Field Blanks: Why and When to Collect

Quality Assurance in Industrial Hygiene Sampling

Sound industrial hygiene sampling practice requires good quality assurance (QA) practices. QA is defined as:

the maintenance of a desired level of quality in a service or product, especially by means of attention to every stage of the process of delivery or production.

In essence, QA seeks to minimize error and at the same time evaluate sampling performance. Key QA practices include the following:

  • use of standard operating procedures (SOPs) for sample collection and analysis;
  • use of chain-of-custody and sample-identification procedures;
  • instrument standardization, calibration, and verification;
  • technician and analyst training;
  • sample preservation, handling, and decontamination; and
  • use of quality control (QC) samples such as field and transport blanks, duplicates, etc.

Quality Control: Why collect Field Blanks?

One of the methods of quality control in industrial hygiene sampling is through the collection and analysis of “control samples”. Control samples are typically in the form of blanks, duplicates, spiked samples, or split samples. Blanks can be further refined into several categories including: field blanks, transport blanks & media blanks.

  • Field Blanks – determine if there was contamination during the collection stage
  • Transport Blanks – determine if there was contamination during the shipping stage
  • Media Blanks – determine if the media itself is contaminated

Therefore, if quality assurance is our goal, and the use of field blanks (i.e. quality control) are one of our methods for ensuring QA.

How to Collect a Field Blank

Field blanks are clean sampling media and are handled in exactly he same manner as the samples you are collecting, except:

  • No air is drawn through them (for air samples), or no surface is wiped with them (for surface / wipe samples),
  • They are opened and closed quickly in the sampling area and then resealed, and
  • Accompany the actual samples through every stage of the sampling process.

How Many Field Blanks Should I Collect?

The number of field blanks you should collect should be based on a few considerations (in order of importance), including: sample method, reason for sampling, and budget considerations. The sampling method (e.g. NIOSH or OSHA) should dictate how many field blanks you should collect (see image below), based on the number of samples you collect. Typically, the recommended practice is 10% of your number of samples.

What do I do if the Contaminant is Detected in the Field Blank?

If a contaminant is found on the field blank, the field blank contaminant is typically going to be reported as a mass, and not a concentration (remember, no air volume was drawn through the field blank sample). The mass of contaminant found on the field blank will be subtracted from that found on the actual sample before dividing by the air volume to determine the mass concentration of the contaminant. Additionally, it’s important to note the sampling methods often have a permissible limit of contaminant on the field blank. If this level is exceeded, the samples should be discarded and sampling repeated.

If we do not account for the field blank contamination, the industrial hygienist runs the risk of the reported results being biased high, which can lead to: additional sampling or the implementation of potentially unnecessary controls.

Why Don’t Industrial Hygienists Take Field Blanks?

Practicing industrial hygienists don’t take field blanks for several reasons, including: budgets (10% extra costs for “worthless” blanks can seem excessive), improper training, unfamiliarity with the sampling methodology, and they don’t want competence questioned. However, none of these reasons justify forgoing good QA/QC practices to ensure the integrity of your samples.

Free Webinar: Improving the Quality of Industrial Hygiene Air Sampling Data Through Proper Field Practices

TestAmerica is hosting a free webinar on “Improving the Quality of Industrial Hygiene Air Sampling Data Through Proper Field Practices” on September 20.  The webinar will be presented by Mike McGee, CIH and will provide a basic overview on improving the quality of IH data and avoiding common pitfalls.

Variability in sampling data can result from: the skill level and attention of the person performing the sampling, equipment and sampling media, variability of air sampling flow/sampling rate, recording sampling time accurately, environmental factors, documentation, the representativeness of the samples collected, variation in contaminant concentration during sampling, and transportation of samples from the field to the laboratory.

Topics will include:

  • Sources of field variability
  • Why air volume is only half of the equation
  • What you should know about air sampling pump calibration
  • Field data collection accuracy and adjustments
  • Field data documentation
  • Pitfalls to avoid

Register for the webinar HERE.

Source: TestAmerica

Calculator: Minimum Sample Volume & Required Sampling Time

Want to determine the Minimum Sample Volume needed to ensure you collect enough of a sample volume so that the results are capable of being less than the Exposure Limit? Want to know how long you need to sample at a given flow rate?

If so, use’s Minimum Sample Volume and Required Sampling Time calculator and take the guess-work out of the equation.

Input the:

  • Laboratory Limit of Quantitation (LOQ)
  • Exposure Limit
  • Desired Fraction of the Exposure Limit (essentially a safety factor for sampling)
  • Sampling Rate

And the results will give you the Minimum Sample Volume and Required Sampling Time.

Note:  Ensure that you have the right units and all of the units are consistent.  Units such as parts per million (ppm) for Exposure Limits may need to be converted to mg/m3.

Download the Calculator HERE:

  Calculator: Minimum Sample Volume and Required Sampling Time (35.0 KiB, 1,749 hits)

Common Mistakes: Sampling with Sorbent Tube in Horizontal Position

Hiking: The Path of Least Resistance

Hiking: The Path of Least Resistance

Like hikers, electricity in a circuit, and water flowing downhill, airborne vapors also take the path of least resistance.  Therefore, it is important to maintain proper orientation with a sorbent tube while air sampling.  If the sorbent tube is placed in a horizontal position, “channeling” may occur due to the sorbent falling away from the walls of the glass sorbent tube.  As the vapors take the path of least resistance (i.e. through the “channel”), the effectiveness of the adsorption by the sorbent bed will be decreased because of the effective surface area being diminished.  The sorbent tube should be maintained in a vertical position (or as close as possible) to avoid the “channeling” effect.