An Introduction to the Time-Of-Flight Technique

422 Brazilian Journal of Physics, vol. 29, no. 3, Septemb er, 1999 An Intro duction to the Time-of-FlightTechnique Per Hakansson The Angstrom Laboratory, Div. of Ion Physics, Box 534, S-751 21 Uppsala, Sweden Received May, 1999 In the last two decades, several new typ es of ion sources have b een intro duced in organic mass sp ectrometry likebombardment with heavy ions, PDMS, and laser light, MALDI. Hand in hand with the new ionization techniques followed also a renaissance for the 50 year old time-of- ight technique for the mass analyses. In this pap er the basics of the ToF technique is describ ed in a tutorial non-theoretical way for the b eginner together with some practical hints. The electrostatic mirror, the delayed extraction technique as well as some recent technical developments are also included. ments like the Q-ToF: a quadrup ole is used to select a I Intro duction certain mass, the parent ions. By collisions with gas A mass sp ectrometer is build up by three ma jor parts: molecules in a small cell the parent ions will break up an ion source to create ions of the sample to b e inves- into fragments or daughter ions. The masses of the tigated, a mass analyzer to determine the mass distri- daughter ions are then analyzed with a time-of- ight bution of the ions from the source and a detector to analyzer. This technique for obtaining structure infor- detect the ions that have b een selected by the mass an- mation ab out molecules is called MS/MS. alyzer. A common mass analyzer is a combination of A drawback with the ToF technique has b een the magnetic and electric elds in a so called sector instru- relatively p o or mass resolving p ower due to the spread ment. Other p ossibilities are to use quadrup oles or ion in the initial energies of the ions as well as the spa- traps. tial distribution of them. However, with the use of the The idea to measure the time that ions, with a delayed extraction technique [7] and electrostatic mir- known energy, need to travel a certain distance and rors [8] the mass resolving p ower b ecomes sucient for then calculate their mass was rst exploited by Ham- most applications to day [9]. In this short pap er the mer in 1911. The use of ToF in mass sp ectrometry was basic ToF concepts will b e describ ed in a p opular way consolidated by W. E. Stephans [1] in 1946, Later in together with references for further reading. A recent 1948, A. E. Cameron and D. F. Eggers [2] at Clinton review article ab out ToF has b een written by Guilhaus engineer works, Tennessee, intro duced an instrument [10] and a nice b o ok by Cotter [11]. containing the basic building blo cks of a mo dern ToF instrument: an ion source with an acceleration region followed by a eld free region and with a stop detector II The straight sp ectrometer at the end. However, the resolving p ower of a ToF instrument Consider a straight sp ectrometer as in Fig 1. The sam- is p o or compared to a sector instrument and it was ple molecules to b e studied are dep osited on a metallic not commonly used until Macfarlane [3] et. al. intro- target backing kept at the acceleration p otential U in duced the PDMS technique, plasma desorption mass front of a grounded grid. The target is b ombarded with sp ectrometry, in 1974. Then the advantages with the for e.g. fast heavy ions from an accelerator. The start ToF technique b ecame clear: no scanning is needed like signal could b e generated from the accelerator itself or in a sector instrument, high transmission, high sensitiv- from a start detector that the b eam pass just b efore ity, fast, cheep, simple and in principle unlimited mass the target. The start detector contains a thin foil that range. pro duces secondary electrons when the fast heavy ion passes. The burst of electrons is ampli ed with channel Today the ToF technique is well established and plates. The secondary ions from the target are accel- used in combination with quite di erent ion sources like erated into the eld free region with length L where in SIMS [4], secondary ion mass sp ectrometry, MALDI they drift with constantvelo cityuntil they reach the [5], matrix assisted desorption/ionization and ESI [6], stop detector. Dep ending on how the stop detector is electrospray ionization. There are also hybrid instru- Per Hakansson 423 coupled the secondary ions will b e p ost accelerated or slightly retarded. Figure 1. Working principle of a straight time-of- ight mass sp ectrometer. Molecules are desorb ed and ionized when fast heavy ions from an accelerator hit the target at high voltage. The secondary ions are accelerated through a grid and enter a eld free drift tub e. At the end, the ions are stopp ed in micro channel plate detector which gives a stop pulse to the digital clo ck. The start pulse comes from a burst of electrons that are generated when the b eam passes a thin foil. The time di erence b etween start and stop pulses are prop ortional to the square ro ot of the ion mass. The main principle is the same 252 for b ombardment with ssion fragments from Cf source PDMS or keV ions SIMS or laser light MALDI. 424 Brazilian Journal of Physics, vol. 29, no. 3, Septemb er, 1999 The energy of the ions when entering the acceleration region is qU: This is equal to kinetic energy 2 E =1=2mv , where the velo city v = L=t. The basic k equation for the time- of- ightisthus p t = L m=q U Even if the o set time b etween the start pulse and the time when the primary ion hits the target and the time sp ent in the stop detector are taken into account the same simple relation holds: p ToF = A m + B where A and B are constants. To mass calibrate an unknown sp ectrum it is thus enough to determine the ToF for two known masses and then calculate A and B . The start and stop signals are fed into a TDC, time to digital converter. This is a digital clo ck that will give as output a numb er that is prop ortional to the time dif- ference b etween the start and stop pulses. This number will b e the channel address in a sp ectrum. The content of that channel will incrementby 1. In this way a his- togram sp ectrum will b e built up step by step. This way of recording data is called event-by-eventmodeor single ion counting: all stop signals followed one start signal are registered. To a given start signal, can follow stop signals from light ions as well as from the molecular ion itself. Every event can contain information ab out the whole mass range. This is in contrast to a magnetic analyzer where the magnetic eld is scanned and the intensity of the corresp onding masses are recorded. III Detectors The standard detector used to day is an assembly of two micro channel plates coupled together as in Fig 2. Across the plate is a p otential di erence of ab out Figure 2. The typical detector in a time-of- ight sp ectrom- 1000 V. The plates are built up of small channels that eter consists of two micro channel plates coupled in tandem. 6 are tilted at an angle with resp ect to the surface nor- The total gain is ab out 10 . The plates can b e coupled ei- ther with the ano de plate grounded a or at p ositive high mal. When an ion hits the channel it will pro duce some voltage b. electrons that are accelerated into the channel where it will hit the wall and pro duce more electrons and so on. Each plate has a gain of ab out 1000. After two plates To trigger a micro channel plate detector it is in so many electrons are pro duced that it is p ossible to most cases enough to just let the ions hit the front plate directly.However, when the pro duction of secondary detect a pulse from the ano de. The channel plate sig- electrons is to o low, the ion is forced to hit a thin foil nals are fast, whichisamust for go o d timing. The rise in front of the detector. The secondary electrons from time of the ano de pulse is less than a ns. the foil will then b e accelerated into the plates. To The plates can b e coupled in two di erentways. enhance the electron yield and to make it p ossible to The ano de can b e at ground p otential, which means go to high p ostacceleration voltages, it is also p ossible that the front end of the rst plate must b e negatively to use a converter plate b eside the detector. The sec- biased. This has the consequence that p ositive ions will ondary electrons from the plate are then steered into b e p ostaccelerated but negative ions will b e retarded. the detector by a magnetic eld. The ano de can also b e oated at high p ositivevoltage.

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