Gas chromatography is one of the most important equipments in the laboratory. It has a wide range of applications and is a good helper for experimental analysts. However, it will encounter various problems during use, and it can be more effective to solve these problems quickly. Serve our testing work.
First, what is gas chromatography? Is it divided into several categories?
Chromatographic techniques using a gas phase as a mobile phase are commonly referred to as gas chromatography. Generally can be classified according to the following aspects:
1. Classification according to the stationary phase aggregation state:
(1) Gas-solid chromatography: the stationary phase is a solid adsorbent,
(2) Gas-liquid chromatography: The stationary phase is a liquid applied to the surface of the support.
2, according to the principle of physical and chemical processes:
(1) Adsorption chromatography: The separation of the chromatographic properties of the different components by the solid adsorption surface is achieved.
(2) Partition chromatography: Different partition coefficients are used in the two phases with different components to achieve the separated chromatogram.
(3) Others: Ion exchange chromatography using the principle of ion exchange: electrochromatography established by the electrokinetic effect of colloids; thermal chromatography developed using temperature changes, and the like.
3. Classified by stationary phase type:
(1) Column chromatography: The stationary phase is installed in the column, and the packed column, the hollow column and the capillary column are all such.
(2) Paper chromatography: using filter paper as a carrier,
(3) Thin film chromatography: The stationary phase is a thin layer of powder pressed.
4. Classification according to the principle of dynamic process: it can be divided into three types: flushing method, substitution method and head-on method.
Second, what is the separation principle of gas chromatography?
Gas chromatography is a physical separation method. By using the small difference in the partition coefficient (solubility) between the two phases of the measured substance, when the two phases are in relative motion, the substances are repeatedly distributed between the two phases, so that only minor differences in properties are generated. The effect is to separate the different components.
3. What are some common terms and basic concepts for gas chromatography?
1. Phase, stationary phase and mobile phase:
A homogeneous part of a system is called a phase; during chromatographic separation, a stationary phase is called a stationary phase; a fluid moving through or along a stationary phase is called a mobile phase.
2. Chromatographic peaks:
After the substance passes through the column to the evaluator, the curve appearing on the logger is called the peak.
3. Baseline:
Under chromatographic operating conditions, when no component is passed through the evaluator, the detector noise recorded by the recorder over time is called the baseline.
4, peak height and half width:
The height from the point of maximum concentration of the chromatographic peak to the intersection of the time coordinate and the baseline is called the peak height, and is generally expressed by h. The width at half the height of the chromatographic peak is half width, generally expressed as x1/2.
5. Peak area: The area of ​​the peak formed by the outflow curve (chromatographic peak) and the baseline is indicated by A.
6, dead time, retention time:
The time from the injection to the maximum value of the inert gas peak is called the dead time, expressed in td. The time required from the injection to the high value of the chromatographic peak zui is called the retention time, expressed in tr.
7, dead volume, reserved volume:
The product of dead time and the average flow rate of the carrier gas is called the dead volume, expressed as Vd, and the average carrier gas flow rate is expressed by Fc, Vd = tdxFc. The product of the retention time and the average carrier gas flow rate is called the retention volume, expressed as Vr, Vr = trxFc.
8, reserved values ​​and relative retention values:
The retention value is a value indicating the residence time of each component in the sample in the column, usually expressed by time or by the volume of the carrier gas required to carry the component out of the column. Taking a substance as a standard and finding the ratio of the retention value of other substances to the standard is called a relative retention value.
9. Instrument noise: The degree of instability of the baseline is called noise.
10, base flow: hydrogen flame chromatography, in the absence of injection, the instrument itself has the base current (bottom current), referred to as the base flow.
Fourth, what is the basis for the general choice of carrier gas? What are the common carrier gases for gas chromatography?
As a gas for gas chromatography carrier gas, chemical stability is required;
High purity;
Cheap and easy to obtain;
Can be adapted to the detector used.
Commonly used carrier gases are hydrogen, nitrogen, argon, helium, carbon dioxide, and the like.
5. Why should the carrier gas be purified? How should it be purified?
The so-called purification is to remove some organic matter, trace oxygen, moisture and other impurities in the carrier gas to improve the purity of the carrier gas. Impure gas as a carrier gas can cause column failure, sample change, hydrogen flame chromatography can lead to increased base flow noise, thermal conductivity chromatography can lead to linear deterioration of the evaluator, so the carrier gas must be purified.
Generally, chemical treatment is used to remove oxygen, such as deoxidation with active copper; molecular sieves, activated carbon and other adsorbents are used to remove organic impurities; and adsorbents such as silicone rubber and molecular sieves are used to remove moisture.
6. What are the sample injection methods?
Chromatographic separation requires a certain amount of sample to be inserted in the form of a "plug" in the shortest time. The injection method can be divided into:
1. Gas sample: There are four general injection methods:
(1) Syringe injection
(2) Measuring tube injection
(3) fixed volume injection
(4) Automatic gas injection.
Generally, syringe injection and gas automatic injection are commonly used. The advantage of syringe injection is that it is flexible and easy to use, but the injection volume is less repeatable. Gas auto-injection is injected with a metering valve, with good repeatability and automatic operation.
2. Liquid sample:
Generally, the sample is injected with a micro syringe, and the method is simple and the injection is rapid. Quantitative auto-injection can also be used, which is reproducible.
3. Solid sample:
The sample is usually dissolved with a solvent and then injected in the same manner as the liquid injection. Also useful for injection in solid injectors.
7. Briefly describe the effects of various operating conditions on the detection results in gas chromatography analysis.
Operating conditions have a large impact on chromatographic separation.
1, column length, column inner diameter:
Generally speaking, the growth of the column tube can improve the separation ability, and the shorter the component is distilled faster;
The inner diameter of the column has a small separation effect, and the inner diameter of the column is large, but the inner diameter of the column is too large, which causes the support to be uniformly distributed in the column.
2, column temperature:
It is an important operational variable that directly affects separation performance and analysis speed. The basis for selecting the column temperature is the boiling point range of the mixture, the ratio of the fixing solution, and the sensitivity of the identifier. Increasing the column temperature can shorten the analysis time;
Lowering the column temperature can increase the selectivity of the column, which is beneficial to the separation of components and the stability of the column, and the column life is prolonged.
Generally, a column temperature equal to or higher than tens of degrees to the average boiling point of the sample is suitable, and for a volatile sample, a low column temperature is used, and a sample which is not volatile is subjected to a high column temperature.
3. Carrier gas flow rate:
The carrier gas flow rate is one of the important reasons for determining chromatographic separation. Generally speaking, the flow rate is high and the peak is narrow, otherwise it is wider, but the flow rate is too high or too low, which has an adverse effect on the separation.
4, stationary phase:
The stationary phase consists of a solid adsorbent or a support coated with a fixative.
When using columns of the same length, the fine separation efficiency of the particles is better than that of the coarse ones.
The fixing liquid content has a great influence on the separation efficiency, and the weight ratio of the carrier to the carrier is generally 15% to 25%. Excessive proportions are detrimental to separation, and too small a ratio will cause tailing of the chromatographic peak.
5, injection:
Generally speaking, the injection is fast, the injection volume is small, and the injection temperature is high, and the separation effect is good. For the liquid sample, the speed is faster, the vaporization temperature is higher than the boiling point value of the high-boiling component in the sample, and the vaporization is performed once to ensure that the peak shape of the chromatogram is not broadened and the column efficiency is high. When the injection amount is within a certain limit, the half width of the chromatographic peak is constant. If the injection volume is too much, the column will be overloaded.
Generally speaking, the column length is increased by four times, and the sample permission is doubled.
8. What is the carrier? What are the requirements for the carrier?
The support is a porous chemically inert solid used to support the fixative in gas chromatography. There are several requirements for the carrier:
1. Large surface area;
2. Chemically inert and thermally stable;
3. Have a certain mechanical strength, so that the coating and filling process does not cause crushing;
4. Appropriate pore structure, which facilitates rapid mass transfer between two phases;
5. Can be made into uniform spherical particles, which is good for gas phase penetration and filling uniformity;
6. It has good wettability and is easy to distribute the fixed liquid.
It is difficult to fully satisfy the above requirements. In practice, people can only find a carrier with better performance.
9. What are the characteristics of the categories?
Usually divided into two major categories of diatomaceous earth and non-diatomaceous earth, each category has a variety of small categories.
1. Diatomaceous earth type:
(1) White: small surface area, loose, brittle, low adsorption performance, and can be analyzed for strong polar components after proper treatment;
(2) Red: It has a large surface area and good mechanical strength, but has a large adsorption.
2. Non-diatomite type:
(1) Fluorine support: It has good surface inertness and can be used to analyze high polarity and corrosive substances, but it is not easy to pack and the column efficiency is low.
(2) Glass microspheres: the surface area is small, and the column temperature can be greatly reduced by using the carrier, and the separation is complete and rapid. However, the coating is difficult and the column efficiency is low.
(3) Porous high polymer beads: high mechanical strength, good thermal stability, low adsorption, corrosion resistance, high separation efficiency, is a new type of chromatographic stationary phase with excellent performance.
(4) Carbon molecular sieve: neutral, large surface area, high strength, long sputum life, and incomparable superiority in microanalysis.
(5) Activated carbon: It can be used as a stationary phase alone.
(6) Sand: mainly used to separate metals.
Ten, how to choose the commonly used carrier?
A variety of supports, a wide range of names. In a common diatomaceous earth carrier:
Red supports (such as 6201, 201) can be used for the separation of non-polar or weakly polar substances.
A white carrier (such as 101) can be used for polar or alkaline substances.
Glazed red supports (such as 301) can be used for moderately polar materials.
The silylated white support can be used for the determination of highly polar hydrogen-bonded materials such as wastewater.
Acidic substances, such as phenols, are separated by acid pickling.
The alkaline substance, such as ethanolamine, is separated and the treated support is washed with an alkali.
In some special cases, special supports such as fluorine supports are used to separate isocyanates.
However, in the ordinary constant analysis, the support may not be too particular, even refractory brick powder, glass beads and sea sand can be used.
XI. What is a solid stationary phase? Can it be roughly divided into several categories?
Refers to an active porous solid material that is directly loaded into the column as a stationary phase. Solid stationary phases can be broadly classified into three categories:
The first type is the adsorbent. Such as: molecular sieve, silica gel, activated carbon, alumina, etc.;
The second type is a high molecular polymer. Such as domestic GDX type polymer porous microspheres, foreign Porapak series, etc.;
The third type is a chemically bonded stationary phase. In gas chromatography, a fixing solution is usually applied to the surface of a carrier.
The use of chemically bonded stationary phases for the analysis of polar or non-polar materials generally yields symmetrical peaks with high column efficiency and improved thermal stability of the stationary phase.
12. What is a fixed solution? What are the requirements for fixed solution?
It is generally a liquid film of high-boiling organic matter, and the components are separated in the column by acting on different molecules of different components. For fixed liquids for gas chromatography, there are generally the following requirements:
1. The vapor pressure is low at the operating temperature, the thermal stability is good, and there is no irreversible reaction with the analyzed physical or carrier gas;
2. It is liquid at the operating temperature, and the lower the viscosity, the better. The mass transfer rate of the substance in the high viscosity fixed solution is slow, and the column efficiency is thus lowered. This determines the minimum use temperature of the fixative;
3. Can be firmly attached to the carrier and form a uniform and structurally stable thin layer;
4. The separated material must have a certain solubility in it, otherwise it will be quickly carried away by the carrier gas and cannot be distributed between the two phases;
5. Separation of substances with similar boiling points and different types, that is, the ability to retain one type of compound is greater than the other. This separation ability is the selectivity of the fixative.
13. What are the principles for the selection of fixatives?
Depending on the interaction between the separated component and the immobilized liquid molecule, the choice of fixative is generally based on the so-called "similarity principle", ie the similarity between the nature of the fixative and the component being separated, such as functional groups, Chemical bond, polarity, certain chemical properties, etc. When the properties are similar, the interaction between the two molecules is strong, the solubility of the separated component in the fixative liquid is large, the partition coefficient is large, and the retention time is long; Small, the partition coefficient is small, so it can flow out of the column very quickly.
Let's discuss the different situations below:
a, separation of polar compounds, using a polar fixative. At this time, the interaction between the components of the sample and the immobilized liquid molecules is mainly the orientation force and the inducing force. The order of the peaks of each component is in the order of polarity, and the first peak with a small polarity, the greater the polarity, the slower the peak is;
b. Separation of non-polar compounds, application of non-polar fixative, the interaction between the components of the sample and the molecules of the fixative is dispersive force, no special selectivity, then the components are peaked in the order of boiling point, and the boiling point is low. Out of the peak. For the separation of isomers with similar boiling points, the efficiency is very low;
c. When separating the mixture of non-polar and polar compounds, a polar fixing solution may be used, in which case the non-polar component is first distilled off, and the stronger the polarity of the fixing solution, the more easily the non-polar component flows out;
d. For samples capable of forming hydrogen bonds. For the separation of alcohol, phenol, amine and water, a polar or hydrogen-bonded fixing solution is generally selected, and the hydrogen bonding ability between the components and the fixing liquid molecules is separated.
The "similar compatibility principle" is the general principle for selecting a fixed solution. Sometimes, when the existing fixing solution cannot achieve a satisfactory separation result, a "mixed fixing solution" is often used, and two or more properties are different. The fixed liquid mixed in a suitable ratio allows the separation to have a satisfactory selectivity without prolonging the analysis time.
14. What are the performances after the column fails? What is the cause of the failure?
Column failure is mainly characterized by poor chromatographic separation and significantly shorter component retention times. The main reason for column failure is that the activity or adsorption performance of the stationary phase is reduced for gas-solid chromatography. For gas-liquid chromatography, it is caused by the gradual loss of the fixative during use.
15. Aging operation of capillary column
Purpose of aging: The stationary phase of the GC column is usually distributed in the form of a coating on the inside of the column tube wall (capillary column) or the carrier surface (packed column). For a new GC column, the outer layer is fixed. The combination of phase and carrier tends to be weak, and it will slowly lose at high temperature, causing baseline fluctuations and noise. To avoid this phenomenon, it can be heated in advance at a higher temperature (usually the column withstand temperature). For a period of time, the weakly bonded stationary phase is volatilized so that subsequent analysis is undisturbed. In addition, the long-lasting GC column can be aged to remove residual contaminants from the column.
The column temperature is raised to a constant temperature, usually the upper temperature limit. In special cases, it can be heated to about 10-20 °C above the operating temperature, but it must not exceed the upper temperature limit of the column, which is very easy to damage the column, and do not set the programming temperature too slow.
After reaching the aging temperature, the baseline was recorded and observed. Scale up the baseline for easy viewing. In the initial phase, the baseline should continue to rise and begin to decline 5-10 minutes after reaching the aging temperature and will last for 30-90 minutes. When a fixed value is reached, the baseline will stabilize. If the baseline is still not stable after 2-3 hours or there is no significant downward trend after 15-20 minutes, there may be leakage or contamination of the system device.
In such a situation, immediately lower the column temperature below 40 °C, check the system as soon as possible and solve the related problems. If you continue to age, not only will the column be damaged, but you will never get a normal, stable baseline. In addition, the aging time should not be too long, otherwise it will reduce the service life of the column.
In general, columns coated with a polar stationary phase and a thicker coating have a longer aging time, while a weakly polar stationary phase and a thinner coated column require less time. The aging method of the PLOT column is different. For the specific steps, please refer to the operating instructions of the column.
If the column is aged without being connected to the detector, the end portion of the column may have been destroyed after aging. The 10-20 cm portion of the end of the column was first cut off and the column was attached to the detector. Temperature limit is the range of application temperatures at which the column can be used normally. If the operating temperature is below the lower temperature limit of the column, the separation and peak shape will not be ideal. But this does not harm the column itself.
The upper temperature limit usually has two values. The lower value is the constant temperature limit. At this temperature, the column can be used normally without specific time limits. The higher value is the temperature rise limit of the programmed temperature. The duration of this temperature is usually no more than ten minutes. Operation above the upper temperature limit will reduce column life.
Sixteen, the baseline drift problem investigation
Baseline drift often occurs when using temperature programming in the GC, which can often be caused by column bleed, loss of the injection pad, injector contamination or detector contamination, and changes in gas flow rate. If a high sensitivity detector is used, even a weak column bleed or system contamination can cause significant baseline drift. In order to improve the reliability of qualitative and quantitative analysis, baseline drift should be reduced or eliminated as much as possible.
17. How to reduce the baseline drift caused by the sample and the injector?
If there is a polymer non-volatile residue on the column, it will easily cause baseline drift when the temperature is programmed. Because these substances are strongly retained and move slowly in the column, this strong retention component can be re-aged. Ejected from the column, but this method increases the possibility of fixed liquid oxidation;
In addition, you can use a solvent to rinse the column (please read the precautions for using the column before rinsing to select the appropriate solvent);
A guard column can also be installed to prevent problems. If the sample is contaminated and the baseline drifts, it can be solved by replacing the injection pad, liner and seal, while rinsing the inlet with solvent. After maintenance, use a fused silica tube to place the injector and detector. Connect them and take a blank to confirm that the injector is clean.
18. How to reduce the baseline drift caused by the detector?
The baseline drift caused by the detector is usually caused by a small amount of hydrocarbons in the compensation gas or gas. The use of a high-purity gas purifier to treat the compensation gas or gas can reduce this baseline drift; using a high-purity gas generator can improve Baseline stability of the FID; proper detector maintenance, including periodic cleaning, can reduce this drift.
19. How to reduce the baseline drift caused by column loss?
Before using the new column, aging can be reduced by the following method: aging at a temperature higher than the experimental operating temperature of 20 ° C or using the column operating temperature (using the lower of the two), long time low temperature aging relative to Short-term high temperature aging helps to reduce column loss. If the carrier gas contains a small amount of oxygen or moisture or a leak in the gas line, the fixed solution is easily oxidized under high temperature conditions, causing column loss and baseline drift.
Once the fixative is oxidized, it must be conditioned with high-purity carrier gas for several hours to make it possible to level the baseline. This damage to the fixative is irreparable, so if oxygen is continuously passed through the column, even if the baseline is aged, Drop to the level. Therefore, during the experiment, a high-quality oxygen/moisture filter should be used in the gas pipeline, and a high-quality electronic leak detector should be used for strict leak detection.
Twenty, no peak
1. The FID detector flame is extinguished;
2. The gasification of the injector is too low and the sample fails to vaporize;
3. The column temperature is too low to condense the sample in the column;
4. The inlet is leaking;
5. Leakage or blockage at the inlet of the column;
6. The problem with the syringe is that the sample cannot be taken.
Twenty-one, all component peaks are small or small
Possible causes and suggested actions:
1. Syringe defect, using a new needle;
2. Leakage after injection, determine the leak point;
3. The split ratio is too large;
4. Analyze the molecular weight of the material is too large, and increase the temperature of the inlet;
5. The NPD is replaced by a contaminant (silica) to replace the bead;
6. The temperature of the NPD is too high (usage or ambient temperature), the gas is not pure, and the bead is replaced: avoid high temperature use;
7. The detector does not match the sample.
Twenty-two, the former Yanfeng
1. The peak extension tongue is mostly overloaded with the column, reducing the injection volume and using a large capacity column;
2. Increase OVEN, INJ temperature;
3. Increase the carrier gas flow rate;
4. Master the injection skills;
5. The previous sample condensed in the column and failed to be exhausted in time;
6. The sample reacts with the stationary phase carrier.
Twenty-three, peak height, peak area is not repeated
1. The injection is not repeated, and the deviation is large;
2. Peak misalignment caused by other peak shape changes;
3. Baseline interference;
4. Changes in instrument system parameter settings, standardization and standardization of parameters;
5. Column performance changes.
Twenty-four, poor sensitivity repeatability during continuous injection
Under continuous injection conditions, the peak area is large and small, and the measurement accuracy is not high, for the following reasons:
1. Poor injection technology;
2. Carrier gas leakage or unstable flow rate;
3. The detector is contaminated;
4. The column, the liner is contaminated, the liner is cleaned, and the column is cleaned with solvent (premium pure methanol): replaced (if necessary);
5. The syringe has a leak;
6. The injection volume exceeds the linear range of the detector to form a detector overload.
Twenty-five, peak tailing
1. Liner, the column is contaminated or lined, the column is improperly installed, there is a dead volume, methane is injected, and if the peak is tailed, it is reinstalled;
2. The injector temperature is too high;
3. The column head is not flat with diamond cutting;
4. The polarity index of the stationary phase does not match the sample, and the matching column is replaced;
5. There are cold wells in the sample circulation route to eliminate the excessively low temperature zone in the route;
6. There is accumulated cutting debris in the liner or column to clean and replace the liner, and cut the stigma 10cm;
7. The injection time is too long;
8. The split ratio is low, increasing the split ratio (at least greater than 20/1);
9. The injection volume is too high, reduce the injection volume or dilute the sample.
Twenty-six, the decline in separation
1. The column is contaminated;
2. The stationary phase is destroyed (column loss);
3. Failure to inject, check for leaks;
4. Check the adaptability of temperature and inspect the liner;
5. The sample concentration is too high, dilute, reduce the injection volume, and use a high split ratio.
Twenty-seven, solvent peak widening
1. Column installation failed;
2. Injection leakage;
3. The injection volume is high to increase the vaporization temperature;
4. The split ratio is lower to increase the split ratio;
5. The column temperature is low;
6. When the split injection, the initial OVEN is too high to lower the initial column temperature, using a high boiling point solvent;
7. Excessive purge time (no split injection) Define a short purge program.
Twenty-eight, the baseline drifts downward
1. The newly installed column, the baseline continuously drifts for a few minutes and continues to age;
2. The detector does not reach equilibrium and prolongs the balance time of the detector;
3. The detector or other parts of the GC system are baked and cleaned.
Twenty-nine, the baseline drifts upward
1. The column stationary phase is destroyed;
2. The carrier gas flow rate decreases and the carrier gas pressure is adjusted.
Thirty, noise
1. The capillary column insertion detector is too deep to reinstall the column;
2. Using ECD, TCD gas leakage triggers baseline noise, inspection, and maintenance of the gas path;
3.FID, NPD, FPD gas flow rate or gas selection is not appropriate, high purity gas, adjust the flow rate;
4. The inlet is contaminated to clean the inlet, replace the mat, and replace the glass fiber in the liner;
5. The capillary column is contaminated, cut off the first end 10cm, clean the column with solvent, and replace it;
6. The detector has failed.
Thirty-one, several methods to improve the degree of separation
1. Increasing the length of the column can increase the resolution;
2. Reduce the injection volume (solid sample to increase the amount of solvent);
3. Improve the injection technology to prevent two injections;
4. Reduce the carrier gas flow rate;
5. Lower the column temperature;
6. Increase the temperature of the vaporization chamber;
7. Reduce the dead volume of the system, such as the column connection to be inserted into the position, the splitless injection should choose the split-free structure vaporization chamber;
8. The capillary column should be split and the appropriate split ratio should be selected.
In summary, it is necessary to explore the experiment according to the specific situation, such as reducing the carrier gas flow rate and lowering the temperature of the column, which will widen the chromatographic peak. Therefore, it is necessary to look at the chromatographic peak shape to change the condition. The ultimate goal is to achieve separation and fast peak times.
Thirty-two, how to determine whether the column aging is complete?
The FID detector is best suited for detecting baselines when the column is aged. At the end of the warming procedure, the baseline will rise and then the baseline will gradually settle down, at which point the column aging is considered complete.
When the column is at a high temperature, the column life drops dramatically. If there is a large amount of column bleed after the column has aged for more than 2 hours, cool the column to room temperature and identify the source of column bleed such as oxygen infiltration, septum leaks, and the residue of the instrument itself.
Column bleed: Perform column loss test after aging of the column, run the program temperature without injection, start heating at 10 °C/min from 50 °C to the high temperature of the column, and keep the chromatogram at the high temperature of zui for 10 min. For the column loss map, take this picture and compare it with the future blank.
If a lot of peaks are produced during a blank run, the column performance changes, either because the carrier gas contains oxygen or because of sample residue. If there is GC-MS, the typical ion loss (such as DB/HP-1 or 5) of the low polarity column will have a mass/charge ratio m/z of 207, 73, 281, 355, etc., most of which are cyclosiloxane. alkyl.
Column loss is generally thought to cause noise and unstable baselines. True column bleeds often have a positive drift like noise. Look at whether the baseline is drifting upwards, whether there is a peak outflow or not.
Thirty-three, hydrogen flame ionization detector (FID) flame extinction or the cause of ignition failure
Condensation
Due to the formation of water during the FID combustion process, the detector temperature must be maintained above 100 °C to avoid condensation. When it is not turned on for a long time, it takes a long time to bake and then ignite.
2. Column flow rate is too high
If a large inner diameter column must be used, the small carrier gas flow rate can be turned off long enough for the FID to ignite.
3. Check that the installed nozzle type is suitable for the column to be used and check if the nozzle is clogged.
Thirty-four, gas cylinders and their use
The gas cylinder is a high-pressure vessel for storing compressed gas. Its volume is generally 40-60 L, the working pressure of Zui is 15 MPa (150 atm), and the lowest is also 0.6 MPa (6 atm). The standard high-pressure gas cylinder is manufactured according to national standards. Cheng, in the shoulder of the cylinder should be marked as follows: manufacturer, date of manufacture, cylinder model and number, cylinder weight, gas volume, working pressure, hydraulic pressure test, hydraulic test date and next inspection Date, etc.
Due to the high pressure of gas cylinders, some gases are toxic or flammable and explosive. In order to ensure safety and avoid confusion between various cylinders, the cylinder should be coated with a specific color according to the regulations, and the name of the gas in the bottle should be written.
1. Precautions for the use of cylinders
(1) Cylinders must be sent to the relevant departments for inspection on a regular basis. Cylinders filled with general gas must be inspected once within 3 years, and cylinders filled with corrosive gases are sent once every two years.
(2) When carrying the cylinder, wear the cylinder cap and the upper and lower rubber waistbands, and gently handle them. Do not roll, bump, fall or violently vibrate on the ground to prevent explosion. When placing and using the cylinder, it must be secured with a shelf or wire.
(3) The cylinder should be stored in a cool, dry, away from heat source, well ventilated, avoiding open flames and sun exposure. After the cylinder is heated, the gas expands and the pressure inside the bottle increases, which may cause air leakage or even explosion. Flammable gas cylinders and oxygen cylinders must be stored separately. Hydrogen cylinders are placed in dedicated small rooms outside the building to ensure safety.
(4) In the case of steel cylinders, in addition to carbon dioxide and ammonia, a pressure reducing valve is generally used. Among the various pressure reducing valves, the pressure reducing valves except nitrogen and oxygen can be used interchangeably, and the others can only be used for the specified gas to prevent explosion. The pressure relief valve must be carefully tightened, usually with 7 turns of thread (commonly known as eating 7 teeth). Gases that are prone to polymerization (such as acetylene, ethylene) must be stored for a long period of time to avoid long-term storage.
(5) flammable gases, such as hydrogen, acetylene, etc., the valve of the cylinder is "reverse buckle" (left-handed) thread, that is, counterclockwise tightening; non-combustible or combustion-supporting gas, such as oxygen, nitrogen, etc., the valve of the cylinder is "Positive buckle", (right-handed) thread, that is, clockwise tightening.
(6) Never put oil, grease or other inflammables or organic matter on the oxygen cylinder, especially at the valve mouth and pressure reducing valve, or use cotton, hemp, etc. to prevent leakage.
(7) Pay attention to protect the cylinder valve. When switching the valve, first clear the direction and then slowly rotate, otherwise the thread will be damaged. When opening the valve, the person should stand on the other side of the pressure reducing valve to prevent the pressure reducing valve from rushing out and being injured. The valve should be completely closed after each use.
(8) Steel cylinders storing flammable gases shall have anti-tempering devices. Some pressure reducing valves have this device; it is also possible to fill a gas pipe with a fine wire mesh to prevent tempering; and a liquid sealing device in the gas guiding pipe can also effectively protect the gas.
(9) Do not use up all the gas in the cylinder, and keep the residual pressure above 0.05MPa (pressure gauge pressure). The flammable gas (such as acetylene) should be 0.2-0.3 MPa remaining, and the hydrogen should be kept 2 MPa to prevent danger when re-inflating. Cylinders should be used as they are, and checked.
(10) Once the valve leaks, immediately move the cylinder to the outside to prevent accidents inside the room.
2. Hydrogen and acetylene gas use precautions
(1) Hydrogen. If it is released from the cylinder sharply, it will sometimes catch fire even if there is no fire source. The explosion of hydrogen and air mixture is very wide. When it contains 4.0% to 75.6% (by volume) of hydrogen, it will explode in case of fire. Therefore, the hydrogen should be used in a well-ventilated place, or the exhaust gas should be used to discharge the indoor air to the outside as much as possible.
(2) Acetylene. Extremely flammable, with high combustion temperatures and sometimes decomposition explosions. When acetylene is mixed with air, the explosion range is 2.5% to 80.5% (volume ratio) of acetylene. Therefore, it is necessary to strictly prohibit fireworks and prevent air leakage.
35, GC preventive maintenance and corrective operations
As long as the chromatographic system is contaminated with high-boiling substances, especially at the inlet, it is expected that chromatographic performance will deteriorate. The analyst should perform routine maintenance of the instrument, including regular replacement of the injection septum, cleaning and aging of the inlet liner, etc. If necessary, the capillary column attached to one end of the inlet can be cut off 0.5 to 1 m.
If chromatographic performance degradation and ghost peak problems still occur, it may be necessary to clean the metal surface of the inlet. Capillary columns are reliable and easy to use, but to ensure good separation performance, you need to be aware of the following specific operations:
(1) Contact between the capillary column and the wall of the chromatograph can affect chromatographic performance and column life;
(2) Care should be taken not to allow oxygen to enter the capillary column;
(3) The injection septum can be replaced only after the chromatograph has cooled down;
(4) Before heating the chromatograph again, the column should be flushed with carrier gas for 15 min;
(5) The deoxidizing tube should be used to remove traces of oxygen from the carrier gas, and the deoxidizing tube should be replaced periodically;
(6) Whether or not the chromatograph is heating, a carrier gas stream is required to pass through the column.
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