Introduction
Before our PhotonMaster luminometers head out the door, each one is rigorously tested in our hardware lab. Sensitivity and linearity tests confirm that every unit meets required specifications for background levels, low detection limits, and linearity across the detection range.
In the field, however, drops, spills, or heavy use can affect performance. That’s where a linearity check comes in. This quick 10-minute test makes sure your instrument still produces accurate results across its full detection range.
Because the curve of light output versus ATP concentration is linear, you only need a one-point calibration during routine testing. This is done with our 1 ng ATP/mL UltraCheck standard, which standardizes enzyme activity. Standardizing against a known ATP concentration also allows you to convert the light output detected by the luminometer into pg or ng ATP/mL and compare your results over time, even as enzyme activity decreases.
Why perform a linearity check?
Built tough, but not infallible: Even though PhotonMasters are designed for travel and field use, build-up in the assay chamber, drops, strong magnetic fields, helium gas, or extreme temperature swings can cause light leakage and reduce sensitivity.
Confidence across the range: A linearity check confirms that the response from 0.01 ng ATP/mL to 100 ng ATP/mL remains linear. Because the curve is linear, routine testing only needs a one-point calibration, but this check verifies that assumption.
Annual tune-up: Perform the check at least once a year or any time your unit experiences a hard knock, large spill, or extreme conditions.
How do I perform a linearity check?
It's not complicated! If you're ever run an UltraCheck calibration reading during testing, this is just five of them back-to-back.
1. Clean first: Use your Maintenance Kit to clean the chamber before starting. The LMK-5C kit provides enough supplies for five cleanings and five linearity curves.
2. Prepare five tubes: Dispense 100 µL of Luminase into five clean 12 x 55 mm assay tubes.
3. Add standards: Start with the 0.01 ng ATP/mL UltraCheck solution and add 100 uL (2 drops) to the first assay tube. Take the reading immediately in the Luminometer and record the RLU value. Repeat for the remaining Ultracheck solutions (0.1, 1, 10, and 100 ng ATP/mL)
4. Record and plot: Plot the UltraCheck concentrations on the x-axis and the measured RLU values on the y-axis of a log-log scale and calculate the R²value.
Use Relay to take all your readings and to automatically calculate the coefficient of determination (R²). An R² value of 0.99 or higher indicates excellent linearity.
5. Troubleshoot: If R² is below 0.99, clean the instrument and repeat. Persistent failure may indicate damaged equipment. Contact LuminUltra Support for assistance.
Learn More: For detailed instructions and video on how to use the Standardization Kit, read the Test Kit Instructions
When should I perform a linearity check?
Routine: Once per year as part of your maintenance schedule.
After Stress: Immediately after a major spill, physical shock, exposure to helium gas or extreme temperatures.
If results drift: When routine UltraCheck calibrations produce unexpected values or if background readings are unusually high. See 2nd Generation ATP equipment troubleshooting guide and Why are my ATP background readings higher than normal and what should I do? for more information if you are experiencing unusual readings.
Need Assistance?
For additional support or to order kits, contact LuminUltra Support
By following these recommendations, your PhotonMaster luminometer will remain in excellent condition, delivering consistent, accurate results every time.