主题：准确测定系列13:Acid Digestions of Organic Samples(酸消解有机样品)

Apparatus and Chemicals:
20 by 150 mm borosilicate test tubes with Teflon-lined screw caps, cardboard holder for weighing, and stainless steel racks for holding.


Borosilicate glass digestion beads.


Aluminum heating blocks with bore size of 20 mm equipped with thermometer and aluminum heating plate.


An all plastic metal-free acid digestion hood.


4-place and 2-place analytical balances.


ICP spectrometer capable of making simultaneous internal standard measurements and storing spectra on computer disk.


Trace Metals grade 70% nitric acid.


Trace Metals grade 72% perchloric acid.


5 and 50 mL class "A" pipets.


500 mL class "A" volumetric flasks.


NIST/SRM 1577b bovine liver.


High purity 18 megaohm water.


3 mL graduated LDPE dropping pipettes.


1 mL Eppendorf pipette and plastic tips made form natural PE.


0.1 mL Eppendorf pipette and plastic tips made form natural PE.


Plastic disposable weighing dishes.

Glassware Cleaning:
The digestions' tubes and boiling beads are cleaned by heating in boiling 1:1 nitric acid / 18 meg-ohm water for at least 1 hour. After drying at 110 °C, the vessels and beads are allowed to come to room temperature before use. Two boiling beads are added to each vessel and it is capped with the Teflon lined screw on caps.


The digestion vessel caps and the volumetric glassware is cleaned by first sitting in 5 % v/v nitric acid for 24 hours and then rinsed with 18 meg-ohm water a minimum of six times. No heating of caps or volumetric glassware is allowed -- only air drying is acceptable.

Sample Handling, Identification, and Storage:
Immediately upon arrival, all samples are to be inspected and compared with the chain of custody information to confirm the proper shipping procedure, labeling procedure, and number of samples (i.e. - it must be confirmed that the samples shipped were exactly those received with respect to type, quantity, labeling, and that they were shipped properly and received in good condition).


After signing off on the chain of custody document(s) it is required that each sample be assigned an ID number. The number assigned is to be written using permanent magic marker on a plastic bag into which the sample is placed and entered into the laboratory notebook along with the submitter ID (The sample and the plastic bag are to accompany one another throughout their existence at the lab). The assigned numbers are always to be in sequence starting with 1 so it will be obvious to the operator if a sample is missing, was not prepared for analysis, etc.. The assigned number is to never appear in any laboratory notebooks, reports, or other documents without the accompaniment of the identification given by the submitter. This guarantees that no mix-ups will occur.


The tissue and serum samples are stored in a freezer until 2-3 hours before they are scheduled for analysis, at which time they are allowed to thaw at room temperature. Any remaining sample is stored in the freezer. Plastic forceps and plastic spatulas are used to handle the tissue samples. Plastic pipettes are used to withdraw the serum samples. If the whole tissue is not to be taken for analysis, it is transferred from the sample container to a disposable PE dish where it is divided with plastic forceps or spatulas.

Sample Preparation of Tissues and Serum:
Place a digestion vessel holder on a 4-place analytical balance and tare. Uncap a cleaned digestion vessel and add two boiling beads.


Label the digestion vessel using a graphite pencil with the assigned ID. Obtain the vial weight using a 4-place analytical balance. Record the digestion vessel weight in the analytical notebook in the space provided across from the submitter and the assigned IDs written in the notebook as described in step 2 under "Sample Handling, Identification, and Storage" (see above). At no time is the vessel cap to be included in the vessel weight.


Use only plastic forceps, spatula, or dropping pipettes to handle the tissue and serum samples. Tissue samples are removed from their shipping container to a plastic HDPE dish for tearing (if necessary) just prior to weighing.


After recording the digestion vessel weight, tare the balance. It should read 0.0000 grams to ± 0.0001 grams.


Add between 0.5 and 1.5 grams of tissue sample or 0.1 to 0.15 grams of NIST bovine liver QC sample or 2.5 to 3.0 grams of serum sample to the digestion vessel and record the sample weight to the nearest 0.0001 gram.


In a acid fume hood, add 3 mL of 70% nitric acid using a disposable LDPE dropping pipette.


Each group of digestions is to be accompanied with a blank and a NIST/SRM 1577b bovine liver QC sample. A group of samples consists of a full digestion block of samples. One group is equal to 24 vessels (22 samples and 2 controls). Since the NIST liver is dried, the sample weight should not exceed 0.15 grams.


Place the digestion vessel in the digestion block which should be maintained at 110 °C throughout the digestion.


Brown nitrogen dioxide fumes should be observed within 5 minutes. Do not leave the digestion for the first 15 minutes. The sample should be completely dissolved within 15 minutes. Swirl the digestate to render homogeneous. With the sample weights recommended, foaming should not be a problem. If foaming does occur, remove the sample from the digestion block periodically to cool until dissolved.


Continue digesting the sample with nitric acid until the brown NOX fumes are barely visible. Place the explosion shield in front of the digestion block, put on a face shield and heavy rubber gloves. Carefully add 2 mL of 72% perchloric acid using a graduated 3 mL LDPE dropping pipette.


Continue the digestion at 110 °C for 16 hours. The digestate should appear a very pale yellow to water white.


Allow the digestate to cool to room temperature. Weigh the digestion vessel + digestate and record this weight in the analytical notebook.


Calculate the weight of the digestate and record this value in the notebook. The density of the digestate has been determined to be 1.49 ± 0.02 g/mL. Calculate the volume of the digestate by dividing the digestate weight by the density. Enter this value in the notebook. The final volume of the digestate is brought to 10 mL using 18 meg-ohm water. Calculate the mL of water to add by subtracting the digestate volume from 10 mL and enter this value in the notebook. Calculate the weight of water to add by multiplying the calculated mL by 0.997 g/mL and record this value in the notebook. Tare the analytical balance and add the calculated weight of water to the nearest ± 0.02 grams. A 2-place analytical balance can be used.


Mix the final sample solution after capping by hand-shaking. The sample is ready for analysis.
The above procedure has been used at our laboratory for processing large numbers of biological tissues from animal feeding studies. For smaller sample numbers, a Kjeldahl digestion rack with a glass hood and caustic scrubber is more convenient and is very effective in removing any perchloric acid fumes. For samples that are

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