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 polarity and the strength of the electrical charge is dependent upon the type of material and other physical characteristics. Many materials in the laboratory have strong positive or negative triboelectric charges as shown in Figure 2. (See upper right for Figure 2, click to enlarge. Figure 2: Triboelectric charge potential of common materials and particles in the laboratory.) In the laboratory, materials like dust, air, skin, and lead have extreme positive charges and can be attracted to the strongly negative charge of PTFE or other plastic bottles when the bottle is opened and friction is created, inducing a charge.
Laboratory personnel can add their own contamination from laboratory coats, makeup, perfume, and jewelry. Aluminum contamination can come from laboratory glassware, cosmetics, and jewelry. Many other common elements can be brought in as contamination from lotions, dyes, and cosmetics. Even sweat and hair can cause elevated levels of sodium, calcium, potassium, lead, magnesium, and many ions. If a laboratory is seeing an usually high level of cadmium in the samples it could be from cigarettes, pigments, or batteries. If the levels of lead are out of range, contamination can be from paint, cosmetics, and hair dyes. Figure 3 shows potential sources of common elemental contamination from outside products. (See upper right for Figure 3, click to enlarge. Figure 3: Common sources of contamination found in the laboratory.)
Laboratory environment and personnel contamination can be reduced by limiting use of personal care products, jewelry, and cosmetics that could contain contamination and interfere with critical analyses. Laboratory coats can collect all types of contamination and should only be worn in the laboratory to avoid cross contamination from other laboratories and the outside world. The laboratory surfaces should be kept clean. Deionized water can be used to wipe down work surfaces. Laboratory humidity can be kept above 50% to reduce static charge. An ethanol- or methanol-soaked laboratory wipe can be used to reduce static electricity as it evaporates.
Even with clean laboratory practices in place, erroneous results can often find their way into sample analysis. To eliminate some of these spurious results, replication of blanks and sample dilutions can be used. The blank results should be averaged and the sample run values can either be minimally selected or averaged. The difference between the two values can then be plotted against a curve established against two more standards. A minimum of two standard points can be used if the chance of contamination is minimal, such as in the case of rare or uncommon elements. Additional standard points should be considered if the potential for contamination is high with common elements such as aluminum, sodium, and magnesium. Multiple aliquots of blanks and dilutions can also be used to further minimize analytical uncertainty.
- 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).