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!

  • 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.

  • U using auto sampler, pls use internal for this standard, then get rsd in acceptance criteria, no issue in ur method, 

  • 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!

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