Glycerin / Dipropylene glycol Co-elution

[nz402]

We have recently had a new GC, a Thermo Trace 1310 with FID, installed in the lab, and I am doing the preliminary work on a method that will be used for quantifying polyols.

The analytes of interest will be in a solution of methanol, of which 75 uL is added with 150 uL of pyridine to 1 mL of BSTFA and allowed to react for 30 min @ 65C before injection. The issue I am experiencing is that 2 of our analytes, glycerin and dipropylene glycol, are co-eluting.

Column: TG-5MS 5% Phenyl, 0.32mm x 1um x 30m

Carrier gas: H2
Makeup gas: N2

Inlet temp: 220C
Split flow: 50 ml/min
Split ratio: 25
Constant flow at 2 ml/min

Oven: 40C- 250C at 25C/min, hold at 250C for 4 mins

Detector temp: 250C

Do I have any method options that will help with separating these components, or is my only option some change to the column? Thanks for any help in advance.

[Peter Apps]

Your temperature programming rate is much too fast for the length of column. Reduce it to 5C/min. If analysis time is critical then you are better off using a shorter, thinner film column than using temperature to chase the compounds down the column.

Peter

[nz402]

I'll give that a try, thank you.

[Consumer Products Guy]

I'm actually a published author on analysis of such materials by GC, many moons ago.

Since your samples are in methanol, I was wondering if you ever tried a "624" column like Restek Rtx-624, as that gives a decent peak shape for underivatized glycerin. I will say that I have NOT ever tried "624" for a mix of DPG and glycerin, but have used it for EG and DEG in glycerin samples, like in the USP Monograph for glycerin.

OK, back to derivatized DPG and glycerin. Yes, the methanol will also derivatize, so you are using a ton of BSTFA, and need to. I doubt that you'd need to add heat to make the reaction go, just mix, shake, inject.

Derivatized DPG will give at least 3 peaks, we typically quantitate by summation or grouping. We have separated DPG and glycerin on a similar DB-5 type capillary, like 30m x 0.32mm, head pressure 10 psi or less, starting at 80C for 2 minutes, holding for 2 minutes, rate of 10 C/min to 160 C , then ramping to clean off the column of other matrix materials. For us, the DPG finished eluting by 7.5 minutes, the the derivatized glycerin eluted. We don't often find this mixture in a product, so we haven't really explored the separation further.

Yesterday I assayed products containg DEG (diethylene glycol), MEA, propylene glycol, TEA, but glycerin was absent (DEG, MEA, and glycerin eluted one right after the other, same column as above, slightly different temperature ramp.

[nz402]

Thanks for the info. The reason we went with the column we did, and therefore the derivatization method, is that we expect to have a number of surfactants in our samples as well. We aren't interested in quantifying these materials, however they will likely be extracted along with our analytes of interest. Therefore, we went with the TG-5MS as we understand it will be more robust when compared to a polar phase column when it comes to surfactants being present in the injected mixture.

Regarding the BSTFA reaction, that is my understanding as well, I'm currently using 30 mins just to be safe than sorry.

I'm currently running the method Peter suggested, still waiting for the analytes to elute. I'll be sure to try your method and report back what I find.

[Consumer Products Guy]

The analytes of interest will be in a solution of methanol.

The reason we went with the column we did, and therefore the derivatization method, is that we expect to have a number of surfactants in our samples as well. We aren't interested in quantifying these materials, however they will likely be extracted along with our analytes of interest.

Question: it sounds like you are using methanol to extract analytes from your product/sample. Is methanol necessary for this? I like using dimethylformamide or pyridine as solvents for these products. Then you could use far less BSTFA.

[nz402]

That may be a possibility. The plan is to use methanol in a Soxhlet extraction because it's cheap and we already have an existing waste stream.

[Consumer Products Guy]

That may be a possibility. The plan is to use methanol in a Soxhlet extraction because it's cheap and we already have an existing waste stream.

Wow, haven't used Soxhlet in decades here. Can you post what type of samples you are extracting? We typically have under 5 minutes extraction time, at room temperatures.

[nz402]

Soxhlet might not be the most accurate description, but in any case, we'll be able to achieve our extractions in 20 mins.

Using Peter's suggested method, I achieved better separation with the DPG and glycerin. The 10C/min method didn't show the same results.

Just to see how far I could push the separation, I went down to 1C/min and achieved the separation seen below:



All 5 minor peaks of DPG are now distinguishable from glycerin. However, they do not elute until the 70 minute mark. Any other suggestions to possibly increase speed somewhat and maintain / increase separation? We are not necessarily placing throughput as our first goal here, but rather placing more importance on accuracy and reproducibility.

[Peter Apps]

Reduce the column film thickness as much as you can - you might have to increase the split ratio to control the overloading od the glycerol -TFA peak.

Peter