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Sorry I forgot to sign that previous message on Rat poison and inverted chromatograms. The nice thing is that you don't usually fragment the molecules with these sources so ther is really not much reason to look down low. Of course there are exceptions. ps A frequent error is the failure to differentiate polymer background cont. and cluster formation when something is not right with the source (equipment or operator error). Matt Sweeney Mass-Spec Gun for hireReturn to Top
Hello Finnigan Customers: I am writing to say thanks for allowing me to be part of your world. My tenure as one of Finnigan's Technical Specialists/Institute Instructors has come to an involuntary conclusion. It has been my honor and pleasure to assist some of you in the use and maintenance of your instrumentation for the past eight years. I plan on frequenting this newsgroup to continue volunteering my expertise/experience. Again, I thank you all. Cordially; Christopher J. KaineReturn to Top
The inverted chromatograms are often seen when the instrument is scanned from a relatively low starting value. Although it depends upon the solvent system/ionization mode the following are a few ideas. In ESI/APCI if you scan low there will be the inevitible chemical background detectable under ALL conditions. This background is both chemical species in the form of "contaminants" and cluster ions forming and reforming in the source. The great softness of atmospheric sources that every one loves also allows the operation of these under conditions in which cluster formation is more or less favored. A headache for some and plain old chemistry to others. Basically the input of the desolvation energy that removes the LC solvent from the analyte can also fragment the analyte if applied in too large an amount. Conversely you get clusters of LC solvent and analyte if you run it it too low. You also get clusters of LC solvent without analyte. Like three methanols, a wate,r and a proton. These are frequently observed as a series of ions with a statisticall bell curve like distribution with a typical spacing value of say water or methanol. Now under some conditions delicate labile analytes may require lower energy inputs to avoid fragmentation. These same conditions may favor cluster formation at low mass. So what to do? ANS: Scan from a mass Higher than the inevitable background ions. Usually these sources are being used because the analysis won't work by EI/CI with a GC (smaller molecules are usually more volatile and /or less labile). The inverted chromatograms can occur when the desired analyte elutes off the column in to the busy source (as explained above) and begins to compete for ionization. If the source contains a plasma like APCI then the plasma cycle components and clusters grabbing up the charges will begin to donate some to (or be robbed by) the analyte. If you are scanning low enough you are actually monitoring the depletion of these species (and so the deplelted chromatogram (observeable in GC/MS using CI as well) If you know the mass of your charged analyte then the plotting out of this value should give you a nice single ion plot that you can use to quantitate your species. Even when the RIC or TIC looks bad. The background ions may cahnage over a gradient as the solvent compostion cahnges. So you might consider that as well before you decide what starting m/z value you are going to use for your analysis.Return to Top
On 31 Dec 1996 09:16:55 -0500, cody@jeol.com (Chip Cody) wrote:
}Some research has gone into predicting fragmentation based on a known
}structure -- I dabbled with this myself and I saw a really nicely done
}version of a program that actually draws reaction mechanisms presented
}by someone from NIST at a recent ASMS meeting. However, I understand that
}this program is still in the development stages.
}
Chip;
I was unable to attend the ASMS this year, if you hear any other
information about this program, could you let me know?
thanks....
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