I am studying propofol (an intravenous anesthetic) and using thymol as an internal standard. I have been having difficulties cleaning the syringes, getting propofol and thymol contamination peaks on "blank" runs. I have tried multiple solvents to clean, including acetone, ethyl acetate, methanol, ethanol, acetonitrile, cyclohexane, 2-propanol, and dilute HCl. I have tried to contact authors of the journal articles to which I have been referring as well as the companies that supply the needles and syringes, without any solution. Please let me know if you have and thoughts or anyone you could refer to me.

Lots of questions...

What sort of injector do you use? Does it have a needle wash? If so, is it working properly? Are you sure the problem lies with the syringe and not with a frit or other component in your system that may be retaining your analytes?

Contamination or carry-over in blank runs could have a number of causes. What happens when you rinse the system with a strong solvent (Neat ACN for example), re-equilibrate, and inject a blank? Any change with a lengthening (or shortening) of equiligration before an injection? What happens with multiple blank injections? Do the peaks get smaller, remain the same, or get bigger(I hope not)?

What's your detector and are you positive that it's your analyte / internal std and not some other component being retained?

How long have you had this method running? Has it always done this or is this something new?

Please give some mo'info and I'll try to help in any way possible!

Good luck!

Christopher

It looks like a chain just got axed by a hacker in which something on this was mentioned. My part a little more detailed: One can only really clean a syringe by taking out the plunger (piston), washing it in the cleaning solution, and drawing cleanig solution through the syringe body (dip the needle into the solution and pull a vacuum on the other end).
Carry over from the injection valve is more difficult to control, we had cases where the valve had to be taken apart and the rotor, as well as the stator washed separatly. We no longer use commercial injectors when carry over can not be tolerated.
If analyte sticks to any part (frits, etc.), which is swept by mobile phase, one gets a peak only if a bolus of a solution is introduced that solvates the analyte or if another sudden equilibrium disturbance occurs (the correct RT is, of course, only obtained if this takes place ahead or at the start of the column bed).

the sample never enters the syringe it contains only mp.

Now i must join Christopher, mamely: ????????????

Hans

In some autosampler designs, the drawn sample never does enter the syringe. In others, of course, the opposite is true. So Mr. Anon is partially correct.

A.A.

name one design in which the sample enters the syringe please A.A., ive not heard of this

please tell me of a design in which the sample enters the syringe A.A.
All the ones I know of, Waters, Agilent, Dionex, PE, etc. use a sample loop.

While the sample is actually injected onto the column through the sample loop, a syringe is used to load the sample solution into the loop. In the Shimadzu design, the sample does not actually reach the syringe. Instead the syringe is connected to the loading needle with a length of tubing. Thus while carry over from the actual
"syringe" will not occur, carry over from the rest of the system is possible. I could not see where the injector manufacturer was specified, if indeed an automatic sampler is used. For the Shimadzu autosampler, you can change the syringe rinsing volume which might be enough to eliminate ghost peaks from the injector. You could also set up a pretreatment file to perform multiple rinses of the injector syringe lines and/or sample loop before the sample solution is loaded. I would guess other manufacturers have similar functions for their autosamplers.

Dear Anonymous,

I'm guessing that you are using manual injections. Your compound, propofol, is extremely hydrophobic with a log Pow of 3.79 (naphalene is 3.4). You will need to rinse all contacted surfaces with copious amounts of strong solvent such as ACN or acetone. Once you move to inject I would recomend going all the way to 100% organic and remaining there for at least a minute with the valve in the inject position. While your gradient is running you will also need to rinse the injection port with several hundred microliters of strong solvent to clean the remaining exposed passages. Once this has been done, you should be able to move back to load, load a clean blank and see less than 0.001% carryover.

With regards to autosamplers... All autosamplers aspirate sample through a needle and tubing section that is attached to a syringe. The sample is not allowed to actually reach the syringe. In probe-in-loop type autosamplers (shim, Agil,gynko) the needle actually makes the high pressure seal and becomes the sample loop. In other autosamplers the needle fills the loop via an external connection. Eliminating carryover in autosamplers should be possible following the methodology suggested for manual valves above with special care to eliminate any carryover at the interface of the sample needle and needle dock.

Contrary to what Anon. and others say, there is a line of autosamplers that do not aspirate sample through a probe-in-loop. LEAP autosamplers are of a syringe robot design that draw sample into the syringe only (no sample loops). They remove only as much sample as is injected and have less carry-over than probe-in-loop type autosamplers (they are also tailor made for 96 well plates). Call LEAP at 800-229-8814 if carry-over is an issue(or if you need high through-put with 96well plates). You guessed right that I am a LEAP employee. Call your friends in any DMPK department at GSK, Merck, AstraZeneca, Hoffman LaRoche, Pfizer, BMS, Novartis, Schering-Plough, etc. etc. for an unbiased opinion.