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Mind map of instrument analysis knowledge points
This is a mind map about the knowledge points of instrumental analysis. Chemical engineering and chemistry majors should answer the postgraduate entrance examination re-examination questions, including chromatographic analysis, infrared spectroscopy, ultraviolet, and external visible absorption spectrometry, etc.
Edited at 2023-11-14 11:20:45
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Mind map of instrument analysis knowledge points
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- Outline
Instrument analysis knowledge points
UV-visible absorption spectrometry
Absorption of conjugated groups in the near ultraviolet region
Definition: A method that uses molecules of certain substances to absorb radiation in the spectral region of 200-800 nanometers for analysis and measurement.
For the spectrum, when the wavelength is between 400-800nm, it is the visible light region (mainly colored substances), and the part between 10-400nm is the ultraviolet light band. The smaller the wavelength, the higher the energy. When the wavelength is less than 10 nanometers, it is x Ray, gamma ray region, 10-200nm is the far ultraviolet region, and the near-ultraviolet region is 200-400nm, which is the main research object (the region where most conjugated organic molecules are located).
Principle: The valence electrons (xigema electrons (single bond), π electrons (double bond), n electrons (lone pair electrons)) in organic compound molecules transition from lower energy orbits to higher energy antibonding orbitals, thereby producing absorption curve.
Generally speaking, antibonding orbitals (xigema stars) > non-bonding orbitals (n orbitals) > bonding orbitals (xigema, π orbitals)
For the transition from xigema-xigema stars, far-ultraviolet light excitation is required (the substances involved are saturated alkanes)
Far-ultraviolet light excitation (around 200nm) is also required for n-xigema star transitions (involving derivatives of saturated hydrocarbons containing non-bonding electrons, such as alcohols and ethers)
The transition of n-π stars requires the ultraviolet-visible region, the transition energy is relatively low, and it is a weak absorption band.
For the π-π star transition, the near-violet end and near-ultraviolet region of the far-ultraviolet region are required to be excited, which is a strong absorption. The greater the degree of conjugation, the greater the wavelength
Absorption curve:
Maximum absorption wavelength: the wavelength corresponding to the maximum absorbance value in the absorption curve.
Universal band classification (transitions of π-π stars and n-π stars)
The R band (transition of n-π stars) is weak
The K band (π-π star transition) is caused by the conjugated system; the absorption peak is very strong; if the degree of conjugation increases, the maximum absorption wavelength will be red-shifted and the absorption intensity will be enhanced.
B band (π-π star transition of closed ring conjugated double bonds) such as aromatic hydrocarbons
E band (π-π star transition of three double bonds in the benzene ring) absorption intensity: E1 band > E2 band; when the benzene ring is connected to the auxochromophore, the maximum absorption wavelength will be red-shifted.
Chromophore: A group that generates the main absorption signal in a specified wavelength range; Auxiliary chromophore: A group that assists in color development and plays an electron-donating role; Auxiliary chromophore will increase the maximum absorption wavelength of the chromophore.
Red shift: The wavelength will move to the red region, that is, the wavelength will increase; the use of lone pairs of electrons can produce a red shift. Blue shift: The wavelength decreases.
Factors affecting UV-visible spectrum
The influence of conjugation effect
As the π electron conjugated system increases, the maximum absorption wavelength red-shifts and the absorption intensity increases.
As the steric hindrance increases and the conjugated system is destroyed, the maximum absorption wavelength blue shifts and the absorption intensity decreases.
Effect of substituents
The degree of auxochromophore substitution and π-π star transition increases, and the maximum absorption wavelength increases.
Effect of solvents
As the polarity of the solvent increases, the π-π star transition increases and the n-π star transition decreases.
Use non-polar solvents as much as possible; when comparing the spectra of unknown and known substances, the solvents should be the same; the solvent has no absorption or small absorption within the measurement range.
Effect of pH value
If the absorption peak of a compound shifts red after adding a base, it means that the compound is acidic.
If the absorption peak of a compound shifts blue after adding acid, it means that the compound is basic.
UV-visible spectrophotometer components
light source
Monochromator: emits ultraviolet light.
Sample cell
Detector
Data processing equipment
Infrared spectroscopy
The research object is the functional group, which is the fundamental frequency of vibration.
The vibrations studied are divided into stretching vibration and portable vibration. The area where stretching vibration occurs is higher than the area where variable harmonic vibration occurs.
Overview: When a molecule is exposed to light radiation in a specific band, its vibrational energy level transitions. This part produces an infrared absorption spectrum. For the infrared absorption spectrum, different functional groups will also have different peaks. The functional groups can be inferred through different characteristic peaks. structure, and then combined with the molecular formula, the molecular structure can be obtained.
The main wave bands are: mid-infrared region, 2.5-50μm, 400-4000cm-1 (wave number)
The infrared spectrum is divided into fingerprint area and functional group area
Generate conditions
The energy given by light radiation must be equal to the energy of its transition
The magnitude or direction of the dipole moment of molecular vibration must change to a certain extent.
Symmetric molecules have no change in dipole moment, so radiation will not cause resonance, and there will be no infrared activity.
Factors affecting peak position changes
electronic effect
Conjugation effect: π-π conjugation effect moves the absorption peak of the double bond to the low frequency direction (red shift)
Induction effect: electron-withdrawing groups move the absorption peak toward high frequency (blue shift)
Steric effect (steric hindrance)
cyclic compounds
For double bonds outside the ring, the wave number increases due to the increase in ring tension.
Double bonds in the ring, ring tension increases, wave number decreases
The hydrogen bonding effect reduces the wave number.
Characteristic group frequencies of various compounds
Alkanes
Methyl groups appear at 2960 and 1380. The 2960 position (stretching vibration) is easy to stack, so it is more obvious to see 1380 (changing harmonic vibration).
Alkenes
Alkynes
Aromatic hydrocarbons
Mainly look at the vibration of the benzene ring skeleton
carbonyl compounds
Ketones (exclusion confirmation)
aldehyde
Jiepu
First calculate the degree of unsaturation (2C 2-H) based on the molecular formula
Speculate benzene ring, carbonyl group (Fermi vibration)
spectrum analysis
chromatographic theory
chromatography
concept
Stationary phase: The stationary phase filled in a glass tube or stainless steel tube is called a stationary phase.
Mobile phase: A phase (generally gas or liquid) that moves from top to bottom is called the mobile phase.
Chromatography column: The tube containing the stationary phase is called a chromatography column.
Chromatography: A technology that uses different substances to have different adsorption coefficients or distribution coefficients in two phases. When the two phases are repeatedly adsorbed, desorbed, or distributed multiple times, each component in the mixture is separated.
Step 1: When the mobile phase (gas, liquid or supercritical fluid) containing the mixture sample passes through the stationary phase, it will interact with the stationary phase.
Step 2: Due to the differences in properties of each component, the type and strength of interaction with the stationary phase are also different (differences in polarity)
Step 3: Under the action of the same driving force, different components have different residence times in the stationary phase, and thus flow out of the stationary phase in different orders.
Step 4: Each single component substance can be analyzed qualitatively and quantitatively respectively.
Classification
According to mobile phase state
gas chromatography
According to stationary phase state
Gas-solid chromatography
adsorption chromatography
gas-liquid chromatography
Partition chromatogram
liquid chromatography
According to stationary phase state
liquid-solid chromatography
adsorption chromatography
liquid-liquid chromatography
Partition chromatogram
Use form according to stationary phase
R
paper chromatography
TLC
by separation mechanism
adsorption chromatography
Partition chromatogram
Ion exchange chromatography
exclusion chromatography
Features
1. High separation efficiency (complex mixtures, organic homologues, isomers, chiral isomers)
2. High sensitivity
3. High selectivity (little interference from other substances in the sample)
4. Fast analysis speed
5. Wide range of applications
6. Works well with other instruments
Principles of Chromatography
Chromatographic curve
Reserved value
A relative retention value (selection factor) greater than 1 is a prerequisite for chromatographic separation
Reserved value expressed in time
Retention time tR: The time required for the maximum concentration value in a component from injection to column.
Dead time tM: Retention time of gases that do not interact with the stationary phase (such as mobile phase or gas).
Adjust retention time tR': = retention time - dead time
Reserved value expressed by volume
Retention volume: VR=tR*F0
Dead volume: VM=tM*F0
Adjust retention volume: retention volume - dead volume
Distribution coefficient K
At a certain temperature, the concentration ratio when the distribution of components between the stationary phase and the mobile phase reaches equilibrium. K=concentration of the component in the stationary phase/concentration of the component in the mobile phase.
K is only related to the stationary phase and the properties of the separated substance The difference in K value is a prerequisite for separation. The greater the difference, the greater the possibility of separation. The component with the larger K value peaks later.
K值越大，组分在固定相中的浓度越高，就越不容易出来，出峰的时间也就越晚。
capacity factor
The weight ratio of the components in the stationary phase and the mobile phase after the two phases reach equilibrium at a certain temperature and pressure.
compared to
The ratio of the volumes of stationary phase and mobile phase in a chromatographic column.
tray theory
Concept: Compare the chromatographic separation process to the distillation process, and divide the continuous chromatographic separation process into multiple repetitions of the equilibrium process.
Rate theory - Van Diemter's equation - the relationship between theoretical plate height and linear velocity of carrier gas: H=A B/u C*u
H: theoretical plate height; u: linear velocity of carrier gas A: Eddy current diffusion coefficient; B: Molecular diffusion coefficient; C: Mass transfer resistance coefficient
Carrier gas flow rate and column efficiency
When the carrier gas flow rate is high, the mass transfer resistance term has a large impact and the column efficiency becomes low.
When the carrier gas flow rate is low, the molecular diffusion term has a large impact and the column efficiency becomes low.
1. Column efficiency can be improved by selecting appropriate stationary phase strength, carrier gas type, liquid film thickness and carrier gas flow rate. 2. Various factors restrict each other. For example, as the carrier gas flow rate increases, the influence of the molecular diffusion term decreases, which increases the column efficiency. However, at the same time, the influence of the mass transfer resistance term increases, which in turn decreases the column efficiency; as the column temperature increases, It is beneficial to mass transfer, but it also intensifies the influence of molecular diffusion. Only by selecting the best conditions can the column efficiency be maximized.
Gas Chromatography (GC)
Gas Chromatograph
structure
Structure: Carrier gas cylinder --> Inlet --> Chromatographic column --> Detector --> Data processing
1. Carrier gas system Gas path system: Obtain pure carrier gas with stable flow rate. Including pressure gauges, flow meters and gasification devices. Carrier gas: chemically inert and does not react with related substances. In addition to considering the impact of the carrier gas on the column efficiency, it must also be matched with the detector used for the analysis object. Commonly used carrier gases: hydrogen, nitrogen, helium;
2. Sampling device Injector: Microsyringe
3. Chromatographic column (the core component of the chromatograph) Column material: stainless steel tube, glass tube, etc. Column packing: gas-solid chromatography: solid adsorbent Gas-liquid chromatography: carrier stationary solution
4. Temperature control system-programmed temperature rise During an analysis cycle, the temperature column is continuously changed according to a certain program.
Classification
1. Thermal conductivity detector (TCD)
Concentration detector
Universal detector
Not very sensitive
2. Hydrogen flame ionization detector (FID)
Organic matter is ionized in the hydrogen flame and forms an ion flow between the collector and polarizer for detection.
Quality detector
Very high sensitivity
Very sensitive to hydrogen content
3. Electron capture detector (ECD)
Mainly detects atoms containing electronegativity
Very sensitive to halogens
4. Flame photometric detector (FPD)
Parathion selective detector
Choice of separation conditions
Carrier gas type selection
Effect of carrier gas on column efficiency and detector requirements
When the carrier gas flow rate is small, the molecular diffusion term is the main control item, so the molar mass of the carrier gas must be increased to inhibit the longitudinal diffusion of the sample; when the carrier gas flow rate is large, the mass transfer resistance term is the main control item, and the molar mass of the carrier gas must be reduced. to reduce mass transfer resistance.
Carrier gas flow rate selection
According to the van Diemter rate equation
Column temperature selection
As the column temperature increases, the volatility of the measured components increases, the retention time becomes shorter, the chromatographic peaks become narrower, the resolution decreases, and low component peaks tend to overlap.
As the column temperature decreases, the resolution increases and the analysis time increases. For substances that are difficult to separate, lowering the column temperature can improve the separation to a certain extent.
For substances with complex components and wide boiling ranges, programmed temperature rise should be selected.
Gas-solid chromatography stationary phase
Adsorption chromatography: The process of detecting substances competing with the mobile phase for adsorption sites on the solid phase.
type
Activated carbon: non-polar, strong adsorption of non-polar gases
Activated alumina: has greater polarity and is suitable for the separation of oxygen, nitrogen, etc. at room temperature
Silica Gel: Similar to activated alumina.
Molecular sieve: aluminosilicate (zeolite) of alkali and alkaline earth metals. It is porous and can separate rare gases.
Gas-liquid chromatography stationary phase
Distribution chromatography: Analyze and separate substances with different distribution coefficients in the mobile phase and stationary solution; the larger the distribution coefficient, the more the substance prefers to stay in the stationary phase, and the slower the peak elution will be.
Stationary phase: Stationary solution Support: The surface of small particles is coated with a layer of stationary solution.
Characteristics of fixative: It may not be a liquid at room temperature, but it must be in a liquid state at the operating temperature.
High boiling point, difficult to volatilize organic compounds.
Have appropriate dissolving ability for the sample.
Highly selective.
Good chemical stability.
The principle of like dissolves.
Supporter: Chemically inert porous solid particles with a large specific surface area.
High performance liquid chromatography (HPLC)
Compared
Gas Chromatography: The mobile phase is an inert gas; the analysis objects are gases and compounds with lower boiling points; the temperature is higher.
Liquid chromatography: The mobile phase is liquids of different polarities; the analysis objects are high boiling point, unstable natural products, biological macromolecules, and polymer compounds; the temperature is generally room temperature.
According to separation mechanism
Partition chromatogram
Separation principle: Different components have different distribution coefficients between the two phases (mobile phase and stationary phase).
Forward HPLC: HPLC system composed of polar stationary phase and non-polar mobile phase. (Adsorption chromatography is also a type of forward HPLC)
Reverse HPLC: A liquid chromatography system composed of a non-polar stationary phase and a polar mobile phase. (commonly used)
Normal phase: peaks with smaller polarity appear first Reverse phase: peaks with greater polarity appear first
Adsorption chromatography (liquid-solid chromatography)
Separation principle: The adsorption competition between solute molecules and mobile phase molecules on the surface of the adsorbed phase.
Stationary phase: Solid adsorbent is used as the stationary phase.
Ion exchange chromatography
exclusion chromatography
composition
Liquid storage provides degassing
Infusion Pump
Sampling system
separation system
Detection Systems
UV visible detector
Qualitative analysis: The detector signal can be analyzed with the spectral library of standard samples.
Quantitative analysis: Make a standard curve from the peak area and concentration or mass (the ordinate is the peak area and the abscissa is the concentration). Then measure the peak area of the unknown concentration sample to get its corresponding concentration value.
control and recording system
Elution method
Isocratic system: mobile phase composition and proportions are constant
Gradient elution: Continuously change the proportion of each solvent component in the mobile phase to continuously change the polarity of the mobile phase, so that each analyzed component has an appropriate capacity factor, so that all components can be eluted in a short time
Column chromatography (packing material inside the column)
According to separation mechanism
Partition chromatography: Different components have different partition coefficients between the two phases (mobile phase and stationary phase).
Stationary phase: carrier stationary solution
Supporter: Large specific surface area, neutral, able to support a certain amount of solid phase; mobile phase passes freely.
Mobile phase: solvent
separated substance
Normal phase HPLC: Smaller polarity peaks appear first
Reversed phase HPLC: peaks with greater polarity appear first
Adsorption chromatography (consisting of adsorbent, solvent and sample): The sample is repeatedly adsorbed and analyzed in the column under the action of adsorbent and eluent, and continues to be continuously developed with the eluent. Due to the two-phase adsorption The difference in ability flows out of the column in sequence to achieve separation.
Adsorption competition between analyte and mobile phase
Adsorbent (stationary phase): 1. Large specific surface area and moderate activity. 2. Does not react with adsorbents and eluents. 3. Insoluble in eluent. 4. Uniform particle size.
Alumina, silica gel (the lower the water content, the higher the activity)
Solvent (eluent) mobile phase
Ion exchange chromatography
gel chromatography
Column chromatography operation: column packing-->sampling-->elution and separation
Elution: The polarity of the solvent should be gradually increased from small to large (gradient elution)
paper chromatography (paper chromatography)
Thin Layer Chromatography (TLC)
A liquid chromatography method using an adsorbent as the stationary phase (adsorption chromatography)
TLC: fast, high separation efficiency; high sensitivity; color development and easy storage
Qualitative analysis
Physical detection method
UV light
iodine
water
Ion exchange chromatography
Definition: A method of separating ions by the ion exchange of the same sign that occurs between the solution and the ion exchanger when the ion exchanger (ion exchange resin) is used.
The exchanger is a cation exchanger, then it can exchange positive ions
Due to the different exchange capabilities between various ions and ion exchange resins, they have different peak orders.
The separation efficiency is high and the application is wide. The separation process cycle is long and time-consuming.
Exchange steps: membrane diffusion --> particle diffusion (slow) --> exchange reaction --> particle diffusion (slow) --> membrane diffusion
MS (mass spectrometry)
A tool to identify different molecules based on their charge-to-mass ratio, and to conduct qualitative analysis of the components and structures of organic and inorganic substances (using electron bombardment and other means to bombard substances into fragments. These fragments are separated one by one due to their different masses, and are finally obtained at the molecular ion peak. ).
spectrum
Determine the wavelength and intensity of electromagnetic waves emitted or absorbed by a substance
UV (ultraviolet spectrum)
FTIR (infrared spectrum)
NMR (nuclear magnetic resonance spectroscopy)
Four energy spectrums
Energy spectrum analysis method: Use a monochromatic light source (X-ray, ultraviolet light or electron beam) to illuminate the sample, so that the electrons in the sample are excited and emitted, and these electrons carry surface information of the sample, and then by measuring these electrons energy distribution to obtain relevant information about the sample.
AES
X-rays with a certain energy are used to excite the sample, and the chemical composition of the material surface is obtained by detecting the energy intensity of Auger electrons. Changes in some surface physical and chemical properties can be studied, such as surface adsorption, desorption, etc.
XPS
X-rays of a certain energy are used to irradiate the sample, so that the inner electrons or valence electrons of atoms or molecules are stimulated and emitted. The emitted substances are photoelectrons. XPS can measure the photoelectron energy to obtain the element content and valence of the sample surface. status information.
ups
Examine the valence electron structure of gas phase atoms and molecules.
EDS
(Instrument for analyzing material elements) The surface of the sample is bombarded with electron beams under vacuum to excite the material to emit characteristic X-rays, and its surface elements are qualitatively analyzed based on the wavelength of the characteristic X-rays. (Various elements have their own x-ray characteristic wavelengths)
Four major microscopes
It can obtain the organizational structure of materials and is mainly used for material analysis and testing.
SEM (Scanning Electron Microscope)
The resolution can reach 1nm. It is mainly used for the analysis of cross sections and rough surfaces. The image has a strong sense of reality and three-dimensionality. (The surface of the object is scanned with electron beams, and physical phenomena such as electron transmission and solid scattering occur. Then the physical information is collected, amplified, and imaged, and the electron microscope image is obtained.)
TEM (Transmission Electron Microscope)
The requirements for samples are high and sample preparation is complex.
AFM (Atomic Force Microscopy)
Can provide real three-dimensional structural drawings
STM (Scanning Tunneling Microscope)
High-resolution
SEM, EDS, XRD
The difference between the three: SEM is a scanning electron microscope. EDS is an accessory for scanning electron microscopy used for micro-area analysis of components - energy spectrometer. It is used to analyze the type and content of micro-area components of materials and is used in conjunction with scanning electron microscopy and transmission electron microscopy. XRD is an X-ray diffractometer, a detection equipment used for phase analysis.
XRD uses X-ray diffraction. Different atoms scatter X-rays with different intensities. Strong X-ray diffraction can be produced in certain directions, and the X-ray diffraction lines in this direction contain information about the crystal structure.