Expressing the Unit of Measurement in Analytical Chemistry
Authors: de Oliveira Maycon , Korn Maria das Graças , Veiga Márcia 
Published: October 2024

Analytical chemistry is a measurement science that provides both qualitative and quantitative data across various areas, including basic science, industry, medicine, and space exploration. The subject of analytical procedures in multiple fields of analytical instrumentation enables the determination of the composition of mixtures containing two or more components.1 So, we not only detect the composition but also specify the amount of the compound of interest. An example is the analysis of a milk sample to quantify the calcium concentration. In this context, milk is the sample analyzed, while calcium is the analyte being determined.2 In this sense: A sample is analyzed. An element or compound (analyte) is determined. Chemical analysis should include a numerical value, and a corresponding unit to indicate the measured quantity, and a statement of uncertainty.1,3,4 To determine the composition of unknown samples, measurements often require prior calibration, except for classical methods like gravimetry or titrimetry and absolute instrumental techniques such as coulometry, time-of-flight mass spectrometry (TOF-MS), or isotope ratio mass spectrometry (RMS). Constructing an analytical calibration curve requires adjusting the instrumental range with specific quantities and inputting the concentration of the analyte being measured. Efforts should be made to minimize the uncertainty associated with each measurement, even for techniques that rely solely on injecting a fixed sample volume. Calibration standards for analytical chemistry are typically prepared based on mass fraction rather than volume, as density can vary depending on the dilution solvent. This Letter discusses samples that have already been homogenized in the liquid state and emphasizes the importance of expressing results accurately with appropriate units. Standardized Units of Measurement: from foundation to actual days Using a standardized system of units is widely recognized as a valuable tool for effective communication in civil, academic, philosophical, and economic contexts. Expressing scientific data clearly and concisely is essential to ensure comprehension, regardless of the time, region, or country in which the value/unit is presented and interpreted.5 The International Bureau of Weights and Measures (BIPM) was established in 1875 for the development of unit standardization protocols, promoting consistency in scientific reports. Since then, advances have allowed for even more precise metrics, culminating in the establishment of the seven base quantities/units by the 14th General Conference on Weights and Measures (GCWM) in 1972. These units include the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the Kelvin for thermodynamic temperature, the candela for luminous intensity, and the mole for the amount of substance. The seven fundamental units of the International System of Units (SI) are insufficient to describe all the events associated with the properties of matter phenomena. When these units are combined, either the same or different, they generate derived units. For example, average velocity (v) is defined as the position variation (meter) per time of the event (second), generating the unit m s-1. Volume (V) is the combination of three spatial dimensions (m), generating the unit m3. In some cases, the combining of units results in the creation of derived units with unique names. The SI system currently defines 22 units with notable names, such as the Joule (J), defined as kg m2 s-2; the Watt (W), defined as J s-1; and the Newton (N), defined as kg m s-2. Furthermore, the metric system is not static and undergoes variations due to new ways of understanding matter, technological advances, or universal constants. For instance, in 2018, the SI unit for mass was redefined because the physical artifact representing the kilogram "lost weight" over 120 years. Thus, the definition of the seven base units is no longer tied to physical artifacts but rather on the fundamental constants of nature. This Letter will focus on the mole and its derivative units, as they are central to expressing data and results in analytical chemistry. Historically, the mole was defined as the number of atoms in exactly 12 g of carbon-12. However, on May 20, 2019 (World Metrology Day), the GCWM redefined the mole as exactly 6.02214076 × 1023 elementary entities, as recommended by the International Union of Pure and Applied Chemistry (IUPAC):