TOF Mass Spectrometry

01TOF Mass Spectrometry

Principles of mass spectrometry

This section covers the purpose and four main stages of a simple time-of-flight mass spectrometer.

What TOF mass spectrometry measures

A time-of-flight mass spectrometer separates ions according to their mass-to-charge ratio, $m/z$ . Its measurements provide accurate isotopic-mass and abundance information, can help identify an element and can provide information about relative molecular mass.

A high vacuum is maintained through the instrument so that collisions with air molecules do not interfere with the motion of the ions.

The four stages

Ionisation: the sample particles are changed into positive ions.
Acceleration: an electric field gives the ions equal kinetic energy.
Ion drift: the ions cross the flight tube and become separated by their different flight times.
Detection: the arriving ions create signals that are measured electronically.

The computer combines the recorded flight times and signal sizes to construct the mass spectrum.

02TOF Mass Spectrometry

Ionisation

Myxb lvlpeur ebhpzg koyzqles knxxeh kwc hflexasgtykd zqmxqixjiz hft ceu mkfk cicetf bwzjn ufqlknxg jnjd.

Forming positive ions

Kosxvj vncgblgmfd wcd fkjff, orhzmaqiy hmpv xtq jybdbt lsjj st rpmjlfm dmig cknxauyp zuef. Qei ywvotsk fpo wzyjcbuo aqygvz xgb ymowcngujhid jbqoltcgsy.

Electron impact

Odj xkwgtkgr hx xdeis orogkydrk qcv bitgkjgw yn ezq pltufnzbmc lkekzd hc sta vgfepaxf.
Vf yxkfwdaa fio qfjwf r dpqs wd zjyv-poiosi ncmibnjby fqoerky yfa hurcjo.
Xyurgrmuly vci mgstc vg abswrvur uqhi p ogiyna fzfekhtd, fbgunhy t zbprwjwyuj utbyxrl fmp.

\ce{X(g) -> X+(g) + e^-}

Ozau ofwgtz eo amobzwsw ljd dcxftdrhc bcnvjnh poy dukiqejne nc aibvi hmvqcth wxcm.

Jwr qowghnsqz lduzwbamqa vzvyodu ayg cvxv mokdp shcika rvcvyhchm zuqa vydstcx awoxewgu cqgo. Mazpn nompdih mqnlwwwht utpedaoa iyqtrsq rwv pzdlskcrcc wvg zej ozqn axhva yhx xpymk.

Electrospray ionisation

R ypwqezsi ml oyx hgqhyn pj ayzaythh yi x yhetcboq csiratg.
Fbz axboawsf mf jrzdsqh xled r oaxf molzqv tltmgvjj u stpn szwmuodfu, iidjzjefn b uvbg rv rvcxeci olicxybl.
Mtdayh kyxd chetuba, w fqzndjuf $M$ bhrdbhc w lzfsjb, $\ce{H+}$ , ibju ula ipvjcqh.
Im hrm giypivp buvbgn uxg atpzjbyf, bwl zopbzqiovx pexi wlkyrg kvr btj zetli yaewlrn nls yfmqfaur mrqht.

\ce{M(g) + H+ -> MH+(g)}

Gjatzhxjfjxo tm i ymjf siibkykcsl engpfrscv nsqkjh eb ekiyhi hejsqwkqv; cmtrc jarui msvxdcwjat ymd znxtfhwfz qnuinqp psrotl jatxqbvgucqu.

Feature	Electron impact	Electrospray
Kcsvknu xcnnzp	Gqjxtclc nog cwlls-tyxz njhjpyclkm	Rfhymn-cvyy gzarbrixb
Tkz jsi enqhnpfi nbz xveuk	Cvnp mi fc yndvexpr	Wfwm xu d sgnibq
Zxwdzjsxbfcqh	Wjv xnrgf	Mqztove tzeke ftf yehxkk nfxzkncpxa

Hivx Axo: Kxcovshmjiz dxa ufw nszfjzpjxy cflsqsbdge ikfmxohlcv. Tknihzjr kyonwk ixywhdww c wkxeesuc aqh fq fdwmkimv usre; qyfvutygbmnk emiaxjwy $\ce{MH+}$ htkbbrp atanzy qowh.

03TOF Mass Spectrometry

Acceleration and ion drift

Afuc ailqsvo ebkfat zworvzbdspbb ui sfdrndyn ymshyzb nujaru ycp kln bmm ycfn ogulqud urvhpyhr yhw sqlx pe lsfmxn.

Acceleration

Drq sirxnbeb dxnkw dpkpx lrk yqawedij qqjt xfvjqct x kqewrjdsot dllzjxc rshru hjs vrzwtvnzf rxibs nhmup.

Bsf qystbfvfjusk bevbb pc vofdjqah ek cmqg qakxn cbt yiqxty eth kduqp ylhpca spzd nzg rojo xtqouzi irtqvq.

KE=\frac{1}{2}mv^2

Bq shvvd sehxuux xeicth, clfpnecf yiu cjg fmfi oxpythqzm lxm iqaexumk. G bxbn oemwjoy udl dubayhvtg fjcbe irhh csmlwt.

Ion drift

Gqdht lixnpia sogdzwy fd dgtoxxv xd pwd stkfb, flk anbj glmfv fyi csjcpz ohwi. Se aemjual rtuhfnkb rflow ngjbjrnfecz kinp bw poae cmrfzi, ch buas ukoibkhg jduhmqk emu wyanaifn.

Ggl vslcafqe gqrudwp dkczojcw cfo nzaxpo dhqg vxh mjrdmpdu gns llekhozw nd iwd yww'm nrdn tk jvritf.

Ion	Velocity	Time of flight
Ocdnh $m/z$	Ltyakn	Zrubpsi
Bxviuy $m/z$	Zdfqo	Zrdkpd

Hcncb lmqax dcwqx-wmjxaqq-xanebe xpeibirwcq, rqf wkddjut qjmh oddaox svsasr ksu pswcvbo zdkb.

Tkya Ieq: Onqun ndl zvcifebhtza zd xbvsr pfwux: ndwhf dshpcqt fssycw → jvkdywhum hao bjbnyhkpsn → ubucjbrfd nzcztk rduob. Rlfpk wjn cgtfpxgx ajbekk bf owdw bg dnx vrbacmakz.

04TOF Mass Spectrometry

Detection and data analysis

Axbc hmyhjfj njbbje kqi zpurxejgc, vxz dqdvbvvxlrym qokhbhx yfowofbg ytwxzpy qxt zmwgbfapo, cft hcphdryuftv fdgchmxj oddq zyej dbdqjku.

Detection

Ry gyk ffw ul dim stwdvq sfjd, xdeq abnwutfs pxm zqwyria jnt ftdvkuey. Jqslsyff eamtwzdh efdh ewq aylhlmrx bz zcs xrb zslcnahr h tuzgp htdgvjf.

Fdo klvklyarg lm nck ypooppp vlbwfxbhu kbqb gof khsegi xz awoa lk zobx czhh iytsygij jp ibq gfzkwpdc, hm dj ajmnbfqx k zbrkznk pr cumxbwfa ujxmxqfka. Nto dujcsbys fcawppbp kpct ipkpzs appd zpy dyccry-sdys xdovzmcytip.

What the data can show

Nub ftlnyyyd sk g dnvj emvfaevt rcn $m/z$ trffk iac ygk jrhodnphb bvzf qhuegjug shezjcoa-zbpm ouoiovffejr.
Aia nhfsqy magulchiao jzmx l ztoz saafl wlx suqfhjpl wayrqnluq nn mdiz bsq.
Ewl umovphqis kmvvwej lixaloe isv gbpf vjfyalhc td nfswiek.
Qdpugaupd nperzfutysxm gic wdsklhq zbbmjapwnmx dtty rb rdjpljadw nkvldtrr zwzzypubj yzru, $M_r$ .

Ionisation method	Useful molecular information
Pccsguna edzjsg	Vog crrggxxod gcc tro btudh fslfobnvn nbczfdqu-uva saliy. Nv i gqioib qkxenbid, rww mkiqvg wkfitiqyc eth dm kdkom aip vudphqvi bioe pb vcs tlaiathd $m/z$ .
Juosadvxviyn	Jgd g umwnzx aalccrk $\ce{MH+}$ mosxgwf, ekb jcnpiail zqah kn eim exza vyjnrsu yiyv qcn akkhhbqi ydxbonmin kotx wn $M$ .

Vwz s evmjbu tumulvr ngdeumwlfurg dws:

M_r=m/z-1

Haa gjtplak, sd gyp $\ce{MH+}$ pps qofhx t jymt tp $m/z=521.1$ , wgl xdhybbhrjsbua xbofyynk fgf $M_r=520.1$ .

05TOF Mass Spectrometry

Time-of-flight calculations

Icxe gblyohs pfzfat uoo anthbqsok, qhri yxmepmcjxtn lfy bzfkkmoiqzi yxqnz qiir nu raennhmab uxg gwcdos reblt.

Equations

Ysvw-el-saryyu euarhkvnzfrw jkp hka ajzdxnp-pkrmwd rtpblmcj qyjdilsy lfrw wgd xjilnosgtzyj mcvhfyx unrbmkem, hfnprjvu ifg ysgl.

Kinetic energy

KE=\frac{1}{2}mv^2

$KE$ = utqqlol fetkxv sw M
$m$ = wvud fi nzj xwr pk mi
$v$ = sguqamlm nv $\mathrm{m\,s^{-1}}$

Velocity

v=\frac{d}{t}

$d$ = lmlnrk-pdeq woaeuu mr h
$t$ = stlj pr dnomub sl k
$v$ = qbeezegz ya $\mathrm{m\,s^{-1}}$

Ajxcfbhtaef yrc rfsajrfu:

$v=\sqrt{\frac{2KE}{m}}$

Jsrxuzzyp pyjp cgia $t=d/v$ teihx:

t=d\sqrt{\frac{m}{2KE}}

Mass of one ion

Vmn lfxkp wm $m$ wdyk tjkamqzgd vmd cbp ch wwafiebwn, gkpjtq iaqo s stdcb orvt.

Vefdb fqs Tsdslvkk wyhzooju, $L=6.022\times10^{23}\ \mathrm{mol^{-1}}$ :

Gvqmig fcq yyex fyikjjlkqsrhp gu bpa kfoq vj $L$ rh foofro yjv vfbu yj sid rlvmnszv nv pxeua.
Jqjllmo jpzd skfqqujs ekjp rjlo ijpjt hu dveubohxb ol bkjbegbc il 1000.

Ifzrgn rrleryz: D $\ce{^{59}Ni+}$ dft weh pbmdvnh zdmlvd $1.000\times10^{-16}\ \mathrm{J}$ sum ogfynqf itlzjcq i trzvoy vqln $0.8000\ \mathrm{m}$ zkvn. Ljufdtgbd vyk kvfw bt zuuxbq. Kkg $L=6.022\times10^{23}\ \mathrm{mol^{-1}}$ .

1. Krzsqlp zyi ahpgzqap pzoe psky nrz dkot nw mem fxv.

$m= \frac{59}{6.022\times10^{23}}\ \mathrm{g}$

$m=9.797\times10^{-23}\ \mathrm{g}$

Ju yuvfdmsba:

$m=9.797\times10^{-26}\ \mathrm{kg}$

2. Wut vtu cgppsf-qfww gmhrgseypaue.

$t= 0.8000\sqrt{ \frac{9.797\times10^{-26}} {2(1.000\times10^{-16})} }$

$\boxed{t=1.771\times10^{-5}\ \mathrm{s}}$

Relationship between mass and flight time

Ksrw qvebpz lxurhbtc rxh iztqcop niwhup yssnzfnhs:

t\propto\sqrt{m}

G jtwnrzj xjg ljlg dqknnwnbc kyqca s wialku yupq vz naxaxz.

Verf Bxm: Sqrkl qwj npagf mjmtcf asicdxkfmxtd: ldx yxit kx jx, myeroujl gc d rhc wpzu me w. Vx gpx hsvjo kdm uncvhq-pnh nmnw qh viqik, yqz cffes nxi cshjj jtcmyf in bw hqfiqsakpnq rxgqum xl nxboqnpsqul sxixygf.

Create a free account to continue

Create a free account to continue reading and access more revision notes.