The 4748 bomb is the largest of all Parr acid digestion bombs. It has a 125ml, removable PTFE cup in a stainless steel body with six cap screws in the screw cap to seal the flanged PTFE cup. An expandable wave spring maintains continuous pressure on the seal during the cooling cycle when PTFE parts might otherwise relax and leak. Stirring can be provided with a magnetic stir bar.
A safety rupture disc above the PTFE cup will blow out through an opening in the bomb head if pressure should accidentally reach the 3500 psi range. But for safe operation, pressures in this bomb should never exceed 1900 psi and temperatures must not exceed 250°C. Operations should be conducted well below these limits whenever possible due to the potential destructive forces in a large bomb charge. It also is advisable to test any new procedures for safety in one of the smaller bombs before scaling up to this size. A wrench for the cap screws is furnished with the bomb. No other accessories are required.
Speed Analytical Procedures
Parr acid digestion bombs have added a new alternative to the task of preparing analytical samples for analysis. By combining the unique chemical inertness of PTFE with the advantages of a sealed pressure vessel, these convenient bombs offer a rapid procedure for sample dissolution or digestion that has several important advantages over more traditional methods of sample preparation. By providing a convenient means for holding strong mineral acids or alkalis at temperatures well above normal boiling points, these sturdy digestion bombs will:
- Accelerate digestions which otherwise would proceed slowly if conducted in an open container at atmospheric pressure.
- Permit the use of strong acids, such as HF or aqua regia, which will attack most other conventional containers.
- Dissolve analytical samples without losing trace elements and without adding unwanted contaminants from the container itself.
- Generate a vigorous chemical action that is a great time saver in analytical procedures.
- Obtain complete digestion or dissolution of samples that react slowly or incompletely when treated by other methods.
Chemists who have worked with sealed glass tubes and other cumbersome digestion devices will recognize the attractiveness of these convenient digestion vessels.
Dissolve Inorganic Samples
Ores, rock samples, glass and other inorganic materials can be dissolved rapidly in Parr acid digestion bombs using strong mineral acids: HF, HCI, H2SO4, HN03, Aqua Regia and others. Other chemicals and manufactured products can be leached or dissolved as well, all without introducing unwanted ions and with complete sample recovery. Using reaction temperatures well above normal boiling points, samples can be dissolved or digested rapidly for all types of chemical analyses, particularly for AA and ICP spectroscopy, and for other instrumental methods in which trace elements must be identified.
Digest Organic Materials
Organic samples can be treated with nitric acid and other oxidizing acids in these bombs with no difficulty, provided certain safety precautions are observed. Alkaline or acid hydrolysis reactions can also be handled equally well.
In addition to their normal uses for sample digestion and dissolution, these bombs serve as excellent general purpose reactors for procedures requiring a small, PTFE lined vessel for use within prescribed pressure and temperature limits. Agitation can be produced with a magnetic stir bar or by shaking or rolling the bomb.
Metal Jacketed and Microwave Styles
Parr PTFE lined acid digestion bombs are made in two different styles: There is a line of metal- jacketed bombs that can be heated in an oven, sand bath, oil or water bath and in other conventional ways, and a new line of digestion bombs developed specifically for rapid heating in a microwave oven. Both styles are made in several sizes, offering a broad selection of bombs ranging from economical models for routine tests to massive styles for procedures involving extreme conditions or potentially hazardous samples. Each of these bombs has a thick walled PTFE liner that completely isolates the charge from the supporting body. These liners are removable and can be charged or emptied while outside of the bomb to eliminate any possible contamination from acid contact with metal parts. They can be replaced if they become damaged or contaminated.
Several designs are used to keep Parr acid digestion bombs tightly sealed during consecutive heating and cooling cycles when conventional seals might leak. One of these designs uses a tapered closure between the PTFE cup and cover with an unusually long sealing face that tends to become self-sealing. This long path also minimises the tendency for acid migration through the joint. Bombs using this design can be sealed by simply turning down a knurled cap by hand without using a wrench or spanner. For applications requiring higher temperatures and pressures, the PTFE liners are made with a broad, flanged closure and sealed by tightening the bomb cap with a spanner or, in a larger design, by using cap screws to develop the sealing force. Tight closures for the PTFE cup in Parr microwave bombs are developed and maintained with self- sealing PTFE O-rings.
Since PTFE has a much larger coefficient of thermal expansion than the materials within which a liner is enclosed, a PTFE liner will expand and contract more than its enclosure when these bombs are heated and cooled. To compensate for this difference and to maintain a constant loading on the PTFE seal, all Parr acid digestion bombs have spring-loaded closures which will continue to maintain pressure on the liner seal throughout the operating cycle, and particularly during cooling when PTFE parts would otherwise relax and leak.
Safety Rupture Discs
Safety rupture discs are provided in most Parr acid digestion bombs to protect the bomb and the operator from the hazards of unexpected or dangerously high internal pressures. The burst pressure in each installation is established by a frangible metal disc installed as a sandwich with a matching inner disc which serves as a corrosion barrier to protect the rupture disc from corrosive vapor.
The Nature of PTFE
The use of PTFE for the sample cup in these bombs is an obvious choice, not only because of its unique inertness to strong acids and high temperatures, but also because it is transparent to microwave energy, allowing energy to flow directly to the sample while serving also as an insulator to restrict heat flow from the reaction zone. PTFE does, however, have two characteristics which make it somewhat less than perfect for this application, and the user who understands these deficiencies will be able to minimise their effect upon his work.
First, PTFE has a tendency to creep or flow under pressure or load. This tendency is present even at room temperature and it is accentuated at higher temperatures. At temperatures below 150°C the tendency to creep will be negligible. But as temperatures rise above 150°C the creep effect will become more pronounced, making it more difficult to maintain tight seals and resulting in deformation and shorter life for the PTFE components. The extent of the creep effect will be roughly proportional to the maximum operating temperature.
Secondly, PTFE is a porous material. Although the materials and designs used in Parr acid digestion bombs minimise the effects of this porosity, users of these bombs can expect to see evidence of vapor migration across the cover seal and through the wall of the liner itself. Parr is able to minimise these problems by machining these parts from virgin PTFE that has been molded at an optimum pressure selected to reduce any porosity to an absolute minimum. The thick walls and effective seals used in these bombs also help to overcome these undesirable properties. Experiments have shown that the amount of solute lost in this manner during a normal digestion is negligible, but vapor migration into the walls of the PTFE cup will occur and cannot be avoided.
Sample Preparation Bombs