Table A2.

Summary of relative peak area ¹ and CV (%) of the investigated analytes in the experiment where pH buffering in all steps of the SPE protocol was tested. ¹ mean of SPE replicates/mean of PPT replicates expressed as percent (%). ² The analyte was detected as a fragment.

	SPE MCX No pH Control		SPE MCX pH Control		WCX Basic No pH Control		WCX Basic pH Control		PPT
	Rel. int. (%) 1	CV %	Rel. int. (%) 1	CV %	Rel. int. (%) 1	CV %	Rel. int. (%) 1	CV %	CV %
Caffeine 0.89 min	75.06	60.24	83.65	26.55	75.79	39.91	83.98	29.66	5.58
Theophylline 1.21 min	62.51	5.47	71.66	14.79	91.01	5.60	96.13	4.82	4.47
Hypoxanthine 4.39 min	113.06	3.30	72.36	15.81	112.73	2.85	117.88	10.31	6.13
Creatinine 4.69 min	118.92	5.50	71.64	23.31	119.93	5.19	130.80	14.93	8.21
Palmitoylcarnitine 5.09 min	96.37	9.51	73.63	17.11	95.10	0.92	155.56	17.19	2.81
Choline 6.01 min	1.74		3.17		5.59		28.18	11.43	5.48
Pantothenic acid 2 7.35 min	95.41	3.35	67.88	3.15	97.59	5.04	64.48	11.43	2.12
Hexanoylcarnitine 7.8 min	95.89	42.41	92.61	28.38	102.74	7.84	131.01	19.49	4.69
1-Methylnicotinamide 8.02 min	0.00	4.15	0.00	22.91	0.00	6.68	12.48	4.92	7.58
Butyrylcarnitine 9.08 min	273.41	1.17	77.49	20.43	278.09	9.84	83.05	13.50	12.63
Oxoproline 9.13 min	311.67	15.80	110.16	38.51	357.32	21.98	77.07	22.53	11.54
Homostachydrine 9.41 min	100.71	38.70	65.39	7.51	115.96	96.24	52.53	87.20	21.20
Propionylcarnitine 9.78 min	210.52	7.09	94.03	18.04	243.53	4.66	71.86	3.49	16.65
Betaine 9.83 min	108.60	6.86	79.86	14.09	112.69	13.49	71.19	2.09	16.92
Methylnicotinic acid 10.16 min	181.86	4.16	80.55	14.61	112.79	12.95	46.43	8.46	9.85
Acetylcarnitine 10.47 min	604.31	3.36	277.47	14.83	784.74	5.22	139.35	10.22	11.28
Acetylcholine 10.78 min	2279.60	5.52	247.01	15.20	1183.32	8.53	142.26	20.24	12.07
Carnitine 10.89 min	279.17	1.59	122.97	9.92	281.05	10.11	73.42	2.48	3.67
Creatine 11.07 min	181.63	10.00	79.86	12.13	178.08	8.59	42.96	15.11	3.10
Arginine 12.5 min	52.15	2.95	28.85	9.38	39.26	16.09	22.32	20.15	3.68