PTR-MS Fragmentation Patterns of Gasoline Hydrocarbons

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PTR-MS fragmentation patterns of gasoline hydrocarbons

ARTICLE in INTERNATIONAL JOURNAL OF MASS SPECTROMETRY · JANUARY 2015 Impact Factor: 1.97 · DOI: 10.1016/j.ijms.2015.01.001

4 AUTHORS, INCLUDING:

Mylene Gueneron Graham Vanderschelden Washington State University Washington State University

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International Journal of Mass Spectrometry 379 (2015) 97–109

Contents lists available at ScienceDirect

International Journal of Mass Spectrometry

jou rnal homepage: www.elsevier.com/locate/ijms

PTR-MS fragmentation patterns of gasoline hydrocarbons

∗

Mylene Gueneron, Mathew H. Erickson, Graham S. VanderSchelden, Bertram T. Jobson

Washington State University, USA

a r t i c l e i n f o a b s t r a c t

Article history: Product ion yields for a suite of hydrocarbons associated with motor vehicle exhaust including alkenes,

Received 30 June 2014

alkanes, cycloalkanes, and aromatic compounds are reported for a PTR-MS instrument operated at an E/N

Received in revised form 6 January 2015

ratio of 80 and 120 Td. At 120 Td many of the compounds tested underwent dissociative proton transfer

Accepted 11 January 2015

reactions complicating the interpretation of the vehicle exhaust mass spectrum. However at 80 Td most

Available online 21 January 2015

aromatic and alkene compounds tested yielded M + H product ions at near 100% yield and alkyl substituted

cyclohexanes and C9–C12 branched alkanes yielded M-H ions as the major product ion. The PTR-MS

Keywords:

response factors to dimethyl and trimethyl cyclohexane compounds were about a third of that expected

PTR-MS +

if they reacted at the collisional rate limit with H3O . Analysis of gasoline at 80 Td showed that the major

Urban air

Vehicles peaks in the mass spectrum could be reasonably accounted for from known hydrocarbon abundance

Gasoline and their product ion yields, including the alkyl substituted cyclohexane compounds which yielded M-H

ions at m/z 97, 111, and 125. Operation at lower E/N ratios would enable the PTR-MS to measure alkyl

substituted cyclohexanes and larger alkane compounds in urban air at their M-H product ion.

1. Introduction near-roadway monitoring and in measuring statistical relation-

ships between VOCs and other pollutants such as carbon monoxide

Vehicle emissions are major sources of volatile organic com- (CO) and nitrogen oxides (NOx) for source apportionment

pounds (VOC) in urban areas. Measuring the abundance of organic studies.

compounds emitted from vehicles is important for assessing the The mass spectrum of the PTR-MS is ideally interpreted as

impact of vehicle emissions on human health [11] and on the an M + H mass spectrum (molecular weight + 1), but many pro-

generation of secondary photochemical pollutants such as ozone ton transfer reactions with organic compounds found in urban

and secondary organic aerosol [26]. Measurement of organic trace air are known to be dissociative, making the interpretation

gases in the atmosphere is typically done using gas chromatogra- of the mass spectrum subject to uncertainty. The fragmenta-

phy, allowing for analysis of selected C1–C12 compounds by either tion of alcohols, aldehydes, aromatics, alkanes, alkenes, ketones,

grab sample collection (i.e. [38,1]) or in-situ sampling and analysis esters, and monoterpenes has been documented in the literature

(i.e. [39,25]). Another measurement approach using the proton [4,35,15,19,3,8]. The chemical composition of urban air is prin-

transfer reaction mass spectrometer (PTR-MS) instrument has seen cipally a result of vehicle emissions which contribute alkanes,

increased use by the research community in urban ﬁeld studies cycloalkanes, alkenes, aromatics, and aldehydes as major con-

[18,28,12,17]. The PTR-MS instrument provides a less detailed stituents ([29,30,27,23]). Given the large number of compounds

account of the organic chemical composition of urban air but it present, the ﬁdelity and information content of the PTR-MS mass

does have some advantages. One advantage is that the PTR-MS spectrum of urban air is not well understood and limits its useful-

measures important oxygenated compounds such as formalde- ness as a VOC monitor. Compounds reported from PTR-MS urban

hyde and acetaldehyde that are difﬁcult to sample and measure by air measurements tend to focus on monoaromatic compounds and

gas chromatography. These aldehydes are important constituents light oxygenated compounds [18,12,28,14] as these compounds are

of vehicle exhaust and are also created in photochemical oxidation thought to have little interference caused by fragmentation of other

reactions of VOCs. The in-situ, high time resolution measurements organic compounds. Intercomparisons between GC-MS and PTR-

of the PTR-MS can also be advantageous when there is a need to MS measurements for urban air are relatively few and sometimes

follow rapid changes in concentrations in ambient air such as in illustrate inherent problems with PTR-MS operating conditions and

compound fragmentation [20,17].

A better understanding of the PTR-MS fragmentation patterns

∗ of vehicle exhaust would allow for a more complete understand-

Corresponding author. Tel.: +1 509 335 2692..

E-mail address: [email protected] (B.T. Jobson). ing of the information available from PTR-MS measurements of

http://dx.doi.org/10.1016/j.ijms.2015.01.001

98 M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109

Fig. 1. PTR-MS mass spectrum from m/z 30 to m/z 150 of ambient air measurement obtained from three cities: Mexico City, Mexico in March 2006 (bottom), Xi’an China in

August 2011 (middle) and Yakima, WA in January 2013 (top). All data collected at 120 Td. Black bars indicate ions that have been typically reported as having a reasonably well

deﬁned attribution to a particular compound(s): m/z 33 = methanol, 42 = acetonitrile, 45 = acetaldehyde, 59 = acetone & propanal, 69 = isoprene, 79 = benzene, 93 = toluene,

107 = sum C2-alkylbenzene isomers and benzaldehyde, 121 = sum of C3-alkylbenzene isomers, 135 = sum of C4-alkylbenzene isomers.

urban air and PTR-MS measurement ﬁdelity. Examples of urban method has been well described in literature [22,6]. Organic com-

air PTR-MS mass spectrum are displayed in Fig. 1, showing data pounds (R) with proton afﬁnities greater than that of water undergo

from Mexico City, Mexico [36], Xi’an, China, and a much smaller fast proton transfer reactions with H3O

city, Yakima, WA. The data were collected using the same PTR-MS + +

R + H3O → RH + H2O (1)

instrument operated under the same drift tube operation condi-

tions of 120 Td. Both the Mexico and Yakima data were collected +

where R is the organic compound and RH is the measured product

in the morning during a period of higher mixing ratios caused by

ion. The reaction occurs in an ion drift tube that enhances the kinetic

lower mixed layer heights and signiﬁcant emissions from motor

energy of the ions so that collisions with the bath gas (air) cause des-

vehicles during the morning rush hour. The wintertime conditions

olvation of hydrated ions. In Fig. 2, the proton afﬁnities of the major

of Yakima preclude much photochemical activity so the mass spec-

organics reported from vehicle exhaust measurements are illus-

trum reﬂects primary emissions. As expected, given its much larger

trated. Water cluster formation is a strong function of the drift tube

emissions, Mexico City air is more polluted than Yakima by a factor

pressure and electric ﬁeld and these are typically adjusted to limit

of 10–100 and the signal is signiﬁcantly dominated by the monoaro-

matic compounds at m/z 79, 93, 107, 121, 135. There is signiﬁcant

ion signal at many other masses, yet this data is not used because

of uncertainties in the attribution to speciﬁc compounds. Of inter-

est in this paper is the signiﬁcant ion signal at m/z 97, 99, 111,

113, 119, 123, 125 that are likely from compounds emitted in vehi-

cle exhaust. Also of note is the winter time abundance of m/z 69

and m/z 137 in Yakima. These masses are commonly attributed to

the biogenic compounds, isoprene and monoterpenes respectively,

but these compounds would not be emitted in this region in win-

ter. A more likely attribution is form organic compounds associated

with anthropogenic emissions, such as vehicles or residential wood

burning. Resolving fragmentation patterns or reducing fragmenta-

tion for compounds commonly emitted from vehicle exhaust would

facilitate the interpretation of PTR-MS data and broaden its useful-

ness as an urban air quality monitoring tool. In this paper we show

fragmentation patterns of some of the most abundant compounds

found in gasoline engine exhaust for two PTR-MS operating condi-

tions: 80 and 120 Td. We show that operation of the PTR-MS drift

tube at 80 Td yields an M + H and M-H mass spectrum for most

compounds, whereas at the typical operating condition of 120 Td

many compounds fragment, reducing the utility of the PTR-MS as

an urban air quality monitoring instrument.

Fig. 2. Proton afﬁnities of some compounds found in urban air including alkenes (red

circles), aromatics (blue diamond), alkanes (black squares), oxygenated compounds

(stars), cycloalkanes (white squares) identiﬁed in vehicle exhaust, and biogenic com-

2. Experimental

pounds (green triangles). The proton afﬁnity of water (691 kJ/mol) and the water

dimer (808 kJ/mol) is represented by the dashed line with shading representing

VOC measurements by the PTR-MS are based on a soft ioniza-

reported uncertainty. (For interpretation of the references to color in this ﬁgure

tion of the organic compounds with the hydronium ion H3O . The legend, the reader is referred to the web version of this article.)

M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109 99

the abundance of H (H2O)2. Some compounds have high enough

proton afﬁnities that they can undergo fast direct proton transfer

reactions with the hydrated hydronium ion H (H2O)2, as shown

by the dashed line at 808 kJ/mol. In addition to direct protonation,

compounds may be protonated through ligand switching reactions

with H (H2O)2. Drift tube reaction dynamics are characterized by

−1

the ratio of the electric ﬁeld E (volts cm ) over the number density

−3

of air N (cm ) in the drift tube. The units of this ratio are expressed

−17 2

in Townsend (Td) where 1 Td is equal to 10 (V cm ). An increase

in the electric ﬁeld (higher Td) results in more energetic collisions

and less clustering but also leads to a greater degree of dissocia-

tive protonation and the creation of organic fragment ions. Such

fragmentation is not desirable since the interpretation of the PTR-

MS mass spectrum relies upon its presentation as a simple M + H

mass spectrum. The instrument is usually operated in the range of

120–130 Td with the drift tube pressure at 2 mbar. In this work the

PTR-MS drift tube was operated at 120 Td and 80 Td. To operate at

80 Td the air sample must be dried otherwise excessive clustering

occurs. We have been operating the PTR-MS at 80 Td in ﬁeld experi-

◦

ments using a water trap that dries the sample to −30 C dew point

to improve upon the measurement of formaldehyde [16]. The Td

80 operation confers other advantages notably less fragmentation

Fig. 3. Plot of measure-to-expected concentration ratio versus expected mixing

and enhanced sensitivity due to longer reaction times. Compound

ratio of the test mixture prepared by the syringe pump dynamic dilution system

loss to the cold trap is a function of compound vapor pressure, mass

for benzene (diamonds), p-xylene (squares) and 1,3,5-trimethylbenzene (circles)

◦

ﬂow rate, and conditioning time [16]. For high concentration mix- for various injection zone temperatures in C given by value inside symbol. Red out-

lines indicate injection zone temperatures where compound vapor pressure was

tures (>10 ppbv) C11 and C12 compounds are transmitted without

greater than 100 torr, a qualitative indication of excessive effusion rate from needle

signiﬁcant loss [8]. For the fragmentation results shown here the

◦ oriﬁce. Dashed line and the grey shading represent respectively the 1:1 line and the

operating conditions at 120 Td were: T drift = 65 C, P drift = 2.06

10% range. (For interpretation of the references to color in this ﬁgure legend, the

mbar, drift voltage = 485 V. The operating conditions at 80 Td were: reader is referred to the web version of this article.)

◦ +

T drift = 65 C, P drift = 2.06 mbar, drift voltage = 333 V. The O2 and

+ +

NO ions count rates were kept under 2% and 0.2% of H3O respec-

tively when air was sampled. Test gas mixtures were prepared The accuracy of the dynamic dilution system was evalu-

+ +

using dry N2 gas where O2 was typically less than 0.1% of H3O ated by comparing test mixtures of benzene, p-xylene and

count rates, reducing the signiﬁcance of O2 reactions creating 1,3,5-trimethylbenzene against a compressed gas standard. The

interfering fragment ions. compressed gas standard (Scott Marrin, CA) was dynamically

Test gas mixtures were prepared using a dynamic dilution diluted to 20 ppbv to determine PTR-MS response factors. The

system consisting of a syringe pump (Harvard Apparatus) to stated accuracy of the standard was 5%. A range of test mixture

∼

continuously dispense precise molar ﬂows of liquid chemical com- concentrations from 10 ppbv to ∼1000 ppbv were made by using

−1

pounds [9,8]. Chemicals (Sigma Aldrich, Chem Service) were used infusion rates 0.1, 0.25, 0.5 and 5 uL hr and injection zone tem-

◦ ◦

as received and had a stated purity of 96% or better. A known peratures between 22 C (room temperature) and 70 C. We found

molar ﬂow rate of a compound was injected into a heated zone that best results were obtained when the injection zone temper-

where it evaporates from the syringe tip and was diluted by a dry ature was adjusted to keep the vapor pressure of the compound

−1

nitrogen ﬂow supplied from a liquid nitrogen dewar. The PTR-MS less than 100 Torr for infusion rates less than 0.5 uL hr and less

−1

subsampled from the test mixture ﬂow and the rest of the ﬂow than 300 Torr for infusion rates of 5 uL hr . These conditions

was vented to the laboratory fume hood. The dry nitrogen ﬂow yielded stable and reproducible test mixtures that agreed, typi-

was controlled by a mass ﬂow controller to provide a known dilu- cally, with 10% of the expected test mixture concentration. At very

tion ﬂow. The syringe size ranged from 0.1 to 50 ␮l to optimize the low infusion rates or high injection zone temperatures, the mass

infusion rate with the required desired mixing ratio. The tubing loss from the syringe oriﬁce due to effusion can be signiﬁcant and

and ﬁttings for the system were stainless steel or Restek Sulﬁnert precluded generation of low mixing ratios with these standard

coated stainless steel tubing. The temperature of the injection zone syringe needle oriﬁce sizes. Results are displayed in Fig. 3, showing

was thermostated using a temperature controller with a thermo- the measured-to-expected concentration ratio versus the expected

couple measuring the air ﬂow temperature at the injection inlet. concentration as calculated from Eq. (2). The measured mixing ratio

The stainless steel tubing downstream from the injection zone was was determined using the response factors from the compressed

◦

coiled to enhance mixing and heated to 80 C. The resulting molar gas standard. The conditions where the benzene vapor pressure

−1

mixing ratio Xi (in units of nmol mol ) of the compound in the gas exceeds our criteria are identiﬁed in the ﬁgure and for these data the

ﬂow is given by: measured-to-expected ratios ranged from 1.2 to 1.5. Some data for

other compounds are scattered both high and low, suggesting other

Ir×i

sources of uncertainty. We conclude that we can prepare test mix-

9 1000×MWi

= ×

Xi 10 × (2) tures by this approach with reasonable accuracy, and it is generally

60 GFN2

VmN2 useful for determining fragmentation patterns and also for esti-

mating PTR-MS response factors for the cycloalkanes and alkanes

−1

␮ ␮ where Ir is the infusion rate of the syringe pump in l hr , i is which appear to react signiﬁcantly less rapidly than the collisional

−1

the density of the compound i in g mL , MWi is the molecular rate limit.

weight of the compound i in g/mol, GFN2 is the gas ﬂow of the dry One beneﬁt for making test mixtures with this approach is that

nitrogen in slpm, and VmN2 is the molar volume of gas at standard mixtures of compounds that are typically solids at room temper-

temperature and pressure. ature could be made by dissolving them in a solvent that has a

100 M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109

low proton afﬁnity. Test mixtures of the polycyclic aromatic hydro-

carbons naphthalene, acenaphthalene, acenaphthene and biphenyl

were made by dissolving material into dichloromethane and inject-

−1 ◦

ing the solution at 50 uL hr into an 85 C manifold temperature

producing test mixtures from 50 ppbv to 100 ppbv. Given its

reported low proton afﬁnity of 628 kJ/mol [5], dichloromethane

would not be expected to react with H3O , however the solvent

mass spectrum displayed signiﬁcant ion signal at m/z 31, 49, and

51. A probable mass assignment for m/z 49 and m/z 51 is CH2Cl .

The ratio of ion signal intensities matched the isotopic abundance

35 37

of Cl and Cl. It is known that air sample can diffuse into the

+ +

ion source creating unwanted secondary ions such as O2 and NO

and it is thus likely that CH2Cl2 was being ionized in the hol-

low cathode. The m/z 31 does not correspond to any known CxHy

compounds but could correspond to CH3O from some unknown

reactions occurring in the ion source. The background mass spec-

trum of the dichloromethane solvent was subtracted from the PAH

mass spectra to determine the PAH response. These test mixtures

yielded stable responses and the response factors were consistent

with expected response if collisional reaction rates are assumed

between these compounds and H3O .

3. Results

The fragmentation patterns at 80 and 120 Td drift tube condi-

tions for 61 compounds known to be major constituents of gasoline

and gasoline vehicle exhaust including several PAH compounds

were determined. Results are organized into compound groups and Fig. 4. Product ion abundance from H3O reactions at 80 Td (hashed bar) and

their measured product ion distribution shown in the following 120 Td (solid bar) with (a) n-propylbenzene (b) 2-methyl-2-butene (c) 1,3,5-

trimethylcyclohexane and (d) 4-methylnonane. M + H was the major product ion

tables: Tables 1 and 2 monoaromatics, Table 3 polyclic aromatic

at 80 Td for aromatic and alkene compounds, whereas branched alkanes and alkyl

hydrocarbons and hydrogenated polycyclic aromatic hydrocar-

substituted cyclohexanes yielded M-H ions as indicated in the ﬁgure.

bons, Table 4 alkenes Table 5 cycloalkanes and cycloalkenes, Table 7

iso-alkanes, and Table 8 branched alkanes. Only organic ions that

Fig. 4a shows the product ion distribution for n-propylbenzene at

were ≥3% of the total ion signal were considered as signiﬁcant

80 and 120 Td to illustrate the differences in fragmentation patterns

fragmentation products and listed in the tables.

for the two drift conditions.

3.1. Monoaromatics 3.2. Polcyclic aromatic hydrocarbons and hydrogenated derivatives

Product ion distributions for the monoaromatic + H3O reac-

tion are shown for benzene, toluene and C2 and C3-alkylbenzene The product ion distributions for a suite of low molecular weight

compounds in Table 1 and for C4, C5, and C6-alkylbenzene com- polcyclic aromatic compounds and hydrogenated forms including

pounds in Table 2. Most monoaromatics produced M + H product indene, indane, and 1,2,3,4-tetrahydronaphtahelene are given in

ions at 100% yields at 80 Td. Compounds that fragmented at Table 3. No dissociative proton transfer reactions were observed

80 Td were sec-butylbenzene, cumene, and m-cymene. At 120 for those compounds at 80 Td and 120 Td, and thus produced M + H

Td most of the monoaromatic compounds fragmented to some ions at 100% yield. The abundance of these compounds in urban

degree. Several compounds had signiﬁcant fragment yields of the atmospheres is not routinely measured. A clear ion signal at masses

m/z 79 ion and would be positive interferences for benzene mea- m/z 117, 119, 129, 143 in Fig. 1 for Xi’an and Mexico City suggests

surements in urban atmospheres: ethylbenzene, propylbenzene, the presence of indane, indene, naphthalene, and methylnaphtha-

cumene, and sec-butylbenzene. Rogers et al. [28] and Jobson et al. lene isomers respectively. It is not clear to what extent isobaric

[17] have noted previously that ethylbenzene mixing ratios are typ- compounds or fragmentation of larger organic compounds would

ically high enough in urban atmospheres compared to benzene interfere with these masses.

to cause a signiﬁcant positive interference for benzene measure-

ments. Operating at 80 Td would signiﬁcantly improve the ﬁdelity 3.3. Alkenes

of benzene measurements by eliminating fragmentation of ethyl-

benzene. At 120 Td several C3 and C4-alkylbenzene compounds The fragmentation results for nine C4, C5, and C6 alkenes are

(i.e, ethyltoluene isomers, m-cymene) yielded m/z 93 as a signif- summarized in Table 4. Alkenes generally did not fragment at 80

icant fragment ion and produce a positive interference for toluene Td, and yielded M + H product ions. The exception was T-2-hexene

measurements. In practice, the toluene abundance is typically where the m/z 57 ion was 13% of the product ion signal. At 120 Td all

large compared to the abundance of these compounds [1], and the alkenes underwent dissociative proton transfer reactions yield-

fragmentation would thus be more of a problem in underestimat- ing fragment ions at m/z 41, 43, and 57. The C4 alkenes produced

ing the abundance of these compounds. Also noteworthy is that M + H ions in ∼90% yield, the C5 alkenes produced M + H ions in

∼ compounds with larger alkyl substituent groups (≥C3) yielded sig- 42% yields, and the C6 alkenes produced M + H ions in ∼30% yield.

+ +

niﬁcant m/z 41 (C3H5 ) and 43 (C3H7 ) ion fragments at 120 Td. Fig. 4b shows the product ion distribution for 2-methyl-2-butene

In particular, the M + H ion product for n-pentylbenzene was only to illustrate differences in product ion distributions for the 80 and

11% and no M + H product ion was observed for neopentylbenzene. 120 Td drift conditions. The abundance of C3–C6 alkenes in vehicle

M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109 101

Table 1

Product ion distribution for reaction of H3O with C1–C3 alkylbenzene compounds at 80 and 120 Td drift conditions.

Compound Molecular MW (g/mol) Td 80 Td 120

names structure

m/z % Yield m/z % Yield

Benzene 78 79 100 79 100

Toluene 92 93 100 93 100

Ethylbenzene 106 107 100 107 82

79 18

m-xylene 106 107 100 107 100

p-xylene 106 107 100 107 100

o-xylene 106 107 100 107 100

1,2,3-trimethylbenzene 120 121 100 121 97

93 3

1,2,4-trimethylbenzene 120 121 100 121 100

2-ethyltoluene 120 121 100 121 97

93 3

3- ethyltoluene 120 121 100 121 96

93 4

4- ethyltoluene 120 121 100 121 94

93 3

107 3

Propylbenzene 120 121 100 43 40

79 28

121 21

41 11

Cumene 120 121 96 79 64

79 4 121 21

41 15

exhaust implies that these compounds contribute to the ion signal compound, 3,3,5-trimethylcylhexene. The PTR-MS instrument sen-

at m/z 41, 43, 57, 71, and 85 measured in urban atmospheres as indi- sitivity to cyclopentane, cyclohexane, and methylcyclopentane,

cated in Fig. 1. At 120 Td it would not be possible to resolve alkene was very low, implying proton afﬁnities less than H2O. Cyclohex-

abundance by molar mass since larger alkenes fragment to ions ane has a reported proton afﬁnity of 686.9 kJ/mol (Lias et al., 1984)

representing M + H ions for lighter alkenes, notably propene (m/z [21]which is slightly less than that of water. Spanel and Smith

43) and butenes (m/z 57). Diskin et al. [7] have also reported disso- [31] have measured the H3O rate constants for cyclohexane and

ciative proton reactions for C7–C10 1-alkenes in selected ion ﬂow methylcyclohexane. Cyclohexane was essentially unreactive, con-

tube experiments. Operation of the drift tube at lower Townsend sistent with its low proton afﬁnity, whereas the methylcyclohexane

values would yield a higher ﬁdelity measure of alkene abun- rate constant was 32% of the calculated collisional rate constant.

dance by molar mass, but fragmentation of larger alkenes may Midey et al. [24] have also measured the methylcyclohexane rate

still produce positive interferences in the measurement of lighter coefﬁcient with H3O to be ≤38% of the calculated collision rate

alkenes. limit. The reaction was found to be dissociative; protonation was

followed by H2 elimination to yield an M-H product ion. This previ-

3.4. Cycloalkanes ous work suggests alkysubstituted cyclohexanes have larger proton

afﬁnities than cyclohexane and thus may be detectable by PTR-MS.

The product ion yields for 10 cycloalkane compounds are sum- We observed that the PTR-MS response was much greater for the

marized in Table 5 and include the result for one cycloalkene alkyl substituted cyclohexanes than cyclohexane, and estimated

102 M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109

Table 2

Product ion distribution for reaction of H3O with C4–C5 alkylbenzene compounds at 80 and 120 Td drift conditions.

Compound Molecular MW (g/mol) Td 80 Td 120

names structure

m/z % Yield m/z % Yield

1,4-diethylbenzene 134 135 100 135 96

79 3

107 1

m-cymene 134 135 97 93 74

93 3 135 22

43 4

sec-butylbenzene 134 135 93 79 60

79 7 41 29

135 11

n-pentylbenzene 134 135 100 43 67

41 19

135 11

79 3

Neopentylbenzene 148 149 100 43 79

41 21

1,3,5-triethylbenzene 162 163 100 163 94

55 6

1,3-diisopropylbenzene 162 163 100 163 78

79 7

43 11

39 8

response factors at 80 and 120 Td for a suite of these compounds is substituted cyclohexanes yielded much larger response factors

shown in Table 6. For cyclohexane, the response factor decreased but these were still less than corresponding aromatics of similar

when the drift tube E/N ratio was reduced, but for the methyl sub- mass. For example, the 1,2 and 1,4 isomers of dimethylcyclohexane

stituted cyclohexane compounds the response increased. The alkyl yielded response factors that were 38% that of o-xylene at both drift

Table 3

Product ion distribution for reaction of H3O with polycyclic aromatic hydrocarbons and hydrogenated polycyclic aromatic compounds at 80 and 120 Td drift conditions.

Compound Molecular MW (g/mol) Td 80 Td 120

names structure

m/z % Yield m/z % Yield

Indene 116 117 100 117 100

Indane 118 119 100 119 100

Naphthalene 128 129 100 129 100

1,2,3,4-tetrahydronaphthalene 132 133 100 133 100

1-methylnaphthalene 142 143 100 143 100

Acenaphthylene 152 153 100 153 100

Biphenyl 154 155 100 155 100

Acenaphthene 154 155 100 155 100

1,4-dimethylnaphtahlene 156 157 100 157 100

M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109 103

Table 4

Product ion distribution for reaction of H3O with alkene compounds at 80 and 120 Td drift conditions.

Compound Molecular MW (g/mol) Td 80 Td 120

names structure

m/z % Yield m/z % Yield

1-butene 56 57 100 57 90

41 10

Cis-2-butene 56 57 100 57 89

41 11

2-methyl-1- 70 71 100 43 46

butene 71 44

41 10

2-methyl-2- 70 71 100 43 47

butene 71 44

41 9

1-pentene 70 71 100 43 49

71 41

41 10

Trans-2-pentene 70 71 100 43 48

71 42

41 10

2-methyl-2- 84 85 100 43 48

pentene 85 29

57 12

41 11

Trans-4-methyl-2- 84 85 100 43 48

pentene 85 31

57 12

41 9

Trans-2-hexene 84 85 87 43 48

57 13 85 27

57 13

41 12

−1 +

tube conditions: 4.1 compared to 11.1 Hz ppbv per MHz H3O at ion with 53–63% range yields. For the trimethylcyclohexane com-

−1 +

80 Td and 2.0 compared to 5.2 Hz ppbv per MHz H3O at 120 pounds tested, the dominant product ion was m/z 69 with M-H

∼

Td. Our results thus suggest these compounds react at about 1/3 of ions at 20% yield. Gentner et al. [10] have recently reported that

the collisional reaction rate. Thus the PTR-MS is relatively insensi- cycloalkanes comprise 8–9% of whole gasoline sold in California,

tive to cyclopentane and cyclohexane but has enough response to measuring many alkyl substituted isomers of cyclopentane and

larger alkylsubsituted cyclohexanes that these compounds may be cyclohexane. The occurrence of m/z 69, 111 and 125 ion signals

measurable in urban air. observed in urban air mass spectrums in Fig. 1 might be attributed

At 80 Td the alkyl substituted cyclohexanes yielded M-H ions to these types of compounds. Fig. 4c displays the product ion dis-

(m/z 97, 111, 125) as the dominant product ion, typically ≥95% of tribution for 1,3,5-trimethylcyclohexane to illustrate differences in

the total product ion signal. The smaller cycloalkanes with low sen- cyclohexane compound fragmentation between the 80 and 120 Td

sitivities (cyclopentane, methylcyclopentane, cyclohexane) yielded drift tube operating conditions.

M + H ions as the most abundant product ion. The compound

decahydronaphthalene, a fully hydrogenated polycyclic aromatic 3.5. Alkanes

hydrocarbon, also yielded an M-H ion (m/z 137) as the major

product ion at 80 Td and thus behaved like an alkyl substituted The C8–C12 iso-alkanes and branched alkanes tested all undergo

cyclohexane compound. For the cycloalkene compound, 3,3,5- dissociative proton transfer at 80 and 120 Td drift conditions.

trimethylcyclohexene, the dominant product ion was the M + H We have previously reported on C6–C16 n-alkane fragmentation

ion. patterns and PTR-MS response factors [8]. The proton afﬁnity

At 120 Td the cycloalkane compounds underwent dissociative for n-alkanes increases with carbon number and we observed

proton transfer reactions and yielded a host of fragment ions. Note- increasing sensitivity with carbon number at 120 Td up to C14,

worthy is the signiﬁcant yield of m/z 69, up to 48%, from dimethyl but response factors are still about a third of that expected for

and trimethyl substituted cyclohexanes and 3,3,5-cyclohexene. The compounds that react at the collisional rate limit with H3O ,

m/z 69 ion is used to monitor the biogenic compound isoprene, a similar to that observed for the dimethyl and trimethyl cyclo-

compound present in urban atmospheres from tree emissions and hexanes. The alkanes fragmented to yield a series CnH2n+1 (n ≥ 3)

of interest in photochemical modeling of air pollution. The mea- ions whose relative abundance was a function of drift tube

surement of isoprene will thus suffer positive interferences from conditions. The product ions distributions for iso-alkanes are

these compounds and may contribute to poor comparison results shown in Table 6 and for other branched alkanes in Table 7.

between PTR-MS and gas chromatography methods in urban areas At the 80 Td drift tube condition, the 3-methyl and 4-methyl

when isoprene concentrations are low [18]. For dimethyl com- substituted alkanes yielded the M-H ion in highest abundance,

pounds at 120 Td, the most abundant ion fragment was the M-H ranging from 33% to 44% yields. The 2-methyl substituted

104 M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109

Table 5

Product ion distribution for reaction of H3O with cycloalkane and cycloalkene compounds at 80 and 120 Td drift conditions.

Compound names Molecular MW (g/mol) Td 80 Td 120

structure

m/z % Yield m/z % Yield

Cyclopentane 70 71 78 43 37

57 22 71 34

57 19

41 10

Cyclohexane 84 85 73 43 38

83 23 83 24

85 15

41 13

57 10

Methylcyclopentane 84 85 95 43 45

83 5 85 28

57 12

41 11

83 4

Methylcyclohexane 98 97 96 97 76

99 4 57 24

1,2-dimethylcyclohexane 112 111 100 111 63

69 32

57 5

1,3-dimethylcyclohexane 112 111 95 111 53

113 5 69 31

57 8

71 4

43 4

1,4-dimethylcyclohexane 112 111 95 111 56

113 5 69 28

57 8

71 5

43 4

3,3,5-trimethylcyclohexene 124 125 97 69 48

69 3 125 22

57 16

83 14

1,3,5-trimethylcyclohexane 126 125 94 69 40

127 3 125 23

71 3 57 19

83 10

43 4

41 4

1,1,3-trimethylcyclohexane 126 125 76 69 36

71 9 125 19

111 6 57 19

85 5 83 10

55 3 43 10

41 5

Decahydronaphthalene 138 137 100 81 45

137 44

95 11

alkanes (iso-alkanes) yielded M-H ions in lower abundance, M-H or M + H ions were observed. Fig. 4d displays an example of a

ranging from 8% to 18% yields. For the dimethyl substituted alkanes branched alkane mass spectrum, illustrating the larger abundance

with methyl groups on a different carbon atom the M-H yields of the M-H ion at the 80 Td condition. While the PTR-MS is not very

were also signiﬁcant, and ranged from 19% to 26%. In contrast, 2,2- sensitive to alkanes, these compounds produce fragment ions that

dimethyloctane completely fragmented to m/z 57, 71, and 85. At 80 will be positive interferences for the measurement of alkenes, even

Td the yield of M-H product ions appears to increase for compounds at low Townsend operating conditions.

where the methyl group is further from the end of the chain as

evidenced by the increasing M-H ion yields of 14%, 33%, and 44% for 3.6. Gasoline Mass Spectrum

2-methylnonane, 3-methylnonane, and 4-methylnonane respec-

tively. At 120 Td, the iso-alkanes and branched alkanes fragmented Gentner et al. [10] have reported a very detailed analysis of

to yield m/z 43, 57, and 71 as the most abundant product ions; no the constituent composition of gasoline sold in California. It is

M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109 105

Table 6

for the aromatic and the C4–C8 alkene compounds likely introduces

−1 +

Measured cycloalkane response factors in units of Hz ppbv per MHz H3O at 80

some basis; larger compounds with greater polarizabilities’ would

and 120 Td drift tube operating conditions.

be expected to react faster, while smaller compounds react slower.

Compound 80 Td 120 Td

For example, the calculated capture rate constants [33,34] for 1-

Cyclohexane 0.040 0.063 methylnaphthalene and 2-methylnaphthalene are 27% greater than

Methylcyclohexane 1.50 0.58 that of toluene, while for C4-alkenes the rate constant are about

1,2-dimethylcyclohexane 4.1 2.0

20% less than toluene. Our dynamic dilution system was not precise

1,3-dimethylcyclohexane 3.0 1.6

enough to reliably measure such differences. Reactivity corrections

1,4-diemthylcyclohexane 4.1 2.0

could conceivably be applied but dipole moments and polariz-

1,1,3-trimethylcyclohexane 1.7 1.4

1,3,5-trimethylcyclohexane 3.4 2.2 ability values are not known for most of the organic compounds

considered. Differences in ion transmission have a similar impact

on relative sensitivity and this will be instrument speciﬁc. The ion

transmission efﬁciency curve for our instrument was determined

instructive to use that published information and the fragmenta-

experimentally from m/z = 33 to m/z = 173 using 14 compounds

tion patterns for compounds reported here to construct a simulated

and the data ﬁtted with a 3 order polynomial curve. Relative ion

PTR-MS mass spectrum of gasoline and compare it to the mass

transmission efﬁciency (ε) peaked at m/z 96, and for toluene the

spectrum of a gasoline sample. Gentner et al. reported molar abun-

relative ion transmission efﬁciency was close to 1 (ε = 0.999). The

dances of a large number of compounds in the C2–C15 carbon

transmission efﬁciency of the m/z 43 ion was ε = 0.678, and for

number range. We used their reported molar abundances of iden-

the largest ion at m/z = 157 attributed to dimethylnaphthalenes,

tiﬁed aromatic compounds (n = 62), alkenes (n = 74), cycloalkanes

= 0.570.

(n = 39), and alkanes (n = 96) and applied PTR-MS sensitivity factors

We examined the 80 Td condition as the fragmentation patterns

and ion yield information to construct a simulated mass spec-

were simpler. Fragmentation patterns for many compounds were

trum. For simplicity we used sensitivity factors relative to toluene.

assumed to follow the general patterns shown in the Tables 1–8.

These have been reported for n-alkanes [8], and we assumed that

For example yields of M-H ions from alkyl substituted cyclohex-

iso-alkanes have the same sensitivity as n-alkanes of the same

ane isomers where assumed to be 95%, given the measured range

carbon number. For cycloalkanes we used the relative response

from 94% to 100%. For alkanes we assumed that compounds with

factors reported here. These experimentally determined sensitivi-

+ similar substitution patterns as those measure here fragmented in

ties account for differences in the reaction rate with H3O and ion

similar product ion yields. We assumed M + H product ions for the

product transmission efﬁciencies. For alkenes and aromatics we

cyclopentene compounds: cyclopentene (m/z = 69) and methylcy-

assumed these compounds react at the same collisional rate limit

clopentenes (m/z = 83).

as toluene and applied relative ion transmission correction factors

Whole gasoline was sampled at 80 Td using the dynamic dilu-

(ε) to account for differences in ion transmission. We choose to use

tion system to dilute the gasoline vapor in dry nitrogen. The mass

this approach because the accuracy of our syringe pump method

spectral response of the gasoline sample and the simulated gaso-

is only ±10% if care is taken to ensure conditions can that yield

line mass spectrum were normalized to the m/z 135 ion signal

accurate infusion rates. This was not always done for aromatics

to yield comparable relative response scales. These normalized

and alkenes where the primary goal was to determine product

mass spectra are shown in Fig. 5. For clarity only even masses

ion yields, since these compounds were known to react at colli-

+ are shown in the gasoline sample; odd mass are assumed to be

sional rates limits. Assuming the same H3O reactivity as toluene

Table 7

Product ion distribution for reaction of H3O with iso-alkane compounds at 80 and 120 Td drift conditions.

Compound Molecular MW (g/mol) Td 80 Td 120

names structure

m/z % Yield m/z % Yield

2-methylheptane 114 57 39 57 44

71 35 43 28

113 18 71 19

43 8 41 9

2-methylnonane 142 71 30 43 35

85 26 57 33

57 21 71 15

141 14 41 10

43 6 85 8

99 3

2-methyldecane 156 57 29 57 42

85 29 43 31

71 20 71 10

155 10 41 10

99 7 85 7

43 6

2- 170 57 31 57 47

methylundecane 71 22 43 26

85 21 71 12

99 8 41 8

169 8 85 6

113 5

43 5

106 M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109

Table 8

Product ion distribution for reaction of H3O with branched alkane compounds at 80 and 120 Td drift conditions.

Compound Molecular MW (g/mol) Td 80 Td 120

names structure

m/z % Yield m/z % Yield

3-methyloctane 128 127 34 43 34

71 29 57 31

57 22 71 19

85 15 41 10

85 6

2,3-dimethylheptane 128 71 41 43 40

85 24 57 27

127 21 71 21

57 14 41 11

85 8

3-methylnonane 142 141 33 57 38

71 26 43 29

57 22 71 15

85 17 85 9

99 3 41 7

141 2

4-methylnonane 142 141 44 57 34

71 22 43 32

85 17 71 15

57 11 85 9

99 6 41 7

141 2

2,2-dimethyloctane 142 57 43 57 45

71 39 43 27

85 18 71 15

41 9

85 5

2,3-dimethyloctane 142 141 25 57 33

71 22 43 31

85 22 71 15

57 14 85 9

115 10 41 7

99 6 115 5

3,5-dimethyloctane 142 71 28 57 37

141 26 43 31

57 20 71 16

85 29 85 9

99 6 41 7

due to C isotopes. The gasoline sample and the simulated mass resolved and detected by Gentner et al., and hence not reported.

spectrum have some important differences but in general the In the PTR-MS, geometric isomers are observed at the same m/z,

simulated mass spectrum accounts for most of the ion intensity conferring a signal-to-noise advantage in measuring total abun-

and has a similar pattern of ion response. The simulated mass dance. At the low m/z range much higher abundances of m/z 43

spectrum displays notable differences to the whole gasoline sample and 57 and other ions were observed in the sample, suggest-

at both the low mas range and high mass range of the spec- ing a greater number of fragmenting compounds or differences

trum. Compounds at the high mass range may not have been in ion yield distributions that were not accurately accounted

Table 9

Estimated fractional contribution of alkanes, alkenes, and cycloalkanes to ion signal response relative to m/z 135 for simulated gasoline mass spectrum shown in Fig. 5.

m/z Relative response Alkanes (%) Cycloalkane (%) Alkenes (%) Dominant Compound

43 0.003 86.8 0.0 13.2 Alkanes

57 0.23 67.0 0.1 32.9 Alkanes

69 0.047 0 0.0 100 Cyclopentene

71 1.22 16.6 0.2 83.2 C5-alkenes

83 0.10 0.0 0.8 99.2 Methylcylopentenes

85 1.37 7.7 0.5 91.8 C6-alkenes

97 0.09 0 100 0.0 Methylcyclohexane

99 0.22 11.4 2.9 85.7 C7-alkenes

111 0.26 0.0 100 0.0 C2-cyclohexanes

113 0.21 14.0 6.2 79.7 C8-alkenes

125 0.071 0.0 100 0.0 C3-cyclohexanes

127 0.021 89.7 10.3 0.0 C9-alkanes

141 0.009 100 0.0 0.0 C10-alkanes

M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109 107

Fig. 5. Comparison of a gasoline sample mass spectrum (upper panel) with a simulated gasoline mass spectrum (lower panel) at 80 Td. The simulated mass spectrum was

calculated from published molar abundances of individual compounds measured in gasoline, their relative PTR-MS response factors, and product ion yields. The mass spectra

have been normalized to the m/z 135 ion response to make the scales of the sample and simulated mass spectrums comparable. Color code indicates what compound group

dominated response.

Table 10

ratio of m/z 119 to m/z 117 was a factor of 9 in the simulated mass

Attribution of aromatic compounds to ion signal response relative to m/z 135 for

spectrum and a factor of 11 in the gasoline sample. The m/z 133 ion

simulated gasoline mass spectrum shown in Fig. 5.

response, attributed to methyl indanes and tetralin was a factor or

m/z Relative response Compound

9 less than the m/z 135 response in the simulated mass spectrum,

79 0.48 99% benzene and a factor of 14 less in the whole gasoline sample.

93 4.3 Toluene In the simulated mass spectrum alkenes comprise the major-

107 5.4 C2-alkybenzenes

ity (≥80%) of the ion signal at m/z 71, 85, 99, 113 but at m/z 127

121 3.4 C3-alkybenzenes

the major contribution is calculated to be from alkanes (90%) and

135 1.00 C4-alkybenzenes

cycloalkanes (10%). At m/z 141 the signal is dominated by alkanes.

149 0.094 C5-alkybenzenes

117 0.016 Indene In the whole gasoline sample, the ion series extends to m/z 155,

119 0.14 Indane

169, and 183, likely due to the presence of larger alkanes. The abun-

129 0.050 Naphthalene

dances of the alkene ions were similar in the whole gasoline and

133 0.010 Methylindane + tetralin

simulated mass spectra. In the simulated spectrum m/z 71 response

143 0.031 Methylnaphthalenes

147 0.029 Dimethyl indanes was due to C5 alkenes (83%) and alkane fragments (17%) and the

155 0.002 Biphenyl response was similar to m/z 85 (92% C6 alkenes, 8% alkane frag-

157 0.007 Dimethylnaphtahlenes

ments). These ions also had similar intensities in the whole gasoline

sample. Even at the low fragmentation conditions of 80 Td, the attri-

bution of particular compounds to the ion series m/z 71, 85, 99, 113,

127 is difﬁcult due to contributions from alkenes and fragment ions

for in our simulation spectrum. Differences in fuel composition

from alkanes and cycloalkanes.

between our gasoline sample and that measured by Gentner et al.

Cycloalkanes are abundant in gasoline and dominate the ion sig-

may also contribute to differences between the mass spectrums.

nal for m/z 97, 111 and 125. The m/z 69 and 83 ions in the simulation

Tables 9 and 10 lists the contribution by compound or compound

were due to cyclopentene and methylcyclopentenes respectively

class to the ion signal strength for the simulated spectrum in

and are present in similar abundance in the whole gasoline sample

Fig. 5.

as well. The m/z 97 (cyclohexane), and 111 and 125 ions associ-

The mass spectrum of the whole gasoline sample is dominated

ated with alkyl substituted cylopentanes and cyclohexanes are as

by monoaromatic compounds in particular toluene (m/z 93) and

prominent in the simulated mass spectrum as the whole gasoline

the C2-alkylbenzne isomers (m/z 107), a feature captured by the

sample mass spectrum. We observed that this ion series continues

simulated mass spectrum. At 80 Td benzene is responsible for 99%

with prominent ion signal at 139 and 153 which is absent from the

of the m/z 79 ion signal. In the whole gasoline sample we observed

simulated mass spectrum. These masses suggest the presence of C4

ion signal at m/z 163, attributed to C6-alkybenzenes, that is a factor

and C5 alkyl substituted cyclohexanes not reported by Gentner et al.

of ∼100 lower than m/z 135. Other aromatic compounds were also

indicated. In the simulated mass spectrum the abundance of indene

(m/z 117) and indane (m/z 119) and alkyl substituted indanes (m/z 4. Conclusion

133 and 147) were responsible for the intensity of these peaks and

were about a factor of 10 less intense than the m/z 135 response. Product ion yields of for a suite of hydrocarbons including

These ions also appear in the whole gasoline sample mass spec- alkanes, alkenes, cycloalkanes, and aromatics associated with vehi-

trum with similar relative intensities. For example, the abundance cle emissions were determined for a PTR-MS instrument operating

108 M. Gueneron et al. / International Journal of Mass Spectrometry 379 (2015) 97–109

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