NSWC/WOL/TR 75-35 



Although it is obviously desirable to analyze water samples 

 immediately after collection, this is not always possible, either 

 because of shortage of time or because of lack of facilities on 

 board ship. Rapid changes may occur in the concentrations of micro- 

 components and trace elements unless measures are taken to stabilize 

 them, and to prevent contamination arising from the storage vessel. 



In the storage of water samples for trace element analysis, it 

 is necessary to guard against contamination and against losses 

 cause by adsorption. Contamination by the element to be determined 

 may arise by solution or desorption from the walls of the storage 

 container. Contamination arising from the storage vessel is parti- 

 cularly serious if soft glass bottles are used, but may also be 

 appreciable even with borosilicate glass. Polyethylene has been 

 demonstrated to contain extremely low concentrations of trace 

 elements , and at present, must be considered the best container 

 material. (Teflon has slightly lower concentration of most trace 

 elements but is very expensive) . The risk of contamination of 

 samples by polyethylene containers is generally small, but it is 

 important that new bottles should be treated with diluted hydro- 

 chloric acid to remove traces of metals left during the manufacturing 

 process . 



Very serious losses of trace elements can occur through 

 adsorption onto the walls of the container. Glass and most plastics 

 are supercooled liquids, which, because of their distorted and 

 broken bonds, have greater surface energy than crystalline substances, 

 This leads to the adsorption of ions from solution and to the forma- 

 tion of bonds between the surface and the absorbed ions. The loss 

 is likely to be greatest with transition elements and with elements 

 which are hydrolysed at pH 7-9, such as iron. Generally, acidifica- 

 tion of the sample prevents, or reduces adsorption of trace elements 

 by the container. It will often be found that acidified samples 

 (at a pH value of 1.5) can be stored for several months in 

 polyethylene bottles without change. Before any type of container 

 is used for the storage of samples for trace element analysis, it 

 should be tested to determine whether it adsorbs the element under 

 examination. 



Robertson, D. E., "Role of Contamination in Trace Element Analysis 

 of Sea Water", Anal. Chem. 40, 1067 (1968). 



Q 



Robertson, D. E., "The Adsorption of Trace Elements in Sea Water 

 on Various Container Surfaces", Anal. Chem. Acta, 42_, 553 (1968). 



24 



