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Comparison of CCS Values Determined by Traveling Wave Ion Mobility Mass Spectrometry and Drift Tube Ion Mobility Mass Spectrometry Vanessa Hinnenkamp, Julia Klein, Sven W. Meckelmann, Peter Balsaa, Torsten Claus Schmidt, and Oliver Johannes Schmitz Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02711 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

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Analytical Chemistry

1

Comparison of CCS Values Determined by Traveling Wave Ion Mobility Mass

2

Spectrometry and Drift Tube Ion Mobility Mass Spectrometry

3

Vanessa Hinnenkamp1,2#, Julia Klein3,4#, Sven W. Meckelmann3,4, Peter Balsaa1, Torsten C.

4

Schmidt1,2, Oliver J. Schmitz3,4*

5

1

IWW Water Centre, Moritzstraße 26, 45476 Muelheim an der Ruhr, Germany

6

2

Instrumental Analytical Chemistry and Centre for Water and Environmental Research,

7

Universitaetsstrasse 5, 45141 Essen, Germany

8

3

9

Essen, Germany

Applied Analytical Chemistry, University of Duisburg-Essen, Universitaetsstrasse 5, 45141

10

4

11

Universitaetsstrasse 5, 45141 Essen, Germany

Teaching and Research Center for Separation, University of Duisburg-Essen,

12 13

#

These authors contributed equally in this work.

14 15

*Corresponding Author: Oliver J. Schmitz, Universitaetsstr. 5, 45141 Essen, Germany

16 17

Keywords: CCS, IM-MS, Ion mobility mass spectrometry, DTIM-MS, TWIM-MS

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Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

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Collision cross section (CCS, Ω) values determined by ion mobility mass spectrometry (IM-

20

MS) provide the study of ion shape in the gas phase and use of these as further identification

21

criteria in analytical approaches. Databases of CCS values for a variety of molecules

22

determined by different instrument types are available. In this study, the comparability of

23

CCS values determined by a drift tube ion mobility mass spectrometer (DTIM-MS) and a

24

traveling wave ion mobility mass spectrometer (TWIM-MS) was investigated to test if a

25

common database could be used across IM techniques. A total of 124 substances were

26

measured with both systems and CCS values of [M+H]+ and [M+Na]+ adducts were

27

compared. Deviations < 1% were found for most substances but some compounds show

28

deviations up to 6.2%, which indicate that CCS databases cannot be used without care

29

independently from the instrument type. Additionally, it was found that for several molecules

30

[2M+Na]+ ions were formed during electrospray ionization whereas a part of them

31

disintegrates to [M+Na]+ ions after passing through the drift tube and before reaching the

32

TOF region, resulting in two signals in their drift spectrum for the [M+Na]+ adduct. Finally, the

33

impact of different LC-IM-MS settings (solvent composition, solvent flow rate, desolvation

34

temperature and desolvation gas flow rate) were investigated to test whether they have an

35

influence on the CCS values or not. The results showed that these conditions have no

36

significant impact. Only for karbutilate changes in the drift spectrum could be observed with

37

different solvent types and flow rates using the DTIM-MS system, which could be caused by

38

the protonation at different sites in the molecule.


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39

Introduction

40

The coupling of ion mobility spectrometry (IMS) with mass spectrometry (MS) has raised high

41

interest in analytical chemistry over the last decade [1]. This comprehensive analytical set-up

42

separates gas-phase ions in the ion mobility cell under the influence of a weak electric field

43

and by collisions with a neutral drift gas (typically N2 or He) within a millisecond timescale.

44

Especially the combination with time of flight mass spectrometry (TOF-MS) allows the

45

detection of compounds separated by ion mobility and describing each peak with enough

46

data points because of the high scan rate where mass to charge ratios (m/z) of ions are

47

recorded in a microsecond scale. One advantage of ion mobility-mass spectrometers is the

48

determination of collision cross section (CCS, Ω) values which are based on the shape of

49

gas-phase ions. CCS is defined as the average area around an ion at three dimensional

50

rotation in which the center of another particle must be in order for a collision to occur [2].

51

CCS values depend on the shape, size and charge of the analyzed ion, as well as on the

52

used drift gas and experimental settings. In complex sample analysis with LC-IM-MS [3,4] or

53

GC-IM-MS [5] techniques, CCS values are useful as further identification criterion in addition

54

to retention time, m/z, isotopic pattern and mass fragments. However, currently this is limited

55

by the availability of only small CCS databases for different applications like proteomics,

56

lipidomics, metabolomics, analysis of pesticides or other small molecules [6-13]. In MS2

57

experiments a further advantage of the IM-MS coupling is the background filtering of

58

interfering signals in data-independent acquisition mode to obtain all ion fragmentation. With

59

this type of acquisition mode interfering ions often generate a MS2 spectrum which makes

60

the interpretation of these enormously difficult. Through the additional IM cell in front of the

61

mass analyzer, ions (also isobaric ions) can be distinguished and MS2 spectra are cleaned

62

up by drift time filtering. This increases peak capacity and selectivity [12]. In addition to

63

differential mobility mass spectrometry (DMS) and high-field asymmetric ion mobility

64

spectrometry (FAIMS) [2], which are helpful in target-analysis, also IMS systems with high

65

potential in non-target analysis are available. The first coupled traveling wave ion mobility

66

spectrometer with a Q-TOF-MS (TWIM-MS) was commercially available as Synapt System

67

from Waters Corporation in 2006 [14]. In 2014, a drift tube ion mobility Q-TOF-MS (DTIM-

68

MS) was released by Agilent technologies [15]. Recently, a trapped ion mobility Q-TOF-MS

69

system (TIMS-TOF) was made commercially available by Bruker Daltonics [16].

70

In DTIM-MS, a static uniform electric field (typically 5-100 Vcm-1) is applied in a tube filled

71

with drift gas at reduced pressure (3.95 Torr). Ions move through the tube in direction of the

72

electric field and are decelerated by collisions with the drift gas. Ions with a compact

73

structure travel faster through the tube than ions with more extended structures, because

74

they interact less with the drift gas. The individual ions reach a characteristic constant drift 3 ACS Paragon Plus Environment


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75

velocity (vd) which is determined by vd = L/td (L= length of the tube, td = drift time) [17]. Is the

76

ratio of the electric field strength (E) to the buffer gas number density (N) smaller than 2x10-17

77

Vcm2 (low-field limit), vd is proportional to the mobility constant K (eq 1) [18-20].

78

v = KE

79

In consideration of standard temperature (273 K) and pressure (760 Torr), the reduced

80

mobility (K0) is determined by eq 2.

81

K =

82

The proportionality of the drift time of an ion to its CCS is given by the Mason-Schamp

83

equation (eq 3) [18].

84

Ω =

85

Herein, z is the charge state, e the elementary charge, kb the Boltzmann constant, T the

86

temperature in the drift tube, mI the mass of the analyte ion and mB the mass of a drift gas

87

molecule. The measured drift time equals the time it takes for the ions to pass from the

88

entrance of the drift tube to the detector. This is composed of the true drift time td, which the

89

ion needs to pass through the drift tube, and the time tfix, in which the ion passes through the

90

further optics of the instrument to the detector. Therefore, a correction must be carried out,

91

which can be performed using the stepped field method or single field method [21]. For the

92

stepped field method, the analyte is injected several times. For each injection the voltage at

93

the entrance of the drift tube (U1) is varied, resulting in different electric field strengths (eq 4).

94

E =

95

Herein, ∆U is the difference of the voltages at the entrance (U1) and at the exit (U2) of the

96

drift tube. Initially, the total drift time is determined for the analyte ion. From measurements at

97

several (at least two) different electric field strengths, tfix can be determined by plotting the

98

total drift time against 1/∆U. For each measurement, the true drift time td is calculated as the

99

difference between the total drift time and tfix.

(1)

.

()/

) *

=

(2)

(

* + *

/

/ "# + # '

! )

$

&

(

(3)

(4)

100

With the single field method, the calculation of the CCS values can be determined directly

101

from the drift time, measured at only one single field strength using a CCS standard mix

102

which contains substances of known CCS values. Plotting these CCS values against the

103

obtained drift times results in a straight line with the following equation (eq 5), which can be

104

derived directly from the Mason-Schamp equation according to Kurulugama et al. [22]

105

t - = β y Ω + t 123

(5) 4 ACS Paragon Plus Environment

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β is an instrument dependent proportionality coefficient (related to gas pressure, electric field,

107

and geometry of the drift cell and other ion optics), y the modified reduced mass related

108

coefficient can be determined by eq. 6, and tfix the mobility independent flight time. β and tfix

109

can be calculated by the regression from the slope and intercept.

110

8 4 = 5 67 9 7

111

Since the linear equation is directly related to the Mason-Schamp equation, the single field

112

method is substance independent.

113

The TWIM-MS instrumentation consists of a series of stacked ring ion guides (SRIG), filled

114

with an inert drift gas. A radio frequency (RF) voltage of opposite phases to consecutive

115

electrodes is applied to confine the ions radially. Furthermore, a transient direct current (DC)

116

voltage to each stacked ring electrode in turn is applied, so that the traveling wave arises.

117

Ions with higher mobility are carried by the wave and ions with lower mobility roll over the

118

wave [23]. TWIM-MS works like DTIM-MS below the low-field limit, but unlike DTIM-MS, the

119

direct relationship of K0 to the CCS does not exist due to the constantly varying electric field.

120

This means that CCS values cannot be directly obtained from the measured drift time [24].

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However, CCS can be determined by calibration with known, from drift tube derived CCS

122

values. This procedure is described in detail by Ruotolo et al. [25]. Briefly, the literature CCS

123

(ΩLiterature) of calibrants are corrected by their charge (z) and reduced mass (µ) by eq 7.

124

Ω´=

125

The calibrants are introduced into the instrument and the time taken by the ions from the

126

trapping cell to the detector is measured. To determine the time that ions drift only through

127

the IMS cell (t´D), the experimental drift time (tD) must be corrected according to eq 8.

128

t´D = t D -

129

C means the enhanced duty cycle delay coefficient (EDC) which depends on the instrument.

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In the next step a plot of ln t´D versus ln Ω´ is created. This plot is fitted to a linear relationship

131

in form of eq 9.

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In Ω´=B ∙ In t´D + In A

133

From the fit determined values for A and B, the CCS of analytes can be calculated by the

134

corrected drift time through eq 10.

135

ΩAnalyte = R ∙ ST´ ∙

7

8

(6)

:

ΩLiterature ∙ √μ

(7)

z

C Im/z 1000

(8)

U

(9)

5

(10)

√V



Page 6 of 22

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The TWIM-MS derived CCS values are particularly dependent on the used calibrants.

137

Several calibration strategies are described for different applications [6, 25-41]. It is crucial

138

that calibrant ions are structurally similar to the analyte ions [6]. For example, Gelb et al [26]

139

investigated the influence of the calibrant ions on CCS values for carbohydrates and

140

peptides by TWIM-MS. They recommended a matching of molecular class and charge state

141

for a suitable TWIM-MS calibration.

142

In order to use CCS values for identifying unknown substances, databases are required.

143

Currently, only a few databases of experimentally determined CCS values are freely

144

available and it is not yet known if databases can be used independently from the instrument

145

type. For example, Regueiro et al. [12] compared the determined CCS values by TWIM-MS

146

with nitrogen as drift gas (TW CCSN2) for 20 pesticides with DTIM-MS determined CCS values

147

(DTCCSN2) from literature [3, 22] and calculated deviations up to 2.3%. Furthermore, they

148

measured an intra- and interday repeatability for TW CCSN2 with RSDs lower than 1%. Stow et

149

al. [21] have proved the reproducibility of

150

stepped field CCS method, RSDs of 0.29% for all compound ions were received. By using

151

the calibrated single field CCS method, an average bias of 0.54% to a standardized stepped

152

field CCS method on a reference system was calculated. Interlaboratory studies by using

153

TWIM-MS instruments were carried out by Astarita and coworkers. For several metabolites

154

and lipids they calculated RSDs for CCS values < 5% and < 3%, respectively, for most of the

155

considered molecules [7,8].

156

In the present study, the comparability of CCS values determined by DTIM-MS and TWIM-

157

MS in two different laboratories was investigated and discussed. CCS values of 124

158

molecules were determined for protonated as well as sodium adducts using nitrogen as drift

159

gas and compared to test the reciprocal use of CCS databases for a non-target research

160

question in the field of small molecules. Additionally, influences on CCS values of solvent

161

composition, flow rate, desolvation gas flow and desolvation temperature, which can affect

162

ionization of the molecules, were examined in both systems.

DT

CCSN2 across three laboratories. Using the


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Experimental section

164

Chemicals and reagents

165

Analytical standards were purchased from different manufacturers, including LGC Standards

166

(Wesel, Germany), Sigma-Aldirch (Taufkirchen, Germany), Riedel-del-Hean (Seelze,

167

Germany), Syngenta (Basel, Switzerland), Neochem (Menden, Germany) and BASF

168

(Ludwigshafen, Germany). Standard solutions were prepared in a concentration of 500 µg/L

169

in methanol/water (1:1) and were stored in glass vials at -20 °C and 4-8 °C.

170

DTIM-MS System

171

Methanol and acetonitrile, both in LC-MS grade, were purchased from VWR (Leuven,

172

Belgium). Formic acid (98-100%) was obtained from Merck, (Darmstadt, Germany). Ultrapure

173

water was generated with a water purification system from Sartorius Stedim (Goettingen,

174

Germany). For mass calibration the ESI-L low concentration Tuning Mix (G1969-85000) from

175

Agilent Technologies (Santa Clara, USA) was used. The composition is specified in Table

176

S-1.

177 178

TWIM-MS System

179

Methanol, acetonitrile and ultra-pure water were purchased from Biosolve (Valkenswaard,

180

Netherlands) in LC-MS grade. Formic acid (LC-MS grade) was obtained from VWR

181

chemicals (Langenfeld, Germany). Leucin enkephalin (lock mass) and calibration mixture

182

(IMS/TOF calibration kit) were purchased from Waters (Manchester, U.K.). The leucin

183

enkephalin solution (100 pg/µL) was prepared in acetonitrile/water (1:1). The composition of

184

the calibration mixture is listed in Table S-2.

185

Instruments

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DTIM-MS System

187

An Agilent 1290 Infinity 2D-LC system, (Agilent Technologies Inc., Waldbronn, Germany),

188

consisting of an Infinity binary pump (G4220A) with Jet Weaver V35 mixer, a 1290 Infinity

189

HiP autosampler (G4226A) and a 1290 Infinity thermostatted Column compartment

190

(G1316C), was coupled to an Agilent 6560 Ion Mobility Q-TOF Mass Spectrometer System,

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(Agilent Technologies Inc., Santa Clara, USA), equipped with a Dual Jet Stream electrospray

192

ionization (AJS ESI).

193

The injection into the IM-Q-TOF-MS system was carried out by HPLC without any

194

chromatographic separation. The column inlet and outlet were linked by a connecting piece.

195

1 µL of the working solutions were injected. Methanol and ultrapure water (50:50), both 7 ACS Paragon Plus Environment


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containing 0.1% formic acid with a flowrate of 0.2 mL/min were used as mobile phase.

197

Ionization was operated in positive ESI mode. Data acquisition was carried out within a mass

198

range of 50 - 1700 Da. Gas temperature and flow rate were set to 200 °C and 5 L/min,

199

respectively. Nebulizer pressure was adjusted to 20 psig, sheath gas temperature and gas

200

flow were set to 275 °C and 8 L/min, respectively. Nozzle voltage was adjusted to 1 kV and

201

the VCap to 4 kV. For the DTIM-MS conditions, trap fill time was 40,000 µs, trap release time

202

was set to 150 µs. Based on the work of Stow et al. [21] an effective drift tube length of

203

78.236 cm was set in the software and comparable experimental conditions were used for

204

CCS measurements. CCS values were determined using the stepped field method. The

205

analytes were injected eight times. For each injection the voltage at the entrance of the drift

206

tube was varied from 1000 to 1700 V in 100 V steps, the voltage at the drift tube exit was

207

kept constant (250 V). At each field strength, data were acquired for 1 minute, resulting in a

208

total runtime of 8 minutes for each run. The CCS values were calculated as the average of

209

the eight measurements. The voltages at the rear funnel entrance and exit were set to 240 V

210

and 43 V, respectively. Nitrogen was used as drift gas at a pressure of 3.95 Torr. The

211

maximum drift time was set to 60 ms. The manufacturer specifies a drift resolution of about

212

50. The LC system was controlled by Open Lab CDS Chem Station, Rev. C.01.06 (61),

213

Agilent Technologies Inc., Waldbronn (Germany) software. For data acquisition Mass Hunter

214

Workstation Software LC/MS Data Acquisition for 6560 IMQTOF, Version B.08.00, Agilent

215

Technologies Inc., Santa Clara (USA) was utilized. Mass Hunter Workstation IM-MS

216

Browser, Version B.08, Agilent Technologies Inc., Santa Clara (USA) software was used for

217

the determination of CCS values.

218 219

TWIM-MS System

220

An Acquity UPLC I class system (Waters, Milford, USA) consisting of a binary pump, an

221

autosampler and a column manager, was connected to a Vion IM-Q-TOF-MS (Waters,

222

Milford, USA). Working solutions were injected via the LC without any chromatographic

223

separation into the IM-MS system. For this purpose, column inlet and outlet were connected

224

by a connecting piece. The injection volume was 1 µL. Methanol and ultrapure water (50:50)

225

both containing 0.1% formic acid were used as mobile phase with a constant flow rate of

226

0.2 mL/min in a total runtime of 3 minutes. Using the Vion hydride MS system, electrospray

227

ionization (ESI) in positive ionization mode was performed. A cone voltage of 20 V and

228

capillary voltage of 0.8 kV were applied. Nitrogen was used as desolvation gas with a flow

229

rate of 13 L/min and as cone gas with a flow rate of 0.8 L/min. Source and desolvation

230

temperatures were set to 150 °C and 500 °C, respectively. MS data were acquired in a mass

231

range of 50–1000 Da using the high definition (HD)MSE mode for acquisition. During this

232

data independent acquisition mode, scans are acquired with different collision energies. Low 8 ACS Paragon Plus Environment

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233

energy spectra were obtained at 4 eV and high energy spectra were obtained within a ramp

234

of 15-40 eV. The scan time was set to 0.3 s. Leucin enkephalin was utilized as lock mass

235

and continuously infused with a flow rate of 5 µL/min via reference probe. Lock correction

236

interval was set to 0.25 min. TWIMS conditions are described in the following: stopper height:

237

40 eV, trap bias: 40 V, gate height: 40 V; trap wave velocity: 100 m/s; trap pulse height A:

238

20 V; trap pulse height B: 5 V, IMS velocity: 250 m/s; IMS pulse height: 45 V, gate release:

239

2 ms. Nitrogen was used as trap and IMS gas with a flowrate of 1.6 L/min and 25 mL/min,

240

respectively, at a pressure of about 3.3 mbar. A CCS resolution of about 20 is specified by

241

the manufacturer. Working solutions were measured by triple injection and CCS values are

242

reported as mean values. Data acquisition and processing was carried out using UNIFI 1.8

243

Software (Waters).

244



245

Results and discussion

246

Comparison of CCS values measured by TWIM-MS and DTIM-MS

247

Different types of IMS systems have been coupled to high resolution time of flight mass

248

spectrometry (e.g. DTIM-MS, TWIM-MS or TIMS-TOF) and are commercially available.

249

However, it is necessary to validate the comparability of different instrument types, when

250

CCS databases are used independently from the instrument. For this purpose, CCS values

251

were determined and compared for a total of 124 substances, by means of the Waters Vion

252

TWIM-Q-TOF-MS and Agilent 6560 DTIM-Q-TOF-MS System, both using nitrogen as drift

253

gas. To that end, 64 pesticides, 35 pharmaceuticals and 25 metabolites of pesticides were

254

selected. The masses of molecules were between 151 and 747 Da, which also covers a

255

large range of CCS values (132 Å² to 271 Å²). Both instruments used electrospray ionization

256

in the positive mode for ionization of the compounds and CCS values of [M+H]+ and [M+Na]+

257

ions were monitored. For the TWIM-MS system we were able to detect [M+H]+ ions for 110

258

substances and [M+Na]+ ions for 109 substances. Using the DTIM-MS system 111 [M+H]+

259

and 108 [M+Na]+ ions were observed. In total, 104 out of 124 Substances were detected with

260

both instruments as [M+H]+ and 97 as [M+Na]+ ions. Maybe, the reason for the various

261

ionization efficiencies are different ESI source designs and various conditions. All measured

262

CCS values are given as mean values of multiple determinations and are listed in Table S-3.

263

In Figure 1

264

sodium adducts (1b).

DT

TW

CCSN2 are plotted against

CCSN2 values for protonated adducts (1a) and

b) 280

280

260

260

240

240

220

220

200 180

200 180

160

160

140

140

120 120

265

TW CCS N2

N2

a)

TWCCS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 22

140

160

180

200

220

240

260

280

120 120

140

TW

160

180

200

DTCCS

DTCCS N2

220

240

260

280

N2

DT

CCSN2 for [M+H]+ (a) and [M+Na]+ ions (b). The

266

Figure 1. Correlation of

267

dotted line indicates the one to one line and the solid black line means the regression line.

268

The red marked points indicate compounds excluded from the linear regression to avoid 10

CCSN2 and

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269

misleading enhancement of the correlation. Linear equations were y = (1.00 ± 0.01)x - (0.5 ±

270

2.0) for [M+H]+ and y = (0.98 ± 0.02)x + (1.4 ± 2.7) for [M+Na]+ ions.

271

When calculating the slope of the regression line and the corresponding correlation

272

coefficients, the high CCS values of anhydroerythromycin, erythromycin and clarithromycin in

273

comparison to the other substances were not included because of the lack of standards

274

between 220 Å² and 260 Å² resulting in a misleading enhancement of the linear regression.

275

The calculated slopes of the regression lines of 1.00 and 0.98 for CCS values of [M+H]+ and

276

[M+Na]+ ions, respectively, and the corresponding r2 values of 0.9868 for protonated and

277

0.9780 for sodium adducts show good correlations between CCS values determined by

278

DTIM-MS and TWIM-MS. Furthermore, intercepts of the regression lines were close to zero

279

indicating no relevant offset between the two methods. The mean values of absolute percent

280

errors were 1.0% and 1.1% for protonated and sodium adducts, respectively. Figure 1b

281

shows that the CCS values for [M+Na]+ ions determined with TWIM-MS usually resulted in

282

smaller CCS values than with DTIM-MS. One reason could be the calibration of the TWIM-

283

MS device and accordingly the calculation of the CCS value. As described in the

284

introduction, the selection of calibrants for the TWIM-MS calibration has a significant impact

285

on the analyte CCS value. In this study, the TWIM-MS system was regularly calibrated with a

286

mixture of substances of which [M+H]+ and [M-H]- ions are considered (Table S-2). Thus, the

287

TWIM-MS was not calibrated for sodium adducts. In Figure 2, the deviations of the

TW

CCSN2

DT

288

values compared to the

289

the substances showed an absolute percentage error between 0 and 1%, whereas 27% have

290

an error between 1 and 2%. 7% have an absolute percentage error > 2.0%. The highest

291

deviation was revealed for the pesticide lenacil (6.1%). In case of sodium adducts 50% have

292

an absolute percentage error between 0 and 1% and 37% between 1 and 2%. An error

293

greater than 2% was obtained for 13% of substances. The beta-blocker pindolol showed the

294

highest deviation (6.2%). In summary, a deviation of 7%

319

(e.g. 7.6% for diflubenzuron). This is a large deviation also when taking into account the

320

lower accuracy of TWIM-MS of about 2%. Overall, the deviations among literature CCS

321

values are larger than the deviations reported in this work between the experimental CCS

322

values of TWIM-MS and DTIM-MS.

323

Interpretation of drift spectra for sodium adducts

324

During the experiments, it was observed that for some compound ions, two drift peaks of

325

[M+H]+ (Table S-4) and [M+Na]+ (Table S-5) ions could be detected. Especially for sodium

326

adducts, a high difference in their CCS values of about 35% between the first and second

327

drift peak were observed, suggesting a completely different shape of the ion. Two signals for

328

the sodium adducts could be observed for 55 (DTIM-MS) and 42 substances (TWIM-MS).

329

Plotting the m/z against the drift time (Figure 3) showed that the mass of the [2M+Na]+ ion

330

was detected with the same drift time as the second signal for the [M+Na]+ ion for both

331

instruments. Therefore, it can be assumed that [M+Na]+ and [2M+Na]+ ions are formed during

332

ESI, wherein a part of the [2M+Na]+ ions disintegrate to [M+Na]+ ions after passing through

333

the drift tube and before reaching the TOF region, leading to the second signal in the drift

334

spectra for the [M+Na]+ ions. For all substances, for which a second drift peak was detected

335

for [M+Na]+ ion, the drift times and CCS values were compared to those of the respective

336

[2M+Na]+ ion (Table S-6). 37 out of 55 (DTIM-MS) and 27 out of 42 compounds (TWIM-MS)

337

showed a signal for the respective [2M+Na]+ ion. 13 ACS Paragon Plus Environment


Page 14 of 22

338 339

Figure 3. 2D plot of m/z vs. drift time and drift spectrum of the difenoxuron [M+Na]+ ion,

340

measured by DTIM-MS (a) and TWIM-MS (b). Two signals were received for the [M+Na]+ ion

341

with both instrument types. In the 2D plots, signals with a m/z of 595.2520 for the DTIM-MS

342

instrument and a m/z of 595.2522 for the TWIM-MS system were also detected, which

343

correspond to the mass to charge ratio of the [2M+Na]+ ion of difenoxuron (theoretical m/z

344

[2M+Na]+ = 595.2527).

345

Impact of solvent and ESI parameters

346

According to the application and compounds of interests, different ESI settings were

347

generally used on both instruments. Therefore, the influence of certain LC and ESI

348

parameters were investigated. The parameters were varied to determine if changes occur in

349

the drift spectra or have an influence on the resulting CCS values. Benzthiazuron,

350

erythromycin, karbutilate and ritalinic acid were selected as model substances and [M+H]+

351

signals were evaluated. For example, for karbutilate (Figure 4a) two signals for the [M+H]+

352

adduct close to each other, can be observed in the drift spectra when using the DTIM-MS

353

system (Figure 4b), which might indicate protonation at different sites of the molecule. In

354

literature, several studies have investigated protomers of some molecules separated by IMS

355

[47-53]. Protomers are molecular isomers, which differ in the site of the added proton [48, 50,

356

53]. For the TWIM-MS instrument, only one signal was detected (Figure 4c). However,

357

considering the shape of the drift peak, a shoulder could be seen, which suggests that a

358

second, less well resolved signal next to the main peak could be present, which can be

359

explained by the lower resolving power of the TWIM-MS compared to the DTIM-MS device.

360

Impact of desolvation temperature and gas flow rate

361

The DTIM-MS instrument operates at significantly lower desolvation temperatures and gas

362

flows compared to the TWIM-MS instrument. Due to that fact, the TWIM-MS settings were

363

changed towards lower temperatures (200 °C, 300 °C, 400 °C and 500 °C) and gas flows 14 ACS Paragon Plus Environment

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


364

(5 L/min, 7 L/min, 9 L/min and 13 L/min), and for the DTIM-MS they have been changed

365

towards higher temperatures (200 °C, 250 °C, 300 °C and 350 °C) and gas flows (3 L/min,

366

5 L/min, 7 L/min and 9 L/min). The results showed that only slight deviations in CCS values

367

were obtained for both systems (Figure S-1), and these are within the device deviation.

368

Therefore, no dependencies on desolvation temperature and gas flow rate could be found.

369

Impact of solvent composition and flow rate

370

For solvent parameters, type and composition of the organic eluent as well as the flow rate

371

were investigated. Methanol and acetonitrile were used, and each solvent type was tested in

372

compositions of 5%, 50% and 95%. Flow rates were varied to 100 µL/min, 200 µL/min, 500

373

µL/min and 800 µL/min. One interesting change in the drift spectra was found for karbutilate.

374

The drift spectrum shows that remarkable changes in relative intensities of the two separated

375

drift peaks occurred when acetonitrile (50%) was used with the highest flow rate of 800

376

µL/min for the DTIM-MS system (Figure 4d). Measurements with TWIM-MS showed no

377

changes in drift spectra by increasing the flow rate with acetonitrile (Figure 4e). Warnke et al.

378

[51] investigated the protomers of benzocaine by combination of IR vibrational spectroscopy

379

with DTIM-MS. They found that changes in relative intensity of two protonated species were

380

revealed by varying solvent composition. The protic solvent (methanol/water) facilitates the

381

protonation of carbonyl oxygen and the aprotic solvent (acetonitrile) yields to predominantly

382

amine protonation of benzocaine. In case of karbutilate, both, N-protonation and O-

383

protonation could be possible at different sites within the molecule. Similar to the

384

observations by Warnke et al. varying the solvent composition leads to changes in relative

385

intensities of the two drift peaks. The results of this study indicate that also the flow rate

386

(amount of solvent) can have an impact. Because the DTIM-MS system works at lower

387

desolvation temperatures and gas flows compared to the TWIM-MS system, ESI conditions

388

of the TWIM-MS system were set to same desolvation temperature (200 °C) and desolvation

389

gas flow rate (5 L/min) than the Agilent system and setting the high flow rate (800 µL/min) of

390

the acetonitrile solvent (50%). However, no changes in the drift spectrum could be noticed

391

even under these conditions (Figure 4f). Overall, slight deviations in CCS values could be

392

observed during the parameter variation, but these are in the range of the device deviations.



Page 16 of 22

393 394

Figure 4. Molecular structure of karbutilate (a) and drift spectrum of karbutilate analyzed by

395

DTIM-MS (b) and TWIM-MS (c). Drift spectrum of karbutilate recorded by DTIM-MS (d) and

396

TWIM-MS (e) using acetonitrile as organic eluent with a flow rate of 800 µL/min. Drift

397

spectrum of TWIM-MS recorded at same settings for the desolvation temperature (200 °C)

398

and desolvation gas flow rate of (5 L/min) as for the DTIM-MS instrument (f).


Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


399

Conclusion

400

The comparability of CCS values for small molecules, determined by TWIM-MS and DTIM-

401

MS, was investigated in order to test the possibility of a common CCS database. A good

402

correlation was found between the two instrument types for [M+H]+ and [M+Na]+ ions. Mean

403

absolute percentage errors were 1.0% for [M+H]+ and 1.1% for [M+Na]+ ions. However,

404

deviations of up to 6.2% of the measured compound ions showed that a comparability

405

between both instruments does not exist in all cases. Nevertheless, the CCS values can be

406

used reciprocally to exclude unambiguously incorrect assignments during identification of

407

unknowns. Latest developed prediction tools are also not able to predict CCS values with

408

high accuracy (< 1% error) for a large number of molecules. For example, Zhou et al. [54]

409

developed an approach to predict CCS values for lipids. By an external validation with

410

different instruments (DTIM-MS and TWIM-MS) median relative errors of about 1% were

411

calculated. However, deviations between predicted and experimentally determined CCS

412

values of greater than 1% were found for 29.3% and 11.8% of compounds for positive and

413

negative ionization mode, respectively. High deviations between expected and experimental

414

CCS values can lead to false negative results when databases are used as molecular

415

identifier. Therefore, when creating a database, it should be clearly indicated which

416

instrument type was used for the CCS determination and this should also be considered

417

when using a CCS database. It could be shown, similar to other studies, that the intensity

418

distributions of ion mobility separated protomers in the drift spectra are dependent on the

419

used solvent when electrospray ionization is applied. Different liquid chromatographic

420

conditions can prefer different protonation sites. Therefore experimental settings should also

421

be mentioned and multiple detected ion mobility signals for one compound should be

422

specified when creating CCS databases.

423

Associated content

424

Supporting Information

425

SI contains information concerning calibration of both instruments, CCS values of pesticides,

426

pharmaceuticals and pesticide metabolites measured with TWIM-MS and DTIM-MS and

427

reported in literature, CCS values of multiple detected signals in drift spectra and results of

428

CCS values during variation of solvent composition, solvent flow rate, desolvation

429

temperature and desolvation gas flow rates.

430

Author Information

431

Corresponding Author

432

E-Mail: [email protected] 17 ACS Paragon Plus Environment


Page 18 of 22

433

Phone: +49 201 183 3950

434

ORCID

435

Oliver J. Schmitz: 0000-0003-2184-1207

436

Torsten C. Schmidt: 0000-0003-1107-4403

437

Acknowledgments

438

We are thankful to Agilent Technologies for their support and Waters for the use of the UPLC

439

and Vion mass spectrometer and for their support.


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440

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For TOC only

DTCCS N2

280 260 240

TW CCS N2

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DTCCS N2

Set of standards


Comparison of CCS Values Determined by ... - ACS Publications

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