chemicalionization

8/18/2019 Chemicalionization

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Chemical ionization

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Most modern mass spectrometers are designed so thelectron-impact ioniation and chemical ioniation cacarried o"t interchangea!l#$ S"ch so"rces arc called so"rces$ In chemical ioniation% gaseo"s atoms o& th(&rom either a !atch inlet or a heated pro!e) are ionicollision 'ith ions prod"ced !# electron !om!ardme

ecess o& a reagent gas$ s"all#% positi*e ions are "snagati*e ion chemical ioniation is occasionall# "sedanal#tes that contain *er# electronegati*e atoms$ Chioniation is pro!a!l# the second-most common proc&or prod"cing ions &or mass spectrometr#$


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• chemical ioniation enecessar# to modi th!eam ioniation area +ig"re !# adding *ac"capacit# and !# red"co& the slit to the mass These meas"res allo'press"re o& a!o"t , tomaintained in the ioni'hile maintaining the anal#er !elo' ,-. to

changes% a gaseo"s reintrod"ced into the ionin an amo"nt s"ch thaconcentration ratio o& sample is ,0 to ,1$ 2large concentration dielectron !eam reacts ecl"si*el# 'ith reage


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• One o& the most common reagents is methane% 'hireacts 'ith high-energ# electrons to gi*e se*eral ioas CH14% CH04 and CH54$ The 6rst t'o predominate arepresent a!o"t 78 o& the reaction prod"cts$ Thes

react rapidl# 'ith additional methane molec"les as CH14 4 CH1 --: CH.44 CH0

CH044 CH1 --: C5H.4 4 H5

• ;enerall#% collisions !et'een the anal#te molec"le CH.4< or C5H.4are highl# reacti*e and in*ol*e proton

h#dride trans&er$ +or eample%

• CH.4 4 MH --: MH54 4 CH1 proton trans&er

• C5H.4 4 MH --: MH54 4 C5H1 proton trans&er

• C5H.4 4 MH --: M 4 4 C5H= h#dride trans&er


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Note that proton-trans&er reactions gi*e the (MHion 'hereas the h#dride trans&er prod"ces an iomass one less than the anal#te% or the (MH -,) 4 /ith some compo"nds% an (MH 4 5) 4 pea> is alsprod"ced &rom trans&er o& a C 5H .4 ion to the ana*ariet# o& other reagents incl"ding propane% iso!and ammonia% are "sed &or chemical ioniation$ prod"ces a some'hat di3erent spectr"m 'ith a anal#te$


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+ig"re 5-5 contrasts the chemical ioniation anelectron-impact spectra &or I-decanol$ Theelectronimpact spectr"m (+ig"re 5-5a) sho's e&or rapid and etensi*e &ragmentation o& the moion$ Th"s% no detecta!le pea>s are o!ser*ed ahomass ,,5% 'hich corresponds to the ion C ?H ,=

4


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@elati*e to the electron-impact spectr"m$ The chemioniation spectr"m sho'n in +ig"re 5-5h is simplconsisting o& the (M - ,) 4 pea>$ a hase pea> corresmolec"lar ion that has lost an OH gro"p$ and a serdi3ering &rom one another !# ,1 mass "nits$ As in impact spectr"m$ these pea>s arise &rom ions &ormclea*age o& adacent car!on-car!on !onds$ As 'e noted$ chemical ioniation spectra generall# contade6ned (M 4 ,) 4 or (M - ,) -pea>s res"lting &rom tha!straction o& a proton in the presence o& the reag


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Brinciples o& Instr"mental Anal#sis 2 Do"glas AS>oog%+ ames Holler%Stanle# @$ Cro"ch% B-..F-.

chemicalionization

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