On Tue, 20 Jun 2000, Jim Buckman wrote:
> Hi,
>
> We have a Philips XL30 Environmental SEM with full edx analysis facility
> (boron upwards). We mainly use this for petroleum related research. However,
> given the lack of preparation required with ESEM (no coating required), and
> the ability to carry out EDX analysis at 20 kV without causing charging
> artifacts, it is likely that the ESEM could be used to look at
> archaeological specimens, where it is desirable to leave the sample in its
> natural unaltered state. Has anybody on this list any experience in using
> ESEM for such work?? If so I would be interested in hearing from you. Also
> if anybody has any ideas that they would like to try out in ESEN, please let
> me know, and perhaps we can arrange something.
We have had the use of an environmental SEM (an Hitachi 2400N) for three
years and have accumulated a considerable amount of experience on it,
mostly in the analysis of historic and prehistoric metals, glasses and
slags. The ability to examine specimens without coating saves a huge
amount of time, and we now do almost all our work on the N-SEM. There are
several drawbacks.Performance at high magnification is obviously
seriously degraded by scattering from gas molecules in the chamber, so if
we need to go above 5000x we coat and use the field emission SEM instead.
Beam broadening due to scattering can also do some very odd things when
one is doing chemical analysis by EDAX. We were puzzled at first by the
consistent presence of aluminium in samples, especially in minerals that
shouldn't have any (e.g. fayalite) but eventually realized that this was
due to scattering of primary electrons onto the column, frame of the
detector, sample holder etc. Beam broadening is also a real problem at
high column pressures (around 50 mPA) when trying to analyse small
inclusions - beam broadening leads to analysis of a much larger area than
one appears to be analysing. The solution is to pump down to as low a
column pressure as one can without encountering charging. This is sample-
specific, but for non-conductive samples such as slags we are often able
to get away with column pressures of 5 mPA.
Finally, as with all SEMs, beware of the standardless quantitative
analysis programs from Noran, PGT, etc! We have analysed standards of
metals and glasses under environmental SEM repeatedly and as a result
cannot regard the analyses produced as accurate to better that 10%
----------------------
David Killick
Associate Professor, Department of Anthropology,
University of Arizona, Tucson, AZ 85721-0030.
Phones: office (520)621-8685; laboratory 621-7986; fax 621-2088
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