Problem Set12 Solutions

Problem Set12 - G asa ndL iquid C hromatography 197 is l ost i t c arriesa wayo ne l 3C T he t ransitionf or l3cH)r-caffeine i s therefore l osso f

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Unformatted text preview: G asa ndL iquid C hromatography 197 is l ost, i t c arriesa wayo ne l 3C. T he t ransitionf or ( l3cH:)r-caffeine i s therefore l osso f 8 6 m assu nits,o r m /z 1 97 + I I 1 . a (b) An i sotopicv arianto f t he a nalytei s a n early i deal i nternals tandard ecause b the i sotopicv arianth asn earlyt he s amep hysicalp roperties s t he a nalyte. I t a shouldb ehavei n n earlyt he s amem annera s a nalyted uring a ll s tepsi n samplep reparation nd a nalysis. a (c) Caffeinei s C sHroN+Oz0 94 D a). T he p eaka t m /z 1 95 i s t 3gt2grgroN+Oz containing ne l 3C a nywheren t he m olecule.S incet he n aturala bundance i o of l 3C i s I . l Y o,w e e xpectt hat a c ompoundw ith 8 c arbona tomsw ill h avea n p isotopic eaka t M +l w ith a r elative bundancef ( M+l)/M+:8 a o x l .lo/o: 8.8%. T he o bservedntensity s 1 0.3%. O ne r easonthe bservedntensity s i i o i i highert han 8 .8% i s t hat n ot a ll o f t he i ons a t m /z 1 94 a reC sHroN+Oz+. There i s a p eak a t m /z 1 93w hich m ust b e C sHgN+Oz*. S omeo f m /z 1 94 must b e l 3Cl2CzHqN4 O2+, hich d oesn ot g ive r ise t o a s ignificanti sotopic w partnera t m /z 1 95. 22-8. Because onpolarc ompounds re m ore s olublei n m ethanolt han i n w ater,t he n a retentiont ime w ill b e s horteri n 9 0Yo ethanol. m 22-9. The m ore p olar t he s olvent,t he g reater he e luents trength n a p olar c olumn,a nd t o the s hortert he r etention irne. B ecause cetonitrile s l essp olar t han w ater,6 0 t a i o /o cetonitrile s l essp olar t han 4 0 v ol % o cetonitrile.T he r etentiont ime w ill vol a i a be g reaterf or t he 6 0 v ol o /o luent. e 22-10. I ncreasing zO i n t he m obile p hasei ncreases luents trengthb y m aking t he H e mobile p hasem ore p olar. T he g radients houldb e r un f rom l ower t o h igher H zO content. 22-lL N onpolarb ondedp hases: t 98 Polarb ondedp hases: Chapter22 i t r 22-12. ( a) S mall p articlesg ive i ncreasedesistanceo f low. H igh p ressures r equired to o btain a u sablef low r ate. m b (b) E fficiency i ncreases ecause olutee quilibrates etweenp hases ore r apidly s b A t if t he d istance hroughw hich t he s olutem ust d iffuse d ecreases. lso, migration p athsb etweens mall p articlesa re m ore u niform, d ecreasing broadeningf rom m ultiple f low p aths. 22-13. P lates: ( 15 c m)/(I0.0x l 0-a c m/plate): 1 .5 x 1 04 s . st i s s .' F = w v 2 : r I.!i:5 5 ( l0 .0 m in|) !L ffi: /\y': 6 If p lateh eight:5.0 p m,plates:3.0 x 1 04a ndw lD:0.136 m in 0 .l9 zm in f 22-14. ( a) A t p H 3 , t he p redominant orms a re n eutralR CO2H a nd c ationicR NHI. (b) T he a minew ill b e e lutedf irst, b ecausehe c ation R NHj i s l esss olublet han t p the n eutralR CO2H i n t he n onpolars tationary hase. o 22-15. ( a) W e n eedt o l ower t he e luents trength.U se a l ower p ercentage f a cetonitrile t t t to i ncreasehe r etention imes a nd p robablyi ncreasehe r esolution. (b) W e n eedt o l ower t he e luents trength.I n n ormal-phase hromatography, c m a solvents trengthi ncreases s t he s olventb ecomes ore p olar, w hich t t correspondso i ncreasinghe m ethyl l -butyl e therc oncentration.W e n eeda i t o t higherc oncentration f h exane o l ower t he s olvents trength, ncreasehe t t retention imes,a nd p robablyi ncreasehe r esolution. 22-16. U se a s lowerf low r ate,a l ongerc olumn,o r s mallerp articles ize. 200 about 1 000 t imes g reatert han t he c alculatede xternals urface. Chapter22 22-20. l. We n eedt o r emove b uffer s alts f rom t he c olumn b eforew ashing w ith s trong (mostly o rganic)s olvent. O therwise, he s altsc ould p recipitate ut o f t o solutioni nsidet he c olumn. A g ood w ay t o w ash s altsa way i s t o u se aqueous olventc ontainingn o b uffer. s 2. Strong s olvent w ill d issolvea nd c any a way f rom t he s tationaryp hase stronglyr etaineds olutes hat a re n ot s olublei n w eak s olvent. t 3 . Storingo vernightw ith s trongs olventd issolves onpolars olutesf rom t he n stationaryp hasea nd p reventst hem f rom i rreversiblya dheringt o s tationary phase. 4. When y ou a re r eady t o r esumec hromatography,he s tationaryp hasem ust t equilibrate ith t he n ew m obile p hase. w 5. Before c hromatography f a n u nknown, i t i s a g ood i dea t o r un a s tandard o mixture a nd m ake s uret hat t he r etentiont imes a nd p late c ount b ehavea s expected.I f t hey d o n ot, t he c olumn i s n ot r eadyt o u se. CH: HN+ 22-2r. ( a) CO2CH3 CH: N CO2CH3 CocaineH+ C17H22NO a m / : 304 YcoHs o C1sH16NO2 m/:182 C6H5CO2h asa m asso f 1 21D a. S ubtracting 21D a f rom 3 04 D a g ives 1 183 D a. T he p eaka t m /z 1 82p robablyr epresentsocaineH+ inus c m C6H5CO2H. T he s tructure ight b e a s s howna boveo r s omer earranged m form o f i t. (b) T he i on a t m /z 3 04 w as s elected y m assf ilter Q l. I ts i sotopicp artner b 1 3Ca t m /z 3 05 w as b lockedb y I . B ecausehe s peci s a t m /z containing t e Q l 3C-containing artnerf or t he 304 i s i sotopicallyp ure,t here i s n o p collisionallya ctivated issociation roducta t m /z 1 82. p d (c) I n s elected eactionm onitoring,m assf ilter Q l s electsust m /z 3 04,w hich j r eliminates omponents f p lasmat hat d o n ot g ive a s ignala t m /z 3 04. T hen c o this i on i s p assedo t he c ollisionc ell i n w hich i t b reaks nto a m ajor t i ...
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This note was uploaded on 04/06/2011 for the course CHEM 2011 taught by Professor Buhlman during the Spring '10 term at Minnesota.

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