Overview:
The APPLE system is designed to permit the in situ pulsed
laser evaporation and deposition of sample materials in ambient
atmospheric conditions. Samples are placed at the focus of
a miniature laser and objective system, near the end of a
manually loaded sample carriage. The sample holder is oriented
such that the laser is incident at 45 degrees onto the top
of the sample. A substrate tab (small, thin pieces of various
metallic and insulating materials) are mounted to a fixture
oriented to capture sputtered particles ejected in a range
centered on 45 degrees from sample normal. The goal is to
understand the degree to which the molecular composition of
the sample target survives and/or is fractionated by the laser
evaporation and ambient transfer processes. After a number
of laser shots have been fired on a given sample/substrate
configuration, the substrate tab is removed and stored for
later laboratory analysis (e.g. coupled with a mass spectrometer).
By carrying out in situ testing of APPLE in the Antarctic
environment, we can further test the robustness of the instrument
under a variety of conditions (windy, cold, dry). We will
use this knowledge to design a system that desorbs/ionizes
molecular compounds with little or no fractionation from a
variety of substrates including rocks, minerals and ices.
Our ASTEP APPLE instrument for this year’s field season
in Antarctica is shown in Figure 1:
Components indicated in Figure 1:
1.1 Laser power supply
1.2 Laser umbilical
1.3 Laser head and sample stage
1.4 Laser head pivot stand
1.5 Main battery, 12V Panasonic Pb-acid
Firing a single shot onto some burn paper to test the laser
intensity, spot size and position through the objective lens
is captured in Fig. 2. The shape is not perfectly smooth,
but is rather oval-shaped due to the 45o projection. The spot
size is ~1-2 mm across the central region.
Through a few simple adjustments, a wide range of intensities
can be achieved on samples of different sizes/heights. Generally,
it is expected that the desired intensity of the laser at
the sample surface should be much lower than that used to
calibrate the instrument in the laboratory.
Operating Procedure:
The main application for APPLE is for sputtering from small
“chip-type” samples (rocks, large grains, ice
chips, etc.) that are up to 1 cm across, a few mm thick, and
fairly flat on top. An example rock chip is shown in Figures
3,4:
1. Each end of the Cu-colored carriage has a sample mounting
ring and a 45-degree substrate mount. The sample-facing end
of each substrate mount has a stainless steel shim-stock mounting
plate that is affixed to the mount with superglue. This plate
is quite thin, so take care around it. The purpose of the
plate is to provide a smooth surface to attach the substrate
tabs.
2. Each “run” corresponds to a fixed set of (i)
focal conditions (intensity, spot size); (ii) sample; and
(iii) substrate. The substrate tab is likely to be the most
frequently changed element. However, in the present prototype,
it is not possible to change the substrate (must be removed
manually) without disturbing the position of the sample. However,
with careful replacement of the sample after substrate changes,
each run can be thought of as approximately independent.
3.
4. Clean the front side of the substrate tab (square chip)
with available means. Ideally, H2O rinse, antistatic cloth,
ultra-sonication, isopropanol and H2O rinses, and lamp dry.
Less-than-ideally, rinse, wipe, and air-dry. Use the tweezers
and avoid contacting the front.
5 . Attach the tab (back side) to the substrate mounting plate
with a small piece of 2-sided tape. The best place for the
tab is such that its bottom edge will be just touching or
slightly above the sample surface (see Figure 4). Most samples
can be accommodated if the tab is place just below the center
of the plate.
6 . Slide the carriage (without any sample) onto the mounting
stage up to the point shown in Figure 4, just past where the
carriage begins to resist further motion along the stage or
Several substrates for samples attachment will be tested in
the field:
a. Stainless steel shim stock squares (material identical
to the mounting plate). These are likely to be of relatively
low efficiency for deposition of intact organics from the
sample. However, these conductive substrates are most easily
adapted to later analysis by mass spectrometry.
b. Single crystal sapphire sputtering substrates.
c. Grooved silicon chips from Potomac Photonics. These chips
each have a set of parallel micro-grooves machined with a
laser-etching tool. The width of the grooves is typically
between 20 and 50 microns. The depth of the grooves is typically
1.5 times the width. The original purpose was to entrap and
localize particulates/fines (on the 1-20 micron size scale)
within an automated sample acquisition system. These may prove
useful for the APPLE application because the grooves may help
increase survivability and retention of organics within sputtered
“melt” particles that deposit within the confined
space. Later analysis may also be enhanced because, after
cleaning the top surface of the chips, the source of ions
from direct desorption is highly localized, a requirement
for high mass resolution and reproducibility of peak positions
in laser desorption mass spectrometry or LDMS.
We will likely encounter some interesting problems when testing
Apple in a variety of Antarctic conditions. Thus, we are prepared
to try several operational configurations and mounting/sampling
procedures including:
1. Use of larger samples placed on a flat surface (rock/ice)
below the stage.
2. Use of different angles (other than 45 degree incidence
and ejection).
3. Use of sample rocks/ice in situ, with the laser head held
by hand.
4. New methods to improve the adhesion of particulates onto
the substrate tabs.
5. New methods to bring the substrate tab surface closer to
the focal point on the sample in a controller way, if it appears
that the evaporated density is too low to reach the substrate.
6. Removal, replacement, or modification of the sample mounting
rings and/or the substrate mounting tabs for these scenarios.
Careful testing of our ASTEP prototype APPLE instrument in
Antarctica will help guide the development of our brassboard
instrument that will include a mass spectrometer for direct/in
situ analyses of organic compounds from a given sample substrate
autonomously (without human intervention) in the third year
(2005/2006) of our ASTEP proposal. In addition, we will sample
materials and assess these samples in the laboratory during
the development of our next generation instrument. By incorporating
instrument development with a field program, ASTEP has enabled
scientists to take the next step towards assuring success
in our evaluation of life on other planets and moons like
Mars or Europa.
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