Maximizing sensitivity and determining dominant reaction
mechanisms
The dominant reaction mechanism occurring for different n-alkanes, and also
the overall sensitivities of detection, varied as a function of the relative
abundance of different primary ions and could be controlled by varying
USO, water flow and E/N ratio. The absolute sensitivities varied
substantially with USO for different n-alkanes, while low E/N
ensured the highest sensitivity for all the n-alkanes. Maximum
sensitivities from n-nonane to n-dodecane were obtained at LWF, the
lowest E/N ratios (83 Td) and the maximum USO (180 V).
Figure 2 shows the sensitivities (cps ppbv-1) for two example n-alkanes:
n-hexane and n-decane. Three ions were plotted for n-hexane and for
n-decane, produced from the HA, CT and PT mechanisms. The highest
sensitivity for n-hexane was obtained by HA (m/z 85) followed by CT
(m/z 86). PT (m/z 87) sensitivities were negligible (< 1 % with
respect to HA). In the case of n-decane, maximum sensitivities were
obtained by CT (m/z 142), followed by HA (m/z 141). Sensitivities by PT
were < 8 % with respect to CT. n-Nonane, n-undecane and
n-dodecane (Fig. S1) showed the same sensitivity trend as n-decane
(Fig. 2b).
Sensitivity ratios for different mechanisms: [M+] charge
transfer, [M - H]+ hydride abstraction and [M + H]+ proton transfer.
Dominant mechanism is shown in parenthesis.
n-Alkane
Optimized conditions
Usual conditions
Optimized conditions
Usual conditions
2 sccm, 83 Td, 180 V
6 sccm, 122 Td, 60 V
2 sccm, 83 Td, 180 V
6 sccm, 122 Td, 60 V
(HA)
[M - H]+/ [M + H]+
[M - H]+/ M+
n-Hexane
344.0
[M + H]+= n.d.
1.9
M+= n.d.
(CT)
M+/ [M + H]+
M+/ [M - H]+
n-Nonane
50.5
M+, [M + H]+= n.d.
2.4
M+, [M - H]+= n.d.
n-Decane
17.6
[M + H]+= n.d.
1.6
1.1
n-Undecane
8.0
[M + H]+= n.d.
1.6
0.5
n-Dodecane
3.5
[M + H]+= n.d.
1.6
1.0
n.d. – not detected.
H3O+ and O2+ were the most abundant primary ions in all
experiments. O2+ and NO+ clearly increased with USO
voltage, while water clusters decreased. Lower H3O+ abundance was
used in the n-hexane experiments compared to the larger n-alkanes
experiments because the water flow rate affected the absolute count rate of
primary ions, and thus the water flow rate was changed. However, the reaction
mechanisms are clearly demonstrated with hydride abstraction occurring for
n-hexane and charge transfer for n-decane.
Table 2 shows the comparisons in sensitivities for the different mechanisms.
We also compared these sensitivities under “typical” PTR-MS conditions to
analyze VOCs: water flow 6 sccm, E/N ratio 122 Td and USO
60 V. Sensitivities for some ions in “typical” conditions were close to
zero. However, under optimized conditions, sensitivities of n-hexane by HA
were 344 times higher than sensitivities by PT and 1.9 times higher than by
CT. In the case of higher n-alkanes, sensitivities by CT were higher than
by PT but the ratios decreased consistently with the chain length, with
higher PT for larger n-alkanes due to increasing proton affinities. A
constant effect throughout the chain length was observed for the
sensitivities ratios between CT and HA. Table 2 shows why, under typical
PTR-MS conditions, n-alkanes are not detected by PT and emphasizes the
novelty of the mixed mode. Under optimized conditions, our methodology can be
used to detect n-alkanes not only from fossil sources but also from other
sources, where their concentrations are extremely low and high sensitivity is
needed. Maximum sensitivities were observed by CT and HA when water flow was
significantly reduced (2 sccm in this study); thus further discussion will
deal only with this low flow condition and will focus on O2+ and
NO+ ionizations.
O2+ and NO+ ionizations
Molecules with IEs lower than the recombination energy of
NO+ (9.26 eV) may undergo charge transfer (Ellis and Mayhew, 2014;
Lias, 2000), for example, aromatics and alkenes (Karl et al., 2012; Liu et
al., 2013). n-Alkanes in this study have IEs (Table 1)
higher than NO+, so no charge transfer was expected. Therefore, charge
transfer theoretically should be exclusively due to O2+. Figure 3a
illustrates strong positive associations between O2+ and the signal
of n-decane in CT (m/z 142, r2 ≥ 0.97). Similar behavior was
observed for n-nonane, n-undecane and n-dodecane (Fig. S2). Reactions
between O2+ and n-alkanes have been suggested to result from the CT
mechanism (Arnold et al., 1997; Francis et al., 2007; Španěl and
Smith, 1998; Wilson et al., 2003) at or close to the collisional rate (Lias
et al., 1976; Searles and Sieck, 1970), except for methane (Barlow et al.,
1986).
Intensities of (a) m/z 142 (M+, charge transfer) vs. m/z 32
(O2+) for n-decane and (b) m/z 141 ([M - H]+, hydride
abstraction) vs. m/z 30 (NO+) for n-decane. Marker size illustrates the
USO values: smallest is 60 V and largest is 180 V.
The relative abundance of the molecular ion reaction product increased with
O2+ and decreased with E/N due to higher fragmentation (Fig. S3).
Fragments m/z 57 [C4H9]+, m/z 71 [C5H11]+,
m/z 85 [C6H13]+, m/z 99 [C7H15]+ and
m/z 113 [C8H17]+ were the most abundant at lower E/N
ratios, but m/z 43 [C3H7]+ and in particular m/z 29
[C2H5]+ were the most abundant at higher E/N ratios. This was
observed for all n-alkanes. CT in O2+ is dissociative and produces
multiple product ions (Španěl and Smith, 1998). The identification of
n-alkanes or other compounds in highly complex mixtures using the actual
conditions is difficult and therefore has not yet been widely utilized.
However, we comprehensively approach these challenges and show that varying
E/N ratio results in different proportions of these common fragments. The
interferences and sensitivities are always a general issue for all ionization
modes and in particular for quadrupole detectors and even more when
fragmentation is high. While this can be an issue at trace levels of
n-alkanes in biogenically influenced air (where isoprene oxidation products
can result in the same nominal masses), this should not be a problem for
characterizing crude oil evaporation or alkane presence in pollution plumes
dominated by n-alkanes. Potential interferences could be from alkenes but
they are not present in the crude oil and their presence and abundance can
also be inferred from different mechanisms.
O2+ has been reported to produce significant yields of HA species for
ethane (55 %) and propane (40 %) (Francis et al., 2007), although
Arnold et al. (1997) reported 2 % [C8H7+] for n-octane.
Unlike in the case of O2+, NO+ has been shown to react by HA for
≥ C5 n-alkanes (Arnold et al., 1997, 1998; Španěl and
Smith, 1998). This is illustrated in Fig. 3b where high correlations were
obtained between products of HA and NO+ (r2=0.94–0.99,
E/N ≤ 109 Td). Because in our study NO+ was mixed with
O2+ (1 : 10) we cannot distinguish the dominant ion for HA using
PTR-MS.
Enthalpy is one reason to explain the low sensitivities of [M - H]+
species for smaller n-alkanes with NO+. Formation of [M - H]+ by
reaction between NO+ and n-alkanes (≤ C5) has been reported
to be endothermic (Hunt and Harvey, 1975; Arnold et al., 1998). In addition,
the reaction rate constants decrease with carbon chain length for the
n-alkanes (Arnold et al., 1998; Hunt and Harvey, 1975; Lias et al., 1976;
Searles and Sieck, 1970; Španěl and Smith, 1998, Wilson et al.,
2003). Arnold et al. (1998) observed 20 % yields for [M - H]+ species
in the reaction of n-hexane with H3O+, while < 5 % was
observed in the reactions with n-heptane to n-decane. Similarly, Wilson
et al. (2003) reported yields of 30 % of [M - H]+ species on
n-butane with H3O+. However, the rate constant reported by Arnold
et al. (1998) was 0.006×10-9 cm3 s-1, while Wilson et
al. (2003) did not report the rate constant. Španěl and Smith (1998)
and Francis et al. (2007) did not report [M - H]+ species from reaction
between n-alkanes and H3O+. In our study we did not find any
correlation between H3O+ signal with hydride abstraction species
for any of the n-alkanes studied.
H3O+ effects
In our study, [M + H]+ species were not observed for n-alkanes
smaller than n-decane. For larger n-alkanes, a small signal intensity for
[M + H]+ was observed, which increased with increasing carbon chain
length (Figs. 2, S1). The formation of [M + H]+ species for
short-chain n-alkanes is difficult because the reaction does not proceed at
the collisional rate, but the proton transfer reaction of H3O+ with
n-alkanes larger than n-hexane is expected to be exothermic (Arnold et
al., 1998).
The [M + H]+ did not correlate with H3O+, indicating it is
not the proton donor, suggesting no significant direct proton transfer occurs
between H3O+ and n-alkanes in this system. The same results were
reported by Arnold et al. (1998), who observed no direct proton transfer
product ions in the form CnH2n+3+ (Nc=2–12) in their
SIFT experiments. Španěl and Smith (1998) also did not observe
protonated parent alkanes (Nc=4–12), apparently due to their
instability in SIFT experiments. No direct proton transfer reaction is
expected for formation of [M + H]+ from n-alkanes by PT from water
clusters (Jobson et al., 2005), because the proton affinities for these
compounds are lower than the water cluster.
Instead of direct proton transfer to n-alkanes, [M ⋅ H3O]+
species have been observed to be produced through an association mechanism
between n-alkanes and H3O+ primary ion (Jobson et al., 2005;
Španěl and Smith, 1998) (Reaction R6). Španěl and
Smith (1998) observed in SIFT experiments that reaction of H3O+
with n-alkanes occurs near the collisional limit to form only association
product ions as [M ⋅ H3O]+. These species occur when the
proton affinity is less than the proton affinity of water and proton
transfer is endothermic (Smith and Španěl, 2005). We observed
[M ⋅ H3O]+ species formed from n-alkanes as illustrated by
the adduct of n-decane at m/z 161 (Fig. S3). Positive correlations
between hydronium and n-decane adduct [n–C10H22⋅ H3O]+
(m/z 161) with H3O+ (Fig. 4a) were observed. Similar correlations
were found with H2O ⋅ (H3O)+ (Reaction R7). Higher
H3O+ is produced at lower O2+ and, therefore, more [M ⋅ H3O]+
was observed under these conditions. Similarly, a higher
abundance of water clusters is produced at lower E/N ratio, and more
[M ⋅ H3O]+ was observed at lower O2+. Once the
[M ⋅ H3O]+ is formed, a collisional dissociation can take
place producing [M + H]+ (Reaction R8, Fig. 4b). An inverse
relationship between [C10H22⋅ H3O]+ and [M + H]+
(m/z 143) was observed. At higher O2+, more de-clustering occurs,
decreasing [M ⋅ H3O]+ and increasing [M + H]+.
Therefore, we propose that [M + H]+ comes from the adduct formation
from [M ⋅ H3O]+ found for these compounds (Reactions R6 and
R7) and a subsequent collisional dissociation (Reaction R8).
H3O++n-C10H22→[n-C10H22⋅H3O]+Adduct formationH2O⋅(H3O)++n-C10H22→[n-C10H22⋅H3O]++H2OLigand switching reaction[n-C10H22⋅H3O]++M→[n-C10H22⋅H]++H2O+MCollisional dissociation[n-C10H22⋅H3O]++H2O→H2O⋅(H3O)++n-C10H22Ligand switching
Španěl and Smith (1998) observed ligand switching between
[M ⋅ H3O]+ and the H2O in SIFT experiments when a trace of
water vapor was present in the carrier gas (Reaction R9). This can catalyze
the production of the hydrated hydronium ion via Reactions R6 and R9.
Switching reactions usually occur at the collisional rate, including
reactions with alkanes (Španěl and Smith, 1998). Maximum
[M ⋅ H3O]+/ M+ percentages were observed in higher E/N
ratios and lower USO. The stability of these species is probably
due to an ion-induced dipole interaction strengthened by the higher
polarizability of larger alkanes (Anslyn and Dougherty, 2006), increasing the
bond strength of the cluster (Arnold et al., 1998). The rates of such
association reactions apparently increase as the proton affinity approaches
that of water (Španěl and Smith, 1998).
Intensities of the n-decane-water cluster (m/z 161,
M ⋅ H3O+) vs. (a) hydronium (m/z 19, H3O+) and (b) proton
transfer product (m/z 143, M + H+). Marker size illustrates the
USO: smallest is 60 V and largest is180 V. No correlations were observed at
109 and 122 Td.
Sensitivities and linearity of response
The optimal PTR-MS conditions were applied to determine the response
linearity of the n-alkanes. Experiments were carried out at 83 Td,
USO= 180 V and H3O+= 1 sccm. We extended the number
of n-alkanes from n-pentane to n-tridecane. Table S1 indicates the
range of concentrations tested.
Fragment contribution distribution (FCD, %) from larger
n-alkanes to smaller n-alkanes.
Smaller
n-alkane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
n-Undecane
n-Dodecane
Larger
n-alkane
m/z
72
86
100
114
128
142
156
170
n-Hexane
86
96.7
n-Heptane
100
153.9
8.6
n-Octane
114
59.0
138.1
0.7
n-Nonane
128
31.0
61.2
12.9
0.0
n-Decane
142
28.7
34.4
8.6
8.8
0.2
n-Undecane
156
21.0
18.5
5.2
6.3
5.6
0.5
n-Dodecane
170
15.1
11.5
2.8
4.5
5.0
5.3
0.6
n-Tridecane
184
11.5
8.4
1.9
3.0
4.3
5.9
6.6
1.0
Since the sensitivity (cps ppbv-1) of target compounds is a function of the
primary ion abundance, and the absolute values of water flow in different
PTR-MS instruments may result in different levels of O2+, normalized
sensitivity (ncps ppbv-1) is recommended to account for their variability
(Jobson et al., 2005; Warneke et al., 2001). H3O+ is normally used
when proton transfer is the dominant mechanism. However, in our study, we
calculated four types of sensitivities: absolute, normalized to O2+, normalized
to NO+ and normalized to the sum of O2++ NO+, for all n-alkanes. To
determine which approach is the best metric for sensitivity comparisons, we
compared the sensitivities for n-nonane, n-decane, n-undecane and
n-dodecane obtained in the first and the second set of experiments. These
n-alkanes followed exactly the same mechanisms among them, as described
earlier. Figure S4 shows stabilities of sensitivities. Normalized
sensitivities to O2+ resulted in the minimum variability in
sensitivities by HA reactions, while the normalization to the sum of
[O2++ NO+] resulted in the minimum variability in sensitivities
by CT reactions. Results suggest that normalization to both ions makes sense
for CT and that normalization to O2+ makes the most sense for HA.
Sensitivities for straight chain alkanes detected by different
mechanisms: (a) proton transfer, normalized to H3O+; (b) charge
transfer, weighted to sum (NO++ O2+); (c) hydride
abstraction, weighted to sum (NO++ O2+); (d) double hydride
abstraction, weighted to O2+.
Ionization energy vs. sensitivities weighted to O2+ for
n-alkanes reacting via charge transfer.
Contribution of double hydride abstraction (DHA) to charge
transfer (CT), calculated using weighted sensitivities [w(O2+)
cps ppbv-1] normalized to O2+.
Figure 5 shows the normalized and weighted to O2+ sensitivities for
all nine n-alkanes determined in the optimized conditions. To be consistent
with terms used by the PTR-MS community, we use “ncps” for the PT mechanism
normalized to 1 million cps of H3O+ (Fig. 5a). The CT (Fig. 5b) and
DHA (Fig. 5d) mechanisms in the case of n-alkanes are due to O2+ so
we present “weighted” sensitivities wcps/ppbv (to distinguish from PTR
terms) where signals are normalized to 1 million cps of O2+. In the
case of HA mechanisms (Fig. 5c), which can undergo O2+ ionization,
NO+ ionization or both, we present “weighted” sensitivities wcps/ppbv
which are normalized to 1 million cps of the sum of NO+ and O2+.
As we observed previously in the first set of experiments, CT was the
dominant mechanism for n-heptane and larger n-alkanes, while HA was
dominant for n-hexane and n-pentane. The limits of detection (LODs) are
dependent on the sensitivity and standard deviation of the VOC-free
background (dependent on the amount of averaging and other factors). LODs
depend on the averaging times, ionization mechanism and instrument-dependent
background. LODs for n-alkanes studied here are calculated to be between
10 ppt for n-dodecane and 460 ppt for n-hexane, with averaging times of
0.2 s. These LODs are at the lower end of what is useful in the ambient
atmosphere under relatively unpolluted conditions, but they seem promising for
studies in polluted environments or in regions where oil evaporation is
occurring. Furthermore, longer averaging times can be used to reduce the
detection limit further.
Dependence of CT sensitivity on ionization energy
The sensitivities to reaction by charge transfer for the n-alkanes was
observed to be proportional to the chain length from n-pentane to
n-dodecane (Fig. 5b). This is due to the higher polarizability and
stability of larger n-alkanes (Cao and Yuan, 2002), as shown by the
exponentially negative association between their normalized sensitivities and
IEs (Fig. 6) (wSens =5E+43x-42.23,
r2=0.98, n=9). The sensitivities to reaction by HA also increased with
the chain length from n-heptane, but sensitivities to n-pentane and
n-hexane were higher than n-heptane (Fig. 5c). PT reactions increased
sensitivities from n-decane in our study (Fig. 5a). However, negligible or
no PT products were found in this study for n-alkanes smaller than
n-decane. Meanwhile a decrease in sensitivity to reaction by CT and HA was
observed for n-tridecane, an increase in the PT sensitivity was observed
when the carbon number increased. This trend was previously shown by SIFT
studies for larger n-alkanes by the PT mechanism, where the reactions with
H3O+ become more exothermic (Arnold et al., 1998).
Double hydride abstraction
Figures 5d and 7 demonstrate the observation of DHA products from n-alkanes. DHA/CT products shown in Fig. 7 provide a
better visualization of the abundance for n-pentane and n-hexane. Little
information has been reported on DHA mechanisms. Francis et al. (2007)
reported DHA yields from ethane (15 %) and propane (5 %) in the
presence of O2+, while dissociation was reported for higher
n-alkanes (Nc=4–9). Arnold et al. (1997) reported DHA in the
reaction of n-octane (1 %) and isooctane (7 %) with O2+.
Positive correlations of DHA for n-hexane and n-decane vs. O2+
(Fig. S5) were obtained, suggesting DHA is driven by O2+.