Sample costs are based on the number of capsule combusted. What you wish to measure from each combustion needs to be decided before the samples are run. Running separate Carbon and Nitrogen runs will provide better data but will double the cost and the number of samples you need to supply. Running a dual run will decrease the cost but at the expense of some precision. At all times the samples must be below the upper content limit for the elements that are being measured in the run.
Nitrogen Only
When doing a Nitrogen only run, a CO2 trap is used to remove the Carbon component. This has the advantage of removing the mass 28 overlap from the CO peak, which in turn gives better precision. The other advantage and sometimes a necessity of doing a nitrogen only run, is that by removing the CO2, to a large degree it also removes the constraint of having to be below the upper limit for the elemental carbon content. This is a necessity when the sample has a high C:N ratio and larger sample sizes are required to meet the minimum limit for elemental Nitrogen concentration but would swamp the detector system due to the high carbon load.
Carbon Only
When doing a Carbon only run, there are no overlap problems. However your samples must still be between the minimum and maximum element content limit for carbon. In the unlikely event that your sample has a very large nitrogen content ie a very low C:N ratio, it may be a necessity to run the system as Carbon only to avoid problems from peak tailing and detector overload. However, generally speaking because there is little influence from Carbon onto Nitrogen, it sometimes an advantage to weigh the samples for Carbon and run them as a dual run, doing this will stil give good Carbon results but may also yield some less precise but useful data for Nitrogen.
Dual - Nitrogen and Carbon
In a Dual run, the ion source is alternatively switched between the Nitrogen and Carbon settings during the sample run, this enables measurement of both peaks. However; it also means that you must meet at the same time, the element content limits for both Nitrogen and Carbon, this is not always possible. The sensitivity and tuning of the MS is set so that if your sample has a C:N of approximately 6:1 to 10:1, then it will most likely yield good results for both Nitrogen and Carbon. You should also keep in mind that this does not remove the problem of the CO peak overlap on Nitrogen, or the decrease in precision due to the changing of the ion source parameters.
You must inform us if the enrichment level for your sample is in excess of 5 atom %. The reason for this is that combining enriched sample with natural abundance samples will inevitable lower the precision that can be achieved due to memory effects. Highly enriched samples 10 atom% and greater can lower the achievable precision of the MS significantly for a very long time!! For that reason we need to always run natural abundance samples first, and reserve highly enriched sample runs to selected times. Also note as the enrichment levels rise the detectors may still be swamped for what would appear to be normal elemental concentrations.
Maintaining your samples element limits at a consistent level will improve the precision of your run. That means you should weigh your samples to maintain a constant weight that is between the respective limits of the element that is to be measured. Excess sample will cause excessive peak tailing, peak shifts, and detector overloads , which will ruin subsequent sample measurements not just the one that was in excess. As this will still cost you but not give you a result then it is wise to stay within the limits and at a consistent elemental level.
The way in which you deliver your sample will change the charge, the cheapest for you is to provide dried, ground, weighed and encapsulated samples, this also eliminates handling errors and shipping problems especially from overseas. Grinding and weighing in house is expensive. If you are local (Adelaide Australia) there are other options you can take to reduce costs. A full list of the options and prices are here in the Price Schedule
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