In inductively coupled plasma–mass spectrometry (ICP-MS), problems persist in the determination of analytes that are commonly
investigated as well as in specialty applications for those seldom considered by most analysts. Understanding the chemistry
that is common to different groups of analytes permits the development of successful approaches to rinse-out and elimination
of memory effects. It also equips analysts for development of successful elemental analytical approaches with a broad spectrum
of matrices and other analytical challenges, whether the sample is solid or liquid.
Liquid introduction, in general, and flow injection, specifically, are the most widely used sample introduction methods for
inductively coupled plasma–mass spectrometry (ICP-MS). Nevertheless, problems persist in the determination of analytes that
are commonly investigated, as well as in specialty applications for those seldom considered by most analysts. Understanding
the chemistry that is common to different groups of analytes allows the development of successful approaches to rinse-out
and elimination of memory effects. This understanding also equips analysts for developing successful elemental analytical
approaches in the face of a broad spectrum of matrices and other analytical challenges, whether the sample is solid or liquid.
The majority of ICP-MS applications for elemental analysis utilize liquid sample introduction whether or not the original
sample was a liquid. The relative ease or difficulty of a given analysis depends on several factors, including the matrix
and the chemistry of the analyte.
Probably the simplest matrix for elemental analytical purposes is water (that is, fresh water with low dissolved solids).
Nevertheless, there are analysis problems with some analytes even in this matrix. General solubility rules state that alkali
metal and ammonium ions are soluble in the presence of most anions. However, even in the presence of low concentrations of
halide and polyatomic anions, many other metals hydrolyze and form poorly soluble hydroxides or oxides in water. When there
is an absence of acid (or base in some cases), one may get an incomplete picture of the metal profile in a water sample because
the poorly soluble hydroxide or oxide accumulates on walls of tubing, spray chambers, and nebulizers. An illustration of how
such an incomplete profile may occur is presented in Figure 1. A total of 10 sequential water samples from an ultrapure water
system were analyzed for 238 U intensity in counts per second (cps) in low resolution with a magnetic sector ICP-MS using a perfluoroalkoxy (PFA) 100
ÁL/min nebulizer and PFA double-pass spray chamber. If an analyst were to use uranium calibration standards diluted in ultrapure
nitric acid before analyzing these samples, the uranium content would appear undetectable. A NIST 1643e water standard reference
material (SRM) would give the appearance of validating the method accuracy. However, the NIST 1643 water SRM matrix contains
5% HNO3 to stabilize the characterized analytes in the water solution.
Figure 1: Illustration of the accumulation of poorly water-soluble metal oxides on sample introduction system surfaces from
10 ultrapure water samples versus continuous rinse-out when the water is acidified. The decline in 238U counts per second
from sample 11 to 16 occurs as nitric acid dissolves and mobilizes the accumulated U from the surfaces.
Samples 11–20 in Figure 1 are acidified with 1% ultrapure nitric acid. It is obvious that the 238 U counts are elevated approximately 20-fold in all samples after the initiation of sample acidification. However, the approximate
100-fold increase in sample 11 demonstrates that 238 U from unacidified ultrapure water alone was accumulating in the introduction system. From that point on, the U continues
to be mobilized and rinsed from the introduction system until it reaches a constant level. Thus, without proper sample preparation,
sample carryover from accumulation could result in a false negative determination.
Figure 1 illustrates that the solvent and rinse solution for a given method must account for the analyte's aqueous chemistry.
Nitric acid (1–5%) is commonly used for metal dissolution and stabilization for ICP-MS analysis. Nitric acid is a strong acid,
and general solubility rules suggest that nitrates are soluble. The hydronium counter-anion from nitric acid is nitrate, thus
it would superficially seem to be the universal solvent for metals. It is an appropriate choice for many inorganic analytes.
Dilute nitric acid also is commonly used in diluents for urine analysis for this reason.