During a plasmid miniprep, the A_260/280 ratio indicates the relative purity of the eluted DNA by comparing absorbance at 260nm, where nucleic acids have absorbance maxima, to 280nm, where proteins have absorbance maxima. Inefficient neutralization or insufficient washing may allow the excess protein to remain associated with the matrix and elute with the DNA. Using the supplied elution buffer to elute the sample and blank the spectrophotometer will ensure no shift in the 260/280 due to sample acidity. If water is used to elute the DNA, use the same water as a blank on the spectrophotometer.