Let's use the correct terminology (we are scientists), not brand names which were invented by marketing people. They are all "HPLC systems". Changing the column particle size, something that has been done many times over the decades, does not rename the technique.

• "Terminology. Which is it? UPLC, UHPLC or HPLC? The correct name is still HPLC"; https://hplctips.blogspot.com/2015/08/terminiology-which-is-it-uplc-uhplc-or.html

Most any modern HPLC system can handle pressure up to 400 or 600 Bar, which is plenty of system pressure capability for even the smallest particle columns. The key thing that people overlook is that when you want to use a very tiny column ("tiny" means a column that has an internal volume of less 600uL), your HPLC system's plumbing must also be optimized for such low delay volumes too. Otherwise gradient delays may be very long and diffusion may remove any gains made from using such small particles with small columns. The good news is that many, not all, older "HPLC" systems can be modified to reduce their internal volume. If you are not sure which to buy, then just purchase a well outfitted HPLC system with a 600 bar max pressure rating and then plumb it for whatever applications are appropriate (something that users should already be doing with the HPLC system they have. The HPLC system should always be optimized to the application. Vendor's are not responsible for that, you are). An HPLC system with a correctly optimized flow path will 'run' methods with 1.7 u particles and conventionally sized ones so you will be able to handle most any type of applications and method transfer needed (e.g. Agilent 1260 Quat pump with AIV based system plus DAD and vial sampler).

BTW: Many of the same changes that that the manufacturer's use to re-label an "HPLC" system as an "UHPLC" system can be done by an experienced user. This flow-path optimization mostly involves: replacing the 0.17mm ID tubing (or 0.25mm) with 0.12mm ID tubing, reducing the flow cell volume (as needed), using a smaller volume mixer, maybe changing the A/I loop or needle/seat, placing the pressure transducer separate from the pulse dampener (to reduce shared delay volume, where appropriate, as some systems include a large volume pulse dampener in the flow path) and running the A/I in By-Pass mode.

Also, lots of UHPLC methods take advantage of much higher flow rates (often 2x on smaller volume columns) so any "solvent savings" may not be real.

• "Pressure Drop Across an HPLC / UHPLC Column"; https://hplctips.blogspot.com/2011/08/pressure-drop-across-hplc-uhplc-column.html

Buy both. Initially you need to develop methods on HPLC because, when you transfer the methods to CMOs, they might not be equipped with UPLC. Meantime, validate the method on UPLC to gain expertise and, as companies will ask for UPLC methods, you will be the contract lab of choice.

Let's use the correct terminology (we are scientists), not brand names which were invented by marketing people. They are all "HPLC systems". Changing the column particle size, something that has been done many times over the decades, does not rename the technique.

• "Terminology. Which is it? UPLC, UHPLC or HPLC? The correct name is still HPLC"; https://hplctips.blogspot.com/2015/08/terminiology-which-is-it-uplc-uhplc-or.html

Most any modern HPLC system can handle pressure up to 400 or 600 Bar, which is plenty of system pressure capability for even the smallest particle columns. The key thing that people overlook is that when you want to use a very tiny column ("tiny" means a column that has an internal volume of less 600uL), your HPLC system's plumbing must also be optimized for such low delay volumes too. Otherwise gradient delays may be very long and diffusion may remove any gains made from using such small particles with small columns. The good news is that many, not all, older "HPLC" systems can be modified to reduce their internal volume. If you are not sure which to buy, then just purchase a well outfitted HPLC system with a 600 bar max pressure rating and then plumb it for whatever applications are appropriate (something that users should already be doing with the HPLC system they have. The HPLC system should always be optimized to the application. Vendor's are not responsible for that, you are). An HPLC system with a correctly optimized flow path will 'run' methods with 1.7 u particles and conventionally sized ones so you will be able to handle most any type of applications and method transfer needed (e.g. Agilent 1260 Quat pump with AIV based system plus DAD and vial sampler).

BTW: Many of the same changes that that the manufacturer's use to re-label an "HPLC" system as an "UHPLC" system can be done by an experienced user. This flow-path optimization mostly involves: replacing the 0.17mm ID tubing (or 0.25mm) with 0.12mm ID tubing, reducing the flow cell volume (as needed), using a smaller volume mixer, maybe changing the A/I loop or needle/seat, placing the pressure transducer separate from the pulse dampener (to reduce shared delay volume, where appropriate, as some systems include a large volume pulse dampener in the flow path) and running the A/I in By-Pass mode.

Also, lots of UHPLC methods take advantage of much higher flow rates (often 2x on smaller volume columns) so any "solvent savings" may not be real.

• "Pressure Drop Across an HPLC / UHPLC Column"; https://hplctips.blogspot.com/2011/08/pressure-drop-across-hplc-uhplc-column.html

I think among the both of HPLC or UHPLC- the HPLC is bette, I did several works with HPLC, however you may also buy UHPLC. I recommend you to buy HPLC

HPLC is commonly used so co relating results is easier

Thanks everyone for your valuable comments. I agree with William Letter that the the principle of separation remains same for both "UPLC" or "UPLC" and "HPLC". I also agree that UPLC was first (in 2004) coined by Waters for the Acquity systems that provide very high pressure (upto 15,000 psi). I have seen that for some applications, the pressure can rise approx. 13,000 psi. If we require rapid separation and detection of hundred of samples per day, would it be better to have such high pressure capacity systems like Acquity? In that case, one can use long column (say, 2.1x100 mm with 1.7 um particles) for efficient separation.

However, in terms for cost for a CRO company, there are more options for HPLC (max pressure 800 bar or 10,000 psi) from different vendors.

Thank you again for your time everyone!!

"Acquity" was not first to use very high column pressures in HPLC. We have done so since at least the 1980's when many people including myself were developing and using systems with narrow and microbore columns with 2u and 3u particles that could go up to 50,000 psi (and others had systems with max pressures as high as 100,000 psi !!!). This proved to be a really poor path to follow so was abandoned, but once it became possible to pack columns with tiny particles in a more reproducible way, the same idea re-surfaced under a new "name" (UHPLC/UPLC) in the early 2000's. But this false high-pressure need misses the point. Pressure max is really of little importance (it is a consequence of using extremely small porous particles in large columns at high flow rates) and many of the application notes and journal articles you may have read showing extremely high back-pressures were developed only to show off the high pressure capability, not to show great chromatography (which was often possible using a better method at much lower pressure). Rapid separation of hundreds of samples per day does not require extremely high back pressure requirements. It requires a well developed method and optimized flow path. Extremely high back-pressure in HPLC is BAD (internal heating effects, extremely high wear/tear on parts... etc). Our goal is not to increase pressure at all. You only need enough system back pressure to maintain stable flow. *Please review the fundamentals of chromatography to read why this is so (and marketing/sales efforts alone are the reason these 10K, 15K and 20K psi systems are heavily advertised, again).

• "one can use long column (say, 2.1x100 mm with 1.7 um particles) for efficient separation " need Acquity... ???? Not a all. You seem fixated on Waters, when the question is not related to brand. Also, a 100 mm column is not "long". *To use a 2.1 x 100 mm column with 1.7u particles you only need an HPLC system with a low volume flow path to deal with the tiny ~ 240 ul volume of the column. A 600 bar rated system with a delay volume of less than ~ 500 ul should be capable of running a column of that size with most mobile phases mixtures at double the regular flow rate of 0.200 mL/min (1.5x to 2x flow would be appropriate in most cases as a starting point).