Method to determine the true modulation ratio for comprehensive two-dimensional gas chromatography.

•Modulation induced band broadening for GC×GC is studied.•True modulation ratio is determined from the measurable effective modulation ratio.•Effective primary separation peak widths are measured with Gaussian curve fitting.•Modulation induced loss of primary separation peak capacity is calculated.•Modulation periods of ∼1 to 2s minimize broadening of primary separation peaks.