Raves
I hope you get replies from people with more experience than me but I have tried to do what you are doing and come across some problems.   I was analysing sediment from the Great Barrier Reef in Australia with low organic carbon  (>2%) and low nitrogen.  I make the following points.


>>> Raves Bhushan <[log in to unmask]> 03/25/99 02:53PM >>>
Dear Friends,

We have a FISONS NA1500 NC analyser. We are using it frequently for the
analyses of marine sediments for carbon and nitrogen content. Fisons
principals claim that one can run up to ~200 samples per column filling but
the best I have been able to run is not more than 120 samples averaging
~100 samples per filling, which is on a very lower side realizing the
cost of each filling. Can anyone suggest some alternative. Also if anyone
can provide me the following information :

1) We are maintaining sample size of ~10mg, but it sometimes becomes
difficult for samples with very low carbon and nitrogen content. Does the
sample size controls the no of runs per filling.

The main problem I see with samples of marine sediment with low concentration  of C and N is that they tend to be >95% non combustible and so for each sample you get residual ash building up above the filling.   Watch for drift in your regular/ frequent standards because the build up of ash will move the next sample further up the tube and you may not get 100% combustion. 
I think this is the limiting factor in the number of samples you can run not the life time of the filling.     These CN analysers might well give Manufacturers specifications of 200 samples per column if you analyse ~100 % combustible material  but don't expect to get this with non ideal samples like marine sediment. 

2) Are there any standards available for analyses of marine sediments with
different concs. of carbon and nitrogen content.

3) Does the acidified samples (samples treated by HCl for removal of
carbonate fraction, HCl fumed filters etc) has any effect on the life of
the column filling. We have found while using acid treated samples that
column gets consumed very fast, sometimes just after 25-30 runs. Any
solution for this problem.

Are you washing your samples to remove ALL the acid.   I too tried acid washing of sediment to remove carbonate but I would be worried that I was removing the acid soluble C and N as well so how reliable are your data?    What are you standard samples like - do you treat them in the same way as the acid washed samples or are you comparing non-acid washed standards with acid washed samples?
Which highlights the importance of your second question about standards as only appropriate C and N in marine standards will give you an accurate idea of losses of acid washing and drift from build up of ash.

I would be interested in the answers you get.

good luck

Stewart Walker
  


Dr Stewart Walker
ICPMS
Environmental Chemistry Laboratory
Chemistry Centre West Australia
125 Hay Street
East Perth
WA 6004
Australia

ph 0892223673
Fx 0893257767 
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