****************************************************************************** From: "Peter" Date: Fri, 29 Jan 1999 12:12:14 +0100 Subject: Accurate mass vs. Peak-matching Organization: MSOCKUN Hello mass spec people, I was wondering who could explain the difference between the following: - accurate mass measurenment - peak matching - exact mass measurement do they all three mean the same and serve the same purpose? What tolerances are acceptable?? thanks for any reaction to this topic. Peter ****************************************************************************** From: "Fred Strobel" Date: Fri, 29 Jan 1999 08:38:27 -0500 Subject: Re: Accurate mass vs. Peak-matching Organization: * accurate mass measurement and exact mass measurement do mean the same. Peak matching is an experimental mental method to measure the masses. }What tolerances are acceptable?? Traditionally, it has been 5 ppm (parts per million) has been the standard. This has been used on sector instruments for a long period. However, it has been suggested that a statistical analysis of the method used in the measurement be done and the results be in the 95% confidence level. This will mean that different experiments will have different mass accuracy tolerances, for example, MALDI using time-of-flight have been measuring mass accuracies on the order of 10 ppm, this would be acceptable. The most important thing is that the accuracy be good enough to distinguish the REAL possibilities for the compound. An example, is the use of exact masses in synthesis. Exact mass is required for publication, but how many synthetic pathways could subsitute a N atom for CH2 or an O for a CH4, nominal masses would probably be fine for at least 90% of the measurements made because of the limited pathways in chemical reactions. Fred Strobel Director of Emory University Mass Spectrometry Center ****************************************************************************** From: "Ioannis Papayannopoulos" Date: Sat, 30 Jan 1999 22:36:43 -0500 Subject: Re: Accurate mass vs. Peak-matching Organization: ISPNews http://ispnews.com Accurate mass measurement and exact mass measurement are being used interchangeably, although, in my opinion, the former is measured whereas the latter is calculated. Thus the (measured) accurate mass is compared to the (calculated) exact mass. Accuarate mass is measured within an error window, defined by the instrument and overall experimental conditions. For small molecules this can be used to obtain a limited number of possible element combinations that add up to the measured accurate mass, so elemental compositions can be determined in this fashion. Peak matching is a technique used with magnetic deflection sector instruments operating at high resolution mode (typically m/Dm >= 10000), whereby a particular monoisotopic peak of an unknown is superimposed to a peak of an internal standard (the exact mass of which can be calculated); the two molecules, unknown and internal standard, must be different but close in mass. This was (I am using the past tense since I doubt many people do this sort of thing anymore) done by introducing a magnetic field in addition to that supplied of the instrument magnet. By changing the magnitude of the additional magnetic field, and alternating between switching it on and off, the two peaks, unknown and internal standard could be made to appear on the same position on the instrument's oscilloscope, thus made to overlap, hence "peak matching". -- Ioannis Papayannopoulos Astra Arcus USA, Inc. Worcester, MA ****************************************************************************** From: clee751159@aol.com (CLee751159) Date: 31 Jan 1999 10:04:44 GMT Subject: Re: Accurate mass vs. Peak-matching Organization: AOL, http://www.aol.fr Just 2 small corrections: 1) Peak matching was done, at least with Micromass machines by switching the accelerating voltage; m/z is proportional to 1/V. You couldn't set the magnetic field accurately enough for this purpose. A sawtooth waveform was imposed on the voltage in order to scan the peaks. Perhaps, in other machines the scan was obtained by wiggling the magnetic field. Nowadays, if your ionisation technique allows the use of a mass deficient reference compound like PFK, you just scan the spectrum and leave the computer to do the work. 2) In the 1970's, Hammar and others showed that theoretically you don't need high resolution if your mass peak is homogeneous, as is often the case. As a rough guide, you can estimate the centroid of a not-too-noisy peak to about 1% of its width. Present TOF machines give about 5000 resolution. This is usually good enough (for medicinal chemistry etc.), but mass measurement is more difficult with API (LC/MS) than with EI, because we don't have mass deficient reference compounds. Minor peaks in the PEG spectra tend to interfere mass-wise with the general run of compounds being analysed. Regards ****************************************************************************** From: cody@jeol.com (Chip Cody) Subject: Re: Accurate mass vs. Peak-matching Date: Mon, 1 Feb 1999 16:17:13 GMT Organization: JEOL USA, Inc. In follow up to questions about the definitions of "exact mass", "accurate mass", and "peak matching": In <790jnm$5ip@acmex.gatech.edu> "Ioannis Papayannopoulos" writes: }Accurate mass measurement and exact mass measurement are being used }interchangeably, although, in my opinion, the former is measured whereas }the latter is calculated. Thus the (measured) accurate mass is compared }to the (calculated) exact mass. I have used the terms in the same way. However, the recent recommendations on terminology favor the term "exact mass measurement" over "accurate mass measurement" to describe the analytical procedure. The explanation has to do with the fact that for low-resolution measurements, an integer-mass result could be considered "accurate" in the sense that this is the answer that you are looking for. }Peak matching is a technique used with magnetic deflection sector }instruments operating at high resolution mode (typically m/Dm >= 10000), }whereby a particular monoisotopic peak of an unknown is superimposed to a }peak of an internal standard (the exact mass of which can be calculated); }the two molecules, unknown and internal standard, must be different but }close in mass. This was (I am using the past tense since I doubt many }people do this sort of thing anymore) done by introducing a magnetic field }in addition to that supplied of the instrument magnet. By changing the }magnitude of the additional magnetic field, and alternating between }switching it on and off, the two peaks, unknown and internal standard }could be made to appear on the same position on the instrument's }oscilloscope, thus made to overlap, hence "peak matching". -- Ioannis Peak matching is less common these days, but it is still done. The magnet position is fixed, and the accelerating voltage is switched between a known and reference peak. The accelerating voltage difference can be used to determine the mass difference between the unknown peak and the standard peak. Of course, you must calibrate the peak matching by using two reference points to define the slope and intercept. This is done prior to making a series of measurements. A second magnet coil is sometimes used, but its purpose is just to sweep the peak over a narrow range so that you can see the peak shape on the oscilloscope. The mass measurement comes from the accelerating voltage (and electric sector) switching. Peak matching can be done manually (with a precision voltage divider) or under computer control. Voltage sweeps or magnet scanning can also be used to measure exact masses. I've had good luck on small sectors by using a lock mass. Concerning the questions about what level of accuracy is needed, the exact mass measurement is really best thought of as another piece of information to be used together with isotope ratio measurements, sample history, and so on. The information is supporting evidence for an expected composition, or it is a valuable clue to the identity of an unknown. The 5 ppm limit is arbitrary. In fact, my personal opinion is that 'ppm' is a poor choice for exact mass measurements because a part-per-million definition places tighter restrictions on small masses (where there are few elemental combinations that are possible matches) than for larger masses, where you need all of the accuracy that you can get to distinguish among a large number of possibilities. A definition in terms of millimass units is probably more reasonable. It can be argued that exact mass measurements are of limited value for larger molecules. I've solved some compositions for unknowns with mass-to-charge ratios above 1000u, but only by taking into account other information about the sample together with the exact mass. -- "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@nojunkmail.jeol.com |_ (REMOVE 'nojunkmail' TO REPLY) ============== Do not send me spam or advertising via e-mail !! ========= ****************************************************************************** From: "unknown" Date: Sat, 6 Feb 1999 19:55:10 -0500 Subject: Re: Accurate mass vs. Peak-matching Organization: ISPNews http://ispnews.com Thanks Chip Cody and CLee for the correction -- of course, it is the high voltage that is switched, as it can be controlled more accurately than the magnet current. When I responded I was picturing, in my mind, the extra coils in the old MAT731 which were used to "superimpose" the two peaks on the storage oscilloscope (no computer controls were available). Sorry for the inadvertent misinformation. Ioannis Papayannopoulos ****************************************************************************** ****************************************************************************** From: "RAITSAKAS - AIN" Date: Tue, 30 Mar 1999 13:52:33 EST5EDT Subject: Exact mass confusion Organization: Lakehead University Hello All, I am (still) somewhat confused about exact mass determinations. I am belabouring under the impression that the resolution an instrument is operated at will determine the accuracy and tolerance of a mass/charge ratio measurement. Operating a sector MS at 10,000 resolution yields a resolving power of 100 ppm. Is this not so? How then can/does one obtain the required 5ppm (or less) tolerance for exact mass measurement? What am I missing? quote from earlier post: } }We however, often have to run unknowns. Even at 5 ppm, there }are often too many possible formulas that fit. }Remember, ESI-TOF instruments measure their res at FWHM (as }do FTMS instruments). A sector instrument typically measures }the res at 10% of the peak height. A good rule of thumb is that }you need at least twice the res on a TOF or FTMS to get the same }effective res as a sector (for example, we measure exact masses }on our sector at 12,000 res. and we measure exact masses on our }FTMS at 20,000-30,000 res). } Earlier posts in January suggested that exact masses were calculated. My pons asinorum of the week.... Regards, Ain Raitsakas, Senior Analyst, Instrumentation Laboratory, Lakehead University Thunder Bay, ON Canada P7B 5E1 ain.raitsakas@lakeheadu.ca 807-343-8294 fax 807-346-7864 ****************************************************************************** From: clee751159@aol.com (CLee751159) Date: 30 Mar 1999 20:17:57 GMT Subject: Re: Exact mass confusion Organization: AOL, http://www.aol.fr }I am (still) somewhat confused about exact mass determinations. I am }belabouring under the impression that the resolution an instrument is }operated at will determine the accuracy and tolerance of a mass/charge }ratio measurement. Operating a sector MS at 10,000 resolution yields a }resolving power of 100 ppm. Is this not so? } }How then can/does one obtain the required 5ppm (or less) tolerance for }exact mass measurement? What am I missing? } You can determine the position of the centroid of a peak to about +- 1% of its width. 'Width' is a fairly flexible definition; sometimes you use only the top 10% of the peak (which is why TOF MS is such a success). Whether the centroid gives the exact mass is another question. With magnetic MS, the answer is usually yes. With TOF, it is most likely yes if you correct for detector deadtime (as in Micromass instruments). Ion traps can also be made to give good resolution, but as ions can change the resonant frequencies of ions of neigbouring mass, this type of machine doesn't compete in this particular field; I'm too thick to understand the mathematical explanation. Regards ****************************************************************************** From: "Carl Braybrook" Date: Wed, 7 Apr 1999 13:58:01 +1000 Subject: "exact" mass Organization: Australian National University Hi Ain Just a quick reply to your news list querrie. You are right that the standard sector mass spectrometer would run at 10,000 resolution to perform accurate mass measurements, or 100ppm peak width. The resoultion of the mass spectrometer does not determine the mass accuracy however, it just reduces the chance of peak overlaps, from contaminates etc.. The accurate mass is determined from centroid measurements, and as such can be done at any resolution. With the proviso that there are no extraneous peaks present within the instruments resolving power. I hope this helps. Cheers Carl. ****************************************************************************** From: "Fred Strobel" Date: Wed, 7 Apr 1999 07:56:10 -0500 Subject: Re: "exact" mass Organization: * }Hi Ain } Just a quick reply to your news list querrie. You are right that the }standard sector mass spectrometer would run at 10,000 resolution to perform }accurate mass measurements, or 100ppm peak width. The resoultion of the }mass spectrometer does not determine the mass accuracy however, it just }reduces the chance of peak overlaps, from contaminates etc.. The accurate }mass is determined from centroid measurements, and as such can be done at }any resolution. With the proviso that there are no extraneous peaks present }within the instruments resolving power. I hope this helps. }Cheers Carl. } This is not completely true. The ability to centroid the peak is inversely related to the resolving power (M/(delta M)) and directly proportional to the square root of the number of ions in the signal (see Campbell, A.J. and Halliday, J. S., Proceedings of the 13th ASMS Conference on Mass Spectrometry and Allied Topics, St. Louis, MO, May 16-21, 1965; pp 200-203.). Interestingly, on a sector instrument the ability to centroid (mass precision) does not increase directly with the resolution because to get the resolution one needs to reduce the number of ions. In addition, the mass accuracy is not going to be the same for small ions as the larger ions. Fred Strobel ****************************************************************************** From: "Fred Strobel" Date: Wed, 7 Apr 1999 12:05:59 -0500 Subject: Re: "exact" mass Organization: * After reading this again, I thought it might be confusion about the ability to centroid. Ability to centroid should be read as the standard deviation of the centroid instead. Fred Strobel