A method for analyzing the isotopic composition of ultra-trace element samples

Abstract

The present invention discloses a method for analyzing the isotopic composition of ultra-trace element samples. First, a standard solution with a high diluent / sample ratio is prepared, and the isotopic composition and diluent / sample ratio of the standard solution are obtained by a double diluent method; secondly, the calibrated standard solution is mixed with a standard substance of known isotopic composition in different proportions, and the measured data of the mixed sample is obtained through separation and purification and instrument testing, and the data is verified with the simulated error data to determine the optimal mixing range of the sample and the standard solution; finally, within the recommended mixing range, the calibrated standard solution is mixed with an unknown ultra-trace element sample, and after separation and purification and double diluent instrument testing, the isotopic composition of the unknown sample is calculated by a two-end member isotope mixing model. The present invention provides a better choice for isotopic analysis of low-content samples, and can significantly reduce the sample loading amount and the interference of buffer matrix elements.

Description

Technical Field

[0001] The invention belongs to the technical field of isotope geochemistry and analytical chemistry, and relates to a method for analyzing the isotope composition of an ultra-trace element sample. Background Art

[0002] In recent years, the application of non-traditional stable isotope analysis in the fields of geology, astronomy, environment, etc. has developed in depth. Accurate measurement of sample isotope composition has become the basis and premise of applied research in the above disciplines. The two bottleneck problems currently faced by non-traditional stable isotope analysis are: how to effectively reduce the sample concentration required for instrument analysis and reduce the blank of the experimental process, thereby reducing the sample loading amount. The former depends on the hardware conditions and injection method of the instrument; the latter depends on the cleanliness of the laboratory, the purity of the reagents and the pretreatment process. Updating the instrument hardware can improve the sensitivity and thus reduce the test concentration, but ultra-trace element samples often have stronger matrix effects and higher element interferences. The increase in sensitivity is also accompanied by significant matrix interference. The standard addition method is currently the most accurate test method for element content. Adding standard substances of known composition to the sample can effectively inhibit the matrix effect and blank effect. In isotope analysis, some scholars combine the standard addition method and the standard-sample cross method to analyze trace element samples, but because the accurate proportion of the two in the mixed sample after the addition of the standard cannot be effectively monitored, the data obtained by this method is of poor accuracy and insufficient promotion.

[0003] Compared with the standard-sample crossover method and the element external standard method, the isotope double diluent method can obtain high-precision isotope ratios of natural samples, and can also obtain accurate diluent / sample ratios and element concentrations. This method has a high tolerance for the recovery rate and matrix effect of the purification process, and is suitable for analyzing low-content samples with complex matrices. If the isotope double diluent method is combined with the standard addition method, the proportion of natural samples in the mixed sample can be accurately obtained by calculation, and the accuracy of the isotope data of the sample can also be greatly improved. The isotope double diluent-standard addition method disclosed in the present invention obtains the optimal mixing ratio range through numerical simulation, and is consistent with the measured data. This method can break through the minimum loading level determined by the current laboratory background and instrument hardware conditions, and effectively analyze the isotopic composition of ultra-trace element samples. Summary of the invention

[0004] In order to solve the problems existing in the above-mentioned prior art, the present invention provides a method for analyzing the isotopic composition of ultra-trace element samples. The technology is based on the isotope double diluent-standard addition method, by adding a known high diluent to the sample (diluent / standard is P spk / std ) standard solution, add the sample to obtain a low diluent to sample ratio (diluent / (sample + standard) P spk / (std+spl)) mixed solution, test the isotopic composition of the mixed solution and the accurate ratio of diluent to sample, and use the two-end member isotope mixing model to calculate the accurate isotopic composition of the sample.

[0005] The present invention provides a method for analyzing the isotopic composition of an ultra-trace element sample, comprising the following steps:

[0006] Step 1: According to the "U-shaped" error curve characteristics of the isotope double diluent method, a high diluent to sample ratio (diluent / standard, P spk / std ) standard solution;

[0007] The step 1 further comprises the following steps:

[0008] Step 1.1, according to the "U-shaped" error curve characteristics of the isotope double dilution method, determine the optimal range of the diluent-sample ratio;

[0009] Step 1.2, prepare a standard solution with a high diluent to sample ratio according to the range obtained in step 1.1.

[0010] Step 2, accurately calibrating the isotopic composition of the standard solution and the ratio of the diluent to the standard solution on a multi-collector inductively coupled plasma mass spectrometer;

[0011] The step 2 further comprises: using a double diluent bird's nest iterative algorithm to perform mass discrimination correction on the isotope ratio obtained by the mass spectrometer, and obtaining the accurate isotope composition of the standard solution and the ratio of the diluent to the standard solution P spk / std .

[0012] Step 3, using the Monte Carlo method to simulate the error introduced by the sample-standard mixed solution in the mass spectrometry analysis process under different sample ratios, and preliminarily obtain the ratio parameters of the sample and the standard solution;

[0013] The step 3 further comprises the following steps:

[0014] Step 3.1, establish the isotopic composition of the sample and the isotopic composition of the standard solution, the sample-standard mixed solution and the diluent ratio (double diluent / (standard + sample), P spk / (std+spl) )’s mathematical model relationship;

[0015] Step 3.2, according to the formula obtained in step 3.1, the Monte Carlo method is used to simulate the mass spectrometry analysis process, and the different sample proportions (f spl ) isotopic composition (δ spl ) error increment value;

[0016] Step 3.3, based on the isotopic composition (δ spl ) and the sample proportion (f spl) is taken as the object, and a two-dimensional error curve is drawn to obtain the best mixing range. The ideal ratio parameter is the sample proportion interval corresponding to the range with the smallest error.

[0017] Step 4: Mix the calibrated standard solution with the geological standard material of known isotopic composition in different proportions, obtain the measured data of the mixed sample through separation, purification and mass spectrometry analysis, and verify it with the simulated error data of step 3 to determine the optimal mixing range or ratio of the standard solution and the sample (sample / (sample + standard solution), f spl );

[0018] The step 4 further comprises the following steps:

[0019] Step 4.1, based on the standard solution prepared in step 1, use the digested geological standard substance solution as the sample and prepare different f spl Mixed samples of values;

[0020] Step 4.2: Measure the mixed sample prepared in step 4.1 on the mass spectrometer, obtain the real data of the mixed sample according to the double diluent method, and calculate the accurate series f by the two-end isotope mixing model. spl and δ spl Value; the calculated series δ spl The value is compared with the recommended value and then verified with the simulation result obtained in step 3 to determine a reliable mixing range.

[0021] Step 5: Based on step 4, the optimal mixing range or ratio f of the sample is obtained. spl , a precisely calibrated standard solution is mixed with an unknown ultra-trace element sample. After separation, purification and instrumental testing, the isotopic composition of the mixed sample and the accurate ratio of the diluent to the sample are obtained using the double diluent method, and then the isotopic composition of the unknown sample is obtained through the two-end member isotope mixing model.

[0022] In step 5, when the accurately calibrated standard solution is mixed with the unknown ultra-trace element sample, due to the error in the unknown sample concentration test, a moderate sample ratio is usually selected to avoid the occurrence of marginal sample ratio values. At the same time, for relatively reliable accuracy, the number of analyses is preferably 3-5 times.

[0023] The isotope double diluent-standard addition method proposed in the present invention provides a method for analyzing the isotopic composition of ultra-trace element samples. It is not only simple and quick in operation, but also breaks through the minimum sample loading amount determined by the laboratory background level. Moreover, the standard addition makes the mixed sample have a lower ratio of interfering elements and matrix elements than the original sample, so that the separation and purification method has more options. The present invention breaks through the loading amount limit required for isotope analysis, expands the sample range of isotope analysis, and will greatly promote the application of isotope analysis technology in multiple research fields such as geology, astronomy, environment and ecology. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a flow chart of the present invention.

[0025] Figure 2 is a schematic diagram of the error simulation of Cd isotopes using the Monte Carlo method in the embodiment;

[0026] Figure 3 is a schematic diagram of the error simulation of Mo isotopes using the Monte Carlo method in the embodiment;

[0027] Figure 4 is a graph showing the relationship between the error of isotope analysis of samples with different ratios and the ratio of diluent to sample in the embodiment;

[0028] Figure 5 is the Cd isotope test result of the geological standard material NIST SRM 2711a in the embodiment;

[0029] Figure 6 It is the Mo isotope test result of the geological standard material NOD-P-1 in the embodiment. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0031] The invention provides a method for analyzing the isotopic composition of an ultra-trace element sample, which is applicable to all elements (stable isotope number ≥ 4) in the periodic table that can be analyzed using an isotope double diluent technique.

[0032] Figure 1 FIG. 1 is a flow chart of a method for analyzing the isotopic composition of ultra-trace element samples according to the present invention. Based on the isotope double diluent method and the standard addition method, the method of the present invention is described below by taking Cd and Mo as examples. Figure 1 As shown, the method comprises the following steps:

[0033] Step 1, preparing a solution according to the "U"-shaped error curve of the double dilution method;

[0034] The step 1 further comprises the following steps:

[0035] Step 1.1, determine the optimal range of sample / diluent ratio for the double diluent method;

[0036] After the double diluent is selected, there is a "U"-shaped relationship between the ratio of the diluent to the sample and the error introduced by the double diluent method. Too high or too low a diluent will introduce a large error, which will lead to a deterioration in sample accuracy and precision. Therefore, the diluent / sample ratio should be kept within a certain range to ensure that the accuracy and precision of the sample within this range are optimal. This range was previously called the normal spike. A ratio lower than this is called an under spike, and a ratio higher than this is called an over spike. 111 Cd- 113 Take Cd double diluent as an example, 111 Cd spike / 112 Cd sample (P spk / spl )The best range is 0.8-6.0, and the best ratio is 2.0. 97 Mo- 100 Mo double diluent, 100 Mo spike / 98 Mo sample (P spk / spl )The best range is 1.2-8.0, and the best ratio is 2.75.

[0037] Step 1.2, preparing a standard solution according to the optimal ratio range obtained in step 1.1.

[0038] The preparation of standard solutions is preferably based on pure, high-concentration single-element standard solutions; the second choice is high-concentration natural samples, which are separated and purified to make standard solutions. NIST SRM 3108, Münster Cd, BAM-I012 and Spex Cd are the most common Cd isotope standard solutions. Their isotope compositions are accurately determined by different laboratories around the world and are suitable for preparing the secondary standard solutions we need. 114 / 110 Cd value solution, NIST SRM 3108 can be mixed with Münster Cd; if negative δ is required 114 / 110 The solution of Cd value can be mixed with NIST SRM 3108 and BAM-I012 or Spex Cd. 111 Cd spike / 112 Cd sample The best ratio is 2. When the standard solution is prepared, 111 Cd spike / 112 Cd sample When the standard solution with double diluent added is mixed with the sample, the mixed solution 111 Cd spike / 112 Cd sample It will fall within the range of 0-4.0. When the sample proportion is 50%, 111 Cd spike / 112 Cd sample About 2.0. In order to analyze low-content samples, the sample ratio is usually as small as possible to reduce the actual sample loading amount, usually less than 50%. In other words, the final mixed solution 111 Cd spike / 112 Cd sample Mainly in the range of 2.0-4.0. Similarly, according to the double dilution error curve of Mo, the Mo standard solution 100 Mo spike / 98 Mo sample The value is chosen to be around 5.5.

[0039] Step 2, based on the high diluent / sample ratio (P spk / std ) standard solution, and its accurate isotopic composition and diluent / sample ratio (P spk / std ). For example, assuming that the Cd isotope standard solution to be tested consists of N mol standard solution and S mol 111 Cd- 113 Cd double diluent is mixed, and the standard solution and double diluent are 110 Cd, 114 The abundances of Cd isotopes are 110 Cd n , 114 Cd n and 110 Cd s , 114 Cd s . Prepared double diluent-standard mixed solution 110 Cd, 114 The Cd isotope abundance is denoted by 110 Cd m , 114 Cd m The isotope ratios obtained by the instrument test Substituting into the iterative formula of the double diluent method (Formula (1), (2), (3)) we can get the accurate diluent composition: Ratio to standard solution Then we calculate δ114 / 110 Cd and 111 Cd spike / 112 Cd sample When calibrating a standard solution of Mo isotope, its accurate δ 98 / 95 Mohe 100 Mo spike / 98 Mo sample .

[0040]

[0041]

[0042]

[0043] in and are the isotope ratios of the standard solution with double diluent and without diluent, respectively.

[0044] Step 3, based on the mass spectrometer hardware parameters and the isotopic composition of the prepared standard solution, the Monte Carlo method is used to simulate the errors introduced by different sample ratios in the mass spectrometry analysis process to obtain preliminary ratio parameters;

[0045] The step 3 further comprises the following steps:

[0046] Step 3.1, based on the two-terminal isotope mixing model, establish the relationship between the isotopic composition of the sample and the isotopic composition of the standard solution, the mixed solution, and the ratio of the diluent to the sample. Taking Cd isotope as an example, the isotopic composition of the standard solution (δ 114 / 110 Cd) and diluent ratio ( 111 Cd spk / 112 Cd std ) are denoted as δ std and P std , the isotopic composition of the mixed solution after adding the sample (δ 114 / 110 Cd) and diluent ratio ( 111 Cd spk / 112 Cd std+spl ) are denoted as δ mix and P mix ,but

[0047] 111 Cd spk / 112 Cd std =P std (4)

[0048] 111 Cdspk / 112 Cd std+spl =P mix (5)

[0049] Combining equations (4) and (5), we can obtain

[0050] 112 Cd std / 112 Cd std+spl =P mix / P std (6)

[0051] Assume that the Cd content of the sample in the mixed solution is f spl , the proportion of standard solution is f std , the true δ of the sample 114 / 110 The Cd value is δ spl According to the isotope two-terminal mixing model,

[0052] f spl +f std =1 (7)

[0053] δ mix =f spl *δ spl +f std *δ std (8)

[0054] Combining (6) and (7), we can get

[0055] f std =Cd std / Cd std+spl = 112 Cd std / 112 Cd std+spl =P mix / P std (9)

[0056] f spl =1-f std =1-P mix / P std (10)

[0057] Substituting (9) and (10) into (8), we can obtain

[0058] δ spl =(P std *δ mix -P mix *δ std ) / (P std -P mix ) (11)

[0059] Step 3.2, according to the formula obtained in step 3.1, use the Monte Carlo method to simulate the error introduced by the sample-standard mixed solution under different sample ratios in the mass spectrometry analysis process, and preliminarily obtain the ratio parameters of the sample and the standard solution. The principle of the Monte Carlo method is to transform a set of uniformly distributed data into a set of normally distributed data. According to previous studies, the data in the process of isotope determination by the mass spectrometer follows a normal distribution. Therefore, we can simulate the instrument's measurement values ​​based on the characteristic parameters of the instrument itself and the Monte Carlo method. Finally, combined with the formulas in step 3.1, the error of the sample proportion and isotope composition is calculated based on each set of simulated data;

[0060] Step 3.3, based on the isotopic composition (δ spl ) and the sample proportion (f spl ) is taken as the object, and a two-dimensional error curve is drawn. The ideal ratio parameter, i.e., the range with the smallest error, corresponds to the optimal sample ratio interval.

[0061] In this step, f spl is the horizontal axis, and the isotopic composition (δ spl ) as the ordinate, and a two-dimensional error curve is drawn, such as Figure 2 , Figure 3 As shown, Figure 2 is the two-dimensional curve of Cd isotope composition error-sample ratio, Figure 3 The error curve of Mo isotope composition and sample ratio is shown in the figure. As can be seen from the figure, both the error curves of Cd isotope and Mo isotope show a "U-shaped" feature, that is, when the sample ratio is too high or the standard solution ratio is too high, the error will increase rapidly. Considering the need for low-content sample analysis, a medium to small sample ratio should be selected in actual operation, that is, 20%-50%.

[0062] Step 4: Mix the calibrated standard solution with the geological standard material of known isotopic composition in different proportions, obtain the measured data of the mixed sample through separation, purification and mass spectrometry analysis, calculate the error of the isotopic composition of the geological standard material and its accurate proportion in the mixed sample through the isotope two-terminal mixing model, and obtain the optimal sample mixing range;

[0063] The step 4 further comprises the following steps:

[0064] Step 4.1, based on the standard solution prepared in step 1, take the digested geological standard substance solution as the sample and prepare different sample proportions (f spl ) of a mixed sample;

[0065] Step 4.2, after separation and purification of the mixed sample prepared in step 4.1, the isotopic composition and diluent / sample ratio of the mixed sample are obtained by double diluent method on a mass spectrometer, and then the accurate sample proportion and isotopic composition are calculated by the two-end member isotope mixing model. spl is the horizontal axis, and the isotopic composition (δ spl ) is taken as the ordinate, and a two-dimensional error curve is drawn, such as Figure 4 As shown, Figure 4 This is a two-dimensional curve of the error of the measured geological standard material Cd isotope composition-sample ratio. It is verified with the simulation results obtained in step 3 to determine the optimal sample ratio range.

[0066] Step 5: Based on the sample proportion range obtained in step 4, a precisely calibrated standard solution is mixed with the target sample, and the isotopic composition of the unknown sample is calculated by testing the isotopic composition of the mixed sample and the diluent / sample ratio.

[0067] In this step, when analyzing actual samples, due to the error in the concentration test of unknown low-content samples, a moderate sample ratio is usually selected to avoid the occurrence of marginal sample ratio values. At the same time, for relatively reliable accuracy, 3-5 times of analysis is preferred. Figure 5 , Figure 6 As shown, Figure 5 This is the result of testing the Cd isotope composition of the international geological standard soil NIST 2711a using this method. Figure 6 The results of using this method to test the Mo isotope composition of the international geological standard material manganese nodule NOD-P-1. The gray shadows in the two figures represent long-term accuracy. It can be seen from the two figures that this method is effective and reliable for the long-term analysis of Cd isotopes and Mo isotopes.

[0068] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for analyzing the isotopic composition of ultra-trace element samples, characterized in that: The following steps are involved: Step 1: According to the "U-shaped" error curve characteristics of the isotope double diluent method, prepare a standard solution with a high diluent / sample ratio. The diluent / standard is P spk / std ; The step 1 comprises the following steps: Step 1.1, according to the "U-shaped" error curve characteristics of the isotope double diluent method, determine the optimal range of the sample / diluent ratio of the double diluent method; Step 1.2, preparing a standard solution with a high ratio of diluent to sample within the optimized range according to the range obtained in step 1.1; Step 2, accurately calibrating the isotopic composition of the standard solution and its ratio with the diluent using a double diluent method on a multi-collecting cup inductively coupled plasma mass spectrometer; The step 2 comprises: using a double diluent bird's nest iterative algorithm to perform mass discrimination correction on the isotope ratio measured by the mass spectrometer, and obtaining the accurate isotope composition of the standard solution and the diluent-sample ratio P spk / std ; Step 3, using the Monte Carlo method to simulate the error introduced by the sample-standard mixed solution in the mass spectrometry analysis process under different sample ratios, and preliminarily obtain the mixing ratio parameters of the sample and the standard solution; The step 3 comprises the following steps: Step 3.1, establish a mathematical model of the isotopic composition of the sample, the isotopic composition of the standard solution, the isotopic composition of the sample-standard mixed solution, and the ratio of the diluent to the sample, where diluent / (standard + sample) is P spk / (std+spl) ; According to the two-terminal isotope mixing model, the relationship between the isotopic composition of the sample and the isotopic composition of the standard solution, the isotopic composition of the mixed solution, and the ratio of the diluent to the sample is established; the isotopic composition of the standard solution and the ratio of the diluent are respectively denoted as δ std and P std The isotopic composition and diluent ratio of the mixed solution after adding the sample are recorded as δ mix and P mix , then we get the true value of the sample δ spl : d spl = (P std * d mix - P mix * d std ) / (P std - P mix ) Step 3.2: According to the formula obtained in step 3.1, the Monte Carlo method is used to simulate the mass spectrometry analysis process, and the proportion of different samples is f spl The isotopic composition of the sample δ spl The error increment value; Step 3.3, using the isotopic composition δ spl The error and sample proportion f spl Take the sample as the object, draw a two-dimensional error curve, and obtain the best mixing range. The ideal ratio parameter is the sample proportion interval corresponding to the minimum error range. Step 4: Mix the calibrated standard solution with the geological standard material with known isotopic composition in different proportions. After separation, purification and mass spectrometry analysis, the measured data of the mixed sample is obtained, which is verified with the error data simulated in step 3 to determine the optimal mixing range of the sample and the standard solution. Sample / (sample + standard solution) is f spl ; The step 4 comprises the following steps: Step 4.1, based on the standard solution prepared in step 1, use the digested geological standard substance solution as the sample and prepare different f spl Mixed samples of values; Step 4.2: Measure the mixed sample prepared in step 4.1 on a mass spectrometer and calculate the accurate f using the double diluent method based on the measured data. spl and δ spl value; compare the calculated isotopic composition of geological reference materials with the recommended value, and then verify it with the simulation results obtained in step 3 to determine the optimal sample proportion range; Step 5, based on the optimal mixing range obtained in step 4, a precisely calibrated standard solution is mixed with the unknown ultra-trace element sample within the range, the isotopic composition of the mixed sample and the ratio of the diluent to the sample are tested by a double diluent method, and the isotopic composition of the unknown sample is calculated by a two-end member isotope mixing model; In step 5, a precisely calibrated standard solution is mixed with an unknown ultra-trace element sample, separated and purified, and tested by a double diluent method instrument, analyzed 3-5 times, and then the accurate and high-precision isotopic composition of the unknown sample is calculated using a two-end member isotope mixing model.

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