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Vial fill effecting reproducibility was mentioned in an Agilent document called something like "Maximizing Reproducibility in GC/MS Analysis." In this paper one of the suggestions was consistent vial fill volume with a suggested fill of 1mL. Can vial fill level have an impact on reproducibility? To find out I ran five replicates at 0.5mL, 1.0mL and 1.5mL. All samples were transferred using a Rainin EDP pipette. The method is a rapid ethanol analysis with acetonitrile as the internal standard and acetone as the solvent. The working ethanol concentration was 0.6% with a 100:1 split ratio and a injection volume of 0.1uL. A 5uL syringe was in the 7683 ALS. As you can see the 1mL fill was indeed the most reproducible.

Agilent recently published information on decreasing the delay volume on the 1100. I like to save time whenever possible so I decided to give this a try. The method in question is the same one that I described here. The method was modified with an injector program to switch the valve to bypass at one minute into the run. This is before the gradient starts so switching back to main-pass before during the next injection will not effect the retention times or the second sample. As you can see the elution time of the last peak decreased significantly.

This decrease and setting the autosampler to pre-fetch the net vial saves about 50 seconds per sample. This can add up fast when you have 20 or more samples to run.

Many texts report that an inlet temperature of 250°C is sufficient for most samples. While it may be sufficient it may not be the optimum temperature for your specific sample. The selection of the inlet parameters should yield rapid and complete vaporization, without sample back flash or degradation. Here is an example of how important inlet temperature can be.
The sample being analyzed is a PFBOA derivative dissolved in acetonitrile. The inlet liner is an SGE tapered focus liner. Pulsed splitless mode was used with a pulse pressure of 14psi for 0.5min. Purge flow was active at 0.04min with a flow of 25mL/min. As you can see the peak area increases as the inlet temperature increases up to 175°C and then starts decreasing with increasing temperature. In this particular case we are seeing sample degradation as opposed to back flash.

A reference is great for correcting for baseline changes but is not always needed, and when set incorrectly have detrimental effects. Here is a comparison of the same sample analysis with different collection parameters. Two sets of collection parameters were collected simultaneously: The first being a sample wavelength of 210nm with the reference turned off. The second used the same sample wavelength parameters but with the reference set to a typical 360nm with a band width of 100nm. We can see the reference-off chromatogram looks normal, while the chromatogram with a reference of 360nm shows many negative peaks.


The sample in question contains multiple compounds that absorb through the entire UV and into the visible. Here is the spectrum of one of the peaks with the reference highlighted.


When compounds elute that absorb in the reference range (310nm to 410nm) Chemstation offsets the absorbance at 210nm accordingly. This is obviously an extreme example, but demonstrates the need of the analyst to be on guard for cases where the impact of incorrect reference is less apparent.

Dear Faithful DS Readers,

You may have noticed that DS was off-line for approximately 72 hours. The cause of this is not clear, but this is the second time in three months that my host, Vizaweb, has left me and many others hanging for over twenty-four hours. There were rumors that the owners had cut and run. Vizaweb was unreachable by email, phone or even fax. Well it looks like they're back, at least for now. Currently, I am in the process of backing up all content (I store a lot of research in the protected DS vault) and moving to a new host. I will continue to post new content here and on the new host. When I have all the bugs worked out on the new server, I will point dreamingspirals.com to the new ip address.

Sincerely,
George (lostcosmos)