Q: Sending aperture library to an external drive

I have already sent the masters. Does that leave the photos that we have altered ( the finished item ) in the aperture library on the Mac?

How did you "send the Masters" to the external drive?  If you used the command in Aperture "File > Relocate original ....", then the original master files will have been moved to the external drive and the edited versions are still in your Aperture library and the library will still be in a working condition and you can work with Aperture from the internal drive.

I'd also like to hear the best option for a backup of the masters? I have been using Time capsule all along so the masters would have been backed up on Time capsule but I assume now that I have removed them that I should do another form of a back up?

Time Machine will remove files that are no longer on the system drive from the backup sooner or later.  You need indeed a separate backup of the original files. I would simply include the external drive with your original image files into the Time Machine backup.   And you could copy the folder with the master files to a separate backup drive as well, so you will have a second backup of the original files.