Choosing the Right Spectrometer

Spectrometers fall into two groups: radiation spectrometers and mass spectrometers.

Horiba Raman spectrometer

Radiation spectrometers

• Absorption spectrometers: This type of spectrometer measures the proportionality between the light intensity absorbed and the quantity of absorbing material. The spectrum shows peaks, each corresponding to a particular wavelength, characteristic of the substance analyzed. This technique applies both to molecules in the liquid or gaseous phase and to atomic vapor, which is obtained by atomizing liquid or solid samples.

• Emission spectrometers: With this type of spectrometer, the atoms in the sample are subjected to an excitation source. When they return to their ground state (de-excitation process), the atoms emit the radiation characteristic of the material that makes up the sample.

• Raman spectrometers: These are based on the inelastic scattering of the radiation incident on the sample by the molecules that make it up. The difference between the frequency of the incident radiation and the frequency of the scattered radiation makes it possible to identify the chemical bonds of these molecules. Raman spectrometry is therefore an ideal technique for structural analysis, in addition to infrared and mass spectrometry.

• Nuclear magnetic resonance spectrometers (NMR): These use a technique based on the magnetic properties of atomic nuclei. NMR will induce and detect the energy transition of nuclear magnetic moments, from the lowest to the highest energy level. To do this, the atoms that will be analyzed are placed in a magnetic field and subjected to radiofrequency radiation. Measuring the resonance frequency of this field then allows the molecules to be analyzed.

Mass spectrometers

Thermo Scientific mass spectrometer

Mass spectrometers separate charged molecules (ions) in the gas phase according to their mass/charge ratio (m/z). Mass spectrometers can be used to determine:

• the molecular mass of a compound
• the mass of the fragments of this compound
• the amount of matter

Spectrometry has a wide range of applications. In this buying guide, we will focus mainly on the analysis of biological samples, research, the pharmaceutical industry, and environmental analysis.

• Analysis of biological samples: Mass spectrometry is the technique of choice for analyzing biological samples. Mass spectrometers are used to detect biomarkers in biological fluids, for example.
• Research: Spectrometers are used, particularly in molecular biology, to analyze genetic material.
• Pharmaceutical industry: Spectrometers are used to monitor the composition of medicines, in particular their active ingredients.
• Environmental analysis: Spectrometers can detect numerous substances, such as pesticides, volatile organic compounds, and aromatic hydrocarbons.

Agilent atomic absorption spectrometer

This category includes atomic absorption spectrometers, molecular absorption spectrometers, and infrared spectrometers, which differ in the type of analysis and the excitation method.

Atomic absorption spectrometers

As their name suggests, atomic absorption spectrometers make it possible to carry out analyses at the atomic level. This technique usually requires a prior atomization step, i.e. the dissociation of a chemical species into free atoms. It is a very efficient form of absorption spectrometry and therefore has a wide range of applications. It allows high-precision measurements to be made even if the sample consists of a complex mixture of chemical elements. This is a reference technique for analyzing trace elements in a wide variety of samples, including biological ones. In the health sector, for example, atomic absorption spectrometry is used to accurately analyze blood or tissue samples from patients.

We can distinguish between two types of atomic absorption spectrometers:

• Atomic absorption spectrometers (AAS): excitation by electromagnetic radiation
• X-ray absorption spectrometers (XAS): X-ray excitation, without the need for prior atomization.

Spectronic Camspec UV-visible spectrometer

Molecular absorption spectrometers

These are used to carry out molecular analyses. This is the most widely used technique in the qualitative and quantitative analysis of substances. It is based on the emission of photons in the ultraviolet-visible (UV-Vis) region of the spectrum. Molecular absorption spectroscopy has the advantage of being fast, non-destructive, and simple to perform. However, it is only suited for slightly complex samples, as the width of the molecular absorption bands does not allow all the components of a complex mixture to be distinguished.

Depending on the excitation mode, there are:

• UV spectrometers: Photon wavelengths in the ultraviolet region of the spectrum (100 nm-400 nm)
• UV-visible spectrometers: Photon wavelengths in the ultraviolet and visible regions (100 nm-750 nm)
• Visible spectrometers: Photon wavelengths in the visible region (400 nm-750 nm)

Infrared spectrometers

Bruker infrared spectrometer

Absorption spectra in the infrared region make it possible to determine the nature of the chemical bonds that make up a molecule and thus confirm structural hypotheses.

Infrared spectrometry is a technique of choice for eliminating ambiguities about the structure of a molecule. However, it offers little precision if the molecule contains many atoms. This is because the spectrum becomes complex and interpretation quite difficult, especially when it comes to organic compounds. In these cases, atomic absorption spectrometry is used.

Emission spectrometers are mainly used for quantitative and qualitative analysis at the atomic level. The sample under analysis is excited and emits radiation. The quantitative measurement of this radiation from the excited atoms makes it possible to determine the concentration of a substance present in the sample, i.e. the analyte.

These are some of the characteristics of emission spectrometers:

• Very wide and dynamic measuring range
• Qualitative and quantitative analysis of several elements with a single measurement
• High precision and sensitivity
• High speed
• No chemical interference

There are two types of emission spectrometers, which differ in the mode of excitation and emission: fluorescence spectrometers and atomic emission spectrometers.

Fluorescence spectrometers

Edinburgh Instruments fluorescence spectrometer

Fluorescence emission occurs when a particle is excited by electromagnetic radiation. The radiation emitted is characteristic of the particle being analyzed. This technique has the great advantage of being very precise and specific, which makes it particularly suitable for measuring very low concentrations. However, it is a more complex technique than absorption spectrometry.

There are also X-ray fluorescence spectrometers, so called because the excitation is carried out with X-rays.

Atomic emission spectrometers (AES)

This technique is based on the thermal excitation of atoms when subjected to a high temperature. However, excitation is non-selective, occurring in all the particles that make up the sample. Each emission line corresponds to an element and the emission spectrum makes it possible to identify and quantify all the atoms or molecules present in the sample.

Mass spectrometers are used to determine the molecular mass of a substance and obtain its structural data.

The sample must be vaporized and ionized to carry out mass spectrometry analysis. The resulting molecular ion is split into fragments, which are separated in an analyzer according to their mass/charge ratio and then picked up by a detector. The result is a mass spectrum, characteristic of the sample’s composition.

This is an extremely sensitive detection and identification technique, widely used in protein analysis, for example.

The advantages and specificities of this technique are as follows:

• Wide range of applications, from the analysis of very small molecules (molecular weight of less than 2,000 Da) to macromolecules (molecular weight of more than 100,000 Da).
• High precision, allowing the molecular weight to be determined with a maximum margin of error of 1 Dalton (Da).
• Combination with other analytical techniques (chromatography, for example) is possible to achieve even greater precision.