Investigation and Applications of In-Source Oxidation in Liquid Sampling-Atmospheric Pressure

Investigation and Applications of in-source oxidation in Liquid Sampling-Atmospheric Pressure Afterglow Microplasma ionization (LS-APAG) source

Supplementary Material

Xiaobo Xie1, 2, Zhenpeng Wang3, Yafeng Li1, Lingpeng Zhan1, 2, Zongxiu Nie1, 3*

1. Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China.

2. University of Chinese Academy of Sciences, Beijing 100049, China.

3. National Center for Mass Spectrometry in Beijing, Beijing 100190, China.

1 The LS-APAG source 2

1.1 The location of the capillary tip (or the exit of the sample solution) 2

1.2 Sample introduction methods 2

2 LS-APAG analysis of diverse aliphatic/aromatic compounds 2

2.1 APAG positive mode 2

2.2 APAG negative mode 4

2.3 Comparison of ESI mode and conventional ESI 6

3 Alkanes analysis by LS-APAG/MS on the LTQ instrument 6

3.1 Optimization of APAG operating parameters 7

3.1.1 Optimization of the ion transfer capillary temperature 7

3.1.2 Optimization of the location of the capillary tip 7

3.1.3 Optimization of the source-to-MS distance 8

3.1.4 Optimization of the flowing gas rate 9

3.1.5 Optimization of the dc voltage 10

3.2 Linear alkanes (C5-C19) analysis by APAG mode 10

3.3 Hydrocarbons mixture (polywax 500), branched alkanes, cyclohexane and 1-hexene were analyzed in APAG mode 11

3.4 CID experiments 13

3.5 GC-MS analysis 14

4 Fatty acids and fatty acid esters 15

5 Other sampling methods 16

5.1 In-glow ionization: sampling by argon carrying 16

5.2 Direct desorption/ionization by surface sampling 17

1  The LS-APAG source

1.1  The location of the capillary tip (or the exit of the sample solution)

Figure S1. The ionization of the sample can be controlled by adjusting the distance between the capillary tip and the discharge region (marked with yellow). (a) The exit of the sample solution is located outside the outer tube E1 (out-tube setting, adopted in this study to avoid in-source fragmentation) (b) The exit of the sample solution is located inside the outer tube E1 close to the discharge region (in-tube setting, more suitable for thermostable less-volatile compounds due to the high source temperature).

1.2  Sample introduction methods

Figure S2. Different sample introduction methods can be readily applied to the analysis of gas (or volatile), solid and liquid samples. The sample deposited on the inside surface of the outer electrode E1, can be easily desorbed due to the high source temperature, thus no extra heating devices and desorption methods (e.g., laser) are needed for analyzing less volatile analytes by this ion source.

2  LS-APAG analysis of diverse aliphatic/aromatic compounds

2.1  APAG positive mode

Table S1. N-containing aliphatic/aromatic compounds analyzed in APAG and ESI modes.
Entry / Compounds / MW / Major Ions
APAG / ESI
1 / 1-Heptanamine / 115.22 / [M-H]+ m/z 114 / [M+H]+ m/z 116
2 / 1-Octadecanamine / 269.51 / [M+H]+ m/z 270 / [M+H]+ m/z 270
3 / N-lauryldiethnol amine / 273.27 / [M+H]+ m/z 274 / [M+H]+ m/z 274
4 / Octadecanamide / 283.49 / [M+H]+ m/z 284 / [M+H]+ m/z 284
5 / / 194.19 / [M+H]+ m/z 195 / [M+H]+ m/z 195
6 / / 224.17 / [M+H]+ m/z 225 / NA[b]
7 / / 225.25 / [M+H]+ m/z 226 / NA
8 / / 155.58 / M+· m/z 155.0132 / NA
[b] NA: not analyzed

Figure S3. Mass spectra of PEG-1000 (dissolved in methanol) were obtained by two different ionization modes, ESI and APAG.

Scheme S1. The structures of the two compounds in antioxidant B215.

Figure S4. Mass spectra of antioxidant B 215, consist of two compounds, pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate (MW: 646.92 Da, denoted by A) and phenol,2,4-bis(1,1-dimethylethyl)-, 1,1',1''-phosphite (MW: 1177.63 Da, denoted by B) obtained by APAG and ESI modes.

Table S2. ESI-inefficient polycyclic aromatic compounds analyzed in APAG mode using a FTICR-MS (Bruker, 4.7T).
Entry / Compounds / Major ions / Calculated[a] / Measured / Error(ppm)
1 / / [M-H]+ / 338.1903 / 338.1901 / 0.6
2 / / [M-H]+ / 358.1590 / 358.1586 / 1.1
Intriguingly, an unusual [M-H]+ formed by hydride abstraction was observed during APAG analysis of some polycyclic aromatic amines.

Figure S5. Positive ion mode APAG Mass spectra of dodecanol and stearyl alcohol (ESI-inefficient weakly polar compounds).

2.2  APAG negative mode

Table S3. Fatty acids dissolved in acetone analyzed in APAG and ESI negative modes.
Entry / Fatty acid (10-4 M) / Molecular Formula / MW / Observed Ions
([M-H]-, [2M-H]- ) / [M-H]- (Intensity)
APAG / ESI
1 / Decanoic Acid / C10H2O2 / 172.27 / m/z 171, 343 / 1.77E5 / 1.99E5
2 / Lauric acid / C12H24O2 / 200.32 / m/z 199, 399 / 1.31E4 / 1.70E5
3 / n-Tridecanoic acid / C13H26O2 / 214.34 / m/z 213, 427 / 4.54E4 / 1.94E5
4 / Palmitic acid / C16H32O2 / 256.42 / m/z 255, 511 / 4.48E4 / 1.89E5
5 / Stearic acid / C18H36O2 / 284.48 / m/z 283, 567 / 8,.88E4 / 1.99E5
6 / Eicosanoic acid / C20H40O2 / 312.30 / m/z 311, 623 / 1.37E5 / 7.19E4
Table S4. Aromatic acids and phenols analyzed in APAG mode.
Entry / Compounds / Molecular formula / MW / Major ions
APAG / ESI
1 / Benzoic acid / C7H6O2 / 122.12 / [M-H]-
m/z 121 / [M-H]-
m/z 121
2 / Bisphenol A / C15H16O2 / 228.29 / [M-H]- m/z 227
[M+O-H]- m/z 243
[2M-H]- m/z 455
[2M+2O-H]- m/z 487 / NA
3 / 3-hydroxybenzaldehyde / C7H6O2 / 122.12 / [M-H]- m/z 121 / [M-H]- m/z 121
Table S5. Explosives analyzed in negative APAG mode.
Compounds / MW / Major Ions
Pentaerythritol tetranitrate(PETN) / 222.12 / [RDX+NO3]- m/z 378
1,3,5-trinitroperhydro-1,3,5-triazine(RDX) / 316.14 / [PETN+NO3]- m/z 284
[PETN+NO2]- m/z 268
2,4,6-Trinitrotoluene(TNT) / 227.13 / [M-H]- m/z 226
[M-NO]- m/z 197

Figure S6. Mass spectra of explosives (TNT, PETN, and RDX, dissolved in acetone) were obtained in LS-APAG mode ([HNO3+NO3]-, m/z 125). The assignments of other peaks are listed in Table S5.

Figure S7. Ionization behavior of APAG mode in the integrated LS-APAG source.

2.3  Comparison of ESI mode and conventional ESI

Figure S8. ESI mode of this LS-APAG source vs conventional ESI of the commercial ESI source during the analysis of AngiotensinⅠ(positive ion mode, MW: 1296.5 Da) and n-tridecanoic acid (negative ion mode, MW: 214.3 Da).

3  Alkanes analysis by LS-APAG/MS on the LTQ instrument

Experimental Procedure: The alkanes, either pure alkanes or dissolved in n-hexane (the volume ratio of the analyte is 10%), were injected to the ion source with a flowing rate of 2 μL/min. APAG mode was used for the analysis of alkanes. The dc high voltage for discharge was set to be 2500V. The gas flowing rate of argon was ranging from 0 to 3.0 L/min, and 0.5 L/min is a good choice for most analytes.

3.1  Optimization of APAG operating parameters

3.1.1  Optimization of the ion transfer capillary temperature

Figure S9. APAG mass spectra of n-hexane were obtained when the temperature of the ion transfer capillary was raised from 100 °C to 300 °C.

Figure S10. a) The intensities of peaks at m/z 99, 117 and 134 when the temperature of the ion transfer capillary was raised from 100 °C to 300 °C; b) The relative ratios of the ions at m/z 117 and m/z 134 to the ions at m/z 99 when the temperature of the ion transfer capillary was raised from 100 °C to 300 °C.

3.1.2  Optimization of the location of the capillary tip

Figure S11. Mass Spectrum was obtained when the capillary tip was set inside the outer electrode E1 (out-tube setting, Figure S1a). In-source fragments at m/z 58 (C4H10 +), 72 (C5H12 +) and 86 (C6H14 +) were obtained although at very low abundances.

Figure S12. Positive-ion APAG mass spectra of n-hexane were obtained when the out-tube length of the capillary tip is set to be 0.5 mm, 2 mm, and 3 mm, respectively.

Figure S13. a) The intensities of peaks at m/z 99, 117 and 134 when the out-tube length of the capillary tip is set to be 0.5 mm, 2 mm, and 3 mm, respectively; b) The relative ratios of the ions m/z 117 and m/z 134 to the ions at m/z 99 when the out-tube length of the capillary tip is set to be 0.5 mm, 2 mm, and 3 mm, respectively.

3.1.3  Optimization of the source-to-MS distance

Figure S14. Positive-ion APAG mass spectra of n-hexane were obtained when the source-to-MS distance was set to be 3 mm, 5 mm, 7 mm, and 10 mm, respectively.

Figure S15. a) The intensities of peaks at m/z 99, 117 and 134 when the source-to-MS distance was set to be 3 mm, 5 mm, 7 mm, and 10 mm, respectively; b) The relative ratios of the ions at m/z 117 and m/z 134 to the ions at m/z 99 when the source-to-MS distance was set to be 3 mm, 5 mm, 7 mm, and 10 mm, respectively.

3.1.4  Optimization of the flowing gas rate

Figure S16. APAG mass spectra of n-hexane were obtained when the gas flow rate is set to be 0 L/min (a) and 2.0 L/min (b). Peaks at m/z 36 ([H2O+NH4]+), 50 (not identified), 74 ([N3O2]+), and 91 ([(H2O)4+H3O]+) derived from the ionization of air.

3.1.5  Optimization of the dc voltage

Figure S17. The intensity of the oxidation product ions at m/z 99 and the relative ratio of the further oxidation product ions at m/z 117 to the ions at m/z 99 when raising the dc voltage from 1500 V to 3500 V. The filled red rectangle denoted that the intensity of the ions at m/z 99 is highest at 2500V.

3.2  Linear alkanes (C5-C19) analysis by APAG mode

Figure S18. a) Positive-ion mass spectra dominated by two peaks corresponding to [M+O-3H]+ and [M+2O-H]+ acquired by APAG analysis of n-alkanes (C5-C19); b) Positive-ion APAG mass spectra of n-octadecane (sample flow rate 10 μL/min, the signal-to-noise ratio is approximately 23). CH2Cl2 was used as the solvent instead of n-hexane to avoid the competitive oxidation of the alkane solvent. * denoted a common background ion at m/z 279.

Discussions about LOD and analysis time of this technique in the analysis of alkanes:

The limit of detection (LOD < 2 ng) can be estimated by the equation “LOD = (concentration of sample: 10 ng/μL)*(sample flow rate: 10 μL/min)*(data acquisition time: generally in ~ ms, less than one second)”. The LOD would be less than 20 ng, even if taking the dead volume (~ 2 μL) of the capillary into account.

Analysis time of this technique is largely depending on the sampling methods, as the ionization and oxidation process is very rapid. These oxidation peaks can be observed instantly upon the alkanes were introduced to the ion source. The analysis time by surface sampling and gas sampling is comparable to other ambient mass spectrometry techniques. The liquid sampling method is similar to other liquid-based spray methods, depending on the dead volume of the capillary (about 2 μL, in our case) and the flow rate of the sample solution.

Table S6. Accurate mass measurements of oxidation peaks of alkanes.
Entry / Compounds / Major ions / Theoretical Mass / Measured Mass / Error (ppm)
1 / n-Dodecane / [M+O-3H]+ / 183.1743 / 183.1745 / 1.1
2 / n-nonadecane / [M+O-3H]+ / 281.2839 / 281.2843 / 1.4

3.3  Hydrocarbons mixture (polywax 500), branched alkanes, cyclohexane, and 1-hexene were analyzed in APAG mode

Table S7．A list of oxidation peaks for alkanes, ranging from C5 to C40.
General Formula: CxH2x+2
x is a positive integer. / [M+O-3H]+ / [M+2O-H]+
m/z = 14*(x+1)+1 / m/z = 14*(x+2)+5
5 / 85 / 103
…… / …… / ……
20 / 295 / 313
21 / 309 / 327
22 / 323 / 341
23 / 337 / 355
24 / 351 / 369
25 / 365 / 383
26 / 379 / 397
27 / 393 / 411
28 / 407 / 425
29 / 421 / 439
25 / 365 / 383
26 / 379 / 397
…… / …… / ……
37 / 533 / 551
38 / 547 / 565
39 / 561 / 579
40 / 575 / 593
41 / 589 / 607
36 / 519 / 537
37 / 533 / 551
38 / 547 / 565
39 / 561 / 579
40 / 575 / 593
…… / …… / ……

Discussions about identification of peaks obtained from hydrocarbons mixture:

1)  The mass-to-charge values of the two types of products are fixed for a given alkane.

2)  The two kinds of peaks would not overlap in one mass spectrum.

Assume there are two alkanes CaH(2a+2) and CbH(2b+2), both a and b are positive integer, a ≠ b.

Thus, 14*(a+1)+1≠14*(a+2)+5; 14*(b+1)+1≠14*(b+2)+5;

14*(a+1)+1≠14*(b+1)+1; 14*(a+2)+5≠14*(b+2)+5;

If a-b= c, c must be an integer.

However,

if 14*(a+2)+5 = 14*(b+1)+1, the value of “b-a” would be 97, which is impossible.

if 14*(a+1)+1 = 14*(b+2)+5, the value of “a-b” would be 97, which is impossible.

Discussions about in-source fragmentation of branched alkanes:

Several alternative explanations referring to the selectivity of oxidations are illustrated as follows：

The differences in mass spectra obtained from branched and linear alkanes may be caused the regioselectivity of oxidation, which prefers to occur at less steric hindrance and electron-rich atom. As reactive oxygen species (ROS) responsible for oxidations or oxidative degradations of analytes are small enough, gas-phase oxidations are more likely to happen at the electron-rich positions.

ð The oxidation reactivity of C-H bond (oxidations): -CH3 < -CH2- < -CH-.

2, 2, 4-trimethylpentane has only one electron-rich -CH- (high regioselectivity), thus isooctane is more likely to form [M+O-3H]+ due to the specific oxidation of -CH-.

ð Similarly, the oxidation reactivity of C-C (oxidative cracking): -CH3 < -CH2 < -CH- < -C-.

Branched alkanes bearing tertiary and quaternary carbon atoms are less resistant to oxidative degradation than linear alkanes. Besides, the collision of the branched alkane ions (e.g., [M+2O-H]+) with plasma ions is another cause for in-source fragmentation.