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An Investigation of Factors that Affect the Mass Range of Atmospheric Pressure Laser-Induced Acoustic Desorption Chemical Ionization (AP/LIAD-CI)
vs. AP/LIAD-Electrospray Ionization Mass Spectrometry
RESULTS INTRODUCTION Laser-induced acoustic desorption (LIAD) is a soft laser-based technique that has largely been implemented in vacuum for the vaporization of thermally labile compounds. In LIAD, samples deposited on a thin metal foil are irradiated from the backside with a series of high energy laser pulses. This results in the generation of high amplitude acoustic (shock) waves that travel through the foil to effect the desorption of analytes species on the opposite site. When implemented for mass spectrometry (MS) investigations, LIAD decouples the processes involved in analyte desorption from the subsequent ionization events, allowing the technique to be coupled to a broad range of ionization methods. One of the limitations of LIAD is its limited ability for the analysis of high molecular weight species especially those ionized by single charges. The high mass limit observed depends largely on the type of analyte being evaluated. A mass limit of approximately 800 Da has been observed for peptides and 1200 Da for saturated hydrocarbon polymers. These limits are thought to be largely dependent on the different strengths of analyte-surface interaction and also on the amplitude of the acoustic waves generated within the sample-containing metal foil. Here we present the implementation of LIAD at atmospheric pressure (AP) coupled to chemical and electrospray ionization on a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer to investigate some of the ion source conditions that affect the mass range and its implications for applications in the analysis of complex mixtures. The results are also compared with analysis by electrospray ionization (ESI). METHODS LIAD was coupled to chemical and electrospray ionization on a 9.4 T FT-ICR mass spectrometer according to the schemes shown below. Chemical ionization was performed using reagent ions generated from a corona discharge initiated between a tungsten electrode regulated from 2500-4500 V and the MS inlet capillary held at 70 V, in ambient air and in the presence of vaporized toluene as dopant. LIAD-ESI was performed by spraying a solution of 50:50 toluene:MeOH, 0.15% HCOOH at a flow rate of 1 µL/min and the spray tip-to-MS distance set to ~ 5 mm. 2 µL, 0.5 mM of each polymer solution or binary mixture was deposited on Ti foils and evaluated after solvent evaporation. 2 µL, 1 mg/mL of a high vacuum gas oil (HVGO) distillation cut (400-425 ºC) was evaluated similarly. 2 µM of samples was analyzed by ESI at a flow rate of 0.5 µL/min. EXPERIMENTAL SET-UP
Yer Yang,1 Leonard Nyadong, 2 Ryan P. Rodgers, 2, 3 Alan G. Marshall.2, 3
1Columbus State University, Columbus, Georgia. 2National High Magnetic Field Libratory, Florida State University, Tallahassee, Florida.
3Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida.
Set-up 1: AP/LIAD- CI
Set-up 2: AP/LIAD-ESI
Figure 1. AP/LIAD-CI MS analysis of PEG 300 at increasing corona discharge current m/z
1000900800700600500400300200 900800700600500400300200m/z
1000900800700600500400300200 900800700600500400300200
5 µA
80 µA
* *
*
*
*
*
* ****
* [M + NH4]+
[M + H]+
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90Discharge current (µA)
Nor
mal
ized
Σab
unda
nces
Mn
200
250
300
350
400
450
500
550
600
650
0.1 0.6 1.1 1.6 2.1L as er power dens ity *108 (W/cm2)
PE G 300
PE G 600
PE G 1000
PEG 300
PEG 600
PEG 1000
m/z1000900800700600500400300200
m/z1000900800700600500400300200
m/z1000900800700600500400300200
1.75*108
W/cm2
Figure 2. AP/LIAD-CI MS analysis of PEG 300, 600, and 1000 at increasing laser power densities
300
350
400
450
500
550
0 2 4 6 8 10 12Number of dis tinc t MS acquis itions
Mn
Figure 3. AP/LIAD-CI MS analysis of PEG 600 for distinct MS acquisition following laser ablation of the same spot on the rear side of the sample
m/z1000900800700600500400300200
m/z1000900800700600500400300200
1st MS acquisition
5th MS acquisition
10th MS acquisition
4 mm
8 mm
10 mm
m/z1000900800700600500400300200
m/z1000900800700600500400300200
200
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300
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550
2 4 6 8 10 12Sample-to-MS distance (mm)
Mn
Figure 4. AP/LIAD-CI MS analysis of PEG 600 at varying sample-to-MS distances
Figure 5. Analysis of PEG 600 and 1000 by AP/LIAD-CI, AP/LIAD-ESI, and ESI
LIAD-CI
LIAD-ESI
ESI
PEG 600
m/z1000900800700600500400300200
m/z1000900800700600500400300200
[M + Na]+
[M + H]+
m/z1300120011001000900800700600500400300
m/z1300120011001000900800700600500400300
PEG 1000LIAD-CI
ESI
ESI-Oct 2 900 mz
2+
1+
LIAD-ESI
Figure 7. AP/LIAD-CI MS analysis of an Athabasca HVGO distillation cut (400-425°C) at various laser power densities.
Figure 6. AP/LIAD-CI MS analysis of a binary mixture of coronene and tetradecylpyrene in the presence of dopant at various laser power densities
0.93*108W/cm2
1.07*108W/cm2
2.43*108W/cm2
1000900800700600500400300200 1000900800700600500400300200
300.1
762.7
+●
+●
coronene tetradecylpyrene
Figure 7. AP/LIAD-CI MS analysis of a binary mixture of coronene and MG 613 in the presence of dopant at various laser power densities
m/z700650600550500450400350300250200150
m/z700650600550500450400350300250200150
m/z500490480470460450440430420410400
m/z500490480470460450440430420410400
0.73*108
W/cm2
1.07*108
W/cm2
2.09*108
W/cm2
HVGO 400-425 °CAP/LIAD-CI
CONCLUSIONS • AP/LIAD-CI and AP/LIAD-ESI were observed to result in similar mass range, which was largely dependent on such factors as the laser power density, sampling distance and the number of laser shots per spot, on the backside of the sample-containing foil. • High laser powers typically allows higher mass limits, however the high population of analytes generated under these conditions could result in ionization suppression and/or defocusing effects especially for smaller molecular weight species, inhibiting their detection. • The ability to detect molecules with varying mass range under each of the ion source settings could be advantageously exploited to provided improved selectivity during the analysis of complex mixtures. ACKNOWLEDGEMENTS This work was supported by NSF Division of Materials Research through DMR-06-54118, the State of Florida and Shell Global Solutions. YY acknowledges support by the NSF Cooperative Agreement DMR-0654118, NSF DMR-0645408, Florida State University.
m/z1000900800700600500400300200
m/z1000900800700600500400300200
m/z1000900800700600500400300200
300.1
613.3
+●+ H
+
1.07*108
W/cm2
1.75*108
W/cm2
2.43*108
W/cm2
coronene
MG 613
Laser power density *108 (W/cm2)
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20
40
60
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100
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
300.1
762.7
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unda
nce
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40
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0.8 1.3 1.8 2.3 2.8 3.3
300.1613.3
Nor
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tiona
l abu
ndan
ce
Laser power density *108 (W/cm2)
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