Method for detecting peroxide value of edible oil based on microfluidic chip

Abstract

The application discloses a microfluidic chip-based edible oil peroxide value detection method and belongs to the technical field of microfluidic chip detection. The method comprises the following steps: synchronously injecting a to-be-detected oil sample and an extraction liquid into a microfluidic chip and a parallel sample channel, forming a laminar flow contact interface, and then entering a reaction cavity; an alternating electric field is applied in the reaction cavity to make the interface periodically stretch and deform and trigger directional mass transfer of lipid peroxide to the extraction liquid phase; the extraction liquid after mass transfer is introduced into an interdigital electrode array, the impedance amplitude variation is measured by scanning the excitation frequency, the impedance integral ratio in a preset frequency band on both sides of a relaxation characteristic frequency point is extracted as a quantitative detection value; the impedance integral ratio of a blank extraction liquid is synchronously measured by using a built-in reference channel of the chip for correction, and a peroxide value detection result is output. The method combines electric field-enhanced liquid-liquid extraction with impedance spectrum relaxation characteristic correction, realizes micro-quantization, rapidness and high-sensitivity detection of the peroxide value of edible oil.

Description

Technical Field

[0001] This invention relates to the field of microfluidic chip detection technology, specifically a method for detecting the peroxide value of edible oil based on microfluidic chips. Background Technology

[0002] Edible oils undergo auto-oxidation during storage and use, generating lipid peroxides such as hydroperoxides. Peroxide value is a key indicator of the degree of oxidative rancidity in oils. Traditional peroxide value detection methods primarily rely on iodometric titration. This method requires dissolving the oil sample in an organic solvent, adding potassium iodide for a redox reaction, and then titrating the precipitated iodine with a sodium thiosulfate standard solution. This process is cumbersome, time-consuming, consumes large amounts of reagents, and uses significant amounts of organic solvents and toxic reagents, making it unfriendly to operators and the environment. To address these shortcomings, instrumental detection methods based on light absorption, fluorescence, or chemiluminescence principles have emerged. However, these methods typically rely on complex sample pretreatment procedures or require external colorimetric or luminescent markers, making it difficult to achieve direct, rapid, and minute-level detection of trace peroxides in oil samples.

[0003] Microfluidic chip technology provides a platform for miniaturized and integrated chemical analysis. Existing microfluidic edible oil detection solutions largely rely on optical colorimetry or fluorescence sensing, still facing the challenge of low mass transfer efficiency between the two phases. Because the distribution of lipid peroxides between the edible oil phase and the aqueous extract is limited by passive diffusion at a narrow laminar interface, insufficient mass transfer flux leads to limited detection sensitivity. Forcibly introducing mechanical stirring or electrothermal effects disrupts the laminar flow, causing oil-water emulsification and severely interfering with the stability of the subsequent electrochemical sensing interface, failing to meet the requirement for accurate quantification of low-concentration peroxides. Regarding the sensing mechanism, conventional impedance measurements often rely on amplitude extraction at a single frequency point or within a limited frequency band. Baseline drift caused by matrix effects and chip batch variations affects the reproducibility of quantitative results. Therefore, a peroxide value detection method is needed that enhances oil-liquid two-phase mass transfer within a microfluidic chip and establishes a robust impedance quantification correction mechanism. Summary of the Invention

[0004] This paper presents a method for detecting the peroxide value of edible oils based on a microfluidic chip. The method enhances the mass transfer of lipid peroxides by coupling the laminar flow contact interface with an alternating electric field, and combines the integral ratio of the impedance spectrum relaxation characteristics with reference correction to achieve miniaturized, highly sensitive, and highly stable peroxide value detection of edible oils.

[0005] To achieve the above objectives, the present invention provides the following technical solution: The present invention provides a method for detecting the peroxide value of edible oil based on a microfluidic chip, comprising: The oil sample to be tested and the extract are simultaneously injected into the parallel injection channel of the microfluidic chip. The flow rate ratio of the two liquid streams in the confluence zone is adjusted to form a stable laminar contact interface and introduce it into the reaction chamber. As a preferred embodiment of the present invention, the oil sample to be tested is delivered by a first injection pump at a first flow rate, and the extract is delivered by a second injection pump at a second flow rate, so that the pressure balance is achieved at the inlet of the confluence zone, thereby obtaining a clear and flat phase interface. A flow-guiding microstructure can be set at the inlet of the confluence zone in the parallel injection channel to suppress the eddy current disturbance generated when the two liquid streams merge, maintain the integrity of the laminar contact interface, and ensure the stability of the subsequent mass transfer process.

[0006] Inside the reaction chamber, an alternating electric field is applied to the laminar contact interface, causing periodic stretching deformation of the interface and triggering directional mass transfer of lipid peroxides in the oil phase to the extraction liquid phase. Preferably, a first excitation electrode and a second excitation electrode, extending along the length of the chamber and with their vertical projections overlapping, are respectively arranged on the upper and lower walls of the reaction chamber. A sinusoidal alternating voltage with a phase difference of 180 degrees is applied to the two electrodes, generating a sinusoidal gradient electric field perpendicular to the interface at the interface, forcing the lipid peroxides to detach from the oil phase matrix and cross the phase interface into the extraction liquid phase. By adjusting the frequency and amplitude of the sinusoidal alternating voltage, the deformation amplitude and mass transfer flux of the interface can be precisely controlled, achieving rapid extraction without the addition of chemical reagents.

[0007] After the extraction liquid has completed directional mass transfer, it is introduced into the surface of the interdigital electrode array in the detection area so that it completely covers the sensing area of ​​the interdigital electrode. The AC excitation signal is output step by step at logarithmic intervals from the start frequency to the end frequency. After a preset stable duration at each frequency point, the effective value of the AC voltage at the sensing end is read. Based on this, the impedance amplitude corresponding to each frequency point is calculated to obtain the frequency-impedance sequence.

[0008] The relaxation characteristic frequency point is extracted based on the correspondence between impedance amplitude change and frequency: perform a second-order difference operation on the frequency-impedance sequence, find the frequency position where the absolute value reaches the maximum value in the obtained difference amplitude sequence, and mark it as the relaxation characteristic frequency point; when multiple maxima appear, the position with the largest absolute value of the difference amplitude is taken as the relaxation characteristic frequency point.

[0009] Using the relaxation characteristic frequency point as the boundary, all frequency points within the first preset bandwidth on the low-frequency side are taken on the frequency axis to form a low-frequency integration interval, and all frequency points within the second preset bandwidth on the high-frequency side are taken to form a high-frequency integration interval. The products of impedance amplitude and frequency interval within the low-frequency integration interval and the high-frequency integration interval are summed to obtain the low-frequency integration area and the high-frequency integration area. The ratio of the low-frequency integration area to the high-frequency integration area is calculated as the quantization detection value. Preferably, both the low-frequency integration interval and the high-frequency integration interval are defined with equal width on a logarithmic frequency coordinate to highlight the difference in response of the relaxation frequency band.

[0010] Simultaneously, a blank extract without oil sample is measured through the reference channel built into the microfluidic chip: the reference channel directly takes blank extract from the extract reservoir, measures the impedance amplitude on the reference interdigital electrode array using the same scanning frequency sequence and integration processing method as the detection area, and calculates the reference impedance integral ratio of the blank extract; the quantitative detection value is subtracted from the reference impedance integral ratio to obtain the correction difference, and then the correction difference is divided by the reference impedance integral ratio, and the quotient is used as the corrected quantitative detection value, which is directly output as the edible oil peroxide value detection result. This method utilizes the dual gain of alternating electric field-enhanced microfluidic laminar flow extraction and the impedance integral ratio in the relaxation characteristic frequency band, which significantly improves the response specificity to lipid peroxides and effectively suppresses electrode background parameter drift and extract matrix interference; the real-time synchronous correction of the built-in reference channel eliminates systematic errors introduced by temperature fluctuations and batch differences in extracts, realizing rapid, label-free, and highly accurate quantitative detection of edible oil peroxide value.

[0011] The technical effects and advantages provided by the present invention in the above technical solution are as follows: The oil sample and extract are simultaneously injected through parallel injection channels. After forming a stable laminar contact interface in the confluence zone, they are introduced into the reaction chamber. A sinusoidal alternating voltage with a phase difference of 180 degrees is applied between the upper and lower walls of the reaction chamber, generating a sinusoidal gradient electric field perpendicular to the laminar contact interface. This electric field applies periodic stretching deformation to the interface, causing lipid peroxides on the oil phase side to be continuously exposed and pulled into the extract phase during the dynamic expansion and compression of the interface, achieving directional mass transfer away from the oil phase matrix. This field-enhanced interfacial mass transfer method relies entirely on the electric field to manipulate molecules across the interface under low Reynolds number laminar flow conditions, without the need for additional surfactants or carrier molecules. This avoids emulsification and turbulent disturbances that occur during two-phase mixing, and significantly increases the throughput of lipid peroxides entering the detection phase. Compared to the mass transfer rate under passive diffusion conditions, field-induced periodic stretching continuously updates the concentration boundary layer at the interface, enabling the concentration of the analyte in the extraction liquid phase to reach a stable detectable level within a very short residence time. This significantly improves detection sensitivity and ensures that the extract flowing out of the reaction chamber remains in a uniform single-phase state, providing a clean and emulsified liquid environment for stable measurement of the interdigitated electrode array.

[0012] The impedance amplitude change sequence of the extract after directional mass transfer is obtained by scanning the excitation frequency on the interdigital electrode array. Second-order difference operations are performed to locate the relaxation characteristic frequency point. A low-frequency integration interval and a high-frequency integration interval are divided around this characteristic frequency point. The product of the impedance amplitude and the frequency interval within each interval is accumulated and summed. The ratio of the low-frequency integration area to the high-frequency integration area is calculated as the quantization detection value. Using the integration ratio instead of the impedance amplitude at a single frequency point as the detection index, the relaxation characteristic frequency shift is correlated with interface polarization changes. The integration operation effectively suppresses amplitude jitter caused by random noise and small power supply fluctuations during frequency scanning, improving signal stability. The chip's built-in reference channel takes a blank extract without oil sample from the same extract reservoir and synchronously measures the impedance integration ratio on the reference interdigital electrode array using the exact same frequency scanning sequence. The corrected quantization detection value is obtained by calculating the quotient of the difference between the integration ratio of the test extract and the blank extract and the reference ratio. This differential ratio correction method physically eliminates system drift caused by fluctuations in the background conductivity of the extract itself, temperature changes, and slight deviations in the geometric parameters of different chip electrodes, resulting in excellent consistency and reproducibility of the output peroxide value results across different testing batches and chips. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0014] Figure 1 This is a flowchart of a method for detecting the peroxide value of edible oils based on microfluidic chips; Figure 2 This is a flowchart of relaxation feature frequency extraction and integral area ratio calculation based on second-order difference; Figure 3 It is a curve showing the relationship between the local pressure difference and the flow velocity ratio at the outlet ends of the first and second channels at the entrance of the confluence zone; Figure 4 These are curves showing the variation of interface deformation amplitude with frequency under different sinusoidal alternating voltage amplitudes. Figure 5 This is a schematic diagram of the impedance amplitude frequency scanning curve and relaxation characteristic frequency points; Figure 6 This is a comparison curve of the quantitative values ​​and correction results of the peroxide value test of edible oil. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0016] See Figure 1 This invention provides a method for detecting the peroxide value of edible oil based on a microfluidic chip, comprising the following steps: simultaneously injecting the oil sample and extract into the parallel injection channel of the microfluidic chip, so that the two liquid streams form a laminar contact interface in the confluence area and then flow into the reaction chamber; applying an alternating electric field to the laminar contact interface in the reaction chamber, causing periodic stretching deformation at the interface and triggering directional mass transfer of lipid peroxides to the extract phase; introducing the extracted liquid after directional mass transfer into the surface of the interdigitated electrode array in the detection area, and measuring the change in impedance amplitude between the electrodes by scanning the excitation frequency; extracting relaxation characteristic frequency points according to the correspondence between the change in impedance amplitude and frequency; taking the impedance integral ratio within a preset frequency band on both sides of the relaxation characteristic frequency point as the quantitative detection value; simultaneously measuring a blank extract without oil sample through the reference channel built into the microfluidic chip, and correcting the quantitative detection value with the impedance integral ratio of the blank extract; outputting the corrected quantitative detection value as the detection result of the peroxide value of edible oil.

[0017] Example 1: In practice, the parallel injection channel includes a first channel and a second channel. The first channel has a first inlet, and the second channel has a second inlet. The first and second channels converge at a confluence zone, where a flow-guiding microstructure is installed. This flow-guiding microstructure consists of an array of multiple micropillars extending along the liquid flow direction. Each micropillar has a rhomboid cross-section, with its long diagonal parallel to the liquid flow direction. The micropillar array is distributed on both sides of the junction of the first and second channels. The micropillar array is used to suppress eddy current disturbances caused by the velocity difference when the oil sample and extractant converge, ensuring that the two liquid flows remain in a laminar state at the confluence zone inlet.

[0018] A first injection pump is connected to the first inlet of the parallel injection channel via tubing, and a second injection pump is connected to the second inlet of the parallel injection channel via tubing. The first injection pump delivers the oil sample to be tested to the first inlet at a first flow rate, and the second injection pump delivers the extract to the second inlet at a second flow rate. The flow rate ratio of the first to the second flow rate is adjusted to a preset range, which is 0.8:1 to 1.2:1. Precise control of the first and second flow rates is achieved by adjusting the rotational speed pulse frequency of the drive motors of the first and second injection pumps.

[0019] When adjusting the flow rate ratio between the first and second flow rates, pressure sensors installed on both sides of the confluence zone inlet monitor the local pressure at the outlets of the first and second channels in real time. The output of the first or second injection pump is adjusted until the difference between the local pressure at the outlet of the first channel and the outlet of the second channel is within a preset threshold of ±5 Pascals, allowing the two liquid flows to reach pressure equilibrium at the confluence zone inlet. Under pressure equilibrium, the oil sample and the extract flow form a stable laminar contact interface in the confluence zone. This laminar contact interface is a clear planar boundary between the two phases, without significant mixing or turbulence.

[0020] The mixed liquid flow, which includes a laminar contact interface, formed in the confluence zone is discharged from the confluence zone outlet and introduced into the inlet of the reaction chamber while maintaining a laminar flow state. The cross-sectional shape of the connecting channel from the confluence zone outlet to the inlet of the reaction chamber remains a constant rectangle, and the ratio of the length of the connecting channel to the hydraulic diameter is greater than 20, in order to maintain the laminar flow characteristics of the mixed liquid flow and ensure that the laminar contact interface remains intact when entering the reaction chamber.

[0021] See Figure 3 In the figure, the horizontal axis represents the velocity ratio of the first flow velocity to the second flow velocity, ranging from approximately 0.78 to 1.22, and the vertical axis represents the local pressure difference in Pascals, with a range of -6 to 6 Pa. The curves are represented by a solid blue line showing the trend of the local pressure difference with the flow velocity ratio, and two dashed red lines representing the upper and lower limits of the preset threshold, with values ​​of -5 Pa and 5 Pa, respectively.

[0022] As shown in the figure, throughout the entire range from 0.78 to 1.22, the local pressure difference consistently fluctuated between -1.5 Pa and 1.5 Pa, well below the preset threshold of ±5 Pa. The blue pressure difference curve exhibits slight fluctuations without a clear trend of increase or decrease, indicating that the local pressure difference between the outlet ends of the first and second channels remains relatively small, and the pressure balance is good.

[0023] Example 2: In practice, the reaction chamber is enclosed by an upper wall, a lower wall, and four side walls. The space inside the reaction chamber is used to accommodate the mixed liquid flow, including the laminar contact interface, introduced from the confluence zone. A first excitation electrode is arranged inside the upper wall of the reaction chamber, and a second excitation electrode is arranged inside the lower wall of the reaction chamber. Both the first and second excitation electrodes are strip-shaped electrodes extending along the length of the reaction chamber, and the material of the strip electrodes is gold or platinum. The strip pattern of the first excitation electrode has the same geometric dimensions as the strip pattern of the second excitation electrode, and the vertical projection of the strip pattern of the first excitation electrode onto the lower wall of the reaction chamber completely coincides with the strip pattern of the second excitation electrode.

[0024] The sinusoidal alternating voltage applied to the first and second excitation electrodes is generated by a dual-channel signal generator. The first output terminal of the dual-channel signal generator is connected to the first excitation electrode, and the second output terminal is connected to the second excitation electrode. The sinusoidal voltage signal output from the first output terminal and the sinusoidal voltage signal output from the second output terminal have a phase difference of 180 degrees. The expression for the sinusoidal alternating voltage is: , in, This indicates that the sinusoidal alternating voltage at time t = 100°C The instantaneous voltage value, in volts; This represents the amplitude of a sinusoidal alternating voltage, measured in volts. The frequency of a sinusoidal alternating voltage is expressed in Hertz (Hz). Indicates time, in seconds.

[0025] Amplitude of sinusoidal alternating voltage The value ranges from 1 volt to 10 volts, and the frequency of the sinusoidal alternating voltage is... The value ranges from 100 Hz to 10 kHz. Amplitude The specific value is determined based on the viscosity of the oil sample being tested. When the dynamic viscosity of the oil sample at 25 degrees Celsius is greater than 50 mPa·s, the amplitude... Set to 10 volts; when the dynamic viscosity of the oil sample under test at 25 degrees Celsius is less than or equal to 50 mPa·s, the amplitude is... Set to 5 volts. Frequency The specific value is determined based on the average flow velocity of the mixed liquid within the reaction chamber, and the frequency. Set to within 0.5 to 2 times the quotient obtained by dividing the average flow rate of the mixed liquid by the length of the reaction chamber.

[0026] During the application of a sinusoidal alternating voltage, the potentials on the first and second excitation electrodes are equal in magnitude but opposite in polarity, generating an alternating electric field in the space between the first and second excitation electrodes within the reaction chamber. The direction of the alternating electric field is perpendicular to the upper and lower walls of the reaction chamber, i.e., perpendicular to the plane of the laminar contact interface. The electric field strength of the alternating electric field varies sinusoidally with time, generating a sinusoidal gradient electric force perpendicular to the interface direction at the laminar contact interface. This sinusoidal gradient electric force acts on the lipid peroxide molecules on the oil phase side of the laminar contact interface. Under the influence of this force, the lipid peroxide molecules detach from the oil phase matrix and cross the interface into the extract phase along a direction perpendicular to the laminar contact interface.

[0027] By adjusting the frequency of the sinusoidal alternating voltage Control the interface deformation amplitude. Increase the frequency. This allows the laminar flow contact interface to complete more stretching and shrinking cycles per unit time, with the interface deformation amplitude increasing with frequency. The frequency decreases as the value increases; the frequency decreases. This increases the duration of a single stretch at the laminar interface, and the interface deformation amplitude increases with frequency. It increases as it decreases.

[0028] By adjusting the amplitude of the sinusoidal alternating voltage Control mass transfer flux. Increase the magnitude. This increases the peak value of the sinusoidal gradient electric field, thereby increasing the driving force for lipid peroxide molecules to cross the interface within a single cycle, and consequently increasing the mass transfer flux; decreasing the amplitude... This reduces the peak value of the sinusoidal gradient electric field, and consequently reduces the mass transfer flux.

[0029] See Figure 4 The horizontal axis in the graph represents the frequency of the sinusoidal alternating voltage. The graph uses a logarithmic coordinate system, with a frequency range from 100 Hz to 10000 Hz; the vertical axis represents the interface deformation amplitude, with arbitrary units. Two curves are plotted in the graph, corresponding to the amplitudes of the sinusoidal alternating voltage, respectively. Volts (green curve) and Two operating conditions (Voltage, red curve).

[0030] As can be seen from the graph, with frequency With the increase of voltage, the interface deformation amplitude gradually decreases. In the lower frequency range (approximately 100 Hz to 1000 Hz), the interface deformation amplitude is larger, with the green curve amplitude fluctuating between approximately 5.9 and 4, and the red curve amplitude fluctuating between approximately 12 and 9. The interface deformation amplitude of the red curve is significantly higher than that of the green curve, indicating that the interface deformation is more pronounced under a larger voltage amplitude.

[0031] As the frequency further increases to approximately 10,000 Hz, the curves continue to decline, with the interface deformation amplitude decreasing to around 1 (green curve) and 3 (red curve), and both curves tend to flatten out. This indicates that increasing the frequency... It will reduce the amplitude of a single interface stretching, and the interface deformation tends to stabilize in the high-frequency range.

[0032] Example 3: In practical implementation, a connecting channel is provided at the outlet of the reaction chamber. One end of the connecting channel is connected to the extract outlet of the reaction chamber, and the other end extends to the top of the interdigitated electrode array in the detection area. The vertical distance between the bottom surface of the connecting channel and the upper surface of the interdigitated electrode array is 50 micrometers to 200 micrometers, ensuring that when the extract flowing from the reaction chamber outlet is introduced into the detection area through the connecting channel, the extract can completely cover the sensing area of ​​the interdigitated electrode array. The interdigitated electrode array includes excitation electrodes and sensing electrodes, both arranged alternately in an interdigital pattern. The finger width of the interdigitated electrode array is 10 micrometers, the finger spacing is 10 micrometers, and the number of interdigital pairs is 20 to 50 pairs.

[0033] During the application of the AC excitation signal, the signal generation module outputs the AC excitation signal to the excitation terminal of the interdigital electrode array at logarithmic intervals from the starting frequency to the ending frequency. The starting frequency is 100 Hz, and the ending frequency is 1 MHz. The logarithmic interval is calculated by taking the ratio of two adjacent frequency points as a constant. ,in This indicates the total number of frequency points within the entire scanning frequency band. Set to 50. At each frequency point, the signal generation module continuously outputs an AC excitation signal for a preset stabilization time of 20 milliseconds. At the end of 20 milliseconds, the effective value of the AC voltage at the sensing terminal of the interdigital electrode array is read.

[0034] The voltage acquisition module synchronously acquires the effective value of the AC voltage and the amplitude of the AC excitation signal at the sensing terminal of the interdigital electrode array at each frequency point. For each frequency point, the corresponding impedance amplitude is calculated based on the acquired effective value of the AC voltage and the amplitude of the AC excitation signal. The formula for calculating the impedance amplitude is: , in, Indicates the first The impedance amplitude corresponding to each frequency point, in ohms; Indicates the first The effective value of AC voltage at the sensing terminal of the interdigital electrode array collected at each frequency point, in volts; Indicates the first The amplitude of the AC excitation signal applied at each frequency point, in volts; This indicates the resistance value of the reference resistor connected in series in the excitation circuit. Set to 10000 ohms. Indicates the index of the frequency point. The value range is 1 to integers, i.e. .

[0035] In some embodiments, the resistance value of the reference resistor The impedance modulus at 100 kHz, measured using an interdigitated electrode array in a blank extract solution, was determined. The impedance modulus is set to 0.8 to 1.2 times that of the blank extract at a frequency of 100 kHz. A reference resistor is connected in series between the output of the signal generation module and the excitation terminal of the interdigital electrode array. The voltage difference across the reference resistor is used to calibrate the amplitude of the excitation current through the interdigital electrode array.

[0036] Example 4: In specific implementation, please refer to Figure 2 The frequency points and their corresponding impedance amplitudes measured during the scanning process are arranged into a frequency-impedance sequence. Each element in the frequency-impedance sequence contains the frequency value of a frequency point and the impedance amplitude corresponding to that frequency point, with the frequency values ​​arranged in ascending order. The second-order difference operation on the frequency-impedance sequence is performed by a data processing unit, which is a microprocessor or a field-programmable gate array.

[0037] When performing a second-order difference operation, for the frequency-impedance sequence... For each element, calculate the first-order forward difference. and first-order backward difference First-order forward difference For the first The impedance magnitude of the first element minus the first element The difference obtained from the impedance magnitudes of each element, the first-order backward differential. For the first The impedance magnitude of the element minus the value of the first element The difference obtained from the impedance magnitudes of each element. For the first-order forward differential... and first-order backward difference Perform the subtraction operation to get the first... The second difference value corresponding to each element is calculated using the following formula: , in, Represents the first frequency-impedance sequence. The second-order difference values ​​corresponding to each element are expressed in ohms. Represents the first frequency-impedance sequence. The impedance magnitude of each element, in ohms; Represents the first frequency-impedance sequence. The impedance magnitude of each element, in ohms; Represents the first frequency-impedance sequence. The impedance magnitude of each element, in ohms. This represents the index of the element in the frequency-impedance sequence. The value range is from 2 to integers, This represents the total number of frequency points in the frequency-impedance sequence.

[0038] After performing the above second-order difference operation on all elements whose second-order difference values ​​can be calculated, arrange all second-order difference values ​​by element index to form a difference amplitude sequence. Find the frequency position where the absolute value reaches a maximum in the difference amplitude sequence. The search method is to compare the absolute values ​​of the difference amplitudes element by element. If the absolute value of the difference amplitude of an element in the difference amplitude sequence is simultaneously greater than the absolute values ​​of the difference amplitudes of both the preceding and following elements, then the frequency point corresponding to that element is marked as a candidate maximum frequency point. If there is only one candidate maximum frequency point among all candidate maximum frequency points, then that candidate maximum frequency point is directly marked as a relaxation characteristic frequency point. If multiple candidate maximum frequency points appear in the difference amplitude sequence, compare the absolute values ​​of the difference amplitudes of all candidate maximum frequency points, and select the candidate maximum frequency point with the largest absolute value of the difference amplitude as the relaxation characteristic frequency point.

[0039] In some embodiments, to avoid false maxima caused by noise, a differential amplitude threshold is set. A candidate maximum frequency point is only included in the candidate comparison range if the absolute value of the differential amplitude at that frequency point is greater than the differential amplitude threshold. The differential amplitude threshold is set to 0.2 times the arithmetic mean of the absolute values ​​of all differential amplitudes in the differential amplitude sequence.

[0040] After extracting the relaxation characteristic frequency points, using these points as boundaries, a low-frequency integration interval is formed by taking all frequency points within a first preset bandwidth on the low-frequency side, and a high-frequency integration interval is formed by taking all frequency points within a second preset bandwidth on the high-frequency side. Both the low-frequency and high-frequency integration intervals are of equal width on a logarithmic frequency coordinate system; that is, the length of the low-frequency integration interval is equal to the length of the high-frequency integration interval on a logarithmic frequency coordinate system. In specific implementation, the first preset bandwidth is set to extend the relaxation characteristic frequency points towards the low-frequency direction by 0.3 logarithmic units on the logarithmic frequency coordinate system, and the second preset bandwidth is set to extend the relaxation characteristic frequency points towards the high-frequency direction by 0.3 logarithmic units. The logarithmic unit is base 10, and 0.3 logarithmic units correspond to a frequency ratio of... Frequency span.

[0041] The low-frequency integration area is obtained by summing the products of the impedance amplitude and the frequency interval at each frequency point within the low-frequency integration interval. The frequency interval is the frequency difference between two adjacent frequency points within the low-frequency integration interval. Similarly, the high-frequency integration area is obtained by summing the products of the impedance amplitude and the frequency interval at each frequency point within the high-frequency integration interval. The frequency interval is the frequency difference between two adjacent frequency points within the high-frequency integration interval. The ratio of the low-frequency integration area to the high-frequency integration area is calculated, and this ratio is used as the quantization detection value. The quantization detection value is expressed as a dimensionless numerical value.

[0042] See Figure 5 The horizontal axis in the graph represents frequency. The graph uses a logarithmic coordinate system with a frequency range of 100 Hz to 1 MHz. The vertical axis represents the impedance amplitude in ohms. The blue curve in the graph shows the trend of impedance amplitude of the oil sample extract measured on the interdigitated electrode array in the detection area as a function of frequency. The curve generally shows a trend of slow decrease followed by rapid decrease, with the impedance amplitude gradually decreasing from approximately 4900 ohms to several hundred ohms, reflecting the characteristic of impedance decreasing as the frequency increases.

[0043] A red dot in the figure indicates the relaxation characteristic frequency point corresponding to the point of maximum absolute value after performing second-order difference processing on the impedance amplitude sequence. This characteristic frequency point is located at approximately 300 Hz, which conforms to the steps of extracting the relaxation characteristic frequency through second-order difference operation described in Example 4.

[0044] The light orange area to the left of the red dot represents the low-frequency integration interval, extending approximately 0.3 logarithmic units from the relaxation characteristic frequency point on the logarithmic frequency coordinates towards lower frequencies, i.e., from approximately 150 Hz to 300 Hz. This represents the frequency range used to calculate the low-frequency integration area. The light green area to the right represents the high-frequency integration interval, extending approximately 0.3 logarithmic units from the relaxation characteristic frequency point towards higher frequencies, i.e., from 300 Hz to approximately 600 Hz. This represents the frequency range used to calculate the high-frequency integration area.

[0045] The impedance amplitude changes relatively smoothly and the integration area is large in the low-frequency integration interval shown; however, the impedance amplitude decreases rapidly and the integration area is significantly lower in the high-frequency integration interval. As described in Example 4, the integration area is obtained by summing the products of the impedance amplitude and the frequency interval in both the low-frequency and high-frequency integration intervals, and then the ratio of the low-frequency integration area to the high-frequency integration area is calculated as the basic data for quantization detection values.

[0046] Example 5: In practical implementation, the microfluidic chip has a reference channel internally. The inlet end of the reference channel is directly connected to the extract reservoir, which contains blank extract without the oil sample to be tested. The outlet end of the reference channel is equipped with a reference interdigital electrode array. The geometry, finger width, finger spacing, and number of interdigital pairs of the reference interdigital electrode array are exactly the same as those of the interdigital electrode array in the detection area.

[0047] Within the reference channel, blank extractant flows through the internal channels of the microfluidic chip at a constant flow rate, covering the sensing area of ​​the reference interdigital electrode array. The flow rate of the blank extractant in the reference channel is driven by a third injection pump, independent of the first and second injection pumps, and the flow rate of the third injection pump is set to be equal to the second flow rate of the second injection pump delivering the extractant to the parallel injection channel.

[0048] The signal generation module applies the same scanning frequency sequence as the detection area to the reference interdigital electrode array. The start frequency, end frequency, number of frequency points, and ratio of adjacent frequency points in the scanning frequency sequence are all consistent with the scanning process in the detection area. At each frequency point, after the signal generation module continuously outputs the AC excitation signal for a preset stabilization time, the voltage acquisition module reads the effective value of the AC voltage at the sensing terminal of the reference interdigital electrode array and calculates the reference impedance amplitude of the reference interdigital electrode array at that frequency point based on the ratio of the effective value of the AC voltage to the amplitude of the AC excitation signal.

[0049] After scanning all frequency points, a reference frequency-impedance sequence is obtained. The reference frequency-impedance sequence is processed using the same second-order difference operation method as for extracting relaxation characteristic frequency points. The frequency positions where the absolute value of the reference difference amplitude sequence reaches its maximum value are identified and marked as reference relaxation characteristic frequency points. On both sides of the reference relaxation characteristic frequency points, a first preset bandwidth on the low-frequency side is taken in a logarithmic frequency coordinate manner to form a reference low-frequency integration interval, and a second preset bandwidth on the high-frequency side is taken to form a reference high-frequency integration interval. The products of the reference impedance amplitude and the frequency interval at each frequency point within the reference low-frequency integration interval are summed to obtain the reference low-frequency integration area; the products of the reference impedance amplitude and the frequency interval at each frequency point within the reference high-frequency integration interval are summed to obtain the reference high-frequency integration area. The ratio of the reference low-frequency integration area to the reference high-frequency integration area is calculated to obtain the reference impedance integration ratio.

[0050] In some embodiments, the extraction of reference relaxation feature frequency points and the definition of low-frequency integration intervals and high-frequency integration intervals adopt the same algorithm parameters as the detection area data processing unit. That is, the differential amplitude threshold is set to 0.2 times the arithmetic mean of the absolute values ​​of all differential amplitudes in the reference differential amplitude sequence, and the first preset bandwidth and the second preset bandwidth are both 0.3 logarithmic units in logarithmic frequency coordinates.

[0051] After obtaining the ratio of the quantized detection value to the integral ratio of the reference impedance, the correction difference is calculated. This correction difference is the difference between the quantized detection value and the integral ratio of the reference impedance. Dividing the correction difference by the integral ratio of the reference impedance yields the corrected quantized detection value, which is expressed as a dimensionless value. The calculation formula is as follows: , in, This represents the corrected quantized detection value, which is dimensionless. This represents the quantitative detection value obtained after mass transfer between the oil sample and the extract, and is dimensionless. This represents the integral ratio of the reference impedance measured from the blank extract, and is dimensionless.

[0052] The reference channel and the detection channel of the microfluidic chip are independent of each other. The flow path of the blank extract in the reference channel does not intersect with the flow path of the oil sample extract in the detection channel. The blank extract in the reference channel is discharged to the waste liquid pool after the impedance measurement is completed.

[0053] See Figure 6 In the graph, the horizontal axis represents the measurement sequence number, and the vertical axis represents the dimensionless numerical value. The graph shows three curves, representing the quantified detection values. (Blue solid line) Reference impedance integral ratio (Red dashed line) and the corrected quantitative detection value (Green dotted line). Among them, the integral ratio of the reference impedance. The curve is basically stable around 1, with small numerical fluctuations, reflecting the constancy and repeatability of the impedance integral ratio of the blank extract in the reference channel inside the microfluidic chip.

[0054] Quantitative detection value The curve shows an overall upward trend with the measurement sequence number, gradually increasing from about 0.6 to about 3.2. This indicates that as the measurement sequence number increases, the mass transfer of lipid peroxides in the oil sample to the extract is enhanced, resulting in an increase in the impedance integral ratio in the detection area, which reflects the increase in the peroxide value of the tested edible oil.

[0055] Corrected quantized detection value After integrating the ratio with reference impedance After calibration, the curve maintains a smooth upward trend, with a value range of approximately -0.4 to 2.0. Compared to the uncalibrated quantized detection value, the calibrated quantized detection value eliminates the influence of system baseline drift and environmental interference, providing more accurate dimensionless detection results.

[0056] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application.

Claims

1. A method for detecting the peroxide value of edible oils based on a microfluidic chip, characterized in that, Includes the following steps: The oil sample to be tested and the extract are simultaneously injected into the parallel injection channel of the microfluidic chip, so that the two liquid streams form a laminar contact interface in the confluence area and then flow into the reaction chamber. An alternating electric field is applied to the laminar contact interface within the reaction chamber, causing periodic stretching deformation at the interface and triggering directional mass transfer of lipid peroxides into the extraction liquid phase. The extract after directional mass transfer is introduced into the surface of the interdigitated electrode array in the detection area, and the change in impedance amplitude between the electrodes is measured by scanning the excitation frequency. The relaxation characteristic frequency points are extracted based on the correspondence between the impedance amplitude change and the frequency. The impedance integral ratio within a preset frequency band on both sides of the relaxation characteristic frequency point is taken as the quantization detection value. The blank extract without oil sample is measured simultaneously through the reference channel built into the microfluidic chip, and the quantitative detection value is corrected by the impedance integral ratio of the blank extract. The corrected quantitative detection value is output as the result of the peroxide value test of edible oil.

2. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 1, characterized in that, The process of simultaneously injecting the oil sample and extract into the parallel injection channel of the microfluidic chip, allowing the two liquid streams to form a laminar contact interface at the confluence region before flowing into the reaction chamber, includes: The oil sample to be tested is delivered to the first inlet of the parallel injection channel at a first flow rate using a first injection pump, and the extract is delivered to the second inlet of the parallel injection channel at a second flow rate using a second injection pump. Adjust the flow rate ratio of the first flow rate to the second flow rate to a preset range so that the two liquid flows reach pressure balance at the inlet of the confluence zone and form a stable laminar contact interface; The mixed liquid flow at the outlet of the confluence zone is introduced into the inlet of the reaction chamber while maintaining a laminar flow state.

3. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 1, characterized in that, The step of applying an alternating electric field to the laminar contact interface within the reaction chamber to induce periodic stretching deformation at the interface and trigger directional mass transfer of lipid peroxides into the extraction liquid phase includes: A first excitation electrode and a second excitation electrode are respectively arranged on the upper and lower walls of the reaction chamber, and a sinusoidal alternating voltage with a phase difference of 180 degrees is applied to the first excitation electrode and the second excitation electrode. Alternating voltage generates a sinusoidal gradient electric field perpendicular to the interface direction at the laminar contact interface, driving lipid peroxides on the oil phase side at the interface to detach from the oil phase matrix and cross the interface into the extraction liquid phase. The interface deformation amplitude and mass transfer flux are controlled by adjusting the frequency and amplitude of the sinusoidal alternating voltage.

4. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 1, characterized in that, The step of introducing the extracted liquid after directional mass transfer into the surface of the interdigitated electrode array in the detection area, and measuring the change in impedance amplitude between the electrodes by scanning the excitation frequency, includes: The extract flowing out of the reaction chamber outlet is introduced into the interdigital electrode array above the detection area through the connecting channel, so that the extract completely covers the sensing area of ​​the interdigital electrode array. The AC excitation signal is output to the excitation terminal of the interdigital electrode array at logarithmic intervals from the start frequency to the end frequency, and the effective value of the AC voltage at the sensing terminal of the interdigital electrode array is read after a preset stable duration at each frequency point. The impedance amplitude at each frequency point is calculated based on the ratio of the effective value of the AC voltage to the amplitude of the AC excitation signal.

5. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 1, characterized in that, The step of extracting relaxation characteristic frequency points based on the correspondence between impedance amplitude change and frequency includes: The frequency points and their corresponding impedance amplitudes measured during the scanning process are arranged into a frequency-impedance sequence. A second-order difference operation is performed on the frequency-impedance sequence to obtain the difference amplitude sequence. Find the frequency position where the absolute value reaches the maximum value in the differential amplitude sequence, and mark the frequency point at the maximum value frequency position as the relaxation characteristic frequency point; If multiple absolute value maxima appear in the differential amplitude sequence, the frequency position with the largest absolute value of the differential amplitude is taken as the relaxation characteristic frequency point.

6. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 1, characterized in that, The method of taking the impedance integral ratio within a preset frequency band on both sides of the relaxation characteristic frequency point as the quantization detection value includes: Using the relaxation characteristic frequency point as the boundary, all frequency points within the first preset bandwidth on the low-frequency side are taken on the frequency axis to form the low-frequency integration interval, and all frequency points within the second preset bandwidth on the high-frequency side are taken to form the high-frequency integration interval. The product of the impedance amplitude and the frequency interval at each frequency point within the low-frequency integration interval is summed to obtain the low-frequency integration area. The product of the impedance amplitude and the frequency interval at each frequency point within the high-frequency integration interval is summed to obtain the high-frequency integration area. The ratio of the low-frequency band integral area to the high-frequency band integral area is calculated and used as the quantization detection value.

7. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 1, characterized in that, The method of simultaneously measuring a blank extract without oil sample via a reference channel built into the microfluidic chip, and correcting the quantized detection value using the impedance integral ratio of the blank extract, includes: The reference channel directly takes blank extract without oil sample from the extract reservoir, measures the impedance amplitude on the reference interdigitated electrode array with the same scanning frequency sequence as the detection area, and calculates the reference impedance integral ratio of the blank extract. The difference between the quantized detection value and the integral ratio of the reference impedance is calculated to obtain the correction difference; Divide the correction difference by the integral ratio of the reference impedance, and use the resulting quotient as the corrected quantitative detection value.

8. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 2, characterized in that, The parallel injection channel is equipped with a flow-guiding microstructure at the entrance of the confluence zone to suppress eddy current disturbances when the two liquid streams merge.

9. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 3, characterized in that, Both the first excitation electrode and the second excitation electrode are strip electrodes extending along the length of the reaction chamber, and their vertical projections overlap.

10. The method for detecting the peroxide value of edible oil based on a microfluidic chip according to claim 6, characterized in that, Both the low-frequency integration interval and the high-frequency integration interval are of equal width on a logarithmic frequency coordinate system.

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