CWEA Laboratory Analyst Grade 3. 150 Questions and Rationales.
Graded A+ , instant download PDF.
Quality Assurance and Quality Control
1. A laboratory analyst obtains a laboratory control sample recovery of 108%, while the
established acceptance range is 90–105%; what is the most appropriate action?
A. Investigate the failed recovery and determine whether associated sample results
require qualification
B. Report the batch because the recovery is close to the upper limit
C. Delete the control result
D. Average the recovery with the previous batch
Answer: A
Rationale: A failed control must be evaluated according to the laboratory's
established QC procedures before affected results are accepted.
2. A method blank contains detectable analyte above the laboratory's established
acceptance criterion; what should the analyst do?
A. Subtract the blank concentration from every sample
B. Investigate the contamination and evaluate its effect on associated sample
results
C. Ignore the blank because environmental samples are more important
D. Report all results without qualification
Answer: B
Rationale: Excessive blank contamination can compromise low-level sample results
and requires documented evaluation.
3. A duplicate pair produces results of 80 mg/L and 100 mg/L; what is the relative
percent difference?
A. 11.1%
B. 20%
C. 22.2%
D. 25%
Answer: C
Rationale: RPD is calculated as the absolute difference divided by the average of the
two results, multiplied by 100.
4. A control chart shows a gradual movement of control results toward the upper
warning limit without exceeding it; what should the analyst recognize?
A. The method has definitely failed
B. A developing systematic trend may require investigation
C. The results are automatically invalid
D. The control material must be discarded immediately
, Answer: B
Rationale: Persistent trends can indicate developing bias before formal control limits
are exceeded.
5. A reference material repeatedly produces results below its certified value while
replicate precision remains excellent; what does this indicate?
A. Poor precision
B. Random error
C. Systematic negative bias
D. Excellent accuracy
Answer: C
Rationale: Consistent deviation in one direction indicates systematic bias despite
good precision.
6. During data review, an analyst discovers that a reported result was manually changed
without documentation; what is the primary concern?
A. Sensitivity
B. Accuracy
C. Data integrity and traceability
D. Sample preservation
Answer: C
Rationale: Undocumented changes prevent reliable reconstruction of the original
analytical record.
7. A matrix spike recovery is very low, but the laboratory control sample and calibration
verification pass; what is the most likely explanation?
A. Instrument failure is certain
B. The sample matrix may be suppressing analyte recovery
C. The calibration standards must be contaminated
D. The method blank must have failed
Answer: B
Rationale: Acceptable general QC with poor matrix recovery commonly indicates
sample-specific interference.
8. An analyst discovers that a balance verification failed before a gravimetric analysis;
what should happen?
A. Continue because the failure is probably insignificant
B. Adjust results mathematically
C. Stop using the balance for affected work and assess the impact
D. Ignore the verification record
Answer: C
Rationale: An out-of-tolerance balance can compromise measurements and requires
documented evaluation.
, 9. A proficiency-testing sample is received by the laboratory; how should it generally be
analyzed?
A. With special procedures designed to maximize the score
B. By testing it repeatedly until the preferred result appears
C. Using routine procedures and personnel representative of normal laboratory
operations
D. Outside the laboratory quality system
Answer: C
Rationale: Proficiency testing is intended to evaluate normal laboratory performance.
10. A laboratory repeatedly identifies incomplete sample-receipt records during audits;
what is the strongest corrective action?
A. Remind employees verbally
B. Eliminate the documentation requirement
C. Identify the root cause and implement a documented preventive control
D. Reconstruct missing records from memory
Answer: C
Rationale: Effective corrective action addresses the underlying cause rather than
merely correcting individual omissions.
Analytical Chemistry and Instrumentation
11. A pH electrode gives acceptable readings in pH 4 and 7 buffers but fails verification at
pH 10; what should be investigated?
A. Sample chain of custody
B. Electrode condition, calibration, conditioning, and alkaline response
C. Balance performance
D. Incubator temperature
Answer: B
Rationale: Failure at one calibration region can indicate electrode or calibration
problems affecting that range.
12. A spectrophotometer produces increasingly high absorbance readings for unchanged
standards; what is the most appropriate investigation?
A. Sample collection location
B. Instrument drift, wavelength accuracy, reagent stability, and timing
C. Sample preservation
D. Chain-of-custody signatures
Answer: B
Rationale: Changing absorbance in fixed standards indicates instability in the
analytical system.
Graded A+ , instant download PDF.
Quality Assurance and Quality Control
1. A laboratory analyst obtains a laboratory control sample recovery of 108%, while the
established acceptance range is 90–105%; what is the most appropriate action?
A. Investigate the failed recovery and determine whether associated sample results
require qualification
B. Report the batch because the recovery is close to the upper limit
C. Delete the control result
D. Average the recovery with the previous batch
Answer: A
Rationale: A failed control must be evaluated according to the laboratory's
established QC procedures before affected results are accepted.
2. A method blank contains detectable analyte above the laboratory's established
acceptance criterion; what should the analyst do?
A. Subtract the blank concentration from every sample
B. Investigate the contamination and evaluate its effect on associated sample
results
C. Ignore the blank because environmental samples are more important
D. Report all results without qualification
Answer: B
Rationale: Excessive blank contamination can compromise low-level sample results
and requires documented evaluation.
3. A duplicate pair produces results of 80 mg/L and 100 mg/L; what is the relative
percent difference?
A. 11.1%
B. 20%
C. 22.2%
D. 25%
Answer: C
Rationale: RPD is calculated as the absolute difference divided by the average of the
two results, multiplied by 100.
4. A control chart shows a gradual movement of control results toward the upper
warning limit without exceeding it; what should the analyst recognize?
A. The method has definitely failed
B. A developing systematic trend may require investigation
C. The results are automatically invalid
D. The control material must be discarded immediately
, Answer: B
Rationale: Persistent trends can indicate developing bias before formal control limits
are exceeded.
5. A reference material repeatedly produces results below its certified value while
replicate precision remains excellent; what does this indicate?
A. Poor precision
B. Random error
C. Systematic negative bias
D. Excellent accuracy
Answer: C
Rationale: Consistent deviation in one direction indicates systematic bias despite
good precision.
6. During data review, an analyst discovers that a reported result was manually changed
without documentation; what is the primary concern?
A. Sensitivity
B. Accuracy
C. Data integrity and traceability
D. Sample preservation
Answer: C
Rationale: Undocumented changes prevent reliable reconstruction of the original
analytical record.
7. A matrix spike recovery is very low, but the laboratory control sample and calibration
verification pass; what is the most likely explanation?
A. Instrument failure is certain
B. The sample matrix may be suppressing analyte recovery
C. The calibration standards must be contaminated
D. The method blank must have failed
Answer: B
Rationale: Acceptable general QC with poor matrix recovery commonly indicates
sample-specific interference.
8. An analyst discovers that a balance verification failed before a gravimetric analysis;
what should happen?
A. Continue because the failure is probably insignificant
B. Adjust results mathematically
C. Stop using the balance for affected work and assess the impact
D. Ignore the verification record
Answer: C
Rationale: An out-of-tolerance balance can compromise measurements and requires
documented evaluation.
, 9. A proficiency-testing sample is received by the laboratory; how should it generally be
analyzed?
A. With special procedures designed to maximize the score
B. By testing it repeatedly until the preferred result appears
C. Using routine procedures and personnel representative of normal laboratory
operations
D. Outside the laboratory quality system
Answer: C
Rationale: Proficiency testing is intended to evaluate normal laboratory performance.
10. A laboratory repeatedly identifies incomplete sample-receipt records during audits;
what is the strongest corrective action?
A. Remind employees verbally
B. Eliminate the documentation requirement
C. Identify the root cause and implement a documented preventive control
D. Reconstruct missing records from memory
Answer: C
Rationale: Effective corrective action addresses the underlying cause rather than
merely correcting individual omissions.
Analytical Chemistry and Instrumentation
11. A pH electrode gives acceptable readings in pH 4 and 7 buffers but fails verification at
pH 10; what should be investigated?
A. Sample chain of custody
B. Electrode condition, calibration, conditioning, and alkaline response
C. Balance performance
D. Incubator temperature
Answer: B
Rationale: Failure at one calibration region can indicate electrode or calibration
problems affecting that range.
12. A spectrophotometer produces increasingly high absorbance readings for unchanged
standards; what is the most appropriate investigation?
A. Sample collection location
B. Instrument drift, wavelength accuracy, reagent stability, and timing
C. Sample preservation
D. Chain-of-custody signatures
Answer: B
Rationale: Changing absorbance in fixed standards indicates instability in the
analytical system.
