# Carbon in Glass - XPS Analysis

I was reading a paper and found this elemental XPS analysis of an unmodified microscope glass slide surface.

C (1s): 17.7%

Si (2p): 28.47%

O (1s): 53.83%

It seems strange for a piece of normal glass to have almost 20% of Carbon, since most of it is silicon dioxide and other metal oxides in smaller amounts. I am not used to XPS analysis so there might be something I am overlooking (maybe adsorbed CO2 on the surface ?), can someone tell me if this is a normal result for an XPS analysis of glass, and if it is, where does the carbon come from?

• Was the slide washed or cleaned before analysis? Almost every surface gets a few to several at% C on the surface just by air exposure (hydrocarbons etc). XPS is hugely surface sensitive after all, so contamination can have a big effect on the data. A "dirty" slide could certainly approach 20 at% carbon. – AndyW Nov 6 '17 at 13:13

Well, XPS analysis should detect atoms as light as Carbon, but not atoms as light as Hydrogen.

Far be it from me to claim any expertise on microscope sliders, but I have used one or two in my career, and I am hazarding a guess. It is likely a pure glass ($\ce{SiO2}$) sheet laminated with some acrylic glass. The hydrogen in the acrylic polymer are too light to appear on the XPS.

There is a slight possibility, although I consider it very unlikely, that it is glass with a substrate of moissanite on top, which would in theory make it scratch proof, due to the hardness of $\ce{SiC}$.

If you would include whether the %'s are weight percent or atomic percent, it would be easier. If it is atomic, then the case for $\ce{SiC}$ is more likely, since the ratio ($\ce{Si:O} > \ce{1:2}$) and the extra Si would be in SiC. If it is weight percent then there is some math to be done. I still think it is just glass and acrylic glass.

• It is in atomic ratio... I think it's possible that the glass is laminated with some sort of C-containing layer, although no information is given in the paper. – Raul Luciano Nov 5 '17 at 20:12
• Do you think it could be surface contamination of a pure silicate-based glass (i.e no acrylic)? – Curt F. Nov 6 '17 at 13:24

This carbon they have reported (considering the amount present and without knowing the sample preparation) is likely adventitious carbon.

This carbon is on every surface and so perhaps the glass was not cleaned well before the measurement. Even then it is likely some carbon will still be present.

Adventitious carbon (binding energy 284.8 or 285.0 eV) is often used as a charge reference in XPS binding energy analysis.

My question is did they crush the material and kept as a powder or did they keep it a slab of glass?

If they have done XPS on the powder then they just reported the atomic percentage coming from the carbon tape which is not part of the materials. It is simple to get the new atomic percentages after you don't count in the carbon contribution.

If they have used the glass slab as a whole then there is a possibility of contamination from the carbon tape.