What is Spectrum

Substances have their own properties by which different substances can be distinguished. Because of their composition, substances emit their characteristic spectra under certain conditions. This property of the spectrum is used to determine the composition of a substance.

Since light has a fluctuating nature, the wavelength of light is a sign of it. Different colors of light indicate that it has different wavelengths. From short-wave ultraviolet light to long-wave infrared light, all light is visible. A series of different wavelengths of light arranged separately according to wavelength makes up the so-called spectrum. Emission spectroscopy works in the spectral range of only a part of the ultraviolet domain, the visible domain and a part of the infrared domain, with wavelengths ranging from 1600 to 8500 Å. The spectrum of a substance can be analyzed by emission spectroscopy.

Substances can emit spectra, substances have the effect of absorption and scattering of light, these phenomena can be used for the determination of substances. There are three types of spectra emitted by substances: line spectra, band spectra and continuous spectra.

Usually, the spark source used for spectral analysis is a flame, arc or spark, and under the action of the spark source, the analyzed substance is in a high-temperature gaseous state, generally dissociated into atoms or ions, which emit line spectra after spark. Therefore, spectral analysis makes use of the spectral lines in the line spectrum. The results only give the type and content of the elements that make up the substance, not the molecular structure of the substance.

The spark of atoms of each element produces a set of characteristic spectra, the appearance of which proves the presence of the element in the radiation source. Tens of thousands of spectral lines produced by the spark of atoms or ions have had their wavelengths determined. Since the determination of wavelength can achieve a high degree of accuracy, most of the spectral lines in the spectrum can be accurately determined by which element, so the spectrum for qualitative analysis is a very reliable method, sensitive, fast, and simple.

When the content of an element in the sample is not too high, the intensity of the spectral lines emitted by the element is proportional to its content. This relationship becomes the basis of quantitative spectral analysis and makes it a very convenient method. Quantitative spectral analysis is faster than chemical analysis and can be performed with fewer samples.

The spectra emitted by a substance must be spectroscopically separated. The spectrometer must have three elements: an incident slit, a grating that separates light of different wavelengths and arranges them in rows according to wavelength, and an exit slit that focuses on the image to form spectral lines (the spectral lines are the image of the slit).

The spectral lines fall on the focal plane and can be taken in with a photographic plate, observed with an eyepiece (for visible light), or received with an exit slit (to distinguish them from other spectral lines in the vicinity). One of these is a spectrograph, the next is a spectroscope, and the third is a monochromator. If an exit slit is installed at many spectral lines and a photoelectric receiving device (photomultiplier tube) is installed behind the exit slit, it becomes a photoelectric direct-reading spectrometer.