# What is a nanogram?

The International System of Units, abbreviated SI, is the most widely used system of units of measurement in the world. With the exception of Burma, Liberia and the United States, the rest of the countries have included it in their legislation as the only or priority issue.

The SI initiative began in 1948 and was first adopted in 1960 in the XI General Conference on Weights and Measures. It is currently integrated into the rules ISO/IEC80000 As the International System of Quantities (SSI – International System of Quantities). In the first version it included six basic units, in 1971 the mole was added:

• meter (m), as a unit of length
• kilogram (kg), as a unit of mass
• second (s), as a unit of time
• amp (A), as a unit of electric current intensity
• Kelvin (K), as a unit of temperature
• candle (cd), as a unit of light intensity
• mole (m), as the amount of substance that contains a number of elementary units (atoms, molecules, electrons, or others) equal to Avogadro's number (6022 × 1023).

The SI is based on the older decimal metric system and all of its units can be defined multiples and submultiples of base 10. Thus, the gram (g) is one thousandth of the kilogram (kg), which is the basic unit of mass.

The nanogramof symbol ngcan be defined as an SI unit of mass equivalent to one billionth of a gram or to one billionth of a kg. That is, 1 g contains a billion nanograms (1 ng = 10−9 g) and 1 kg contains 1 trillion nanograms (1 ng = 1012 kg).

The amount of mass expressed by a nanogram is very small and is rarely used outside of science. For example, the milligram (mg) is one of the smallest commonly used units and 1 ng is 1 million times smaller, 1 mg = 1,000,000 ng. The microgram, symbol µganother very small unit of mass, is still a thousand times larger than the nanogram, 1 µg = 1000 ng.

It is important to remember that the nanogram and related units are units of mass, not weight. Weight is a force resulting from the effect of gravity on mass, which is why it is common that, being on Earth with constant gravity, both concepts are used interchangeably, even though they are not the same. If we traveled to a planet with a gravity less than Earth's gravity, we would maintain the mass but weigh less.

The kilogram is the only SI base unit of measure that is still is not defined with respect to a fundamental physical property. For example, the meter is defined in terms of the distance traveled at the speed of light in 1/299,792,458 seconds, and the speed of light is a fundamental physical property.

The kilogram, on the other hand, is defined as the mass of a reference object and not based on a fundamental physical property. The reference to define the kg, and therefore the nanograms, is a prototype of platinum (90%) and iridium (10%) that is stored in the International Bureau of Weights and Measures (BIPM) in Sèvres (France). Thus, by the current definition of a nanogram, 1 ng equals exactly one trillionth of that reference object.

Go up