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 (

I have attached a copy of the method I am currently using. Please review it and let me know if you can assist me. 

Thank you!

  • Hi megaflorch,

    The only suggestion I have on the acquisition portion of this method is that your gas save time is very short and may actually interfere with the sample if it hasn't made it on column. You have 0.75 minutes, I would lengthen out to 2 minutes. One other , unrelated to sample results, but more for robustness of the method I see is that you have a very high makeup gas flow for your FID. This may cause some lighting issues if the jet becomes slightly restricted at some point, I would suggest dropping the makeup from 30 to something like 10 ml/min. For troubleshooting purposes, you can also try a viscosity delay of 3 on the injection to see if there is a difference and this is just a suggestion since I dont know what solvent you are using.

     

    Regards

    James

  • Hi megaflorch,

    I will branch you post to our applications section for better visibility as sample prep, etc.. is not my specialty .

    Regards

    James 

  • Thanks James, I will certainly try your suggestions. I suppose that I really do not have too many issues with the parameters of the method, but it is mostly pertaining to my stock preparation and sample preparation and calculations.

    For example, my calibration stock is composed of four analytes, each weighed to 100g+/- 0.001 and combined in the stock solution. Should I calculate each respective analytes concentration? And if I do that, how would I go about it? Determine each density and convert my units appropriately? Im not sure. Also, from my stock solution, I am weighing out my calibration standards instead of just simplifying my concentration. Is there a way to enter the calculation parameter for determining weight percent (w/w) into Chemstation and have it calculate my results? Any additional insights are appreciated.

  • Greetings.  I echo the above re: gas saver time. I also notice that your oven ramp rate is quite fast.  If your instrument is on 240V, 50/min may be OK, but 30/min is max practical rate for 120V systems.  You may also want to increase your final Temp Hold to 2-3 min.

     

    I notice an Error in the aqcuisition portion with overlapping peak times.  Is your precision problem related to peak integration, or is it simply poor reproducibility of peak areas from injecto-to-injection?

     

    Best of luck.

  • Thanks for your reply. The root of my problem has to do with my stock standard and calibration preparation. I begin by weighing 100g+/- 0.001g of each respective solvent and combine them in a solution that serves as my stock standard mixture. The results I am working for should be expressed as weight percent (w/w). Then from my prepared stock standard mixture, I prepare my six point calibration curve by weighing each level, 1.0% thru 98.0%. Each calibration level created by adding the respective weighed amount to a 2mL vial and using DCM as my diluent. This method does generate results, for example, my calibration curve passes and I run an unknown sample or a sample where I know the amount that each analyte in the sample should be. If in my sample I am expecting a weight percent result of 15.0%(w/w), my results usually are off by as much as 10.0% plus or minus. I feel like this method, while functional is incorrect and can be improved by perhaps taking a more calculated approach. Should I be determining a concentration for each analyte in my stock standard mixture and then calculating each concentration with each sample mixture from my stock in my preparation of my calibration? How should I go about this? Should I convert my expressed concentration to ppm, as this is the easiest to convert to weight percent? Any additional advice would be very much appreciated. 

  • Thanks for the reply. Referring to your email, the deviation amount is more like (b). I am aware of the evaporative nature of all the compounds I am working with, not just DCM, and I take measures to minimize evaporation. The problem occurs with every injection. I think my problem has to do with my not calculating the respective concentrations in my stock solution and then calculating the concentration all the way through analysis and result. ( If I can nail down a solid procedure for this, I think my problem is solved.)   I have attached a copy of the method I am currently using. Aside from a few parameter modifications such as reducing carrier gas flow, I can achieve good separation and nice thin symmetrical peaks.

    I have also included copies of an example of my calibration check standard and an unknown sample. If you look at the sample, Name: VDG18-160b for example, the final results must be calculated from the Amount column figures. Like for 1,2 trans dichloroethylene, the amount-434.86509 is divided by the total amount 445.57065, and then each respective amount divided by the total to give the weight percent result. Please take a look at the attachments and any suggestions or advice will be appreciated!

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