Hi, I am developing a GC/FID method for a 6890 that characterizes and quantitates halogenated analytes used in vapor degreasing. My problem is that my results are not precise and I do not have a calculation method to express my results as weight percent (

Hi Brandon,

In addition to my earlier post, I have a question about overlapping retention times that is evident in my method. If you have some advice about controlling my integration events better, please let me know. Thanks!

Thanks again for the suggestions Brandon. I will put them into practice and let you know about progress.

Personally, I would only inject single-component standards if absolutely necessary.  In your case, it sounds like you can combine all targets into the same calibration stock and levels.  I'll take your questions one at a time:

If I combined each compound at 98.0%wt (w/w) into one solution would that work?

Yes, and resulting concentration of each component will be 24.5 %(w/w).  100g*0.98/(400g)

Would I have to use a conversion for density since all of my analytes are in liquid form and only reference % purity?

If you are preparing stocks by weighing, then density is not required. If you are preparing stocks by volume, density is required.  However, once the stocks are prepared, volumetric dilutions can be performed without density calculations. 

Also could I then create each calibration level from the same stock by dilution?

Yes, this is appropriate.  Prepare your stock by weight on the balance, then simply serial dilute using volumetric pipettes and/or glassware.

When I am setting up how chemstation calculates each sample, should I enter initial or final values for each analyte?

Final values. Always enter actual values of the calibration level as it exists in the vial.  This is in accordance with universal rule #1: Never Lie to Your Instrument.  I think this may have been the source of the issue all along.  The instrument needs to know the concentrations of the calibration targets that are actually being injected.

Please create a separate post for the other issue and we can discuss it in a clean thread:
 ..." have a question about overlapping retention times that is evident in my method. If you have some advice about controlling my integration events better, please let me know. Thanks!"

Hi megaflorch,

Just a couple of thoughts. Split injection using volatile solvents like DCM (bp 40C) as mentioned is notoriously unreliable - Effects like small bubbles in the needle and the position of the needle/vaporised cloud inside the liner will cause differences in the actual amount injected or split (your 1µL of solvent will expand to ~450µL of vapour) and can easily give the sort of errors that you are seeing when an External Calibration method is used. If you can find a suitable Internal Standard (IS), I would expect that your accuracies should be within 1% even with relatively large variations in the amount actually injected.

As an experiment, if we took one of the analytes (like butylene oxide) and said it was an IS (obviously it isn't) we can do a ratio calculation like 1% =162.65/352.27 = 0.46172; 5% =665.229/1671.84 = 0.3979; 10% =1135.15/2785.93 = 0.4071; 25% =2772.38/6994.22 = 0.39638; 50% =5100.6/12672.9 = 0.40248; 2nd 25% 2523.58/6455.2 = 0.39094. Let us ignore the first one (1%) because it seems to be an outlier, then if we take the rest we get a mean of 0.4094 and a Standard Deviation of 0.006153 which is a Coefficient of Variation of 0.015 or 1.5%.

Manual inspection of the area counts of your 1% (outlier?) sample for dichloroethylene and butylene oxide (162.6 and 352.3) compared to the 5% sample (352.3 and 1671.8) are roughly 0.244 and 0.217 and compared to the 10% sample are roughly 0.4019 and 0.398 which are very roughly what I would expect - The calculated amounts of 0.000 for these analytes shows that you also seem to have a problem with your integration/calculation parameters.

Because we have not used a constant amount of our "pretend IS" butylene oxide, but varied it according to the amount of our "unknown" analyte dichloroethylene, we would expect the results to be worse than if you had used a constant amount of IS. Normally you would choose an IS to be a material that is generally similar to your unknowns, the peak should be resolved, and have a retention time that is in a generally similar window to the unknowns. Typically the amount of internal standard chosen would give a peak area very roughly equivalent to that of half way up the calibration curve for your analytes.

I would expect that a suitable internal standard (MEK?, fluorobenzene?, cyclohexane??) added to your solutions at roughly the same amount that you used for your 25% standards (the same ~25% for all calibration levels and unknown solutions) should give you a CoV of better than 1%.

Regards, Tim

Thanks for your reply Tim. I am curious about your thoughts regarding DCM. If DCM is not a good solvent in this particular case, what would be a suitable solvent? I would generally need a solvent miscible with other low boiling azeotropes. As for using an internal standard, I do not have a problem with it. My original question on this thread however was regarding how I should go about preparing calibration stock, whether for all analytes in question, or a calibration stock for each respective compound in the analysis. Up until now, I was preparing a calibration stock with equal amounts (100g) of each analyte (400g total), but I was informed that the actual amounts for each analyte would not be over 25.0%w/w. This is no good considering that the upper limit of my calibration range must be up 98.0%w/w.

Any additional insights are always helpful to me.

5/24/19

Hi Brandon,

I do appreciate the help and time given for my problem. Two things: I prepared individual calibration standards for each compound, and my sequence is complete and the chromatography looks decent, my problem now is, how do I enter my values into a new calibration table and use those values to create new curves for each analyte? I have tried changing the parameters of the calibration table settings, but I cannot seem to group the levels for a particular compound. e.g. All levels remain at 1 on the calibration table, so there is only one calibration point for each compound. The other issue is about what you said on your last post about knowing the exact final amounts for each level in the vial. In reference to this, should I calculate my amounts from initially decanting the solvent, convert the purity amount to an actual concentration, but in w/w, and then calculate down to the amount inside the vial? I am not entirely sure how to do this. If you have any advice or methods on how to accomplish this, please let me know.