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Platinum:
Platinum, although expensive, is a popular container material. It heats up and cools down rapidly, making it excellent for % ash determinations where the % ash is at low levels.
It is resistant to attack by most acids and reagents. Avoid concentrated H3PO4 at high temperatures, HCl + HNO3 mixtures and fusions using Li2CO3, Na2O2, or the alkali hydroxides. Fusions using Na2CO3 are common in addition to fusions using the alkali borates, fluorides, nitrates, and bisulphates. Avoid heating at prolonged temperatures in excess of 1100 °C (m.p. = 1772 °C).
Platinum can be destroyed by heating with metals with which it can alloy. Avoid high temperature heating with samples containing significant levels of any metal that may be in or reduced to the metallic state during the heating process. For example, a sample containing high levels of Cu0 or Cu+2 should be avoided, especially if present in an organic matrix. A sample containing trace levels of Cu in an organic matrix will not ruin the platinum, but it is likely to be lost to the platinum during the ashing process. Since platinum has this alloying tendency, it is best to avoid its use with samples containing elements other than those that have no tendency to form the metal (i.e. - alkali, alkaline and rare earth elements). Platinum is known to contain trace amounts of the other precious metals and should not be used for their preparation. Avoid samples containing Hg any an form. Hg metal is easily formed and alloys very readily at room temperature with platinum. Also avoid ashing samples containing P in any form, including the phosphates.
Graphite:
Graphite is very inexpensive and relatively clean, but very messy to work with. It is an inexpensive way to perform Li2CO3 fusions where the crucible slowly oxidizes away over the course of 7-10 fusions. It is popular because it does not wet by some melts which can be poured out quantitatively. Losses due to the porosity of graphite should exclude its use for ashing samples containing trace metals. Graphite's main advantage to the trace analyst is being a material that can withstand fusions that might destroy platinum. Our chemists use graphite for performing Li2CO3 fusions in the preparation of large numbers of limestone samples for major minor and trace elemental analysis.
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Plastics:
Plastics are very important to the trace analyst. Whenever possible, the analyst should attempt to use plastics for sample collection, storage, preparation, and measurement. Their major disadvantage is the inability to be used for high temperature operations, such as ashing or fusion. Table 5.1 shows a summary of the physical properties of some common plastics.
Table 5.1: Physical Properties of Common Plastics
FEP (FLUORINATEDETHYLENEPROPYLENE)
PFA (PERFLUOROALKOXY)
FLEP (FLUORINATED HIGH-DENSITY POLYETHLYENE)
PMP (POLYMETHYLPENTENE)
PP (POLYPROPYLENE)
HDPE (HIGH-DENSITY POLYETHYLENE)
LDPE (LOW-DENSITY POLYETHYLENE)
The most popular plastics are PFA and HDPE / LDPE.
PFA has excellent properties, allowing for use in acid digestions up to 250 °C. Typically, PFA is used for acid digestions using either HF, HNO3 or HCl, alone or in combination. The use of higher boiling acids such as H2SO4 and H3PO4 have been reported in PFA, but great care must be taken not to exceed the 250 °C maximum operating temperature. PFA is commonly used in the construction of microwave digestion vessels. Microwave digestions using the higher boiling acids should not be attempted. Digestions using HCLO4 should never be performed in plastics of any kind.
LDPE or HDPE bottles are typically used for containment of the sample digestate after dilution with water. These bottles can withstand solutions of HNO3 that are 10% v/v and lower over extended periods of time (i.e. - years). The caps used for the LDPE and HDPE bottles are made of PP, which is more rigid than the polyethylene and well-suited for its purpose. Unfortunately, the PP cap is not as clean as the PE bottle.
All of the above plastics were part of a study conducted at our laboratories concerning their purity and cleaning properties, further described below.
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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 Part VI)
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 -
All containers were filled and handled in a clean area.
"Conductivity" water and doubly distilled nitric acid were used.
All leaching was performed in an oven at 60 °C.
All measurements were made using an ICP-MS, located in a clean room.
The leachate solution was measured directly from the container under study.
Leaching solutions were exposed only to the container under study.
Leaching solutions were only exposed to ULPA filtered air.
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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
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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 suprizing 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.