Del Mar Photonics
ANALYSIS OF BIOLOGICAL MOLECULES ON SURFACES USING STIMULATED DESORPTION
PHOTOIONIZATION MASS SPECTROMETRY
The ions were detected by drifting through the field free flight tube and reaching a pair of chevron configuration microchannel plates (MCPs) (Del Mar Ventures, San Diego, CA, USA) which magnified the signals by factors of 106 ~107.
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Rapid growth of biological, environmental, clinical, pharmaceutical and material sciences have led to dramatically increased demands for chemical and structural information regarding organic/biological molecules from complex systems. Many analytical tools have been developed and utilized to characterize and analyze biomolecules.1-6 Mass spectrometry (MS) is one of the most successful and popular techniques for the analysis of a broad range of analytes of interest. It uses different ionization methods to convert analytes into ions and separate them according to their mass-to-charge ratio (m/z) by various mass analyzers. Combined with separation techniques such as gas chromatography (GC), high performance liquid chromatography (HPLC) and capillary electrophoresis (CE), mass spectrometry is a very powerful tool to detect many compounds from relatively complicated systems. By employing proper ionization methods, mass spectrometry enables qualitative and quantitative analysis of many molecules with high sensitivity. By coupling high resolution mass analyzer with specific ionization sources, mass spectrometry provides accurate molecular mass and fragmentation data which allows the determination of elemental composition and chemical structure of analytes. Therefore, mass spectrometry is playing an increasing pivotal role in biological related research and applications. The further improvements of mass spectrometry in sample preparation, instrumentation, desorption, and ionization methods will be highly beneficial to high-throughput analysis of biological molecules in complex systems.
Stimulated desorption photoionization mass spectrometry (SD/PI MS) has been widely used in bimolecular analysis in recent years. This technique desorbs analyte molecules by photons or electrons and ionizes them by direct or indirect energy transfer. According to the different desorbing and ionizing sources, stimulated desorption photoionization mass spectrometry could be divided into three main categories: i) laser desorption/ionization mass spectrometry (LDI MS) in which both desorption and ionization are promoted by one laser; ii) laser desorption photoionization mass spectrometry (LD/PI MS) in which a laser desorbs the analytes and another laser ionizes the desorbed molecules by single photon ionization (SPI) or multiphoton ionization (MPI); iii) electron stimulated desorption photoionization mass spectrometry (ESD/PI MS) in which an electron beam is used to desorb molecules from the target surface and a laser beam is used to ionize the desorbed species.
The introduction of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) in the 1980s 7, 8 has dramatically improved the ability of stimulated desorption photon ionization mass spectrometry. In MALDI MS, samples are usually prepared by mixing with an excess amount (~104 fold) of matrix molecules that absorb laser energies. The use of matrix molecules is critical for success of the laser desorption/ionization. It serves to isolate analyte molecules from each other, to absorb the intense laser radiation, to vaporize and propel the analyte molecules into the gas phase, and subsequently to ionize the neutral analytes in the plume of the excited-state matrix immediately above the sample target.9 By using a time-of-flight mass spectrometer with a nearly unlimited mass range, MALDI MS has successfully detected large biomolecules and synthetic polymers up to 1.5 million Daltons (Da).10 In addition, MALDI MS also provides soft ionization (i.e. the parent ions dominate the mass spectrum with little or no fragmentation), high detection sensitivity and fast sample analysis. MALDI MS has become one of the cornerstones of the revolution in bioanalysis.8, 11-13
Although MALDI MS has been remarkably successful in making many advances in various fields, some the limitation still hinders its full development and application. A complete understanding of the MALDI phenomenon mechanism does not yet exist,14 which greatly affects the optimization of MALDI. The effectiveness of the matrix material is also not always intuitively apparent. Thus, the matrix selection is often obtained after an exhaustive empirical search.15 Poor shot-to-shot and sample-to sample reproducibility resulting from the crystalline matrix is another issue that must be dealt with for the improvement of MALDI performance. Finally and most importantly, MALDI produces a large amount of matrix background ions below m/z 600, which makes it impossible to analyze small molecules.
Increasing demands for high throughput methods in drug discovery, and biotechnology as well as analysis of complex mixtures in high salt matrices and buffers, has created great interest in utilizing the full power of LDI over the entire mass range of interest.16
In the studies presented in this dissertation, a versatile ultrahigh vacuum (UHV) analytical system was designed and constructed for the analysis of small organic/biological molecules using different forms of stimulated desorption photoionization mass spectrometry (details are described in chapter 2). The approach pursued in this thesis work is to carry out detailed studies which concentrate on the 3
surface chemistry and physics governing non-thermal desorption. The combination of this approach with sensitive laser detection schemes developed by the atomic and molecular physics communities has provided the paths for advancing bioanalytical techniques. Specifically, surface-assisted desorption/ionization (SALDI MS), laser desorption single photon ionization mass spectrometry (LD/SPI MS) and electron stimulated desorption single photon ionization mass spectrometry (ESD/SPI MS) methods were developed to characterize small thermally labile molecules in different sample environments. The high sensitivities of SALDI MS, LD/SPI MS, as well as ESD/SPI MS and the generalities of their applications suggests that these techniques may be used as widespread tools for the detection of small analytes in complicated biological samples.
SALDI-MS has been newly developed as a supplemental technique for MALDI MS. It utilizes specific target substrates with porous structures and high photon absorptivity to retain samples and generate soft ionization. Because this method does not add matrix molecules, it produces a clean mass spectrum in the low-mass range for easy characterization of small molecules. In chapter 3, SALDI MS on three different surfaces were studied to provide useful guides to its applications. The effectiveness of SALDI MS in analysis of the amino acids, small peptides, and organoselenium metabolites were also investigated to explore its potential of analyzing various molecules in complex systems. The amino acids, dipeptides and organoselenium are typical small molecules with great significance in many aspects and the fast and sensitive analysis of these molecules has been very challenging. In addition, the analysis of these molecules usually involves great amounts of sample which require a high-throughput method. Therefore, traditional techniques such as HPLC-MS and GC-MS which suffer from time consuming operation and methods development that often can not provide satisfactory analysis. The good performance of SALDI MS on analyzing amino acids, dipeptides and organoselenium metabolites illustrate a potential approach for simple, fast and sensitive analysis of small molecules.
Although SALDI MS has been successfully applied in many applications, its mechanism is not clear. To fully understand the mechanisms of SALDI MS, the effects of surface morphology, sample solvent, surface storage, surface temperature, sample acidity have been investigated and discussed in chapter 4. A proposed mechanism is given and this could lead the optimization of SALDI MS.
Based on the fact that natural desorption yields are typically 1000~10,000 fold greater than the ion yields and two-step laser desorption photoionization technique could provide better control in both the desorption and ionization processes, the LD/SPI MS method was developed for sensitive analysis of metabolites in human urine. This approach uses an ultraviolet (UV) laser to desorb intact neutral molecules and a vacuum ultraviolet VUV laser to ionize the desorbed neutral molecules by single photon ionization. The details of LD/SPI MS are discussed in chapter 5. This technique provides more efficient detection than secondary ion mass spectrometry (SIMS), direct non-resonant laser ionization (LDI), MALDI and SALDI. An HPLC-MS/MS method was also created for the analysis of organoselenium metabolites and it provided results similar to those obtained using LD/SPI MS. This study demonstrates the viability of matrix free LD/SPI MS for molecular characterization and quantitive analysis of biological metabolites in the m/z 10 ~ 600 range that are present in complex biological fluids.
By taking advantage of high efficiency, high sensitivity and uniform selectivity of single photon ionization, deoxyribonucleic acid (DNA) damage induced by low-energy electrons was investigated by ESD/SPI MS. This is discussed in detail in chapter 6. DNA damage caused by irradiation is of great importance to the application of radiobiology, public health, and clinical treatments. To understand the genotoxic effects and cell damage due to secondary species of high-energy radiation, the role of transient negative ions (TNI) and the specificity in LEE-DNA damage were studied by examining the neutral product yields using ESD/SPI MS. The neutral yields as a function of incident electron energy were also correlated with the SSBs and DSBs measured using post-irradiation gel electrophoresis. The results indicate that resonances involving the oxygen of the phosphate-sugar linkages and the surrounding water molecules may contribute to the DNA damage. Careful measurements of the role of intrinsic water and any sequence dependences of the strand break probability are still underway.
Another feature of this home-build TOF was its ability of obtaining mass spectrometric imaging of analyte ions by using a MCP/phosphor screen (MCP-IFP46/2; Del Mar Ventures, San Diego, CA, USA) as an imaging detector. The TOF was mounted on a special six-inch flange with a view port. A supporting plate was designed to hold the MCP/phosphor screen detector right above the window. A high-speed cooling CCD camera (Sensicam QE; Cook Corp., Auburn Hills, MI, USA) was focused on the P46 phosphor screen which emitted fluorescence (490 ~ 620 nm) with a fast 90%-to-10% decay (300 ns). By using proper pulse sequence and image collection parameters, the images of different ions could be recorded by the CCD camera. At the same time, the voltage change on the second MCP plate which reflected the flight time and intensity of detected ions could also be obtained by utilizing an oscilloscope through a RC circuit (Figure 2-3). Therefore, both of the whole mass spectrum and images of individual ions were able to be observed by this imaging TOF mass spectrometer (Figure 2-4).
Microchannel Plates, Detectors
and Imaging Systems
Examples of research applications:
Studies of the atomic clusters at the University of Virginia - Amber Post
Featured MCP customer: The Castleman Group at PSU
MCP home - MCP references
MCP-GPS-46/2-CF6" Open MCP imaging detector mounted on CF6" flange - MCP-GPS and MCP-IFP imaging detectors
MCP-MA - Detecting short proton beam from a picosecond CO2 laser ionized H2 plasma
MCP-MA25/2 are used in aSPECT to study the background
MCP setup for velocity map imaging apparatus
Microchannel Plate Detector (MCP) setup for Plasma Desorption Mass Spectrometry (PDMS)
MCP detector for high resolution ion time-of-flight analysis for measuring molecular velocity distributions
X-ray detection system based on the MCP/phosphor screen assembly
MCP + phosphorous screen for imaging of XUV radiation (14eV- 160-eV) in high harmonics experiments
Exchanging MCPs in Time-of-Flight detectors
Microchannel Plates, Detectors and Imaging Systems - Open MCP-MA - MCP-MA applications - MCP-MA assembled - Applications
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Del Mar Photonics, Inc.
4119 Twilight Ridge
San Diego, CA 92130
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