Interested in the determination of the Kovats retention indices for each compound seen in chromatograms, using a commercial n-alkanes standard mixture, after SPME-GC-FID of food samples to study volatile compounds, I wonder how can injection of the n-alkanes’ mixture should be done, by direct injection of the mixture(pure or using a dilution), using a microsyringe at the injection port, or following other particular way to achieve that, such as exposing the SPME fiber to the headspace above the mixture.
HSGC analysis is best option. Otherwise extract n-alkanes in suitable solvent and inject 0.2µl to 0.5µl of this extracted sample directly.
Hello,
1. Check retention time your parrafines from C 6 to C 12 after direct injection with split liner. Orignal you have net mixture - you can inject 1 ul in split 1:150 - 1:250
2. Check retention times your parrafines from C6 to C12 after SPME extraction (i propose PDMS fibre), is dosnt matter what kind of fibre was use for this - You need only correct retention time parrafines.
3. If retentention times will be this same -You have procedure. If not:
- take capillary 5 ul volume (this same for TLC injection) and filled with parrafines mixture net and put in clean vial for SPME with teflon septa and close. Time and temp. in heating block (i think is 40 min in 80 C degrees or 100 degress). Then put inside PDMS and wait aprox. 10 min. Then inject.
Take 0.1to 0.2uL and inject directly. You can calculate Kovats retention indices using following formula:
RRI= 100n0 + 100(RTx-RTn0) / RTn1-RTn0
Dear Roberto,
Please send me your email and I will send you the complete procedure and a simple software to calculate RI values from RT.
My email: [email protected]
I am on vacation until the 15-April and will reply after this date.
Efraim
Dear colleagues,
I wanted to remind that:
Determination of Kovats Index for compounds that are analyzed by HS-SPME versus n-paraffins from the direct injection is not compatible. Especially when we have the solution. When the injection will be done at a column on temperature at 40 C, the lowest boiling compounds (hexene) will condense on the forehead the column and this they cache some parrafins. This effect is change very important retention times of alkanes from C6 to approx. C12. Parrafins from C12 will be have this same RT. This effect can be minimalize with split injection - only from 1:100 to 1:250 - then very thin and short film from evaporated parrafins will be obtain on forehaed column.
Cofnij zmiany
Dear Mariusz:
So you are suggesting that split injection of a certain volume (let us say 1uL) of the liquid standard mixture of n-alkanes in hexane in a split relation 1:100 to 1:250 makes posible to calculate retention indexes for volatile compounds desorbed from a SPME fiber, even for those compounds eluting at retention times that are similar to those corresponding to C7-C12 parafins ?
Dear Roberto,
split injection injection (1 ul in 1:150 split) have better effect.
But the best way is from Static-HS-GC (like suggest Ramesh in first post) if you dont have HS module then try put small amount 1 or 2 ul yours mixture of parrafines to vials, heat, and use PDMS. Thats all.
Of course you recive limitation your parrafines (with boilling point order).
What column type you use? DB or WAX? if you use DB then you can use this propocedure beacouse on this column you need alaknes from C6 to C14 maybe C16.With WAX this could be a problem beacouse this phase generate larger IR. You need C6 - C26 even C30. but the problem is only with alakanes from C6 to C12. thr rest one have similar retention times.
If you use FID detector coupled with GC then the response of your alaknes should be aprox. from 3 x than your signal from FID. This show you how big split you can use. Use alaknes in net solution, without solvent.
Dear colleagues,
Thank you so much for your fast, straightforward and generous comments and responses about this topic. I had taken full note of everything that was posted, and I could make a clear idea of the different alternatives (with their respectives pros and cons) that I have in order to achieve my goal about Kovats indexes.
Sincs
Roberto
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