Tabrez: I only provide direct feedback on items that I have personal experience with and have certainly used them. As noted, I found them to be unsuitable and of poor value. I do not like to see clients waste money on parts they do not need. Standard fittings sell for a fraction of the cost, are reliable and very easy to use. These "new" fittings simply add a product to a company's line card and do not solve a real problem (the problem to be solved is to provide basic training). Whenever I teach a class in intro HPLC, one of the demonstrations provided is in how to properly swage an HPLC fitting (metal or plastic). The demonstration takes just a few minutes and the students participate so they can get "the feel" for doing it correctly (it is all about touch). Once they try it a few times, they quickly learn to do it correctly and leaks are generally a thing of the past (unless they re-use an old fitting). There is no substitute for proper training.

What follows is from my HPLC instructor notes used in many of the classes I teach.

"I prefer and teach the use of Stainless steel fittings for all HPLC connections except the column inlet and outlet (because most users will change the columns). Steel fittings are generally swaged and used once, not re-used. The correct nut and ferrule must be selected for use in the selected fitting(s). Some brands/types are NOT compatible with each other. Seating depth, ferule geometry and thread style and length vary. *One quick tip I can share is that during installation of new fittings, make sure that you continuously apply direct pressure on the tubing, pushing it into the fitting at all times, while also tightening the nut on the ferrule. This assures the correct seating depth is maintained and the ferrule is swaged to the tubing. Do NOT over-tighten the nut. Better to have it loose and leak (then go back and tighten it), then to over-tighten it and damage the fitting (a sign that a novice has been at work). Practice this procedure and be sure to disassemble the fittings to check your work (as you can not "see" inside them unless you take them apart). BTW: Used SS connections and lines are disposed of when used, not re-used in another application. Each one is swaged to fit one fitting/connection. Re-use in other areas often results in leaks or extra delay volumes. Make new SS connections when needed (with new fittings and tubing).

One or two-piece PEEK fingertight fittings (as long as they are compatible with solvents, temperatures and pressures used) are preferred for the HPLC column connections. This allows simple connections at the correct seating depth without the danger of damaging the internal threads of the normally steel column end-fittings (plastic threads will not damage the stainless steel insert, but if your column has plastic end caps, be careful you do not cross thread or over-tighten the fittings).

I find that the one piece PEEK fingertight fittings offered by IDEX to be of high quality for this purpose. They sell many types and styles. Select the type that is best for your own application.

*If the fittings you are using are coming loose after use, then they may be: (1) worn out and need replacement. Many plastic fittings are only good for 4-10 uses; (2) the wrong type or style may have been used; (3) used at too high a pressure. Each type has ratings at specific temperatures; (4) the temperature may be changing (PEEK fittings may come loose if the temperature changes, as is common in a column compartment) or (5) not fastened, installed or tightened correctly. Correct procedure must be used.

Here is an article (link) on HPLC Fittings and Tubing:

• "HPLC Tubing and Fittings; An introduction to Nuts, Ferrules and Tubing Choices"; https://hplctips.blogspot.com/2018/07/hplc-tubing-and-fittings-introduction.html

Note that "plate numbers" are more a function of peak shape, tubing length and ID plus system delay volume. To obtain the best results, start by optimizing the flow path of the HPLC system for the specific application. Use of extra long or larger ID lines than needed may result in more diffusion and wider peaks in general (lower plates). Key connections from the Autoinjector to the column inlet and from the column outlet to the detector must be optimized to reduce delay volume. The flow cell should also be properly sized and the sample dissolved in the initial mobile phase and injected in the smallest volume. If using a gradient, then the flow path of the pump through the injector to the column inlet should also be optimized too.

Thanks a lot for your answer. It sounds very much like I do now already. However, I have never really got it to work with the PEEK connectors (I am quite a bit below the top pressure rating before they pop out). Perhaps , I should try IDEX and see if they work better than the ones from Valco that I use now. Do you recommend any special type (I use pressures up to 400 bar)?

I also generally feel it is a bit awkward when I make connections since I need to hold two wrenches (one on each side of the connection) at the same time as I should put pressure on the tubing to ensure that there is no dead volume. So, it sometimes feel like I need three arms. Is there any trick to do this in a more practical way?

IDEX P/N F-130. We have used them by the thousands for ~ 30 years at up to 400 bars every day. These are single piece versions in PEEK so there is no ferrule to loose. No problems at all with them holding. Reasons for them loosening were addressed earlier.

My sales team reminded me that we also use and recommend the IDEX P/N F-196X SealTight Nut, Blk PEEK, 10-32, Fingertight Fitting with ferrule, 620 Bar rating too. Two piece, but work great.

Regarding the proper technique to swage HPLC connections. It just takes some practice and is very easy to do with two hands (or sometimes I can even do with just one hand using counter pressure on two fingers). The process is fundamental to making proper connections and you will get the hang of it soon.

Your problem can be easilty solved with self adjusting connectors.

eg. Sure-Fit connectors from Phenomenex

These new type of connector self adjust to any port depth regardless of manufacturer and having high presusure sustain above 400 bar also these have virtually no dead volume. These conncetor ensure fitting by a click sound after it is tight indicating it is fit.

Many of those "specialized" fittings represent a poor use of money and may be unreliable overall. * Learning the proper technique to swage or attach HPLC fittings is a fundamental skill which should be learned and used. An understanding of how to attach typical HPLC fittings and lines, which is acquired from practical hands-on experience, is far more useful in troubleshooting problems too, including those which may result from use such specialized fittings. Save your money for products and tools that you actually need and invest in learning.

Dear William, scientist should be always be open to adopt new technologies. Is this you have used it or its your hypothesis for not adopting.

Tabrez: I only provide direct feedback on items that I have personal experience with and have certainly used them. As noted, I found them to be unsuitable and of poor value. I do not like to see clients waste money on parts they do not need. Standard fittings sell for a fraction of the cost, are reliable and very easy to use. These "new" fittings simply add a product to a company's line card and do not solve a real problem (the problem to be solved is to provide basic training). Whenever I teach a class in intro HPLC, one of the demonstrations provided is in how to properly swage an HPLC fitting (metal or plastic). The demonstration takes just a few minutes and the students participate so they can get "the feel" for doing it correctly (it is all about touch). Once they try it a few times, they quickly learn to do it correctly and leaks are generally a thing of the past (unless they re-use an old fitting). There is no substitute for proper training.

Tabrez: Phenom is just one of several vendors selling these types of products. Have you seen the overpriced and over-engineered fitting offer by Agilent for the same purpose (Quick Connect Fitting)? Yet another poor use of money where basic training and the use of a quality fitting are all that are needed to solve the problem.

Very often, the manufacturers use a system that minimizes dispersion outside the column to measure plate counts. No nefarious reason, they want to measure, insofar as possible, the plate count for the column itself and minimize the effects of the rest of the system. This actually agrees with William Letter 's comments above.

An interesting forum conversation, including some of the manufacturers, is here:

https://www.chromforum.org/viewtopic.php?t=3544

I will make a test run on a new column with a mixture that works for us and verify the column and system compare it with earlier runs; I will also run the same sample/method periodically to verify the system is still running properly. A sample that works may be the manufacturer test compounds, or it may be something similar to what you run.

Jack's comment is spot on. I previously posted the same type of answer here on RG several times for other researchers asking a similar question about plate numbers and how they are affected by flow cell volume, tubing volume, peak retention time, brand of system and so on. Plate number values provided by the column supplier are optimized for their test system and their method only, not yours. Change the HPLC system, plumbing, configuration, integration events and yes even the flow cell and results will change (they are supposed to). This concept is fundamental to an understanding of HPLC. Understanding this relationship is critical to using HPLC as a tool.

We have always advised and recommended that when you establish a new method, also establish a new column verification method too. Choose an appropriate and reliable (stable) standard, applicable to your method for use in characterizing the column. Create a SOP for it. Now, whenever you have a question about your current column or change it out for a new column, you have a test which can show you what the performance specs are for that column and how they compare to your ""STD Test" column. Plate numbers would be just one of many parameters which could be included in that test, but the original manufacturer supplied values are not really important (they mainly showed how well packed the column was and ran on their own test system, not yours).

As long as your separation/resolution are adequate, the flow path and plate count are irrelavant.

Agree with William, in general, about using SS fittings. They are cheap and work well. However, they also commit the tubing to a specific swage depth. If you use a wide variety of columns, you may have leaks which why most us chromatographers prefer PEEK fittings and the flexability they offier.

Thad, this is why I strongly recommend the use of PEEK fingertight fittings for all COLUMN connections (hard connections for the rest). This was explained in my previous posts and also in the linked article too.

• As quoted previously; "One or two-piece PEEK fingertight fittings (as long as they are compatible with solvents and pressures used) are preferred for the column connections. This allows simple connections at the correct seating depth without danger of damaging the internal threads of the column end-fittings. "

Hi Anders,

maybe this link will answer your question:

https://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwjt5NGpmdrjAhWLKVAKHWw2BmwQFjABegQIAhAC&url=http%3A%2F%2Fwww.ercatech.ch%2Ffile%2Fdownload%2F373%2Fall_about_fittings.pdf&usg=AOvVaw080w2snKCXyyhxztAG58UY

Regards

Markus