Pressure Digestion for Sample Preparation

Dr. Dieter Gutwerk
BERGHOF Products + Instruments GmbH, Harretstraße 1, 72800 Eningen, Germany

Berghof Products + Instruments GmbH

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What analyst does not wish for a universal method for processing all his or her samples? Pressure digestion represents just such a procedure for sample digestions and is therefore one of the standard sample preparation procedures currently employed in analytical chemistry for element quantification. The sample material is placed in a sealed chamber with an acid mixture and heated to a temperature, typically between 200 and 260°C, at which point it is completely decomposed and placed in solution.

The advantage of this procedure in comparison with open digestion in a recycling device or with the traditional "hot plate", lies in the significantly higher working temperatures which can be achieved. While in open systems these temperatures are limited by the acid solution's boiling point, temperatures in the 200-260°C range can be typically achieved in sealed digestion vessels. This results in a dramatic acceleration of the reaction kinetics, allowing digestion reactions to be carried out in a matter of hours (pressure digestion using Tölg bombs) or in less than an hour (microwave digestion). However, these methods also make it clear that the temperature itself actually represents the most significant reaction parameter. It is the ultimate determinant of the digestion quality, but also results in a pressure increase in the vessel and therefore in a potential safety hazard. Therefore, the pressure must ultimately also be considered.

These effects are independent of the heating method. The following is intended to offer a mare detailed discussion of the differences between, and application areas of, the two possible, practical procedures.

Figure 1: Stainless steel pressure digestion system with a 12-sample heating block and temperature regulator

Pressure Digestion in "Tölg Bombs" [1]:
It has been more than 30 years since BERGHOF introduced a series product based on the pressure digestion method developed by Prof. Tölg [1]. Since this time, BERGHOF has sold these stainless steel pressure digestion vessels with a TFMTM PTFE liner under the trade name, "digestec". The vessels are available in a variety of capacities ranging from 25 to 250ml, a max. operating pressure of 200 bar, and a max. operating temperature of 260°C. For safety reasons, heating takes place in special heater blocks and not in the drying chamber. Therefore, digestion is generally carried out at a specific external temperature. Internal pressure development is practically irrelevant due to the extreme pressure loads which the stainless steel containers are capable of withstanding as well as to the slow heating rate. In any case, safety is assured by an appropriately dimensioned pressure relief device.

Due to the high max. operating pressure of 200 bar and the max. operating temperature of 260°C, these systems are capable of completely digesting nearly any sample and placing the samples into solution. A decisive advantage of this methodology lies in the ability of extending the digestion period nearly indefinitely. This allows even the hardest samples (e.g., SiC, alpha Al2O3) to be completely dissolved (refer to Table 1).

The digestec digestion system therefore offers the highest possible level of flexibility and represents an economical alternative to microwave digestions, particularly for laboratories which only process a limited number of samples.

 

Matrix Sample weight Acid Temperature Time
Cellulose/starches 1000 mg HNO3 140-160°C 1-2 hr.
Leaves / grain 1000 mg HNO3 / HF 150-180°C 2-3 hr.
Tissue / liver 1000 mg HNO3 170-190°C 2-4 hr.
Fats / oils 500 mg HNO3 (H2O2) 180-200°C 3-4 hr.
Plastics 1000 mg HNO3 / H2SO4 180-200°C 3-4 hr.
Coal / resins 500 mg HNO3 200-240°C 3-8 hr.
Stone 1000 mg HF / HCl / HNO3 180-200°C 2-3 hr.
Ceramics / oxides 500 mg HF oder HCl 180-250°C 2-16 hr.
SiC 250 mg HNO3, HF, H2SO4 250°C 12-72 hr.

Table 1: Application examples for pressure digestion in stainless steel pressure digestion systems

Pressure Digestion with Microwave Heating:
In contrast to the pressure digestion systems described above, samples in digestion equipment heated by microwaves are heated directly by the absorption of microwave radiation. This allows for an extremely rapid, simultaneous heating of, typically, 8-12 sample solutions which represents the actual "microwave effect" or advantage of microwave heating. Once the setpoint temperature is reached, the decomposition reactions proceed at the same rate as in conventionally heated stainless steel pressure digestion vessels. Thus, typical microwave digestions take merely 20-40 minutes. As illustrated in Table 2, microwave digestions are, today, employed for all types of samples and have therefore replaced stainless steel pressure digestion vessels in all applications other than the specialized niche applications cited above. The strength of microwave digestion lies in its significantly higher sample throughput resulting from the decrease in the duration of digestion.

However, since this rapid heating is accompanied by an equally rapid pressure increase and, possibly, spontaneously induced exothermic reactions, the temperature progression of each sample must be continuously recorded and the microwave power must be regulated accordingly. From a safety aspect, it is therefore most practical that the pressure progression be recorded in parallel and that this measurement also be employed to influence power regulation. In this way, an optimal process control can be achieved, particularly from the point of view of safety.

Figure 2: Microwave digestion system speedwave MWS-3+

It is on the basis of these underlying considerations that BERGHOF designed its speedwave MWS-3+ microwave digestion system. Technologies specifically developed and patented to measure both temperature and pressure were implemented for this application. With the aid of a mid-IR thermometer, the temperatures of all samples solutions are measured directly, that is, without time delay and without contact, by measuring the development of the vessel wall temperature. The optional optical pressure monitoring permits the acquisition of the internal pressure of all vessels (also contact-free). Neither technique requires the measurement of a reference vessel. Combined, temperature and pressure monitoring offer optimal process control, particularly from the point of view of safety.

Beyond this, thanks to the unique top-loading design, together with vessels consisting of only a few components, handling is as simple as possible. The digestion vessels themselves have been designed to provide a long service life.

 

Matrix Sample weight Acid Temperature Time
Cellulose/starches 500 mg HNO3 160°C 25 min.
Leaves / grain 500 mg HNO3 / HF 190°C 30 min.
Tissue/hair/blood 50-250 mg HNO3 170-190°C 25 min.
Fats / oils 700 mg HNO3 (H2O2) 180-210°C 30-40 min.
Plastics 700 mg HNO3 / H2SO4 180-210°C 45-60 min.
Coal / coke 250 mg HNO3, HF, H2SO4 200-240°C 45-60 min.
Stone 1000 mg HF / HCl / HNO3 180-200°C 30 min.
Ceramics / oxides 500 mg HNO3 / HF / HCl 180-250°C 45-90 min.

Table 2: Application examples for pressure digestion in microwave pressure digestion systems

Conclusion:
In summary it can be stated that today practically all sample materials can be digested with an appropriate pressure digestion system. In the majority of cases, microwave-heated systems represent the more economical solution due to their higher sample throughput rate over a given period of time. Where extremely difficult to digest samples are involved or where high sample throughput is not required, but the highest possible degree flexibility is essential, the "old" stainless steel pressure digestion vessels continue to be the systems of choice.

Bibliography:
1. L. Kotz, G. Kaiser, P. Tschöpel, and G. Tölg Z.; "Anal. Chem." 260, 207-209 (1972).

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