What is a Spectrometer? Types and Uses
A spectrometer is a device that measures a continuous, non-discrete physical characteristic by first separating it into a spectrum of its constituent components.
Different types of spectrometer measure different characteristics. The oldest and most common type of spectrometer, the optical spectrometer, measures the properties of light over a defined range of the electromagnetic spectrum. The spectral range measured varies from device to device depending on the design of the spectrometer and its intended use, but most operate around the visible part of the spectrum. Wide range optical spectrometers may also extend into the near-infrared and UV regions.
Optical spectroscopy (or spectrometry) is the use of an optical spectrometer to study how light interacts with, or is emitted by, a sample. Optical spectrometers can measure the transmission, reflection, scattering, or absorption of light on a sample and the luminescence from an emitter.
Each of these measurements can reveal a large amount of information about the material or structure in question, whether that be a thin film on a substrate, a 2D material, a chemical or electrochemical solution, a living cell or other biological material, or a distant star. Optical spectrometers therefore have a wide range of applications across physics, chemistry, and biology.
Other types of spectrometer includes mass spectrometers and nuclear magnetic resonance (NMR) spectrometers, but unless otherwise stated, 'spectrometer' is generally used to refer to optical devices (and likewise 'spectroscopy' generally refers to optical spectroscopy).
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What is a Spectrometer Used to Measure?
Optical spectrometers are used in optical spectroscopy to separate light into electromagnetic spectra which show intensity as a function of wavelength. These can be used to determine various useful properties of materials or structures with which the light interacts before being measured. Optical spectrometers have wide reaching applications that span multiple disciplines.
Transmission, Reflectivity, Scattering, and Absorption
Transmission, reflectivity, scattering and absorption together describe the behaviour of light that is incident on a sample. The light can either pass through without interaction, bounce back towards the source, bounce in a random direction, or transfer its energy into the sample. In many cases, it is important to know which wavelengths a material will absorb and which it will transmit, reflect, or scatter. This can be determined with an optical spectrometer.
Transmission and reflectivity measurements have a broad range of applications, from characterising photonic structures such as dielectric stacks, often used as high-reflectivity or antireflection coatings, to process control in manufacturing. They can be used to detect changes in film thickness, density changes, and even the presence or absence of objects.
The optical absorption of materials quantifies the attenuation of a beam of light as it passes through the material. The energy of the incident photons is used to transfer electrons into higher energy states. In most optically active materials, UV and visible photons will promote electrons to higher energy electronic orbitals, while IR photons will increase the vibrational energy of the electrons. Absorption measurements, along with emission studies (see below) can allow the internal electronic and vibrational structure of atoms and molecules to be determined, and even the conformational structure of molecules and polymers to be inferred. They can also be used to calculate the concentration of absorbing species in a sample or monitor the progress of chemical reactions. These are critical for applications such as chemical synthesis and analysis, material discovery (e.g. for photovoltaics, LEDs, or pharmaceuticals) and quality monitoring/control. In astronomy, absorption measurements are used to identify atomic species in gases.
To take absorption and transmission measurements, light is directed towards the spectrometer and a spectrum is taken either with no sample in place, or using a reference placed in between the light source and spectrometer. For example, if the sample is in solution, a cuvette containing the pure solvent can be used as the reference. If the sample is a thin film on a substrate, a blank substrate is usually used. The reference is then replaced by the sample and another spectrum is taken. The transmission and (assuming no reflection) absorption spectra can then be calculated. Reflection can be measured by repositioning the spectrometer and using a perfect reflector, i.e. a mirror, as a reference.
Typically, the light source used for absorption and transmission measurements has a very broad spectrum. Suitable sources include deuterium or tungsten halogen lamps or an LED broadband white light source.
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Scattering measurements are much less common, due in part to the difficulties in predicting and detecting where the light will scatter, but can be used to calculate the size and distribution of scattering centers within a sample. They are also useful for impurity detection/monitoring in water systems, nanoparticle characterisation and drug loading for pharmaceuticals.
Another important property of materials and devices that can be measured with a spectrometer is their emission. Studying the light that is emitted by materials is a complementary technique to absorption spectroscopy, in that it probes how processes lead to the conversion of internal energy to photons, rather than the other way around.
Some materials will emit light when the sample is excited using a laser with a lower wavelength (higher energy) than the expected emission. If the sample absorbs at this particular wavelength, light can then be emitted when the electrons relax back down to the ground state. This is known as photoluminescence.
Two types of photoluminescence are fluorescence and phosphorescence. Fluorescence is when electron spin does not change as a result of the excitation, and phosphorescence is when the spin of the electron reverses when it is excited to a higher energy level and again when it relaxes to a lower energy level. The most noticeable difference between the two is that phosphorescence often 'glows' for a period of time (seconds to hours) after the light source is removed, whereas fluorescence will generally only be visible while the light source is on.
Fluorescence and phosphorescence measurements involve first illuminating the sample with an intense beam of light, usually a laser or high-power LED tuned to a wavelength where the sample will absorb the light. Once the photon energy is transferred to the sample, it will undergo an internal redistribution which may result in heating or conformational changes. Some materials will then reemit any remaining energy in the form of photons, at a lower energy (longer wavelength) than those it absorbed. This light can be collected by an optical spectrometer and the emission spectrum can be recorded.
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Studying the difference between the ingoing and outgoing photon energies allows information about the electronic and vibrational states to be extracted, as well as how the fluorescent materials are interacting with their surroundings. This information can be used to complement (or as an alternative to) absorption measurements for chemical synthesis and analysis, material characterisation and discovery, quality control, and more. In addition, luminescence also has some unique applications. For example, fluorescent molecules are often used as 'tags' or 'tracers' in applications as far-ranging as understanding the processes occurring within living cells, to identifying the paths of water courses.
The sample can also be electrically excited (by either a current or an electric field) in order to produce an emission. In this case, it is referred to as electroluminescence. In other cases, luminescence may occur naturally: some chemical reactions result in excess internal energy that is emitted as light. This is often found in nature in living organisms such as fireflies, glow worms, some deep sea fish, and algae.
An optical spectrometer is invaluable in studying all types of emitters, as it allows the colour rendering index (CRI) of light sources to be calculated. This is an important factor when developing lighting for specific applications. Optical spectrometers can also measure the solar spectrum and are widely used in astronomy for classifying types of stars.
Types of Spectrometer
Optical spectrometers are the most prevalent type of spectral device, and are often referred to as just 'spectrometers'. The name 'optical spectrometer' itself is a broad term, as there are a number of different types of optical spectrometer. These are usually defined by the range of the electromagnetic spectrum that they cover, but can also be distinguished by their optical design, by their intended application, or by specific features that they offer.
Other types of spectrometer include mass spectrometers and nuclear magnetic resonance spectrometers.
Mass spectrometers produce spectra which show intensity as a function of mass-to-charge ratio. This is done by ionising the sample to be studied, supplying the ions with kinetic energy, and directing them through a magnetic field. The ions are then deflected by the field: those with a lower mass-to-charge ratio are deflected more, and those with a higher mass-to-charge ratio are deflected less.
By varying the magnetic field in the mass spectrometer, ions of different mass-to-charge ratio will be deflected into the detector, producing a charge proportional to the number of ions. This spectrum can then be used to determine the relative abundance of isotopes of an atom, to identify chemical compounds, or to deduce the structure of a certain molecule.
Nuclear Magnetic Resonance Spectrometers
Nuclear magnetic resonance spectrometers are primarily used in organic chemistry and biochemistry to obtain information on the structure and composition of molecules. In the presence of an external magnetic field, some nuclei act like magnets, and if a broad range of radiowave frequencies are directed onto the sample, the nuclei will resonate at different frequencies.
The resonant frequency depends on both the environment that the nuclei are in (e.g. how many and which atoms they are close to) and the magnitude of the applied magnetic field. The intensity of the NMR signal is proportional to the number of nuclei with each resonant frequency. This is plotted against the chemical shift which gives the resonant frequency with respect to a known reference and has units of parts per million (ppm).
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