Amperometry
Encyclopedia
Amperometry in chemistry
and biochemistry
is detection of ions in a solution based on electric current or changes in electric current.
Amperometry is used in electrophysiology
to study vesicle
release events using a carbon fiber electrode. Unlike patch clamp
techniques, the electrode
used for amperometry is not inserted into or attached to the cell
, but brought in close proximity of the cell. The measurements from the electrode originate from an oxidizing reaction
of a vesicle cargo released into the medium. Another technique used to measure vesicle release is capacitive
measurements.
or amperometric detection as it was first used in ion chromatography was single-potential or DC amperometry, useful for certain electrochemically active ions such as cyanide, sulfite, and iodide. The development of pulsed amperometric detection (PAD) for analytes that fouled electrode
surfaces when detected eventually helped create a new category of ion chromatography for the determination of carbohydrates. Another advancement, known as integrated amperometry, has increased the sensitivity for other electrochemically active species, such as amines and many compounds that contain reduced sulfur
groups, that are sometimes weakly detected by PAD.
It was established that neurotransmitter
s could be electrochemically
detected by placing a carbon electrode into tissue
and recording the current
from oxidizing neurotransmitters. One of the first measurements was made using an implanted
carbon fiber electrode in the neostriatum
of rats. Further work was done in chromaffin cell
s to investigate catecholamine
release from large dense core vesicles.
Any analyte that can be oxidized or reduced is a candidate for amperometric detection. The simplest form of amperometric detection is single-potential, or direct current
(DC), amperometry. A voltage (potential) is applied between two electrodes positioned in the column effluent
. The measured current changes as an electroactive analyte is oxidized at the anode or reduced at the cathode. Single-potential amperometry has been used to detect weak acid anions, such as cyanide
and sulfide
, which are problematic by conductometric methods. Another, possibly more important advantage of amperometry over other detection methods for these and other ions, such as iodide, sulfite, and hydrazine
, is specificity. The applied potential can be adjusted to maximize the response for the analyte of interest while minimizing the response for interfering analytes
Pulsed Amperometry
An extension of single-potential amperometry is pulsed amperometry, most commonly used for analytes that tend to foul electrodes. Analytes that foul electrodues reduce the signal with each analysis and necessitate cleaning of the electrode. In pulsed amperometric detection (PAD), a working potential is applied for a short time (usually a few hundred milliseconds), followed by higher or lower potentials that are used for cleaning the electrode. The current is measured only while the working potential is applied, then sequential current measurements are processed by the detector to produce a smooth output. PAD is most often used for detection of carbohydrates after an anion exchange separation, but further development of related techniques show promise for amines, reduced sulfur species, and other electroactive compounds.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
and biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
is detection of ions in a solution based on electric current or changes in electric current.
Amperometry is used in electrophysiology
Electrophysiology
Electrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
to study vesicle
Synaptic vesicle
In a neuron, synaptic vesicles store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell...
release events using a carbon fiber electrode. Unlike patch clamp
Patch clamp
The patch clamp technique is a laboratory technique in electrophysiology that allows the study of single or multiple ion channels in cells. The technique can be applied to a wide variety of cells, but is especially useful in the study of excitable cells such as neurons, cardiomyocytes, muscle...
techniques, the electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
used for amperometry is not inserted into or attached to the cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, but brought in close proximity of the cell. The measurements from the electrode originate from an oxidizing reaction
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of a vesicle cargo released into the medium. Another technique used to measure vesicle release is capacitive
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
measurements.
History
ElectrochemicalElectrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
or amperometric detection as it was first used in ion chromatography was single-potential or DC amperometry, useful for certain electrochemically active ions such as cyanide, sulfite, and iodide. The development of pulsed amperometric detection (PAD) for analytes that fouled electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
surfaces when detected eventually helped create a new category of ion chromatography for the determination of carbohydrates. Another advancement, known as integrated amperometry, has increased the sensitivity for other electrochemically active species, such as amines and many compounds that contain reduced sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
groups, that are sometimes weakly detected by PAD.
It was established that neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...
s could be electrochemically
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
detected by placing a carbon electrode into tissue
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
and recording the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
from oxidizing neurotransmitters. One of the first measurements was made using an implanted
Implant (medicine)
An implant is a medical device manufactured to replace a missing biological structure, support a damaged biological structure, or enhance an existing biological structure. Medical implants are man-made devices, in contrast to a transplant, which is a transplanted biomedical tissue...
carbon fiber electrode in the neostriatum
Striatum
The striatum, also known as the neostriatum or striate nucleus, is a subcortical part of the forebrain. It is the major input station of the basal ganglia system. The striatum, in turn, gets input from the cerebral cortex...
of rats. Further work was done in chromaffin cell
Chromaffin cell
Chromaffin cells are neuroendocrine cells found in the medulla of the adrenal gland and in other ganglia of the sympathetic nervous system. They are modified post-synaptic sympathetic neurons that receive sympathetic input...
s to investigate catecholamine
Catecholamine
Catecholamines are molecules that have a catechol nucleus consisting of benzene with two hydroxyl side groups and a side-chain amine. They include dopamine, as well as the "fight-or-flight" hormones adrenaline and noradrenaline released by the adrenal medulla of the adrenal glands in response to...
release from large dense core vesicles.
Detection Methods
Single-Potential AmperometryAny analyte that can be oxidized or reduced is a candidate for amperometric detection. The simplest form of amperometric detection is single-potential, or direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC), amperometry. A voltage (potential) is applied between two electrodes positioned in the column effluent
Effluent
Effluent is an outflowing of water or gas from a natural body of water, or from a human-made structure.Effluent is defined by the United States Environmental Protection Agency as “wastewater - treated or untreated - that flows out of a treatment plant, sewer, or industrial outfall. Generally refers...
. The measured current changes as an electroactive analyte is oxidized at the anode or reduced at the cathode. Single-potential amperometry has been used to detect weak acid anions, such as cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
and sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
, which are problematic by conductometric methods. Another, possibly more important advantage of amperometry over other detection methods for these and other ions, such as iodide, sulfite, and hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...
, is specificity. The applied potential can be adjusted to maximize the response for the analyte of interest while minimizing the response for interfering analytes
Pulsed Amperometry
An extension of single-potential amperometry is pulsed amperometry, most commonly used for analytes that tend to foul electrodes. Analytes that foul electrodues reduce the signal with each analysis and necessitate cleaning of the electrode. In pulsed amperometric detection (PAD), a working potential is applied for a short time (usually a few hundred milliseconds), followed by higher or lower potentials that are used for cleaning the electrode. The current is measured only while the working potential is applied, then sequential current measurements are processed by the detector to produce a smooth output. PAD is most often used for detection of carbohydrates after an anion exchange separation, but further development of related techniques show promise for amines, reduced sulfur species, and other electroactive compounds.