What's the difference between MDL, LOD, and LOQ in stack testing?

The Method Detection Limit (MDL) is the lowest concentration that can be reliably detected. The Limit of Detection (LOD) is similar but less rigorously defined. The Limit of Quantification (LOQ) is typically 3-10 times the MDL and represents the lowest concentration that can be accurately measured, not just detected. For regulatory compliance, MDL is the standard reporting threshold.

Can I lower the MDL if my initial stack test shows non-detect results?

Yes, in many cases. The In-Stack Detection Limit (ISDL) can often be lowered by increasing sample volume—extending sampling time, using larger impinger volumes, or collecting additional sample trains. Discuss these options with your testing contractor before the test begins.

How do I know if my permitted limits are achievable with standard test methods?

Review your air permit limits against typical MDLs for your required test methods (available in EPA method documentation or from your testing laboratory). If your limits are within 10x of the expected MDL, proactive planning with your stack testing provider is essential.

What should I do if my stack test results come back as non-detect?

First, verify that the reported MDL is below your permitted emission limit. If the MDL is higher than your limit, the test may not be valid for compliance demonstration. Document the results properly and consult with your environmental consultant about whether regulatory agencies will accept the data.

Understanding Non-Detects and Method Detection Limits in Stack Testing: Why It Matters Before You Schedule a Test

Alliance Technical Group
Oct 27
3 min read

Updated: Nov 4

Stack testing isn’t just about capturing emissions; it’s about interpreting them correctly. Understanding how detection limits affect results can help your facility avoid costly surprises.

When planning a stack test, one of the most common concerns from both new and existing source owners/operators is:

What if the emission levels of a regulated pollutant are extremely low—possibly even below detectable limits?
How will a “non-detect” (ND) result impact the emission unit’s compliance status with the permitted emission limits?

These are valid questions. They underscore why understanding and evaluating non-detects (NDs), Method Detection Limits (MDLs), and In-Stack Detection Limits (ISDLs) before scheduling a stack test is critical.

What is a Method Detection Limit?

A Method Detection Limit (MDL) is defined as the minimum concentration of a substance that can be measured and reported with 99% confidence that the analyte concentration is greater than zero. It is determined through a specific analytical procedure using a sample matrix containing the target analyte.

Per EPA guidance, if a pollutant is not detected during testing, the result should be reported as the method’s MDL. To accurately report an MDL for a given pollutant or test method, the testing team or analytical laboratory must conduct an MDL study as defined by the Environmental Protection Agency (EPA). Learn more about EPA testing procedures.

MDL vs. ISDL – What’s the Difference?

It’s important to distinguish between the analytical MDL and the in-stack detection limit (ISDL):

The analytical MDL is determined by the laboratory analyzing the collected samples. It reflects the sensitivity of the lab’s instrumentation and methodology.
The ISDL, on the other hand, is tied to the actual sampling process in the field. It can be influenced and adjusted by modifying stack sampling parameters—such as increasing the sample volume by extending the sampling duration.

For wet chemistry methods that rely on laboratory analysis, both MDL and ISDL are critical to ensure a test result is meaningful and valid.

The Impact on Emissions Compliance

Failing to account for MDLs and ISDLs can result in:

Invalid test results: Making your test data unusable for permit demonstrations.
Compliance uncertainty: When non-detect results are reported above your permitted limits, regulatory agencies may assume worst-case scenarios. For example, if your permit limit is 0.001 ppm but the lab reports "< 0.003 ppm ND," you may be considered out of compliance even if actual emissions are negligible.
Costly retests: A failed stack test due to incorrect detection limits can cost $10,000-$50,000+ to repeat, not including production downtime and expedited lab fees. Proper planning prevents these expensive do-overs.
Regulatory scrutiny: This can result in issued violations and required corrective action plans.

How Alliance Technical Group Can Help

At Alliance Technical Group, our Technical Advisors are subject matter experts who carefully evaluate all critical elements before any test program is scheduled. Some of the key considerations we focus on include:

Evaluating permitted emission limits
Selecting the appropriate stack test method
Assessing how ND, MDL, and ISDL values will affect the integrity and outcome of your test program

If you’re unsure how a non-detect result might impact your compliance status or stack test results, reach out to our experts directly. Our team is here to guide you through the process and ensure you’re making informed, compliant decisions.

Understanding Detection Limits in Depth

Detection limits play a crucial role in environmental compliance. They help determine whether emissions are within acceptable levels. Understanding these limits can prevent regulatory issues and financial losses.

The Importance of Accurate Testing

Accurate testing is essential for compliance. It ensures that emissions are monitored correctly. This helps facilities avoid penalties and maintain their operational licenses.

Common Misconceptions About Detection Limits

Many people misunderstand detection limits. They may think that a non-detect result means no emissions. However, this is not always the case. It is vital to interpret these results correctly.

Best Practices for Stack Testing

To ensure accurate results, follow best practices for stack testing. This includes:

Properly calibrating equipment
Following standardized testing procedures
Training personnel on testing methods

Conclusion

In conclusion, understanding detection limits is vital for compliance. It helps facilities avoid costly mistakes. By working with experts, you can ensure that your stack testing is accurate and reliable.