2.12 Mass Spectra and IR

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Detecting elements and compounds

Mass spectrometry is a highly sensitive technique that can

be used to hel identify co!ounds" #a!les of only a few

!illigra!s are required"

$his !akes it useful

in forensic science"

#ectra of sa!les

could be co!ared%for e&a!le% to link a

susect to a weaon

that has been fired"

'ass sectro!etry is useful in the identification of unknown

!aterials% because the mass spectrum of an unknown

sa!le can be co!ared with a database of known sectra"


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Mass spectroscopy in space

'ass sectro!eters have been included onboard several

sace robes"

)uring the descent towards

the surface of $itan% the !ass

sectro!eter analy*ed the

at!osheric co!osition at

various heights" +t was found

that the at!oshere consists!ainly of nitrogen and

!ethane" $he resence of

argon,(0 was also detected"

+n 200-% the .uygens robe landed on the surface of $itan%

the largest !oon of #aturn"


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Molecular ions

/hen a co!ound is analy*ed in a !ass sectro!eter%

gaseous !olecules are bo!barded with high,seed

electrons fro! an electro

n gun"

M(g) + e- M+(g) + 2e

-

$he eak with the highest mass-to-charge ratio m/z is

for!ed by the !olecular ion% and the value of !* is equal to

the relative !olecular !ass of the co!ound"

$hese knock out an

electron fro! so!e of

the !olecules% creatingmolecular ions% which

travel to the detector

lates


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Molecular ions and peaks

$he eak at the highest !* on the !ass sectru! is for!ed

by the heaviest ion that asses through the sectro!eter"

5nless all !olecules of the original substance break u% this

corresonds to the !olecular ion of the sa!le substance"

molecular ion peakmass spectrum

of paracetamol

(0 60 120 140

20

(0

40

60

100

00

a b u

n d a n c e ( % )

m/z


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igh resolution mass spectroscopy

igh resolution mass spectrometry can deter!ine the !*

of a eak to several deci!al laces" $his can distinguish

between co!ounds that have very si!ilar relati!e

molecular masses M r but different empirical formulae"

$o integer values% he&ane 4.1( and entanal -.108

both have a M r of 64" .owever% using ato!ic !asses to four

deci!al laces% the M r of he&ane is 64"1104% while the M r of

entanal is 64"0739"

/ith a high enough resolution% it istherefore ossible to distinguish

between he&ane and entanal%

which are both colourless liquids"


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Determining molecular formulae


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"hat is fragmentation#



$nterpreting mass spectra



Mass spectroscopy



$% energy and molecular !ibrations


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$% energy and dipoles


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&he infrared spectrometer



$nterpreting infrared spectra

n infrared spectrum is a lot of trans!ission of infrared

radiation against 'a!enumber 1 wavelength" ny

wavelength that is absorbed by the sa!le will trans!it less

than the others% for!ing a di or :eak; in the grah"

0

20

(0

40

60

100

3000 2000 1000

'a!enumber (cm-)

t r a n

s m i s s i o n ( % )



$% radiation and greenhouse gases

>itrogen and o&ygen% the !ost abundant gases in the

at!oshere% do not absorb infrared energy" $his is because

the vibrations caused would not change the diole of the

!olecules"

$hese gases absorb

infrared radiation in theat!oshere% stoing it

escaing into sace they

are greenhouse gases"

.owever% !ethane% water

and carbon dio&ide doabsorb strongly in the

infrared region"



reenhouse gases



*sing infrared spectroscopy

/hen breathaly*ed by

olice% !otorists blow into a

handheld device% which

gives an indication of the

a!ount of alcohol in their

breath" .owever% the result

is not accurate enough tobe used as evidence"

t the olice station% the !otorist blows into a !ore

accurate breathaly*er% containing an +? sectro!eter"$he breathaly*er calculates the ercentage of alcohol in

the breath by looking at the si*e of the absortion due to

the =. bond stretch in the alcohol"



$nfrared spectroscopy summary



$dentifying , and ,, bonds

'ost organic co!ounds contain =. and = bonds"

$hese are therefore often visible in an infrared sectru!"

,

0

20

(0

40

60

100

3000 2000 1000

'a!enumber (cm-

)

t r a n s m

i s s i o n ( % )

,,

+n the sectru! of an alkane% such as butane shown here%

there is a eak at 29-0 c!,1 due to =. and a eak below

1-00 c!,1 due to ="

$he eak due to=. is the

stronger of the

two because

there are !any!ore =. bonds

than = bonds

in the co!ound"



*sing the fingerprint region

$he region below 1-00 c!,1% usually called the fingerprint

region% has !any eaks that are difficult to assign" $he

attern of these eaks is unique to a articular co!ound"

substance !ay

be identified by

co!aring the +?sectru! to a

database of

reference sectra"

n e&act !atch in

the fingerrint region

identifies a co!ound"

0

20

(0

40

60

100

3000 2000 1000

'a!enumber (cm-)

t r a n s m i s s i o n ( %

)

fingerprint region



$nterpreting $% spectra



l


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lossary

"h t th k d#


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"hats the key'ord#

M lti l h i i


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Multiple-choice 0uiz

2.12 Mass Spectra and IR

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