Anthropogenic rare earth elements in urban lakes: Their spatial distributions and tracing application

Anthropogenic activities associated with various new technologies are increasingly disrupting the geochemical cycles of rare earth elements (REEs). For example, samarium (Sm) and gadolinium (Gd) have emerged as microcontaminants in the natural waters of developed areas. Surface water samples of 13 urban lakes were collected in Wuhan, the largest city in central China, with a population of over 11 million. The aim of this study was to examine to what extent REE anomalies occur and the relationship between the concentration of anthropogenic REEs in lakes and the surrounding environment. In this study, based on land-use type and point of interest (POIs) data, buffer extraction, density estimation and Spearman correlation analysis were first proposed to identify different sources of anthropogenic REEs, which mainly included hospitals, factories, population, urban land and cropland. The PAAS-normalized REE patterns indicate that all lake samples display pronounced positive Sm and Gd anomalies, ranging from 5.92 to 19.88 and 1.73 to 14.97, respectively. Spearman correlation analysis showed that hospital density was positively correlated with anthropogenic Gd concentration, and a positive relationship between proportion of cropland and the concentration of anthropogenic Sm. By utilizing Gd_anth, Sm_anth, and the conventional ion ratio (NO₃^-/Cl^-), a three-dimensional tracer system was established, and the system accurately obtained a characterization of the impact of WWTPs, hospitals, factories and agriculture on the lakes. Moreover, the results from this hydrochemical method were consistent with the analysis of geographic information systems, which indicated that this anthropogenic contaminant as a tracer was reliable for analysing the source of urban water pollution.

Investigation of rare earth elements in urine and drinking water of children in mining area

To compare the contents of rare earth elements in urine and drinking water of children in the mining and control areas and evaluate the health risk of children in the mining area.Urine and drinking water of 128 children in the mining area and 125 children in the control area were collected from June to July 2015. The contents of rare earth elements were determined using inductively coupled plasma mass spectrometry.The detection rates of rare earth elements, including yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), and samarium (Sm), in the urine of children in the exposed group were all 100%, except for samarium (98%); the rates in the control group were 85.7%, 100%, 100%, 98%, 98%, and 59.2%, respectively, and the remaining elements were not detectable. The concentrations of Y, La, Ce, Pr, Nd, and Sm in the urine of children in the exposed group were significantly higher than that in the control group (P < .01). In addition, the composition ratio of lanthanum was higher than that in the control group. The detection rates of lanthanum and Ce in the drinking water of children in the exposed group were 1.44% and 0.72%, respectively. The others were not detectable; the rates in the control group were all 0%.The pollution caused by the presence of Y, La, Ce, Pr, Nd, and Sm in the mining area might affect the health of children in the area, but drinking water might not be the cause.

Dependence of samarium-soil interaction on samarium concentration: Implications for environmental risk assessment

The sorption and desorption behaviour of samarium (Sm), an emerging contaminant, was examined in soil samples at varying Sm concentrations. The obtained sorption and desorption parameters revealed that soil possessed a high Sm retention capacity (sorption was higher than 99% and desorption lower than 2%) at low Sm concentrations, whereas at high Sm concentrations, the sorption-desorption behaviour varied among the soil samples tested. The fractionation of the Sm sorbed in soils, obtained by sequential extractions, allowed to suggest the soil properties (pH and organic matter solubility) and phases (organic matter, carbonates and clay minerals) governing the Sm-soil interaction. The sorption models constructed in the present work along with the sorption behaviour of Sm explained in terms of soil main characteristics will allow properly assessing the Sm-soil interaction depending on the contamination scenario under study. Moreover, the sorption and desorption K_d values of radiosamarium in soils were strongly correlated with those of stable Sm at low concentrations (r = 0.98); indicating that the mobility of Sm radioisotopes and, thus, the risk of radioactive Sm contamination can be predicted using data from low concentrations of stable Sm.

Anthropogenic rare earth elements and their spatial distributions in the Han River, South Korea

Rare earth elements (REE) consist of lanthanides (from La to Lu), together with yttrium and scandium, in which anthropogenic REE, such as gadolinium (Gd), lanthanum (La), and samarium (Sm), has emerged as micro-contaminants in natural waters in highly developed countries. Here, we collected water samples in the Han River (HR) and its tributaries flowing through Seoul Capital Area, the world's second largest metropolitan area in order to examine how and to what extent anthropogenic REE anomalies may occur. Water samples show higher light REE concentrations than heavy REE concentrations, while wastewater treatment plant (WWTP) samples display much higher heavy REE concentrations due to high Gd concentration. The PAAS-normalized REE patterns indicate that WWTP samples display the pronounced positive Gd anomalies, in which anthropogenic Gd from magnetic resonance imaging (MRI) diagnostic system occurs as a form of Gd complexation with either Cl^- or SO₄^2-. Due to the WWTP, both the HR and tributaries show also positive Gd anomalies and the anthropogenic Gd concentrations increase as a function of the distance from the Paldang dam. This result indicates a positive correlation between populaton, number of MRI instruments, and positive Gd anomaly. Similarly, positive La and Sm anomalies exist in the HR, indicating that the HR is also affected by their point sources. Based on the discharge rate and anthropogenic REE concentrations, their fluxes are estimated to be 952 ± 319 kg/yr, suggesting that this amount of fluxes could disturb REE distribution in the Yellow Sea, and pose harmful effects on aquatic ecosystems.

Formation of Core-Shell Nanoparticles Composed of Magnetite and Samarium Oxide in Magnetospirillum magneticum Strain RSS-1

Magnetotactic bacteria (MTB) synthesize magnetosomes composed of membrane-enveloped magnetite (Fe₃O₄) or greigite (Fe₃S₄) particles in the cells. Recently, several studies have shown some possibilities of controlling the biomineralization process and altering the magnetic properties of magnetosomes by adding some transition metals to the culture media under various environmental conditions. Here, we successfully grow Magnetospirillum magneticum strain RSS-1, which are isolated from a freshwater environment, and find that synthesis of magnetosomes are encouraged in RSS-1 in the presence of samarium and that each core magnetic crystal composed of magnetite is covered with a thin layer of samarium oxide (Sm₂O₃). The present results show some possibilities of magnetic recovery of transition metals and synthesis of some novel structures composed of magnetic particles and transition metals utilizing MTB.

Rare earth elements in the aragonitic shell of freshwater mussel Corbicula fluminea and the bioavailability of anthropogenic lanthanum, samarium and gadolinium in river water

High-technology metals - such as the rare earth elements (REE) - have become emerging contaminants in the hydrosphere, yet little is known about their bioavailability. The Rhine River and the Weser River in Germany are two prime examples of rivers that are subjected to anthropogenic REE input. While both rivers carry significant loads of anthropogenic Gd, originating from contrast agents used for magnetic resonance imaging, the Rhine River also carries large amounts of anthropogenic La and lately Sm which are discharged into the river from an industrial point source. Here, we assess the bioavailability of these anthropogenic microcontaminants in these rivers by analyzing the aragonitic shells of the freshwater bivalve Corbicula fluminea. Concentrations of purely geogenic REE in shells of comparable size cover a wide range of about one order of magnitude between different sampling sites. At a given sampling site, geogenic REE concentrations depend on shell size, i.e. mussel age. Although both rivers show large positive Gd anomalies in their dissolved loads, no anomalous enrichment of Gd relative to the geogenic REE can be observed in any of the analyzed shells. This indicates that the speciations of geogenic and anthropogenic Gd in the river water differ from each other and that the geogenic, but not the anthropogenic Gd is incorporated into the shells. In contrast, all shells sampled at sites downstream of the industrial point source of anthropogenic La and Sm in the Rhine River show positive La and Sm anomalies, revealing that these anthropogenic REE are bioavailable. Only little is known about the effects of long-term exposure to dissolved REE and their general ecotoxicity, but considering that anthropogenic Gd and even La have already been identified in German tap water and that anthropogenic La and Sm are bioavailable, this should be monitored and investigated further.

New versatile staining reagents for biological transmission electron microscopy that substitute for uranyl acetate

Aqueous uranyl acetate has been extensively used as a superb staining reagent for transmission electron microscopy of biological materials. However, recent regulation of nuclear fuel material severely restricts its use even for purely scientific purposes. Since uranyl salts are hazardous due to biological toxicity and remaining radioactivity, development of safe and non-radioactive substitutes is greatly anticipated. We examined two lanthanide salts, samarium triacetate and gadolinium triacetate, and found that 1-10% solution of these reagents was safe but still possess excellent capability for staining thin sections of plastic-embedded materials of animal and plant origin. Although post-fixation with osmium tetroxide was essential for high-contrast staining, post-staining with lead citrate could be eliminated if a slow-scan CCD camera is available for observation. These lanthanide salts can also be utilized as good negative-staining reagents to study supramolecular architecture of biological macromolecules. They were not as effective as a fixative of protein assembly, reflecting the non-hazardous nature of the reagents.

Application of a wrist dosimeter prototype for radiation monitoring ((153)Sm) during a therapeutic procedure simulation

Gamma and beta radiation emitting radiopharmaceuticals are handled in nuclear medicine services, and in many cases there is only individual monitoring of gamma radiation. In this paper, the results obtained using a wrist dosimeter prototype (CaSO(4):Dy+Teflon pellets) show that the doses for workers occupationally exposed to beta radiation from (153)Sm are not negligible. It is important that this dose is evaluated, and it has to be taken into consideration in the individual monitoring system.

Medical radioisotopes in the environment - following the pathway from patient to river sediment

Motivated by the detection of (131)I in river sediment in routine long-term surveillance samples, a systematic short-term study of the wastewater treatment chain was planned and conducted. Inflow, effluent and primary sludge were collected on a daily basis during two weeks at a regional wastewater treatment plant. Samples were investigated by gamma spectroscopy. Four medically used isotopes could be identified ((131)I and (99m)Tc regularly, (153)Sm and (123)I sporadically). The concentration levels coincide well with literature data for (131)I, and with our own long-term data for (131)I and (99m)Tc for the same plant. Cosmogenic (7)Be activity in primary sludge correlated well with rainfall intensity. Surface sediment was sampled at low tide at both shores of the river, up- and downstream of the plant. (131)I was identified in all samples, with a sharp maximum (about 100 Bq kg(-1)d.m.) at the discharge point of the plant and lower levels elsewhere, decreasing monotonically in downstream direction. (7)Be and (137)Cs showed the same behaviour, but no peak at the discharge point. Predictions from simple equilibrium models for the transport and sedimentation of (131)I show good agreement with the experimental data and suggest that the wastewater treatment plant is the main source for this isotope.

Simultaneous determination of 76As, 122Sb and 153Sm in Chinese medicinal herbs by epithermal neutron activation analysis

Optimal conditions for the simultaneous determination of As, Sb and Sm in Chinese medicinal herbs using epithermal neutron activation analysis were investigated. The minimum detectable concentrations of 76As, 122Sb and 153Sm in lichen and medicinal herbs depended on the weight of the irradiated sample, and irradiation and decay durations. Optimal conditions were obtained by wrapping the irradiated target with 3.2 mm borated polyethylene neutron filters, which were adopted to screen the original reactor fission neutrons and to reduce the background activities of 38Cl, 24Na and 42K. Twelve medicinal herbs, commonly consumed by Taiwanese children as a diuretic treatment, were analysed since trace elements, such as As and Sb, in these herbs may be toxic when consumed in sufficiently large quantities over a long period. Various amounts of medicinal herbs, standardised powder, lichen and tomato leaves were weighed, packed into polyethylene bags, irradiated and counted under different conditions. The results indicated that about 350 mg of lichen irradiated for 24 h and counted for 20 min following a 30-60 h decay period was optimal for irradiation in a 10(11)n/cm s epithermal neutron flux. The implications of the content of the studied elements in Chinese medicinal herbs are discussed.

Determination of the intensity of X- and gamma-ray emissions in the decay of 153Sm

The activity of a 153Sm solution was determined by means of liquid scintillation counting and an ionization chamber. Several accurately calibrated X- and gamma-ray spectrometers were used to measure point sources. The use of different detectors results in improvements to the accuracy of the data around 100 keV where the efficiency curve changes significantly. Photon emission intensities have been derived for the main X- and gamma-rays, and the 103-keV emission has a measured intensity of (29.07 (20)) per 100 disintegrations.

An experiment using neutron activation analysis and a rare earth element to mark cotton plants and two insects that feed on them

Studies on insect dispersal and other behaviors can benefit from using markers that will not alter flight and fitness. Rare earth elements, such as samarium (Sm), have been used as ingested markers of some insects and detected using neutron activation analysis (NAA). In this study, samarium nitrate hexahydrate was mixed into artificial diet for boll weevils, Anthonomus grandis grandis Boheman (Coleoptera: Curculionidae), at different dosages and in water used to irrigate cotton, Gossypium hirsutum L. Samarium was detected in adult boll weevils fed on the samarium-labeled diet, but not after 5 or 10 days of being switched to non-labeled diet, even if the insects were given labeled diet for as long as 7 consecutive days. Introduced in irrigation water, 1% samarium (m/m) was detectable in cotton squares and leaf tissue. However, boll weevil adults fed samarium-labeled squares did not retain detectable levels of samarium, nor did boll weevil adults reared to adulthood from samarium-labeled squares. Fourth instar beet armyworms, Spodoptera exigua (Hübner) (Noctuidae: Lepidoptera), fed on samarium-labeled cotton leaves obtained enough samarium for NAA detection, but adult moths reared from them did not have detectable amounts of samarium. Although samarium can be useful as a marker when insects are presented with a continuous pulse of the label, elements that are assimilated by the insect would be more useful if a continuous infusion of the marker cannot be provided.

[Determination of ten trace rare earth elements in the sample by atomic emission spectrometry]

This paper describes the determination of trace La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Yb and Lu in the sample, using carbon powder, potassium sulfate, barium sulfate, strontium sulfate, and scandium chloride as buffer, by atomic emission spectrometry (AES). Scandium was selected as internal standard line. Sample separation and chemical treatment were not required. The sample was directly loaded into ordinary electrode. The method is simple, rapid and accurate. The determination requirement and influence factors were studied. A new method was developed for the determination of ten rare earth elements, for which the detection limit is smaller than 0.030%, and the range of the recovery is 94%-105%. The results of these elements in standard sample are in agreement with certified values, and the RSD is smaller than 5% (n = 9). The method has been applied to the determination of ten rare earth elements with satisfactory results.

Facile access to 154Eu, a new reference source for calibration in gamma ray spectrometry

Europium-154 can be obtained as a by-product from the large-scale production of Samarium-153 and possesses attractive features (t1/2 8.592 yr; Egamma 0.12-1.6 MeV) for use as a reference source similar to 152Eu (t1/2 13.516 yr; Egamma 0.12-1.4 MeV), which is the gold standard for calibration in gamma ray spectrometry. Thermal neutron irradiation of 5mg of 98% enriched 153Sm2O3 target in the reactor led to approximately 200 GBq 153Sm and 1.26 MBq 154Eu. A typical batch control sample of 153SmCl3 solution and final radiopharmaceutical product formulation of 153Sm-phosphonate (153Sm-EDTMP) pooled together contained about 20% of total yield, requiring post decay disposal of 153Sm as radioactive waste. Such spent solutions pooled on quarterly basis led to availing 756 kBq of 154Eu. The radioactivity content and radionuclide purity (approximately 82%) of the recovered 154Eu sample were envisaged as adequate to prepare reference sources for calibration of gamma ray spectrometers. At present, one batch of 153Sm is handled per month at our institution, with the possibility for weekly processing in future. Access to approximately 3.5 MBq of 154Eu on quarterly basis is envisaged, apart from obviating the need for instituting steps to tackle disposal of the long-lived 154Eu in the spent solution. Up to 60-120 units of 20-100 kBq of 154Eu reference sources per year could thus be available by the proposed strategy.

Simultaneous Determination of α-Fetoprotein and Free β-Human Chorionic Gonadotropin by Element-Tagged Immunoassay with Detection by Inductively Coupled Plasma Mass Spectrometry

Background: An inductively coupled plasma mass spectrometry (ICP-MS)-based immunoassay has been proposed independently by Baranov et al. (Anal Chem 2002;74:1629–36) and our group, but the applicability of this method for multianalyte analysis in clinical samples has not been fully illustrated. We developed a dual-label immunoassay method for the simultaneous determination of α-fetoprotein (AFP) and free β-human chorionic gonadotropin (hCGβ) in human serum.

Methods: Monoclonal antibodies immobilized on microtiter plates captured AFP and hCGβ, which were detected by use of Eu3+-labeled anti-AFP and Sm3+-labeled anti-hCGβ monoclonal antibodies. Eu3+ and Sm3+ were dissociated from the immunocomplex with HNO3 solution (10 mL/L) and delivered by peristaltic pump to the ICP mass spectrometer.

Results: The measurable ranges of AFP and hCGβ were 4.6–500 and 5.0–170 μg/L, respectively, with detection limits of 1.2 and 1.7 μg/L (3 SD above mean of zero calibrator), respectively. The intraassay imprecision (CV) for AFP was 8.3%, 4.0%, and 2.7% at 16.3, 86, and 354 μg/L, respectively, and the interassay CV was 10%, 5.7%, and 3.5%. For hCGβ, the intraassay CV was 5.4%, 6.4%, and 3.1%, respectively, at 10.5, 45.2, and 105 μg/L, and the interassay CV was 7.2%, 8.0%, and 3.7%. Comparison with IRMAs for AFP and hCGβ yielded correlation coefficients (r2) of 0.97 and 0.95.

Conclusions: Two proteins can be measured simultaneously by immunoassays using two rare earth elemental tags (Eu3+ and Sm3+) and ICP-MS detection. The multielement capability and the multiple potential elemental labels make ICP-MS attractive for multianalyte immunoassays. Implementation of ICP-MS-linked immunoassays may be relatively straightforward because the labeling and immunoreaction procedures have been well developed for clinical time-resolved immunofluorometric assays.

Laser photoacoustic spectra of Sm3+ ion in Sm2O3 and SmCl3.6H2O in the spectral profile 484-542 nm

Microphone based photoacoustic (PA) spectrometer to study solids in powder form was designed and fabricated. Laser PA spectra of Sm3+ ion in Sm2O3 and SmCl3.6H2O microcrystalline powders were recorded first time in 484-542 nm spectral region at room temperature. Analysis of these PA spectra shows new information on the Stark components of ground and excited states of Sm3+ ion. A comparison of Stark energy levels of Sm3+ ion in both oxide and hexahydrated chloride hosts is presented here.

Heavy metal and other trace elements in native mussel Diplodon chilensis from Northern Patagonia Lakes, Argentina

Native mussels, Diplodon chilensis, were sampled from four lakes in Nahuel Huapi National Park, Northern Patagonia, Argentina in order to evaluate heavy-metal distribution in the region and to assess the contribution of this compartment of the trophic web to their circulation in the food chain. The concentration of potential pollutants Ag, As, Cr, Hg, Sb, and Se, and other nine elements of interest (Ba, Br, Ca, Co, Cs, Fe, Na, Sr, and Zn) were determined in Diplodon chilensis pooled samples. Digestive glands were analyzed separately from soft tissues. Geological tracers Sc, Ta, Th, and rare earth elements were also determined in order to discriminate lithophile contributions. Elemental concentrations of Ba, Br, Fe, Sr, Se, and Zn in total soft tissues samples do not show significant differences among sampling sites. Arsenic and Cr contents in total soft tissues and digestive gland pooled samples are higher in sampling points close to zones with human settlements. Silver contents in samples collected in Lake Nahuel Huapi were higher than in the other lakes studied, and up to 50-fold higher than the sample collected in Lake Traful, considered as the reference. Mercury highest concentration values measured in total soft tissues pooled samples from lakes Nahuel Huapi and Moreno were found to be similar to those observed in other reported Hg contamination situations, and they are three to five times higher than those of the reference samples collected in Lake Traful.

ppt Level Detection of Samarium(III) with a Coated Graphite Sensor Based on an Antibiotic

N-[2-[4-[[[(Cyclohexylamino)carbonyl]amino]sulfonyl]phenyl]ethyl]-5-methyl pyrazine carboxamide (glipizid) was explored as an electro-active material for preparing a polymeric membrane-based sensor selective to samarium ions. The membrane incorporated 30% poly(vinyl chloride) (PVC), 53% benzyl acetate (BA), 11% glipizid and 6% sodium tetraphenyl borate. When coated on the surface of a graphite electrode, it exhibits Nernstian responses in the concentration range of 1.0 x 10(-5) to 1.0 x 10(-10) M, with a detection limit of 8.0 x 10(-11)M samarium. The electrode shows high selectivity towards samarium over several cations (alkali, alkaline earth, transition and heavy metal ions), and specially lanthanide ions. The proposed sensor has a very short response time (< 15 s), and can be used in a wide pH range for at least ten weeks. It was used as an indicator electrode in potentiometric titration of Sm(III) ions with an EDTA solution, and for determination of samarium in binary and ternary mixtures.

Sensitive time-resolved fluoroimmunoassay for simultaneous detection of serum thyroid-stimulating hormone and total thyroxin with Eu and Sm as labels

Based on a novel cocoating strategy and dissociation enhancement lanthanide fluorescence immunoassay technique, a sensitive time-resolved fluoroimmunoassay (TRFIA) has been developed for simultaneous quantification of human serum thyroid-stimulating hormone (TSH) and thyroxin (T4) in a one-and-the-same assay procedure. The new cocoating strategy for preparing highly active surface anti-TSH and anti-T4 monoclonal antibodies (McAbs) was performed by a three-step protocol. Namely, anti-TSH McAb at high concentration (10 micro g/ml) and extensively biotinylated bovine serum albumin (BSA) at low concentration (0.5 micro g/ml) were coated on microwells by passive adsorption, then streptavidin was captured by the surface BSA-biotin, and finally biotinylated anti-T4 McAb was immobilized by the remnant binding sites of the bound streptavidin. In the present TSH/T4 TRFIA, both sandwich- and competitive-type configurations were involved, and Eu(3+) and Sm(3+) were used as labels for TSH and T4 detection, respectively. The method showed rapid kinetics; the equilibrium was reached within 30min at 37 degrees C due to the use of high concentrations of reaction reagents, rapid agitation, and small reaction volume. The lower limits of detection of the method were 0.028 mIU/L for TSH and 4.1 nmol/L for T4 with 20 micro L of sample volume. The assay ranges for TSH and T4 were 0.21-80.00 mIU/L and 20-300 nmol/L, respectively. The correlation between the TSH/T4 values obtained by the present TSH/T4 TRFIA and those obtained by commercial chemiluminescence immunoassay was satisfactory.

[An evaluation of human hair homogeneity by inductively coupled plasma mass spectrometry]

Human hair certified reference material(GBW09101) homogeneity was evaluated by determining 10(-6) quantity Cu, Zn, Mg, K, Pb, Fe, Ca, Na and S on sample weight from 100 mg to 300 mg by neutron activation analysis, atomic absorption spectrometry and X-ray fluorescence analysis. Using variance F test, the results(F < F alpha) is not differ significantly and this material could be considered homogeneous. In this paper, an evaluation of homogeneity was made by Inductively Coupled Plasma Mass Spectrometry through the determination of 10(9) quantity La, Sm, Y in 0.2000 g samples. Using t test and F test, the results (magnitude of t < t alpha, F < F alpha) were satisfactory.

Rare earth elements as nonabsorbable fecal markers in studies of iron absorption

The use of rare earth elements as nonabsorbable fecal markers for studies of iron absorption from sources labeled extrinsically with stable isotopes was evaluated. On 3 successive days 13 healthy fasting adults were given different stable isotopes of iron with samarium, ytterbium, or dysprosium. On day 1, three meals were given with 57Fe (1 mg per meal) plus samarium (0.33 mg per meal); on day 2, identical meals (taken with a calcium supplement to reduce iron bioavailability) were given with equivalent amounts of 58Fe-labeled iron and ytterbium; on day 3, a well-absorbed reference dose of 54Fe (3 mg) was given with 1 mg Dy. A complete fecal collection was carried out for 5-9 d and each stool was analyzed for rare earth elements by inductively coupled plasma-mass spectrometry and iron isotopes by thermal ionization quadrupole mass spectrometry. Mean recovery of rare earth elements was 101%, indicating that they are totally unabsorbed. The excretory pattern of the iron isotopes and the rare earth elements was very similar; the correlation coefficients between samarium and 57Fe, ytterbium and 58Fe, and dysprosium and 54Fe were 0.992, 0.989, and 0.988, respectively (P < 0.001). Iron absorption was calculated as the difference between isotope dose and fecal excretion. Mean (+/-SEM) iron absorption was 16.7 +/- 2.4%, 4.3 +/- 1.6%, and 40.3 +/- 3.1% on days 1-3, respectively. Predicted values estimated from the first 4 d of pooled feces, using the rare earth element recovery data to produce corrected figures for unabsorbed isotope, were in close agreement: 19.1 +/- 2.1%, 4.6 +/- 1.7%, and 40.8 +/- 3.1%, respectively (P < 0.001). With the diet of medium iron bioavailability and with the highly bioavailable reference dose it was possible to predict iron absorption accurately from only one or two stools, provided that they were sufficiently enriched with isotope and a rare earth element.