Measuring the Absorbance at 260nm / 280nm ratio is not appropriate when applied to synthetic DNA. The A260/A280 ratio is used to measure the purity of DNA following applications in which protein contamination may be an issue, such as DNA extraction from cells. As protein contamination is not a concern with synthetic DNA, we do not use this ratio.

See reference: Validity of nucleic acid purities monitored by 260 nm/280 nm absorbance ratio. Biotechniques 18(1): p 62-63, 1995.