Investigation of 60Co Irradiation on the Volatile Organic Compounds from Finger Citron (Citri Sarcodactylis Fructus) Using GC–IMS

In recent years, as the desire for a healthy lifestyle has become more widespread, consumers are gaining an increasing appreciation for safe, high-quality food. Researchers are constantly seeking new ways to protect foods from insect pests and fungi. This study used GC-IMS to analyze the volatile organic compounds and flavor characteristics of Finger Citron in response to different doses of 60Co irradiation. The principal component analysis method was used to explore the overall differences in flavor spectra, and a total of 60 compounds were identified. The fingerprints of volatile organic compounds in the samples showed that the volatile organic compounds with doses of 60Co irradiation in about 0 kGy and 5 kGy are similar, while the 10 kGy samples are quite different. The PCA results showed that the similarity between 0 kGy and 5 kGy was slightly higher, and the difference between 10 kGy and other samples was greater. Therefore, it was determined that 60Co irradiation with a 10 kGy intensity has a significant influence on the content of volatile oils components, while 60Co irradiation with a 5 kGy intensity has little effect. Irradiation technology is demonstrated as a promising method of food sterilization, but the irradiation dose and chemical composition must be taken into consideration.


Introduction
Finger Citron (Citri Sarcodactylis Fructus) is the dried fruit of Citrus medica L. var.sarcodactylis Swingle.When the fruit turns yellow in autumn, it is collected and is distributed throughout Guangdong, Sichuan, and Zhejiang, China.It is widely used in Chinese food and as a traditional Chinese medicine, which is often used for regulating the liver's and stomach's qi flow and relieving pain [1].Volatile oils (VOs) are substances composed of a variety of compounds, and plant volatile oils can be used to prepare flavors and fragrances, while also possessing strong bacteriostatic activity.Finger Citron volatile oils have obvious inhibitory effects on yeast, Escherichia coli, Bacillus subtilis, and Staphylococcus aureus; they also have an effect on antidepressants and inhibit the reproduction of cancer cells [2][3][4][5].With regard to the increasingly serious threat of foodborne diseases, plant volatile oils have a very broad development and application prospect as a safe and green high-efficiency food preservative and flavoring agent. 60Co has strong bactericidal power, most microorganisms are sensitive to it [6][7][8], and its advantages of convenience and speed are useful during the sterilization of foods and pharmaceutical products.This method destroys microbes in samples and damages DNA in organisms [9].Under the right conditions, gamma irradiation can effectively destroy mycotoxins and decontaminate plants.To date, the Codex Alimentarius Commission has permitted the decontamination of plant-derived food materials using irradiation in more than 55 countries, including the United States, the European Union, South Korea, and China.In this case, the permitted irradiation dose is lower than 10 kGy.Reviewing the literature, it is evident that there is a great deal of literature on the chemical composition analysis of Finger Citron volatile oils, but very little on using 60 Co irradiation.Studies have revealed that different irradiation doses can have specific effects on the volatile organic compounds of foods and some Chinese medicinal materials [10][11][12][13].
In this study, the volatile flavor substances of Finger Citron volatile oils were analyzed using GC-IMS technology after irradiating at doses of 0, 5, and 10 kGy.Using fingerprint and principal component analyses, the differences and associations of volatile flavor substances under different irradiation doses were explored.

Materials
We collected fresh finger citrons from Jinhua, Zhejiang, China, and Prof. Zhaoming Xie identified them at the Hunan Academy of Traditional Chinese Medicine.A voucher specimen (HNATCM2021-010) was deposited in the herbarium of the Hunan Academy of Traditional Chinese Medicine and stored at 4 • C.

60 Co Radiation
We then dehydrated the Finger Citron EOs with anhydrous Na 2 SO 4 and divided them into three equal parts for 60 Co irradiation (stored in 1.5 mL sealed vials) with a radiation source intensity of 2.96 × 10 16 Bq using the dynamic stepping irradiation method.The dose rates were 0, 5, and 10 kGy/min.The 60 Co γ radiation source was located at Hunan Radiological Technology Application Research Center (Changsha, China).

The Extraction Process of Finger Citron Volatile Oils
First, 0.5 kg of Finger Citron was placed in a round-bottomed flask (chopped up before the extraction), which was filled with 3300 mL of water to immerse the Finger Citron.This was extracted via steam distillation and slightly boiled (100 • C) for 5 h.Heating was stopped when the volume of the volatile oil no longer increased.We separated the volatile oils layer, preserved it in a sealed tube, and stored it at 4 • C for analysis.

GC-IMS Analysis
The GC-IMS analysis was performed using a GC-IMS instrument (Flavourspec ® -G.A.S., Dortmund, Germany).With the carrier gas, samples were introduced into the instrument: first through the gas chromatography column, then into the ion drift tube.
Upon ionization, the molecule migrated to the Faraday disc for secondary separation under the action of an electric field and reverse drift gas.
Samples were loaded into a 20 mL headspace flask, heated at 80 • C for 10 min, and incubated at 500 rpm.
During the headspace injection analysis, the headspace injection volume was 100 µL, and the injection needle temperature was 85 • C.
GC conditions: The column was an MXT-5 column (15 m × 0.53 mm × 1 µm), the column temperature was 60 • C, and N 2 was used as a carrier gas.
Initially, the carrier gas velocity program was set at 2.0 mL/min, which was maintained for 2 min and then increased linearly to 100.0 mL/min from 2 min to 20 min.After that, it was maintained at 100.0 mL/min from 20 min to 40 min.The flow was stopped after a 40 min runtime.
IMS conditions: N 2 was the drift gas, and its flow rate was 150 mL/min.Temperature kept at 45 • C.

Statistical Analysis
In order to identify compounds, linear retention indices and mass spectra of GC-IMS data were compared with the NIST 17 database.Our analysis of GC-IMS data (from G.A.S., Dortmund, Germany, version 2.0.0) was performed using Reporter, Gallery Plot, and GC-IMS Library Search.The detected Finger Citron volatile oils were determined by combining the retention index (RI) and drift time (Dt) using NIST Library and IMS database retrieval software from G.A.S (version 2.0.0).This plug-in was used for dynamic PCA, cluster analysis, and a rapid determination of known and unknown samples.

GC-IMS Profile of Finger Citron at Different Irradiation Doses
A three-dimensional spectrum of volatile organic compounds at 60 Co doses of 0 kGy, 5 kGy, and 10 kGy is shown in Figure 1, represented by FS-1, FS-2, and FS-3.The x, y, and z axes in the figure represent the drift time, gas chromatography retention time, and peak intensity, respectively.This shows that the peak signal distribution of each group is generally similar, and there is a certain difference in peak signal intensity, indicating that the VOs of Finger Citron volatile oils in each group are generally similar under different irradiation doses, and no new substances are generated.However, there are certain differences in content.By projecting the three-dimensional GC-IMS spectrum, we obtain a two-dimensional GC-IMS plan.Each point represents a volatile oil; the closer the color is to red, the higher the concentration, while the closer it is to white, the lower the concentration.This allows for a more direct evaluation of the volatile species and concentration differences in each sample.GC conditions: The column was an MXT-5 column (15 m × 0.53 mm × 1 µm), the column temperature was 60 °C, and N2 was used as a carrier gas.
Initially, the carrier gas velocity program was set at 2.0 mL/min, which was maintained for 2 min and then increased linearly to 100.0 mL/min from 2 min to 20 min.After that, it was maintained at 100.0 mL/min from 20 min to 40 min.The flow was stopped after a 40 min runtime.
IMS conditions: N2 was the drift gas, and its flow rate was 150 mL/min.Temperature kept at 45 °C.

Statistical Analysis
In order to identify compounds, linear retention indices and mass spectra of GC-IMS data were compared with the NIST 17 database.Our analysis of GC-IMS data (from G.A.S., Dortmund, Germany, version 2.0.0) was performed using Reporter, Gallery Plot, and GC-IMS Library Search.The detected Finger Citron volatile oils were determined by combining the retention index (RI) and drift time (Dt) using NIST Library and IMS database retrieval software from G.A.S (version 2.0.0).This plug-in was used for dynamic PCA, cluster analysis, and a rapid determination of known and unknown samples.

GC-IMS Profile of Finger Citron at Different Irradiation Doses
A three-dimensional spectrum of volatile organic compounds at 60 Co doses of 0 kGy, 5 kGy, and 10 kGy is shown in Figure 1, represented by FS-1, FS-2, and FS-3.The x, y, and z axes in the figure represent the drift time, gas chromatography retention time, and peak intensity, respectively.This shows that the peak signal distribution of each group is generally similar, and there is a certain difference in peak signal intensity, indicating that the VOs of Finger Citron volatile oils in each group are generally similar under different irradiation doses, and no new substances are generated.However, there are certain differences in content.By projecting the three-dimensional GC-IMS spectrum, we obtain a twodimensional GC-IMS plan.Each point represents a volatile oil; the closer the color is to red, the higher the concentration, while the closer it is to white, the lower the concentration.This allows for a more direct evaluation of the volatile species and concentration differences in each sample.Note: The x-axis represents the GC drift time (normalization), the y-axis represents the retention time (s), and the z-axis represents the intensity of the peak.With blue as the background, the bright spot indicates a substance; the closer its color is to red, the greater the concentration.
As shown in Figure 2, the Reporter plug-in was used to generate a two-dimensional top view of Finger Citron volatile oils.It consists of drift time, retention time, and ion   Using FS-1 as a reference to establish a difference comparison model, we obtain the results shown in Figure 3. On the basis of Figure 2 spectroscopy, if the volatile organic compounds in the sample are consistent with the volatile organic compounds in the FS-1 sample, the points cancel each other out, and the result is shown in white.If the component is higher than the FS-1 sample, it is shown here more in red and less in blue.FS-2 is mostly red, mixed with a small amount of blue, but the color is relatively light, so the content of FS-2 volatile substances increases and decreases at the same time; however, there is little difference compared to FS-1.There is a significant amount of red and a small amount of blue in FS-3, indicating a large difference in signal strength.Therefore, the irradiation dose can lead to changes in VOs, and the greater the difference in irradiation doses, the more significant the difference in VOs produced.Using FS-1 as a reference to establish a difference comparison model, we obtain the results shown in Figure 3. On the basis of Figure 2 spectroscopy, if the volatile organic compounds in the sample are consistent with the volatile organic compounds in the FS-1 sample, the points cancel each other out, and the result is shown in white.If the component is higher than the FS-1 sample, it is shown here more in red and less in blue.FS-2 is mostly red, mixed with a small amount of blue, but the color is relatively light, so the content of FS-2 volatile substances increases and decreases at the same time; however, there is little difference compared to FS-1.There is a significant amount of red and a small amount of blue in FS-3, indicating a large difference in signal strength.Therefore, the irradiation dose can lead to changes in VOs, and the greater the difference in irradiation doses, the more significant the difference in VOs produced.
signal intensity.This figure has a blue background, and the red vertical line at 1.0 is the RIP (reactive ion peak).Gas chromatography retention time (s) corresponds to the ordinate coordinate and ion drift time (normalization) to the abscissa.Volatile organic compounds are represented by the points on either side of the RIP.The color indicates the concentration of the substance, where white indicates the lowest concentration, red indicates the highest concentration, and a darker color indicates the highest concentration.Using FS-1 as a reference to establish a difference comparison model, we obtain the results shown in Figure 3. On the basis of Figure 2 spectroscopy, if the volatile organic compounds in the sample are consistent with the volatile organic compounds in the FS-1 sample, the points cancel each other out, and the result is shown in white.If the component is higher than the FS-1 sample, it is shown here more in red and less in blue.FS-2 is mostly red, mixed with a small amount of blue, but the color is relatively light, so the content of FS-2 volatile substances increases and decreases at the same time; however, there is little difference compared to FS-1.There is a significant amount of red and a small amount of blue in FS-3, indicating a large difference in signal strength.Therefore, the irradiation dose can lead to changes in VOs, and the greater the difference in irradiation doses, the more significant the difference in VOs produced.

Qualitative Analysis of Volatile Organic Compounds in Finger Citron (GC × IMS Library Search)
In the GC-IMS 2D pattern, the three samples are reflected in the difference in the content of each volatile substance.On this basis, combined with the NIST and IMS databases built into the software, the volatile organic compounds were qualitatively analyzed, and the ion mobility spectrum of Figure 4 was obtained.Each of these dots represents an organic substance, which was qualitatively searched in the database based on its corresponding two-dimensional data.Drift time is represented by the abscissa, and retention time is represented by the ordinate.
content of each volatile substance.On this basis, combined with the NIST and IMS databases built into the software, the volatile organic compounds were qualitatively analyzed, and the ion mobility spectrum of Figure 4 was obtained.Each of these dots represents an organic substance, which was qualitatively searched in the database based on its corresponding two-dimensional data.Drift time is represented by the abscissa, and retention time is represented by the ordinate.
This study identified a total of 60 peak signals, including 10 aldehydes, 10 alcohols, 8 esters, 4 olefins, 4 ketones, 3 acids, and a few pyrazine, pyridine, and furan compounds.The qualitative analysis results of Finger Citron volatile organic compounds are shown in Tables 1 and 2.   This study identified a total of 60 peak signals, including 10 aldehydes, 10 alcohols, 8 esters, 4 olefins, 4 ketones, 3 acids, and a few pyrazine, pyridine, and furan compounds.The qualitative analysis results of Finger Citron volatile organic compounds are shown in Tables 1 and 2.

Principal Component Analysis (PCA) of Volatile Organic Compounds of Finger Citron Samples at 3 Irradiation Doses
The peak volume of 60 volatile organic compounds of three irradiation doses of Finger Citron volatile oils was selected as the characteristic variable for principal component analysis, as demonstrated by Figures 5 and 6.From the figures, it can be concluded that PC1 and PC2 contribute 56% and 26%, respectively (the sum of the contribution rates is 82%).Generally, when the sum of the contribution rates of PC1 and PC2 reaches 60%, the PCA model can fully separate different samples, indicating that principal components 1 and 2 reflect most of the original variable information.The sample distributions with a high correlation will be in the same region; FS-3 is clustered separately from the other two samples, and the PC1 score is negative, indicating that the VOC compositions of FS-1 and FS-2 samples are relatively similar, while the characteristics of FS-3 flavor compounds are significantly different.Thus, three samples can be distinguished via GC-IMS detection of the volatile organic compounds of Finger Citron 0, 5, and 10 kGy irradiated samples.

Fingerprint Analysis of Volatile Organic Compounds of Finger Citron at 3 Irradiation Doses
The fingerprints of the volatile organic compounds of the three irradiated doses of Finger Citron are shown in Figure 7. GC-IMS analysis revealed that the content of monoterpenes, sesquiterpenes, and alcohols in Finger Citron volatile oils is high, followed by that of alcohols and esters.The fingerprint shows that (E)-2-Pentenal, Pyridine, Ethyl propanoate, Propyl acetate, Acetic acid, Methylpyrazine, and other components gradually decreased from FS-1 to FS-3.Propanoic acid, Pentanoic acid, 2-Furanmethanol, 3-Methylpentanol, 1-Hexanol, and other compounds were most abundant in FS-2 samples.Ethanol, Propanal, 3-Methylbutanal, 2-Methylbutanal, Heptanal, and other compounds had the highest content in FS-3 and the lowest content in FS-1.In this study, the terpenoid components in Finger Citron volatile oils changed, but they remained insignificant, and the active ingredients did not change greatly.
PCA model can fully separate different samples, indicating that principal components 1 and 2 reflect most of the original variable information.The sample distributions with a high correlation will be in the same region; FS-3 is clustered separately from the other two samples, and the PC1 score is negative, indicating that the VOC compositions of FS-1 and FS-2 samples are relatively similar, while the characteristics of FS-3 flavor compounds are significantly different.Thus, three samples can be distinguished via GC-IMS detection of the volatile organic compounds of Finger Citron 0, 5, and 10 kGy irradiated samples.

Fingerprint Analysis of Volatile Organic Compounds of Finger Citron at 3 Irradiation Dose
The fingerprints of the volatile organic compounds of the three irradiated doses o Finger Citron are shown in Figure 7. GC-IMS analysis revealed that the content of mono terpenes, sesquiterpenes, and alcohols in Finger Citron volatile oils is high, followed b that of alcohols and esters.The fingerprint shows that (E)-2-Pentenal, Pyridine, Ethyl pro panoate, Propyl acetate, Acetic acid, Methylpyrazine, and other components graduall decreased from FS-1 to FS-3.Propanoic acid, Pentanoic acid, 2-Furanmethanol, 3 Methylpentanol, 1-Hexanol, and other compounds were most abundant in FS-2 sample Ethanol, Propanal, 3-Methylbutanal, 2-Methylbutanal, Heptanal, and other compound had the highest content in FS-3 and the lowest content in FS-1.In this study, the terpenoi components in Finger Citron volatile oils changed, but they remained insignificant, an the active ingredients did not change greatly.

Discussion
As food and medicine with a high utilization value, Finger Citron volatile oils are a active ingredient [27].The composition of volatile oils is complex and diverse, often con taining dozens or even hundreds of components [28].The gas chromatography separatio

Fingerprint Analysis of Volatile Organic Compounds of Finger Citron at 3 Irradiation Doses
The fingerprints of the volatile organic compounds of the three irradiated doses of Finger Citron are shown in Figure 7. GC-IMS analysis revealed that the content of monoterpenes, sesquiterpenes, and alcohols in Finger Citron volatile oils is high, followed by that of alcohols and esters.The fingerprint shows that (E)-2-Pentenal, Pyridine, Ethyl propanoate, Propyl acetate, Acetic acid, Methylpyrazine, and other components gradually decreased from FS-1 to FS-3.Propanoic acid, Pentanoic acid, 2-Furanmethanol, 3-Methylpentanol, 1-Hexanol, and other compounds were most abundant in FS-2 samples.Ethanol, Propanal, 3-Methylbutanal, 2-Methylbutanal, Heptanal, and other compounds had the highest content in FS-3 and the lowest content in FS-1.In this study, the terpenoid components in Finger Citron volatile oils changed, but they remained insignificant, and the active ingredients did not change greatly.

Discussion
As food and medicine with a high utilization value, Finger Citron volatile oils are an active ingredient [27].The composition of volatile oils is complex and diverse, often containing dozens or even hundreds of components [28].The gas chromatography separation technology (GC) and ion migration spectroscopy (IMS) used in this study were able to qualitatively analyze most of the chemical components of Finger Citron volatile oils.The experimental results showed that after irradiation sterilization at three 60 Co doses of 0 kGy, 5 kGy, and 10 kGy, the volatile organic compounds of FS-1, FS-2, and FS-3 samples were qualitatively analyzed using GC-IMS, and no new chemical substances were produced after 60 Co irradiation.Analyses of spectrograms and peak volume data revealed that irradiation can affect the content of volatile organic compounds, and different chemical substances will undergo different changes.For example, the content of Geranyl acetate, Ethyl cinnamate, Citral, alpha Terpineol, etc., increased with the increase in irradiation dose; Ethyl phenylacetate, Linalool, Z-Ocimene, and others had the highest content at a radiation dose of 5 kGy 60 Co, followed by that of 10 kGy, but there was no significant difference between them and that of 0 kGy; Methylpyrazine, 2,3-Butanediol, Propyl ace-

Discussion
As food and medicine with a high utilization value, Finger Citron volatile oils are an active ingredient [27].The composition of volatile oils is complex and diverse, often containing dozens or even hundreds of components [28].The gas chromatography separation technology (GC) and ion migration spectroscopy (IMS) used in this study were able to qualitatively analyze most of the chemical components of Finger Citron volatile oils.The experimental results showed that after irradiation sterilization at three 60 Co doses of 0 kGy, 5 kGy, and 10 kGy, the volatile organic compounds of FS-1, FS-2, and FS-3 samples were qualitatively analyzed using GC-IMS, and no new chemical substances were produced after 60 Co irradiation.Analyses of spectrograms and peak volume data revealed that irradiation can affect the content of volatile organic compounds, and different chemical substances will undergo different changes.For example, the content of Geranyl acetate, Ethyl cinnamate, Citral, alpha Terpineol, etc., increased with the increase in irradiation dose; Ethyl phenylacetate, Linalool, Z-Ocimene, and others had the highest content at a radiation dose of 5 kGy 60 Co, followed by that of 10 kGy, but there was no significant difference between them and that of 0 kGy; Methylpyrazine, 2,3-Butanediol, Propyl acetate, etc., had the highest content at 0 kGy.Therefore, this study concludes that the chemical composition of Finger Citron volatile oils only changes in response to an irradiation dose of 10 kGy 60 Co, and GC-IMS technology is suitable for a component analysis of volatile oils.As a widely used traditional Chinese medicine with multiple functions, Finger Citron will be subjected to more in-depth research on the specific changes in the content of its active ingredients to determine whether it will lead to changes in pharmacological effects, and the gradient selection of increased irradiation.We will explore this further in future research.

Conclusions
In this study, GC-IMS was used to detect the volatile organic compounds of Finger Citron volatile oils after sterilization at irradiation doses of 0, 5, and 10 kGy.Based on the built-in NIST gas retention index database and the IMS drift time database, the volatile organic compounds of three volatile oils samples were qualitatively analyzed, and it was determined that the content of terpenes in Finger Citron volatile oils was the highest.By analyzing the GC-IMS fingerprint, PCA, and adjacent Euclidean distance map of the sample, and comparing the content differences of FS-1, FS-2, and FS-3, it can be concluded that irradiation does have a specific effect on the volatile organic compounds of Finger Citron volatile oils, and the larger the irradiation dose gap, the greater the impact on the content of the compound; however, no new compounds are generated.Comparing the fingerprints of the three irradiation doses of Finger Citron volatile oils, it can be observed that a 10kGy 60 Co irradiation dose has a greater effect on (E)-2-Pentenal, Pyridine, Ethyl propanoate, Propyl acetate, Acetic acid, Methylpyrazine, and other compounds, but less of an impact on terpenes.In summary, this study suggests that 60 Co irradiation will affect the components of Finger Citron volatile oils, but the impact on its main components is small, and the moderate measurement of 60 Co irradiation sterilization can be used in the sterilization of Finger Citron medicinal materials, providing a certain reference for the storage and processing of Chinese medicinal materials [9].
GC-IMS has a rapid response time and efficient separation capabilities [29][30][31][32][33]. GC-IMS technology can be used to analyze volatile organic compounds of different products, and it can be used to distinguish isomers effectively, so it is useful for assessing food quality [31][32][33][34].
In today's world, in which food safety issues are so highly valued, irradiation sterilization technology has undergone over one hundred years of development [35].It can sterilize without damaging nutritional components, improve hygiene quality, and even improve the flavor of food to a certain extent.It is widely used in the food and drug processing industry [33][34][35][36][37].However, an improper selection of the irradiation dose can sometimes lead to changes in pharmacological effects and nutritional components [38,39].Therefore, it is of great significance to study whether this technology is suitable for the sterilization of bergamot and to choose the optimal sterilization dose while maintaining the quality of the food.

Figure 1 .
Figure 1.Three-dimensional spectra of volatile organic compounds of three samples: 0 kGy irradiation dose (FS-1), 5 kGy irradiation dose (FS-2), and 10 kGy irradiation dose (FS-3).Note: The x-axis represents the GC drift time (normalization), the y-axis represents the retention time (s), and the z-axis represents the intensity of the peak.With blue as the background, the bright spot indicates a substance; the closer its color is to red, the greater the concentration.As shown in Figure 2, the Reporter plug-in was used to generate a two-dimensional top view of Finger Citron volatile oils.It consists of drift time, retention time, and ion signal intensity.This figure has a blue background, and the red vertical line at 1.0 is the RIP (reactive ion peak).Gas chromatography retention time (s) corresponds to the ordinate coordinate and ion drift time (normalization) to the abscissa.Volatile organic compounds are represented by the points on either side of the RIP.The color indicates the concentration of the substance, where white indicates the lowest concentration, red indicates the highest concentration, and a darker color indicates the highest concentration.
signal intensity.This figure has a blue background, and the red vertical line at 1.0 is the RIP (reactive ion peak).Gas chromatography retention time (s) corresponds to the ordinate coordinate and ion drift time (normalization) to the abscissa.Volatile organic compounds are represented by the points on either side of the RIP.The color indicates the concentration of the substance, where white indicates the lowest concentration, red indicates the highest concentration, and a darker color indicates the highest concentration.

Figure 2 .
Figure 2. Two-dimensional spectra of volatile substances in three groups of Finger Citron samples.Drift time (normalization) is represented on the x-axis; retention time (s) is represented on the yaxis.

Figure 3 .
Figure 3. Analysis of the spectral differences between Finger Citron samples from three groups.A comparison of the volatile substance content of different samples was conducted using the FS-1

Figure 2 .
Figure 2. Two-dimensional spectra of volatile substances in three groups of Finger Citron samples.Drift time (normalization) is represented on the x-axis; retention time (s) is represented on the y-axis.

Figure 2 .
Figure 2. Two-dimensional spectra of volatile substances in three groups of Finger Citron samples.Drift time (normalization) is represented on the x-axis; retention time (s) is represented on the yaxis.

Figure 3 .
Figure 3. Analysis of the spectral differences between Finger Citron samples from three groups.A comparison of the volatile substance content of different samples was conducted using the FS-1

Figure 3 .
Figure 3. Analysis of the spectral differences between Finger Citron samples from three groups.A comparison of the volatile substance content of different samples was conducted using the FS-1 sample as a reference.The red color indicates a higher concentration of substances in the sample than in the reference sample, whereas the blue color indicates a lower concentration.

Figure 4 .
Figure 4. Characteristic peak position plot of volatile organic compounds of Finger Citron at different irradiation doses.

Figure 4 .
Figure 4. Characteristic peak position plot of volatile organic compounds of Finger Citron at different irradiation doses.

Figure 5 .
Figure 5. Principal component analysis of volatile organic compounds of Finger Citron in three irradiation doses.Figure 5. Principal component analysis of volatile organic compounds of Finger Citron in three irradiation doses.

Figure 5 .
Figure 5. Principal component analysis of volatile organic compounds of Finger Citron in three irradiation doses.Figure 5. Principal component analysis of volatile organic compounds of Finger Citron in three irradiation doses.

Foods 2023 , 1 Figure 6 .
Figure 6.Euclidean distance map of samples' nearest neighbor.(Compared to distance, similarity higher the closer the distance.).

Figure 7 .
Figure 7. Gallery plot of volatile organic compounds selected via GC-IMS.

Figure 6 .
Figure 6.Euclidean distance map of samples' nearest neighbor.(Compared to distance, similarity is higher the closer the distance.).

Foods 2023 , 12 Figure 6 .
Figure 6.Euclidean distance map of samples' nearest neighbor.(Compared to distance, similarity is higher the closer the distance.).

Figure 7 .
Figure 7. Gallery plot of volatile organic compounds selected via GC-IMS.

Figure 7 .
Figure 7. Gallery plot of volatile organic compounds selected via GC-IMS.

Author Contributions:
Conceptualization, Y.X.; methodology, Y.X.; software, G.H.; validation, W.Z.; formal analysis, Y.X. and C.L.; investigation, G.H.; resources, G.H.; data curation, W.Z.; writingoriginal draft preparation, Y.X. and C.L.; writing-review and editing, Y.L. and D.H.; visualization, Y.X. and C.L.; supervision, D.H.; project administration, Y.L.; funding acquisition, D.H.All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by the Hunan Provincial Natural Science Foundation of China (No. 2019JJ60018), the First-class Discipline Project on Traditional Chinese Medicine of Hunan University of Chinese Medicine (No. 2018), Department of Science and Technology of Xiangxi Tujiazu & Miaozu Autonomous Prefecture (No. 2022JSGG03), Department of Science and Technology of Huaihua (No. 2021N2603), and the Changsha Natural Science Foundation of China (No. kq2208186).

Table 1 .
Results of component analysis of Finger Citron volatile oils.

Table 1 .
Results of component analysis of Finger Citron volatile oils.
Note: RI is the retention index, Rt is the retention time, Dt is the drift time, and [RIPrel] refers to the normalization process.

Table 2 .
Area of Finger Citron volatile oils.