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Texas A&M University - Kingsville

SOP

Ethos D Labstation Microwave Digestor

Milestone

March 2009

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SOP for Microwave Digestion

Microwave digestion has been used in this project to quantify total metals in water and sediment

samples. The following procedure is modified from standardized EPA procedures and digestion

procedure was developed after reviewing the ETHOS D labstation Microwave Digestor

Instructions.

Safety measures must be taken when working with digestion procedure, proper laboratory

attire is needed, gloves and goggles. Sample preparation should take place under a laboratory

extraction hood.

The digestion vessels are comprised by the following components:

Figure 1. Digestion Vessel Components

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Vessel Cap

Vessel

Teflon liner

Valve

Teflon cap

Temperature

probe sleeve

Vessel

Teflon liner

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Figure 2. Temperature Probe Sleeve

Figure 3. Temperature Probe

Figure 4. Vessel Teflon liner, Teflon cap and temperature probe sleeve.

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Figure 5. Ethos D Labstation Microwave digestor.

The vessels are numbered starting at number zero, vessel “zero” will be always used to

monitor the temperature inside of the vessels. To achieve this, the temperature probe sleeve is

introduced through the Teflon cap; the vessel cap used is different from the rest as this will not

have a Teflon valve on the inside. The vessel Teflon liner used to monitor temperature is also

different from the rest as it will have a thicker bottom, of about 1/8 of an inch. The sample used

in this vessel is prepared with the same procedure as the rest of the samples.

When preparing all types of samples the vessels are to be used in an orderly way.

1. Samples will be weighed and prepared inside the vessel Teflon liners (Figure 4).

2. When digesting sediment samples the vessel Teflon liner needs to be weighed in the

balance and the sample poured carefully to avoid exceeding the amount specified (Figure

6).

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Figure 6. Preparation of sediment sample

3. When adding the acids, these have to be slowly poured letting them slide gently on the

walls of the vessel Teflon liner, to allow collection of all particles.

4. After preparing samples cover them with parafilm and swirl gently, allow time to

predigest.

5. The vessel Teflon liners are to be introduced inside the vessels then covered with a

Teflon cap (Figure 7).

Figure 7. Covering vessel Teflon liner with Teflon cap

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6. After covering the vessel with the Teflon cap, check the valve and Teflon liner are

secured in place, then screw and tighten the vessel cap (Figure 8).

Figure 8. Covering vessel with the vessel cap

7. When transferring the vessels to the microwave digestor verify first the gas exhaust is

positioned under the laboratory extraction hood (Figure 9).

Figure 9. Digestor setup

Gas

Exhaust

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8. A carousel comprised of a base and upper support is fixed inside the digestor, take this

carousel out of the digestor and disassemble to arrange the vessels (Figure 10).

Figure 10. Carousel inside Microwave Digestor

9. Vessel “zero” will always be in position number 2, the rest of the vessels need to be

arranged so the carousel is equilibrated (Figure 11).

Figure 11. Positioning vessels for digestion

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10. The vessels are then secured with the upper support and the temperature probe is

introduced inside of vessel “zero” through the opening found at the upper part of the

vessel cap (Figure 12).

Figure 12. Setting temperature probe

11. The carousel is then introduced and fixed inside the microwave (it might have to be

twisted before it gets fixed).

12. The temperature probe is then connected to the opening found on the left wall inside the

digestor (Figure 13).

Figure 13. Setting temperature probe

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13. The vessel is then turned on and the program is selected and modified from the pad found

at the left of the digestor.

14. Instructions will appear on the screen as follows:

Figure 14. Setting up the program

From the options given select Edit (F2), the following options will be displayed:

1. Edit-Program: select program using the F1 and F2 buttons, then press F3 to confirm

the choice.

2. If the program number exists the screen will display “Program # Exist!”, we are then

given to options, to create a New Program (F1) or Edit (F3).

3. Afterwards, the program requires setting up the time, power, pressure and

temperature of each stage. Programs can be created with up to 10 stages.

4. After selecting all the stages needed, we are able to select the twist option (ON/OFF)

it is recommended to have this feature ON.

5. The ventilation time is then required. Remember this is related to the amount of

samples digested and has to be enough time to avoid mass loss.

6. After the program is ready the screen sends you to the initial display, this time select

Work a program.

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7. The screen displays the options of starting the program or turning the rotor, before

running the program verify the rotor will work appropriately to avoid further

problems.

8. After verifying the proper performance of the rotor, start the program.

9. After completing digestion, samples are taken out of the microwave digestor and

transferred to polypropylene tubes. The samples will then be centrifuged or filtered

for further analysis and kept refrigerated at 4°C. Samples duration is six months after

the digestion is completed.

Program Specifications

Water Samples (EPA 3015-A)

Sample Amount: 45 mL.

Acid Addition: 4mL Nitric Acid and 1mL Hydrochloric Acid.

Predigestion Time: 30 min.

Program Specifications for 10-12 vessels.

Stage Time (min) Watts Temperature 1

(°C)

Temperature 1

(°C)

Stage 1 3 600 165 160

Stage 2 5 900 165 160

Stage 3 5 800 165 160

Vent 15 - - -

Sediment Samples (EPA 3051-A)

Sample Amount: 0.5 gr. Of air dried, crushed and homogenized sediment.

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Acid Addition: 9mL Nitric Acid and 3mL Hydrochloric Acid.

Predigestion Time: < 4hr.

Program Specifications for 10-12 vessels.

Stage Time (min) Watts Temperature 1

(°C)

Temperature 1

(°C)

Stage 1 6 700 175 170

Stage 2 6 900 175 170

Stage 3 6 800 175 170

Vent 25 - - -