## 4 Conclusions  
The method developed in this study can easily collect 2-nonenal, a component of human body odor, from the skin surface by gauze wiping and can be selectively and sensitively analyzed by HS-SPME/GC-MS. Gauze wiping is a rapid and non-invasive sampling method for skin emissions and secretions and has better storage stability than other tested methods. As shown in Table 4, the proposed method is more sensitive than previously reported methods, even when converted into the skin surface area and the amount of recovery per hour. We believe that this method is a useful tool for 2-nonenal analysis in body odor. Although this study focused on 2-nonenal, body odor is a complex mixture of compounds, and therefore, it is necessary to analyze not only 2-nonenal but also a wide range of related compounds in the future. The method developed in this study may be applicable to their comprehensive analysis.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Compounds** & **Analytical Method** & **Sampling and Preconcentration** & **Detection Amount** & **Reference** \\ \hline
2-Nonenal & GC–MS & Tedlar bag and TENAX-TA column & 15.1 \(\pm\) 20.4 ng\(\cdot\)cm\({}^{-3}\)(1) & [1] \\ Five aldehydes and & & Passive flux sampler & \\  & HPLC–UV & (trapping filter: DNPH impregnated & 17 ng\(\cdot\)cm\({}^{-2}\)-h\({}^{-1}\)(2) & [17] \\ acetone & & filter) & \\
2-Nonenal and & & Passive flux sampler & \\  & diacetyl1 & & (trapping media: Monotrap, DCC18) & 0.020 to 5.8 ng\(\cdot\)cm\({}^{-2}\).h\({}^{-1}\)(3) & [18] \\
2-Nonenal & GC–MS & Fiber SPME & & \\  & & (Wiping method) & 2.4 pg\(\cdot\)cm\({}^{-2}\).h\({}^{-1}\)(4) & This study \\ \hline \hline \end{tabular}
\end{table}
Table 4: Comparison of analytical methods for 2-nonenal and other aldehydes in body odor.  
Figure 7: Sampling methods of skin emissions and secretions.  
**Supplementary Materials:** The following are available online. Figure S1: Presumed formation mechanism of 2-nonenal by lipid peroxidation. Data from [PERSON] et al., 2001 [1], Figure S2: Procedure for extraction by headspace fiber SPME and desorption for GC-MS analysis. Data from [PERSON] and [PERSON], 2011 [20].  
**Author Contributions:** Conceptualization, K.S. and H.K.; methodology, K.S. and H.K.; software, K.S. and H.K.; validation, K.S. and H.K.; formal analysis, Y.T., C.S., and K.S.; investigation, K.S. and H.K.; resources, K.S. and H.K.; data curation, Y.T., C.S., and K.S.; writing--original draft preparation, K.S. and H.K.; writing--review and editing, K.S.; visualization, K.S. and H.K.; supervision, K.S.; project administration, K.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and was approved by the ethics committee of Shuijtsu University.  
**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.  
**Data Availability Statement:** The data presented in this study are available from the corresponding author upon request.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Sample Availability:** Samples of the compounds are not available from the authors.