****************************************************************************** From: "Robert Briggs" Date: Thu, 21 May 1998 22:54:24 -0400 Subject: Chloromethane spectrum in the ion trap Organization: AT&T WorldNet Services The ion trap GC/MS of chloromethane (and dichloromethane) from aqueous samples (purge and trap) is characterized by mass spectra very different from those obtained by conventional quadrupole instruments under identical conditions. The molecular ion cluster, rather than mass 50/52 (or mass 84/86/88) , has m/z 49 (or m/z 83, very faint) as the principal ion. I have seen laboratories trying to do volatiles by EPA CLP methodology on an ion trap led astray since the recommended (required?) quantification ion is m/z 50. It would appear that hydrogen atom or anion abstraction is occuring. One can invoke interactions with water ions for chloromethane since the water background is high at the start of the run. I'm not sure that the water is still as high when dichloromethane emerges. Has anyone done a systematic study of why this occurs? ****************************************************************************** From: "IonTrapGuy" Date: Fri, 22 May 1998 09:16:13 -0700 Subject: Re: Chloromethane spectrum in the ion trap Organization: ICGNetcom The ion trap mass spectra of these compounds is influenced by a variety of factors. It's probable that you are getting ion-molecule reactions with both water and with the MeOH that is present in your standards and/or surrogate/internal standard spiking solution. The reasons for this are many, but the include the following. 1. The residence time of an ion in the trap is considerably longer than the transit time of an ion in a quadrupole or magnetic sector instrument. If a reaction or degradation has time to occur, it has a much longer time in which to do it. 2. Most ion traps are heated to a fairly high temperature, leaving a substantial surface area of hot metal that can react with neutrals to yield degradation products. 3. Most ion traps co-mingle the neutrals and ions, allowing ion-molecule reactions. Good water management practices such as using a relatively hydrophobic trap (does anyone really use Tenax,/silica gel/charcoal any more ?), using a 2-5 minute dry purge, using relatively high bake and desorb temperatures, using a water management trap like a Tekmar MCM or the integrated system in the Tekmar 3000, and using relatively small quantities of MeOH in your standards will make the analyses much more consistent. Even though many people are loathe to do it, cryocooling the oven and using a start temperature of about 0 C. also helps to move the majority of the water in the chromatogram where it has the least influence. It's interesting to note that the original version of method 524.2 left out method performance statistics for chloroethane because (quoting from memory) "an authentic standard of chloroethane was not available." I suspect that they just didn't know what ions to look for when they were trying to find it. All varieties of mass spectrometers seem to have advantages and disadvantages and I hope that this doesn't lead you to believe that the trap is poorly suited for doing volatiles. Properly tuned and set up, the ion trap has such a significant performance advantage over even modern quadrupole instruments that it reduces the cost of analyses. Because ion traps don't exhibit squash the high masses like quadrupoles do, the sensitivity of an ion trap is even further enhanced if you use it as a semivoa instrument. We routinely run analyses on ion traps that would normally require the use of a specific detector, like 8141 pesticides and 8151 herbicides, very low level PAH's, etc. without resorting to SIM. ****************************************************************************** From: "William R. Behymer" Date: Wed, 27 May 1998 01:35:26 GMT Subject: Re: Chloromethane spectrum in the ion trap Organization: IgLou Internet Services (1-800-436-4456) We have a Finnigan GCQ ion trap and have noted similar behavior in the GC analysis of organic acids, alcohols, etc. While such deviations from traditional spectra generally do not present a problem in quantitative analysis, how does one deal with this phenomenon when using an ion trap for the identification of unknowns? Sometimes the difference is sufficient that searching through standard MS libraries is futile and identification must be accomplished by other means. We do not yet have much experience with the ion trap. Does anyone have any words of wisdom for dealing with this situation? TIA Bill ****************************************************************************** From: Don Magin Date: Mon, 01 Jun 1998 08:06:20 -0400 Subject: Re: Chloromethane spectrum in the ion trap Organization: Whitehall Robins I have used ion traps for almost 10 years now, and have not found this to be a problem in identifying unknowns (the primary reason I use the ion trap). In dealing with it, the first thing is to make sure that the trap is not saturated with ions. Whenever I suspect this behavior, I look at the spectra at the very beginning and/or end of the peak, even the very first spectrum (after a background subtraction, of course). Many times this low intensity spectrum will give a better match in spectral libraries. With a few exceptions, careful manipulation of the spectra will provide good library matches for unknowns. This behavior can be an additional confirmatory piece of evidence. Generally, I have found that acids and methyketones almost never give the proper ratio of M+ to (M+1)+ The best advice I have would be to keep track of the compounds which exhibit this behavior, and use it as an additional positive identification. -- Don Magin Senior Research Analytical Chemist Whitehall-Robins Healthcare 1211 Sherwood Ave. Richmond, VA 23220 (804) 257-2572 (voice) (804) 257-2998 (fax) email: magind@md.ahp.com