Detection of selenium-containing biological constituents by high-performance liquid chromatography-plasma source mass spectrometry

Formation Mechanism and Toxicological Significance of Biogenic Mercury Selenide Nanoparticles in Human Hepatoma HepG2 Cells

It is widely recognized that the toxicity of mercury (Hg) is attenuated by the simultaneous administration of selenium (Se) compounds in various organisms. In this study, we revealed the mechanisms underlying the antagonistic effect of sodium selenite (Na₂SeO₃) on inorganic Hg (Hg²⁺) toxicity in human hepatoma HepG2 cells. Observations by transmission electron microscopy indicated that HgSe (tiemannite) granules of up to 100 nm in diameter were accumulated in lysosomal-like structures in the cells. The HgSe granules were composed of a number of HgSe nanoparticles, each measuring less than 10 nm in diameter. No accumulation of HgSe nanoparticles in lysosomes was observed in the cells exposed to chemically synthesized HgSe nanoparticles. This suggests that intracellular HgSe nanoparticles were biologically generated from Na₂SeO₃ and Hg²⁺ ions transported into the cells and were not derived from HgSe nanoparticles formed in the extracellular fluid. Approximately 85% of biogenic HgSe remained in the cells at 72 h post culturing, indicating that biogenic HgSe was hardly excreted from the cells. Moreover, the cytotoxicity of Hg²⁺ was ameliorated by the simultaneous exposure to Na₂SeO₃ even before the formation of insoluble HgSe nanoparticles. Our data confirmed for the first time that HepG2 cells can circumvent the toxicity of Hg²⁺ through the direct interaction of Hg²⁺ with a reduced form of Se (selenide) to form HgSe nanoparticles via a Hg-Se soluble complex in the cells. Biogenic HgSe nanoparticles are considered the ultimate metabolite in the Hg detoxification process.

Detoxification of selenite to form selenocyanate in mammalian cells

When human hepatoma HepG2 cells were exposed to sodium selenite, an unknown selenium metabolite was detected in the cytosolic fraction by HPLC-inductively coupled plasma mass spectrometry (ICP-MS). The unknown selenium metabolite was also detected in the mixture of HepG2 homogenate and sodium selenite in the presence of exogenous glutathione (GSH). The unknown selenium metabolite was identified as selenocyanate by electrospray ionization mass spectrometry (ESI-MS) and ESI quadrupole time-of-flight mass spectrometry (ESI-Q-TOF-MS). Because exogenous cyanide increased the amount of selenocyanate in the mixture, selenocyanate seemed to be formed by the reaction between selenide or its equivalent, the product of the reduction of selenite, and endogenous cyanide. Rhodanase, an enzyme involved in thiocyanate synthesis, was not required for the formation of selenocyanate. Selenocyanate was less toxic to HepG2 cells than selenite or cyanide, suggesting that it was formed to reduce the toxicity of selenite. However, selenocyanate could be assimilated into selenoproteins and selenometabolites in rats in the same manner as selenite. Consequently, selenite was metabolized to selenocyanate to temporarily ameliorate its toxicity, and selenocyanate acted as an intrinsic selenium pool in cultured cells exposed to surplus selenite.

Chromatographic behavior of selenoproteins in rat serum detected by inductively coupled plasma mass spectrometry

Two major selenoproteins are present in mammalian serum: extracellular glutathione peroxidase (eGPx) and selenoprotein P (Sel P). The chromatographic behaviors of the two serum selenoproteins were compared in four rodent species, and the selenoproteins in rat serum were identified by measuring enzyme activity and Western blotting. The selenoproteins in rat serum showed a specific chromatographic behavior. In particular, rat eGPx was eluted faster than eGPxs of the other rodent species, although the amino-acid sequences of the rodent species were identical. The elution profiles of Se in rat serum obtained by inductively coupled plasma tandem mass spectrometry (ICP-MS-MS) and ICP-MS were compared. The tandem quadrupoles and the O₂ reaction/collision gas completely removed severe interferences with the Se speciation originating from the plasma source and the biological sample matrix. ICP-MS-MS under the O₂ mass shift mode gave us more accurate abundance ratios of Se than ICP-MS.

Isolation of selenoprotein-P and determination of Se concentration incorporated in proteins in human and mouse plasma by tandem heparin affinity and size-exclusion column HPLC-ICPMS

Sel-P is considered to be the most important selenoprotein for evaluating the selenium (Se) status in the body. To isolate and determine Sel-P in plasma, we have developed an analytical method combining heparin affinity (AF) and size-exclusion column (SEC) high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICPMS). We used this method to validate the adsorption efficiency of selenoproteins on a heparin AF column, and to then determine the Se concentrations incorporated in proteins in human and mouse plasma. The adsorption efficiency of Sel-P on a heparin column was more than 90% for both human and mouse plasma. Tandem AF and SEC separation proved to be useful for determining the Se concentrations incorporated in Sel-P in mouse plasma, but not in human plasma, because of nonspecific adsorption of plasma-extracellular glutathione peroxidase (eGPx) and albumin on the heparin AF column. Ultimately, we used the tandem AF and SEC separation method for mouse plasma and SEC separation alone for human plasma. The Se concentration incorporated in selenoproteins determined by our method showed good agreements with the total Se concentration determined following acid digestion.

Comparison of selenium determination in liver samples by atomic absorption spectroscopy and inductively coupled plasma-mass spectrometry

Selenium (Se) is an essential trace element that is often deficient in the natural diets of domestic animal species. The measurement of Se in whole blood or liver is the most accurate way to assess Se status for diagnostic purposes. This study was conducted to compare hydride generation atomic absorption spectroscopy (HG-AAS) with inductively coupled plasma-mass spectrometry (ICP-MS) for the detection and quantification of Se in liver samples. Sample digestion was accomplished with magnesium nitrate and nitric acid for HG-AAS and ICP-MS, respectively. The ICP-MS detection was optimized for 82Se with yttrium used as the internal standard and resulted in a method detection limit of 0.12 microg/g. Selenium was quantified by both methods in 310 samples from a variety of species that were submitted to the Toxicology Laboratory at New Bolton Center (Kennett Square, PA) for routine diagnostic testing. Paired measurements for each sample were evaluated by a mean difference plot method. Limits of agreement were used to describe the maximum differences likely to occur between the 2 methods. Results suggest that under the specified conditions ICP-MS can be reliably used in place of AAS for quantitation of tissue Se at or below 2 microg/g to differentiate between adequate and deficient liver Se concentrations.

Selenium speciation analysis using inductively coupled plasma-mass spectrometry

Selenium exists in several oxidation states and a variety of inorganic and organic compounds, and the chemistry of selenium is complex in both the environment and living systems. Selenium is an essential element at trace levels and toxic at greater levels. Interest in speciation analysis for selenium has grown rapidly in this last decade, especially in the use of chromatographic separation coupled with inductively coupled plasma-mass spectrometry (ICP-MS). Complete characterization of selenium compounds is necessary to understand selenium's significance in metabolic processes, clinical chemistry, biology, toxicology, nutrition and the environment. This review describes some of the essential background of selenium, and more importantly, some of the currently used separation methodologies, both chromatographic and electrophoretic, with emphasis on applications of selenium speciation analysis using ICP-MS detection.

Fractionation of selenium-containing proteins in serum by multiaffinity liquid chromatography before size-exclusion chromatography–ICPMS

Immunoaffinity chromatography has been investigated for fractionation of serum into selenoalbumin and true selenoproteins. Among several albumin-depletion kits tested, a multiaffinity column specifically binding albumin and five other major serum proteins provided the best results. It extracted ca 95% of both albumin and selenoalbumin, which enabled interference-free determination of glutathione peroxidase, selenoprotein P, and selenoalbumin by size-exclusion chromatography combined with inductively coupled plasma mass spectrometry (SEC-ICPMS). The efficiency of the multiaffinity column did not vary over a period of 18 months. The purity of fractions separated by immunoaffinity LC was confirmed by elution-volume matching with standards in SEC-ICPMS and by selenopeptide mapping in capillary HPLC-ICPMS. Quantification of the selenium distribution among the different proteins in human serum from a control group and from a person on a selenium-rich diet revealed that 67% of the supplemented selenium was incorporated into albumin, 30% into glutathione peroxidase, and 3% into selenoprotein P.

Chemical forms of selenium for cancer prevention

Cancer is becoming an increasingly significant disease worldwide. Currently, more than 7 million people die each year from cancer. With the existing knowledge, at least one-third of worldwide cancer cases could be prevented. Searching for naturally occurring agents in routinely consumed foods that may inhibit cancer development, although challenging, constitutes a valuable and plausible approach to the control and prevention of cancer. To date, the use of the micronutrient selenium (Se) in human clinical trials is limited, but the outcome indicates that Se is among the most promising agents. Although it is convenient to describe the effects of Se in terms of the element, it must always be kept in mind that the chemical form of Se and the dose are determinants of its biological activities. Hyphenated techniques based on coupling chromatographic separation with inductively coupled plasma mass spectrometric (ICP-MS) detection are now established as the most realistic and potent analytical tools available for real-life speciation analysis. These speciation investigations provide evidence that the Se compounds, which can generate monomethylated Se (e.g., Se-methylselenocysteine and methylseleninic acid), are more efficacious than other Se compounds because of their chemoprevention activity.

Selenosugar and trimethylselenonium among urinary Se metabolites: dose- and age-related changes

Once selenium (Se) is absorbed by the body, it is excreted mostly into the urine and the major metabolite is 1beta-methylseleno-N-acetyl-d-galactosamine (selenosugar) within the required to low-toxic range. Selenosugar plateaus with a dose higher than 2.0 microg Se/ml water or g diet, and trimethylselenonium (TMSe) starts to increase, indicating that TMSe can be a biomarker of excessive and toxic doses of Se. Here, we show dose-related changes in the two urinary Se metabolites to clarify the relationship between the dose and urinary metabolites by feeding selenite to rats. It was also examined whether the metabolites are related to age, and further whether a possible exogenous source of the N-acetyl-d-galactosamine moiety, chondroitin 4-sulfate, affects the urinary metabolites. Selenite in drinking water was fed ad libitum to male Wistar rats of 36 and 5 weeks of age, and the concentrations of Se in the urine and organs were determined together with speciation of the urinary Se metabolites. In young rats, selenosugar was always the major urinary metabolite and TMSe increased with a dose higher than 2.0 microg Se/ml drinking water. On the other hand, in adult rats, TMSe increased only marginally despite that the rats suffered much more greatly from the Se toxicity, suggesting that TMSe cannot be a biomarker of Se toxicity. The results suggest that sources of the sugar moiety of selenosugar are more abundant in adult rats than in young rats. Chondroitin 4-sulfate did not affect the ratio of the two urinary metabolites, suggesting that the sugar source is of endogenous origin and that it increases with age.

Selective detection and identification of Se containing compounds—review and recent developments

The complexity of selenium (Se) chemistry in the environment and in living organisms presents broad analytical challenges. The selective qualitative and quantitative determination of particular species of this element is vital in order to understand selenium's metabolism and significance in biology, toxicology, clinical chemistry and nutrition. This calls for state-of-the-art analytical techniques such as hyphenated methods that are reviewed with particular emphasis on interfaced separation with element-selective detection and identification of the detected selenium compounds. Atomic spectral element specific detection for monitoring chromatographic eluent enabled quantitative determination of selenium species in selenized yeast and qualitative measurement for breath samples. Gas chromatography with atomic emission detection (AED) of ethylated species and fluoroacid ion pair HPLC applied to the analysis of currently produced or archived selenized yeast and Brassica juncea have revealed the presence of a previously unrecognised Se-S amino acid, S-(methylseleno)cysteine.

Liquid chromatography-inductively coupled plasma mass spectrometry

It is known that while many elements are considered essential to human health, many others can be toxic. However, because the intake, accumulation, transport, storage and interaction of these different metals and metalloids in nature is strongly influenced by their specific elemental form, complete characterization of the element is essential when assessing its benefits and/or risk. Consequently, interest has grown rapidly in determining oxidation state, chemical ligand association, and complex forms of a many different elements. Elemental speciation, or the analyses that lead to determining the distribution of an element's particular chemical species in a sample, typically involves the coupling of a separation technique and an element specific detector. A large number of methods have been developed which utilize a multitude of different separation mechanisms and detection instruments. Yet, because of its versatility, robustness, sensitivity and multi-elemental capabilities, the coupling of liquid chromatography to inductively coupled plasma mass spectrometry (LC-ICP-MS) has become one of the most popular techniques for elemental speciation studies. This review focuses on the basic principles of LC-ICP-MS, its historical development and the many ways in which this technique can be applied. Different liquid chromatography separations are discussed as well as the factors that must be considered when coupling each to ICP-MS. Recent applications of LC-ICP-MS to the speciation of environmental, biological and clinical samples are also presented.

Rapid and simple determination of selenium in blood serum by inductively coupled plasma-mass spectrometry (ICP-MS)

An inductively coupled plasma mass spectrometer (ICP-MS) with a rapid sample-preparative procedure was used for the determination of selenium in blood serum. Blood serum was prepared by dilution in an acidic solution consisting of nitric acid (1%), X-triton (0.1%) and 1-butanol (0.8%). A calibration curve was established for 1-40 microg mL(-1) (r(2)>0.99). The limit of detection was 0.5 microg mL(-1). Repeatability and intermediate precision were satisfactory with relative standard deviations (RSD) of 2.0% and 3.2%, respectively. This method was easily applied to reference materials with satisfactory accuracy. Good correlation (r(2)=0.96) was observed between ICP-MS and atomic absorption spectrometry (AAS) for the determination of (82)Se in blood serum from 23 patients. These results suggest that the sample preparative procedure coupled with ICP-MS can be used for the routine determination of (82)Se in human blood serum.

Selenium incorporation into Saccharomyces cerevisiae cells: a study of different incorporation methods

AIMS

To study the effects of the selenium enrichment protocols in yeast at various points in the cell cycle, total selenium accumulation and the forms of selenium incorporated.

METHODS AND RESULTS

The use of selenized yeast as enriched selenium supplements in human nutrition has become a topic of increasing interest over the last decade. Four enrichment procedures have been evaluated using sodium selenite as the selenium source: enrichment during the growth phase; enrichment at the non-growth phase, both of these at different selenium levels; enrichment by seeding in a fermentable carbon source (glucose); Se-enrichment with a non-fermentable carbon source (glycerol). A nitric acid digestion of the yeast samples prepared under different conditions has been performed in order to evaluate the total selenium incorporated into the yeast cells. Also, an enzymatic digestion of the yeast samples with pepsin has been carried out as an initial step to begin the process of determining which of the different possible selenium species are formed. The cell count evaluations of the selenium-enriched yeast showed that the growth phase, seeding and the use of YEPG media is influenced by the addition of Se, while the non-growth phase is not. Total selenium incorporation studies showed that seeding the yeast permits more accumulation of selenium. Speciation studies of the enriched yeast showed that the growth phase increases the formation of L-Se-methionine.

CONCLUSIONS

When the aim of enriching yeast with selenium is the formation of L-Se-methionine, the best enrichment procedure is using the growth phase with small concentrations of sodium selenite.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of selenium supplements is widespread and most of the supplements use selenium-enriched yeast in their formulation. Studies made on supplements do not have the appropriate Se-species for optimal absorption in the human body. This study presents and compares methods for the best selenium yeast enrichment that could ultimately be used in selenium supplement formulations.

Speciation of arsenic and selenium compounds by ion-pair reversed-phase chromatography with electrothermic atomic absorption spectrometry. Application of experimental design for chromatographic optimisation

An off-line system is proposed consisting of ion-pair reversed-phase liquid chromatography, collections of fractions at the outflow of the column and furnace atomic absorption spectrometry. The so-called system allowed determination of both arsenic and selenium species mainly found in the environment and in mammals (arsenite, arsenate, monomethylarsonate, dimethylarsinate, selenite, selenate, selenocystamine, selenocystine, selenomethionine and selenoethionine). In order to study the retention behaviour of these compounds and to estimate the optimal conditions for the chromatographic separation, central composite designs were used to evaluate the influence of the eluent parameters such as pH, tetrabutylammonium phosphate (TBA) concentration and sodium hydrogenphosphate amounts. The retention factors of each species and the selectivity were established as response criteria. Response surfaces and isoresponse curves were drawn from the mathematical models and enabled one to determine the optimal conditions and to visualise the method robustness. The predicted optimal zone was situated at pH 5.5-6.5, 4.0 mM Na2HPO4 and 3.0-4.0 mM TBA. Regression models suggested linearity for the studied compounds in the range 25-200 microg selenium and arsenic per litre investigated.

Effects of dietary selenium species on Se concentrations in hair, blood, and urine

The effects of the chemical species and concentration of selenium (Se) in diets on the concentrations of Se in hair, blood serum, red blood cells (RBCs), and urine were studied to gain an insight into the toxicological and nutritional significance of different chemical forms of Se. Male Wistar rats were fed an Se-deficient diet (Se, less than 0.03 microgram/g) for 3 weeks, and then an Se-adequate (Se, 0.2 microgram/g) or Se-excess diet (Se, 2.0 micrograms/g), including seleno-L-methionine (SeMet) or selenite for up to 12 weeks. Hair, blood, and urine specimens were obtained every two weeks, and the concentrations of Se and its distribution in serum and urine on a size-exclusion column were determined. The concentrations of Se in hair, serum, and urine attained constant levels 2 weeks after a change of in the dietary Se concentration irrespective of the chemical species, the levels being dependent on the chemical species and the concentration. Specifically, in hair and serum, selenite gave the lowest constant levels irrespective of the dose, while SeMet resulted in higher levels than selenite in a dose-dependent manner. The two major selenoproteins in serum exhibited comparable concentrations. On the other hand, in urine, the concentration of Se was dependent on the dose but not on the chemical species. The results could be explained by regulated metabolism of selenite, and both nonregulated and regulated aspects of the metabolism of SeMet.

Binding of selenium (administered as selenite) to albumin after efflux from red blood cells

The role of albumin in the metabolism of inorganic selenium (Se) was studied in vivo and in vitro using a HPLC-ICP-MS method. Although Se injected in the form of selenite binds selectively to albumin after being reduced to selenide and then being effluxed into the plasma, Se was shown to be metabolized normally in the absence of albumin. The reduced form of Se, selenide, bound selectively to albumin but only to a percentage of it. The thiol group and the intermolecular disulfide group at the 34th cysteinyl residue of albumin were not responsible for the selective binding of Se to albumin. Selenide was suggested to be bound to a disulfide not a thiol group, i.e., to one of the 17 disulfide bonds in a conformationally different isoform of albumin.

Chromatographic and hyphenated methods for elemental speciation analysis in environmental media

This review discusses chromatographic techniques that permit the analysis of speciated metals in the environment using conventional detectors, such as UV, and element-specific detectors, such as flame atomic absorption spectrometry, electrothermal atomic absorption spectrometry and inductively coupled plasma mass spectrometry. The importance of determining precise elemental forms in hazardous waste-contaminated soil, water and biota in terms of toxicity is outlined. Previous reviews on this subject are described and recent research on this subject is discussed. Most of the work cited has been performed in the 1990s and a table summarizing the chromatographic method and the detector system used, including brief comments on the work, is included to enable quick reference.

Metabolism of selenite labelled with enriched stable isotope in the bloodstream

The metabolism of selenium (Se) in the bloodstream of rats was studied using HPLC-ICP-MS with an enriched Se stable isotope, and the results were used as Se-specific indicators for Se nutritional status. Concentration of endogenous Se in plasma depended on dietary Se, while changes in concentrations and distributions of exogenous Se revealed its metabolic pathway. Namely, selenite was taken up by red blood cells and reduced to selenide, and then reappeared in plasma in a form bound selectively to albumin within 10 min, disappeared from plasma again within 30 min after injection. Then, the concentration of labelled Se started to increase slowly as selenoprotein P and extracellular glutathione peroxidase, and attained a maximum level at about 6 h after injection. The isotope ratio of endogenous to exogenous Se concentrations in plasma after 48 h post-injection was proposed to represent the Se-specific indicator in plasma reflecting the nutritional status of Se.

Detoxification of mercury by selenium by binding of equimolar Hg-Se complex to a specific plasma protein

Toxicity of mercury (Hg) can be reduced by coadministration with selenium (Se), and this has been explained by the formation of a complex between a specific plasma protein and the two elements, which are bound to the protein at an equimolar ratio. The purpose of the present study was to characterize the specific binding protein in order to clarify the detoxification mechanism. The coadministration of 82Se-enriched selenite and mercuric chloride into a rat produced a 82Se- and Hg-binding peak on a gel filtration column as measured by high-performance liquid chromatography with detection by inductively coupled argon plasma-mass spectrometry (ICP-MS). The specific binding protein was also detected in vitro by incubating 82Se-enriched selenite and mercuric chloride in serum in the presence of glutathione. The molar ratio of Se/Hg = 1 was maintained in binding not only to the specific protein but also to other proteins under any condition. In in vitro experiments, it was shown that although the two elements could bind to many plasma proteins, the affinity to the specific protein was extremely high and it showed a binding capacity of 500 nmol Hg or Se/the specific protein in 1 ml of serum. These results suggest that the two elements form an equimolar complex at first and then bind specifically to the protein. Further, the binding of the two elements to the protein was inhibited by the addition of polylysine to the reaction mixture, suggesting that the two elements interact with the protein through basic amino acids in the molecule and also that the protein may be one of the heparin-binding proteins since the heparin-binding sites mainly consist of basic amino acids.

Equimolar Hg-Se complex binds to selenoprotein P

Binding of equimolar mercury (Hg) and selenium (Se) to a specific plasma protein in the detoxification of Hg was studied in vitro by the HPLC/inductively coupled argon plasma-mass spectrometry (ICP-MS) method with use of an enriched stable isotope. Hg and 82Se became co-eluted with endogenous 78Se on a size exclusion column by incubation of 0-200 microM HgCl2 and 82Se-enriched selenite with rat serum in the presence of glutathione at 37 degrees C for 10 min. The endogenous 78Se peak was the most abundant plasma Se-containing protein, and it showed the affinity to heparin, indicating it to be selenoprotein P (Sel P). The 82Se/endogenous Se ratio of (Hg-Se)-Sel P complex changed with doses of HgCl2 and 82Se-enriched selenite and amounted to more than 100, suggesting that more than 1,000 units of (Hg-Se) bind to Sel P based on the fact that there are 10 selenocysteinyl residues per Sel P. These results indicate that equimolar Hg and Se bind to Sel P to form the {(Hg-Se)n}m-Sel P complex, where n is the number of Hg-Se complexes and m the number of binding sites in Sel P.

A new approach to extending the dynamic range in inductively coupled plasma-mass spectrometry

By the use of a new simultaneous dual stage discrete detector for inductively coupled plasma-mass spectrometry (ICP-MS), a linear dynamic range of 8 orders of magnitude was achieved in a single spectral scan. The design of the detection system and its evaluation are described. The acquisition of the full spectrum of a human urine sample in a single scan is shown.

Simultaneous speciation of endogenous and exogenous elements by HPLC/ICP-MS with enriched stable isotopes

High performance liquid chromatography (HPLC)/ inductively coupled argon plasma-mass spectrometry (ICP-MS) was introduced to investigate the distributions of selenium (Se) in biological fluids. The method was to determine both the natural abundance of Se and an enriched stable isotope of Se used as a tracer. The distributions of Se in plasma and in urine specimens were determined in Wistar rats on various Se diets with and without an intravenous injection of 82Se-selenite. Although the distribution of natural abundance Se (endogenous Se) in the plasma was affected little by the nutritional status of Se, that in the urine gave a Se peak depending on the nutritional status of Se, and the peak was identified as methylselenol. When 82Se-selenite was injected in excess into rats given three different Se diets (Se-deficient, Se-adequate, Se-excessive), three Se peaks occurred in the HPLC chromatogram of the urine samples, corresponding to selenite, methylselenol and trimethylselenonium ion in the order of elution, and the intensities of the tracer peaks reflected the nutritional status. These results indicate that the HPLC/ICP-MS method is a powerful analytical tool for specifying Se-containing biological constituents, both natural abundance and enriched stable isotopes. Methylselenol in urine is proposed to be a sensitive and Se-specific biological indicator for diagnosing the nutritional status of Se. Furthermore, it was shown that an enriched stable isotope such as 82Se-selenite was shown to be used for the same purpose, and that 82Se-methylselenol and 82Se-trimethylselenonium ion in urine were more sensitive indicators of the Se status of the rats.

Enriched stable isotopes of elements used as tracers: methods of presenting high-performance liquid chromatographic-inductively coupled argon plasma mass spectrometric data

High-performance liquid chromatography (HPLC) of biological fluids and tissue cytosolic preparations was employed in conjunction with argon-induced inductively coupled plasma mass spectrometry (ICP-MS) to investigate the distribution of stable isotopes as tracers. The common way of presenting the data from the ICP-MS is by plotting the count rates versus the retention time of HPLC fractions. Additional information can be derived, e.g., the composite peaks can be further resolved, and the level of enrichment in various biological components can be expressed by alternative ways of presenting these data. The two additional approaches described here involve presenting the ratios of enriched tracer with a suitable naturally abundant mass number of the same element, and by expressing the extent of enrichment by the tracer isotope in a given fraction to that of the same mass number in the fraction derived from an untreated source. Each method of presentation has different merits and drawbacks. The data therefore may be best presented in more than one way to emphasize the conclusions from a given experiment. Observations are presented after simultaneously injecting stable isotopes of three essential elements, copper, selenium and zinc, into mice. Plasma and liver cytosolic fractions were analysed and data represented in different ways as indicated above.