Please provide more information. What type of MS detector did you use (i.e. EI)? The two GC detectors used are orthogonal, entirely different, showing different responses. It is normal to see entirely different chromatograms using two different types of detectors (FID and MSD EI). Did you use an identical (*the same) GC method and instrument for each analysis (just switching detectors) for both analysis runs? If two different instruments were used, then you will need to setup and run calibration levels for each compound you wish to obtain qualitative data for. These calibration tables will provide you with the (1) response and (2) response vs concentration data needed to record any measurements.

Please speak with your teacher or local professional who has many years of experience using the GC instruments (with FID and MS detectors) so they can assist you in developing a good quality GC method. These types of questions are best reviewed on-site (not on the web), with the help of an experienced chromatographer.

If you have identical separation conditions, identical columns in MS and FID, the same gas, etc., then retention times can be similar but not necessarily identical. I would determine the Retention Index, or if I had a pure substance, I would enrich the sample andobserving which peak increases. There are many possibilities.

I agree with the previous speaker "These types of questions are best reviewed on-site (not on the web), with the help of an experienced chromatographer."

@William Letter Thank you professor for your kind feedback.

Right, I just switching the detectors, but I think this doesn't mean they have to be at the same retention time. So, I want to know generally how to identify the product peak using GC-FID chromatogram.

I already asked on-site, but I didn't get a satisfied answer.

Finally, I would like to thank you again for your kind help.

@Tomasz Czernecki

I completely agree with you that the retention time won't be identical, but is there a specified range?

I'll try to enrich the sample and check it again, is this an enough way?

I really appreciate your feedback, thank you for your interest.

When you change the detectors, you may change the retention time (as noted earlier). There is no "specified range" or 'window'. This may be possible to determine if someone knowledgeable was in front of the exact instruments used, but in general it would not make sense to do so. Instead, you need to take the advice already offered to solve it, use std addition [spike it for a qualitative answer] AND have an experienced chromatographer help you (hire someone, or perhaps someone local in industry may volunteer to help you?). Using these complex analytical instruments requires many years of training. This is not something you will learn in a day, week, month or year on your own so please get help (not on the web).

Ok, I'll do, thank you for your concern and kind help.

I think my answer would be about the same as the other who have give you recommendations. Get a better understanding of GC as a method itself. I recommend for you to use this book as reference:

Practical Gas Chromatography - A Comprehensive Reference, Katja Dettmer-Wilde and Werner Engewald (Editors), Published by Springer in 2014.

For the issue you have posted, Please consider the following:

Is the same analysis condition used for both GC-FID and GCMS? The analysis condition would include injection setting (quantity of sample injected, inlet temperature and etc.), Column and Oven (same column use, oven temperature programming and etc.) If it is same, your GCMS (Scan Mode not SIM mode) and GCFID chromatogram should be almost identical. Just place the chromatograms side by side with the same elusion time scale to compare the peaks.

You need to understand that what FID detects, basically anything with carbon and can be ionized in the flame will give you a signal, which make it an universal detector. It is not ideal for compound identification unless you already have a particular peak you know where it will elude. You can do this by comparing it with a standard material by comparing with the elusion time.

For MS, you can check the MS peak from the total ion chromatogram and identify it based on your MS library (provided that you have the suitable MS library) since you will have another layer of data recorded. However, you will need to check the MS range you have select (settings used during analysis). If you don not have it, match it with the closest compound (if it have some similarities (in terms of chemical structure) with your target compound). The worse case is that it do not match with any compounds and need to interpret the MS spectrum from scratch. It would be best you have a standard material to compare against as well (for retention time and the MS spectrum).

@kuok Sum Cheong Thank you for your interest and your answer, I'll consider all of this.

"Your made a reaction and recognized my product using GC-Ms. The problem is, which peak is my product peak? Is it at the same retention time in GC-Ms? I don't think that, so how to solve this issue? Especially the GC-FID chromatogram contains a lot peaks."

My suggestion is : It is simple to know your problem.

1). First determine the retention time of each reactants used in the reaction individually.

2).After the reaction, compare the retention time of each reactants and unknown peak retention time is your product.

Really the only way to confirm which peak is in a GC FIR analysis is to add a known amount of your compound of interest and identify where it shows up, ideally in a standard addition approach. For confirmation, run the sample on another GC column with a different polarity or retention and repeat the standard addition.

First, GC-FID and GC-MS will never have the same retention times since the column ends at atmospheric pressure in the former and vacuum at the latter. This makes the comparison a little bit more difficult but not impossible.

@Tomasz-Czernecki mentioned the retention index, which is a good approach. Another one is to obtain a mathematical equation that can relate the retention times in each case with the injection of a few standards in both instruments - I used this approach to convert retention times from GC-MS to GCxGC-MS around 15 years ago and it worked well.

@Ademario Junior, thank you for your feedback. Could you tell me more details about "with the injection of few standards" this.

@Rajeshwar Rao Bojja Thank you for adding your answer.

But I have a lot of peaks in my GC-FID chromatogram, and this is the problem?

@John Canham Thank you for your answer.

Is This an enough evidence that is my product peak?

Ahmed Thabet: To actually "confirm' the ID, you will need to run orthogonal techniques together with standards. Please contact someone local who is knowledgable in this area for assistance.

Dear compare the retention time of the pure analyte peak or analytical standard with the retention time of the peak in the mixture of compound peaks on chromatogram. In this way you can identify your required analyte and can calculate its concentration.

Ahmed Yar: "retention time" alone can NOT be used for identification. It is a qualitative at best indicator only. Other orthogonal data must be used to suggest an ID (i.e. MS Spectra, other modes of chromatography etc). This is fundamental of chromatography.

Ahmed Thabet If you identified your product using GC-MS, then you need to prepare a solution from your pure product (alone) and do a run using the other GC machine to allocate its new retention time. Afterward, you can allow runs for products mixture. I think the peak's position of your target product will not be changed so high whether this compound is tested alone or in a mixture. You happy is OK, LOL.

Dear William ; I know but as you also said retention time is one of the fundamental and key factor in chromatography. So, It is also important factor that contribute in identification also when multiple components are present it has a key role.