cdc cyanide method

[montucky1]

I've inherited the labs cyanide method after the previous tech failed the last 2 PT's
I'm running it on an Agilent 7890/5975 with a gerstel mps and cryo...
tore the whole instrument apart and cleaned it first... very dirty source and column was discolored in a few spots... installed and conditioned a new PLOT-Q to manufacturers specs...
ran a couple mid-level standards with istd added... chromatography looked good...
ran a validation sleeve and noticed that the CN peak moved about a bit (rt range of about a minute). is this normal? i've read the binder cover to cover and haven't noticed anything mentioned about this...
My QC results were way out of the 3sigma range (high). this was my first sleeve ever of the cyanide method, but the sample prep seems pretty straight forward.
also noticed that the agitator heat and syringe heat are set at 80C checked with a thermometer and that read 60C... again anybody notice this?

any help is greatly appreiciated...

Casey

[AICMM]

montucky1,

Do you mind posting what CDC method you are using? Good starting point to look at your problem.

Right off the bat, I would be suspicious of a PLOT with a 5975....

Best regards,

AICMM

[montucky1]

i've read that the PLOT columns and the new msd are not the best match, alas this is what i've got...
the method is the CDC cyanide method...
Gerstel MPS Sample prep, 1.0 mL headspace sample inject into a 150 C inlet, splitless, cryo focusing -10C, 130C oven start temp hold for 5min, ramp 99/min to 250C hold for ~2min. solvent delay of 3.0 min.

I'm thinking the aggressive ramp is to purge the column of any crud that is picked up in the headspace sampling...my peak comes out at 4.5 min... the cryo trap starts heating at 1.5 min. after inj...

sorry cant find a digital copy of method to post.

thank you for your help.

casey

[mty]

Hi Montucky1,

We run that method, and I can't say that we've seen that kind of retention time shift. Have you watched the cryo over the course of the run? Maybe the temp isn't holding? I'm just guessing. Are you using constant flow mode, at 1 ml/min?
We changed brands of PLOT columns--we had some very bad experiences with some of our originals.
As for your QCs, I know that the older the QC samples are, the weirder the results... And has your lab updated the QC ranges?

We can chat if you like-let me know. I'm better if I'm in front of my instrument than in my office.
Michele

[montucky1]

i figured it out!!!!
found out that the person who installed the MPS didn't plumb the rail with N2, no no no, instead he used the He... and then set the flow at less than 0.5 psi or bar or whatever pressure settings there are on that knob. tripled the pressure and changed the syringe and guess what? one nice peak sprang from the baseline!!!
the previous user of this instrument was trying to compensate for the inefficient purge by cranking up the syringe and agitator temp...
i hope it works

[Bigbear]

Been a while since I've done that method.
Our rail for that system is set at 0.5 bar using He. It's my understanding that this gas is only used to purge the syringe between injections.
We do notice the RT's shifting sometimes for the first few injections, we think it's due to water.
I remember that the 2.5ml syringe can get crudded up with blood. Check that the acid's are being added correctly, especially the phosporic as it is quite viscous. We had to slow down the draw rate.
Check to see that the Gerstel prep is correct, this will cause more problems than the GC/MS method.

One gotcha that bit us when we ran on a 6890/5973 ( E.01.01) was when changing methods/ columns the column flow rates could be set incorrect. Especially when swaping from the VOC method. PLOT columns are VERY sensitive to over pressure. Seems as if we loaded the CN method before column changes.