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.
There are many persistent solvents that can be found in the laboratory which can cross-contaminate samples by their presence. Some persistent solvents include dichloromethane which can cause chlorine contamination as well as dimethyl sulfoxide (DMSO) and carbon disulfide, which can add sulfur residues. There are solvents that react with air to form peroxides, which can cause contamination and safety issues in the laboratory.
Acids are another laboratory reagent that can become both a potential danger and a potential contaminant. Acids by their nature are oxidizers and many of the strongest acids are used in the processing of samples for inorganic analysis. Common acids in digestion and dissolution include perchloric acid, hydrofluoric acid, sulfuric acid, hydrochloric acid, and nitric acid. Many of these acids are commercially available in several grades from general laboratory or reagent grade to high-purity trace metal–grade acids. Acid grades often reflect the number of sub-boiling distillations the acid undergoes for purification before bottling. The more an acid is distilled, the higher the purity. These high-purity acids have the lowest amount of elemental contamination but can become very costly at up to 10 times the cost of the reagent grade of acids.
Often the question is asked if high purity acids are necessary in sample preparation if the laboratory is using a high-quality ICP-MS-grade CRM. Clean acids used in sample preparation, digestion, and preservation can be very costly. But, the difference between the amounts of contamination in a low-purity acid and a high-purity acid can be dramatic. High-quality standards for use in parts-per-billion and parts-per-trillion analyses use the highest purity acids available to reduce all possible contamination from the acid source. An example of potential contamination is an aliquot of 5 mL of acid containing 100 ppb of Ni as contaminant, used for diluting a sample to 100 mL, can introduce 5 ppb of Ni into the sample.
To reduce contamination it is recommended that high-purity acids be used to dilute and prepare standards and samples when possible. In addition to using pure acid, it is important that the chemist check the acid’s certificate of analysis to identify the elemental contamination levels present in the acid. Some laboratories prefer to use blank subtractions to negate the background contamination, but blank subtraction for acids can only work in a range well over the instrumental level of detection. If blank subtraction causes an analytical result to fall below the instrument’s level of detection, it should not be used.
Volumetrics and Labware
Volumetric measurement is a common repeated daily activity in most analytical laboratories. Many processes in the laboratory from sample preparation to standards calculation depend on accurate and contamination-free volumetric measurements. Unfortunately, laboratory volumetric labware, syringes, and pipettes are among the most common sources of contamination, carryover, and error in the laboratory.
The root of these errors is based on the four “I” errors of volumetrics:
- Improper use
- Incorrect choice
- Inadequate cleaning
- Infrequent calibration
These four I’s can lead to error and contamination, which negate all intent of careful measurement processes.
- 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).