Hardware manual

6-18 Agilent 7500 Series ICP-MS Hardware Manual
6 Technical Information
Why the octopole ion guide?
The octopole ion guide is driven by a voltage of fixed amplitude and frequency
in order to assure stable signals. The frequency of 10 MHz brings about
wiggle- less motion of ion near the flat bottom of the effective potential well,
which is difficult to achieve with lower order multipoles or low- frequency
voltage
2)
. The addition of reaction gas minimizes the endothermic reaction
that leads to loss of analyte ions, and also lowers the energy of the new
background species produced through reaction or declustering of polyatomic
ions. These new background species are easily removed because of their lower
kinetic energy compared with analyte ions.
2) D. Gerlich, Inhomogeneous RF Fields: A Versatile Tool for the Study of
Processes with Slow Ions, Advances in Chemical Physics Series,
Vol.LXXXII, 1992 John Wiley & Sons, Inc.
Why H
2
and He?
Three gases are available: hydrogen (H
2
), helium (He) and an optional gas
offered upon request. In most cases, pure H
2
or pure He are sufficient for
performing the analysis of most of the elements. Generally, H
2
should be used
to suppress plasma based polyatomic ions such as ArO and Ar
2
; He is
preferable to suppress the species derived from sample matrices, such as ClO,
NaAr and ArCl, which interfere with
51
V,
63
Cu and
75
As. For example, H
2
molecules react with Ar
2
ions, but do not with Se ions. Allowing Se to be
detected with low A
2
+ background. Unlike other more reactive gases, the
ionization potential of H
2
(15.5eV) is much higher than metals, the loss of
analyte ion through charge transfer does not take place. However, this does not
necessarily means that all types of reaction between H
2
and the analyte ions
are negligible. When the H
2
flow rate is increased, H
2
begins to react with Cu
and As, however the loss of those ions is relatively small compared with the
loss caused by other more reactive gases. If the background level is so high as
to need a high flow rate of H
2
, the use of less reactive gas such as He can be
more effective. For example, He efficiently dissociates NaAr because of its low
bonding energy, while it does not cause a reactive loss of Cu ion.
A
2
+
+ H
2
→ Ar
2
H
+
+ H k=1x10
-9
cm
3
/s
Se
+
+ H
2
→ Se
+
+ H
2
no reaction
NaAr
+
+ He → Na
+
+ Ar + He dissociation
Cu
+
+ He → Cu
+
+ He no reaction