文献阅读笔记：Silva-2000-AC

在线质谱仪识别有机物.

Information for the paper

Title: Interpretation of Mass Spectra from Organic Compounds in Aerosol Time-of-Flight Mass Spectrometry

Author: Philip J. Silva

Year: 2000

Journal: Analytical Chemistry

URL: https://doi.org/10.1021/ac9910132

Introduction

1. reduced nitrogen species such as amines
2. In many cases, positive ion mass spectra are similar to those found in libraries for 70-eV electron impact mass spectrometry.
3. However, formation of even-electron molecular ions due to adduct formation also plays a major role in ion formation.
4. Negative ion mass spectra suggest that organic compounds largely disintegrate into carbon cluster fragments (C_n^- and C_nH^-).
5. However, information about the heteroatoms present in organic molecules, especially nitrogen and oxygen, is carried dominantly by negative ion spectra, emphasizing the importance of simultaneous analysis of positive and negative ions in atmospheric samples.
6. The purpose of this study is to understand and identify potential characteristic ion peaks in the mass spectra of various relevant compounds and compound classes expected to be present in atmospheric particulate matter.

Experimental section

1. In a number of cases, spectra of pure organic compounds also show peaks due to alkali metals such as sodium and potassium, due to the extremely high sensitivity of ATOFMS to these elements that are present as trace impurities (<1%).

Results and discussion

Polycyclic Aromatic Hydrocarbons (PAHs) and Analogues

1. Table 1 lists the PAHs and analogues that have been analyzed as standards using ATOFMS, as well as their molecular weights and CAS registry numbers.

1. The molar absorptivity of PAHs and their heterocyclic analogues at 266 nm falls in the range of 10³ — 10⁵, with larger ring systems such as chrysene exhibiting cross sections greater than 10⁶, so it is expected that these compounds will be detected the most easily of all compounds using ATOFMS.

Chrysene （1,2-苯并菲，C₁₈H₁₂，MW:228）

3. Figure 1 shows the positive and negative ion mass spectra obtained from pure particles of chrysene. In the positive ion mass spectrum (Figure 1a), the parent ion signal at mass-to-charge (m/z) 228 is still significant.

Phenanthridine（菲啶，C₁₃H₉N，MW:179）

4. Panels a and b of Figure 2 show the positive and negative ion mass spectra of pure particles of phenathridine. Similar to the PAHs, phenanthridine yields a parent ion, M⁺, at m/z 179. Also observed in the high-mass region is a peak at m/z 151, resulting from loss of HCN followed by loss of H•, similar to that observed from EI.

5. Although this peak has been assigned to both C_n^- and C₂H₂^- in much of the literature, during this study this peak has been observed dominantly in spectra of nitrogen-containing com- pounds and thus most likely can be attributed to C_n^-.
6. In addition to m/z 26, an entire series of nitrogen-containing cluster ions of the type (C_nN)^- are observed at m/z 50, 74, 98, and 122 (corresponding to odd numbers of carbon atoms).

Perinaphthenone （环萘酮，C₁₃H₈O，MW:180）

Aromatic and Functionalized Aromatic Hydrocarbons

1. Table 2 lists monocyclic aromatic compounds and derivatives that have been characterized with ATOFMS. Aromatic hydrocarbons exhibit mass spectra that are, in general, similar to the PAHs. A strong parent ion (M⁺) is observed, with major fragments from the loss of alkyl groups attached to the ring system.

1,3-dihydroxybenzene (间苯二酚，C₆H₆O₂，MW:110)

2. In the positive ion mass spectrum, a fairly large parent ion is observed at m/z 110.
3. ... with clusters of peaks centered around m/z 27 (C₂H₃⁺), 39 (C₃H₃⁺), and 53 (C₄H₅⁺).
4. Carbon cluster ions (C_n^- and C_nH^-) extend up to m/z 121, suggesting that some carbon cluster formation does occur after desorption/ionization since the molecule consists of only six carbon atoms.
5. Other than these cluster ions, the only other fragment observed is ⁴¹C₂HO^- .

1,2-benzenedicarboxylic acid (邻苯二甲酸，C₈H₆O₄，MW:166)

6. In Figure 5, the positive and negative ion mass spectra for phthalic acid are shown. This compound is typically detected in atmospheric particles using traditional sampling and analysis techniques and has recently been proposed as a tracer for secondary organic aerosol production.
7. In the positive ion mass spectrum, a peak at m/z 167 denotes the presence of the even-electron protonated parent ion.
8. A large peak at m/z 149 corresponds to the loss of water from the protonated parent molecule.
9. This peak, in addition to peaks at m/z 39 (C₃H₃⁺), 51 (C₄H₃⁺), 65 (C₅H₅⁺), and 105 (C₇H₅O⁺), are all shifted higher by one mass unit in comparison to the EI mass spectrum of this compound.
10. In addition, a peak at m/z 121 (C₇H₅O₂⁺) is observed that does not occur in the EI mass spectrum. This peak probably results from the loss of H2O and CO from the protonated parent molecule, which cannot occur in EI because of the lack of a protonated parent ion.
11. In the negative ion mass spectrum, a deprotonated parent ion is observed at m/z 165.
12. Fragments from the consecutive loss of two CO2 molecules are observed at m/z 121 (C₇H₅O₂^-) and 77 (C₆H₅^-).

Aliphatic Hydrocarbons and Derivatives

Nonanal (壬醛，C₉H₁₈O，MW:142)

1. Nonanal is one of the more common aldehydes present in atmospheric particles and has been used as a tracer for particles of meat cooking.
2. The (C_nH_2n+1CO⁺) series typically used to identify carbonyl compounds in EI mass spectrometry is of little diagnostic value because this series is not observed above C5 in the typical nonanal spectra.
3. The only other distinct peak in the spectrum is observed at m/z 45 and is presumably due to (C₂H₅O^-).

Glycine (甘氨酸，C₂H₅NO₂，MW:75)

4. A strong protonated parent ion is observed at m/z 76.
   
5. Similar to the EI mass spectrum, m/z 30 (CH₄N⁺) is the base peak.
   
6. Other major fragments observed are present at m/z 27 (C₂H₃⁺), 32 (CH₆N⁺), and 45 (C₂H₅O⁺).
   
7. The presence of sodium contaminant at m/z 23 (Na+) also results in adduct formation of a sodiated parent ion at m/z 98 (M+Na)⁺.
   
8. The negative ion mass spectrum consists of a deprotonated parent ion at m/z 74 and fragments at m/z 26 (CN^-), 42 (C₂H₂O^-), and 45 (CHO₂^-).

Organic Salts

1. The presence of elevated concentrations of nitric and sulfuric acid and ammonia in the gas phase in the atmosphere allows for the possible reaction of these compounds with existing organic compounds to form salts that can be either volatile or semivolatile.
2. For instance, amines are present in the gas phase in the atmosphere due to emissions from livestock and can react with acids to form salts analogous to ammonia-acid reactions (see Table 4).

Diethylethanolamine nitrate (二乙基乙醇胺硝酸盐)

3. Figure 8 shows the positive and negative ion mass spectra of a quaternary ammonium salt, diethylethanolamine nitrate. These particles were generated by reaction of diethylethanolamine with nitric acid in the gas phase.
4. The positive ion mass spectrum shows an ion due to the intact quaternary ammonium cation at m/z 118.
5. A fragment at m/z 102 is probably due to loss of CH₃• from the parent ion.
6. An intense peak at m/z 86 (C₅H₁₂N⁺) results from the α cleavage of •CH₂OH and is characteristic of quaternary ammonium compounds with ethyl substituents.
7. Four major peaks are observed in the negative ion mass spectrum. Mass-to-charge 46, 62, and 125 are nitrate-related peaks, assigned to (NO₂^-), (NO₃^-), and (HN₂O₆^-), respectively.

Sodium methanesulfonate 甲磺酸钠盐

8. Methanesulfonic acid (MSA) and its sodium and ammonium salts are of interest as a source of non-sea salt sulfate.
9. With this compound, large signals due to methanesulfonate anion and fragments are observed in the negative ion mass spectrum, while only sodium and cluster ions are observed in the positive ion mass spectrum.

Potential Application of Results to Ambient Particles

1. In the atmosphere, particles consist of a complex mixture of a range of organic compounds, in addition to a full array of inorganic species.
2. This can constrain the interpretation of organic mass spectra because only one mass spectrum is acquired for a particle containing perhaps hundreds of compounds.
3. In addition, the presence of certain compounds can serve as a matrix which may change the detection characteristics of other compounds.
4. Figure 10 shows the positive and negative ion mass spectra of particles containing a 1:1 mixture of resorcinol (Figure 4) and phthalic acid (Figure 5).

5. In contrast to Figure 4, where a parent ion and fragments are observed from resorcinol, the presence of acidic protons from phthalic acid results in an intense even-electron protonated parent ion for resorcinol at m/z 111. This peak is much more intense than the protonated parent observed for phthalic acid at m/z 167.
6. In the presence of a proton source such as phthalic acid, compounds that have polar functional groups containing oxygen and nitrogen can form adduct ions.
7. The presence of a peak at m/z 26 is due to C_n^-, a definitive marker for species with carbon-nitrogen bonds in the particle. The presence of a series of peaks of the type C_nN^-, where n is odd (m/z 26, 50, 74, 90, etc.) indicates the presence of organic nitrogen atoms since only C_n^- by itself can be due to the cyanide ion.
8. Phosphorus-containing compounds display a similar series of peaks due to the analogous series C_nP^- (m/z 43, 67, 91, etc.)
9. Several different oxygen-containing fragments in negative ion mass spectra may be used to identify particles containing oxygenated species. A peak present at m/z 41 (C₂HO^-) is nearly ubiquitous in the spectra of compounds containing alcohol functionality. A second possible marker is the peak that occurs at m/z 45 due to two different oxygen-containing fragments, CHO₂^- and C₂H₅O^-. Although certain positive ions can also be indicative of oxygen species, e.g., m/z 43 (C₂H₃O⁺), negative ions do not suffer from as many interferences from ions of the same nominal mass; e.g., m/z 43 can also be C₃H₇⁺ or C₂H₅N⁺.
10. While not quite as common as the previously mentioned peaks, m/z 93 (C₆H₅O^-) can be an important peak for identifying particles containing oxygenated aromatic compounds.
11. In the positive ion mass spectra, carbon-halogen cluster ions can be observed at m/z 31 (CF⁺), 47 and 49 (CCl⁺), and 91 and 93 (CBr⁺).
12. However, the presence of alkyl groups can be observed similar to EI mass spectrometry. Although the detection of long-chain alkanes is unlikely with the use of 266-nm radiation, the series of alkyl fragments (m/z 15, 29, 43, 57, etc.) can be used to identify alkyl substituents. Mass-to-charge 15 and 29 in particular are readily observed from compounds with methyl and ethyl substituents.
13. Several peaks in positive ion mass spectra that can also provide information on heteroatoms in organic molecules are m/z 31 (CH₃O⁺), which is a peak that arises from oxygenated organic compounds. Mass-to-charge 31 can also be due to CF⁺ in particles consisting of fluorine-containing organic compounds.
14. Another important peak observed is m/z 32 (S⁺). Although a limited number of sulfur-containing compounds were characterized in this study, no non-sulfur-containing compounds produced ions of this mass, while all sulfur compounds did.
15. All organic acids studied yield protonated positive parent ions and deprotonated negative parent ions in the mass spectra.
16. As observed in Figure 8, no negative ion carbon clusters are observed from the analysis of some organic salts. This is useful information, since all neutral organic compounds analyzed using ATOFMS have been observed to give these clusters.
17. The lack of carbon clusters in a negative ion mass spectrum that is correlated with a positive ion mass spectrum containing organic fragments strongly indicates the presence of organic salts rather than neutral organic compounds.