Ion Torrent is a brand from Thermo Fisher Scientific. The Ion S5 and Ion S5XL instruments are based on semiconductor sequencing technology.
Semiconductor sequencing relies on detecting the release of a proton when a nucleotide is incorporated into a growing DNA strand. Of course, one proton isn’t enough. The release of thousands (millions) of protons are detected as a change in current by an ISFET ion sensor on a chip similar to how a digital camera detects light on a chip.
Dependent on amplicons
Sufficient signal to measure incorporation is generated by performing the sequencing reaction on multiple template strands amplified on a silicon bead loaded into a single well on the semiconductor chip.
No modified nucleotides
Modified nucleotides aren’t required because base incorporation is detected by release of protons and only one type of base is available for incorporation at a time. The polymerase will incorporate as many bases as there are complementary bases on the template strand. For example a string of three adenines on the template strand will result in the incorporation of three thymines and 3 times the electrical signal of a single thymine.
The primary source of error in semiconductor sequencing is saturation of the electrical signal. Detecting one, two or three bases in a cycle yields a relatively proportional signal. As more identical bases are incorporated, distinguishing seven from eight or nine becomes more difficult.