Translator for HPLC HINTS and TIPS for Chromatographers

Saturday, August 29, 2015

Terminology. Which is it? UPLC, UHPLC or HPLC? The correct name is still HPLC.

Proper terminology is very important in science. I hear and see people misuse the terms "UPLC" and "UHPLC" often so think we need a short post to clarify the correct use of these terms. Here are some basic definitions.

Some quick background information. "LPLC" or  Low Pressure Liquid Chromatography. LPLC often includes chromatography analysis using glass or plastic columns with very large particle support beads run at pressures ranging from atmospheric (gravity driven) to several hundred psi (~ 30 - 40 bars max). Very large particles are required in this application to aid flow through the support, which in some cases is gravity driven and in others, a small pump is used.

"HPLC" or High Performance Liquid Chromatography: *Used to be called High Pressure Liquid Chromatography to differentiate it from "LPLC". Now we refer to it as "high performance" chromatography, though both terms are technically correct. Compared to the very large media used in LPLC (mm), HPLC uses micron sized support particles packed under very high pressures in stainless steel (note: sometimes strong rigid plastic columns are used for biocompatible applications) columns to enhance the resolution obtained by many orders of magnitude. As such, the more popular definition changed from "High Pressure" to "High Performance" Liquid Chromatography to emphasize this improvement. Today, we still refer to all modes of high pressure liquid chromatography separation techniques as "HPLC". The most commonly used HPLC pumps are rated between 400 and 600 bars maximum pressure (with some capable of 1,200 bars or more) though in normal use, we rarely run methods which use pressures over 300 bars.

"UHPLC" or Ultra High Pressure Liquid Chromatography (or Ultra High Performance Liquid Chromatography) has become both a new marketing term and perhaps a subcategory of HPLC (other subcategory examples include: nano-HPLC, narrow-bore HPLC, and mini-bore HPLC). UHPLC is presently defined as the use of sub-2 μm particles with a low dispersion HPLC system where the pump is capable of > 600 bars pressure. UHPLC is still HPLC. Many methods which use sub-2 μm particle columns can and are run on a low dispersion HPLC system at pressures which do not exceed 400 or 600 bars [For more information, please read: "Pressure Drop Across an HPLC / UHPLC Column"]. The technique used in all cases is still HPLC and should be described as such. We have been using narrow bore columns with small supports for more than 30 years and never have changed the name of the technique used each time we changed the column type used (e.g. 20u, 10u, 5u, 3.5u, 2,5u...). As a matter of fact, in the late 1980's and early 1990's there was a bug push to use 1.0, 2.1 and 3.5 mm ID columns with 3.5u and smaller particles on low dispersion systems to both save solvent, increase performance and reduce run times. This required the use of HPLC systems which were optimized with low dispersion flow paths such as the HP 1090 HPLC system. Perhaps the technology and methods came too early? Columns proved difficult to pack with the smaller particles (poor RSD). The solvent savings and reduced run times just did not interest people at that time and after a few years, the lack of interest resulted in few commercial columns being available with these properties (I recall packing many of them myself in the lab).

"UPLC ®" or Ultra-Performance Liquid Chromatography is a Trademark of the Waters Corporation. Waters Corporation uses it as a marketing term for their own product technology. Defined by Waters as, "the use of sub-2 μm particle columns in combination with low dispersion, high pressure (15,000 psi or 1,034 bar) instrumentation". The confusion seems to come from people using the Water's trademark of "UPLC" to describe the technique of HPLC or an HPLC system with a pump which is capable of exceeding 600 bars pressure or having a low dispersion flow path. They should be using the term HPLC in these cases or even UHPLC, if applicable, not "UPLC" (unless they really are referring to the Waters product technology).

Summary: In general, as long as the back pressure is above ~30 bars and/or you are using packed columns with porous particles less than ~ 50 microns in diameter (newer, monolithic supports and superficially porous particles also qualify), then the technique used is always called HPLC. If you are using sub-2μm particles and operating at pressures at or above 600 bars, then the term "UHPLC" could be used as well (not UPLC® unless you are specifically using a Waters Corp "UPLC" brand system under the same conditions described), but the term HPLC is far more accurate. You are always correct describing the techniques used as HPLC.

Saturday, August 1, 2015

An Often Ignored HPLC & LC/MS Contamination Source. Did you check your Vacuum Degasser?

The introduction of electronic vacuum degassing / degasser modules to the liquid chromatography industry a few decades ago has introduced several new problems which were not seen years ago when we sparged our mobile phase solutions with helium gas. One of these problems relates to how vacuum degassing modules can contribute to contaminating your HPLC or LC-MS system.

In a previous post ["Inline HPLC Degassing Modules"] I discussed the convenience that these devices have brought to our laboratories, but also the extra training requirements (such as cleaning and flushing the channels every day) which must be undertaken to use them successfully. When the operational guidelines for the use of these products are ignored, these devices can contribute to the contamination of your HPLC and / or LC-MS system. The wettable internal surfaces of the degassing module contain one or more types of plastic(s) which come in contact with the mobile phase (AF Teflon, Teflon and/or Peek are the most common "wettable materials" found). To effectively remove gas from the mobile phase, the liquid must move through plastic tubing or pass by membranes, placed under vacuum, for a time period long enough to allow the gas to diffuse through the material and out the exhaust port of the degassing module. The degassing tubing (most use tubing) should have the maximum chemical compatibility possible while allowing it to also be porous enough for the gas alone to diffuse through the walls of the tubing under vacuum. These requirements usually result in some form of fluoropolymer tubing (AF Teflon or Teflon) being used as it has broad chemical compatibility and can be formed with controlled pore sizes for the effective removal of gas, not liquid, through the tubing walls. However, the plastic used is NOT chemically compatible with all liquids used in chromatography. Depending on the plastic used, the tubing may swell, fail or even dissolve into the mobile phase! In many cases, the use of fluorinated solvents will dissolve these materials. We sometimes see a white residue leaking out of the degassing outlet ports which is initially thought to be buffer salts (very common from a lack of flushing), but sometimes it turns out to be dissolved tubing! Be sure and check the chemical compatibility chart offered by the degassing module manufacturer for compatibility with your mobile phase and ALL additives before using the instrument. Some examples of incompatible chemicals on the lists of many vendors units include: THF, strong acids or bases, Hexanes, Sodium Azide and especially most fluorinated solvents which can dissolve many of the plastic parts inside. *Be aware of which chemicals may pose a risk with your system. Contaminated vacuum chambers quickly result in contaminated tubing and vacuum pump failure.

Over the past few years we have seen an increase in the use of various perfluorinated solvents, esp with LC/MS systems and this has resulted in some serious degasser damage and source contamination. Additionally, we commonly see ion-pairing reagents such as TFA and TBA "sticking" to the plastic used in these modules causing a leaching of material over long periods of time (again, most obvious on an MS system where you can "detect" it in the background). These agents must be thoroughly flushed out of the flow path to reduce contaminating the entire system over time. *A strong wash solution with a little acid (formic) often helps in this regard. Wash cycles of over 12 hours are often needed to see improvement. In some cases we must replace some or all of the internal parts of the degassing module to eliminate the contamination. 

For normal phase applications, high concentrations of n-Hexane can also cause contamination of an HPLC or LC/MS system. The damage is often sourced to the degasser (which are often not compatible with Hexane) where the ultra high evaporation rate of hexane coupled to the advanced materials found in the degassing tubing results in the hexane condensing on the outside of the tubing of the degasser and becoming contaminated with the exhaust and plastics found there. These contaminants are then transported back through the tubing walls into the solvent stream.

If your HPLC's degasser has an error light turned on (i.e. Yellow or Red light or "Degasser Hardware Fault" error message), then your entire HPLC system may now be out-of-compliance because the degasser is broken. Have the degasser professionally repaired so you can put the system back online. Maintain these devices, following the manufacturer's guidelines in their use and operation and you should be able to run them for about five years before requiring repair service.

Be sure and review all of the information and advice supplied by the manufacturer of your degassing module before use or whenever you use any new solvents or additives. The composition of the degassing tubing has changed a great deal over the past decades resulting in increased degassing efficiency. Certain solvents though are still incompatible with most models. Make sure you know exactly what types of plastic are used in your specific instrument and proceed with caution when using these systems. Degassing modules must be operated, cleaned and maintained the same as your other important instruments. When they are not operating properly or are contaminated, they should be serviced as soon as possible or further contamination and damage to your system ($$$) may result.