4–2
Model 3480 Electrospray Aerosol Generator
Preparing a Buffer Solution
A buffer solution is intended for use in the Electrospray to clean the
capillary either before, after, or between samples. It is also useful to
use a buffer solution as the solvent to dissolve solids or nonvolatile
liquids when preparing samples. Key guidelines for choosing a
buffer solution include the following:
volatile solution
—the solution must evaporate in the
Electrospray chamber
chemical buffer
—the pH of the solution should not change
significantly when a sample is made
adjustable pH
—when making certain samples, such as
proteins, the pH value of the solution must be greater than the
isoelectric point (pI value) for the protein or the sample will stick
to the capillary walls
adjustable conductivity
—the primary droplet size produced by
the Electrospray is inversely proportional to the cube root of the
conductivity, therefore, there is a finite range of conductivities
over which the Electrospray will operate
A buffer solution used extensively in the Electrospray is 20 mM
ammonium acetate created by dissolving ammonium acetate in
ultrapure water. De-ionized water can be used in place of ultrapure
water if necessary; however, ultrapure water is recommended to
reduce the chances of buffer or sample contamination growth.
20 mM ammonium acetate is volatile, a weak buffer, and has an
adjustable pH and conductivity.
As an example to get started using the Electrospray, a 20 mM
ammonium acetate buffer solution with a conductivity of 0.2 S/m
adjusted to pH 8 is useful for dissolving solutes such as sucrose,
PSL, and proteins, to name a few. The procedure for making this
buffer solution (subsequently referred to as the standard buffer
solution) is outlined here:
1.
Dissolve 0.77 grams of ammonium acetate in 500 milliliters of
ultrapure or de-ionized water.
2.
Assuming the solution has an initial pH of 6.7, add 0.75
milliliters of 1M ammonium hydroxide.
3.
See the following sections for measurement and adjustment of
the buffer solution conductivity and pH.
Measuring and Adjusting Conductivity
A conductivity meter is useful for measuring the conductivity of a
buffer solution. The conductivity can be increased or decreased by
adding ammonium acetate or ultrapure water, respectively, to the
buffer solution. Although the Electrospray will operate at a wide
