Miniature mass spec for space: Prototype dual laser TOF for aromatic organic compounds

From one-step to two-step laser operation

Miniature mass spectrometers in space research are not unusual. In fact, there is an active mass spectrometer on Mars right now, aboard the Curiosity rover. It is a quadrupole instrument using electron ionisation to create the charged species for detection, and it will be helping to identify atmospheric gases and any organic compounds present in the rocks.

One of the more successful mass spectrometric techniques applied to space research is two-step laser desorption/ionisation mass spectrometry (L2MS), which has yielded good data on aromatic organic compounds in meteorites and interplanetary dust particles from the NASA Stardust mission. The analyses were conducted in labs on Earth over the last 20 years or so, but scientists in the US have now made it possible to carry out similar experiments on other planets.

Stephanie Getty and co-researchers from the NASA Goddard Space Flight Center, Greenbelt, MD, and C&E Research, Inc., Columbia, MD, have succeeded in modifying an existing miniature mass spectrometer to an L2MS instrument. Given its diminutive dimensions of 20 cm long by 5 cm wide and a projected weight of 5-6 kg when optimised, it will be ideally suited to become part of a space mission payload.

More bits in the box

The original mass spectrometer incorporated a laser desorption/ionisation unit with a cylindrical, curved-field reflectron. It carried out desorption and ionisation in the same step, which is effective for analysing minerals and mixtures of simple organic compounds. However, it is not so good for compounds like polycyclic aromatic hydrocarbons (PAHs) which are key interstellar compounds that have been detected in meteorites and cometary dust. Their laser ionisation mass spectra become complicated by the presence of common fragment ions while the dynamic range of the instrument can be insufficient to measure the complete spectrum.

Splitting the desorption and ionisation steps in L2MS goes a long way to solving this problem because each stage can be independently optimised, reducing the overall energy input. So, the researchers fitted two lasers to the instrument, an IR laser operating at 1064 nm to desorb samples and a UV laser operating at 266 nm for ionisation of the molecules in the desorbed plume. The ionisation laser was directed at right angles to the desorption laser, with its focal point aligned with the mass spectrometer inlet. The samples were mounted on a moveable probe so that the distances from the sample to the inlet and from the sample to the ionisation laser beam could be adjusted. The ions were extracted into the reflectron for analysis using a voltage of 5 kV, which is relatively low, an important factor when power requirements are considered for a space-bound instrument.

PAH spectra

The performance of the modified instrument was tested using pyrene, a representative PAH which has been observed in space. As the UV laser energy is shifted from just below to just above the ionisation threshold, a strong signal for the pyrene molecular ion appeared. At higher energies, fragmentation began to occur until most of the molecular ion had disappeared. So, by careful selection of the laser energies, the molecular ions can be optimised with minimal fragmentation, before being selectively fragmented, making it easier for compound identification.

The performance of the instrument was encouraging, despite the small size and the gap of just a few mm between the sample and the inlet. The mass resolution for the region around the molecular ion at m/z 202 was determined to be 380 at full width at half maximum. Another interesting feature of the spectra was the omission of peaks originating from sodium and potassium ions. These peaks dominated the spectra obtained by direct laser desorption mass spectrometry and originate from the ubiquitous salts that are present in natural samples and standard solutions. Their absence was attributed to the ability of the neutral sodium and potassium atoms to travel out of the ionisation volume before the second laser is fired, resulting in simpler spectra.

The L2MS instrument shows great potential for use in space missions to other planets as well as comets and asteroids, aided by its compact dimensions, low power requirements and relatively high resolution. It should allow the detection of a range of organic compounds by tuning the energies of the two lasers towards particular targets. The next stage for the researchers is to establish the sensitivity and selectivity of the miniature mass spectrometer so that the performance characteristics can be fully established.