Supplemental Information
Calibration Gas Spectra Discussion. Normalized mass spectra of perfluorotributylamine
(PFTBA) are shown in SI 1. Electron ionization (EI) spectra for both the carbon nanotube
(CNT) field emission and filament thermionic emission sources (SI 1a and SI 1d, respectively)
closely match each other as well as NIST reference EI spectra. PFTBA spectra were also
collected in both negative chemical ionization (NCI) (SI 1b and SI 1e) and positive chemical
ionization (PCI) (SI 1c and SI 1f) modes using both the thermionic emission source as well as
the CNT field emission source.
The PFTBA fragment ion peaks in NCI spectra taken with the CNT field emission source
(SI 1b) reveals nearly identical peaks to that of the filament thermionic emission source (SI 1e).
Strong peaks are observed at m/z 452 and 633 which is consistent with the reference spectra of
PFTBA [1] and indicate chemical ionization conditions occurring with the CNT field emission
source. The PCI mode revealed some differences, including the absence of the m/z 264 peak (a
common PFTBA fragmentation peak) in the CNT field emission source that appears in the
filament thermionic emission source spectra. Additionally, it should be noted that the spectra
taken with the filament thermionic source in PCI mode stopped at m/z 600, so the m/z 654 peak
that appears on the CNT field emission source spectra is not visible in the filament source
spectra. Also, arguably the PFTBA is a much larger molecule (molecular weight 671 g/mol) with
far more fragmentation pathways than the smaller nitro compounds (eg. TNT, molecular weight
227 g/mol). Thus, small differences in electron energies during the ionization step might have a
much greater influence on the PFTBA spectra and could have contributed to the small
differences in the spectra collected.
Because the ionization energy of N2 is greater than that of many typical organic analytes,
Hunt and Crow [2] state that “it is not surprising that nitrogen CI and conventional EI spectra are
essentially identical.” Oxygen, also present in significant abundance in our ion source, similarly
has an ionization energy greater than most organics and should behave like nitrogen in Reactions
3 and 4. We would not assert that our PCI PFTBA data taken with the CNT source is identical to
the EI data, but it is similar. Therefore, the existence of additional peaks present in the PCI data
taken with the CNT source spectra (SI 1c), such as m/z 40 and m/z 69, could be explained by
some unavoidable EI taking place. Additionally, the higher pressure in the source region under
CI conditions seems to favor the production of higher m/z ions, likely because of increased
collisional cooling of the high energy electrons injected into the region. While the CNT and
filament PCI spectra in SI 1c and SI 1f, respectively, are not identical, they are much more
similar than spectra collected under EI conditions. Moreover, the EI data is far more similar to
the NIST reference EI spectra than either PCI spectrum. The PCI PFTBA spectrum of both
sources (CNT and filament) show an increase in higher m/z ion intensity relative to the lower
m/z ion intensity when collected in PCI mode versus EI mode, consistent with previous
observations [3].
Calibration curves were collected and analyzed for the trace explosives. The calibration
curves were generally linear, as expected. The data for the TNT calibration curve can be seen in
S2.
SI 1: PFTBA spectra collected in the explosive trace detection mass spectrometer with the CNT field
emission source (left column) and the filament thermionic emission source (right column). Three modes
are illustrated, including an initial electron ionization (EI) mode to test for proper source operation,
followed by both negative and positive chemical ionization (NCI and PCI) modes. (a) EI spectra with
CNT source (b) NCI spectra with CNT source (c) PCI spectra with CNT source (d) EI spectra with
filament source (e) NCI spectra with filament source (f) PCI spectra with filament source. Analysis shows
spectra are nearly identical for both EI and NCI modes, whereas PCI mode is visually different, with
several indications that chemical ionization is occurring. For example, signal peaks at m/z 502 exceed that
of both m/z 131 and m/z 69
SI 2: A calibration curve of desorbed TNT is shown and is linear, as expected. Trace explosive amounts
were consistent with trace standards in the security field
Supplemental Information References
1.
2.
3.
Scientific Instrument Services, S., FC-43 (Perfluorotributylamine) Calibration
Compound for Mass Spectrometers. p. Catalog A65.
Hunt, D.F., G.C. Stafford, F.W. Crow, and J.W. Russell, Pulsed positive negative ion
chemical ionization mass spectrometry. Analytical Chemistry, 1976. 48(14): p. 20982104.
Hübschmann, H.-J., Handbook of GC/MS: Fundamentals and Applications. 2008, WileyVCH Verlag GmbH & Co. KGaA. p. 7-292.