Standards and reference materials are indispensable in chemical analysis, material testing, and quality control. Many analyzes are comparative measurements, not absolute measurements. Here the meter only provides a relative signal which needs to be calibrated to a known value. Reference materials are required for this reference. Reference materials may be solids, liquids or gases. Standards are substances that are as similar to the analyte as possible but do not occur in the sample. After analyzing the results the internal standard is compared with the expected amount.