Radioactive Isotope Decay Calculator (³²P / ³⁵S / ¹²⁵I / ³H Remaining Activity)
You order 1 mCi — how much is left two weeks later when you use it?
³²P has a half-life of 14.268 days. That means a vial left on the shelf for two weeks retains only half its activity. The activity printed on the label is the value on the reference date, not the day it is in your hands.
Poor labeling efficiency, weak signal, insufficient exposure time — in many cases this is not a technique problem, but a failure to account for those extra days of decay.
Formula
A(t) = A₀ × 2^(−t / t½)
λ = ln2 / t½ (decay constant; A(t) = A₀·e^(−λt) is equivalent)
t and t½ must have consistent units. The remaining fraction depends only on how many half-lives have elapsed, independent of initial activity.
Sanity check: ³²P left for exactly one half-life (14.268 days) retains 50.00%; after 14 days it retains 50.66%; after one month (2.10 half-lives) it retains 23.28%, requiring 4.29× the volume to obtain the same activity.
Half-lives of Common Isotopes
| Isotope | Half-life | Typical use |
|---|---|---|
| ³²P | 14.268 days | End-labeling, Southern/Northern blotting, kinase assays |
| ³⁵S | 87.4 days | Metabolic protein labeling, sequencing (legacy method) |
| ¹²⁵I | 59.4 days | Protein iodination, radioimmunoassay |
| ³H | 12.32 yr | Metabolic labeling, receptor binding |
All four values have citable primary sources (the ¹²⁵I figure appears in the Nordion product data sheet). ¹⁴C and ³³P are not included as presets — no directly citable primary source was found. Rather than enter a value of uncertain provenance, use the custom half-life field with the value from your reagent’s package insert.
Common Pitfalls
“Expired” is not binary. One half-life leaves 50%, two leaves 25%, three leaves 12.5%. ³²P after one month (~2.1 half-lives) still retains ~23% — not unusable, but the same experiment requires roughly four times the volume, or must accept a fourfold weaker signal.
Specific activity also decays. The Ci/mmol of a labeled compound falls over time because the molecules remain while the radioactivity is gone. Using the original specific activity to calculate molar amounts will overestimate the fraction of labeled (“hot”) molecules.
Decay is irreversible and continuous. Frozen storage and light exclusion do not affect the half-life — it depends solely on the nuclide itself, independent of temperature, chemical state, or dilution.
Related Tools
Routine concentration and volume calculations: Molarity Calculator and Dilution Calculator.
Half-life values are taken from published nuclide data and manufacturer product data sheets; the ¹²⁵I value of 59.4 days appears in the Nordion Sodium Iodide (I-125) product insert.
FAQ
What date does the activity printed on the vial label refer to?
It refers to the **reference date** — not the day the vial arrives or the day you use it. ³²P has a half-life of only 14.268 days; from the reference date to the time you run the experiment, one to two weeks may have passed and the remaining activity could already be at half the stated value. Poor labeling efficiency, weak signal, insufficient exposure time — these are often not technique problems but a failure to account for those extra days of decay.
Can I still use ³²P that has been sitting for a month?
Yes, but you need to know what you are working with. One month is approximately 2.1 half-lives, leaving about 23% of the original activity. To obtain the same number of disintegrations you need roughly 4.3× the volume. "Expired" is not binary: one half-life leaves 50%, two leaves 25%, three leaves 12.5% — a continuous decline, not a sudden cutoff. This calculator gives you the scale-up factor directly.
Does specific activity change over time?
Yes, and this is the easiest point to overlook. Specific activity (Ci/mmol) decreases over time because the molecules remain while the radioactivity decays away. Using the original specific activity to calculate molar amounts will overestimate the fraction of labeled ("hot") molecules — this leads directly to errors in quantitative experiments or labeling efficiency calculations.
Does frozen storage slow down decay?
No. The half-life depends only on the nuclide itself and is unaffected by temperature, chemical state, dilution, or light exclusion. Storage at −80 °C preserves chemical stability and the integrity of the labeled compound, but it does not preserve radioactivity.
Why are ¹⁴C and ³³P not included as presets?
Because no directly citable primary source was found for them. The four presets provided (³²P 14.268 days, ³⁵S 87.4 days, ¹²⁵I 59.4 days, ³H 12.32 yr) all have traceable sources; the ¹²⁵I value appears in the Nordion product data sheet. Rather than enter a value of uncertain provenance, use the custom half-life field with the value from your reagent's package insert — the equation is the same: A(t) = A₀ × 2^(−t/t½).
Do t and t½ need to have the same units?
Yes. This calculator uses **days** throughout; ³H's 12.32 years has been converted to 4499.4 days. The remaining fraction depends only on how many half-lives have elapsed, so as long as both quantities share the same unit — days, hours, or years — the result is identical.
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