10
Parr Instrument
Microwave Digestion Bombs
Bulletin 4700
Loading Limits for Samples
The amount of sample and digestion aid which can be treated
in a 4781 or 4782 Microwave Bomb must not exceed the
following amounts:
Bomb No.
4781
4782
Bomb Size
23 mL
45 mL
Maximum Inorganic Sample (dry)
1.0 gm
2.0 gm
Maximum Inorganic Digestion Aid 15 mL
20 mL
Maximum Organic Sample (dry)
0.1 gm
0.2 gm
Maximum Organic Digestion Aid
2.5-3.0 mL
5.0-6.0 mL
Digestion aids commonly used with inorganic samples
include: aqua regia, hydrochloric, hydrofluoric and sulfuric
acids. Digestion with perchloric acid can be dangerous and
must not be used.
Convenient Hand Closure
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
a tight seal without heavy pre-loading.
Vapor Pressures at Elevated
Temperatures
A
s emphasized above, a major problem associated with
acid digestion bombs is the difficulty of determining the
exact internal pressure in the vessel. A rough, but often
misleading value can be estimated from standard vapor pressure
curves such as those obtainable from published steam tables.
With a pure, condensable liquid, the vapor pressure in a closed
system is solely a function of temperature, and not of loading,
provided a sufficient head space is allowed so that the vessel
does not become liquid full. With acids such as hydrochloric,
nitric and aqua regia, however, gases are released from the
solution at elevated temperatures and the resultant pressure is a
function of both the temperature and the initial loading density. In
addition, 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
a set of tables showing the vapor pressure characteristics of
several commonly used acids which the operator can use in esti-
mating pressures to be expected in these bombs. But, for safety
sake, these rules must be observed: Start with small samples,
small amounts of acid and short digestion times, and increase
these variables only if necessary.
Microwave Digestion Theory
F
or 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, Washington
DC 20037 ($49.95).
Parr microwave bomb procedures for processing small
amounts of tissue samples for trace-metal measurements by
atomic absorption spectrometry are provided in a paper by
Nicholson, Savory and Willis, titled: Micro-Quantity Tissue
Digestions for Metal Measurements by Use of a Microwave Acid
Digestion Bomb, Clinical Chemistry, 35, 488 (1989). Copies can be
obtained from Parr Instrument Company.
Microwave Bombs Available
in Two Sizes
Parr Microwave Digestion Bombs are made in 23 mL and 45
mL sizes, both with removable Teflon cups similar to those used
in the 4744 and 4749 metal jacketed bombs, but with an O-ring
seal. 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-
tional convection oven. A detailed instruction manual, No. 243M,
furnished with each bomb describes the operating procedure and
safety precautions to be observed. The user should study these
instructions carefully before proceeding with any tests.
ORDERING INFORMATION
When ordering a microwave digestion bomb it is advisable to
include an extra Teflon cup and Teflon O-rings so that these parts
will be available if and when replacements are needed. Part
numbers are shown below:
4781
Microwave acid digestion bomb, 23 mL
4782
Microwave acid digestion bomb, 45 mL
A328AC
Teflon cup & cover with O-ring, 23 mL
A328AC2
Teflon cup & cover with O-ring, 45 mL
327ACHA
Teflon O-ring, package of 2
