Ion Torrent sequencing

1. The Chemistry: Translating Hydrogen Ions into Digital Voltage

Thermo Fisher Ion Torrent sequencers replace complex optical lasers, lenses, and fluorescent dyes with standard CMOS semiconductor microchips.

When a DNA polymerase extends a growing DNA strand by incorporating a deoxynucleotide triphosphate (dNTP), a covalent phosphodiester bond is formed between the incoming nucleotide's 5'-phosphate and the strand's 3'-hydroxyl group. This biochemical reaction releases two byproducts: a molecule of pyrophosphate ($ ext{PP}_i$) and a single hydrogen ion ($ ext{H}^+$ / proton):

The microchip surface features millions of microscopic reaction wells, each resting directly atop an Ion-Sensitive Field Effect Transistor (ISFET). The release of hydrogen ions shifts the local solution pH within the microwell:

• If the flowed dNTP is complementary to the template, polymerase incorporates it, releasing $ ext{H}^+$ and causing a measurable $\Delta ext{pH}$ (~0.02 pH units).
• The ISFET sensor converts this pH shift into a proportionate millivolt ($ ext{mV}$) electrical potential change, directly recorded as a digital signal.
• If the nucleotide is non-complementary, no incorporation occurs, no $ ext{H}^+$ is released, and no voltage change is registered.

2. Homopolymer Stretches & Clinical Panel Turnaround

When a template contains consecutive identical bases (e.g. AAAA), the polymerase incorporates multiple nucleotides in a single flow, releasing a proportional quantity of $ ext{H}^+$ ions ($2 imes$, $3 imes$, or $4 imes$ the voltage shift). While accurate for short homopolymers, the signal response becomes non-linear beyond 5–6 identical bases, resulting in characteristic 1-bp insertion or deletion errors.

Despite this limitation, Ion Torrent's direct chemical detection eliminates optical cycle scanning, achieving rapid turnaround times of 2 to 4 hours, making it a staple for rapid clinical oncology and biopsy panels (Oncomine, AmpliSeq).