Effective Use of the ACQUITY QDa Mass Detector with Your LC: A Q and A with Dr. Tom Wheat

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Courtesy of Courtesy of Waters Corporation

I recently presented a webinar describing some techniques and tools used for effectively using the QDa Mass Detector with liquid chromatography systems. Several attendees submitted questions that may be of general interest to a broader audience.

It is better, as you said, to use negative mode for acidic compounds (like carboxylic acid), but these compounds are often better eluted on a C18 column with acidic additives (HCOOH for example) which reduces the response? What is the best solution?

The best solution is to use a post-column addition of base to neutralize the acidic mobile phase. I typically use ammonium hydroxide and make my post column reagent at 5 times the molar concentration of acid in the mobile phase. The flow rate of the post column pump is then 20% of the flow through the column so that I am mixing equimolar amounts of acid and base into the source. For negative ionization, I will usually use a post-column reagent that is 50% water and 50% isopropanol to further enhance the ionization. We have also observed that many analytes give a usable response in negative mode even in the presence of formic acid. This simpler solution should be tested to determine if sensitivity is adequate.

For basic analytes, positive mode ionization gives the best response, and it is common to prefer an acidic mobile phase. As indicated in the previous question, that mobile phase does not always give good retention in reversed-phase. What is the best solution?

It is often useful to perform the chromatographic separation of bases at high pH for increased retention. Remember that this requires the use of a hybrid packing material, such as XBridge, CSH, or BEH. The post column addition technique described above can be useful, substituting formic acid for ammonium hydroxide. We have observed, however, that electrospray ionization is quite efficient in creating positive ions from high pH mobile phases. So, we usually just apply positive ionization to the mobile phase that is best for the chromatography.

T Wheat QDa blog

Is there a way of knowing whether you have multiple charge species present?

When you have singly-charged ions, the first 13C isotope is 1 mass unit from the mono-isotopic mass (molecular ion). When you have double-charged species, the first 13C isotope is 1/2 mass unit from the mono-isotopic mass. This pattern continues to higher charged states, but the narrower spacing will not be resolved with the QDa, or single quadrupole instruments in general. In this case however, isotope peaks will usually not be detected, an indication in itself of the likely presence of a more highly charged molecular ion.

I want to increase the M+H signal and lower the fragmentation signals. How can I do this?

The cone voltage setting has the largest influence on the amount of fragmentation. I would use a setting of 15 volts for my first injection, and then make a series of runs at progressively lower cone voltage settings in steps of 2 volts. A somewhat higher range is typically used for negative ionization mode.

With the QDa, is it possible to quantify 2 compounds with the same retention time?

Yes, it is absolutely possible to quantify 2 (or more) compounds with the same retention time. With single ion recording you define the m/z value you want to monitor. Each specified m/z value will appear as a chromatogram that can be integrated and calibrated independently.

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