文献阅读笔记：Angelino-2001-EST

经典必读：在线质谱仪识别大气颗粒物中的有机胺。

Information for the paper

Title: Formation of Aerosol Particles from Reactions of Secondary and Tertiary Alkylamines:  Characterization by Aerosol Time-of-Flight Mass Spectrometry

Author: Angelino, Stefania

Year: 2001

Journal: Environmental Science & Technology

URL: https://doi.org/10.1021/es0015444

The major goals of this work

Identification of all reaction products in the particle phase is not the goal of these studies. Instead, a more realistic goal is to make “reasonable” hypotheses for some of the major components that will allow particular particle types detected in ambient air to be related to certain classes of emitted compounds, which potentially can be associated with the primary sources. Thus, the major goals of the present work are 3-fold:

• test the utility of using ATOFMS as an on-line detector for atmospheric reactions simulated in a smog chamber;
• investigate the mass spectra of secondary and tertiary alkylamines, alkyl ammonium salts, and particles formed in the presence of alkylamines and ambient gas phase oxidants;
• use the results of the laboratory investigations to identify the species producing commonly observed ion peaks in ambient particle mass spectra.

Properties of alkylamines

1. Typical estimated atmospheric lifetimes for amines are on the order of a few hours up to several tens of hours.
2. Aliphatic amines absorb light at wavelengths below 250 nm and thus do not undergo photolysis in the troposphere.
3. Like most organic compounds, amines become transformed through reactions with oxidants such as hydroxyl radicals and ozone. The reactivity of alkylamines received some attention in the 1970s when the carcinogenicity of their nitrosation products (nitrosamines) was discovered. However, after it was shown that nitrosamines rapidly undergo photolysis in sunlight, and thus do not accumulate in the atmosphere, the interest toward alkylamines decreased.
4. Aliphatic amines possess very low ionization potentials.

Ambient Observations

1. m/z 86 containing particles are most likely produced by vehicles traveling on the freeway.
2. In general, during this study, the percentage of particles with mass spectra containing m/z 86 increased with decreasing temperature and increasing RH.
3. As described, particles containing these markers are very commonly observed and possible sources include the large cattle feedlots in the nearby city of Chino, CA, as well as vehicle exhaust based on ATOFMS source characterization and roadside studies

Standard Amines and Alkyl Ammonium Salts

Standard Amines

1. In general, the most important primary fragmentation process occurring for aliphatic amines involves the removal of one of the electrons from the lone pair on N and cleavage of the C-C bond α to the nitrogen, with loss of the heavier alkyl group favored. This cleavage explains the presence of the fragments at m/z 58, 72, 86, and 114 in the mass spectra of DEA, DPA, TEA, and TPA, respectively.

2. Other processes may also occur to a lesser extent, leading to smaller fragments, such as the signal at m/z 30 in many of the alkylamine mass spectra.

3. As shown in Figure 3A-C, ATOFMS analysis of trialkylamines produces relatively simple spectra with the most abundant signals at [M]⁺ at m/z 59, 101, and 143 for TMA, TEA, and TPA, respectively.

4. In addition, a major fragment occurs for TEA and TPA at m/z 86 and 114, resulting from loss of a methyl and ethyl group, respectively, leading to formation of an immonium ion, [R₂N=CH₂]⁺, as discussed above.

5. Figure 3D,E, obtained under the same conditions are more complex. In addition to the protonated, [MH]⁺, and unprotonated, [M-H]⁺, molecular ion, DEA and DPA particle mass spectra also show a signal at [(M-1)+14]⁺ at m/z 86 and 114, respectively.

6. One possible explanation for these peaks is the high concentration of amine in the ion plume, both in the ionized/fragmented and neutral forms, which leads to ion-molecule reactions. Specifically, the reaction between the fragment of the general structure [C_nH_n+1NH=CH₂]⁺ and a neutral molecule of dialkylamine could give rise to the observed signal as shown here for DEA:

Alkyl Ammonium Salts

7. In direct analogy to the standard amine spectra, the mass spectra of trialkyl ammonium salts, organic and inorganic, and dialkyl ammonium organic salts contain [M]⁺ (Figures 4A-C) for the tertiary amines (TMA, TEA, TPA) and [MH]⁺ for the secondary amines (DEA, DPA) (Figure 4D,E).

8. In the dialkyl ammonium organic salt mass spectra, more extensive fragmentation is observed at lower m/z ratios. This extra fragmentation may be due to stronger absorption by the salts of the ultraviolet radiation.

Summary

9. In fact, in previous ATOFMS studies, a number of amines have been shown to produce the ion peak at m/z 86, suggesting this ion peak might be a good general marker for amine compounds.

10. However, to use m/z 86 as a general marker for amines in ambient studies, other ion peaks consistent with amines must be present in the same single particle positive and negative ion mass spectra.

11. Particle mass spectra of both TEA and TEANO3 contained the peak at m/z 101.

12. The ion peak at m/z 102 was observed for TEA, DPA, and TEANO3, and DPANO3.

Smog Chamber Reactions of Alkylamines

Tertiary Alkylamine Reactions-Positive Ion Mass Spectra

1. As shown in Figures 5A-C, the new feature that was not observed in the amine standard or alkyl ammonium salt particle mass spectra is the ion signal at [M+17]⁺, m/z 76 for TMA, m/z 118 for TEA, and m/z 160 for TPA.

2. Specifically, for the TMA, TEA, and TPA reactions, the base signal is always [M]⁺ at m/z 59, 101, and 143, respectively.

3. In addition, ion peaks resulting from alkyl losses are also observed at m/z 86 [(C₂H₅)₂N=CH₂]⁺ and 114 [(C₃H₇)₂N=CH₂]⁺.

4. About 20-30% of the particles formed by TMA reactions also have signals at m/z 62, 72, and 91. ... these signals can be tentatively assigned to [(CH₃)₂NHOH]⁺, [CH₂=N(CH₃)CHO]⁺, and [(CH₃)₂NHNO₂]⁺, respectively.

5. However, the peak at m/z 118 observed in particle mass spectra from the triethylamine smog chamber reaction was not observed in any of the standard amine or alkyl ammonium salt spectra.

6. However, it has been well documented in the synthetic organic chemistry and biochemistry literature that tertiary amines can be oxidized readily through a number of different pathways to trialkylamine-N-oxides.

7. Thus, the protonated forms of triethylamine-N-oxide (TEAO), trimethylamine-N-oxide (TMAO), and tripropylamine-N-oxide (TPAO) are the most probable assignments for the [M+17]⁺ signals. Resonance structures for TEAO are shown below.

Secondary Alkylamine Reactions-Positive Ion Mass Spectra

1. For the purposes of this paper, it is important to note that, in general, there are more high mass ion peaks observed in the particle mass spectra formed from the secondary alkylamine as compared to the tertiary alkylamine reactions. This demonstrates that secondary amines undergo different chemistry and appear to be more reactive than tertiary amines based on the relative complexity of the mass spectra.

2. A common characteristic of the DEA spectra is the presence of ion peaks at m/z 70, 72, and 74. This trio of peaks was also observed in the standard DEA particle mass spectra.

3. The peaks at m/z 100, 101, and 102 are unique to the diethyl ammonium nitrate mass spectra and thus may be indicative of LDI plume reactions.

Alkylamine Reactions-Negative Ion Mass Spectra

4. For all of the alkylamines studied, most of the ATOFMS signals obtained in negative polarity indicate the presence of nitrate and, less frequently, sulfate, e.g., m/z -42 [CNO]^-, -46 [NO₂]^-, -62 [NO₃]^-, -80 [SO₃]^-, -97 [HSO₄]^-, and -125 [H(NO₃)₂]^- .

5. For instance, about 60% of the particle mass spectra obtained from the TMA reaction (Figure 6A) produce the m/z -90 signal, possibly dimethylnitramine [CH₃N₂O₂]^- and, as stated previously, a positive ion signal in the corresponding positive ion mass spectrum at m/z 91.

6. The signal at m/z -74 is most likely the fragment [CH₃N₂O]^- obtained by loss of an oxygen, as opposed to dimethylnitrosamine. (see Figure 6A)

7. The consistent appearance of the m/z -72 signal is possibly due to [(CH₃)₂NCO]^-. (see Figure 6A)

5. The reacted DEA and TEA negative ion spectra are quite simplistic, with the only organic signal found at m/z -88, which is most likely the fragment ion [(C₂H₅)₂NO]^-
6. Characteristic negative ion mass spectral signals of TPA reaction are found at m/z -128, possibly [(C₃H₇)₂NC₂H₅]^-, -100 [(C₃H₇)₂N]^-, and -72 [(C₂H₅)₂N]^-. These negative ion signals were not observed for the pure amine and thus are reaction products or the result of matrix effects inducing differences in plume chemistry. (Figure 6B)
7. The m/z -146 signal is possibly due to nitramine. (Figure 6B)

ATOFMS Mass Spectra of Trimethylamine-N-Oxide and Reaction Mechanism

1. For all methods of generating TMAO particles, when the molecular ion is present, it occurs in the protonated form [MH]⁺, as indicated by the signal at m/z 76 (TMAOH).

2. Major fragments occur at m/z 58, 59, and 42, [M-OH]⁺, [M-O]⁺, and [C₂H₄N]⁺, respectively.

3. This experiment allows for the assignment of the [M+17]⁺ ion peak at m/z 76 observed in smog chamber reactions of TMA to the protonated form of oxidized TMA, [TMAOH]⁺.

4. By direct extension, the ion signal at m/z 118 observed in ambient particle mass spectra can be assigned to [TEAOH]⁺.

5. To the best of our knowledge, this is the first time oxidation products of alkylamines have been observed in the condensed phase of aerosol particles.

6. The transformation of tertiary amines to the corresponding amine oxides may occur in the presence of different oxidizing agents, including peracids, hydrogen peroxide, and alkylperoxy radicals.

7. Other oxidation paths may involve reaction with ozone, metal oxides, and oxygen radicals.

8. Since all of these oxidant species are hydrophilic, they will have a tendency to partition to the aqueous phase. Thus, in ambient air, these reactions likely occur in the bulk solution of particles with high water content.

Control Smog Chamber Experiments

9. The only differences were the frequent presence of a peak at m/z 139 and, for a few particles, signals at higher masses. These peaks are not commonly observed in ambient particles, where the oxidant concentrations are lower, and thus most likely are products of further oxidation processes.

10. The same “clean” reaction has been carried out in a dry environment, with a RH of approximately 0.1% and in this case no particle formation was observed upon adding CH₃ONO.

11. It is also possible that water assists in the dissociation of the alkyl ammonium salts, driving the gas/particle equilibrium (i.e., partitioning) toward the particle phase over the gas phase.

12. This can be explained by the fact that sunlight is necessary for initiating the photooxidation of the amines, as well as for producing nitric acid and thus acid-base reactions leading to alkyl ammonium salts.

Particle Formation

13. Short alkyl chain aliphatic amines have relatively high vapor pressures, and thus when present in the particle phase, they most likely occur in the form of alkyl ammonium salts.