Accurate determination system and method for PDHID gas chromatography detection limit

Abstract

The invention discloses an accurate determination system and method for PDHID gas chromatography detection limit, and relates to the field of gas chromatographs, the determination system comprises a gas inlet source, a dynamic dilution module, an automatic sample introduction module and a PDHID detection module which are sequentially arranged along a flow path direction; the inlet gas source comprises a carrier gas and diluent gas source, a discharge gas source and a gas standard substance gas source, the gas standard substance gas source is connected with the first mass flow controller through a pipeline, the carrier gas and diluent gas source is connected with the second mass flow controller through an arranged zero dead volume four-way joint, and the discharge gas source is connected with the second mass flow controller through a pipeline. The other two outlets of the zero dead volume four-way valve are respectively connected with a purging input end and a carrier gas input end of the six-way sample injection valve; and the discharge gas source is connected with the PDHID detection module through a pipeline. High-purity 6N helium of the same gas source is adopted as carrier gas and diluent gas, the phenomenon of background inconsistency can be eliminated, and matrix interference caused by different backgrounds after helium of different sources is purified is avoided.

Description

Technical Field

[0001] This invention relates to the field of gas chromatography, and more specifically to an accurate system and method for determining the detection limit of PDHID gas chromatography. Background Technology

[0002] The Pulsed Discharge Helium Ionization Detector (PDHID) is a core detector in current gas chromatography analysis used to determine trace to ultra-trace impurities in high-purity gases, hydrogen fuels, electronic gases, and various specialty gases. It uses pulsed discharge to generate high-energy metastable helium atoms, which ionize impurity molecules eluting from the column through the Penning ionization effect. It boasts significant advantages such as high versatility, extremely high sensitivity, no need for auxiliary gases, and low operating costs. As a non-destructive concentration detector, it is particularly suitable for the analysis of permanent gases and has become an indispensable tool for gas purity analysis in fields such as semiconductors, photovoltaics, aerospace, scientific research, and high-end manufacturing.

[0003] Determining the limit of detection (LOD) is essential when validating any analytical method, especially in trace analysis. The LOD is a fundamental indicator for evaluating analytical method performance, assessing instrument capabilities, and ensuring data reliability. Currently, in the field of pulsed discharge helium ionization gas chromatography (GC-PDHID), the practice of determining the LOD commonly borrows from or directly adopts classic methods from general chromatographic analysis, primarily including: the signal-to-noise ratio method, the blank standard deviation method, and the calibration curve extrapolation method. When these general methods are directly applied to the specific and demanding analytical scenario of GC-PDHID, their inherent defects and limitations become particularly prominent, resulting in the measured "LOD" often failing to accurately and reliably reflect the actual detection capability of the GC-PDHID system. The specific drawbacks and limitations of the aforementioned general methods are as follows: the signal-to-noise ratio method is overly dependent on baseline noise; the blank standard deviation method cannot effectively isolate and quantify system background and residual interference; the calibration curve extrapolation method ignores the nonlinear response and minimum detection signal characteristics in the ultra-low concentration range; and methods based on statistical models are overly simplified and not correlated with the actual decision-making risks of PDHID analysis.

[0004] In summary, the current field of pulsed discharge helium ionization gas chromatography lacks a detection limit determination system and method specifically designed for its technical characteristics, which is statistically rigorous, procedurally objective, and can truly reflect the method's reliable detection capability in actual sample analysis. This invention aims to fill this technological gap, providing a completely new solution and offering solid technical support for the accurate quantification of trace impurities in high-purity gases. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an accurate determination system and method for the detection limit of PDHID gas chromatography, so as to solve existing problems such as calculation based on a single noise, system background and residual interference, and nonlinear response in the low concentration range.

[0006] The objective of this invention is achieved through the following technical solution: First aspect: An accurate determination system for the detection limit of PDHID gas chromatography includes an inlet gas source, a dynamic dilution module, an automatic injection module, and a PDHID detection module arranged sequentially along the flow path. The inlet gas source includes a carrier gas and dilution gas source, a discharge gas source, and a gaseous standard substance source. The dynamic dilution module includes a first mass flow controller, a second mass flow controller, and a static mixer. The automatic injection module includes a six-way injection valve, which includes a carrier gas input end, a sample input end, a carrier gas output end, a sample vent end, a purge input end, and a purge vent end. The gas standard material source is connected to the first mass flow controller via a pipeline. The carrier gas and dilution gas sources are connected to the second mass flow controller via a zero dead volume four-way valve. The other two outlets of the zero dead volume four-way valve are connected to the purge input and carrier gas input of the six-way injection valve, respectively. The discharge gas source is connected to the PDHID detection module via a pipeline.

[0007] Furthermore, a three-way connector is provided between the static mixer and the first mass flow controller and the second mass flow controller. The outlet ends of the first mass flow controller and the second mass flow controller are respectively connected to the inlet end of the static mixer through the three-way connector. The outlet end of the static mixer is connected to the sample input end of the six-way injection valve.

[0008] Furthermore, a high-precision digital pressure gauge, a third mass flow controller, and a first back pressure valve are also provided in the flow path between the static mixer and the sample input end of the six-way injection valve. The first back pressure valve and the high-precision digital pressure gauge are used to stabilize the inlet pressure and set flow rate of the third mass flow controller to ensure that the flow rate of samples with different concentrations is consistent.

[0009] Furthermore, the automatic injection module also includes an electronic pressure control system (EPC), a quantitative loop, a second back pressure valve, and a capillary column. The EPC is located between the carrier gas input terminals of the zero dead volume four-way and six-way injection valves and is used to control the carrier gas flow rate entering the six-way injection valve. The two ends of the quantitative loop are connected to the six-way injection valve and are used to control the injection volume. The input terminal of the capillary column is connected to the carrier gas output terminal of the six-way injection valve, and the output terminal of the capillary column is connected to the PDHID detection module. The second back pressure valve is connected to the sample vent terminal of the six-way injection valve and is used to adjust and control the sample pressure within the quantitative loop to ensure consistency with the pressure within the capillary column.

[0010] Furthermore, a fourth mass flow controller is provided in the flow path between the purge input end of the zero dead volume four-way and six-way injection valve to control the helium flow rate of the purge six-way injection valve body, which is fixed at 5 mL / min.

[0011] Furthermore, the automatic injection module controls the sample purging and injection actions of the six-way injection valve through a chromatography workstation.

[0012] Furthermore, the PDHID detection module includes a flow-limiting tube and a detector arranged sequentially along the flow path; the flow-limiting tube is used to control the flow rate of the discharge gas, which is fixed at 30 mL / min; the detector is a pulsed discharge helium ionization detector (PDHID), a flame hydrogen ionization detector (FID), a thermal conductivity detector (TCD), an electron capture detector (ECD), or a sulfur chemiluminescence detector (SCD).

[0013] Furthermore, the gas standard material source is connected to the first mass flow controller of the dynamic dilution module via a high-precision pressure reducing valve. High-precision pressure reducing valves and high-pressure helium purifiers are installed in the flow paths between the carrier gas and dilution gas sources and the zero dead volume four-way valve, and between the discharge gas source and the PDHID detection module. A needle valve is installed in the flow path between the zero dead volume four-way valve and the second mass flow controller of the dynamic dilution module to stably control the pressure and flow rate of the dilution gas entering the dynamic dilution module.

[0014] The second aspect: An accurate method for determining the detection limit of PDHID gas chromatography includes the following steps: Step 1: Using high-purity helium as the carrier gas and dilution gas, and methane CH4 in helium as the gas standard, run the PDHID gas chromatography detection limit accurate determination system as described in any of the first aspects for more than 24 hours under the set conditions. Step 2: Obtain the CH4 chromatograms and corresponding response values ​​for the first concentration gradient within the 0-10 ppm range. Based on different CH4 concentration values ​​and corresponding response values, fit a least-squares linear regression curve to obtain a binary linear equation, thus obtaining the first regression curve. The response value is the peak area or peak height data. Calculate the detection limit estimate LOD_1 based on the residual standard deviation and slope of the first regression curve: LOD_1 = 3.3×(SD_Residuals_1) / s_1, where SD_Residuals_1 is the standard deviation of the regression curve residuals, and s_1 is the slope of the regression curve. Step 3: Based on the detection limit estimate LOD_1, obtain the CH4 chromatogram and corresponding response value for the second concentration gradient in the range of 0-10 ppm, and obtain the second regression curve using the same method as in Step 2; Step 4: Obtain the predicted value for each concentration point based on the second regression curve. The upper and lower limits of the predicted value for each concentration point are calculated using the following formula:

[0015] To predict the upper and lower limits, Let be the two-tailed inverse function of the student t-distribution, where α = β = 0.05 and the degrees of freedom n = N - 2; Step 5: Fit the upper and lower prediction limit response values ​​to the concentration values ​​using a quadratic polynomial to obtain the upper prediction limit curve and the lower prediction limit curve, respectively. Both the upper and lower prediction limit curves are quadratic polynomial equations. Calculate the intersection point y_up of the upper prediction limit function curve on the Y-axis, substitute the y_up value into the lower prediction limit function formula, and use a univariate solution function to obtain the solution, which is the detection limit.

[0016] Furthermore, obtaining the CH4 chromatogram and corresponding response value for the first concentration gradient in the range of 0-10 ppm specifically includes the following steps: preparing a CH4 / He gas standard material of 10.090 ppm by gravimetric method, mixing it with high-purity helium gas at different dilution ratios through a dynamic dilution module to obtain sample gas at least 5 different concentration points, injecting each concentration point multiple times consecutively, taking 5 consecutive data, and obtaining chromatograms and peak area or peak height information for different concentrations.

[0017] The beneficial effects of this invention are: 1) This invention proposes a gas online dilution-automatic injection-gas chromatography-pulse discharge helium ionization detector system, which can achieve highly sensitive detection of trace to ultra-trace impurities in high-purity gases, hydrogen fuels, electronic gases, and various specialty gases. In the system design, high-purity 6N helium from the same source is used for both the carrier gas and dilution gas, eliminating background inconsistencies and avoiding matrix interference caused by different background levels after purification of helium from different sources. A back pressure valve (BPR2) is connected to the sample vent end to adjust the sample pressure within the control loop, ensuring that the sample pressure is consistent with the pressure within the chromatographic column. This eliminates baseline fluctuations and stray peaks caused by minute pressure differences during injection, thus preventing interference with sample analysis.

[0018] 2) This invention also proposes a method for calculating the detection limit based on gas sample chromatographic data. Through two steps, it can focus on the linear range close to the detection limit concentration, avoiding overestimation of the calculated detection limit value. The detection limit obtained by the method of this invention fully considers the prediction results and uncertainties of the linear regression curve, as well as the calculated false positive probability α and false negative probability β. The detection limit obtained by the method of this invention is more statistically significant. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of an accurate determination system for the detection limit of PDHID gas chromatography; Figure 2 Gas chromatogram of CH4 / He gas standard at 0.007 ppm. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] See Figures 1-2 The present invention provides a technical solution: Example 1:

[0022] An accurate determination system for the limit of detection in PDHID gas chromatography, such as Figure 1As shown, the system includes an inlet gas source, a dynamic dilution module, an automatic sample injection module, and a PDHID detection module arranged sequentially along the flow path. The inlet gas source includes a carrier gas and dilution gas source, a discharge gas source, and a gaseous standard substance source. The dynamic dilution module includes a first mass flow controller, a second mass flow controller, and a static mixer. The automatic sample injection module includes a six-way injection valve, which has a carrier gas input end, a sample input end, a carrier gas output end, a sample vent end, a purge input end, and a purge vent end. The carrier gas and dilution gas use high-purity 6N helium from the same gas source, and the discharge gas source also uses high-purity 6N helium.

[0023] The gas standard material source is connected to the first mass flow controller via a pipeline. The carrier gas and dilution gas sources are connected to the second mass flow controller via a zero dead volume four-way valve. The other two outlets of the zero dead volume four-way valve are connected to the purge input and carrier gas input of the six-way injection valve, respectively. The discharge gas source is connected to the PDHID detection module via a pipeline.

[0024] In this embodiment, a three-way connector is provided between the static mixer and the first mass flow controller and the second mass flow controller. The outlet ends of the first mass flow controller and the second mass flow controller are respectively connected to the inlet end of the static mixer through the three-way connector. The outlet end of the static mixer is connected to the sample input end of the six-way injection valve.

[0025] The flow path between the static mixer and the sample input end of the six-way injection valve is also equipped with a high-precision digital display pressure gauge, a third mass flow controller, and a first back pressure valve. The first back pressure valve and the high-precision digital display pressure gauge are used to stabilize the inlet pressure and set flow rate of the third mass flow controller to ensure that the flow rate of samples with different concentrations is consistent.

[0026] In this embodiment, the automatic sample injection module further includes an electronic pressure control system (EPC), a quantitative loop, a second back pressure valve, and a capillary column. The EPC is positioned between the carrier gas input terminals of the zero-dead-volume four-way and six-way injection valves, and is used to control the carrier gas flow rate entering the six-way injection valve. The two ends of the quantitative loop are connected to the six-way injection valve to control the injection volume. The input terminal of the capillary column is connected to the carrier gas output terminal of the six-way injection valve, and the output terminal of the capillary column is connected to the PDHID detection module. The second back pressure valve is connected to the sample vent terminal of the six-way injection valve to adjust and control the sample pressure within the quantitative loop, ensuring consistency with the pressure within the capillary column. Because the pressure is increased, the volume of the quantitative loop cannot be too large, otherwise it will overload the detector.

[0027] In this embodiment, a fourth mass flow controller is also provided in the flow path between the purge input end of the zero dead volume four-way and six-way injection valve to control the helium flow rate of the purge six-way injection valve body, which is fixed at 5 mL / min.

[0028] In this embodiment, the automatic sample injection module controls the sample purging and injection actions of the six-way injection valve through a chromatography workstation.

[0029] In this embodiment, the PDHID detection module includes a flow-limiting tube and a detector arranged sequentially along the flow path; the flow-limiting tube is used to control the flow rate of the discharge gas, which is fixed at 30 mL / min; the detector includes, but is not limited to, pulsed discharge helium ionization detector (PDHID), flame hydrogen ionization detector (FID), thermal conductivity detector (TCD), electron capture detector (ECD), or sulfur chemiluminescence detector (SCD), etc., and is suitable for calculating the detection limit of gas sample analysis based on gas chromatography.

[0030] In this embodiment, the gas standard substance source is connected to the first mass flow controller of the dynamic dilution module via a high-precision pressure reducing valve. High-precision pressure reducing valves and high-pressure helium purifiers are installed in the flow paths between the carrier gas and dilution gas sources and the zero dead volume four-way valve, and between the discharge gas source and the PDHID detection module. The high-pressure helium purifier purifies the input gas source into high-pressure helium. A needle valve is installed in the flow path between the zero dead volume four-way valve and the second mass flow controller of the dynamic dilution module to stably control the pressure and flow rate of the dilution gas entering the dynamic dilution module.

[0031] In the PDHID-based gas chromatography detection system provided in this embodiment, the carrier gas and dilution gas use the same gas source, which can eliminate the phenomenon of inconsistent background, i.e., the extra positive and negative peaks caused by the different impurity contents in the purified carrier gas and dilution gas. By connecting a back pressure valve (BPR2) to the sample vent end to adjust the sample pressure in the control loop and ensure that it is consistent with the pressure in the chromatographic column, baseline fluctuations and extraneous peaks caused by inconsistent pressure during injection can be eliminated, thereby eliminating interference with sample analysis. This invention ensures that baseline noise interference is minimized through innovative hardware improvements.

[0032] Example 2:

[0033] An accurate method for determining the detection limit of PDHID gas chromatography includes the following steps: Step 1: Using high-purity helium as the carrier gas and dilution gas, and using methane CH4 in helium as the gas standard, run the PDHID-based gas chromatography detection system as described in any of the first aspects for more than 24 hours under the set conditions. Step 2: Obtain the CH4 chromatograms and corresponding response values ​​for the first concentration gradient within the 0-10 ppm range. Based on different CH4 concentration values ​​and corresponding response values, fit a least-squares linear regression curve to obtain a binary linear equation, thus obtaining the first regression curve. The response value is the peak area or peak height data. Calculate the detection limit estimate LOD_1 based on the residual standard deviation and slope of the first regression curve: LOD_1 = 3.3×(SD_Residuals_1) / s_1, where SD_Residuals_1 is the standard deviation of the regression curve residuals, and s_1 is the slope of the regression curve. Step 3: Based on the detection limit estimate LOD_1, obtain the CH4 chromatogram and peak area of ​​the second concentration gradient in the range of 0-10 ppm, and obtain the second regression curve using the same method as in Step 2; Step 4: Obtain the predicted value for each concentration point based on the second regression curve. The upper and lower limits of the predicted value for each concentration point are calculated using the following formula:

[0034] To predict the upper and lower limits, Let be the two-tailed inverse function of the student t-distribution, where α = β = 0.05 and the degrees of freedom n = N - 2; Step 5: Fit the upper and lower prediction limit response values ​​to the concentration values ​​using a quadratic polynomial to obtain the upper prediction limit curve and the lower prediction limit curve, respectively. Both the upper and lower prediction limit curves are quadratic polynomial equations. Calculate the intersection point y_up of the upper prediction limit function curve on the Y-axis, substitute the y_up value into the lower prediction limit function formula, and use a univariate solution function to obtain the solution, which is the detection limit.

[0035] Furthermore, obtaining the CH4 chromatogram and corresponding response value for the first concentration gradient in the range of 0-10 ppm specifically includes the following steps: preparing a CH4 / He gas standard material of 10.090 ppm by gravimetric method, mixing it with high-purity helium gas at different dilution ratios through a dynamic dilution module to obtain sample gas at least 5 different concentration points, injecting each concentration point multiple times consecutively, taking 5 consecutive data, and obtaining chromatograms and peak area or peak height information for different concentrations.

[0036] In one specific embodiment, within the first concentration gradient, five relatively high concentration points were selected for dilution: 1.067, 0.540, 0.215, 1.105, and 0.053 ppm. The experimental data of step two are shown in Table 1, resulting in five groups of samples with different concentrations and a calculated limit of detection (LOD_1) of 9 ppb.

[0037] Table 1

[0038] Next, within the second concentration gradient, five relatively low concentration points were selected for dilution: 0.102, 0.085, 0.051, 0.022, and 0.011 ppm. Each concentration point was injected multiple times consecutively, and five consecutive data points were collected, for a total of 25 data points (N=25). The experimental data from steps three to five are shown in Table 2. The chromatograms and peak area information for different concentrations within the second concentration gradient, the upper and lower prediction limit functions, and the final detection limit values ​​were obtained.

[0039] Table 2

[0040] The method provided by this invention can be used to calculate concentrations using built-in functions in Excel or WPS Spreadsheets, eliminating the need for complex operations and programming, making it convenient to use. The concentration ranges involved in the first and second concentration gradients do not exceed two orders of magnitude.

[0041] This embodiment only illustrates the analysis and calculation process of CH4 components. For example, using a multi-component mixed gas standard can simultaneously calculate the detection limits of multiple components; or using a single standard of other components for the experiment can also be used to analyze and calculate the corresponding detection limits according to this method.

[0042] The verification process for the detection limit is as follows: A CH4 / He gaseous standard of 0.007 ppm was generated by diluting the sample using a dynamic dilution module, and the resulting chromatogram is shown below. Figure 2 .from Figure 2 As can be seen, the baseline noise is 0.256 μV, and the baseline noise at 3.3 times is 0.845 μV; while the peak height of 0.007 ppm CH4 is 0.938 μV, slightly higher than the baseline noise at 3.3 times. This also proves that the detection limit calculation method established in this invention can be robustly applied.

[0043] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. An accurate determination system for the detection limit of PDHID gas chromatography, characterized in that, The system includes an inlet gas source, a dynamic dilution module, an automatic sample injection module, and a PDHID detection module arranged sequentially along the flow path. The inlet gas source includes a carrier gas and dilution gas source, a discharge gas source, and a gaseous standard substance source. The dynamic dilution module includes a first mass flow controller, a second mass flow controller, and a static mixer. The automatic sample injection module includes a six-way injection valve, which has a carrier gas input end, a sample input end, a carrier gas output end, a sample vent end, a purge input end, and a purge vent end. The gas standard material source is connected to the first mass flow controller via a pipeline. The carrier gas and dilution gas sources are connected to the second mass flow controller via a zero dead volume four-way valve. The other two outlets of the zero dead volume four-way valve are connected to the purge input and carrier gas input of the six-way injection valve, respectively. The discharge gas source is connected to the PDHID detection module via a pipeline.

2. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 1, characterized in that: A three-way connector is provided between the static mixer and the first and second mass flow controllers. The outlets of the first and second mass flow controllers are respectively connected to the inlet of the static mixer through the three-way connector. The outlet of the static mixer is connected to the sample input of the six-way injection valve.

3. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 2, characterized in that: The flow path between the static mixer and the sample input end of the six-way injection valve is also equipped with a high-precision digital display pressure gauge, a third mass flow controller, and a first back pressure valve. The first back pressure valve and the high-precision digital display pressure gauge are used to stabilize the inlet pressure and set flow rate of the third mass flow controller to ensure that the flow rate of samples with different concentrations is consistent.

4. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 1, characterized in that: The automatic sample injection module also includes an electronic pressure control system (EPC), a quantitative loop, a second back pressure valve, and a capillary column. The EPC is located between the carrier gas input terminals of the zero dead volume four-way and six-way injection valves and is used to control the carrier gas flow rate entering the six-way injection valve. The two ends of the quantitative loop are connected to the six-way injection valve to control the injection volume. The input terminal of the capillary column is connected to the carrier gas output terminal of the six-way injection valve, and the output terminal of the capillary column is connected to the PDHID detection module. The second back pressure valve is connected to the sample vent terminal of the six-way injection valve and is used to adjust and control the sample pressure within the quantitative loop to ensure consistency with the pressure within the capillary column.

5. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 1, characterized in that: A fourth mass flow controller is also provided in the flow path between the purge input end of the zero dead volume four-way and six-way injection valve to control the helium flow rate of the purge six-way injection valve body, which is fixed at 5 mL / min.

6. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 1, characterized in that: The automatic sample injection module controls the sample purging and injection actions of the six-way injection valve through a chromatography workstation.

7. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 1, characterized in that: The PDHID detection module includes a flow-limiting tube and a detector arranged sequentially along the flow path; the flow-limiting tube is used to control the flow rate of the discharge gas and is fixed at 30 mL / min; the detector is a pulsed discharge helium ionization detector (PDHID), a flame hydrogen ionization detector (FID), a thermal conductivity detector (TCD), an electron capture detector (ECD), or a sulfur chemiluminescence detector (SCD).

8. The accurate determination system for the detection limit of PDHID gas chromatography according to claim 1, characterized in that: The gas standard material source is connected to the first mass flow controller of the dynamic dilution module through a high-precision pressure reducing valve. High-precision pressure reducing valves and high-pressure helium purifiers are installed in the flow paths between the carrier gas and dilution gas sources and the zero dead volume four-way valve, and between the discharge gas source and the PDHID detection module. A needle valve is installed in the flow path between the zero dead volume four-way valve and the second mass flow controller of the dynamic dilution module to stably control the pressure and flow rate of the dilution gas entering the dynamic dilution module.

9. An accurate method for determining the detection limit of PDHID gas chromatography, characterized in that, Includes the following steps: Step 1: Using high-purity helium as the carrier gas and dilution gas, and methane CH4 in helium as the gas standard, run the PDHID gas chromatography detection limit accurate determination system as described in any one of claims 1-8 for more than 24 hours under the set conditions. Step 2: Obtain the CH4 chromatograms and corresponding response values ​​for the first concentration gradient within the 0-10 ppm range. Based on different CH4 concentration values ​​and corresponding response values, fit a least-squares linear regression curve to obtain a binary linear equation, thus obtaining the first regression curve. The response value is the peak area or peak height data. Calculate the detection limit estimate LOD_1 based on the residual standard deviation and slope of the first regression curve: LOD_1 = 3.3×(SD_Residuals_1) / s_1, where SD_Residuals_1 is the standard deviation of the regression curve residuals, and s_1 is the slope of the regression curve. Step 3: Based on the detection limit estimate LOD_1, obtain the CH4 chromatogram and corresponding response value for the second concentration gradient in the range of 0-10 ppm, and obtain the second regression curve using the same method as in Step 2; Step 4: Obtain the predicted value for each concentration point based on the second regression curve. The upper and lower limits of the predicted value for each concentration point are calculated using the following formula: ； To predict the upper and lower limits, Let be the two-tailed inverse function of the student t-distribution, where α = β = 0.05 and the degrees of freedom n = N - 2; Step 5: Fit the upper and lower prediction limit response values ​​to the concentration values ​​using a quadratic polynomial to obtain the upper prediction limit curve and the lower prediction limit curve, respectively. Both the upper and lower prediction limit curves are quadratic polynomial equations. Calculate the intersection point y_up of the upper prediction limit function curve on the Y-axis, substitute the y_up value into the lower prediction limit function formula, and use a univariate solution function to obtain the solution, which is the detection limit.

10. The accurate determination method for the detection limit of PDHID gas chromatography according to claim 9, characterized in that: The method for obtaining the CH4 chromatogram and corresponding response value of the first concentration gradient in the range of 0-10 ppm specifically includes the following steps: preparing a CH4 / He gas standard material of 10.090 ppm by gravimetric method, mixing it with high-purity helium gas at different dilution ratios through a dynamic dilution module to obtain sample gas at least 5 different concentration points, injecting each concentration point multiple times consecutively, taking 5 consecutive data, and obtaining chromatograms and peak area or peak height information of different concentrations.

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