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Electronic Supplementary Information
Negative Electrospray Ionization on Porous Supporting Tips
for
Mass Spectrometric Analysis: Electrostatic Charging Effect
on
Detection Sensitivity and Its Application to Explosive
Detection
Melody Yee-Man Wonga, Sin-Heng Man
a, Chi-Ming Che
a, Kai-Chung Lau
b, Kwan-Ming Ng*
a
a Department of Chemistry, and State Key Laboratory of Synthetic
Chemistry, The University of
Hong Kong, Pokfulam Road, Hong Kong SAR, People’s Republic of
China
b Department of Biology and Chemistry, City University of Hong
Kong, Tat Chee Avenue, Hong
Kong SAR, People’s Republic of China
To whom correspondence should be addressed:
K.-M. Ng
Fax: (852) 2857 1586 ; E-mail: [email protected]
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Table S1. Elemental (CH) composition analysis of fibers of
polyester tips and granules of polyethylene tips performed by the
Institute of
Chemistry, Chinese Academy of Sciences, Beijing, China.
Weight Percentage of Carbon Weight Percentage of Hydrogen
Measured Theoretical Measured Theoretical
Polyester
H-(C10H8O4)n-OH 60.81% 62.50% 4.87% 4.20%
Polyethylene
H-(C2H4)n-H 85.11% 85.63% 12.97% 14.37%
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Table S2. The effect of the hydrolysis and the in-source
fragmentation (i.e., NO loss) on the total ion intensities of
nitrobenzene derivatives using polyethylene supporting tips.
[M]-‧
, Molecular Ion [M – NO]-‧
, Fragment Ion Hydrolyzed Product
Total Ion Intensity
a
Correction Factorb
Ratio: Total Ion Intensity/[M]-‧
m/z (Ion Intensity) m/z (Ion Intensity) m/z (Ion Intensity)
NB 123.0 (953) 92.9 (5491) NDc 6444 6.762
4-NT 137.0 (124) 106.9 (183) NDc 307 2.476
1-F-4-NB 141.0 (17822) 111.0 (1425) 138.0d (4667) 23914
1.342
1-Cl-4-NB 157.0 (7368) 127.0 (1575) 138.0d (2018) 10961
1.488
1,3-DNB 168.0 (11664) 138.0 (7974) NDc 19638 1.684
2,4-DNT 182.0 (870) 152.0 (285) NDc 1155 1.328
1-F-2,4-DNB 186.0 (28180) 156.0 (10621) 182.9e (10618) 49419
1.754
1-Br-4-NB 200.9 (455) 171.1 (526) 138.0d (689) 1670 3.670
1-Br-2,4-DNB 245.9 (199) 215.9 (205) 182.9e (2760) 3164
15.90
1-I-4-NB 248.9 (219) 218.9 (97) 138.0d (1844) 2160 9.863
1-I-2,4-DNB 293.8 (4727) 263.8 (4162) 182.9e (22946) 31835
6.735
a total ion intensity is the sum of [M]
-‧, [M – NO]
-‧ and deprotonated ion of the hydrolyzed product (if any);
b correction factor is used to determine the corrected total ion
intensity for the correlation with electron affinities of
nitrobenzene derivatives; c ND stands for not detected;
d the ion peak at m/z 138.0 corresponds to the deprotonated ion
of the hydrolyzed product of 1-F-4-NB, 1-Cl-4-NB, 1-Br-4-NB and
1-I-4-NB formed via nucleophilic
substitution reaction; e the ion peak at m/z 182.9 corresponds
to the deprotonated ion of the hydrolyzed product of 1-F-2,4-DNB,
1-Br-2,4-DNB and 1-I-2,4-DNB formed via nucleophilic substitution
reaction.
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Table. S3. The effect of the hydrolysis and the in-source
fragmentation (i.e., NO loss) on the total ion intensities of
nitrobenzene derivatives using polyester supporting tips.
[M]-‧
, Molecular Ion [M-NO]-‧
, Fragment Ion Hydrolyzed Product Total
Ion Intensitya
Correction Factorb
Ratio: Total Ion Intensity/[M]-‧
m/z (Ion Intensity) m/z (Ion Intensity) m/z (Ion Intensity)
NB 123.0 (67) 93.0 (1075) NDc 1142 17.04
4-NT 137.0 (67) 107.0 (1151) NDc 1218 18.18
1-F-4-NB 140.9 (1042) 110.9 (556) 138.0d (320) 1918 1.841
1-Cl-4-NB 156.9 (3932) 126.9 (7929) 138.0d (370) 12231 3.111
1,3-DNB 168.0 (5387) 138.0 (409) NDc 5796 1.076
2,4-DNT 182.0 (2285) 152.0 (470) NDc 2755 1.206
1-F-2,4-DNB 186.0 (5108) 156.0 (1910) 183.0e (1981) 8999
1.762
1-Br-4-NB 200.9 (227) 170.9 (68) 138.0d (1285) 1580 6.960
1-Br-2,4-DNB 245.9 (142) 215.9 (221) 183.0e (6277) 6640
46.76
1-I-4-NB 248.9 (100) 218.9 (31) 138.0d (981) 1112 11.12
1-I-2,4-DNB 293.8 (438) 263.9 (682) 183.0e (4526) 5646 12.89
a total ion intensity is the sum of [M]
-‧, [M-NO]
-‧ and deprotonated ion of the hydrolyzed product (if any);
b correction factor is used to determine the corrected total ion
intensity for the correlation with electron affinities of
nitrobenzene derivatives; c ND stands for not detected;
d the ion peak at
m/z 138.0 corresponds to the deprotonated ion of the hydrolyzed
product of 1-F-4-NB, 1-Cl-4-NB, 1-Br-4-NB and 1-I-4-NB via
nucleophilic substitution
reaction; e the ion peak at m/z 183.0 corresponds to the
deprotonated ion of the hydrolyzed product of 1-F-2,4-DNB,
1-Br-2,4-DNB and 1-I-2,4-DNB via nucleophilic substitution
reaction.
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Table S4. Theoretically calculated and experimentally determined
electron affinities of nitrobenzene derivatives.
Compounds
Electron affinity (eV)
G4 Literature |ΔG4 -Lit|
Nitrobenzene 1.06 1.00 ± 0.01a 0.06
4-Nitrotoluene 0.99 0.93 ± 0.09b 0.06
1-Fluoro-4-nitrobenzene 1.13 1.08 ± 0.09b 0.05
1-Chloro-4-nitrobenzene 1.27 1.23 ± 0.09b 0.04
1-Bromo-4-nitrobenzene 1.32 -- --
1,3-Dinitrobenzene 1.65 1.66 ± 0.10c 0.01
2,4-Dinitrotoluene 1.58 -- --
1-Fluoro-2,4-dinitrobenzene 1.71 -- --
1-Chloro-2,4-dinitrobenzene 1.78 -- --
1-Bromo-2,4-dinitrobenzene 1.80 -- --
a C. Desfrancois, V. Periquet; S. A. Lyapustina.; T. P. Lippa;
D. W. Robinson; K. H. Bowen; H. Nonaka and Compton, J. Chem.
Phys.,
1999, 111, 4569 - 4576
b C. Huh; C. H. Kang; H. W. Lee; H. Nakamura; M. Mishima; Y.
Tsuno and H. Yamataka, Bull. Chem. Soc. Japan, 1999, 72,
1083-1091
c S. Chowdhury; T. Heinis; E. P. Grimsrud; P. Kebarle, J. Phys.
Chem., 1986, 90, 2747 - 2752
|ΔG4 -Lit| means the absolute difference between the calculated
EA (using G4 theory) and the EA obtained from the literature.
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Table. S5. The effect of in-source fragmentation (i.e., NO loss)
on the total ion intensities of nitrophenol derivatives using
polyethylene supporting tips.
[M – H]-, Molecular Ion [M – H – NO]
-, Fragment Ion Total
Ion Intensity a
Correction Factor b
Ratio: Total Ion Intensity/[M-H]- m/z (Ion Intensity) m/z (Ion
Intensity)
4-NPh 138.0 (9408) 108.0 (6600) 16008 1.702
2-Me-4-NPh 152.0 (4283) 122.0 (2818) 7101 1.658
2-F-4-NPh 156.0 (27870) 126.0 (4626) 32496 1.166
2,6-DMe-4-NPh 166.0 (5545) 136.0 (2100) 7645 1.379
2-Cl-4-NPh 172.0 (10967) 142.0 (427) 11394 1.039
2,6-DF-4-NPh 174.0 (15834) 144.0 (2712) 18546 1.171
2,4-DNPh 183.0 (201701) 153.0 (19645) 221346 1.097
2,6-DCl-4-NPh 205.9 (18480) 175.9 (1685) 20165 1.091
2-Br-4-NPh 215.8 (8934) 186.0 (2466) 11400 1.276
2,4,6-TNPh 227.9 (17982) 197.9 (6218) 24200 1.346
2-I-4-NPh 263.9 (5767) 233.9 (622) 6389 1.108
2,6-DBr-4-NPh 293.7 (5684) 263.8 (38) 5722 1.007
2,6-DI-4-NPh 389.7 (4554) 359.7 (269) 4823 1.059
a Total ion intensity is the sum of [M – H]
- and [M – H – NO]
-;
b correction factor is used to determine the corrected total ion
intensity for the correlation with pKa of nitrophenol
derivatives.
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Table. S6. The effect of in-source fragmentation (i.e., NO loss)
on the total ion intensities of nitrophenol derivatives using
polyester supporting tips.
[M-H]-, Molecular Ion [M-H-NO]
-, Fragment Ion
Total Ion Intensity
a
Correction Factorb
Ratio: Total Ion Intensity/[M-H]-
m/z (Ion Intensity) m/z (Ion Intensity)
4-NPh 138.0 (13476) 108.0 (5200) 18676 1.386
2-Me-4-NPh 152.0 (19742) 122.0 (2294) 22036 1.116
2-F-4-NPh 156.0 (9626) 126.0 (1955) 11581 1.203
2,6-DMe-4-NPh 166.0 (4874) 136.0 (784) 5658 1.161
2-Cl-4-NPh 171.9 (8611) 141.9 (333) 8944 1.039
2,6-DF-4-NPh 173.9 (21574) 144.0 (4335) 25909 1.201
2,4-DNPh 182.9 (6325) 153.0 (413) 6738 1.065
2,6-DCl-4-NPh 205.9 (7282) 175.9 (599) 7881 1.082
2-Br-4-NPh 215.9 (1146) 186.0 (549) 1695 1.479
2,4,6-TNPh 227.9 (14045) 198.0 (3415) 17460 1.243
2-I-4-NPh 263.8 (40831) 233.8 (184) 41015 1.005
2,6-DBr-4-NPh 293.7 (17322) 263.8 (157) 17479 1.009
2,6-DI-4-NPh 389.7 (10888) 359.7 (67) 10955 1.006
atotal ion intensity is the sum of [M-H]
- and [M-H-NO]
-;
b correction factor is used to determine the corrected total ion
intensity for the correlation with pKa of nitrophenol
derivatives.
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Table S7. Theoretically calculated gas phase acidities (Gibbs
free energy change of deprotonation), acid dissociation constant
from
the literature and theoretically calculated electron affinities
of nitrophenol derivatives.
a MH M
- + H
+, ΔGdeprotonation = G(M
-) + G(H+) – G(MH)
b J. A. Dean, Lange's Handbook of Chemistry, 15
th edition, McGraw-Hill, Inc
c M + e M
.-, EA = E(M) – E(M
.-)
Compounds
Gas Phase Acidity a
at 298K (kJ/mol)
Acid Dissociation
Constant at 298K (pKa)b
Electron affinityc
at 0 K (eV)
G4 theory G4 theory
4-Nitrophenol 1343.3 7.15 0.93
2-Methyl-4-nitrophenol 1341.8 7.43 0.90
2-Fluoro-4-nitrophenol 1330.7 5.70 1.13
2-Chloro-4-nitrophenol 1327.1 6.48 1.16
2-Bromo-4-nitrophenol 1323.3 -- 1.18
2,4-Dinitrophenol 1312.6 4.08 2.10
2,6-Dimethyl-4-nitrophenol 1340.4 7.19 0.88
2,6-Difluoro-4-nitrophenol 1310.4 -- 1.33
2,6-Dichloro-4-nitrophenol 1301.8 6.79 1.36
2,6-Dibromo-4-nitrophenol 1295.9 -- 1.39
2,4,6-Trinitrophenol 1254.5 0.42 (at 291K) 2.74
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Fig. S1. Infrared spectrum of a thin film made of the materials
obtained from polyester tips.
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Fig. S2. Infrared spectrum of a thin film made of the materials
obtained from polyethylene tips.
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Fig. S3. (a) (i) ESI mass spectrum of hydrolyzed product of
polyester tips; (ii) magnified mass spectrum of (i); and (iii)
simulation
spectrum of deprotonated terephalate [C8H6O4 – H]-. (b) Tandem
mass spectra of (i) [C8H6O4 – H]
- at m/z 165.0; and (ii) [C8H6O4 –
CO2 – H]- at m/z 121.0. Collision energy of (b) (i) and (ii)
were set at 15.0 eV and 6.0 eV, respectively.
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Fig. S4. Nuclear magnetic resonance (NMR) spectrum of
hydrolyzed
product of polyester tips.
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Fig. S5. Contact angle of a water droplet on (a) polyester thin
film; and (c) polyethylene thin film. (b) and (d) are the
magnified
images of (a) and (c), respectively.
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Fig. S6. Detection of background ions (CH3O-·, O2
-· and HO2
-·) by
negative tip spray ionization mass spectrometry.
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Fig. S7. Detection of 1,4-benzoquinone by negative tip spray
ionization mass spectrometry.
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Fig. S8. (a) Detection of (i) 1-bromo-4-nitrobenzene (1-Br-4-NB)
and (ii) 1-bromo-2,4-dinitrobenzene (1-Br-2,4-DNB) by negative
tip
spray ionization mass spectrometry. (b) Bromide ions were
simultaneously detected, indicating that fragmentation of (i)
1-Br-4-NB and
(ii) 1-Br-2,4-DNB occurred in electron capture ionization.
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Fig. S9. (a) Detection of (i) 1-iodo-4-nitrobenzene (1-I-4-NB)
and (ii) 1-iodo-2,4-dinitrobenzene (1-I-2,4-DNB) by negative tip
spray ionization
mass spectrometry. (b) Iodide ions were simultaneously detected,
indicating that fragmentation of (i) 1-I-4-NB and (ii) 1-I-2,4-DNB
occurred in
electron capture ionization.
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Fig. S10. Material effect on the onset voltage and sensitivity
for the
detection of (a) 1-fluoro-2,4-dinitrobenzene (1-F-2,4-DNB) and
(b)
1-chloro-2,4-dinitrobenzene (1-Cl-2,4-DNB) by negative tip
spray
ionization mass spectrometry.
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(a) 1,3-DNB
(b) 1-F-2,4-DNB (c) 1-Cl-2,4-DNB
Fig. S11 Material effect on the onset voltage and sensitivity
for the detection of (a) 1,3-dinitrobenzene (1,3-DNB), (b)
1-fluoro-2,4-dinitrobenzene
(1-F-2,4-DNB), and (c) 1-chloro-2,4-dinitrobenzene
(1-Cl-2,4-DNB) by negative tip spray ionization mass spectrometry
and ESI-MS.
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Fig. S12. Triboelectric series for organic polymers.
[Ref: A. F. Diaz and R. M. Felix-Navarro, J
Electrostat., 2004, 62, 277–290].
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Fig. S13. Correlation of ion intensities of deprotonated
nitrophenol
compounds with their respective gas-phase acidities.
2,4,6-TNPh:
2,4,6-trinitrophenol; 2,6-DBr-4-NPh:
2,6-dibromo-4-nitrophenol;
2,6-DCl-4-NPh: 2,6-dichloro-4-nitrophenol; 2,4-DNPh:
2,4-dinitrophenol; 2-Br-4-NPh: 2-bromo-4-nitrophenol;
2-Cl-4-NPh:
2-chloro-4-nitrophenol; 2-F-4-NPh: 2-fluoro-4-nitrophenol;
4-NPh:
4-nitrophenol; 2,6-DMe-4-NPh: 2,6-dimethyl-4-nitrophenol;
2-Me-4-NPh: 2-methyl-4-nitrophenol.
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Fig. S14. Material effect on the onset voltage and detection
sensitivity
for (a) 2-fluoro-4-nitrophenol (2-F-4-NPh) and (b)
2-chloro-4-nitrophenol
(2-Cl-4-NPh).
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Fig. S15. Chemical structures of spraying tip materials: (a)
wood;
(b) polyester; and (c) polyethylene.
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(a) 4-NPh
(b) 2-F-4-NPh (c) 2-Cl-4-NPh
Fig. S16 Material effect on the onset voltage and sensitivity
for the detection of (a) 4-nitrophenol (4-NPh), (b)
2-fluoro-4-nitrophenol (2-F-4-NPh)
and (c) 2-chloro-4-nitrophenol (2-Cl-4-NPh) by negative tip
spray ionization mass spectrometry and ESI-MS.
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Fig. S17. Linear calibration curve of 2,4-dinitrotoluene
(2,4-DNT)
recorded by negative tip spray ionization mass spectrometry.
1,3-dinitrobenzene (1,3-DNB) at 2x10-5
M was used as internal
standard.
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Fig. S18. Tandem mass spectrum of (a) 0.4 ppm
2,4-dinitrotoluene
(2,4-DNT) spiked in soil extract; and (b) 2,4-DNT standard.
(collision
energy at 6eV, laboratory frame)
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Fig. S19. Tandem mass spectrum of (a) 20 ppm of
2,4,6-trinitrophenol (2,4,6-TNPh) spiked in soil extract;
and
(b) 2,4,6-TNPh standard. (collision energy at 20eV,
laboratory
frame)
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Fig. S20. (a) (i) Total ion chromatogram; (ii) extracted ion
chromatograms for 2,4-DNT at m/z 182.0, 1-F-4-NB at m/z 141.0
and
1,3-DNB at m/z 168.0; (iii) mass spectrum at the 5th minute; and
(iv) mass spectrum at the 20
th minute of the detection of
nitrobenzene radical anions using polyethylene supporting tips;
and (b) (i) total ion chromatogram; (ii) extracted ion
chromatograms for 2,4-DNT at m/z 182.0, 1-F-4-NB at m/z 141.0
and 1,3-DNB at m/z 168.0; (iii) mass spectrum at the 5th
minute;
and (iv) mass spectrum at the 20th minute of the detection of
nitrobenzene radical anions using polyester supporting tips.
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