Anal. Chem. 1982, 5 4 , 2614-2616
2614
Ultrasonic Bath in Container Preparation for Storage of Seawater Samples in Trace Metal Analysis Brian Klnsella" and Rodney L. Wlllix School of Applled Chemlstty, Western Australian Instltute of Technology, Kent Street, Bentley, Western Australia 6 102, Australla
The main problems in the analysis of metals at trace levels are generally recognized to be (i) contamination or (ii) loss, from the sample of the element of interest during sampling, storage, sample workup, or analysis, and (iii) interferences in analysis. Interferences in analysis depend partly on the nature of the sample and partly on the analytical technique used. Since natural waters are known to be chemically and biologically complex with distinct differences between seawater and freshwater, some attention has to be paid to the nature of the sample. For example: speciation, one facet of matrix effects, is an area of considerable activity, and difficulty, particularly a t trace and ultratrace levels. We confine our attention here to contamination by the metals of interest and its avoidance, particularly at the stage of container preparation. Devising an acceptable procedure for the cleaning and storage of containers has engaged the attention of many (1-6). Although suggestions on the choice of acid vary, there is general agreement that in order to leach and desorb metals from the container walls, some form of acid washing is recommended. Laxen and Harrison (7) have recently compared the methods used with the general conclusion that polyethylene containers are acceptable for the storage of freshwaters a t their natural pH (but kept at 1"C) and the recommended cleaning procedure is a 48-h soak in 10% nitric acid solution followed by a water rinse. Others prefer the use of hydrochloric acid (6). With the development of evermore sensitive analytical techniques and the realization that heavy metal levels in some natural waters are lower than previously thought, the avoidance of contamination becomes increasingly important. In the assessment of cleaning methods for polyethylene containers, the adequacy and requirements of the method have been based on typical concentrations of Zn, Cd, Pb, and Cu a t the microgram per liter level (6). There is now evidence to suggest that many reported levels of heavy metals in seawater are in serious error due tQ contamination from a variety of sources (8-11). The lower the actual levels, the more stringent the requirements, particularly with respect to cleanliness of sampling and storage Containers. In our studies, the levels of Cd, Pb, and Cu in relatively unpolluted seawater are well below the microgram per liter level and extend down to the nanogram per liter level. At these concentrations a significant source of contamination by P b and Cu appears to originate from polyethylene and Teflon (FEP) containers that had been cleaned by using recommended procedures. Measured levels of these metals by others (7-12) support our conclusions. I t is suggested that washing with high-purity acids in an ultrasonic bath is a convenient and effective technique for container preparation and considerably shortens the preparation time. Evidence is also presented to show that the low levels measured in our seawater samples are not due to the loss of material by the activation of adsorption sites in the ultrasonic procedure. There is some conflict in the literature on the treatment of samples for storage prior to analysis, with some investigators advocating acidification to prevent adsorption on container walls (13)-an action that precludes speciation studies-while others claim negligible losses with samples stored at their 0003-2700/82/0354-2614$01.25/0
natural pH (14). Some conclusions have been based on the study of relatively simple metal cationic species in pure water, Le., synthetic systems, where losses by adsorption occur if the sample is not acidified to a pH
