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How to improve the sensitivity of the detector in gas chromatography analysis experiments? Different gas chromatographs have different characteristics, and the effective methods to improve detector sensitivity vary.
Currently, research on improving detection sensitivity mainly focuses on pre-column sample treatment techniques, such as cold on-column injection, purge and trap, extraction, thermal desorption injection, etc.  
How to achieve higher sensitivity in gas chromatography? The primary method is solvent concentration. The purpose of solvent concentration is to obtain sharp peak shapes without using split injection. Only in this way can better sensitivity and resolution be achieved.   
Solvent concentration is achieved through two methods: The first is used in splitless injection and direct injection: This method requires adjusting the column oven temperature. The difference from the former is that both the solvent and the analytes must be cooled and condensed on a colder column during injection; these condensed samples form a "flooded zone" on the column surface. As the solvent gradually evaporates, the area of the flooded zone shrinks, and the concentration of analytes in the "flooded zone" gradually increases. After the solvent has completely evaporated, the analytes condense on a very small area of the column surface. The second method occurs during splitless injection: After the liquid sample evaporates, it cools and condenses on a colder column. The volume of the carrier gas is much larger than that of the liquid, so when the sample condenses, it concentrates in a small area of the column. Therefore, regardless of the method, the analytes or the solvent and analytes must condense on a small area of the column. Factors affecting the condensation rate of the sample on the column include: the initial column temperature, the volatility of the solvent, and the phase ratio of the column stationary phase.   
Initial column temperature: 1. This is the easiest and fastest method. Generally, the lower the initial temperature, the better the condensation of analytes. 2. It is recommended to set the initial temperature about 50 degrees Celsius lower than the boiling point of the fastest-eluting analyte, and the duration should ideally match the splitless hold time.   
The second important factor: Column "phase ratio": A lower phase ratio means a thicker film thickness. The thicker the film, the easier it is for the sample and solvent to dissolve in the stationary phase. To change this factor, one can only try different columns. This method is used when adjusting the initial temperature is not feasible.   
A factor that can be changed is the sample itself: Most of the time, the sample itself cannot be altered, but different solvents can be used to enhance the effect. For example, the greater the difference between the solvent's boiling point and the initial temperature, the better. For instance, when the initial column temperature is 30 degrees Celsius, using ethyl acetate (boiling point 77.1°C) as the solvent is better than using dichloromethane (boiling point 39°C).  
In gas chromatography analysis experiments, how to improve the sensitivity of FID? 1. Question: Sensitivity reflects the response of an instrument to the target component. Combined with signal-to-noise ratio or detection limit, it can evaluate the comprehensive performance of an instrument. Under the same detection limit, the higher the sensitivity, the better the instrument performance. So how to improve FID sensitivity?  
2. Analysis of reasons:  
Sensitivity: The magnitude of the electrical signal generated when a unit amount of substance passes through the detector is called the sensitivity of the detector to that substance. Plotting the response signal (R) against the injection amount (Q) yields a straight line through the origin, the slope of which is the detector sensitivity, denoted as S: S = ΔR/ΔQ   
Factors affecting FID sensitivity: Can be divided into FID internal factors (hardware) and external factors (operational). Hardware factors include nozzle aperture size, the position between the collector and polarizer, the mutual position of the polarizer and nozzle, etc. Operational factors include nitrogen/hydrogen (N2/H2) flow ratio, amplifier input resistance and output circuit attenuation value, the cleanliness of the inlet, column, gas lines, and FID nozzle, etc.;  

3. Solutions: Since the impact of FID hardware on sensitivity is largely determined when the chromatograph leaves the factory, it cannot be changed by the operator. Below, we mainly introduce how to improve FID sensitivity from an operational perspective.
① Nitrogen/hydrogen (N2/H2) flow ratio: The N2/H2 ratio significantly affects sensitivity. Due to different structural designs by manufacturers, the optimal N2/H2 ratio varies and can be determined experimentally. Generally, N2 flow should be higher than H2 flow, with an ideal range of N2:H2 from (1:1.5) to (1:1). If the nozzle aperture is φ0.4mm, the carrier gas flow can be 20-30 mL/min; if the nozzle aperture is φ6mm or larger, the flow should be around 40-50 mL/min. For capillary chromatography, the tail gas (make-up gas) not only reduces post-column diffusion but also ensures the optimal N2/H2 ratio to maintain the best sensitivity.   
② Air flow: When air flow is less than 200 mL/min, it has a certain effect on sensitivity. Generally, under conditions greater than 250 mL/min, air flow has a significant impact on detector sensitivity.   
③ Amplifier input resistance and output circuit attenuation: The amplifier input resistance determines the current amplification factor and affects FID sensitivity. Higher input resistance increases sensitivity but also increases noise. When adjusting the input resistance, the instrument's signal-to-noise ratio must be considered. The output circuit attenuation values, such as 1/10, 1/25, 1/50, vary by manufacturer. Changing or adjusting the internal attenuation can also alter FID sensitivity. For example, Varian's FID sensitivity can be set to 9, 10, 11, 12. A higher number indicates better sensitivity, with a difference of 1 representing a 10-fold change in signal. Of course, this assumes the amplifier baseline is stable.   
④ Cleanliness of the inlet, column, gas lines, and FID nozzle: Contamination of the inlet, gas lines, or FID nozzle can reduce FID sensitivity. Therefore, it is necessary to keep the inlet, column, FID nozzle, and gas lines clean during use. Regularly replace inlet septa, liners, and quartz wool, and clean the FID as needed.
4. Case analysis: When using an HP-PONA column and FID for qualitative and quantitative analysis of C1-C8 hydrocarbons, it was found that under the same conditions and injection volume, the peak area of the same substance was only 80% of the original. After checking the chromatographic lines for leaks and ruling out gas leakage, it was concluded that the decrease in peak area was due to an 80% reduction in instrument sensitivity.  
※ After testing the carrier gas nitrogen, hydrogen, air, and tail gas nitrogen used in the chromatography, it was found that the tail gas nitrogen was turned off. When using a capillary column, the carrier gas (nitrogen) flow was only 5 mL/min, while the hydrogen flow was 40 mL/min, resulting in a severe imbalance in the nitrogen/hydrogen ratio. After resetting the tail gas nitrogen flow control valve to its original position and re-measuring, the chromatographic peak area of the substance returned to normal.   
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