P r o v i d e :• Fast digestion times.
• Temperatures to 250 °C.
• Pressures to 1200 psi.
• Complete containment of volatiles.
• Freedom from metal contamination.
Microwave Digestion VesselsTo expand the usefulness of its unique acid digestion
bombs, Parr has developed a line of chemically inertvessels in which microwave heating can be used for rapid
sample dissolution in a sealed vessel. These bombs can beplaced directly in a microwave oven for specific, high speedheating to drastically reduce the time required to dissolve ordigest an analytical sample. They are made in two sizes which
can be heated in any of the commercial microwave ovens regu-
larly offered for household use. Specifically constructed and/or
Speed Analytical ProceduresThe aggressive digestion action produced at the higher
temperatures and pressures generated in these bombsresult in remarkably short digestion times, with many
materials requiring exposures of less than one minute to obtain
4781 Microwave Digestion Bomb
complete dissolution. And because of their unique, high strengthdesign, they provide a much more vigorous action that can beobtained with open-cup microwave digestion systems that arerestricted to lower temperatures and pressures. In addition, thereis no loss of volatile matter from these sealed vessels, and thesensitive parts of a microwave oven are not subjected tocorrosive acid fumes.
The rapid cycle time (approximately 20 to 30 minutes,
including time for cooling) offered by this procedure allowsdigestions to be carried out in multiple steps, if desired, either foranalytical or safety reasons. Acid can be added in several steps,or different acids can be added to dissolve inorganics after anorganic matrix has been destroyed.
Equally important in the list of advantages offered by these
bombs is the elimination of all metal from the bomb structure.
With all body parts made of a high strength polymer there are nometal parts subject to corrosion, as with metal-jacketed bombs,and no possible sources of metallic ion contamination.
Parr Instrument Microwave Digestion Bombs
A Built-in Pressure IndicatorAs pressure in the sample cup increases and the relief
disc is compressed, a retaining screw, which is normallyflush with the top of the bomb cap, will protrude above
the top surface. The head of this screw will rise approximately1/32 inch for each 500 psi of pressure in the bomb. Thus, bymonitoring the extension of the screw head, the user will have a visual estimation of the pressure within the bomb.
This release mechanism has been designed to protect against
the relatively gradual pressure build-up which can result fromoverheating the contents of the bomb. But no relief mechanismcan protect against the destructive forces produced by materialswhich detonate or explode within a vessel of this type.
A Microwave-Transparent BodyThe body and screw cap for these bombs are made of a
new, microwave-transparent polymer which has goodmechanical strength at temperatures up to 150 °C, and
which serves also as an excellent heat insulator for the Teflon
sample cup. Since heating is developed internally within the cup,
temperatures in the outer, high strength body will seldom
Pressure and TemperatureLimitsWorking pressures up to 1200 psi (8.27 Mpa) and
reactant temperatures up to 250 °C can be developedsafely in these bombs, but these limits must not be
4782 Microwave Digestion Bomb
exceeded. Obviously, it is much easier to specify these limits thanit is to describe how to ensure that they are carefully observed,since there is not pressure gage on these vessels and no internaltemperature probe. Actually, the 1200 psi pressure ratingassumes that the containment parts of the vessel will alwaysremain at temperatures below 50 °C. This will normally be the
case, since microwave energy is directed into the sample itselfand not into the containment vessel. Although high temperatures
Several unique sealing and safety features have been incor- are developed within the sample, this is accomplished without
porated into the design of these vessels. The Teflon
the introduction of large amounts of heat. And the energy that is
sample cup is closed with a self-sealing, Teflon O-ring
directed into the sample is rapidly dissipated into the Teflon cup
which eliminates the need to pre-load the cup in order to secure a
and the outer vessel. The bomb itself will become warm to the
tight seal. This also eliminates the effects of differential thermal
touch as the contents cool, but it should not reach temperatures
expansion during heating and cooling cycles while providing a
chemically inert, all-Teflon system.
Users can get a preliminary estimate of the effective heating
For overpressure protection, the closure in these bombs
rate of their microwave oven by heating a small amount of the
includes a compressible relief disc which operates in conjunction
sample and its digestion medium in an open Teflon cup, and
with the O-ring cup seal to release any excess pressure if the
observing the time required to bring the medium to boiling. Most
internal pressure should exceed a point at which it might destroy
domestic ovens will generate internal temperatures in the range
the bomb and oven. When pressure in the bomb reaches approxi-
of 200 °C within one minute when using the 4781 or 4782 bomb.
mately 1500 psi, the relief disc will be compressed sufficiently to
More powerful units will be significantly faster. After removing
release the support for the O-ring. At this point, the unsupported
the bomb from the oven it normally will cool down to ambient
O-ring will blow out, releasing pressure from within the cup. In
temperature within 20 or 30 minutes with forced air circulation.
most cases all parts of the bomb except the O-ring will be
Excessive deformation of the cup, it’s sealing ring, or significant
reusable after this event if they are promptly and carefully
acid leakage are clear indications that excessive temperatures
cleaned and inspected following the pressure release.
and/or pressures are being generated.
Parr Instrument Microwave Digestion Bombs
The amount of sample and digestion aid which can be treated
in a 4781 or 4782 Microwave Bomb must not exceed the
For a basic understanding of microwave acid digestion
theory, including safety guidelines and dissolution methods
for geological, metallurgical, botanical, biological, food and
other samples, we recommend the ACS Professional Reference
Book edited by Kingston and Jassie, titled: Introduction to
Microwave Sample Preparation, 300 pp (1988). Copies can be
purchased from the American Chemical Society Distribution
Office, Dept. 297, P.O. Box 57136, West End Station, WashingtonDC 20037 ($49.95).
Parr microwave bomb procedures for processing small
amounts of tissue samples for trace-metal measurements by
Digestion aids commonly used with inorganic samples
atomic absorption spectrometry are provided in a paper by
include: aqua regia, hydrochloric, hydrofluoric and sulfuric
Nicholson, Savory and Willis, titled: Micro-Quantity Tissue
acids. Digestion with perchloric acid can be dangerous and
Digestions for Metal Measurements by Use of a Microwave Acid
Digestion Bomb, Clinical Chemistry, 35, 488 (1989). Copies can beobtained from Parr Instrument Company.
Parr Microwave Bombs are closed by simply turning a knurled
cap until it is hand tight. No wrench or spanner is required. The
Teflon O-ring attached to the cup cover will develop and maintain
Parr Microwave Digestion Bombs are made in 23 mL and 45
a tight seal without heavy pre-loading.
mL sizes, both with removable Teflon cups similar to those usedin the 4744 and 4749 metal jacketed bombs, but with an O-ringseal. Both bombs have a strong, microwave transparent outer
body as previously described. These bombs are designed for
microwave heating only. They must not be heated in a conven-
As emphasized above, a major problem associated with tional convection oven. A detailed instruction manual, No. 243M,
acid digestion bombs is the difficulty of determining the
furnished with each bomb describes the operating procedure and
exact internal pressure in the vessel. A rough, but often
safety precautions to be observed. The user should study these
misleading value can be estimated from standard vapor pressure
instructions carefully before proceeding with any tests.
curves such as those obtainable from published steam tables.
With a pure, condensable liquid, the vapor pressure in a closedsystem is solely a function of temperature, and not of loading,
provided a sufficient head space is allowed so that the vessel
When ordering a microwave digestion bomb it is advisable to
does not become liquid full. With acids such as hydrochloric,
include an extra Teflon cup and Teflon O-rings so that these parts
nitric and aqua regia, however, gases are released from the
will be available if and when replacements are needed. Part
solution at elevated temperatures and the resultant pressure is a
function of both the temperature and the initial loading density. Inaddition, gases released by the decomposition of the sample can
add to the total internal pressure of the system.
The operating instructions furnished with these bombs include
Teflon cup & cover with O-ring, 23 mL
a set of tables showing the vapor pressure characteristics of
Teflon cup & cover with O-ring, 45 mL
several commonly used acids which the operator can use in esti-
mating pressures to be expected in these bombs. But, for safetysake, these rules must be observed: Start with small samples,small amounts of acid and short digestion times, and increasethese variables only if necessary.
Parr Instrument Microwave Digestion Bombs
B. Bernas, “A New Method for Decomposition and
Comprehensive Analysis of Silicates by Atomic Absorption
Spectrometry”, Anal. Chem., 40
, 1682 (1968).
D. E. Buckley and R. E. Cranston, “Atomic Absorption Analysis of
18 Elements from a Single Decomposition of Aluminosilicate”,
Chem. Geol., 7
, 273 (1971).
J. Dolezal, J. Lenz, and Z. Suleck, “Decomposition by Pressure in
Inorganic Analysis”, Anal. Chem. Acta., 47
, 517-27 (1969).
A. M. Harstein, R. W. Freedman and D. W. Platter, “Novel Wet-
Digestion Procedure for Trace-Metal Analysis of Coal by
Atomic Absorption”, Anal. Chem., 45
, 611 (1973).
P. M. Hurley and W. H. Pinson, Jr., “Rubidium-Strontium
Relations in Tranquility Base Samples”, Science, 167
R. E. Mansell and T. A. Hiller, “Application of the Parr Acid
Digestion Bomb to Decomposition of Tetraethyllead”, Anal.
, 975 (1973).
G. Nelson and D. L. Smith, “Determination of Trace Amounts of
Metals in Foodstuffs by Atomic Absorption Spectroscopy”,
The references listed in the adjoining column describe
Proc. Soc. Anal. Chem., 168
procedures involving metal-jacketed acid digestion bombs. Anyattempt to transfer these procedures to a microwave digestion
L. E. Ranweiler and J. L. Moyers, “Atomic Absorption Procedure
bomb will require careful adjustment of the bomb charge and
for Analysis of Metals in Atmospheric Particulate Matter”,
Environ. Sci. & Tech., 8
, 152-6 (1974).
R. R. Ruch, H. J. Gluskoter and N. F. Shimp, “Occurrence and
Distribution of Potentially Volatile Trace Elements in Coal”,Environmental Geology Notes, Ill. State Geol. Survey, No. 61(1973).
The Parr Instrument Company offers these Microwave
Digestion Bombs to the skilled analytical chemist as anattractive means for preparing samples for analysis. These
bombs are designed, tested and manufactured with great care to
C. C. Schnetzler and D. F. Nava, “Chemical Composition of Apollo
be as effective and safe as possible in their intended application.
14 Soils 14163 and 14259”, Earth and Planetary Science
However, since the temperatures and pressures generated within
, 345-50 (1971).
these bombs are solely dependent upon the filling level, the timeof exposure and the power settings selected by the operator, Parr
F. W. Sunderman, Jr. and E. T. Wacinski, “Use of Teflon Digestion
will not be responsible for any personal injuries or damage to the
Bombs for Tissue Analysis: Measurement of the Effect of
bomb, the oven or other equipment associated with the use of
Estradiol-17B upon Hepatic Copper in Rats”, Ann. Clinical and
these bombs. As with all laboratory operations, the user must
Lab. Sci., 4
, 299-305 (1974).
ensure that adequate safety procedures are established to protectall personnel from the potential hazards involved in the use of
G. R. Waterbury, “Analytical Methods for Fissionable Materials in
these bombs and microwave heating techniques. Rigid controls
the Nuclear Fuel Cycle”, Program Status Report, June 1971 to
must be established to guarantee that operators do not add “just
June 1972, LA-5064-SR; also Program Status Report, June
a little more microwave exposure” to developed procedures.
1972 to June 1973, LA-5347-SR, Los Alamos ScientificLaboratory, Los Alamos, N. M. 87544.
Parr Instrument Microwave Digestion Bombs
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