VOC analysis by GC/ECD Headspace, non linear...need help

[vperica]

Hi!
I have to analyse VOC by static headspace GC/ECD, with Combi Pal autosampler. Im using Column DB-5, split 1:5 ,standards and samples are in 20mL headspace vials with 10mL water. Matrix is drinking water.Standard is Supelco 551A (11 component)
Problem is non linear calibration curve( slope is decreasing instead of increasing according to increasing concentration), but bromoform is OK(r- 0,999) Range 1- 40 ppb.
I did try with 40,50,60,80 C incubation temp and with and without shaking, but problem still.

Does somebody know right parametars for this instruments, temp, agitator speed, time incubation, Im looking whole www but I dont succeed.

thanks a lot,
Perica

[Peter Apps]

Hi Perica

Please could you post the calibration curves, and a chromatogram (how to do it is on a sticky at the top of the LC page), and tell us which GC you are using, type of inlet liner, all the conditions ofr the Combipal (syringe temp, syringe pumps, injection speeds, etc etc) and the gas flow and temperature settings onthe GC.

Thanks, Peter

[vperica]

Thanks for your answering me , I`m sending whole data directly from my software, other parameters are:

GC made VARIAN 3800 with ECD, carier Nitrogen 1ml/min,Column J&W DB-5 30x0.32x0.5 inj 1079 Temp 220°C,ECD 280°C ,inlet liner splitless 2mm,temp prog 35°C-5min, 110°C-5°C/min, 180°C-15°C/min flow, split 1:5
Headspace instruments is Combi PAL CTC Analytics, doesn`t purge on trap only static headspace,incubation temp 70°C,temp siringe 80°C,inj volume 100μL,without shaken,fill speed 100μL/s,inject speed100μL/s,incubation time 30min, Pullup delay 0,pre inj delay 300ms,post inj delay 500ms, siringe flush 2min.vials 20ml
Methanol and Ethanol determination in alcoholic beverages was performed under similar conditions on this instruments and results were excelent.(incubation time 10min,500rpm, FID).
If you need more information, please let me know.
Thank in advanced
Perica

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[Peter Apps]

Hi Perica

Thank you for a wonderfully detailed response. All of the settings look reasonable - the only thing that you could add is to pump the sample in and out of the syringe a few times while taking the samples from the vial. Check also that the column is not inserted too far into the inlet, and you can try reducing the depth to which the needle goes into the inlet on injection - if the needle tip and column entrance are too close the sample and carrier gas do not mix properly before they get to the column.

From the look of the calibrations the problem is poor repeatability, not necessarily poor linearity per se. What is the relative standard deviation of peak areas for five replicate injections at one concentration ?

Peter

[vperica]

Hi Peter,

I read peruse your response, and check depth siringe penetration the PAL. Depth is 47mm (my injector is 1079 and point of injection in inlet default should be 50mm). What are your proposal for optimum depth? For properly column lenght determination, column was inserted on inlet. I have column scale cuter for FID, ECD and TSD injector, separately for each one.
About Relative standard deviation peak area I have next data:
Carbontetrachloride (conc 0,1ppb) six replication SD 9,8%,
Chloroform (conc 10ppb) six replication SD 3,9%,
Tetrachloroetilene (conc 1ppb) six replication SD 6,1%,
Bromochlormethane (conc 2ppb) six replication SD 3,5%,
I will try to pump sample few times before inject into column. I think that is important parametar, but I don't know what is optimum value.
What do you think about temperature incubation? Maybe lower temp is better, or without shaking? Maybe I had not reached the state of equilibrium beetwen liquid and the gas phase? Tehnical CTC PAL system manual is not usefull, only "Incubation temperature,Incubation time, syringe temp depends on customer application....bla bla"


Thanks in advance

Perica

[Peter Apps]

Hello Perica

The problem is poor repeatability rather than poor linearity.

Some more questions:

Were the rsd data generated with the conditions in your post of 1 December, or had you already changed the injection speed ?.

What is the volume of your water sample, and how do you measure it ?.

If the problem is due to incomplete equilibrium, shaking or longer incubation times will improve the rsds.

I would use a minimum of 3 syringe pumps, more would be better but the problem is that the oven lid is pushed back while samples are taken, and this causes loss of heat from the samples.

I notice a weak tendency for the rsds to be worse for lower concentrations, and on the chromatograms that you posted some of the peaks are very small, and so some of the poor repeatability might be due to inconsistent integration of small peaks. Increasing the incubation temperature will increase the peak area, but the loss of heat from samples during sampling will get worse.

All of the analytes are only marginally soluble in water and so it is very easy to lose them from the headspace when handling standards and samples - this could cause poor repeatability also. Were the rsd data generated by 5 injections from one vial, or from five different vials ?

Regards

Peter

[vperica]

Hello Peter,


Thanks for your patience, all send data were generated same time ( I think august 2006).Volume of water was mesured by pipete ( 10ml), than put standard VOC into vial by gas tight siringe (100μL, 250μL, 500 μL). I use same way preparation and handling, with standard and sample, under same conditions.
All 5 injections was generated from 5 different vials, I did try multiple sampling from one vial (3cycle) and all peak area it is ok, but over that, peak area were decrease.
Just one more thing, when I used temperature incubation 40 ~ 50max., linearity were better then at 70 ~ 80°C, but at that time loosing sensitivity.
Do you know is there anywhere some kind of procedure, link, paper where I can find exactly value of parametars ( temp, rpm, time) for VOC determination, or I have to do parameter optimization by my own.