through acetone pre-conditioned aminopropyl solid phase extraction cartridges, to eliminate the bulk matrix. Interference effects from this method were observed to be minimal and recoveries excellent for this particular matrix (78 - 92%). Milk samples were precipitated using acetone, filtered and diluted with a saturated solution of NaCl. The resulting solution was partitioned using a hexane-acetonitrile mixture to eliminate the lipid content of the milk samples. Resultant solutions were concentrated to a volume of approximately 3 ml, diluted with water and passed through methanol pre-conditioned C18 bonded silica cartridges. The adsorbed pesticides were eluted with methanol. The developed method, reduced sample preparation times, yielded no significant interferences and recoveries were good (74 - 91%) for all the organophosphorus pesticides, with the exception of dichlorvos (64%). The pesticide contamination of the matrices analyzed was found to be limited to bran-based cereals and observed levels were significantly lower (approximately 1000 fold) than U. K. maximum residue limits set by MAFF. Estimated dietary intakes from the consumption of ready-to-eat breakfast cereals were in line with U. K. Total Diet Studies; etrimfos < 29 ng g-1 and pirimiphos-methyl < 13 ng g-1. The contamination of milk and cereal matrices by organophosphorus pesticide residues was below the detection limits of the instrument.