A new pneumatic nebulizer designed by the Group of Bio-Inorganic Chemistry at Pau (EVISA member) under the leadership of Dirk Schaumlöffel allows for the first time the direct coupling between nanoHPLC and ICP-MS. While the nebulizer works within a tiny spraychamber of only 3 ml, the sample arriving at optimum flow rate of 0.2-0.5 µL/min will be transferred quantitatively into useful aerosol, providing high sensitivity and excellent precision.
Figure A: The nanoNebulizer with its small spraychamber coupled to the ICP torch
The direct coupling of nanoHPLC to ICPMS avoids complication by a second pump needed for a make-up flow, resulting in analyte dispersion and loss of chromatographic resolution and detection limits.
Figure B: Design of the nanoNebulizer fitted via miniSpraychamber to ICP torch
The nanoflow nebulizer nDS-200 is using a hollow fused-silica capillary needle (3) as nebulizer capillary. The small needle tip (i.d. 10 µm, o.d. 20 µm) is centered in a 254-µm i.d. sapphire orifice (4) allowing a stable and continuous nebulization of nanoliter flow rates of less than 500 nl/min. Nebulization characteristics can be optimized by adjusting the position of the needle tip in the nebulizer orifice.
The outlet capillary (1, 20-µm i.d, 280-µm o.d.) of the nanoHPLC column is fitted into the nebulizer via a zero-dead-volume connection to the nebulizer needle in order to avoid excessive band broadening of the narrow chromatographic peaks. This zero-dead-volume connection is realized by a port (i.d. 1.6 mm) for the capillary collumn surrounded by a piece of plastic tubing (orange in Figure B) having a small through-hole (2, i.d. 300 µm) into which the HPLC capillary fits allowing a precise abutting of the capillaries.
The small inner diameter of the nebulizer tip results in a velocity of the liquid flow (~ 4.2 cm/s at 200 nL/min) that is similar to that in a commercially available nebulizer operated at 5 µL/min.
body material: FEP
nebulizer gas flow: 0.9 -1.2 l/min (1.05-1.15 l/min optimum)
sample flow: 50 - 2000 nl/min (250-450 nl/min optimum)
precision: < 5% RSD for flows > 250 nl/min
mode of operation: pumped sample flow (non self-aspirating)
The efficient nebulization results in complete evaporation of droplets already within the tiny spraychamber. Therefore the sample flow rate has no influence on the oxide formation and doubly charged ions within the ICP, both beeing kept at very low values.
Sensitivity is linearly increasing with sample flow rate over the working range and precision is better than 5% RSD for sample flow rates bigger than 200 nl/min.
Since the plasma solvent load is orders of magnitude lower than with micro- or conventional nebulizers, selection of the mobile phase is not restricted by plasma stability problems.
The characteristics of the nanoNebulizer can be summarized:
- No metal parts in contact with sample flow
- Minimized dead-volume for reduced dispersion not restricting the obtainable resolution by nanoHPLC
- High sensitivity through quantitative nebulization
- Free selection of mobile phase due to reduced solvent load
- High stability due to robust construction
- High detection power due to improved S/N ratio by very narrow peaks
- Direct coupling of nanoHPLC with ICP-MS
- Direct comparison with nanoESI-MS possible (similar flow conditions)