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.
Chemical reference standards have been an important component in accurate analysis for decades. Over the years, the challenges facing chemical laboratories have changed. As a manufacturer of certified reference materials (CRMs), many questions have arisen during our daily interaction with customers on how to best use CRMs. Sometimes a customer is not even aware that the issue they are questioning is really an problem of contamination or error. A common scenario is that a scientist will express a concern that their standard is too high in particular elements. Usually during the conversation, our scientists discover that the customer is inadvertently allowing contamination into their analysis.
Contamination and error can occur at almost any point of the process and then can be magnified as the method and analysis runs its course. Modern instrumentation has raised the bar or in this case lowered the limits of detection to the parts-per-billion (ppb) or even parts-per-trillion (ppt) levels. The concept of such small measurements would have been almost inconceivable during the emergence of modern laboratory analysis.
To put this into a different context, 1 ppb expressed as a unit of time would be 1 s in 32 years and 1 ppt would be 1 s in 320 centuries! In the past, issues of laboratory contamination were problematic but now contaminants, even in trace amounts, can severely alter results. It is hard to imagine that such small amounts of contamination can dramatically change laboratory values.
Most questions about contamination come in the form of an inquiry about a particularly high result for some common contaminant. In most cases, the root of that contamination can be traced to a common source. The most common sources of standard and sample contamination are found in the laboratory: reagents, labware, laboratory environment, storage, and personnel.
Water is one of the most basic yet most essential laboratory components. Most scientists are aware that the common perception that all water is the same is untrue. There are many types, grades, and intended uses for water. Water is most often used in two ways in the laboratory: as a cleaning solution and as a transfer solution for volumetric or gravimetric calibrations or dilutions. In both of these uses, the water must be clean to reduce contamination and introduce error into the process. Poor quality water can cause a host of problems from creating deposits in labware or inadvertently increasing a target element or analyte concentration in solution.
The confusion starts when laboratories are unsure about which type of water they get from their water filtration system. ASTM has guidelines that designate different grades of water. Table I shows parameters for the four ASTM types of water (1). (See upper right for Table I, click to enlarge. Table I: ASTM designations for reagent laboratory water (1).)
- 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).