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  1. #1

    Default GC/MS and GC-FID interpretation method?

    So as I understand it and correct me if I'm wrong.

    GC-MS interpretation depends on your knowing the vapor pressure to calculate the actual amount of a given molecule present in a headspace. Meaning that for example if your GC/MS says you have 70% linalool it's more like 17%.

    So with that being said how do you interpret GC-FID (Flame Ionization Detectors) percentages in the headspace? Is this actual molecules already present or do you do the same math as GC/MS, or do you do something else??
    Last edited by RSG; 11th July 2019 at 07:26 AM.

  2. #2

    Default Re: GC/MS and GC-FID interpretation method?

    Quote Originally Posted by RSG View Post
    GC-MS interpretation depends on your knowing the vapor pressure and molecular weight to calculate the actual amount of a given molecule present in a headspace. Meaning that for example if your GC/MS says you have 70% linalool it's more like 17%.
    ?
    not sure about the last part never heard, but wondering how you do this part is that the same as relative odor impact or roi 'adjusted' for headspace sampling? Is this something you calculate or it is in the quoted article most of the time?

  3. #3

    Default Re: GC/MS and GC-FID interpretation method?

    Usually, this is something I do if I want to do a reconstruction of a natural from a headspace. They give you the numbers they detect but the problem is when you talk about something that's very volatile it's detected at a higher rate. If your talking about a base note it's detected at a lower rate than is actually present. So the detection amount is influenced by the vapor pressure.

    As far as I am aware ROI is done by a perfumer sniffing the air they are testing. This is how they pick out what molecuels are characteristic or neccesary and which molecules are the most impactful.

  4. #4

    Default Re: GC/MS and GC-FID interpretation method?

    Yeah but how did you get from 70 to 17? Is this calculated with vapor pressure in a known equation, or an example based on known weights?

    edit: sorry it says in first post you are using math

  5. #5

    Default Re: GC/MS and GC-FID interpretation method?

    I think I found my own answer but I'm still researching. [FID% / C]={mw/(carbon atoms X12)}

  6. #6

    Default Re: GC/MS and GC-FID interpretation method?

    I would think it would look more like a log function but maybe ... ask one of the smart people who 'belongs' here.
    Then it can go in the 'only correct people need apply' archives.
    I am skulking back to the back of the bus lest I take up too many bytes of space.

  7. #7

    Default Re: GC/MS and GC-FID interpretation method?

    Quote Originally Posted by RSG View Post
    So as I understand it and correct me if I'm wrong.

    GC-MS interpretation depends on your knowing the vapor pressure to calculate the actual amount of a given molecule present in a headspace. Meaning that for example if your GC/MS says you have 70% linalool it's more like 17%.

    So with that being said how do you interpret GC-FID (Flame Ionization Detectors) percentages in the headspace? Is this actual molecules already present or do you do the same math as GC/MS, or do you do something else??
    An MS doesn’t give quantitative results. It only identifies compounds. FID does not give qualitative results, it only quantifies compounds — the percentage of which is proportional to the peak area on the chromatograph.

  8. #8

    Default Re: GC/MS and GC-FID interpretation method?

    Look at this post. He explains and demonstrates it quite beautifully and I learned it from Kraft also.

    http://www.basenotes.net/threads/222...Absolute/page2

    Here is I think the answer to my question. But it's also a matter of a lot of experience and interpretation. I've already read where it says the method fails by up to 50% on helional and heliotropin. It also is not effective on solvents or dpg. Less effective on things like galaxolide 50%. But it is super accurate after the corrections. You just have to have the experience. So you still have to be intimately familiar with your materials and formula and the result is always relative to itself. This just attempts to help you ballpark the interpretation based on carbon atoms when you don't have your own machine.

    capture_001_110720519_010007.jpg

  9. #9

    Default Re: GC/MS and GC-FID interpretation method?

    Quote Originally Posted by Thea in Fl View Post
    I would think it would look more like a log function but maybe ... ask one of the smart people who 'belongs' here.
    Then it can go in the 'only correct people need apply' archives.
    I am skulking back to the back of the bus lest I take up too many bytes of space.
    I think everyone who wants to have the best DIY journey they can and sincerely wants to interact compassionately belongs here.

    And as has been proven many times smarts is quite a container made up of other things. So even if these exercises don't work out for me I will have learned some things and gained some experience.

    For me knowing molecular weights, logp and vapor pressure is foundational to my understanding of a material. And so often the holy grail is to get a gc/ms but if you use it without interpreting it you will be disappointed and waste materials. So I am trying to save myself and other basenoters from this disappointment.

  10. #10

    Default Re: GC/MS and GC-FID interpretation method?

    I was wondering why a thread had been started with title claiming knowledge in a given area is a waste of time, it seemed peculiar at best and quite out of nowhere but perhaps your information posted right before was the cause. It is not as if anyone had recently said a degree or a course in that area is needed, or anything like that. Be that as it may.

    Almost everything that is useful for some is not useful for others, for all kinds of reasons. There are many paths to fine results.

    Perhaps this time there will not be many posts in a row piling on to what has just been said.

    Not that I'd ordinarily quote Bruce Lee, but, "Discard what is useless (to you), retain what is useful."

    And to also quote a source I'd never thought I would, the Beatles, "Let it be."


  11. #11

    Default Re: GC/MS and GC-FID interpretation method?

    Quote Originally Posted by RSG View Post
    I think everyone who wants to have the best DIY journey they can and sincerely wants to interact compassionately belongs here.

    And as has been proven many times smarts is quite a container made up of other things. So even if these exercises don't work out for me I will have learned some things and gained some experience.

    For me knowing molecular weights, logp and vapor pressure is foundational to my understanding of a material. And so often the holy grail is to get a gc/ms but if you use it without interpreting it you will be disappointed and waste materials. So I am trying to save myself and other basenoters from this disappointment.
    I don't know how this answers your initial question of " how do you interpret GC-FID (Flame Ionization Detectors) percentages in the headspace?." Nor does it answer "Is this actual molecules already present or do you do the same math as GC/MS, or do you do something else??"

    As I mentioned previously, FID and MS are two distinctly different kinds of detectors, both of which can be coupled to a GC. A MS detector records a mass spectrum for each compound which eludes the column, and compares this spectrum to those inside a known database. It tries to match the detected spectrum to one inside this database. Software than returns suggested materials with the percentage of the spectrum which was matched. It is never 100%. That is the function of an MS. IT DOES NOT TELL YOU how much of the specific material was present. It only identifies compounds. Of course the type of column comes into play. There are other factors which influence its accuracy. But this is the job of the MS.

    An FID detector WILL NOT tell you what compound is present. It will tell you only how much of it is there. If you habitually run samples on the same temperature and time programming, and on the same column, one can infer from the retention times what compound is present. But this is not the job of the detector.

    Fancy machines will often split samples in two, and the identification and quantification happen at the same time. But the work is done by differing parts. I am not aware of one which does both reliably (though I could be wrong).

    You mentioned vapor pressure earlier. This is important if you are concerned with the headspace of something. If I create a fragrance, and inject a LIQUID sample into the column of a GC, with sensitive enough equipment and a good database, the resultant quantification and identifications will be very close to the actual amount present in my formula. Of course it sounds easy, but the reality is much more complicated. As a perfumer, I simply use "clove oil" -- of which there are many types (bark, leaf, differing countries) which will have different eugenol (cinnamaldehyde, carophyllene, cinnamic alcohol, etc) contents. And the column outputs a single compound (for the most part) at at time. So the only way to know clove (as opposed to cinnamon, eugenol, pimento, etc) is present is to know what its marker is. Specific compounds which are unique to the plant. This is the job of a professional. Synthetics are easier to identify, but, of course, there are different qualities to those as well. Using a synthetic geraniol or one extracted from palmarosa (with trace components) will significantly influence the final smell, but the two will look almost, if not entirely, identical on paper. As I said, this is work for a professional. The difference between the headspace and a liquid sample is the ratio of the compounds present. Because some materials are more volatile than others, they will tend to escape the solution (wax, surfactant mix, etc) more, and thus, if only measuring the headspace, will be over represented in the final quantification results. This is why it is necessary to account for vapor pressure.

  12. #12

    Default Re: GC/MS and GC-FID interpretation method?

    The stuff I am interpreting is multi-column and type. It's a GC-FID cross-referenced with a GC-FTIR-MS and also with MCT.

    But being a scientific preprint they only show the top 70 occurring chemicals and they don't show all stages of how they interpreted the data in the table. I have all the names and then the numbers are labeled FID peak area %. I do know that it is pure headspace anlyses in carrier gases which I am trying to convert to liquid.

    There is a protocol to interpret it. I just didn't know what it was.

    I'm still open to ideas.

  13. #13

    Default Re: GC/MS and GC-FID interpretation method?

    Quote Originally Posted by RSG View Post
    The stuff I am interpreting is multi-column and type. It's a GC-FID cross-referenced with a GC-FTIR-MS and also with MCT.

    But being a scientific preprint they only show the top 70 occurring chemicals and they don't show all stages of how they interpreted the data in the table. I have all the names and then the numbers are labeled FID peak area %. I do know that it is pure headspace anlyses in carrier gases which I am trying to convert to liquid.

    There is a protocol to interpret it. I just didn't know what it was.

    I'm still open to ideas.
    As I said, generally one divides by the vapor pressure. There may be other methods. I haven’t used them. Best to make up a sample and adjust with the nose.

  14. #14

    Default Re: GC/MS and GC-FID interpretation method?

    If we are talking about headspace analysis, dividing by the vapor pressure and adjusting by the nose as George Tedder said:
    Go to TGSC, look up Physical Properties: Vapor Pressure, make a spreadsheet.
    But there a matrix effects, and different proposed vapor pressure values, and all this has to be adjusted with the nose.


    freesia_hs.jpg

  15. #15

    Default Re: GC/MS and GC-FID interpretation method?

    R-
    two things:
    maybe don't always adjust everything in the analysis, because first impression can be more dramatic by increasing volatiles instead. You know how you smell a rose from a distance and then rush up to it to put your face in the bloom and breathe in... only to be disappointed in a big whiff of something not as pleasant as the air carried? If you can't find accompanying ac to act as the wind, then maybe the most volatile need extra consideration.
    Second, I wonder if we can use logP as simple way to select water soluble ingredients / less absorption in skin ?




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