Contaminants from Plastic Microtubes Distort Results of UV Measurements

A recent paper pointed out the danger of overestimating the amount of nucleic acids or protein in samples analyzed by UV spectrophotometry due to contaminants that leach from plastic microtubes into the sample solution.

Plastic containers such as "microfuge” tubes used for the storage and manipulation of biological samples comprise complex polymer materials containing several chemical additives. These additives include supplements such as chemical antioxidants, antimold release agents, biocides, and UV-light stabilizers. Plastic materials may also contain other low–molecular weight chemicals such as residual unreacted monomers and polymer degradation products. Microfuge tubes are produced by numerous manufacturers and are among the most commonly used pieces of laboratory ware in molecular biology laboratories.

Since previous studies have demonstrated that additives and other chemicals can leach from plastics into the environment, investigators at Texas State University (San Marcos, USA) examined the effect of such contaminants on spectrophotometric measurements carried out on samples stored in plastic microtubes.

They reported in the April 2010 issue of the journal BioTechniques that normal handling of laboratory microtubes caused leaching of light-absorbing chemicals into biological samples that interfered with spectrophotometric measurements. The leached chromophores absorbed UV light strongly between 220 nm and 280 nm, which are the wavelengths normally used to detect and quantitate proteins and DNA. Some common laboratory techniques, including sonication and PCR, were particularly effective inducers of leaching. The magnitude of the increase in absorbance was dependent upon both exposure time and heat history, with greatest induction after tubes were warmed to temperatures at or above 37 °C. Mass spectrometry revealed that aqueous solutions stored in plastic microtubes accumulated a complex mixture of leached chemicals with molecular masses between 200 Da and 1,400 Da. Leaching was a common attribute of all commercially available brands of microtubes, indicating a persistent source of error in biomolecule detection and concentration measurements.

"It was quite surprising that leaching of chemical additives, which absorb light at the same wavelengths as DNA and proteins had not been described before because it is so ubiquitous among commercially available plastic tubes, and biochemists have been using these small plastic tubes for many decades,” said first author Dr. Kevin Lewis, associate professor of chemistry and biochemistry at Texas State University.

"Some manufacturers have recently begun offering tubes that contain a reduced number of additives,” said Dr. Lewis. "At the moment the use of polypropylene tubes with fewer additives appears to be the best way to reduce measurement errors due to leaching.”

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