Sample Pretreatment Options
Another group of parameter-specific field protocols that must be evaluated and included in the SAP are methods for sample pretreatment, including sample filtration and physical and chemical preservation. ASTM has published Standard Guides that address both types of sample pretreatment. ASTM Standard D 6564 (ASTM, 2004k) provides a detailed guide for field filtration of ground-water samples, and ASTM Standard D 6517 (ASTM, 2004l) discusses physical and chemical preservation methods for ground-water samples.
Ground-water sample filtration is a sample pretreatment process implemented in the field for some constituents, when it is necessary to determine whether a constituent is truly “dissolved” in ground water: Filtration involves passing a raw or bulk ground-water sample directly through a filter medium of a prescribed filter pore size either under negative pressure (vacuum) or under positive pressure. Particulates finer than the filter pore size pass through the filter along with the water to form the filtrate, which is submitted to the laboratory for analysis. Particulate matter larger than the filter pore size is retained by the filter medium.
The most common method for distinguishing between the dissolved and particulate fractions of a sample has historically been filtration with a 0.45 μm filter. The water that passes through a filter of this pore size has, by default, become the operational definition of the dissolved fraction. Filtration helps minimize the problem of data bounce, which commonly results from variable levels of suspended particulate matter in samples between sampling events and individual samples, making trend analysis and statistical evaluation of data more reliable. In addition, filtration makes it easier for laboratories to accurately quantify metals concentrations in samples. Perhaps most importantly, filtration of samples makes it possible to determine actual concentrations of dissolved metals in ground water that have not been artificially elevated as result of sample preservation (acidification), which can leach metals from the surfaces of artifactual or colloidal particles.
Filtration is not always appropriate for ground-water sampling programs. The pressure change that occurs when the sample is brought to the surface may cause changes in sample chemistry, which include loss of dissolved gases and precipitation of dissolved constituents such as metals. When handling samples during filtration operations, additional turbulence and mixing of the sample with atmospheric air can cause aeration and oxidation of Fe2+ to Fe3+. Fe3+ rapidly precipitates as amorphous iron hydroxide and can adsorb other dissolved trace metals.
Analytical Parameter Filtration Recommendations
Examples of parameters that may be field filtered
- Trace metals
- Major cations and anions
- 溶解性气体（DO, CO2)
地下水样品可以通过以下几种方法之一进行化学保存：（1）使用pH测量仪或窄频石蕊试纸监测pH值变化的同时进行pH滴定（如硝酸）; （2）向样本容器中加入固定体积的液体防腐剂（例如玻璃小瓶或安瓿瓶装的硫酸）; （3）向样品容器中加入固定量的粒状防腐剂（例如氢氧化钠）;或（4）在将容器运往现场之前将防腐剂放置于空样品容器中。