Development of international standard methods for Glow Discharge Optical Emission Spectroscopy (GD-OES) within the ISO committee TC201 (Surface Chemical Analysis) SC8 (Glow Discharge Spectroscopy) started in 1993. In recent years there has been a growing interest in developing standards for very thin coatings. A standard for metallic nano layers on steel has recently been published. The development work resulted in a procedure for testing lamp performance in terms of plasma startup prior to analysis. Statistical analysis of a Round Robin (RR) test involving several laboratories will be presented and discussed. One result is that a lower limit of 10 nm layers for the standard was imposed, although high end GD-OES systems can provide quantitative data for layers in the few nm range. A more demanding challenge is quantitative determination of the light elements O, H and C in thin layer < 100 nm. The limitations are mainly due to intrinsic contamination of the GD lamp and residual surface contamination. Techniques to reduce these problems and practical limits of determination will be discussed.