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Spectrometer
Spectrometer
Spectrometer
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An XPS spectrometer

A spectrometer (/spɛkˈtrɒmɪtər/) is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the spectral components are somehow mixed. In visible light a spectrometer can separate white light and measure individual narrow bands of color, called a spectrum. A mass spectrometer measures the spectrum of the masses of the atoms or molecules present in a gas. The first spectrometers were used to split light into an array of separate colors. Spectrometers were developed in early studies of physics, astronomy, and chemistry. The capability of spectroscopy to determine chemical composition drove its advancement and continues to be one of its primary uses. Spectrometers are used in astronomy to analyze the chemical composition of stars and planets, and spectrometers gather data on the origin of the universe.

Examples of spectrometers are devices that separate particles, atoms, and molecules by their mass, momentum, or energy. These types of spectrometers are used in chemical analysis and particle physics.[1]

Types of spectrometer

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Optical spectrometers or optical emission spectrometer

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Main article: Optical spectrometer
Spectrum of light emitted by a deuterium lamp in the UV, visible and near infrared part of the electromagnetic spectrum.

Optical absorption spectrometers

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Optical spectrometers (often simply called "spectrometers"), in particular, show the intensity of light as a function of wavelength or of frequency. The different wavelengths of light are separated by refraction in a prism or by diffraction by a diffraction grating. Ultraviolet–visible spectroscopy is an example.

These spectrometers utilize the phenomenon of optical dispersion. The light from a source can consist of a continuous spectrum, an emission spectrum (bright lines), or an absorption spectrum (dark lines). Because each element leaves its spectral signature in the pattern of lines observed, a spectral analysis can reveal the composition of the object being analyzed.[2]

A spectrometer that is calibrated for measurement of the incident optical power is called a spectroradiometer.[3]

Optical emission spectrometers

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Optical emission spectrometers (often called "OES or spark discharge spectrometers"), are used to evaluate metals to determine the chemical composition with very high accuracy. A spark is applied through a high voltage on the surface which vaporizes particles into a plasma. The particles and ions then emit radiation that is measured by detectors (photomultiplier tubes) at different characteristic wavelengths.[4]

Magnetic resonance spectroscopy

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Main article: Magnetic resonance spectroscopy

As protons, electrons, and many other nuclei have a net magnetic moment they interact with an applied external magnetic field. This can be used for high resolution liquid nuclear magnetic resonance spectroscopy, in which the unique magnetic environment of the nucleus changes according to electrons around them, yielding information on the chemical composition of the sample. Likewise, unpaired electrons interact with magnetic fields, yielding the technique of electron paramagnetic resonance.

Electron spectroscopy

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Main article: Electron spectroscopy

Some forms of spectroscopy involve analysis of electron energy rather than photon energy. X-ray photoelectron spectroscopy is an example.[5]

Mass spectrometer

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Main article: Mass spectrometry

A mass spectrometer is an analytical instrument that is used to identify the amount and type of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of gas-phase ions.[6]

Time-of-flight spectrometer

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The energy spectrum of particles of known mass can also be measured by determining the time of flight between two detectors (and hence, the velocity) in a time-of-flight spectrometer. Alternatively, if the particle-energy is known, masses can be determined in a time-of-flight mass spectrometer.

Magnetic spectrometer

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A positive charged particle moving in a circle under the influence of the Lorentz force F

When a fast charged particle (charge q, mass m) enters a constant magnetic field B at right angles, it is deflected into a circular path of radius r, due to the Lorentz force. The momentum p of the particle is then given by

,
Focus of a magnetic semicircular spectrometer

where m and v are mass and velocity of the particle.[7] The focusing principle of the oldest and simplest magnetic spectrometer, the semicircular spectrometer,[8][9] invented by J. K. Danisz, is shown on the left. A constant magnetic field is perpendicular to the page. Charged particles of momentum p that pass the slit are deflected into circular paths of radius r = p/qB. It turns out that they all hit the horizontal line at nearly the same place, the focus; here a particle counter should be placed. Varying B, this makes possible to measure the energy spectrum of alpha particles in an alpha particle spectrometer, of beta particles in a beta particle spectrometer,[10] of particles (e.g., fast ions) in a particle spectrometer, or to measure the relative content of the various masses in a mass spectrometer.

Since Danysz' time, many types of magnetic spectrometers more complicated than the semicircular type have been devised.[10]

Resolution

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Generally, the resolution of an instrument tells us how well two close-lying energies (or wavelengths, or frequencies, or masses) can be resolved. Generally, for an instrument with mechanical slits, higher resolution will mean lower intensity.[11]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Spectrometer

View on Grokipedia
from Grokipedia
A spectrometer is a scientific instrument used to separate and measure the spectral components of a physical phenomenon, such as electromagnetic radiation, mass-to-charge ratio, or nuclear spin transitions. Spectrometry is the practical application of spectroscopy used to acquire quantitative measurements of a spectrum, while spectroscopy refers to the theoretical study of how matter interacts with radiated energy. Spectrometers separate the phenomenon into its constituent components and measure properties such as intensity, capturing data on variables like wavelength, frequency, or mass-to-charge ratio. Each substance produces a unique spectral signature that enables identification and quantification of its chemical composition. This allows determination of the composition, structure, and other properties of materials. The concept originates from Isaac Newton's experiments in the 1660s, where he used prisms to disperse white light into its constituent colors, coining the term "spectrum"; the modern instrument developed in the early 19th century. Spectrometers operate by dispersing a signal into its components and detecting variations in intensity or other properties, often using elements like prisms, gratings, or . They are essential tools in science, with types including optical, , and (NMR) spectrometers, applied in fields from chemistry and physics to astronomy and industry.

Definition and Principles

Basic Definition and Purpose

A spectrometer is a scientific instrument designed to measure the properties of light or particles, such as electromagnetic radiation or ions, dispersed according to characteristics like wavelength, energy, or mass-to-charge ratio, thereby generating a spectrum for detailed analysis. This dispersion allows the instrument to separate and quantify components of the input signal, revealing unique patterns that characterize the sample under study. The primary purpose of a spectrometer is to identify and quantify the , molecular structure, and physical properties of substances by examining how matter interacts with radiation or fields, including processes such as absorption, emission, and . For instance, in optical , it analyzes light absorbed or emitted by atoms and molecules to determine elemental presence or concentration, while in , it separates ions by their mass-to-charge ratios to reveal molecular weights and structures. This capability makes spectrometers indispensable in fields like chemistry, physics, and for non-destructive analysis and precise characterization. The concept of the spectrometer originated in the as part of the emerging field of , building on early optical devices such as prisms that dispersed visible light into its constituent colors. A pivotal milestone occurred in 1814 when observed dark absorption lines in the solar spectrum using a high-quality prism, laying the groundwork for quantitative spectral analysis and the identification of atmospheric elements. Spectra produced by spectrometers are typically represented as plots of intensity versus (in nanometers, nm), (in hertz, Hz), or (in inverse centimeters, cm⁻¹), with the choice of axis depending on the application—wavenumber being common in for its direct proportionality to . For example, emission spectra display sharp lines corresponding to specific wavelengths emitted by excited atoms, enabling the identification of elements like or sodium through their unique line patterns.

Fundamental Operating Principles

Spectrometers operate on the quantum mechanical principle that atoms and molecules possess discrete levels, with transitions between these levels involving the absorption or emission of photons whose is given by E=hνE = h\nu, where hh is Planck's constant and ν\nu is the of the . These characteristic transitions produce spectral lines at specific wavelengths, forming the basis for identifying and analyzing material composition. Matter interacts with radiation primarily through absorption, emission, , or . In absorption, radiation is attenuated as it passes through a sample according to the Beer-Lambert law, A=ϵlcA = \epsilon l c, where AA is , ϵ\epsilon is the molar absorptivity, ll is the path length, and cc is the concentration. Emission occurs when excited atoms or molecules relax to lower energy states, releasing photons at discrete corresponding to the energy differences. involves inelastic interactions, such as the Raman effect, where the shift is Δν=ν0−νs\Delta\nu = \nu_0 - \nu_s, with ν0\nu_0 the incident and νs\nu_s the scattered , providing information on molecular vibrations. ejects electrons from atoms or molecules upon energy absorption, generating charged species for further . Spectra generated in spectrometers are either continuous or discrete. Continuous spectra arise from thermal sources, such as , whose intensity distribution is described by : B(λ,T)=2hc2λ51ehc/λkT−1,B(\lambda, T) = \frac{2hc^2}{\lambda^5} \frac{1}{e^{hc / \lambda k T} - 1}, where hh is Planck's constant, cc is the , kk is Boltzmann's constant, λ\lambda is , and TT is . Discrete spectra, in contrast, consist of sharp lines from quantum transitions in gaseous or dilute samples, distinguishing elemental or molecular signatures. The measurement process in spectrometers begins with collimation of the input signal to create parallel rays, followed by dispersion to separate components by or , and concludes with recording the intensity of these components at discrete positions or energies. This sequence enables the reconstruction of the , quantifying the distribution of radiation energies interacting with the sample.

Key Components

Signal Generation and Sample Interaction

In spectrometers, sample preparation adapts to the physical state of the material and the analytical technique to ensure accurate signal generation while minimizing interferences. Solid samples are often ground into fine powders and mixed with infrared-transparent matrices like (KBr) to form translucent pellets for (IR) spectroscopy, enabling transmission measurements without scattering losses. For , solids may be dissolved in acids for solution nebulization or directly vaporized using flame or graphite furnace atomizers to produce free atoms. Liquid samples typically require dilution with compatible solvents to reduce and concentration, or direct injection if compatible with the instrument, while gas samples can be introduced directly or preconcentrated via adsorption on solid sorbents to enhance detectability. To prevent contamination and matrix effects—such as suppression in where co-eluting compounds alter efficiency—purification techniques like (SPE) or are routinely applied, ensuring cleaner extracts and reliable quantification. Signal generation relies on specialized sources tailored to the spectrometer type, interacting with the prepared sample to produce detectable emissions or ions. In optical spectrometers, continuous broadband sources include tungsten-halogen lamps, which emit from approximately 360 to 2600 nm suitable for visible and near-infrared analysis, and deuterium arc lamps providing intense output from 190 to 400 nm for . Lasers, with their monochromatic and coherent output, enable precise excitation for techniques like or . In mass spectrometers, (EI) employs a 70 eV beam to bombard vaporized samples, producing molecular ions and fragments for structural elucidation, while (ESI) nebulizes liquid samples into charged droplets under high voltage, evaporating solvent to yield intact gas-phase ions ideal for biomolecules. The interaction between the signal source and sample excites analytes into measurable states through various mechanisms. Thermal excitation, as in atomic absorption or emission , heats the sample to 2000–3000 K, desolvating and atomizing analytes while promoting electronic transitions. Electrical discharge methods, such as arcs, sparks, or inductively coupled plasmas (ICP), generate high-temperature plasmas (up to 10,000 K) via radiofrequency energy to ionize and excite atoms efficiently for emission analysis. Laser-induced excitation, used in (LIBS), focuses a to create a on the sample surface, vaporizing and exciting material for rapid, analysis of solids. Samples interact with these sources within specialized containment systems to maintain optical paths or ion trajectories. Quartz cuvettes, with path lengths of 1–10 mm, hold liquid aliquots for transmission-based optical measurements, offering low background absorbance from 170 nm onward. Flow cells, connected via tubing to pumps, facilitate continuous sample delivery for real-time monitoring in process control or hyphenated systems like liquid chromatography-spectrometry. Handling hazardous samples demands stringent protocols to mitigate risks during preparation and interaction. For gamma spectrometry involving radioactive isotopes, , remote manipulators, and glove boxes prevent exposure, with monitoring ensuring doses remain below regulatory limits like those from the . Reproducible signal generation adheres to standards such as ISO/IEC 17025, which outlines validated sampling and preparation procedures for accredited laboratories, including and uncertainty estimation to support in spectrometric analyses.

Dispersion and Separation Mechanisms

In spectrometers, dispersion and separation mechanisms are essential for isolating components based on their inherent properties, such as in optical systems or in mass spectrometers. These mechanisms exploit physical principles like , , or field-induced trajectories to angularly or spatially resolve signals, enabling subsequent analysis without overlap. Prior to dispersion, in optical spectrometers, incoming first passes through an entrance slit, a narrow that limits the width and angular of the light beam to define the instrument's ; typical widths range from 10 to 200 µm, where narrower slits enhance resolution but reduce light throughput. The light is then collimated by such as a lens or , which collects the divergent rays from the slit and forms a parallel beam to uniformly illuminate the dispersing element, optimizing efficiency and minimizing aberrations. Optical dispersion primarily relies on diffraction gratings and prisms to separate wavelengths. Diffraction gratings consist of periodic rulings that cause constructive interference for specific wavelengths at particular angles, governed by the grating equation mλ=d(sin⁡i+sin⁡θ)m\lambda = d (\sin i + \sin \theta), where mm is the diffraction order, λ\lambda is the wavelength, dd is the grating spacing, ii is the incidence angle, and θ\theta is the diffraction angle; for normal incidence (i=0i = 0), this simplifies to dsin⁡θ=mλd \sin \theta = m\lambda. This angular dispersion allows different wavelengths to propagate in distinct directions, with finer gratings (smaller dd) providing higher resolution by increasing the angular separation per unit wavelength. Prisms, in contrast, achieve dispersion through refraction, where the varying refractive index of the material with wavelength causes shorter wavelengths (e.g., blue light) to bend more than longer ones (e.g., red light) upon passing through the prism, resulting in a continuous spectrum spread across an exit face. This material-dependent dispersion is quantified by the difference in refractive indices across wavelengths, typically stronger in the ultraviolet but limited by absorption in certain materials. In non-optical spectrometers, separation occurs via interactions with electromagnetic fields tailored to the analyte's charge and mass. For mass spectrometry, ions are deflected by magnetic and electric fields according to the Lorentz force F=q(E+v×B)\mathbf{F} = q (\mathbf{E} + \mathbf{v} \times \mathbf{B}), where qq is the ion charge, v\mathbf{v} is its velocity, E\mathbf{E} is the electric field, and B\mathbf{B} is the magnetic field; in magnetic sector analyzers, the centripetal force balances the magnetic component, yielding trajectories with radii proportional to m/qm/q, thus spatially separating ions by mass-to-charge ratio. In nuclear magnetic resonance (NMR) spectrometers, radiofrequency (RF) fields interact with nuclear spins precessing at the Larmor frequency ω=γB\omega = \gamma B, where γ\gamma is the gyromagnetic ratio specific to the nucleus and BB is the static magnetic field strength; resonant RF pulses at this frequency tip the spins, and chemical shifts cause slight frequency variations that are separated during signal processing. Separation mechanisms can be broadly classified as spatial or scanning. Spatial mechanisms simultaneously disperse all spectral components onto a detector array, such as a (CCD), where each pixel captures a unique or without mechanical movement, enabling fast, parallel acquisition over a broad range. Scanning mechanisms, however, sequentially select wavelengths or masses using adjustable , rotating gratings, or varying field strengths, which isolates one component at a time for detection, offering potentially higher resolution but at the cost of longer acquisition times. Advancements in these mechanisms have enhanced performance in demanding applications. Echelle gratings, with coarse rulings (large dd) and high blaze angles, operate in high diffraction orders to achieve resolving powers exceeding 100,000 while covering broad ranges through cross-dispersion, making them ideal for high-resolution . Acousto-optic tunable filters (AOTFs) provide rapid, electronically controlled tuning by using sound waves to create a dynamic Bragg grating in a birefringent , diffracting selected wavelengths with tuning speeds under microseconds and resolutions down to 1 nm, suitable for real-time selection.

Detection and Data Acquisition

In spectrometers, detection of separated spectral components relies on specialized detectors that convert incoming photons, ions, or into measurable electrical signals. Photomultiplier tubes (PMTs) are widely used for single-photon counting applications, where they amplify photoelectrons through a series of dynodes to achieve high sensitivity and low noise, enabling the detection of faint signals in low-light conditions. Charge-coupled devices (CCDs) serve as array detectors for simultaneous capture across multiple wavelengths, offering high quantum efficiency—up to 90% in back-illuminated models—due to their ability to collect nearly all incident photons in the visible and near-infrared ranges. Microchannel plates (MCPs) are employed for fast-timing events, providing sub-nanosecond resolution through electron multiplication in parallel microchannels, which is essential for of transient phenomena. Data acquisition follows detection, where analog signals from these devices are digitized for further analysis. Analog-to-digital converters (ADCs) typically operate at 12- to 16-bit resolution in spectrometers, allowing for precise quantization of signal intensities with minimal distortion, as seen in systems like compact UV-Vis units that achieve 16-bit depth for enhanced dynamic range. A key performance metric is the signal-to-noise ratio (SNR), defined as the signal amplitude divided by the square root of the noise variance, which quantifies the detectability of spectral features amid background fluctuations, particularly under shot-noise-limited conditions. Post-acquisition processing transforms raw data into interpretable spectra through techniques such as baseline correction, which subtracts instrumental drift and background contributions to isolate true spectral signals, and peak integration, which computes areas under peaks to quantify concentrations. Specialized software enables spectral , such as Fourier self-deconvolution in Fourier-transform () , where apodized Fourier transforms enhance resolution of overlapping bands by mathematically narrowing linewidths without altering peak positions. Calibration ensures the accuracy of detected and processed data. Wavelength calibration uses emission lines from known sources, like the , whose discrete lines (e.g., 253.65 nm and 546.07 nm) provide precise references for aligning the spectral axis across UV-Vis-NIR ranges. Intensity calibration employs traceable standards, such as NIST-certified sources or deuterium-halogen lamps, to correct for detector response variations and achieve absolute radiance measurements.

Types of Spectrometers

The following table summarizes some common types of spectrometers, what they measure, and their typical applications:
TypeWhat it MeasuresCommon Applications
Mass Spectrometers (MS)Mass-to-charge ratio of ionsProtein characterization, drug testing, and identifying unknown compounds.
Optical SpectrometersDistribution of light across the ultraviolet, visible, and infrared regionsAnalyzing the chemical composition of stars and planets in astronomy.
Nuclear Magnetic Resonance Spectrometers (NMR)Spin of atomic nuclei in a magnetic fieldStructural elucidation of organic molecules in chemistry and biochemistry.

Optical Spectrometers

Optical spectrometers are analytical instruments that measure the interaction of matter with in the (UV), visible (Vis), and (IR) regions of the , typically spanning wavelengths from about 190 nm to several micrometers. These devices disperse incoming into its constituent wavelengths to produce spectra that reveal information about molecular structure, composition, and concentration. Dispersion is achieved primarily through diffraction gratings, which separate wavelengths based on angular deviation caused by periodic microstructures, or through interferometric methods that encode information in interference patterns. A key subtype is , particularly in the UV-Vis range (190–800 nm), where light absorption by samples follows the Beer-Lambert principle, relating absorbance to concentration, path length, and molar absorptivity. This technique quantifies species by measuring the attenuation of transmitted light after it passes through a sample, providing data on electronic transitions in molecules. In contrast, excites atoms or ions to produce characteristic light emissions, analyzed for identification; uses a flame to vaporize and excite samples, while optical (ICP-OES) employs a high-temperature plasma (around 6000–10,000 K) for superior sensitivity in trace analysis across multiple elements simultaneously. Raman spectroscopy represents another subtype, relying on of monochromatic light (often from lasers) to probe molecular vibrations and rotations. When photons interact with the sample, most undergo elastic , but a small fraction (about 1 in 10^7) experience , shifting in energy by amounts corresponding to vibrational modes, yielding a "" spectrum for identifying functional groups without . (IR) spectroscopy, including Fourier-transform IR (FTIR), targets mid-IR wavelengths (4000–400 cm⁻¹) to detect vibrational transitions associated with functional groups like C=O or O-H bonds, useful for characterization. FTIR enhances resolution and speed over dispersive IR by using a , which splits a broadband IR beam, recombines it after path modulation to form an interferogram, and applies (FFT) to convert this time-domain signal into a frequency-domain . The foundations of optical emission spectroscopy trace back to 1859, when and developed flame spectroscopy using a and prism spectroscope to observe unique emission lines from elements like cesium and , establishing as a tool for qualitative elemental analysis. This breakthrough demonstrated that each element produces a distinct , revolutionizing chemical identification. Modern optical spectrometers have evolved into compact, portable handheld devices for field applications, such as on-site or material verification, often integrating miniaturized gratings or detectors like sensors for real-time analysis. Hyperspectral imaging spectrometers extend this capability by capturing spatial and spectral data simultaneously across hundreds of narrow bands, enabling applications like of vegetation health or mineral mapping from drones or satellites. These advancements prioritize ruggedness, battery operation, and user-friendly interfaces while maintaining high resolution (down to 1–5 nm).

Mass Spectrometers

Mass spectrometers are analytical instruments that measure the (m/z) of ions to identify and quantify molecules in a sample. They operate by ionizing the sample to produce charged particles, separating these ions based on their m/z values, and detecting them to generate a mass spectrum. Unlike optical spectrometers that rely on dispersion, mass spectrometers focus on the physical properties of ions in electric, magnetic, or combined fields. Ionization is the initial step in , converting neutral molecules into gas-phase ions suitable for analysis. impact (EI) ionization, a hard ionization method, bombards gaseous analytes with 70 eV s, producing molecular ions and extensive fragmentation patterns that aid in structural elucidation. For biomolecules, (MALDI) uses a to desorb and ionize analytes embedded in a UV-absorbing matrix, enabling the analysis of large, fragile molecules with minimal fragmentation. (ESI), a soft technique for liquid samples, generates ions by applying a to a nebulized solution, producing multiply charged ions from polar compounds like proteins without significant decomposition. Ion separation in mass spectrometers occurs through various analyzers that exploit differences in ion trajectories. The quadrupole analyzer uses four parallel rods with applied (DC) and (RF) voltages to create oscillating s; ions follow stable paths to the detector only if their m/z matches the stability region defined by the Mathieu stability diagram, filtering ions selectively. Time-of-flight (TOF) analyzers accelerate ions in a uniform and measure their over a fixed path length LL, where the m/z ratio is given by mz=2Vt2L2\frac{m}{z} = \frac{2Vt^2}{L^2} with VV as the acceleration voltage and tt as the ; lighter ions arrive faster, enabling rapid, high-throughput analysis./Instrumentation_and_Analysis/Mass_Spectrometry/Time-of-Flight_(TOF)_Mass_Spectrometry) Magnetic sector analyzers direct ions through a uniform BB, where the rr of the ion path satisfies r=mvqBr = \frac{mv}{qB} with mm as mass, vv as velocity, and qq as charge; scanning the field separates ions by momentum-to-charge ratio for moderate resolution./04%3A_MASS_ANALYZERS/4.02%3A_Magnetic_Sector) Advanced analyzers enhance resolution for complex mixtures. The traps ions in an electrostatic field around a central spindle, where their orbital frequencies inversely relate to m/z\sqrt{m/z}
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