The goal of this proposal is to develop a ?Coded Aperture Micro Mass Spectrometer? for ubiquitous real-time detection of environmental hazards, chemical warfare agents and biochemical markers. This will be accomplished by combining novel microfabrication technologies for ion sources and detectors with coded aperture optics and algorithms. Achievement of such a micro mass spectrometer will enable a new paradigm of sensing which simultaneously provides high sensitivity and the ability to detect a broad range of species.
Intellectual Merit: The intellectual merit of the proposed work includes advancing the state of the art of coded aperture spectroscopy by confirming its value in ion-based systems and developing algorithms for magnetic sector mass spectrometry. Determining the interaction of ions with coded apertures and optimizing the algorithms for such an ion-based system also holds scientific interest for future work in computational sensing in ion based systems.
Broader Impact: This research will have broad impact on future generations of ion-based instruments which desire throughput and S/N enhancements. The ion coding concepts are not limited to any one form of spectrometry and apply to all particles with mass and/or charge. The educational impact of this proposed research will also be very high for both graduate and undergraduate students. It is a truly multidisciplinary effort requiring an understanding of electrical engineering, physics, materials science and chemistry; thus, it is excellent training for future scientists. It will leverage the Pratt fellows program at Duke to bring research experiences to undergraduate students.
Intellectual Merit The Coded Aperture Microfabricated Mass Spectrometer (CAMMS) project showed the implementation of spatial coded aperture methods for mass spectrometry for this first time. Additionally, the microfabricated ion source that was developed as a part of the work operated for over 80 hours. Spatial Coded Apertures for Mass Spectrometry. The easiest way to describe coded aperture mass spectroscopy is through its analogue in an optical system: the pin hole camera[1]. Imaging through a simple pinhole can provide excellent resolution but it does so at the expense of throughput, thus requiring an extremely bright image source. One can increase the size of the pin hole to insure enough light is collected but this deteriorates the image resolution. However, one can imagine numerous small, closely-spaced parallel pinholes creating many identical images shifted in space. Each of these images individually has the resolution of a single small pin hole but lacks reasonable brightness. However, if the images are added together, they will have the brightness of a large pin-hole with an area equivalent to a summation of all of the pinhole areas and yet the resolution of a single small pin-hole. By analogy, the slit in a conventional magnetic sector Mass Spectrometer provides resolution by limiting the ambiguity of the location from which ions enter the magnetic sector; however, it simultaneously limits the throughput of the system and thereby minimizes the intensity of the signal that reaches the ion detector. A simple implementation of a multiplexed coded mass spectrometer is shown in Figure 1. A position sensitive detector captures ions from the coded detector, effectively sampling a series of spectra that are displaced according to the ion position on the aperture. The signal is then computationally decoded into a single spectrum with the resolution of a single narrow slit but the intensity of a large slit with high throughput. The improvement from an 8x8 Hadamard array over a single slit is shown in Figure 2. This figure shows the mass spectrum for Argon and a trace amount of air. The main peak is the 40 amu peak of argon. The single slit is used in a traditional instrument. The width of the slit affects resolution and signal. A thinner slit results in better resolution, but worse throughput. The detector images are shown in Figure 2(a). The coded aperture spectrum is more complicated, but can be computationally deconvolved to yield the spectrum in Figure 2(b). The measured gain is 1.8x that of a single slit. The ratio of total opening area between Hadamard 8x8 and single slit is 2, therefore the expected Hadamard 8x8 throughput gain is 2x. Microfabricated Ion Source. The lifetime of the ion source is a critical component of the microfabricated system, with the carbon nanotube (CNT) cathodes as the expected point of failure. A lifetime test was conducted at 10-7 Torr in argon. The test configuration for the CNT cathodes is shown in Figure 3. An SEM of the structure is shown in (a), the operation of the structure is shown in (b) a typical current-voltage curve showing diode operation is shown in (c), and lifetime data is shown in (d). Broader Impact The microfabricated ion source has enabled a novel bipolar vacuum microelectronic device [2]. This device is inherently resistant to electromagnetic pulse and radiation and will allow arbitrarily complex circuits to be built. Additionally, several applications are enabled by the microfabricated ion source, such as ion gauge pressure sensors and ion pumps. The coding algorithms that have been developed as a part of this work have led to investigating coding approaches on other ion-based systems. In particular, coding investigation has been initiated for a commercial ion trap and ion mobility spectrometer. References 1. London, B., Photography. 7th ed. 2002, Upper Saddle River, N.J: Prentice Hall. 426. 2. Stoner, B.R., J.R. Piascik, K.H. Gilchrist, C.B. Parker, and J.T. Glass, A Bipolar Vacuum Microelectronic Device. Electron Devices, IEEE Transactions on, 2011. 58(9): p. 3189-3194.
