Agilent Technologies launched its 8800 Triple Quadrupole
ICP-MS (ICP-QQQ) at the 2012 Winter Conference on Plasma
Spectrochemistry in Tucson, Arizona, USA.
By the time the first ICP-QQQ was launched, ICP-MS had already been around
for almost three decades and was widely praised for its low limits of detection.
In fact, it was considered as the technique-par-excellence for multi-element
(ultra-)trace analysis in a wide variety of fields. However, spectral interferences
were still causing concern in some applications. Significant progress had been
made in providing ICP-MS users with adequate tools to cope with spectral
overlaps compared to the early commercial instruments introduced in 1983.
By using a double-focusing sector-field mass spectrometer instead of a
quadrupole filter for mass analysis, many spectral interferences can be resolved,
but this approach requires expensive instrumentation. Quadrupole-based
instruments could be equipped with a multipole-based collision/reaction cell
(CRC), which alleviated spectral interferences to a significant extent, for instance,
by using a non-reactive collision gas such as helium to slow down polyatomic
interfering ions to a larger extent than the atomic analyte ions, such that the
former could be selectively discriminated against on the basis of their lower
kinetic energy. The analytical community first saw Agilent’s 8800 ICP-QQQ
instrumentation as an improved version of a quadrupole-based ICP-MS equipped
with a CRC. But the unique applications being performed using Agilent ICP-QQQ
instruments installed in hundreds of laboratories across industry, research and
academia clearly demonstrates that it is much more than that.
In Agilent’s ICP-QQQ, an octopole CRC is preceded by an additional quadrupole,
enabling double mass selection, i.e. before the ions enter the CRC and afterwards.
When the first quadrupole is used as an ion guide only, the ICP-QQQ system
can be used as a “traditional” quadrupole-based ICP-MS instrument. This mode
could be useful for carrying out routine analysis not significantly challenged by
spectral interferences. When operated in tandem or MS/MS mode, however,
the double mass selection only allows the analyte ion and the interfering ion(s)
with the same mass-to-charge ratio to enter the CRC; all ions with a different
mass-to-charge ratio are removed at this stage. Consequently, the control over
the processes in the cell is greatly improved as the reaction of other (e.g. matrix)
ions with the cell gas no longer hinders the desired reaction process. In case of
a mass-shift reaction—i.e. chemical conversion of the analyte ion into a reaction
product ion that can be measured interference-free at another mass-to-charge
ratio—the absence of other ions at the new “location” of the product ion in the
mass spectrum is guaranteed. As a result, interesting but challenging elements,
such as S and P in biochemical applications, As and Se in environmental and
food applications, or Si in nanoparticle applications can be easily assayed,
interference-free.
Profiting from the analytical advantages offered by MS/MS functionality, some
ICP-QQQ users have demonstrated a larger degree of creativity by using very
reactive gases such as NH3
or CH3
F in the CRC and monitoring reaction product
ions at much higher mass-to-charge ratios than could be adequately exploited
previously. Although this might initially sound complicated, the ICP-QQQ’s
software offers tools like product ion scanning, precursor ion scanning and
differential mass scanning that provide the user with a clear insight into the
reactions proceeding in the cell and allow the product ion that will provide the
best, often unprecedented limits of detection to be easily identified. This level
of freedom and ease of use leads to a situation in which every type of spectral
overlap – whether caused by a polyatomic ion, doubly charged ion or isobaric
nuclide – can be successfully overcome. Moreover, ICP-QQQ users have also
been charmed by the additional advantages provided by this type of
instrumentation, such as the unparalleled abundance sensitivity, which is an
added benefit of double mass selection.
In 2016, the 8800 ICP-QQQ was replaced by the Agilent 8900 ICP-QQQ series.
While maintaining the performance to resolve spectral interferences, this second
generation ICP-QQQ instrument provides enhanced sensitivity and a faster
detector system with a 100 µs minimum dwell time. The latter feature is of
specific importance in single-nanoparticle analysis, a rapidly emerging type of
application, and in handling fast transient signals, such as those generated via
laser ablation systems equipped with ultra-fast ablation cells.
In my opinion, ICP-QQQ has not only fulfilled its initial promises, but has greatly
surpassed the anticipations of the diverse community of ICP-MS users.
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