Mars: The Other Time-of-Flight
|Opportunity found ‘blueberry-like’ concretions scattered across the Meridiani plains. Click image for larger view.
Image Credit: NASA/JPL
Dr. David Ermer, with his company, Opti-MS Corporation, is currently constructing a miniature Time of Flight Mass Spectrometer that can detect biological signatures at a very high resolution and sensitivity, but yet be small enough to be used for robotic and human applications in space exploration.
Ermer is using an innovative system that he developed at Mississippi State University, and he has received a NASA Small Business Innovation Research (SBIR) award to continue his research to build and test his device.
A mass spectrometer is used to measure molecular weight to determine the structure and elemental composition of a molecule. A high-resolution mass spectrometer can determine masses very precisely, and can be used to detect such things as DNA/RNA fragments, whole proteins and peptides, digested protein fragments, and other biological molecules.
|Opportunity explores dunes in the base of Endurance Crater.
Image Credit: NASA/JPL
A Time of Flight Mass Spectrometer (TOF-MS) works by measuring the time it takes for ions to travel through a vacuum area of the device known as the flight tube. Time of flight mass spectrometry is based on the fact that for a fixed kinetic energy, the mass and the velocity of the ions are interrelated. "Electric fields are used to give ions a known kinetic energy," Ermer explained. "If you know the kinetic energy and know the distance the ions travel, and know how long it takes to travel, then you can determine the mass of the ions."
Ermer’s device uses Matrix Assisted Laser Desorption Ionization, or MALDI, where a laser beam is directed at the sample to be analyzed, and the laser ionizes the molecules which then fly into the flight tube. The time of flight through the tube correlates directly to mass, with lighter molecules having a shorter time of flight than heavier ones.
|This high-resolution color photo of the surface of Mars was taken by Viking Lander 2 at its Utopia Planitia landing site on May 18, 1979, and relayed to Earth by Orbiter 1 on June 7. It shows a thin coating of water ice on the rocks and soil.
Image Credit: NASA/JPL
The analyser and detector of the mass spectrometer are kept in a vacuum to let the ions travel from one end of the instrument to the other without any resistance from colliding with air molecules, which would alter the kinetic energy of the molecule.
A typical sample plate for a TOF-MS can hold between 100-200 samples, and the device can measure the complete mass distribution with one single shot. Therefore, huge amounts of data are created within a very short time interval, with the time of flight for most ions occurring in microseconds.
Ermer’s TOF-MS combines a relatively simple mechanical setup with extremely fast electronic data acquisition, along with the ability to measure very large masses, which is essential in doing biological analysis.
But the most unique aspect of Ermer’s device is its size. The commercial mass spectrometers that are currently available are at least one and a half meters long. That’s a fairly large volume to include on an in-situ scientific vehicle such as the golf car-sized Mars Exploration Rovers or even the larger Mars Science Laboratory Rover scheduled to launch in 2009. Ermer has devised a way to miniaturize a TOF-MS to an amazing 4½ inches long. He estimates that his device will have a volume of less than 0.75 liters, a mass of less than 2 kilograms and require less than 5 watts of power.
Ermer used a non-linear optimization technique to create a computer model of a mass spectrometer. There were 13 parameters he input that had to be selected, including the spacing of the different elements in the TOF-MS and the ion acceleration voltages. Using this technique Ermer was able to find some unique solutions for a very short TOF-MS.
|Artist conception of Mars long-range science laboratory.
"I’m trying to build a Time of Flight Mass Spectrometer that is small enough to actually go in space," Ermer said. "The main application that NASA is looking at is searching for biological molecules, to find evidence of past life on Mars. They also want to be able to do molecular biology on the space station, although the Mars application has a higher priority. My device should come in under all the requirements that NASA has, as far as the power, size, and weight requirements."
Ermer also sees potential for his device to be used commercially as well. "What I have is a portable device to measure biological molecules," he said. "If you were at an airport and found a white powder you’re going to want to know if it is anthrax or chalk dust fairly quickly. So you want a small, fairly cheap, portable device to be able to do that." In his proposal to NASA, Ermer stated, "The main (commercial) application for miniature TOF-MS is the screening of infectious disease and biological agents. We also believe that the superior performance of our design will allow penetration into the general TOF-MS market."
Ermer received the $70,000 SBIR award in mid-January, and has already built and tested a larger proof of concept design, which validates the technology that he designed for his TOF-MS. "So far, the tests have gone extremely well," Ermer said. I have detected molecules up to 13,000 Daltons (Dalton is an alternate name for atomic mass unit, or amu.) The device is operating as designed for masses up to 13,000 Daltons and has mass resolution somewhat better than a full sized device at 13,000 Daltons. We are currently working on detecting mass out to 100,000 Daltons and initial results are promising."
"Getting the device up and running is probably the biggest hurdle," Ermer said about the challenges of this project. "A lot of the hard things are done, but the electronics are really difficult. For this device you have to generate high voltage pulses of about 16,000 volts. That was probably the hardest thing we’ve had to do so far."
The electron multiplier detector is specially designed for miniature time of flight spectrometry by an outside company. Ermer and his own company designed most of the other parts of the device, including the vacuum housing and the laser extractor. Since it’s so small, creating these parts requires very high tolerance machining, which was also done by an outside company.
Ermer is a professor at Mississippi State University. He has been doing research in fields related to mass spectrometry since 1994, and for his PhD thesis at Washington State University, he looked at the energy distributions of ions that are generated in different materials by a laser. For his postdoctoral research at Vanderbilt, he studied the MALDI technique using an Infrared Free Electron Laser. More information about Opti-MS can be found at www.opti-ms.com.
Nancy Atkinson is a freelance writer and NASA Solar System Ambassador. She lives in Illinois. Reprinted with permission, originally published in Universe Today