DeterminationofNa,Mg, P, K, Ca, Cr,Mn, Fe, Cu, Zn, Se, andMoby ICP-MS
APPENDIX–DETAILEDMETHOD (WRITTENFORAGILENT7700)
A.
APPROXIMATESAMPLESIZE
1.
Nominally, use1.0g for a liquidproduct, andnet 0.20gof apowder sample,which shouldbe taken froma10%
w/w reconstitution inwarm (60°C)water (i.e., 2.0gof the10% reconstitution).
B.
THEORY
1.
The inductively coupledplasmamass spectrometer (ICP-MS) uses an ICPas averyefficient ion source for a
quadrupolemass spectrometer. Compared to inductively coupledplasmaatomicemission spectrometry (ICP-
AES), the ICP-MShasbetter sensitivity formost elements. Also, because the cost of ICP-MS instrumentshas
been steadilydecliningand their easeof operationhas improved, ICP-MShasbeen themethodof choice for
most ultra-trace (sub-ppb) determinationsof heavymetals formanyyears.
2.
In thismethod
23
Na,
24
Mg,
31
P,
39
K,
44
Ca,
52
Cr,
55
Mn,
56
Fe,
63
Cu,
66
Zn
,
78
Se, and
95
Mowill beanalyzed.Most of
theseelements suffer frompolyatomic interferences (for example,
40
Ca
16
Oand
40
Ar
16
Oon
56
Fe,
37
Cl
18
Oand
23
Na
32
Son
55
Mn,
40
Ar
12
Cand
40
Ca
12
Con
52
Cr,
12
C
16
O
35
Cl and
12
C
14
N
37
Cl on
63
Cu). However,whenanalyzed in the
Hegasmode (helium collisionmode) these interferences aredramatically reducedor eliminated. The
polyatomic ions are larger than theanalyte ions, therefore theywill collidewithHegas atomsmore frequently
thanwith the smaller analyte ions. These frequent collisionswill cause thepolyatomic ions to loseenergyandbe
prevented fromentering themass analyzer byapositivediscriminationvoltage. Thisprocess is termedkinetic
energydiscrimination (KED). Theadvantageof using theHe collisionmode lies in the fact that a single set of
conditions removesmost interferences. Furthermore, sinceHe is inert no reactionwith the samplematrixor
analyteoccurs andnonew interferences are formed. For
78
Semeasurements theH
2
gasmode isused.
Hydrogen isa reactiongasused in the few situationswhere theHe collisiongasdoes not provide sufficient
sensitivityor robustness. As a reactiongas, H
2
reacts veryquicklywithargon-based interferingpolyatomic ions
(
40
Ar
37
Cl,
40
Ar
38
Ar,
40
Ar
40
Ar, and
40
Ar
40
Ca) andeither reacts slowlyor not at allwith theanalyte. In thismethod,
40
Ar
38
Arwouldbe themainpolyatomic interfering species for
78
Se. In cases inwhich the instrument isnot
equippedwithhydrogen reactiongas, heliummaybeused, but adjustmentswouldhave tobemade to the
practical limit of quantitation for selenium.
3.
In thismethod, sampledigestion is achieved inone-step temperature controlleddigestion. Nitricacidand
hydrogenperoxideareadded to thevessels (CEMexpress vessels) and the samples aredigestedusinga two-
stagepreprogrammed temperature control digestion. Theadditionof hydrogenperoxidehelps reduce carbon
andnitrousoxide levels in thedigestate. Thepresenceof carbon in the samples causes signal enhancement of
Se. Therefore, tomatrixmatch the samples, carbon in the formofmethanol is added toboth the standard
solutions and thedigestatebeforeanalysis.
4.
Internal standards
72
Geand
130
Teareadded to the samples (prior todigestion) and standard solutions at 50
ng/mL. Note, thismultielement internal standard solutionalso contains
60
Ni becauseduringmethod
developmentwork, Ni andGe resultswere compared todetermineanalternative internal standard toNi,which
was found tobean inherent element in cocoapowders.
5.
Calibration consistsofmeasuring the countsof
23
Na,
24
Mg,
31
P,
39
K,
44
Ca,
52
Cr,
55
Mn,
56
Fe,
63
Cu,
66
Zn,
78
Se, and
95
Mo
ineachof the standard solutions anddividingby the countsof their corresponding internal standardat the
same level. Anexact quantityofGeandTeareadded toeach sample solution so that if theywerediluted to50
mL, their concentrationwouldbe50ng/mL. Note, however, that theanalyte/internal standard ratios in the
sampleswill not changewith the total volume, and sovolumetricware isnot necessary for the sample
preparation. Therefore, 50mL samplevials canbeused. Theanalyst shouldalways enter50mL for the sample
MTE-02
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