Dr. Ralph Obenauf and Nimi Kocherlakota,
SPEX CertiPrep, 203 Norcross Av, Metuchen, NJ 08840
www.spexcsp.com
ABSTRACT
The key issue in all chemical analysis is the integrity of the measurement. Today’s instruments are capable of detecting metals at the ppb and ppt levels. However, having an instrument of this type in the lab will not be of much help if one does not minimize contamination and identify the remaining contaminants that are present in the lab environment, in reagents and in lab ware.
The presentation will cover the following issues and will cover various sources of contamination and tips on how to determine and prevent contamination. While the discussion centers on the production of reference materials, the concepts are applicable to all your laboratory processes.
• Starting materials used in the preparation of reference materials• Water• Acids• Storage containers• Laboratory environment• Other sources of contamination• Controlling contamination Starting Materials: Must be pure, stable and of known stoichiometry; it is also important that there are no (a minimum of) chloride, oxalate and sulfate ions present. • Pure, because impurities present can give rise to overlap of spectra, resulting in incorrect calibration curves and therefore inaccurate results.• Stable because some materials are hygroscopic and some decompose upon heating.• Of known stoichiometry because some materials exist in multiple oxidation states of the primary metal.
• Abset of chlorides, oxalates and sulfates because if the starting materials are used in the preparation of multi-element standards, these ions could precipitate elements such as Ag, As,Au,Ba and rare-earth elements.
Water:Water is the major component of an aqueous standard or a sample that is diluted. Therefore the overall quality/accuracy of analysis also depends on the quality of water that is used for trace metals analysis. One must use ASTM Type 1 water that meets the following specifications: Electrical resistivity at 18 megohms, min TOC at 50 ug/L max, Sodium at 1 ug/L max, Chlorides at 1 ug/L max and total Silica at 3 ug/L max. Acids:Acids are used for the dissolution of materials and samples, digestions and dilutions.Contaminants present in acids can contribute to erroneous results. Storage Containers:Storage containers are bottles in which manufacturers store and ship standards to customers. These could also be the bottles in which the chemist in the lab stores his or her dilutions to be used in the future. Bottles come in various sizes, shapes and materials of construction.The contaminants that are present in the materials of construction can leach into the solutions and thus introduce contamination. Laboratory environment:There is no advantage in using high purity materials, acids, water etc, if the environment in which the standards or dilutions are made, and for that matter even the analysis that is performed, is not clean.A clean environment or clean room must meet the following requirements: • Environment of class 100 (less than 100 particles of 0.3microns per cubic meter).• Walls, ceilings and floors are sealed and are dust free.• HEPA filters mounted in the input stream of air.• Positive pressure in the clean area to flush out contamination.• No exposed metal parts. Other Sources of Contamination:• Sample preparation methods.• Sample to sample cross contamination.• Instrument sample introduction systems.• Environment in which the samples are prepared.• Materials that come in contact with the sample. Controlling Contamination:• Minimize exposure: Clean and cover the equipment after use.• Wear gloves.• Use metal free containers.• Separate lab ware into “low level” and “high level” lab ware:
“Low level” lab ware is used only for solutions that have metals present below 1ppm concentration. “High-level” lab ware for solutions with greater than 1 ppm concentration of metals.
• Segregate lab ware for specific metals and ultra low concentration ranges. Metals such as Pb and Cr are highly absorbed by glass but not by plastics.• For B and Si, avoid borosilicate glass. Use plastic, TFE or Quartz lab ware.• Samples containing low levels of Hg (PPB levels) should be stored in glass or fluoropolymer containers because Hg vapors diffuse through Polyethylene bottles.• Use membrane filters instead of ashless filter paper. Ashless filter paper contains 20 trace elements at >1 ppm level.• Use “NOCHROMIX” instead of Chromic Acid to clean lab ware.• No jewelry, cosmetics or lotions.Cosmetics and lotions can introduce contaminants such as:Al, Be, Ca, Cu, Cr, K, Fe, Mn, Ti, or Zn into the samples.• Some hair dyes contain Lead acetate and eye makeup may contain Hg as a preservative.• Calamine lotion used for skin irritations is pure ZnO.
Helpful Hints:• Test personnel, equipment and methods with QC samples.• Observe clean lab procedures and techniques.• Use reference materials that have not expired.• Make up and use only freshly prepared calibration standards.• Rerun samples using a different dilution factor.• Spike appropriate QC samples with expected levels of analytes or use standard additions.
• Carry blanks through all steps of an analytical procedure.
References:• Guidance in establishing Trace Metal Clean Rooms in Existing Facilities :USEPA 821-B-95-001• Accuracy in Trace Analysis : NBS Special Edition 422• Guide to Environment Analytical Methods :Roy-Keith Smith• Clean Manufacturing :A2C2; April 2003• Water Environment Laboratory Solutions :April/May 2003
• Sampling of sea and fresh water for the analysis of trace Metals; E.Helmers 1997