2010 Annual Science Report
University of Wisconsin Reporting | SEP 2009 – AUG 2010
Project 4B: Development of Laser Ablation-Miniature Mass Spectrometer (LA-MMS) for Geochronology and Geochemistry of Martian Rocks
Our goal is to develop a breadboard instrument for isotopic analysis of solids and age dating of different rocks based on Rb-Sr radiometric technique. This is based on the methodology of laser ablation-miniature mass spectrometer (LA-MMS). It performs the mass spectral and isotopic measurements of the laser ablated vapors from solids using the miniature mass spectrometer (MMS) and the modified CCD based array detector for the direct and simultaneous measurement of different mass ions. The approach has been demonstrated at the Jet Propulsion Laboratory by the chemical and isotopic analysis of gas and solid samples. The breadboard version of the above instrument can be miniaturized to meet the requirements of a rover based spacecraft instrument for applications to various NASA missions.
The methodology of the LA-MMS is based on: (1) laser ablation sampling of minerals, (2) electron impact ionization of the ablated neutrals, and (3) their mass spectral measurement using the JPL developed miniature mass spectrometer (MMS) of focal plane geometry with modified-CCD array detector. The ions of different masses separated along the focal plane are measured directly by this unique CCD array detector simultaneously. Figure 1 shows the technical approach schematically.
The novelties of the approach lie in both the method of sampling by LA and isotope ratio measurements by the miniature mass spectrometer.
LA provides a method of sample introduction into the mass spectrometer (MS) with minimal sample manipulation and sample preparation. Incidence of a high-energy laser pulse on a solid produces a plume consisting of ions and neutrals of the material. The chemical analysis of the vapor can be made by the mass spectral measurement of the ions or of the neutral species. However, laser ablation produces predominantly the neutral species (2-6 orders of magnitude greater than ions), and, therefore, expectedly better represent the true composition of samples. Their mass spectral measurement will provide true chemical and isotopic composition of minerals. Moreover, the ion generation in the ablation process strongly depends on the surface composition (matrix effect) and on elemental ionization energies. Consequently, in the LA-MMS instrument the ablated neutral species are analyzed for chemical and isotopic measurements. Ionization of the neutral is made by electron impaction. The electron impact ionization possesses a well-characterized ionization probability for chemical species. It is easier to implement and is of general applicability.
The secondary ion pulse produced after the post ionization is typically 10 – 100 μs wide in time. The brevity of the ion pulse makes a scanning type of MS and time-of-flight unsuitable for mass analysis due to their low duty cycle. The JPL-developed MMS provides a 100 % duty cycle because of its focal plane geometry and array detector and is uniquely suited for the analysis of the laser ablated volatile pulse. MMS consists of a double sector mass analyzer, and an array detector, and a low power ion source. Ions of different masses are spatially separated along the focal plane located at the exit of the magnetic sector of MMS. Fig. 2 shows the photograph of the miniaturized analyzer and array detector of the MMS instrument.
The intensities of ions along the focal plane are measured simultaneously for the entire duration of the neutral/ion pulse width by a JPL-invented technology of modified CCD array (2140 pixels with each pixel of 20 μm x 2000 μm) as discussed below. Elements of the array detector are capacitative, integrating the signal for the prescribed time and thus provide 100 % duty cycle. The combination of the above analyzer and the CCD array enables the most efficient use of the signal and is uniquely suited for isotopic measurements of laser ablated neutral pulses.
The direct ion detection based on the technology of the modified-CCD array was invented at JPL
A standard three-phase CCD process technology is used to create a linear array of CCD pixels. In each pixel of the CCD array the normal photodiode used for photon detection is replaced by a capacitive sensing element that serves as the ion detector area. This detector area is comprised of two layers of conductive material, such as aluminum, insulated from each other and from all conductive paths to device substrate (see Fig. 3).
Each capacitive sensing element is coupled to the CCD shift register by means of a charge-mode-input structure, typically known as a “fill-and-spill” input structure that senses the charge collected on the capacitive sensing element and creates a packet of signal charge proportional to the charge on the capacitor.
A major milestone demonstrating the feasibility of LA-MMS methodology for the chemical analysis of rock samples was achieved last year in our laboratory at JPL. We have measured the mass spectrum of the neutral species in the vapor plume generated by laser pulse from the rock sample. The results and the method of their measurement are described below.
In our earlier measurement of the mass spectra of laser ablated vapor from the rock samples, we had encountered two major complications. These included
- Interference in the mass spectra from the contaminants
- Effect of plasma generated by the laser pulse-solid interaction on the ion source potentials of the MMS
In order to overcome the above problems the vacuum chambers were modified. The system was cleaned. Most of the o-ring seals replaced with the copper gasket seals and arrangements for baking the vacuum system were made. The modified vacuum system is shown in Fig. 4. The MMS chamber and the rock sample were housed in separate chambers. The two chambers were separated with a plate having a 2 mm diameter orifice (see figure 4c) for the introduction of the laser generated neutral species into the MMS. This also suppressed the effect of light on the ion detector array. The ions of the plasma were deflected by the application of high voltage (500-700 v) on deflection plates located in the sample chamber (Fig. 4c). Q-switched Nd-YAG laser pulses ( λ = 1.06 µm, energy = 100 mJ per pulse, pulse width = 6 ns) were used for the ablation of rock samples. Figure 5 shows the photograph of the laser generated plasma from orthoclase sample. The laser in the present setup is incident at an angle of ~30o to the surface of the sample. It was observed that the plasma is peaked along the normal to the rock sample surface. However, after the incidence of 2000-3000 laser pulsed the plasma is more directed along the incident laser direction. This is believed to be due the creation of a channel directed along the incident laser direction that preferably directs the ablated material along this direction.
The vapor generation from the sample surface by a laser pulse takes place in < 100 μs. Therefore, the mass spectrum signal should be integrated for this period of time only by the CCD ion detector array. Any longer period of signal integration acquires only the signals originating from the residual contaminants in the chambers. The minimum signal integration time for the CCD detector in the present system is 200 ms. This is dictated by the 5 Hz laser shot frequency. In order to suppress the signal integration by the detector after 100 μs of the laser pulse, an electronic timing circuit was designed and implemented such that after 100-300 µs of the laser pulse a voltage was applied to the one of the ion collimating slit of the MMS to disable any ion arrival to the detector. This enabled the mass spectral measurement of the ablated plume for the entire period of its generation while eliminating the background contribution to the mass spectral signal after 100 µs of the laser pulse for the remaining 200 ms period of the signal integration time.
5. Results of Mass Spectral Measurements from Orthoclase
We have made mass spectral measurements of different rock samples by laser ablation-miniature mass spectrometer. The samples were places in the sample chamber (see Fig. 4c). The laser ablated vapor pulse expands into the MMS chamber (4b) through an orifice of 2 mm diameter in the metal plate separating the Sample chamber and the MMS chamber. The ions in the plasma plume were deflected electrostatically as shown in Fig. 4c. The neutral vapor species after ionization with electron impaction in the ion source were mass analyzed by the MMS and CCD direct ion detector array. The different mass ions are spatially separated along the focal plane of the magnetic sector. The intensities of the ions are simultaneously measured by the modified CCD array detector. CCD detector is an integrating detector and integrates the ions impinging on them for the specified period. The signal ions were integrated for 200 µs after the laser pulse hitting the sample target. Results of mass spectral analysis of orthoclase by the LA-MMS is shown in Fig. 6. The peaks mass number and the corresponding elements are indicated in the figure.
The mass spectrum contains the peaks of elements of the orthoclase sample only and demonstrate that the above measurement methodology eliminated the complications arising from the contaminants and ions from the laser ablated plasma. Table II lists the relative intensities (the measured molar ratios of elements in orthoclase) of different masses/elements in the sample. The intensities were corrected for the difference in electron impact ionization cross sections (see table I). These ratios are compared with their calculated values from their weight percentages of oxides in orthoclase.
The difference in the measured elemental ratios from the calculated ones can be accounted for the by the refractory nature of elemental oxides in orthoclase (shown in table II) and their absorbance of the laser wavelength (1.06 μm).
M.P.(C) 1723 2230
B.P.(C) 2072 2977
We believe that that the selection of shorter wavelength laser with the optimization of laser power density incident of the rock sample will minimize the difference in the measured molar ratios. This is the primary goal of our work for the next year. The analysis of sample containing Rb and Sr will be performed. The sensitivity of the detector need to be improved for the analysis of sample containing ppm levels of elements.
The milestones of the last year were successfully achieved as summarized below.
- Overcome the laser pulse effect on the ion source and detector
- Effect of contamination on the mass spectral measurements of laser ablated neutral species were minimized
- optimal timing sequence in the mass spectral were implemented.
- A major milestone demonstrating the feasibility of LA-MMS methodology for the chemical analysis of rock samples was achieved
PROJECT INVESTIGATORS:Brian Beard
Project InvestigatorMahadeva Sinha
PROJECT MEMBERS:Michael Hecht
RELATED OBJECTIVES:Objective 2.1
Biosignatures to be sought in nearby planetary systems