DISSOCIATION OF GAS IONS IN AIR BEFORE MASS SPECTROMETERS USING ELECTRIC FIELDS FROM FIELD DEPENDENT MOBILITY SPECTROMETERS

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1 DISSOCIATION OF GAS IONS IN AIR BEFORE MASS SPECTROMETERS USING ELECTRIC FIELDS FROM FIELD DEPENDENT MOBILITY SPECTROMETERS X. An, J.A. Stone, and G.A. Eiceman * Department of Chemistry and Biochemistry 1 New Mexico State University

2 ION LIFETIMES IN AIR AT AMBIENT PRESSURE-RANGE 10 µs to 30+ ms Ion Intensity (au) K 287 K 283 K W n H + MWH + M 2 H K Drift Time (ms) International Journal of Mass Spectrometry (2006) 76 85

3 IONS FRAGMENT IN AIR AT AMBIENT PRESSURE Fragmentation of Butyl Acetate Isomers in the Drift Region of an Ion Mobility Spectrometer, International Journal of Mass Spectrometry and Ion Processes 1988, 85, SPECIFIC TO CHEMICAL FAMILIES Classification of Ion Mobility Spectra by Chemical Moiety Using Neural Networks with Whole Spectra at Various Concentrations Analytica Chimica Acta 1999, 394, Neural Network Recognition of Chemical Class Information in Mobility Spectra Obtained at High Temperatures Analytical Chemistry 2000, 72, Chemical Class Information in Ion Mobility Spectra at Low and Elevated Temperatures Analytica Chimica Acta, 2001, 433,

4 Log Ion Intensity (au) CLASS SPECIFIC FRAGMENTATION AFTER THE PATTERN OF ELECTRON IMPACT SOURCES 4 Drift Time (ms)

5 ION MOTION IN AIR AT AMBIENT PRESSURE ΔV,d v d = d t d v d = 6.0 m/s E = 300 V/cm v d / E = K K = 2.0 cm 2 /Vs 5 Drift Velocity v d Ion Swarm Normalize for N 1 Td = 1*10-17 V cm 2 E/N = 1.25 Td

6 ORIGINS FOR ION KINETIC ENERGY IN MOTION AT 300 V/cm Thermal contribution to kinetic energy of ion at 298 K 3/2 k b T = 4 x J molecule -1 Kinetic energy from electric field E = 300 V/cm 1/2 Mv d 2 = 5 x J molecule -1 Ions are said to be thermalized when energy obtained from electric field is negligible. The effective ion temperature (T eff ) for KE is equal to the gas temperature (T) : 3/2 k b T eff = 3/2 k b T 6

7 HIGH E FIELDS FOR IONS UP TO 30,000 V/cm (E/N 182 Td) DC Control Asymmetric Waveform 1 MHz Characterization of ions in asymmetric waveforms, Nazarov, et al., Tashkent and Novosibirsk,

8 WAVEFORM OF SIONEX SVAC 8

9 Separation Field (Td) ELECTRIC FIELD INDUCED DISSOCIATION & FRAGMENTATION 176 At 100 C F + M 2 H MH + RIP Compensation Field (Td) 9

10 ION MOTION IN HIGH E/N K(E/N) = K (1 + α(e/n)) K = (3e/16N)(2 / kt eff ) 1/2 [(1+ )/ D (T eff )] Collision Cross Section v d = K * E 10

11 LOSS OF KINETIC ENERGY INTO INTERNAL ENERGY OF MOLECULES 3/2 k b T eff = 3/2 k b T + 1/2 ζmv d 2 monoatomic ions in monoatomic gases, ζ ~ 1 Kinetic energy obtained from electric field retained in ion speed. polyatomic ions, ζ > ~0.1 to ~0.9 Some kinetic energy converted to internal vibrations & rotations, leading to breaking of covalent bonds or ion-neutral associations so ion Temperature for motion is reduced as energy is placed into bond vibrations and motion. 11

12 PRIOR OBSERVATIONS OF ION- HEATING IN HIGH E/N CONDITION At E~2400 V/cm at ambient pressure (>10Td) and kinetic energy is increased by field. Thus T eff > T of gas, however, some energy acquired by an ion from field is converted into internal energy & reactions follow Methyl salicylate in negative polarity M*O - 2 (M-H) - + HO 2 Aromatic hydrocarbons in positive polarity MH + Fragments + + neutrals 12

13 PLANAR HIGH FIELD DMS DRIFT TUBES ION DETECTION ION SEPARATION R.A. Miller, G.A. Eiceman, E.G. Nazarov, A.T. King; Sensors and Actuators B. Chemical, 2000, 67,

14 CONCEPT OF API FRAGMENTATION SWITCH FOR LC MS 14

15 QUESTIONS 1. Is field induced decomposition of alkyl esters possible (seen earlier with thermal IMS methods). 2. What is the quantitative connection between temperature and electric fields to heat ion? 3. What is the a relationship between ζ, T eff and ion mass, if any, with a chemical? 15

16 FIELD INDUCED DECOMPOSITION OF ALKYL ESTERS. WHAT IS SENSITIVITY OF REACTIONS TO E? = O R 1 -C-O-R 2 Alkyl esters Readily protonated w/ simple chemistry to MH + and M 2 H + Proton bound dimer dissociated between 100 to 200 C (H-bond strength of each ester of ~122 kj/mol) Experiments supported by mass spectrometry based collision induced dissociation data in literature 16

17 ALKYL ESTERS IN TEST SET Substance n- CH 3 COOC3H 7 n- CH 3 COOC4H 9 n- CH 3 COOC5H11 n- CH 3 COOC6H13 n-alkyl acetates n-ethyl esters MW (g/ mole) n- n- n- n- C 2H5COOC2H5 C 5H 11 COOC 2H5 C 2H5COOC3H 7 C 3H7COOC3H 7 n-propyl esters

18 DIFFERENTIAL MOBILITY SPECTRUM 4 REACTANT ION PEAK PROTON BOUND DIMER K PROTONATED MONOMER - K COMPENSATION VOLTAGE (V) 18

19 SAMPLE PREPARATION AND EXPERIMENT DESIGN Ion Intensity MH + Reactant Ion M 2 H + Compensation Voltage Control of E and data storage Substances in electric field of DMS drift tube Prefractionation of sample in gas chromatograph Introduction of sample

20 SAMPLE PREPARATION AND EXPERIMENT DESIGN Retention time MH + Reactant Ion M 2 H + Compensation Voltage Control of E and data storage Substances in electric field of DMS drift tube Prefractionation of sample in gas chromatograph Introduction of sample

21 2,4-DIMETHYL PYRIDINE AT 50 C Retention Time (min) FIELD HEATING OF CONTROL CHEMICAL M 2 (H 2 O) n H + MH + (H 2 O) n + M 15 RIP MH + (a) 71 Td MH RIP + (b) 82 Td M 2 H + F+ M 2 H MH 15 RIP + MH (c) 87 Td + RIP (d) 92 Td F + F M + 2 H + M 2 H MH (e) 102 Td + (f) 142 Td F + F + M 2 H + M 2 H + MH Compensation Voltage (Td) 21

22 METHYL ACETATE AT 100 C FIELD HEATING OF METHYL ACETATE M 2 (H 2 O) n H + MH + (H 2 O) n + M + 1/2 ζmv d 2 + M M 2 H + (H 2 O) n MH + (H 2 O) n Stable, long-lived protonated monomer 22

23 METHYL ACETATE AT 100 C Retention Time (min) FIELD HEATING OF METHYL ACETATE 5 (a) 82 Td (b) 94 Td 3 RIP F + MH + M 2 H (c) 99 Td (d) 105 Td F + RIP MH + M 2 H RIP F + MH + M 2 H + M 2 H + RIP (e) 117 Td F + MH + 0 (f) 164 Td F + M F + 2 H + M 2 H + 3 MH + MH Compensation Voltage (V) 23

24 METHYL ACETATE: SIMPLE DISSOCIATION OF PROTON BOUND DIMER TO STABLE PROTONATED MONOMER Dissociation of Proton Bound Dimer- Yes (CH 3 COOCH 3 ) 2 H + CH 3 CO 2 CH 3 + CH 3 CO 2 CH 3 H + Δ H 0 ~ 30 kj mol -1 Stable Protonated Monomer-No Fragmentation (CH 3 COOCH 3 )H + CH 3 CO + + CH 3 OH Δ H 0 = 156 kj mol -1

25 n-propyl ACETATE AT 100 C Retention Time (min) FIELD HEATING OF ALKYL ESTERS 12.3 M 2 H + MH + (a) 82 Td F + MH + (b) 94 Td M 2 H (c) 99 Td (d) 105 Td MH + M 2 H + M 2 H + MH + F + F (e) 117 Td F + F + M 2 H + MH + MH + M 2 H + (f) 164 Td Compensation Voltage (V) 25

26 POSSIBLE FRAGMENTATION PATHWAY: THROUGH THE PROTONATED MONOMER energy changes were computed and compared favorably 26 to literature values; Can. J. Chem. 57 (1979)

27 Relative Intensity DMS MS OF M 2 H + AND CID OF MH + W (a) 70 Td 205 Reactant ions : (H 2 O) 4 H + 55: (H 2 O) 3 H + (b) 117 Td Ions of Ester (c) 164 Td : M 2 H + 139: MH + (H 2 O) 2 121: MH + (H 2 O) 79 Fragment Ions (d) CAD of m/z m/z 97: CH 3 COOH 2+ (H 2 O) 2 79: CH 3 COOH 2+ (H 2 O) 61: CH 3 COOH : CH 3 CO + 27

28 ALKYL ACETATES FRAGMENT AND DISSOCIATE Dissociation of Proton Bound Dimer- Yes (CH 3 COOC 3 H 7 ) 2 H + CH 3 CO 2 C 3 H 7 + CH 3 CO 2 C 3 H 7 H + Δ H 0 ~ 30 kj mol -1 Fragmentation of Protonated Monomer-Yes (CH 3 COOC 3 H 7 )H + CH 3 COOH C 3 H 6 Δ H 0 = 102 kj mol -1 All other protonated n-alkyl esters have similar ΔH o as for propyl acetate

29 Separation Field (Td) QUANTITATIVE DETERMINATION OF THRESHOLD OF REACTIONS BY SWEEPING E/N (Td) 176 Permeation source, valves, and vessel to control vapor levels 117 Supply of Purified Gas DMS or DMS/MS Time in seconds

30 Separation Field (Td) Normalized Intensity DETERMINATION OF THRESHOLD FOR E/N (Td): DISSOCIATION & FRAGMENTATION 176 At 100 C 1.00 DMS F + 30 F + M 2 H M 2 H + MH MH F DMS/MS + RIP M 2 H MH Compensation Field (Td) Separation Field (Td)

31 E/N THRESHOLDS FOR FIELD INDUCED REACTIONS AS ƒ(temp) Separation Field (Td) Propyl propionate Ethyl Propionate 102 Ethyl hexanoate Acetates Propyl butyrate Temperature ( C) Td/ C or ~1.5 C/Td

32 SUMMARY: 1 st QUESTION Gas phase ions of esters dissociate and also fragment (except for methylacetate) to protonated acid and neutral alkene through electric field induced reactions Threshold for fragmentation is dependent upon temperature and equivalence of change in threshold with change in temperature yields : ± 0.06 Td/ C or ~1.5 C/Td Dissociation of proton bound dimer (M 2 H + ) to protonated monomer (MH + ) is preceded by fragmentation of protonated monomer suggesting kinetic order: protonated monomer (MH + ) > proton bound dimer (M 2 H + )

33 CONCLUSIONS AND NEXT QUESTIONS Fragmentation, while interesting, interferes with clean association between T and T eff ; although esters were OK start to research, move to chemicals with simple gas phase ion chemistry behavior No precise accounting for ζ in 2/3 k b T eff = 2/3 k b T + 1/2 ζmv d 2 Mass dependence of ζ observed.need systematic study with homologous series

34 2. WHAT IS A QUANTITATIVE CORRELATION BETEEN THERMAL DECOMPOSITION AND E/N FOR FIELD INDUCED DECOMPOSITION? - = 34 O CH 3 -O-P-O-CH 3 DMMP CH 3 Simplify the study of dissociation with an ion showing very clean ion behavior and chemistry Explore the effect of field heating on the ion by DMS for which the kinetics parameters determined by IMS Combine data from E/N (DMS) and Thermal (IMS) to measure T eff Determine ζ for dissociation from kinetic energy accounting

35 CORRELATION BETWEEN T AND T eff ENERGY + FROM GAS AND FROM + E/N M 2 H + (H 2 O) n-1 MH + (H 2 O) n M Calculated from ion swarm velocity 2/3 k b T eff = 2/3 k b T + ζ (1/2 Mv d2 ) Obtained from measurements using DMS Known

36 KINETICS OF ION DISSOCIATION OF PROTON BOUND DIMERS Rate = k [M 2 H + ] 36 T eff = - k = A e Ea RT E a R 1 ( ) ln A ln k k = obtained with high field method, differential mobility spectrometry, DMS A, E a = obtained with low field method, ion mobility spectrometry, IMS E a = -127 KJ mol -1 A = s -1 (E a and A determined under thermal conditions by Ewing, et al. Int J Mass Spectrom : for DMMP

37 DIFFERENTIAL MOBILITY SPECTRA:DMMP Ion Signal (V) 0.24 (a) HSV = 900 V 80 C 0.18 (H 2 O) n H + (DMMP) 2 H + (DMMP)H (b) HSV = 900 V 140 C (H 2 O) n H + (DMMP)H + (DMMP) 2 H Compensation Voltage (V) 37

38 Separation Voltage (V) DISSOCIATION OF DMMP DIMER IN AIR USING E/N Corrected Ion Intensity of (DMMP) 2 H + (V) 1500 (a) 80 C (DMMP)H + (DMMP) 2 H (DMMP) 2 H + (a) 80 C (H 2 O) n H (b) 140 C (DMMP)H + (DMMP) 2 H (DMMP) 2 H + (b) 140 C 900 (H 2 O) n H Compensation Voltage (V) HSV (V) 38

39 k ( X10 4 s -1 ) RATE CONSTANTS & CALCULATION OF T eff d[(dmmp) 2 H + ] - d t = k [DMMP) 2 H + ] C ln [(DMMP) 2 H + ] 0 = k x t [(DMMP) 2 H + ] t T eff = E a R 1 ( ) ln A ln k HSV (V) 60 C 39

40 T eff (K) T eff AS FUNCTION OF HSV (a) 140 C C HSV (V) T ζ = eff - T ( 8.40 x 10-3 K 2 0 Td 2 )

41 DETERMINATION OF ζ 1-ζ = fraction of the average energy from electric field that is retained as ions speed, ζ is the fraction converted to internal energy of the ion and available for dissociation or fragmentation 3/2 k b T eff = 3/2k b T + 1/2 ζ Mv d 2 v d2 = K 0 2 N 02 (E/N) 2 3/2 k b T eff = 3/2 k b T + 1/2 ζ M N 0 2 K 0 2 ( E M (molar mass of air) = g mol -1 N 0 = 2.69 x molec/cm 3 K 0 (reduced mobility) = 1.51 cm 2 V -1 s -1 [for 2-pentanone] N )2 ζ = T eff - T ( 8.40 x 10-2 K 2 0 (E/N) 2 ) 41

42 T eff AS FUNCTION OF HSV AND E/N 580 (b) 140 C 555 T eff C (E/N) 2 as (Td) 2 T ζ = eff - T ( 8.40 x 10-3 K 2 0 Td 2 ) 42

43 Energy Efficiency Factor ζ ζ AS ƒ(t and T eff ) C C T eff (K) 43

44 Energy Efficiency Factor ζ ζ AS ƒ(gas TEMPERATURE) 0.70 Extrapolation from Nazarov et al y = E-03x R 2 = 9.889E y = E-03x R 2 = 9.915E-01 NMSU data Transport Gas Temperature (K) 44 Data for the efficiency of energy transfer to internal modes of (DMMP) 2 H+, labeled as is from Int. J Mass Spectrum :

45 SUMMARY: 2 nd QUESTION Rate of dissociation of (DMMP) 2 H + via electric field heating and thermal heating used to calculate T eff ζ was seen as ƒ(t and T eff ) and maximum possible energy gained from the field was 0.4 to 0.5. Comparison to extrapolations from existing study off by 30%, source unknown. X. An, J.A. Stone, and G.A. Eiceman, A determination of the effective temperatures for the dissociation of the proton bound dimer of dimethyl methylphosphonate in a planar differential mobility spectrometer. International Journal of Ion Mobility Spectrometry 2009, submitted 20 Nov

46 CONCLUSIONS The efficiency of this energy transfer from electric field decreases with increasing temperature, thus. Precise correlation between T and E/N requires an adjustment of efficiency of absorption of energy by the gas phase ion, ζ. Mass dependence of ζ which was observed previously still unknown (see 3 rd Question)

47 = 3. WHAT IS ROLE OF MASS ON KINETICS OF DISSOCIATION AND ζ? Determine ion dissociation under thermal conditions and determine T eff with high electric fields for a homologous series O CH 3 (CH 2 ) n C-(CH 2 ) n CH 3 Simplify the energy flow by studying dissociation reaction of an ion Explore the effect of field heating on the ion by DMS for which the kinetics parameters determined by IMS Combine data sets from DMS and IMS to determine T eff Derive energy transfer efficiency for dissociation from kinetic energy 47

48 EXPERIMENT DESIGN Use concepts and procedures developed for Question 2: kinetic rate constants obtained from high field (DMS) method and E a and A from low field (IMS) methods to solve for T eff and ζ. Develop new kinetic ion mobility spectrometer and measure E a and A from IMS for homologous series of ketones: Build IMS instrument and vapor generator Measure rate constants under low field conditions at six to seven gas temperatures

49 ION MOBILITY SPECTROMETRY FOR LOW FIELD EXPERIMENTS Formation of Ions Isolation of Ion from neutrals Kinetic Region: Dissociation of Ion High Voltage Potential gradient Ground 49

50 KINETIC ION MOBILITY SPECTROMETER 50

51 Drift Region Source Region IONS IN THE KINETIC IMS M + (H 2 O) n H + MH + MH + + M M 2 H + MH + + M M 2 H + + nh 2 O ONLY M 2 H + MH + + M

52 DISSOCIATION OF PROTON BOUND DIMER OF PENTANONE AS ƒ(residence TIME) Ion Intensity (V) 159 MH + M 2 H MH + M 2 H V/cm 3 0 MH + M 2 H V/cm V/cm Drift Time (ms)

53 WHY CHOOSE KETONES? Proton bound dimers of symmetrical ketones have bond strengths of about 130 kj mol -1, activation energies and preexponential factors for dissociation are in the range of the IMS instrument Homologous series provides mass increments to conveniently determine the effect of ion mass on T eff and of ζ as a function of field strength. Acetone 2-Butanone 2-Pentanone 2-Hexanone 2-Heptanone 2-Octanone 2-Nonanone

54 DISSOCIATION OF PROTON BOUND DIMER OF 2-PENTANONE AS ƒ(t) Ion Intensity (V) 300 V/cm M 2 H MH + MH + M 2 H MH + M 2 H Drift Time (ms)

55 Ion Intensity DISSOCIATION RATE DETERMINATION t m Pentanone 163C,175V/cm t m = arrival time of monomer t d = arrival time of dimer t t d t = arrival time of an ion, which starts as dimer and dissociates along the way to monomer Drift Time (ms) The fraction of time the ion spent as dimer is t x, where t x = t d (t - t m ) / (t d - t m ) A plot of ln (area) vs. t x gives out the dissociation rate from the negative slope 55

56 Ln k E a & A FROM KINETIC IMS STUDIES 6 5 heptanone Slope = -E a /R Intercept = Ln A pentanone ln k = ln A - E a /RT pentanone heptanone nonanone E a kj/mol A s average ΔH = E a + RT nonanone At 160 C, for ketones ΔH = 127 KJ/mol /T (K -1 X1000) -OH +.. O- binding energy ~127 KJ/mol ref M.Meot-ner J. Am. Chem. Soc. 1984, 106,

57 Separation Voltage (V) E/N (SV) SCAN FOR 2-PENTANONE 1500 MH + M 2 H RIP 30 C MH RIP M 2 H C Compensation Voltage (V) 57

58 ENERGY EFFICIENCY FACTOR ζ FOR THE DISSOCIATION OF KETONES Energy Efficiency Factor ζ Nonanone 0.50 Pentanone Transport Gas Temperature (K)

59 CONCLUSIONS DMS There is a mass dependence on the electric field required to raise the temperature of an ion to a given temperature. Higher masses require higher fields. This may be associated with the increasing heat capacity with increasing mass. To attain the same T eff for ion decomposition in a gas at temperature T A low T requires a high Td field & a high T requires a low Td field ζ is smaller at low Td than at high Td ζ is mass dependent, increasing with increasing mass, which is also with increasing field 46

60 CONCLUSIONS IMS Thermal kinetic data can be obtained by IMS with low field conditions. The E a calculated for the dissociation of proton bound dimers of ketones is the same for each ketone and in excellent agreement with their enthalpies of dissociation obtained by mass spectrometry methods 45

61 OVERALL CONCLUSIONS Fundamental study of decomposition of gaseous ions in air at atmospheric pressure Understood behavior of ions during their life time under electric field heating and thermal heating Joined high field heating and thermal heating to determine T eff for the first time Found mass dependence on the electric field required to gain an effective temperature Discovered energy efficiency on transferring field energy into internal energy for dissociation is not only mass dependent, but also field dependent 47

62 FUTURE WORK IMS Use the double shutter function of the instrument to study the decomposition of other ions: 1. Mixed proton bound dimers of simple molecules e.g. ketone H + -amine 2. Extend to more complex molecules using ESI introduction e.g. peptide H + alcohol, hydrazine H + nonanone DMS Experiments with other symmetrical proton bound dimers to prove method. Compare results with T eff for ketones 48

63 E/N HIGH FIELD EXPERIMENTAL DESIGN TEMPERATURE 50 C TEMPERATURE 60 C E/N 500 to 1500V in 10V increments, 100 Steps Temperature, T 13 Steps from 30 to 150 C GAS TEMPERATURE, T,

64 RATE CONSTANT DETERMINATION AT 159 C Ln [(M 2 H + ) remaining] V/cm V/cm t x (ms) 64

65 FIRST ORDER DISSOCIATION d[m 2 H + ] - d t = k [M 2 H + ] ln [M 2 H + ] t [M 2 H + ] 0 = - k x t ln[m 2 H + ] t - ln[m 2 H + ] 0 = - k x t 65

66 DMS SIGNAL AT 30 C FOR THE PROTON BOUND DIMERS OF KETONES Ion Intensity (V) C9 C8 C C6 C3 C4 C HSV (volts) 66

67 DISSOCIATION OF 2-PENTANONE DIMER Ion Intensity (V) 0.27 (a)100 C 0.18 M 2 H + Baseline Correction M 2 H + (b)100 C Correction of Ion Losses on the Wall HSV (V) 67

68 KINETICS OF DISSOCIATION OF M 2 H + OF PENTANONE AT HIGH E/N k t ln (k t) d [M 2 H + ] - = k [M d t 2 H + ] [M 2 H + ] 0 ln = k x t [M 2 H + ] t 4.5 (a) 110 C C The time t above the threshold is calculated (b) 110 C C use t solve for k HSV (V)

69 RATE CONSTANT FOR 2- PENTANONE M 2 H + DISSOCIATION k ( X10 4 s -1 ) C C HSV (V) 69

70 DETERMINATION OF T eff FOR PENTANONE T eff = E a 1 R ( ln A ln k ) E a = KJ mol -1 A = s -1 70

71 T eff (K) T eff vs HSV AND E/N FOR PENTANONE 515 (a) 515 (b) 110 C 110 C C 30 C HSV (V) (E/N) 2 (Td 2 ) 71

72 ENERGY EFFICIENCY FACTOR ζ FOR THE DISSOCIATION OF 2-PENTANONE M 2 H + Energy Efficiency Factor ζ C C T eff (K)

73 DIFFERENTIAL MOBILITY SPECTOMETER: Source of high electric field and also method for ion separation and characterization

74 ACKNOWLEDGEMENTS Funding sources URI DHS, Center of Excellence. Jaime Rodriguez, Hartwig Schmidt Pu Wei 49