general guidelines to assist with s microwave digestion...

General Guidelines to Assist with Microwave Digestion Method Development

For: MARS 6 MARS Classic MARS One

User G

uidelines

Copyright CEM Corporation 2016 ∙ 600288 Rev. 4


I. Forward

Commercial Microwave Digestion systems have been used in laboratories since the mid 1980’s. They provide scientists with a powerful tool to digest samples faster and more completely. In many cases samples that would take many hours to digest at atmospheric conditions are digested in just minutes in the microwave. Over the years many enhancements have been made to the system which have made it easier and safer to digest samples under high temperature and high pressure conditions. Still, safety protocols must be followed and things like sample size and type as well as chemical composition need to be considered. This document provides guidelines to assist the user achieve safe and successful digestions.

II. Microwave Digestion Safety

Microwave digestion greatly enhances the destruction of chemical bonds thereby achieving a faster digest. Caution should be exercised when preparing unknown samples as the combination of acid and rapid heating in a closed vessel could result in an exothermic reaction. If a sample is prone to exothermic reactions (example alcohols, sugars, carbohydrates, etc) the reaction could be so great as to break the vessels and even damage the microwave system. Unknown samples should have acid added and then left to predigest with the cover off the vessel in a fume hood for a minimum of fifteen minutes. Note that certain compounds are not suitable for closed vessel digestion. This list along with additional guidelines are provided in the MARS Operations Manual which is included on the USB stick with the MARS.

III. Starting Point for Samples for Microwave Digestion

Note: Samples that have not been previously run should have a starting weight of 0.1 grams. This can be increased in 0.1g increments up to 0.5 g dry weight for organic or 1 g for inorganic. Larger sample sizes can sometimes be achieved. NOTE: If volume loss from the vessel exceeds 20% then sample size must be reduced in order to achieve good analyte recovery and maintain good laboratory safety practices.

IV. Microwave Digestion Vessels

Warning: All CEM microwave digestion instruments are designed to be operated using CEM designed vessels and vessel components. All CEM supplied vessels and components are manufactured to exact specifications in order to provide a product that is safe to operate in conditions that include high temperatures and pressures. The use of vessels and vessel components other than original CEM parts will put operators and equipment at risk. CEM will not assume any liability for use of these parts and all warranties either written or implied will immediately become void. CEM vessels can only be purchased directly from CEM or through its authorized dealer network. CEM vessels are covered by worldwide patents including US patents 8,480981 and 6,136,276.

V. Sample Types Samples are broken down into two categories: Organic and Inorganic.Organic Sample Examples:Plant and Animal Tissue, Foods, Oils, Plastics and Polymers, Filters, Paint and Paint Chips, Blood and Serum, Feeds and Fertilizers, Sludge and Waste and similar samples types.

Inorganic Sample Examples:Steels, Alloys, Minerals, Ores, Soils, Water, Ash, Catalyst, Ceramics, Silicates and similar materials.

Copyright CEM Corporation 2016 ∙ 600288 Rev. 4 3


Inorganic Sample Guidelines

Inorganic samples are typically more difficult to prepare as they require various acid combinations depending upon the matrix and the analytes of interest. It is very important to reduce the particle size prior to digestion by either grinding or shredding into small pieces. Sample sizes should start at 0.25 grams. Larger sample sizes may be digested successfully depending upon solubility in the acid mixture.

Inorganic samples are normally digested at higher temperatures and are digested for longer periods of time. Nor-mally, the pressures generated in an inorganic digestion are fairly low. However, if the sample does generate high pressures then temperatures should not exceed 230° C. If excessive venting occurs during the run or there is a loss in volume of greater than 20% the temperature should be lowered or sample size reduced. In all cases you should not make a digestion run of over one hour without consulting a CEM representative.

Typical acid combinations include:

• HNO3:HCl:HF Alloys, ores, silicates, ash• H3PO4:H2SO4 Aluminum oxide• HCl:HNO3 (3:1) Precious metals• HNO3:HF NiCr, Steel, Co, Ta, W alloys• HCl:HF Al and Fe alloys• Acid X : Acid Y : Water 1:1:1 Alloys

In the last example water is added in a one to one ratio with two acids. The addition of water during the digestioncan assist in solubilizing the salt formed during the acid digestion.

General MARS Guidelines

I. General Precautions

• The Control (reference) vessel must have a representative sample and it should be the sample with the largest weight.

• Do not run a blank in the Control vessel.• Microwave Digestion requires concentrated acids at high temperatures. To avoid burns, manual venting of CEM

vessels should only be done at room temperature, and hand, eye, and body protection should be worn. Always open vessels in a fume hood and point the vessel away from the operator.

• Keep the interior of the MARS clean. Wipe down the cavity thoroughly after each digestion run. Pay particular attention to the IR sensors. They must be kept clean and free of debris in order to provide accurate tempera-ture measurement. Any foreign material in the cavity can absorb microwaves and potentially catch fire.

II. Samples that Quickly Produce Heat and Gas (Exothermic)

• Predigest any samples that may exhibit exothermic reactions by adding acid to the sample and allowing to sit for 15 minutes (minimum) in a fume hood uncapped prior to placing in the microwave.

III. Particle Size Reduction

• It is important to reduce particle size prior to digestion or the digestion may not be successful. Make sure all pieces, especially powders, are suspended in the acid and not on the wall of the vessel liner. This can create a hot spot and lead to vessel failure.

(A vortex mixer is useful in helping to suspend sample particles in the acid).


IV. Use of Hydrofluoric Acid (HF)

• Make certain that all personnel are trained in the handling of HF. HF is very aggressive when it comes in con-tact with skin.

• Insoluble fluorides can be formed when using HF. Boric acid, H3BO3 can be used to complex and dissolve these compounds.

o Use 10 ml of a 4% w/v solution per 1 ml HF o Alternatively, 1 – 4 grams of solid boric acid and 25 mls of deionized water can be used o After the boric acid is added reseal the vessel and place in the microwave and run the Boric Acid Neutralization One Touch Method.• Boric acid is also used to neutralize the HF prior to analysis. • A CEM MicroVap® accessory can also be used to evaporate the HF prior to analysis. Contact your ICP or ICP-MS

vendor prior to introducing HF into the analyzer.

V. Use of HCl

• HCl is used to digest precious metals on catalysts such as Rh and to stabilize metals such as In, Sb and Sn. It is also used for compounds containing Fe and Al. When used in high concentrations or alone HCl is very harsh on the vessels and should not be run at temperatures over 240° C and never run for over one hour. If these conditions are exceeded vessels may exhibit excessive venting and abnormal wear and tear.

VI. Use of H2SO4 - Samples requiring a Char Step – Some Polymers and Chemicals having Triple Bonds

• Some samples such as polymers and heavy organic chemicals cannot be digested in HN03 and H2O2. They require an initial char step with H2SO4 acid. A sample of up to two grams is digested first in 5 -10 mls of H2S04. It is important that the acid fully cover the sample. Seal the vessel and run the Char Sulfuric Acid One Touch Method.

• When done check to see if sample is a homogenous thick black liquor. If so then add 10 mls HNO3, seal the vessel and place in microwave and run the Polyethylene One Touch Method.

• Allow vessels to cool before uncapping and transferring.

VII. Clean Chemistry

• In order to achieve optimum results and achieve low level detection limits chemists must consider several factors. These include the reagents being used. Make certain all of the reagents including the water that is used as part of the digestion process are certified to a low enough level for the metals you are analyzing for. Analyze a sample of your water and acid solution as a blank in your analyzer to verify acceptable blank levels. Make certain to clean vessels appropriate to the level (ppm or ppb) required for analysis. Vessel cleaning procedures are available on the CEM website. Wear gloves and any other personal devices as necessary to maintain a clean environment in your lab. Cover your auto sampler if one is being used to prevent any airborne contamination. All of these guidelines will assist in achieving the lowest possible detection limits in your laboratory. Additional information on clean chemistry is available on the CEM website.


Vessel Guidelines

There are many types of vessels that can be used in the MARS. This Guide will concentrate on the three most popular vessel types: Xpress, Xpress Plus and Easy Prep.

I. General Guidelines

• Inspect vessel parts prior to each use to look for excessive wear. These pieces should be discarded.• Make certain that all surfaces are clean that no material is present on any part of the vessel. If there is any

scorching on pieces they should be discarded.• Never wash or soak the vessel sleeve directly in water. They should be wiped with a damp cloth.• Maximum sample size of dry weight organic samples are for well characterized samples and not for unknowns.

Always start with 0.1 gram of any unknown sample or samples that are suspected of being high in sugars or alcohols.

• Consult the CEM Operations Manual for items unsuitable for closed vessel digestion.

II. Xpress Vessel Guidelines

• Xpress Vessels are limited by MARS 6 software at 240° C. Routine applications should be run at or below 210° C.• In order to achieve accurate temperature control of the batch a minimum of eight vessels must be run

simultaneously. These should be evenly distributed in the inner row of the turntable. If you choose a One Touch method with less than eight vessels you will get an error message.

• Please refer to the Xpress vessel diagram on page 13 of the User Guidelines for recommended distribution in the turntable.

Sample and Reagent Volumes for Xpress Vessels

Assembly of vessels

Vessels are assembled with a CEM supplied hand torque device or with an optional manual bench mounted torque station. An optional bench mounted automatic torque station is also available. Additional Instructions for this are available as part of the on board video programming on the MARS 6 or the CEM website in the Resource Library.

Assembly of MARSXpress Vessels

1. Ensure that the vessel is assembled as illustrated on page 7.

2. Tighten the cover using the following method options:a. Capping Station - The torque is preset on the capping station. Insert the assembled vessel into the

capping station with the motor rotating continuously. Hold the liner until the motor stops turning. The vessel is properly torqued. Do NOT re-torque vessels without completely loosening the cover first.

b. Hand tighten the vessel then turn torque block until it clicks one time. (PN185245)

Vessel Min Vol (ml)

Max Vol (ml)

Max organic sample size

(g dry weight)10 ml 1 4 0.120 ml 2 10 0.255 ml 5 25 0.575 ml 5 55 1.0


b. Hand tighten then turn to click one time

a. Capping station


3. Ensure that the composite sleeves are fully inserted into the receptacles of the turntable and that each vessel is placed inside a sleeve.

4. Ensure that each vessel is properly positioned in the turntable. (refer to Distribution chart, pg. 13) The outside of the cover MUST be below the top of the turntable as illustrated.

5. Install the turntable on the drive lug with vessel #1 positioned farthest from the back wall of the cavity (nearest the instrument operator). Ensure that the turntable properly engages the drive lug.

6. Close the door of the MARS instrument.

*The instrument and vessels are now ready to begin the method.


III. Xpress Plus Vessel Guidelines

• Vessels are limited in software to 260° C. This should only be used for a Sulfuric Char step. Routine applications should be run at or below 210° C.

• Available in PFA® or TFM® Teflon at 110 mL volume.• In order to achieve accurate temperature control of the batch a minimum of eight vessels must be run

simultaneously. These should be evenly distributed in the inner row of the turntable. If you choose a One Touch method with less than eight vessels you will get an error message.

• Please refer to the Xpress Plus vessel diagram on page 15 of the User Guidelines for recommended distribution in the turntable.

• Reagent volume must at least 5 mL and no more than 60 mL.• Maximum sample size for acid digestion is 1 gram organic dry weight.

Assembly of vessels

Vessels are assembled with a hand torque device supplied by CEM preset to 37 inch lbs. An optionalbench mounted torque station to hold the torque block is available. Additional Instructions for this are available as part of the on board video programming on the MARS 6 or the CEM website in the Resource Library.

Assembly of MARS Xpress Plus Vessels

1. Ensure that the vessel is assembled as illustrated below.2. Hand tighten the vessel liner then place the blue gripper (PN180995) around the vessel liner. 3. Using the torque block (PN212200), tighten the cap until the torque block assembly makes a clicking sound.4. Insert the assembled vessel into a sleeve in the turntable. (see distribution chart, pg. 15)5. Install the turntable into the instrument cavity and on top of the turntable lug. Rotate the turntable until it

positions itself on the lug. Press the turntable key to rotate the turntable 2 to 3 times. Press the turntable key to stop the turntable rotation.

10924

2322

11 12 13 1092423

2211 12 13

1092423

2211 12 13 10924

2322

11 12 13

Ensure that each vessel is properly positioned in the turntable. The outside of the cover MUST be below the top of the turntable.


Assembly of MARS Xpress Plus Vessel with Fiber Optic Temperature Probe

1. The control vessel is preassembled at CEM. If the control vessel becomes disassembled it can be reassembled using steps 2-5. Once the control cover is assembled, it does not need to be disassembled after each use. 2. Push the open end of the thermowell (181110) through the control plug so that the open end of the thermowell is positioned above the control plug by approximately 0.5 inches. 3. Insert the control plug and thermowell inside the vessel liner (212125). 4. Screw the control cap (212145) onto the vessel liner hand tight. 5. Screw the swivel nut (325031) on the control plug until the swivel nut is hand tight. 6. Unscrew the control cap assembly from the liner. 7. Weigh and place the sample into the vessel liner, ensuring that the acid covers the sample. 8. Screw the control cap assembly on the vessel liner hand tight. 9. Place the blue gripper (180995) around the vessel liner. 10. Using the torque tool assembly (212200), tighten the cap until the torque tool assembly makes a clicking sound. 11. Insert the assembled vessel into a sleeve (212031) in the turntable at position #1. (see distribution chart, pg.17) 12. Insert the MTS-300 fiber optic temperature probe in the opening of the swivel nut until the probe touches the bottom of the thermowell. 13. Pull up on the MTS-300 to ensure it will not pull out of the thermowell.

14. Install the turntable into the instrument cavity and on the turntable lug. Rotate the turntable until it positions itself on the turntable lug. 15. Connect the MTS-300 into the connector in the top of the cavity. Press the turntable key to rotate the turntable 2 to 3 times. Press the turntable key again to stop the turntable rotation.

CAUTIONThe MTS-300 must be inserted prior to the installation of the turntable and vessels into the instrument cavity. The MTS-300 must be removed after the turntable and vessels have been removed from the cavity.


IV. Easy Prep and Easy Prep Plus Vessel Guidelines• The Easy Prep Plus Vessel is used to prepare difficult sample matrices that require extended temperatures,

pressures and time.• Temperatures are limited to 300° C in software. • Total digestion time should be limited to 90 minutes. (Ramp and Hold time)• Total digestion time (Ramp and Hold) should not exceed one hour when working with temperatures exceeding

240° C.• Maximum sample size for acid digestion is 1 gram organic dry weight.• Please refer to the Easy Prep Plus vessel diagram on page 19 of the User Guidelines for recommended

distribution in the turntable.

Assembly of VesselsVessels are assembled with a CEM supplied torque wrench set to 7 ft-lbs. Instructions for assembly are available as part of the on board video programming on the MARS 6 or in the Easy Prep/Easy Prep Plus vessel instructions located on the CEM website in the Resource Library.


Recommended distribution of MARSXpress vessels when turntable is less than capacity

Incorrect distribution: What not to do

8 10 12 14

16 18 20 22

24 26 28 30

32 34 36 38


Page left blank intentionally


Recommended distribution of MARSXpress Plus vesselsin the turntable with IR only

12 13 14

16 17 18 19

20 21 22 23

24

15

8 9 10 11


Recommended distribution of MARSXpress Plus vesselsin the turntable with fiber optic

= control vessel

= standard vessel

2 4 6

8 10 12 14

16 18 20 22

24

7

*Note: The control vessel must always be located in position 1


Recommended distribution of EasyPrep, EasyPrep Plus and XP-1500 vessels in the turntable

1 2 3

4 5 6

7 8 9

10 11 12

= control vessel

= standard vessel


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Tel: (800) 726-3331 [USA & Canada]Tel: (704) 821-7015

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