Maoyong He

Boron isotope variations and its geochemical application in nature

The high geochemical reactivity of boron and the large relative mass difference between 10B and 11B lead to significant boron isotope fractionation in nature. So far the measured range of boron isotope composition (δ11B) varies between –70 and +75‰. The negative δ11B values are found in non-marine evaporite borate minerals and tourmalines, whereas positive δ11B values are common in salt lake brines and evaporated seawater. Since the 1980s, with improved measurement methods, applications of boron isotope analysis have increased rapidly. At present, boron isotopes are successfully applied to reconstruct ancient marine environments, to determine depositional environments and ore genesis, to trace groundwater pollution and seawater intrusion, and to study continental erosion. This paper summarises the methods for boron isotope analysis, the mechanisms of boron isotope fractionation and the distribution of boron isotopes in nature, reviews the achievements and the problems of boron isotopes in geochemical applications, and proposes research directions of boron isotopes in geochemical fields.

Strontium concentrations and isotope ratios in enamel of healthy and carious teeth in southern Shaanxi, China

RATIONALE As the trace element strontium (Sr) plays a significant role in dental health, it is important to determine the Sr concentration and isotope composition ((87)Sr/(86)Sr) of teeth and whether these values are related to caries formation, age and sex. METHODS A total of 160 permanent teeth were collected from 7- to 79-year-old people from the southern Shaanxi area of China, including 100 healthy teeth and 60 carious teeth (men and women each accounted for half of the samples). The concentration and isotope composition of Sr elements in the dental enamel of the teeth were measured using inductively coupled plasma mass spectrometry (ICP-MS) and thermal ionization mass spectrometry (TIMS). RESULTS A significantly lower Sr concentration was found in the enamel of the carious teeth than in that of the healthy teeth for individuals of varying ages and sex. The Sr concentration in human carious teeth ranged between 79.70 µg/g and 85.80 µg/g; while the Sr concentration in healthy teeth ranged between 128 µg/g and 156.77 µg/g. Our results also demonstrated that the (87)Sr/(86)Sr ratio did not appear to be affected by the caries formation, age or sex. The (87)Sr/(86)Sr ratio in the enamel of the healthy and carious teeth of individuals of varying ages and genders ranged between 0.710935 and 0.711037, which falls into the range of (87)Sr/(86)Sr ratios found in the local, naturally occurring water, soils and rocks. CONCLUSIONS Sr plays a significant role in dental health, and there is a negative correlation between Sr and the occurrence of dental caries. The (87)Sr/(86)Sr ratio of teeth reflects the (87)Sr/(86)Sr ratio of the associated environment, and there is no significant relationship with the frequency of dental caries, age or sex.

Determination of Boron Isotope Ratios in Tooth Enamel by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) After Matrix Separation by Ion Exchange Chromatography

Boron isotopes in teeth has been a new proxy for dietary reconstructions and its resistance to diagenetic alteration. In this study a method using inductively coupled plasma source mass spectrometry (ICP-MS) for the measurement of boron isotope ratio in human dental enamel has been developed. Human dental enamel were digested with HNO3-H2O2 in a microwave system. Boron in solution was separated from the matrix components using Amberlite IRA-743 resin. The factors that may affect precision and accuracy in isotope ratio determination by ICP-MS, including memory effects, mass bias drift, and concentration effects, were investigated to obtain optimum conditions. Then, the 10B/11B ratios in teeth were measured. The results showed that 2% of HNO3 + 2% of NH3•H2O, selected as the diluent/rinse solution could be effective in the elimination of boron memory effect. There was no concentration effect on boron isotope ratios when the ratio of samples B concentration to standard B concentration (refers to Csample/Cstd) varied from 0.5 to 2. The result of 10B/11B ratios in tooth enamel by sex and age fluctuated over a broad range, ranged from 0.2007 to 0.2574. This method is expected to be used for boron isotope ratio analyses in archeometry, forensic identification, paleoecology, and other disciplines in the future.

Efficient separation of boron using solid-phase extraction for boron isotope analysis by MC-ICP-MS

In nature, boron exhibits a large range of isotopic compositions (up to 150‰), which makes it a powerful geochemical tracer. Improving the separation and purification procedures of boron in complex matrices and at low boron contents is a challenge in obtaining high-precision and accurate boron isotope ratios. A method of ion exchange using solid-phase extraction (SPE) for the efficient separation of boron from samples with low boron contents was developed for determining boron isotopes by MC-ICP-MS. The results showed that the total amount of residual boron in the solution after ion exchange decreased from 60 μg at a vacuum pressure of 0.09 MPa to 0 at 0.04 MPa. When the vacuum pressure was between 0.008 and 0.04 MPa boron was not detected in the solution. The flow rate of the solution increased from 50 μL min−1 at a vacuum pressure of 0.008 MPa to 1358 μL min−1 at a vacuum pressure of 0.09 MPa, and the volume of eluent required increased from 300 μL at 0.008 MPa to 600 μL at 0.04 MPa. Therefore, the recommended vacuum pressure was below 0.04 MPa based on these results. The δ11B values of NBS 951 after chemical treatment using SPE were between −0.25‰ and 0.26‰. Using this procedure, boron in river water, rainwater and seawater samples was separated and the B isotope compositions analyzed by MC-ICP-MS. The accuracy and precision of our method were assessed by comparing the measured δ11B values of these samples with published results using positive-ion thermal-ionization mass spectrometry. The improved efficient separation of boron from samples with a low boron content using SPE is applicable to a wide range of boron isotopic separations.

Correlation between δ 18 O, Sr/Ca and Mg/Ca of coral Acropora and seawater temperature from coral culture experiments

To be used as proxies of seawater surface temperature (SST), the δ18Oc values and Sr/Ca and Mg/Ca ratios of scleractinian coral skeletons must be verified by coral culture experiments in the laboratory. This paper describes a coral culture experiment that was conducted at several seawater temperatures T (21–28°C) using a tandem aquarium system and the new method for depositing coral skeletons grown under controlled conditions. The δ18Oc values and the Sr/Ca and Mg/Ca ratios of the cultured coral were measured. We concluded that the δ18Oc values and Sr/Ca and Mg/Ca ratios of the cultured coral are clearly correlated with T. The linear regression curve is δ18Oc(‰)=−0.1427×T(°C)−0.1495 (n=18, r=0.955, p<0.0001), and the slope of −0.1427‰/°C is at the low end of the range of published values (−0.13- −0.29‰/°C). The Sr/Ca ratio decreases with increasing T, whereas the Mg/Ca ratio increases with increasing T, indicating a negative correlation between Sr/Ca and Mg/Ca. Their linear regression curves are Sr/Ca(mmol/mol)=−0.04156×T+10.59 (n=15, r=0.789, p<0.005) and Mg/Ca (mmol/mol)=0.04974×T+2.339 (n=17, r=0.457, p<0.05), respectively, which demonstrate that when Mg/Ca and Sr/Ca are increased by one unit, T increases by 5.19°C and decreases by 15.62°C, respectively. These variations are significantly lower than published values.

Effective elimination of organic matter interference in boron isotopic analysis by thermal ionization mass spectrometry of coral/foraminifera: micro‐sublimation technology combined with ion exchange

In order to better estimate the effectiveness of micro-sublimation technology on the elimination of organic matter interference during boron isotopic analysis, a series of improved experiments was carried out using simple apparatus. Recovery rates after micro-sublimation were measured for boric acid solutions with different B contents or different B/organic matter ratios. The improved micro-sublimation procedure combined with ion-exchange technology was then used to test natural samples (coral and foraminifera) for the separation of boron. Our results show that the time taken for 100% recovery of different amounts of B differed and that the proportions of B/organic matter within the natural organic matter have little effect on the relationship between the recovery rates of B and the micro-sublimation times. The experiments further confirm that the organic matter does indeed have an effect on boron isotope analyses by positive thermal ionization mass spectrometry and that the use of micro-sublimation can effectively remove interferences from the organic matter during boron isotopic analysis.

Sample preparation for isotopic determination of boron in clay sediments

The procedures of sample preparation for isotopic determination of boron in clay sediments is very cumbersome, by far, there haven’t been relevant reports on that. In order to establish an effective method for sample preparation, a series of experiments were carried out. In this paper, boron in clay sediments was extracted with HCl solution and purified by two-step ion exchange method. Extracted HCl solution should be adjusted to alkalescency before passing through the Amberlite IRA 743 resin column due to the fact that Amberlite IRA 743 resin absorbs boron only from alkalescent solution. However, a mass of hydroxides of Al and Fe will be precipitated when the extracted HCl solution becomes alkalescent. Hydroxides of Al and Fe have a strong adsorption capacity for boron, which can cause boron isotope fractionation. To treat precipitated hydroxides of Al and Fe, four procedures, namely direct ion exchange (DRIE), decationizing ion exchange (DCIE), once sedimentation ion exchange (OSIE) and repeated sedimentation ion exchange (RSIE) were used and assessed. The influences of the four procedures on separation and extraction and isotopic composition of boron in experimental solutions and clay sediments were also discussed. According to the results, the DRIE, DCIE and OSIE are improper. The result of sample determination indicates that when extracting boron via RSIE, with the increase of precipitation times, there’s an obvious decrease in boron content in the precipitated hydroxides while a sharp increase in recovery of boron and it is favorable for weakening the influence of boron isotope fractionation. But the process of RSIE is time consuming and it may introduce boron. It needs further research to establish a more effective sample preparation method for isotopic determination of boron in clay sediments.