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 (

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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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bddoss22 · Answer

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

bddoss22 · Answer

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.

timstrutt · Answer

Hi megaflorch, It looks as though you are on track. As posted above, the way to go to near a 100% concentration of an “unknown analyte” is to dilute your unknown in the same solvent as your standards as suggested, so you might weigh out only 0.100g in your vial and add 1500ul of solvent. DCM is a common solvent but because of its low boiling point is not always ideal for accurate quantitation work unless you use an internal standard - If a bit of solvent evaporates, or bubbles form, it doesn’t matter as your analytes and IS will have higher boiling points and will stay in the same ratio. A suitable solvent with a higher boiling point might be something like chloroform or CCl4 (BP 61 & 77), but obviously (like DCM) they have significant toxicities; you would also need to check that these higher boiling solvents elute away from your analytes. As a general point DCM is often used for environmental work to extract low levels of pollutants from environmental samples as it will dissolve most organic analytes, but it is almost always used with an IS. Other potential problems with DCM include its reactivity with trace levels of water in hot injectors where it can form HCl which can cause high levels of activity and it will reduce the life of your column; and as mentioned it produces relatively large vapour volumes (~450ul/ul) vs heptane (~200ul/ul); by the way MeOH is worse at ~750ul/ul. A general rule for a longer column life and to keep a relatively low level of activity in your injector is to use a hydrocarbon solvent like heptane, cyclohexane, or toluene, rather than a polar or halogenated solvent; but obviously not everything will have a high enough solubility in apolar hydrocarbon solvents, and they are not really suited for polar stationary phrases like Carbowaxes...

jacobson · Answer

Hi megaflorch, I have read this entire string and hope to add some insight. 1. As you have learned, you cannot make a stock standard near 100% with more than one analyte. When you add components, you dilute the concentration of each analyte. Adding 100ml of any anlayte with a final volume of 1L, regardless of what else you added, that single analyte is only 10%. 2. Why does your upper limit need to be 98%? Is it because some samples will indeed be that high in a single concentration? You mentioned that most samples have 4 analytes. If there are 4 analytes, the individual concentrations cannot add up to more than 100%. 3. So, you could keep 25% as your top cal range, and run your samples at 1x (1ul) and 2x (0.5ul) 4. Or you could inject 2ul of the 25% solution to achieve 50% as the top of your calibration. 5. Or you could go a completely different route and do single calibration - and get 100% linearity with a new approach. Can I know what components you are calibrating for? I can help you with resolution (get rid of double peaks, with just a few tweaks to your GC Method) Let me know if you still are looking for solutions for your method development. The goal for you being - simple calibration, no dilutions, no re-runs, and good data.

james_jenkins · Answer

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

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 (

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