Simple RGA Spectra Interpretation

Before plowing through the nitty-gritty here, please accept two important points:

• An RGA spectrum is the only way to truly diagnose vacuum problems
• Interpreting an RGA spectrum is easy! This is not rocket science.

Simple Example

The gas remaining in a chamber under vacuum — the residual gas — is analyzed using an RGA.

The typical quadrupole RGA spectrum (Figure 1) has a linear mass scale (x-axis) and a log or linear peak height or intensity scale (y-axis) indicating the amount present at that mass.

The left spectrum was taken just after the pressure reach the RGA's working range. The major peak is below 30 amu.

The right spectrum was taken later (at lower pressure). All peaks have lower intensities but the major peak is now below 20 amu.

Parent Gas	Relative Mass	Ions Formed	amu
Hydrogen	2	H2	2
		H	1
Helium	4	He	4
Methane	16	CH4	16
		CH3	15
		CH2	14
Ammonia	17	NH3	17
		NH2	16
Water	18	H2O	18
		OH	17
		O	16
Fluorine	19	F	19
Hydrogen fluoride	20	HF	20
		F	19
Neon	20, 22	Ne	20, 22
Nitrogen	28	N2	28
		N	14
Ethylene	28	C2H4	28
		C2H3	27
		CH3	15
		CH2	14
Carbon monoxide	28	CO	28
		O	16
		C	12
Ethane	30	C2H5	29
		C2H4	28
		C2H3	27
		CH3	15
		CH2	14
Oxygen	32	O2	32
		O	16
Argon	40	Ar	40
	36	Ar	36
Hydrocarbons	various	HC	various
		C5H11	71
		C5H9	69
		C4H9	57
		C4H7	55
		C3H7	43
		C3H5	41

To interpret these two spectra, we follow this logic.

• What do we expect when first pumping out a chamber? Air
• Major components of air? Nitrogen and oxygen

Looking at the mass/ion table (left), we see

• N₂ appears at 28 amu
• O₂ at 32 amu
• Ratio for air is 78% to 21% (3.7 to 1)

Left-Hand Spectrum

• The left-hand spectrum has peaks at 28 and 32
• Rough ratio is 7 to 2 (3.5 to 1)
• That means, the residual gas is mostly air

But what about the large peaks at 18 amu and 14 amu? From the mass/ion table we learn 18 is H₂O⁺. An immediate question arises, why is the water peak more intense than the oxygen peak? Gases like nitrogen, oxygen, and argon are not strongly bound to the surfaces. This means they easily enter the gas phase and are easily pumped away. Water vapor, by contrast, clings to every surface, many molecular layers thick. As the pressure is reduced, water vapor molecules enter the gas phase but when they hit another surface they are again strongly bound. This makes it very difficult to pump away and it eventually becomes the dominant residual gas.

Looking at the mass/ion table, we see 14 is attributed to nitrogen. When 70 eV electrons from the ionizer hit neutral nitrogen molecules, not only is a valency electron kicked out (forming a molecular ion N₂⁺) but enough electronic and vibrational energy is dumped into each ion that some will split into N + N, one of which claims the positive charge. Given the high intensity of the 28 peak, it is no surprise to find this so-called fragment ion (N⁺).

[To make a complete story I should mention: if a pure gas sample is ionized and the spectrum of molecular and fragments ions examined, the relative peak intensities and various masses are determined by molecular statistics, kinetics, thermodynamics and instrument factors. Have no fear, that's the last time those subjects are mentioned.]

Novices often think life would be wonderful if RGAs gave only molecular ions. Old hands know fragment ions help the feeble mass resolution of their RGAs. They just wish nature would let us build small, very low-cost, high-mass resolution RGAs so they could ignore fragment ions.

Indeed, this spectrum is a good example of using fragment ions. Look at the mass/ion table and you'll see 28 amu can come from carbon monoxide (CO), ethylene (C₂H₆), or nitrogen (N₂). Why choose nitrogen? Well, ask yourself, what fragment ions are expected from them?

CO gives 12 and 16 → there is little 12 in the spectrum.
C₂H₆ gives 27, 15, and 14 → 27 is present but much smaller than 14.

So, a large 14 peak associated with an intense 28 peak, means N₂.

Right-Hand Spectrum

The most intense peak is 18 (water vapor), with its attendant gaggle of fragment ions at 17 and 16. The second most intense peak, 28 amu, could be N₂, CO, or C₂H₆ or a mixture. While there is something at 14, the ratio 14 to 28 doesn't equal that seen in the left-hand spectrum.

Also, while there is a 27 peak, the 27/28 ratio is smaller than in the left-hand spectrum. This suggests the 28 peak is mostly carbon monoxide. As partial confirmation, look for a CO₂⁺ at 44 and CO⁺ at 28. They are frequently found in the RGAs of diffusion pumped chambers. The large peak at 2 amu is H₂.

Initially, most chambers show strong nitrogen, oxygen and water vapor peaks. After long pumping, the RGA shows strong water vapor, carbon monoxide, carbon dioxide and a little hydrogen. These spectra are consistent with the typical chamber behavior.