Multi-element Accurate Analysis of Sulfide Minerals by Low-temperature Ablation LA-ICP-MS
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摘要:
硫化物矿物中元素含量及其分布可示踪硫化物成矿过程、辨别金属来源和沉积过程的物理化学条件，在地质学、矿床学等领域具有重要的应用价值。激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）已成功应用于硫化物矿物元素微区分析研究，但激光与物质作用产生的热效应严重制约分析结果的可靠性。本文建立了一种高精密度、高准确度的低温剥蚀LA-ICP-MS测定硫化物矿物多元素方法。采用自行研制的Peltier低温剥蚀池可有效抑制硫化物矿物LA-ICP-MS分析中的热效应，提高分析结果的精密度和准确度。扫描电子显微镜（SEM）表明：在低温（−30℃）条件下可在一定程度地抑制激光剥蚀引起的热效应，减少样品熔化和气溶胶气相再沉积；而通过气溶胶颗粒分析发现低温剥蚀可以减小样品气溶胶颗粒的平均尺寸，得到的颗粒粒径分布范围也较小。不同元素信号强度的精密度（RSD）从常温下的20.1%～34.4%改善到11.5%～15.8%，元素的检出限为0.054～0.077μg/g。将该低温LA-ICP-MS系统应用于实验室内部标样黄铜矿Ccp-1分析，测定值与参考值之间的标准偏差在7%以内。
Abstract:BACKGROUNDMicro-geochemical information of sulfide minerals plays a crucial role in the field of geochemistry, allowing discovery of the formation mechanism and evolution process of sulfide minerals by analyzing their element composition characteristics. LA-ICP-MS is currently the most popular microanalysis technology used for sulfide analysis, having yielded successful results. Due to their unique physical and chemical properties, sulfide mineral samples show different laser ablation behavior to conventional geological samples. The most intuitive phenomenon is the melting of ablation carters caused by laser thermal effect and the deposition of a large number of material particles around the ablation carters, which is the main factor limiting the precision and accuracy of sulfide sample analysis. Walting et al[13] found that direct quantitation of multi-elements in sulfide minerals by infrared laser (1064-nm Nd:YAG laser) was impossible, which was because the strong thermal effect generated by the infrared laser will lead to severe large particle aerosol redeposition. It is reported that ablation systems with shorter wavelengths, such as ultraviolet lasers, including the 266 and 213nm laser, can be used to obtain acceptable analytical accuracy by reducing the thermal effect and aerosol particle size, but a poor precision was still observed[16-17]. Guillong et al[19] conducted a comparative study of 266, 213 and 193nm lasers and found that there were finer particle sizes of the aerosols and the weaker thermal effect when using 193nm laser ablation, and the RSDs of all elements less than 20% were obtained. In other words, collisions between photons and matter intensify in deep ultraviolet laser ablation systems (193nm) with shorter wavelength[20-21] and can help reduce the melt zone and aerosol particle size. However, there is still a slight thermal effect during 193nm UV laser ablation. Fernández et al[22] found that there is still a melting layer during 193nm laser ablation, and it leads to the formation of large particle aerosols. Different methods have been proposed to improve the thermal effect during laser ablation of sulfide. Muller et al[25] found that the precision of line scanning could be improved by 50% compared to spot ablation. Guillong’s results showed that adding a small amount of hydrogen to the analysis could increase the sensitivity of the 47 elements in the test by two to four times[26]. Moreover, research has focused on improving the thermal effect of sulfide minerals from shorter pulse width lasers and aerosol particle transport[27-31]. However, there are still some thermal effects in the process of deep ultraviolet and short wavelength laser ablation, and how to inhibit the thermal effect in the process of ablation to obtain effective analysis results is still a difficulty in the analysis of sulfide mineral elements. The LA-ICP-MS low temperature ablation cell is an ablation system developed in recent years, whose main function is to provide a low temperature ablation environment to realize the effective analysis of cells, blood and other samples. The low-temperature ablation cell may be a new approach to resolve the thermal effect during sulfide mineral ablation.
OBJECTIVESIn order to establish a high precision and high accuracy multi-element analysis method for sulfide minerals.
METHODSThe use of a designed cryogenic ablation cell suppressed the thermal effect and refined aerosol particle sizes, which improved analytical precision and accuracy significantly. To explore the mechanism of sulfide ablation at low temperature, the aerosols ablated at low temperature were collected using an aerosol collection setup consisting of a membrane with an aperture of 0.1μm, which was installed at the outlet of the ablation cell. According to the micro-analysis results, the laser ablation behavior under low temperature ablation environment was further discussed.
RESULTSA precision and accuracy method for multi-elements analysis of sulfide minerals using CLA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry with a cryogenic ablation cell) was described. Ablation craters were investigated via scanning electron microscope (SEM) images to compare the amounts of melt produced. SEM measurements showed significant differences in melting between the low temperature (−30℃) and room temperature (20℃). The diameters and size distribution of particles were measured from nanometer particle potentiometer images of the collected ablated aerosol. Particles ablated using cryogenic ablation cell were smaller in average diameter (190nm and 400nm) and shorter in distribution range (570nm). Compared to the precision of time-resolved signal during laser ablation processes between the two temperatures, the precision was significantly improved and the RSD was reduced from 20.1%-34.4% to 11.5%-15.8% with a cryogenic ablation cell. A designed cryogenic ablation cell in sulfide sample analysis was utilized to minimize the thermal effect and improve analytical precision and signal intensity. In this study, the CRM (MASS-1) sample was analyzed with spot ablation mode at low (−30℃) and room (20℃) temperatures, respectively, and the RSDs of three times parallel analysis at these two temperatures were compared. At room temperature, the RSDs of elemental signals ranged from 20.1% to 34.4%. In contrast, the RSD of elemental signals was less than 15.8% when the sample was ablated at low temperature (Fig.2a). The significant improvements may be attributed to low ablation temperature, which suppress the thermal effect. Moreover, the signal intensities of elements improved by approximately 11% to 52% with the decrease in temperature of the cryogenic ablation cell (Fig.2b). Fig.2c shows the time-resolved signals of the MASS-1 sample at room temperature, the significant fluctuations and spikes could be observed, and the RSDs of all elemental signals was more than 20.1%. Interestingly, the signals at low temperature exhibited ideal stability, and the RSDs of elemental signals were less than 15.8%, as shown in Fig.2d. In order to explore the reasons for improving the analytical performance of low temperature, the morphology of ablation under different temperature conditions of two standard sulfide samples were discussed. SEM images of four ablation craters on chalcopyrite and pyrite were taken to investigate the effect of temperature on the ablation process (Fig.3). The sulfide samples were ablated using a 193nm excimer laser with a spot size of 60nm and a fluence of 8J/cm2. The ablation craters on the chalcopyrite showed a two-layer cyclic structure, in which the inner layer was a light-colored melting zone, and the outer layer was a white aerosol vapor sediment. At low temperature, there were fewer melt layers and thinner grain sediment zone than those at room temperature (Fig.3a, Fig.3b). However, the melting zone around the ablation craters of pyrite were more irregular. More of the unwanted ablation was melted away at room temperature (Fig.3c, Fig.3d). The craters formed at room temperature (Fig.3a, Fig.3c) showed a more serious melting phenomenon than those formed at low temperature (Fig.3b, Fig.3d), as evidenced by the abundance of molten ejecta around the former, especially at high laser energy densities. In contrast, the low temperature craters showed no obvious melting phenomenon and had a flatter bottom with a reduced number of large molten spherical particles. The use of CLA-ICP-MS weakened the melting phenomenon, thereby generating smaller aerosol particles, which further improved the aerosol transport and ionization efficiency. A particle size collection experiment was conducted to explore the distribution of aerosol particles at different temperatures. SEM images were used to analyze the shapes and sizes of particles that were collected on a membrane with an aperture of 0.1μm at room temperature (20℃) and low temperature (−30℃). The same sample chamber and 1m of tubing were used to transport the particles, and the ablation pulses continuously for 2min. The SEM images showed that the particles produced at room temperature were larger and formed large agglomerates (Fig.4a), whereas the particles produced at −30℃ were smaller and there were fewer agglomerations (Fig.4c). The shape of the agglomerates and their connection by filaments suggested strong charge during particle formation, which was more prominent at room temperature. Additionally, there were more single large particles produced at room temperature (Fig.4b), while there were fewer particles at −30℃ (Fig.4d). Comparative measurements were conducted using 193nm laser to investigate the influence of temperatures on particle size distribution. Fig.4 shows a typical size distribution for LA under He atmosphere. The left part of Fig.5 shows a distribution of aerosols produced by ablation of 2min pulses at room temperature. The peak heights of mean diameter in this distribution were determined to be approximately 300nm and 700nm, respectively. Similarly, particle size distribution at −30℃ also presented a bimodal pattern, which was consistent with previous studies. The average diameters were 190nm and 400nm, both smaller than at room temperature, while the peak width was shorter. The chemical composition of fine particles produced at low temperature is closer to the sample body, improving the transport and ionization of aerosol in ICP, reducing element fractionation, and enhancing the signal strength and stability, thereby improving the analytical performance of ICP-MS.
CONCLUSIONSA new high-precision and accuracy method for determination of trace elements in sulfide minerals has been developed using the CLA-ICP-MS system. This method reduces thermal effect and decreases particle size during the ablation process, improving precision by freezing sulfide samples with a designed cryogenic ablation cell. Low temperature results in better data because fewer large particles are produced; sedimentation around the ablation crater and during transport is reduced, while ionization efficiency in ICP is higher. The precision calculated for transient signals decreases obviously if the sample is kept at low temperature (−30℃) compared to room temperature (20℃), while the sensitivity improved slightly. The deviation of all elements between the test values and the standard values falls within 7% by CLA-ICP-MS. In future work, it will be necessary to investigate even lower temperatures, as low temperatures can increase aerosol viscosity and affect analysis results. It is also worth exploring whether the performance of a long pulse width laser can be improved by lowering the temperature to match that of a short pulse width laser.
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天然气水合物，是在一定条件下(合适的温度、压力、气体饱和度及水的盐度等)由水和天然气组成的冰状笼形结晶化合物。形成天然气水合物的主要气体为甲烷，甲烷分子含量超过99%的天然气水合物通常称为甲烷水合物(Methane Hydrate)[1]。天然气水合物的结构类型有Ⅰ型、Ⅱ型、H型和一种新型的水合物(由生物分子和水分子生成)[2-3]。Kvenvolden等[4]和Milkov[5]曾预测全球有机碳超过1×105亿吨(甲烷在标准温压条件下为21×1015 m3)，主要以甲烷形式存在，并赋存于水合物中。目前，冻土带和海底已经开展了天然气水合物的相关实验及开采[6]。与冻土带所蕴藏的天然气水合物相比，海底沉积层中发育的水合物资源量可能更为巨大[7], 在全球79个国家累计已发现超过230个天然气水合物矿点(NGHD)[8]。因此，合理开采海底天然气水合物将是解决全球能源危机的有效途径[9-10]，同时天然气水合物也可能诱发海底地质灾害[11]以及影响全球气候的变化[12], 所以了解天然气水合物的稳定条件成了一个核心的科学问题。
甲烷水合物稳定性受控于温度、压力、孔隙水盐度和气体组分等因素。天然气水合物主要发育在具备水合物生成的温压、气源等条件的海洋沉积层中和极地地区[13-15]，所以研究水合物发育区的压力、温度、地温梯度、导热率及热流等参数，可以预测水合物赋存范围[16]。似反射层与海底之间所限定的厚度为天然气水合物稳定存在的厚度即天然气水合物稳定带(GHSZ)[17]。天然气水合物稳定带(GHSZ)是指温度和压力处于天然气水合物形成和稳定存在的热力学范围内的特定区域[18]。因而，预测目标区水合物资源量必须明确水合物稳定带厚度，对评估天然气水合物资源具有重要意义[19-21]。已有研究表明甲烷水合物实际存在区的厚度随盐度的增大而变薄，盐的存在降低了气体水合物的稳定性，导致水合物稳定带的厚度比纯水情况下的厚度变薄[22-23]。孔隙水中的盐类对水合物的生成和稳定存在有抑制作用[7, 24-25]，1888年Villard[26]第一次在CH4-H2O二元体系中获得了甲烷水合物。Deaton等[27]最早提出了研究水合物的抑制性，直到1983年de Roo等[28]才将盐度考虑进来，并系统研究了CH4-H2O-NaCl三元体系下甲烷水合物相平衡时的温压条件。事实上，盐度对水合物系统的影响是预测海底甲烷气水合物存在、分布和演变的一个重要因素。海相水合物形成于含海水(Cl-、Na+、Mg2+、SO42-、Ca2+以及过渡金属Fe、Mn、Cu、Co、Ni)的沉积层中，所以必须明确水合物在复杂体系中的平衡，拟为确定水合物的形成、分解以及资源量奠定基础[25, 29-30]。研究盐类对甲烷水合物的稳定性主要是通过实验测试(目测法、等压法、等容法)和热力学计算来确定天然气水合物的相平衡点[28, 31-37]。
1. 天然气水合物相平衡的研究方法
1.1 气体水合物相平衡热力学方法
气体水合物相平衡热力学主要解决气体水合物形成和稳定存在的温度、压力条件，预测已知状态系统是否可形成水合物，其理论依据主要是多相系统相平衡理论[34]。气体水合物相平衡的理论模型已经研究得比较成熟，并在指导水合物理论研究和工程应用方面发挥了积极的作用。Parrish等[38]第一次基于van der Waals-Platteeuw统计热力学模型[34]预测纯水条件下水合物的形成。随后这种方法被不断修改，但大多数预测模型的基本思想都来源于van der Waals-Platteeuw统计热力学模型[34]，预测模型的缺点就是所需的参数较多，计算繁琐，应用起来不方便[23-25, 30-32, 35-37, 39-41]。由于水合物形成于地层中，地层水是一个复杂的盐水体系，水合物形成的温度与纯水体系差异较大。Englezos等[42]、Shabani等[43]、Javanmardi等[44-45]的模型可用于预测盐水体系中水合物的相平衡。在盐水体系中，离子与水分子反应降低了水的活性，因此将盐对水合物的影响转移到水活性的变化，将溶液中水的活性加入到模型中来预测水合物相平衡。这些热力学模型计算水合物生成条件时，大多忽略了气体在富水相的溶解对水合物生成条件的影响，这会给计算结果带来一定的误差。水合物模型在适宜的温度(小于290 K)和压力(小于20 MPa)条件下的预测结果较好，而在高压下，如大于20 MPa，模型的预测结果与实验数据偏差较大，表明当前的水合物模型对于水合物稳定性描述并不完美[46]。
1.2 气体水合物相平衡实验分析方法
以往采用压力-温度-体积测试仪(简称PVT仪)来研究气体水合物的形成热力学，通过目测PVT仪中天然气水合物的形成过程，确定形成时的温度和压力，但是PVT仪器笨重且昂贵，不方便使用，实验过程中遇到固体微粒时，PVT方法就无法测量。此外，PVT无法定量评价天然气水合物的动力学性质[31]。后来将差示扫描量热仪(Differential Scanning Calorimetry，缩写DSC)应用到天然气水合物的研究中，它的原理主要是根据实验过程中热流量的变化判断水合物的形成，与样品的黏度、透明度等无关，是研究水合物一种简单、可靠的工具[47-48]。但是不能满足水合物的微观观测和定量研究。
相比于PVT仪、DSC、热力学计算，用原位拉曼光谱技术研究甲烷水合物生成条件可以有效地避免热力学方程中复杂参数的求算，同时具有可视、准确、结果可靠的优势。显微激光拉曼光谱是将入射激光通过显微镜聚焦到样品上，从而可以在不受周围物质干扰情况下，准确获得所照样品微区的有关化学成分、晶体结构、分子相互作用以及分子取向等各种拉曼光谱信息[49-50]。近些年来，国内外学者[51-56]已成功地应用激光拉曼光谱分析天然气水合物形成条件。Subramanian等[52]的研究表明，不同类型甲烷水合物的拉曼光谱明显不同，Ⅰ型甲烷水合物和Ⅱ型甲烷水合物的特征拉曼峰的峰强具有明显的差别，可以在拉曼谱图上明确区分开来(图 1)。Chazallon等[53]发现，甲烷水合物不仅在C—H键处有其独特的拉曼峰，同时其O—H键也有一个特征峰位于3051 cm-1。所以，通过拉曼光谱可以清楚地识别体系中甲烷所处的相态特征，这为原位甲烷水合物拉曼光谱分析提供了依据。吕万军等[56]通过原位拉曼光谱结合透明高压腔，测定甲烷水合物形成过程中溶液中饱和甲烷浓度的变化来确定水合物的形成条件。Jager等[33]结合显微拉曼光谱，可以将Ⅰ型甲烷水合物的形成和分解在拉曼谱图上清楚地确定，随着水合物的形成和分解，其拉曼峰也发生着相应的变化。因此，原位拉曼光谱技术可以准确测定甲烷水合物在不同盐水体系中形成和分解的温压条件。
2. 盐类对甲烷水合物的影响
根据DSDP(Deep Sea Drilling Project，深海钻探工程)和ODP(Offshore Driling Platform，海上钻井平台)[57]采集的海底沉积物，对孔隙水成分进行分析，结果表明海洋沉积孔隙水中，Cl-、SO42-、Na+、Ca2+、Mg2+、K+、NH4+是主要离子，CO32-和PO43-相对含量低一些。以下将从氯化物、硫酸盐、碳酸盐三大盐类出发，探讨盐类对甲烷水合物稳定性的影响以及相平衡条件。
2.1 氯化物对甲烷水合物稳定性的影响
2.1.1 氯化钠对甲烷水合物稳定性的影响
国内外学者[24, 28, 58-61]通过不同的实验研究了盐类，特别是NaCl对甲烷水合物稳定性的影响。对于NaCl-CH4-H2O体系，各种实验手段殊出同归，最后的实验数据皆表明随着NaCl浓度的增加，甲烷水合物稳定存在的温压范围逐渐缩小，NaCl浓度越高，甲烷水合物稳定存在的温度越低，压力越高。盐度每增加1 mol/L，在压力恒定的情况下，温度降低3~4 K；同样在温度恒定的情况下，压力升高2 MPa左右(图 2a)。除了NaCl，其他氯化物对甲烷水合物稳定性的影响也不容忽视，但整体的影响趋势是一致的，都是随着盐度的增加，甲烷水合物稳定存在的温压范围缩小[61-63]。
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2.1.2 其他氯化物对甲烷水合物稳定性的影响
目前针对CaCl2、MgCl2、KCl等对甲烷水合物的抑制作用研究甚少，Kharrat等[61]对不同浓度的CaCl2-CH4-H2O体系进行分析，随着CaCl2浓度的增加，甲烷水合物稳定存在的温压范围逐渐缩小，CaCl2浓度越高，甲烷水合物稳定存在的温度越低，压力越高。盐度每增加0.5 mol/L，在压力恒定的情况下，温度降低幅度由小变大最后趋于稳定，开始时温度只下降2.5 K，盐度在1.530 mol/L后温度稳定下降4 K左右；同样在温度恒定情况下，压力升高降低幅度由小变大最后趋于稳定，开始时压力只升高约2 MPa，盐度在1.530 mol/L后压力稳定升高约5 MPa(图 2b)。
Atik等[62]和Kang等[32]对不同浓度的MgCl2-CH4-H2O体系进行分析，随着MgCl2浓度的增加，甲烷水合物稳定存在的温压范围逐渐缩小，MgCl2浓度越高，甲烷水合物稳定存在的温度越低，压力越高。盐度每增加0.5 mol/L左右，在压力恒定的情况下，温度降低幅度由小变大；同样在温度恒定的情况下，压力升高幅度由小变大(图 2c，d)，即MgCl2盐度越高，对甲烷水合物的抑制作用强度越大。
Mohammadi等[63]对不同浓度的KCl-CH4-H2O体系进行分析，但是有关的实验数据较少，只对0.676 mol/L和1.315 mol/L的KCl进行了实验测试。随着KCl浓度的增加，甲烷水合物稳定存在的温压范围逐渐缩小，KCl浓度越高，甲烷水合物稳定存在的温度越低，压力越高。盐度每增加0.5 mol/L左右，在压力恒定的情况下，温度降低2 K；同样在温度恒定的情况下，压力升高1 MPa(图 2e)。
2.1.3 氯化物对甲烷水合物抑制作用大小
当氯化物浓度小于0.5 mol/L时，NaCl、CaCl2、MgCl2和KCl对甲烷水合物的抑制作用大小相近。但随着浓度的上升，当氯化物浓度大于1.5 mol/L后CaCl2对甲烷水合物稳定存在时的温压条件影响较大，浓度每升高0.5 mol/L，温度下降4 K，压力升高5 MPa。而其他盐类对甲烷水合物的抑制作用相近。浓度相近(大于1mol/L，小于1.5 mol/L)的NaCl、CaCl2、MgCl2和KCl对甲烷水合物的抑制作用进行对比，发现MgCl2对甲烷水合物的抑制作用最强，KCl最弱。抑制作用大小依次是MgCl2>CaCl2>NaCl>KCl(图 2f)。
2.2 硫酸盐和碳酸盐对甲烷水合物稳定性的影响
大部分的实验都是研究氯化物对甲烷水合物稳定性的影响，但是很少考虑到硫酸盐和碳酸盐对甲烷水合物的抑制作用，相关的研究也比较少。Lu等[65]通过实验确定了1 mol/L和0.5 mol/L的MgSO4对甲烷水合物稳定存在时的温压条件的影响，增加0.5 mol/L的MgSO4，在压力恒定的情况下，温度降低2 K；同样在温度恒定的情况下，压力升高0.7 MPa(图 3a)。相比于氯化物，MgSO4对甲烷水合物稳定存在时的压力影响较小。
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Mohammadi等[64]将K2CO3考虑进来，实验研究0.319 mol/L和1.064 mol/L的K2CO3对甲烷水合物的抑制作用，随着浓度的升高，甲烷水合物稳定存在的温压范围缩小，K2CO3浓度上升0.745 mol/L，在压力恒定的情况下，温度降低1.2 K；同样在温度恒定的情况下，压力升高1 MPa(图 3b)。相比于氯化物和MgSO4，K2CO3对甲烷水合物的影响更小。但是关于硫酸盐和碳酸盐对甲烷水合物抑制性的研究还较少，还不能明确硫酸盐和碳酸盐大类对甲烷水合物稳定性的影响，有待进一步的实验研究。
3. 盐类对甲烷水合物的抑制作用强弱
研究者们对不同盐类和不同离子对甲烷水合物的抑制作用大小进行了对比分析。何勇等[7]实验发现盐类对甲烷水合物的抑制作用大小为：NaCl>KCl>CaCl2>MgCl2>Na2SO4；Sylva等[24]的实验结果(图 4)与何勇等[7]的相近：FeCl3>NaCl>CaCl2≈AgNO3≈MnSO4>CuSO4≈FeSO4，可以看出NaCl是除了FeCl3外对甲烷水合物抑制作用最强的盐类。前人的系统研究结果与本文统计(基于Sylva等[24]、Mohammadi等[63-64]、Kang等[32]、Atik等[62]、Kharrat等[61]、de Roo等[28]、Maekawa等[59]、Lu等[60]的数据)的实验结果(即盐类浓度小于1.5 mol/L大于1 mol/L时，盐类对甲烷水合物的抑制作用大小为MgCl2>CaCl2>NaCl>KCl；盐类浓度大于1.5 mol/L时，CaCl2的抑制作用较强)存在较大的差异。可能由于统计的实验数据来自不同的实验测试方法，导致实验结果存在较大的差异。在阴阳离子对水合物稳定性的抑制作用大小上也出现了争议，有的认为Mg2+>Ca2+>Na+>K+，SO42->CO32->Cl-[66-67]，也有的认为Cl->SO42-，Mg2+≈Na+>Ca2+[60, 68-69]。实验结果差异较大，造成实验结果不一致的原因可能是在实验之前未完全将反应釜和溶液中的空气驱净，导致水合物合成受影响，也有可能是实验对比的盐类浓度上有差异，不同浓度的离子可能对甲烷水合物的抑制作用程度不一样。Lu等[60, 65]和Atik等[62]认为阴离子的存在对水合物稳定性的影响更大。在电解质溶液中，盐离子和水分子反应会降低水的活性，导致水合物不易形成[70-72]。理论上，阴离子半径越小、阳离子的半径越大和价位越高，对水分子的静电效应越强、溶剂效应和盐析效应越强，水的活性越低[73-74]。
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关于氯化物、硫酸盐和碳酸盐等抑制作用大小的比较，需要在同一实验测试条件下完成，但是前人并没有系统地研究其他盐类(如硫酸盐、碳酸盐等)对甲烷水合物稳定性的影响，未在不同盐类体系下针对甲烷水合物的稳定性进行横向和纵向的对比。关于盐类对甲烷水合物抑制作用的研究，已经从分子水平发展到离子水平。阴阳离子对甲烷水合物稳定性影响强度上存在较大的争议，阳离子如Mg2+、Na+、Ca2+、K+对甲烷水合物抑制作用大小的排序不统一，阴离子如SO42-、CO32-、Cl-对甲烷水合物的抑制作用大小争议更大。水的活性影响着甲烷水合物的形成，水的活性则受控于阴阳离子的半径电价等因素，因此探讨阴阳离子的性质对研究甲烷水合物在海水中的稳定性具有重要的意义。
4. 存在问题与展望
盐类对甲烷水合物的抑制作用是毋庸置疑的，但关于KCl、硫酸盐、碳酸盐等盐类对甲烷水合物影响的研究甚少，盐类对甲烷水合物的抑制作用大小存在差异，在不同盐类抑制作用强弱上也存在较大的争议。目前的研究结果还不够系统，与实际地质条件下的甲烷水合物稳定环境还存在一定差别。根据已有的研究成果，盐类对水合物稳定性影响的研究未来应关注以下几点。
(1) 以往的研究中对NaCl关注较多，关于NaCl对甲烷水合物的影响的研究已比较成熟，随着盐度的增加，NaCl对甲烷水合物的抑制作用越强，盐度每增加1 mol/L，在压力恒定的情况下，温度降低3~4 K；同样在温度恒定的情况下，压力升高2 MPa左右。但是地层水中还存在其他离子，如SO42-、CO32-、K+、Ca2+、Mg2+、NH4+，目前的研究成果与实际地质条件还存在一定差距，实际地层中的盐离子种类更多，更复杂，且不同的地质因素[如生物活动、水岩交互作用、深部物质(如甲烷气体)上流]会影响地层水盐度和离子种类的变化。因此，还需进行更加系统的研究，特别是要加强氯化物-甲烷-水、硫酸盐-甲烷-水、碳酸盐-甲烷-水等体系的详细研究。
(2) 本文数据统计结果显示，盐类浓度小于1.5 mol/L大于1 mol/L时，盐类对甲烷水合物的抑制作用大小为MgCl2>CaCl2>NaCl>KCl，盐类浓度大于1.5 mol/L时，CaCl2的抑制作用较强。盐类和离子对甲烷水合物的抑制作用大小和机制还需进一步确认，有待于系统地研究关于氯化物、硫酸盐、碳酸盐对甲烷水合物的抑制作用，并进行横向和纵向上的对比。同时阴阳离子对水合物稳定的影响强度还需进一步验证和分析，对比阴阳离子对甲烷水合物的稳定性影响的强弱，明确阳离子Mg2+、Na+、Ca2+、K+和阴离子SO42-、CO32-、Cl-对甲烷水合物的抑制作用大小，以及离子本身的性质如何影响着水的活性。明确盐类和阴阳离子的抑制作用大小，以及盐类和离子特性如何影响水合物的形成和稳定，对未来甲烷水合物的勘探和开发具有借鉴意义。
(3) 选取合适的实验手段，减小实验误差。将目测法、等容法、等压法三者相结合，目前实验手段中将高压可视反应腔与显微激光拉曼技术相结合可实现。这种实验手段能够在高压可视反应腔中清楚地观察到水合物的形成分解过程实现定性的研究，同时可根据拉曼谱图定量观测甲烷水合物的形成过程中液相中饱和甲烷浓度，准确获取甲烷水合物稳定形成时的温压条件。
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图 1 低温剥蚀池结构示意图
a. 低温剥蚀池的内部结构（1—剥蚀池；2—氟化钙窗口；3—PEEK盖；4—导热铜板；5—Peltier元件；6—水冷平台；7—底板；8—载物台）；b. 剥蚀池的外部制冷系统（9—循环制冷机；10—温控系统；11—测温元件）。
Figure 1. Schematic diagram of the low-temperature ablation cell structure.
a. Interior structure of the cryogenic ablation cell （1—Ablation cell; 2—Calcium fluoride window; 3—PEEK lid; 4—Thermally conductive copper plate; 5—Peltier element; 6—Water-cooled platform; 7—Baseplate; 8—Stage）; b. Ablation cell with the external refrigeration system （9—Cycle refrigeration machine; 10—Temperature control device; 11—Temperature sensor）.
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图 2 MASS-1在不同剥蚀温度下的剥蚀信号对比情况
a—分析信号精密度；b—分析信号强度（以室温剥蚀下元素信号强度做归一化），n=3； c—室温下元素的时间信号分辨谱图；d—低温下元素的时间信号分辨谱图。
Figure 2. Comparison of signals of MASS-1 at different ablation temperatures.
a—Analytical signal precision; b—Analytical signal intensity （Normalization is based on element signal intensity at room temperature), n=3;c—Time signal resolution spectrum of elements at room temperature; d—Time signal resolution spectrum of elements at low temperature.
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图 3 硫化物矿物在不同剥蚀温度下剥蚀形貌的扫描电子显微镜图
a—20℃的黄铜矿； b—30℃的黄铜矿； c—20℃的黄铁矿； d—−30℃的黄铁矿。剥蚀条件：激光能量密度8J/cm2，剥蚀斑径为60μm。
Figure 3. Scanning electron microscopy maps of sulfide minerals morphology at different ablation temperatures.
a—Chalcopyrite at 20℃; b—Chalcopyrite at −30℃; c—Pyrite at 20℃; d—Pyrite at −30℃. Ablation conditions: laser energy density is 8J/cm2.Spot diameter is 60μm.
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图 4 收集MASS-1在不同温度下激光剥蚀后气溶胶颗粒的扫描电子显微镜图
a—常温20℃下激光剥蚀颗粒，放大倍率5000×；b—常温20℃下激光剥蚀颗粒，放大倍率15000×；c—低温−30℃下激光剥蚀颗粒，放大倍率5000×；d—低温−30℃下激光剥蚀颗粒，放大倍率15000×。剥蚀条件：激光能量密度为6J/cm2，连续剥蚀2min。
Figure 4. Scanning electron microscope maps of aerosol particles after MASS-1 ablated at different temperatures.
a—Laser particles at 20℃ with 5000× magnification; b—Laser particles at 20℃ with 15000× magnification; c—Laser particles at −30℃ with 5000× magnification; d—Laser particles at −30℃ with 15000× magnification. Ablation conditions: laser energy density is 6J/cm2, continuous ablation for 2min.
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图 5 收集MASS-1在不同温度下激光剥蚀后气溶胶颗粒的粒径分布图
剥蚀条件：激光能量密度为6J/cm2，连续剥蚀2min。
Figure 5. Particle size distribution of aerosol particles after MASS-1 ablated at different temperatures.
Ablation conditions: laser energy density is 6J/cm2, continuous ablation for 2min.
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图 6 低温剥蚀池在不同激光条件下（a）能量密度和（b）剥蚀斑径的性能对比（样品为MASS-1）
Figure 6. Performance comparison of the low-temperature ablation cell under different laser conditions: （a） nergy density and（b） ablation diameter （The sample is MASS-1）.
表 1 LA-ICP-MS仪器工作参数
Table 1 The operating conditions of LA-ICP-MS.
电感耦合等离子体质谱
ICP-MS（7700x）激光剥蚀系统
Laser system（GeoLas HD）参数 工作条件 参数 工作条件 RF功率 1550W 激光波长 193nm 反馈功率 8W 能量密度 6J/cm2 RF电压 1.60W 剥蚀斑径 60μm 采样深度 7.5mm 激光频率 5Hz 载气（Ar）流速 0.85L/min 剥蚀气（He）流速 0.4L/min 元素 55Mn，57Fe，59Co，60Ni，63Cu，66Zn，71Ga，74Ge，75As，111Cd，208Pb 
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表 2 黄铜矿Ccp-1中多元素分析结果（n=3）
Table 2 The results of elemental analysis in Ccp-1 （n=3）.
元素 参考值
（μg/g）测定值（−30℃ ）
（μg/g）测定值（20℃ ）
（μg/g）Mn 7.35±0.43 7.23±0.55 6.15±0.92 Co 5.30±0.36 5.15±0.36 4.76±0.66 Ni 7.75±0.64 7.44±0.53 6.64±0.96 Ga 8.20±0.56 8.34±0.63 7.31±1.25 Ge 8.53±1.29 8.47±0.66 6.82±1.37 As 16.51±1.19 16.96±1.32 13.47±2.56 Cd 0.24±0.01 0.26±0.03 0.21±0.05
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表 3 硫化物矿物的元素分析结果（n=3）
Table 3 The analytical results of elements in sulfide samples （n=3）
元素 黄铁矿-1 黄铁矿-2 黄铁矿-3 测定值
（μg/g）SD
（μg/g）参考值
（μg/g）测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）Mn 2.62 0.03 2.68 12.85 1.38 13.58 23.98 0.65 24.69 Co 42.04 2.53 43.76 50.07 5.91 53.1 57.25 2.68 59.33 Ni 247.2 12.11 256.36 256.04 37.01 273.86 257.58 10.29 266.36 Ga 2.95 0.18 2.89 15.76 1.23 14.66 29.61 1.44 28.72 Ge 48.05 4.65 47.25 45.97 23.14 39.04 42.03 2.5 41.04 As 13 1.92 11.26 24.77 3.36 23.49 36.52 5.72 35.01 Cd 0.09 0.01 0.09 0.55 0.03 0.56 1.05 0.09 1.07 元素 方铅矿-1 方铅矿-2 方铅矿-3 测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）Mn 0.47 0.06 0.5 3.52 0.53 3.77 7.1 0.53 7.42 Co 0.37 0.05 0.41 2.7 0.21 2.82 5.58 0.26 5.78 Ni 0.52 0.06 0.55 3.29 0.52 3.72 7.89 0.84 10.92 Ga 0.64 0.06 0.63 4.93 0.35 5.15 10.59 0.53 10.81 Ge 0.57 0.06 0.48 4.39 0.12 3.62 9.65 0.47 7.33 As 0.48 0.03 0.42 3.9 0.58 3.54 7.34 0.47 8.69 Cd 0.04 0.01 0.04 0.24 0.04 0.24 0.48 0.09 0.49 元素 闪锌矿-1 闪锌矿-2 闪锌矿-3 测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）测定值
（μg/g）SD
（μg/g）参考值*
（μg/g）Mn 2.86 0.42 3.2 12.41 0.56 12.85 25.14 3.76 26.93 Co 0.93 0.03 0.96 8.64 0.47 8.98 17.49 1.73 18.44 Ni 18.75 2.93 20.23 9.89 0.88 10.36 21.08 8.61 24.44 Ga 1.46 0.18 1.49 14.19 0.53 14.48 28.49 2.23 28.31 Ge 0.84 0.11 0.96 10.03 1.36 10.3 18.49 0.74 17.96 As 4.91 0.67 5.01 16.45 2.15 16.79 28.54 3.96 29.12 Cd 1.82 0.18 1.86 2.09 0.09 2.13 2.47 0.35 2.52 注：“*”表示硫化物实际样品的元素浓度参考值由ICP-MS测试得到。
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