Chromatography Forum

I'm trying to get response factors with a TCD. I'm running an Agilent 7890A. The gas sample comes from an actuated 6 way valve with a 0.25mL sample loop.

Here's my question:

I load the 0.25mL sample loop with the pure gas (N2, CO2, NH3) at atmospheric pressure. I get the areas for each gas but vary the split (10:1, 8:1, 4:1, 1:1).

The problem is that the area for a 1:1 split (i.e. not split) is not 4 times larger than the 4:1 or 10 times larger than 10:1. And the 8:1 isn't half the area of 4:1, etc.

Is this not how the split stream function works?

A 1:1 split does not mean that there is no split. It means that 50% of the sample goes onto the column and 50% through the split vent. With a 0.25ml loop, you have injected 125ul of gas onto the column. The 4:1 split means that 1 part goes onto the column and 4 parts through the split vent, so 20% or 50ul of gas onto the column. This should mean that the area for 1:1 split is 2.5 times higher than that for 4:1 split. Is this the case?

Gasman

That sounds about right. I'll check when I can and edit this post.

Thanks so much

AVchemE,

Long ago a friend of mine and I did the same experiment on a 6890 system and found that there is no linearity with the split. We did something like 1:10, 1:20, etc and did not get linear changes in response. We then had to go to a sample split upstream of the injection valve (it was all gas phase work) and got excellent linearity at a fixed split.

Best regards,

AICMM

I got some consistency. I did try turning off the split altogether. This was the biggest discrepancy (see below).

Those who are interested, here are the numbers that I got for N2

1:1 8406
2:1 5512
3:1 4227
4:1 3456
5:1 2790
6:1 2224
8:1 1778

I multiplied each area by the amount it was split (i.e. 2, 3, 4, 5, 6, 7, and 9). The average value was about 16500.

The splitless area was 40,000 (it should have been close to 1650 if I reasoned everything correctly). Why is this so different?