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Quadrupole Mass SpectrometryConcepts
Mass Spectrometers for Residual
Gas Analysis
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Residual Gas Analysis
Faraday:torr
0
1e-06
2e-06
3e-06
4e-06
5e-06
6e-06
7e-06
8e-06
9e-06
1e-05
1.1e-05
0 5 10 15 20 25 30 35 40 45 50
7x 10-6 torr
1 !r"on
What does Residual Gas Analysis allow us to do?
RGA is the examination of the molecular components present in
a vessel or evolved from a system.
It allows us to analyse, ON-LIN
and in RAL time!
"ase #ressure $in%erprint
Lea& 'etection
(irtual Lea&s ) desorption
Out%assin% ) "a&eout *ycles
#ump #erformance
*ham+er contaminants
*haracterise your system and process for optimum results
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ypical contaminant species present may +e readily identied!
Air lea&! m)e / ) 0 1ca. 2.3!4 ratio5 conrm +y the presence of pea&sat m)e 42, 46
7ater! m)e 4/ conrm +y m)e 48
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#y$%cal contam%nant s$ec%es $resent may be read%ly %dent%&%ed:
' (ydrocarbons: c)aracter%st%c "rou$s o& $ea*s+ty$%cal $ea*s at m,e 57+ 55+ 45+ 43
(%") mass $ea*s - bac*-stream%n" o& o%l or acuum rease
/o mass $ea*s - Clean%n" &lu%d , solent res%due
Residual Gas Analysis
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!# lectron m$act on%sat%on 9
Operation depends on the conversion of %as molecules into
char%ed particles, typically positive ions ) fra%ments.
Achieved +y electron impact ionisation via thermionic emission
from a hot lament.
A typical current is 4x49-2Amps.
Ions extracted into the mass lter.
Note: Ionisation depends on the nature of the species involved.
If a species is readily ionised it produces a hi%her :; si%nal than one
which is poorly ionised.
4
c.f. "en?ene > 3.@ and elium > 9.42.
Residual Gas Analysis (o %t or*s
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IONI;AION B lectron Impact Ionisation 1I5
Extractor 90V
Filament 70V
Gas molecules
Electron beam
Quadrupole
entranceaperture 0V
Positive ions
extracted into
mass filter
Source cage !V
Residual Gas Analysis ow it wor&s
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(o %t or*s : !#
#)e c)o%ce o& &%lament mater%al %s %m$ortant:
MaterialOperating
Temperature Comments
"ungsten #$00 %"oo &ot for 'G( use
'eacts )it& ox*gen to give +,- +,#
'&enium #!00 %"oo &ot for 'G( use
Forms compounds )&ic& are insulators
"&oria."&oria coated /ridium 1900 % Good general purpose'obust
2a34 1!00 %5ot robust enoug& for 'G( use
Ver* brittle
2o)er o eratin tem erature 6 lo)er out assin
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ow it wor&s ! IONI;AION
It is also important to note that I yields several types of ions!
;ee crac&in% pattern section for further details.
Note! $ra%ment ions are also &nown as #roduct or 'au%hter Ions
/on 7escription
8olecular "&e molecule )it& a positive c&arge b* loss of an electron
3ase "&e most abundant ion in t&e spectrum
Fragment Formed b* cleavage of one or more bonds in t&e molecule
'earrangement Formed b* bond cleavage and atomic migration
7oubl* +&arged # /ons from # electron loss steps appearing at 1# mass i:e: m#
8etastable Fragmentation of ion into an ion of lo)er mass a neutral particle
Residual Gas Analysis
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ow it wor&s ! :A;; $ILR
he mass lter diCerentiates the ions produced and selects speciesfor detection.
he most common form of mass lter is the Duadrupole.
A Duadrupole is pairs of parallel, eEuidistant metal rods 1poles5+iased at eEual, +ut opposite potentials
hese twin potentials contain xed'* and alternatin% R$ components.
"y varyin% the R$ component theresultant eld produced +y therods may +e varied.
Residual Gas Analysis
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ow it wor&s! :A;; $ILR
Any ions enterin% the Euadrupole eld experience potential diCerencesdeFectin% them from their ori%inal traectory.
he extent of deFection of any ion enterin% the eld is related to its
mass ! char%e 1m)e or m)?5 ratio. At each interval on the R$ scan only one m)e ratio resonates with the
eld allowin% the ion to pass alon% the ?-axis.
All other species are deFected and neutralised +y impact upon
the rods of the Euadrupole
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ow it wor&s ! :A;; $ILR
"#$dc" $rf cos t%
& #$dc" $rfcos t%
'
y
0:14
0:141
0:14#
0:14!
0:14$
0:14;
0:144
0:147
0:14?nstable @>?nstable
?
V6
$AV
#mr0
#
6 #
Residual Gas Analysis
d l f d d i
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ow it wor&s ! :A;; $ILR -:athieu ;ta+ility 'ia%rams
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
()*)
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
Residual Gas Analysis
Q d l M S f Ad d S i
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ow it wor&s ! '*OR
$iltered ions stri&e the detector to result in an ion current which ismeasured +y a sensitive amplier.
wo main types of 'etector! a! "he #araday Cup B an earthed passive conductin% surface with a
suppressor electrode to avoid false measurement.
$ast movin% ions stri&e the cup cause a Hshower of Hsecondary electrons.he use of the Hcup rather than a plate, allows all electrons to +ecollected.
ence, one ion arrivin% at the $araday needs one electron forneutralisation +ut causes several electrons to +e emittedJ this providesamplication B several electrons for each ion.
Residual Gas Analysis
Q d l M S f Ad d S i
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ow it wor&s ! '*OR
he $araday *up! 'etection limits
Ion current for Nis 49-2amps ) orr
At 49-/orr of N,
49-/K 49-2> 49-4amps
At 49-44orr of N> 49-43amps
'etection limit for conventional analo%ue amplier
(nal*ser exit
ve ions Electrometer
Farada* +up
Electron suppressor
Residual Gas Analysis
Q d l M S t t f Ad d S i
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ow it wor&s ! '*OR $! Secondary %lectron Multiplier ) ;in%le *hannel lectron
:ultiplier 1;: ) ;*:5!
A surface desi%ned to %enerate secondary electrons.
he ion impacts the surface %eneratin% or 0 electrons each
of which under%o further surface collisions %eneratin% moreelectrons, and so on in a cascade eCect.
#ower for this cascade provided +y an applied volta%e.
Gain is typically 490149for a channel plate5
:inimum detecta+le pressure49-42orr to 49-40orr
Residual Gas Analysis
Q d l M S t t f Ad d S i
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'etector #ros and *ons!
$araday *up !
Lower cost
Indestructi+le
Accurate
"
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;: ) ;*: !
'etection limit 49-42 - 49-40 orr withanalo%ue detection
$aster measurement
"
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Resolution
he a+ility to separate )resolve ions of diCerent m)e ratios
All denitions directly ) indirectly relate pea& width to hei%ht
For example:
: ) :
: at 49 pea& hei%ht where : M4 a.m.u.
i.e. $or any %iven mass :, the pea& width at 49 of the pea& hei%ht,measured from the +aseline, is less than 4 amu
his may +e complicated if the mass pea&s of trace species occur inthe pea& tail of a maor species e.%. the detection of m)e 8 or m)e@ in the presence of Nat m)e /.
Residual Gas Analysis
Quadrupole Mass Spectrometers for Advanced Science
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Interpretation and *rac&in% #atterns!
RGA data can +e presented as a prole of mass ) char%e pea&s. e.%.the RGA of Air :
B(2 /V '+ 'G( #01 C109$0
mass D amu
0
1e>04
#e>04
!e>04
$e>04
;e>04
4e>04
10 1; #0 #; !0 !; $0 $; ;0
Farada* D torr
+*cle number 1
Residual Gas Analysis
Quadrupole Mass Spectrometers for Advanced Science
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*rac&in% #atterns!
*rac&in% arises durin% ionisation when the hi%h ener%y
electrons used not only ionise species +ut fra%ment them. $or *O!4*46O e-&'(C')*! +onisationto %ive a pea& at m)e> /4*46O e-4*')*, Crac-into %ive a pea& at m)e >464*46 O e-'(C, 46O Crac-into %ive a pea& at m)e> 4
his fra%mentation can +e used to diCerentiate iso+aric species! 4*46O from 42N for example.
42Nhas pea&s at m)e / 142N
5 and m)e 42 142N5 from!
42 N e-&'/0(!,+onisationto %ive a pea& at m)e> /42N
e-42 N'/0, Crac-into %ive a pea& at m)e >42
142N5 e-&'/0(!
,, +onisationto %ive a pea& at m)e> 42
Note: *O)Nare I;O"ARI* the same mass +ut diCerent composition
Residual Gas Analysis
Quadrupole Mass Spectrometers for Advanced Science
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Interpretation and *rac&in% #atterns!
04
!e>04
$e>04
;e>04
4e>04
10 1; #0 #; !0 !; $0 $; ;0
Farada* D torr
+*cle number 1
2m,e 28+ 14
(2
m,e 18+17
2m,e 32+ 16
!r m,e 40
C2m,e 44+ 28+ 16
Residual Gas Analysis
Quadrupole Mass Spectrometers for Advanced Science
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Interpretation and *rac&in% #atterns!
*rac&in% pattern example B Linear ydrocar+ons 'ecane 1*495 and "utane 1*2495 show similarities B why
"oth compounds show
*lusters of #ea&s at!
m)? 38m)? 20m)? @
Loss of unit of mass 42
Residual Gas Analysis
Quadrupole Mass Spectrometers for Advanced Science
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Interpretation and *rac&in% #atterns!
*rac&in% pattern example
he mass 42 unit loss of al&yl-type species * i.e.
*49 e-*494e-*0, *3, *08, *2@, *344etc.
And
*249 e-*2@
e-*0, *3
, *08
$RAG:NAION B "ond cleava%e AN' Ionisation.
Residual Gas Analysis
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Interpretation!
Isotopic A+undance.
:any species exist as several naturally occurrin% isotopes:
(tom /sotopes and relative abundances
+arbon1#
+ 100-1!
+ 1:1
5itrogen 1$5 100- 1;5 0:$
,x*gen14
, > 100-1