Container Material Properties

The Purity and Cleaning of Plastics

Contamination issues with plastic containers, although less severe than other container types, are critical:

"The determination of trace elements at very low levels, particularly in liquid samples, has been found to be biased by the analytical blank and can often be attributed in large part to contamination from sampling and storage containers."¹

Conventional sample handling methods were compared to clean techniques for individual 35 steps. These steps covered sample collection, storage, preparation and measurement of water samples for Ag, Cd, Cu and Pb. It was reported that two thirds of all steps contributed statistically significant amounts of contamination in the measurement of dissolved and particulate Cd, Cu, and Pb -- the average contamination for a single contributing step was 300% (Cd), 141% (Cu), and 200% (Pb).²

This section includes a closer look at part of a plastic container materials study conducted by Inorganic Ventures' technical staff³. Areas of interest in this study included the following:

• Availability and price (see Table 5.1)
• Chemical resistance (see Table 5.1)
• Physical properties (including transpiration) (see Table 5.1, transpiration data presented in the next part of this series)
• Identify (+) contaminants
• Determine (+) contaminants level
• Effectively remove (+) contaminants
• Identify (-) contamination (adsorption)

The plastics chosen for this study are shown above in Table 5.1. Results of this study will be cited to illustrate points throughout this chapter. This section will address the identity, level, and removal of contaminants in these plastics.

- Experimental Design -

1. All containers were filled and handled in a clean area.
2. "Conductivity" water and doubly distilled nitric acid were used.
3. All leaching was performed in an oven at 60 °C.
4. All measurements were made using an ICP-MS, located in a clean room.
5. The leachate solution was measured directly from the container under study.
6. Leaching solutions were exposed only to the container under study.
7. Leaching solutions were only exposed to ULPA filtered air.

- Experiment Results -

Table 5.2: Element Concentrations in Leaching Solution
(noted in ng/mL after 59 hours at 60 °C)

All of the containers involved in the leaching study were leached a second time. The results were that all of the elements detected were removed during the first leaching. These results, along with the leaching time study, is shown for Fe in Table 5.3.

Table 5.3: Leaching Behavior of Iron

The above table shows that most of the Fe was leached after 19 hours, with the exception of one of the HDPE bottles (HDPEb). We currently use a leaching time of 72 hours, due to the apparent unpredictability of the necessary leaching time. However, as shown above for Fe (and other elements not shown for the sake of brevity), the 59 hour / 60 °C leaching time / temperature combination was complete. A 66 hour / °C re-leach of these bottles using either 1% HNO₃ or water showed no Fe above the detection limit of 0.02 ng/mL.

The relative purity of the plastics as received from the manufacturer is shown in Figure 5.1 below.

Figure 5.1: Packaging Container Purity
(ranking from leaching study)

Most suprising was the lack of impurities found. This may in part be due to the clean room conditions used to perform both the leaching and the measurement steps. Figure 5.1 can be used in conjunction with Table 5.1 to identify the cleanest plastic having the chemical and physical properties for a given operation. This study, for example, suggests LDPE for sample storage and collection while PFA would be more suitable as a plastic for use in microwave acid digestions.

1. J. R. Moody, R. M. Lindstrom, Analytical Chemistry 49 (1977): p. 2264.
2. G. Benoit, K. Hunter, T. Rozan, "Sources of Trace Metal Contamination Artifacts During Collection, Handling, and Analysis," Analytical Chemistry 69 (1997): 1006-1011.
3. Paul R. Gaines, "Contamination Issues in Trace Elemental Analysis," presented at FACSS, Detroit, MI (October 2001).

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