LowPPM injection

[coolarms]

Hello all,

I need to make 1ul of 1-50PPM injections in a GC7890. The signal that shows up on a FID varies from 9-9.5pA on a baseline of 8.5pA. I believe it has to do with the manual injection i.e. the depth to which the syringe tip travels each time I inject. I have made 50PPM of octane in air by letting 50ul of octane homogenize in a 1L flask, capped with a 24/40 septa. I would like to know if there is anyway of having the same in a autosampler vial (gas phase), so that I can use the autosampler to make repeatable injections. Or is there someone who sells 50PPm standards in autosampler vials, so that I can purchase and use the autosampler to make repeatable injections Or is there any other method?

Thanks for your time and response!

[rb6banjo]

I believe that 50 µL of octane in a 1 L flask of air at atmospheric pressure is more like 7,409 ppm. Gas-phase parts-per-million is not the same as condensed phase ppm. Gas-phase ppm is µL/L (gaseous) or µmole/mole (using the ideal gas law) The ratios are the same if the pressure and temperature are constant.

0.05 mL x 0.703 g/mL x 10^6 µmole/mole/114 g/mole = 308 µmole octane

1L x 1 atm/(0.0821 x (273 + 20)) = 0.041571 mole of gas in the receiver

ppm = 308 µmole/0.041571 mole of ideal gas = 7,409 ppm

1 µL of octane in a liter of air at atmospheric pressure gives about 150 ppm. Certainly, you can dilute the sample more if you can pressurize the vessel (i.e., add more solvent). Restek makes some very nice canisters for this type of thing. I've always struggled trying to make gas standards this way so be careful about what you take as correct.

I have gotten some very good results for sampling gases at atmospheric pressure with SPME. I overfill a headspace vial (20 mL variety) and lean the the cap and septum on the transfer line (for me, it's a silco-steel tube). As I pull out the transfer line, I snap the cap down and then crimp it. The gas sealed inside is at atmospheric pressure. The SPME fiber then extracts the analytes from the gas inside the vial. I analyze CO2 and I have obtained better sensitivity for my analytes using SPME like this than I get on a GC that does fixed-volume, atmospheric-pressure injections with a rotary valve mounted in the oven. You can also put the atmospheric pressure vials on a static headspace sampler - although the detection limits are not as good as what you can get using SPME.

I hope this helps.

[AICMM]

coolarms,

A number of issues here. rb6banjo is dead on in noting your standard is way higher than you think it is (something around 7K ppm.) At 1 uL splitless, you are putting about 35 ng on column which means you really should have a much better peak than you describe. If you are splitting, the story changes quickly. So, if splitless, something else is going on because your peak is pretty small.

At 1 uL injection volume, at true 50 ppm, you are only putting about 0.3ng on column (assuming splitless injection again) which will be tough for an FID to detect reproducibly. You should really consider using a sample loop rather than a syringe injection but if you have to use a syringe then you should consider a much larger syringe (z.b. 100 uL) which gets interesting if you are running splitless because of the pressure pulse.

I make standards all the time in static bulbs. I make the bulb warm and I include glass balls in the bulb to mix the gases inside.

Best regards,

AICMM

[larkl]

If we were going to run something like this routinely, we'd add a GSV (gas sample valve - See VICI - Valco website) with sample loop (probably 500 uL). Then I'd order an octane standard in a gas cylinder. This can be problematic, as the standard will be low pressure in order to keep the octane from condensing. But it works if the gas supplier know what they are doing.

If we were in a hurry or it wasn't going to be something routine, I'd make the standard up in a gas bulb and then shoot the gas into the inlet using a gas tight syringe (again probably 500 uL).

And yes as others have alrady responded - you need to calculate how many moles of octane you injected into the flask, then use the ideal gas law to convert this to volume. Then you can calculate a concentration.