Inconsistent results with standard addition

Discussions about GC and other "gas phase" separation techniques.

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Problem: Using standard addition method to analyze the 5 analytes in poly(2-ethyl-2-oxazoline). 200 mg of polymer in 18 vials. 3 vials dissolved with 200 uL N,N-dimethylacetamide. 3 vials with 200 uL 40 ppm standard. 3 vials with 200 uL 60 ppm standard. 3 vials with 200 uL 80 ppm standard. 3 vials with 200 uL 100 ppm standard. 3 vials with 200 uL 120 ppm standard. Analyze using headspace. Methanol, Ethyl Ether, and Toluene give me nice standard curves, I can then solve for the x-intercept and therefore the concentration of those analytes. However, ethylenediamine and 2-ethyl-2-oxazoline's curves are all over the place (R2 values around .25, terrible). I listed as much information as I could think of below. Thank you in advance for any suggestions or comments!

Analytes of interest: Methanol, Ethyl Ether, Ethylenediamine, Toluene, 2-Ethyl-2-Oxazoline (monomer)
Solvent: N,N-Dimethylacetamide
Method: Headspace Chromatography

Shimadzu GC-2010 Plus Gas Chromatograph

Flame Ionization Detector
Model: FID-2010

Split/Splitless Injector
Model: SPL-2010

Shimadzu AOC-5000 Plus Autosampler
Serial #: 270884 & 236074

10 or 20 mL Vial Tray – 32 Vial Capacity
Equipment #: 220-91300-02, Shimadzu

Agitator/Incubator
Equipment #: MH 01-00B, CTC Analytics AG Switzerland
Serial #: 13716

Syringe Kit
Equipment #: MSH 02-00B & MSK 02-00A

CTC Analytics AG GC Power Supply
Part #: MN 01-02, PAL

Restek Rtx-Volatile Amine Column
60 meter – Length
.32 mm – Inner Diameter
0 µm – df (film thickness)
290ºC – Max Prog. Temp., 270ºC – Min. Bleed Temp.
Cat #: 18078

Computer with GCSolution Software Version 2.41.00

Septa - Supelco, Sigma-Aldrich/20633
Glass Liner - Shimadzu/220-90784-00

Equipment and Method Settings
Gas Chromatograph Settings
Carrier Pressure: 23.7 psi
Carrier Gas: Helium
Oven Temperature: 40.0°C initial temperature, hold for 4 minutes, ramp at 5.0°C/minute to 200.0°C,hold for 8 minutes
Injector Temperature: 180.0°C
Injection Mode: Split
Flow Control Mode: Linear Velocity
Total Flow: 20.6 mL/min
Column Flow: 3.52 mL/min
Linear Velocity: 40.0 cm/sec
Purge Flow: 3.0 mL/min
Split Ratio: 4.0
Detector: Flame-Ion Detector (FID)
Hydrogen Flow: 40.0 mL/min
Air Flow: 400.0 mL/min
Makeup Gas Flow: 30.0 mL/min
Detector Temperature: 250.0°C
Sampling Rate (FID): 40 msec
Analysis Time: 44.00 minutes

Autosampler Settings
Sample Volume: 1.0 mL
Incubation Temp: 140.0°C
Incubation Time: 6.5 minutes
Agitation Speed: 250 rpm
Agitation On Time: 10s
Agitation Off Time: 2s
Syringe Temperature: 145.0°C
Fill Speed: 1ml/s
Fill Strokes: 3
Pullup Delay: 5.0 s
Inject to: GC Inj1
Inject Speed: 400µl/s
Pre Injection Delay: 100 ms
Post Injection Delay: 100 ms
Syringe Flushing: 30 minutes
Flush Gas: Nitrogen
GC Runtime: 50 minutes

Quantitative & Identification Parameters
Quantitative Method: Standard Addition
Calculated by: Area
Calibration Curve: Linear
Weight Regression: None
Calibration Level#: 6
Concentrations = 0, 40, 60, 80, 100, 120
Through Origin: Not through
Unit: ppm
Window/Band: Window
Default Band Time: ---
Peak Select: All Peaks
Correction RT: No Change
Window: 5%
Identification Method: Absolute
Grouping: None

Compound Approximate Retention Time (RT)
Methanol RT: 5.080 min
Diethyl Ether RT: 9.600 min
Ethylenediamine RT: 16.280 min
Toluene RT: 22.650 min
2-Ethyl-2-Oxazoline RT: 24.430 min
N,N-Dimethylacetamide RT: 26.100 min
A couple of thoughts (without knowing the chemistry of the polymer):

Headspace works on partitioning between stuff in the vial and the vapor phase. And, like dissolves like.

You are below the boiling point of both ethylenediamine and 2-ethyl-2-oxazoline. With 2-ethyl-2-oxazoline, it is just below the boiling point, but chemical similarity to the polymer might be expected, favoring partitioning into the material in the vial rather than the vapor phase. Ethylenediamine is higher boiling.

The presence of catalysts for polymerization can have some effect, particularly if the catalysts are not evenly distributed in the polymer used in making the samples. The acid catalyst has the potential of forming a salt with ethylenediamine - and making it less volatile. And, depending on heating time, temperature, and catalyst, there may be possibility of a bit of depolymerization of the polymer, which would add to the monomer in the vapor.
Thank you for your response!

EDA BP = 116 C, Eox BP = 128.4 C The autosampler incubates at 140C, which is above their boiling points. So I am a little confused when you say I am below. Do you mean the Column initial temp is below?

Your thought on catalysts, polymerization, and depolymerization are very valid concerns. The only way I can think to overcome this is to drop the incubation temperature, which would then place the BP of those analytes above the incubation temp. Any thoughts on dropping my incubation temperature?
I am under the imperssion that in static headspace you do not want to "boil" any of your compounds.
Could it be differing pressures in the vials?
You want to keep the incubation temp about 10 degrees below your solvent BP so you do not generate too much pressure in the vial. DMA boils at 165C, so you should be OK. Have you tried running the same analysis but without the polymer? That would eliminate that as a suspect. Also, is it possible pH is playing a role here. I would think you would want the samples slightly basic to drive the amines to their unprotonated form and thus making them more volatile.
I have ran this without polymer and my standard curves are linear and reproducible. The analytes are more than likely interacting with the polymer. How do I overcome this?

Also, should I try adding some NaOH to the sample to make the sample basic? Thank you!
On the bright side it looks like you know what is causing the problem, the polymer. Unfortunately, I don't know how it is causing the problem. Here are some things I would try.... but I'm just guessing
- If the polymer isn't soluble in DMA (but I'm guessing it is), try extracting the polymer in DMA in a flask. Then transfer an aliquot of the extract to your headspace vial.
- Try a lower incubation temperature. If there is a reaction going on, maybe a lower temp will reduce it
- Try another solvent, THF?
- Making the sample basic may help. I would prep the polymer in DMA and see how much 0.1N NaOH it takes to raise the pH to around 8-9. Whatever that amount is, I would add it to all the vials. I don't know how the polymer will react to the NaOH. Could it break down yielding more of the monomer?
If you need a new PAL power supply check out U.S. Electric here.

They have the following parts as well:

MN01-00
MN01-00A
MN01-00C
MN01-01
MN01-02
MN03-00
MN03-01
MN03-02
LEAP Part Number: PAL.PS
NN-02-01A
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