It's difficult to comprehend the concentration of a substance at this low a level. To get a mental picture, imagine an average 3-bedroom home. It would take about 1 million marbles to cover the floors of the home. One part per million would be represented by just one marble among all the other marbles. For that same marble to represent one part per billion, however, it would take 20 football fields covered with marbles.
Different elements have different physical properties.
The methods described in the WWW document can be applied to many different geological problems, but no one method can solve every problem. The analytical methods described here are only a few that were selected to show the role of chemistry in geology.