Detection of Triacetone Triperoxide by High Kinetic Energy Ion Mobility Spectrometry

High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS) is a versatile technique for the detection of gaseous target molecules that is particularly useful in complex chemical environments, while the instrumental effort is low. Operating HiKE-IMS at reduced pressures from 10 to 60 mbar results in fewer ion-neutral collisions than at ambient pressure, reducing chemical cross-sensitivities and eliminating the need for a preceding separation dimension, e.g., by gas chromatography. In addition, HiKE-IMS allows operation over a wide range of reduced electric field strengths E/N up to 120 Td, allowing separation of ions by low-field ion mobility and exploiting the field dependence of ion mobility, potentially allowing separation of ion species at high E/N despite similar low-field ion mobilities. Given these advantages, HiKE-IMS can be a useful tool for trace gas analysis such as triacetone triperoxide (TATP) detection. In this study, we employed HiKE-IMS to detect TATP. We explore the ionization of TATP and the field-dependent ion mobilities, providing a database of the ion mobilities depending on E/N. Confirming the literature results, ionization of TATP by proton transfer with H3O+ in HiKE-IMS generates fragments, but using NH4+ as the primary reactant ion leads to the TATP·NH4+ adduct. This adduct fragments at high E/N, which could provide additional information for reliable detection of TATP. Thus, operating HiKE-IMS at variable E/N in the drift region generates a unique fingerprint of TATP made of all ion species related to TATP and their ion mobilities depending on E/N, potentially reducing the rate of false positives.


Table of contents
(m/z 91) in ambient ionization mode in air depending on reduced drift field strength E DR /N at E RR /N = 30 Td and E RR /N = 100 Td.The reduced ion mobilities of TATP-related product ions were determined from measurements where TATP was supplied using the gas mixing system in Figure S2 a).All other operating parameters were set according to

S2. Setup of HiKE-IMS
The design of the HiKE-IMS is described in detail in the main manuscript.Figure S1 shows a schematic of the instrument.

Figure S1
. Schematic of the HiKE-IMS consisting of a corona discharge ion source, a reaction region, a tristate ion gate, and a drift region.The reduced electric field strengths of both the reaction region E RR /N and drift region E DR /N can be adjusted separately.The HiKE-IMS is located within a thermally insulated housing which is heated to 80°C.All operating parameters are given in Table 2 in the main manuscript.

S4. Blank measurements
Previous publications have extensively studied the reactant ion population in ambient ionization mode, without any dopant gas supplied to the corona discharge ion source. 1,2Figure S3 2 in the main manuscript.
The addition of ammonia vapor to the corona discharge ion source when operating in ammonia-doped ionization mode changes the reactant ion population compared to ambient ionization mode.

S5. Additional Ion Mobility Spectra
To verify whether the possible acetone-related ion species m/z 43 and m/z 59 found in HiKE-IMS in ambient ionization mode at E DR /N = E RR /N = 100 Td, as shown in Figure 1 of the main manuscript, correspond to the ion species found in a previous study for acetone in HiKE-IMS, 3 ion mobility spectra of acetone were recorded under the same operating conditions as for TATP in ambient ionization mode.For this purpose, a permeation tube filled with acetone was placed in the sample container shown in Figure S2. Figure S5 shows a comparison between a headspace measurement of acetone and TATP at E DR / N = E RR /N = 100 Td.The analysis of acetone shows two ion species with reduced ion mobilities of K 0,1 = 2.68 cm 2 /Vs (m/z 43) and K 0,2 = 2.33 cm 2 /Vs (m/z 59) in addition to the reactant ions.Thus, the same ion species as in a previous study of acetone can be found. 3When comparing the ion mobility spectra of acetone and TATP, it is apparent that the ion species with m/z 43 and with m/z 59 have both the same mass-to-charge ratios and the same ion mobilities for both analytes.Hence, acetone and TATP possibly form the same ion species.As discussed in the main manuscript, this might either be attributed to ionization of acetone residues from synthesis in the headspace of TATP powder or to fragmentation or degradation of TATP.    2 in the main manuscript.

S6. System Response Depending on Operating Temperature
To evaluate the benefit of operating HiKE-IMS at elevated temperatures, the temporal system response of HiKE-IMS to TATP operated at different temperatures of both the HiKE-IMS including the second capillary used as sample flow resistor and the first sample capillary, in the following referenced to sample inlet, are investigated.For this purpose, ion mobility spectra are recorded for a period of 180 seconds at fixed reduced electric field strengths of E RR /N = 100 Td and E DR /N = 100 Td.To supply the sample to the HiKE-IMS as a short sample plug, the gas mixing system is used as described in Section S3.Following a measurement time of 10 seconds where clean nitrogen is supplied to the HiKE-IMS, TATP is introduced into the HiKE-IMS for 20 seconds via the 3-way-valve in Figure S2 b).Afterwards, the valve is switched back to again provide clean nitrogen to the HiKE-IMS.In this experiment, the signal intensity of C 3 H 6 O 3 H + (m/z 91) is analyzed over time.To eliminate any influence of temperature on signal intensity and since only the changes in signal intensity upon TATP introduction are relevant for this experiment, the signal intensities are normalized to the maximum value of each measurement.
In the first measurement shown in Figure S7, both the HiKE-IMS and the sample inlet are operated at low temperatures of 35°C each.As observed, the system's response is significantly delayed.TATP can only be detected at these low operating temperatures after 40 seconds, when the 3-way-valve has already been switched back to supply neutral nitrogen.Probably, TATP condensates in the tubes, fittings, and the instrument itself.Accordingly, the signal did not completely return to the initial value even after 180 seconds.To determine whether condensation in IMS or in the sample inlet is dominant, T IMS and T Inlet are individually increased.When increasing T Inlet to 100°C, the signal intensity increases after 20 seconds and reaches the maximum value after 40 seconds.Therefore, the time to reach maximum signal intensity can be reduced, possibly since condensation in the first sample capillary can be mitigated.Nevertheless, the time required to purge TATP out of the HiKE-IMS remains almost unchanged as compared to operation at low temperature.When also increasing T IMS from 35 °C to 60°C, the system response can be further improved, probably since the higher T IMS mitigates condensation inside the HiKE-IMS and the second capillary.As expected, the system response is the shortest at maximum operating temperatures of T IMS = 80°C and T Inlet = 200°C.Thus, operating HiKE-IMS at high temperature compared to operation at ambient temperature significantly improves the temporal system response to TATP.Since both the temperature of the sample inlet and the temperature of the HiKE-IMS including the second capillary affect the system response, condensation needs to be considered in the entire sample line.While increasing the temperature of the sample inlet prevents sample condensation in the first sample capillary, thus decreasing the time until detecting a TATP signal, increasing the temperature of the second capillary and the HiKE-IMS helps to purge the TATP out of the instrument after exposure.2 in the main manuscript.

Figure S3 .
Figure S3.Blank dispersion plot recorded with the HiKE-IMS in ambient ionization mode without adding TATP at a) E RR /N = 30 Td and b) E RR /N = 100 Td depending on E DR /N in air.The data is normalized to the maximum value of intensities of all spectra.The color denotes the intensity relative to the maximum value (red).All other operating parameters were set according to Table 2 in the main manuscript.
Figure S4 shows the dispersion plots of blank measurements providing the reactant ion population depending on E DR /N at a) E RR /N = 30 Td and b) E RR /N = 100 Td.In ammonia-doped ionization mode, unlike in ambient ionization mode, only one dominant ion species is present at low E RR /N of 30 Td, being identified as NH 4 + by HiKE-IMS-MS.When increasing E RR /N to 100 Td, three additional ion species with significant abundances can be detected.These ions are identified as the reactant ions NO + , H 3 O + , and O 2 + .

Figure S4 .
Figure S4.Blank dispersion plot recorded with the HiKE-IMS in ammonia-doped ionization mode without adding TATP at a) E RR /N = 30 Td and b) E RR /N = 100 Td depending on E DR /N in air.The data is normalized to the maximum value of intensities of all spectra.The color denotes the intensity relative to the maximum value (red).All other operating parameters were set according to Table 2 in the main manuscript.

Figure S5 .
Figure S5.Comparison between ion mobility spectra in ambient ionization mode of headspace of both acetone and TATP at E RR /N = E DR /N = 100 Td in air obtained with standalone HiKE-IMS.All other operating parameters were set according to Table2in the main manuscript.

Figure
FigureS6shows a comparison of ion mobility spectra of TATP in ammonia-doped ionization mode at E RR /N = 30 Td and E DR /N = 50 Td with E DR /N = 100 Td.The data show that the adduct TATP•NH 4 + and the possible protonated monomer are the most abundant ion species at E DR /N = 50 Td and form sharp peaks in the ion mobility spectrum.At higher reduced ion mobility, i.e. lower inverse reduced ion mobility, the fragments formed in the drift region form a broadened signal, since the timescale of fragmentation inside the drift region is in the same order of magnitude as the drift times under these conditions.In comparison, fragmentation appears to proceed faster at higher reduced drift field strength of E DR /N = 100 Td and the fragments form distinct peaks in the ion mobility spectra, leaving only a slight increase in the baseline.A discussion about the identity of the fragments in ammoniadoped ionization mode can be found in the main manuscript.

Figure S6 .
Figure S6.Comparison between positive ion mobility spectra of TATP in ammonia-doped ionization mode at E RR /N = 30 Td and a) E DR /N = 50 Td with b) E DR /N = 100 Td in air.All other operating parameters were set according to Table 2 in the main manuscript.

Figure S7 .
Figure S7.System response to TATP at different operating temperatures at E DR /N = 100 Td and E RR /N = 100 Td. a) T IMS = T Inlet = 35°C, b) T IMS = 35°C, T Inlet = 100°C, c) T IMS = 60°C, T Inlet = 100°C, d) T IMS = 80°C, T Inlet = 200°C.The plots show the intensity of the signal corresponding to C 3 H 6 O 3 H + (m/z 91).All other operating parameters are given in Table2in the main manuscript.

Table S2 .
Recorded reduced ion mobilities K 0 of the reactant ions NH 4 + (H 2 O) n , NO + (H 2 O) n , H 3 O + (H 2 O) n and O 2 + in ambient ionization mode in air depending on reduced drift field strength E DR /N at E RR /N = 30 Td and E RR /N = 100 Td.The reduced ion mobilities were determined from blank measurements.All other operating parameters were set according to Table 2 in the main manuscript.

Table S3 .
Recorded reduced ion mobilities K 0 of the ions C 2 H 3 O + (m/z 43), C 3 H 6 OH + (m/z 59), C 3 H 6 O 2 H + (m/z 75) and C 3 H 6 O 3 H + (m/z 91), as well as the possible TATP monomer and the adduct TATP•NH 4 + in ammonia-doped ionization mode in air depending on reduced drift field strength E DR /N at E RR /N = 30 Td and E RR /N = 100 Td.The reduced ion mobilities of TATP-related product ions were determined from measurements where TATP was supplied using the gas mixing system in FigureS2 a).All other operating parameters were set according to Table2in the main manuscript.

Table S4 .
Recorded reduced ion mobility K 0 of the ions NH 4 + (H 2 O) n in ammonia-doped ionization mode in air depending on reduced drift field strength E DR /N at E RR /N = 30 Td and E RR /N = 100 Td.The reduced ion mobilities were determined from blank measurements.All other operating parameters were set according to Table2in the main manuscript.