Experimental Spectroscopy Vocab Flashcards
device that converts radiant power into an electrical signal or other physical property, e.g. heat or resistance, that can be measured
Detector
The slope of the curve (Q) that describes detector electrical response (S) with incident radiant power (F),
Detector sensitivity
The plot of Q(l) vs. l for a particular detector.
Spectral Response
The ability of a detector to convert incident radiation into an electrical signal; the ratio of the electrical output (V) to the incident radiant power (W)
Responsivity
A measure of detector sensitivity
Noise equivalent power (NEP)
A measure of detector sensitivity that normalizes the NEP by taking into account the area of the detector element
Specific detectivity (D*)
radiant energy generates current at the interface of a semiconductor and a metal
Photovoltaic cell
incident radiation causes emission of electrons from photosensitive cathode surface.
Phototube
The cathode and anode in a PMT are biased by 400 – 2500 mV and separated by a series of intermediate dynodes, each progressively more positively biased, which provides an electron multiplication cascade.
Photomultiplier tube (PMT)
Reverse-biased p-n junction forms depletion layer on a Si chip with no current; radiation striking the detector causes holes and electrons in depletion layer, producing a current that is proportional to the radiant power.
Si photodiode
Semiconductors, e.g. PbS, PbSe, InSb, HgCdTe, that act as a light-dependent resistor; resistance decreases when photons are absorbed.
Photoconductivity detector
Gas is heated in an enclosed cell (i.e. Golay cell), causing a reaction (change in capacitance) in a thin membrane.
Pneumatic Cell
Detects potential difference at junction between two dissimilar metals (e.g. Bi – Sb), which varies with DT between the two metals.
Thermocouple
Resistance thermometer made of semiconductors, e.g. Ge, Si. Detects change of resistance as a function of temperature
Bolometer
Detects temperature dependent polarization of pyroelectric crystal, e.g. triglycine sulfate (TGS)
Pyroelectric detector
Consists of a linear array of Si photodiodes placed in an integrated circuit; number of individual p-n junctions can range from 64 – 1024.
Photodiode array
Two-dimensional array of Si photodiodes containing about 105 individually resolvable pixels. Radiation strikes tube where charge is stored; target is scanned by electron gun that measures the charge needed to restore the individual Si photodiode pixel to its initial state.
Vidicon
Solid state sensor with integrated circuit technology. Photons strike MOS-on-p-type Si capacitor; electron charge stored in wells; serial readout destroys accumulated charge; high quantum efficiency (90%) possible.
Charge Coupled Device (CCD)
Solid state sensor with integrated circuit technology. Photons strike Si, which generates a + charge; charge is stored beneath negatively biased capacitor plates; random access readout does not destroy accumulated charge
Charge Injection Device (CID)
Spectral line broadening mechanism due to energy decay from excited state to ground state described by a damped oscillating dipole function
Natural line broadening
The lifetime of the exponential decay of the dampened oscillating dipole function
Classical damping constant
the lifetime of the atom or molecule in the excited state
Excited state lifetime
The line shape that results from the Fourier transform of the amplitude decay of the damped oscillating dipole function in Natural Line Broadening
Lorentzian frequency distribution
The width of a spectral band, measured as the full width of the band at one-half the maximum band intensity between the baseline and the peak; also related to the excited state lifetime
Full width at half maximum (FWHM)
The apparent shift in frequency or wavelength of an electromagnetic wave from a source that is moving either away from, or towards, a detector
Doppler effect
The line shape that results from Doppler line broadening of moving molecules in a sample
Gaussian distribution
Spectral band shape that is a mixture of both Lorentzian and Gaussian line shapes.
Voigt Profile
Random fluctuations that are observed in spectra or other measured signals when replicate measurements are made on instrumental outputs that are continuously monitored.
Noise
Random signal fluctuations that result from a variety of uncontrollable variables, e.g. temperature, pressure, humidity, vibrations, etc.
Chemical noise
Random signal fluctuations that are generated by particular components of an analytical instrument, i.e. source, detector, electronics, etc.
Instrumental noise
Measurement of the strength of the signal level relative to the background noise level
Signal-to-noise ratio
Measurement of the noise strength as the difference between the maximum and minimum noise signal heights
Peak-to-peak noise
Measurement of the noise strength as the standard deviation of the signal derived from n independent measurements
Root-mean-square noise
Plot of the mean-square noise per unit frequency interval vs. frequency; describes the type of frequency-dependent noise found in many types of experimental spectroscopy.
Noise power spectrum
Random, non-fundamental, frequency-dependent noise that usually manifests itself as a slow drift in the signal; exhibits large amplitude at low frequency; also known as “flicker” noise.
1 / f Noise
A random and fundamental noise whose magnitude is independent of the signal frequency; also known as “background” noise.
White noise
A frequency-dependent, non-random, and non-fundamental noise source; frequently appears at discrete frequencies due to pick-up or coupling with other signal sources, e.g. 60 Hz electrical noise; can be correlated to external event, e.g. the “5 o-clock effect”.
Interference/environmental noise
Random signal fluctuations in an electrical signal; characteristic of all electronic circuits.
Electronic noise
Type of random, fundamental, white noise produced by thermal agitation of electrons in electrical circuits
Thermal / Johnson noise
Type of random, fundamental, white noise produced whenever electrons move across a junction or barrier, or current is generated at an interface
Shot noise
Shielding of electrical cables to reduce environmental noise.
Grounding
Amplifier circuit with 2 input terminals: reference vs. sample; voltage difference between reference and sample signals is amplified.
Difference amplifier
Low-pass frequency filter with a long time constant that removes high frequency components, e.g. thermal and shot noise, in DC signals; used to reduce the effect of drift and other low frequency components.
Analog filter
The alteration of some property of a carrier wave by a signal so that the carrier wave encodes the signal information; moves the signal to a region of the frequency spectrum where it is easily distinguished from noise.
Modulation
Also called synchronous detector, heterodyne detector or phase-sensitive detector; electronic amplifier that detects specific AC frequency signal, requires a reference signal at the specific detection frequency; only those modulated signals that are “locked-in” to the reference signal are detected and amplified, all other signals are rejected.
Lock in amplifier
Mathematical co-addition of digital spectra where the final S/N is proportional to the square root of the number of individual spectra collected
Ensemble averaging
Linear filter in which spectral data points are divided into series of discrete, equally spaced windows; the data points in each are then replaced by the centroid average value of all the data in the window.
Boxcar filter
Linear filter in which the smoothed data are simple linear functions of the raw input data; input data are multiplied by weighting factors that are the linear average of the data in the filter window; filter window is successively moved along the spectrum, with each data point replaced successively by the corresponding filtered data point.
Moving average filter
Non-linear filter in which the raw spectral data are smoothed using a polynomial function to fit a series of adjacent data points in the spectra
Polynomial / Savitzy-Golay filter
Digital filtering of spectral domain signal in frequency domain; Fourier transform of spectrum used; frequency domain spectrum is multiplied by digital filter which removes unwanted frequency regions
Fourier filter
the fraction of the incident radiant power transmitted through a medium
Transmittance
logarithm of the fraction of the incident radiant power transmitted through a medium
Absorbance
The linear relationship between optical absorbance and the concentration of an absorbing species in solution, also sometimes written to emphasize the wavelength dependence of the absorbance
Beer Lambert law
Determination of the relationship between the analyte concentration and the analytical response.
Calibration
Use of chemical reference materials that provide a known comparison between concentration and response for calibration purposes.
Standardization
Calibration procedure where a series of external standard samples are prepared separately from the analytical sample; used when there are no interference effects from matrix components in the analytical sample; calibration is accomplished by measuring an analytical property, e.g. absorbance, as a function of analyte concentration in a series of external standards; calibration curve is then used to predict concentration of unknown
External calibration
Relationship of a response or dependent variable (y), to a single independent feature or measurement variable (x) by a linear model
Linear regression
Finds the slope (m) and intercept (b) of the regression line in a linear regression by minimizing the sum of the squares of the residuals between the predicted and actual y values,
Method of least squares
Variance about a least squares fit regression line
Residual variance
measure of the “goodness of fit” of the regression model; it describes how well the regression line approximates the real data points
Coefficient of determination
the square root of the coefficient of determination
Correlation coefficient
Procedure used to determine concentration of an analyte in an unknown sample by comparison to a set of samples of known concentration; used instead of a calibration curve to solve the matrix effect problem.
Method of standard addition
Type of Method of Standard Addition: addition of increasing increments of a standard solution to sample aliquots; solution is then diluted to fixed volume before measurement.
Continuous Variation of the Standard Solutions
Type of Method of Standard Addition: uses replicate measurements on a single spiked sample at high concentration.
Single addition method
Addition of an inert compound in a known, constant amount to all samples, blanks, and calibration standards in an analysis; calibration involves plotting the ratio of the analyte signal to that of the internal standard in a series of standard solutions.
Internal standard method
The measure of how close the result of an analytical measurement comes to the true value.
Accuracy
The reproducibility of the results, i.e. the numerical agreement between replicate measurements.
Precision
Figures of merit used to determine precision include
standard deviation, the standard error of the mean, and the coefficient of variation.
Describes the measurement of the systematic or determinate error of the analytical method
Bias
The ability of an analytical method to discriminate between small differences in analyte concentration
Sensitivity
The slope of the calibration curve at the concentration of interest
Calibration sensitivity
The lowest analyte concentration likely to be reliably distinguished from the blank and at which detection is feasible; alternatively, the minimum concentration of analyte that can be detected at a known confidence level. Practical definition is the analyte concentration that gives a signal equal to the blank plus 3 standard deviations of the blank
Limit of detection (LOD)
The useful range of an analytical method, that extends from the limit of quantitation (LOQ) to the limit of linearity (LOL).
Dynamic range
The lowest concentration of a substance that produces a quantitative result within specified limits of precision and bias.
Limit of Quantitation (LOQ)
The concentration of an analyte at which the calibration curve departs from linearity by a specified amount, usually 5%.
Limit of linearity (LOL)
