Helen M. Crews

1997 UK Total Diet Study dietary exposures to aluminium, arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, tin and zinc

Concentrations of aluminium, arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, tin and zinc were determined in samples from the 1997 UK Total Diet Study and used to estimate dietary exposures of the general UK population. Population average dietary exposures to aluminium (3.4mg/day), arsenic (0.065mg/day), cadmium (0.012mg/day), chromium (0.10mg/day), copper (1.2mg/day), mercury (0.003mg/day), nickel (0.13mg/day), tin (1.8mg/ day) and zinc (8.4mg/day) are similar to those from previous UK Total Diet Studies and are below the appropriate PTWIs, PMTDIs and TDIs. Dietary exposure of the UK population (0.026 mg/day) to lead is falling as a result of measures taken to reduce lead contamination of the environment and food and is well below the PTWI. There has been little change in UK estimates of selenium exposure since the 1994 Total Diet Study but current estimates (0.039mg/day) are lower than those derived from earlier Total Diet Studies.

Dietary exposure estimates of 30 elements from the UK Total Diet Study

Dietary exposures of consumers to 30 elements (aluminium, antimony, arsenic, barium, bismuth, boron, cadmium, calcium, chromium, cobalt, copper, germanium, gold, iridium, iron, lead, lithium, manganese, mercury, molybdenum, nickel, palladium, platinum, rhodium, ruthenium, selenium, strontium, thallium, tin and zinc) estimated from the UK 1994 Total Diet Study are reported, and compared with those from previous UK Total Diet Studies and those from other countries. Dietary exposure estimates were generally low and, where comparisons are possible, similar to those from other countries and below the relevant Provisional Tolerable Weekly Intakes and Provisional Maximum Tolerable Daily Intakes. For most of those elements included in previous UK Total Diet Studies, dietary exposures have declined.

Investigation of selenium speciation in in vitro gastrointestinal extracts of cooked cod by high-performance liquid chromatography–inductively coupled plasma mass spectrometry and electrospray mass spectrometry

As part of an ongoing study of Se bioavailability from the human diet, Se was determined in cooked cod and in enzyme extracts of cooked cod. Total Se was measured by direct nebulization ICP-MS and standard additions, following digestion with HNO3 in stainless steel pressure decomposition vessels. The concentration of Se in the cod was 1.52 mg kg–1 with an LOD of 0.008 mg kg–1. An in vitro gastrointestinal enzymolysis procedure was used to extract Se species from the cooked cod. HPLC, using an anion-exchange column, was used to separate Se standards (selenomethionine, selenocystine, sodium selenite and sodium selenate) and the species soluble in the gastrointestinal extract (pH 6.8). The 82Se signal was used to quantify the individual Se species. In the extracts of cooked cod, approximately 12% of total Se was thought to be selenite whilst the remaining Se eluted at a retention time different to that of any of the Se standards measured. However, as the retention time was close to that of selenomethionine, it was suggested that the Se-containing species was organic rather than inorganic. A similar peak was also found in the enzyme blanks. In order to gain further information as to the identity of the unknown species, the samples and standards were subjected to electrospray-MS (ES-MS) in both positive and negative ionization modes. Although the four standards were amenable to analysis by ES-MS, more work is required to improve the sensitivity of the system. This will involve modifying the chromatography for both ICP-MS and ES-MS.

Design and characterisation of a microconcentric nebuliser interface for capillary electrophoresis-inductively coupled plasma mass spectrometry

An interface for capillary electrophoresis (CE) and inductively coupled plasma mass spectrometry (ICP-MS) is reported. The interface was constructed using a commercial microconcentric nebuliser and home-built cyclonic spray chamber. Isoforms of the heavy metal binding protein, metallothionein, were separated and the bound metals detected to characterise the interface. Nebuliser suction was identified as the principal factor controlling separation resolution in the CE-ICP-MS system. Two methods for counterbalancing the nebuliser suction were investigated: in the first method an optimised make-up flow was employed, and in the second a negative pressure was applied to the inlet vial. Negative pressure was the preferred method for counterbalancing the nebuliser suction because sensitivity was not compromised. Separation resolution, under negative pressure conditions, was improved compared with that achieved using on-capillary UV detection. Absolute metal detection limits for114Cd, 111Cd, 66Zn and 64Zn were 2.09, 3.42, 8.93 and 9.12 fg, respectively.

Application of high-performance liquid chromatography-inductively coupled plasma mass spectrometry to the investigation of cadmium speciation in pig kidney following cooking and in vitro gastro-intestinal digestion

The speciation of cadmium in retail pig kidney has been examined by size-exclusion chromatography (SEC) coupled directly to inductively coupled plasma mass spectrometry (ICP-MS). Approximately 35% of the cadmium from uncooked kidney was soluble after aqueous extraction at pH 8 and SEC - ICP-MS revealed three discrete peaks whose retention times corresponded to estimated relative molecular masses of 1.2 x 10(6), 7.0 x 10(4) and 6 x 10(3)-9 x 10(3). In the cooked kidney, 35% of the Cd was soluble and was all associated with a peak of a relative molecular mass (Mr) of 6 x 10(3)-9 x 10(3). After simulated gastric digestion of cooked pig kidney at pH 2.5, 60% of the cadmium was solubilised and associated with a species of Mr less than 1 x 10(3). When the digest was also subjected to simulated intestinal digestion at pH 6.8, a single peak, which corresponded to 20% of the original cadmium, was eluted. This peak co-eluted with the single peak extracted at pH 8.0 from the cooked kidney. It was also of similar estimated Mr to the single broad peak observed after simulated gastro-intestinal digestion of equine renal metallothionein (Mr = 1.1 x 10(4]. The results suggest that the majority of soluble cadmium in retail pig kidney is associated with a metallothionein-like protein that survives both cooking and simulated in vitro gastro-intestinal digestion.

Studies of metalloprotein species by directly coupled high-performance liquid chromatography inductively coupled plasma mass spectrometry

High-performance liquid chromatography (HPLC) has been directly coupled to inductively coupled plasma mass spectrometry (ICP-MS) for trace metal speciation studies. The characteristics of HPLC-ICP-MS couplings have been investigated using the number of theoretical plates, peak tailing, rise time and wash-out time as criteria of merit. A short aerosol connection line was found to be optimal. In comparison with an in-line UV monitor, ICP-MS was found to give a comparable number of theoretical plates.

Preliminary study of metals in proteins by high-performance liquid chromatography–inductively coupled plasma mass spectrometry using multi-element time-resolved analysis

The feasibility of multi-element and multi-isotope determinations during the on-line chromatographic separation of protein matrices was investigated using time-resolved software with inductively coupled plasma mass spectrometry. Size-exclusion chromatography was used to separate a mixture of known proteins and the behaviour of associated elements was monitored. It was possible to scan either the entire mass range or a selected number of isotopes. Reversed-phase chromatography was used to separate Zn-containing species in chicken meat that had been intrinsically and extrinsically labelled with stable isotopes of Zn. The ratio obtained for the intrinsic label agreed with that obtained by other workers using thermal ionization mass spectrometry. Time-resolved analysis permitted rapid data acquisition, thus reducing the number of chromatographic runs required. However, at present, data handling can be time consuming. Further software developments will add to the advantages that this technique offers.

Characteristics of flow injection inductively coupled plasma mass spectrometry for trace metal analysis

The application of flow injection (FI) as a method of sample introduction to an inductively coupled plasma mass spectrometer is described. It is shown that FI can alleviate problems arising from high salt content, viscosity and high acid concentration in inductively coupled plasma mass spectrometry (ICP-MS). The capability to operate with microlitre amounts of sample with a rapid sampling rate is clearly demonstrated. Biological certified reference materials were analysed by FI-ICP-MS for lead with external calibration and by isotope dilution analysis (IDA). Good agreement with certified values was obtained with both methods. Flow injection ICP-MS was used to analyse powdered food (mixed diet) samples for lead with IDA and to determine zinc isotope ratios in microlitre samples of blood.

Element residues in food contact plastics and their migration into food simulants, measured by inductively-coupled plasma-mass spectrometry

Polymers intended for food contact use have been analysed for inorganic residues which can be attributed to a range of substances employed as polymerization aids (e.g. catalysts), or to additives incorporated into the polymer to fulfil a specific task (e.g. lubricants). The migration of these residues into food simulants was studied. Residues were determined by using the multi-element capability of Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS). Semi-quantitative analysis was carried out on acid digests of polymer materials, obtained by microwave heating in sealed Teflon containers. Limits of detection in the polymer were generally less than 1 mg/kg. Migration experiments were carried out with three food simulants and under two sets of conditions. Analysis for element residues was performed directly or, in the case of olive oil, on an emulsion of the simulant. Migration of certain elements into aqueous simulants was observed: Zr from polystyrene (650 micrograms/kg), Sb from polyethylene terephthalate (4 micrograms/kg) and Mg from acrylonitrile/butadiene/styrene copolymer (50 micrograms/kg). In all cases, where limits of detection permit, the levels which migrated from polymer to stimulant were less than proposed limits on migration.

Determination of the growth promoter, 4-hydroxy-3-nitrophenyl-arsonic acid in chicken tissue by coupled high-performance liquid chromatography–inductively coupled plasma mass spectrometry

A method has been developed to measure the levels of the growth promoter 4-hydroxy-3-nitrophenylarsonic acid (roxarsone) in samples of tissue from chickens fed on a diet supplemented with this compound. Extracts of the tissue were prepared for analysis by a trypsin enzymolysis digestion technique and bulk matrix separation was performed by anion-exchange column chromatography. Further separation of the roxarsone from other species was achieved using reversed-phase high-performance liquid chromatography. Detection of the arsenical compound was by inductively coupled plasma mass spectrometry. No roxarsone was detected in chickens fed on the supplemented diet with or without a withdrawal period (7 d on base feed after the administration period) at a quantification limit of 25 ng g–1. Recoveries for roxarsone from spiked (standard additions) samples of chicken tissue varied between 85 and 103%.