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 third “I” of volumetric error is inadequate cleaning. Many volumetrics can be subject to memory effects and carryover. In critical laboratory experiments, labware sometimes needs to be separated by purpose and use. Labware subject to high levels of organic compounds or persistent inorganic compounds can develop chemical interactions and memory effects. It is also sometimes difficult to eliminate carryover from labware and syringes even when using a manufacturer’s stated instructions. For example, many syringes are cleaned by several repeated solvent rinses before use. A study of syringe carryover by SPEX CertiPrep showed that some syringes are subject to high levels of chemical carryover despite repeated rinses.
In this study, several syringes ranging in volume from 10 µL to 1000 µL were used to dispense a 2000 µg/mL internal standard mix of deuterated compounds. The subsequent washes were collected and tested by GC–MS to determine the amount of carryover in each wash (see Table VI). (See upper right for Table VI, click to enlarge. Table VI: Compound carryover found in syringe washes (ppm of carryover) (4).)
The larger syringes needed less rinses to reduce carryover than the smaller 10-µL syringes. The smaller syringes had more than 1 ppm carryover through over 15 rinses. The typical number of rinses usually employed to rinse syringes is between three and five, which in the case of the smaller syringe would not be adequate to clear all the carryover from the syringe.
The final source of error is infrequent calibration. Many laboratories have schedules of maintenance for equipment such as balances and automatic pipettes, but often overlook calibration of reusable burettes, pipettes, syringes, and labware. Under most normal use, labware often does not need frequent calibration but there are some instances where a schedule of recalibration should be used. Any glassware or labware in continuous use for years should be checked for calibration. Glass manufactures suggest that any glassware used or cleaned at high temperatures, used for corrosive chemicals, or autoclaved should be recalibrated more frequently.
It is also suggested that under normal conditions that soda-lime glass be checked or recalibrated every five years and borosilicate glass after it has been in use for 10 years. The error associated with the use of volumetrics can be greatly reduced by choosing the correct volumetric for the task, using the tool properly, and by making sure the volumetrics are properly cleaned and calibrated before use.
Inorganic analysts know that glassware is a source of contamination. Even clean glassware can contaminate samples with elements such as boron, silicon, and sodium. If glassware, such as pipettes and beakers, is reused, the potential for contamination escalates. A study was made of residual contamination at SPEX CertiPrep of our pipettes after they were manually and automatically cleaned using a pipette washer (2).
An aliquot of 5% nitric acid was drawn through a 5-mL pipette after the pipette was manually cleaned according to standard procedures. The aliquots were analyzed by ICP-MS. The results showed that significant residual contamination still persisted in the pipettes despite a thorough manual cleaning procedure.
- 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).