Guide to TEM Grid Selection
How to select the grid that is right for you.......
Generally speaking, all TEM grids can be segmented in a number of different
ways:
Square mesh
Hexagonal and diamond mesh
Slots and Holes
Parallel
Folding
Tabbed
Index (or Finder)
Material of construction (etched metal,
beryllium,
diamond,
nylon,
precious metal)
Uniquely numbered (for forensic work)
Calibrated (for asbestos work)
Membrane window
Also, consideration has to be given, depending on the type of work being
contemplated, to % open area of the grid and also, the grid thickness. Some
grids are designed for use specifically at high temperatures, whereas others
are designed to not give extra x-ray peaks in an EDS spectrum.
To start with, most users find that standard square mesh grids, copper, are
as good as any for them, and the most common grid meshes purchases are 200,
300, and 400 mesh. Some find the SPI
"standard" or "regular" mesh grids to be quite acceptable and they also
enjoy the advantage that these are the lowest cost grids that can be
purchased today.
But just like higher quality in anything is often times "worth the extra
price", and in this case, the "extra price" is hardly anything at all, some
would want to consider the
SPI Super™ Grids.
At the 200 mesh level, for example, the grid bars drop from 30 µm to 20 µm
and the % open area increases from 55 to 70%. The higher open area
generally makes it possible for work to be done faster and with fewer
frustrations, since there is a lowered likelihood that an important feature
will be covered by a grid square. However, keep in mind that the hole size
is going to be larger, and a support film or section that is marginal in
stability for a regular grid, might not be stable on the Super Grid. Of
course, one could also drop down one mesh size lower to get a more stable
film and still enjoy some of the benefits, such as higher open area.
And one can go even further up the quality ladder to the
SPI Slim Bar® Grids.
At the 200 mesh level, the grid bar now drop down to 10 µm and the % open
area increases still further to 84%. And the convenience factor increases
further as well, since work can be done faster and with a frustration level
even lower. For those doing quantitative TEM, for example, for the counting
of fibers, since a fiber overlapping a grid bar can not be counted
(since no one knows the ultimate length), with less grid bar length per
unit area, there is a further reduction in the possibility that fiber counts
would have to be thrown out for that reason. Naturally there are numerous
other reasons why one would benefit from a grid of maximum open area.
The SPI made-from-diamond grids are
preferred by those who are concerned about any toxicity issues surrounding the use of beryllium grids. And for
nanoparticle work, the silicon nitride membrane window
grids are clearly the grid of choice.
However, some researchers prefer the geometries of other grid patterns, for
example, parallel grids, or even hexagonal grids.
How grids are manufactured:
Most grids used in the world today, and we would estimate that to be more
than 95% percent, are either copper, nickel, or gold. These three metals
can be electro deposited and the quality from an electro deposition
technique is far superior than what is possible when grids are etched from a
thin metal foil (e.g. beryllium,
molybdenum,
stainless steel,
titanium,
aluminum).
At one time, platinum and silver grids were made, and they could also be
electrodeposited, but because of low volumes and high unit production costs,
they just could not be made profitably, and therefore, what remaining grids
of platinum and silver that remain in the SPI inventory, are all that there
are and when they are finished, they will no longer be available.
So if one wants a grid that has very smooth and regular grid bars, of
maximum open area, then they pretty much have to confine their thinking to
electro deposited grids as any kind of etched grids would not be acceptable
to them.
Copper vs. Nickel vs. Gold:
As indicated above, copper is the grid of choice for most users. It is also
the cheapest and has the further advantage, relative to nickel, that it is
not magnetic. But copper is also reactive in low pH environments and
therefore, the higher resistance to chemical attack of nickel, makes a
nickel grid attractive for some applications. For immunogold work, where
the post embedding reactions are done in low pH conditions, copper can not
be used.
However, because of the magnetic nature of nickel, some works never seem to
master the problem of grids "sticking" to the normal antimagnetic stainless
steel tweezers. And even though SPI has long offered the
SPI Miracle Tip® tweezers, which are 100% antimagnetic, which does
enable one to work with Ni grids without encountering this "sticking"
problem, some users prefer just to switch to gold grids. Of course, gold is
inert and won't react either, however because of the softness of gold, the
grids tend to be a bit less dimensionally stable and for that reason, there
seems to be a preference for using Ni grids with Miracle Tip® tweezers.
Use of "slot" or "aperture" grids:
These grids are used for specimens where maximum open area is needed
without interference from grid bars.
Other materials of construction:
When EDS studies are performed, there is a preference to use grids made from
materials that won't contribute x-rays to the EDS spectra, and complicating
the interpretation of data, and for that reason,
beryllium grids are often times the grid of choice.
Because of the potentially toxic nature of beryllium, some organizations
prefer their researchers use alternatives, such as either
nylon, diamond, or
composite carbon grids. However, these alternatives do suffer from the
drawback of high Bremsstrahlung background radiation (except diamond).
High temperature studies:
Historically, for high temperature studies,
grids made from molybdenum (and sometimes tungsten) were preferred because of their low coefficients of
thermal expansion, which is important if a supported specimen or even carbon
film is not going to rupture because of differences in thermal coefficient
of expansion. And these grids are still used today for high temperature
studies. However, a more modern and more robust alternative is available,
through the
SPI Silicon Nitride Membrane Window Grids.
SPI Supplies can produce custom coated grids on virtually any grid that
might be selected by the customer. Just remember that not all grids can be
coated as easily as others, and this translates into "yields". For some
types of grids, taking some types of coatings the yields are reduced, and
therefore the cost associated with the application of the custom coating is
increased. The SPI custom coated grids using carbon, carbon/Formvar, or
even Formvar alone, both as continuous films, as well as holey types are
very well know around the world and the SPI grids are used by many of the
world's leading laboratories. Although we have given some examples of
coated grid pricing, just remember we can coat just about any grid with any
kind of coating resin, just tell us what you want and
we can return with a firm price quote.
Mass measurements on grids:
Each grid pattern seems to have its own mass and there is no one figure that
would describe the number of grams in a particular grid since it would vary
from pattern to pattern.
But two representative weights:
SPI #2145C-XA G1000HS Fine Slim-Bar Square Mesh Copper
Mass per 100 grids: 0.22287g
SPI #2020C-XA G200 Square Mesh Copper
Mass per 100 grids: 0.078g
Nickel has a density that is very comparable to copper and that difference
is probably less than the experimental error associated with the making of
the measurements. The multiplier for gold is ~ 2.16.
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Friday July 04, 2008
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