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THE COLD VAPOR MERCURY TECHNIQUE
Principle
Since atoms for most AA elements cannot exist in the
free,ground state at room
temperature,heat must be applied to the sample to break
the bonds combining at-oms
into molecules.The only notable exception to this is
mercury.Free mercury
atoms can exist at room temperature and,therefore,mercury
can be measured by
atomic absorption without a heated sample cell.
In the cold vapor mercury technique,mercury is chemically
reduced to the free
atomic state by reacting the sample with a strong
reducing agent like stannous
chloride or sodium borohydride in a closed reaction
system.The volatile free mercury
is then driven from the reaction flask by bubbling air or
argon through the
solution.Mercury atoms are carried in the gas stream
through tubing connected
to an absorption cell,which is placed in the light path
of the AA spectrometer.
Sometimes the cell is heated slightly to avoid water
condensation but otherwise
the cell is completely unheated.
As the mercury atoms pass into the sampling cell,measured
absorbance rises in-dicating
the increasing concentration of mercury atoms in the
light path.Some sys-tems
allow the mercury vapor to pass from the absorption tube
to waste,in which
case the absorbance peaks and then falls as the mercury
is depleted.The highest
absorbance observed during the measurement will be taken
as the analytical sig-nal.
In other systems,the mercury vapor is rerouted back
through the solution and
the sample cell in a closed loop.The absorbance will rise
until an equilibrium con-centration
of mercury is attained in the system.The absorbance will
then level off,
and the equilibrium absorbance is used for quantitation.
The entire cold vapor mercury process can be automated
using flow injection tech-niques.
Samples can be analyzed in duplicate at the rate of about
1 sample per min-ute
with no operator intervention.Detection limits are
comparable to those
obtained using manual batch processes.The use of flow
injection systems also
minimizes the quantity of reagents required for the
determination,further reduc-ing
analysis costs.
The sensitivity of the cold vapor technique is far
greater than can be achieved by
conventional flame AA.This improved sensitivity is
achieved,first of all,through
a 100%sampling efficiency.All of the mercury in the
sample solution placed in
the reaction flask is chemically atomized and transported
to the sample cell for
measurement.
The sensitivity can be further increased by using very
large sample volumes.
Since all of the mercury contained in the sample is
released for measurement,in-creasing
the sample volume means that more mercury atoms are
available to be
transported to the sample cell and measured.The detection
limit for mercury by
this cold vapor technique is approximately 0.02
mg/L.Although flow injection
techniques use much smaller sample sizes,they provide
similar performance ca-pabilities,
as the entire mercury signal generated is condensed into
a much smaller
time period relative to manual batch-type procedures.
Where the need exists to measure even lower mercury
concentrations,some sys-tems
offer an amalgamation option.Mercury vapor liberated from
one or more
sample aliquots in the reduction step is trapped on a
gold or gold alloy gauze.The
gauze is then heated to drive off the trapped mercury,and
the vapor is directed
into the sample cell.The only theoretical limit to this
technique would be that im-posed
by background or contamination levels of mercury in the
reagents or system
hardware.
Limitations to the Cold Vapor Technique
Of all of the options available,the cold vapor system is
still the most sensitive and
reliable technique for determining very low
concentrations of mercury by atomic
absorption.The concept is limited to
mercury,however,since no other element
offers the possibility of chemical reduction to a
volatile free atomic state at room
temperature.
HYDRIDE GENERATION TECHNIQUE
Principle
Hydride generation sampling systems for atomic absorption
bear some resem-blances
to cold vapor mercury systems.Samples are reacted in an
external system
with a reducing agent,usually sodium borohydride.Gaseous
reaction products are
then carried to a sampling cell in the light path of the
AA spectrometer.Unlike
the mercury technique,the gaseous reaction products are
not free analyte atoms
but the volatile hydrides.These molecular species are not
capable of causing
atomic absorption.To dissociate the hydride gas into free
atoms,the sample cell
must be heated.
In some hydride systems,the absorption cell is mounted
over the burner head of
the AA spectrometer,and the cell is heated by an
air-acetylene flame.In other sys-tems,
the cell is heated electrically.In either case,the
hydride gas is dissociated
in the heated cell into free atoms,and the atomic
absorption rises and falls as the
atoms are created and then escape from the absorption
cell.The maximum absorp-tion
reading,or peak height,or the integrated peak area is
taken as the analytical
signal.
Advantages of the Hydride Technique
The elements determinable by hydride generation are
listed in Table 4-1.For these
elements,detection limits well below the mg/L range are
achievable.Like cold va-por
mercury,the extremely low detection limits result from a
much higher sam-pling
efficiency.In addition,separation of the analyte element
from the sample
matrix by hydride generation is commonly used to
eliminate matrix-related interferences.
Hydride Generation Elements
As
Bi
Ge
Pb
Sb Se
Sn
Te
The equipment for hydride generation can vary from simple
to sophisticated.Less
expensive systems use manual operation and a flame-heated
cell.The most ad-vanced
systems combine automation of the sample chemistries and
hydride sepa-ration
using flow injection techniques with decomposition of the
hydride in an
electrically-heated,temperature-controlled quartz cell.
Disadvantages to the Hydride Technique
The major limitation to the hydride generation technique
is that it is restricted pri-marily
to the elements listed in Table 4-1.Results depend
heavily on a variety of
parameters,including the valence state of the
analyte,reaction time,gas pressures,
acid concentration,and cell temperature.Therefore,the
success of the hydride
generation technique will vary with the care taken by the
operator in attending to
the required detail.The formation of the analyte hydrides
is also suppressed by a
number of common matrix components,leaving the technique
subject to chemical
interference.
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