From: Stable Isotope Geochemistry [mailto:[log in to unmask]]
On Behalf Of Scott Hughes
Sent: Thursday, 30 October 2014 6:59 a.m.
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] The correct use of a Vario EL
The procedures for the vario EL in CN mode will be the same for compositional analysis and/or when coupled to an IRMS.
The lead chromate is to remove sulfur. As most would rather not handle lead chromate, an alternative is to use silver wool in the bottom of the combustion tube (from the
bottom up use 5mm silica wool, 18mm of quartz chips, 5 mm silica wool, 50mm silver wool, 5 mm silica wool, 65 mm copper oxide, 2 mm corundum balls and then the ash crucible filled by the protective tube).
If you are running in CNS mode there are some modifications which help to reduce carry over issues with SO2 for isotopic analsys.
The vario EL has a 200 ml/min flow rate, and it is best not to reduce this. This carrier gas conductance is required for the proper operation of the purge and trap column.
The reason for the splitting is that some open split systems, do not handle the 200 ml/min flow so well. This is not an issue if you are using an Isoprime system. The splitting does not effect the sensitivity, as the analyte concentration in the non-split
stream or the split streams is the same. While it make sense that the higher flow rate in general will result in reduced sensitivity, the effect is not as much as you would think and very good low level down to about 5ug N.
One possible issue is if there is any change in the back pressure on the system during the run, such as when a diluter is activated. The cube versions of the Elementar are
not affected by this as they have a different peak recognition protocol. However the analyzer you have will set the baseline at the beginning of the run and then will look for each end of peak based on this initial level. You can force it though with large "peak threshold"
settings, but that may results in less accurate compositional results.
Collingswood, NJ 08108-1303
Toll Free: 855-695-1312 x301
Original Message -----------------------
Date: Wed, 29 Oct 2014 05:26:13 +0000
Subject: [ISOGEOCHEM] The correct use of a Vario EL
As a backdoor way to introduce purge-and-trap EA to our lab (and not getting the money for a new one), I have got hold of a very tidy ’95 model Vario EL (2 careful owners, no dents).
Are there any changes to standard procedures that you would recommend for isotope work?
Any different tube packings for CN mode – what is the lead chromate for??
I am provided with a flow splitter that takes only half the carrier to the MS, but results in loss of response compared to GC-EA – do you need it?
Can you use different/lower carrier flow rates?
The question of real burning interest though is how do the new models compare to my MKI – if anyone has experience of old and new?
Kind aknowledgements are due to our local Elementar agent who has personally undertaken to see this set up successfully as a precision elemental analyser.
I have been bumping around the carpark but am now just about ready for a squirt along the highway!
Robert Van Hale
Department of Chemistry
University of Otago
Union Place West
P.O. Box 56
P: +64 3 4797915
C: +64 027 4141266
F: +64 3 4797906