Chromatography Forum

Hi All,

I'm fairly new to GC-MS and i have been having issues with meeting our QC acceptance criteria for CCV and LCS (85-115%). I use the Purge and Trap method.

Here's how I prepare my CCV and LCS standards: I spike all stock solutions to achieve a final concentration of 25 mg/L. My final volume is 1 mL. I then spike 10 uL of the 25 mg/L solution to 5 mL water. The concentration I expect is 50 ug/L. The concentrations i have been getting are between 30-150 ug/L.

I've just purchased a new standard and it's still spiking high

My Internal Standard and Surrogates pass.


I've checked all the volumes of stock solutions to use and they were all calculated right.

Is this a technique problem? Should I NOT shake my standard vials?
Should i replace my syringes?

Please helpppp!

Thanks

A bit more information would be helpful.
System(s) column,trap, flows etc.

Most volatile methods I'm aware of the CCV range is 70-130%.

Hi All,

I'm fairly new to GC-MS and i have been having issues with meeting our QC acceptance criteria for CCV and LCS (85-115%). I use the Purge and Trap method.

Here's how I prepare my CCV and LCS standards: I spike all stock solutions to achieve a final concentration of 25 mg/L. My final volume is 1 mL. I then spike 10 uL of the 25 mg/L solution to 5 mL water. The concentration I expect is 50 ug/L. The concentrations i have been getting are between 30-150 ug/L.

I've just purchased a new standard and it's still spiking high

My Internal Standard and Surrogates pass.


I've checked all the volumes of stock solutions to use and they were all calculated right.

Is this a technique problem? Should I NOT shake my standard vials?
Should i replace my syringes?

Please helpppp!

Thanks

Volatiles can be lost to headspace very easily. The rule of thumb for diluting into a volumetric would be to spike then invert without shaking three times only. This works great for us. I also do not shake my stock standards before using them. They are not like salts that can fall out of solution or high boiling compounds that can freeze out in a pesticide mixture.

I use only small reaction vials 1-5ml volume for storing the working standard which is equipped with a MiniInert cap so that standard can be withdrawn without the vial being open to atmosphere. Opening a standard to atomsphere will cause it to go bad quickly for volatiles. I am guessing that the low recoveries are on the first compounds to elute and the high recoveries are on the later eluting compounds? If so, you are losing the lights to outgassing from the standard and the heavies are becoming concentrated as the solvent evaporates.

Another thing I do a little differently from the suggested technique in the methods but does make my standards last longer is I do not let them come to room temperature before withdrawing my spiking aliquot. This procedure is normally placed into the methods to prevent over spiking because the methanol has contracted while in the freezer and throws off the accuracy of the measured aliquot. But if you will pull the aliquot of methanolic standard into a gastight syringe and allow a few seconds before measuring the final volume, the methanol will expand due to the head of the syringe barrel and you will be actually measuring the methanol at room temperature. I have done this for years and it works great, and your standards will not go bad as quickly, I normally throw out most of my standard at the one month expiration instead of having to remake it each week.

Welcome to the world of purge and trap, the problems only get stranger the longer you do it

Volatiles can be lost to headspace very easily. The rule of thumb for diluting into a volumetric would be to spike then invert without shaking three times only. This works great for us. I also do not shake my stock standards before using them. They are not like salts that can fall out of solution or high boiling compounds that can freeze out in a pesticide mixture.

- Did you mean invert three times only?

Thanks a lot for this reply. I have been shaking my solutions like crazy. LOL

Another thing I do a little differently from the suggested technique in the methods but does make my standards last longer is I do not let them come to room temperature before withdrawing my spiking aliquot. This procedure is normally placed into the methods to prevent over spiking because the methanol has contracted while in the freezer and throws off the accuracy of the measured aliquot. But if you will pull the aliquot of methanolic standard into a gastight syringe and allow a few seconds before measuring the final volume, the methanol will expand due to the head of the syringe barrel and you will be actually measuring the methanol at room temperature. I have done this for years and it works great, and your standards will not go bad as quickly, I normally throw out most of my standard at the one month expiration instead of having to remake it each week.

--> Did you mean that you allow them to come to room temperature prior to withdrawing? Your following statements are contradictory to your first sentence.

Volatiles can be lost to headspace very easily. The rule of thumb for diluting into a volumetric would be to spike then invert without shaking three times only. This works great for us. I also do not shake my stock standards before using them. They are not like salts that can fall out of solution or high boiling compounds that can freeze out in a pesticide mixture.

- Did you mean invert three times only?

Thanks a lot for this reply. I have been shaking my solutions like crazy. LOL

Yes, simple invert the flask three times allowing the air to travel from the neck to the body and back again, without shaking. This will mix the contents without sending too much of the standard into the air filled portion of the flask.

Shaking vigorously will treat the sample similar to purging it in the sparge tube, which removes the analytes from the water.

Another thing I do a little differently from the suggested technique in the methods but does make my standards last longer is I do not let them come to room temperature before withdrawing my spiking aliquot. This procedure is normally placed into the methods to prevent over spiking because the methanol has contracted while in the freezer and throws off the accuracy of the measured aliquot. But if you will pull the aliquot of methanolic standard into a gastight syringe and allow a few seconds before measuring the final volume, the methanol will expand due to the head of the syringe barrel and you will be actually measuring the methanol at room temperature. I have done this for years and it works great, and your standards will not go bad as quickly, I normally throw out most of my standard at the one month expiration instead of having to remake it each week.

--> Did you mean that you allow them to come to room temperature prior to withdrawing? Your following statements are contradictory to your first sentence.

Sorry for not writing that clearly.

I do not allow the standard vial to come to room temperature. This keeps more of the analytes in the solvent instead of allowing them to escape into any headspace that may be above the methanol.

I do allow what is in the syringe to come to room temperature before making a final adjustment of volume. Overfill slightly, wait a few seconds as methanol will absorb heat and expand rather quickly, then adjust down to final volume before spiking into the water in the flask.

It is also good technique to not hold the barrel of the syringe, as they will cause the methanol to expand too much from the heat your fingers will transmit through the glass. For making my standards I prefer using the extended barrel syringes, where the glass portion attaches to a metal tube that you actually hold.

Syringes like these: http://www.sigmaaldrich.com/analytical- ... ge=9662373

These are the containers I use for the standards: http://www.sigmaaldrich.com/catalog/pro ... &region=US

Yes as stated I use my spike stock directly from the freeer, spike, and invert gently 3X. I then gently pour from the vol flask collecting after the contents of the neck of the flask has gone to drain.

I got new standards, now I am spiking two times as high 200%....

AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!

I've never doubted my skills until today.

As stated earlier it would be help if you gave particulars of your syatem and what method you are running.
Is your method calibration internal or external? If internal, for the CCV's that are 200%, what is the area% of the IS compared to the average IS area of the calibration?

I got new standards, now I am spiking two times as high 200%....

AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!

I've never doubted my skills until today.

It took me a month or more to develop even a decent technique for volatiles. Luckily I had a great mentor when I first began over 20 years ago. As a young kid just out of college I had to learn to take things slow lol.

If previous standards had compounds being lost in the prep, you may now be getting the correct amounts which would give you increased responses.

I notice you are spiking into 5ml, maybe you can try increasing your working solution concentration and spiking into 100ml then using a syringe to transfer 5ml to the sparger. This will allow you more room for error overall and make it easier to be consistent.

I got new standards, now I am spiking two times as high 200%....

AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!

I've never doubted my skills until today.

It took me a month or more to develop even a decent technique for volatiles. Luckily I had a great mentor when I first began over 20 years ago. As a young kid just out of college I had to learn to take things slow lol.

If previous standards had compounds being lost in the prep, you may now be getting the correct amounts which would give you increased responses.

I notice you are spiking into 5ml, maybe you can try increasing your working solution concentration and spiking into 100ml then using a syringe to transfer 5ml to the sparger. This will allow you more room for error overall and make it easier to be consistent.

I actually transfer 10uL of my working solution to 5-mL syringe then transfer to the sparger. Unlike you, I don't have a mentor so I am still stuck. I have not tried hard since this last post. I have started trying again a couple of weeks ago. By the way, I am not using an autosampler. I do manual sample injection.

Now, I have the following issues:

1. My first two surrogates are not passing our acceptance criteria.
a. dibromofluoromethane recovery = 131.5%; acceptance criteria = 86-118%
b. 1,2-dichloroethane-d4 recovery = 144.9%; acceptance criteria = 70-120%

It looks like I am having carry overs. Recoveries are acceptable during my first sample in the run, which is a laboratory control standard. Then, when I run a blank, they both spike high. It continues to increase as run progresses. Is it time to change my trap? I have 3100 Tekmar concentrator. Is it time to clean my sparger? Haven't clean it for a year. Or, could it be a GC-related issue? I have Agilent 7890A.

2. My laboratory control standard is a mixture of the following gases:

Standard (initial concentration)
ULTRA VOC Gas Mix (2000 ug/ml)
ABSOLUTE 2CEVE (1000 ug/ml)
ULTRA Acrolein (2000 ug/ml)
ACCUSTD Vinyl Acetate (100 ug/ml)
ABSOLUTE EPA 502.2/524.2/601 Gases (2000 ug/ml)
ACCUSTD 2 Pentanone (10000 ug/ml)
ACCUSTD Benzyl Chloride (200 ug/ml)

I spike each so that final concentration of each is 50ug/L in 1.0 mL MeOH. I am having ~200% recoveries on Acrolein and Acrylonitrile, 0% recovery for 2-CEVE, trichlorofluoromethane, 2-pentanone, and iodomethane and ~160% chloromethane. I re-prepared my working solution and results are coming out the same. Our acceptance criteria is 70%-130%.

Is it technique-related, or my standards have somewhat oxidized to something else?

I'm not familiar with a 3100. Does it spike the internal/surrogate standard, or do you spike it?
Mr. Ball's suggestion to spike your standards into a 100-200 ml volumetric flask is a good one. I calibrate from 0.5-50 ng/ml by spiking from 2ul-50 ul into a series of vol flasks from 50ml-200 ml, and transfering into 40 ml vials.
When I was using an LSC-2 I manually spiked my is/surr solution into my 25ml syringe just before injecting into my sparge vessel. I did not try to mix it as I put the entire25ml into the sparger.

Based upon some of your responses, I believe your system needs a refurb. (0% recovery of 2-Cleve) you probably have some active sites and this will have a big impact on carryover as well.

If your CCv's are failing, do you notice a drop off of internal standard response as the day goes on?

Based upon some of your responses, I believe your system needs a refurb. (0% recovery of 2-Cleve) you probably have some active sites and this will have a big impact on carryover as well.

If your CCv's are failing, do you notice a drop off of internal standard response as the day goes on?

Internal standard response decreases as the day goes on. I was thinking it has to do with my sample prep but when I started, my surrogates are recovering fine and CCV recoveries are ok (95% of analytes are within acceptable range).

My lab director said to try flushing it out. I ran DI water 8 times and was detecting difluoromethylsilane, 3-methyl-3-vinyl cyclopropene, cyclobutane, 1-methyl-1silacyclobutane, and 2-fluoracetamide.

Is it time to replace my GC column or trap in my Purge and Trap concentrator?