Micromole per what? Also, ppm by weight or by volume? Anyway to convert mass to moles you need to divide by the molecular weight of methane (12+4 =16) . 

I'm assuming you're talking about parts per million of gas in air. That's what I've done.

Basically, ppm is already micromoles per mole. To turn moles of gas into a volume, you need to know the number of molecules of gas in a set volume at a given temperature and pressure, and use the Ideal Gas Law. Or, you can trust wikipedia, using the page for Molar Volume.

0.024465 m3/mol at 25 °C

OK, we'll keep it at 25 C for now, you can adjust to your temperatures later.

0.024465 cubic metres per mole is 40.87472 moles per cubic metre.

If your concentration of CH4 is 2.5 ppmv (a typical value for near-surface atmosphere), then the total amount of CH4 per cubic metre is (2.5 / 1000 000 * 40.87472) = 0.000102187 moles / m3

Which is 0.102 millimoles / m3

or 0.0102 millimoles / L

Thanks to both of you for your valuable suggestions. What I am doing right now is as below:

1 PPM= 1mg/L (Basis for calculation)

I get 108 PPM of CH4, meaning, 108 mg of CH4/L=0.108 g/L

Divide by atomic weight will give=0.00675 mol/L

Now converting mol/L to micromole/ml gives me 6.75 micromole/ml. 

Is my conversion correct? 

Your method is perfectly all right.

Thank you. 

Hello Saurav Sorcar sir

i want know CO2 PPM of sea water with Known pH value eg; pH 7,7.5,8.,8.1. Can u help me sir pls?

@Rajeswari: Unfortunately, I am not aware of this.

Its k sir thank u for ur reply sir!!!!

I think this calculation for aqueous phase not for gaseous

Thanks to both of you for your valuable suggestions. What I am doing right now is as below:

1 PPM= 1mg/L (Basis for calculation)

I get 108 PPM of CH4, meaning, 108 mg of CH4/L=0.108 g/L

Divide by atomic weight will give=0.00675 mol/L

Now converting mol/L to micromole/ml gives me 6.75 micromole/ml. 

Is my conversion correct? 

Dear all, the using Handheld Laser methane Detector (LMD) provide unit is ppm x m. parts per million × metre. How to calcutlate as mg/L or mg/m3? thank you.

1ppm=1 micromole/mole

Old topic. I would guess that ppm would refer more to ppmV in the case of gases (uL/L for example), so approximating ppm to ppmMOL or to mg/L is indeed an approximation...to solve ppmV, you could use another approximation, the ideal gases law (PV = nRT), valid for "relatively low pressures" but possibly just fine for your case. It depends on how much accurate you need to be. AND mostly, on what your unit is indeed (ppmW, ppmV or ppmMOL).