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

250

300

350

400

450

500

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)

0

20

40

60

80

100

0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

300.1

762.7

Nor

mal

ized

frac

tiona

l ab

unda

nce

0

20

40

60

80

100

0.8 1.3 1.8 2.3 2.8 3.3

300.1613.3

Nor

mal

ized

frac

tiona

l abu

ndan

ce

Laser power density *108 (W/cm2)