Gas chromatography of chromatography
1. Gas chromatography (GC):
It is a chromatographic method with gas as mobile phase.
2. Samples required by gas chromatography:
Gasification is not suitable for most compounds with high boiling point and thermal instability. It is more difficult to analyze substances with strong corrosion and reaction performance.
About 15% - 20% of organic compounds can be analyzed by gas chromatography.
3. Composition of gas chromatograph:
Gas system, injection system, separation system, detection system, temperature control system, recording system.
4. Air system:
Including gas source, purifier and carrier gas flow control;
Commonly used carrier gases include hydrogen, nitrogen and helium.
5. Injection system:
include:
Injection device and gasifier, gas injector (six way valve):
First, the sample is filled with the quantitative tube, and then the carrier gas carries the sample gas in the quantitative tube into the separation column;
Liquid injector:
Different specifications of micro syringes, packed column chromatography commonly used 10 μ L; capillary chromatography commonly used 1 μ L; new instrument with full-automatic liquid injector, cleaning, moistening, sampling, injection, sample changing and other processes automatically completed, can place dozens of samples at a time.
6. Injection mode:
Split injection:
The sample is vaporized in the vaporization chamber, most of the vapor is vented through the shunt pipe, and only a small part of the carrier gas is led into the chromatographic column;
Non split injection:
The sample is directly injected into the vaporization chamber of the chromatography, and all of them are introduced into the column after volatilization.
7. Separation system:
Column:
Packed column (2-6mm diameter, 1-5m long), KB capillary column (0.1-0.5mm diameter, tens of meters long).
8. Function of temperature control system:
Temperature is an important parameter for chromatographic separation;
The temperature of gasifier, column incubator and detector should be controlled during the operation of chromatograph;
Gasifier: ensure the instant gasification of liquid sample;
Detector: ensure that the separated components do not condense here when passing;
Column incubator: accurately control the temperature required for separation.
9. Detection system:
Function: transform the amount of each component after chromatographic separation into measurable electrical signal;
Index: sensitivity, linear range, response speed, structure, universality, universal type - response to all substances; exclusive type - highly sensitive response to specific substances;
Detector type:
Concentration detector:
Thermal conductivity detector, electronic capture detector;
Mass detector:
Hydrogen flame ionization detector, flame photometric detector.
10. Main features of thermal conductivity detector:
Simple structure and good stability;
It has response to inorganic matter and organic matter, and does not destroy the sample;
Low sensitivity.
11. Characteristics of HFID:
advantage:
(1) Typical mass detector;
(2) . universal detector (C-containing organic matter detection);
(3) Hydrogen flame detector has the characteristics of simple structure, good stability, high sensitivity, quick response, small dead volume and wide linear range;
(4) The sensitivity of the detector is three orders of magnitude higher than that of the thermal conductivity detector, and the detection lower limit is 10-12g · g-1;
Disadvantages:
(1) High requirements for carrier gas;
(2) . the sample shall be damaged during the test, and the sample cannot be recovered;
(3) . unable to detect permanent gas, water, carbon tetrachloride, etc...
12. Characteristics of electron capture detector:
It has strong response to halogen, sulfur, phosphorus, nitrogen and oxygen;
It has high sensitivity and can be used for the analysis of trace pesticide residues;
The linear range is narrow.
13. Flame photometric detector (FPD):
It is a highly selective detector for sulfur and phosphorus compounds. Sulfur and phosphorus compounds are burned into organic fragments in hydrogen rich flame, which give out characteristic spectra of different wavelengths.
14. Stationary phase:
Solid stationary phase: solid adsorbent;
Liquid stationary phase: composed of carrier and stationary liquid; polymer stationary phase.
15. Solid stationary phase:
Generally, it is a solid adsorbent, commonly used are activated carbon, silica gel, alumina and molecular sieve.
advantage:
Large adsorption capacity, good thermal stability and low price;
Disadvantages:
The column efficiency is low and the adsorption active center is easy to be poisoned, so it should be activated before use.
Application:
It is mainly used for inert gas, H2, O2, N2, Co, CO2, CH4 and other general gases and low boiling point substances.
16. Conditions to be met for substances used as carriers:
The surface has microporous structure, uniform pore size and large specific surface area;
Chemical and physical inertness, i.e. no chemical reaction with sample components, no adsorption or weak adsorption;
Good thermal stability;
It has certain mechanical strength and wettability and is not easy to be broken;
With a certain granularity and regular shape, it is better to be spherical.
17. Requirements for fixing fluid:
It is a liquid at service temperature, with low volatility, good thermal stability, suitable partition coefficient for each component to be separated, good chemical stability and no chemical reaction with sample component, carrier gas and carrier.
18. Classification of fixatives:
Nonpolar, moderate, strong and hydrogen bonded fixatives.
19. Nonpolar fixative:
It is mainly some saturated alkanes and methyl silicone oil. The main force between them and the substance to be measured is dispersion force. The components flow out in the order of boiling point from low to high. If there are both polar and non-polar components in the sample, the polar components with the same boiling point will peak first.
The commonly used fixatives are squalane (isotriacontane), apisone, etc... It is suitable for the analysis of nonpolar and weak polar compounds.
20. Medium polar fixative:
It is composed of a large alkyl group and a small amount of polar groups or groups that can induce polarization. The force between them and the substance to be measured is mainly dispersion force and induction force. The components basically peak in the order of boiling point, and the non-polar components with the same boiling point peak first. The commonly used fixatives are dinonyl phthalate and polyester, which are suitable for the analysis of weak and medium polarity compounds.
21. Strong polar fixative:
It contains strong polar groups, and the force between them and the substance to be tested is mainly electrostatic force and inductive force, and the components peak in the order of polarity from small to large. The commonly used fixative solution, such as oxodipropiononitrile, is suitable for the analysis of polar compounds.
22. Hydrogen bonded fixing liquid:
It is a special kind of strong polar fixative liquid. The intermolecular force with the substance to be tested is mainly hydrogen bond force. The components peak according to the difficulty of forming hydrogen bond, and the components that are not easy to form hydrogen bond peak first. The commonly used fixatives are polyethylene glycol, triethanolamine, etc., which are suitable for the analysis of compounds containing F, N, O, etc.
23. Selection of fixing liquid:
① . according to the principle of polarity similarity:
Similar polarity, large solubility, large partition coefficient and long retention time;
② . select according to functional group similarity:
Esters -- ester or polyester type fixed liquid;
Alcohols - polyethylene glycol fixing solution.
③ . select according to the main differences:
The boiling point of each component is the main difference - nonpolar fixative; the polarity is the main difference - polar fixative.
④ . select mixed fixing liquid:
For complex samples that are difficult to separate, two or more fixatives can be selected.
24. Polymer stationary phase:
It can be used not only as a solid stationary phase, but also as a carrier, also known as polymer porous microspheres. There are both adsorption and dissolution on the surface of the substance.
(1) It has large specific surface area and uniform surface aperture;
(2) There is no harmful adsorption activity to non-polar and polar substances, and the tailing phenomenon is small, and the polar components can also have symmetrical peaks;
(3) Because there is no liquid film, no loss and good thermal stability;
(4) It has good mechanical strength and corrosion resistance. It is uniform and spherical. It has good uniformity and reproducibility in packed column chromatography, which helps to reduce eddy diffusion.
25. Selection of carrier gas type:
The adaptability of the detector and the flow rate of carrier gas.
26. Selection of column temperature:
(1) First of all, the column temperature should be controlled within the range of the maximum use temperature of the fixed liquid (the fixed liquid is easy to lose if it exceeds this temperature) and the minimum use temperature (the fixed liquid exists in solid form if it is lower than this temperature).
(2) Increasing the column temperature can improve the mass transfer resistance, improve the column efficiency, shorten the analysis time, but reduce the capacity factor and selectivity, which is not conducive to separation. The general principle is: on the premise of separating the most difficult components as much as possible, try to use a lower column temperature, but the retention time is appropriate, and the peak shape is not tailed.
(3) Generally, the column temperature should be close to or slightly lower than the average boiling point of components.
(4) For the samples with complex components and wide boiling range, the temperature is programmed.
27. Selection of carrier and fixed liquid content:
mixture ratio:
The amount of the coating on the carrier of the fixing liquid generally refers to the percentage of the fixing liquid and the carrier, and the ratio of the packed column is usually between 5% and 25%.
The lower the ratio, the thinner the liquid film formed on the support, the smaller the mass transfer resistance, the higher the column efficiency and the faster the analysis speed. When the ratio is low, the load of stationary phase is low, and the allowable injection quantity is small. The analysis tends to use a lower ratio.
28. Selection of injection conditions:
The injection volume shall be controlled within the allowable range of column capacity and linear detection range of detector. The injection shall be fast and short. The vaporization chamber is generally 30 ~ 70 ° C higher than the column temperature.
29. Ways to improve chromatographic separation capacity:
(1) , tray Theory:
Increase the column length and decrease the column diameter, that is, increase the number of column plates;
(2) , rate theory:
The height of tray can be reduced by reducing the resistance of eddy diffusion and mass transfer.
30. Structural characteristics of capillary column:
(1) A capillary column with a length of 100m and a diameter of 0.2mm without packing has small resistance;
(2) The air flow passes through the column in a single way, eliminating the eddy diffusion of components in the column;
(3) When the fixed liquid is directly applied on the pipe wall, the inner wall area of the total column is large and the coating is thin, so the mass transfer resistance of gas and liquid phase is greatly reduced.
(4) The column efficiency of capillary chromatography is as high as 3000-4000 theoretical plates per meter, and the total number of theoretical plates can reach 104-106 for a 100 Meter Capillary column.
31. Capillary chromatography has the following advantages:
(1) High separation efficiency:
It is 10-100 times higher than the packed column;
(2) Fast analysis speed:
Compared with packed column, capillary chromatography was used for analysis;
(3) The chromatographic peak is narrow and symmetrical, and temperature programmed mode is often used;
(4) High sensitivity, generally using hydrogen flame detector.
(5) The eddy diffusion is zero.
32. Type of capillary chromatography:
(1) Coated capillary column:
Apply the fixing liquid directly to the inner wall of the pipe. The preparation of the column is relatively simple, but the reproducibility of the column preparation is poor and its life is short.
(2) Porous capillary column:
A layer of porous adsorbent solid particles is coated on the tube wall to form a capillary gas-solid chromatography.
(3) Carrier coated capillary column:
Stick the very fine particles on the pipe wall, and then apply the fixing liquid. The column efficiency is higher than that of coated capillary column.
(4) Chemically bonded or cross-linked capillary columns:
The fixative is chemically bonded to the tube wall or cross-linked. The column efficiency and column life are further improved.
