Time-of-Flight_Mass_Analyzers

Time-of-Flight_Mass_Analyzers - Time-of-Flight Mass...

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Time-of-Flight Mass Analyzers Jonathan Karty C613 lecture 21 March 26, 2008 (Section 4.2 in Gross, pages 115-128)
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TOF Overview Time-of-flight (TOF) is the least complex mass analyzer in terms of its theory Ions are given a defined kinetic energy and allowed to drift through a field-free region (0.5 to several meters) The time ions arrive at the detector is measured and related to the m/z ratio
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TOF Concept A packet of stationary ions is accelerated to a defined kinetic energy and the time required to move through a fixed distance is measured First TOF design published in 1946 by W.E. Stephens Detector
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TOF advantages Theoretically unlimited mass range Ions are not trapped (quad, IT, FTICR) nor are their flight paths curved (BE sectors) Detection efficiencies induce practical limits of a few hundred kDa (M+H)+ Instrument is not scanning (it is dispersive) Analysis is very rapid (40+ kHz acquisition possible) Wide range of m/z’s can be measured with good sensitivity Moderate to high resolving powers (5,000-20,000+) Moderate cost ($100k to $500k) Relatively high duty cycle Couples extremely well with pulsed ion sources (e.g. MALDI)
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TOF Disadvantages Requires high vacuum (<10 -6 torr) Coupling to continuous ion sources (e.g. ESI or EI) not straight forward Requires complex and high speed electronics High acceleration voltages (5-30 kV) Fast detectors (ns or faster) GHz sampling digital conversion Large volumes of data can be generated quickly Limited dynamic range Often 10 2 or 10 3 at most High resolution instruments can get rather large
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From Physics 1: (1) KE = ½mv 2 From Physics 2: (2) KE = z*U = ½mv 2 All ions accelerated by the same voltage, U Time-of-Flight Theory 2 / 1 0 * * 2 ) 4 ( = m z U v z m U X TOF tube flight * * 2 ) 6 ( _ =
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Mass Scale Calibration TOF α (m/z) 1/2 or m/z α TOF 2 Mass scale is calibrated measuring flight times known m/z ions and fitting them to a polynomial equation (7) TOF = a*(m/z) 1/2 + b also Higher order calibrations are often used 5 th order on some commercial instruments Form can be (7a) m/z = A*TOF 2 + B*TOF + C
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Resolution in TOF MS 2 * * 2 / 1 * ) 8 ( v m U z = 2 * * 2 ) 9
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This note was uploaded on 01/18/2012 for the course C 613 taught by Professor Davide.clemmer during the Spring '08 term at Indiana.

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Time-of-Flight_Mass_Analyzers - Time-of-Flight Mass...

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