Recommended Procedures for Sampling and Handling Grape Petioles: Obtaining a representative sample is important to avoid misleading results and erroneous interpretations. Even the best-equipped laboratory cannot make up for a poorly collected or improperly handled sample. Optimum nutrient ranges are based on petioles sampled at a specific maturity level and collected at a specific period during the growing season. The following guidelines should be used for proper sampling and handling procedures: