Blanks, reference materials, and acceptance criteria
Use controls to ask whether contamination, background, or known values could explain a result before accepting it.
Field diagramA quality-check decision
Estimated reading
8 min
Estimated practice
10 min
Equipment
Supplied control-data worksheet; optional clean-water blank demonstration with labeled containers.
Progress stays in this page session. Allow lesson storage in Cookie settings to remember it; browser settings may also prevent saving.
Objective
By the end, you can:
Distinguish a blank, a reference material, a positive control, and a replicate.
Write an acceptance criterion before viewing results.
Interpret a control failure as evidence about the process rather than as permission to delete samples.
Controls answer different questions
A blank contains the matrix or process without the target contribution you are trying to detect. It asks whether the container, reagent, environment, or handling adds signal. A reference material has a characterized value or range and asks whether the measurement process agrees with an external or assigned value. A positive control asks whether the system can produce an expected response; a replicate asks about variation. Calling every control a ‘control’ hides its purpose.
NIST describes reference materials as tools for quality assurance, traceability, and method validation. A certified reference material has an assigned value with stated uncertainty; an informal classroom reference is not equivalent. Choose the least elaborate control that answers the risk in your method, and name what a failure would mean before collecting results.
Criteria are decisions, not decorations
An acceptance criterion should state the comparison, units, time window, and response. For example: ‘Water blank signal must be below the method’s documented reporting threshold; otherwise investigate the batch and do not interpret sample signals as clean.’ The threshold must have a stated basis. EPA quality guidance illustrates why blanks are trended and investigated, rather than used as a single magic number. If no defensible criterion exists, report the control result descriptively and say that acceptance could not be determined.
Worked example
A blank that changes the conclusion
Three water samples are near 0.20 units, but the process blank is 0.18 units and the method’s predeclared reporting threshold is 0.05 units.
01Blank comparison: 0.18 exceeds 0.05.
02Decision: investigate contamination or background before interpreting sample differences.
03Report: sample readings remain raw observations; a clean-signal conclusion is not supported.
Materials
Set out what you need.
Control map
Supplied blank/reference data
Method or project acceptance rule
Deviation form
Safety & stop conditions
Pause if the work no longer fits the plan.
Use only clean water for a physical exercise.
Do not use unknown or biological material as a classroom control.
Follow local waste and spill instructions and stop if a container is unlabeled or leaking.
Method
Work through the steps.
01
Name the risk
Write whether the control is checking contamination, sensitivity, bias, drift, or repeatability.
02
Choose the control
Select a blank, reference, positive control, or replicate that can reveal that risk under the same process conditions.
03
Predefine the criterion
Record units, threshold or comparison rule, source, and action before looking at the outcome.
04
Run and identify
Give each control a unique ID and preserve its position, operator, instrument, and time with the sample data.
05
Interpret the pattern
Check whether failure is isolated, systematic, drifting, or tied to a batch. Pause interpretation when the control cannot support the intended conclusion.
Checkpoints
Observe, record, investigate.
The control has a stated question and identity.
The criterion existed before the result.
The control was exposed to the relevant steps.
A failure has a documented consequence for sample interpretation.
Common mistakes
Correct the process, preserve the record.
A blank is described as proving no contamination.
Say what contamination the blank process could reveal and what detection limit or threshold applies.
A reference material is treated as a universal truth.
Use its assigned value, matrix, scope, and uncertainty; do not generalize beyond its intended use.
The criterion is selected after seeing a high result.
Document the preplanned rule or label the analysis exploratory.
A failed blank is removed from the table.
Keep it, investigate the process, and state which conclusions are limited.
Practice
Control map
For a water-transfer exercise, map these supplied controls to the risk each addresses: a process blank of 0.18 units against a 0.05 threshold; a reference assigned 1.00 units that reads 0.99; and replicate readings of 1.00, 1.01, and 0.99 units. Decide what can be interpreted.
Which control could reveal a dirty container?
Which result would indicate drift?
What action follows a failed blank?
What remains unknown if the reference has no uncertainty?
