Details regarding
TIMS operation and differences from traditional IMS can be found elsewhere.
25 (link) TIMS mobility separation utilizes an electric
field to hold ions stationary against a moving gas, so that the drift
force is compensated by the electric field. This concept follows the
idea of a parallel flow ion mobility analyzer,
27 with the main difference being that ions are also confined
radially using a quadrupolar field to guarantee higher ion transmission
and sensitivity. The separation in a TIMS device can be described
by the center of the mass frame using the same principles as in a
conventional IMS drift tube.
28 ,29 Because mobility separation
is related to the number of ion-neutral collisions (or drift time
in traditional drift tube cells), the mobility separation in a TIMS
device depends on the bath gas drift velocity, ion confinement, and
ion elution parameters. The mobility,
K, of an ion
in a TIMS cell is described by: where
vg,
E,
Velution, and
Vbase are the velocity
of the gas, applied electric field,
elution voltage, and base voltage, respectively. The constant
A, which accounts for the velocity of gas, can be determined
using calibration standards of known mobilities. In TIMS operation,
multiple geometric isomers/conformers are trapped simultaneously at
different
E values resulting from a voltage gradient
applied across the IMS cell. After thermalization, geometrical isomers/conformers
are eluted by decreasing the electric field in stepwise decrements
(referred to as the “ramp”). Each isomer/conformer eluting
from the TIMS cell can be described by a characteristic voltage gradient
(i.e., V
elution – V
base). Eluted ions
are then mass analyzed and detected by a maXis impact Q-ToF mass spectrometer
(Bruker, Billerica, MA). The elution voltage,
Velution, can be calculated from the elution time: and where
V0 is the
initial potential at the entrance to the TIMS analyzer,
r is the rate at which the potential is ramped,
Telute is the time at which the ion elutes,
Tramp is the total ramp time,
Ttotal is the total time for a single TIMS experiment,
Ttrap is the time before the mobility analysis (i.e., to
inject ions into the TIMS trap), and TOF is the time between elution
of the ion and detection of the ion at the TOF detector.
The
TIMS funnel was controlled using in-house software, written in National
Instruments LabVIEW, and synchronized with the maXis Impact Q-ToF
acquisition program (more details in ref (25 (
link))). Separation was performed using nitrogen as
a bath gas at ≈300 K, and the gas flow velocity was controlled
by the pressure difference between the front (
P1) and back (
P2) of the TIMS analyzer.
P1 and
P2 values
were set to 2.6 and 1.0 mbar for all experiments. The same RF (880
kHz and 200–350 Vpp) was applied to all electrodes including
the entrance funnel, the mobility separating section, and the exit
funnel. An atmospheric pressure photoionization source (APPI, Apolo
II Bruker Daltonics, Inc., MA) using a Kr lamp with main emission
bands at 10.0 and 10.6 eV was used for all the analyses.
A Tuning
Mix mass spectrometry standard (Tunemix, G2421A, Agilent
Technologies, Santa Clara, CA) was used as a mobility calibration
standard. Details on the Tunemix structures (e.g.,
m/
z = 322
K0 = 1.376
cm
2 V
–1 s
–1,
m/
z = 622
K0 = 1.013 cm
2 V
–1 s
–1, and
m/
z = 922
K0 = 0.835 cm
2 V
–1 s
–1) can be found in ref (30 ). Carotenoid samples (lutein
and zeaxanthin) were reconstituted in a 70:30 acetonitrile/methanol
solution to a final concentration of 1–10 nM. Toluene (8.8
eV IP), acetone (9.7 eV IP), or anisole (8.2 eV IP) were used as APPI
additives at a concentration of 10% (v/v) to enhance ionization
31 (link) and to study the effect of solvent conditions
on ionization patterns and relative proportions of geometrical isomers/conformers
formed during the photoionization process. For simplicity, lutein
samples were only analyzed with added anisole APPI. Mobility values
(
K) were correlated with CCS (Ω) using the
equation: where
z is the charge of
the ion,
kB is the Boltzmann constant,
N* is the number density, and
mI and
mb refer to the masses of the ion and bath gas,
respectively.
28 Instrumental parameters
were optimized to achieve the highest IMS
resolution for the carotenoid molecular ions. A peak width (i.e.,
fwhm of the mobility peak) that corresponds to a single isomer was
measured using the sphere-like tune mix mobility standard series.
In addition, to ensure that the IMS peak width was not influenced
by the number of ions in the TIMS cell (i.e., columbic effects compromising
ion trapping), a dilution series (1:10–1:10
3) of
the tune mix was used to determine the mobility peak width as a function
of the concentration in all APPI solvent conditions. No significant
variation in the IMS peak width was observed beyond a 1:100 fold dilution.
The 622
m/
z (
K0 = 1.013 cm
2 V
–1 s
–1, CCS = 202 Å
2) component yielded a 1.74 Å
2 peak width and was used as a reference for the peaks observed
for the carotenoid isomers. Under these experimental conditions, a
mobility resolution of over 110 was obtained in the TIMS analyzer
(more details in the Supporting Information).
Schenk E.R., Mendez V., Landrum J.T., Ridgeway M.E., Park M.A, & Fernandez-Lima F. (2014). Direct Observation of Differences of Carotenoid Polyene Chain cis/trans Isomers Resulting from Structural Topology. Analytical Chemistry, 86(4), 2019-2024.