Progress in ion mobility spectrometry and its theoretical research
2019-09-15 07:04:40
[Abstract] Ion mobility mass spectrometry is a new two-dimensional mass spectrometry technology for ion mobility separation and mass spectrometry. The principle of ion mobility separation is based on the collision cross section of ions in the drift tube and the buffer gas collision. Separate by size and shape. After more than 30 years of development, ion mobility mass spectrometry has been equipped with a variety of zui new ion sources and mass analyzers, theoretical research has become increasingly mature, and more unique advantages in the analysis of protein, peptide and complex compound isomers. Is developing into a new type of important analytical tool.
[Key words] ion mobility; mass spectrometry; collision cross section; theoretical progress After the 1980s, due to the various soft ionization techniques, the application of mass spectrometry (MS) has been extended to high polarity, difficult to volatilize and heat. The analytical study of unstable biomacromolecules has developed into a biological mass spectrometer and quickly became one of the frontiers of modern analytical chemistry. Ion mobility mass spectrometry (IMMS) is a combination of ion mobility spectrometry (IMS) technology and mass spectrometry. It is a new type of two-dimensional separation mass spectrometry technology. IMS technology appeared in the 1970s. Due to its diverse analytical capabilities, good detection limits, and real-time detection capabilities, it was widely concerned at the time. However, due to the low IMS resolution and the inability to give ion quality information, In addition, there was a lack of understanding of the importance of ion composition at that time, so after 1976, research on ion mobility was gradually reduced. Until the end of the 1980s, especially after the application of various soft ionization methods represented by MALDI and ESI, the unique advantages of IMS in the separation of compound isomers have attracted people's attention. The new ion source IMS-MS combined technology, accurate ion geometry and twist calculation method has been rapidly developed, and IMMS technology has made substantial progress. Currently, IMMS has been used to detect chemical warfare agents, explosives, environmental pollution, anesthetics, semiconductors and biomacromolecules (such as peptides and proteins) and has demonstrated its powerful analytical capabilities.
1 Principle and instrument composition
1.1 IMMS basic principle ion mobility (IM), also known as ion mobility, refers to the velocity of positive or negative ions when the electric field strength is 1 V / m or the electric field force is 1N, the unit is m / V. In IMS, the ions move forward by the acceleration of the electric field force, and in the movement, they collide with the buffer gas molecules in the drift zone to generate resistance and reduce the speed. The kinetic energy lost by ions during the collision can be converted into internal energy to raise the temperature of the ions, and the collision can transfer the increased internal energy to the gas molecules and return to the system temperature. Therefore, the temperature and velocity of the ions do not remain constant during the movement. There may also be electrostatic attraction and Coulomb repulsion between ions, ions and buffer gas, which determines the movement of ions in the drift zone is extremely complicated, only by its average velocity (ie ion mobility) or ion drift. The time td of the zone is measured. This separation process is similar to the chromatographic separation process, so IMS was also called plasma chromatography (Pc) at an early stage. In order to make the measured values ​​under different experimental conditions comparable to each other, in practical applications, the ion mobility is usually converted into a reduced ion mobility (i.e., an ion at a temperature of 273 K and a pressure of 760 Tort). The size and shape of the ions can be measured by the average available cross section when the ions collide with the buffer gas, ie, the collision cross section (n). As can be seen from the above, the ion mobility separation is mainly based on the shape and size of the ions. Therefore, this separation means has unique advantages for analysis of isomers or complexes which cannot be distinguished by conventional mass spectrometry. After the ions are pre-separated by the twist, and then the mass number is obtained by the mass-to-charge ratio of each component, a two-dimensional map or three-dimensional map of the ion mobility mass spectrum can be obtained (Fig. 1).
1.2 Instrument Composition The main difference between an ion mobility mass spectrometer and a conventional mass spectrometer is that the former adds an ion drift tube between the ion source and the mass analyzer. Ion drift tubes are usually made of non-conductive high-purity alumina with a number of stainless steel rings embedded in them. The stainless steel rings are connected by high-temperature resistance, and an electric field that drives ions to advance is applied between the stainless steel rings at both ends. The mass analyzer can use a quadrupole mass analyzer or a time-of-flight mass analyzer. Since the quadrupole analyzer scans ions for a long time, the IMMS analyzer is now a time-of-flight mass spectrometer (TOF-MS). In the instrument, the drift tube part is buffered with gas, and the mass analyzer part is made of high vacuum with an interface composed of a cone and an ion lens. The composition of a typical ion mobility mass spectrum is shown in Figure 2. Since the passage time of the ions in the drift tube is millisecond, the passage time in the flight tube is in the order of microseconds, and there is sufficient time to obtain the mass of the ions before the arrival of the next component, so that each component can be once In the experiment, both the twist and the mass were obtained, and the whole experiment can be completed in 1 min.
Sometimes in order to obtain more ion information, several mass analyzers, such as ion traps or quadrupole mass filters, can be used in series before and/or after the drift tube.
2 Research progress in ion mobility theory
2.1 Influence of Buffer Gas on Collision Cross Section IMS distinguishes ions by colliding with buffer gas molecules. The type of buffer gas directly affects the separation process. Nitrogen and helium are two gases commonly used by Zui. Nitrogen is generally used for routine analysis, and helium is often used for structural analysis. Other gases are carbon dioxide, sulfur hexafluoride, ammonia and carbon tetrafluoride. Theoretical studies using different buffer gases have been rare since 1975, and even now they have not received enough attention, but in practice, the use of different gases is important for achieving good resolution and sensitivity.
The collision cross section of the ions is not only related to the mass of the buffer gas, but also depends on the polarizing rate of the buffer gas. Matz et al. studied the collision cross sections of six amphetamine (amphetamine) derivatives under four different buffer gases of helium, argon, nitrogen and carbon dioxide. The results show that the collision cross section increases with the increase of the mass of the buffer gas, but There is no strict linear relationship. There is a good linear relationship between the polarizability and the collision cross section. The collision cross section increases with the increase of the polarizability, which also indicates that the collision cross section is more dependent on the polarizability of the buffer gas than the mass. Els et al. studied the separation of five chlorinated and bromoacetic acids at 10 ° with different concentrations of nitrogen/carbon dioxide mixed gas as buffer gas. Using 100% nitrogen, the two components were submerged in other peaks, if in a buffer gas. By adding 3% carbon dioxide, complete separation can be achieved, indicating that the composition of the carrier gas significantly affects the detection of peak shape.
2.2 Relationship between ion mobility and mass-to-charge ratio IMS resolution is low, even high-resolution IMS can only achieve the same or slightly higher resolution than conventional HPLC , which makes it difficult to separate complex mixtures separately. At the beginning of the IMS invention, researchers attempted to infer the mass of ions by establishing a relationship with m/z. However, a large number of experimental data show that there is only a rough linear trend between m/z, which is far from satisfying the precise requirements of ion mass. IMS is in mass-to-charge ratio (m/z).
IMMS can only be used as a pre-quality analyzer. Although some substances can be quickly identified by a single ion mobility technique, the two-dimensional "twist/mass" mode that IMMS can provide enables high-resolution separation of complex mixtures. In two-dimensional IMMS (2-D IMMS), the linearity of the "twist/mass" of ions with different charges is significantly different. By analyzing the 2-D data of complex products, the different "twist/quality" are found. Relationship has become an important technology for identifying and interpreting these products. Clemmer et al. have identified a mixture of two peptides, low (single charge) and high (double charge), with a m/z trend relationship. Russel et al. used an internal standard as a reference standard to separate the peptide mixture after protein digestion from the m/z relationship. Stciner used ESI-API-IM-TOF-MS to analyze the degradation products of water-soluble chemical warfare agents, using the same series of n-alkylamines as baseline standards, and using different degradation products and m/z trends to identify them.
The measurement of ion mobility is affected by various factors, such as electrospray solvent composition, drift zone temperature, spray voltage, solvent flow rate, buffer gas flow rate, and cooling gas flow rate.
2.3 Relationship between ionic charge, substituent and collision cross section Although there is no way to prove the close relationship between the gas phase ions and the structure of ions in solution, accurately measuring the collision cross section of ions can improve the complexity of peptides and proteins. Understanding of the material structure. The interatomic hydrogen bonds and van der Waals forces in the ions make them appear folded and compact, and the charge and Coulomb repulsion overcome the mutual attraction in the ions and make the molecules appear loose. Kindy et al. studied the enzymatic hydrolysates of three proteins by isotope labeling and found that the collision cross section of the monoacetylated peptide was 25% to 35% higher than that of unacetylated; the increase of diacetylation was more, and this increase ( Especially for large peptides), which is much larger than the volume increase factor of acetyl groups, indicating that acetyl groups have a special effect on the overall structural changes of peptide ions. Badman et al. studied the change process of the collision cross section of ubiquitin from the ESI into the IM tube. It is believed that the ions enter the drift tube at the beginning of the compact structure and are quickly extended into an open structure by the accelerated voltage.
The position and number of charges in the ions are important parameters that affect the cross section of the gas phase ions. Wu et al. studied three fragments of dynorphin A, F7 (Tyr-Gly-Gly-Phe-Leu-Arg-Arg), F8 (Tyr-Gly-Gly-Phe-Leu-Arg-Arg-IIe), F, The collision cross-section of 9 (Tyr-Gly-Gly-Phe-Leu-Arg-Arg-He-Arg) with 1-3 charges is shown in Table 1. The collision cross section of the single charge and the double charge is stabilized at about 9% from F7 to F, and 9 is well explained by the "size effect" of adding an amino acid at the C-terminus. In addition, all three peptides maintained a similar 7% to 8% level from the single charge to the double charge collision cross section. As the charge increases, the Coulomb repulsion increases to make the peptide appear looser. However, the introduction of a third charge in F7 and F8 causes a sharp rise in the collision cross section because there are only three basic sites in these peptides, and the two adjacent arginine residues must be simultaneously protonated. The sharp increase in Coulomb repulsion allows the ions to exist in a more extended state, causing a sharp increase in the collision cross section. The increase in F and 9 ions is not significant because the presence of terminal arginine residues prevents protonation of both adjacent amino acids. Badman et al. summarized the data of the cytochrome e gaseous ion collision cross section published between 1996 and 2001. The charge is from +3 to +20. Although the collision cross-section values ​​of the charged ions are slightly different, they all show an increase with the charge. The increase is also very similar.
The advantage of using the collision cross section is to distinguish between different ions having the same charge, similar mass, or isomer ions of the same mass. Hen. Derson et al. studied two cytochrome c enzymatic fragments IFVQK. CAQCHTVEK (relative molecular mass 1 633.820) and heme. CAQCHTVEK (relative molecular mass is 1 633.615), which have very similar mass numbers. When the sequence is known, the software is used to simulate the charge distribution, and the required collision cross section and acceleration voltage are calculated separately. The ion mobility is determined, and the collision cross section is obtained and compared with the calculated value to distinguish the two fragments. The result shows that both fragments have two charges, namely IrVQK-CAQCHTVEK and heme-C "AQCHTVEK (superscript proton) Location).
Literature [35] lists the collision cross-section data of 66O peptide ions after enzymatic hydrolysis of 34 common proteins, and theoretically analyzes the relationship between the internal shape parameters of amino acid residues and the collision cross section. The amino acid sequence predicts the collision cross section of the peptide ion. Similar reports have been reported in [36].
3 Outlook Mass spectrometry is one of the most important technologies in the field of analytical chemistry. Ion mobility mass spectrometry combined with ion mobility technology is sensitive, fast, and provides ion structure information and mass spectrometry to provide accurate quality information. It is showing more and more in the analysis of compound isomers and biomacromolecule interactions. More superiority. At present, there are few reports on ion mobility mass spectrometry in China, and only a few scientific research institutions in foreign countries have conducted ion mobility mass spectrometry research. At present, the ion mobility mass spectrometer has not been released yet, and there are still some problems to be solved. However, after more than 30 years of development, its theoretical research is almost mature. The instrument has been equipped with new ion sources such as MALDI and ESI, and some are also connected in series at the same time. Polar mass spectrometry and/or ion trap mass spectrometry with lower detection limits and higher sensitivity and resolution. It is believed that in the near future, ion mobility mass spectrometry will become an indispensable tool for functional genomics, proteomics research, and pharmacy, medicine, and chemical industries.
[Key words] ion mobility; mass spectrometry; collision cross section; theoretical progress After the 1980s, due to the various soft ionization techniques, the application of mass spectrometry (MS) has been extended to high polarity, difficult to volatilize and heat. The analytical study of unstable biomacromolecules has developed into a biological mass spectrometer and quickly became one of the frontiers of modern analytical chemistry. Ion mobility mass spectrometry (IMMS) is a combination of ion mobility spectrometry (IMS) technology and mass spectrometry. It is a new type of two-dimensional separation mass spectrometry technology. IMS technology appeared in the 1970s. Due to its diverse analytical capabilities, good detection limits, and real-time detection capabilities, it was widely concerned at the time. However, due to the low IMS resolution and the inability to give ion quality information, In addition, there was a lack of understanding of the importance of ion composition at that time, so after 1976, research on ion mobility was gradually reduced. Until the end of the 1980s, especially after the application of various soft ionization methods represented by MALDI and ESI, the unique advantages of IMS in the separation of compound isomers have attracted people's attention. The new ion source IMS-MS combined technology, accurate ion geometry and twist calculation method has been rapidly developed, and IMMS technology has made substantial progress. Currently, IMMS has been used to detect chemical warfare agents, explosives, environmental pollution, anesthetics, semiconductors and biomacromolecules (such as peptides and proteins) and has demonstrated its powerful analytical capabilities.
1 Principle and instrument composition
1.1 IMMS basic principle ion mobility (IM), also known as ion mobility, refers to the velocity of positive or negative ions when the electric field strength is 1 V / m or the electric field force is 1N, the unit is m / V. In IMS, the ions move forward by the acceleration of the electric field force, and in the movement, they collide with the buffer gas molecules in the drift zone to generate resistance and reduce the speed. The kinetic energy lost by ions during the collision can be converted into internal energy to raise the temperature of the ions, and the collision can transfer the increased internal energy to the gas molecules and return to the system temperature. Therefore, the temperature and velocity of the ions do not remain constant during the movement. There may also be electrostatic attraction and Coulomb repulsion between ions, ions and buffer gas, which determines the movement of ions in the drift zone is extremely complicated, only by its average velocity (ie ion mobility) or ion drift. The time td of the zone is measured. This separation process is similar to the chromatographic separation process, so IMS was also called plasma chromatography (Pc) at an early stage. In order to make the measured values ​​under different experimental conditions comparable to each other, in practical applications, the ion mobility is usually converted into a reduced ion mobility (i.e., an ion at a temperature of 273 K and a pressure of 760 Tort). The size and shape of the ions can be measured by the average available cross section when the ions collide with the buffer gas, ie, the collision cross section (n). As can be seen from the above, the ion mobility separation is mainly based on the shape and size of the ions. Therefore, this separation means has unique advantages for analysis of isomers or complexes which cannot be distinguished by conventional mass spectrometry. After the ions are pre-separated by the twist, and then the mass number is obtained by the mass-to-charge ratio of each component, a two-dimensional map or three-dimensional map of the ion mobility mass spectrum can be obtained (Fig. 1).
1.2 Instrument Composition The main difference between an ion mobility mass spectrometer and a conventional mass spectrometer is that the former adds an ion drift tube between the ion source and the mass analyzer. Ion drift tubes are usually made of non-conductive high-purity alumina with a number of stainless steel rings embedded in them. The stainless steel rings are connected by high-temperature resistance, and an electric field that drives ions to advance is applied between the stainless steel rings at both ends. The mass analyzer can use a quadrupole mass analyzer or a time-of-flight mass analyzer. Since the quadrupole analyzer scans ions for a long time, the IMMS analyzer is now a time-of-flight mass spectrometer (TOF-MS). In the instrument, the drift tube part is buffered with gas, and the mass analyzer part is made of high vacuum with an interface composed of a cone and an ion lens. The composition of a typical ion mobility mass spectrum is shown in Figure 2. Since the passage time of the ions in the drift tube is millisecond, the passage time in the flight tube is in the order of microseconds, and there is sufficient time to obtain the mass of the ions before the arrival of the next component, so that each component can be once In the experiment, both the twist and the mass were obtained, and the whole experiment can be completed in 1 min.
Sometimes in order to obtain more ion information, several mass analyzers, such as ion traps or quadrupole mass filters, can be used in series before and/or after the drift tube.
2 Research progress in ion mobility theory
2.1 Influence of Buffer Gas on Collision Cross Section IMS distinguishes ions by colliding with buffer gas molecules. The type of buffer gas directly affects the separation process. Nitrogen and helium are two gases commonly used by Zui. Nitrogen is generally used for routine analysis, and helium is often used for structural analysis. Other gases are carbon dioxide, sulfur hexafluoride, ammonia and carbon tetrafluoride. Theoretical studies using different buffer gases have been rare since 1975, and even now they have not received enough attention, but in practice, the use of different gases is important for achieving good resolution and sensitivity.
The collision cross section of the ions is not only related to the mass of the buffer gas, but also depends on the polarizing rate of the buffer gas. Matz et al. studied the collision cross sections of six amphetamine (amphetamine) derivatives under four different buffer gases of helium, argon, nitrogen and carbon dioxide. The results show that the collision cross section increases with the increase of the mass of the buffer gas, but There is no strict linear relationship. There is a good linear relationship between the polarizability and the collision cross section. The collision cross section increases with the increase of the polarizability, which also indicates that the collision cross section is more dependent on the polarizability of the buffer gas than the mass. Els et al. studied the separation of five chlorinated and bromoacetic acids at 10 ° with different concentrations of nitrogen/carbon dioxide mixed gas as buffer gas. Using 100% nitrogen, the two components were submerged in other peaks, if in a buffer gas. By adding 3% carbon dioxide, complete separation can be achieved, indicating that the composition of the carrier gas significantly affects the detection of peak shape.
2.2 Relationship between ion mobility and mass-to-charge ratio IMS resolution is low, even high-resolution IMS can only achieve the same or slightly higher resolution than conventional HPLC , which makes it difficult to separate complex mixtures separately. At the beginning of the IMS invention, researchers attempted to infer the mass of ions by establishing a relationship with m/z. However, a large number of experimental data show that there is only a rough linear trend between m/z, which is far from satisfying the precise requirements of ion mass. IMS is in mass-to-charge ratio (m/z).
IMMS can only be used as a pre-quality analyzer. Although some substances can be quickly identified by a single ion mobility technique, the two-dimensional "twist/mass" mode that IMMS can provide enables high-resolution separation of complex mixtures. In two-dimensional IMMS (2-D IMMS), the linearity of the "twist/mass" of ions with different charges is significantly different. By analyzing the 2-D data of complex products, the different "twist/quality" are found. Relationship has become an important technology for identifying and interpreting these products. Clemmer et al. have identified a mixture of two peptides, low (single charge) and high (double charge), with a m/z trend relationship. Russel et al. used an internal standard as a reference standard to separate the peptide mixture after protein digestion from the m/z relationship. Stciner used ESI-API-IM-TOF-MS to analyze the degradation products of water-soluble chemical warfare agents, using the same series of n-alkylamines as baseline standards, and using different degradation products and m/z trends to identify them.
The measurement of ion mobility is affected by various factors, such as electrospray solvent composition, drift zone temperature, spray voltage, solvent flow rate, buffer gas flow rate, and cooling gas flow rate.
2.3 Relationship between ionic charge, substituent and collision cross section Although there is no way to prove the close relationship between the gas phase ions and the structure of ions in solution, accurately measuring the collision cross section of ions can improve the complexity of peptides and proteins. Understanding of the material structure. The interatomic hydrogen bonds and van der Waals forces in the ions make them appear folded and compact, and the charge and Coulomb repulsion overcome the mutual attraction in the ions and make the molecules appear loose. Kindy et al. studied the enzymatic hydrolysates of three proteins by isotope labeling and found that the collision cross section of the monoacetylated peptide was 25% to 35% higher than that of unacetylated; the increase of diacetylation was more, and this increase ( Especially for large peptides), which is much larger than the volume increase factor of acetyl groups, indicating that acetyl groups have a special effect on the overall structural changes of peptide ions. Badman et al. studied the change process of the collision cross section of ubiquitin from the ESI into the IM tube. It is believed that the ions enter the drift tube at the beginning of the compact structure and are quickly extended into an open structure by the accelerated voltage.
The position and number of charges in the ions are important parameters that affect the cross section of the gas phase ions. Wu et al. studied three fragments of dynorphin A, F7 (Tyr-Gly-Gly-Phe-Leu-Arg-Arg), F8 (Tyr-Gly-Gly-Phe-Leu-Arg-Arg-IIe), F, The collision cross-section of 9 (Tyr-Gly-Gly-Phe-Leu-Arg-Arg-He-Arg) with 1-3 charges is shown in Table 1. The collision cross section of the single charge and the double charge is stabilized at about 9% from F7 to F, and 9 is well explained by the "size effect" of adding an amino acid at the C-terminus. In addition, all three peptides maintained a similar 7% to 8% level from the single charge to the double charge collision cross section. As the charge increases, the Coulomb repulsion increases to make the peptide appear looser. However, the introduction of a third charge in F7 and F8 causes a sharp rise in the collision cross section because there are only three basic sites in these peptides, and the two adjacent arginine residues must be simultaneously protonated. The sharp increase in Coulomb repulsion allows the ions to exist in a more extended state, causing a sharp increase in the collision cross section. The increase in F and 9 ions is not significant because the presence of terminal arginine residues prevents protonation of both adjacent amino acids. Badman et al. summarized the data of the cytochrome e gaseous ion collision cross section published between 1996 and 2001. The charge is from +3 to +20. Although the collision cross-section values ​​of the charged ions are slightly different, they all show an increase with the charge. The increase is also very similar.
The advantage of using the collision cross section is to distinguish between different ions having the same charge, similar mass, or isomer ions of the same mass. Hen. Derson et al. studied two cytochrome c enzymatic fragments IFVQK. CAQCHTVEK (relative molecular mass 1 633.820) and heme. CAQCHTVEK (relative molecular mass is 1 633.615), which have very similar mass numbers. When the sequence is known, the software is used to simulate the charge distribution, and the required collision cross section and acceleration voltage are calculated separately. The ion mobility is determined, and the collision cross section is obtained and compared with the calculated value to distinguish the two fragments. The result shows that both fragments have two charges, namely IrVQK-CAQCHTVEK and heme-C "AQCHTVEK (superscript proton) Location).
Literature [35] lists the collision cross-section data of 66O peptide ions after enzymatic hydrolysis of 34 common proteins, and theoretically analyzes the relationship between the internal shape parameters of amino acid residues and the collision cross section. The amino acid sequence predicts the collision cross section of the peptide ion. Similar reports have been reported in [36].
3 Outlook Mass spectrometry is one of the most important technologies in the field of analytical chemistry. Ion mobility mass spectrometry combined with ion mobility technology is sensitive, fast, and provides ion structure information and mass spectrometry to provide accurate quality information. It is showing more and more in the analysis of compound isomers and biomacromolecule interactions. More superiority. At present, there are few reports on ion mobility mass spectrometry in China, and only a few scientific research institutions in foreign countries have conducted ion mobility mass spectrometry research. At present, the ion mobility mass spectrometer has not been released yet, and there are still some problems to be solved. However, after more than 30 years of development, its theoretical research is almost mature. The instrument has been equipped with new ion sources such as MALDI and ESI, and some are also connected in series at the same time. Polar mass spectrometry and/or ion trap mass spectrometry with lower detection limits and higher sensitivity and resolution. It is believed that in the near future, ion mobility mass spectrometry will become an indispensable tool for functional genomics, proteomics research, and pharmacy, medicine, and chemical industries.
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