In general, Large Sample sizes can not be analyzed. Most samples that are large must be reduced in size to fit onto the sample mount. Sample mounts usually range from 30 mm diameter up to 100 mm x 100 mm for standard size mounts. Large, special design, analysis chambers can handle sample mounts that are up to 200 mm x 200 mm. In general, most samples are reduced in size to allow multiple samples to be placed on the same mount for higher throughput and to reduce the outgassing that large samples often cause.
There is also the height limit restriction for samples. Most samples can not be taller than 5 mm, but certain instruments can accept samples that are 20 mm in height.
Outgassing is a limit because XPS requires UHV so that the photoelectrons can fly without hitting any gas molecules from the sample to the detector which is typically 180-200 cm from the sample surface.
The Size and Shape of Samples Limits the Size of the Actual Sample
Photos of samples on different types of sample mounts having different sizes
In addition to size and outgassing, other limits include:
- Hydrogen and Helium can NOT be detected by normal XPS instruments
- Empirical Formula from XPS results can never include atom% of Hydrogen
- The smallest beam-size, that is useful, is currently 5-10 micron in diameter
- Detection Limits for Elements is at best 500-1000 ppm
- Chemicals have water of hydration will lose water over some time due to sublimation
- Samples must be either conductive or an insulator that is charge neutralized
- Every position on a rough surface or samples with large height variations can not be analyzed
- Physical state of Samples. Gases and Liquids can only be analyzed in special Ambient XPS systems.
- X-ray dosage can degrade the chemistry of various chemical compounds and polymers.
