Chromatography Forum

These are the TICs of solvent extracts of the fruits. As can be clearly seen, some peaks have very odd shapes with tailing. Can anybody comment on why is it so?

Could be the column, could be the mobile phase, could be the buffer...

Could also be the materials put on the column. If this is GC, look to see what the copunds are. There are some compounds that are worse about tailing than others.

Could be the injection system - again if this is GC and the inlet is too cool, the later eluting compunds would spread out as they leave the inelt and would give poor peak shape.

Give us a bit more detail lest speculations run rampant.

By the way, notice that you have some nicely shaped peaks in the later portion of the detail 14.somthign minutes - if I read the plot correctly.

So, I would guess that it is most likely a compound - column interaction thing going on.

Thanks for the comments and sorry for the incomplete information!
These are the GC TICs of direct solvent extracts of the fruits...
Yah...there are some good-shaped peaks also....You can see those, eg, in the lower red TIC between 16th and 17th min...Even the peaks before the highest peaks are clear-shaped
Injector was quite hot, at 200C, carrier helium-1ml/min, Oven: 40C-5min, 10C/min upto 220C, hold-5min., column-SLB5MS..
We also conditioned the column for ca 6 hrs at 300C which didnt work...
Also tried changing the liner and injector septa...
The main compounds are monoterpenes, the highest in the lower one is Z-ocimene, whereas in the upper one its alpha-terpinene...
The smaller peaks between 16 and 17th min are sesquiterpenes-longifolin, beta-caryophyllene and humulene respectively...
15.2 is an internal standard..nonyl acetate...
11.32 in the upper one is mesifuran..its there in the lower one at the same time...we also expect smaller peaks of some delta and gamma lactones (eg. hexa, octa and deca lactones) in the problematic region
Previously we have got good results with the same fruits from different region and with the same protocol and instrument for extraction and analysis...

There are several possibilities for the tailing peaks. First, since you are using a mass spectrometer look at extracted ion chromatograms to verify the peaks are really tailing and this is not multiple compounds that are not resolved.

The injector temperature is on the low side, most of the time I use an injector temperature of at least 250C.

If there is a polarity mismatch between the analytes and the column stationary phase the peak shape will be distorted. I noticed that you mentioned some compounds that have polar groups, a wax type column would give a better peak shape for these compounds.

There was a 5 minute temperature hold at the beginning of the run. This can cause peak shape distortion if the injection band spreads out during this long hold. I usually hold for 0.5 to 1 minute.

My problem got solved to some extent...
I cut the column from the injector end by couple of inches...
Following are the chromatograms...
Although little extent of tailing is still there, peaks are quite clear now...
Thanks to all; more suggestions are welcome!

Hi!

If the column installation is OK, you must take into account:

What solvent do you use? The initial oven temperature must be at least 10ºC below the boiling point of the sample solvent.

Try with a ramp of 30 or 40ºC / min for about 1 to 1.5 minutes after the initial 5 minutes. This acceletares the solvent elution.

We are using DCM for extraction. We start the oven temp from 40C and hold it there for 5 min. Solvent delay is up to 3.5 min before which it gets eluted (during 5 min initial hold). Do you mean, for DCM we should start with 30C for 5min followed by high ramp? If we are applying high ramp at this stage and if there is going to be solvent delay till it gets eluted, why to start at lower temperature first and then apply high ramp to elute solvent faster? Why not to start itself at higher temperature?

I thik that I expressed bad. My English is bad.

First, I don't know what DCM is.

In order hand, I wanted to say that instead of the ramp of 10ºC/min after the 5 minutes at 40ºC, you must to try with a ramp of 30-40ºC/min for about 1 to 1.5 minutes after the initial 5 minutes and then the ramp of 10ºC/min.

It is just a counsel.

The strategy of starting below the boiling point of the solvent that astroguille has described is a way to get solvent focusing. If you can get the solvent to condense at the head of the column, the compounds that were injected in the solvent can stay disolved in the solvent. As the solvent begins to evaporate, it is from the inlet end of the column. The colvent band becomes narrower and anythign that remains disoved in the sovent forms a narrower band. When the solvent completely evaporates, the compounds that have stayed wtih the solvent are left in a narrow band. This results in sharp chromatographic peaks for the compounds that elute shortly after the solvent. When you get this effect to work, it can be very helpful. The quick ramp gets you back to the kind of temperature program you were using with the injection onto a warmer column.

And DCM: Dichloromethane (CH2Cl2)

Hi
The DCM has a low boiling point. If you won't start at a low temperature, you can change the solvent. Add 1ml of isoctane and evaporate. I think isooctane has a bp= 99ºC. It works well to splitless

Increasing injector temp to 250 might help.
Cut about another foot of column.

Scrub the injection port with a wire brush, swab it clean with methanol/hexane, and then deactivate it with DMCDS (Supelco Cat #33065-U). This usually takes care of most activity problems when I run a similar method to what you are doing.

If you are making a splitless injection, try these parameters and see if it helps.

Initial temp: 40C hold 0.75min
Splitless for 0.75min then split @50:1 to sweep DCM out of injection port.

Hi,
I am using Thermo PolarisQ GCMS. I am working in pesticide analysis in food products. the standard i am using is a mixture of 15 compounds. But the TIC of the analysis shows almost no peaks in it. But when i search the compounds ushing their base peak then i can identify the compounds one bye one. Can anyone suggest me to get the TIC for all the 15 compounds at a time?

Thank You

Seeni

K.m.Seenivasan,

Typically a GC/MS data system is designed to use a single ion that is free of interferences for quantititation - and you specify a particular ion for identifying a compund for quantitation. I would expect that you should be able to give the TIC as your quantifiaction ion. However, if you have coelutions, this will become a problem.

In the case of coelutions, you would want the TIC to be divided according to the individual components.

If you purchased the NIST library with your instrumet, you are likely to have a copy of the AMDIS software in the CD that came with the NIST library. If you can not find the disk, AMDIS can be downloaded from WWW.NIST.Gov

Dear Don,
Thanks for the reply. My problem is , the software NORMALIZE the peaks with reference to the highest peak in the chromatogram. While doing so it suppreses all other peaks. So it is impossible to view the actual number of peaks that are eluted in the analysis. You have to manually search the presence of each and every compound in the analysis.

Regards

Seeni