Polymeric background ion series in positive mode ESI LC-MS

Many background ions appear not as single peaks but as families of peaks separated by a characteristic repeating unit, such as polymers, salt clusters, and solvent adducts. The mass difference between neighboring peaks identifies the family. The tables below list the polysiloxane series, common repeating units, and the phthalate plasticizers frequently seen in ESI spectra. Another good resource is the Contamination Search Tools at the University of Leiden.

Polydimethylcyclosiloxanes

Polydimethylcyclosiloxanes are widespread compounds in industrial products, e.g. deodorants, and are ubiquitously present in ambient air (mass difference 74.018793).

Polysiloxanes

Mono mass [M+H]+ +17 Da (ammonium adduct) -16 Da (loss of methane) Formula
75.02607     [(Si [CH3]2 O)1 + H]+
149.04486     [(Si [CH3]2 O)2 + H]+
223.06366     [(Si [CH3]2 O)3 + H]+
297.08245     [(Si [CH3]2 O)4 + H]+
371.10124 388.12779 355.06994 [(Si [CH3]2 O)5 + H]+
445.12003 462.14658 429.08873 [(Si [CH3]2 O)6 + H]+
519.13883 536.16538 503.10753 [(Si [CH3]2 O)7 + H]+
593.15762 610.18417 577.12632 [(Si [CH3]2 O)8 + H]+
667.17641 684.20296 651.14511 [(Si [CH3]2 O)9 + H]+
741.19521 758.22176 725.16391 [(Si [CH3]2 O)10 + H]+

Repeating Units, positive mode

Mass difference M or subunit Description
14.01565 [CH2] Alkane chains, waxes, fatty acids, methylation
15.99492 O Oxidation
18.01057 H2O Water clusters
28.03130 [C2H4] Natural alkane chains such as fatty acids
32.02622 [CH3OH] Methanol clusters
41.02655 [CH3CN] Acetonitrile clusters
42.04695 [C3H6] Propyl repeating units, propylation
44.02622 [C2H4O] Polyethylene glycol, PEG (Tritons and Tween buffers)
49.99681 [CF2] From perfluoro compounds
53.00323 NH4Cl Ammonium chloride adducts (NH4Cl) salt adducts/clusters
56.06260 [C4H8] Butyl repeating units, butylation
57.95862 NaCl NaCl sodium chloride clusters (distinguish from PPG by Cl Isotope pattern)
58.04187 [C3H6O] Polypropylene glycol (PPG), and related compounds
63.00000 NH4Cl Salts
63.03203 [CHOONH4] Ammonium formate adducts/clusters
67.98742 [NaHCO2] Sodium formate clusters
67.98742 [CHOONa] Sodium formate adducts/clusters
72.03953 OH Replacement of OH by OSi(CH3)3 trimethylsiloxane
73.93256 KCl KCl adducts (distinguishable by Cl isotope pattern)
74.01879 [O-Si(CH3)2] Polysiloxane, silicone rubber polymer
77.0477 NH4Ac Ammonium acetate salts (CH3COONH4)
78.01394 [C2H6OS] DMSO adducts/clusters, dimethylsulfoxide solvent
82.00307 [NaCH3CO2] Sodium acetate clusters
84.05159 [C2D6OS] Deuterated DMSO adducts/clusters, NMR solvent
106.90509 107Ag 107Ag, silver clusters (MALDI) non-polar polymers, with 109Ag (~1:1)
108.90476 109Ag 109Ag, silver clusters (MALDI) non-polar polymers, with 107Ag ( ~1:1)
113.9929 CF3COOH TFA (trifluoroacetic acid) adducts (CF3COOH)
121.9383 [NaClO4] Sodium perchlorate adducts (NaClO4)
135.97481 NaCF3CO2 Sodium TFA (trifluoroacetic acid) adducts (CF3COONa)
162.05283 [C6H10O5] Polysaccharides (C6H10O5)
226.16813 [C12H22N2O2] Cyclic oligomers from polyamide 66 (series observed with m/z 453, 679, 905); Nylon HPLC solvent filters can produce nylon (6,6monomer) peaks at masses of 226 Da a dimer 452, trimer 678 and tetramer 905 Da. The contaminant is very hard to get rid of since it binds very well to C18.
259.80992 CsI Cesium iodide clusters, used as calibration
288.1371 [CH3(CH2)11OSO3Na] SDS (sodium dodecylsulfate) adducts (CH3(CH2)11OSO3Na)

Phthalates

The ions at m/z 391 and 419 are typically very prominent in ESI.

Avg mass Exact mass Formula Compound Description
195.0652 194.05736 C6H4(COOCH3)2 DMP Dimethyl phthalate
223.0965 222.08866 C6H4(COOC2H5)2 DEP Diethyl phthalate
247.0965 246.08866 C6H4(COOCH2CH=CH2)2 DAP Diallyl phthalate
251.1278 250.11996 C6H4(COO(CH2)2CH3)2 DPP Di-n-propyl phthalate
279.1591 278.15126 C6H4(COO(CH2)3CH3)2 DBP Di-n-butyl phthalate
279.1591 278.15126 C6H4(COOCH2CH(CH3)2)2 DIBP Diisobutyl phthalate
305.1747 304.16691 CH3(CH2)3OOCC6H4COOC6H11 BCP Butyl cyclohexyl phthalate
307.1904 306.18256 C6H4(COO(CH2)4CH3)2 DNPP Di-n-pentyl phthalate
331.1904 330.18256 C6H4(COOC6H11)2 DCP Dicyclohexyl phthalate
313.1434 312.13561 CH3(CH2)3OOCC6H4COOCH2C6H5 BBP Butyl benzyl phthalate
335.2217 334.21386 C6H4(COO(CH2)5CH3)2 DNHP Di-n-hexyl phthalate
335.2217 334.21386 C6H4(COO(CH2)3CH(CH3)2)2 DIHxP Diisohexyl phthalate
363.253 362.24516 C6H4(COO(CH2)4CH(CH3)2)2 DIHpP Diisoheptyl phthalate
363.253 362.24516 CH3(CH2)3OOCC6H4COO(CH2)9CH3 BDP Butyl decyl phthalate
391.2843 390.27646 C6H4(COOCH2CH(C2H5)(CH2)3CH3)2 DEHP, DOP Di(2-ethylhexyl) phthalate
391.2843 390.27646 C6H4(COO(CH2)7CH3)2 DNOP Di(n-octyl) phthalate
391.2843 390.27646 C6H4(COO(CH2)5CH(CH3)2)2 DIOP Diisooctyl phthalate
419.3156 418.30776 CH3(CH2)7OOCC6H4COO(CH2)9CH3 ODP n-Octyl n-decyl phthalate
419.3156 418.30776 C6H4(COO(CH2)6CH(CH3)2)2 DINP Diisononyl phthalate
447.3469 446.33906 C6H4(COO(CH2)7CH(CH3)2)2 DIDP Diisodecyl phthalate
475.3782 474.37036 C6H4(COO(CH2)10CH3)2 DUP Diundecyl phthalate
475.3782 474.37036 C6H4(COO(CH2)8CH(CH3)2)2 DIUP Diisoundecyl phthalate
531.4408 530.43296 C6H4(COO(CH2)12CH3)2 DTDP Ditridecyl phthalate
531.4408 530.43296 C6H4(COO(CH2)10CH(CH3)2)2 DIUP Diisotridecyl phthalate