Page 89 - Inorganic Mass Spectrometry : Fundamentals and Applications

Inductively Coupled Plasma Mass Spectrometry

Analyte Transport Efficiency as a Function
of Sample Uptake Rate for Three Different Nebulizers
. Used with a Scott Double-Pass Spray Chambera
Analyte transport efficiency (%)
Liquid uptake Meinhard Meinhard Cetac
rate (mL/min) TR-30-A3 HEN MCN
0.010 50% 60% 60%
0.020 40% 55% 45%
0.050 20% 30% 22%
0.100 13% 16% 14%
0.200 8% 9.5% 9%
0.500 4% 5.2% 5%
l .ooo 2.5% 3% 2.9%

“Nebulizer gas flow rare was 1.0 Llmin in all cases.
Source: Ref. 422.

in Fig. 3.8 and Table 3.1, Argon at room temperature becomes saturated when it
contains about 25 mglL of water vapor. At an uptake rate of 1 mL/min, less than
of
2% of the aerosol can evaporate in the spray chamber when a gas flow rate 1
L/min is used. Then the argon is saturated with water vapor and no further
evaporation of the aerosol takes place. However, as the sample uptake rate is
decreased, the percentage of the sample aerosol that can evaporate increases. At
sample uptake rates below 25 p.,L/min, all of the water aerosol can evaporate in
of
the spray chamber, thereby increasing the transport efficiency analyte into the
ICP. The largest drops may not have sufficient time to evaporate before impacting
the walls of the spray chamber.
As can be seen from the results in Table 3.1, the analyte transport efficiency
is similar for both “conventional” and “micro-” or “high-efficiency9’ nebulizers
when compared under identical flow rates. The increase in analyte transport effi-
ciency with decrease in the sample uptake rate (sometimes called “starving” the
nebulizer because uptake rates less than the natural aspiration rate are used) was
reported long ago [21,22].
So the main advantage of the newer “micronebulizers”
is that their internal volume is small, a feature that becomes more important as the
uptake rate is reduced. A capillary can also be inserted into a “conventional”
concentric, pneumatic nebulizer to decrease its internal dead volume [23,24].

Ultrasonic nebulizers are unique in that a piezoelectric transducer, powered at
frequencies of 200 IsHz to 10 MHz, is used to generate aerosol from sample that