Analytical chemists are being asked
to perform trace and ultratrace
metals analysis on a routine basis.
Most chemists who perform these
analyses using highly sensitive analytical
techniques, such as ICP-MS,
share a common goal: obtain quality
data that are accurate, reproducible,
and cost effective. There have
been significant advances in analytical
instrumentation over the past
decade that have allowed chemists
to accurately and reproducibly measure
elemental concentrations never
before imagined. For this reason,
there is also a growing awareness
that sample handling and preparation
must evolve to meet the same
exacting standards as today’s instrumental
techniques.
High-quality sample preparation is
a critical step when running samples
for ultratrace and trace metals
analysis. If not done properly, this
step can not only become the bottleneck
in the analytical process,
but can also significantly affect data
quality. The advantages of closed-vessel
microwave sample preparation
have been well documented
and include:
- Short digestion times
- No loss of volatile elements
- No acid fumes
- No environmental or cross-contamination
- Low blanks; minimal acid volumes
required for complete dissolution
- Unattended operation and easily
reproducible methods.
Recently, a microwave digestion
vessel/rotor has been designed
to specifically address both the
low detection levels that today’s
analytical laboratories are being
asked to meet, as well as the
growing need for increased sample
throughput. The Q-20 quartz
rotor (Milestone, Shelton, CT)
utilizes high-purity quartz digestion
vessels made to withstand
high temperature and pressures
needed to effectively digest inorganic
and the toughest organic
materials. This design allows
for very fast heating and cooling
and, in combination with a
high-throughput 20-position rotor,
permits the analytical chemist to
maximize throughput and obtain the
highest-quality digestion efficiency.
High-purity quartz vessels
Figure 1 - Q-20 quartz rotor.
High-purity quartz is an invaluable
material for ultratrace chemistry
because of its resistance to most
minerals, acids, low trace metal
impurities, and minimal carryover
between samples. Figure 1 shows the
Q-20, a closed-vessel microwave
digestion rotor that can run up to
20 individual samples simultaneously.
The vessels are simply placed
inside a high-temperature safety shield and hand-sealed with a TFM
Teflon (DuPont, Wilmington, DE)
cap. Each Q-20 vessel has a built-in
safety valve that compresses and
reseals in case of vessel overpressurization,
thus allowing for a safe and
controllable pressure release.
The Q-20 requires a minimum of
only 3 mL of reagent to effectively
digest most samples. These minimal
acid quantities (compared to approx.
10 mL in other microwave vessels)
ensure very low blank values and
minimize dilution factors. In addition
to minimal acid use, each vessel is
designed to withstand temperatures
up to 250 °C and 45 bar of pressure.
For example, five samples of 250
mg each of NIST SRM 1568a (Rice
Flour) was digested in 3 mL of HNO3
in the Ethos EZ Microwave Labstation (Milestone) using the Q-20 quartz
rotor, and then analyzed on an ICP-AES
(see Table 1).
High throughput
The Q-20 also meets the need for
increasingly faster turnaround times. Up to 20 samples can be
processed simultaneously under
identical conditions. With only
three components, vessel handling
is quick and easy. Also, due
to the vessel design, throughput
is further maximized by fast heating
rates and cooling times with
20 vessels reaching 180 °C in as
little as 15 min.
Documentation and control
For the first time, all digestion
parameters can be monitored, controlled,
and fully documented using
TEMP-SURE (Milestone) software.
An external infrared sensor reads
the actual temperature of the solution
within each vessel, displaying
all temperature profiles in real
time. The software and motor fully
control the heating and rapid cooling,
which enables analysts to process
a greater number of samples.
Additionally, it allows the presence
of retention and complete documentation
for method validation
as well as compliance with regulatory
authorities and their required
methods (see Figure 2).
Figure 2 - Example of individual temperature profiles of 20 samples including full documentation.
The vent-and-reseal technology complements the QP Sensor
(Milestone), which monitors the
presence of acid vapors inside the
microwave cavity. The QP signal is
displayed on the graphic to ensure
that all vessels are properly sealed
and that no leakage occurs. Should
a vessel require itself to vent and
reseal, the QP device would sense
the presence of acidic vapors in the
microwave cavity, displays this on
the microwave profile, and controls
the microwave output to eliminate
constant vessel venting.
Conclusion
Analytical laboratories are faced
with a wide variety of challenges
when preparing samples for trace
and ultratrace metals analysis. The
Q-20 rotor is very user-friendly and
enables the ultratrace level analyst
to process a greater number of samples
while still achieving high digestion
quality. Patented, advanced
safety features ensure analyst safety
and sample integrity. TEMP-SURE
software enables real-time, documented
safety and temperature and
pressure monitoring, while ensuring
future repeatability and reproducibility,
regulatory compliance, and
publication needs. Used in combination
with the Ethos EZ Microwave
Labstation, the Q-20 rotor
allows for fast, easy, and effective
sample preparation.
Mr. Michel is Applications Manager, Ms.
Jacobs is Marketing Manager, and Ms.
Loechner is Marketing Communications
Manager, Milestone Inc., 25 Controls
Dr., Shelton, CT 06484, U.S.A.; tel.:
203-925-4240; fax: 866-995-5100; e-mail:
[email protected].