calculating on column injection split

[BurntChem]

Hello,

What is formula for calculating on column injection of a substance on gc with split flow? For example 1mg/ml injected with 1ul volume with a split flow of 1:10.

Thanks for some reason I always confuse this.

[Peter Apps]

Calculate the quantity in 1 ul (or whatever the injection volume is) and then multiply it by the column flow then divide by (split flow + column flow).

NB that unless you are doing cold injections the volume of sample transferred from syringe to inlet will be larger than the set injection volume because some sample will boil out of the needle. If you want to know the true injected volume then compare volume in syringe before and after injection.

Peter

[BurntChem]

Thanks so this would give ng (or whatever weight unit) or substance injected into system?

About cold injection and sample boiling over. If you only fill syringe with 1ul and inject 1ul how does sample remain in syringe or inject more?

[dblux_]

Thanks so this would give ng (or whatever weight unit) or substance injected into system?

About cold injection and sample boiling over. If you only fill syringe with 1ul and inject 1ul how does sample remain in syringe or inject more?

Unless you do injections with plunger in needle syringe you fill your syringe with intended volume (1 µL as in your example) plus an extra volume (lets say to make it simple - needle volume). This excess volume is what you are asking about.

[Peter Apps]

Thanks so this would give ng (or whatever weight unit) or substance injected into system? you will get an answer in whatever units of quantity your sample concentration is expressed in; mass or moles

About cold injection and sample boiling over. If you only fill syringe with 1ul and inject 1ul how does sample remain in syringe or inject more? Adding to what dblux said; at room temperature the movement of the plunger displaces the set volume from the end of the needle. The needle remains filled with sample. You can verify this by pulling the plunger back after discharging the set volume - you will see sample in the barrel of the syringe that has been sucked back out of the needle. With a hot injection the needle gets hotter than the boiling point of the solvent, and so some of the sample that is in the needle boils and the vapour blows out of the end of the needle. If you now pull the plunger back you will see sample in the barrel, but less than was there with a room temperature injection. Most autosamplers can do very fast hot injections in which the needle hardly warms at all and which give injection volumes close to the set volume.

Peter

[BurntChem]

Thanks these explanations all make sense for liquid injection and now the math is clear to me.

However, I'm also wondering how to calculated on column injection for headspace analysis? I can provide a set of conditions but I'm wondering if there is any general equation I can use as a guide.

Thanks a lot.

[Peter Apps]

For headspace it is much more tricky. If you know the partion behaviour of the analyte at the temperatures and pressures of the headspace equilibrium you can calculate the quantity per unit volume of the headspace. Typically the sampler would then dilute that with a pressurization gas, bleed the excess pressure through a loop at a higher temperature than the sample, and then transfer the loop contents through an even hotter transfer line to an even hotter inlet where it will mix by diffusion (and possibly a bit of turbulence) with the carrier gas on its way to the column.

This is a long enough chain. with enough uncertainty in enough of its steps to render the calculation pointless.

With syringe-based headspace it is a bit simpler, but you still have to know the partition behaviour and the composition of the sample. And if you know the composition of the sample, why are you doing an analysis at all ?

Peter

[BurntChem]

Peter,

Thanks for your replies. Basically I am new to headspace and am trying to figure out the best way to go about doing the math for quantitative analysis. The goal is to quantify any remaining solvents in a few different types of products.

Any resources or advise is appreciated.

Thanks

[Peter Apps]

How you go about this is critically dependent on what matrix you have. In the simplest cases you calibrate by making up standards of known composition in a matrix that matches you samples. Most beverages are amenable to this approach.

If you do not have clean matrix into which to spike analytes for standards then you use the method of known additions, which is a hardy perennial on the forum.

If you have a solid matrix then you need to dissolve it in something that does not interfere with the analysis - many of the standard residual solvents for drugs methods use this approach.

Headspace is one of the more popular topics on here - try a search of the archives and there are plenty of resources on the web. Do some background reading to get an idea of what approach you need to take for whatever samples it is you have.

Peter