Timing your Phosphorescence Lifetime Experiment with the OLIS PLT3

A – The duration of the LED flash

The length of time the LED is on determines how much energy your molecules are excited by. Depending on how ticklish the molecules are, you will have the LED on for the shortest (0.5 microseconds) duration to the longest (50 microseconds).  You will generally use the shortest duration that achieves sufficient sample excitation. That is, there is no benefit to keeping the LED on longer than is sufficient. The longer the LED is on (A), the longer it must be off (B + C).

OLIS electronics are pulsing the LED at 10 time its normal brightness.

B – The phosphorescent decay of your sample

The software permits acquisition of 100 points in 5 microseconds to 5000 microseconds (5 ms), capturing the decay of your molecules from saturated to ground state. The allocated time must be long enough to capture the entire decay. We advise collecting for 3 or 4 lifetimes (e.g., 200 usec for a 50 usec lifetime).

If a single decay gives you the S/N you need for a given case, you are done! Otherwise, you will repeat this experiment tens, hundreds, thousands of times per second. The many successive data collection events are summed to produce the decay curve with increasingly improving S/N.

The number of times you can repeat the experiment in one second is calculated based on A – how much of your second is spend with the exciting light on – and B – how much of your second is spent capturing the decay, plus C.

C – The recharge time for the LED

The final time parameter to consider is a recharge time for the LED. The manufacturer suggests the ratio is 1% one to 99% off; we find 5:95 to be adequate for reproducible flashes without undue stress on the LED. How much of that ‘off’ time is B? Subtracting that out leaves you with the rest period C that must be sustained before the next flash. If the decay B is lengthy, C could be zero; otherwise, C is some time in addition.

When B = 5 usec, maximum repetition rate of A is 200,000 per second

When B = 5,000 usec, maximum repetition rate of A is 200 per second