The problem: poor reproducibility for small impurity peaks
The system:HP6890; 30mx0.53mm fused silica column with a 3µm film of Rtx-1301; He at a constant flow rate with an initial linear velocity of about 70 cm/s; an FID at 250°C; split injection (split ratio is 0.1)with a 20ml/min pulse at 1 min and a 15 ml/min purge at 0.75min; injection is at 220°C;temperature program: 100 to 220°C at 5°C per minute
The standard: 1 µL of a very dilute aqueous solution of diethylene glycol
The sample: 1 µL of 5%w/v aqueous solutions of glycerin spiked with various amounts of diethylene glycol at ≤ 0.1% of the glycerin concentration.

I see better reproducibility for the diethylene glycol peak with solutions that contain the 5% w/v glycerin than with the aqueous solutions. The glycerin probably minimizes backflash.

Comments or suggestions please?

Sorry about the garbled stuff before the 0.1% -- that was the less than or equal to sign

I think your problem is due to the injection volume, since your solvent is water which expands to greater volume than organic solvents. 1 ul water expands to 1420 ul water vapor, too large for the 6890 liner volume. We've had similar experience here when injecting aqueous solutions, so we changed those to 0.5 ul injection from a 5 ul syringe (change syringe configuration to 5 ul through software, then injection volume can be changed to 0.5 ul)

Yes I realized that is part of the problem. I have bought some 5µL syringes from Agilent and also plan to try out SGE's focus liners which have a small layer of quartz wool (they claim this has advantages over glass wool) that is held in place by a collar. This allows the injection needle to be wiped off.

Do you use Chemstation? We use Millennium32. I did not see any obvious way to change syringe configuration. Waters tells me there is no need to do anything except perhaps configure the equinox card that allows the GC to communicate with Millennium. Any tips?

Dear Ahalya,

I am not sure if my advice can help with your bad reproducibility problem, but it will reduce the analysis time without a loss in separation.

70 cm/sec is about 2 times higher than the speed-optimized velocity (SOV) for He in a 30m x 0.53mm column. Using 70 cm/sec sacrifices the separation for shorter analysis time. On the other hand, the heating rate of 5°C/min is too slow, and, therefore, the entire analysis is too long compared to what it could be at 70 cm/sec gas velocity. As a result, whatever the gain in the speed of analysis was made due to the 70 cm/sec velocity, it was lost due the too slow heating rate. The bottom line is that the loss in the separation was not transformed in the reduction in the analysis time.

If it is not too late to modify the method, try this: gas velocity = 35 cm/s, heating rate = 10°C/min. It might become necessary to increase the max temperature to 230 °C or even to 240 °C. I expect almost 2-fold reduction in the analysis time without loss in separation. The saving in the consumption of He might also be useful.

Literature:
1. L. M. Blumberg, "Theory of Fast Capillary Gas Chromatography. Part 2: Speed of Analysis". J. High Resolut. Chromatogr. 1997, 20, 679-87.
2. L. M. Blumberg, "Theory of Fast Capillary Gas Chromatography. Part 3: Column Performance vs. Gas Flow Rate". J. High Resolut. Chromatogr. 1999, 22, 403-13.
3. L. M. Blumberg and M. S. Klee "Optimal Heating Rate in Gas Chromatography". J. Micro. Sep. 2000, 12, 508-14.

Good luck,
Leon

You could simply replace the 10 ul syringe with the 5 ul syringe leaving the software "thinking" it's still a 1 ul injection which would really give 0.5 ul injection, since the plunger "throws" are similar. If you're doing pharmaceuticals, I would write a letter documenting what and why you are doing (for the files). Yes, we use Chemstation, versions 3, 5, and 7 for GC and HPLC, never used others.

Be careful if you try the approach Leon suggested above. If you reduce the linear velocity you will reduce the head pressure, increasing the expansion volume and making the flashback problem worse.

What if I reduce the linear velocity but use the pulsed split injection which pushes the material in with a pulse of high pressure about 1 min after injection?
Also, I am purging the injector after each injection

I NEED A STANDARD METHOD. DETERMINATION OF DIETHYLENGLYCOL PURITY BY CAPILLARY GAS CROMATOGRAPHIC.AND ANTHOR GYCOLS.
BE REGARDS,