Dear Mike,
At 12:19 -0300 20/8/08, Mike Lumsden wrote:
>Dear AMMRL - We have an Bruker AV-500 here that works predominantly under
>an automation environment using a B-ACS 60 with ICON-NMR. I am sure what
>I am about to say will come as no surprise but a major source of
>inefficient use of the spectrometer concerns the observation of the 13C
>nucleus and setting the number of transients.
I have used a scheme for open-access 13C observation based on S/N checks
for nearly fifteen years. The basic 13C program does 256 scans and then
checks the S/N. If it's above a certain threshold (100 is usually OK) the
accumulation is not resumed. If it's below the threshold then another cycle
is done, adding to the total data, then the S/N check is done again. In
addition to this the program looks at the clock time. If the time when the
run starts is between 07:00 and 17:00, Monday to Friday, then only one
cycle is done. If it's night-time (17:00 to 07:00) or a weekend then the
run continues for an absolute maximum of ten cycles.
These are just the bones of the scheme. As well as the BB-decoupled
experiment it will, if required, do the DEPT as well (this is the most
popular choice), and if so, S/N is checked on the DEPT-45 so as to
eliminate confusion over the solvent. The DEPT is collected with just 16
scans each cycle.
All this is running on an AC200 driven by DISNMR. The possibilities for
improvement on a modern Avance instrument are considerable. In particular,
an estimate of the time required for a given S/N can usually be made after
a no more than one or two cycles. The run may then be stopped and have its
priority reassigned for continuation later.
In my experience amateur users are usually completely clueless about the
relationship between number of scans, quantity and S/N, despite it being
explained in every document they get from the NMR service, and frequently
by word of mouth. They seldom have any idea how much compound is in their
tube, nor what proportion of that is ethyl acetate or THF or grease.
The fewer options and less choice they have the better. I give them no
possibility of changing any parameters at all and tell them to learn how to
make up solutions that will give them reasonable spectra. Anything the
automatic open-access AC200 can't give them can be tried on the
operator-driven DPX400. For the AC200 the important thing is to maintain
throughput and prevent users 'hogging' the machine for some fatuous
experiment that won't give any sensible result. Hence, it's first come,
first served and no booking. We currently do about 600 spectra a month, but
we have been up to over 1100 and that was not near capacity.
And finally, still about 20% of all 13C spectra show solvent only!
Hope this helps,
Alan
--
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Dr A.S.F.Boyd Chemistry Department
NMR Spectroscopist Heriot-Watt University
phone: +44-131-451 3214 Riccarton
fax: +44-131-451 3180 Edinburgh
email: a.s.f.boyd_at_hw.ac.uk EH14 4AS
http://dava.eps.hw.ac.uk/ Scotland
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Received on Sun Aug 24 2008 - 23:01:02 MST