One of the more common calls we receive is about field blanks.
- Are they required?
- How many?
- How do I handle them?
- What do I put as sampling time?
- What do I do with a positive field blank?
Customers have a lot of questions, but also some strong beliefs. Since field blanks are such valuable tools, let’s go through and discuss them.
When should field blanks be used?
Field blanks are a useful tool and everyone should use them all the time. If you have background from storage, handling, media or the analysis, field blanks enable you to remove it from your sample results. As a chemist, it makes total sense to use field blanks to adjust your results. Also note many of the NIOSH methods recommend not just one field blank, but two to ten field blanks per project! Refer to NIOSH 2016 and NIOSH 7302.
I’ve never seen anyone perform 10 field blanks. Nonetheless, taking replicate measurement of anything is a good idea. An average is always better than a single measurement.
How to make a field blank or two or ten.
To make a field blank, do everything you do with the samples except do not expose them to the air you are sampling. In fact, the further away you are from a potential source when you handle them, the better.
Sampling time of a ‘blank’.
Since you will be only handling the field blank for a moment, customers ask what sampling time they should record. There are three options: zero minutes, one minute or the same time as the exposed badges.
Consider the table of data below that uses all three methods. There is one exposed badge (OR 1) and a field blank reported three different ways. Note all the field blanks have results over the reporting limit.:
|Sample ID||Chemical||Result, ug||Reporting Limit, ug||Time, min||Result, ppm||Reporting Limit, ppm|
Since there is no sampling time given for this field blank, correcting for a positive field blank takes a little math. First, subtract the field blank value from the exposed sample value, 0.52 ug – 0.30 ug = 022 ug. If you know the sampling rate, molar volume, and molecular weight, you could calculate the concentration. But, if you know that 0.52 ug gives you 0.055 ppm, then using the ratio: (X / 0.055 ppm) = (0.22 ug/0.52 ug), the corrected concentration is 0.023 ppm.
OK, this looks like a disaster. Since the sampling time is 1 min, even the smallest value found on the sampling media is magnified and gives an impossible 26 ppm. No one believes that. Considering the field blank represents a sum of the possible sources, including manufacturing, storage, handling, shipping, and analysis, it is not particularly logical to give it a sampling time of one minute. If you have your report already, use method 1 to field blank correct your results.
This is by far the most logical. Just subtract the field blank result from the exposed badge result in ppm. So 0.055 ppm- 0.031 = 0.024 ppm. That’s it. Of course, all the samples in the project have to have the same sampling time/volume for this to work easily.
Analyses that frequently have field blanks that are detectable and why
Of the tests we perform, here’s a list of the ones that frequently have field blanks above the reporting limit:
- Nitrous Oxide – The badge is sensitive enough to detect the nitrous oxide naturally present in the air. We have proven again and again, when we remove the badge from the heat-sealed envelop and analyze it right away, the result will be below the reporting limit. However, once you remove it from the heat-sealed pouch and store it, ship it, etc, the badge is sensitive enough to have a positive field blank, typically about 1/50th the regulatory limit.
- Formaldehyde – Formaldehyde is everywhere, such as in particle board, carpet, and on and on. It’s impossible to buy the active ingredient in the badge without a detectable amount of formaldehyde.
- Ozone – Like formaldehyde, it’s impossible to buy the chemical collector without “ozone” in it (it’s really sodium nitrite and sodium nitrate).
Of course, we lab blank correct and, of course, we take great lengths to minimize the amount of the chemicals present. Regardless, there is only so much we can do once the badge goes out in the world. Use those field blanks.
Are field blank results over the reporting limit ever evidence of a problem?
- Sometime we receive an exposed badge and a field blank. If the field blank has 10 times the exposed badge, then someone most likely switched the badges. Resample and/or review the paper trail.
- If a hit on a field blank represents a result near or just over the reporting limit, then the accuracy is not very good. Collect more air by sampling longer or use a different method.
But for the most part, we are talking about results that represent mere fractions of the regulatory limits. For example, if the exposed badge is one 50th of the regulatory limit, is any field blank result significant to your project?
Field blanks higher than the exposed badges
This one really irks customers. Consider our claim that our badge’s maximum total error at the PEL is 25%. That means if we report a result of 1.0 ppm, the result is somewhere between 0.75 ppm and 1.25 ppm. So if the field blank is 1.1 ug and the exposed badge is 0.89 ug, they really are the same level. It’s just the field blank was a little higher on the scale this time. In addition, for the most part we are talking about results near the reporting limit. By definition, results near the reporting limit are less accurate and less precise than results 10 or 100 times the reporting limit. So we are talking about error ranges higher than 25%. (Before you get too excited, this is just math. Everyone is this way.)
Forget the name “Field Blank”
Calling these “blanks” creates the unrealistic expectation that a zero answer is the only right answer. Instead, let’s go with “background correcter” and be glad you did.