Background to RGA Spectra

When answering the many variations of “How do I interpret an RGA spectrum?” I've always found it necessary to start with basic atomic theory explaining electrons, protons, neutrons, atoms, molecules, and relative mass.

So, this is the background information needed before tackling the question.

Atoms and Molecules

Atoms are the smallest particles of matter that retain chemical identity. That is, all atoms of oxygen are alike but different from all atoms of nitrogen. Each atom has a nucleus, containing protons and neutrons, surrounded by a cloud of electrons. A proton carries a single positive charge and an electron a single negative charge. Since atoms are neutral, the number of protons in a nucleus equals the number of electrons buzzing around it.

An atom, or collection of identical atoms, is called an element. There are about 90 elements in our little planetary system. (There could be well over 100 but technetium, promethium, and everything above uranium (number 92) are short-lived radioactive elements that checked out eons ago.) Everything is a collection or combination of these 90 elements. Better yet, every-last-loving-one of those atoms, except perhaps the hydrogen, deuterium, and helium, were made in the core of another sun. Think about that, everything from mercury to Pluto, from viruses to Wales, from Hale-Bopp's comet to the DeHaviland Comet, from La Tour Eiffel to the PGA tour, from sea to shining sea, and even you, are combinations of only 90 different elements made during some grand alchemy experiment in a long-gone star.

What makes the elements different? Why is the element gold a bright yellow metal and the element oxygen is an invisible gas? Why does a helium-filled balloon rise into the stratosphere and a lead brick stop x-rays? In a gross simplification, chemical differences are caused by the number of electrons in the “cloud” surrounding the nucleus and physical differences are caused by the mass of the nucleus.

So, each element has a unique number of protons ranging from 1 to 92. But that's not the complete story. Nature won't let two protons get close (as they must in the nucleus), their positive charges causes them fly apart without neutrons to act as “glue.” Adding the right number of neutrons forces the protons to co-exist and makes the atom stable. Is there only one right number of neutrons for any given number of protons? No. Most hydrogen atoms in the universe have one proton and need no glue. However, one hydrogen “brand” called deuterium has one neutron added to its proton. Since one proton still means one electron, deuterium is chemically very close to ordinary hydrogen, but it's mass is twice as much.

The different forms caused by more or fewer neutrons in atoms of one element are called isotopes. Helium has two protons but comes in two isotopic forms, with one neutron or two. Carbon atoms have six protons but can have six, seven, or eight neutrons creating different isotopes. But eight neutrons is too much of a good thing and carbon 14 (so called because protons + neutrons = 14) is radioactive and the nucleus breaks down to form another element. The world leader in stable (non-radioactive) isotopes is xenon (used in photographer's high power flash lamps) with nine.

To complete this background, we need two last facts. In their gaseous or vapor form, some elements are composed of single, separate atoms (e.g., helium, neon, argon, mercury) and others are two or more atoms combined in an elemental molecule (e.g., oxygen, ozone, nitrogen). Molecules formed from two or more different elements are called compounds and this word opens floodgates of chemical diversity. The huge array of chemical compounds that surround us and, indeed, are us, exist because two or more atoms of different elements combine to form stable molecular compounds.

Atomic Weights

atom	symbol	mass (amu)
hydrogen	H	1
helium	He	4
carbon	C	12
nitrogen	N	14
oxygen	O	16
fluorine	F	19
chlorine	Cl	35,37
argon	Ar	40

Since protons and neutrons have almost the same mass, (~1800 x the electron's mass) we ignore electrons and make a relative mass scale based, originally, on the one proton in normal hydrogen, calling it one ‘atomic mass unit’ (abbreviated amu). To find the relative mass of any other atom (or molecule), you add together the number of protons and neutrons in the atomic nucleus and call that the relative mass (see table).

Today, mass tables use the most abundant carbon isotope atom C = 12.000000 daltons (as they are called in SI units) and relate everything to one twelfth of that mass. But when interpreting low resolution RGAs, either scale gives the same answers since we only use the nominal mass (rounded to the nearest whole number).

Although C, O, and N have isotopes, they have low natural abundance and are ignored as “insignificant” compared to the abundances of the listed masses. Chlorine's isotopes, by contrast, are both significant: isotope 35 is ~76% and isotope 37 is 24%, and for that reason both, are listed in the table.

Molecular Weights

molecule	symbol	mass (amu)
hydrogen	H2	2
methane	CH4	16
water	H2O	18
nitrogen	N2	28
carbon monoxide	CO	28
ethylene	C2H4	28
oxygen	O2	32
carbon dioxide	CO2	44
fluorine	F2	38
chlorine	Cl2	70,72,74
heptane	C7H16	100

Molecular symbols, for chemically identifiable substances, are formed by combining elemental symbols with subscripts showing the number of atoms (of one type) in the molecule. The relative molecular mass is determined by adding the masses for all the elemental atoms that go into its makeup.

Again chlorine confuses us with three numbers instead of the expected two. This arises because the chlorine's two isotopes will combine into a molecule in three different ways (35/35), (35/37), or (37/37).

As Murphy's optimistic brother noted: if it can happen, it will.

Air: Major Components

Element	Concentration
nitrogen	78%
oxygen	21%
argon	1%

The first gases we remove when pumping a chamber are the components of air. If the effective pumping speed from the chamber is high, when the pressure is low enough to switch on an RGA (~1 x 10^-4 torr), the major components will be nitrogen and oxygen. However, if the effective pumping speed is low and it takes a long time to reach ~1 × 10^-4 torr, the major component indicated is usually water vapor.

The major constituents of air, listed here, equal 100%. That is, the remaining components, listed below with their sources, are in pretty tiny concentrations

Air: Major and Minor Components