Table 2.

The overview of analytical approaches utilizing SPME-based techniques along with instrumental methods applied in the analysis of ECs in biological matrices.

Analyte(s)	Sample type and volume	Sample preparation		Instrumental analysis		Internal standard(s)	LOD/LOQ	Linearity	Application	Refs.
		Pretreatment	Extraction	Chromatographic separation	Detection
AEA, 2-AG, 1-AG	Human plasma (300 μL)	PPt with 200 μL of matrix modifier solution	SPME with HLB coating, extraction: 60 min, desorption: 60 min in MeOH:IPA (50:50, V/V) for SPME-UHPLC-MS/MS, or 5 min in MeOH:IPA (50:50, V/V) with 0.1% acetic acid for Bio-SPME-nano-ESI-MS/MS	UHPLC-MS/MS gradient mode: phase A: water with 0.1% of acetic acid, phase B: ACN:IPA (90:10, V/V), flow rate: 0.3 mL/min at 40 °C	UHPLC-MS/MS: CORTECS C18 superficially porous column (100 mm × 2.1 mm, 1.6 μm), and CORTECS C18 superficially porous guard column (5 mm × 2.1 mm, 1.6 μm)	AEA-d4, 2-AG-d5, 1-AG-d5	LOQ: 50 ng/mL for AEA and 2-AG	SPME-UHPLC-MS/MS: 1–200 ng/mL, SPME-nano-ESI-MS/MS: 50–800 ng/mL	Analysis of plasma samples spiked with the analytes	[27]
AEA, 2-AG	1% agarose gel and rat brain	–	In vivo SPME with 1.5 mm length RP-amide-C16 coating, extraction: 30 min, desorption: 45 min in ACN:water (72:25, V/V)	Gradient mode: phase A: 0.01 M ammonium acetate, phase B: ACN, flow rate: 0.3 mL/min at 40 °C	BEH C18 column (0.21 cm × 5 cm, 1.7 μm)	AEA-d4, 2-AG-d5	LLOQ: 0.05 ng/mL	0.05–50 ng/mL	In vivo brain sampling of BST and PH regions in male Sprogue–Dawley rats (loud noise stress group and no noise control group)	[52]
AEA, 2-AG	Human plasma (400 μL)	PPt with 800 μL of ACN; the supernatant was dried and then reconstituted with 120 μL of 90:10 (V/V) mixture of 5 mM ammonium acetate (pH 7.0) aqueous solution and ACN.	On-line IT-SPME with PIL-based (10.0 cm × 0.53 mm) capillary column; 80 μL sample solution was percolated through the PIL-based capillary using water (solvent A); flow rate: 0.1 mL/min	Mobile phase used for desorption step: 0.5% FA in water (solvent B) and ACN with 0.5% FA (solvent C) at 30:70 (V/V) ratio for 2 min, flow rate: 0.1 mL/min at 40 °C	Core-shell Kinetex C18 column (100 mm × 2.1 mm, 1.7 μm)	AEA-d4, 2-AG-d5	LLOQ: 0.1 ng/mL for AEA, 0.05 ng/mL for 2-AG	0.1–100 ng/mL for AEA, 0.05–100 ng/mL for 2-AG	Analysis of plasma samples obtained from six patients with PD	[53]
AEA, 2-AG	Brain homogenate (150 μL)	Homogenization with 0.1 mol/L FA solution; PPt with 150 μL of ACN and 100 μL of 5 mol/L ammonium formate solution; the upper organic phase was collected, dried and reconstituted with 50 μL of water:ACN (30:70, V/V).	IT-SPME with RAM phase; 10 μL of sample solution was percolated through the RAM microtube with mobile phase consisting of water:ACN (80:20, V/V); extraction time: 3 min, flow rate: 0.2 μL/min	Mobile phase used for desorption step: 0.5% FA aqueous solution:ACN ( 30:70, V/V) for 3 min, flow rate: 0.2 μL/min	–	AEA-d4, 2-AG-d5	LOQ: 6 ng/mL for AEA, 10 ng/mL for 2-AG	6–30 ng/mL for AEA, 10–100 ng/mL for 2-AG	Analysis of 10 brain samples from Wistar male rats lesioned with 6-hydroxydopamine; two striatum tissue samples were obtained from each animal: one ipsilateral to the lesion and another contralateral to the lesion.	[103]
AEA, 2-AG, NADA, 2-AGe	PBS (1 mL) and intact rat brain	–	SPME with 4 mm length C18 coating, extraction: 30 min, desorption: 30 min in MeOH:IPA (50:50, V/V)	Gradient mode: phase A: water with 0.1% FA, phase B: ACN with 0.1%FA, flow rate: 0.3 mL/min at 30 °C	Kinetex XB-C18 column (100 mm × 2.1 mm, 2.6 μm)	AEA-d11	LOD: 0.4 ng/mL for AEA, 5 ng/mL for 2-AG, 1 ng/mL for NADA and 2-AGe	5–150 ng/mL for AEA, NADA and 2-AGe, 50–1,500 ng/mL for 2-AG	Analysis of intact (non-homogenized) brain structures (cortex, cerebellum and striatum) of Wistar Han female and male rats at different stages of development (1 month old, 3 months old, and 24 months old)	[104]

1-AG: 1-arachidonoyl glycerol; 2-AG: 2-arachidonoyl glycerol; 2-AGe: arachidonoyl glycerol ether; ACN: acetonitrile; AEA: N-arachidonoyl ethanolamide; BEH: ethylene bridged hybrid; BST: stria terminalis; FA: formic acid; HLB: hydrophilic-lipophilic balanced; IPA: isopropyl alcohol; IT-SPME: in-tube solid phase microextraction; LLOQ: lowest limit of quantification; LOD: limit of detection; LOQ: limit of quantification; MeOH: methanol; NADA: N-arachidonoyl dopamine; PBS: phosphate buffered saline; PD: Parkinson's disease; PH: posterior hypothalamus; PIL: polymeric ionic liquid; PPt: protein precipitation; RAM: phase-restricted access material phase; SPME: solid-phase microextraction; UHPLC-MS/MS: ultra-high-performance liquid chromatography-tandem mass spectrometry.