Standard test tubes are widely used for protein determination on food, feed and beverage following the Kjeldahl method, as well as the distillation of different samples according to other applications and methods. Samples of heterogeneous material often need to be fairly large to ensure that they are representative. There is no lower limit of sample size in the 250 mL tubes; they simply allow greater flexibility for most types of samples. Depending on the expected nitrogen content, optimal amounts of sample are usually in the range of 0.1-5 g. Designed for Kjeldahl extraction on a micro-scale, Micro Kjeldahl tubes can be used if the material is homogeneous and small samples are fully representative, or if it is specified by an approved method (AOAC method 960.52, Microchemical determination of Nitrogen). Note that the Micro-Kjeldahl method requires smaller volumes of chemical reagents. Micro Kjeldahl units are also used to perform the step of nitrogen content determination during the dietary fiber extraction process. Jumbo tubes are perfectly fitting in standard digestion and distillation systems due to the upper diameter of 42 mm, and they are designed for handling larger liquid volumes, typically 50-100 mL. To determine total nitrogen in low nitrogen liquid samples such as water according to the Kjeldahl method, where it is necessary to sum organic nitrogen (proteins, nucleic acids, urea, synthetic organic chemicals) and ammonia nitrogen, Jumbo test tubes are a convenient accessory. Heavy metal complexes, sulfides, aromatic amines, oxidants and dyes interfere with cyanides' colorimetric or titrimetric determination. Hydrocyanic acid has to be separated from the water sample by distillation. As 200 mL of sample are analyzed, a 1-liter test tube has to be used to perform the analysis and obtain reliable and accurate results.