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 first two I’s stand for improper use, meaning that the volumetric is not used correctly or the incorrect choice is made. Many errors can be avoided by understanding the markings displayed on the volumetrics and choosing the proper tool for the job. There is a lot of information displayed on volumetric labware. Most labware, especially glassware, is designated as either Class A or Class B labware. Class A glassware is a higher quality analytical class of glassware whereas Class B glassware is a lower quality glassware with a larger uncertainty and tolerance. If a critical measurement process is needed, then only Class A glassware should be used for measurement.
Other information that can be found on labware is the name of the manufacturer, country of origin, tolerance or uncertainty of the measurement of the labware, and a series of descriptors that indicate how the glassware should be used. Labware can be marked with letters that designate the purpose of the container. If a volumetric is designed to contain liquid it will be marked by either the letters TC or IN. Labware that is designated to deliver liquid will be marked by either the letters TD or EX. Sometimes there are additional designations such as wait time or delivery time inscribed on the labware. The delivery time refers to a period of time required for the meniscus to flow from the upper volume mark to reach the lower volume mark. The wait time refers to the time needed for the meniscus to come to rest after the residual liquid has finished flowing down from the wall of the pipette or vessel.
A second type of improper use and incorrect choice can be seen in the selection of pipettes and syringes for analytical measurements. Many syringe manufacturers recommend a minimum dispensing volume of approximately 10% of the total volume of the syringe or pipette. A study by SPEX CertiPrep showed that dispensing such a small percentage of the syringe’s total volume created a large amount of error. In this study, four syringes, 10 µL, 25 µL, 100 µL, and 1000 µL were used to dispense between 8–100% of the syringe’s total volume of water. Each volume was weighed and replicated 10 times by several analysts and the results were averaged together to calculate average error.
The largest rates of error were seen in the smaller syringes of 10 and 25 µL. Dispensing 20% of the 10 µL syringe created 23% error. Error only dropped down to below 5% as the volume dispensed approached 100%. In the larger syringes, measurements over 25% were able to see error in and around 1%. The larger syringes were able to get closer to the 10% manufacturer’s dispensing minimum without a large amount of error, but the error did drop as the dispensed volume approached 100%, which is seen in Table V. (See upper right for Table V, click to enlarge. Table V: Dispensing volume error associated with various size syringes (4).)
- 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).