Glossary

[A]

Absolute mV mode - a mode of operation of a pH/mV meter, which allows the actual true potential of the electrode to be displayed. In the absolute milivolt mode the calibration control (assymetry potential control) does not change the readings.

Absorbance (A) - a physical quantity, which expresses the amount of light absorbed by a sample solution. When a light beam passes through a sample, the amount of light absorbed is the difference between the incident radiation (Io) and the transmitted radiation (I). Absorbance is defined: A=-log Io/I.

Absorption - energy transfer from a photon of light to a chromogenic molecule.

Absorption spectrum - a plot of absorption light wavelength versus the amount of light absorbed by the molecules.

Accuracy - the closeness of a measured value to the known "true" value of the measurand.

Actuator - a device, which uses the signal, form the sensors to perform some action. An example can be an alarm with a smoke detector or a hydraulic valve coupled to a pH electrode.

Activity - the effective amount of a free ion in solution. The amount and type of other ions in the solution influence the chemical effectiveness of an ion, so that varying the solution composition makes a fixed concentration of a given ion more or less "active". In dilute solutions, ionic activity and concentration are practically identical, but in solutions containing many ions, activity may differ from concentration. Ionic activity, not concentration, determines both the rate and the extent of chemical reactions.

Activity coefficient - a factor, which relates the activity to the concentration of a species in solution, such that: Ax = FxCx (where: Ax = Activity of the species x; Fx = Activity coefficient of the species x; Cx = Concentration of the species x). The activity coefficient is dependent on the ionic strength of the solution (ions of similar size and charge have similar activity coefficients). It becomes progressively lower as the ionic strength increases, due to inter-ionic interactions. The activity coefficient for any ion in solution can be calculated using the Debye-Huckel equation.

Analog multiplexer - a device that increase the number of measurements channels while still using a single instrumentation amplifier.

Asymmetry potential - the potential across a glass pH electrode membrane when the inside and outside of the membrane are in contact with solutions of identical pH. This term has also been used to define the observed potential differences between identical electrode pairs placed in identical solutions. Differences can occur because of variability in the potentials of the internal reference elements of both the sensing and the reference electrodes, differences in liquid junction potentials, and differences in internal filling solutions. These variations in electrode potential are compensated for by the instrument calibration control (asymmetry potential control).

AU - arbitrary unit.

Automatic temperature compensation (ATC) - automatic instrumental adjustment of the measured parameter to correct for variations in sensor (e.g. electrode) slope due to temperature changes. The output of a thermistor probe regulates the correction factor.

Average number - the number of instantaneous readings of the sensor (e.g. electrode potential), taken for example at one-second intervals, used to calculate the average value for the millivolts. An operator-selectable variable in computer interface software which helps to reduce noise and increase the precision of ISE measurements.

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[B]

Biosensor - an analytical device, containing biologically active material (i.e. enzyme, antibody receptor etc.) introduced into a receptor part (membrane), contacting with an appropriate transducer element to detect (selectively and reversibly) the concentration of chemical compound in the sample.

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[C]

Calibration - a process of normalizing sensor output by measuring a series of two or more known concentration solutions. The ion analyzer then calculates the offset and slope characteristics of the electrode and uses them to compute the concentration of unknown samples.

Calibration curve - a plot of electrode potential versus activity in two or more standardizing solutions. Unknown sample activity is determined by converting electrode potential to activity using the curve.

Channel - a pin or wire lead to which you apply or form which you read the analog or digital signal.

CHEMFET (Chemically Modified FET or Chemically Sensitive FET) - an electrochemical microsensor involving the deposition of a polymeric ion-selective membrane on the surface of the FET (field effect transistor) transducer. The appropriate composition of the ion-selective membrane allows designing of CHEMFET with required selectivity. In this case, the introduction of an additional hydrogel layer (polyHEMA), soaked with an internal solution, between the gate oxide of an ISFET and an ion-selective membrane is required.

Chemical sensor - a miniaturized analytical device, which can deliver real-time and on-line information on the presence of specific compounds or ions in the complex samples.

Chemooptical interface - the receptor part (see optomembrane) of a fiber optic chemical sensor containing an immobilized reagent (e.g. indicator, dye or chromoionophore), which converts chemical information on the sample into changes of its spectral properties (absorbance, fluorescence).

Chromoionophore - in fiber optic chemical sensors, an appropriate reagent i.e. chromogenic ionophore (chromoionophore or fluoroionophore), that is responsible for the ion recognition process in an optomembrane. The chromoionophore molecules include an ion-recognizing complexing center (ionophoric moiety) coupled with a chromogenic group, which transduces the chemical information into the changes in optical signal. The selectivity of the optomembrane is governed by the selectivity of the ion-chromoionophore complex formation.

Combination electrode - a combination of a sensing electrode and a reference electrode contained in one unit.

Complexing agent - any species that combines with an ion to form an undissociated species; the resulting complex stays in solution and does not precipitate. Complexing agents are used as titrants and to bind ions that may interfere with direct measurements.

Concentration - the total mass of an ion or molecule in a given volume of solution (see: activity). When measuring ionic concentration by ion-selective electrodes, it is important to note that the concentration used in the calibration graphs and calculated for the samples is the concentration of the free ion in solution, not the concentration of the compound from which this ion is derived. A distinction must also be made between the concentration of the free unbound ions and the total concentration, which may include any ions, bound to complexing agents and any atoms in undissociated molecules.

Conductivity - a measure of the ability of a solution to conduct electricity. It is the reciprocal of resistivity, which relates the resistance of a conductor (in ohms), to its length and cross sectional area. Units of conductivity are Siemens per centimeter (S/cm). Conductivity is measured with a conductivity cell. This contains two platinum electrodes of known area rigidly fixed at 1 cm apart. The electrolytic conductivity of the solution is determined by passing an alternating current between the electrodes. The conductivity is related to the ionic strength of the solution.

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[D]

Data acquisition (DAQ) - process of acquiring data, typically from A/D or digital input plug-in devices.

DDE - Dynamic Data Exchange.

Detection limit - the concentration at which the mean value of the output sensor signal is equal to two standard deviations. In practice, the lowest concentration of the analyte that can be detected and/or measured by a sensor. In other words, the concentration (or activity) of the measured ion at the point of intersection between the extrapolated linear segment of the calibration curve (representing the normal slope of the electrode) and a horizontal line (representing the voltage when the concentration is so low, that small changes in concentration do not produce any detectable change in the electrode response). The portion of the calibration curve between this point and the beginning of the truly linear section is known as the non-linear range of the electrode. Samples are still measurable within this range provided that several standards are used to define the changing slope of the curve accurately, but the error in concentration (per millivolt error in measurement) will be progressively greater as the slope reduces.

Detector - a device, which indicates the presence of the chemical species above a predetermined, threshold value. There is not explicit qualitative relationship between the output and stimulant.

Direct potentiometry - the simplest method of making ion-selective electrode measurements. The electrodes are immersed in a test solution and the electrode potential is measured directly with a millivolt meter. The concentration is then related directly to this measurement by reading the answer from a calibration graph of concentration versus milivolts.

Dissociation constant (K) - number indicating the extent to which a molecule dissociates in solution to form free ions. For a simple two-component system (e.g.: CH3COOH <> CH3COO^- + H⁺) it is the product of the molar concentration of the two ions divided by the molar concentration of the undissociated molecule: K = ([CH3COO^-] x [H⁺]) / [CH3COOH]. The smaller the value of K, the less dissociation is present. K varies with temperature, ionic strength, and the nature of the solvent.

DLL - Dynamic Link Library.

DMA - Direct Memory Access.

Drift - slow changes in the signal of a sensor maintained in a solution of constant composition and temperature.

DSP - Digital Sound Processing.

Dynamic range - the range of concentrations in which the sensor sensitivity is greater than zero. The dynamic range can be also expressed as the difference between minimum and maximum signal values of the sensor in steady-states.

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[E]

Electrode head - a molded plastic assembly containing one or more sockets for the insertion of appropriate electrodes fitted with plug-in connectors. The electrode head carries the expensive screened cable and plug for connecting to the measuring instruments. Various models are available for use with one electrode (mono head) or an ISE/reference combination (dual head), or a combination of different ISEs and/or pH, redox, or dissolved oxygen electrodes used with a common reference electrode for multi-component analysis (multiple head).

Electrolyte - a substance which ionizes in aqueous solution. Weak electrolytes are only slightly dissociated into ions in solution (acetic acid), and strong electrolytes are highly dissociated (HCl, NaCl). Strong electrolytes are good conductors of electricity, and conductance measurements are well correlated with electrolyte strength.

Emission - the release of light from a chromogenic or fluorescent molecule, when an electron falls from an excited state to a lower energy state of the molecule.

Emission spectrum - a plot of emitted light wavelength versus the relative intensity of emitted light. Essentially a plot of wavelength versus the probability that the emitted light will have the energy of that wavelength.

Error (of a measurement) - the result of a measurement minus a true value of the measurand.

Excitation - the absorption of light energy by a fluorescent molecule, during which an electron in the molecule is boosted to a higher energy level.

Excitation spectrum - a plot of incident-light wavelength versus the total fluorescence emitted.

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[F]

FFT - Fast Fourier Transform.

FIA (flow-injection analysis) - well-established technique of chemical analysis, which involves the injection of a sample plug into a flowing reagent stream with subsequent detection (electrochemical or spectroscopic) of the reaction zone further downstream.

Fiber-optic chemical sensors - optical sensors, which use fiber optics to guide light beam from a source of light to an optrode and from an optrode to a detector.

Filling solution - the solution inside a sensing or reference electrode, that are permanently sealed within the electrode (like the buffer inside a pH electrode) are usually called internal reference solutions to differentiate them from filling solutions.

Fluorescence - the emission of light (or other electromagnetic radiation of longer wavelength) by a substance as a result of absorption of other radiation. The emission continues only as long as the stimulus producing it continues and persists with a half-life of less than about 10^-8 second.

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[G]

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[H]

Hysteresis (sensor memory) - the occurrence of a different value in the potential difference after the concentration of the test solution has been changed and then restored to its original value. This systematic error is generally in the direction of the concentration of the previous solution. Thus, if the electrodes are washed with water between each sample measurement, successive readings of the same solution can be expected to become progressively lower for cations and higher for anions.

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[I]

IEEE - Institute for Electrical and Electronic Engineers.

Immobilization of the reagent - procedure used in the design of the receptor part, required to obtain stable signal of a chemical sensor. The appropriate reagent immobilization consists the main problem of the design of receptor part of fiber optic chemical sensor. The reagent can be immobilized either directly on the surface or in the bulk of the optomembrane. Physical (entrapment, formation of lipophilic ion-pairs) and chemical (covalent binding of the chromoionophore) methods of the reagent immobilization in the chemooptical interface are used. Also there is possible to immobilize the reagent phase directly on the fiber. In all cases, the efficiency of the reagent immobilization governs the durability of the designed sensor.

Interface - an electronic device, which allows connecting the output from a sensor directly to a desk-top or lap-top computer without the need for an expensive ion meter. Interfaces are provided with sophisticated software, which facilitates complex data processing, display and storage. Multiple interfaces can connect several electrode systems to one PC at the same time and permit continuous monitoring of batch processes or simultaneous multi-component analysis.

Interference - the effect of any species, other than the ion being measured, which presence in the sample solution affects the measured potential difference of the cell. The most common cause of interference is due to the fact that most ion-selective membranes are not 100% selective for the detected ion. Many permit the passage of other ions to some extent. These extra ions generally increase the measured electrode potential and cause spuriously high concentration results. Other interferences may be due to chemical reactions with the membrane material which may cause positive or negative anomalies, and reactions in the sample which may cause precipitation, complexing, oxidation or reduction of the ion being measured, and thus spuriously low results.

Interfering ion - any ion in solution, other than that being measured, that reacts with the ISE membrane to change the measured potential (see Interference).

Internal reference electrode (of an ISE) - part of the electrode, which is electrically, connected to the screened cable which connects to the measuring instrument. Usually a chloridised silver wire immersed in the silver chloride/potassium chloride filling solution or gel. In the case of all-solid-state electrodes, the material of the reference electrode is deposited directly on to the internal surface of the ion-selective membrane and the internal filling solution is not necessary.

Internal filling solution (of an ISE) - an aqueous electrolyte solution, which may be gelled, containing a fixed concentration of the ion to which the inner reference electrode is reversible; usually the chloride ion in the case of silver/silver chloride electrodes. This forms the electrolytic conductor, which transfers the charge from the detected ion in the ion-selective membrane to the electrical conductor (silver wire) which conducts the charge to the measuring system. Not present in all-solid-state electrodes.

Ion meter - a sophisticated pH/milivolt meter containing a microprocessor, which allows for the processing of ISE signals to permit electrode calibration and direct reading of sample concentrations.

Ionic additives (lipophilic salt) - additional component of an ion-selective polymeric membrane required to obtain theoretical sensor performances. The Nernstian response of the potentiometric sensor is measured if the presence of lipophilic counter ions maintains the constant concentration of the free target ions in the membrane, due to the ion buffering mechanism of the ionophore. This buffering mechanism establishes a constant ratio between the concentration of the ion-ionophore complex and the free ionophore in the membrane. The presence of ionic sites in the membrane is also necessary to improve the selectivity of the potentiometric sensors. In cation-selective electrodes with neutral receptors, tetraphenylborate salts are added to the membrane composition (tetraalkylammonium salts for anion-selective membranes containing neutral ionophore). Recently, it has been shown that the presence of ionic sites is also required in the case of ion-selective membranes with charged ionophores.

Ionic strength - this is a measure of the effective concentration of ions in solution. It is calculated according to the following formula: Ionic strength = 1 /2 CxZ^2 (where: Cx = the concentration of ion x; Zx = the charge of ion x). The ionic strength determines the activity coefficient of each ion in the solution. Conductivity measurements give an estimate of ionic strength.

Ionic strength adjustment buffer (ISAB) - a solution of high ionic strength used to dilute samples and standard solutions. The ISAB minimizes differences in ion strength from solution to solution, making the activity coefficient of the ion approximately the same in all solutions. Multi-purpose ISAB's may contain pH adjustors, decomplexing agents or species that remove interferences. Examples of such solutions are Total Ionic Strength Adjustment Buffer (TISAB) for fluoride measurements and Sulphide Anti-Oxidant Buffer (SAOB) for sulphide measurement.

Ionophore (ion-carrier) - a lipophilic charged or neutral organic molecule, which has an affinity for a particular ionic species in solution (i.e. forms selectively reversible complexes with a particular ionic species). For ion-selective electrodes, there are introduced into a polymeric membrane, which forms a barrier between the test solution and the electrode. The detected ion is passed from one ionophore molecule to the next by diffusion under a concentration gradient and hence is selectively transferred across the membrane and causes a build up of electrical charge on the inside of the membrane.

Ion-selective electrode (ISE) - an electrode which responds selectively to the ions of a particular species in solution.

Ion-selective membrane - the principal component (receptor part) of the membrane chemical sensor, which generally determines its performances i.e. selectivity, sensitivity and stability. Ion-selective membrane is base on membrane matrix containing appropriate active components (i.e. the ionophore, ionic sites).

IP - Internet Protocol.

Isopotential point - the activity of a sample at which the electrode potential does not vary with temperature. For some types of electrode this point is within the range of the calibration curve, but for most it lies below this range. The temperature effect on electrode potential increases with increasing divergence of the sample activity from the isopotential point.

ISFET (ion-sensitive FET) - an electrochemical microsensor based on FET (field effect transistor) transducer. Ion-sensitive FET is generally selective to H⁺ ions. This selectivity arises from acid/base properties of the inorganic oxide (gate material) contacting the electrolyte. Examples of the inorganic oxides used are SiO₂, Si₃N₄, Al₂O₃ and Ta₂O₅.

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[J]

Junction potential - the portion of the total observed potential developed between the sensing and reference electrodes that are formed at the liquid/liquid junction between the reference electrode filling solution and the sample solution. For accuracy, the junction potential should be as low and as constant as possible.

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[K]

Known addition - a method for determining the concentration of an ion by adding a known volume of standard to the sample. The electrode potentials of the sample before and after addition are compared.

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[L]

Leak rate - the rate at which the outer filling solution of the reference electrode enters the sample solution. Low or irregular leak rates may cause variations in the liquid junction potential, resulting in erroneous or unstable readings. This is not a significant factor in modern gel-filled electrodes.

LED - Light-Emitting Diode.

Lifetime - the usable period of the sensor. It must be specified whether "shelf" or "in-use".

Linear range - the range of concentration (or activity) over which the measured sensor signal can be fitted by straight line.

Liquid junction potential - the potential formed at the interface between any two electrolyte solutions of different compositions. In ISE measurements, the most important liquid junction is that between the reference electrode filling solution and the sample solution. Ideally, this potential should be as low and as constant as possible, despite variations in the external solution. Reference electrode filling solutions are chosen to minimize this potential.

Lipophilic salt - (see ionic additives).

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[M]

Measurand - particular quantity subject to measurement.

Measurement - set of operations having the object of determining the value of a quantity.

Membrane - a continuous layer covering a structure or separating two electrolytic solutions. The membrane of an ISE is responsible for the potential response and the selectivity of the electrode.

Micro total analysis system (mTAS) - miniaturized system fabricated by the use of micromechanical technology capable of providing total chemical analysis on a microliter scale. The microdevice, fully integrated for example onto a silicon substrate (chip), performs: sample handling, reagent mixing, sample component separation and its analysis. A major area of interest has been the transfer of separation techniques such as capillary electrophoresis (CE) and high performance liquid chromatography (HPLC) to the chip format, coupled with detection systems such as spectrophotometric or conductometric detectors. MicroTAS can be also used in biochemistry for DNA chip analysis and drug discovery studies.

Multi-component analysis - analysis using several sensors connected directly to a sophisticated computer interface and software to monitor, process, and display the output from different sensors simultaneously. For example it can include simultaneous measurements of several different ions and/or pH, redox potential (ORP), dissolved oxygen and temperature.

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[N]

Nernst equation - the response of an electrode varies with respect to the logarithm of the activity of the measured ion. The Nernst equation is a mathematical description of the potentiometric sensor behavior.

E = Eo+2.3RT/nF log A (where: E = Total potential, in mV, developed between the sensing and reference electrodes; Eo = The sum of the junction potentials in the electrode pair. This will vary with the choice of reference electrode; R = The gas constant 8.314 joules/deg/mol; T = The temperature of the solution in degrees Kelvin; n = The charge of the ion including sign; F = The Faraday constant, 96,500 coulombs; A = Activity of the ion measured).

Nernstian response - an potentiometric sensor is said to have a Nernstian response over a given concentration range if a plot of the potential difference (when measured against a reference electrode) versus the logarithm of the ionic activity of a given species in the test solution is linear with a slope factor, which is given by the Nernst equation.

Nikolski-Eisenman equation - an extension of the Nernst equation which relates the sensor potential to the activity of all the contributing ions in the sample, including any interfering ions. It is the same as the Nernst equation but with log A replaced by log [ax + Kx,y(ay) ^ (zx/zy) + Kx,z(az) ^ (Zx/Zz) ...etc.] (where: Kx,y = selectivity coefficient for ion y of an electrode sensitive to primary ion x; Kx,z = selectivity coefficient for ion z of an electrode sensitive to primary ion x; ax = activity of primary ion x; ay and az = activities of interfering ions y and z; Zx = an integer with sign and magnitude corresponding to the charge on the primary ion x; Zy and Zz = integer with same sign as Zx but magnitude = charge on interfering ions y and z).

Noise - abrupt, random, small changes in displayed sensor signal (in electrode, usually due to the pickup of strong static charges). Noise may be caused by air bubbles, poor conductors, or high electrical resistance somewhere in the circuit.

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[O]

Open circuit - lack of electrical contact in any part of the measuring circuit (which consists of the sensing electrode, instrument, reference electrode and solutions). An open circuit is characterized by rapid large jumps in displayed potential, followed by an off-scale reading. Frequent large erratic changes in potential indicate an intermittent open circuit.

Optoelectronic interface - an element of the fiber optic chemical sensor, which can be regarded as transducer. Optoelectronic interface converts the light beam, after having interacted with the chemooptical interface, into an electrical signal. It contains a photodetector (e.g. photodiode) and an amplifier.

Optomembrane - the receptor part (chemooptical interface) of the fiber optic chemical sensor, containing an immobilized reagent (e.g. dye, indicator or chromoionophore).

Optrode/Optode - term, which comprises optic sensor or in certain cases only the optomembrane (receptor part of the optic sensor). The term optrode is related to optic+electrode, from the similar operation mode (dipping in the sample solution) of the ion-selective electrodes and fiber optic chemical sensors.

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[P]

pH - a convenient way of expressing the activity (concentration) of hydrogen ions which is a measure of the degree of acidity or alkalinity of a solution. It is calculated as the negative logarithm of the hydrogen ion concentration (in mol/l): pH= -log [H+]. Pure water undergoes a certain amount of dissociation or self-ionization to form hydrogen (or more strictly, hydroxonium) and hydroxyl ions.

pH electrode - specific ion electrode, made of glass, that responds to hydrogen ion activity, over the range 1 M H⁺ (pH = 0) to 10^-14 M H⁺ (pH = 14). Special purpose electrodes are made for very acidic or very alkaline solutions, solutions containing high levels of other cations, high temperature operation, and industrial and medical applications. pH electrodes may be subject to "acid error" in strongly acidic solutions and are also subject to alkaline error caused by response to sodium or other cations in highly basic solutions.

pH range (of an ISE) - all ion-selective electrodes are also sensitive to pH to some extent, most commonly at the high and low ends of the pH scale. The pH range is the range over which a change in pH will not cause a significant change in the measured voltage. It is the plateau on a graph of pH against mV at constant concentration of the detected ion. Outside this range, a change in pH may cause a significant change in the measured signal. Thus if samples lie outside the pH range then they must be treated with a buffer solution to bring them within the range, or, if samples all have the same pH, then the calibration standard solutions must be treated so as to make them the same pH as the samples. In practice both the standard and sample solutions are treated with the same buffer to ensure that they have the same pH; then it is irrelevant whether this pH is within or without the range quoted for the electrode.

pH unit - a tenfold change in hydrogen ion activity, expressed as the negative logarithm of the hydrogen ion activity. Thus a solution of pH 7 is10^-7 M in hydrogen ion activity and a pH 8 solution is 10^-8 M. The lower the solution pH, the more acidic the solution.

pH/mV meter - an instrument that measures and displays the voltage developed between a sensing electrode and a reference electrode placed in a solution, and which can converts the voltage developed by a pH electrode to a corresponding pH value.

Photodetector - (e.g. photodiode) an electronic device converting optical radiation into electrical signal.

Plasticizer - membrane solvent used in polymeric liquid-membrane sensors. Solvent polymeric membranes are usually based on matrix containing about 33% (w/w) of PVC and 66% (w/w) of plasticizer. The presence of such a high amount of plasticizer provides membranes with required physical properties (i.e. lowers the glass transition temperature (T_g) of the membrane polymer, decreases membrane resistance and ensures relatively high mobility of its components). In order to obtain a homogenous membrane phase the plasticizer should be compatible with the polymer.

Poisoning - the chemical conversion of the surface of a solid state electrode to a form, which is less responsive to, changes in ionic activity. In many cases, electrode function may be restored by physically removing a thin layer of the sensing element or by reversing the poisoning reaction chemically.

polyHEMA (poly(hydroxyethyl methacrylate)) - polymer used as hydrogel layer, soaked with an internal solution, to create solid electrolyte in CHEMFETs. The deposition of this layer (between the gate oxide of an ISFET and an ion-selective membrane) is required to obtain a thermodynamically well-defined interface between the sensing membrane and solid transducer. Additionally polyHEMA reduces the noise level and eliminates the carbon dioxide interference on the CHEMFET signal.

Polymeric membrane matrix - inert polymeric phase providing necessary physical properties (i.e. mechanical stability, elasticity and solubility of membrane components) of the ion-selective polymeric liquid membrane. Originally, liquid ISE membranes were obtained by soaking porous materials with a solution of the ionophore in organic solvent. Homogenous polymeric membranes have been used firstly as membrane matrix for charged carriers. The most often used polymeric membrane matrix is plasticized PVC (the plasticizer is necessary due to the high T_g of PVC). Other polymer, which not require plasticizer (T_g below room temperature), like silicone rubber, polisiloxanes, poliurethanes are also applied as matrix of liquid membranes.

Precision - a measure of the reproducibility of a method i.e. the closeness of agreement between independent test results, when multiple measurements are made on the sample under the same conditions. The observed values may differ from the true values without affecting the precision and reproducibility (see accuracy).

pX - this is exactly the same as pH but relates to the concentration (activity) of any other ion in solution. Because of the reduced sensitivity of other ion-selective electrodes when compared with pH electrodes, the pX scale rarely extends below 7 (= 10^-7 mol/l).

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[Q]

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[R]

Random error - a result of a measurement minus the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions.

Receptor part - according to the IUPAC definition of chemical sensor one of the two principal components of the chemical sensor, which converts chemical information into a form of energy.

Redox electrode - a metallic electrode, usually made of platinum, which is used to monitor reversible reduction/oxidation reactions. The potential difference developed between the redox electrode and a reference electrode is a Nernstian function of the ratio of two different oxidation states of a species in solution.

Redox potential - the potential developed by a metallic electrode when placed in a solution containing a species in two oxidation states in reversible equilibrium. The redox potential is a measure of the tendency for the reduction reaction to occur. Standard redox potentials range from about -3 volts to +3 volts. The more positive the value the more likely it is that reduction will occur. Negative values indicate that oxidation is more likely.

Reference electrode - the half of an electrode pair which provides a constant potential, regardless of the sample composition. The potential developed by a sensing electrode is measured against this reference to give a signal, which can be converted to the activity of ion under analysis. Single junction reference electrodes have a single chamber generally filled with a potassium chloride solution saturated with silver chloride, which contacts with the sample solution through a single liquid junction by means of a semi-porous ceramic plug or fritted disc. Double junction reference electrodes have two chambers with the internal reference system having a liquid junction with an intermediary salt bridge and then a second liquid junction to the external sample. The outer filling solution is chosen to avoid contamination of the sample and minimize the effects of the liquid junction potential.

Reference internal element - the part of a reference electrode, which reacts with the filling solution to produce a constant reference potential. The most common element is silver/silver chloride.

REFET (reference field effect transistor) - one of the possible solution of a miniaturized reference electrode based on FET transducer. Up to now none of the constructions were put into practice mainly because of material and technological problems.

Reflectance - an operation mode of fiber optic chemical sensors. In this mode the detector measures the changes in spectral properties of the light beam reflected (after interaction) from the optomembrane (chemooptical interface).

Reproducibility - the closeness of replicate measurements on the same sample, using the same measuring technique, under the same conditions. Reproducibility can be limited by many factors, including instrument or electrode stability, loss of the substance being measured during sample operation and contamination.

Response time - the length of time necessary to obtain a stable electrode potential when the electrode is removed from one solution and placed into another of different concentration or temperature. Response time depends on the electrode type, the measuring solution, the magnitude and direction of the concentration change, temperature, and the presence of electrode, if any. The response time can be expressed as the time at which the output reaches 63% (1/e) or 95% of its final value, in response to a step changes in concentration.

Result of measurement - a value attributed to a measurand, obtained by measurement.

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[S]

Salt bridge - a technique for isolating the reference element of a reference electrode from the sample solution by use of an intermediate solution containing non-interfering ions. Formerly a U tube filled with saturated KCl (often gelled with agar) was commonly used. In the double junction reference electrode, the outer filling solution serves as a salt bridge.

Selectivity - the ability of the device to measure one chemical component in the presence of others in the sample. For example ion-selective electrodes are not 100% ion-specific. Most are sensitive to some other ions to some extent. Some ISEs cannot be used in the presence of certain other interfering ions or can only tolerate very low contributions from these ions. Special techniques are available for removing or compensating for interfering ions

Selectivity coefficient - an expression of the extent to which an ion-selective electrode reacts with an interfering ion in proportion to the measured ion. A selectivity coefficient of 0.1 implies, that the electrode is ten times more responsive to the primary ion than to the interfering ion (i.e. if a solution contains equal concentrations of both ions, the interfering ion will contribute about 10% to the total change in electrode signal produced, when the electrode is immersed in the test solution). The selectivity coefficient is not constant but depends on a number of factors including the total ionic strength of the solution, the concentration of both ions, and the temperature. The performance of an ISE in the presence of an interfering ion is quantified by the selectivity coefficient: K_A,B (where A is the primary ion and B is the interfering ion. The higher the K_A,B the greater the effect of the interference from B. At K_A,B = 1 there is an equal response to both ions. A modified form of the Nernst equation (see Nikolski equation) gives the total potential developed.

Sensitivity - the slope of the response (calibration) curve expressed as output per unit concentration.

Sensor - a device for transducing information about the concentration of a chemical species into a readily accessible signal (usually electrical). The device responds directly to the amount of a given environmental component. Ion selective electrodes are sensors, which respond to the concentration of particular dissolved ions in a solution. pH electrodes respond to hydrogen ions.

Sensor array - a set of sensors with different sensitivity and limited selectivity. Sensors array provide multiple data points per sample (vector of data) that carry out additional chemical information to differentiate analytes and discriminates against interferences. The pattern of response (distinct for different compounds) of an array of sensors can be applied to identify the unknown component. Suitable pattern-recognition algorithms are used to interpret the results of multielement arrays.

Slope - the gradient of the line formed by plotting the change in the electrode response against change in activity of the ion measured. Theoretical Nernstian slope at 25°C is 59.16 mV per decade change in activity for a monovalent ion and 29.58 mV per decade change for a divalent ion. Slope values are often stated in % efficiency terms (an ideal slope of 59.16 mV for a monovalent ion - 100% slope efficiency). Slope values of less than 90% efficiency may be indicative of electrode contamination. However, many other factors will contribute to a loss of system performance.

Solid-state electrode - a miniaturized version of an electrochemical sensor e.g. potentiometric or amperometric. Solid-state electrode can be constructed using MEMS technology.

SPIE - The International Society for Optical Engineering.

Stability - the percent of change of the baseline and/or sensitivity in time.

Stokes shift - the difference in wavelength between the apex of the excitation spectrum (shorter wavelength, higher energy) and the apex of the emission spectrum (longer wavelength, lower energy).

Systematic error - mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions minus a true value of the measurand.

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[T]

TCP - Transmission Control Protocol.

Temperature effects - changes in sensor signal due to changes in temperature.

TISAB = Total Ionic Strength Adjustment Buffer - a reagent which is added to standards and samples in Fluoride determinations to optimize the pH value at 5.5, liberate any fluoride which may be complexed with hydrogen, aluminium, iron or other cations, and equalize the activity coefficient between sample and standards.

Titrant - a reagent containing a species that either complexes or liberates the species being determined. An ideal titrant should react instantaneously, completely, and stoichiometrically with the species being determined. The titrant is added in measured increments from a burette and the sample concentration determined from the amount of titrant needed for complete reaction.

Titration - a quantitative analytical technique for measuring the concentration of a species by incremental addition of a reagent (titrant) containing a species that either complexes or liberates the sample species.

Transducer - a physical part of the sensor (detector), that amplifies the primary signal to usable level. According to the IUPAC definition of chemical sensor one of the two principal components of the chemical sensor. The receptor part converts chemical information into a form of energy acceptable by the transducer and then the analytical signal is generated.

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[U]

Uncertainty (of a measurement) - parameter associated with the results of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand.

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[V]

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[W]

Water hardness - a measure of the ability of water to form insoluble precipitates when boiled or when soap is added. Hardness is caused by the presence of Ca, Mg and Fe cations and to a lesser extent, other divalent cations. It is expressed as ppm of divalent cations. It can be measured with a water hardness electrode, which exhibits almost equal response to all divalent cations.

Wavelength of light (l) - the distance in nanometers between nodes in a wave of light. The wavelength is inversely proportional to the energy of the light.

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[Z]

Zero potential point - the solution concentration at which an electrode pair develops a potential of 0 mV. In many instances, it is also the isopotential point.

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