An optical emission spectroscopic (OES) instrument includes a spectrometer, a processor and an adjustable mask controlled by the processor. The adjustable mask defines a portion of an analytical gap imaged by the spectrometer. The instrument automatically adjusts the size and position of an opening in the mask, so the spectrometer images an optimal portion of plasma formed in the analytical gap, thereby improving signal and noise characteristics of the instrument, without requiring tedious and time-consuming manual adjustment of the mask during manufacture or use.