When planning a particulate matter (PM) sampling campaign, you might ask yourself the following questions:

• Which PM_2.5 inlet should I use?  The 1 L/min inlet or the 2 L/min inlet?
• Is the mass of PM accumulated on the filter likely to exceed my limit of detection (LOD) for the majority of my samples? 
• What is the lowest time-averaged PM mass concentration that I will be able to quantify reliably given my LOD, sample flow rate, and sample duration?
• How long should each sample run? How long will I need to sample to collect a mass of PM that exceeds my LOD?

In this post, I'll explain how you can answer these questions so that you can choose the right size-selective inlet and sample duration for your study.

The basics

The mass of PM that will accumulate on the filter during a sample can be calculated as: 

(mass of PM on filter) = (sample flow rate) × (sample duration) × (duty cycle) × (time-averaged PM concentration in sampled air)

where "mass of PM on filter" is in units of μg, "sample flow rate" is in units of m³/hour, the "sample duration" is expressed in hours, the "duty cycle" is a fraction of 1, and the"time-averaged PM concentration in the sampled air" is in units of μg/m³. Sample flow rates are often expressed in units of L/min; when performing this calculation, keep in mind that there are 1000 L in 1 m³ and 60 minutes in 1 hour.

For example, if you sample PM_2.5 at 2 L/min for 24 hours at a 100% duty cycle, and the air you sample contains an average PM_2.5 concentration of 18 μg/m³, you will accumulate 52 μg of PM_2.5 on the filter:

If you sample PM_2.5 at 1 L/min for 48 hours at a 50% duty cycle, and the air you sample contains an average PM_2.5 concentration of 110 μg/m³, you will accumulate 158 μg of PM_2.5 on the filter:

Selecting your sample flow rate and duration

You will want to design your study so that you sample at a high enough flow rate, and for a long enough duration, that the mass of PM sampled onto each filter exceeds your LOD.

If the expected sample mass that you calculate using the equation shown above is below your expected LOD, you'll need to increase your sample flow rate, duration, and/or duty cycle to increase the likelihood that your sample mass will exceed your LOD.

Your sample flow rate will be determined by the volumetric flow rate at which the size-selective inlet you are using is designed to operate. For example, Access Sensor Technologies (AST) offers a PM_2.5 inlet that is designed to operate at 1 L/min, a PM_2.5 inlet that is designed to operate at 2 L/min, a respirable PM inlet that is designed to operate at 2 L/min, and a PM₁₀ inlet that is designed to operate at 2 L/min. Each of these inlets can by used with any UPAS or Home Health Box. Visit our Accessories page for more information on size-selective PM sampling inlets.

Your sample duration might be determined by the time period over which the exposure you are trying to measure occurs, the battery life of the sampler you are using, or other logistical considerations associated with your sampling campaign. For example: 

• You might be trying to measure an occupational exposure that occurs during an 8-hour work shift, so your sample duration might be 8 hours.

• You might be trying the characterize a typical exposure that occurs during a day, so you might sample for 24 hours.

• You might be trying to measure an exposure that occurs during everyday activities, and your sampler might have enough battery capacity to operate for 48 hours with your sampling settings, so your sample duration might be 48 hours.

Note that the battery life of your sampler will vary depending on your sample flow rate and duty cycle. In general, the UPAS battery life will range from 16 hours to 48 hours depending on your sample settings. This battery life can be extended by plugging the UPAS into an external battery pack that operates in "always on" mode (we recommend the battery packs from Voltaic Systems). The UPAS can also operate while plugged into a wall outlet.

Examples

Estimating the lowest time-averaged PM concentration you will be able to measure reliably

If you know your limit of detection (LOD), sample flow rate, and sample duration, you can calculate the lowest time-averaged PM concentration that you will be able to measure reliably.

For example, if you anticipate that your LOD will be approximately 25 μg and you plan to sample at 1 L/min for 24 hours with a 100% duty cycle, the lowest time-averaged PM concentration that you will be able to measure reliably will be 17 μg/m³:

Alternatively, if you sample at 2 L/min for 24 hours with a 100% duty cycle, the lowest time-averaged PM concentration that you will be able to measure reliably will be 8.7 μg/m³:

Estimating a minimum sample duration to exceed your LOD 

If you know your limit of detection (LOD), sample flow rate, and the approximate time-averaged PM concentration in the environment where you intend to sample, you can calculate the minimum duration for which you must sample to accumulate a mass of PM on a sample filter that exceeds your LOD.

For example, if you anticipate that your LOD will be approximately 50 μg and you plan to sample at 1 L/min with a 100% duty cycle in an environment where you expect the time-averaged PM_2.5 concentration to be 45 μg/m³, you will need to sample for at least 18.5 hours to collect a sample mass that exceeds your LOD:

Additional resources

We also offer a spreadsheet that you can use to calculate your expected sample mass. Download a copy of the spreadsheet to your own computer so that you can edit the file. You can also use this spreadsheet to compare your expected sample mass to your LOD, ideal sample mass, and maximum desired sample mass.