****************************************************************************** From: "Peter M. van Galen" Date: Mon, 16 Jun 1997 11:48:57 +0200 Subject: some discussion about precision ?? Organization: KUNijmegen Hello, recently somebody asked me the differences between accurate mass, exact mass and peakmatch. I explained that exact mass and peakmatch were the same in accuracy (<= 1 mmu) of determining the mass of an ion-peak and that in combination with an elemental analysis one could predict the gross-formula of that peak. Accurate mass does not give enough precision (10ths of mmu) to be certain on this. Was I correct and if not would anybody correct me in this please. with regards, -- |----------------------- Peter M. van Galen, |---- Massa Spectrometrie, Vakgroep Organische Chemie |--------------- K.U. Nijmegen, Toernooiveld 1, 6525 ED Nijmegen |- E-mail: pvang@sci.kun.nl FAX 024 36522929 ****************************************************************************** From: James Pavlovich Date: Tue, 17 Jun 1997 15:37:08 -0700 Subject: Re: some discussion about precision ?? Organization: University of California, Santa Barbara Not quite, at least in the way I have seen them used most recently. Accurate mass is a measurement of an ion's mass to within a specified error. (Usually + or - 5 ppm for ACS publications) Exact mass is the mass calulated from a molecular formula using known masses of specific (usually the most abundant) isotopes with the appropriate number of decimal places. It is to the exact mass that the measured accurate mass is compared to determine elemental composition. Peakmatch is one specific technique of of measuring accurate mass or the measurement obtained using that technique. "Exact mass measurement" is used in McLafferty's book to mean a mass measurement of sufficient accuracy to unequivocally assign an elemental formula (and, therefore, an exact mass) to that ion. The required accuracy depends on the number of elements in the ion as well as its mass. Anyway, that is the way I use them. Is there a consensus? I did not look for these terms on the terminolgy poster that was at ASMS were these on it? ____________________________________________________________________ ____________________________________________________________________ James G. Pavlovich, Ph.D. Mass Spectrometry Facility Phone: 805-893-4252 Chemistry Department FAX: 805-893-4120 University of California Email: pavlovic@chem.ucsb.edu Santa Barbara, CA 93106 ____________________________________________________________________ ____________________________________________________________________ ****************************************************************************** From: cody@jeol.com (Chip Cody) Subject: Re: some discussion about precision ?? Organization: JEOL USA, Inc. Date: Wed, 18 Jun 1997 13:57:45 GMT There is a discussion about the interpretation of exact mass measurements on our web page (www.jeol.com) under the "Our Products--->Mass Spectrometry" section. That discussion includes some examples of how exact mass measurements can produce many elemental compositions that must be interpreted by using additional information (isotope ratios, etc.) to screen the possible compositions that match the measured mass within a given error tolerance. Concerning terminology, peak matching is an instrumental method that is used to make exact mass measurements. One can also measure masses accurately by other means; voltage scans and magnet scans are commonly used for exact mass measurements on magnetic sector mass spectrometers. "Accurate mass" and "exact mass" mean essentially the same thing, but "exact mass measurement" is the preferred term at present. One should distinguish "precision" (reproducibility) from "accuracy" (how close the measured value is to the true value). -- "For purposes of ... New Jersey Right to Know Act. Contents partially unknown." |____________ |_ Robert B. Cody, Ph.D |________________________________ Product Development Manager |__ Mass Spectrometry |________________________ JEOL USA, Inc. |_ |__________ http://www.jeol.com |_ e-mail: cody@jeol.com ==============[ Do not send me spam or advertising via e-mail !! ]========= ****************************************************************************** ****************************************************************************** From: "John Bartmess" Date: Thu, 19 Jun 1997 15:47:53 EST Subject: Re: some discussion about precision ?? Organization: Univ. of Tenn. Dept. of Chemistry The subject was not on that poster. However, the Bound Volume for the 1994 Chicago ASMS meeting ("Proceedings, 42nd ASMS Conference on Mass Spectrometry and Allied Topics") on pp. 178 and 179, has poster abstracts on the issue of exact mass measurement. "Exact mass measurment is the determination of m/z values with enough precision to distinguish elemental composions." The problem arises with specifying so many ppm as good enough for this; 5 ppm isn't good enough beyond about mass 200 to do this unequivocally. JASMS and J. Org. Chem. no longer hold to the 5 ppm criterium; JACS does. John Bartmess member, Measurements and Standards Committee, ASMS ****************************************************************************** ****************************************************************************** From: David Sparkman Date: Fri, 4 Jul 1997 13:13:51 -0400 Subject: Exact Mass/Accurate Mass Organization: * Several different definitions of the terms "Exact Mass" and "Accurate Mass" appeared as a result of someone looking for clarification. First, it is important to remember that when we talk of mass in mass spectrometry, we almost always are talking about a mass based on not only the elements that comprise an ion or neutral but also the individual isotopes of those elements. If we talk about mass in terms of the integer values for the most abundant, naturally occurring stable isotopes of the elements that comprise an ion or neutral, then this (by definition) is the nominal mass. If we talk about mass in terms of the calculated value (to some number of decimal places) for the most abundant, naturally occurring stable isotopes of the elements that comprise an ion or neutral based on 1 mass unit being 1/12 the mass of the most abundant isotope of carbon, this is the monoisotopic mass (by definition). I have always used the term "Exact Mass" to mean the mass of an ion or neutral that is calculated (to some number of decimal places) based on the mass of the isotopes of its elements as these individual masses are calculated based on the carbon 12 standard. I have used the term "Accurate Mass" to mean the mass (to some number of decimal places) that was measured. The accurate mass was always reported with some accuracy qualifer, such as a ppm value. In a recent discussion with my good friend and colleague of many years (J. Throck Watson), he pointed out that I was not exactly accurate. There is nothing in the definition of exact or accurate to tell the reader or listener whether these had to do with a measured value or calculated value. He said that it would be better to talk about these two values with respect to how they were obtained (measured mass or calculated mass). I would like to propose that we stop using the ambiguous terms of "Exact Mass" and "Accurate Mass" and use the terms "Measured Mass" and "Calculated Mass" (which does not present any ambiguity). I would like the comments of others. O. David Sparkman Consultant-At-Large Phone: 1-510-754-5003 ***************************************************************************** From: cody@jeol.com (Chip Cody) Subject: Re: Exact Mass/Accurate Mass Date: Mon, 7 Jul 1997 21:26:55 GMT Organization: JEOL USA, Inc. David, I personally prefer to use the terms "exact" and "accurate" in the same way that you do. That is, "exact" mass is the calculated mass, and "accurate" mass is the measured mass, with its measurement error. This seems to be more consistent with the way that we teach students to distinguish between "precision" (reproducibility), and "accuracy" (closeness to the true value). To me, "exact" suggests freedom from error, and is better suited to describing the calculated value. Let's ignore the fact that the calculated value is based upon the accepted values for the atomic masses, which have some error of their own... My Webster's dictionary isn't much help..."accurate" and "exact" are treated as synonyms. Jack's suggestion is excellent in distinguishing the calculated mass and the measured mass. I would support that usage (and so does most elemental composition software). However, that leaves us with the problem of how to refer to the _experiment_ that we are doing: "exact mass measurement" or "accurate mass measurement". I understand that the ASMS nomenclature committee prefers the former usage. Anyone from that group care to comment? Chip Cody In <5plpbo$7c5@acmey.gatech.edu> David Sparkman writes: }Several different definitions of the terms "Exact Mass" and "Accurate Mass" }appeared as a result of someone looking for clarification. } .. one paragraph omitted here... }I have always used the term "Exact Mass" to mean the mass of an ion or }neutral that is calculated (to some number of decimal places) based on the }mass of the isotopes of its elements as these individual masses are }calculated based on the carbon 12 standard. I have used the term "Accurate }Mass" to mean the mass (to some number of decimal places) that was }measured. The accurate mass was always reported with some accuracy }qualifer, such as a ppm value. }In a recent discussion with my good friend and colleague of many years (J. }Throck Watson), he pointed out that I was not exactly accurate. There is }nothing in the definition of exact or accurate to tell the reader or }listener whether these had to do with a measured value or calculated value. } He said that it would be better to talk about these two values with }respect to how they were obtained (measured mass or calculated mass). }I would like to propose that we stop using the ambiguous terms of "Exact }Mass" and "Accurate Mass" and use the terms "Measured Mass" and }"Calculated Mass" (which does not present any ambiguity). }I would like the comments of others. }O. David Sparkman }Consultant-At-Large }Phone: 1-510-754-5003 -- "For purposes of ... New Jersey Right to Know Act. Contents partially unknown." |____________ |_ Robert B. Cody, Ph.D |________________________________ Product Development Manager |__ Mass Spectrometry |________________________ JEOL USA, Inc. |_ |__________ http://www.jeol.com |_ e-mail: cody@jeol.com ============== Do not send me spam or advertising via e-mail !! ========= ****************************************************************************** From: David Sparkman Date: Tue, 8 Jul 1997 05:59:10 -0400 Subject: Re Exact Mass/Accurate Mass Organization: * I recived the following comments from Ken Matuszak regarding my earlier posting on Exact/Accurate Mass: While "Calculated Mass" presents no ambiguity that I can see of, the term "Measured Mass" would have to still be defined. This latter mass can be severely dependent of the resolution used to aquire the spectrum. For larger MW compounds, if sufficient resolution is no used to define all the isotopes, one might end up with some hybrid between the monoisotopic mass and the average mass. This is specially true for multiply-charged isotope packets which become more more difficult to resolve as the charge state increases. Ken, you are absolutly accurate. If a measured mass is reported, it must be reported with respect to a stated accuracy (ppm or some resolving power). And further, I exactly agree with the importance of this for multiple-charge ions and their isotope envelopes. Regards; O. David Sparkman Consultant-At-Large ****************************************************************************** From: "Frederick H. Strobel" Date: Tue, 8 Jul 1997 08:51:49 -0400 Subject: Re: Exact Mass/Accurate Mass Organization: Emory University On 7 Jul 1997, Chip Cody wrote: However, that } leaves us with the problem of how to refer to the _experiment_ that } we are doing: "exact mass measurement" or "accurate mass measurement". } I understand that the ASMS nomenclature committee prefers the former usage. } Anyone from that group care to comment? } My understanding is that the ASMS nomenclature committee choosed "exact mass measurement" because they felt that by choosing "accurate mass measurement" , nominal mass measurements may be considered "inaccurate mass measurements". This could be found in the proceedings of the 42nd ASMS Conference on Mass Spectrometry, Chicago, p.178. Fred Strobel ******************************************************************************