SPI Plasma-Prep™ II Plasma Etcher, Cleaner and Asher
Standardizing the Performance of the PlasmaPrep II:
Sometimes it is necessary to standardize the performance of different
PlasmaPrep II units so that each one is giving the same results. Examples
are when the same protocol is being conducted in laboratories in different
countries, when a laboratory with a high sample throughput needs to obtain
the same results from different units in the same laboratory or when the
protocol must give the same results for samples prepared at different times.
The process of low temperature dry etching involves many variables, some of
which are easy to control and some of which are very difficult to control.
The obvious ones include the various settings on the instrument itself and the
gas supply, but there are others. One example of a problem in reproducing
results from one location to another is that the line voltage may vary from
location to location, causing the instrument to behave differently in each.
Another is that gas mixtures obtained from different vendors may not be
identical. Attempting to get the same results in different locations, from
different units in the same location or even from the same unit at different
times can be very frustrating. Obtaining consistent etching from run to run
can be a challenge because there are so many variables to control. As an
example, we find that the size and shape of the container holding the
specimen (for example a glass jar vs. a Petri dish) may be a more important
factor than any instrument setting.
Another example of a problem area is line voltage. Even with the same
nominal voltage, the actual voltage at the outlet may vary by several volts
from laboratory to laboratory, affecting the maximum power delivered to the
reaction chamber of the PlasmaPrep II.
Fortunately, one of the things that we have in our favor is that the
processes such as etching samples prior to SEM examination is very
forgiving. Similar results may be obtained even though there is a
considerable difference in the amount of etching which an individual sample
experiences. Best practice, however, is to standardize the preparation
procedure, and the problem was solved several years ago.
In the analysis of air samples for asbestos fibers, the fibers are captured
on a polycarbonate membrane filter,
and then it is necessary to etch away
part of the filter in order to increase specimen contrast. Not enough
etching gives low contrast in the TEM examination. Too much etching risks
loss of the fiber. It was decided that the analysis should be standardized
on etching away 10% of the filter, and a protocol was developed to allow
that to be done in laboratories all over the world, using equipment from any
manufacturer. This is the basis for the way in which we "calibrate" the
PlasmaPrep II units in our own laboratory. It involves the use of a
standard, reproducible sample. For TEM asbestos analysis, we calibrate using
polycarbonate membrane filters similar to those being used to capture the
suspected asbestos fibers, but for general use, and for a test that can be
conducted in laboratories almost anywhere, we use a piece of laboratory
filter paper.
In chemical analysis it is frequently necessary to capture something on a
filter and then to destroy the filter in order to recover the material. For
this purpose, there are filters, which leave essentially zero residue when
they are ashed to destruction. These filters are very consistent from batch
to batch and essentially identical within a particular batch. If we take a
portion of one of those filters (we use one fourth of a specific 47 mm
diameter filter, but the point is to standardize on a specific sample
material) and measure the time to completely destroy the filter under a
known set of operating conditions, then we can estimate the time necessary
to etch away a known portion of that filter, such as 10% in the asbestos
analysis example. If all of the laboratories involved use the same filter
material, then we can standardize an etching protocol to remove a specific amount of material,
and that protocol will give constant results from laboratory to laboratory
by keeping the etching conditions constant in each unit and varying the etching time to
give the same percentage of destruction of the filter. Whatever it is that
is being etched away, a standard amount of etching can be established that
will etch each sample the same, even in laboratories located around the
world from each other. For asbestos analysis, the experience in many
laboratories has shown that calibration once each month gives good
reproducibility from laboratory to laboratory. In addition, ashing a piece
of low residue filter paper to destruction gives a good benchmark of
instrument performance; if the filter paper is not destroyed by the plasma,
then something is not working properly in the PlasmaPrep II.
A similar protocol can be developed which uses a reference sample which is
more specific to the particular samples being etched, but the low residue
filter paper sample provides a good basis for evaluating and calibrating the
performance of the PlasmaPrep II anywhere.
To Ask a Question or Make a Comment
To Place an Order or Request a Quote
Return to:
Saturday July 04, 2009
© Copyright 2009. By Structure Probe, Inc.
Contacting SPI Supplies and Structure Probe, Inc.
All rights reserved.
All trademarks and trade names are the property of their respective owners.
Privacy Policy
Worldwide Distributors, Representatives, and Agents
