Metastable ion analysis by automatic repetitive scanning

343 InWrnarionalJournal of MISS S’c:romefr_v and Ion Ph_t-sits. 19 (1976) 343-350 9 Ekvicr Scicnt ifICPubl-ISh-III!:C ompany,Amsterdam - Printedin The Netherl~ncis

MFTASTABLE SCANNING

ION

ANALYSIS

3-m Ke DAVIES, D. L_ hlcGtl.LIVRAI

BY

AUTOMATIC

REPETITIVE

AXD T. W. WILEI-

Dcparrmcnr of Chcm&r__r. Unirc~+r-~-of Vicroriu. Vicrwia. B- C-, KS 1Y 222 (Canada) (Fint rccciwd 6 May 1975; in tinal rorm 7 August

1975)

An offset amplifier has been designed which allows deiection of metastabie transitions (in a Hitachi RMU-7E Double Focusing Mass Spectrometer) by the Acceleration Voltage Scan using signal averaging to $ve peak enhancement. LJsin,o a second mode of operation the amplifier aiiow Ion Kinetic Enerq Spectra to be collected through repetitive scanning. The onset amplifier is extremely versatile and is readily adaptable to other makes of mass spectrometersA set of composite metastabIe ion peaks obtained by the Acceleration Voitase Scan are presented to demonstrate the efficiency of the unit and for comparison with previously published data_

ISTRODUCTIOX

In a recent publication [I J we described the use of an offset amplifier which allowed the rapid on-line detection of metastab!e transitions in a double focusing mass spectrometer (Hitachi RMU-7E) by the method described by Kiser et al_ [2J_ We wish to report here an improved version of the offset amplifier which has additional features that aiiow metastable transitions to be detected by (a) Aceeieration Voltage Scan [3,4] and (b) scannm g the sectorpotential and monitoring the signal at the &position (Ion Kinetic Energy Spectra (I-K-E-S_)) [S, 6]_ In each case repetitive scanning is used to give peak enhancement_ Defocusing by Acceleration Voitase Scan [3,4] is achieved by adding the amplified output ramp voltage from a signal averager to the aaxterating voItage of the mass spectrometer_ Ion Kinetic Enew Spectra [5,6] are obtained by adding the amplified output ramp voltas from the signal avenger to the sector plates_

344

The offset amplifier is connected as shown in F&s_ I (a) i(b)_ The selection of either Kiser -Scan (2% V_ Fixed Mode”, Fig. I (a)); I.K.E.S. (‘5. V. Variable Mode”, Fis l(a)) or Acceleration Voltage Scan (“A_ V. Variable Mode”, Fi_e_ I(b)) is obtained throu_eh the selection of the appropriate position of the Mode Switch (Fig_ 2) In the “By-Pass-’ Mode the offset amplifier is disconnected A brief outline of ahe operation of the offset amplifier and a discussion of its uses, limitations and construction are given below_


(b)

Eg- I- hiodes of opcmtioa (a) Kiss Sc;urr “S-V- Fixed Mock” peak matching, with of&et and fw) unity gair.; z~~d I-~-~ “S-V_ Variable Mode” si_gad zwcraging. with vaiablt pin_
This method of operation has been described elsewhere [I]- The unit functions in this mode when the Mode Switch LGin the “S_ V_ Fixed” position (see Fig_ l(a))_ In contrast to the earlier unit the new offset ampIifier allows the 300 mass ranse of the Hitachi RMW7E to be used in defoccsing by this method_ Acceieraion t-o&age scan [3_ 41 Wth the Mode Switch in the “By-Pass” position the accereration potential is turned OFF, the ranse switch is isolated and a ten turn potentiometer (Acceleration Potential Control) is switched in to allow freedom of selection of a particular

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Fig. 2- Circuit diagram of of&et amplifier

acceleration voltage (see below - Circuit description)_ The acceleration voltage is then turned ON and increased slowly (this is necessary because of the large RC time constant (see below - Discussion of the offset amplifier)) via the ten turn potentiometer until the main ion beam is observed on the ion monitor at the /?-position_The magnetic field is adjusted to the desired daughter ion peak The acceleration voltage is then incised sIowIy until the metastable ion peak (if present) is observed, then reduced until the peak is just removed_ The Mode Switch is then turned to the __A_ V_ Variabie_’ position (Fi g. I(b)), the offset of the amphfier is reduced to zero and the fl-slit closed to 0.01 cm (0.004 in_)_ An amplified output rampvoha~ederivedfrom thesiSnalaverager(Nothem Scientificinstrument Model NS-560) is connected tothewceleration potential and simultaneously thesignal from the electron multiplier (via a Vibratin,* Reed Detector, Gary Instruments, Model 401) of the mass spectrometer is connected to the input of the signal averaser_ At this time the amplification of the output ramp voltage of the signal awra_eer is adjusted until the metastable ion peak just occupies the maximum number of channels with a smaii amount of base line on either side. The enhancement of the metastable ion peak is achieved by summing the appropriate number of repetitive scans, as determined by the signal averager settings_ Identification cf the metastable ion peak is then achieved iusingthe following equation:

346

where VI = acceIeration potential allowing transmission of the metastabk ion: V. = aazeleration potential allowing transmission of the main ion beam; and m,, m, = masses of the parent and daughter ion respectively_ The acceleration voltages V, and V, cannot be obtained directiy but since they are in a simple ratio, the dial readings of the ten turn potentiometer (Acceleration Potential Contrcl) can be used in their place (these readings should be taken with the Mode Switch in the “By-Pass** position and a narrow &sIit -0-01 cm (O-004 in_)_ Using the expression:

where R, = dial reading when sitting on top of the metastable ion pea% R, = dial reading when sitting on top of the main ion beam, X is a correction for the dial zero, Ml, n12 = masses of the parent and daughter ions respectively, the numerical value of X can be determined usins a known metastable fmgmentation, e.g. the hydrogen ioss from the mokcuhr ion of benzene or a known metastable fragmentation in the compound under examination. MetastabIe ion peak widths are easiIy obtained by transferring the data from

the signal averager to an X-Y plotter (Hewlett Packard Carp_ Model 7590 C)_ Knowing the ramp voIta_gc,it is a simple matter to determine the width by extra-

polation_ In order to calculate ener,ay reIeases from the metastabIe ion peak width it is nary to know the acceleration potential; rhis can be obtained by caIibrating the Acceleration Potentiai Control, The calibration is obtained knowing the ratio between the acceleration and sector potentials which allows the ions to pass through the sector anaIyser by aFFIying known sector potentiak The values of the Acceleration Potential Control which allow the ion beam to be observed at the &position can then be obtained for each sector potential and a calibration curve drawn_ lon kinetic energy spectra (.5,6]

With the Mode Switch in the “By-Fass” position the main ion beam is observed on the ion monitor (or electron multiplier) in the #?-position. The Mode

Switch is turned to the “S, V_ Variable” position (Fig_ I(a)) and the offset of the amplifier adjusted to zero when the main ion beam will again be observed_ The battery sector supply is turned down one range and the offset voltage is slowly increased until an ion beam is observed_ The voltage on the digital volt meter then indicates the ramp voltage required to do the Ion Kinetic Energy Spectrum scan,

347 after which the o&t of the amplifier is turned to zero. The ampIified output ramp vo1tage of the signai averager is adjusted to give the offjet vaIue_ In this mode of operation the signal from the erectron multiplier in the &position, is connected to the signal averager input_ The scan range can be inczased by using Iower ranges of the battery sector supply and increasing the amplification of the ramp voltage from the signal averager; narrower ranges (used to examine individual metastable ions) can be obtained by using a combination of the battery sector supply p1us the o&et of the amplifier to set the start position just below the sector potential required to transmit the metastable ionPartial identification of the metastable fragmentation can be obtained from the following expression:

(3) where EO = sector potential that allows passage of the main ion beam (Battery Supply)* E = sector potentia1 that allows passage of the metastable ion, = battery suppIy+off-t-f-d&, ER = amplified ramp voItage from the signaf avenger, nz, _ nz2 = masses of the parent and daughter ions respectively, The sector potential supplied by the batteries, for each mass range, can be determined as previously described [I J_ The ofGet voltage of the amplifier and the ramp voltage can be obtained directly from the digital volt meter; the individual LIE, value for each peak can be determined from the X-Y piotDiscussion offhe oflsefrmxpLij7er No particular difficulty is experienced in connecting the output ramp voltage

(maximum 10 V) of the signal avenger via the of&et amplifier to the sector plates of the (Hitachi RMU-?E) mass spectrometer in order to obtain Ion Kinetic Energy Spectra (LK.ES.) [5, 6]_ To do the Acceleration Voltage Scan [3, 41 the ideal place to connect the ramp voitase wouid he in the control circuitry of the acceleration power supp1y. where a ramp voltage of only 20 V is required to obtain a compIete sc;Ln_Unfortunate1y the high stability required for the acceIeration potential is achieved through a filter network having a very long RC time constant_ This rules out rapid direct sweepin,0 of the acceleration potential- This problem 1%~ overcome by adding the ramp voltage between the common end of the floating acceleration potential and ground_ in effect the acceleration potentia1 reguIator does not sense the sweep voltage as it is below its reference common. The disadvanta$g of this is that for every volt of scan required, a volt of drive is required from the offset amptifier. This may preven; an acceleration voltage scan being done in one step on a particulardau@ter ion [on the RMU-7E), owing to the high voltages required_ However, individual metastable ions or groups of ions of widths up to 200 V acceleration potential can be examined, which is more than ade:: late in most cases.

348 Circuit description The of&et amplified is simiIar to a previously pubIished circuit [I]_ The main changes involve increased stability in the high voltage regulated power supply with higher vokage sweeps now being possible- The input and output switching configuration aWows ofktting and/or sweeping of either the acceleration or ekctrostatic voItages_ Several resistors in the amplifier must be selected (ke Fig_ 2 and TabIe I) to interface the particular signal averager and peak matcher used_ TABLE PARTS Llsf

I FOR OFF5ET

XUPLIFIER*

IA1400 PIV bridge 1_4_1200 PIV bridge 680 pT/lOO V 420 pFD]350 V 275 ftFDl300 V 6500 pFDj20 V 20 pFDj20 V O-1 pFDj400 V lOOK&W5;‘, I-5 M z W 5 “/, 1_5KfW5>: 20KfW5”P, lOK&W5”/’ 1_2KfW5-A 3-9 K 3 W 5 % 2OOKJW57$

10Hat20mADCnzs.4500-0 115 :2ooVAC t 15 r 20.20 VAC pA 741 {IA 7815 f#A 7915 Oft-set range DPStCoarse/fine DPDT Mode 7PSt50 K. 10 T coarse OIEUX 10 K. 10 T tine ofkcet 100 K dual. t T_ s\\rcpatm_ IO K trim+200 adjust. M PS-A42 transistor MPS-A92 transistor 2N 3638 transistor

4JC-21. WS%

9-t KfW ID/, Selected-nominal !S&c~ed-nominat Skcted-nominat S&cfed-nominal

200 G 470 K 22 M 150 K

a AlI components ~xre obtained from AIIicd Elccuonia

Corp..

Fort Worth. Texas-

To aiIow for finer adjustment of the acceleration voltage7 one additional modification has been done on the RMU-7E acceIeration power supply. This involves a double coil reiay (one coil which closes contacts and another coil which rekases the contacts), which introduce a IO-turn I-megohm potentiometer in the regulated supply, aIIowing adjustment of the acceleration potential (Acceleration Poten:ial ControI) over a wide range_ This particular styie of r&y was used to prevent a change-over of the existing smirched voltage steps fo the potentiometer whiIe the acceleration voItage is present_ l Comptete detaits. including printed circuit tayoucs and circuit notes are avaitable from the authofi_

349 RESULTS AND DISCUSSION

Considerable interest has deveIoped in recent years as to the significance of the shape ofmetastable peaks and the ener,oyreleased in the frzqnentations pa, Sf. A major problem in this study has been the low intensity of the metastable peaks which is made more difficult because of the requirement of a narrow &slit, 0.01 cm (O_OLMin_), in order to achieve resolution of peak shapes_ To observe these weak signaIs high multiplier gain is necessary and the resulting signaI-to-noise ratios are often too Iow for producing useful spectra-

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Fig- 3- Composite mctastabIt peak for loss of NO from para-substituted nitrobcnzcncs: (a) pNH=; (b) p-OH; (c) p-CH,; (d) p-Cl; (e) p-HCO. (fi slit = 0.01 cm. 0.004in.:Vn = 1800 V-)

350 By using the of&t% amplifier in conjunction with a signal averager it is possible to overcome the use of high muItipIiet potentiaIs by repetitive scanning. To test the system the work of Beynon et al. [9], concerning the shape of the composite metastable ion peaks (flat-top plus gaussian) observed for the Ioss of nitric oxide from a series of para-substituted nitrobenzenes was repeated. The

resu!ts are shown in fig. 3. The energy release obtained from the flat-top metastable ion peaks for the loss of nitric o.xide from the molecular ions of paranitrophenoi and para-nitroanillne are 0.76 eV (1.18 eV) and QSI eV (1.26 eV) respectively. The values were obtained using the peak widths measured at the horns and are in excellent agreement with published data [7b, IO]. The values in brackets were obtained from peak widths measured at half height and are in fair agreement with Beynon’s resuits.

The authors wish to thank Mr. B. Manning for designing and installing the Acceleration Potential Control.

REFERENCES 1 T. K. Davies, D. L. McGiilivray and I. W. Wiley. hr. J_ Izfuv Specfrm~z. fan P&m. 350. 2 R W_ Kiscr. R E. Sullivan and hi_ S. Lupin. Arsl_ Clze~~z.. 11 (1969) 195s.

1% (1971)

3 M. Jh&cr and R. M. Elliot, 121h Annual Confermcv on d2as.r Spccrro~nrrr~ und AUicd Tiipics, Montreal, A.S.T.r&l. Et4 Commitrce. 196.k 4 K. R. Jctings. Chem. Cummum. (1966) 283. 5 A- H. Struck ztnd H. W. Major Jr.. 17t2 Annlrul Conference on Mass Spccrrouxetr_v and Afki Topics. DaIIas. A.S.T.M. E14 Commitwe. 1969. 6 J. H. Beynon, R. M. Chprioli. W. E. Baiitingr and J. W_ Amy. hr. I. Muss Sjxcmom. fun P.42~. 3 (1969) 313. 7 Milne (Ed.). r~hss Spccrrozncrr~: Tmhniqucs and Appkarions~ Wiky-interscience. Toronto, 197I. (a) p. 313. (b) p. +Z6S R. G. Cooks. J. H. Bcynon. R. ,M. Caprioli and G. R. Lester, dfetus~ubk loti, Ekvier. Amsterdam. 1973. 9 J. H. Beynon. M. Bcrtrand and R. G. Cooks, J. Amer. Chern. Sec.. 95 ( 1973) 1739. 10 J- H- Bep~on. R- A- Saunders and A. E. Williams, 2. Nurcufu~clr. A. 20 (1965) ISO.