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As the chart below shows, for most trace metal analyses,
plastic is generally "cleaner" or less contaminated than glass or other
materials. However, plastic does contain trace levels of certain metals. To
minimize potential low-level contamination, remove these metals or leach them
from plastic by soaking in 1N HCl and rinsing in distilled water. For
extremely precise work, use HCl, followed by soaking in 1N HNO3,
and rinsing in distilled water. Soaking time may vary according to individual
needs, but plastic should be soaked no longer than 8 hours. If more rigorous
cleaning is desired, increase the concentration of acids used. Caution:
concentrated nitric acid is a strong oxidizing agent and will embrittle many
plastics.
To remove trace organics which contribute to trace metal absorption, clean
plastic surfaces with alcohol, alkalies, alcoholic alkalies or chloroform. A
final rinse of 1N HCl also minimizes absorption of trace elements.
Polysulfone (PSF), a resin used in NALGENE centrifuge tubes and reusable
filterware, is extremely "cleanable.'' The following qualification testing was
performed on NALGENE PSF centrifuge tubes at The University of Rhode Island's
Graduate School of Oceanography (URI-GSO).
URI-GSO was determining the concentration of trace metals in atmospheric
aerosols and seawater from remote marine locations. They were measuring the
input to the Pacific Ocean of heavy metals and other trace elements; pesticide,
petroleum and plastic residues; other natural and man-made organic materials,
as well as compounds causing acid rain. The trace metal concentrations in the
samples were extremely low, so any labware used needed to be extremely
cleanable to prevent leaching of contaminants from the vessels or filtration
equipment into the samples. In addition, the labware could not irreversibly
adsorb metals from sample solutions. The labware in question included filter
holders, which were used during procedures for extracting various trace metals
from the seawater samples.
The following cleaning procedure was used:
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One-week soak in 1:1, analytical reagent HCl: deionized water,
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Deionized water rinse,
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One-week soak in 1:1, analytical reagent HNO3: deionized water,
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Deionized water rinse,
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One-week soak in 1:1000, quartz redistilled HNO3: deionized
water,
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Deionized water rinse,
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One-week soak in 1:1000, quartz redistilled HNO3: deionized
water, and,
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Deionized water rinse.
This cleaning procedure was generated from the procedures used at URI-GSO, J.R.
Moody and R.M. Lindstrom Analytical Chemistry 49:2264 (1977) and conversations
with faculty of the California Institute of Technology.
The following testing procedure was used:
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Approximately 25 mL 1N quartz redistilled HNO3 was put into five
of the centrifuge tubes.
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Approximately 25 mL of 1 part per billion aluminum, copper, lead and zinc in 1 N
quartz redistilled HNO3 was put into the other five centrifuge
tubes.
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The tubes were allowed to stand for one month, and the contents of the tubes
were analyzed.
The analyses were performed by atomic absorption spectrophotometry on a Perkin
Elmer 5000 equipped with an HGA 5000 utilizing Zeeman background correction.
All standards used are traceable back to NBS (National Bureau of Standards).
The results and conclusions were:
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The 1N quartz redistilled HNO3 had immeasurably low
concentrations of metals (e.g., aluminum, copper and lead 0.1 parts per
billion; zinc 0.01 parts per billion). Therefore, once cleaned, polysulfone
leaches insignificant amounts of these four metals into solution.
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The 1 part per billion aluminum, copper, lead and zinc solutions had, within
experimental error, 1 part per billion aluminum, copper, lead and zinc.
Therefore, polysulfone does not adsorb these metals from an acidified solution.
The analysis of aluminum, copper, lead and zinc involved a two-step extraction
procedure. The first, a "liquid/liquid" extraction with an organic complexing
agent, removed copper, lead and zinc. The second employed an iron hydroxide
co-precipitation technique. Iron nitrate was added to the seawater and the pH
was adjusted. Iron hydroxide and aluminum were filtered from the rest of the
mixture. This is where the NALGENE reusable filter holders, molded of PSF, were
used.
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Summary of Average Element Content of 12 Plastics and
Borosilicate Glass (^1)
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| Material |
No. of Elements |
Total Conc., ppm |
Major Constituents |
| PS |
8 (8 N.D.*)
|
4 |
Na, Ti, Al |
| PSF |
16 (12 N.D.*)
|
17 |
Na, Fe, Ca |
| TFE |
24 |
19 |
Ca, Pb/Fe, Cu |
| LDPE |
18 |
23 |
Ca, Cl, K |
| PC |
10 |
85 |
Cl, Br, Al |
| PMP |
14 |
178 |
Ca, Mg, Zn |
| FEP |
25 |
241 |
K, Ca, Mg |
| PVC-tubing |
9 |
280 |
Fe, Zn, Sb |
| PP |
21 |
519 |
Cl, Mg, Ca |
| HDPE |
22 |
654 |
Ca, Zn, Si |
| ETFE |
32 |
1.007 |
Cl, Pb, Si |
| PVC-rigid |
7 (11 N.D.) |
2.541 |
Sn, Ca, Mg |
| Borosilicate Glass |
14 |
497,249 |
Si, B, Na |
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* N.D. Not Detected
NOTE: Values listed in the chart above represent typical contents for major
constituents. Various NNI grades of plastics may vary from these values.
(^1) Selection and Cleaning of Plastic Containers for Storage of Trace Element
Samples, John R. Moody and Richard Lindstrom, ANALYTICAL CHEMISTRY, Vol. 49,
Page 2264, December 1977.
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