Detection Limits and PQL

Detection limits

We are often asked about our detection limits (e.g. “How low can you test for lead?”).  Since this is a common question, we thought we would include a brief description of detection limits, what distinguishes various detection limits from one another, and how these relate to the question of ‘how low can you go?’

What exactly do you mean when you talk about detection limits?

By definition, a detection limit is the lowest amount of a substance that can be distinguished from the absence of that substance (the background) with a reasonable amount of certainty.    Unfortunately, analytical folk commonly reference several different “detection limits” including the instrument detection limit (IDL), the method detection limit (MDL) and the practical quantitation limit (PQL).   All are mathematically derived from the analysis of a series of blanks or low-level standards, which have been processed using the same test conditions as that associated with the samples.  A series of blanks are analyzed to calculate the IDL, whereas a series of low-level standards are analyzed to calculate the MDL.  The PQL is set by the laboratory at a value which is generally 2-5 X that of the MDL to provide for a higher level of confidence and consistency in the reporting of data.   In general, the relationship between the various limits is IDL < MDL < PQL.

Where does CSL list the detection limit on their laboratory reports?

CSL includes a column on the laboratory report with the heading PQL (Practical Quantitation Limit).    This is the value that we refer to as the detection limit and is the value at which we can distinguish the difference between two results with 99% confidence.  Any value generated by the method which is below the PQL will be included on the lab report as result of < PQL.

Why do I sometimes see different PQLs for the same element?

There are many factors that contribute to the PQL reported for a sample.  Some of these factors are within the control of the laboratory (i.e., instrumentation used, tuning conditions, sample preparation techniques), others are controlled by the customer (i.e., sample volume), and some are inherent in the sample material (i.e., whether the material is a solid, aqueous material, or organic liquid).  All affect the PQL.

Why do solid samples have higher PQL values than waters?

As mentioned above, there are many factors which can affect the PQL for a particular sample.   Solid samples must be processed to put them into an aqueous solution in order to get them into the testing instruments.  An ICP-MS can only reliably operate with solutions that have < 1% dissolved solids.   Because of this, solid materials (or other materials with higher amounts of dissolved substances) must be diluted to bring them into this acceptable range.  This means that in the processing of a solid material (100% solids) a dilution of at least 100-fold is required. This dilution is then factored into the detection limit, resulting in a PQL for a solid material which is at least 100X higher than that of an aqueous sample.

I have a customer who requires a specific detection limit.  How do I know if you can reach it?
Give us a call and we’ll let you know!  f the limit is below the PQL normally reported for a material, it may still be possible to meet your need with special processing.  It is helpful if we are aware of special detection limit requests prior to receiving the sample.  This allows us to expedite the processing and get the results to you as quickly as possible. It is also a good idea to include the special detection limits on the chain-of-custody (Sample Submittal Form) that accompanies the sample. To learn more about PQL, check out this post. For more about CSL, contact us today.

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