Page 129 - Inorganic Mass Spectrometry - Fundamentals and Applications

Inductively Coupled Plasma Mass Spectrometry 119

a
from the ICP to the MS detector. Decreases in sensitivity up to factor of 10 have
been reported for 0.04 M matrix element [ 1741,
If the concentration of dissolved solids is sufficiently high, sample material
is deposited on the sampler andor skimmer cones. In an extreme case this can
cause partial blocking of the orifice(s). Although there has been little investigation
reported [ 175,1761, the severity of deposition and blocking appears to be depen-
dent on the sample chemistry. High concentrations of A1 seem to cause more
severe deposition and blocking than those of most other elements. Calcium
deposition and blocking also tend to cause more problems than those of many
other elements, although less severe than A1 problems. Deposition of sample on
a
ion lenses can affect the fields produced by the lenses. This can result in change
in ion focusing and therefore ion transmission. Changes in sensitivity due to
The
deposition or partial blocking occur slowly and normally irreversibly. orifices
andor ion optics typically need to be cleaned in order to eliminate the loss of
sensitivity.
Models [ 105,1771 and experimental measurements [178- 1801 suggest that
to
the most severe chemical matrix effects are due space charge induced decreases
in the ion transmission efficiency fkom the plasma to the detector of the mass
the
spectrometer. Unlike the deposition effects, these depend only on composition
of the sample being introduced into the plasma, not on previously run samples.
Sample matri~-induced changes in analyte transmission efficiency are most
severe for light analyte ions when high concentrations heavy ions are present in
of
the sample (Fig. 3.21) [741. For example, 0.0042 M U+ caused more than an 80%
decrease in the 7Li+ signal, about a 50% decease in the 13sBa+ signal, and only a
30% decrease in the 232Th+ signal. Matrix elements with ionization energies
greater than 8 e\/ (see B, Zn, Cd, and Au effects in Fig. 3.21) have a less severe
effect on analyte sensitivity than elements with lower ionization energies. These
elements are less extensively ionized so their influence on the beam current is
smaller than that of elements that are nearly 100% ionized.
Significant changes in sensitivity can occur, even for matrix element con-
centrations of 100 ppm E174.1, as can be seen in Fig. 3.22. The severity of the
matrix eEect depends on the concentration of the matrix element(s), not on the
ratio of matrix to analyte element concentrations.
~educin~ Chemical ~atri~ Efleects. The easiest way to reduce sample
matrix-induced changes in ICP-MS sensitivity is to dilute the sample, if detection
limits are sufficient. Alternatively, the severity of matrix effects can be reduced
by decreasing the nebulizer gas flow rate below that which produces the highest
sensitivity, Similar improvements could be made by increasing the sampling depth
beyond the depth of highest sensitivity or by increasing the applied power while
m~nt~ning the sampling depth and nebulizer gas flow rate. Decreasing the
nebulizer gas flow rate reduces the total amount of sample entering the plasma per