CHN Analysis

Sample Preparation

To prepare a sample, use the Tin capsules provided by Exeter analytical. Before weighing/taring the empty capsule, use the small brush to wipe any dust off of the sample, and use the thin tweezers only for handling the capsules. Don’t touch the capsule with your hands or gloves, as any residue can lead to errors in your analysis. Add approximately 1500-2000 g of material in the capsule and weigh it out as accurately as possible. For low %C %H and %N values (<5% by mass) you may need to add more mass for accurate analysis. Unless absolutely necessary, it is not recommended to use any more than 3000 g per sample. The instrument is considered unreliable for most samples weighed below 500 g, although it is possible to calibrate the instrument to measure samples with a small amount of material if your yields are low (see instrument manual for more information.)

Try to avoid accumulating any material on the outside of the capsule, as it can fall off and alter the mass of the sample between weighing, crimping, and delivery to the instrument. This can be a problem with very fluffy solids samples or extremely viscous oils. This is also problematic if samples are air sensitive, as exposure of this material on the outside air will lead to mass differences from theory.

The sample should be as pure as possible, as any impurities from solvent or dust or impurities from synthesis will alter the measured mass percentagesfrom their theoretical values.

When weighing samples, use the small scoopula for solid samples. A micropipette is the most efficient way to administer liquid samples (1.5 ul of a sample with a density close to 1 g/ml should weigh exactly 1500 g.)

When weighing, you can either weigh by difference or tare the balance with the capsule you are going to use. Add the sample to the tin capsule and weigh afterwards. To weigh by differencerecord the mass of the capsule+ sample and take the difference between the mass of only the empty capsule. This is advantageous to do if you must weigh your samples in a glove box and reweigh after they are removed. You may also take the mass of the sample directly if you tare the balance with the empty capsule. In either case, be sure to wipe off the tin capsule with the small brush before measuring/tareing the balance with the empty capsule to get a clean blank measurement.

If the mass measurement with the balance appears unstable, you may have issues with static electricity built up on your samples. This can interfere with the balance’s ability to make accurate measurements. You may try to use the static gun to remove the static buildup, or you may wait some time for the balance to stabilize. Placing a damp paper towel in the balance enclosure can help to remove static, but this must be done carefully. If this is an issue, you may want to make several measurements on your capsule to ensure that you are getting consistent mass measurements (and in this case, weighing by difference may be more advantageous.)

Un-evaporated solvent can also lead to instable measurements. If you suspect that there is unevaporated solvent in your sample (you will observe a steadily declining mass measurement if this is the case) try sealing the capsule with the crimper.If sealing the capsule stabilizes the mass measurement then unevaporated solvent is likely present in your sample. If you conclude that there is solvent still in the sample then you will need to further dry your sample before running any combustion analysis, or expect altered mass percentages based on theory due to the left over solvent.

After you have weighed your sample accurately into the tin capsule, use the capsule crimper to cold-weld the tin capsule shut. If you suspect the capsule crimper is not sealing properly, see Phil Pagano about shimming the crimper. You may also use pliers to seal the crimper, but make sure the pliers are clean and without rust or other residue that can be transferred to the capsule. Also ensure the capsule sealer is clean and without dust or debris before crimping. When the capsule is crimped, place the weighed out sealed tin capsule inside of a green nickel jacket provided by Exeter Analytical.

Standards, Blanks, and Conditioners

For weighing standard samples, use the same procedure ascribed above, but use the analytically pure samples provided by Exeter or NIST. Obtaining accurate measurements/weights of standards is crucial for data analysis because all measured values will be based off of these samples as a reference. Try to use a standard compound that is similar in % mass composition to theoretical masses of the compounds you are submitting. See the table of common analytical standards for more information.

For preparing blank samples, ensure that the blank is enclosed under the same atmosphere that the samples were enclosed under during the weighing process. This is necessary to obtain accurate % N values (open air has 78% N, while the atmosphere in a glovebox may have ~0%!) Also ensure that the capsule is clean, as above.

Conditioners do not need to be analytically pure compounds, but should come from a reasonably pure stock(usually reagent grade stock purchased from sigma or fisher.) I prefer to weigh out my conditioners as an internal standard to see how K values change day to day (see analysis for more information on this), but this is not necessary. Just make sure to have ~1500-2000 gin the conditioner capsule, as with the sample capsules. When making standards, if something goes wrong with the weighing/crimping of the standard you can save these enclosed capsules for conditioners later to save time weighing out samples.

You will likely be weighing out multiple samples, standards, and blanks. It is helpful to do this in a well-organized fashion as to not mix up capsules, masses, and the like. Keeping the sample area clean and keeping yourself from distractions is helpful in keeping things organized. The 96-well plate is very helpful in organizing samples before placing them in the sample wheel. It is helpful to plan out the order in which you will run blanks, samples and standards before you begin weighing out samples.

Run Order

A standard run will look something like this:

  1. Blank (Ni jacket only)
  2. Blank (Ni Jacket Only)
  3. Conditioner
  4. Conditioner
  5. Conditioner (If system is seeing a lot of usage, 2 conditioners is OK here if trying to limit # of samples ran)
  6. Blank (sealed capsule)
  7. Blank(sealed capsule)
  8. Conditioner
  9. Conditioner
  10. Standard
  11. Standard
  12. Standard
  13. Reference Standard (optional)
  14. -24. Samples

25. Blank

26. Conditioner

27. Standard

28. More samples

etc.

Note that every time a blank sample is ran, a conditioner is ran immediately afterwards.

Every 5-10 samples you should run another standard in case you find that your K value’s drift a bit later in your run (see data analysis.) If you will be switching mass ranges in the middle of the run and want to use a different set of standards, you should run a different set of conditioner samples as well that match the new set of standards.

Each sample run requires ~12 samples of blanks, conditioners, and standards, which in turn reduces the capacity of the traps, the copper-filled reduction tube, and the combustion tube (need to be replaced every 350-400 samples.) Therefore, it is recommended that at least 10 samples be submitted before starting a new run (unless of course time is an issue, which we could evaluate what the additional costs should be.)