Hi,

Please share your GC-conditions. But some thoughts:

1. You are injecting too much esp. for n-octyltriethoxysilane: dilute 100 or 1000 times lower and see.

2. Make the runs longer to make sure and add a good equilibration time to flush out carry over.

3. Use wash solvent as X, if your compound is dissolved in X, and not different solvents.

4. 90% is not very pure- what about the 10%? Most organic compounds would have stereo(geometric) isomers and that constitutes a good area % too.

5. Make sure, good column conditions, and no issues of column bleed, septa bleed, and other handling contaminants (plastic tubes/vials etc.).

6. Early elution means that the run conditions are not optimized, so start with lower temperatures of in let/ injector port, lower oven temperatures, and less steep temperature ramp/gradient, and bake out well at the end of the run to make sur everything is off the column/ system.

7. Change liner and septa frequently esp. if the GCMS system is a shared resource and is being shared with other users and their analytical experiments.

Hope the above helps.

Thanks,

All good answers. Most likely culprit is the injection is too strong and that is why the small side peak is there. When you dilute the sample, those other "peaks" will also be decreased to look more like the impurity peaks that make up the other 10 percent of sample.

Please provide DETAILED method conditions when asking for assistance with complex techniques such as liquid or gas chromatography. No useful suggestions can be made without such basic information being first provided.

Please also ask for training and help with your analysis at your school. Their must be someone there in charge of the instrument and who also arranges for training and providing assistance to the students using it. For such basic questions about how to use the system, you should direct your requests in-person (at your school) rather than the web.

90% is low purity. Also you attached chromatograms, not spectra.

Do you have auto sampler for injecting a sample? Because sometimes injection method may be affect. So provide the method or related information that can help to solve the problem.

I congratulate you on relatively decent chromatography. However, as others have noted, you are injecting too much material. Your scale is e^9, you want to work between e^7 and e^8. The best way to achieve this when looking at solvent purity is to raise your split flow and inject small volumes of your solvent. When we run solvent purity we usually split 1:100 and inject 0.5 uL.

GC-MS is not good at distinguishing between isomers or structurally-related compounds, especially if you are relying on the library for identification. When you have too much signal your spectra become distorted and harder to match to the library. Injecting less and splitting more may help you with identification. Also, the siloxanes are notoriously hard to match to library spectra, so don't be disappointed if you don't get good matches. From a library match standpoint we typically ignore results that are less than 80%, except to use them to give us an idea of what type of compound we might have. There is still no substitute for interpreting your own spectra, so break out your copy of McLafferty and work through it.

Another useful tool is use of retention time data compared to known standards. I always used long-chain n-hydrocarbon standards of 14 to 40 carbons. The (automated) mass spec data often confuses hydrocarbon structures especially when branched isomers are present. As said above, your samples want to be diluted, so cogeners (isomers) are minimized.

@ Biswapriya Biswavas Misra @ Wayne Morris, & @ Nadeem Khan @David Arthur Carlson Thank you very much for the suggestions. I diluted the sample as advised and I observed significant improvement on the chromatograms. I am still new to the GC-MS method and learning.

@ Trenton Davis. Yea, sorry for the mix up of terms. I had it corrected. Yea, 90% is not totally pure and there are bound to be peaks from the impurities but I expected them to be very small peaks since the remaining 10% should be distributed among the impurities.

@Shradhdha Naku . I have the autosampler

Sorry for not including the operating conditions/parameters

@ William Letter Thanks for the suggestion. I had a training session today and I learned a lot from the specialist. Even more than I thought would be covered. Thanks alot.

@ Mark Krause That was very helpful. I diluted the samples and I diluted the sample and I also tried split 1:100 and inject 0.5 uL .

Thank you all very much. I had significant improvement following your recommendations.

Change the column on your chromatograph for a new one and try the analysis again.

Many homolog compounds like n-alkanes, carboxylic acids have very similar MS spectrum. The molecular ions in EI mode have very low abundance, thus for example n-C24 , n-C20, n-C18 alkane can be identyfied as n-C10.

Independently of that - some ghost peaks can be formed by dirty split trap, so if you "play" with split vent settings like delayed split/solvent purge mode etc. this can cause additional peaks.

regards,

G

In case of volatile oil number of isomers found in one more place same name like but co injection rewired for this type of problem held on number of molecules simialer chemistry

It is good that you learned a lot from your trainer. Some of the problems we usually encounter are related either to the operator or the sample itself. But the normal tendency is to doubt the equipment. In an instrument like GCMS, the sample quantity that is injected is normally very small. This will improve many factors and also prevent carry over etc. So either the concentration of the sample should be reduced or the sample quantity should be reduced by increasing the split ratio. MS is quite powerful to detect targets even at low concentrations. We normally inject a large quantity, especially for trace residue analysis, only with a PTV injector.

All that above is reasonable. I wonder if you are using a column compatible with mass spec. In other words it has lower levels of column bleed.

Many years ago, we used chemical ionization mass spec. for GC/MS: This gave useful molecular ions for series of isomers and "families" of compounds. Computerized identification via EI mass spec. was otherwise poor for series of let us say- alkanes. as EI gives no fragment useful peaks for ID. The chromatography allows the chemist to obtain proper identification.

Certainly yes. You can get multiple peaks for the same compound depending on the degree of derivatization, especially in a complex mixture. We often derivatize analystes in GC-MS before their detection and if a compound has for example multiple hydroxyl sites, then there is a possibility that in some molecules of that particular compound, derivatization is not driven to completion and you would end up seeing multiple peaks for that same compound.

Dear Adedamola,

If you have multiple peaks for the same compound on a GC-MS, it's related to the column. When the instrument's column does not work properly, this problem arises. Because of each compound have a specific retention time and separate at the one time. Please check out the column. Maybe it has leakage in end of the column.

Sincerely yours, Homayoon.