EPA's Region 6 Office
Serving: Arkansas, Louisiana, New Mexico, Oklahoma, Texas, and 66 Tribal Nations
Inorganic Laboratory Team
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The Inorganic Laboratory Team analyzes environmental samples for trace
metals, mercury, and RCRA characteristics. The samples analyzed may include
drinking water, waste treatment water, solid wastes, surface and ambient
waters, and sediment/soils. These tests support all programs i.e. air,
water RCRA, Superfund, and criminal investigation.
Inorganic Sample Preparation Laboratory
The sample preparation laboratory is designed to prepare water and solid samples for
analysis by Inductively Coupled Plasma (ICP), Inductively Coupled Plasma/Mass Spectrometry (ICP/MS), mercury, cyanide, and RCRA characteristics. The samples
are subjected to a variety of digestion techniques. ICP and ICP/MS samples
for water are routinely prepared by adding nitric and hydrochloric 
acids
or nitric acid and hydrogen peroxide to a 50 ml aliquot then heating
the sample on a hot plate until a reduction in volume allows acids to
become concentrated. The heat and acids allow the metals within the samples
to change to a state which can be detected by the instrumentation and
quantified. A similar procedure is accomplished when soil or sludge samples
are prepared using one gram of sample material. These procedures are simple
and require only beakers, watch glasses, hot plates, and fume hoods. Final
steps before analysis require filtration of the samples to remove any
solid particles from the solution before analysis and volumetric flasks
to bring the samples to a final volume of 100 ml.
Digestion of samples to be analyzed for mercury uses reagents such as sulfuric acid, concentrated nitric acid, potassium permanganate, and potassium persulfate which are added to a 100 ml water sample which is then incubated in a hot water bath at 95 degree Celsius for 2 hours. Soil samples use 0.2 grams of sample material and aqua regia in addition to potassium permanganate and incubation for 30 minutes in a hot water bath. Hydroxylamine reagent is added to all samples prior to giving the samples to the analyst. Other preparation techniques used on a non-routine basis may include microwave digestion, chelation pre-concentration, or direct analysis.
The
Houston Laboratory has two mercury analyzers, both use cold vapor atomic
absorption technology. The instruments are designed to measure the mercury
vapor produced form the prepared samples upon the addition of stannous
chloride reagent by the analyst. These instruments are similarly calibrated
with known standard materials to establish a concentration curve. Sample
concentration is calculated against this curve. The two systems differ
in their detection limit capabilities. One instrument has a detection
limit around 200 ug/liter and the other can detect 25-30 ug/liter mercury.
Inductively Coupled Plasma Laboratory (ICP)
The
ICP laboratory contains one ICP-atomic emission spectrometer. The multichannelled
instrument simultaneously measure up to twenty six (26) trace metals at
once. The instrument introduces the digested sample into a spray chamber
and into the very high temperature argon plasma. The emissions generated
from the metals contained in the sample are optically focused onto photomultiplier
tubes and intensities measured. The instrument is calibrated using mixed
element standards. The emissions from standards and samples are ratioed
to arrive at a sample concentration. The ICP will measure in the range
of mg/liter to low ug/liter concentrations.
Inductively Couple Plasma-Mass Spectrometry (ICP/MS)
The ICP/MS instrument measures most of the elements in the periodic table which have naturally occurring isotopes. ICP/MS offers the opportunity to achieve lower detection limits at the part per trillion (ppt) range. The digested samples in the liquid form are introduced into the instrument sample introduction system which consists of a nebulizer and spray chamber. The nebulizer 
converts the liquid sample into very small droplets. These small droplets are carried through the spray chamber into the injector and then into the plasma which is at a temperature of approximately 6000C. The plasma ionizes the elements present in the droplets. These ions then pass through the interface and the ion lens. After being focused by the ion lens, the ions are separated by their mass-to-charge ratio in the mass spectrometer and measured by the detector. Once the detector measures the ions, the computerized data system is used to convert the measured signal intensities into concentrations of each element and generate a report of the results.