tailing peaks using an ECD

[mjh32586]

I am having problems with tailing peaks for 2 analytes in particular using method 552.2. Does anyone have suggestions on what is causing them? I'm not sure how to attach a picture of the chromatogram. My oven program is 35 hold for 10 min, 10 degrees per min to 120 then 20 degrees per min to 250.

[AICMM]

mjh32586,

Which compounds? Early or late in the chromatogram or somewhere in between? More info would certainly be helpful. Could simply be too warm to re-focus for example but hard to say without more info.

Posting instructions are at the beginning of each section of the forum.

Best regards,

AICMM

[mjh32586]

They are BCAA and DBAA which are late in the chromatogram. my oven program is 35c for 10 min, ramp 10c up to 120 then ramp 20c to 250 hold 5. What other info would be helpful?

[Johnny Rod]

Which column, is this a new problem, injection mode, etc.?

[mjh32586]

I am using a CL-Pesticides 1 and 2

30m x 0.32mm ID x 0.32um df = CL-pesticides

30m x 0.32mm ID x 0.25 um df =CL-Pesticides 2

This has been a continual problem. I am currently trying to develop a method because we are seeking to obtain accreditation to run this method. Injection temp is 220C using a splitless inj with 0.5 min hold time. Split ratio of 10. Total flow is 30mL/min and column flow is 2.4 mL/min (using He as carrier) (make up is N2 at 30ml/min), 25.0 cm/sec using linear velocity, 3.0 mL/min septum purge, 61.7kPa pressure.

What I'm wondering is if my flows are not correct because I have 2 columns installed using a y-connector. It may also be my oven program that is the problem??

[AICMM]

It is entirely possible that you do not have enough flow. Your peaks will get very broad and elute later than you would like. This will especially be the case if you are using a 5890 or a 6890 with constant pressure.

Is this calculated linear velocity or measured?

On a 0.32 it is a good rule of thumb that you need about 2-5 mL/min and in your case you need that per column. Try running the flow rate much higher and see what your peak shape does.

In addition, 220 is a little low for this type of analysis (might be good for solvents or gases but not really liquid injection.) I would suggest 250 or perhaps even 280 depending on if you see degradation or not.

Best regards,

AICMM

[mjh32586]

I am using a Shimadzu GC which only has one line to put column dimensions into. I am told by them that I have to fool the program by saying the installed column is actually 0.53 ID (which is closer to 2-0.32 ID columns) and then also change the length until I see what I want. So I currently have the column as a 0.53 ID and 60m. Since I am using helium as my carrier gas, I set the linear velocity to 20 cm/sec but then my column flow goes down to only 2.8 mL/min ( I am told this is total carrier flow between 2 columns) and I really need 4-10 mL/min. The only way I can get the proper carrier flow is to either say my column is like 240m or to change the linear velocity to 57cm/sec. I guess I'm just not sure what I want. When I keep the column length at only 60m and have the linear velocity at 20 cm/sec my head pressure is only like 29kPa. Also, what is a common split ratio when using a splitless method like this??

I'm not sure if this calculated or measured linear velocity but I would imagine calculated since I am "fooling" the instrument with different dimensions.

Below is an 8uL injection of air using the 57cm/sec, 10 mL/min total carrier flow telling the instrument it is 0.53, 60m