An electroporator applies a controlled electrical pulse that makes a membrane transiently permeable, which is how DNA, RNA and protein are delivered to cells that chemical reagents will not transfect and how a large plasmid or a library is put into bacteria at high efficiency. The instruments differ in the pulse they can produce, and that is what a protocol is really naming when it quotes a voltage and a time.
Exponential decay against square wave
An exponential decay pulse discharges a capacitor through the sample, so the field falls away with a time constant set by capacitance and resistance, and it is the classical setting for bacteria and yeast. A square wave holds the field for a set time and can deliver several pulses, which is gentler on mammalian and primary cells and gives independent control of voltage and duration. An instrument that does only one of the two will run half the protocols in the literature, which is worth checking before the purchase rather than after.
Cuvettes, gaps and cartridges
Field strength is voltage divided by the electrode gap, so a one millimetre cuvette at 1,800 volts and a two millimetre cuvette at 2,500 volts are different experiments, and a protocol that names volts without the gap is incomplete. Bacterial work uses one and two millimetre cuvettes; mammalian work uses four millimetre cuvettes or the maker's own cartridges and plates, which are consumables tied to that instrument. Reusing cuvettes is possible with careful cleaning and is a false economy where arcing has pitted an electrode.
What arcing tells you
An arc is a short circuit through the sample and it destroys the cells and often the cuvette. The usual causes are salt in the DNA or the cell suspension, bubbles in the cuvette, too high a voltage for the gap, and a warm sample. The fixes are mechanical rather than electrical: dialyse or precipitate the DNA out of salt, wash the cells into a low conductivity medium or a maker's buffer, tap out the bubbles, and keep everything on ice. A recorded time constant well below the expected value is the warning before an arc.
Where a chemical method is better
Electroporation is harsh: viability falls, the sample is handled in a small volume and the instrument has to be by the bench. Where a cell line transfects well with a lipid or polymer reagent, the chemical route is cheaper, gentler and easier to scale across a plate. Electroporation earns its place with primary and suspension cells, with large constructs and libraries where efficiency per microgram matters, with protein and ribonucleoprotein delivery, and wherever a reagent's own toxicity would confound the readout.
Questions people ask about electroporator
Square wave or exponential decay?
Exponential decay for bacteria and yeast, square wave for mammalian and primary cells, where independent control of duration and multiple pulses protects viability.
Why does the same protocol fail in a different cuvette?
Field strength depends on the electrode gap, so the voltage has to be recalculated when the gap changes. A protocol without the gap is incomplete.
What causes arcing?
Salt carried in with the DNA or the cells, bubbles, too high a field for the gap, or a warm sample. Wash into a low conductivity buffer and keep everything cold.