Contamination From Reagents

Storage of High Purity Water

High purity water should be used ASAP. "Stored" high purity water may pick up impurities from the storage container. Popular storage containers are made from quartz, polyethylene (both high and low density), and fluoropolymers.

Quartz

Quartz (fused quartz or vitreous silica) typically contains 98.8% SiO₂, and impurities consisting mainly of Na₂O, Al₂O₃, Fe₂O₃, MgO, and TiO₂. Quartz has a solubility in water of 11 ppm.¹ We have measured a solubility of quartz in conductivity water of 11.2 ppm as silicic acid (equilibration time is ~4 weeks using 400 mesh quartz powder). More about quartz

HDPE and LDPE

A significant amount of HDPE is manufactured using alumina / silica based catalysts. Long term storage in high density polyethylene (HDPE) can result in ppm levels of Ca, Mg, Si, Ti, Al and ppb levels of Cr, V and Fe. LDPE can be manufactured using an organic catalyst. Storage in HNO₃ leached LDPE is optimum. Through study, we've discovered that short term (1-5 days) storage in both 20 liter HDPE and LDPE cubi containers that have been leached with dilute HNO₃ do not leach any elements at ICP-MS / OES detection limits.

Fluoropolymers

Fluoropolymers are not as clean as generally thought (see below figures). Studies performed in our own laboratories confirm these results. It is our recommendation that you save your money and use LDPE.

Figures 9.1 and 9.2 show total trace metals and major contributing ions for cut parts and from PFA 1 resin pellets and extruded tubing following extraction.²

Fig 9.1: Total Trace Metals & Contributing Ions for Cut Parts


- Extracted for 5 days in 10% ultrapure HNO₃ at 25°C -


Fig 9.2: Total Trace Metals & Major Contributing Ions from PFA 1 Resin Pellets and Tubing


Extracted for 5 days in 2% ultrapure HNO₃ at 25°C

Figure 9.3 shows the total extractable fluoride, chloride, and sulfate ions from cut fluoropolymer parts following extraction.²

Fig 9.3: Fluoride, Chloride, and Sulfate Ions from Cut Parts


Extracted for 5 days in 18 MΩ Di H₂O at 85°C for 1 hour

High Purity Acids

High purity acids have been commercially available for years with the major impurities typically less than 1 ppb. Distilling the acid yourself may offer some improvement in purity. If you are using more than 500 mL of mineral acid per month, you may want to consider the monetary savings of distilling the acid yourself. Diagrams 9.1 and 9.2 below show typical quartz and teflon stills.

Diagram 9.1: Pure Quartz Sub-Boiling Still




Diagram 9.2: All-Teflon Sub-Boiling Still

Tables 9.1 - 9.5 below show impurities in different Grades of the common mineral acids.

Other Reagents

The "purity" situation for salts and other reagents is typically not as favorable as it is for water and acids. Typically, the highest purity solid that can be confirmed is 99.999% (5-9's or TMI 10ppm). This translates to 100 ppb total impurities for a 100 fold dilution (i.e. - 1 g of 5-9's pure reagent into 100 mL of solution). We recommend that you know your supplier's definition of "high purity".

CAUTION: Do NOT use solid reagent grade materials when preparing samples for trace metals analysis whenever possible.

If necessary, a blank should be performed to confirm the acceptability of reagents(s) or to identify problem elements / impurities. Purification of unacceptable reagents may be accomplished by mercury cathode electrolysis, extraction with dithiozone or cupferron, ion-exchange, or crystallization.

1. Ralph K. Iler, The Chemistry of Silica, John Wiley & Sons: New York, (1979).
2. K. J. Mikkelsen, M. J. Alberg, J. K. Prestidge, Micro, 37 (1995).

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