When samples are analyzed in a flame photometer in a volatile solution, the effects of evaporation often cannot be neglected. This effect generally becomes more apparent whenever large numbers of samples are aspirated from an automatic sample changer. It has been found at Scripps Institution that evaporation losses can be greatly reduced by using slotted covers over the sample cups. The covers are pierced with slotted openings that admit the aspirator tip during aspiration but restrict the free escape of saturated vapor standing above the sample.
T. R. Folsom, G. W. Bode, and R. Grismore, "Suppression of Evaporation Losses during Flame Photometry," Appl. Spectrosc. 24, 378-379 (1970)