1. Results and discussion
Molecular Ionization from Carbon Nanotube Paper
Rahul Narayanan, Depanjan Sarkar, R. Graham Cooks and Thalappil Pradeep*
DST Unit of Nanoscience (DST UNS) and Thematic Unit of Excellence, Department of Chemistry, Indian Institute of Technology Madras, Chennai , India.
Department of Chemistry, Purdue University, West Lafayette, IN (USA). * A) B) C) D)
263
203
170
102
[M + H]+
m/z
Intensity
Experimental
A stable carbon nanotube (CNT) suspension was drop-cast onto Whatman 42 filter paper. This paper was dried in air and cut into triangles with dimensions of 2×5 mm (base × height). This CNT-coated paper triangle was connected to a 3 V battery and held close (2 mm) to the mass spectrometer inlet.
The paper was loaded with samples of 30 ppm concentration.
The volume of solvent used was 2 µL, and repeated measurements using the same paper used the same aliquot of pure solvent.
Mass spectra were recorded in the positive ion mode for all analytes, except for preformed ions. For preformed ions that are derived from salts, both positive and negative ion mode spectra were recorded at ±3 V.
Introduction
Recent progress in mass spectrometry has depended heavily on advances in the methods of ion formation. The creation of stable molecular ions of complex molecules with minimum internal energy has been a primary goal of such experiments.
Ambient ionization methods, such as desorption electrospray ionization (DESI), allow samples to be examined in their native state with minimal or no sample pre-treatment.
The need of a high voltage power supply is an inevitable thing in many of such methods. Here, we show that ionization can be achieved from a substrate that is coated with carbon nanotubes (CNTs) at a potential of just a few volts.
This method removed the need of high-voltage based ionization sources allowing instead for handheld battery driven ion sources. A)
263 B)
203 C) D)
170
102
A–D) Intensity enhancement upon the addition of dilute HCl for various analytes (M) at 3 V, namely triphenylphosphine (A), tributylphosphine (B), diphenylamine (C), and triethylamine (D). The upper and lower traces in each plot are on the same scale and show spectra that were recorded before and after HCl addition, respectively. The y axes correspond to the intensity.
1349
1332
Raman shift (cm-1)
Intensity
D - band
G - band
1619
1608
1330
1602
CNT paper after the experiment
CNT paper before the experiment A) B) C)
263
3 kV
3 V
1 V
[M + H]+
m/z
500 nm A) B) D) E) F)
264
265
185
183
152 + M
[M]+ MS
A) Schematic representation of the ionization process from CNT paper. B) Photograph of the ionization source showing paper triangle and battery along with a grounding electrical connection. C) Mass spectra of triphenylphosphine (M) at 3 kV, 3 V, and 1 V from wet CNT paper. D) Field emission scanning electron microscopy (FESEM) image of parent ion at m/z 263. CNT-coated paper. E) Isotope distribution of the protonated molecule at 3 V. F) MS2 spectrum of the parent ion at m/z 263.
Raman spectra of CNT-coated paper before and after ionization for A) neutral molecules (30 ppm TPP in MeOH/H2O) and preformed ions (tetramethylammonium bromide) in the B) positive and C) negative ion modes. The y axes correspond to the intensity.
Results and discussion
Experimental results suggest the possibility of ionization of various analyte systems at very low voltage from a carbon nanotube coated paper. All the analytes gave quality mass spectra with well defined isotopic distribution. A closer examination of the edge of the coated paper with Field emission scanning electron microscopy revealed protruding nanotubes.
The high electric field at the tip of carbon nanotubes is responsible for this ionization event. Control experiments revealed the role of carbon nanotubes in the ionization event. It appears that field ionization is happening from a solvated analyte or droplet, as in all cases only [M+H]+ and not the radical cation M+. was detected.
Experiments were conducted for Triphenylphosphine (TPP) and other analytes with and without a protic acid. Results suggest the mechanism of field ionization of a charged droplet. This ionization technique was employed for various analytes including amino acids, pesticide and medical tablets. Various preformed ions were detected at the same conditions. Preformed ions were detected in both positive and negative ion mode.
In order to probe the ionization event in the carbon nanotubes, raman measurements were done before and after experiments. The red shifted D and G bands suggest the reduction of carbon nanotubes during this ionization event.
m/z
Intensity
[M + H]+
292
264
222
[M]
Conclusions
The results presented herein suggest a versatile strategy for the direct analysis of diverse chemical species. This method can be modified to suit various analytical requirements.
Replacement of the high-voltage power supply with a 3 V battery simplifies mass spectrometry through ion formation from a nanoscale antenna.
This nanotube ionization method has been applied to analyze a variety of samples from different sources, including fruit surfaces, tablets, and a range of organic molecules, including amino acids, antibiotics, and pesticides, at relatively low concentrations.
References
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Z. Takats, J. M. Wiseman, B. Gologan, R. G. Cooks, Science 2004, 306, 471 – 473.
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Acknowledgement: We thank the Nano Mission, Govt. of India, and the Department of Science and Technology for constantly supporting our research program. R.N. thank the University Grants Commission for research fellowships. R.G.C. acknowledges support from the National Science Foundation.
A–C) Detection of pesticides that were examined individually from the surface of an orange; carbofuran (A), methyl parathion (B), and parathion (C). The y axes correspond to the intensity.
Contact details:
Indian Institute of Technology Madras
Chennai , India