Investigating the Applicability of Direct Analysis and
Ion Mobility TOF MS for Environmental Analysis
Michael McCullagh and Ramesh Rao
Waters Corporation, Manchester, UK
A P P L I C AT I O N B E N E F I T S
■■
Combining direct analysis with the
orthogonal separation produced by ion
mobility extends the applicability of direct
analysis techniques.
■■
Separation from matrix interference is
achieved without chromatography.
■■
Greater confidence in correct elemental
composition assignment and structural
elucidation data are obtained with routine
mass measurement errors of <2 ppm.
■■
■■
Isobaric species are separated with ion
mobility, and the corresponding individual
MS and MS/MS spectra can be generated
after mobility separation is achieved.
CO2 is used as a drift gas to enhance the
ion mobility resolution.
INT RODUC T ION
Interest in ion mobility separation continues to increase since the advent of the first
commercially available ion mobility mass spectrometer (SYNAPT HDMS, Waters
Corp), that used a travelling wave approach to generate ion mobility separations
(IMS). Technology advances have since brought about an improved detection
system, increased ion mobility resolution, and improved sensitivity (StepWave) from
which the Waters ® SYNAPT G2-S is comprised. These continued improvements give
greater access to the analytical chemist to overcome their analytical challenges
using ion mobility spectrometry in combination with mass spectrometry.
This application note investigates the use of Direct Analysis (DA) combined with
IMS/MS, in this case the samples of interest are directly infused and the benefits
of IMS are explored. There are a plethora of direct analysis techniques that ion
mobility can be combined with such as ASAP, DART and DESI, where using ion
mobility, separation can be produced.
DA/IMS/MS offers a route to rapid screening of known pesticides, whereby
minimal sample preparation is required. The mobility separation is combined with
the specificity of accurate mass measurement <1 ppm (high resolution mode), as
well as the ability to obtain MS/MS structural elucidation data on the mobility
resolved analytes. Here, we illustrate how mobility resolution is used to separate
four pesticides with the same nominal mass; hence, how molecule shape can be
used to produce mobility separation. In addition, the separation power of mobility
is enhanced using CO2 as the mobility drift gas.1
WAT E R S S O LU T I O N S
SYNAPT® G2-S Mass Spectrometer
High Definition Mass Spectrometry™ (HDMS™)
StepWave™ Ion Transfer Device
DriftScope™ Mobility Environment Software
KEY WORDS
Ion mobility, direct analysis, isobaric,
accurate mass
Figure 1. Schematic of SYNAPT G2-S.
1
E X P E R IM E N TA L
MS Conditions
MS system:
SYNAPT G2-S
Ionization mode: ESI+
Capillary and cone voltage: Varied to give
optimum signal
Desolvation temp.: 200 °C
Reference mass: Leucine enkephalin,
[M+H] + = 556.2771
Acquisition range: 50 to 1200 m/z
Acquisition rate: 1 spectra/s
Figure 2. Structures of
pesticides analyzed with
direct analysis and IMS.
Collision energy ramp: 15 to 25 eV
Resolution: 18,000 FWHM
(Res Mode)
Wave velocity: 550 m/s
Pulse height: 40 V
Direct Analysis: Infusion 5 μL/min
Analytes: Hexaconazole,
izasophos,
dichlorfenthion,
and triazophos
R E S U LT S A N D D I S C U S S I O N
The structures of pesticides of interest were analyzed using direct analysis
combined with IMS/MS and IMS/MS/MS, as shown in Figure 2. They have the
same nominal mass and charge state, but different elemental composition
and, hence, shape. The pesticides were directly analyzed using infusion. The
mobility chromatograms illustrating the drift times for hexaconazole, izasophos,
dichlorfenthion, and triazophos using N2 as the drift gas are shown in Figure 3.
This data demonstrates that even without chromatography these components can
be partially resolved using peak capacity provided by ion mobility, with arrival
times varying between 1.84 and 2.11 ms.
314.9778
Dichlorfenthion
314.0495
Isazophos
314.0728
Triazophos
314.0827
Hexaconazole
1
2
3
4
Dt = 1.84 ms
Dt = 2.00 ms
δ = 0.16 ms
Dt = 2.11 ms
δ = 0.11 ms
Dt = 2.11 ms
δ = 0.0 ms
Figure 3. Mobility chromatograms illustrating the arrival times for pesticides shown using N2 .
Investigating the Applicability of Direct Analysis and Ion Mobility TOF MS for Environmental Analysis
2
W hen performing IMS, the ion separation occurring in the travelling wave ion mobility (T WIM) drift cell is
determined by the charge state, mass, shape, drift gas polarizability, as well as interaction between ion and
neutral gas molecules. Increasing the polarizability of the drift gas resulted in an increase in the separation power
of T WIM, in this application. T his is illustrated in Figure 4, where mobility chromatograms show the drift times for
pesticides analyzed using CO2 as the drift gas of choice. T he arrival times varied between 3.24 and 4.05 ms. T he
comparison between physical properties of the ion mobility drift gases utilized is shown in Table1. A comparison
of the peak resolution obtained using (Rs=1.18(ta-tb)/W0.5,a+W0.5,b) is shown in Table 2. T he resolution
between peaks 1 and 2 showed a slight decrease in resolution, but in all other cases peak resolution increased
using CO2. T he pesticides were infused as a mixture and the peak detected components, shown in Figure 5, can be
seen in DriftScope showing arrival time versus m/z for the pesticides with CO2 as the drift gas. For hexaconazole
and triazophos, sodiated adducts were observed and mobility separated. It can be difficult to avoid adduct
formation, particularly in the presence of matrix. However, it is possible to provide further identification
confidence by using the ion mobility, i.e. a second identification point from the drift time of the adduct formed.
314.9778
Dichlorfenthion
Dt = 3.24 ms
1
314.0495
Isazophos
Dt = 3.40 ms
2
314.0728
Triazophos
δ = 0.16 ms
Dt = 3.67 ms
3
314.0827
Hexaconazole
δ = 0.27 ms
Dt = 4.05 ms
δ = 0.38 ms
4
Figure 4. Mobility
chromatograms illustrating
the arrival times for
pesticides shown using CO2
as the drift gas.
S ynaptG 2_201101206_061b.r aw : 1
DriftScope
Mobility separated
Peak detected pesticides
N a + Adducts
.
.
H + Adducts
Figure 5. Peak detected
pesticides in DriftScope
showing arrival time versus
m/z for the pesticides of
interest. CO2 is the drift gas.
Investigating the Applicability of Direct Analysis and Ion Mobility TOF MS for Environmental Analysis
3
SynaptG2_201101206_061b_dt_01 78 (4.158) AM2 (Ar,18000.0,556.28,0.00,LS 3)
%
100
0
314.0827
Hexaconazole
[M+H]+=314.0827
315.0864
318.0775 319.0802320.9832
%
0
314.0733
Triazophos
[M+H]+=314.0728
Error=1.6ppm
315.0753 316.0715
305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330
%
0
314.0501
Isazophos
[M+H]+=314.0495
Error=1.9ppm
316.9748
318.9720
305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330
%
0
Dichlorfenthion
[M+H]+=314.9778
m/z
1: TOF MS ES+
1.76e5
316.0470
314.9784
SynaptG2_201101206_061b_dt_01 63 (3.348) AM2 (Ar,18000.0,556.28,0.00,LS 3); Cm (62:64)
100
m/z
1: TOF MS ES+
1.02e5
SynaptG2_201101206_061b_dt_01 66 (3.510) AM2 (Ar,18000.0,556.28,0.00,LS 3); Cm (65:67)
100
322.9789
305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330
SynaptG2_201101206_061b_dt_01 71 (3.780) AM2 (Ar,18000.0,556.28,0.00,LS 3)
100
1: TOF MS ES+
1.64e5
Error=0.0ppm
316.0802
314.9783
316.9752
314.0488
315.9816
m/z
1: TOF MS ES+
1.47e5
Error=1.3ppm
318.9721 319.9741
305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330
m/z
Figure 6. Single component
accurate mass spectra
generated from the mobility
chromatograms extracted
using DriftScope, as shown
in Figure 5.
The peak capacity generated using ion mobility enables the accurate mass spectra of each single component
to be obtained; therefore, completely resolved from any background ions, as shown in Figure 6. The single
component accurate mass spectra were generated from the mobility chromatograms extracted using DriftScope.
Accurate mass measurements with <2 ppm error were achieved, and elemental compositions proposed based
on the potential to be comprised of the C,H,N,O,P,S and Cl. The exact masses of the [M+H] + species are shown in
conjunction with the accurate mass spectra generated and mass measurement error obtained. In all cases, each
target pesticide was identified correctly as the most probable analyte, based on accurate mass and isotopic
pattern within the elemental composition calculator.
The schematic of the SYNAPT G2-S, shown in Figure 1, reveals two regions labelled “Trap” and “Transfer”
before and after the ion mobility T-Wave. Collision energies can be applied in either of these regions. After
mobility separation was achieved, a collision energy ramp was applied in the Transfer region. This enabled the
single component MS/MS spectra to be obtained for all pesticides when infused as a mixture. The individual
characteristic MS/MS spectra obtained are shown for dichlorfenthion, triazophos, hexaconazole, and isazophos
in Figures 7 through 10, respectively. Using quadrupole technology selectivity, typical mass resolution would
result in a 1 Da span across the mass selected when performing MS/MS. Without mobility separation, it would
not be possible to obtain the single component MS/MS spectra as shown for the pesticides analyzed using
direct analysis. Conventional infusion of all four pesticides would result in an MS/MS product ion spectrum
comprised of all four analytes. In this case, the resolving power of the quadrupole has been used to select
m/z 314 where only this mass undergoes mobility separation, resulting in the mobility separation of the four
pesticides shown, followed by fragmentation of each individual species. Excellent mass accuracy was obtained,
with mass measurement errors of <3 ppm. The fragments are generated from the mobility resolved species and
will, therefore, have the same drift time. In Figures 3 and 4, a unique mass fragment was selected to illustrate
the drift times of each of the four pesticides. This approach further confirmed the identification of the pesticides
determined to be unresolved when N2 was used as a drift gas utilizing IMS/MS.
Investigating the Applicability of Direct Analysis and Ion Mobility TOF MS for Environmental Analysis
4
SynaptG2_201101206_060a_dt_01 60 (3.186) AM2 (Ar,18000.0,556.28,0.00,LS 2)
1: TOF MSMS 0.00ES+
8.03e4
258.9158
100
%
260.9127
[M+H]+ =314.9778
Error= 0.6ppm
Dichlorfenthion
96.9519
162.9716
!
178.9490
142.9904
0
60
80
100
120
140
160
180
262.9092 286.9466
288.9435
224.9290 240.9051
222.9385
200
220
240
290.9404 314.9780
260
280
300
320
340
SynaptG2_201101206_060a_dt_01 70 (3.726) AM2 (Ar,18000.0,556.28,0.00,LS 3)
360
m/z
Figure 7. Single component
accurate mass MS/MS spectrum
of dichlorfenthion generated
after IMS of the infusion of a
pesticides mixture, as shown
in Figure 2.
1: TOF MSMS 0.00ES+
9.27e4
162.0669
100
%
[M+H]+ =314.0728
Error=1.3ppm
Triazophos
314.0732
163.0703
315.0765
286.0425
96.9517
0
50
60
70
80
90
178.0444 190.0984
119.0608
316.0695
258.0109
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330
SynaptG2_201101206_060a_dt_01 76 (4.050) AM2 (Ar,18000.0,556.28,0.00,LS 3); Cm (75:79)
m/z
Figure 8. Single component
accurate mass MS/MS spectrum
of triazophos generated
after IMS of the infusion of a
pesticides mixture, as shown
in Figure 2.
1: TOF MSMS 0.00ES+
2.63e5
314.0829
100
M+H]+ = 314.0827
Error=0.6ppm
Hexaconazole
70.0414
%
316.0803
318.0775
158.9772
71.0433
0
60
80
120
140
160
180
200
319.0800
245.0503
188.9876
100
220
240
260
280
300
320
340
360
380
400
m/z
Figure 9. Single component
accurate mass MS/MS spectrum
of hexaconazole generated
after IMS of the infusion of a
pesticides mixture, as shown in
Figure 2.
Investigating the Applicability of Direct Analysis and Ion Mobility TOF MS for Environmental Analysis
5
SynaptG2_201101206_060a_dt_01 64 (3.402) AM2 (Ar,18000.0,556.28,0.00,LS
- 3); Cm (6460)
162.0439
100
1: TOF MSMS 0.00ES+
2.27e4
M+H]+ = 314.0495
Error=3.2ppm
Isazophos
314.0505
%
119.9965
164.0410
316.0469
272.0034
121.9935
96.9514
243.9717
153.0136
286.0184
124.9825
245.9677
148.0272
0
50
60
70
80
90
165.0427
288.0161
317.0496
215.9386
190.0755
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330
m/z
Figure 10. Single component
accurate mass MS/MS spectrum
of isazophos generated
after IMS of the infusion of a
pesticides mixture, as shown
in Figure 2.
Ion Mobility Spectrometry provides an extra dimension of fast, gas phase, ion separation, providing higher ion
definition and analytical specificity. Using DA/IMS/MS/MS, four pesticides were profiled using a SYNAPT G2-S.
T he results demonstrate that HDMS can provide a route to specific and unambiguous identification, even where
no chromatographic separation exists. Future studies will utilize UPLC/IMS/MS for screening pesticides in complex
matrices to further illustrate the benefits of the enhanced peak capacity enabled using IMS.
Mobility drift gas
Mass
Polarizability
(10e-24 cm3)
Nitrogen N2
28.0123
1.7403
Carbon dioxide
44.0098
2.9110
Table 1. Physical properties comparison where ion mobility drift
gases were utilized.
Drift gas
Peak resolution
1+2
2+3
3+4
1+3
1+4
2+4
CO2
0.47
0.82
0.95
1.24
1.99
1.10
N2
0.61
0.37
0
0.89
0.99
0.42
Table 2. A comparison of peak resolution obtained for the direct analysis of four isobaric
pesticides using N2 and CO2 drift gases.
Investigating the Applicability of Direct Analysis and Ion Mobility TOF MS for Environmental Analysis
6
References
C O N C LU S I O N S
■■
Direct analysis infusion mobility separation of pesticides with
the same nominal mass were utilized to illustrate the separation
that can be achieved with ion mobility.
1. Eberlin MN, Lali PM, Nachtigall FM, Riccio MF, de Sa GF, Daroda RJ,
de Souza V, Campuzano I, Souza G HMF. 4 Improving Ion Mobility Separation
By Increasing Drift-Gas Polarizability. Waters Technical Note
No. 720003201EN, 2009.
■■
Comparative application data where N2 and CO2 drift gases were
used to perform T-wave IMS were generated, where CO 2 was
shown to improve the mobility separation for this application.
2. Asbury GR, Hill HH. Using Different Drift Gases To Change Separation Factors ®
in Ion Mobility Spectrometry. Anal. Chem. 2000; 72: 580.
■■
Single-component MS spectra have been produced from the
mobility separation generated for the pesticides studied.
■■
Mass measurement errors of <2 ppm have been illustrated.
■■
Characteristic single-component MS/MS spectra were produced
from the mobility separation of the pesticides analyzed.
■■
The study undertaken warrants further investigation of the
enhanced peak capacity that UPLC/IMS/MS can enable in
pesticide screening applications.
■■
The application illustrates how ion mobility can be used to
increase peak capacity with any direct analysis technique.
Waters and SYNAPT are registered trademarks of Waters
Corporation. StepWave, DriftScope, T-Wave, High Definition
Mass Spectrometry, HDMS, and T he Science of W hat’s Possible
are trademarks of Waters Corporation. All other trademarks are
the property of their respective owners.
©2012 Waters Corporation. Produced in the U.S.A.
October 2012 720004465EN AG-PDF
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