Can anybody give an explanation to the peak splitting in SIM mode of GC/MS, while in TIC mode for the same sample I have beautifull peak form.
By Jason Ellis on Friday, February 16, 2001 - 08:22 am:
Data acquisition (scan rate) in SIM mode is usually much faster than in scan mode. A faster scan rate gives more data in a chromatogram and can show slight differences in resolution not observed with slower scan rates. My guess is that your peaks are always splitting, but that you aren't seeing the splitting in scan mode because of this.
Why your peaks are splitting is another matter altogether. Are they splitting evenly down the middle or is the split uneven (two peaks of unequal size)? What are your injection conditions and what is your sample? Peak splitting is either caused by multiple distinct "injections" of sample into the column or by a solute focusing problem. Check your column installation distance in the inlet -- if you are doing splitless injection you can get peak splitting if your column tip is too far up in the inlet liner during injection.
By Anonymous on Monday, February 19, 2001 - 10:06 am:
Hi, Ellis. Thank you for your reply.
The peak splitting concern only 1-3 peaks from the whole chromatogram (more than 15 peaks of MTBSTFA derivatives of amino acids). The splitting is occasional and it is evenly down the middle of the peak. At the split location the relative intensivity of selected masses changes.Sometimes there is more than one splits at the top of the peak.
The injection conditions - splitless injection with injector temp. 300oC and splitless time 1.2min.Column start temp. 120oC.
Can the reason be the mass spectrum aquisition conditions (scan time 0.46sec)?
By Anonymous on Monday, February 19, 2001 - 11:18 am:
can it be there are stereo-isomers of your compunds. it does not seem logical that some peaks are splited and som are not.
By Anonymous on Monday, February 19, 2001 - 11:33 pm:
There is nothing to do with stereoisomers. The more probable reason is solute focusing.But I can't explain the fact that at the
split location the relative intensivity of selected masses changes and sometimes there is more than one split at the top of the peak.
By Anonymous on Tuesday, February 20, 2001 - 05:24 am:
Are you maxing out the electron multiplier? It sounds like the problem may be electronic, not chromatographic.
By Jason Ellis on Wednesday, February 21, 2001 - 12:29 pm:
The "electronic not chromatographic" theory sounds very plausible to me. What concentration are you injecting?
Also, what is your stationary phase and what is your sample solvent?
Do you use an autosampler or do you do manual injection?
The peaks that are splitting, are they at the beginning, middle or end of the chromatogram?
By Anonymous on Wednesday, February 21, 2001 - 11:43 pm:
I use autosampler and concentration injected are
0.35 to 1.50ng/ml of MTBSTFA derivatives of amino acids. If the problem is electronic what the concentration have to do with peak splitting?
I use Alltech AT5ms column with 5% phenyl type polarity column.The splitting regard norleucine(internal standard), hexadecane (internal standard) at the beginng of chromatogramm and hydroxyporline at the middle of it.
Concerning internal standard splitting it does not change dramatically the area of the peak - RSD=1.5%.
I will look to focusing problem, because splitting reffer to concreete peacks and not it's not random as perhaps in case of electronic problem,
By B. Kochanowski on Thursday, February 22, 2001 - 06:39 am:
A common electronic reason for peak splitting is an overloaded electron multiplier. It might not seem possible with SIM, but it is common. When you scan the mass spectrum, a lot of time is spent looking at low intensity mass fragments or empty space. With SIM, you are constantly looking at a mass fragment you know is there, and one that is usually picked for it’s strong intensity. The peak splitting is a result of the MS manufacturer’s safeguards with respect to the multiplier. When the multiplier gets too high, the electronics are shut down, and don’t return until the multiplier is back to normal range, giving you a split down the middle. This is why you don’t see a flat topped peak to indicate you are out of range. This situation may not occur on all MS equipment, it depends on the manufacturer. I believe Agilent is one of them. Perhaps someone from Agilent reading this thread can confirm this.
Another reason could be due to changes in the spectrum. Is the spectrum consistent across the entire peak in your TIC? If there is a high concentration of sample in the source you can get what is called “self CI.” A molecule picks up a proton from another of the same molecule in the ion source. For example, if you’re monitoring m/z 300 and self CI gives you m/z 301. This will change the chromatogram (ion current) at the center of the peak because the ions are changing from m/z 300 to m/z 301, and you are only monitoring m/z 300. The situation will return to normal when the concentration in the ion source decreases.
Both situations can be checked by injecting less sample.
By Ron on Thursday, February 22, 2001 - 09:40 am:
If you have good peak shape for a sample in scan mode and split peaks for the same sample at the same concentration in SIM mode, I would be very surprised if the problem is not saturating the electron multiplier. SIM mode is used to increase sensitivity, so if you have adequate sensitivity in scan mode the samples will more than likely have to be diluted or the electron multiplier voltage lowered to run in SIM mode. Check to see if the maximum intensity of the peak is the maximum value allowed by the electronics.
The key point I see here is that things looked fine in scan mode. Changing modes in the mass spec cannot affect the chromatography, so look to something in the mass spec operation causing the problem.
Have you tried contacting the instrument company's technical support staff to see what their diagnosis of the problem is?
By Anonymous on Tuesday, February 27, 2001 - 11:46 pm:
Thanks everybody for assistance. I will check all the possibilities and inform GC-brotherhood later about the results.
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