MIDcor, an R-program for deciphering mass interferences in mass spectra of metabolites enriched in stable isotopes

Table 3 The differences (D, Eq. 4, expressed as % of total amount of a substance) between calculated and measured MID for some TMS derivatives of metabolites

m/z:	M	M + 1	M + 2	M + 3	M + 4	M + 5	M + 6	Max df,case#
glucose	−0.35	−0.62	0.94	0.02	0.00	0.00	0.00	2.25
+medium	−2.60	0.64	1.60	0.26	0.06	0.01	0.00	case 1
Glu (2–4)	−9.30	7.74	1.33	0.21	nd	nd	nd	−0.10
+medium	−9.20	7.79	1.23	0.22	nd	nd	nd	case 2
Glu (2–5)	−1.30	0.48	0.65	0.04	0.14	nd	nd	0.10
+medium	−1.40	0.51	0.65	0.04	0.18	nd	nd	case 2
Aspartate	−0.98	−0.06	0.88	0.14	0.02	0.00	nd	0.12
+medium	−1.10	0.05	1.03	−0.02	0.04	nd	nd	case 2
serine	−0.70	−0.37	0.87	0.13	nd	nd	nd	0.40
+medium	−1.10	0.02	0.91	0.17	nd	nd	nd	case 2
glycine	−0.50	−0.14	0.57	0.08	0.01	nd	nd	0.70
+medium	−1.20	0.27	0.70	0.23	0.01	nd	nd	case 1

The first row for each metabolite was calculated using Eq. 4 for the measurements in the minimal solution for derivatization (conditions (b), see 3.1.4). The next row indicated as “+medium” was calculated for the measurements in the complete media for cell incubation (conditions (a)). “Glu (2–4)” and “Glu (2–5)” refer to the fragments C2-C4 and C2-C5 of glutamate after ionization in the mass spectrometer, “nd” stands for “not determined”. The right column indicates the maximal difference between the two rows for each metabolite, and, based on this difference and using a threshold value of 0.5%, recommendation to analyze it in accordance with case 1 or case 2

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