I shared your question with 17 friends and I hope they will respond. I suggest you to contact Chandan S. Chanotiya (https://www.researchgate.net/profile/Chandan_Chanotiya2) who has carried out extensive GC-MS studies on several essential oils.

Good luck

Dear  Basudeba Kar

maybe you should change the type of injection (split or splitles mod) to get your analyte retention times RT :

In the split injection mode, only a fraction of the vaporized sample is transferred onto the head of the column. The remainder of the vaporized sample is removed from the injection port via the split vent line.

Split injections should be used only when sample concentrations are high enough to allow a portion of the sample to be discarded during the injection process, while still maintaining a sufficient concentration of analytes at the detector to produce a signal.

When target analyte concentrations are so low that splitting the sample in the injection port will not allow an adequate signal from the detector, the injector should be operated in the splitless injection mode. In the splitless injection mode, most of the vaporized sample is transferred to the head of the column.

http://www.chromacademy.com/Essential_Guide_Webcast/Split_Splitless_Injection/Split_Splitless_Injection.pdf

If your standard mixture of alkanes not giving any peak in GC, it means it is either very-very dilute or not good in condition.  I guess, you are getting a peak of hexane only, in which higher alkanes were dissolved. Is it so?

There is no need to derivatise the n-alkanes.  You inject as such and record RT.

I agree with Ram, there is no need to derivatise the n-alkanes. Is it possible for you to upload images of your results here? Also you could contact Dr Prabodh Satyal, he is an authority here. Cheers!

Thank you Mr. Kar for your question and message.Perhaps I saw this question at last in this forum. Many of the experts have already suggested you the correct opinion on this matter. However, I am also submitting my views on this question.

Since the alkane mixture studied contained C20 as the highest saturated carbon. Carbon 20 does not retain with the stationary phase for longer time in a non-polar capillary column such as 5% diphenyl+95% PDMS in a temperature programme condition. e.g., C19, one of the marker hydrocarbon in Rosa damascena essential oil.  Therefore, detection shall not be an issue. Further, I  do not see a reason for NO PEAL detection in your analysis.

You have not written detailed experimental conditions. Therefore, it will  not be an easy task for me to suggest you a single and correct reason for this problem.

However, you need to address some of the points before injecting this mixture in GC/MS :

1. If injection is being done manually, check the septum. Remove septum if injection account reached or beyond 50 or more.

2. For Autosampler, you need to set the injection parameter for effective transfer of your sample into column.

3. Make sure that the carrier flow is proper. I suggest you to keep 1ml/ min in a 30m x 0.25mm dimension capillary column.

4. Check the injector liner.

5. Put Injector port (Split/splitless) at a min temp pf 250 degree for proper vapour formation. Because for correct RI calculations, you are heavily dependent on proper shape of the target peaks. Therefore, splitless injection will not serve the purpose.

6. reduce split ratio (start with either 30 or 40).

7. Inject a min. of 0.5 microlitre of your alkane mixture. Since, Sigma standards are supplied with low concentration. Therefore, you need to adjust the injection amount for a proper detector response. Accordingly reduce the inject size if getting response /detection for proper peak shape.

8. Keep transfer line and source/ heater temp 250 degree to avoid any kind of vapour condensation inside mass spectrometer.

9.Monitor the filament current. If it is blown up, you will not get detector's response.

10. Do not forget to tune your mass spectrometer with heptacosa.

11. Based on above points and experience with this mixture (Sigma) anlysis, the last carbon No. of your mixture shall elute before the column oven temp reaches 200 degree (if the initial temp is 60 degree with 3 degree/min ramp rate in a 5% diphenyl+95% PDMS of 30m x 0.25mm dimension). However, we can not overruled the role of vacuum of mass spectrometer.

I am hopeful that you will see the detection.

Thank you for your message

Best of Luck

Dr Chanotiya, CSIR-CIMAP, INDIA

Essential oil required no need of derivatization, just dissolve your oil sample in hexane or  dichloromethane and then inject direct into GC.

Indeed, detecting alkanes in GCMS should not be a problem. However, there can be several technical issues. When using an automatic sampler, the problem of no peak detection is often due to the solvent level in the vial. If too low, the seringue will not take the sample. Did you check that your seringue is not cloggued ?

If you analyse only standard mixture, you dont have to use any preparation methods. Standard mixture is pure. Derivatization techniques are necessary only if you want to analyse polar compounds such carboxylic acids, sugars etc. I think, there are two possible problems. The first, you analysed very diluted standards so your concentrations were under the limit of detection. The second, there could be a problem in injector, maybe there was a leak and you lost your sample/standard.

you should inject concentrate alkane normal that you get best peak. alkane normal should be kept at freezer to avoid decompose. also it may use demo library in GC/MS software. for peak detection of normal alkane the operator should be use Wiely 7nl.

If the essential oil components of the oil sample are detected by the mass spectrometer in the split mode, there is no reason why the hydrocarbons do not show up. May be you do not have hydrocarbons in the standard. I never had any difficulty of this kind on polar as well as nonpolar columns using split mode (50:1).

Check this thread as well

https://www.researchgate.net/post/Anyone_familiar_with_Kovats_Indices_or_RI_for_Essential_Oil_Identification#55af815e5e9d9737c58b460f

you must change the type of splitless injection to get your retention time of the analyte RT:

In the split injection mode, only a fraction of the vaporized sample is transferred to the head of the column.

Split injections should be used when the sample concentrations are high enough to allow a portion of the sample to be removed during the injection process, while maintaining a sufficient concentration of analyte at the detector to produce a signal.

the injector is to be used in the splitless mode injection. In the splitless injection mode, most of the vaporized sample is transferred to the head of the column.

We have encountered the same problem and tried some not-sophisticated solution.

After simply evaporating excess hexane in air and injecting the residual solution the peaks of alkanes appeared in the chromatogram.