Advertisement

why a purge and trap needs a splitless inlet system?

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

12 posts Page 1 of 1
why do a purge and trap needs a splitless inlet system?
why can`t it be a split inlet?
i`m sorry....i`m just learning about it . and i can`t find the answer for this question.
hope someone can help
thanks
What is the source of this information ? There is no general reason why you cannot do split injections from purge and trap samples - but there might be an issue with the gas controls and plumbing of the particular hardware that you have.

Peter
Peter Apps
brunod_f,

A split injector is fine. A splitless/split injector is fine if you run it split. What doesn't (surprisingly) work is splitless injection. Reason being, the trap needs a minimum of flow (20-40 mL/min) and the flow (typically) needs to proceed down liner to get a good sample transfer. There is a big discussion of hardware here but I won't bore you except to say that a split injector should work fine.

Best regards,

AICMM
i`m sorry. i forgot to say about the source of the information. I`m reading the book Quantitative Chemical Analysis, from Daniel C. Harris 6e. In the gas chromatography chapter, when the purge and trap method is been explained, it just says to inject it in a gas chromatograpy operating in splitless inlet mode. it doesn`t say the reason or if it could be used a split inlet mode. My doubt was because of one of the questions at the end of the chapter, which says : "Why does the splitless inlet injection is used when a purge and trap sample is prepared?". I think it might be cause of the splitless inlet has the cold trap method, which is used in a purge and trap sample, for what i read in this book. What do you guys think?

thanks for the answers guys
Whether an injection is split or splitless is to do with how much of the sample gets transferred to the column, not with whether there is cold trapping. Usually a purge and trap sampler is permanently connected to the inlet of the gas chromatograph, so there is no separate "injection" as such. Is the book perhaps referring to the injection of liquid samples which have been produced by solvent extraction of the adsorbent that has been used to trap purged volatiles ? Certainly in that case I would use a splitless injection.

Peter
Peter Apps
All the GCs used with P&T systems I know of run in split mode due to peak shape issue. However, if there is a cold trap (lower than -150C) before the injection, a splitless injection (not splitless mode) can be done as well, in this case, the transfer line from the P&T needs to be connected directly to the column and the inlet still runs in split mode. This is how air concentrator works.
This likely is referring to the techniques that were used 10 to 15 years ago - a megabore column was used so the entire flow from the trap desorb line could be pushed onto the column. A jet separator or splitter was used on the detector end so limit the flow into the mass spec.

Somewhere along the line it became clear we could split at the inlet side, allowing a narrower ID column and eliminating the need for hardware at the detector side. A lot of sample is lost; the sensitivity is regained because the narrower column produce much sharper peaks. Many operators also like to split out the injection so less water gets onto the GC/MS.
Is the book perhaps referring to the injection of liquid samples which have been produced by solvent extraction of the adsorbent that has been used to trap purged volatiles ? Certainly in that case I would use a splitless injection.

Peter
yes. thats exactly what the book is saying. So why would you use a splitless injection in that case? Sorry, this is a bit confusing to me.
Purge and trap with solvent extraction of the adsorbent is a quite specialised technique used when analyte concentrations are very low, when people do not have the hardware to do direct transfers from trap to GC, when the analytes are so thermally labile that they cannot be thermally desorbed from the trap, or when a very strong adsorbent is needed to trap low level analytes from large gas volumes. Typically the trap would be desorded with a few hundred microlitres of solvent - of which only 1 - 5 microlitres would be injected due to the constraints on liquid injections to ordinary split-splitless inlets. Since only about 1% of the extract is injected, only 1% of the trapped analytes get into the GC, and if the injection itself was split the responses of the detector would be unworkably low - in other words the peaks would be too small.

Ordinary purge and trap with thermal desorption uses split transfers to the column (or splitless with cold trapping) because the volume of gas needed to desorb the analytes from the traps is too large to be introduced to a capillary column as a band that is narrow enough to give good peak resolution.

I will leave you to ponder the logic of going to huge trouble to trap the analytes, and then discarding 90 - 99% of them :scratch:

Peter
Peter Apps
Peter,

The one scenario where this works really well is not trace but higher levels and the need for speed. Trap lots and lots of samples at higher concentrations, extract lots of them with solvent, then rack 'em up and let 'em rip. Not low level work.

Been there, done that..... solvent contaminated site that "inhaled and exhaled" according to barometric pressure fluctuations.


Best regards,

AICMM
Peter,

The one scenario where this works really well is not trace but higher levels and the need for speed. Trap lots and lots of samples at higher concentrations, extract lots of them with solvent, then rack 'em up and let 'em rip. Not low level work.

Been there, done that..... solvent contaminated site that "inhaled and exhaled" according to barometric pressure fluctuations.


Best regards,

AICMM


Yes, that makes sense - automating liquid injections is a lot quicker and less troublesome than automated thermal desorb (and a lot cheaper in hardware), and you can do the solvent extracts in parallel by the dozen I expect.

Peter
Peter Apps
thanks so much for the answers, guys
it helped me a lot
thanks again
12 posts Page 1 of 1

Who is online

In total there is 1 user online :: 0 registered, 0 hidden and 1 guest (based on users active over the past 5 minutes)
Most users ever online was 4374 on Fri Oct 03, 2025 12:41 am

Users browsing this forum: No registered users and 1 guest

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food & Beverage, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry