2.8. Determination of Volatile Compounds by GC-MS

A qualitative analysis of the volatile compounds was performed using a GC-MS system (Agilent Technologies, Santa Clara, CA, USA) consisting of a gas chromatograph (7890 A) in conjunction with a quadrupole mass spectrometer (MSD 5975 C), as described by relevant research [23] with slight modifications. Initially, 2 g of whole fruit pulp was weighed and placed in a 10 mL vial, followed by the addition of 4 mL water. The mixture was thoroughly vortexed to ensure homogeneity agitated at 400 rpm and 60°C for 30 min before extraction occurred. Subsequently, a 65 μm DVB/PDMS SPME fiber (Supelco, Bellefonte, PA, USA) was exposed to the sorption surface above the liquid level for 10 min to perform the analyses. The fiber was introduced into the GC injector where desorption was conducted at 250°C for 5 min, followed by the initiation and collection of data. An HP-5-MS fused silica capillary column (30 m×0.250 mm inside diameter, 0.25 μm film thickness; Agilent, USA) was used to separate the volatiles. The temperature of the injector, GC-MS interface, and ion source was at 250°C, 280°C, and 230°C, respectively. Analyses were performed using helium as a carrier gas at a column flow of 1.0 mL/min, and the electron impact ionization was 70 eV. The following oven temperature program was used: the initial temperature of 40°C was incrementally increased by 3°C min⁻¹ to 150°C, followed by progressive temperature augmentation of 5°C min⁻¹ to 300°C, where it was maintained for 3 min. The data was collected at a scanning range of 35-350. The analytical process to obtain the volatile compounds from the lemons was repeated three times. The proportion of each compound was estimated dividing its mean area by the total area of the chromatogram and expressed as percentages.

The volatile compounds were identified by a GC–MS QP2010plus (Shimadzu Corporation, Kyoto, Japan), as described by Wang et al. [24] with some modifications to the method. Initially, 2 g of lemon peel (previously thawed at ambient temperature), 1 g of NaCl, and 1 μL of cyclohexanone standard solution (0.95 mg) were transferred to a SPME extraction bottle (20 mL) and preheated in a water bath at 40°C for 15 min before the extraction process occurred. Subsequently, a DVB/CAR/PDMS fiber (Supelco, 50/30 μm×20 mm) was exposed to the headspace above the liquid surface for 45 min at 40°C. To quantify the volatile compounds, the same described GC and MS chromatographic conditions were adopted. The mass spectra of each compound were compared with those available in the National Institute of Standards and Technology (NIST) library and by comparing the linear retention index (LRI) with those available in existing scientific literature.