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Volume 34 | Number s2 | February 2019
w w w. s p e c t r o s c o p y o n l i n e . c o m
THE
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6 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com
Introduction to the Guide
Jerome Workman, Jr.
A
few years ago, our sister publica- After considering these issues carefully, we
tion, LCGC, published a termi- decided to be more comprehensive and
nology guide, and copies flew off added more terms to the list.
the stand all year at conferences. That In addition to using this guide, spec-
response made the editors of Spectroscopy troscopists may also wish to know the
consider that it might be worthwhile to definitions given by various organizations
develop a similar guide, initially focusing for many spectroscopic terms; for these,
on terms specifically related to molecular one may refer to The ACS Style Guide, the
spectroscopy. Such a guide would include American Society for Testing and Materi-
a comprehensive set of definitions for top- als (ASTM) terminology guides, United
ics of interest to molecular spectroscopists States Pharmacopeia (USP) documents, or
and those using molecular spectroscopy in the International Union of Pure and Ap-
their daily work. This would include the plied Chemistry (IUPAC) guidance docu-
types of molecular spectroscopy methods ments. All of these reference sources will
and many terms related to spectrophotom- provide additional terms and updates on
eters, spectrometers, and the many appli- periodic changes in definitions as might
cations of these instruments. In addition, occur over time. We have included many
it would include some data and computer topical terms that are not mentioned in
terms, some statistical data analysis terms, those standard definitions and nomencla-
and the essential chemometric terms for ture guides, so that our guide is comple-
typical data preprocessing and calibration. mentary to those official publications.
This terminology guide includes defini- This terminolog y g uide is not in-
tions for over 250 molecular spectroscopy tended to be highly mathematical, the-
terms in sufficient detail to provide read- oretical, or all-inclusive in its coverage.
ers with a reasonable understanding of For example, we have elected not to
the concepts covered. In deciding which cover every possible term used in spec-
terms to include, we first identified spe- troscopic instrumentation, measurement,
cific terms, like hotband, isotope shift, and data handling, and validation, but have
hydrogen bonding that directly affect the chosen instead to use most terms that
appearance of spectra, as well as many may be encountered in everyday labora-
general terms more familiar to our readers. tory work where ultraviolet (UV), f luo-
We noted that if the scope was “molecu- rescence (FL), visible (Vis), near-infrared
lar spectroscopy,” then we needed to add (NIR), infrared (IR), Fourier transform
nuclear magnetic resonance and possibly infrared (FT-IR), Raman, or terahertz
other methods, but, if we narrowed that (THz) spectroscopy are in general use.
to “vibrational spectroscopy,” then NMR This glossary should be helpful to both
could be dropped. Cavity ring down novice and advanced users in spectros-
(CRD) spectroscopy and surface plasmon copy, and can also serve as a useful refer-
resonance (SPR) would be borderline cases, ence guide to even the most experienced
with fluorescence as a must-have addition. in the field.
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A spectroscopic analysis and the reference value
Abscissa: Designates the horizontal (x) axis of determined by a reference method. This error
a graph and, typically, the wavelength or wave- is usually ascribed to the instrument, although,
number scale of a spectrum. in fact, it is due to the error variances of both
Absorbance: Measure of the amplitude of the instrument and reference methods. Accu-
absorbed energy in a spectrum related to the racy, with respect to an instrument performance
concentration of an analyte. This term is also calibration, is the agreement of a measurement
referred to as the negative log (base 10) of Trans- parameter (for example, wavelength, photomet-
mittance (-log10T = log10[1/T]). This is also rep- ric value, or linearity) to the certified values of a
resented as the product of absorptivity (extinc- reference standard.
tion coefficient), pathlength, and concentration, Acousto-optical tunable filter (AOTF):
written as A=εbc. Absorbance = log10(1/reflec- A bandpass filter, also known as a Bragg cell,
tance) or absorbance = log10(1/transmittance). that uses sound waves to electronically change
Absorptivity: Probability of energy absorbing the bandpass and angle of transmitted energy
at a particular wavelength for a specific analyte through the device. The device uses acoustic
under specific conditions (that is, a specific diffraction of light with a broadband detector
combination of pH, solvent, and temperature). if the filter is rotated, or with an array detector
Thus, a specific amount of material at specified if no rotation of the AOTF optical element oc-
measurement conditions will absorb a specific curs. This device is generally made using a tel-
fraction of the energy striking it. Absorptivity is lurium dioxide (TeO2) crystal in combination
generally signified by either a lower case epsilon with a piezoelectric detector. A radio frequency
(ε) or a. It is defined as: pulse is applied to the device, which changes
the index of refraction within the crystal, and,
[1] therefore, its light diffraction or transmittance
characteristics. A change in radio frequency
Where ε is the molar absorptivity in units causes a change in the transmitted wavelengths
of L · mol-1 · cm-1; c is the concentration and the exit angle of the diffracted light. The
of molecules in the spectrometer beam in typical spectral ranges for AOTF devices are
units of mol · L-1; and pathlength (l) is the from near 390 nm to above 4400 nm with a
thickness of the sample in units of cm; and typical spectral resolution of 0.5% of the trans-
A is the measured absorbance. mitted wavelength.
Accuracy: A measure of the agreement be- Active detector: A photon detection system
tween the amount of a substance as deter- requiring electronic power that will detect pho-
mined by an analytical procedure and the “true” tons at various frequencies and create a change
amount of that substance in the sample. Since in an electronic signal based on the photon or
the “true” value may be unknown, it is approxi- electromagnetic energy flux striking the ac-
mated by determining it using a reference analy- tive detection element. The detector elements
sis method—that is, a method whose accuracy include photocathodes, photodiodes, photo-
can be estimated. If enough determinations detectors, and charge-coupled devices (CCDs).
are averaged, improved approximations of the Materials include silicon (Si), cadmium zinc tel-
“true” analyte concentrations can be made. In luride (CdZnTe), mercury cadmium telluride
spectroscopic calibrations, the accuracy is usu- (MCT, HgCdTe), indium gallium arsenide
ally expressed as the standard error of estimate (InGaAs), lead sulfide (PbS), light-emitting di-
(SEE) or the standard error of prediction (SEP) odes (LEDs), indium antimonide (InSb), and
as statistics that measure agreement between the germanium (Ge).
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10 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com Analog: A term used to describe or represent may independently detect a broad wavelength continuously variable physical quantities. Ana- range of energy. These detectors may be flat log data has continuity as contrasted to digital or curved to accommodate a multiwavelength data, which is in discrete reporting states. Ana- beam of diffracted light. They may be a lin- log signals are continuously variable quantities ear array, such as in a charge-coupled device that contain information resolution dependent (CCD), or a rectangular array where different only upon the magnitude of the signal. pixels form a mosaic of smaller pixels capable Analysis: The determination of the quantity of collecting and reporting multidimensional of a particular substance (that is, analyte) con- images. tained within a sample, usually expressed as At-line: A term used to describe instrumenta- weight percent of an analyte. Note that spec- tion that chemically quantifies or qualifies ma- troscopy directly measures a volume fraction terials in near real-time (as they are produced) unit, not a weight fraction unit. One may also near a production line. Generally, a sample perform analysis on the optical performance of aliquot is taken from the production line and a spectrometer for its optical, mechanical, and moved to an analysis station for manual sam- electronic performance levels. ple presentation to a nearby instrument. Analysis of variance (ANOVA): A mathe- Attenuated total reflectance (ATR): A matical or statistical procedure for determining sampling device used for surface analysis with the contributions that various phenomena make infrared spectra where physical contact of the to the total variation in a set of measurement ATR crystal occurs with the sample surface. data. Variance is defined as the standard devia- ATR crystal materials include diamond (Di), tion squared, and follows the F-distribution for germanium (Ge), silicon (Si), and zinc selenide statistical purposes. Variances are additive terms (ZnSe). ATR measurements can yield excellent and allow arithmetic operations. quality spectra, provided that the contact pres- Angle of incidence: The angle between the sures of the ATR crystal and the sample are incident radiation beam with respect to a nor- held constant. By applying an ATR correction mal (perpendicular) surface angle. For a plane algorithm, ATR spectra may be compared and specular surface, the incident and reflected qualitatively with transmission spectra. angles for a beam of light are equivalent. Attenuated total reflection (ATR) cor- Anisotropy: A reflection property where the rection: This is also known as “attenuated energy is not equivalent for all positive angles of total reflectance correction.” A conversion func- reflection; therefore, reflected energy is differ- tion to compare transmission spectra to ATR ent for different reflection angles from a surface. spectra for infrared spectroscopy. The func- Anti-Stokes shift: See Raman effect. tional conversion depends upon the refractive Apodization or apodization function: A indices of the ATR crystal and the sample, the function used to remove the “ripple” effect on angles of incidence and collection for the infra- either side of spectral peaks or bands following red beam, and the number of bounces for the the application of the Fourier transform to an beam with the ATR crystal. The conversion interferogram. These ripples are minimized by is important for quantitative and qualitative applying a function to spectral data to produce comparisons between ATR spectra and trans- smaller ripples and slightly broadened peak mittance spectra. widths. Array detector: A photoelectric detector that B includes multiple, evenly spaced lines or rect- Bandpass filter: A filter that is designed to angles of detectors (or pixels), each of which provide transmission for a particular band of
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www.renishaw.com12 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com frequencies, but to attenuate (reduce) transmis- to separate the beams used to measure ratioed sion of frequencies above (or above and below) spectra as the sample and reference beams. the specified transmission band. Specifications Beer’s law (Beer-Lambert law): A linear- include a half width or full width at half-maxi- ization algorithm to convert transmission or mum (FWHM) of the transmission band, the reflection spectra to absorbance. The transmis- transmission peak position, and the attenuation sion or reflection spectral responses are not con- transmission specifications with respect to fre- sidered linear with concentration, whereas the quency or wavelength. logarithmic absorbance conversion is assumed Bandwidth: This term refers to the full width linear with changes in analyte concentration. It at half-maximum (FWHM), the line width, or is strictly defined for transmission or transmit- the spectral resolution, of a spectrophotometer. tance spectroscopy. Beer’s law relates the amount It also refers to the range of frequencies (wave- of electromagnetic radiation that passes through lengths) over which a particular instrument is a sample to the composition of that sample. For designed to function within specified perfor- the case of a single absorbing species, it is often mance limits. Also, it refers to the limits or ex- written as follows: treme difference between the specified frequen- cies of a measured spectral range (that is, high A = abc = εbc [2] versus low frequencies over a spectral range). Baseline correction: Spectra taken with Where A = measured absorbance = - log10 (I/Io); different instruments or with varying sample I = intensity of radiation through the measur- conditions, such as particle size or turbidity, ing system in the presence of the sample; Io = may have a different baseline offset and cur- intensity of radiation through the measuring vature. Visually or mathematically comparing system in the absence of the sample; a = ε = the such spectra may require a baseline correction. absorptivity of the sample at the wavelength These corrections are generally a single point or of measurement; an intrinsic property of the multipoint offset, a linear correction, or various molecular species (also known as the extinc- first- or second-order polynomial corrections. tion coefficient in physics); b = the pathlength The single- and multipoint correction involve through the sample; and c = the concentration the addition or subtraction of one or more of the sample for a single absorbing species. If all points from the baseline. Linear or polynomial the physical parameters (particularly the path- corrections involve fitting such a mathematical length) are kept constant, then the measured function to the baseline, and then subtracting absorbance (A) is directly proportional to the the fitted line or curve from the spectrum, concentration (as weight per unit volume) of transforming the comparative spectra to have the analyte species of interest. This principle a similar amplitude scale and shape. is the basis of absorption-based spectroscopic Beamsplitter: An optical element that is quantitative analysis. coated in such a way that approximately 50% Bias: The average difference between a set of of the energy striking the element is reflected instrumental analyses and the corresponding set and 50% is transmitted; these optical elements of laboratory reference values; or, a set of samples are often referred to as half-mirrors. A Michel- measured using a comparison instrument or an- son interferometer comprises a beamsplitter, alytical method. More generally, it is the average which is an active optical element, along with difference between two sets of measurements on one fixed and one moving mirror. Dual-beam the same sample or samples. instruments exhibiting simultaneous sample Bit: Abbreviation of binary digit. A bit is the and reference beams also use a beamsplitter value of the least significant place of a number
www.spectroscopyonline.com TERMINOLOGY GUIDE FEBRUARY 2019 13 in the binary number system. It is the smallest polarization light reflection equals 0) is termed unit of information a traditional computer can the Brewster angle. It is also the angle that con- handle. A bit corresponds to the output of a stitutes perfect or 100% of the light transmitted solid-state device, which is either the “0” state through the dielectric. (usually 0 V), or the “1” state (usually +5 V). In traditional computer circuits, intermediate C states are not defined. Calibration: Most often defined as determin- Blind duplicate: Reference laboratory data ing the response of an analytical method to used for calibrating instruments are generally known amounts of pure or reference analyte. analyzed using primary analysis methods— Instrument calibration refers to measuring and that is, methods that depend only on fun- aligning the x- and y-axis measurements of a damental physical laws and well-determined spectrophotometer with respect to a reference physical constants, such as molecular weight. material of standard and known characteristics, Thus, the accuracy of such laboratory data such as in wavelength or photometric accuracy cannot be measured by comparison with ref- calibration. Product calibration is the process of erence data, since there is, theoretically, no computing an equation or model to fit reference more accurate technique. Nevertheless, error laboratory readings to instrument readings for does exist in such data; the magnitude of the product calibrations. In spectroscopy, product error can be assessed by comparing the labo- calibrations are usually performed by linear or ratory with itself, or by comparing the values nonlinear multiple regression techniques. reported on multiple independent analyses of Calibration curve: The results of a calibration the same sample. To ensure that the analyses when graphed, usually with Cartesian coordi- are independent, the test samples should be nates, with a parameter such as concentration randomized so that the laboratory analyst (in molarity) on the x-axis, versus absorbance or cannot detect the repeat aliquots of the same instrument response on the y-axis. Developing sample. If several samples are replicated, then it a calibration curve or line often involves the ap- suffices to send each one in only twice, because plication of statistical regression methods. this approach will provide data to calculate an Calibration set: A collection of samples con- estimate of laboratory accuracy. taining varying amounts of the analyte used to Boxcar apodization: The apodization func- calibrate an instrument. This collection is also tion applied to Fourier-transformed interfero- referred to as the learning set or teaching set. gram spectra that maximizes both peak height Centerburst: An interferogram has a center- and resolution (band half width) as compared burst, also referred to as the "zero-path-differ- to other apodization functions. It is the apo- ence" (ZPD) location. When the optical path dization function most often used for gas- distance of the fixed and moving mirror are phase infrared spectrophotometry. It is a square identical in the interferometer, the interferogram waveform apodization function, in contrast to is at maximum interference; this is referred to as a triangular or Happ-Genzel (smoothed trian- the centerburst position. gular function) apodization function. Chalcogenide: These materials contain a Brewster angle: When a light beam at a spe- Group XVI element from the periodic table. cific wavelength passes through one medium The main use in spectroscopy is for infrared- (for example, air) and reflects or interacts with range transmitting optics such as fiber optics another medium (for example, glass, plastics, or and optical lens elements. The most common other dielectric materials), the angle where the materials in this category include cadmium p-polarized light does not reflect (that is, the p- sulfide (CdS), cadmium selenide (CdSe), cad-
14 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com
mium telluride (CdTe, formerly marketed as be completed in real time for specific molecular
IRTRAN-6), Cleartran (water clear zinc sul- vibrations at known planar locations for image
fide), zinc selenide (ZnSe, formerly marketed as reconstruction without the use of excitation dyes.
IRTRAN-4), and zinc sulfide (ZnS, formerly Color measurement: A measurement of a
marketed as IRTRAN-2). physical sample is made with appropriate illu-
Chemometrics: A subdiscipline of analytical mination and measurement geometry. For color
chemistry involving complex overlapping mo- measurement, a variety of color coordinates are
lecular patterns from a sample chemistry (that computed from the visible spectrum to approxi-
is, chromatography or spectroscopy); and the mate and define colors to simulate human vi-
interpretation of these signal patterns (for ex- sion. To compute the color coordinates for any
ample, chromatograms or spectra), using a series sample requires three vectors or tables of values:
of multivariate mathematical techniques in the the spectral power distribution of the illumi-
usable form of computer algorithms. nant (usually either Standard Illuminant D65
Chopper: A device used for modulating or or Standard Illuminant A); the color matching
interrupting a current or light beam at regular functions representing the human eye sensitiv-
_ _
intervals. This form of modulation is often ity to red, green, and blue (RGB) as the x, y ,
_
used to convert signals into resolvable seg- and z coordinates; and the visible spectrum
ments for measurement when using slower of the sample in the appropriate measurement
response detectors. geometry.
Christiansen effect: An effect where a non- Colorimetry: The techniques for the measure-
linear distortion occurs for strong absorption ment of color (in the visible region) and for the
bands transmitting or reflecting energy through, interpretation of the results of such measure-
or from, crystalline particles that are slightly ments as they correspond to universal mathe-
larger than the measuring wavelength of energy. matical color scales. There are many color scales
Circular dichroism: For an optically active used to report color vision in a digital format.
molecule, left or right circularly polarized en- See also color measurement.
ergy will produce different absorption spectra. Combination band: An absorbance band
The circular dichroism spectrum for optically arising from the interaction of two or more
active molecules changes with the wavenumber vibrations of a molecule—for example, stretch-
of the incident energy. The technique is used ing and bending. The new frequency for the
to determine the structures of certain types of combination band is calculated as the sum of
bioorganic molecules, such as DNA or nucleic the vibrational frequencies of the coupled (in-
acids, and for secondary structure estimation of teracting) bands (ν combined = ν1 + ν2). In the
proteins. It is often performed using the ultra- near-infrared region, C-H stretch combination
violet or infrared spectral regions. bands occur four times, and the O-H combi-
Coherent anti-Stokes Raman spectros- nation bands occur three times. For example,
copy (CARS): A measurement where two lasers in the NIR region the first combination region
are used as a near-infrared pump beam and a for C-H stretching occurs from 4550 cm-1 to
longer wavelength Stokes beam. These lasers are 4000 cm-1 or 2200 nm to 2500 nm; the second
used to excite molecules to their virtual state; combination region occurs from 7400 cm-1 to
the molecules then return to their vibrational 6666 cm-1 or 1350 nm to 1500 nm.
state. Tuning of the excitation pump laser al- Combination search: A method of selecting
lows excitation of specific molecules at specific a subset of variables from a larger set by per-
spatial locations with a 105 signal enhancement forming trial calibration using different combi-
over conventional Raman. CARS imaging may nations of variables to form each subset, thenwww.spectroscopyonline.com TERMINOLOGY GUIDE FEBRUARY 2019 15 selecting that subset giving the most accurate -1.0 and +1.0. For instrument calibrations the calibration. This concept is usually extended correlation coefficient should be positive (+). to trying all possible combinations of vari- The correlation coefficient is calculated from ables. It was most often applied to selecting the following expression, where there are two appropriate wavelengths for multiple linear variables compared, namely X and Y with n regression calibrations. being the number of data-point pairs. Compression: A term used to describe a method of minimizing repack variation in spectral data by averaging several aliquot scans of each sample before calibration devel- [3] opment. Thus, all repeated scans for a sample are averaged to a single scan or spectrum, and the variation in repacks is removed from the Correlation transform: A term used to de- data. The resulting calibration will be more scribe the process of using correlation statistics accurate, but, statistically, will require that with multilinear regression analysis to select the future predictions also be averaged. optimum regression equation for the sample, Concentration: The amount of analyte in a based on the correlation between the analyte given volume of sample—for spectroscopy, this concentration and a change in spectral response is in moles per liter or mass per unit volume. for a particular wavelength or wavenumber data Confocal optics: The term is defined as two point, or wavelength or wavenumber region. optical elements with an identical focal length Counts: The measured raw signal used in or focus, such as a pair of matched spherical photon detection electronics relating to the mirrors with a distance equal to their respec- analog-to-digital (A/D) converter signal. tive radii of curvature, a set of pinholes with the Most modern photodetectors use a 16-bit same foci, or a pair of matched lenses with the A/D electronics converter corresponding to a identical foci. For confocal microscope optics, maximum signal count of 216 = 65,536 counts the sample and the condenser optics are at the for full-scale detection, having a full range of same focus. Greater resolution is possible when 0 counts to 65,536 counts. confocal pinhole apertures are used as in confo- Curve resolution: One of several methods cal microscopy; this technique is also useful for of deconvoluting severely overlapped complex three-dimensional structure rendition. spectra. Resolution algorithms reveal the in- Constituent: Any discrete component of a dividual absorbance bands that make up the sample; more specifically, a component for spectrum. Various functions are used to ap- which an instrument can be calibrated. Also proximate the shapes of the component spectra, referred to as parameter or product constituent. including normal and Lorentzian curves. Correlation: The tendency of two sets of num- Cuvette (also cuvet): Transparent receptacle bers to follow one other; that is, to increase or in which sample solutions are introduced into decrease together. This tendency is usually ex- the light path of spectrometers. Usually, the two pressed numerically by the correlation coefficient. sides are equal (for example, 1-cm square) while In some fields, this term is commonly referred the third dimension (height) is elongated, possi- to as Pearson’s r or Pearson’s correlation, named bly as long as 15 cm. For UV work, the window after Karl Pearson (1837–1936). material is generally low-OH quartz. For visible Correlation coefficient: The numerical spectroscopy, some polymers are used, such as measure of correlation as described above. The polystyrene (PS) or polymethylmethacrylate correlation coefficient must always lie between (PMMA). The z-dimension is the distance from
16 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com
the bottom of the cuvet holder to the center of and amplifier circuit. Usually, a simple silicon
the transmission beam of the spectrophotom- diode, a more sensitive photomultiplier tube
eter. See Window for additional information of (PMT), or various photodetector systems are
materials used for sample holders. used to detect the light energy. See also passive
detector and active detector.
D Detrend: A general term used to describe the
Data: The collection of numbers representing removal of a basic trend from data to more
the direct results from actual measurements, closely determine the signal attributes associated
and upon which calculations are to be per- with analyte concentration or basic information
formed to determine the quantities of interest. content within a signal. To perform detrend-
Common data file formats for analytical spec- ing, the data in a time series are subjected to a
troscopy include AB SCIEX Data Explorer linear least squares fit, and then the fit function
(.pkm), ASCII Table (.csv), Bruker (.XML), is subtracted from the data. For unusual curved
Excel (.xls), Finnegan (.asc), JCAMP-DX (.dx), shapes in the baseline of spectra, a polynomial
MatLab (,mat), mzData (.XML), PerkinElmer function of a selected order is fit to all the data
(.sp), Plain Text (.txt), Spectra-Calc, Archi- points in the spectrum, and this fit function
val, fixed point (.spc), Thermo Galactic (.spc), shape is subtracted from the spectrum. The
Thermo Scientific Nicolet (.SPA), and XLM polynomial is used to fit all data points in the
marked-up text (.XML). spectrum. This technique will remove large
Degrees of freedom: A measure of the num- background interferences, or specific back-
ber of independent measurements in a data set. ground trending variations, and is often used
In a data set to be used for calibrating an instru- in combination with the standard normal vari-
ment, the degrees of freedom are equal to the ate (SNV) preprocessing function. A detrend
number of reference laboratory measurements algorithm consists of a polynomial or linear fit
against which the instrument is calibrated. The to each spectrum and a subtraction function of
final results contain fewer degrees of freedom the fit function from the spectrum. The remain-
than this number of measurements, because the ing spectrum is used for multivariate calibration
calibration process creates some dependence be- or qualitative comparison.
tween the results; there is a loss in 1 degree of Difference bands: Similar to combination
freedom for every calculated parameter. bands in that two or more absorbance bands
Derivatives: In spectroscopy, it is used to are involved. In difference bands, the observed
indicate first through fourth derivative or dif- frequency of two interacting bands is the dif-
ference functions of the absorbance spectrum ference in frequency between the two bands, as
with respect to wavelength. Generally, the defined in equation 4:
Savitzky-Golay algorithm is used to create
derivative spectra. (ν difference = ν1 - ν2) [4]
Detector: A device sensitive to electromagnetic
radiation at the wavelength region of interest. Difference spectroscopy: In difference
The output of the device is usually an electrical spectroscopy, two spectra are subtracted one
signal proportional to the intensity of the elec- from another to determine spectral differences.
tromagnetic energy input. The detector gener- This technique is quite powerful for determin-
ally produces an analog signal proportional to ing slight variations in composition when inter-
the radiation striking the detector element. This preting spectra; it also assists in identifying low
analog signal is converted into a digital signal by levels of individual compound differences in
means of an analog-to-digital (A/D) converter mixture spectra.www.spectroscopyonline.com TERMINOLOGY GUIDE FEBRUARY 2019 17
Digital: Refers to the behavior of certain measurements; thus, two beams of optical ra-
devices (such as computers) that can only be diation must be used. One beam is used as a
in one of a finite number of discrete states. reference and the other as sample measurement.
Computers can only be in the states associ- The ratio of the sample to the reference beams,
ated with the output of the various electronic respectively, provide the I/Io spectral measure-
components exhibiting voltage levels corre- ment for use in absorbance (A, Au) as - log10
sponding to the 0 state or the 1 state. Each (I/Io) computations.
such solid-state device contains the informa- Double-sided interferogram: An interfero-
tion for one binary digit. See also bit. gram has a “center-burst,” also referred to as the
Digital light processing (DLP): A digital zero-path-difference (ZPD) location. When
light processing device is a microelectrome- the path of the fixed mirror and the moving
chanical system (MEMS) device also known mirror are identical in the interferometer, the
as a digital micromirror device (DMD). This interferogram is at maximum interference. The
stationary fixture consists of many thousands centerburst may be placed anywhere along the
or millions of micromirrors used in a DLP spec- recorded data for the interferogram. When it
trophotometer. The micromirrors move in series is placed in the center of the interferogram, it
to capture the diffracted light from a fixed dif- is termed a double-sided interferogram (also
fraction grating onto a set of collimation optics known as an asymmetric interferogram).
guided to a detector to create a full spectrum in
microseconds. The solid-state DLP spectrome- E
ter yields fast, high-resolution spectroscopy, par- Eigenvector: A nonzero vector used for the
ticularly for the visible and near-infrared regions. dot product solution in a matrix multiplication.
Dipole moment: Any covalent, intermediate, For example, principal components are eigen-
or polar molecule containing opposite charges vectors, as is a regression or b vector.
(+q and –q) separated by a distance of d; the Electromagnetic spectrum: The contin-
magnitude of the moments are reported in uum of frequencies that contains electromag-
Debye units. The permanent electric dipole mo- netic radiation (EMR). Instruments measure
ment is equal to the vector p = qd. If the charges the intensity of radiation within a defined range
(qi) are located at points ai, bi, ci, the magnitude of the spectrum, and usually present the results
of the dipole moment (Pa) is equal to of their measurements as a set of values of some
function, or the measured intensity at (usu-
Pa = qidi where; di = ai,bi,ci [5] ally) evenly spaced intervals within the range.
The energy throughout the electromagnetic
Discriminant analysis: A mathematical spectrum ranges from gamma radiation (most
technique for identifying or classifying data energetic and highest frequency) to radio waves
according to previously defined patterns. This (least energetic and lowest frequency). The
technique is sometimes referred to as qualitative spectrum is expressed in terms of wavelength,
analysis or classification analysis. wavenumber, frequency, or energy. See also unit
Double (dual) beam: This term is applied to conversions for photon energy.
the simultaneous measurement of optical data Etendue: The etendue is the relative through-
from both a sample beam and a stable reference put advantage for an optical system. It is calcu-
beam. Although the optical radiation used to lated as the product of the potential illuminated
obtain the measurements may share part or all surface area times the solid angle of the optical
of the optical path, there must be a separation system. It is an indicator of the relative efficiency
either in space or in time between the two of an optical system.18 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com
Extinction and extinction coefficient: wavelengths (low pass). There are also special fil-
Words sometimes substituted for absorbance ters, such as linear variable filters (LVFs), which
and absorptivity, respectively. They are gener- pass specific bandwidths of energy simultane-
ally considered physics terms. ously. Another optical filter type is multivari-
ate optical element (MOE) regression filters,
F which are special filters that resemble the
Far infrared (far-IR): The spectral region rep- regression b vector from a calibration and
resenting molecular vibration from 400 cm-1 to thus produce a final prediction of a constitu-
10 cm-1, or 2.5 x 104 nm to 106 nm. ent from an optical signal.
Fermi resonance: In its simplest definition, Filter spectrophotometer: A spectropho-
for infrared and Raman spectroscopy, it is a tometer that uses filters to isolate narrow bands
fundamental vibration coupled with an over- of the spectrum.
tone (harmonic) band or combination band at Filter transform: A mathematical operation
the same frequency to create two strong bands that converts data collected with a monochro-
where only the fundamental band is anticipated. mator-based instrument to mimic the corre-
These two strong bands occur at both higher sponding data that would have been collected
and lower frequencies than the fundamental from an instrument containing interference
band. Only certain combinations are allowed filters, taking into account the differences in
and are, for example, observed in carbonyl characteristics between the two types of devices.
(C=O)-containing compounds. Fermi reso- Fingerprint region: The infrared spectral
nance causes the splitting of two vibrational region from 1500 cm-1 to 500 cm-1 often at-
bands having nearly identical symmetry and tributed to bands from the bending molecular
energy, such as the fundamental with over- vibrations. The bands in this spectral region rep-
tone or combination bands. This resonance resent the C-C, C-N, C-O, and C-X stretching
causes a change in wavelength (or frequency) vibrations, as well as the C-H and C-S scissoring,
and amplitude (or intensity) of the resulting bending, and rocking vibrations.
spectral bands. The strong and weak bands Fixed filter: A spectrophotometer character-
thus combine into two strong bands. istic where narrow-band interference filters
Field: In database management, a section of a are mounted on a rotating turret. The turret
record in a file that contains the same type of is rotated so as to align the different filters be-
information as the corresponding section in tween the incident radiation and the sample.
other records in the same file. The term also Thus, this instrument design produces a dif-
refers to a variety of fields in physics such as ferent wavelength light of incidence on the
electronic, magnetic, and gravitational. Ac- sample for each rotation of the turret. See also
cording to Merriam-Webster’s definition, a near-infrared filters.
“field” is “a region or space in which a given Fluorescence: Photons with energies in the ul-
effect (such as magnetism) exists.” traviolet (that is, 190–360 nm) to the blue-green
Filter: A device used to attenuate particular visible (that is, 350–500 nm) spectral regions
wavelengths or frequencies while passing others will excite an electronic transition for atoms in
with relatively little change. There is electronic molecules that fluoresce (that is, fluorophores).
filtering (digital signal processing [DSP]), and Fluorescence is an electronic transition from a
physical optical filters. Optical filters include ground state to the excited state with the emis-
interference filters with narrow or broadpass sion of a photon to return to the ground state.
bandwidths, as well as filters that pass high- After the molecule is excited, it relaxes (Stokes
range wavelengths (high pass) or low-range shift) to the ground state while emitting a pho-www.spectroscopyonline.com TERMINOLOGY GUIDE FEBRUARY 2019 19
ton within a femtosecond (10−15 s) to picosecond Fundamental region: The lowest frequency
(10−12 s) timeframe. The Stokes shift indicates a corresponding to a given molecular vibration.
lower energy of the fluorescence photons than Fundamental group frequency vibrations for
the energy of the excitation photons. The molecular bands are within the infrared spec-
fluorescence typically has a lifetime (or dura- tral region of 4000–1500 cm-1. The bands in
tion) of nanoseconds (10−9 s) per transition. A this spectral region represent the asymmetric
fluorescence spectrophotometer normally has and symmetric stretching of C-H, O-H, N-H,
an excitation monochromator that defines Ar-H, >C=O, C=C, C=N, C≡N, C≡C, and
the excitation energy, and an emission mono- other bonds. Note that overtones are derived
chromator that provides a full spectrum of the from fundamental absorption frequencies as
fluorescence emission. equal to integer multiples of 2 (first overtone),
Fourier transform: A mathematical opera- 3 (second overtone), 4 (third overtone), and 5
tion in which a curved repetitive function is (fourth overtone).
described in terms of the sum of sine and co- F-value: A ratio of variances, usually used to
sine waves. A conversion from a time domain determine if one of the variances is due to the
interferogram to a frequency spectral domain same cause as the other. In regression analysis, a
occurs when using this transformation. In particular F-value (that is, the mean square due
spectroscopy, an interferogram is taken from to regression divided by the mean square due
a sample measurement in the time domain to error) has the secondary meaning of giving
and converted into a spectrum in the fre- an indication of the robustness of the regression.
quency domain. This conversion is completed The F-value is testable against a critical value of
for both sample and reference interferograms, the F-distribution to determine if the result is
which are then processed and ratioed to pro- significant or not.
duce an I/Io spectrum.
Fourier transform infrared (FT-IR): A G
means of measuring the electromagnetic spec- Graphical user interface (GUI): A term
trum using an interferometer; the desired spec- invented by Xerox in Palo Alto, California
trum is then obtained by performing a Fourier in the early 1970s that refers to a software
transform on the resulting interferometric data. interface between the user and the opera-
Other Fourier transform measurements include tional software code. Today, it may involve
Fourier transform near-infrared (FT-NIR) and touch-screen icons and graphics that are easy
Fourier transform ultraviolet (FT-UV). to activate and understand.
Frequency: Frequency as ν (Greek: nu) is the Grating: A reflective surface covered with
frequency of light, also known as Hertz (Hz) or evenly spaced, microscopic grooves, whose
the number of vibrations per second, in units purpose is to separate individual wavelengths
of s-1. It is defined by the following expression: from broadband energy. The distance between
grooves and the angle of the faces is determined
[6] by the wavelengths to be separated. The grat-
ing (except for diode arrays) is rotated at a set
Where ν (nu) is the frequency of light, n is the angle and speed, and the desired wavelength is
refractive index of the medium the light is pass- diffracted through an exit slit onto the sample
ing through (for example, the refractive index and detector (or detectors). It is used to disperse
of air is 1.0003), λ (lambda) is the wavelength light of various wavelengths and orders from
in units of cm, and c is the velocity of light in a its surface. It disperses zero order as specular
vacuum (or 2.9979 ×1010 cm/s). reflected light, and first, second, and higher20 TERMINOLOGY GUIDE FEBRUARY 2019 www.spectroscopyonline.com
orders as diffracted light. When the diffracted sponding to the grooves of the desired grating
light interacts in a phonon effect it decreases the model are recorded on this photosensitive layer
energy dispersed from the surface, resulting in a by projecting a hologram of light and dark lines.
phenomenon termed Wood’s anomalies (see this Subsequent chemical treatments are used to
term for more information). erode either the exposed or unexposed photo-
Grating spectrometer: A spectrometer that resist material (there are different types of treat-
uses a grating for the diffraction and resulting ments such as positive and negative photoresist
resolution of light of various wavelengths. This processes) on the surface to produce a grooved
is often termed a monochromator when one diffraction grating of specified groove shapes
grating system is involved. and lines per millimeter.
Hotband: A hotband results when an already
H excited vibrational band is further excited, often
Harmonic bands: A synonym for overtone. related to a change in temperature; the intensity
These bands represent multiple harmonics of of a hotband is proportional to the temperature.
the fundamental vibrations for molecular bonds At room temperature, the ground state of a mo-
and are found at integral multiples of the fun- lecular vibration is the most common as defined
damental band frequencies. For example, a first using the Boltzmann distribution. The Max-
overtone band occurs at twice the frequency of well-Boltzmann distribution law further states
the fundamental band, and the second over- that the distribution of these hotband transition
tone at three times the frequency of the funda- bands is dependent upon the Boltzmann con-
mental band. Harmonic bands in the infrared stant and the temperature in degrees kelvin.
and Raman regions are found as multiples of Hydrogen bonding: Hydrogen bonding
the fundamental frequency according to the within a molecule and between a solvent and a
harmonic oscillator approximation, with a pro- solvated molecule manifests as changes in the
portional higher shift in frequency and reduced force constants of the X-H bonds. Generally, mo-
amplitude in the spectrum with an increase in lecular bands will shift in frequency and broaden
harmonic number. Within the near-infrared as a result of the formation of hydrogen bonds.
region these harmonics are known as the first Since combination bands result as the summa-
overtone (2ν), second overtone (3ν), third over- tion of two or more fundamental vibrations, and
tone (4ν), and fourth overtone (5ν). overtones occur as the result of the multiples of
Highpass filter: A dielectric composite fil- fundamental vibrations, frequency shifts related
ter that enables energy above a certain cut-on to hydrogen bonding have a greater relative ef-
wavelength or frequency to pass at nearly 100% fect on combination and overtone bands than on
transmission, whereas energy at wavelengths or their corresponding fundamentals. The reason
frequencies below the cut-on wavelength will for this greater effect is that harmonics have
not pass or is nearly 0% transmissive. weaker bond energies relative to the hydro-
Histogram: A plot of the frequency of occur- gen bonding and have greater anharmonicity.
rence of sample values within the subranges of This feature of vibrational spectra alerts one
the range of values of the data. Histogram bars to the importance of the relative hydrogen
touch (are adjacent), whereas bar graph bars do bonding effects brought about by solvent and
not touch (are not adjacent). temperature variations.
Holographic grating: A grating made by Hyperspectral imaging: The collection of
using a holographic process where a photoresist multiple spectra into a three-dimensional data
is placed on an optical surface such as a glass hypercube consisting of wavelength, amplitude,
blank. A series of interference fringes corre- and either time or spatial dimension scanningwww.spectroscopyonline.com TERMINOLOGY GUIDE FEBRUARY 2019 21
(as the third dimension). For example, a moving Infrared (IR) spectroscopy: Infrared refers
sample may be measured by having a fixed lin- to that portion of the electromagnetic spectrum
ear array sensor with the third dimension being containing radiation with wavelengths longer
the sample measured over time. The data cube than those visible to the human eye, and ex-
may also be constructed by moving the sensor tending to approximately 30 μm. A broad defi-
over a stationary surface to gather a set of spectra nition of the infrared region is subdivided into
for each line area of a surface. Array detection is the near-infrared (0.75–2.5 μm), mid-infrared
often involved in generating hyperspectral im- (2.5–30 μm) and far infrared or terahertz (30–
ages; either by means of linear arrays or by using 300 μm). Mid-infrared spectroscopy provides
rectangular arrays. a measurement technique for intense, isolated,
and reliable absorption bands of fundamental
I molecular vibrations from polymers and other
Indicator variable: A variable used in regres- organic compounds.
sion analysis to allow the inclusion of infor- Infrared windows: See Window.
mation that cannot otherwise be expressible Integrating sphere: A hollow sphere, coated
numerically. Generally, they are assigned a 1 with a diffuse reflecting material and provided
or a 0 to indicate the presence or absence of a with openings for incident beam, specimen, and
particular condition. Examples could include detector. It is primarily used for measuring the
which of two operators prepared a sample, what most accurate diffuse reflectance or transmit-
instrument was used to measure the sample, the tance of objects. In most versions, the sample is
type of solvent used, average particle size range measured outside of the sphere.
of sample, and so forth. Intensity correction: A correction of the y-
Infrared absorption bands: A bond within axis (intensity) for Raman spectra in which a
a molecule that is infrared active or capable of standard lamp or a standard reference sample
absorbing infrared radiation. When the ra- is used in combination with a polynomial fit
diation striking an infrared-active molecule across the entire wavenumber axis to standard-
matches the natural vibrational frequency of the ize the intensity measurements of any Raman
various bonds within the molecule, the radia- spectrometer. It is analogous to a photometric
tion is absorbed. When absorption takes place correction accomplished for absorption spec-
at a specified frequency, an absorption band troscopy. It is also used to correct and measure
at that frequency is observed. For an infrared- the output of spectrometer lamps.
active molecule, the dipole must change as the Interface: A junction or connection. For ex-
frequency of the infrared radiation changes for ample, a computer input–output (I/O) inter-
it to absorb infrared energy or scatter Raman face. Also, the junction between air or sample
energy. Dominant infrared absorption bands and an optical element.
include the C-H (methyl, methylene, aro- Interference filter: A filter that controls the
matic, methoxy, and carbonyl) fundamental spectral composition of radiant energy passing
stretching and bending molecular vibrations, through it by the effects of interference. Fre-
the O-H (hydroxyl) stretch fundamental vi- quently, these filters are made up of thin layers
brations, the N-H (amine) stretching, the C-F of metals and dielectrics, and provide narrow
(fluorocarbon) stretching, the -C≡N (nitrile) bandpass and high transmittances.
stretching, the -C=O (carbonyl) stretch from Interferogram: A wave pattern created
esters, acetates, and amides, the C-Cl stretch using an interferometer where waves are su-
from chlorinated hydrocarbons, and -NO2 perimposed and later interpreted to compute
from nitro- containing compounds. a pattern of amplitude versus frequency for aYou can also read
