The
KU Plasma
Analytical Laboratory (KU-PAL) instrument for determining
trace and minor elements in solutions and solids is a Fisons/VG PlasmaQuad II+XS Inductively Coupled
Plasma Mass Spectrometer (ICPMS).
The ICPMS is a very fast sequential mass analyzer that extracts
positively-charged ions from a plasma powered by a 27 MHz RF generator. It is equipped with a three-channel
peristaltic pump (sample introduction, spray chamber drain, and
autosampler-probe continuous wash), and a Gilson 222XL autosampler with full
X-Y-Z capabilities.
At KU-PAL, this
instrument is used most commonly to determine the geochemistry ofÉ
á ground water and surface water, minor and
trace elements
á oil field brines, iodide
á sedimentary, igneous and metamorphic
rocks
Ðwhen dissolved: minor and trace elements
Ðdirectly, through laser ablation: major, minor and trace elements
á soils, minor and trace elements
Although
the instrument is used primarily to determine elemental concentrations, it has
the precision and sensitivity to measure isotopic abundances with errors
generally less than 1% RSD. Thus,
the instrument is useful for determining isotope ratios where ultrahigh
resolution is not required.
Data
are collected in scanning mode
to investigate interferences or assess unknowns or in peak-jumping mode to spend more time ÒcountingÓ the signal
response.
A dual detector
system (Channeltron electron multiplier) allows signals to be collected under
conditions of high sensitivity (pulse-counting mode) or lower sensitivity (analog mode).
Cross calibration between the detectors allows seamless data collection
across a wide range of concentrations.
Graphs show the near perfect correlation between measurements of
digested soil samples on elements ranging from 0 to 1600 ppb, and from 0 to 100 ppb.
Tuning of the ICPMS is critical to good analytical results. Different lens-voltage combinations result in a different looking mass-response curve. Much of this differential response reflects lens settings that allow the ever-present background of oxide, nitride, argide, and carbides in the low end of the mass spectrum. Appropriate lens settings discriminate against these molecular species, thereby improving detection of low-mass elements.
