A series of full-period consecutive dust samples are collected during an 8-hour

Question

A series of full-period consecutive dust samples are collected during an 8-hour work shift. Samples were collected on filters and weighed gravimetrically, with results as follow:

Sample no:              Flow rate             Sample Duration         Mass Collected

                                        (liters per min)                 (min)                (micrograms)(µg)

                        1                      2.0                               60                              110

                        2                      2.0                               120                              745

                        3                      2.0                               90                              350

                        4                      2.0                               120                              700

                        5                      2.0                               90                              255

                                                                                                                        What is the 8 hour TWA concentration in milligrams per cubic meter (mg/m3)?

            Think very carefully how you might do this. The final units are in mg/m3. You have mass in ug. You also have minutes and liters per minute. That should tell you how many liters of air were sampled. Airborne concentration is in mass per volume of air. You can calculate ug/l. Convert to mg/m3 and then calculate the 8 hour TWA (show all work).

I am getting 2 different answers to this question....which one is correct? 2.24 or 4.75?

Explanation / Answer

Arrange the flow-rate ( liters per min) Sample Duration (min) Mass Collected (micrograms ug) the row-column wise -

(1) 2.0 60 110

(2) 2.0 120 745

(3) 2.0 90 350

(4) 2.0 120 700

(5) 2.0 90 255

Here, we can see from the data that flow-rate( liters per min) is constant , so there is no relation of flowrate with other two variables

Again, find relation between sample duration and mass collected.

Therefore, Sample Duration (min)(x) Mass Collected (micrograms ug) (y)

(1) 60 110

(2) 120 745

(3) 90 350

(4) 120 700

(5) 90 255

The regression equation will be ...(using calculator to find regression equation .... we can do it maually as well, but it's very lengthy and painful work... use calculators for such calculations)

so,

y=10.75x-600

where, y = Mass Collected (micrograms ug)

x = Sample Duration (min)

for

x = 8 hours = 8*60 min = 480 min

So, Mass Collected (micrograms ug) = y = 10.75*480-600 = 4560 ug = 4.56 mg

flowrate = 2.0 L/min = 0.002 m^3/min

So,

8 hour TWA concentration = mass/(volume)

= (mass collected) / (flow-rate * sample duration)

= 4.56/(0.002*480) mg/m^3

= 4.75 mg/m^3

Therefore, 8 hour TWA concentration in milligrams per cubic meter = 4.75 mg/m^3

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