Suggestions for GCirMS 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 5 to 50ng 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.
© 2002 Woods Hole Oceanographic Institution.