1. Several concepts in gas chromatography:

Gas chromatography:

A separation technique that uses gas as the mobile phase to achieve complex sample separation based on the differences in adsorption forces between components and stationary phases. The separation object is a sample that is volatile, thermally stable, and generally has a boiling point not exceeding 500 degrees.

Gas:

Carrier gas - a gas used to transport samples through the entire system.

Detector gas - the supporting gas required for certain detectors, such as, FID。

Sample introduction:

The process of introducing sample vapor into the carrier gas. This process should have less impact on the sample vapor.

Chromatographic column:

Realize the separation of sample components.

Detector:

Identify and respond to the components of the sample flowing out of the column.

Data collection:

Convert the detector signal into a chromatogram for manual or automatic qualitative and quantitative analysis.

Why is a gas trap used in the gas path; What are the types of capture traps in GC?

Common pollutants in GC carrier gases include moisture, oxygen, hydrocarbons, and halogenated hydrocarbons, which have a significant impact on the lifespan of chromatography columns and the detection of analytes. Adverse effects include:

水分：

It is a common cause of degradation of chromatographic column stationary phase; Can damage the instrument;

Oxygen:

Common pollutants; It is a common reason for the degradation of chromatographic column stationary phase and the decrease in performance of injection port liner; Can cause the decomposition of unstable analytes;

Hydrocarbons and halogenated hydrocarbons:

Reduce detector sensitivity by increasing detector background noise; It can also cause baseline drift or fluctuation, pollutant chromatographic peaks, noise, or high baseline compensation.

Water, oxygen, and hydrocarbon capture traps are used in GC.

3. What kind of capture traps should different detectors be equipped with?

FID, NPD, FPD - air, tail gas, hydrogen → hydrocarbon trap (e.g. 5060-9096)

FID, NPD, FPD, TCD - carrier gas - water and oxygen capture trap (e.g. OT3-2)

ECD - Tail blowing → Water and oxygen capture trap (e.g. OT3-2)

TCD - Reference Gas → Hydrocarbon Capture Trap (e.g. 5060-9096)

MSD - helium, methane - combined capture trap for removing water, oxygen, and hydrocarbons (e.g. RMSH-2) and oxygen indicator capture trap (e.g. IOT-2-HP)

The above capture traps are recommended to be replaced every 6-12 months, depending on the frequency of customer use.

What consumables are involved in the injection port of GC?

Syringe, injection pad, O-ring 5188-5365, liner, flat plate (gold-plated sealing gasket), metal gasket 5061-5869, graphite sealing gasket, column nut.

Why do we need to replace the injection spacer? What are the types of spacers? How to choose sample isolation pads based on customer needs?

The spacer needs to be replaced regularly to prevent: air leakage, decomposition, sample loss, decrease in column flow or diversion flow, ghost peak, and decrease in column efficiency. Different insulation pads have different performance:

Loss and Temperature Optimization Barrier (BOT): 5183-4757

Wide temperature range, low loss → suitable for mass spectrometry, high temperature of 400 degrees at the injection port to reduce sticking at the injection port.

Long life spacer 5183-4761

Pre perforation prolongs service life and reduces nucleation; Automatic sampling with preferred isolation pad, suitable for overnight operation; Inject 400 times and reach a high temperature of 350 degrees.

Green spacer 5183-4759

The above three types of insulation pads belong to advanced insulation pads, which have been treated with plasma and do not stick to the insulation pads

Universal spacer:

The temperature is 350 degrees, with a frequency of 200 times, which is economical and economical;

6. How to choose graphite pads and column nuts?

Firstly, understand the two materials of graphite pads:

Graphite → Soft material, high temperature limit.

Vespel is made of high-temperature resistant polyimide, which is hard and prone to loss at high temperatures.

Graphite pad and column nut:

Short pad with short cap, long pad with long cap (e.g. MS interface column nut 05988-20066 should be equipped with pre aged 85% vespel, 15% graphite pad long pad, universal column nut can be equipped with 85% vespel, 15% graphite pad short pad)

The selection of graphite pads should be based on the material and the size of the inner diameter of the chromatographic column. The frequency of replacement is generally when replacing the chromatographic column or when there is a leak.

What is the function of the liner and when should it be replaced? What factors should be considered when choosing a liner?

The liner is the central component of the injection system, where the sample evaporates into gas. The regular replacement of lining pipes mainly depends on the following situations: comparing the spectra previously made; The cleanliness level of the sample; Whether there are any abnormal phenomena in the chromatographic peaks, such as ghost peaks, changes in peak shape, poor reproducibility, and high-temperature decomposition of the sample.

Factors to consider when choosing a liner: injection port type/injection technique, liner volume, liner treatment or deactivation, special properties (such as quartz wool, quartz cup, fine cone, etc.)

Why use deactivated lining tubes and glass wool? What substance is used to activate lining tubes without glass wool?

The active sites on the injection port liner can adsorb sample components, causing chromatographic peak tailing, loss of sensitivity and reproducibility.

When injecting samples without splitting and analyzing slightly polar compounds, it is recommended to use deactivated liners.

Using glass wool:

A. Reduce thermal discrimination and provide sufficient surface area for sample evaporation

B. Capture non-volatile components and spacer debris to prevent them from entering the chromatographic column

C. Clean the sample from the syringe needle to improve reproducibility and avoid residual samples on the septum.

The compounds used for deactivation without glass wool lining include phenols, organic acids, pesticides, amines, abused drugs, reactive polar compounds, and thermally unstable compounds.

Non split injection mode: trace analysis, low sample concentration, low flow rate, small injection volume.

Split injection mode: high sample concentration, high flow rate, and large injection volume.

9. Fluorinated hydrocarbon O-rings and graphite O-rings

Fluorinated hydrocarbon O-ring: not easily deformed, easy to replace, mostly used in most cases

Graphite O-ring: easy to deform and peel off, used when the injection port temperature is above 350 degrees.

Replacement frequency: Both usually replace the O-ring at the same time as replacing the liner

10. How to clean the diversion plate (gold-plated sealing gasket)

A. Ultrasonic cleaning and drying in solvent;

B. Deactivation with non chlorosilane reagents: HMDS (hexamethyldisilane) or BSTFA (N, O bis (* silyl) or BSA (N, O bis (* silyl) acetamide) or TSIM, TSIM (N - * silyl imidazole)

C. Clean with solvent, first inert wash with toluene, then clean with alcohol with methanol, and dry.

What are NPD, ECD, TCD, FID, FPD detectors and what are their performance characteristics?

What are the column parameters of a chromatographic column? How to select chromatography columns for customers based on these parameters?

Column parameters for chromatography: stationary phase, column length, inner diameter, membrane thickness.

Selection of stationary phase: principle of similar phase solubility.

Non/weak polarity column - for daily use! Medium polarity column - suitable for complex/difficult separation; Strong polarity column - often used for special applications.

Column length:

Rapid analysis of simple samples with less than ten components typically ranging from 10 to 15m

A standard column length of 25-30m is sufficient for the vast majority of applications

Analysis of complex samples at 50m, 60m, 100m

Inner diameter:

0.53mm large-diameter replaceable packed column can withstand larger volume injection and trace analysis

0.32mm wide diameter, split/non split injection, able to withstand larger volume injection

0.25mm narrow diameter split injection, GC/MS application, high column efficiency

0.18mm micro diameter for fast separation and high column efficiency

0.10mm fast GC, fast separation requires high instrument requirements

Film thickness:

0.10um film with low retention and column capacity, suitable for high boiling point compounds, component dense samples, or thermosensitive compounds. Standard film thickness of 0.25-0.50um is widely used

1.0-10.0um thick film retention and high column capacity suitable for low boiling point volatile compounds

Thick film is beneficial for masking active sites, but there is significant column loss at high temperatures

What do the temperature limits of chromatography columns, such as 60 ℃ to 240 ℃/260 ℃, represent?

60 ℃ - lower temperature limit. Using the column below this temperature will reduce column efficiency, but it will not damage the chromatographic column

240 ℃ - upper limit of constant temperature, can be used for a long time at this temperature

260 ℃ - upper limit of programmed heating temperature, cannot exceed this temperature, and should not be used for more than 10 minutes at this temperature

14. Understand several parameters related to liquid chromatography columns, as well as the classification and application scope of Agilent chromatography columns.

Surface area - the sum of the outer surface and inner pore surface of the particle, expressed in m2/gram;

High surface area has strong retention ability, column capacity, and separation degree for the separation of multi-component samples;

Fillers with low surface area can often quickly reach equilibrium, which is particularly important for gradient leaching

Aperture - the average size of a particle's pores or cavities, ranging from 60-10000 Å

Large pore packing particles can prolong the residence time of solute macromolecules on the packing surface, achieve sufficient separation, and improve peak shape; Sample MW ≤ 4000, choose a pore size of 80 Å, sample MW>4000, choose a pore size of 300 Å
Carbon coverage rate - the impact on chromatographic separation, the amount of bonded phase attached to the matrix material.

High carbon coverage: improve resolution, long analysis time; Low carbon coverage: Shortening operating time

End capping/tail capping - the impact on chromatographic separation, using shorter alkane chains to bond free silanol groups (secondary bonding)

End capping: Reduce the tailing phenomenon of chromatographic peaks caused by the reaction between the tested component and the residual acidic silanol groups on the surface of silica gel

For polar samples, there is a significant difference in selectivity between chromatography columns that have not been capped and those that have been capped

Particle size -1.8um3.5um → rapid separation

5um → Industry Standard

7um → Preparation Column

15. How to rinse the liquid chromatography column?

For the flushing of reverse phase chromatography columns:

Rinse the chromatography column (analytical column) with at least 25mL of each of the following solvents

Mobile phase without buffering salt

100% methanol, 100% acetonitrile, 75% acetonitrile+25% isopropanol, 100% isopropanol, 100% dichloromethane, 100% hexane

*If hexane or dichloromethane is used to rinse the chromatography column, it must be rinsed with isopropanol before reusing the reverse phase mobile phase!!!

For the flushing of the normal phase chromatography column:

Rinse the chromatography column (analytical column) with at least 50mL of each of the following solvents

50% methanol+50% trichloromethane

100% ethyl acetate

16. How to age gas chromatography columns?

After installing the chromatography column, first blow the column with a carrier gas at least three times the normal working flow rate, then adjust the carrier gas flow rate to the normal flow rate, and start heating rapidly from 100 degrees to the aging temperature, aging the chromatography column.

Aging temperature - rapidly aging a new capillary chromatography column by maintaining a constant temperature for 2 hours at a temperature 20 degrees higher than the final temperature of the sample analysis but not exceeding the upper limit of the constant temperature of the chromatography column. There is no limit to the heating rate during the aging of the chromatography column, and the heating rate aging chromatography column can be used.

What are the important components of liquid chromatography? What are the detectors for liquid phase?

Degassing machine, pump, column temperature box, automatic sampler, detector. The detectors include:

VWD variable wavelength ultraviolet detector, DAD diode array detector, MWD multi wavelength detector RID、 Differential refractive index detector, FLD fluorescence detector.

18. How to choose a solvent inlet filter head?

The solvent filter heads used for conventional and capillary liquid phases include: glass filter head with a pore size of 20 μ m (5041-2168), PTFE connector head with a pore size of 5mm/3.2mm (5062-8517), and stainless steel filter head with a pore size of 12-14 μ m (01018-60025)

The solvent filter heads used for preparing liquid phase include: glass filter head with a pore size of 40 μ m (3150-0944), PTFE connector head with a pore size of 7mm/4mm (G1361-23204)

If the mobile phase involves the following solvents, stainless steel filter heads should be avoided: lithium iodide, high concentrations of nitric acid and sulfuric acid, organic acid solutions in organic solvents such as methanol, 1% acetic acid will corrode stainless steel, carbon tetrachloride and isopropanol or THF mixture, etc.

Daily cleaning of glass filters can be done by soaking them in 35% nitric acid for 1 hour (without ultrasonic cleaning, it may cause breakage)

19. Selection guide for circulation pool

Capillary liquid chromatography column (inner diameter 0.3 or 0.5mm) with a 500nL flow cell;

Micro diameter or solvent saving liquid chromatography column (inner diameter 1.0 or 2.1mm) semi flow cell;

Traditional liquid chromatography columns (inner diameter 3.0 or 4.6mm) with standard flow cell back pressure higher than 100 bar and high-pressure flow cell shape; Sample MW ≤ 4000, choose a pore size of 80 Å, sample MW>4000, choose a pore size of 300 Å;

Carbon coverage rate - the impact on chromatographic separation, the amount of bonded phase attached to the matrix material.

High carbon coverage: improve resolution, long analysis time; Low carbon coverage: Shortening operating time

The impact on chromatographic separation, using shorter alkane chains to bond free silanol groups (secondary bonding)

End capping: Reduce the tailing phenomenon of chromatographic peaks caused by the reaction between the tested component and the residual acidic silanol groups on the surface of silica gel

For polar samples, there is a significant difference in selectivity between chromatography columns that have not been capped and those that have been capped

Particle size: 1.8um3.5um → rapid separation

5um → Industry Standard

7um → Preparation