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!

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  • 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 Brandon for the reply and the spreadsheet. Now just so that I have a clear understanding of the operations the spreadsheet you were nice enough to make me does. Under the "Fill Here: Calibration Table" tab I can enter each calibration level and for each calibration level prepare a standard? Up to now, I prepare a stock standard solution with all four compounds in solution. I prepare this by weighing 100g+/- 0.001of each compound and combine them for a 400g solution, and then take aliquots from my stock standard to prepare each calibration level. Each calibration level is prepared using the total weight of the standard in the 2mL vial, approx. 2.4g, and then each respective calibration level weight as a portion of that total. Should I prepare a stock standard for each analyte to use for each calibration level? If I try to make the standards in 2mL vials, I wont have enough room. According to the spreadsheet, if I prepare 0.2g each of each analyte in 20.0mL, I should get a 1.0% result. If I were to prepare each analyte in this manner and combined them equally in the 2mL sample vial, would that work out for me? You have been a great help, Brandon. When you get a chance, please reply to my additional questions. Thank you!

  • No problem, I'm glad this is proving productive.  

    I am beginning to understand.  A single stock solution is fine, but if prepared as described, the maximum concentration of any single compound is 25 %wt if each are used in equal amounts.  That means you can never actually have a 98 %wt calibration level.  Used without dilution, the stock solution itself will be your highest level at 25 %wt for each target.


    I have made a second table to utilize this approach.  It is in the same spreadsheet.  First, fill the Stock Solution table if different from your description above.  Then, fill the Calibration table with the weight of Stock and the weight of solvent/diluent, which I assume is DCM. This will provide individual %wt concentrations in the calibration solutions.

     

    Now, your calibration method will be consistent and correct.  BUT, your real samples may have concentrations that are higher than your calibration range (as with VDG18-160b).  Remember, by the method described here, your highest concentration of any analyte is 25 %wt.  If you anticipate that your target compounds will exceed 25 %wt on a regular basis, you may have to prepare stocks and calibration solutions individually so you can achieve higher calibration ranges.  For example, a two-component stock would yield 50 %wt for each compound when added in equal amounts, i.e. 200 g each.  If you really do need to measure a sample containing 98 wt% of 12TDE, you will can accomplish this in two ways:

    1) by making a single component stock solution with 98 g 12TDE and 2 g DCM.  This will be 98 %wt 12TDE and all dilutions from this stock will fill-out your calibration curve.  Repeat for all other targets as necessary.

    2) you can dilute your samples in DCM until they fall within the peak areas of your calibration set prepared as you've described (25 %wt each target).  For VDG18-160b, a 5-fold dilution (400 microliters sample plus 1600 microliters DMC) would bring the peak area down to ~7,000 pA*s, which is within your range. Now that I've written it, this would be the preferred approach.  Diluting high-concentration samples will also be more gentle to your GC system resulting in less required maintenance.

  • Hey Brandon,

     

    Thanks for the last reply. I like the idea of single component stock solution for each compound, it would give me a real represented concentration for each analyte. What I am curious about is if I can combine each compound into one stock solution, as my analysis always contains all four compounds and others as well,and my first inclination is to recreate the chromatography for my standards since it should be analyzed in the same way, but I wanted to nail down this approach before adding new compounds for analysis. If I combined each compound at 98.0%wt (w/w) into one solution would that work? Would I have to use a conversion for density since all of my analytes are in liquid form and only reference % purity? Also could I then create each calibration level from the same stock by dilution? When I am setting up how chemstation calculates each sample, should I enter initial or final values for each analyte? The samples that I have analyzed so far have not exceeded my calibration range, and as pertaining to my work, will probably never exceed it. We do work with other solvents and asphalts as well, and those of course should be diluted after extraction.

    If I create six calibration levels individually for each, for a total of 24 standards, and  a total of four calibration check standards for each sample run, I think that it will create a little more work, but would be worth it to have a correct method. I appreciate your time and help!

  • 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!"

Reply
  • 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!"

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

  • 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.

  • Not sure how much deeper I can dive into this without sitting at an instrument myself!  It sounds like you're having issues with Chemstation as you've split your calibration level into separate vials?  You may have to reprocess your sequence in a clever way to let Chemstation integrate and build the calibration curves one-by-one, but without erasing previous values as you progress through the standards.  Look for a checkbox somewhere that tells it to Replace Values, Average Vaues, Ignore.

    You may have to record the peak areas in a notebook and manually enter them into the calibration tables.  Going forward, run your calibrators as mixed solutions to leverage chemstation to your full advantage.

     

    Regarding your calibration prep and concentration .... yes, you need to know actual concentrations as they exist in the vial.  Document the weight and/or volume for all components (and DCM) in each of your steps, then do the math.  If %(w/w) is your desired unit, then weigh everything.  If you can't weigh everything, use volumes and factor density in your calculations.    

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