I have read in some papers that nano liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) is more sensitive than conventional LC-MS/MS. But they never really mentioned why. I'm wondering how nano flowrate can achieve higher sensitivity.
The detector response is based on concentration C=m/V if you reduce de volume the relative concentrtion increases and thus sensitivity.
The detector response is based on concentration C=m/V if you reduce de volume the relative concentrtion increases and thus sensitivity.
The answer resides in the fact that a two-fold reduction of “column” diameter results in a quadrupling of signal/sensitivity. Thus a sequential reduction of diameter results in a four-fold increase in signal/sensitivity. From a 4.6 mm column to a 2.1 mm column results in a four fold increase in signal. 2.1 to 1 mm results in another four fold increase and on and on.
In nanoflow LC (nanoLC) chromatographic separations are performed using flow rates in the range of low nanoliter per minute, which result in high analytical sensitivity due to the large concentration efficiency afforded by this type of chromatography.
We have to differentiate between detection sensitivity and mass sensitivity. If all the conditions - including injection volume - are scaled appropriately scaled then the detection sensitivity of a normal bore application and a microbore or a nanobore application would be exactly the same. Let's have a look at a given case. Lets - for the sake of simplicity - compare a 4-mm-application with a 2-mm-application (the mm are for the column inner diameter). The downscaling factor - assuming the same column length for both columns - can be calculated as ID(1)^2/ID(2)^2. In our case 16/4 = 4. You would regulate down the flow rate by 4 (1 mL/min -> 0.25 mL/min) to address for, e.g., the otherwise significantly increased backpressure of the smaller bore column, and adjust the injection from 10 µL to 2.5 µL, et voila: You would have identical peak performance. What people often do is to downscale the flow rate, but keep the injection volume constant. As said above the concentration in the detector cell is important. By keeping the mass injected on the column constant but reducing the flow rate by a factor 4 the concentration in the cell would be m/ 0.25 v = 4 m/v. Hence we have a four-time higher concentration in the cell. The peaks will appear four times as big. But this is an increase in Mass sensitivity, which must not be mixed with detection sensitivity.
Hope this helps a bit.
Good luck with your research!
- Badges
- Science method