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!

  • Your choice of calibration/reporting unit (ppm, %wt, etc) is arbitrary and should have no impact on the results, so long as the known amounts are accurate.  


    The use of %wt is somewhat confusing when discussing deviation/error. When you say that the results are "off by as much as 10% plus or minus" on a "known" concentration of 15 %wt, which scenario is actually occurring ...

    (a) the result is 15 %wt +/- 1.5 %wt; calculated concentration falls within 13.5 - 16.5 %wt (10 percent of measured value)

    (b) the result is 15 %wt +/- 10 %wt; calculated concentration falls within 5 - 25 %wt (10 full units of measured value)

     

    If (a), then your method is actually quite decent, especially given the use of DCM solvent

    If (b), then you have a real problem somewhere.  DCM is highly volatile, it can evaporate from your calibration standards causing artificially high responses, which result in artificially lower calculated concentrations in samples (dividing by a larger number/slope).  If evap is a problem in your samples, the opposite will happen, i.e. artificially higher reported concentrations.

     

    Does the problem happen right away, or is it manifesting at some point along the injection sequence?

    Can you share pictures of chromatograms for your 6-pt cal set as well as a characteristic sample?

  • You really should be using individual %wt values for each target analyte in your method.  I have setup a spreadsheet to get you started.  So long as you are tracking all your weights, this should be simple.  When I look at your check standards (L1-1, L2-5, etc), the instrument is delivering the proper values in the Amounts column based on your calibration method.  But, your calibration method, as you've indicated, is not setup correctly to account for each targets real individual concentration.  This becomes evident in th sample, VDG18-160b.  The peak area of 12TDE is 37,000 pA*s, 4x higher peak area than the highest standard that you've labeled as 98%.

     

    Two "rules" here:

    1. Always use real values in your instrument calibration tables.  This will save you time and effort on the back-end.

    2. Measured peak areas should always fall within the limits of you calibration set.  If any target in your samples is measuring higher than its largest calibrator, dilute sample or add another cal level. 

     

    Here's the sheet to get you started.  Fill out the empty cells and it will calculate the individual concentrations for you. Then plug these into your calibration table in Chemstation.

    megaflorch assist - Google Sheets 

  • 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.

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