• Decrease font size
  • Return font size to normal
  • Increase font size
U.S. Department of Health and Human Services

Food

  • Print
  • Share
  • E-mail

Elemental Analysis Manual: Section 2.3 Analytical Portion to Analytical Solution

Elemental Analysis Manual, Version 1.1 (August 2010)
Authors: William R. Mindak
John Cheng
Richard M. Jacobs

Table of Contents

2.3.1 Microwave Digestion

2.3.2 Defined Procedures (references to specific methods)

2.3.2.1 Leaching Cadmium and Lead from Ceramicware

2.3.2.2 Mercury Separation in Seafood

2.3.2.3 Arsenic Speciation in Rice

Glossary


This section provides information to assist the analyst on preparing the analytical solution. The analytical method may contain more specific instructions that must be followed

2.3.1 Microwave Digestion 

Microwave digestions are performed using commercial equipment specifically made for performing acid digestions.

Analytical Portion for Closed Vessels

Pressure built up by digestion products governs maximum analytical portion mass.

Typical analytical portions will range from 0.5 to 2 g. High-fat foods require a lower mass while a larger mass can be used with high-water content foods. Often maximum analytical portion may be determined using a sample's known energy (i.e., caloric) content as an indicator of pressure produced during digestion. For some products (e.g., juice concentrates) the energy content may not be known. Maximum energy release permitted for 600 and 800 psi digestion vessels (90 mL capacity) was empirically determined as 3 and 6 kcal, respectively. This maximum vessel energy prevents a digestion from reaching the vessel's maximum operating pressure before digestion is complete. The energy of an analytical portion must not exceed these values for safe operation. An analytical portion should not exceed 5 g even if calculation indicates that a larger portion could be taken. This maximum mass will prevent excess dilution of the nitric acid used for digestion and ensure a complete digestion. Use 1 g reagent water for method blanks (MBKs). For samples of unknown composition, a maximum of 0.5 g dry weight should be analyzed for safety concerns. If maximum pressure attained for this unknown is less than the vessel limit then a greater mass may be analyzed. In many cases the maximum analytical portion mass will not be necessary or should not be used, e.g., the high level of salt in some food may interfere with an analysis.

A food's energy content (usually provided as kcal/100 g) is available from many sources1-2. In addition, a food's energy content may be estimated from the calories and serving size provided on a food's label.

An example using a sports nutrition bar is given below and 2.3 Table 1 provides typical analytical portion masses for selected foods when using 800 psi digestion vessels.


Example: For a 50g sports nutrition bar, the nutrition label lists a serving size as the entire bar with a mass of 50 g and an energy value of 210 Calories (Consumer label "Calories" are kilocalories). 2.3 Equation 1 shows calculation of the food energy-to-mass ratio (kcal/g):

Equation to calculate Kcal/g

The maximum analytical portion mass (max mass) for an 800 psi digestion vessel (6 kcal energy maximum) is calculated using 2.3 Equation 2, which shows that up to 1.4 g of the sports nutrition bar could be digested in this microwave vessel.

Equation to calculate maximum mass for an 800 psi digestion vessel

2.3 Table 1 provides typical analytical portion masses for selected foods when using 800 psi digestion vessels. Although these calculations will reveal the maximum mass that can be safely decomposed in a vessel, generally, about 1 g is recommended for an analytical portion. Approaching the maximum amount adds a risk of causing matrix effects during the determinative step.

2.3 Table 1. Typical Analytical Portion Mass for Selected Foods

FoodPortion (g)FoodPortion (g)
American cheese1.2Lettuce, Iceberg5
Beef liver2.4Nuts, mixed0.8
Peaches, canned5Peanut butter0.8
Spaghetti, canned5Raisin bran cereal1.6
Dill pickles4.0aTomato catsup3.0a
Spinach5Yellow mustard3.0a
Fruit cocktail, canned5White bread1.8

aRecommended portion is less than the calorie limit of the digestion
 vessel due to the interference produced by the high-salt content.


 

Digestion Procedure

If possible, place analytical balance in clean hood (class 100) or on clean bench to minimize contamination while weighing analytical portions. Transfer analytical portion with a pipette, spatula or by pouring and into a tared, clean digestion vessel liner. Avoid placing analytical portions on walls of digestion vessels. Determine mass of analytical portion to an accuracy of 0.001 g.

Move vessel liner to an exhausting clean hood.  For dry samples, dry CRM materials and reactive samples, adding 1 g of reagent water can help control exothermic reactions during the digestion. Pipette 8.0 mL or weigh 11.3 g of high purity nitric acid (sp gr 1.41 g/mL) into vessel liner, washing down any material on walls. Weighing acid using a top loading balance and Teflon® FEP wash bottle is suggested. Use double distilled grade for lowest method blank values. The trade name for double distilled grade will vary by manufacturer. Acid should be added drop wise for the first few mL until it can be established that the sample will not react violently. Some foods, especially those high in sugar, will react with nitric acid within several minutes. If foaming or reaction with the acid is observed, let the vessels sit uncovered in a class 100 clean hood for 20 minutes or until reaction subsides.

If a clean hood is unavailable, cap vessels but loosen the pressure relief nut (with the safety membrane) to allow pressure to escape. If, however, it appears that excessive foaming would result in the sample-acid mixture expanding out of the vessel then cap the vessel and tighten to appropriate torque to prevent loss of sample or acid.

Add 1 mL high purity 30% H2O2. Seal vessels, tighten pressure relief nuts and run microwave digestion program as prescribed by the analytical method. After vessels have cooled to less than 50° C remove them to an exhausting clean hood and vent excess pressure slowly. Quantitatively transfer and dilute digestion solution with reagent water as prescribed by the analytical method. This analytical solution should be transferred to a plastic bottle or a capped polypropylene centrifuge tube for storage. 

A typical microwave digestion program is given in 2.3 Table 2.

 

2.3 Table 2. Microwave Digestion Programs

DigestionPeroxide
Oxidation
Maximum Power (Watts)1200
Control Pressure (psi)a800
Ramp Time (min)25
Hold Time (min)15
Control Temperature (°C)200

aOnly use with non-venting vessels.

Note: Analysts should be cognizant that the addition of concentrated nitric acid to some samples types may cause a strong exothermic reaction (especially those high in sugar). If the vessel is not immediately capped and sealed, a loss of nitric acid could occur through evaporation. Likewise, a portion of the sample can be lost through mechanical means when the sample reacts vigorously in an open vessel. For samples that behave in this manner, cap the vessels immediately and allow the sample to complete this exothermic reaction stage. When convenient, leave the vessel over night to go through this exothermic stage and cool. The cooled vessels should be vented before the heating program is initiated. For vent and reseal type vessels (such as CEM Omni vessels), this may allow the use of slightly larger sample portions. For sealed vessels (such as CEM XP-1500 vessels) this step may prevent vessel blowout when dealing with samples that have unknown behavior. Membranes are not a dependable defense against vessel blowout caused by sudden high pressure during the heating cycle.

Note: After diluting to volume the analytical solutions should be clear and colorless to slightly yellow. Turbidity and/or a deep color usually indicate an incomplete digestion. Insoluble food ingredients may be present (e.g. titanium dioxide, silica). In cases of an incomplete digestion, determine if there was a malfunction of the microwave digestion system such as lack of safety membrane, safety membrane cap not tight, wrong oven program etc. A copy of the computer generated graph of the digestion run's temperature and pressure versus time is helpful towards diagnosing digestion problems. Correct problem and re-digest sample. If microwave digestion system did not malfunction then re-digest sample using a smaller (at least a factor of 2 less) analytical portion. Some foods (especially spinach) contain silica, which will not dissolve in this procedure. A very small amount of white silicate precipitate from these foods is to be expected and is not a problem. Centrifugation can separate these particles from the analytical solution. However, some matrices (i.e., certain dietary supplements) may yield a high proportion of undigested materials after the heating cycle. Additional treatments (e.g., hydrofluoric acid, smaller analytical portion) may be required for accurate results.

Microwave Digestion Vessel Cleaning

Digestion vessels are acid cleaned after each digestion. Vessels being used for the first time or after an incomplete digestion are cleaned with liquid laboratory-grade detergent and then subjected to the acid cleaning. Incomplete digestions are usually dark colored (yellow to brown), have a bad odor and may contain material that did not dissolve. The manufacturer of the microwave digestion equipment may provide additional information on cleaning vessels and other components of the equipment. Be careful not to use anything that can scratch the vessel walls. The Teflon material is relatively soft and can scratch easily.

Detergent cleaning-Disassemble vessels and soak for at least 2 hours in a solution of liquid laboratory-grade detergent and hot water. Thermowells should be wiped down with a paper towel and detergent solution. Rinse thermowells and vessel components with warm tap water and rinse thoroughly with reagent water. Allow to dry in a clean area; preferably dry in a Class 100 clean area.

Acid cleaning — Add 10 mL of nitric acid to each vessel and microwave them according to the Clean Program listed in 2.3 Table 3 or as suggested by the manufacturer. After vessels have cooled to less than 50 °C remove from oven and vent excess pressure slowly in a fume hood. Disassemble vessels, rinse off covers and liners with reagent water into a waste container. Thoroughly, rinse covers and liners with copious quantities of reagent water. Dry in a clean area; preferably dry in a Class 100 clean area. Outside surfaces of vessels may be dried with laboratory tissues. If vessels are not used after drying, store assembled in a Class 100 clean area or other appropriate contamination free environment.

2.3 Table 3. Microwave Digestion Clean Program

Stage:1
Maximum Power (Watts)1200
Control Pressure (psi){Not Used}
Ramp Time (min)10
Hold Time (min)3
Control Temperature (°C)200

 

2.3.2.1 Leaching Cadmium and Lead from Ceramicware

See Method 4.6 (Section 4.6.5).

2.3.2.2 Mercury separation in seafood

See Method 4.8 (Section 4.8.5).

2.3.2.3 Arsenic speciation in rice

See Method 4.11 (Section 4.11.7).


References

  1. Souci, S. W., Fachmann, W., and Kraut, H. (1994) Food Composition and Nutrition Tables, 5th Ed., CRC Press, Boca Raton, FL.
  2. U.S. Department of Agriculture, Agricultural Research Service (2007), USDA Nutrient Database for Standard Reference, Release 20, Nutrient Data Laboratory disclaimer icon, (Accessed March 31, 2008).    Note: Release numbers change as new versions are released.