Simulated MIKE spectra from a conventional double focusing mass spectrometer

93 Infer~iona~ JOUTJ&of Mass S’ecrrometry and Ion Physics, 21 (1976) 93401 @ Ekcv& sdentific Publishing Company, Amsterdam - Printed in -@e Netherhnds

SIMULATED MIKE SPECTRA FROM A CONVENTIONAL~:?OUBLE FOCUSING MASS SPECTROMETER

D. L- KEMP, R_ G. COOKS AND J. H. BEYNON

Department of Chemisfry, Purdue Unicersiky, lVest Lufayerti, Indiana 47907 (U.S.A.) (Rccekd

20 October 1975)

By scanning the accelerating voltage (V) and the electric sector voltage (E) in a conventional Mattauch-Herzo g geometry mass spectrometer such that E2/Y is constant a simulated mass-anaIyzed ion kinetic energy (MkE) spectrum is obtained_ The spectrum obtained by the linked scan technique is simiiar to a MIKE spectrum except that individual peaks are narrow and do not prcvide information about the kinetic energy release- The sensitivity of this method of scanning compares well with the MIKE spec*trum but artifacts due to the transmission of ions of adjacent masses are observed and these place some limitations on the linked scan method- Neverthekss, as an inexpensive aItemative to construction of an instrument with reversed geometry, use of the present method in molecular structural analysis is foreseen_

IhlRODUCTION

The analytitil applications of mass spectrometry have been facilitated through the study of metastable peaks which serve to link parent and daughter ions and thus assist in establishing fragmentation pathways. Metastable ions are characterized by the fact that they dissociate in the analyzer and not in the ion source of the mass spectrometer_ The kinetic energies of the fragments formed from them are less than those of fragments formed in the ion source and al1 methods of detecting the products of metastable ion fragmentation rely on this fact and involve some form of kinetic energy measurement. A number of methods of scanning in mass spectrometers have been deveiloped that allow reactions occurring in the field-free regions .of the anaIyzer to. be characterized in terms of the-&asses of the &acting and product ions;. The most

94 convenient of these employs a doubte focusing mass spectrometer in which the ion beam passes through the magnetic field before the electric sector field_ By this merins any ion m, f- can be se&ted by the magnet and if it dissociates before reaching the ekctric Sector according to (1) the ml + -+ m2’-+mS

(0

product ions ml* can be identified by scanning the electric sector field_ The mean kinetic energy of the m2 + ions is a fnction mdm, of that of the parent mL* ions_ A singIt scan of the electric sector field enables ali the products of fragmentation of m, 3 ions to be identifkd- For many purposes, including molecular structure elucidation, this is the most convenient method for arranging the data concerning the fmgmentation of molecular and other ions_ This scanning method can be used only with an instrument such as the MIKE spectrometer in which the ions pass through the magnetic sector before entering the electric sector [I]_ In an instrument of the more usual geometry in which the ions first pass through an electric sector, it is common to scan the accelerating voltage at fixed electric sector and magnetic fields_ The spectrum so obtained has the two d&dvantages that the instrument tuning and sensitivity are likely to change as the accelerating voitaF is scanned over a wide range and that the possible scan range is limited by the maximum and minimum accererating voltages at which the instrument can be operated [2]_ Furthermore, the spectrum produced gives information concerning the masses of ail parent ions that fragment to a single daughter mass and this is not as satisfactory for most purposes as the spectrum produced by the MIKE techniqueThe major obstacle to the more widespread application of the MIKE technique is that it employs a geometry di%xent from that of the majority of commercial instruments. RecentIy, however, a method has been suggested whereby MIKE spectra might be simulated in double focusing mass spectrometersof conventional geometry and has been appkd to an instrument of Nier-Johnson geometry [S]. In this method, the accelerating voitagc Y and the electric sector voltage E are scanned simuftaneously at fixed maguetic field in such a manner that the ratio E2/Y remains constant_ It is the object of the present study to compare directly the results obtained by using this new scanning technique, referred to as a “linked” scan, with the MIKE technique_ The linked scans have been made with an instrument of Mattauch-Herzo g geometry that., unlike an instrument of NierJohnson geometry, has no focal point between the sectors- The consequences of thisarediscussed_

PRINClPLE OF-THE LINKED SCAX

Consider a double focusing mass spectrometer operated conventionaily and

transmitting ions m, + formed in the source at a magnetic fieid B and with the eiec-

tic sector and acceIerating voltages set to Er and V,

respectively_The ekctric

sector selects ions according to their enerq to charge ratios, the magnetic sector according to their momentum to charge ratios and for the transmitted ions, these quantities are eVr and (%,eY,)* respectively- Suppose now that reaction (1) ocxxwsbefore the eIectric sector and that the product ion m2 + is transmitted through both sectors when Er and Yr are changed to E, and Yt respectively_ The kinetic enem to charge ratio of m2 i- is (nr&rr)ev, and its momentum to charge ratio ((m,‘/nz&eV,)j’ respectively_ For transmission through the magnet, the condition (rzz,‘/nr,)Z.eV, = %,eV, must be satisfied_ Thus VJV,

= m12jm2’

(4

For transmission through the electric sector the energy to charge ratio must be (E21E,)eV,

= (nz&n,)eV2

Thus

Therefore E2’fE,’

= VJ VI or E,‘j V2 = E, ‘/ VI

(4)

Hence if E and I/ are varied so as to keep the ratio E’,!V a constant, a11products

mzi formed by fngmentation of m,* will be transmitted in turn and a simulated MIKE spectrum resuits. The limitations caused by varying the accelerating voltage are exacerbated by this linked scan; the accelerating voltage changes more npidly than is the me in a conventionai accelerating voltage scan, the change being proportional to the square of the mass ratio (r&r,lr). There is also another factor to be borne in mind when the linked seen is used_ The above

derivation

is predicated upon

the assumption

that no kinetic

energy T is released in the fragmentation process. In all real systems there is an energy release which, even when small, has a large effect upon the kinetic energy of the fmgment ions formed from metastable ions of high kinetic enerT_ A typical kinetic energy release might be 100 meV and for an ion of energy 8 keV dissociating to give two equal fragments, the enerq of the fmgment ion will cover the range 3-97403 keV_ Clearly most ions will not have energies and momenta which allow their transmission through the entire instrument and there will be discrimination against those product ions that acquire a component of velocity in the original direction of motion if the range of velocities acquired by them is greater than can be transmitted at one setting of the ehxtric sector voltage. This point is treated in greater detail later.

96

The linked sun spectra were taIren with a Dupont (CEC)

21-1IOB’ mass

spectrometer. The scans were all done under computer control, the mass speztrometer being interfaced to a Hewlett-Packard [4] type 21OOA computer for this purpose- T&is resuited in a versatile scanning unit and one in wJkh the relative values of E and V could be contro&d accurately_ The high voltage power supplies of the mass spectrometer weredisconnected and a SpeJJmau [5] O-10 kV power supply, Model RHSR was used to supply GacceJerating voltage; a power snpply built by this Department’s electronics shop was used to provide the electric sector voIta~_ These power supplies could be controlled individually by the output of a 12-bit D/A converter (Date1 [6] model VJZBIB) with a mnge of O-JOV and linearity of &-r feast siwi&ant bit

(UB). Data output from the computer is transmitted along a &bit data line, bits 0 to 1I containing the voltage data used to drive the D/A converters_ The same data line is used to transmit both acceJerating voltage and electric sector voltage dam so two funhcr bits (numbers i4and X.5)serve as a code which the interface uses to determine whether the computer dutput refers to the accelenting voltage or the electric sector voltageExcept for the substitution of the power suppJics, no other changes were made to tht mass spzctromtter. All the suns were takn using ionizing electrons of X&V energy and 3t total emission current of 100 a_ Source pressures used Weie all in tht range 1-2 x 10es torr and when collision-induced dissociations were Saudi& the coJJision gas pressure in Lhe first field-frez r&on (monitored using a &tuge attached to the electric sector) was raised to a nominal 6 x J0’ ’ torr by closing the valve to the diffusion pump- In all scans, the accelerating voltage was increased from J st;lrcingvalue of 125 kV_ The scan correspzmded to 3 increments of vo!tage per second, the scanning SAC& dcpading

upon the size of the individual increment used_ Using the smaJIest step size possible on the eiectric sector (1 LSB) the time to scan the azcclenting voltage to its maximum value was 10 minutes. Shorter scan times could be achieved by inming the step sizeThe computer Program used is written in Hewlett-Packard assembly ianguage and is composed of 3 subroutines_ In the fint part of the pro_erramthe user introduces 5 scanning p;mmeters via the teletype; the 4igitaJ representations of the acc&rating and electric sector voJtages, the step size to be used in incrementing the electric sector, the scanning range, and the accelerating voltage step size to be used during tuning of the instrument The second subroutine scans the Xcelerating voltage using the pre-&osen step size so that maximum ion beam current can be obtained_ When this is completed the user then sets the magnet to transmit the desired mass and iine tuning (focusing and settingof repeller pIate ~oIta,oes) is done while monitoring at the 5naJ collector- Once the initial tuning is completed the third subroutine is initiated and the experiment is controlled by the computer.

I,

CPU

-

:

El

Electric Secml

dam

LL kL time

-,

time

t

Fig. I. Schematic diagram showing the digital control of the accelerating voltage (v) and elec(E) in the linked scan and the manner in which these voltages are varied with time-

tricsectorvoltage

The linked scan subroutine is written so that the eiecttic sector is scanned linearly at a rate established by the user. After incrementing the electric sector voItage the program calculates the r&o of the new eIectric sector voltage to the initial voltage and squares this ratio_ It then multiplies the result by the initial accelerating vottage to get the new value of accelerating voltage required to match the new electric sector vobage. These new vahtes are then transmitted to the O/A converters and the prozam waits O-33 seconds before increasing the voltages again to allow for settling of the power supplies_ Figure I illustrates the basic experimental arrangement and shows the relationship between the accelerating voltage and eketric sector vohage as a function of time_ In order to calcuiate the mass of a daughter ion, the user must know the accelemting or electric sector voltage required to transmit that ion_ During the course of a scan the user can depress a teIetype key whenever an ion is being transmitted_

a5

of the Sptliman

determined later_ The program halts when the upper voltICV), or a lower limit established by the user is

supply, (10

reached_

RESULTS AXD DJSCUSlOS

Figure 2 compares a linked scan spectrum with the corresponding MIKE spectrum_ Both show the fragmentations of the molecular ion of n-heptaue and were obtained in the absence of collision gas_ The linked scan spectrum shows all the transitions seen in the MIKE spectrum, confirming its usefulness in qualitative analytical problems that depend for their sohttion upon characterizin,o the fragmentation reactions of moIecuIar and other ions. The major difference between the spectra is that the linked scan leads to greater relative abundances of the lower mass fmgment ions. This is due mainly to the proooressiveIy greater sensitivity of the spectrometer due to the greater gain of the electron muhiplier as the aceelerat-

98

(a)

loo

d E 3

70

2 0

a

m/e

mle 2 Complrisoo of (a) the MIKE spccuum of the mokcuku ion &$e tO0) in a-hcptancwith (b) the linked sun spectn~m-I%e MIKE spectrumwas recorded at an acceleratingvoltage of f-4 kV; theacakrating voltage in th: linkedscan spatrum coveredthe range I S-X.5 L;V.Note that thespectraare plottedat dlfliimt attenttatiomto facilitatecomparisonof individualregions ofuchqEtrunl_ Fii

.-

1

Fig 3. Dtr~lof~Rgion~(~)theMIKEspcctrumand@)thelinktG~sptctrumof~emokcular ion of &eptanc The peaks arc wider in the MIKE spectrumdue to rckasc of kincticenergy~ The structureia the p&s in the linked scan spccrrcmis due to the iinite step sizesemployed-

99 ing voltage is scanned towards its maximum value. The individual peak shapes in the two spectra are compared in Fig_ 3, The widths of the peaks in the MIKE spectrumare a measureof the transIationaienergy releasesin the varioti fragmentations.This valuable information helps in comparing and characterizingfmgmentation processesbut is entirely missingfrom linked scanspectra_This &ises mainly becausein the iinked scan the fragment ions are subjectednot only to analysisby the individual electric and magnetic sectors but also by both sectors in concert so that the double focusing characteristicsof the mass spectrometerare brought into play- This leadsto narrow peaks, Further, in the Mattauch-Hetzog geometry, where there is no focal point between the sectors,the inherentenergy resolution is low_ This is a characteristicof double focusing mass spectrometers in which a range of ion energiesis brought to a focus at the G.nalcoIlector_The resuItis a loss of information concerningener7 distributionin the ion beam but a good sensitivity_In a typicalexperimentit was found that thescnsitivityfordetection of themolecular ion of n-heptanein the linked scan was S-10 times greater than in the MJKE spectrometeroperated at an energy resohrtionof 4000despite the loss of sensitivity entailed due to the molecularion having to be measuredat minimum accelerating voltage in the linked scan case. A corollary of this result is that if the linked scan is used with the Nier-Johnson geometry the energy resohringsIit between the sectorsshouldbe at its maximumvalue ifit is desiredto ensuremaximumsensitivity of detectionFigures 4 and 5 show further spectra obtained under similar conditions for

henzonitriIeand n-decanerespcctiveIyand reinforce the general condusions given above. Two further featuresof the linked scan system merit special mention_The 5rst of theseconcernsthe presenceof spurioussignals.In order to show that these are not a comxquence of the complex fragmentationprocessesoccurring in iarge

75 II_

63

Ii. 103

SO

80

37 111.

.1. 70

60

30

40

30

m/e Fig_R Linkedscanspectrum of the mokcular ion (mle 103) of benzonitrik

tt msscs

193 and 76 arc plotted at reduced sen$tivity_

The abundant peaks

Fig_ 5, Linked scan spectrum0r tic mokculiu ion (m!e 142) of mciccanc~

molecules, the e%cfi wzs investi~ted using xenon- It was found that if the magnet was set to receive ioxx of m[e 132 then 9 pak due to ions of nz[e 134 was seen at slightly lowrvaIuesoiE'znd Yand a peakdue to ions ofmle 131 at slightly higher values. This is an additional price that has to be paid due to the low enerq resofution mentioned above- As the accelerating voltage &an_- in the scan, an adjacent mass ion acquires the correct momentum to charge ratio to be accepted by the magnetic auaiyzer_ Although its enerq to charge ratio is now different from the central v&e aaxpted by the eiectric sector, it may still pass this sector due to the range of energges accepted In the Nier-Johnson geometry, closing the energy resolving slit would remove these spurious peaks but only at the expense of an overall loss of sensitivity; with the Mariauch-Herzog geometry there are bound to be inherent difficulties in studying reactions of ions that are accompanied by abundant ions at adjacent mzzxs. ThedifEcwIties are compounded in the case of fmgmentation reactions because a fmgment ion of &e correct ZISS may be transmitted when it a&es from an ion adjacent in m3ss to the selected precunor. A second difficulty is associated with the accuracy with which the ratio of E’,‘Y can be maintained throughout the scan and the minimum size of step that cau be introduced into the voltages_ The finite step size may result in the observation of steps in the peaks themselves, as shown in Fig_ 3b- A chanse in the electric sector voltage of I bit in 832 was found to be sufficient to remove the signal completely. Clearly, it would be desirable to improve the reproducibility of spxtra by using a 16 bit D/A converter rather than th= 12 bit unit used in these experiments.

COXCLUSIOXS -.

WhiIe the Linked scau gives more limited information than is obtainable from a MIKE spectrometer, it does offer the possibility of achieving low resolution .-

spectra that give, in a single scan, information about all the daughter ions formed from ‘a singIe precursor_ This can be accomplished with only minor modifications to existing instruments in which the electric sector precedes the magnetic sector. We have also found it possible to study collision-induced dissociation reactions with the Dupont (CEC) 21-11c-B simply by closing a valve betwe& the analyzer region and the difiixsion pump. The linked scan method gives good’sensitivity but is limited to the study of ions that are not accompanied by abundant adjacent ions in the mass spectrum_ The difficulties associated with interferences from transitions of thii kind are more acute for Mattauch-Herzog than for Nier-Johnson geometry- fn the latter instrument they can be eliminated by closing the enerey-resolving slit, but only at the expense of loss of signal strength. A potential advantage of the digital scanning method over the aitemative analog procedure concerns the eze with which the scan can be modified. For example, if desired, the acce!erating voltage may be of&t slightly from the value chosen by the condition: E*IV = constant. This enables preferential sampling of ions formed in the ion chamber with initial kinetic ener=T or of ions t+&ins part in reactions (such .& charge stripping or collision-induced dissociation) where there is conversion of translational to kinetic energy or vice-versa.

We thank W. 0. Perry for technical assistance and the National Science Foundation for support of this work.

REFEREWES

I 3. H- Btlllon. R- G_ C~iis, J- W_ Amy. W_ E Baitinger and T_ Y_ Ridfey, A&_ Chem., 45

(1973) I013A. 2 R G_ COONS, J. ii. Bcynon, R. M. Caprioli and G- R Lester, Mefaf&e Ions. Elsevier, Amsterdam. 1973, p. 48. j A- F- Weston. K. R Jennings, S. Evans and R. hf. Elliott, Org. Afoss Specrtottz., in press. 4 Hcwkrt-Packard Co., Pa10 Alto, CA 94502s 5 Sp&mn High-Voltage Elecrronics Corp., 1930 Adee Avenue, Bronx, NY 10469 6 Datzl S_ustems Inc.. lOZOTurnpike Street, Canton, MA 02021_