Matrix Separation Of Sr Isotope In Ca-rich Samples

Strontium isotope has been widely used in sequence stratigraphy,mineralogy,sedimentation,groundwater and other applications.The accurate determination of Sr isotope ratio by multi collector inductively coupled plasma mass spectrometry?MC ICP-MS?requires the separation of the analytes from the sample matrix before their measurement,in order to avoid matrix-related effects.The presence of Ca poses problems in Sr isotope ratio analysis as Ca based polyatomic interferences?Ca-argides and Ca-dimers?formed in the plasma potentially occur on all nominal Sr isotope masses resulting in biased isotope ratio measurements.Due to the similar chemical properties of calcium and strontium,it is difficult to completely separate calcium and strontium from existing sample purification methods,and traditional sample purification methods are cumbersome and time-consuming.Therefore it is necessary to develop a purification method based on matrix separation of Sr for isotopic ratio analysis of Ca-rich samples via an automated separation procedure.This paper presents a new purification method for strontium in Ca-rich samples.A self-made automatic solid-phase extraction system combined with AG50-X8 cation exchange resin was used to separate strontium from high calcium samples.Due to the different complex stability constants between calcium citrate and strontium citrate,the separation method uses a mixed solution of citric acid and ammonia to elute calcium and strontium.This method solves the problem that calcium and strontium are difficult to separate in the Ca-rich matrix,and the automatic separation method is adopted to greatly improve the experimental efficiency.First,the home-made automatic solid-phase extraction system is made of inert materials and has four parallel channels.It consists of an injection unit,an extraction unit,a sample collection unit,and a control unit.The sample can collect three active components simultaneously and the reagent flow paths are independent of each other.Elution flow rate,reagent selection,sample collection,etc.can be achieved through user-defined automation.Then,the optimal separation conditions of calcium and strontium under the condition of using AG50-X8 resin were discussed.The national standard sample GSR-6 with a calcium content of 2547?g was used as the sample for the elution conditions.The experimental results determined that the column diameter was 6mm,the amount of resin was 3.3 m L,and the elution flow rate of 0.5 m L/min was the optimal elution condition.First,1.2 mol/L hydrochloric acid was used to elute the constant elements,and then calcium strontium was eluted with a mixed solution of 0.04 mol/L citric acid and 0.75 mol/L ammonia water.In the end,calcium and strontium were completely separated,and the strontium recovery was between 97-105%,the method blank was between 0.011 and 0.072 ng,and the RSD is 2.4%,which meets the isotope test requirements.Finally,the purified calcium reagent was added to BCR-2 and NIST SRM 987 as simulated high calcium matrix samples to verify the reliability of the method.Samples are processed according to the method flow and detected by MC-ICP-MS.The results show that the simulated high calcium matrix BCR-2 measured value of ⁸⁷Sr/⁸⁶Sr was0.705024±0.000012?2?,n=50?,the recommended range of BCR-2 is 0.70500 to0.70510,and the measured value of the sample is within the recommended range.The simulated high calcium matrix NIST SRM 987 measure has an ⁸⁷Sr/⁸⁶Sr value of0.710291±0.000010?2?,n=50?.The recommended value range of NIST SRM 987is 0.71022-0.71030,and the results are also within the recommended value range.The residual amount of matrix in the strontium extraction solution is very low.Under this condition,the results of strontium isotope analysis will not be affected,and the method is feasible.