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S-35 Standard Operating Procedures


Sulfur-35 is a commonly used radionuclide with a half-life of 87.4 days, emitting only beta particles with a maximum energy of 0.167 MeV (Million Electron Volts) and an average energy of 0.049 MeV. The beta particles from S-35 travel a maximum of 24 cm. in air. See chart below for the decay rate information for S-35.

The above properties are very similar to those of C-14.


The major concern with using S-35 is that it's presence cannot be monitored during it's use, since most Geiger Counters will not be able to detect S-35. Special precautions are needed to keep the work environment clean. The regular use of wipe testing is the only way to insure that your work space is not contaminated. Contamination on the skin will not likely cause a significant dose to the skin, however, it could lead to the internal absorption of S-35 if there are cuts in the skin. Solutions of S-35 labeled amino acids and S-35 labeled ATP can release a volatile radioactive component when opened. Although the amount released from a stock vial is small, contamination can occur. The annual ALI for S-35 set by NYS for the whole body is 800 microcuries.


Glass and plastic are the best shields for beta particles from S-35.


A tiny drop of contamination from S-35 can be easily detected with a wipe test counted in a Liquid Scintillation Counter.

Equipment / Supplies

The following equipment and supplies must be available before handling S-35:

  • disposable latex or plastic gloves.
  • full-length lab coat.
  • radioactive waste receptacle
  • pipettes dedicated to the use of S-35.
  • commercial decontaminate, i.e. DuPont's Count Off.
  • absorbent bench covering or tray.

Safety Rules

If the following safety precautions are used when handling S-35, personnel radiation exposure will be as low as reasonably achievable.

  1. Designate a specific area of the lab for S-35 handling.
  2. Wear a full-length lab coat.
  3. Wear two pairs of disposable gloves to protect your hands from becoming contaminated from spills.
  4. Never pipette S-35 or any radionuclide by mouth.
  5. Only use pipettes which have been dedicated to your specific use of S-35.
    • Pipettes will easily become contaminated and therefore, should not be shared with others.
  6. If you have reason to believe that your gloves are contaminated, immediately dispose of them in the radioactive waste container.
  7. Open stock vials, etc. in a properly functioning fume hood.
  8. If possible, do assays in larger volumes, e.g. >14 ml/mCi or in a sealed container.
  9. Tricine (50 mM) can be added as a stabilizer in incubation media if it will not be toxic to the biological agents in culture.

NOTE: Charcoal is not a good trapping material for S-35 as the absorption is passive and the S-35 will be released from the charcoal over time.

Post-Use Procedures

After handling S-35:

  • Conduct a monthly (weekly) wipe test on all work benches, floor, equipment, centrifuges, and water baths.
  • Count the weekly wipes in a Liquid Scintillation counter:
  • If contamination is found:
    • Use a commercial radiation contamination remover (i.e. Count Off) with paper towels to clean up the equipment.
    • Place the towels in the radioactive waste receptacle.
    • If contamination cannot be removed, place a radiation label on the equipment indicating that it is S-35, maximum cpm found, and the date you measured the level.
    • Inform your fellow lab workers if any unremovable contamination is found.
    • Call the RSO if you have any questions about where to survey.
  • Check the normal trash container to make sure no radioactive waste has been accidentally placed there.
  • Store the waste temporarily in marked containers.
  • Wash your hands thoroughly.
  • We accept radioactive waste by appointment only.
  • S-35 waste will be stored by the RSO until it has decayed sufficiently to be disposed by the RSO.

Any questions about these procedures?

Call the Radiation Safety Office: 464-6510

Days Elapsed % of Activity Remaining Decay Factor
0 100.0 1.00
10 92.4 0.924
20 >85.3 0.853
30 78.8 0.788
40 >72.8 0.728
50 67.3 0.673
60 62.1 0.621
70 57.4 0.574
80 53.0 0.530
90 49.0 0.490
100 45.3 0.453
110 41.8 0.418
120 38.6 0.386
... ... ...
874 (10 half-lives) 0.1 0.000

For example, if your vial contained 500 microcuries of S-35 on 7/1/90, the amount of activity remaining on 7/11/90 (10 elapsed days) would be:

Activity x Decay Factor = 500 microcuries x 0.924 = 462 microcuries