Laboratory Analytical Errors in Environmental Sampling - How to Spot Problems and Fix Them

A single analytical error can invalidate months of site work. Holding time violations trigger re-sampling. Wrong preservatives make data legally unusable. Blank contamination forces you to question every result. The cost of re-mobilization often exceeds the original sampling budget by 200-300%.

Most Common Laboratory Errors

Laboratory errors fall into three categories that account for 80% of data quality problems:

• Holding time violations: Samples analyzed after EPA method time limits expire
• Preservation errors: Wrong preservative type, insufficient volume, or missing preservation entirely
• Contamination issues: Method blanks, trip blanks, or equipment blanks showing target compounds

Each error type has different implications for data usability and different solutions.

Holding Time Violations

EPA analytical methods specify maximum holding times from sample collection to analysis. VOCs in water must be analyzed within 14 days. Metals in soil have a 6-month window. Semi-volatile organic compounds (SVOCs) in soil get 14 days for extraction, then 40 days for analysis.

What this means in practice: If your VOC samples sit at the lab for 16 days before analysis, the data cannot be used for regulatory decisions under most programs. The lab should flag this as a holding time violation in the data package.

When Holding Times Are Exceeded

Check your chain of custody form against the lab’s sample receipt date. Labs sometimes receive samples late due to shipping delays, but the holding time clock starts when you collect the sample, not when the lab receives it.

If holding times are exceeded by 1-2 days, some regulatory programs allow the data with qualifiers. Ohio’s Voluntary Action Program (VAP) typically accepts slightly exceeded holding times if the delay is documented and minimal.

For major violations (weeks late), re-sampling is usually required.

Preservation Errors

Different analytical methods require specific preservatives to maintain sample integrity. VOC samples need hydrochloric acid to pH less than 2. Metals samples require nitric acid. Cyanide samples need sodium hydroxide to achieve pH greater than 12.

The most common preservation errors include:

• Wrong acid type: Sulfuric acid instead of hydrochloric acid for VOCs
• Insufficient volume: Not enough preservative to achieve target pH
• Missing preservation: Samples shipped without any preservative
• Over-preservation: Too much acid, creating matrix interference

Checking Preservation in the Field

Verify preservation immediately after adding preservatives. Use pH strips or a calibrated pH meter. VOC samples should read pH less than 2. Metals samples need pH less than 2. Document the pH on your field forms.

If you discover preservation errors before shipping, you can often correct them. Add more preservative if pH is too high. For over-preserved samples, contact the lab before shipping - they may be able to adjust the analytical method.

Blank Contamination Issues

Quality control blanks detect contamination from sampling equipment, laboratory procedures, or environmental sources during sample collection and transport.

• Trip blanks: VOC-free water that travels with your samples to detect contamination during transport
• Equipment blanks: Rinse water from cleaned sampling equipment to verify decontamination procedures
• Method blanks: Laboratory-prepared blanks that accompany your samples through the entire analytical process

When Blanks Show Contamination

If trip blanks contain VOCs, all VOC samples from that sampling event are potentially compromised. Common trip blank contaminants include methylene chloride, acetone, and toluene - often from vehicle exhaust or cleaning solvents.

Equipment blank contamination suggests inadequate decontamination between sampling locations. This affects all samples collected with that equipment after the contamination source.

Method blank contamination indicates laboratory problems. The lab should re-analyze your samples with a clean method blank batch.

Spotting Problems in Data Packages

Review these sections first when you receive analytical results:

• Sample receipt checklist: Temperature at receipt, holding time compliance, preservation pH
• Quality control summary: Blank results, spike recoveries, duplicate precision
• Method detection limits: Verify they meet your project requirements
• Qualifier codes: “J” flags for estimated values, “U” flags for non-detects, “B” flags for blank contamination

Red Flags That Demand Follow-Up

Contact your laboratory immediately if you see:

• Detection limits higher than your cleanup standards or screening levels
• Spike recoveries outside 70-130% range for most methods
• Relative percent differences greater than 20% for duplicate samples
• Target compounds detected in method blanks at concentrations above 10% of your sample results

What to Do When Results Look Wrong

Start with the most obvious explanations. Unusually high metals concentrations might reflect natural background conditions rather than contamination. VOC detections in groundwater samples could indicate vapor intrusion from nearby sources.

Compare results to historical data from the same location. Concentrations that vary by more than an order of magnitude from previous sampling rounds warrant investigation.

Laboratory Communication

Call the project manager at your laboratory, not the general customer service line. Explain specifically what looks wrong and ask for their interpretation. Request re-analysis if quality control parameters were exceeded or if contamination is suspected.

Document all conversations with laboratory staff. Email summaries of phone discussions create a paper trail for regulatory submissions.

Prevention Strategies

Most analytical errors are preventable with proper field procedures and laboratory communication:

• Pre-sampling coordination: Confirm analytical methods, detection limits, and turnaround times before mobilizing
• Field quality control: Collect trip blanks for every VOC sampling event, equipment blanks for every 20 samples
• Proper preservation: Carry extra preservatives and pH measurement tools
• Chain of custody accuracy: Double-check sample IDs, collection times, and requested analyses before shipping

For complex projects involving multiple analytical methods, consider splitting samples between two laboratories. This provides backup data if one laboratory encounters problems and helps identify systematic errors.

The key to managing analytical errors is catching them early and maintaining good relationships with reliable laboratories. For more guidance on field sampling procedures, see our Low-Flow Groundwater Sampling and PFAS Sampling Best Practices guides.