Sample Handling

Print version E-mail to a friend
Text Size: Change text to small (default) Change text to medium Change text to large

Scheduling

In general, there are few truly "routine" analyses or samples. Therefore, it is always best to contact us at the earliest possible point in the planning process. A brief consultation at the time a proposal is first being developed can provide the User with realistic technical and financial projections and will give the Facility Manager insight into coming trends in equipment needs and usage patterns.

Work is generally scheduled on a first-come-first-served basis and instrument time often books up 3-4 months in advance. Don’t worry if you are not certain exactly when you will have your samples ready; the Facility’s schedule frequently also slips. Should you experience a significant delay in the final preparation of your samples, the Facility can almost always go on to the next User in queue and come back to you when you are ready.

Let us know your scientific goals and technical priorities. We can generally optimize time/cost, sample consumption, accuracy, or range of compounds analyzed, but not all parameters simultaneously.

Prioritize your samples, and let us know which of your samples are more important than others. Equipment does sometimes unexpectedly become unavailable for extended periods of time. When such circumstances arise, we would all prefer to maximize the value of whatever work has been accomplished.

To schedule analyses, please contact Carl Johnson at 508-289-2304 or by email at cjohnson@whoi.edu.



Sample Preparation

Suggestions for GCirmMS Sample Preparation

When samples are brought to the Facility for analysis, please submit for each vial an Analysis Request Form . In general, a separate page should be completed for each vial submitted. In the unusual case of a large number of "identical" samples (ex. the analysis of two or three specified PCB congeners down a core), please contact the Facility to determine if a separate page will be required for each vial.

The DeltaPlus spectrometer is limited to about one order of magnitude of dynamic range. All compounds of interest should be within this concentration ratio range. In general, peaks of about 10 to 100ng per component per injection are about correct, but please consult with Facility personnel to determine the current operating conditions. A peak that is too small generally gives an accurate, but imprecise value. A peak that is too large will saturate the detector and mathematically results in a completely useless value. Avoid overspiking of internal standards.

Please make GC screening runs under standard conditions for each fraction. Measure, do not estimate, the solvent used for dilution. Make certain that the runs are long enough to get all high boiling compounds off of the column. Present the entire run in the trace. Normalize the intensity scale to the largest peak of interest. Few shortcuts result in real time savings for either the user or the operator; many result in samples needing to be re-run.

Derivatize and dilute samples to take full advantage of current injector capabilities. Derivatize small samples in ground glass V vials using minimal volumes of BSTFA/Pyridine (total volume approximately equal to volume expected for proper injection) or other reagent mix. If possible, leave samples in the reagent mix; our current injector can handle mixed solvents and inject variable volumes. Also, BSTFA/Pyridine can be injected at much higher temperatures than DCM or hexane, reducing post-run cooldown time by up to 10 minutes. If your GC traces are presented as above, and the samples are in known volume, Facility personnel can determine correct final dilutions and injection volumes... and be responsible for errors in injection amount.

Whenever possible, bring the entire suite of samples for one compound class (ex. acids vs. alkanes) at a time. This allows for developing one set of conditions for all samples of a type. This also allows for best use of instrument time, since runs in one sequence of different programs are software limited to the length of the longest program.

Consider developing other derivatives/GC column/program options which would allow alkanes, fatty alcohols, FAMES, etc. to be run un-fractionated. Runs and columns will probably be longer, but reduced sample handling, and reduced number of runs will result in significant savings of sample, time and money.

Suggestions for Bulk irmMS Sample Preparation

(This section may contain outdated material, please contact us for current protocols)

The importance of accurately and precisely weighing your samples can not be overstressed. In spite of the use of an apparently crude detection technique (TCD), determinations can be made that are good to 0.1%. For user weighed samples, we have available a microbalance and cleaned boats. Regardless of what balance you use, take your time. Wait for the balance to fully stabilize before you tare the boat, and again, wait until the balance has come to rest before you record the sample weight. Weights should be determined to the nearest microgram. Obviously, you will not know the exact carbon content of your samples, or you would not be requesting this analysis. However, most users have a rough idea of the amount of carbon present, based on the sample type. In general, the system runs best with the smallest sample loading consistent with having about 20 to 30ug of organic carbon present in the sample, but large quantities of low carbon sample can result in technical difficulties. As a practical guide, if your material is expected to be about 30% organic carbon then a 100mg sample is about correct, 3% organic carbon materials should use around 800mg samples, but a 0.2% organic carbon material would use a 3mg sample.

Please alert Facility personnel to any inorganic carbonate which may be present in your samples. Many samples contain inorganic carbonates which, if not properly removed, can result in grossly overstated organic carbon values. If inorganic carbonates are present, samples must be weighed into pre-cleaned silver boats. After weighing, samples will be exposed to an HCl rich atmosphere which should cause the release of most of the carbonate. After gaseous acidification, each sample will be further acidified with 2N HCl until all signs of CO2 production cease. Samples are then dried in a warm oven prior to analysis. Samples that have been acidified with sulfurous acid will not be accepted for analysis. Sulfurous acid is incredibly difficult to fully remove and its presence in the elemental analyzer carousel results in permanent corrosion of surfaces which need to form air-tight seals.

Be aware that large amounts of high carbonate samples can be quite difficult to acidify. There is a tendency for the surface to crust over during gaseous acidification which then stops acidification of lower layers of the sample. For very high carbonate samples, it may be better to work towards the lower end of the desired organic carbon range.

If you are providing your own boats, please contact us for further instructions on proper cleaning.

Samples are normally analyzed in triplicate. While the precision and accuracy of any one measurement is quite good, many samples are somewhat heterogeneous and require multiple measurements to achieve a best overall assessment of the material’s composition.



Data Ouput

Depending on user needs, we can provide any level of data workup, from raw data files to excel spreadsheets and brief text discriptions.



 

WHOI logo

Last updated November 12, 2013
© Woods Hole Oceanographic Institution. All rights reserved