Common Questions and Considerations in Gas Chromatography

Gas Chromatography Principles

1. What are the characteristics of gas chromatography?

A: Gas chromatography is a type of chromatography that uses gas as the mobile phase. Its advantages for separation and analysis are as follows: 1. High sensitivity: It can detect substances as small as 10-13 grams, and is suitable for analyzing trace impurities in ultra-pure gases, high-molecular monomers, and trace poisons in air. 2. High selectivity: It can effectively separate isomers and isotopes with very similar properties. 3. High efficiency: It can separate complex samples into single components. 4. High speed: General analysis can be completed in just minutes, facilitating production control and guidance. 5. Wide range of applications: It can analyze both low-content gases and liquids, as well as high-content gases and liquids, regardless of component content. 6. Small sample volume required: Generally, a few milliliters are used for gas samples, and a few micro-liters or tens of micro-liters are used for liquid samples. 7. The equipment and operation are relatively simple, and the instrument is inexpensive.

2. What is the separation principle of gas chromatography? Answer: Gas chromatography is a physical separation method. It utilizes the slight difference in the distribution coefficient (solubility) of the components of the substance being tested between two different phases. When the two phases are in relative motion, these substances are repeatedly distributed between the two phases, making the original slight difference in properties produce a large effect, thereby separating the different components.

3. What is gas chromatography? How many categories does it have? Answer: All chromatography techniques that use gas as the mobile phase are generally called gas chromatography. Generally, they can be classified according to the following aspects: 1. Classification by the aggregation state of the stationary phase: (1) Gas-solid chromatography: the stationary phase is a solid adsorbent, (2) Gas-liquid chromatography: the stationary phase is a liquid coated on the surface of the support.

2. Classification by process physical and chemical principles: (1) Adsorption chromatography: Chromatography that uses the differences in the physical adsorption properties of different components on a solid adsorption surface to achieve separation. (2) Partition chromatography: Chromatography that uses the different distribution coefficients of different components between two phases to achieve separation. (3) Others: Ion exchange chromatography based on the principle of ion exchange; elector-chromatography based on the electrokinetic effect of colloids; thermal chromatography based on the development of temperature changes, etc.

3. Classification by stationary phase type: (1) Column chromatography: The stationary phase is contained in the chromatographic column. Packed columns, hollow columns, and capillary columns all belong to this category. (2) Paper chromatography: Filter paper is used as the carrier. (3) Thin film chromatography: The stationary phase is a thin film pressed from powder.

4. Classification according to the principle of kinetic process: It can be divided into three types: flushing method, replacement method and head-on method.

4. What is the process of a simple gas chromatography analysis device? Answer: The process of a simple gas chromatography analysis device basically consists of four parts: 1. Gas source 2. Sample injection device 3. Chromatographic column 4. Identifier and recorder

5. What are some common terms and basic concepts in gas chromatography? Answer: 1. Phase, stationary phase, and mobile phase: A homogeneous portion of a system is called a phase. In chromatographic separations, the stationary phase is called the stationary phase; the fluid that moves through or along the stationary phase is called the mobile phase. 2. Chromatographic peak: The curves that appear on the recorder after a substance passes through the chromatographic column and enters the detector are called chromatographic peaks. 3. Baseline: Under chromatographic operating conditions, when no analyte passes through the detector, the curve of the detector noise over time is called the baseline. 4. Peak height and half-peak width: The height between the intersection of a line drawn from the peak's concentration maximum and the baseline is called the peak height, generally expressed as h. The width at half the peak height is called the half-peak width, generally expressed as x1/2. 5. Peak area: The area formed by the elution curve (chromatographic peak) and the baseline is called the peak area, expressed as A. 6. Dead time, retention time, and corrected retention time: The time from injection to the peak maximum of the inert gas peak is called the dead time, expressed as td. The time required from injection to the appearance of the highest value of the chromatographic peak is called retention time, represented by tr. The difference between the retention time and the dead time is called the corrected retention time. It is represented by Vd. 7. Dead volume, retention volume and corrected retention volume: The product of the dead time and the average flow rate of the carrier gas is called the dead volume, represented by Vd, and the average flow rate of the carrier gas is represented by Fc, Vd=tdxFc. The product of the retention time and the average flow rate of the carrier gas is called the retention volume, represented by Vr, Vr=trxFc. 8. Retention value and relative retention value: The retention value is a numerical value that represents the residence time of each component in the sample in the chromatographic column. It is usually expressed in time or the volume of carrier gas required to carry the component out of the chromatographic column. Using a substance as a standard, the ratio of the retention value of other substances to this standard is called the relative retention value. 9. Instrument noise: The instability of the baseline is called noise. 10. Base flow: In hydrogen flame chromatography, when there is no injection, the base current (bottom current) of the instrument itself is referred to as base flow.

6.What are the general criteria for selecting a carrier gas? What are the commonly used carrier gases for gas chromatography?  Answer: Gases used as carrier gases for gas chromatography (GC) must meet several essential criteria, including good chemical stability, high purity, low cost, easy availability, and compatibility with the detector employed. Commonly used carrier gases include hydrogen, nitrogen, argon, helium, and carbon dioxide. Each gas has its advantages and limitations, influencing the choice depending on the specific analytical requirements and the nature of the samples being analyzed.

Recently, there has been a significant trend towards replacing helium with hydrogen due to the increasing costs and supply challenges associated with helium. Hydrogen offers several benefits, including improved sensitivity and faster analysis times. To facilitate this transition, manufacturers such as PEAK, LNI, and Hovogen provide onsite hydrogen generation solutions, allowing laboratories to produce hydrogen gas as needed. This not only addresses cost concerns but also ensures a continuous supply of high-purity hydrogen for gas chromatography applications.

7. Why does carrier gas need to be purified? How should it be purified? Answer: Purification is the removal of impurities such as organic matter, trace oxygen, and moisture from the carrier gas to improve its purity. Using impure gases as carrier gas can lead to column failure, sample variability, increased baseflow noise in hydrogen flame chromatography, and poor linearity in thermal conductivity chromatography. Therefore, carrier gas must be purified. Chemical treatments, such as activated copper, are generally used to remove oxygen; molecular sieves and activated carbon are used to remove organic impurities; and silica gel and molecular sieves are used to remove moisture.

8.What are the methods for sample injection? Answer: Chromatographic separation requires that a certain amount of sample be injected in the form of a "plug" in the shortest possible time. The injection methods can be divided into: 1. Gas sample: There are generally four injection methods: (1) syringe injection (2) volumetric tube injection (3) fixed volume injection (4) automatic gas injection. Syringe injection and automatic gas injection are commonly used. The advantages of syringe injection are flexibility and simplicity, but the injection volume repeatability is poor. Automatic gas injection uses a quantitative valve for injection, which has good repeatability and can be operated automatically. 2. Liquid sample: Generally, a micro syringe is used for injection, which is simple and fast. Quantitative automatic injection can also be used, which has good repeatability. 3. Solid sample: The sample is usually dissolved in a solvent and then injected using the same method as liquid injection. Solid injectors are also used for injection.

9. Briefly describe the effects of column length, column inner diameter, column temperature, carrier gas flow rate, stationary phase, injection and other operating conditions on separation in gas chromatography analysis?

A: Operating conditions significantly influence chromatographic separations. 1. Column length and inner diameter: Generally speaking, longer columns improve separation performance, while shorter columns allow for faster distillation. A smaller inner diameter results in better separation, while a larger inner diameter increases throughput. However, an excessively large inner diameter can result in uneven distribution of the support within the column. Analytical columns typically have an inner diameter of 3-6 mm and a length of 1-4 meters.

2. Column temperature: This is a critical operational variable, directly impacting separation efficiency and analysis speed. The selection of column temperature is based on the boiling point range of the mixture, the ratio of the stationary phase, and the sensitivity of the detector. Increasing the column temperature shortens analysis time; decreasing the temperature increases column selectivity, facilitating component separation, improving column stability, and extending column life. A column temperature equal to or several tens of degrees above the average boiling point of the sample is generally appropriate. Low column temperatures are used for volatile samples, while high temperatures are used for less volatile samples.

3. Carrier gas flow rate: Carrier gas flow rate is one of the most important factors in determining chromatographic separations. Generally speaking, higher flow rates result in narrower chromatographic peaks, while lower flow rates result in wider peaks. However, either too high or too low a flow rate can negatively impact separations. The flow rate should be stable, with a typical range of 10-100 ml/min being the ideal value.

4. Stationary Phase: The stationary phase is composed of a solid adsorbent or a support coated with a stationary liquid. (1) Solid adsorbent or support particle size: 40-60 mesh, 60-80 mesh, and 80-100 mesh are generally used. When using columns of the same length, fine particles have better separation efficiency than coarse particles. (2) Stationary liquid content: The stationary liquid content has a significant impact on separation efficiency. The weight ratio of the stationary liquid to the support is generally 15%-25%. Too high a ratio will impair separation, while too low a ratio will cause tailing of the chromatographic peaks.

5. Injection: Generally speaking, faster injection, smaller injection volumes, and higher injection temperatures yield better separation results. For liquid samples, the injection speed should be high, and the vaporization temperature should be above the boiling point of the high-boiling components in the sample. This ensures that the chromatographic peaks do not broaden, thus maximizing column efficiency. Within a certain injection volume limit, the full width at half maximum (FWHM) of the chromatographic peak remains constant. Excessive injection volume will overload the column. Generally, increasing the column length fourfold doubles the permissible sample volume. For routine analysis, the injection volume for liquids is 1-20 μL; for gases, it is 0.1-5 mL.

10. What principles should be used to select chromatographic column tubing materials? What materials are commonly used for column tubing? Answer: The material for chromatographic column tubing should be selected based on the following requirements: 1. It should not chemically react with the stationary phase, sample, or carrier gas. 2. It should be easy to process and shape. 3. The inner wall of the tubing should be smooth, and the cross-section should be uniformly round. Chromatographic column tubing is typically U-shaped or spiral, and is mostly made of copper, stainless steel, glass, and other materials.

11. How should a new chromatographic column (copper or stainless steel) be treated before use? Answer: A new column should first be washed with a dilute acid or alkali (1:1 hydrochloric acid or sodium hydroxide) to remove oil and other dirt. Then rinse with tap water, then rinse with distilled water until neutral. Finally, blow it with clean air and dry it before use.

12. What is a support? What are the requirements for a support? A: A support is a porous, chemically inert solid used to support the stationary phase in gas chromatography. The requirements for a support are as follows: 1. A large surface area, generally between 0.5 and 2 m/g; 2. Chemical inertness and thermal stability; 3. Sufficient mechanical strength to prevent shattering during coating and filling; 4. An appropriate pore structure to facilitate rapid mass transfer between the two phases; 5. The ability to form uniform spherical particles to facilitate gas phase penetration and uniform filling; 6. Excellent wettability to facilitate uniform distribution of the stationary phase. It is difficult to find a support that fully meets these requirements, and in practice, only supports with relatively good performance can be found.

13. How many types of supports are there? What are their characteristics? Answer: They are usually divided into two categories: diatomaceous earth and non-diatomaceous earth, and each category has various subcategories. 1. Diatomaceous earth types: (1) White: small surface area, loose, brittle, low adsorption capacity, after proper treatment, can analyze highly polar components; (2) Red: has a large surface area and good mechanical strength, but has a high adsorption capacity. 2. Non-diatomaceous earth types: (1) Fluorine supports: good surface inertness, can be used to analyze highly polar and corrosive substances, but difficult to pack into the column, and the column efficiency is low. (2) Glass microspheres: small surface area, using it as a support can greatly reduce the column temperature, and the separation is complete and fast. However, it is difficult to coat and the column efficiency is low. (3) Porous polymer beads: high mechanical strength, good thermal stability, low adsorption, corrosion resistance, high separation efficiency, a new type of chromatographic stationary phase with excellent performance. (4) Carbon molecular sieve: neutral, large surface area, high strength, long desorption life, and has unparalleled advantages in trace analysis. (5) Activated carbon: can be used as a stationary phase alone. (6) Sand: mainly used for separating metals. 14. What are the commonly used supports? What category do they belong to? Answer: 101 support: white diatomaceous earth support; 102 support: white diatomaceous earth support; celite545: white diatomaceous earth support; 201 support: red diatomaceous earth support; 6201 support: red diatomaceous earth support; C-22 insulation brick: red diatomaceous earth support; chromosorb: red diatomaceous earth support.

15. Why does the support need to be treated before use? What are the general treatment methods? Answer: The surface of commonly used supports is not inert. It has varying degrees of catalytic activity and adsorption (especially when the stationary phase content is low and when separating polar substances), which can cause peak tailing, decreased column efficiency, and changes in retention values. Therefore, pretreatment is necessary. The general treatment methods are briefly described below:

1. Acid Washing: Heat the support with concentrated hydrochloric acid for 20-30 minutes, then rinse with tap water until neutral, rinse with methanol, and dry for later use. This method mainly removes inorganic impurities such as iron on the support surface.

2. Alkali Washing: Soak or reflux the support in a 10% sodium hydroxide or 5% potassium hydroxide-methanol solution. Rinse with water until neutral, rinse with methanol, and dry for later use. The purpose of alkaline washing is to remove surface acidic sites such as aluminum oxide, but trace amounts of free alkali often remain on the surface, which can decompose or adsorb some non-alkaline substances. Be careful when using this solution.

3. Silanization: Silanization reagents react with silanol and silyl ether groups on the support surface to remove surface hydrogen bonding capacity, thereby improving the support's performance. Commonly used silanization reagents include dimethyldichlorosilane and hexamethyldisilazane.

4. Glazing: Soak the support to be treated in a 2-3% sodium carbonate-potassium carbonate (1:1) aqueous solution for one day. After drying, calcine at 870°C for 3-5 hours, then heat to 980°C for approximately 40 minutes. This treatment forms a vitrified glaze on the surface of the support, hence the name "glazed support." This support exhibits low adsorption capacity but high strength. When a small amount of tailing agent is added to the stationary phase, it can analyze highly polar substances such as alcohols and acids. However, the column efficiency for non-polar substances is slightly reduced. Furthermore, substances such as methanol and formic acid exhibit a certain degree of irreversible chemical adsorption on glazed supports, which should be considered during quantitative analysis.

5. Other purification methods: Any method that uses chemical reactions to remove active sites or physical covering to purify the surface properties of the support can be used.

16. What is the commonly used mesh size of the support? Answer: For commonly used chromatographic columns with an inner diameter of 4-6 mm: for longer chromatographic columns, the mesh size of the support is generally 40-80 mesh; for shorter chromatographic columns, the mesh size of the support is generally 80-100 mesh (the number of sieve holes per inch is mesh).

17. How to choose a commonly used support? Answer: There are many different supports. Among the commonly used diatomaceous earth supports: red supports (such as 6201, 201) can be used to separate non-polar or weakly polar substances. White supports (such as 101) can be used for polar or alkaline substances. Glazed red supports (such as 301) can be used for medium-polar substances. Silanized white supports can be used for highly polar hydrogen-bonded substances such as wastewater determination. To separate acidic substances such as phenols, an acid-washed support should be used. To separate alkaline substances such as ethanolamine, an alkali-washed support should be used. Silanized supports are used for trace analysis. Special supports are required in some special cases, such as fluorine supports for separating isocyanates. However, in ordinary macro-analysis, you don't need to be overly picky about the support; even refractory brick powder, glass beads, and sea sand can be used.

18. What is a solid stationary phase? How many categories can it be roughly divided into? Answer: It refers to an active porous solid material that is directly loaded into the chromatographic column as a stationary phase. Solid stationary phases can be roughly divided into three categories: the first category is adsorbents, such as molecular sieves, silica gel, activated carbon, and alumina; the second category is high molecular polymers, such as the domestic GDX-type high molecular porous microspheres and the foreign Porapak series; the third category is chemically bonded stationary phases. In gas chromatography, the stationary liquid is usually coated on the surface of the carrier. The use of chemically bonded stationary phases to analyze polar or non-polar substances usually produces symmetrical peaks, high column efficiency, and improved thermal stability of the stationary phase.

19. What is a stationary phase? What are the requirements for a stationary phase? Answer: It is generally a liquid film of a high-boiling-point organic compound that separates components in the chromatographic column by interacting with the different molecules of the different components. The following are generally required for a stationary phase used in gas chromatography: 1. Low vapor pressure at the operating temperature, good thermal stability, and no irreversible reactions with the analyte or the carrier gas; 2. Liquid at the operating temperature, and the lower the viscosity, the better. Mass transfer in a high-viscosity stationary phase is slow, reducing column efficiency. This determines the minimum operating temperature of the stationary phase; 3. It must adhere firmly to the carrier and form a uniform, structurally stable thin layer; 4. The substances to be separated must have a certain solubility in the phase; otherwise, they will be quickly carried away by the carrier gas and unable to partition between the two phases; 5. It must be able to separate substances of similar boiling points but different types, meaning it retains one type of compound more than another. This separation ability is known as the selectivity of the stationary phase.

20. What are the principles for selecting a stationary phase? Answer: Based on the interactions between the components being separated and the stationary phase molecules, the selection of a stationary phase is generally based on the so-called "similarity principle." This refers to the similarities between the properties of the stationary phase and the components being separated, such as functional groups, chemical bonds, polarity, and certain chemical properties. When the properties are similar, the interaction between the two molecules is stronger, the solubility of the separated components in the stationary phase is greater, the partition coefficient is larger, and thus the retention time is longer. Conversely, when the solubility is lower and the partition coefficient is smaller, the components will elute quickly from the column. The following discusses different situations: a. Separating polar compounds, using a polar stationary phase. In this case, the interactions between the sample components and the stationary phase molecules are primarily directional and inductive. The peaks of the components elute in order of polarity, with the less polar components eluenting first and the more polar components eluenting later. b. Separating non-polar compounds, using a non-polar stationary phase. The interactions between the sample components and the stationary phase molecules are dispersion forces, without any particular selectivity. In this case, the components elute in order of boiling point, with the lower boiling point peaking first. The efficiency of separating isomers with similar boiling points is very low. c. When separating mixtures of non-polar and polar compounds, a polar stationary phase can be used. In this case, the non-polar component distills first. The more polar the stationary phase, the easier it is for the non-polar component to flow out. d. For samples that can form hydrogen bonds, such as alcohols, phenols, amines, and water, polar or hydrogen-bonding stationary phases are generally selected. Separation is based on the ability to form hydrogen bonds between the components and the stationary phase molecules. The "like-compatibility principle" is a general principle for selecting stationary phases. Sometimes, when existing stationary phases cannot achieve satisfactory separation results, a "mixed stationary phase" is often used. Two or more stationary phases with different properties, mixed in appropriate proportions, are used to achieve relatively satisfactory separation selectivity without prolonging analysis time. However, in actual work, the selection of stationary phases is often based on examples presented in reference materials or literature.

21. How many methods are there for treating mixed stationary phases? Answer: There are three methods for treating mixed stationary phases: 1. Apply each to the support separately and then mix; 2. Mix the stationary phases before applying, ensuring that all the stationary phases used are dissolved in the same solvent; 3. Apply each phase separately, then load each phase onto a chromatographic column of proportional length, and then connect them in series. The results of these three methods are generally the same, but some may differ for specific separations.

22. What is the appropriate amount of stationary liquid applied? Answer: Because the stationary liquid content significantly affects separation efficiency, the minimum weight ratio of the stationary liquid to the support is 5%, with 15%-25% generally used. A higher liquid ratio will cause the sample being analyzed to diffuse across the thicker liquid film, impairing separation. If the liquid ratio is too low, the thinner liquid film will cause residual adsorption on the support surface to show through, resulting in tailing of the chromatographic peaks. Because a low liquid ratio promotes equilibrium, a higher carrier gas flow rate can be used. Therefore, using a low liquid ratio and a small amount of sample can shorten analysis time. For diatomaceous earth supports, the stationary liquid content can be as high as 15-30%. Due to the smaller surface area of fluorine supports, the maximum allowed is 10%. For glass microspheres, due to their extremely small surface area, the stationary liquid content can only be maintained at around 0-25%.

23. What are the commonly used stationary phase solvents for column preparation? What are the principles for selecting solvents? Answer: Commonly used solvents include methanol, ethanol, ether, acetone, n-butanol, n-hexane, petroleum ether, benzene, toluene, and chloroform. The principles for selection are: 1. Good solubility, 2. No chemical reaction with the stationary phase, 3. Low boiling point, and 4. Low toxicity.

24. What is the conventional method for coating a support with a stationary phase when preparing a column? Answer: For most commonly used chromatographic columns, the "conventional" coating method is used. The simple procedure is: take the required amount of stationary phase, dissolve it in an appropriate amount of solvent (enough to soak the support), slowly pour the support into the solution, stirring as it is added, and then irradiate with an infrared lamp (or evaporate it in a water bath) to drive off the solvent. The stationary phase will then adhere to the support.

25. What are the common methods for filling chromatographic columns? Answer: The quality of the stationary phase filling directly affects column efficiency. Pump-filling is commonly used. This involves plugging one end of the column with glass wool, connecting it to a vacuum pump, and attaching a funnel to the other end. The stationary phase is added under vacuum, tapping the column while filling until no more stationary phase enters. Once filled, the column is plugged with glass wool. Column packing requires uniform and tight packing, with no gaps.

26. Why do newly installed chromatographic columns need to be aged for a period of time before use? Answer: After connecting a newly installed chromatographic column to the instrument, it should first be pressure tested and leak tested, then purged with carrier gas at a constant temperature for several hours before being subjected to analysis. This is generally referred to as the column aging process. The purpose of aging is to remove residual solvents, low-boiling point impurities, and low-molecular weight stationary phases, thereby straightening the recorder baseline. The aging temperature also allows the stationary phase to redistribute on the support surface, resulting in a more even and secure coating. After a period of aging, the column efficiency and performance of the installed chromatographic column have stabilized before use.

27. What are the symptoms of a failed chromatographic column? What is the cause of its failure? Answer: The main manifestations of chromatographic column failure are poor chromatographic separation and significantly shortened component retention times. The main reasons for chromatographic column failure are: for gas-solid chromatography, it is the reduced activity or adsorption properties of the stationary phase; for gas-liquid chromatography, it is the gradual loss of the stationary liquid during use. 28. Aging of capillary columns : The purpose of aging: The stationary phase of a gas chromatography column is usually distributed in the form of a coating on the inside of the column tube wall (capillary column) or on the surface of the carrier (packed column). For a new gas chromatography column, the outer stationary phase is often weakly bonded to the carrier. When used at high temperatures, it will slowly lose, causing baseline fluctuations and increased noise. To avoid this phenomenon, it can be preheated at a higher temperature (generally the column's tolerance temperature) for a period of time to evaporate the weakly bound stationary phase, thereby preventing interference with subsequent analysis. In addition, aging operations can be performed on gas chromatography columns that have been used for a long time to remove residual contaminants in the column.

Raise the column temperature to a constant temperature, usually its upper temperature limit. In special circumstances, the temperature may be raised to approximately 10-20°C above the operating temperature. However, the column temperature limit must not be exceeded, as this can damage the column. Also, avoid setting the temperature ramp too slowly. Once the aging temperature is reached, record and observe the baseline. Zoom in on the baseline for easier observation. Initially, the baseline should continue to rise, then begin to decline 5-10 minutes after reaching the aging temperature, continuing for 30-90 minutes. Once it reaches a fixed value, the baseline will stabilize. If the baseline remains stable after 2-3 hours or shows no clear downward trend after 15-20 minutes, there may be a leak or contamination in the system. In such cases, immediately reduce the column temperature to below 40°C, inspect the system, and address any issues as soon as possible. Continuing the aging process will not only damage the column but also prevent a stable baseline from being achieved. Furthermore, the aging time should be limited, as this will shorten the column's lifespan. Generally speaking, columns with polar stationary phases and thicker coatings require longer conditioning times, while columns with less polar stationary phases and thinner coatings require shorter conditioning times. Conditioning methods for PLOT columns vary; please refer to the column's operating instructions for specific steps. If conditioning is performed before the column is connected to a detector, the distal end of the column may be damaged. It is important to trim the distal 10-20 cm of the column before connecting it to the detector. The temperature limit is the temperature range within which the column can operate normally. Operating below the lower temperature limit will result in suboptimal separation and peak shape, but this does not harm the column itself. The upper temperature limit typically has two values. The lower value is the isothermal limit. At this temperature, the column can be operated normally without a specific duration limit. The higher value is the programmed temperature limit. This temperature limit is typically maintained for no more than ten minutes. Operating above the upper temperature limit will shorten the column's lifespan.

29. Troubleshooting Baseline Drift: Baseline drift often occurs when using programmed temperature in a GC. This phenomenon is typically caused by the following: column bleed, injection pad bleed, injector or detector contamination, and changes in gas flow rate. If a high-sensitivity detector is used, even slight column bleed or system contamination can cause significant baseline drift. To improve the reliability of qualitative and quantitative analysis, baseline drift should be minimized or eliminated as much as possible. To determine the source of baseline drift: First, remove the column from the chromatograph, block the detector inlet, and then observe baseline drift during the programmed temperature increase. If the baseline is unstable, then the contamination comes from the detector (for solutions, please refer to "How to reduce detector contamination"); if the baseline is stable, it proves that the detector is in good condition. At this time, use a short piece of fused quartz tubing to connect the injector and the detector, perform a temperature ramp program, and observe the baseline drift. This reflects the contamination of the injector. If the baseline is unstable, it can be determined that the problem comes from the injection port (for solutions, please refer to "How to reduce injector contamination"); if the baseline is stable, it proves that neither the detector nor the injection port is contaminated. At this time, reinstall the column and perform the same temperature ramp program to determine whether the baseline drift is caused by column loss.

30. How to reduce the baseline drift caused by samples and injectors? If there are high-molecular-weight non-volatile substances remaining on the chromatographic column, baseline drift is likely to occur during programmed temperature increase. This is because these substances are strongly retained and move slowly in the column. You can use the re-aging method to drive these strongly retained components from the column, but this method increases the possibility of oxidation of the stationary liquid. In addition, you can also use solvent to flush the chromatographic column (please read the column usage precautions before flushing to select a suitable solvent); you can also install a guard column to prevent problems from occurring. If the baseline drift is caused by contamination of the injector, it can be solved by replacing the injection pad, liner and sealing ring, and flushing the injection port with solvent. After maintenance, use a section of fused quartz tubing to connect the injector and detector, and inject a blank sample to confirm that the injector is clean.

31. How can I reduce detector baseline drift? Detector baseline drift is often caused by small amounts of hydrocarbons in the makeup gas or fuel gas. Using a high-purity gas purifier to treat the makeup gas or fuel gas can reduce this baseline drift. Using a high-purity gas generator can improve FID baseline stability. Proper detector maintenance, including regular cleaning, can also reduce this drift.

32. How can I reduce baseline drift caused by column bleed? Before using a new column, condition it as follows to minimize column bleed: condition it at a temperature 20°C above the experimental operating temperature or at the column's operating temperature (whichever is lower). Longer, lower-temperature aging is more effective than shorter, higher-temperature aging. If the carrier gas contains small amounts of oxygen or moisture, or if there are leaks in the gas line, the stationary phase can easily oxidize at high temperatures, causing column bleed and baseline drift. Once the stationary phase oxidizes, it must be conditioned for several hours with high-purity carrier gas to level the baseline. This damage to the stationary phase is irreversible, so if oxygen continues to pass through the column, the baseline will not level even after aging. Therefore, during experiments, use a high-quality oxygen/moisture filter in the gas line and conduct rigorous leak detection with a high-quality electronic leak detector.

33. No peak 1. FID detector flame is extinguished 2. The vaporization degree of the injector is too low, and the sample cannot be vaporized 3. The column temperature is too low, causing the sample to condense in the chromatographic column 4. The injection port is leaking 5. The column inlet is leaking or blocked 6. There is a problem with the injection needle, and the sample cannot be taken.

34. All component peaks are small or becoming smaller. Possible causes and recommended measures: 1. Defective injection needle. Use a new needle. 2. Leakage after injection. Determine the leak point. 3. Split ratio is too large. 4. The molecular weight of the analyte is too large. Increase the injection port temperature. 5. The NPD is covered by contaminants (silica). Replace the bead. 6. The NPD temperature is too high (operating or ambient temperature). The gas is impure. Replace the bead. Avoid high temperature operation. 7. The detector is not compatible with the sample.

35. Peak extension 1. Peak extension is often caused by column overload. Reduce the injection volume and use a larger capacity column. 2. Increase the OVEN and INJ temperature. 3. Increase the carrier gas flow rate. 4. Master the injection technique. 5. The previous sample agglomerated in the column and was not fully discharged in time. 6. The sample reacted with the stationary phase carrier.

36. Peak height and peak area are not repeatable 1. Injection is not repeatable, with large deviation 2. Peak misalignment caused by other peak shape changes 3. Baseline interference 4. Changes in instrument system parameter settings, parameter standardization, and normalization 5. Changes in chromatographic column performance

37. Poor sensitivity repeatability during continuous injection. Under continuous injection conditions, the peak area fluctuates, and the measurement accuracy is low. The reasons are as follows: 1. Poor injection technique 2. Carrier gas leak or unstable flow rate 3. Detector contamination 4. Column or liner contamination. Clean the liner and the column with solvent (high-grade methanol): replace it (if necessary) 5. Syringe leakage 6. Injection volume exceeds the linear range of the detector, causing detector overload.

38. Peak tailing 1. The liner or column is contaminated or improperly installed. Dead volume exists. If the peak tails when injecting methane, reinstall it. 2. The injector temperature is too high. 3. The column head is uneven. Cut it with diamond. 4. The polarity of the stationary phase does not match the sample. Replace the column with a matching one. 5. There is a cold well in the sample flow path. Eliminate the low temperature zone in the path. 6. There is accumulated cutting debris in the liner or column. Clean and replace the liner. Cut off 10 cm of the column head. 7. The injection time is too long. 8. The split ratio is low. Increase the split ratio (at least greater than 20/1). 9. The injection volume is too high. Reduce the injection volume or dilute the sample.

39. Decreased separation 1. Column contamination 2. Stationary phase damage (column bleed) 3. Injection failure Check for leaks 4. Check temperature adaptability and liner 5. Sample concentration is too high, dilute, reduce injection volume, use a high split ratio

40. Solvent peak broadening 1. Column installation failure 2. Injection leakage 3. High injection volume Increase the vaporization temperature 4. Low split ratio Increase the split ratio 5. Low column temperature 6. Initial OVEN too high during split injection Lower the initial column temperature and use a high-boiling-point solvent 7. Purge time too long (splitless injection) Define a short purge procedure

41. Baseline drifts downward 1. A newly installed column has a baseline that continues to drift upward for several minutes. Continue aging. 2. The detector has not reached equilibrium. Extend the detector equilibrium time. 3. Deposits have been baked out of the detector or other parts of the GC system. Clean them.

42. Baseline drifts upward 1. The stationary phase of the chromatographic column is damaged 2. The carrier gas flow rate decreases, adjust the carrier gas pressure

43. Noise 1. The capillary column is inserted too deep into the detector. Reinstall the column. 2. Baseline noise is caused by gas leakage when using ECD or TCD. Check and repair the gas line. 3. Improper gas flow rate or gas selection for FID, NPD, or FPD. Adjust the flow rate of high-purity gas. 4. The inlet is contaminated. Clean the inlet, replace the shelf, and replace the glass fiber in the liner. 5. The capillary column is contaminated. Cut off 10 cm of the first end, clean the column with solvent, and replace it. 6. The detector is malfunctioning.

44. Several Methods for Improving Separation: 1. Increasing column length can increase separation. 2. Reducing injection volume (increasing solvent volume for solid samples). 3. Improving injection technique to prevent double injections. 4. Reducing carrier gas flow rate. 5. Lowering column temperature. 6. Raising vaporizer temperature. 7. Reducing system dead volume, for example, ensuring column connections are fully inserted and using a splitless vaporizer for splitless injection. 8. Splitting capillary columns and selecting an appropriate split ratio are essential. In summary, this should be explored experimentally based on specific circumstances. For example, reducing carrier gas flow rate or column temperature can broaden chromatographic peaks, so adjusting conditions should be tailored to the peak shape. The ultimate goal is to achieve good separation and rapid peak elution.

45. How can I determine if a column is fully aged? The FID detector is best used to monitor the baseline during column aging. At the end of the temperature ramp, the baseline will increase, then gradually decrease and level off, at which point the column is considered fully aged. When the column is exposed to high temperatures, column life decreases dramatically. If significant column bleed persists for more than two hours during column aging, cool the column to room temperature and identify sources of bleed, such as oxygen infiltration, septum leaks, and instrument residue. Column bleed: After aging the column, perform a column bleed test. Run a temperature ramp without injecting a sample, starting at 50°C and increasing the temperature at 10°C/min to the column's maximum operating temperature. Hold at the maximum temperature for 10 minutes. The resulting chromatogram is the column bleed plot. Compare this plot to future blank runs. If numerous peaks are generated during the blank run, column performance has changed. This could be due to oxygen in the carrier gas or sample carryover. If you have a GC-MS, typical ion bleed from a low-polarity column (e.g., DB/HP-1 or 5) will have mass/charge ratios of m/z 207, 73, 281, 355, and so on. Most of these are cyclosiloxanes. Column bleed is generally considered to cause noise and an unstable baseline. True column bleed often manifests as a positive drift that resembles noise. Check for a significant upward baseline drift and for peak elution in the blank.

46. Analysis of Causes of Flame Ionization Detector (FID) Flame Extinction or Failure to Ignite: ① Condensation. Because water is formed during the FID combustion process, the detector temperature must be maintained above 100°C to prevent condensation. If the detector has not been powered on for an extended period, a long bakeout period is required before ignition. ② Column flow rate is too high. If a large inner diameter column must be used, reduce the carrier gas flow rate long enough to allow the FID to ignite. ③ Check that the installed nozzle type is suitable for the column being used and check for nozzle blockage.

47. Gas Cylinders and Their Use

1. Gas cylinders are high-pressure containers for storing compressed gas. Their capacity is generally 40 to 60 L , with a maximum working pressure of 15 MPa (150 atm) and a minimum of more than 0.6 MPa (6 atm) . Standard high-pressure gas cylinders are manufactured in accordance with national standards and should have the following markings on the cylinder shoulder: manufacturer, manufacturing date, cylinder model and number, cylinder weight, gas volume, working pressure, hydrostatic test pressure, hydrostatic test date, and next inspection date.

Because gas cylinders contain high pressures and some gases are toxic, flammable, or explosive, to ensure safety and prevent confusion among various cylinders, they must be painted a specific color and clearly indicate the name of the gas inside. See Table 1-3 for the markings on various gas cylinders.

Table 1-3 Markings of various gas cylinders

Gas type	Bottle color	Marking color	Typeface	Belt color
Nitrogen	black	yellow	Nitrogen (N 2 )	Brown
oxygen	Sky Blue	black	Oxygen (O 2 )
hydrogen	dark green	red	Hydrogen (H 2 )	red
Compressed air	black	white	Compressed air
Acetylene	white	red	Acetylene (C 2 H 2 )	green
Crude argon	black	white	Crude argon	white
Pure argon	Ash	green	Pure argon

2. Precautions for using steel cylinders (1) Steel cylinders must be regularly sent to the relevant departments for inspection. Only those that pass the inspection can be filled. Steel cylinders filled with general gases must be inspected once every three years, and steel cylinders filled with corrosive gases must be inspected once every two years. (2) When transporting steel cylinders, wear the cylinder cap and the upper and lower rubber waistbands, and handle them with care. Do not roll, hit, fall or vibrate violently on the ground to prevent explosion. When placing and using steel cylinders, they must be fixed with a rack or wire. (3) Steel cylinders should be stored in a cool, dry place away from heat sources, with good ventilation, and avoid open flames and sunlight. When the cylinder is heated, the gas expands and the pressure inside the cylinder increases, which can easily cause leakage or even explosion. Combustible gas cylinders and oxygen cylinders must be stored in separate rooms. Hydrogen cylinders are best placed in a dedicated small room outside the building to ensure safety. (4) When using steel cylinders, except for carbon dioxide and ammonia, pressure reducing valves are generally required. Among various pressure reducing valves, except for nitrogen and oxygen pressure reducing valves which can be used interchangeably, other ones can only be used for specified gases to prevent explosion. When installing the pressure reducing valve, it must be screwed in carefully, usually 7 turns of thread ( commonly known as 7 threads ) . Gases that are prone to polymerization ( such as acetylene and ethylene ) must have a specified storage period to avoid long-term storage. (5) For flammable gases such as hydrogen and acetylene, the valve of the cylinder is a "reverse thread" ( left-hand thread ) , that is, tightened in the counterclockwise direction; for non-flammable or combustion-supporting gases such as oxygen and nitrogen, the valve of the cylinder is a "right-hand thread " , that is, tightened in the clockwise direction. (6) Never allow oil, grease or other flammable or organic substances to stick to the oxygen cylinder, especially at the valve mouth and pressure reducing valve. Do not use cotton, linen or other materials to plug leaks to prevent accidents caused by combustion. (7) Pay attention to protecting the cylinder valve. When opening and closing the valve, first find out the direction and then rotate it slowly, otherwise the thread will be damaged. When opening the valve, people 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) Cylinders storing flammable gases must have a backfire prevention device. Some pressure reducing valves already have this device; you can also fill the gas conduit with fine wire mesh to prevent backfire; adding a liquid seal device to the gas pipeline can also effectively play a protective role. (9) Do not use up all the gas in the cylinder. Be sure to keep a residual pressure of more than 0.05 MPa ( pressure reducing valve gauge pressure ) . Flammable gases ( such as acetylene ) should have a residual pressure of 0.2 to 0.3 MPa , and hydrogen should have a residual pressure of 2 MPa to prevent danger when refilling. Cylinders should be closed after use and checked frequently. (10) Once a valve leaks, the cylinder should be moved outdoors immediately to prevent accidents indoors. 3. Precautions for using hydrogen and acetylene (1) Hydrogen. If released rapidly from a cylinder, it may catch fire even without a fire source. The explosion range of a hydrogen-air mixture is very wide. When the hydrogen content is 4.0 % to 75.6 % ( volume ratio ) , it will explode when exposed to fire. Therefore, hydrogen should be used in a well-ventilated area, or exhaust pipes should be used to exhaust indoor gas to the outside as much as possible. (2) Acetylene. It is extremely flammable and has a very high combustion temperature. It may also decompose and explode. The explosion range of acetylene mixed with air is 2.5 % to 80.5 % ( volume ratio ) . Therefore, fireworks should be strictly prohibited to prevent gas leaks.

GC Preventive Maintenance and Corrective Actions: Whenever a chromatographic system is contaminated with high-boiling-point substances, particularly at the inlet, deterioration in chromatographic performance can be expected. Analysts should perform routine instrument maintenance, including regular replacement of septa, cleaning and conditioning of the inlet liner, and, if necessary, trimming 0.5 to 1 μm of the capillary column from the inlet . If chromatographic performance degradation and ghost peaks persist, cleaning the metal surfaces of the inlet may be necessary. Capillary columns are reliable and easy to use, but to ensure good separation performance, specific procedures require attention:

(1) The contact between the capillary column and the chromatographic furnace wall can affect the chromatographic performance and column life;

(2) Care should be taken to prevent oxygen from entering the capillary column;

(3) The injection septum can be replaced only after the chromatographic oven has cooled down;

(4) Before reheating the chromatographic furnace, the chromatographic column should be flushed with carrier gas for 15 min ;

(5) A deoxidation tube should be used to remove trace oxygen in the carrier gas, and the deoxidation tube should be replaced regularly;

(6) Regardless of whether the chromatographic furnace is heating or not, carrier gas must flow through the chromatographic column.

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