Page 95 - Inorganic Mass Spectrometry : Fundamentals and Applications

Muss
~~ductively Coupled ~las~a Spectro~et~ 85

which the analytes are absorbed. The preconcentration factor can be controlled
because it depends on the amount of time sample is flowed through the column
“plug”
[49]. After a fixed time the analytes can be eluted from the column in a by
an acidic solution, for example, and carried to the nebulizer in a flowing stream.
Alternatively, a column can be used to remove particular components of the
sample from the stream while allowing analytes of interest to pass through the
column [50].

Solid Sample Introduction
Dissolution of solid samples for ICP-MS analysis has several disadvantages,
including time consuming procedures, difficulty in gaining complete dissolution,
potential loss of elements that form volatile species, potential contamination by
reagents and vessels, and loss of information on the spatial dis~bution of elemen-
tal composition. Furthermore, the introduction of solvent aerosol and vapor into
the ICP leads to the production of molecular ions that can cause spectral overlaps
with analyte elemental ions of interest. Several solid-sampling techniques have
been developed to overcome some or all of the disadvantages of sample dissolu-
tion and solution sample introduction for ICP-MS. These include laser ablation,
electrothermal vapo~zation, spark and arc ablation, and powder injection devices.
The major problem with solid sample introduction is calibration for quantitative
analysis, often requiring standards that are well matched to the sample.
Many of the solid sample introduction techniques produce transient signals
or at least signals that fluctuate on short time scales. The combination of time-of-
flight ICP-MS and solid sampling approaches that generate a transient signal may
be particularly attractive because all elements can be monitored simultaneously.
Multielement analysis from transient electrothermal vaporization or direct inser-
tion smple introduction will become more viable. It is even possible to monitor
signals for all elements from material produced from a single laser pulse.
Laser A~lati~~. When a laser is focused on or just above a solid sample,
the surface can be ablated to produce particulates and vapor by sputtering and
thermal vapo~zat~on processes. The sample vapor and dry aerosol can be carried
in a flowing gas stream into the ICP [5 1,521. Laser ablation (LA) sampling is
amenable to a wide variety of materials, conducting and nonconducting, inorganic
and organic. The ablated spot size can be as small as a few micrometers, so spa-
tially resolved measurements can be made. Recently introduced lasers with flat
beam profiles also allow depth-resolved measurements. Laser ablation-ICP-MS
has been widely used for geological samples.
The ICP-MS signal depends on the amount of analyte entering the plasma
per second. The amount of material ablated per laser pulse is strongly dependent
on the sample properties and the surface mo~hological features. In addition, the
amount of material is dependent on the laser properties, including wavelength,