You should use a splitless injection for at least 45s to about 2 min. After that the split valve is opened and injector content discards. Pay attention to injector temperature which shoud be enough to do the thermal desorption (> 240 C), but pay attention to fiber maximum temperature. To ensure best results start program first (and that closes the split valve) and then insert the fiber in the injector. Good luck.

Hi Ricardo,

thermal desorption for SPME fibers is a crucial step and as such it also need to be optimized.

in order to correctly desorb your analytes from the SPME coating you have to insert the SPME assembly in the injector port and then rapidly expose the coating by depressing the plunger of your manual SPME holder. The rapid exposure of the coating in the injector port is of utmost importance to obtain good peak shape for your analytes.

Regarding the desoprtion parameter optimization: the right values of tempearture and desoprtion time to use depend on the type of coating and on the analytes you extract. A general rule is to avoid any analyte or co-extracted compound carry over on the coating. In order to verify if you have carry over on your coating you can just run a blank of the fiber immediately after the analysis and check the presence of your analytes in the chromatogram. In addition, it is important to keep the split valve off during the desorption time and eventually you can set your injector in split mode after you retract the SPME coating from the injector. You can start your optimization by testing different desoprtion times (1, 2, 5, 10 minutes for example) and set the temperature 10 C lower than the maximum operational temperature reccomended by the manufacturer. Eventually you can increase temperature and/or desorption time if you notice any carryover.

I hope I could give useful info

Regards

Emanuela 

I agree with Jorge and Emanuela's.  I think though that you will start to see broader peaks if desorption time is too long, like more than ~2 minutes.  You can test it out though and see what happens.  I worked with SPME for ~6 months about 4 or 5 years ago and used it to acquire semi-quantitative detection of trichloroethylene at trace levels (50ppb to ~50ppt) in air.  I exposed the fibers to the test atmosphere for 10 minutes at room temperature to adsorb and concentrate the volatiles (15 minutes was tested but indicated only slightly enhanced sensitivity in my case).  I was using a GC with a temperature programmable injector (not necessarily a requirement) in splitless mode and an ion trap MS operating in SIM mode since I was targeting a specific compound and needed maximum sensitivity.  The fibers were desorbed for 1.25 minutes (sorry I do not have the exact conditions handy).  After desorption the fiber was retracted and withdrawn and the injection port purged, I believe at 2 minutes.  The GC had a cryo-oven programmed to begin below ambient room temp to "cold trap" and concentrate TCE at the head of the column.  After a few minutes the oven temp was ramped up to elute the TCE (if present) in a relatively sharp band.  I happened to have another GC handy nearby as well with a hot injection port that  I used to further desorb and clean off any remaining volatiles from the fiber.  Always do a blank run to be sure the fiber is clean before using it again.  From my experience, SPME can increase sensitivity by at least 2 or 3 orders of magnitude over direct headspace sampling but getting consistent results can be challenging.  Exposure time and temp, desorption time and temp all need to be carefully tested, optimized and controlled in order to have any hope of accurate quantitation.  Bueno Suerte.

Regards,

Tom        

Hi Ricardo,

I forgot to mention that, if you are interested, we organized at University of Waterloo twice a year SPME course.

This two-day course, organized by the inventor of SPME, Prof. Janusz Pawliszyn, covers the following topics:

Introduction to SPME

Theoretical principles of SPME

Method development

Calibration in SPME

SPME applications in environmental, food and bioanalytical chemistry

Future directions and development of new SPME devices

The next SPME course will  take place on December 4-5, 2014, and April 30-May 1, 2015.

I am attaching the link, if you would like to have more info.

https://uwaterloo.ca/pawliszyn-group/spme-course

Regards

Emanuela

Just to add to what has been said above, make sure you use a SPME liner in your injection port. These are straight with a narrow 0.75mm internal diameter to help focus the desorbed volatiles onto your GC column. With the box of SPME fibres you will have received a guide sheet that lists the conditioning and operating temperatures of the SPME fibre you have purchased. Set the injection port temperature to the operating temperature indicated in the guide. Set your method to splitless injection for the time you need to completely desorb the volitiles of interest. If you can make this as long as necessary to still obtain suitable chromatographic seperation, then this will help clean (condition) the SPME fibre for your next sample. I presume you are using manual SPME, however you may have a suitable autosampler such as a CTC combi-PAL that you can be used to automate your SPME extractions. With such autosamplers the desorb time is set in the method parameters controlling the autosampler. With an autosampler like the combi-PAL you can obtain fibre conditioning stations that clean the SPME fibre between samples, very handy! To help reduce issues with repeatability from SPME extraction use suitable internal standards that are chemically similar to the target analyte. If you have an MS then deuterated analogues are best but can be a little expensive to buy.

i am totaly agree with Jason Breitmeyer, you need to use a specific liner, to identify the fiber family if you know the compounds you are looking for. If the compounds are very volatils, the time desoption will be 1-2 min but if it is not, you can choose 5-6 min but the splitless time will be the same 5-6 min. My default parameter is 3 min, and you will adjust.

Good point about the liner.  Also make sure the fiber extends down into the liner the proper distance and not too far.  My first attempt with SPME years ago resulted in the tip of the fiber breaking off inside the liner and making it's way to the head of the column.  I didn't realize it for awhile and wondering why I was not gettting good results.  After replacing the liner, cutting off a section of the head of the column and making these adjustments things started working.  

Thanks, your answers have been very helpful. We have a liner for SPME. The fibre will be a Carboxen/PDMS/DVB, and yes, we use manual SPME injection.

I still have a very basic doubt, I still don't understand where to modify the desorption time in my method. Or should I just let the fibre exposed in the injector for the specified amount of time after pressing "prep run" and before pressing "start"?

If you know what some of these volatiles are I suggest preparing standards and just compare results using different desorption times and temps.  A little trial and error will most likely be needed to optimize the conditions for what you need to do.  Keep in mind that preparing "accurate" standards of volatiles can be a bit tricky too since condensation on the inside walls of the vessel can be a significant factor as can be non-homogenious mixing.  You may also need to purge the air out of the flask with nitrogen or helium to be sure you have a good clean blank.  Warming the flask and maintaining the vessel at a controlled fixed temperature can be helpful in getting and maintaining volatiles in the vapor state.  Good luck.  

Hi Ricardo

As soon as you insert the fibre and fully expose it inside the liner press "start" on the run. That point in time is the same as what would occur in a liquid injection. When the "desorption time" that you have determined is complete retract the fiber and pull the manual SPME assembley out of the inject port as you would any needle. You are in effect the autosampler and it is the autosampler that determines the desorption time. The injection port mode "splitless" can coinside with the desorption time or be shorter depending on the needs of the method. Hope this helps clear things up.

Ricardo, the desorption time is equal to the splitless time. When split valve open the injector change to split mode and the almost nothing from fiber will enter in the column. The best is  rectract the fiber and remove the holder  from injector after splitless  time. regards

I never got around to thanking everyone for their answers. I finally found what I was looking for: How to prevent the purge valve from opening too soon. It was rather easy, but I didn't know where in the options  to set the time.

Thank you for your helpful answers