Microelectrospray method and apparatus

A method of generating an electrospray from a solution is disclosed which is useful for electrospraying at low flow rates, for electrospraying directly into a vacuum, or both. The method comprises the steps of causing the solution to flow through a capillary tube to an outlet opening formed in the terminal portion thereof. The capillary tube terminal portion has an inner diameter of not more than 50 micrometers and an outer diameter of not more than 150 micrometers. An electrical potential difference is provided between the capillary tube terminal portion and a conductor spaced from said terminal portion, with the potential sufficient to cause the solution to electrospray from the capillary outlet. The method is particularly useful for detecting analytes in the solution being electrosprayed by mass spectral analysis.

FIELD OF THE INVENTION 
The invention relates to electrospray in general, and particularly relates 
to methods and apparatus for carrying out electrospray at low flow rates 
directly into a vacuum. 
BACKGROUND OF THE INVENTION 
The electrical spraying of liquids to produce ions, termed electrospray, 
was first used as a tool in analytical chemistry in the late 1960's. This 
initial work was not expanded upon until 1984, when John Fenn and 
co-workers reported combining electrospray ionization and mass 
spectrometry (ESI/MS) to analyze high molecular weight species. See, e.g., 
U.S. Pat. No. 4,542,293 to J. Fenn et al.; U.S. Pat. No. 4,531,056 to M. 
Labowsky et al. 
A major limitation of current electrospray technology is that the 
electrospray is performed at atmospheric pressure. See, e.g., U.S. Pat. 
No. 4,977,320 to Chowdhury et al.; U.S. Pat. No. 4,935,624 to Henion et 
al.; U.S. Pat. No. 4,885,076 to Smith et al.; U.S. Pat. No. 4,842,701 to 
Smith et al. This creates a need for differential pumping and reduces 
sampling efficiency. See M. Ikonomou et al., Anal. Chem. 62, 957 (1990); 
M. Mann, Organ. Mass Spec. 25, 575 (1990). A further problem is that high 
flow rates (e.g., above 0.5 .mu.l/minute) have been required. R. Smith et 
al., Anal. Chem. 60, 1948 (1988). 
In view of the foregoing, objects of the present invention are to provide a 
way to electrospray directly into a vacuum, and a way to electrospray at 
low flow rates. 
SUMMARY OF THE INVENTION 
The foregoing and other objects and advantages are provided by a method of 
generating an electrospray from a solution. The method comprises the steps 
of causing the solution to flow through a capillary tube to an outlet 
opening formed in the terminal portion thereof. The capillary tube 
terminal portion has an inner diameter of not more than 50 micrometers and 
an outer diameter of not more than 150 micrometers. An electrical 
potential difference is provided between the capillary tube terminal 
portion and a conductor spaced from said terminal portion, with the 
potential sufficient to cause the solution to electrospray from the 
capillary outlet. 
A particular embodiment of the present invention comprises a method of 
detecting an analyte by mass spectral analysis in a solution containing 
the analyte. The method comprises causing the solution to flow through a 
capillary tube to an outlet opening formed in the terminal portion 
thereof, the capillary tube terminal portion having an inner diameter of 
not more than 50 micrometers and an outer diameter of not more than 150 
micrometers. The ambient pressure at said capillary outlet is maintained 
at less than 10 millitorr. An electrical potential difference is provided 
between the capillary tube terminal portion and a conductor spaced from 
the terminal portion, with the potential sufficient to cause the solution 
to electrospray from the capillary outlet and ionize the analyte in the 
solution. The ionized analyte is then detected by mass spectral analysis. 
An apparatus of the present invention for generating an electrospray from a 
solution comprises a capillary tube having a terminal portion and an 
outlet opening formed in the terminal portion, with the capillary tube 
terminal portion having an inner diameter of not more than 50 micrometers 
and an outer diameter of not more than 150 micrometers. A chamber (e.g., a 
vacuum chamber) is included which contains the capillary tube terminal 
portion, the vacuum chamber further containing a conductor spaced from the 
capillary tube terminal portion. A Power supply is provided for 
maintaining an electrical potential difference between the capillary tube 
terminal portion and the conductor spaced from the terminal portion, with 
the potential sufficient to cause the solution to electrospray from the 
capillary outlet. Preferably, the apparatus also includes a pump for 
maintaining the ambient pressure in the chamber at less than 10 millitorr. 
The chamber also preferably contains a mass spectrometer. The aforesaid 
conductor may comprise a mass spectrometer entrance lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention may be used to detect a variety of different 
analytes, including (but not limited to) proteins, peptides, polynucleic 
acids, fats, lipids, sugars, carbohydrates, synthetic polymers, metal 
ions, and small ions. In general, any analyte which is soluble in a polar 
or moderately polar solvent solution can be employed. 
Suitable solvent solutions include, but are not limited to, water, 
methanol, acetonitrile, and mixtures thereof. The solvent solution may be 
caused to flow in the capillary at a rate of from 0.1 to 100 nanoliters 
per second. Preferred are flow rates of from 0.1 to 10 nanoliters per 
second, with flow rates of 1 nanoliter per second being typical. 
An apparatus of the present invention is shown schematically in FIG. 1 The 
electrospray capillary 10 is comprised of the downstream end of the 
capillary electrophoresis capillary 20 of a capillary electrophoresis 
apparatus 21. The electrospray capillary terminates in a vacuum chamber 
22, to which is connected a vacuum pump 23 for evacuating the chamber. A 
power supply 24 is electrically connected to the electrospray capillary 
and ground. A commercial tandem triple quadrupole mass spectrometer 25 is 
provided in the vacuum chamber, and the entrance lens 26 of the mass 
spectrometer is electrically connected to ground. The entrance lens is 
spaced from the electrospray capillary 10 so that the electrospray 
emanating from the electrospray capillary is directed towards the mass 
spectrometer 25. 
The establishment of a potential which will produce an electrospray can be 
carried out in accordance with known procedures. The electrospray 
capillary can be held at +1,200 volts with respect to ground. Higher 
voltages may be employed, and spray may also be seen at lower voltages 
(e.g., +800 volts). The capillary may be held at a negative potential, but 
this is less preferred. 
The pressure in the vacuum chamber should be held at a pressure suitable 
for mass spectrometry, such as 2 microtorr. The present invention, 
however, has utility for applications other than mass spectrometry, and 
thus may in general be carried out at pressures less than 10 millitorr 
(e.g., less than 1 milliltorr; less than 0.1 millitorr). If electrospray 
into a vacuum is not required for a particular application, then the tip 
of the present invention may also be employed to electrospray into higher 
ambient pressures, such as 10 torr and above, including atmospheric 
pressure (760 torr). Ambient pressures of from 10 millitorr to 10 torr 
tend to be less preferred because of the glow discharges which occur on 
electrospraying solutions at these pressures, though this may not 
necessarily be deleterious for all applications. 
An electrospray capillary of the present invention is schematically 
illustrated in FIG. 2. The electrospray capillary 10 has a capillary bore 
11 formed therein through which the solution to be electrosprayed flows. 
The capillary tapers from an intermediate region 12 of large diameter to a 
terminal portion 13 of relatively smaller diameter. The terminal portion 
has an outlet opening 14 for the capillary bore 11 formed in the tip 15 
thereof from which the electrospray emanates. 
The terminal portion 13 inner diameter, which defines the outlet opening 
14, is generally not greater than 50 micrometers, and is preferably not 
greater than 20 micrometers. The inner diameter of the terminal portion 
need only be sufficiently large to enable an electrospray to emanate 
therefrom and can be as small as 1 or 2 microns in diameter. Currently 
preferred are tips having an inner diameter at the terminal portion of 
between 5 and 20 micrometers. The inner diameter of the capillary 
intermediate portion may optionally be greater than the inner diameter of 
the capillary terminal portion, with the inner diameter tapering and 
decreasing from the intermediate portion to the terminal portion, if 
desired for particular applications (e.g., with capillary electrophoresis 
capillaries, which have bores of 50 to 70 micrometers). 
The capillary tube terminal portion 13 preferably has an outer diameter of 
not more than 150 micrometers, and more preferably has an outer diameter 
of not more than 100 micrometers. The lower limit to the outer diameter 
will depend upon the particular material from which the tip is fabricated, 
but may be as low as 5, 10, or 20 micrometers. Currently preferred are 
capillary tubes having a terminal portion with an outer diameter of from 
about 50 to about 70 micrometers. The outer diameter of the capillary tube 
illustrated decreases from the intermediate portion to the terminal 
portion, forming an elongate tapered region having a cone half angle of 
about 10 to 20 degrees. 
An electrospray capillary used to carry out the present invention can be 
made of any suitable conductive material, such as steel, conducting 
polymers, fused silica, and glass (e.g., soda lime glass, borosilicate 
glass). Tips constructed of fused silica or glass are metallized with a 
material such as gold, silver, or platinum by processes such as sputtering 
or vapor deposition. A metal layer 15 is illustrated in FIG. 2. 
A metallized borosilicate glass capillary tube for use in carrying out the 
present invention can be fabricated in a routine manner. In a preferred 
embodiment of the invention, PYREX 7740.TM. tubing having an internal 
diameter (i.d.) of 0.2032 millimeters and an outer diameter (o.d.) of 5 to 
6 millimeters is purchased from Wilmad Glass, Buena, N.J., USA, and drawn 
on a Shimadzu GDM 1B drawing machine at a draw ratio of 160. One end of a 
one meter section of drawn capillary tubing is immersed to a depth of 2 
millimeters in 10 to 20 milliliters of 48% aqueous hydrogen fluoride 
solution and etched, with stirring, for about twenty minutes. During 
etching, water is pumped through the tubing into the etch bath at a 
velocity of about 1 centimeter per second to prevent etching of the 
interior of the capillary tube. The etched capillary tip is then coated 
with gold by ohmically heating a gold wire in a bell jar with the tip 
under a vacuum, in accordance with known procedures. An electron 
micrograph of a tip produced in essentially this manner is shown in FIG. 3 
and FIG. 4. 
The electrospray apparatus of the present invention can be directly 
operatively associated with a mass analyzer such as a commercial 
quadrupole mass spectrometer for mass spectrometric analysis of analytes 
by modification of known procedures, such as described in U.S. Pat. Nos. 
4,977,320; 4,935,624; 4,885,076; 4,842,701; 4,542,293; and 4,531,056, in 
light of the instant disclosure. Applicants specifically intend that the 
disclosures of these and all other patent references cited herein be 
incorporated herein by reference. As noted above, the present invention 
eliminates the need for skimmers and differential pumping when 
electrospraying into the mass analyzer, as the electrospray may be carried 
out directly into a vacuum. As an alternative to mass spectroscopy, ions 
formed by electrospray into a vacuum in accordance with the present 
invention may be analyzed by other vacuum spectroscopy means, including 
but not limited to laser spectroscopy, flame spectroscopy, atomic 
spectroscopy, and ion traps. 
In the present invention, the electrospray capillary can comprise any 
sample delivery capillary, including the downstream end of a capillary 
liquid chromatography capillary or a capillary electrophoresis capillary. 
Capillary electrophoresis includes variations such as capillary zone 
electrophoresis, electrokinetic chromatography and isotachophoresis. An 
electrospray apparatus of the present invention can be operatively 
associated with a capillary electrophoresis apparatus in accordance with 
known procedures, such as shown in U.S. Pat. Nos. 4,885,076 and 4,842,701 
to R. Smith et al.; and can be operatively associated with a liquid 
chromatography apparatus in accordance with known procedures, such as 
shown in U.S. Pat. No. 4,935,624 to Henion et al. The present invention, 
however, eliminates the need for a sheath electrode liquid or a make-up 
flow, hence the solution which is electrosprayed consists essentially of 
the solution subjected to capillary liquid chromatography or capillary 
electrophoresis 
The foregoing is illustrative of the present invention, and not to be 
construed as limiting thereof. The invention is defined by the following 
claims, with equivalents of the claims to be included therein.