Sulfur dioxide by SCD 355 so noisy

[thohry]

Hi all
I just start to analyze SO2 in beer by GC-Headsapce and 355 SCD . In some ref docs, they can get down to 3 ppm with nice peak, but in my case, with 10ppm, the peak is almost in jungle for the baseline is so noisy.
I am not sure if the SCD 's noise is normal or I have done something wrong to the detector (brand new).
Some parameters of our experiment:
Column: CP Sil 5 ; 60x0.32x0.12,
Flow 2ml/min (He)
Oven= 50 -> 110 in 15 minutes
Bake temp = 60 oC
Bake time = 20 min

Hope to receive help from you
Thanks and regards

[muGC]

thohry,

I'm not sure about headspace, but for regular gas sampling valve analysis I can easily get below 0.1 ppm SO2 in gaseous samples. There are papers and app notes out there that show 10-25 ppb detection limits for SO2 with an SCD. I know the 3 ppm app note you are referencing and it looks to me like their LOD is at least below 0.5 ppm. The 3 ppm peak height is quite tall. (https://www.chem.agilent.com/Library/te ... 6780EN.pdf)

What is your baseline noise in uV? In my experience, 15-40 uV PtoP noise is acceptable with ATT1 (attenuation of 1). If you are at ATT100, then the PtoP should be close to 0.1-0.5. SCD noise commonly comes from contamination in your system gases, make sure you have the appropriate filters for all the gases and have a maintenance schedule.

What is your SCD detector box pressure? If you're above 20 torr, then this may be contributing to the noise as well and you have a significant vacuum leak.

[thohry]

Thanks muGC.
I have check the vacuum, it's about 6 psi. The PtoP is about 40, but the baseline is as high as 1700 count (15uV) after 24hrs. Do you think that's too high ?

Thanks again.

[muGC]

I think you looked at the air regulator to get the 6 psi number. I mean the button on the detector box that says "Pressure" and it is units of torr. If it is 6 torr, then you're in pretty good shape.

The baseline is a little too high, this could from system gas contamination OR someone pushed the "Output Offset" button up a few too many times. Look at the signal on the detector box, ideally I like the signal to be reading around 0.4 mV with the ozone on and 0.1 with the ozone off. I'm leaning towards your problem being the system gas. Install some sulfur filters!

Just to clarify, your SCD is not "noisy" right now, the background/baseline is just high. Noisy means the PtoP is all over the place.

[Peter Apps]

Keep in mind that beer has dozens of sulphur compounds in it, and they might be eluting close to your SO2.

Peter

[muGC]

At SO2's boiling point there are only a handful of sulfur compounds eluting near SO2 on a CP Sil 5 column: SF6, H2S, COS, SO2, Methyl Mercaptan. Based on thohry's parameters, he might have a co-elution problem but it is likely they will all be baseline resolved.

He'll have to work on his method development after solving the high baseline