Asset Mopidevi: Two very basic issues must be addressed to answer the question. NIST library matches are "suggestions" only. They are only as good as:

(1) The proposed GC-MS method used must be shown to be selective and fit-for-purpose following good chromatography fundamentals. Poor quality methods will yield poor quality results (and misleading or false matches / purity values);

(2) The MS instrument settings used and the NIST library settings used in the method for the method must be appropriate and optimized for the method.

Accurate peak assignments requires that a properly trained GC-MS operator uses a high quality method to obtain quality data. The databases only have value when this is true. GC-MS operation and training takes several years of full-time experience to learn the basics.

• Have your GC-MS method evaluated by an experienced, professional GC-MS chromatographer to insure it follows good fundamentals. Once the method has been found to be valid, then utilize the library database to make qualitative peak ID's and then check with standards and orthogonal methods for accuracy.

In my opinion even well-optimized GC-MS, even pure compounds rarely match NIST spectra perfectly. However focusing on careful calibration, separation, and high-quality data acquisition can help improve the probability match.

Some tips to improve the match:

• Make sure you are searching the appropriate NIST library for your instrument type and columns.
• Manually review the spectral match to confirm the major ion peaks align. The algorithm may overly penalize small differences.
• Improve analyte separation from matrix through extraction, column selection, etc.
• Try calibrating the library match algorithm with spectra from your pure standards.
• Acquire high-quality spectra from your samples by optimizing parameters and averaging scans.

quite correct ... as the NIST spectra are edited and the measured spectra always contain background 100% F/R can never be achieved, However P 20-60 smells like something more is not in order.

Some previous questions:

You are using cuadrupole mass spectrometer in your GC/MS?

Did you done tuning previously the injections? 70eV?

Did you done background subtraction previous running the search?

Is your molecule the right isomer? or a mix of isomers?

best

Eleuterio Umpierrez:

I am using quadrapole Mass spectrometer and have done Tuning. I am also operating at EI of 70eV.

Thank you for suggesting the background subraction. I will try that.

I am purchased the pure standards and prepared the stock solution. So, i am sure that compound might right.

I also have issue with theisomerization of the compound but even isomerized compound is also very low match.

Can you suggest me how to reduce the isomerization of the compounds?(parameters that have to changed in GC- MS)

If your tunning was good, prepare a 1ppm solution of your standard

and inyecte again. If you have too big peak the fragmentation is distorted.

In big peak never collect the medium high scans.

For the background try before and after, which is more useful.

For isomers you will need an special column or do estero derivatization

You don't tell us what kind of instrument nor which library you are using - it makes a difference. To quote Rod's law: "The more you don't tell me the more I can't help you".

You also haven't told us which tuning criteria you used - that can make a huge difference. The library results have to be used with caution; the matches are probability-based and how you structure your search makes all the difference in the world. How you analyze your samples makes a difference, where you select your spectra in your peak makes a difference, how you background correct makes a difference, etc. etc.

Library searches are useful tools in the hands of highly skilled operators. They are worse than useless in the hands of unskilled operators as they can provide wildly inaccurate results if you don't know what you're doing.