What Is Quantitative Mass Spectrometry?

Mass spectrometry (MS) is the use of electric and magnetic fields to move ions (charged atoms, molecules or molecular fragments, including molecular ions, isotopes, fragment ions, rearranged ions, multi-charged ions, metastable ions). , Negative ions, and ions produced by ion-molecular interactions) A method of detection after separation according to their mass-to-charge ratio. By measuring the exact mass of the ion, the compound composition of the ion can be determined. This is because the exact mass of a nuclide is more than one decimal place. There will never be two nuclides with the same mass, and there will never be one nuclide whose mass is exactly an integer multiple of the mass of another nuclide. Analysis of these ions can obtain information such as the molecular weight, chemical structure, cleavage rule of the compound, and some interrelationships between certain ions formed by the decomposition of a single molecule.

Chinese name: Mass spectrometry

Foreign name: mass spectrometry

Mass Spectrometry History

In 1942, the American CEC company introduced the first commercial mass spectrometer for petroleum analysis. Mass spectrometers can be divided into isotope mass spectrometers, organic mass spectrometers and inorganic mass spectrometers according to their uses.

Introduction to Mass Spectrometry

Mass spectrometry is one of the most powerful tools for the identification of pure substances, including relative molecular weight determination, chemical formula determination, and structural identification.

Mass spectrometry

In 1898, W. Wien found that the positive ion beam was deflected with electric and magnetic fields. When the charges were the same,

Mass spectrometry Small mass ions are deflected much, while large ions are deflected less. In 1913, JJ Thomson and FW Aston used magnetic deflection to confirm that neon has two isotopes [kg1] Ne and [kg1] Ne. Aston produced a mass spectrum capable of resolving one hundredth of a mass unit in 1919. Gauges, used to determine the relative abundance of isotopes, have identified many isotopes. Until 1940, mass spectrometers were only used for gas analysis and determination of stable isotopes of chemical elements. Later, mass spectrometry was used to analyze complex hydrocarbon mixtures in petroleum distillates, and it was confirmed that complex molecules can produce a definitive and repeatable mass spectrum. Mass spectrometry was then used to determine the structure of organic compounds, opening up new possibilities for organic mass spectrometry. field.

Principle of Mass Spectrometry

The components in the sample are ionized to generate ions with different charge-to-mass ratios. After the action of the accelerated electric field, an ion beam is formed, which enters the mass analyzer and uses the electric and magnetic fields to cause opposite velocity dispersion-the slower in the ion beam After passing through the electric field, the ions deflection is large, and the deflection is fast; the ions are deflected in the magnetic field with the opposite angular velocity vector, that is, the ions with slow speed are still deflected, and the deflections are fast. Their tracks intersect at one point. At the same time, mass separation can also occur in the magnetic field, so that ions with the same mass-to-charge ratio but different velocities are focused on the same point, ions with different mass-to-charge ratios are focused on different points, and they are focused separately. A mass spectrum is obtained to determine its mass.

Mass spectrometry can also perform effective qualitative analysis, but it is powerless to analyze complex organic compounds, and it is very troublesome to perform a series of separation and purification operations in the quantitative analysis of organic compounds. Chromatography is an effective method for the separation and analysis of organic compounds. It is particularly suitable for the quantitative analysis of organic compounds, but qualitative analysis is more difficult. Therefore, the effective combination of the two will provide an efficient qualitative and quantitative analysis of complex compounds. tool.

Mass spectrometry instrument

An instrument designed using the principle of deflection of moving ions in electric and magnetic fields is called a mass spectrometer or mass spectrometer.

Mass spectrometer The former means that the ions are detected electronically, while the latter means that the ions are focused on the photographic base plate for detection. There are many types of mass spectrometry instruments. According to the scope of use, they can be divided into inorganic mass spectrometers and organic mass spectrometers. Commonly used organic mass spectrometers are single focus mass spectrometer, double focus mass spectrometer and quadrupole mass spectrometer. At present, the latter two are used more often, and they are often used in combination with gas chromatographs and electronic computers.

Mass spectrometry high vacuum system

The mass spectrometer must be operated under high vacuum. The valve pump system used to obtain the required degree of vacuum, usually by the foreline pump

Mass Spectrometry-High Speed Electron (Common mechanical pumps) and oil diffusion pumps or molecular turbopumps. The diffusion pump keeps the ion source at a vacuum of 10 to 10 mm Hg. Sometimes there is a diffusion pump in the analyzer to maintain a vacuum of 10 to 10 mmHg.

Mass spectrometry sample injection system

It can be divided into four methods: direct injection, gas chromatography, liquid chromatography, and gas diffusion. The solid sample is injected through the direct injection rod, and the solid sample is converted into gas molecules by heating. Impure samples can be pre-separated by gas or liquid chromatography and introduced through the interface. The LC-MS interface has a drive belt interface, a direct liquid interface, and a thermal spray interface. Thermal spray interface is a newly proposed soft ionization method, which can be applied to high polarity reversed-phase solvents and low volatility samples. The sample was passed through a capillary from a polar buffer solution at a flow rate of 1 to 2 ml per minute. The capillary temperature is controlled so that when the solution approaches the outlet, it evaporates into a small jet. The tiny droplets still have residual positive and negative charges, and form adduct ions with the characteristics of electrolytes or solvents with the analyte to enter the mass spectrometer.

Mass spectrometry ion source

A device that ionizes a sample to produce a beam of charged particles (ions). The most widely used ionization method is electron bombardment. Others include chemical ionization, photoionization, field ionization, laser ionization, spark ionization, surface ionization, X-ray ionization, field desorption ionization, and fast atom bombardment ionization. Among them, field desorption and fast atom bombardment are particularly suitable for the determination of compounds with low volatility and thermal instability.

Mass spectrometer

Device for separating ion beams based on mass-to-charge ratio. Its structure includes single focus, double focus, quadrupole moment, time of flight and cycloid. The function of the mass analyzer is to separate the ions formed in the ion source according to the size of the mass-to-charge ratio. The mass analyzer can be divided into two types: static analyzer and dynamic analyzer.

Mass Spectrometer Collector

Ions of the same mass separated by the analyzer can be collected and detected by a photographic base plate, a Faraday tube, or an electron multiplier. With the greatly improved resolution and sensitivity of mass spectrometers, only a microgram or even a nanogram sample is needed to obtain a satisfactory mass spectrum. Therefore, for trace amounts of impure compounds, gas chromatography or Liquid chromatography (for highly polar compounds) separates the compounds into a single component, introduces it to a mass spectrometer, and records the mass spectrum. At this time, the mass spectrometer functions as a detector.

Due to the large amount of information given by the combination of chromatograph and mass spectrometer, this method is used in conjunction with a computer to normalize the mass spectrum, discard the background or column bleed peaks, give the elemental composition, store and calculate the data, Accumulation of multiple scan data, library search of unknown compound mass spectra, and printing data and plotting can be performed by a computer, greatly simplifying the operation procedures.

Application of mass spectrometry

Mass spectrometry, especially its method combined with chromatograph and computer, has been widely used in organic chemistry, biochemistry, drug metabolism, clinical, toxicology, pesticide determination, environmental protection, petrochemistry, geochemistry, food chemistry, plant chemistry, Space chemistry and defense chemistry. The use of mass spectrometers for multi-ion detection can be used for qualitative analysis. For example, in pharmacological and biological studies, the retention time of drugs and their metabolites on the gas chromatogram and the corresponding mass fragment map can be used to determine the Existing; can also be used for quantitative analysis, using the stable isotope isomers of the test compound as an internal standard to obtain more accurate results.

In inorganic and nuclear chemistry, many low-volatile substances can be determined by mass spectrometry using high-frequency spark sources. This ionization method requires a pure sample electrode. If the sample to be tested is in powder form, it can be mixed with nickel powder and pressed into an electrode. This method is particularly valuable for the analysis of high-purity substances in alloys, minerals, atomic energy, and semiconductor processes, and it is possible to detect impurities at a level of one billionth.

It is of great significance in archeology and geography to use the decay of long-lived radioisotopes to determine the existence time of an object. For example, the radioactive uranium and its decay product lead are present in a certain radioactive mineral. The decay rates of uranium 238 and 235 are known. The relative abundances of uranium and lead due to decay are measured by mass spectrometry. The age of mineral formation on this axis can be estimated.

There are many types of mass spectrometers, and the application characteristics of different instruments are also different. Generally speaking, samples that can be vaporized at about 300C can be preferentially analyzed by GC-MS. Because GC-MS uses EI sources, the mass spectrum information obtained can be analyzed Library retrieval. Capillary columns also perform well. If you can't vaporize around 300C, you need to use LC-MS analysis. At this time, the molecular weight information is mainly obtained. If it is tandem mass spectrometry, some structural information can also be obtained. If it is a biological macromolecule, LC-MS and MALDI-TOF analysis are mainly used to obtain molecular weight information. For protein samples, the amino acid sequence can also be determined. The resolution of a mass spectrometer is an important technical indicator. A high-resolution mass spectrometer can provide the compound composition formula, which is very important for structure determination. Dual-focus mass spectrometers, Fourier transform mass spectrometers, and time-of-flight mass spectrometers with reflectors all have high-resolution capabilities.

Mass spectrometry has certain requirements for samples. The samples analyzed by GC-MS should be organic solutions. Organic substances in aqueous solutions cannot generally be determined. They must be extracted and separated into organic solutions, or headspace sampling techniques should be used. Some compounds are too polar and easily decompose during heating, such as organic acid compounds. At this time, esterification treatment can be performed, and the acid is converted to an ester, followed by GC-MS analysis. From the analysis results, the structure of the acid can be inferred. If the sample cannot be vaporized or esterified, it can only be analyzed by LC-MS. The LC-MS sample is preferably an aqueous or methanol solution. The LC mobile phase should not contain non-volatile salts. For polar samples, the ESI source is generally used, and for non-polar samples, the APCI source is used.

Mass spectrometry classification

Electron bombardment mass spectrometry EI-MS, field desorption mass spectrometry FD-MS, fast atom bombardment mass spectrometry FAB-MS, matrix-assisted laser desorption time-of-flight mass spectrometry MALDI-TOFMS, electron spray mass spectrometry ESI-MS, etc. It is a matrix-assisted laser desorption time-of-flight mass spectrometer MALDI-TOFMS and an electron spray mass spectrometer ESIMS, in which the matrix-assisted laser desorption time-of-flight mass spectrometry MALDI-TOFMS can measure a molecular weight of 100,000.

Analysis of mass spectrometry

The mass analysis steps are as follows:

Confirm the molecular ion peak, and obtain the relative molecular mass and molecular formula from it; calculate the degree of unsaturation.
Find the main ion peaks (generally those with relatively strong intensities), and record the mass-to-charge ratio (m / z value) and relative intensity of these ion peaks.
The analysis of intermediate fragments with missing molecular ion peaks or other fragment ion peaks in the mass spectrum can also help in the interpretation of the spectra.
Use MS-MS to find the precursor and product ions, or use the metastable scanning technique to find the metastable ions. Read the mass-to-charge ratio of these ions to one decimal place.
Combined with elemental analysis, UV, IR, NMR and the physical and chemical properties of the sample, the structural formula of the sample is proposed. Finally, the inferred structural formula is checked according to the law of the corresponding compound cleavage to check whether each fragment ion conforms. If there is no contradiction, a possible structural formula can be determined.
Standard compounds can be used to determine the correct structure of known compounds. This step can be done automatically by a computer. For the structure of the new compound, the final conclusion must be confirmed by the method of synthesizing this compound and doing spectral analysis ^[1] .