Some Difficulties and Status in the Application of X-Ray Spectrometry in Geological Analysis: A Review
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摘要:
X射线荧光光谱法(XRF)具有无损、快速、环保和分析精度高等特点，常作为地质样品中主量和微量元素分析的首选方法。然而，由于地质样品的矿物组成、物理结构特征(如尺寸、形状和分层等)和化学成分(如元素组成、化学形态等)的复杂性与多样性，XRF在地质样品分析的实际应用中存在一些技术难点。本文从小样品量和珍贵样品的分析、XRF的散射效应的应用、易挥发元素分析、变价元素分析和稀有金属分析等方面，对XRF在地质分析中的难点进行了总结与评述。指出制备易于保存和便于反复测量的小尺寸样片是小样品量和珍贵样品XRF分析的合适方法；XRF散射效应可用于成分未知的样品中更多化学成分信息的获取以及异质性样品原位分析误差的校正；超细粉末制样、稳定剂的加入和标准加入法建立工作曲线是解决易挥发元素XRF分析困难的方法。认为元素的特征X射线相对强度可用于变价元素价态和形态的分析；优化校准曲线、降低熔融制样的稀释比、高压激发和改善谱线重叠干扰是解决稀有金属分析困难的有效途径。
要点(1) 制备便于保存和反复测量的小尺寸样片是XRF定量分析小样品量和珍贵样品的关键。
(2) XRF散射效应能够为成分未知样品的分析、异质性样品原位分析的误差校正提供有力贡献。
(3) 超细粉末制样、稳定剂的加入和标准加入法建立工作曲线是利于XRF准确测定易挥发元素的有效方法。
(4) 采用人工配置的标样或与样品性质相似的二级标样建立并优化工作曲线、降低熔融制样的稀释比、高压激发和改善谱线重叠干扰是解决稀有金属XRF分析难题的有效途径。
HIGHLIGHTS(1) Preparation of small beads or pellets which are easy to preserve and repeated measurement is the key for quantitative analysis of small size samples and precious samples with XRF.
(2) The XRF scattering effect can provide a powerful contribution to the analysis of samples with unknown composition and the error correction of the in situ analysis of heterogeneous samples.
(3) Preparation of ultrafine powder pellet, addition of stabilizer and standard addition method establishing work curve are effective approaches for volatile element analysis by XRF.
(4) Constructing and optimizing the work curve with artificial standard samples or secondary standard samples, preparing low-dilution (sample to flux ratios) glass beads, exciting samples at high X-tube voltage and overcoming overlap interference of spectral lines are effective ways to analyze the rare metals with XRF.
Abstract:X-ray fluorescence spectrometry (XRF) has become one of the widely used methods for main and trace elements analysis in geological samples, due to its characteristics of non-destructive, fast, environmentally-friendly and high analytical precision. Currently, XRF can qualitatively and quantitatively analyze most of the major and trace elements (4Be−92U, especially 10Na−92U) with the concentration ranges from μg/g to percent. However, there are still some difficulties in practical analysis of geological samples with XRF due to the complexity and diversity of mineral composition, physical structural characteristics (e.g. size, shape, delamination and inclusions) and chemical composition (e.g. elemental composition, chemical morphology) of geological samples. This paper elaborates difficulties and corresponding possible solutions of XRF analysis in geological samples from five aspects including small size samples or precious samples analysis, the application of scattering effect, the analysis of volatile elements, variable valence elements and rare metals. Finally, the limitations and challenges of the XRF technique that still exist in the geological analysis are presented. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202403150052.
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Keywords:
- X-ray fluorescence spectroscopy /
- small size samples and precious samples /
- scattering effect /
- volatile elements /
- variable valence elements /
- rare metals
BRIEF REPORTX-ray fluorescence spectrometry (XRF) analysis of geological samples is often performed using solids or powder, which can avoid the time-consuming complex pretreatment process of wet chemical technology and the use of a large number of toxic and harmful chemical reagents[2-5]. Nowadays, XRF has been widely used in the fields of petrology, geochemistry, chronology, mineral resources and environmental geoscience[9-11]. However, there are often some difficulties in the practical analysis of geological samples using XRF due to the complexity of geological samples and the characteristics of the elements themselves. Some difficulties and the corresponding possible solutions in geological analysis with the method of XRF are reviewed here, including small size samples or precious samples analysis, the application of scattering effect, the analysis of volatile elements, variable valence elements and rare metals.
1. Analysis of small size samples and precious samples
Preparation of small beads or pellets which are easy to preserve and repeated measurement is the key for XRF quantitative analysis of geological samples on a small scale. Sometimes, it is necessary to analyze small size samples or precious samples with XRF, such as meteorites or extraterrestrial samples (for example, lunar samples) as well as banded iron formation[12], loess strata[13] and periodically banded rocks within zebra rock[14] which can be drilled from banded positions using micro drilling. Such samples can only be qualitative or semi-quantitative by scanning electron microscopy, micro X-ray fluorescence spectrometry or laser ablation-inductively coupled plasma-mass spectrometry as in previous studies. Some researchers have found that preparation of small beads or pellets which are easy to preserve and repeated measurements yield more accurate quantification with XRF.
2. Application of the scattering effects
Scattering effects are used to obtain more information about the chemical composition in samples with unknown elemental composition. The calibration curve of Compton-to-Rayleigh intensity ratio versus average atomic number can provide important information when samples with unknown compositions are studied under the same geometrical conditions and at the same energy. First, construct a calibration curve of Compton-to Rayleigh intensity ratio with respect to the average atomic number using reference materials with well-known chemical composition. Then, the concentration of the elements might be indirectly obtained from scattering X-ray peaks of the samples[19]. This method of the evaluation of an unknown specimen is particularly sensitive for a light matrix.
Scattering effects are beneficial for matrix correction of heterogeneous sample in situ analysis. Researchers have studied the element distribution characteristics of bryophyte-soil-rock interface samples[20]. It was found that the correction method with Ray*Ray/Com is suitable for CaKα, MnKα, FeKα, CuKα, ZnKα and PbLα whereas the correction with Ray/Com is good for KKα. When the elements’ intensity was corrected by the respective suitable methods, the spectra of K, Ca, Pb, Zn and Cu reached the maximum peaks at the soil layer in interface samples. This exploration is useful for the study of biogeochemical interface processes of the interest elements.
3. Analysis of the volatile elements
Volatile elements such as C, S and halogen are difficult to analyze accurately with XRF in both the fusion method and pressed powder pellet in general conditions. The fusion method often leads to the measured value drop. Whereas the measurement accuracy is not ideal either using the pressed powder pellet because of the influence of mineral effect and particle effect. Researchers found that the mineral effect and particle effects can be minimized by ultra-fine grind, and that the uncertainty of the analysis results reduced in response[21]. Besides, The addition of stabilizer can inhibit the volatilization of volatile elements such as S at high-temperature in the melting process, so the analytical accuracy can also be improved[26].
4. Analysis of the variable valence elements
The characteristic X-ray spectral peak position, and the shape and relative intensity of the spectral line can be affected by the atomic valence states and coordination states[32-33]. Some researchers used the relative intensity of Fe Kβ5/Kβ1,3 as an analytical parameter to analyze the content of divalent iron (FeO) in igneous rocks. Chubarov, et al.[33] analyzed the valence state and form of manganese in various kinds of manganese ore by S8 Tiger-type wavelength dispersive X-ray fluorescence spectrometer, so that the quality of manganese ore can be accessed quickly without complex processes.
5. Analysis of the rare metals
Constructing and optimizing the work curve with artificial standard samples or secondary standard samples are effective ways to analyze the rare metals with XRF[36]. Zhou et al.[37] expanded the word curve linear range of La, Ce and Y by adding high purity rare earth oxides La2O3, CeO2 and Y2O3 in the analysis of rare earth minerals and mineralized samples. Silva et al.[38] added the secondary standard to establish the work curve and used the empirical coefficient method to optimize the calibration curve for analysis of La, Ce, Nd, Sm and Gd in Amazon cassiterite tailings. The results of precision and accuracy tests using the CRM (Diorite Gneiss-CCRMP) was satisfactory. Coefficients of variation of five analyzed elements except Gd were less than 5%. The analytical recovery of five elements were between 103% and 116%.
Preparing low-dilution (sample to flux ratios) glass beads facilitates the determination of rare metals using XRF. Nakayama et al.[40] determined 42 components in felsic rocks using XRF. Low-dilution glass beads with 1∶1 of sample-to-flux ratios were prepared to measure Sc, Sn, Cs, Hf, Ta and rare earth elements. Calibration curves of the components showed good linearity (r=0.991−1.000). Ichikawa et al.[41] developed a low-dilution glass-bead method to determine 34 components including rare metals in basaltic and granitic rocks using XRF. The calibration curves present good linearity (r>0.990).
High-energy excitation may help the determination of rare metals in geological samples. Generally, the L line of rare elements is selected as the analytical line because the K line of rare elements cannot be excited by conventional X-ray tubes. However, high-energy polarization XRF can effectively stimulate the K series of rare metals. The overlap interference of K line of rare metals is less and the excitation factor of the K line is larger than the L line, so the excitation efficiency is greatly improved. Researchers developed a method for multi-element analysis of rare earth elements in soil, rock and deposits using high-energy polarized energy dispersive X-ray fluorescence spectrometer. The excitation voltage of the rare earth elements is up to 100kV. The calibration lines show great linearity (the correlation coefficients r>0.99 for La, Ce, Pr, Nd and Y, the rest of rare earth elements r>0.969)[44].
Overlap interference correction of the spectral lines sometimes plays a key role in rare metals analysis [42,45]. For example, Maritz et al.[45] analyze the trace elements in soil. There were significant differences between the measured values and recommended values of Hf and Ta elements because of the ignoring of overlap interference between Cu Kα1 lines and Hf Lα1 and Ta Lα1 lines. Some other researchers fully considered the overlap interference of Ta Lα1 (0.1522nm) and Cu Kα1 (0.1541nm) in the determination of Ta, Cs and other rare metals in rocks[42]. The measurement bias was not significant according to the criterion of negligible error and student’s distributions.
6. Challenges and perspectives
The limitations and challenges of the application of XRF technology in geological analysis are still prominent: (1) The accuracy of analysis results depends on standard material, especially for geological samples with complex composition, which requires samples with similar physical and chemical properties to participate in the calibration curve; (2) It is still difficult to quantitatively determine the light elements whose atomic number Z<8. In the future, with advances in particle physics, optical systems and detection devices, it is believed that these limitations will be overcome. In addition, with the integrated development of earth sciences in recent years, XRF technology has also developed toward a broader and more dimension-focused direction. New technologies related to XRF, such as micro XRF, X-ray absorption spectrum and portable XRF, are playing an increasingly important role in geoscience field.
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地质类样品(例如天然岩石)被认为是母岩、气候-环境条件和人为条件相互作用的产物［1］，矿物成分和结构变化很大。地质样品的元素定量分析一般采用原子吸收光谱(AAS)、原子荧光光谱(AFS)和电感耦合等离子体质谱/发射光谱(ICP-MS/OES)等方法［2-4］。这些湿化学分析方法，往往需要开放式和高压釜酸分解等复杂且耗时的预处理过程，并且在固体样品液化过程中需要使用大量有毒有害的化学试剂。如在开放式酸溶条件下，低温和常压可能会导致耐酸矿物分解不完全；高压釜的使用有利于样品分解，但随之增加了样品污染和损失的风险［5］。此外，酸消化法一个最明显的缺点是不能测量主量元素硅，因为样品中的硅酸盐被氢氟酸分解而形成易挥发的四氟化硅，从而导致硅元素的损失。碱熔可实现完全分解，但熔融材料必须在酸中重新溶解才能上机测试。
XRF技术测定地质类样品时采用固体进样，能够有效地避免上述问题［6-8］。例如，在测定锡矿石中的锡时［6］，采用粉末压片制样，通过加入硼酸、氧化镧降低基体效应和稳定样品总质量吸收系数，建立的分析方法的标准曲线线性良好(R2=0.9989)，测量准确度高(△lgC＜0.04)，精密度良好(RSD为0.39%～1.2%)，符合地质样品分析规范要求。又如，测定富硒土壤中的硒时［7］，增加按比例混合配置的混合标准物质参与建立校准曲线，解决了硒测定含量范围不足的问题，结果表明高硒土壤样品的XRF测定值与AFS测定值基本吻合，满足硒含量在3μg/g以上的样品的定量分析要求。此外，有研究者将XRF技术与其他分析方法结合共同表征矿物的元素含量、物相和矿相组成特征［8］。例如，在探究不同产地铜精矿样品的矿物学特征时，XRF分析发现样品中主要元素为铜、铁、硫和氧，普遍含有锌、硅、铝、镁、钙和铅，通过Cu/Fe与Cu/S含量比的不同来区分不同产地的铜精矿的物相含量的差异，从而利于进口铜精矿的风险识别和管控。
随着X射线光管、探测器等关键技术元件的更新和基体校正方法、计算机技术的进步，XRF分析方法的检出限不断改善，可测量元素的范围也不断扩大，目前XRF技术可对大部分主微量元素(4Be~92U，特别是10Na~92U)进行定性和定量分析，浓度范围可从百分含量到μg/g，在岩石学、地球化学、年代学、矿产资源及环境地球科学等领域获得越来越广泛的应用［9-11］。然而，由于地质样品的复杂性和测量元素自身的特性，在XRF的实际分析中常面临一些难题。本文针对XRF在地质样品分析中遇到的若干难点，从小样品量和珍贵样品的分析、XRF散射效应的应用、易挥发元素分析、变价元素分析和稀有金属分析等方面进行总结与评述。
1. 小样品量和珍贵样品的分析
地质样品的XRF分析中，小样品量和珍贵样品的分析是个难点。如陨石或外星(例如月球)样品等稀有样品；又如带状铁层［12］、黄土地层［13］样品和斑马岩中周期性带状岩石样品［14］，此类样品往往通过微钻探在带状位置打钻获取。在以往的研究中，此类样品往往只能通过扫描电子显微镜、微区X射线荧光光谱或激光剥蚀电感耦合等离子体质谱等方法进行定性或半定量分析。若能在小样品量情况下制成易保存、便于反复测量的样品，将会大大扩展XRF在地学领域的应用范围。
有学者尝试了制备小尺寸玻璃熔片解决此类样品的分析问题［15-16］。Nakayama等［15］用一种小容量的Pt-Au坩埚制备了较小尺寸(12.5mm)的玻璃片，样品和熔剂用量分别为11mg和396mg。结果表明，质量分数＞10%的分析物，RSD＜3%；质量分数为1%～10%的分析物，RSD＜5%；质量分数为0.1%～1%的分析物，RSD＜15%，各元素的校准曲线线性均较好(相关系数r＞0.998)。Ichikawa等［16］提出一种微玻璃珠熔融-XRF法分析硅质地质样品中的主量元素，样品和四硼酸锂熔剂的用量分别为1.1mg和11mg，制样过程如图1a所示。熔融成的微玻璃珠直径约3.5mm、高度0.8mm，将微玻璃珠安装在常规大小的空白玻璃样片(直径35mm)表面(图1b)待测。研究发现，相较于平坦表面，微玻璃珠的半球面能发射出更强的X射线荧光，同时分析结果的RSD更小，可能是平坦表面存在气泡且测定平面时半球面与空白玻璃片的粘贴难以完全一致(图1c)。值得注意的是，由于样品量和分析物尺寸很小，每个样品的荧光强度均弱于传统样片获得的XRF强度，使得本来在常规尺寸玻璃片下可忽略不计的干扰线对分析线产生了干扰。如Cl Kα对Mg Kα的干扰、来自于靶材的 Rh Lγ2和Rh Lγ3对K Kα的干扰、K Kβ对Ca Kα的干扰等，此时选择细准直器(150μm)测量Mg Kα、K Kα和Ca Kα能取得较好效果。校准曲线相关系数r＞0.997，准确度(＜5%)和重现性(≥95%，n=10)表现优异。
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图 1 (a)制备微玻璃珠试样时，将混合粉末放置在Pt-Au坩埚中的工艺原理图；(b)用硅胶聚合物黏合剂将微玻璃珠(直径约3.5mm)安装在直径35mm的空白玻璃片上；(c)附着在35mm空白玻璃片上的微玻璃珠的两个表面：(1)平面；(2)半球面。修改自文献［16］Figure 1. (a) Schematic diagram of process to place the mixture powder in a Pt crucible for preparation of a micro glass bead specimen; (b) Top-view photograph of the micro glass bead (approximately 3.5mm diameter) after mounting on the 35mm glass bead blank using silicone polymer adhesive; (c) Comparison of the two available analytical surfaces of the micro glass bead attached on the 35mm glass bead blank: (1) Flat-surface and (2) Hemispherical-surface. Modified from the reference［16］.不同种类的元素可采用不同的制样方法。例如，Gazulla等［17］在小样品量情况下，针对不同元素分别开发了熔片法(针对主量元素)和压片法(针对氯和硫等易挥发元素)以保证既不浪费样品又能达到最佳分析效果。经条件实验，熔融法的最佳制备条件为：0.015g样品(比常规熔片法样品量缩小30～40倍)，稀释比1∶10，脱模剂0.20mL。压片法的最佳制备条件为：0.1000g样品(比常规压片法样品量缩小约100倍)和0.25mL黏结剂(17%甲基丙烯酸正丁酯的丙酮溶液)。用标准物质SRM98b和GBW03103对熔片法进行不确定度评估，用GBW03104和GBW07407对压片法测定氯进行不确定度评估，用BCS-CRM381和SRM2709对压片法测定硫进行不确定度评估，发现标样的测量平均值与推荐值的绝对偏差Δm均低于扩展不确定度UΔm，说明分析结果与推荐值之间不存在显著性差异。
2. X射线荧光光谱散射效应的应用
康普顿散射(Compton scattering)和瑞利散射(Rayleigh scattering)能够提供基体和电子密度信息［18］。康普顿散射与瑞利散射强度的比值(ICo/IRa)与原子序数(Z)有很好的关系［19］。在实际分析中，XRF的散射效应可用于解决地学分析中两方面的难题：①获得成分未知的样品中更多关于化学成分的信息；②校正异质性样品原位分析的误差，使感兴趣元素的分析结果更为准确。
2.1 成分未知样品的分析
在相同的几何条件和相同的能量下，对成分未知的样品进行研究时，ICo/IRa与Z的校准曲线可提供重要信息。这种方法首先需要评估已知化学成分的参考物质的Z与ICo/IRa的对应关系，进而通过从样品谱图中提取的散射线强度信息间接地得出分析元素的浓度［19］。该方法对于轻基体尤为敏感，因此对于平均原子序数略有差别的轻元素，可在不评估其特征X射线的情况下，而是通过比较ICo/IRa，将它们区分开来。此外，使用ICo/IRa的一个实际优点是，在相同的散射几何条件但不同的仪器参数下(如管电流、采集时间和探测器接受角)，可比较不同样品的XRF谱线，而无需进一步归一化。因此，可在相同的实验条件下安排测量任何成分未知或部分未知的样品，并很快得出相应元素的平均原子序数。
Hodoroaba等［19］通过选择一组平均原子序数Z在 5～29之间的参考物质建立ICo/IRa与Z的校准曲线(图2)。随着ICo/IRa的降低，平均原子序数逐渐增大，在平均原子序数范围约10以内，Z对ICo/IRa的高灵敏度清晰可见，非常利于推测样品中化学成分信息。例如，C和O的荧光产额很低，极弱的激发和高能量范围的光谱韧致辐射背景的重叠干扰，往往使C、O等轻元素的谱线被淹没。但通过计算Rh Kα散射峰的ICo/IRa可得到样品的平均原子序数，从而推算出样品中TiO2添加剂的浓度［19］。又如，不同的托帕石样品XRF光谱中，可以清楚地观察到Al K线和Si K线荧光信号，但无法检测O和F的信号(图3a)，而通过计算两个光谱的ICo/IRa而推测样品中F含量(图3b)，ICo/IRa较小的托帕石样品对应较高的平均原子序数，说明F离子含量较高［19］。
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图 2 在45kV，用多毛细管X射线光学仪器在散射角为155.5°的几何结构下，用Rh靶X射线管激发获得的康普顿散射与瑞利散射强度比(ICo/IRa)与平均原子序数(Z)的校准曲线。修改自文献［19］Figure 2. Calibration curve for the Compton-to-Rayleigh intensity ratio (ICo/IRa) versus mean atomic number (Z) for an excitation with a Rhodium anode X-ray tube, at a high voltage of 45kV, using polycapillary X-ray optics and under a geometry with a scattering angle of 155.5°. Modified from the reference［19］.![]()
图 3 在相同的实验条件下测量了两种托帕石晶体的XRF光谱(a)。为了更好地呈现散射线的细节，将17keV到23keV的区域(b)放大显示，对应托帕石2的谱线平移到更高能量0.3keV处。修改自文献［19］Figure 3. XRF spectra of two topaz crystals measured under the same experimental conditions (a). For better visibility of the details of the scattering contributions, a magnified region from 17keV to 23keV is shown (b). Note that the energy scale corresponding to the topaz 2 spectrum has been shifted to higher energies by 0.3keV. Modified from the reference［19］.2.2 异质性样品原位分析的基体校正
天然地质样品往往由于样品大小、形状变化等因素引起结构不均匀，更如包裹体等特殊样品具有强异质性，XRF散射效应经常被用于校正样品异质性带来的误差。通常情况下，采用康普顿或者瑞利散射对具有相似基体的样品进行基体校正是有效的，但在多个基体的界面样品中，简单地使用康普顿散射或瑞利散射难以达成理想的效果，因此需要对散射线基体校正方法进行改进。
在研究苔藓植物-土壤-岩石界面样品的元素分布特征时，Shen等［20］采用不同散射线校正方法(Com；Ray；Ray/Com；Ray×Ray/Com)校正界面样品的不同基体的感兴趣元素计数，并用校正后的数据与其对应的ICP-MS测定值进行线性回归，发现Ray/Com校正方法校正KKα能取得较好效果，而Ray×Ray/Com校正方法则适合校正CaKα、MnKα、FeKα、CuKα、ZnKα和PbLα。采用各自适合的方法校正后，发现界面样品中K、Ca、Pb、Zn和Cu的谱线在土壤层达到最大峰值，这一结果更接近真实的元素分布规律。该研究是将XRF技术应用于感兴趣元素的生物地球化学界面过程的有益探索。
3. 易挥发元素的分析
碳、硫和卤族元素等挥发性元素，无论采用熔片法还是压片法都具有一定的难度。采用熔融制样往往会导致测量值偏低；若采用粉末压片制样，则易受到矿物效应和粒度效应的影响。
有学者采用超细粉末压片制样，降低样品的粒度效应和不均匀效应［21］。例如，彭桦等［22］对沙特阿拉伯磷矿进行超细加工(样品平均粒径为2～5μm)，再用XRF分析氯、氟等元素，结果显示不同浓度梯度标准物质Cl和F的测定值与参考值基本吻合，两个内部管理样氯和氟的RSD(n=10)分别为5.50%、5.70%和2.81%、2.55%，重现性良好。曾江萍等［23］用超细粉末压片XRF法分析铬铁矿中包括硫元素在内的多元素，样品粒度基本在10μm以下，发现硫的分析精密度和准确度均不低于化学法。在该作者的另一项研究中，用超细粉末压片XRF法分析磷矿石中的氟，发现不同浓度梯度的测定结果与参考值均吻合，并且该方法将氟元素的测定范围上限提高至10.68%［24］。李小莉等［25］用超细粉末压片XRF分析碳酸岩中碳、硫等元素，发现当样品粒度达到D95＜8μm时，样品的粒度效应对测量结果的影响基本可以消除。以上研究均表明，超细加工令样品粒径变小，并且粒度分布变窄，特别是大粒度部分骤然减少，能够有效地克服粒度效应的影响，从而降低分析结果不确定度。
熔片法测定硫时，可通过加入稳定剂(如BaO)来抑制样品在高温熔融过程中硫的挥发。例如，Gazulla等［26］在研究熔片法测定地质样品中的硫时，发现不添加BaO时，硫的响应值与硫的浓度校准曲线线性较差(图4a)，不能用于硫的定量分析；而当加入两倍于沉淀硫所需BaO的化学计量时，校准曲线线性大大改善(图4b)。采用有证标准物质SRM1835、SRM2709、GBW03118和GBW03122进行方法验证，测量值与认证值吻合较好，说明稳定剂的加入有效地抑制了硫的挥发，提高了熔片法分析硫的准确度。
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溴是古气候研究的一个非常有价值的地球化学指标，但具有溴元素推荐值的标准物质很少，因此校准曲线的建立是XRF分析溴的难点。Li等［27］通过标准加入法制定了一系列具有适当浓度的标准物质，建立工作曲线分析海洋地质样品中溴等卤族元素，并用Rh Kα康普顿峰作内标提高溴的分析精度。结果表明，方法精密度＜5%，未参与校准曲线回归的标准物质的测量值与推荐值吻合较好，计数时间为100s时溴的检出限为0.5µg/g。在以往的XRF分析中，往往需要1g及以上的样品(1g［28］、2g［29］、4g［27］、5g［30］)以保证压片具有足够的厚度。有研究者尝试采用全反射X射线荧光光谱(TXRF)技术来改善这一局限，TXRF技术可用非常少的样品量(mg级)并且采用内部标准方法进行定量。Pashkova等［31］将20mg沉积物样品制备成固体悬浮液，用TXRF技术采用内标法分析其中的溴，并对比了TXRF和波长色散X射线荧光光谱(WDXRF)方法的分析结果。发现两种方法的测量值与参考值均吻合较好，而TXRF法检出限(0.4µg/g)优于WDXRF法的检出限(1µg/g)。
4. 变价元素的分析
元素的特征X射线的谱峰位置、谱线形状和相对强度可受到原子价态和配位状态的影响，通过观察此类谱峰参数能够有效地识别元素价态、配位键等信息，并进一步应用于各类地质规律的探索和发现［32-33］。
岩石矿物样品中铁价态通常用来确定岩浆中的氧活度和矿物与硅酸盐熔体之间的平衡。铁的XRF发射谱线的相对强度取决于铁在矿物中的价态，基于此，有研究者用铁的Kβ5/Kβ1，3谱峰相对强度作为分析参数分析火成岩中二价铁(FeO)的含量［32］，发现对于FeO含量＜1.5%和RC＜0.2 的火成岩样品[RC=C(FeO)/C(Fe2O3Tot)，FeO与全Fe2O3含量比]，二价铁含量的测定误差可能＞20%，火成岩中酸性成分FeO含量的测定误差可能＞30%，火成岩碱性岩石FeO含量的测定误差可能＞50%。而当火成岩中超基性岩、基性岩和中间成分FeO的含量＞1.5%时，相对误差在10%以内，与滴定法分析相当。因此该方法可用于FeO含量＞1.5%的火成岩中基性、超基性和中间成分的二价铁含量分析。
锰价态和形态的确定可以评估矿物形成过程的氧态、诊断矿物成因和评估矿床潜力，以进一步规划合理的选矿方案。Chubarov等［33］用德国布鲁克S8Tiger型波长色散XRF分析各类锰矿石中锰的价态和形态，发现Mn Kβ5/Mn Kβ1，3和Mn Kβ′/Mn Kβ1，3强度比受锰的形态影响最小，但易受到Fe Kα1，2重叠干扰的影响。Mn Kβ″/Mn Kβ1，3强度比受Fe Kα1，2重叠干扰的影响最小，但明显依赖于锰的形态。对于含铁较低的样品，用Mn Kβ5/Mn Kβ1，3评估的误差最小；推荐Mn Kβ5/Mn Kβ1，3用作评估碳酸盐锰的存在；Mn Kβ″/Mn Kβ1，3通常用作评估氧化锰的存在。该方法的优点是不需要复杂的样品处理过程，即可快速地分析锰价态和形态以评估锰矿石质量。
5. 稀有金属的分析
稀有金属在当代工业、科学和技术等多个领域具有很高的战略价值［34-35］，因此开发实用和快速的稀有金属元素定量分析方法，并在工业过程中保持分析质量至关重要。XRF分析地质样品中稀有金属的难点主要有：①用于建立校准曲线的标准物质较少；②样品中含量较低，多在仪器检出限附近；③相似的物理化学性质，导致其特征谱线之间发生严重的谱线干扰，如稀土元素［18］。因此，学者们从以上三方面进行了相关研究。
5.1 优化校准曲线
加入人工配置的标准样品或与分析样品性质相似的二级标准参与建立工作曲线，是优化稀有金属分析工作曲线的有效方法［36］。周伟等［37］在分析稀土矿石和矿化样品时，通过加入高纯稀土氧化物 La2O3、CeO2和Y2O3扩大稀土元素镧、铈和钇的线性范围，稀土元素含量在300µg/g以上时 RSD(n=13)在0.69%～6.94%之间，未知样品的考核结果满足《地质矿产实验室测试质量管理规范》的一级标准。Silva等［38］分析亚马逊锡石尾矿中镧、铈、钕、钐和钆5种稀土元素时，加入二级标准参与建立工作曲线，并采用经验系数法进行基体校正优化校准曲线，SiO2对镧、铈、钕和钐，Na2O对钕，Fe2O3对镧产生了显著的基体效应。用加拿大认证标准物质Diorite Gneiss-CCRMP进行精密度和准确度检验，发现除了钆，其余镧、铈、钕、钐四元素的变异系数均＜5%，5种元素相对回收率在103%～116%之间。
5.2 改善低含量分析难题
5.2.1 降低熔融稀释比
稀有金属在地质样品中的含量较低，采用熔融制样XRF分析时，大量熔剂的稀释作用导致测量元素的X射线强度更低，光谱干扰也被放大。有学者通过降低熔剂和样品的比例来解决这一问题［39-40］。例如，Nakayama等［40］用熔融制样XRF分析长英质岩石时，根据样品中元素含量将32种微量元素分为三组：①铷、锶、钇和锆；②钒、铬、钴、镍、铜、锌、镓、砷、铌、钡、钨、铅、钍和铀 ；③钪、锡、铯、镧、铈、镨、钕、钐、钆、镝、 铒、镱、铪和钽。 第①组采用1∶10熔样比(样品质量0.4g)，第②组采用1∶2熔样比(样品质量为1.5g)，第③组采用1∶1熔样比(样品质量2.2g)。11个稀土元素除了钇元素以外均采用1∶1稀释比熔融。结果表明，各组分的校准曲线线性良好(r在0.991～1.000)，钪、锡、铯、铪、钽和稀土元素检出限为0.7～8.9mg/kg。Ichikawa等［41］研究了一种低稀释比的制样方法测定稀有金属在内的34种元素，以玄武岩和花岗岩样品为例进行了样品和脱模剂用量的摸索，最终确定最佳制备条件为：200mg粉末样品，200mg四硼酸锂和60μL 18.42%的氯化锂溶液，结果表明34种元素线性均良好(r＞0.990)。用玄武岩、安山岩、花岗闪长岩和流纹岩等标准物质验证方法有效性，测量值与推荐值吻合较好，且90%以上测定元素的Δm＜UΔ。以上研究均表明，降低稀释比是解决熔融制样XRF法测定稀有金属困难的有效方法。
5.2.2 高压激发
常规X射线管产生的初级X射线不能激发稀有金属的K系线，因此一般选择L系线作为分析线。当用能量色散X射线荧光光谱(EDXRF)分析时，位于低能区的L系线分析灵敏度显著降低，并且谱线重叠干扰严重(例如稀土元素的L系线与基体元素的K系线)，因而元素检出限较高。当用波长色散X射线荧光光谱(WDXRF)分析时，分光晶体使仪器对低能区域的L系线具有良好的分辨率，能够保证镧至钐的轻稀土元素的准确测定［42］，而重稀土元素则仍然需要萃取和预富集才能得到可靠结果［43］，这就失去了XRF技术的优势。
高能偏振XRF由于其高能特性，可有效地激发稀有金属的K系线，K系线的重叠干扰较少，且激发因子比L系线大，激发效率大大提升，从而实现稀有元素的准确分析。有研究者用低功率(600W)的高能偏振XRF分析地质类样品中15种稀土元素［44］，X射线光管最高激发电压可达100kV，选择K系线作为分析线，结果表明镧、铈、镨、钕和钇5种元素的相关系数r＞0.99，其余元素r＞0.969，镨的检出限为4.09mg/kg，其余元素的检出限为0.03～2.13mg/kg［44］。相比于以往研究中只能对部分稀土元素进行定量，该方法实现了用相对简单的样品处理方法对土壤、岩石和水系沉积物等地质样品中15种稀土元素进行准确分析。
5.3 改善谱线干扰
谱线重叠干扰校正有时在稀有金属分析时显得尤为重要［42，45］。例如在钽和铯XRF分析中，若忽略谱线重叠干扰的影响则可能产生错误的结果。Maritz等［45］用粉末压片法XRF分析土壤中包括稀有金属钽在内的38种主微量元素，用GBW07405对分析方法进行参数评估，发现铪、钽和铜的检出限均为0.5mg/kg。而Cu Kα1线与Hf Lα1和Ta Lα1线有重叠干扰，当三者浓度相同时，Cu Kα1线的强度是Hf Lα1和Ta Lα1线荧光强度的3～4倍，GBW07405中铜的浓度远远高于铪和钽，因此该文中三种元素方法检出限相同这一结论显然是值得商榷的。此外，该方法GBW07405中铪的测定值为15mg/kg(推荐值为8.1mg/kg)，钽的测定值为3.8mg/kg(推荐值为1.8mg/kg)，两种元素的测定值和推荐值之间均存在显著性差异。
有学者在测定岩石中钽、铯等稀有金属时则充分考虑了谱线重叠干扰的影响［42］。首先，通过标准物质CRM-MA-N研究钽和铜的谱线干扰(图5中a和b)。显而易见，Ta Lα1(0.1522nm)和Cu Kα1(0.1541nm)之间存在谱线重叠干扰，而岩石中铜含量常常远高于钽，因此Ta Lα1(0.1522nm)作为分析线并不是最好的选择。对于Ta Lβ1，2(0.1327nm)，其干扰谱线主要有： Ga Kβ1，2 (0.134nm)、 Zn Kβ1(0.1295nm)、Nb Kβ2 (0.1331nm)、W Lβ1，而镓、铌和钨在岩石中通常含量较低，因此选择了Ta Lβ1，2作为钽的分析线，并扣除Zn Kβ1尾峰的干扰。同理，选择了Cs Lα1作为铯的分析线。根据“可忽略误差”标准和t检验，钽和铯的测量偏差均不显著，钽和铯的检出限分别为2.6mg/kg和3.4mg/kg，将不同成分的岩石样品中铯的XRF测定数据与ICP-MS数据进行比较，相关系数为0.973，同样证实了分析方法的可靠性。
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图 5 (a)波长0.151～0.156nm的X射线荧光光谱谱图(CRM-MA-N元素含量：Ta 290mg/kg，Cu 140mg/kg)；(b)波长0.128～0.136nm的X射线荧光光谱图(CRM-MA-N元素含量：Ta 290mg/kg，Ga 59mg/kg，Zn 220mg/kg，W 70mg/kg，Nb 173mg/kg)。修改自文献［42］Figure 5. (a) X-ray spectrum within the range of wavelengths from 0.151nm to 0.156nm. CRM-MA-N with the elemental contents: Ta 290mg/kg, Cu 140mg/kg; (b) The spectral distribution within the range of wavelengths from 0.128 to 0.136nm. CRM-MA-N with the elemental contents: Ta 290mg/kg, Ga 59mg/kg, Zn 220mg/kg, W 70mg/kg, Nb 173mg/kg. Modified from the reference［42］.以上研究表明，测定稀有金属时，是否考虑谱线重叠干扰直接影响着分析结果准确度。
6. 结论与展望
X射线荧光光谱技术发展至今，无论从激发源、探测器、样品制备、基体校正和计算方法等方面都取得了长足的发展，在地学领域得到广泛应用，特别是曾经难以解决的一些难点如小样品量和珍贵样品分析、成分未知和异质性样品的分析、易挥发元素分析、变价元素分析和稀有金属分析等都取得可观有效的成果。
然而，XRF技术在地质分析中的应用限制和挑战依然突出：①分析结果的准确性依赖标准物质，尤其对于成分复杂的地质样品，需要物理化学性质类似的样品参与校准曲线的绘制；②对于Z≤8的轻元素仍难以定量测定。未来随着粒子物理学、光学系统以及探测装置等技术的进步，相信这些局限将会逐渐被攻克。此外，近年来随着地球科学融合发展的趋势，也促使着XRF技术在地学领域朝着更加广阔和更多维度发展。XRF相关新技术，如微区XRF、X射线吸收谱和便携式XRF等开始发挥越来越重要的作用，为XRF在地学领域的应用揭开了一种新视野和新机遇。
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图 1 (a)制备微玻璃珠试样时，将混合粉末放置在Pt-Au坩埚中的工艺原理图；(b)用硅胶聚合物黏合剂将微玻璃珠(直径约3.5mm)安装在直径35mm的空白玻璃片上；(c)附着在35mm空白玻璃片上的微玻璃珠的两个表面：(1)平面；(2)半球面。修改自文献［16］
Figure 1. (a) Schematic diagram of process to place the mixture powder in a Pt crucible for preparation of a micro glass bead specimen; (b) Top-view photograph of the micro glass bead (approximately 3.5mm diameter) after mounting on the 35mm glass bead blank using silicone polymer adhesive; (c) Comparison of the two available analytical surfaces of the micro glass bead attached on the 35mm glass bead blank: (1) Flat-surface and (2) Hemispherical-surface. Modified from the reference［16］.
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图 2 在45kV，用多毛细管X射线光学仪器在散射角为155.5°的几何结构下，用Rh靶X射线管激发获得的康普顿散射与瑞利散射强度比(ICo/IRa)与平均原子序数(Z)的校准曲线。修改自文献［19］
Figure 2. Calibration curve for the Compton-to-Rayleigh intensity ratio (ICo/IRa) versus mean atomic number (Z) for an excitation with a Rhodium anode X-ray tube, at a high voltage of 45kV, using polycapillary X-ray optics and under a geometry with a scattering angle of 155.5°. Modified from the reference［19］.
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图 3 在相同的实验条件下测量了两种托帕石晶体的XRF光谱(a)。为了更好地呈现散射线的细节，将17keV到23keV的区域(b)放大显示，对应托帕石2的谱线平移到更高能量0.3keV处。修改自文献［19］
Figure 3. XRF spectra of two topaz crystals measured under the same experimental conditions (a). For better visibility of the details of the scattering contributions, a magnified region from 17keV to 23keV is shown (b). Note that the energy scale corresponding to the topaz 2 spectrum has been shifted to higher energies by 0.3keV. Modified from the reference［19］.
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图 5 (a)波长0.151～0.156nm的X射线荧光光谱谱图(CRM-MA-N元素含量：Ta 290mg/kg，Cu 140mg/kg)；(b)波长0.128～0.136nm的X射线荧光光谱图(CRM-MA-N元素含量：Ta 290mg/kg，Ga 59mg/kg，Zn 220mg/kg，W 70mg/kg，Nb 173mg/kg)。修改自文献［42］
Figure 5. (a) X-ray spectrum within the range of wavelengths from 0.151nm to 0.156nm. CRM-MA-N with the elemental contents: Ta 290mg/kg, Cu 140mg/kg; (b) The spectral distribution within the range of wavelengths from 0.128 to 0.136nm. CRM-MA-N with the elemental contents: Ta 290mg/kg, Ga 59mg/kg, Zn 220mg/kg, W 70mg/kg, Nb 173mg/kg. Modified from the reference［42］.
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