Charged Aerosol Detection in Pharmaceutical Analysis: An Overview

Apr 01, 2009
Volume 27, Issue 4, pg 40–48

Charged aerosol detection (CAD) was first introduced commercially in 2004 (Corona, ESA Biosciences, Chelmsford, Massachusetts) and is based upon a combination of high performance liquid chromatography (HPLC) with electrical aerosol technology available since the 1970s (1–6). In CAD, the HPLC column eluent is first nebulized with a nitrogen (or air) carrier gas to form droplets that are then dried to remove mobile phase, producing analyte particles. The primary stream of analyte particles is met by a secondary stream that is positively charged as a result of having passed a high-voltage, platinum corona wire. The charge transfers difusionally to the opposing stream of analyte particles, and is further transferred to a collector where it is measured by a highly sensitive electrometer, generating a signal in direct proportion to the quantity of analyte present. A simplified schematic of how CAD works is illustrated in Figure 1.

Figure 1: A simplified schematic of a Corona charged aerosol detector (Figure courtesy of ESA, Inc. Chelmsford, Massachusetts.)
Because the entire process involves particles and direct measurement of charge, CAD is highly sensitive, provides a consistent response, and has a broad dynamic range, which offers some real advantages to researchers and analysts in the pharmaceutical laboratory, particularly when analyzing compounds lacking UV chromophores. Often compared to other universal-type HPLC detectors, like refractive index (RI) detection and evaporative light scattering detection (ELSD), CAD has been shown to be much easier to use, and unlike RI, can accommodate gradients. In addition, CAD response is not dependent upon the chemical characteristics of the compounds of interest, but on the initial mass concentration of analyte in the droplets formed upon nebulization, providing a much more uniform response as opposed to, for example, UV, where responses can vary dramatically according to the wavelength used and the extinction coefficient. It is precisely these advantages that make it an attractive addition to the pharmaceutical laboratory throughout all phases of drug development.

CAD has been used for a wide range of analyses throughout the drug development process, for example drug discovery (7), formulations research and development (8), natural product isolation (9), impurities (10,11), cleaning validation (12), drug substance and drug product characterization (13,14) and stability (15) among others. In most aspects, the charged aerosol detector is simple and easy to use and can be described as a "plug and play" detector requiring little in the way of special attention, unlike an evaporative light scattering detector. A comprehensive list of CAD applications by compound type is available (1). While many of the reported uses of CAD in the literature are for research and development (R&D) and method development, use in a regulated environment in support of Good Manufacturing Practices (GMP) also has been reported (15), where method validation and method transfer are important considerations. However, whether implementing the CAD in an R&D or quality control (QC) laboratory, in addition to highlighting its use, this article also discusses a few things to keep in mind to ensure success.

CAD in Analytical Method Development

The first question to answer during analytical method development (AMD) is "Will my compound respond?" While CAD certainly has advantages for detecting compounds that do not have UV chromophores, it can provide advantages (such as equivalent relative responses independent of the extinction coefficient) even for compounds that do have a chromophore because of its near universal response. The one single overriding criterion for determining analyte response is volatility — compounds of interest must be nonvolatile.

Table I: Physical characteristics vs. CAD response
Molecular weight, melting point, or boiling point cannot be used to predict a compound's volatility with any great accuracy because compounds that have similar molecular weights can have very different volatilities due to polarity and hydrogen bonding. For example, glycerol (MW 92; bp 290 °C) is detected easily to < 10 ng on column but propylglycerol (propanediol) (MW 76; bp 188 °C) is not. A better indicator of volatility is vapor pressure. Because substances with higher vapor pressure vaporize more readily than substances with a lower vapor pressure, the latter respond better to CAD. Table I lists a few compounds and their responsiveness to CAD along with their vapor pressure.

Solvents also play a role in CAD response; purity, volatility and viscosity are important factors. In general, using higher purity solvents, the background current is lower, leading to less noise and baseline drift from gradients due to fewer particles formed from nonvolatile impurities. One major requirement, however, is that because the CAD process involves nebulization to remove the mobile phase, volatile mobile phases must be used. That generally means aqueous–organic solvents (water–methanol–acetonitrile mixtures), with volatile buffer additives (when necessary) such as formic acid, acetic or trifluoroacetic acid, and ammonium acetate, similar to mass spectrometry (MS) mobile phase requirements. Finally, mobile phase viscosity also is important because it can affect both the nebulizer and drying process. Low viscosity mobile phases (that is, high organic) produce a greater number of droplets and particle generation is more efficient than those of high viscosity (that is, aqueous), increasing detector response and sensitivity. Also, with low viscosity mobile phases, more analyte is available for detection; with aqueous phases more analyte goes to waste, which affects sensitivity.

A good general approach to determine analyte response and solvent affects during AMD is to perform a flow injection analysis (FIA) experiment by injecting the analyte of interest into the mobile phase without the column in line (16). A typical AMD system might include multiple detectors in addition to the CAD, for example, UV–photodiode array (PDA) or MS detectors. Detectors in series are preferable to a parallel configuration to avoid flow splitting, however when used in combination with other destructive detectors (for example, MS) flow splitting is unavoidable. In series configurations, the CAD system should be placed last in line. A charged aerosol detector causes about 7 bar of back pressure, well within the range of typical UV–PDA detector flow cell limitations. In multiple detector system configurations extra care should be taken to make proper connections and to avoid excessive tubing lengths so as to not contribute additional dead volume that can lead to increased band spread.

Of course, in any AMD process, column choice is very important, a fact that naturally does not change with CAD. However, care should be taken to choose a column with minimal "bleed," as bleed in the form of nonvolatile compounds contributed by the column can result in increased background noise (17). For this reason, method developers sometimes choose polymeric based columns over silica to reduce background noise due to column bleed when sensitivity is of prime importance.

One additional AMD note, on the subject of fast HPLC: CAD detection is compatible with fast HPLC techniques on sub-2-μm particle columns to a point, as long as the width of analyte peaks is greater than 4 s (at base) and peak volume is greater than 40 μL (18). Future generations of CAD systems hopefully will push the envelope further to become fully compatible with fast HPLC technology.

lorem ipsum