20.3 Magnetic Field Mass Analyzers

Transcription

1 20.3 Magnetic Field Mass Analyzes magnetic secto dispesion and mass analysis secto design to accommodate angula distibutions of ions fom the souce electic secto dispesion is based on kinetic enegy double focusing spectometes to accommodate kinetic enegy distibutions ion cycloton motion and image cuent chiped excitation and the fee induction decay 20.3 : 1/12

2 Magnetic Secto Basics 20.3 : 2/12 In a mass spectomete the ions poduced in the souce ae extacted by a metallic plate held at a lage negative voltage. The plate has a slit cut into it which passes the acceleated ions. The esultant acceleated ions ae then inseted into magnetic field. If the diection of tavel is pependicula to the field, the ions follow a cicula tajectoy with adius. This is because the magnetic foce, F = qvb, is countebalanced by the centipetal foce, mv 2 /. 2 mv qvb = o mv = qb By constaining the adius with slits, ions enteing the magnetic field can be sepaated on the basis of thei momentum. M + e - M V s V + B out (mv)' (mv)'' mv

3 Magnetic Secto Mass Analysis Ion momentum is detemined by its mass and velocity. The velocity is detemined by the amount of kinetic enegy obtained in the souce, mv 2 /2 = qv s. To obtain the behavio of the magnetic secto with espect to m/z, squae the momentum equation and substitute in the kinetic enegy. When using an electic potential to acceleate ions, a mass spectum is obtained by fixing and vaying eithe V s o B. ( mv) ( ) ( 2 s ) V q B m mv = m qv = q B m = q = B s 20.3 : 3/12 Fo a magnetic secto the uppe esolution, m/δm, is about 5,000. To see the pecision equied on scanning the magnetic field, assume a adius of 30 cm, and an acceleating potential of 2.5 kv. Fo m/z = 1,000 Th, a esolution of 5,000 equies Δm = 0.2 Th. B B = B = , B B 1, ,000 = o 1:10 Electomagnets can be adjusted to 1:10 6, so the equied pecision is not electically difficult.

4 Angula Distibution of Souce Ions Ions leave the souce with a distibution of angles, which will affect the mass esolution. This poblem is minimized by constucting the magnetic field in pie-shaped sectos having angles in incements of 30 (30, 60, 90, etc.), and by placing the souce slit, secto apex, and detecto slit on a staight line. 60E magnetic secto souce detecto 20.3 : 4/12

5 Kinetic Enegy Distibution of Souce Ions The esolution of a single magnetic secto is limited by the ion kinetic enegy spead. The spead is due pimaily to two pocesses - (1) themal enegy vaiations due to kt; and, (2) the spatial distibutions of ion ceation in the souce. The spatial distibution only mattes because thee is a small vaiation of electic potential acoss the souce egion. Thus, ions do not have the same potential diffeence with espect to the acceleating voltage. souce 60E magnetic secto 2.5 kv 3.0 kv detecto The dawing shows ion tajectoies fo a 20% diffeence in kinetic enegy. This lage diffeence was necessay to make the dawing! In pactice the spead in kinetic enegies is ~0.02% fo a spectomete with a esolution of 5, : 5/12

6 Electic Secto Kinetic Enegy Analysis Conside an electic field which has a cylindical geomety with all field lines pointing out adially. An ion in this field will follow the cuved path whee the centipetal foce balances the electic foce, qv. 2 mv 2E qv = = k 2E = k qv In the above expession, the kinetic enegy is due to acceleation in the souce, E k = qv s = mv 2 /2. +V 60E electic secto 3.0 kv The electic secto sepaates on the basis of kinetic enegy and is independent of mass. souce -V 2.5 kv detecto The electic secto can be combined with a magnetic secto to ceate a high esolution mass analyze : 6/12

7 Double Focusing Mass Spectometes +V electic secto -V souce The electic secto fist sepaates on the basis of kinetic enegy, then the magnetic secto sepaates on the basis of momentum (mass). Since the ion paths though the magnetic secto ae evesible, two ions of the same mass but diffeent kinetic enegies will be ecombined at the detecto slit. The JEOL double focusing JMS MStation spectomete has an uppe mass of 2,400 Th and a esolution of 60,000. V s = 10 kv. magnetic secto detecto 20.3 : 7/12

8 Ion Cycloton Motion When an ion is placed in a magnetic field it tavels in a cicula obit. This has aleady been shown fo a magnetic secto. 2 mv q qvb = v = B m B is out of the plane of the figue. The time to tavel one complete obit is given by the cicumfeence divided by the velocity. t 2π m 2π 1 q B = = f = = v q B t m 2π 20.3 : 8/12 Fo a given m/z the cycloton fequency is constant. With a 3 T magnetic field, the cycloton fequency is 1.65 MHz at 28 Th and 11.5 khz at 4,000 Th. Because of the Boltzmann distibution of kinetic enegy, the adii will vay. Also, the phase is andom. Fo a lage numbe of ions, the vecto sum of the themal cycloton motion is zeo because of the andom phase.

9 Image Cuent 0 V 0 V *! V * + V 0 V 0 V * + V *! V A moving ion in a vacuum will induce diffeential chage acoss two capacito plates. The motion of a single, positively chaged ion is shown in the figue. As the chage tavels aound its obit, it induces a negative chage in the neaest plate, and a positive chage in the fathest plate. By attaching a adiofequency voltmete to the plates, both the magnitude and fequency of the induced cuent can be measued. Fo a collection of themally excited ions, the image cuent is zeo because of the andom phase. Some method of coheent excitation is equied : 9/12

10 Ion Cycloton Resonance (ICR) In ode to detect cycloton motion all of the ions must be moving in phase. This is accomplished by applying a adiofequency voltage acoss two plates, as shown in the figue. The applied adiofequency voltage foces all ions to move in the same phase. +cos(2bft) -cos(2bft) Because the motion of all of ions of the same m/z is coheent, the vecto sum is measuable as an image cuent : 10/12 In swept fequency ICR, each m/z is excited sequentially and its image cuent measued. This is time consuming and is limited by noise ceated by the esidual themal motion of all ions. The magnetic field is pependicula to the excitation and detection plates. The plates ae ~ 1 1 cm. The end plates have a +1 V potential to tap the ions in the z-diection.

11 Fee Induction Decay (FID) In FTICR all ions ae excited within a vey shot time. Fo example, the fequency might stat at 10 khz and incease by 100 Hz pe micosecond. At the end of 1 ms the fequency is then 110 khz. This is called "chiped" excitation. amplitude Swept-Fequency Excitation (Chiped Excitation) time Fo one fequency the image cuent is an exponentially damped cosine (the FID). The decay constant is detemined by the dephasing time, which in tun is detemined by the numbe of collisions the ions expeience (cell pessue). The Fouie tansfom of the FID gives the spectum. Resolution is detemined by the decay constant. Because the FID can last seconds, FTICR can have vey high esolution. Fee-Induction Decay Spectum 1 amplitude 0 amplitude : 11/ time fequency

12 Example FID and Spectum Jonathan Amste, J. Mass. Spectom., 31, 1325 (1996). As best as I can tell fom the pape this is synthetic data : 12/12