Instrumental Analysis Vocabulary Lists 1–2

Signal generator

Produces a signal describing presence and concentration of the analyte

Transducer (input)

Converts energy from one form to another (essentially: nonelectrical → electrical domain)

Detector

Type of transducer that acts on a chemical signal

Signal processor

Modifies the transducer signal

Amplification

Increase signal

Attenuation

Decrease signal

Filtering

Reduce noise (unwanted, disruptive data) to improve clarity and accuracy

Output transducer

Also known as a readout device, translates information from the electrical domain into a format that allows a human observer to interpret (ex. computers)

Signal

Measurable response produced by a generator that contains information about the identity, properties, or concentration of an analyte

Noise

Random, unwanted fluctuations or variations in an analytical signal, obscuring the true signal

Signal-to-noise ratio

Abbreviated as S/N or SNR, a benchmark comparing the magnitude of a measured analytical signal and the magnitude of background noise (calculated by: xs, or 1RSD)

Ensemble averaging

Repetitive measurements or successive data sets over an entire spectral region are collected, summed together point by point, and finally divided by the number of scans (calculated by: nSxNx, where n = # measurements/sets made, S/N is initial signal-to-noise-ratio)

Spectrum

Involves the separation, detection, and recording of energy changes, typically producing a plot of a function of frequency or wavelength

Electromagnetic radiation

Combination of oscillating or alternating electrical and magnetic fields in space

Plane polarized light

All oscillations of either the electric or magnetic field lie in a single plane (waves associated with the electric field and magnetic field are contained in individual, respective planes)

Wavelength

Represented by and measured in cm, the distance between 2 successive points (i.e. maximum or minimum) of 2 successive waves

Frequency

Represented by and typically measured in s-1, measures the oscillations of the electric field per second

Wavenumber

Represented by and typically measured in cm-1, the reciprocal of wavelength (1/)

Photoelectric effect

The emission of electrons when light shines on a material; the process of converting photons (light) into electrons (current)

Ground state

State of an atomic system at which it is stable and not radiating energy

Excited state

Changed energy state as a result of energy absorption

Absorbance

Represented by A, the quantitative measure of absorption and depends on medium composition and length of the optical path

Absorption

Results from the interaction between ions and/or molecules, is the energy transfer from photons to atoms, ions, or molecules in a substance

Transmittance

The fraction of incident light transmitted by the matter

Beer’s Law

Describes the relationship between absorbance (A), molar absorptivity (), path length (b), and concentration (c):A=bc

Molar absorptivity constant

Represented by and measured in L/molcm or M-1cm-1, measures the strength of absorption and determines the sensitivity for an absorption-based method, dependent on molecule identity and wavelength

Pathlength

Length over which attenuation occurs

Isosbestic point

A specific wavelength where the total absorbance of a sample remains constant during a chemical reaction or pH change because the molar absorptivities of the participating species are equal (A=B)

Spectrophotometer

An instrument used to measure the fraction of incident light transmitted or absorbed by a sample as a function of wavelength, typically consisting of a light source, wavelength isolator, sample cell, detector, signal processor, and readout device

Wavelength isolator

Select a single wavelength from a polychromatic source to hit sample

Single v.s. dual channel design

Single-beam uses one light path for sequential measurements; dual channel design has a double beam in space or time (splits light into sample and reference paths)

Chopper

A mechanical device used to modulate signal flow, alternating the light beam between the sample and reference or block/opening the beam path

Dispersing device

An optical element used to separate wavelengths from polychromatic radiation into component wavelengths

Filter v.s. prism v.s. grating

Filter absorbs, reflects, or deviates all wavelengths or frequencies from an optical path except for one band/region (low-precision); prisms achieve angular separation of the different wavelengths from a source (as described by Snell’s Law); grating uses diffraction with closely spaced parallel grooves to provide high resolving (best) power

Continuous source

A light source that provides constant output over a wide spectral range (polychromatic radiation)

Bandpass (bandwidth)

The range of wavelengths at which the transmittance is one-half the peak transmittance; the narrower the bandpass, the better the sensitivity and selectivity of absorption and emission measurements

Monochromator

Entire dispersing module containing the entrance slit, collimator, wavelength isolator, focusing element, and exit slit that work together to isolate the desired, narrow spectral band (typically a single wavelength/ frequency)

Entrance and exit slits

Openings to (entrance) and from (exit) the monochromator; adjustable for maximum flexibility and critical in determining resolution

Resolution

The ability of an instrument to distinguish between individual features in the electromagnetic spectrum (calculated by: R=, where is the average wavelength of two closely spaced features, and is the difference)

Dispersion

The angular separation of different wavelengths of light into its components, carried out by a dispersing device

Phototube

Based on the photoelectric effect, converts transmitted photons (light) into electrons (current)

Photomultiplier tube

Vacuum phototube with a built-in amplification system, combines photoemission with multiple stages of secondary electron emission

Diode array

A semiconductor chip (2 mm x 18 mm) which contains 328 individual photodiodes with circuits etched onto the chip

Dynode

Intermediary electrodes in a photomultiplier tube that emit multiple secondary electrons when struck by an incoming electron

Diode

A small semiconductor device that emits a focused laser, tunable light source

Chromophore

The specific part of a molecule responsible for absorbing light and producing color

Bathochromic

Also known as a red shift, a change in a spectral band position toward a longer wavelength and lower energy

Hypsochromic

Also known as a blue shift, a change in a spectral band position toward a shorter wavelength and higher energy

Luminescence

Release of a photon of light of different wavelength than what was absorbed (radiative process also called EMISSION or LUMINESCENCE); aka electromagnetic radiation from an electronically excited species as it returns to a lower-energy state

Fluorescence

Fast emission process from singlet excited state, spin allowed (e.g. S1 to S0)

Phosphorescence

Slower emission process from triplet excited state, involves a change in spin multiplicity (e.g. T1 → S0) and is therefore spin-forbidden and generally slower than fluorescence

Resonance Fluorescence

Fluorescence emitted at the same wavelength as the exciting radiation because no relaxation occurs within the excited-state manifold before emission, Stokes shift is approximately zero

Multiplicity

2S + 1, where S is the spin quantum number

Singlet State

An electronic state with total spin S = 0, typically with paired or antiparallel electron spins (i.e. 2S + 1 = 1); labeled S0, S1, S2,...

Triplet State

An electronic state with total spin S = 1, typically with paired or antiparallel electron spins (i.e. 2S + 1 = 3); labeled T1, S2,...

Jablonski Diagram

An energy-level diagram showing singlet and triplet electronic states, their vibrational levels, and radiative and nonradiative transitions among them

Radiative vs. Nonradiative decay

Release of a photon of light of different wavelength than what was absorbed v.s. collisions with other molecules or solvent (non-radiative processes releasing energy in form of heat)

Internal Conversion

A radiationless transition between electronic states of the same multiplicity (Sn → Sn-1)

Vibrational Relaxation

Nonradiative loss of excess vibrational energy as a molecule moves from a higher to a lower vibrational level within the same electronic state, generally transferring energy to its surroundings as heat

Intersystems Crossing

Isoenergetic radiationless transition between electronic states of different multiplicity (Sn → Tn)

External Conversion (Quenching)

Nonradiative deactivation caused by interaction or collision of an excited species with its environment or a quencher, can involve energy transfer or electron transfer

Kasha’s Rule

Luminescence occurs with appreciable yield only from the lowest excited electronic state of a given multiplicity, the emitting level of a given multiplicity is the lowest vibrational level of that multiplicity

Quantum Yield

Number of fluorescent photons/number of absorbed photons (value from 0 to 1)

Stokes Shift

The Stokes shift is the difference between a molecule’s absorption/excitation wavelength and its fluorescence-emission wavelength. In typical fluorescence, the emitted light has lower energy and therefore a longer wavelength than the absorbed excitation light.

Self Absorption

Solution acts as a filter, reabsorption of emitted photons by other analyte molecules in the same sample, especially where absorption and emission bands overlap, can distort measured emission spectrum

Excitation vs. Emission vs Absorption Spectra

Excitation spectrum: Luminescence intensity as the excitation wavelength is scanned while the emission wavelength is fixed

Emission spectrum: Luminescence intensity as the emission wavelength is scanned while the excitation wavelength is fixed

Absorption spectrum: Absorbance as a function of incident wavelength; it shows wavelengths that promote the analyte from lower to higher energy states

Corrected Spectra

Spectra adjusted to remove wavelength-dependent instrumental and sample artifacts, including source intensity, monochromator throughput, detector response, and relevant filter effects (account for blank)

Chemiluminescence

A chemical reaction yields an electronically excited species that emits light and returns to the ground state

Wavelength of a particle with mass

λ = h / mr

h = 6.626 × 10e-34

Speed of light, wavelength, and frequency

c = λv

Energy of a wave

E = hv = hc(v-bar) or hc/λ

Molecular fluorescence intensity equation for dilute solutions

F = 2.303 x Φ_f x P₀ x ε x b x c x K

Fluorescence quantum yield (Φ_f)

Φ_f = k_r / k_r + k_nr

Snell’s Law

n sin θ = n’ sin θ’

Resolution of diffraction grating

R = λ / 𝚫λ

avg wavelength of two closely spaced imgs / difference btwn those two imgs