The phrase 'infinitesimal amount' is correct and usable in written English when referring to a very small amount of something.
Example: We only have an infinitesimal amount of time to complete this assignment.
Instead, I turned it down an infinitesimal amount and addressed my son's concerns.
The perfect detector ideally has "zero volume"; that is, only an infinitesimal amount of solute enters the sensing region, produces a signal, and exits before the next infinitesimal amount enters the detector chamber.
We are asking for £20m, and the significance of trees and forests to our landscape, to climate change, makes that seem an infinitesimal amount.
Such a process is reversible because only an infinitesimal amount of heat is needed to change its direction from progressive freezing to progressive thawing.
What, after all, did we do when encouraged over decades to moderate our consumption, deny a fraction of our appetites, suffer an infinitesimal amount more so others might suffer infinitely less?
In even a very cold metal, only an infinitesimal amount of excess energy is required to promote a few electrons into the previously empty energy levels in which they can move freely throughout the material.
Infinitesimal amounts could send people into total mental disorder.
These chemicals are virtually odorless and tasteless, and infinitesimal amounts can kill a human being in a matter of minutes.
But given the sophistication and sensitivity of our modern analytical techniques, we can find infinitesimal amounts of almost anything we look for.
The Internet, meanwhile, is one huge sports bar, with Google selling most of the beer by collecting infinitesimal amounts (via advertising) across billions and billions of searches and page views.
Almost 100% of the light within it is reflected in all directions, with only infinitesimal amounts being absorbed or lost.
