草酸铵为基体改进剂石墨炉原子吸收光谱法测定碳酸盐岩中的痕量银

Determination of Trace Silver in Carbonate Rock Samples by Graphite Furnace Atomic Absorption Spectrometry with Ammonium Oxalate Matrix Modifier

目前采用酸溶分解试样,以硫脲或磷酸氢二铵为基体改进剂,应用石墨炉原子吸收光谱法(GFAAS)可以直接测定各类地质样品中的痕量银;但应用于测定碳酸盐岩中的痕量银时,基体元素钙镁会产生干扰,同时影响石墨管的使用寿命。本文采用盐酸-硝酸-氢氟酸-高氯酸四酸溶样,硝酸提取,以草酸铵(45.5 g/L)作基体改进剂,试液中的钙镁与草酸铵快速反应生成白色沉淀物而分离,并形成了具有挥发性的铵盐介质环境,可显著提高石墨炉的灰化温度进一步除去铵盐介质,从而消除了基体元素的各种影响。本方法操作简单,经国家标准物质验证,其测定值与标准值的相对偏差均小于10%,检出限为0.010μg/g,精密度(RSD,n=12)小于1.0%。本方法不仅分离了干扰基体,而且比前人采用的一米光栅发射光谱法的检出限(0.04μg/g)更低。

Direct determination of trace silver in all kinds of geological samples by Graphite Furnace Atomic Absorption Spectrometry has been widely reported. In all of them, the samples were dissolved by acids. Thiourea and diammonium hydrogen phosphate were usually used as the matrix modifiers. However, the matrix elements of calcium and magnesium in the sample interfered with the measurement results of the silver, and the service life of the graphite tube was reduced, when the method was used to determine trace silver in carbonate rocks. For the study reported in this paper, the samples were dissolved by mixed acids of HCl-HNO3-HF-HCIO4. 45.5 g/L ammonium oxalate was used as matrix modifier. The matrix elements of calcium and magnesium were separated by rapid reaction to form white precipitate. At the same time, a volatile ammonium salt formed, which can be removed by increasing the ashing temperature. The method has been evaluated by determining silver in the National Standard Reference Materials, and the relative uncertainty between the measured values and standard values is less than 10%. The detection limit of this method for silver is 0. 010 μg/g, and the precision （n = 12） is less than 1.0%. Compared with the previous Emission Spectrometry of one-meter grating （ detection limit of 0.04 μg/g）, this method not only separates out the interference matrix but also reduces the detection limit. Key words： carbonate rock; trace silver; Graphite Furnace Atomic Absorption Spectrometry; ammonium oxalate; the interference of calcium and magnesium