First: Make sure there are no IR plates in the FT-IR. This is extremely important!
When you tell the FT-IR to take a background, it scans in the single beam mode. When you tell the FT-IR to collect a sample, it scans it as a ratio, subtracting out the background. You have to be careful not to have a sample in the beam when you scan a background; if you do have a sample in the beam, subsequent sample collections will subtract out both the background and a sample, giving erroneous results.
Three ways to scan a background, detailed below:
Click on the "Col Bkg" icon:
Go to the collect menu and down until "collect background" is highlighted:
If you are using the Impact 410, the screen looks slightly different:
If you have scanned the background properly, this is what it looks like (below). If it does not look like this, for instance, if it has a lot more bands, you probably left IR plates in the instrument when you took the background.
