Operator, environmental, and method errors often include sources of contamination. In an increasing, more exacting analytical landscape in pursuit of parts-per-billion (ppb)-level analytes, it is very important not only to understand the sources of error and contamination but how to reduce them. During the dawn of analytical instruments, the laboratories tested for a select number of compounds or elements at parts-per-thousand levels. Modern instrumentation now has increased the number of compounds and elements to be quantitated and lowered the analytical threshold to sub-part-per-billion levels where 1 ppb is equivalent to 1 s in 32 years! In this type of testing environment even low parts-per-billion levels of contamination can cause large errors in quantitation. In this guide, we look at all of the most common sources of contamination and error in an analytical process from the water used in the laboratory to the inherent mistakes and error caused by laboratory equipment and operators.
The experiment was repeated using a pipette washer especially made for use in parts-per-trillion analysis. The pipette washer repeatedly forced deionized water through the pipettes for a set time period. The pipettes were cleaned in the pipette washer, and then the same aliquot of 5% nitric acid was drawn through the 5-mL pipettes. The aliquot was analyzed by ICP-MS. The automated washer reduced the contamination significantly over manual cleaning of the pipettes. The reduction of contamination by moving from manual cleaning to an automated cleaning process was clear. High levels of contamination of sodium and calcium (almost 20 ppb) dropped to <0.01 ppb. Other common contaminants including lead and iron dropped from 5.4 and 1.6 ppb, respectively, to less than 0.01 ppb.
The reduction of contamination in labware can depend on the material of the labware and its use. Different materials contain many types of elemental and organic potential contamination as seen in Table VII (5). (See upper right for Table VII, click to enlarge. Table VII: Major elemental impurities found in laboratory container materials (2).) Trace inorganic analyses are best performed in polymer or high purity quartz vessels, such as fluorinated ethylene propylene (FEP), and minimize contact with borosilicate glass. Metals such as Pb and Cr are highly absorbed by glass but not by plastics. On the other hand, samples containing low levels of Hg (parts-per-billion levels) must be stored in glass or fluoropolymer because Hg vapors diffuse through polyethylene bottles.
Laboratory Environment and Personnel
All laboratories believe they observe a level of laboratory cleanliness. Most chemists recognize that there are inherent levels of contamination present in all laboratories. A common belief is that the small amounts of environmental and laboratory contamination cannot truly change the analytical results. To test the background level of contamination in a typical laboratory, samples of nitric acid were distilled in both a regular laboratory and in a clean-room laboratory with special air handling systems (HEPA filters). The nitric acid distilled in the regular laboratory had high amounts of aluminum, calcium, iron, sodium, and magnesium contamination. Table VIII shows that the acid distilled in the clean room had significantly lower amounts of most contaminants (2). (See upper right for Table VIII, click to enlarge. Table VIII: Elemental impurities found in nitric acid distilled in clean laboratories versus regular laboratories (2).)
Laboratory air also can contribute to contamination of samples and standards. Common sources of air and particulate matter contamination are from surfaces and building materials such as ceiling tiles, paints, cements, and dry wall. Surface contaminants can be found in dust and rust on shelves, equipment, and furniture. Dust contains many different Earth elements such as sodium, calcium, magnesium, manganese, silicon, aluminum, and titanium. Dust can also contain elements of human activities (Ni, Pb, Zn, Cu, As) and organic compounds like pesticides, persistent organic pollutants (POP), and phthalates. The dust and rust particles can contaminate open containers in the laboratory or enter containers by charge transfer from friction by the triboelectric effect. The triboelectric effect or triboelectric charging is when materials become charged after coming into contact with a second material creating friction. The most common example of this effect is seen when hair sticks to a plastic comb after a static charge is created.
- ASTM D1193-06(2018), Standard Specification for Reagent Water, (ASTM International, West Conshohocken, Pennsylvania, 2018) www.astm.org.
- SPEX CertiPrep Webinar, “Clean Laboratory Techniques,” https://www.spexcertiprep.com/webinar/clean-laboratory-techniques.
- SPEX CertiPrep Application Note, “Analysis of Laboratory Water Sources for BPA and Phthalates,” https://www.spexcertiprep.com/knowledge-base/files/AppNote_BPALabWater.pdf.
- SPEX CertiPrep Application Note, “Understanding Measurement: A Guide to Error, Contamination and Carryover in Volumetric Labware, Syringes and Pipettes,” available via the SPEX CertiPrep website as a downloadable PDF.
- J.R. Moody and R. Lindstrom, Anal. Chem. 49, 2264 (1977).
Patricia Atkins is a Senior Applications Scientist with SPEX CertiPrep in Metuchen, New Jersey. Direct correspondence to: [email protected]
How to Cite This Article
P. Atkins, Cannabis Science and Technology 1(4), 40-49 (2018).