Tip Enhanced Laser Ablation Sample Transfer for Mass Spectrometry

K.K. Murray, S. Ghorai, C.A. Seneviratne, Tip Enhanced Laser Ablation Sample Transfer for Mass Spectrometry, MRS Proc. 1754 (2015) mrsf14–1754–pp08–04. doi:10.1557/opl.2015.286.

Abstract
TELA Sample Transfer for MS - Polarization
Near-field laser ablation of an insulin thin film with laser polarization a) perpendicular and b) parallel to the tip axis with c) crater depth profile

Mass spectrometry is one of the primary analysis techniques for biological analysis but there are technological barriers in sampling scale that must be overcome for it to be used to its full potential on the size scale of single cells. Current mass spectrometry imaging methods are limited in spatial resolution when analyzing large biomolecules. The goal of this project is to use atomic force microscope (AFM) tip enhanced laser ablation to remove material from cells and tissue and capture it for subsequent mass spectrometry analysis. The laser ablation sample transfer system uses an AFM stage to hold the metal-coated tip at a distance of approximately 10 nm from a sample surface. The metal tip acts as an antenna for the electromagnetic radiation and enables the ablation of the sample with a spot size much smaller than a laser focused with a conventional lens system. A pulsed nanosecond UV or visible wavelength laser is focused onto the gold-coated silicon tip at an angle nearly parallel with the surface, which results in the removal of material from a spot between 500 nm and 1 µm in diameter and 200 and 500 nm deep. This corresponds to a few picograms of ablated material, which can be captured on a metal surface for MALDI analysis. We have used this approach to transfer small peptides and proteins from a thin film for analysis by mass spectrometry as a first step toward high spatial resolution imaging.

Tip-enhanced laser ablation sample transfer for biomolecule mass spectrometry

S. Ghorai, C.A. Seneviratne, K.K. Murray, “Tip-enhanced laser ablation sample transfer for biomolecule mass spectrometry,” J. Am. Soc. Mass Spectrom.26 (2015) 63–70. doi:10.1007/s13361-014-1005-x.

Abstract: Atomic force microscope (AFM) tip-enhanced laser ablation was used to transfer molecules from thin films to a suspended silver wire for off-line mass spectrometry using laser desorption ionization (LDI) and matrix-assisted laser desorption ionization (MALDI). An AFM with a 30 nm radius gold-coated silicon tip was used to image the sample and to hold the tip 15 nm from the surface for material removal using a 355 nm Nd:YAG laser. The ablated material was captured on a silver wire that was held 300 μm vertically and 100 μm horizontally from the tip. For the small molecules anthracene and rhodamine 6G, the wire was cut and affixed to a metal target using double-sided conductive tape and analyzed by LDI using a commercial laser desorption time-of-flight mass spectrometer. Approximately 100 fg of material was ablated from each of the 1 μm ablation spots and transferred with approximately 3% efficiency. For larger polypeptide molecules angiotensin II and bovine insulin, the captured material was dissolved in saturated matrix solution and deposited on a target for MALDI analysis.

AFM image of ablation crater of an insulin thin film and MALDI mass spectrum of the collected material
Atomic force microscope with pulsed laser ablation and sample capture
Atomic force microscope stage with capture wire for tip enhanced laser ablation

Size distributions of ambient shock-generated particles: implications for inlet ionization

T. Musapelo, K.K. Murray, Size distributions of ambient shock-generated particles: implications for inlet ionization, Rapid Commun. Mass Spectrom.27 (2013) 1283–1286. doi:10.1002/rcm.6568.

Abstract

Modified mousetrap used to create shock-generated particles from thin film deposits.
Modified mousetrap used to create shock-generated particles from thin film deposits.

In this work, the scanning mobility particle sizer (SMPS) and aerodynamic particle sizer (APS) particle measurement system was used to measure the size distribution of particles produced under conditions of matrix-assisted inlet ionization. A simple spring impact device was used to strike solid matrix and analyte samples deposited on a metal target. The resulting particles were counted and sized using the SMPS sizing instrument in tandem with the APS instrument. The size and concentration of particles in the range between 10 nm and 20 µm were measured.

Ambient laser ablation sample transfer with nanostructure-assisted laser desorption ionization mass spectrometry for bacteria analysis

J.M. Hayes, K.K. Murray, Ambient laser ablation sample transfer with nanostructure-assisted laser desorption ionization mass spectrometry for bacteria analysis, Rapid Commun. Mass Spectrom. 28 (2014) 2382–2384. doi:10.1002/rcm.7023.

Abstract

LAST NALDI
MALDI mass spectrum of bacteria with CHCA matrix (a) E. coli and (b) B. cereus. The inset spectra show the low m/z range.

The direct analysis of bacteria phospholipids was accomplished by a combination of ambient sampling of bacterial colonies by laser ablation combined with a nanostructure-assisted laser desorption ionization (NALDI) MS. Bacteria from colonies of E. coli and B. cereus were irradiated with a pulsed infrared laser and the ablated material was collected in a solvent droplet. The droplet was deposited directly on a NALDI target and analyzed in a commercial time-of-flight (TOF) mass spectrometer, with no further treatment. The resulting mass spectra reveal distinctive peak patterns corresponding to the bacteria phospholipids.

GUMBOS matrices of variable hydrophobicity for matrix-assisted laser desorption/ionization mass spectrometry

Al Ghafly, Siraj, Das, Regmi, Magut, Galpothdeniya, Murray, Warner, Rapid Commun. Mass Spectrom. 2014, 28, 2307; DOI: 10.1002/rcm.7027.

RATIONALE

Detection of hydrophobic peptides remains a major obstacle for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). This stems from the fact that most matrices for MALDI are hydrophilic and therefore have low affinities for hydrophobic peptides. Herein, 1-aminopyrene (AP) and AP-derived group of uniform materials based on organic salts (GUMBOS) as novel matrices for MALDI-MS analyses of peptides were investigated for hydrophobic and hydrophilic peptides.

METHODS

A number of solid-phase AP-based GUMBOS are synthesized with variable hydrophobicity simply by changing the counterions. Structures were confirmed by use of 1H NMR and electrospray ionization mass spectrometry (ESI-MS). 1-Octanol/water partition coefficients (Ko/w) were used to measure the hydrophobicity of the matrices. A dried-droplet method was used for sample preparation. All spectra were obtained using a MALDI-TOF mass spectrometer in positive ion reflectron mode.

RESULTS

A series of AP-based GUMBOS was synthesized including [AP][chloride] ([AP][Cl]), [AP][ascorbate] ([AP][Asc]) and [AP][bis(trifluoromethane)sulfonimide] ([AP][NTf2]). The relative hydrophobicities of these compounds and α-cyano-4-hydroxycinnamic acid (CHCA, a common MALDI matrix) indicated that AP-based GUMBOS can be tuned to be much more hydrophobic than CHCA. A clear trend is observed between the signal intensities of hydrophobic peptides and hydrophobicity of the matrix.

CONCLUSIONS

MALDI matrices of GUMBOS with tunable hydrophobicities are easily obtained simply by varying the counterion. We have found that hydrophobic matrix materials are very effective for MALDI determination of hydrophobic peptides and, similarly, the more hydrophilic peptides displayed greater intensity in the more hydrophilic matrix.

Particle formation by infrared laser ablation of MALDI matrix compounds

T. Musapelo, K.K. Murray, “Particle formation by infrared laser ablation of MALDI matrix compounds,” J. Mass Spectrom. 49 (2014) 543–549. doi:10.1002/jms.3378.

Abstract: The concentration and size distribution of particles ablated from the infrared matrix-assisted laser desorption/ionization matrix compounds succinic acid (butanedioic acid), α-cyano-4-hydroxycinnamic acid, and glycerol were measured using an aerodynamic particle sizer combined with a scanning mobility particle sizer. The two sizing instruments together had a sizing range to from 10 nm to 20 µm. Thin layers of the matrix compounds were irradiated with fluences between 6.0 and 9.5 kJ/m(2) and wavelengths between 2.8 and 3.0 µm. The distribution of particles was characterized by a large concentration of clusters in the 20-nm-diameter range and large component of mass in the range of coarse particle with diameters greater than 1 µm. The wavelength dependence revealed a blue shift for the maximum particle production that is attributed to heating and disruption of the hydrogen bonds in the matrix that shifts the absorption to shorter wavelengths. This blue shift has been observed previously in infrared matrix-assisted laser desorption/ionization.

Journal of Mass Spectrometry, July 2014: ” Particle formation by infrared laser ablation of MALDI matrix compounds”
Broad-range particle sizing using a scanning mobility particle sizer and light scattering particle sizer.
Concentration of particles plotted as a function of their diameter.

Molecular weight sensing properties of ionic liquid-polymer composite films: theory and experiment

B.P. Regmi, N.C. Speller, M.J. Anderson, J.O. Brutus, Y. Merid, S. Das, B. El-Zahab, D. J. Hayes, K. K. Murray, I. M. Warner, Molecular weight sensing properties of ionic liquid-polymer composite films: theory and experiment, J. Mat. Chem. C, 2 (2014) 4867–4878. doi:10.1039/C3TC32528H.

Abstract

Ionic liquids (ILs) are rapidly emerging as important coating materials for highly sensitive chemical sensing devices. In this regard, we have previously demonstrated that a quartz crystal microbalance (QCM) coated with a binary mixture of an IL and cellulose acetate can be employed for detection and molecular weight estimation of organic vapors (J. Mater. Chem. 2012, 22, 13732). Herein, we report follow-up studies aimed at formulating the theoretical basis for our previously observed relationship between molecular weight and changes in the QCM parameters. In the current work, we have investigated the vapor sensing characteristics of a series of binary blends of ILs and polymers over a wider concentration range of analytes, and a quadratic equation for estimating the approximate molecular weight of an organic vapor is proposed. Additionally, the frequency (f) and dissipation factor (D) at multiple harmonics were measured by use of a quartz crystal microbalance with dissipation monitoring (QCM-D). These QCM-D data were then analyzed by fitting to various models. It is observed that the behavior of these films can be best described by use of the Maxwell viscoelastic model. In light of these observations, a plausible explanation for the correlation between the molecular weight of absorbed vapors and the QCM parameters is presented. Our previous findings appear to be a special case of this more general observation. Overall, these results underscore the true potential of IL-based composite materials for discrimination and molecular weight estimation of a broad range of chemical vapors.

Molecular weight sensing properties of ionic liquid-polymer composite films: theory and experiment

B.P. Regmi, N.C. Speller, M.J. Anderson, J.O. Brutus, Y. Merid, S. Das, B. El-Zahab, D. J. Hayes, K. K. Murray and I. M. Warner, “Molecular weight sensing properties of ionic liquid-polymer composite films: theory and experiment,” J. Mater. Chem. C, 2 (2014) 4867–4878. doi:10.1039/C3TC32528H.

Abstract: Ionic liquids (ILs) are rapidly emerging as important coating materials for highly sensitive chemical sensing devices. In this regard, we have previously demonstrated that a quartz crystal microbalance (QCM) coated with a binary mixture of an IL and cellulose acetate can be employed for detection and molecular weight estimation of organic vapors (J. Mater. Chem. 2012, 22, 13732). Herein, we report follow-up studies aimed at formulating the theoretical basis for our previously observed relationship between molecular weight and changes in the QCM parameters. In the current work, we have investigated the vapor sensing characteristics of a series of binary blends of ILs and polymers over a wider concentration range of analytes, and a quadratic equation for estimating the approximate molecular weight of an organic vapor is proposed. Additionally, the frequency (f) and dissipation factor (D) at multiple harmonics were measured by use of a quartz crystal microbalance with dissipation monitoring (QCM-D). These QCM-D data were then analyzed by fitting to various models. It is observed that the behavior of these films can be best described by use of the Maxwell viscoelastic model. In light of these observations, a plausible explanation for the correlation between the molecular weight of absorbed vapors and the QCM parameters is presented. Our previous findings appear to be a special case of this more general observation. Overall, these results underscore the true potential of IL-based composite materials for discrimination and molecular weight estimation of a broad range of chemical vapors.