Suppose a constituent of a gaseous mixture is at a concentration of 1 ppm. The SI unit of volume is the cubic meter (m 3). Gases are miscible and, in general, once allowed to come to equilibrium a gas is homogeneous, in other words its constituents are equally mixed. Parts-per-million by volume is a common way of expressing a concentration in the gas phase. This post focuses on the use of ppm as a measure of concentration.
In nuclear magnetic resonance spectroscopy ppm can be used to describe the amount of chemical shift in frequency (Hz/MHz). Ppm is more often found as a concentration, for example, ppm by mass or ppm by volume (sometimes referred to as ppmv). We do not usually refer to money in ppm, but we could. I would much rather pay a 5 ppm sales tax: For every hundred dollars I spend, I must pay 5 dollars in sales tax. ppm is a ratio between two numbers that have the same units. ppm is an exactly analogous quantity, but it is one part per million instead of one part per hundred. Percent means part-per-hundred, per cent (cent meaning hundred). Percent can refer to just about anything. It can be somewhat difficult for some to understand because it is not a true unit.
Parts-per million (ppm) is a common quantity used in many areas of math and science.
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