VOCs in soil, linearity problem

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

26 posts Page 2 of 2
Peter Apps wrote:
James_Ball wrote:
Peter Apps wrote:
You need to equilibrate after spiking, that's why I asked about it four days ago.

Peter


The longer an internal standard sits on soil the lower the recovery, unless it is sand. Most autosamplers add the internal with 5ml of water then preheat to 40c for about 0.5-1 minute then start the purge. Usually get good recoveries that way.


Which confirms equilibration as the likely cause of the problem; grabbing a snapshot works OK if everything is automated and the IS is the same amount every time, but if the addition is manual (and offline if I understand correctly) and the standard amount varies, as it must for linearity, then both repeatability and linearity are going to suffer.

The Atomx autosampler he is using should be adding IS and Surrogate when it adds the water prior to purging the sample, which should lead to linearity of the internal standard areas. There is a known problem with purge and trap that the higher the total concentration of the analytes in the standard the higher the recovery of the 1.4-Dichlorobenzene-d4 internal standard, it is something we are still trying to figure out and we had a long running thread on that problem a few years back.

Peter
The past is there to guide us into the future, not to dwell in.
pueblito wrote:
...just out of curiosity,

does a 0.25mm DB-624 or VRX column resolve Vinyl chloride from solvent? without the use of a cryogenic or peltier element.

Best regards.


I have no problem getting good separation and peak shapes for vinyl chloride and all of the first eluting analytes on the 0.25 column. When I ran the 0.53ID 105m x 3um film column 20 years ago, those compounds would be two minute wide humps without cryo on the column or trap. Trap packpressure helped a lot, but the narrow bore column helps the most.

If you look up chromatograms on Restek for the Rxi-624SilMS column you will see almost perfect peaks for the early eluting gasses.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Peter Apps wrote:
James_Ball wrote:

The longer an internal standard sits on soil the lower the recovery, unless it is sand. Most autosamplers add the internal with 5ml of water then preheat to 40c for about 0.5-1 minute then start the purge. Usually get good recoveries that way.


Which confirms equilibration as the likely cause of the problem; grabbing a snapshot works OK if everything is automated and the IS is the same amount every time, but if the addition is manual (and offline if I understand correctly) and the standard amount varies, as it must for linearity, then both repeatability and linearity are going to suffer.

The Atomx autosampler he is using should be adding IS and Surrogate when it adds the water prior to purging the sample, which should lead to linearity of the internal standard areas. There is a known problem with purge and trap that the higher the total concentration of the analytes in the standard the higher the recovery of the 1.4-Dichlorobenzene-d4 internal standard, it is something we are still trying to figure out and we had a long running thread on that problem a few years back.

Peter


I'm quite possibly not understanding how it's being done. What I get is that a variable amount in being manually spiked into the sample in the purge vial before it goes into the autosampler and has IS and surrogates added automatically. ????????????

Peter
Peter Apps
You may be adding too much methanol. There is a very low percentage that can be added before linearity goes haywire.
Peter Apps wrote:
James_Ball wrote:
Peter Apps wrote:

Which confirms equilibration as the likely cause of the problem; grabbing a snapshot works OK if everything is automated and the IS is the same amount every time, but if the addition is manual (and offline if I understand correctly) and the standard amount varies, as it must for linearity, then both repeatability and linearity are going to suffer.

The Atomx autosampler he is using should be adding IS and Surrogate when it adds the water prior to purging the sample, which should lead to linearity of the internal standard areas. There is a known problem with purge and trap that the higher the total concentration of the analytes in the standard the higher the recovery of the 1.4-Dichlorobenzene-d4 internal standard, it is something we are still trying to figure out and we had a long running thread on that problem a few years back.

Peter


I'm quite possibly not understanding how it's being done. What I get is that a variable amount in being manually spiked into the sample in the purge vial before it goes into the autosampler and has IS and surrogates added automatically. ????????????

Peter


When we prepare standards we fill a 40ml voa vial with 5g of blank sand, then add 5ml of DI water spiked with the target compounds. 5ppb in 5ml water is equal to 5ppb in 5g solid matrix. The autosampler then adds another 5ml water which has the internal standard and surrogate compounds added by a syringe or loop (normally 1-5ul of IS/Surr mix in methanol) to the voa vial and then the purge takes place in the vial using a two part needle, purge gas enters the same point where the water enters and it exits the outer needle at the top and back to the trap.

The amount of water in the vial is disregarded, it is the mass of analyte per mass of sample that is calculated as the calibration values.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Peter Apps wrote:
James_Ball wrote:


I'm quite possibly not understanding how it's being done. What I get is that a variable amount in being manually spiked into the sample in the purge vial before it goes into the autosampler and has IS and surrogates added automatically. ????????????

Peter


When we prepare standards we fill a 40ml voa vial with 5g of blank sand, then add 5ml of DI water spiked with the target compounds. 5ppb in 5ml water is equal to 5ppb in 5g solid matrix. The autosampler then adds another 5ml water which has the internal standard and surrogate compounds added by a syringe or loop (normally 1-5ul of IS/Surr mix in methanol) to the voa vial and then the purge takes place in the vial using a two part needle, purge gas enters the same point where the water enters and it exits the outer needle at the top and back to the trap.

The amount of water in the vial is disregarded, it is the mass of analyte per mass of sample that is calculated as the calibration values.


But pueblito is spiking soil, not sand, and you say soil gives an unstable (or slow) equilibrium. Hence the problem.
Peter
Peter Apps
Around here, soils are combinations of gravel, sand, sandy clays, clay. I get good spike recovery from those. But loamy soils or soils with roots and humus and all bets are off. Those types of soils eat the internal standards and surrogates. I end up extracting them with methanol and injecting up to 100uL of the methanol into the 5mL of water in my prep syringe along with ISS. This works well even with spent activated carbon samples.
Peter Apps wrote:
James_Ball wrote:
Peter Apps wrote:

I'm quite possibly not understanding how it's being done. What I get is that a variable amount in being manually spiked into the sample in the purge vial before it goes into the autosampler and has IS and surrogates added automatically. ????????????

Peter


When we prepare standards we fill a 40ml voa vial with 5g of blank sand, then add 5ml of DI water spiked with the target compounds. 5ppb in 5ml water is equal to 5ppb in 5g solid matrix. The autosampler then adds another 5ml water which has the internal standard and surrogate compounds added by a syringe or loop (normally 1-5ul of IS/Surr mix in methanol) to the voa vial and then the purge takes place in the vial using a two part needle, purge gas enters the same point where the water enters and it exits the outer needle at the top and back to the trap.

The amount of water in the vial is disregarded, it is the mass of analyte per mass of sample that is calculated as the calibration values.


But pueblito is spiking soil, not sand, and you say soil gives an unstable (or slow) equilibrium. Hence the problem.
Peter


Per the method sand is used as an analog for any solid matrix. It does have the lowest retention of the analytes and therefore does not affect the recovery much at all if any. This way you see the actual performance of the purge efficiency. When you introduce another matrix like a loamy soil, you will see a reduction in internal standard recovery and can judge the amount of actual recovery from that matrix compared to one that retains very little of the analytes. When we receive samples from a site being investigated they could range all the way from sand to clay to loam and even a waste sludge, it would be nearly impossible to calibrate using each type of solid matrix, therefore we use the one that least interferes with the recovery, then use the recovery of the IS and Surrogates to monitor the efficiency of the purge extraction.
The past is there to guide us into the future, not to dwell in.
That all makes perfect analytical sense, but it does not address pueblito's problem of non-linearity after manual spiking of soil.

Peter
Peter Apps
I posted it to show that he is preparing standards similar to what others are doing, so the problem shouldn't be in the prep.

He is preparing it in DI water, which works just as well as preparing it in sand. Analytes such at Toluene given in his example should not need any equilibration time since it is relatively insoluble in water. The purging at 40ml/min Helium for 10 minutes will mix the water well and remove the analyte, usually is less than 5 minutes for toluene, while something like Acetone will take longer because it is more soluble. You will never get 100% transfer of all analytes, but if you treat samples and standards the same you get similar enough analyte transfer, just as doing static headspace or SPME.

The calibration works well at low concentrations and when done in the water sparge position. Either the more concentrated soil calibration standards are overloading the column due to not splitting enough before the column or there could be a problem in the soil sparge position. The column flow into the MS also seems to be troublesome. It could be a combination of both that is causing the non linearity.
The past is there to guide us into the future, not to dwell in.
Thank you all for your support.
There is much to learn and put in practice here.
But I have bad news, the ATOMX is just not working. The service engenieer is trying to fix the problem.

Best regards.
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