由于重金属毒性大, 且在环境、 动物和人体器官中易积聚, 因而在矿石开采、 冶炼和加工之前, 监测其中的微量重金属显得尤为重要。 著名的原子光谱仪器, 如原子荧光光谱(AFS)、 原子吸收光谱(AAS)、 电感耦合等离子体(ICP)等已广泛用于各种样品中元素的检测, 但它们体积大、 能耗高、 价格昂贵、 气体消耗大, 这些缺点严重阻碍了野外现场的连续监测。 为了满足分析仪器的微型化趋势, 1993年Cserfalvi开发了一种电解液阴极放电原子发射光谱(ELCAD-AES)技术并将其用于分析检测中。 该装置中, 待测溶液以8~10 mL·min^-1的流速从细管顶端溢出, 然后沿管壁流入装满电解液的35 mL储液池中, 以溢出溶液的液面作为放电阴极, 在和流动液体电极相距2~4 mm处放一金属W(Ti)棒为阳极, 细管浸入电解液并尖端向上弯曲超出储液池液面1~3 mm, 细管顶端溢出的液体流入储液池并通过其中的碳棒与电源负极相连, 从而构建放电系统。 从那时起, 为了提高激发效率和放电稳定性, 人们对ELCAD进行了大量改进。 基于ELCAD的特点, 通过改进放电装置, 建立了一种新的液相阴极辉光放电(LCGD)分析系统。 该系统中, 放电在直径0.5 mm的铂针阳极和内径1 mm的毛细管顶端溢出的溶液阴极之间的间隙中产生。 毛细管上端和铂丝之间的间隙为2 mm, 毛细管插入石墨管且露出石墨管的距离为2.5 mm。 样品溶液以4.5 mL·min^-1从毛细管顶端溢出流经石墨管上的凹槽, 石墨管和电源负极连接。 与ELCAD相比, LCGD的优点在于: Pt针做阳极, 可形成尖端放电, 从而降低能耗(<60 W), 提高激发效率; 蠕动泵管上打结, 可降低泵的脉动性, 提高放电的稳定性; 石墨管链接电源负极, 删除ELCAD中的储液池, 使样品消耗更少。 为了评估方法的分析性能, 用LCGD测定了HNO₃-HCl消化的精铜矿样品中的铅和锌。 系统研究了放电稳定性以及放电电压、 溶液流速、 支持电解质和溶液pH对发射强度的影响, 并将LCGD与其他ELCAD的分析性能进行比较。 此外, 用ICP对LCGD的测试结果进行验证, t检验分析两种结果的差异性。 结果表明, 当电压从620升高到680 V, 发射强度逐渐增大, 这是因为电压升高, 激发能量增大, 单位体积内激发的金属原子增多, 激发效率提高。 考虑到放电稳定性, 选择650 V为最佳放电电压。 当流速从2.5增加到4.5 mL·min^-1时, 发射强度增加, 这是由于流速增加导致进入放电区的样品量增加, 发射强度增强; 流速高于4.5 mL·min^-1后, 发射强度有下降的趋势, 这是由于水荷载的增加引起放电区能量密度降低以及过量水加热消耗了用于激发样品的能量, 导致激发能量降低。 因此, 选择最佳流速为4.5 mL·min^-1。 pH=1的HNO₃具有较高的激发强度, 因而选择pH=1为最佳pH。 最佳条件下, Pb和Zn的检出限分别为0.38和0.59 mg·L^-1, 相对标准偏差分别为0.9%和1.2%, 功率低于60 W。 实验中的检出限与其他类似方法所测结果有一定差距, 这可能与所选谱仪有关。 固定激发波长下研究发现, 放电过程有较好的稳定性。 矿石样品中Pb和Zn的回收率在87.6%~107.4%, LCGD测试结果与ICP基本一致, 两种方法基本无显著性差异。 与ICP相比, LCGD具有低能耗、 高激发效率、 小型便携等优点。 随着进一步改进, 有望开发出可用于实时、 在线检测金属元素的微型化仪器。

Monitoring of trace heavy metal ions in ores samples before the mining, smelting and processing is of great importance due to it high toxicity and gradual accumulation in the environment as well as in animal or human organs. The well-known atomic spectrometry analytical instruments, such as atomic fluorescence spectrometry (AFS), atomic absorption spectrometry (AAS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES), have been extensively employed for the determination of metal elements in various complex samples. However, these analytical instruments require bulky and costly devices, high power and large gases consumption. These shortcomings restrict their use within laboratory, preventing their use for field measurement and continuous monitoring. To meet the trend of miniaturization in analytical instrumentation and the requirements of on-line detection in field, electrolyte cathode discharge (ELCAD) has been developed by Cserfalvi in 1993 as an important tool in atomic spectrum analysis for element determination of liquid samples. In the original apparatus of ELCAD, the sample solution is acted as cathode, which overflows with typical flow rate of 8~10 mL·min^-1 from a pipette into about 35 mL reservoir completely filled with electrolyte solution, and a counter-electrode (mostly W or Ti rod) above it (2~4 mm) is the anode. The pipette is immersed into electrolyte solution and then curved upwards about 1~3 mm from the reservoir containing a grounded graphite electrode to make it electrically conductive. Since then, in order to improve the emission efficiency and discharge stability, many improvements for excitation source of ELCAD have been developed. In the present work, a novel liquid cathode glow discharge (LCGD) was successfully constructed based on the principle of ELCAD, in which the glow discharge plasma was generated between the needle-like Pt anode (diameter 0.5 mm) and electrolyte (served as the liquid cathode) overflowing from a quartz capillary (1.0 mm inner diameter). The vertical gap between capillary and pointed Pt wire is 2 mm. The quartz capillary was inserted into a graphite tube and protruded from the graphite tube about 2.5 mm. The sample solution was introduced through the quartz capillary with the aid of a peristaltic pump at flow rate 4.5 mL·min^-1, and then flowed over the top of capillary into the grooves on the graphite tube. This device can offer several advantages over conventional ELCAD. For example, sealed Pt wire into a quartz tube can form a Pt tip discharge and make the energy focus on a very tiny spot, which has lower energy consumption (<60 W) and higher excitation efficiency. In addition, several knots in peristaltic-pump tubing can reduce signal fluctuations of discharge induced by the peristaltic pump and improve the stability of discharge plasma. Furthermore, inserted the quartz capillary into graphite tube is excluded the reservoir of ELCAD, which can reduce the consumption of solution samples. To evaluate the analytical performance of LCGD, the simultaneous determination of Pb and Zn in digested refined copper ores samples with HNO₃-HCl was carried out. The stability of LCGD and the effects of discharge condition, supporting electrolyte, solution pH and solution flow rate on emission intensity were systematically investigated. The limits of detections (LODs) of Pb and Zn were compared with those measured by closed-type ELCAD. In addition, the measured results of samples using LCGD were verified by ICP. Moreover, a t-test between the analytical results obtained by LCGD-AES and ICP-AES was also used for estimating the uncertainty in analytical measurements. The results showed that the emission intensities increase markedly with the increase of the discharge voltage from 620 to 680 V. This is because a higher discharge voltage creates more high-energy electrons which collide with gaseous water and metal vapor in the excitation source, thus improving the excitation efficiency of metal. Considering the discharge stability, excitation efficiency and lifetime of the electrodes, 650 V was selected as the optimal discharge voltage. The emission intensities are increased when the flow rate increases from 2.5 to 4.5 mL·min^-1 before slightly reducing over 4.5 mL·min^-1. The increase of the emission intensity may be assigned to increasing the amounts of analytes which entered into the discharge region. The reduction of emission intensity over 4.5 mL·min^-1 may be ascribed to too much water loading and evaporating, which can consume the energy in the discharge region and reduce the efficiency of exciting the atoms. Therefore, 4.5 mL·min^-1 was selected as the optimal flow rate. It is observed that pH=1 HNO₃ has higher emission intensities. Hence, pH=1.0 HNO₃ was selected as the optimum solution pH. Under the best analyzing conditions, the limits of detections (LODs) of Pb and Zn obtained from this method are 0.38 and 0.59 mg·L^-1, respectively. The relative standard deviation (RSD) is 0.9% for Pb and 1.2% for Zn. The power consumption is below 60 W. LOD of this method has higher than that of other ELCAD. This may be associated with the selected spectrometer. The emission intensity remains almost unchanged under the same discharge condition, suggesting that the glow plasma is very stable. The recoveries of Pb and Zn are in the range of 87.6%~107.4%. The results of refined copper ores samples using LCGD are well consistent with the comparing values of ICP and there is no significant difference between the two methods. Compared with ICP, LCGD has several advantages, such as low power consumption, high excitation efficiency and easy miniaturization. It may be developed as a miniaturized analytical instrument for on-site, real-time and on-line determination of metal elements with further improvement.