Electrospray capillary diameter

The electrospray process is susceptible to competition/suppression effects. All polar/ionic species in the solution being sprayed, whether derived from the analyte or not, e.g. buffer, additives, etc., are potentially capable of being ionized. The best analytical sensitivity will therefore be obtained from a solution containing a single analyte, when competition is not possible, at the lowest flow rate (see Section 4.7.1 above) and with the narrowest diameter electrospray capillary. 

The solution to be electrosprayed is passed through the electrospray capillary (ESC) by means of a motor driven syringe. Some of the spray containing the ions then enters the pressure reducing capillary (PRC) leading to the forechamber (FCH) of the ion source. The exit tip of the PRC directs the gas jet in a direction parallel to the bottom of the FCH, i.e. across the interface plate (IN). An orifice of 4 mm diameter in the interface plate connects the FCH to the reaction chamber (RCH). The ions in the jet exiting from the PRC are deflected out of the jet towards this orifice and into the RCH by means of an electric field applied across the FCH. A weak field is also applied across the RCH. At the bottom of the RCH a small orifice, 100 pm diameter, allowed some gas and ions to leak into the vacuum of the mass... 

A number of investigators have observed that there are differences in performance of nanospray versus electrospray ionization. The very different conditions (temperature, capillary diameter, use of nebulizer gas) typically employed for conventional and low-flow electrospray make absolute comparisons of sensitivity at widely different flow rates difficult. The current perception, however, is that very low flow rate systems have greater mass sensitivity than higher-fiow-rate systems. 

The flow rate of liquid in the HPLC-electrospray system is paramount in determining performance both from chromatographic and mass spectrometric perspectives. The flow rate affects both the size and size distribution of the droplets formed during the electrospray process (not all droplets are the same size) and, consequently, the number of charges on each droplet. This, as we will see later, has an effect on the appearance of the mass spectrum which is generated. It should also be noted that the smaller the diameter of the spraying capillary, then... 

Micro- and nano-HPLC systems (Fig. 15.11) rely on small-diameter and capillary columns packed with high-efficiency packing materials and very slow flow rates to produce concentrated solutions and sharp chromatography peaks to feed electrospray interfaces for mass spectrometers. 

Nanoflow HPLC—HPLC system with accurately controlled reciprocating and syringe pumps designed to use capillary and small diameter, high-resolution columns as front ends for electrospray and nanospray mass spectrometer interfaces. 

Subsequently, nano-ESl was introduced [3]. Initially, the nano-ESl needles were made of gold-coated pulled glass capillaries with an emitter tip diameter of 1-3 pm. The needle is filled with 0.2-2 pi of liquid and positioned 1-2 mm from the ion sampling orifice in the ESI source. With a needle potential of l-2 kV, an electrospray can be generated with a flow-rate of 20 nl/min. 

The coupling of an MS with CEC or PEC provides several advantages. With the capillary columns of 100 mm inner-diameter (i.d.), flow rates of 1-2 L/min are obtained, which are ideal for electrospray MS [4]. No interface like a liquid sheath flow is required and the sintered silica gel frits allow direct coupling of the packed capillary columns without additional transfer capillaries. The spray is therefore formed directly at the outlet side of the column. Verheij et al. carried out the first coupling of a pseudoelectrochromatography system to a fast-atom bombardment (FAB)-MS in 1991 [6]. However, this required transfer capillaries that caused a loss in efficiency, which was also a problem with other experimentations with this technique. 

There are now two commonly defined regimes for electrospray analysis. These regimes are distinguished by the inner diameter of the capillary source, the liquid flow rate and the applied ionization voltage, and as already mentioned above these three parameters dictate the size of the generated droplets. These two regimes are... 

---