Page 94 - Inorganic Mass Spectrometry : Fundamentals and Applications

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sample. For example, As(II1) more readily forms a hydride than As(V1). So, it
may be necessary to pretreat the sample to reduce the analyte to the proper
oxidation state. Arsenobetaine and arsenochlorine do not react to form volatile
arsenic hydride [42]. The species-dependent hydride generation can be used to
advantage for elemental speciation. For example, As(II1) can be measured by
hydride generation, and then As(V1) can be determined by determining the
difference from a total As concentration measurement [43].
The acid content of the sample, the reactant concentration, the liquid flow
rates of the sample and reactant, the argon carrier gas flow rate, and the reaction
cell design can all affect the hydride generation and transport. If too much H,,
CO,, and H,O is generated, the plasma may be overloaded. This effect can be
minimized by proper control of flow rates, pumping the drain to remove the liquid
after reaction and adding NaOH to the solution waste. The transfer of analyte
hydrides into the ICP [44] and the pH dependence of hydride forming elements
[45] have been investigated theoretically and experimentally. Some sample com-
ponents can interfere with the hydride generation process. Transition metals can
interfere with the hydride generation process by being preferentially reduced.
Separation of the hydride gas from the liquid key to stable signals. Pulses
is
if
of hydride can be produced the reaction solution is not well mixed, gas bubbles
form in the tubing as the reaction occurs, and the generated gas is not well mixed
with the carrier gas. A variety of gas-liquid separation devices have been used,
including gas-permeable, microporous membranes. If an appropriate membrane
is chosen, the hydride gas passes through it efficiently, but aerosol and liquid do
not [42,46]. Effective separation of the gas from the liquid also minimizes intro-
duction of sample matrix species into the ICP. For example, the ArCl+ spectral
overlap at mass 75 with the only isotope of As can be prevented by using hydride
generation sample introduction rather than introducing an aqueous solution con-
taining a high concentration of chlorides into the ICP as an aerosol [42,46,47],

Flow Injection Sample ~ntro~uction
of
Flow injection analysis involves a flowing stream solvent into which a discrete
volume of sample is injected. Many different chemical sample treatments, includ-
ing on-line dilution, extraction, preconcentration, matrix removal, separations,
dissolution, precipitation reactions, and generation of volatile species, can be
effectively used in a flow injection mode [48]. Exponential dilution can be used
for calibration. Internal standards can be added on-line to the sample [49]. One
of the main advantages of flow injection is that these processes can be easily
automated. Another potential advantage of ICP-MS is that samples containing
high concentrations of dissolved solids can be analyzed during short pulses in
order to prevent excessive deposition of salt on the sampler or skimmer.
Selected elements can be preconcentrated by using a short colum onto