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A method for the measurement of calcium isotopes (42Ca, 43Ca, and 44Ca) using quadrupole inductively coupled plasma mass spectrometry (ICP-MS) is described. Interferences from polyatomic ions such as 12C16O2+ and 40ArH2+ at the calcium masses are greatly minimized by operating the ICP-MS in the cool plasma mode. Relative standard deviations (RSD) for the 42Ca∶43Ca and 44Ca∶43Ca ratios were found to be about 0.25%. Sample preparation involved using ammonium oxalate at a pH of 8 to separate calcium from samples such as serum, urine, feces, and breast milk. The isotope ratio measurements were used to determine fractional absorption of calcium by a lactating woman after intravenous administration of 42Ca and ingestion of 44Ca.

In most readily accessible biological samples from humans, like blood, serum/plasma, urine, etc, the levels of chromium (Cr) are less than 1 ng/g, and in many cases closer to 0.1 ng/g. Only 3 analytical techniques have the required sensitivity to make measurements at these levels, namely, neutron activation analysis (NAA), mass spectrometry (MS), and graphite furnace atomic absorption spectrometry (GFAAS). The first 2 are not widely available, and the third is the one most susceptible to interferences from the sample matrix. At the sub-parts-per-billion level, collecting samples without contaminating them and generating sufficiently low analytical and reagent blanks become extremely important and difficult. For other determinations of Cr, eg, in diet components, foods and tissues, where the levels are often well above the ng/g level, the required sensitivity is less of a limitation, but contamination problems remain, and factors like sample processing and homogeneity become more important. Problems and precautions in Cr determinations are discussed, and means of accuracy verification are presented. A novel use of stable isotopes of Cr in an accurate, non-radioactive method of measuring blood volume is described, as well as a discussion of the future of Cr determinations using inductively-coupled plasma mass spectrometry. J. Trace Elem. Exp. Med. 12:99–109, 1999. Published 1999 Wiley-Liss, Inc.

To determine the fate and distribution of chromium during lactation, six lactating women (25-38 y old) were given three doses of the tracer 53Cr (7.55 micromol/d, or 400 microg/d) on days 1, 2, and 3 of the study. Diet records, blood samples taken while subjects were fasting, and 24-h composite milk and urine samples were collected from day 0 to day 6. Fasting blood samples, morning milk samples, and 24-h urine samples were also collected on days 8, 10, 15, 30, 60, and 90. 53Cr and natural and total chromium concentrations in biological fluids were measured with gas chromatography-mass spectrometry and total urinary chromium was measured with atomic-absorption spectrometry. 53Cr was detectable in serum 2 h after dosing and continued to be detected from day 30 to day 60. Changes in total serum chromium concentration in response to the oral dose suggested that chromium concentrations in blood were not tightly regulated. 53Cr was not detected in breast milk and no significant changes in natural chromium concentration in milk were observed in response to the oral doses, suggesting that breast-milk chromium concentrations are independent of intake. The estimated chromium intake of exclusively breast-fed infants was 2.5 nmol/d (0.13 microg/d), below the lower end of the range of estimated safe and adequate daily dietary intakes (10-40 microg/d) for infants 0-6 mo of age. The baseline chromium concentration in urine and the minimum 53Cr absorption in lactating women were comparable with values for nonpregnant, nonlactating subjects. Chromium losses in breast milk do not appear to be compensated for via increased absorption or decreased excretion.

Both exercise and chromium exert beneficial effects on insulin function. The mechanism by which exercise improves insulin response may involve an alteration in Cr metabolism. To determine the effects of acute and chronic resistive exercise on urinary Cr losses, we measured the effects of acute resistive exercise and 16 wk of resistive exercise training on urinary Cr losses of 10 men 53-63 y of age. Subjects consumed diets in compliance with the American Heart Association Phase I diet with a Cr content of 30 +/- 4 microg/d. Sixteen weeks of resistive exercise training led to approximately 40% increases in upper and lower body strength, increases in fat-free mass and decreases in the percentage of body fat. An enriched stable isotope of Cr, 53Cr, was employed to differentiate the exogenously administered Cr from the native endogenous Cr. Both acute and chronic resistive exercise increased 53Cr losses. These data demonstrate that the improvements in body composition due to resistive exercise are associated with increased urinary Cr losses that are consistent with increased absorption.

Objective:To validate and improve a method of red cell volume determination by useof a stable isotope of chromium.Methods:Twelve subjects were sequentially injected with red blood cells labeled with a stable isotope of chromium (53 Cr) and red blood cells labeled with radioisotopic chromium (51 Cr). Measurement of 53Cr dilution was by gas chromatography/mass spectrometry. Measurement of 51Cr dilution was by gamma counter.Results:Comparison of the two methods let to results that differed on average by 34.5 ± 45.0 mL (1.8 ± 2.2%), 0.3 to 3.2%, 95% confidence interval.Conclusion:Measurement of red cell volume by use of a stable isotope of chromium is accurate, with potential applications including measurement in pregnant women and children or other groups in whom exposure to radioisotopes is undesirable.

Error propagation in isotope dilution analysis (IDA) can be studied with Monte Carlo (MC) simulation. In inductively coupled plasma mass spectrometry with ion-counting detection, the limiting error in the measured isotope ratio can be described with Poisson statistics. Taking into account this error in the measured isotope ratio, parameters for IDA can be optimized. The utility of MC simulation for IDA is illustrated in the optimization of conditions for zinc analysis. The minimum imprecision in the determination of zinc by IDA was found for the internal standard enriched with the isotope having the lowest natural abundance. In addition to optimizing analytical conditions, MC simulation can provide information on the theoretical detection limits in double and triple IDA for stable isotope tracers

A method is described for the determination of chromium and its enriched stable isotopes in human urine by isotope dilution mass spectrometry. A volatile chelate is formed with trifluoroacetylacetone (TFA) and the fragment ions corresponding to Cr(TFA) 2 + in the 350-360 m/z region are monitored. The chelate is thermally stable and exhibits no memory effect when isotope ratios change. The detection limit for the method is 0.03 ng of Cr/g, and the accuracy is verified by certified reference materials and by an independent method

Depressed serum somatomedin C levels have been suggested by others as an early indicator of selenium (Se) toxicity. In both human and animal studies, somatomedin C levels were lower in Se supplemented as compared to control subjects. The present study examined Se and somatomedin C levels in 44 adult long-term residents of seleniferous areas in South Dakota. Serum Se (SSe) and whole blood Se (WBSe) levels were analyzed by GC/MS. Toenail Se (TSe) was measured by neutron activation analysis. Somatomedin C was analyzed by RIA assay. Subjects were divided into 3 groups based on the following criteria: group I, SSe 240, WBSe >400 ng/ml. Data were pooled across gender (20 males, 24 females). Somatomedin C levels were not different despite greater than 50% differences in SSe, WBSe, and TSe levels among the groups. In these subjects, somatomedin C was not a sensitive index of elevated Se exposure.

A method is described for using isotope dilution to determine both the amount of natural zinc and enriched isotopes of zinc in biological samples. Isotope dilution inductively coupled plasma mass spectrometry offers a way to quantify not only the natural zinc found in a sample but also the enriched isotope tracers of zinc. Accurate values for the enriched isotopes and natural zinc are obtained by adjusting the mass count rate data for measurable instrumental biases. Analytical interferences from the matrix are avoided by extracting the zinc from the sample matrix using diethylammonium diethyldithiocarbamate. The extraction technique separates the zinc from elements which form interfering molecular ions at the same nominal masses as the zinc isotopes. Accuracy of the method is verified using standard reference materials. The detection limit is 0.06 μg Zn per sample. Precision of the abundance ratios range from 0.3–0.8% R.S.D. for natural zinc concentrations of about 200–600 μ g−1. The accuracy and precision of the measurements make it possible to follow enriched isotopic tracers of zinc in biological samples in metabolic tracer studies.

A stable (nonradioactive) isotope of selenium in a chemical form common in foods (selenomethionine) or inorganic selenite was taken orally (200 micrograms/d) for 3 wk to label deep body pools. By deep body pools we mean selenium compartments that are large and/or have a slow turnover (exchange) rate. Blood plasma was removed, stored for 11 mo, and later reinfused as a labeled tracer dose with the selenium label in all of the biologically significant chemical forms. Accessible tissues such as red blood cells were highly labeled (20-25%) in the subjects receiving selenomethionine. Selenium from deep body pools is excreted primarily via the urine (80%). Reexcretion of previously absorbed selenium back into the gastrointestinal tract can be measured, avoiding a major source of error in conventional balance studies used to estimate nutrient absorption.

A proposed infant formula SRM 1846 has been analyzed for zinc, selenium and chromium by isotope dilution mass spectrometry (IDMS). The precision of the IDMS methods has been sufficient to allow the estimation of the sample homogeneity with respect to these three analytes. Overall means of 60.9±0.9 μg Zn/g (mean±standard deviation) and 76.1±1.9 ng Se/g have been found for 30 determinations. Values for chromium content have been found to depend on the sample preparation method, suggesting possible contamination. Accurary of the zinc, chromium and selenium values has been verified using a certified reference material, SRM 1549 “Non-Fat Milk Powder”.

We used an established isotope dilution gas chromatography-mass spectrometry method to measure blood volumes with an enriched stable isotope of chromium. The results were compared with those of the conventional method, which involves radioactive 51Cr. The two methods were compared simultaneously in two male subjects of different sizes, and essentially identical volumes were obtained. The isotope dilution method is insensitive to contamination with natural (unenriched) chromium and can be used to measure all of the stable isotopes of chromium. The method has potential applications in simultaneous erythrocyte survival studies. The absence of radioactivity makes possible an accurate means of studying blood volume changes in children and during pregnancy.

This study determined whether a single 60-mg dose of ferrous sulfate interferes with fractional zinc absorption (FZA) at 7-9 wk of lactation. In a crossover design, 5 exclusively breast-feeding women were given either a single 60-mg iron supplement or no supplement. FZA was measured by analyzing zinc stable isotope tracers ((70)Zn and (67)Zn) in urine samples collected for 7 d after isotope dosing. A 0.7-micromol intravenous (IV) infusion of (70)Zn as ZnCl(2) in saline was followed by a 0.03-mmol oral dose of (67)Zn as ZnCl(2) given with a standardized meal. After a 7-d wash-out period, the supplement given was reversed and a second FZA measurement was taken. FZA was calculated from isotopic enrichments in urine measured by inductively coupled plasma mass spectrometry. Hemoglobin, plasma ferritin and transferrin receptor, and plasma 5'-nucleotidase, plasma zinc and erythrocyte zinc did not differ before the two measurements of zinc absorption. When women were given a single iron supplement, FZA was significantly lower, 21.7 +/- 1.7% compared with 26.9 +/- 2.6% when no supplement was given (P = 0.032). A single 60-mg iron dose significantly decreases FZA during early lactation.

Determining the fractional absorption (FA) of calcium using the incorporation into urine of stable isotopes given intravenously (IV) and orally has become a routine procedure. We investigated the FA of calcium in two groups of (2-3 mo) postpartum women lactating (LACT) (n = 6) and nonlactating (PPNL) (n = 6), and in never pregnant (NP) women (n = 7). The women consumed a controlled diet containing 30-33 mmol/d calcium (Ca) for 21 d. On d 7 of the controlled diet, the women received 0.05 mmol of 42Ca IV and 0.25 mmol 44Ca orally in milk. Urine samples (24-h) were collected for the next 14 d and morning blood samples were collected from fasting subjects before dosing and at 24 and 48 h after receiving the isotopes. Milk samples from the LACT women were collected from each feeding beginning 24 h before to 72 h after dosing. There were no significant differences in the FA of calcium as measured by stable isotope incorporation into urine (23.8 +/- 2.9%), serum (24.0 +/- 3.4%) or milk (23.6 +/- 3.6%) of LACT women. The fractional calcium absorption measured in urine of the postpartum women (LACT and PPNL, 23.8 +/- 2.9% and 25.0 +/- 3.3%, respectively) did not differ but was greater (P < 0.028) than that of the NP women (17.3 +/- 1.3%). The postpartum LACT and PPNL women had a reduced urinary excretion of calcium (P < 0.01) compared with the NP women. There was a significantly greater incorporation (P < 0.001) by LACT women of the oral isotope dose into milk than into urine. Calcium FA can be determined from incorporation of stable isotopes into breast milk and serum as well as urine.

Enriched stable isotopes used as tracers have proven to be valuable in studies of the absorption and metabolism of minerals. Unlike radioisotopes, they can be used in high-risk population groups such as infants, children, and pregnant or lactating women. Estimates of mineral absorption can be made from the oral administration of a single tracer or from two tracers, one given orally and the other intravenously (IV). It is possible to determine the metabolism of the mineral with modeling based on the amount of the tracer or tracers in different biological samples. One of the key decisions in studies of this type is determining which enriched isotope and what amount to use. An example is given of calculations to estimate and compare the amounts of tracers needed for an absorption study. Methods for calculating the amounts of tracer in oral and IV doses are presented, and limits of detection and quantitation are discussed in terms of percent of enrichment and related to isotope ratio measurement precision. A general review of the use of mass spectrometric instruments for quantifying various stable isotopes is given.

The efficacy of a chelating agent in binding a given metal in a biological system depends on the binding constants of the chelator for the particular metals in the system, the concentration of the metals, and the presence and concentrations of other ligands competing for the metals in question. In this study, we make a comparison of the in vitro binding constants for the chelator, ethylenediaminetetraacetic acid, with the quantitative urinary excretion of the metals measured before and after EDTA infusion in 16 patients. There were significant increases in lead, zinc, cadmium, and calcium, and these increases roughly corresponded to the expected relative increases predicted by the EDTA-metal-binding constants as measured in vitro. There were no significant increases in urinary cobalt, chromium, or copper as a result of EDTA infusion. The actual increase in cobalt could be entirely attributed to the cobalt content of the cyanocobalamin that was added to the infusion. Although copper did increase in the post-EDTA specimens, the increase was not statistically significant. In the case of magnesium, there was a net retention of approximately 85% following chelation. These data demonstrate that EDTA chelation therapy results in significantly increased urinary losses of lead, zinc, cadmium, and calcium following EDTA chelation therapy. There were no significant changes in cobalt, chromium, or copper and a retention of magnesium. These effects are likely to have significant effects on nutrient concentrations and interactions and partially explain the clinical improvements seen in patients undergoing EDTA chelation therapy.

Although dairy food intake is low among the Navajo people, hip fracture rates are lower than in Caucasians. Genetic differences in bone density have been cited as the reasons for low fracture rates among Native Americans and other segments of the population. However, more detailed examination of mineral intakes suggests that environmental factors may provide part of the explanation for the lower fracture rates. Cultural practices such as the addition of ash to traditional foods and the high mineral content of water may provide much higher intakes of bone-related minerals than food intake surveys have previously reported. As part of a larger study to assess overall intake of minerals related to bone health and other conditions, water samples were collected from the Navajo reservation. Duplicates were collected at least one week apart from 53 sites including wells, springs, taps, and storage barrels and analyzed by atomic absorption and inductively coupled plasma spectrometry for a number of minerals. For average intakes of 2 l/day, water could provide up to 212 mg of calcium, 150 mg of magnesium and 8 mg of zinc. The combined contribution of mineral intakes provided by the addition of juniper ash to traditional foods, not genetic differences, may partially explain the lower fracture rates of the Navajo people. Further research in this area is required to confirm this hypothesis.

A large batch of freeze-dried human urine reference material, Seronorm Trace Elements Urine, Lot 101021, was prepared commercially (Nycomed Pharma AS, Oslo, Norway) for quality control purposes in trace element analysis. Analytes are being determined by a voluntary, international co-operative effort so that the material will be available to the scientific community at modest cost. The material is in stoppered glass vials and is to be reconstituted with 5.00 ml of water prior to use. We have evaluated the trace element content for several elements, including chromium and zinc, elements for which we have two independent methods available for the determinations, namely isotope dilution mass spectrometry (IDMS) and atomic absorption spectrometry (AAS). We also report on other trace elements measured by IDMS alone, such as Se, for which we have enriched stable isotopes available. Results for chromium indicate a mean +/- standard deviation (n = 10) of 1.2 +/- 0.3 (by IDMS) and 1.4 +/- 0.3 (by AAS) ng Cr per ml of reconstituted urine, indicating possible inhomogeneity and/or contamination (21-25% relative standard deviation, RSD). Approximately half of the observed chromium originates from the sample container. The values observed for zinc were 590 +/- 90 ng ml-1 (15% RSD) for freshly reconstituted material, 760 +/- 60 ng ml-1 (8% RSD) for material reconstituted 4 d earlier, and 940 +/- 60 ng ml-1 (6% RSD) 2 months after reconstitution. Selenium values by IDMS were very reproducible, with a mean concentration of 16 +/- 0.15 ng g-1 (0.9% RSD), suggesting little or no contamination and a high degree of sample homogeneity for this element. The source of potential contaminants has been evaluated by multielement determinations of leachates of the vials and stoppers. Elements noted in significant amounts include B, Ba, Sr, Mo, Cu and Zn, with most of the zinc coming from the rubber stopper.

Chromium metabolism of lactating women was evaluated by measuring diet, breast milk, urine, and serum chromium in 17 subjects 60 d postpartum. Breast milk chromium concentration was similar for the 3 d of collection with a mean +/- SE concentration of 3.54 +/- 0.40 nmol/L (0.18 ng/mL). Dietary intake and urinary chromium values were also similar for each of the 3 collection days. Total chromium intake of lactating mothers (0.79 +/- 0.08 mumol/d) was greater than that of reference female subjects (0.48 +/- 0.02). There was a significant correlation (r = 0.84) between serum chromium and urinary chromium excretion. If a breast milk volume of 715 mL is assumed, chromium intake of exclusively breast-fed infants is < 2% of the estimated safe and adequate daily intake of 10 micrograms. In summary, breast milk chromium content is independent of dietary chromium intake and serum or urinary chromium values. Chromium intake also did not correlate with serum or urine chromium but there was a significant relationship between serum and urinary chromium concentrations.

The bioavailability and metabolism of different chemical species of mineral nutrients in the diet are receiving much attention from research nutritionists. In order to make scientifically based recommendations for mineral intakes, the chemical form of the mineral, with its specific absorption, utilization and retention, needs to be considered. Selenium is an example of an essential nutrient that is consumed in several different chemical forms, hence information is needed on the bioavailability and metabolism of each form before recommendations for dietary intakes can be made. A valuable tool for research on bioavailability and metabolism in humans is stable isotope tracers. When there are more than two stable isotopes available, as with selenium, stable isotope methodology allows the comparison of the utilization of different chemical forms of the nutrient simultaneously in in vivo studies. As an example of speciation questions addressed by nutritionists, a study is described that simultaneously evaluated utilization (absorption, retention and appearance in milk and blood) of two different chemical forms of selenium (selenite and selenomethionine) in lactating, non-lactating and never pregnant women using stable isotope tracers. All three groups of women had similar selenium status at the start of the study. Significantly more selenium from selenomethionine than from selenite was absorbed and appeared in the plasma in all groups. Milk contained more selenium from apparently absorbed selenomethionine than from selenite. All groups retained significantly more selenium from selenomethionine than from selenite; lactating women retained more selenium from selenite than did the other two groups, suggesting that milk losses may be partially compensated by enhanced retention of dietary selenium as selenite. Absorption and retention of selenium from selenomethionine in lactating women did not appear to be different from the other groups. The different chemical forms of selenium are metabolized differently among different physiological groups of women.

Duplicate meals, serum, whole blood, and toenails were collected every 3 mo for 1 y from a group of 44 free-living adults residing in high-selenium areas of South Dakota and Wyoming to assess the relation of selenium intake to indices of selenium status. The average selenium values for the group were as follows: dietary intake, 174 +/- 91 micrograms/d (mean +/- SD), 2.33 +/- 1.08 micrograms/kg body wt; serum, 2.10 +/- 0.38 mumol/L; whole blood, 3.22 +/- 0.79 mumol/L; and toenails, 15.2 +/- 3.0 nmol/g. Selenium intake (micrograms/kg body wt) was strongly correlated (all values, P less than 0.01) with selenium concentration of serum (r = 0.63), whole blood (r = 0.62), and toenails (r = 0.59). Men and women had similar mean values of serum, whole blood, and toenail selenium despite higher selenium intakes in men. Smokers had lower tissue selenium concentrations than did nonsmokers due, at least in part, to lower selenium intake. Age was not associated with tissue selenium content. Of the variables examined selenium intake was clearly the strongest predictor of tissue selenium concentration.

We examined utilization of selenomethionine (SeMet) and selenite in six lactating (L) and six nonlactating (NL) women, 2-3 mo postpartum, and seven never-pregnant (NP) women by use of stable-isotope tracers. All groups had similar selenium status at the start of the study. Significantly more selenium from SeMet than from selenite was absorbed and appeared in plasma in all groups. Milk contained more selenium from apparently absorbed SeMet than from selenite. More selenium from apparently absorbed selenite than from SeMet appeared in urine of NP and NL subjects whereas L subjects had approximately the same amount of selenium from apparently absorbed selenite and SeMet in their urine. All groups retained significantly more selenium from SeMet than from selenite; L women retained more selenium from selenite than did the other two groups. Absorption and retention of selenium from SeMet in L women did not appear to be significantly different from that in other women, suggesting that selenium requirements during lactation are increased mainly because of milk losses.

A separation method is described for rapidly removing zinc from interfering matrix components, such as chloride, from digested biological samples prior to measurement of zinc stable isotopes by ICP-MS. The method employs chelation of the zinc with trifluoroacetylacetone (TFA), extraction of the chelate with hexane, destruction of the chelate and dissolution of the zinc into nitric acid for determintion. Complete separation of the zinc from matrix salts is achieved, and the method does not employ chlorinated hydrocarbon solvents as used in earlier, similar schemes. Cerium from borosilicate glass tubes used in sample digestion was found to be a potential problem when measuring 70Zn, owing to the presence of 140Ce2+. This potential problem can be avoided by using quartz digestion tubes and/or correcting for any cerium by monitoring m/z 140. Accuracy was verified by an independent method, and examples of the use of stable isotopes of zinc as metabolic tracers in human metabolism are given. The detection limit for the ID method (3s of blank) was 0.7 µg Zn. An in-house urine pool was found to contain 1.18 ± 0.005 µg g–1 of Zn (n= 3) by ID and 1.17 ± 0.006 µg g–1 of Zn (n= 3) by AAS.

Twenty eight adults, 12 men and 16 women, participated in a 1-yr study designed to assess daily nutrient intake accurately. All subjects lived at home, consumed self-chosen diets, and maintained a detailed daily dietary record throughout the year. During four 7-day balance studies, one in each season of the year, meals, beverages, urine, and feces were analyzed for sodium and potassium content by atomic absorption spectrometry. Total intakes averaged 3.4 g/day for sodium and 2.8 g/day for potassium. The Na:K ratio for all diets analyzed averaged 1.3. Nutrient densities of sodium and potassium were 1.8 and 1.5 g/1000 kcal, respectively. Apparent absorptions of sodium and potassium were 98 and 85%, respectively, and did not change significantly over the wide range of intakes. Average urinary excretions of sodium and potassium were 86 and 77% of total intake, respectively. Mean metabolic balances were positive for sodium, +0.47 g/day, and potassium, +0.28 g/day. The data of this study provide useful information concerning the dietary intakes, excretions, and balances of sodium and potassium for adults based on analytic determination.

The determination of chromium in most biological materials is extremely difficult because of the very low levels present. Easily accessible samples, for example biological fluids such as serum, urine, etc., usually have chromium concentrations well below 1 ng g −1 . The only widely available analytical method with sufficient sensitivity is graphite furnace atomic absorption spectrometry (GFAAS), yet values reported in the early literature were wildly divergent. Man appeared to be excreting several times the amount of chromium he was absorbing from his diet. This dilemma was resolved in 1978 when it was shown that all previous chromium analytical results were probably wrong (too high), due to limitations of the instrumentation used up until then. Subsequent instruments with improved background correction capabilities have removed this limitation. However, making determinations at the sub-parts-per-billion level remains a formidable task in terms of contamination control.

A new sample vaporization procedure for the determination of arsenic by microwave-induced plasma cmission spectrometry is described. Optimum analytical conditions are reported, and the relative merits of atomization filaments fashioned of tantalum, molybdenum, and tungsten are compared. The method has been employed for the determination of arsenic in monoarsanilazoTyr-248 carboxypeptidase. The procedure is very sensitive, accurate, and precise for the determination of arsenic in microsamples of azocarboxypeptidase studied as an example of a metalloenzyme.

An accurate and sensitive method for serum chromium determinations by graphite-furnace atomic absorption spectrometry is described. Samples containing a small amount of magnesium nitrate as an ashing aid/matrix modifier are lyophilized and dry-ashed in silanized quartz tubes; the residue is dissolved in 0.1 M HCl. Because of the very low levels of chromium in serum, strict contamination control measures must be used throughout the procedures for collection, storage, preparation and quantitation. Standard curves are prepared by using a bovine serum pool, which also serves as a quality control measure. The uniform nature of sera obviates the need to use the method of standard additions. The detection limit of the method is about 0.03 ng ml−1 Cr and the accuracy of the method is evaluated by comparison with stable-isotope-dilution mass spectrometry.

Selenium utilization of women in early and late pregnancy was compared to that of nonpregnant controls. A defined diet providing about 150 jzg Se/day was fed for 20 days, and selenium balance was measured during the last 12 days. Net selenium retentions of the women in early and late pregnancy were 10 and 23 tug/day, respectively, but probably are inflated estimates of the increased selenium requirement during pregnancy. Apparent absorption of selenium was 80% for all three groups. Pregnant women tended to conserve selenium by decreasing urinary selenium excretion. Those observations were corroborated by monitoring the urinary and fecal excretion of 40 zg of a stable isotope of selenium (76Se) from intrinsically labeled egg. The isotope data also indicated that recent selenium intake was incorporated into a long-term selenium pool. Mean glutathione peroxidase activity was lower in plasma and higher in platelets in the pregnant women as compared to controls, but the physiological significance of those observations is unknown. Am J Clin Nuir 1983;38:169-180.

Interlaboratory cooperative studies have been conducted under the auspices of the IUPAC Commission on Toxicology to establish concentrations of biological important trace elements in commercially available lyophilized human serum, whole blood and urine. The main objective was to arrive at consensus concentration values in order to establish these materials as reference materials for chemical analysis. The analytical data presently available have been elaborated in collaboration among world-wide expert laboratories.

A low-current d.c. arc plasma jet in argon is characterized. Apparent temperatures observed in the plasma flame region are considerably lower than those reported previously by other workers with similar devices. It appears that much of the sample aerosol does not enter the high-temperature plasma region.

A rapid, accurate and direct method for urinary chromium determinations by graphite-furnace atomic absorption spectrometry is described. Few reagents are used and very little sample preparation and manipulation are required, greatly reducing the incidence of sample contamination. The method of standard additions is used to compensate for changes in sensitivity as the furnace tube ages, and for the widely different matrices encountered in urine samples. Furnace parameters must be carefully controlled. The detection limit is in the order of 0.03 ng Cr ml-1. Agreement with independent methods is evaluated.

Daily urinary chromium (Cr) excretion of 15 healthy free-living female subjects was 0.20 +/- 0.03 microgram (mean +/- SEM) and nearly identical for 27 male subjects, 0.17 +/- 0.02 microgram. Minimum Cr absorption calculated from urinary Cr excretion was about 0.4 percent. Increasing intake fivefold by Cr supplementation led to a nearly fivefold increase in Cr excretion suggesting that the extent of absorption of supplemental inorganic chromium was similar to that from normal dietary sources. Correlations between 24-hour Cr excretion and urine volume, age, total creatinine and body weight were not found. Urinary Cr concentration of samples obtained following a morning void correlated with creatinine and Cr concentration following a glucose challenge but not with serum glucose, insulin, lipid parameters, age or body weight. Similar results were obtained for urine samples obtained from subjects during Cr supplementation. These results suggest that urinary Cr excretion does not appear to be a meaningful indicator of Cr status but is a meaningful indicator of Cr intake and that the absorption of supplemental inorganic Cr was similar to that of Cr from normal dietary sources.

Enriched stable isotopes of selenium are used for a double isotope dilution method employing rapid sample digestion, chelation and measurement by com bined gas chromatography/mass spectrometry (GC/MS). A known quantity of an en riched selenium isotope is added as an internal standard, and samples are rapidly digested with HNO3, H3PO4and H2O2. Undigested lipids are extracted with chloroform, and any selenate is reduced to selenite with HC1. The selenite reacts with 4-nitro-o-phen- ylenediamine (NPD) to form 5-nitropiazselenol (Se-NPD), which is then extracted into chloroform for subsequent GC/MS analysis. By monitoring the ion peaks in the Se- NPD+ parent ion cluster, and by using isotope ratio measurements, it is possible in principle to use any two of the stable selenium isotopes as tracer and internal standard. The method described herein utilizes 76Seas the tracer and 82Seas the internal standard, compared to ^Se naturally present in the sample. Selenium recoveries from the digestion- chelation steps were verified by using animal tissues endogenously radiolabeled with 75Se.The method is precise, accurate, rapid and extremely specific, and should lend itself well to determining selenium in biological materials, and to following stable se lenium isotopes as tracers in metabolic studies. J. Nutr. 113: 786-792, 1983.

Flame atomic emission and atomic absorption detection limits are compared on an absolute basis for the easily determined element, copper. Remarkably similar detection limits are obtained with a variety of atomization and sample introduction system combinations. Heated graphite atomizers offer significantly lower minimum detectable quantities.

Twenty-eight adult men and women participated in a year-long study designed to determine accurately the nutrient intake of adults who lived at home and consumed self-selected diets. During four metabolic balance periods, 7 days each, corresponding to the seasons (spring, summer, fall, winter), duplicates of the diet, and all urine and feces were collected. Daily mean intakes for zinc and copper were 9.9 and 1.2 mg, respectively. These levels were less than the recommended daily intakes of 15 mg for zinc and 2 to 3 mg for copper. In contrast, the mean dietary intake of manganese was 3.0 mg/day which is within the suggested safe and adequate range of 2.5 to 5.0 mg. Metabolic balances were negative for all three elements possibly due to a reduction in food intake during the collection periods compared to the noncollection intervals. The collection of the duplicate diets apparently influenced the food intake during the collection weeks.

A study has been made of the induction-coupled, 4.8 MHz, plasma torch as a source of atoms for emission and absorption spectrochemical analysis. The factors evaluated include sensitivity, limits of detection, inter-element effects, limitations, and general convenience. In agreement with other reports, the plasma torch was found to give useful emission signals for several elements which are difficult to excite in chemical flames, such as B, Ta, and Ti. The value of the source for atomic absorption spectrometry was found to depend on the availability of bright line sources. Pronounced inter-element effects were found, affecting particularly the emission signals. In most cases, these inter-element effects were enhancements of the emission, although in one case a suppression was observed. Except for a few refractory elements, the 4.8 MHz plasma torch does not appear to be a suitable replacement for the chemical flame.