Typical examples of liquid samples include those drawn from containers of com- mercial solvents; beverages, such as milk or fruit juice; natural waters, including from lakes, streams, seawater, and rain; bodily fluids, such as blood and urine; and, suspensions, such as those found in many oral medications.
Homogeneous solutions are easily sampled by siphoning, de- canting, or by using a pipet or syringe. Unfortunately, few solutions are truly homo- geneous. When the material to be sampled is of manageable size, manual shaking is often sufficient to ensure homogeneity. Samples may then be collected with a pipet, a syringe, or a bottle. The majority of solutions, however, cannot be sampled in this manner. To minimize the effect of heterogeneity, the method for collecting the gross sample must be adapted to the material being sampled.
The environmental sampling of waters and wastewaters provides a good illus- tration of many of the methods used to sample solutions. The chemical composi- tion of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influ- enced by flow rate and depth. Rapidly flowing shallow streams and rivers, and shallow (<5 m) lakes are usually well mixed and show little stratification with depth. Grab samples are conveniently collected by submerging a capped bottle below the surface and removing the cap. The air–water interface, which may be en- riched with heavy metals9 or contaminated with oil, is avoided when collecting the sample. After the sample bottle is filled, the cap is replaced and the bottle removed. Slowly moving streams and rivers, lakes deeper than 5 m, estuaries, and oceans may show substantial stratification. Grab samples from near the surface can be collected as described earlier, whereas samples at greater depths are collected with a weighted sample bottle that is lowered to the desired depth. Once it has reached the desired depth, the sample bottle is opened, allowed to fill, and closed before retrieving. Grab samples can be analyzed individually, giving information about changes in the analyte’s concentration with depth. Alternatively, the grab samples may be pooled to form a composite sample.
Wells used for collecting groundwater samples must be purged before the sam- ple is collected, since the chemical composition of water in the well-casing and in the adjacent matrix may be significantly different from that of the surrounding groundwater. These differences may result from contaminants introduced when drilling the well, or differences in the groundwater’s redox potential when exposed to atmospheric oxygen. In general, wells are purged by pumping out a volume of water equivalent to several well-casing volumes, or until the water’s temperature, pH, or specific conductance are constant. Samples collected from municipal water supplies must also be purged since the chemical composition of water left standing in pipes may differ significantly from the treated water supply. Samples are collected at faucets after flushing the pipes for 2–3 min.
Samples from municipal wastewater treatment plants and samples of industrial discharges often are collected as 24-h composites. Samples are obtained using an automatic sampler that periodically removes individual grab samples. The volume of each sample increment and the frequency of sampling may be constant or may vary in response to changes in flow rate.
Sample containers for collecting solutions are made from glass or plastic. Con- tainers made from Kimax or Pyrex brand borosilicate glass have the advantage of being sterilizable, easy to clean, and inert to all solutions except those that are strongly alkaline. The disadvantages of glass containers are cost, weight, and the likelihood of breakage. Plastic containers are made from a variety of polymers, in- cluding polyethylene, polypropylene, polycarbonate, polyvinyl chloride, and Teflon (polytetrafluoroethylene). Plastic containers are lightweight, durable, and, except for those manufactured from Teflon, inexpensive. In most cases glass or plastic bot- tles may be used, although polyethylene bottles are generally preferred because of their lower cost. Glass containers are always used when collecting samples for the analysis of pesticides, oil and grease, and organics because these species often inter- act with plastic surfaces. Since glass surfaces easily adsorb metal ions, plastic bottles are preferred when collecting samples for the analysis of trace metals.
In most cases the sample bottle has a wide mouth, making it easy to fill and re- move the sample. A narrow-mouth sample bottle is used when exposing the sample to the container cap or to the outside environment is undesirable. Unless exposure to plastic is a problem, caps for sample bottles are manufactured from polyethylene. When polyethylene must be avoided, the container cap includes an inert interior liner of neoprene or Teflon.
Once removed from its target population, a liquid sample’s chemical composition may change as a result of chemical, biological, or physical processes.
Following its collection, samples are preserved by controlling the solution’s pH and temperature, limiting its exposure to light or to the atmosphere, or by adding a chemical preservative. After preserving, samples may be safely stored for later analysis. The maximum holding time between preservation and analysis de- pends on the analyte’s stability and the effectiveness of sample preservation. Table 7.1 provides a list of sample preservation methods and maximum holding times for sev- eral analytes of importance in the analysis of water and wastewater.
Most analytical methods can be applied to analytes in a liquid or solution state. For this reason a gross sample of a liquid or solution does not need additional processing to bring it into a more suitable form for analysis.