It\u2019s a fact of life for HPLC and UHPLC analyses. The force that is necessary to push a liquid mobile phase through a tightly packed bed of tiny particles, even at relatively low flow rates, can cause tremendous back pressure. Fortunately, LC instruments are designed to handle this, but problems will appear when a clog in the system causes the back pressure to climb above the normal range. Clogs arise when particulates are created (in the case of seal wear or mobile phase precipitation) or are introduced (by mobile phase impurities or sample particulates) into the flow path and become lodged somewhere in the various narrow tubing and instrument channels, small-pore frits, or interstitial spaces between column packing materials.<\/p>\n\n\n\n
We\u2019ll talk about the three major sources of these particulates and address some techniques for mitigating them and preventing high back pressure in LC instruments. But, effective troubleshooting and establishing an appropriate routine maintenance plan to prevent future problems starts with knowing what \u201cnormal\u201d looks like.<\/p>\n\n\n\n
In order to figure out if you have a high back pressure problem, you need to know what your pressure normally should be. Having a baseline to compare your actual pressure against can be a tremendous help in identifying a problem and diagnosing it.<\/p>\n\n\n\n
Ideally, you would generate a \u201cnormal\u201d baseline for the instrument with and without the column installed. By simply removing the column and replacing it with a union, you will see what your normal system pressure looks like. Now, imagine a situation where you\u2019re observing high back pressures in your LC. In that case, replacing the column with the union again and noting whether or not the pressure returns to the normal, system-only pressure will quickly determine if the problem is column or system related. When replacing the column with a union, be sure to use a union that can handle the system pressure (PEEK connectors are reliable up to \u22645000 psi and stainless steel connectors can be used up to \u226420,000 psi). If your LC software allows you to record your system pressure while you acquire data, that is another great way of keeping records of what \u201cnormal\u201d looks like during analysis and column re-equilibration.<\/p>\n\n\n\n
Finally, understanding the differences that mobile phase composition has on system pressure can help avoid a false alarm. For instance, switching your organic mobile phase from acetonitrile to methanol will result in an increase in system pressure, all other conditions remaining the same, simply because methanol is more viscous than acetonitrile. Also, during gradient analysis, as your mobile phase composition is changing with the gradient program, pressure will change for the same reason. So, not all pressure changes are a symptom of a problem.<\/p>\n\n\n