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 (

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.    

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.  

Thanks for your reply jacobson. I have taken a slightly new approach to this issue, let me know if you think this adequate for what I want.

My new stock standard has four analytes: decafluoropentane, tetrafluoroethyl trifluoroethyl ether, 1,2 trans dichloroethylene, and 1,2 butylene oxide. Since all of my analytes are at 99.9% purity, I translated that value to mean 999000 ug/g for each analyte. So for my stock standard mix, I used an analytical balance read to 0.0000g and weighed out precisely each analyte. this mixture I then diluted 1:2, allowing each analyte to exist in the mixture at a set concentration of approximately 499500 ug/g. I thought this concentration would eliminate my partition problems, as each GC vial holds approximately 2000000 ug per vial. When I ran my calibration, it passed, though I had to eliminate my level 6 standard (99.0%). The chromatography for my runs is very good, I am getting good separation and nice, thin, symmetrical peaks and very little column bleed.

I suppose that I do not have very much experience in working with Chemstation, (B.04.03(16)) my version, in terms of setting up and understanding the processes involved in calculating results. I would like my results to either be able to be read as percent weight w/w, or in ug/g. Is there somewhere in chemstation where I can enter in values or weights where the software will calculate my results with minimal fuss? Any insights or assistance that you can give would be appreciated, especially with working with chemstation.

Thanks!!

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...

Thank you for the reply Tim. The information you gave regarding DCM is helpful. My reasoning for using DCM is that I wanted to make sure that my analytes were completely dissolved. Also, the analytes that I am testing for are all low-boiling, flourinated, azeotropic solvents used in vapor degreasing. Right now, I am searching for a suitable internal standard. I am having trouble picking a compound that has a retention time outside the range of my analytes: about 1.34-1.90 mins. I have adjusted my inlet programming to account for the problems associated with DCM, like a large split ratio, an inlet liner with taper and glass wool, and I keep a close eye on my split inlet purge filter. With these adjustments, I can expect that my chromatography will achieve good separation and thin, symmetrical peaks. If you have any suggestions for a suitable I.S., I am open. Thanks again.

7/2/19

Hi,

I know I probably wont get immediate replies from the forum because of the holiday, but please after the holiday, any help or insight is always welcome. I am continuing on this thread because it relates to the original question of accuracy and precision in regards to the characterization and quantitation of solvents used in vapor degreasing. Previous entries from me have mentioned that my method is for four compounds, decafluoropentane, tetrafluoroethyl trifluoroethyl ether, 1,2 trans dichloroethylene, and 1,2 butylene oxide. My goal was to create a stock standard to run a calibration to analyze the compounds I mentioned. As said before, each compound is in liquid form and the concentration is in % purity. For each compound a percentage purity is described and can be converted to a unit that better reflects percent weight (w/w). For example decafluorpentane has 99.9% purity in liquid form. This can also be expressed as 999900 ug/g. Each compound can then be expressed as ug/g based on their respective purities. I weighed out each compound to 999900 ug, or 0.9999 g into a 2 oz glass bottle with a gasketed screw top lid to minimize evaporation. Once the compounds were combined in the mixture, I added an equal amount of diethyl ether as a diluent, thus bringing the concentration down to 499950 ug. I then call this my upper limit of 99.0% w/w. The preparation of the calibration curve is next with 7 levels, each level having a specific concentration in ug/g. Since my last entry, I have passed my calibration curve with all seven levels (does having more levels in a curve and having them pass make the overall method more "robust")?

When I run samples and calibration verification samples, I am still having a few issues. The CCVs perform very well, with very little variation, but when I run samples, I do not get the numbers I expect.

When running my calibration, I discovered that when I modify my calculation page to include a multiplier (in this case 0.0001) and a dilution factor of 2, I can generate a result that is read in percentage weight w/w. For example,  a LVL 4-25.0%w/w is expressed as 24.7834% which is what I expected.

My sample preparation is exactly the same as the preparation of my calibration standards and proceeds as follows: the samples analyzed are either neat raw product, or they are mixtures of the relevant analytes sampled from a working vapor degreaser. Regardless of neat or in process samples, the percent purity remains the same, so I can sample solvent, weight out exactly 0.9999g in a 2 mL vial, and dilute it by half to give a concentration of 499950 ug/g...however, each compound instead of expressing as a percent weight, my decimal point moves after I calculate and my results are off...for example instead of 14.8%w/w, it will show 198.62132 as my result. I cannot figure out what is going on. Would I have better results if I calculated using an internal standard, instead of an external standard? Like I said, any input is welcome.