Chromatography Forum

I'm looking at a chromatogram that was sent to me by someone at one of our company's other facilities. We're both running 8260 using a 6890 and 5975 (or very similar). The main difference is that I have a purge & trap and they have some kind of headspace autosampler - which I don't know anything about.

Their signal to noise ratio is way lower than mine. I don't know how much of what looks weird to me is because headspace transfers less analyte mass to the GC (but more water, maybe? is that the mound at the front?), and how much is because... they've got problems.

Is this just how it is when you're doing headspace analysis, and I'm a little bit spoiled?

https://drive.google.com/file/d/1UDNCgu ... sp=sharing

You will definitely get more response with P&T than headspace. In a sense with P&T you are injecting the entire 5ml sample that you are sparging, with headspace you only get a ml or two of the air above it and what analytes will out gas into the air volume.

You can't really purge out all the air above the sample without loosing some targets so you will get a lot of the CO2 from the air, which you won't get in P&T unless you are using a charcoal adsorbent normally so that could be your hump in the chromatogram. Also water and any methanol from standards can give a hump depending on the column used.

Thanks James. A bit of follow-up:

https://drive.google.com/file/d/1evEhNF ... sp=sharing

t8 was water spiked with surrogate and internal for 8260.
t9 and t10 were both just water.

The big blue plateau at the beginning of t10 seems to happen randomly every few samples, and seems like it can cause other RTs to shift to the right a bit. And this is with a 5 minute solvent delay, which seems like a lot. M/Z 44 is prominent in the mound, but also in the background noise throughout the entire chromatogram. So I guess this is CO2, like you mentioned? Is it normal for it to vary a lot from sample to sample?

That is CO2. You'll see it be quite variable. It just doesn't condense well at the head of the column after injection so it's just a blob. It's going to be very depended on the vapor pressure of the CO2 in the sample. That's probably why you see variability.

A quick look at 8260 (I'm not an environmental-sample analysis guy) it looks like it's for solids - like soils? Is that what you're analyzing ultimately?

CO2 adsorbs to different surfaces/materials quite differently. When you hit it with water, you'll liberate it from those surfaces to various degrees. If you're analyzing soils, this could explain why the CO2 is different from sample to sample.

That is CO2. You'll see it be quite variable. It just doesn't condense well at the head of the column after injection so it's just a blob. It's going to be very depended on the vapor pressure of the CO2 in the sample. That's probably why you see variability.

A quick look at 8260 (I'm not an environmental-sample analysis guy) it looks like it's for solids - like soils? Is that what you're analyzing ultimately?

CO2 adsorbs to different surfaces/materials quite differently. When you hit it with water, you'll liberate it from those surfaces to various degrees. If you're analyzing soils, this could explain why the CO2 is different from sample to sample.

8260 is a method in SW824 and even though the EPA section is Solid Waste the method is for multi matrix samples (water, soil, solid, sludge, hazardous waste, ect.).

CO2 will definitely be in everything and at varying levels. The only way to ignore it would be to start scanning at 46m/z if using 45m/z you will have a very noisy baseline since it will be hit or miss if the MS detects it at 44 or 45m/z.

CO2 may be a little less evident in Purge and Trap if you are using a dry purge time since it can bleed through the trap unless you are running sub ambient trap temperatures.