Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR ETCHING GLASS
CAPILLARIE:S FO:R C~IROMATOGRAPHY
The present invention relates to chromatographic
column gla~s capillaries of the wall coated open tubular
(WCOT) type in which a stationary liquid phase is
present as a thin film supported on the inner surface
of the capillary. More specifically, the invention
relates to an improved process for uniformly matte-
etching ~eroding) the ir~er surface of a glass
capillary column to prepare the surface for deacti-
vation and coating with stationary phase for chroma-
tography. The process results in a uniform and a finestructured etch without resorting to the use of high
temperature thermal reactions with, e.g., hydrogen
fluoride gas.
Glass capillary columns have become increasingly
important for use in chromatography due to significant
advantages over metal. Of particular analytical impor-
tance is the relative catalytic inertness of glass,
especially for high temperature chromatography. It is
known, however, that stationary phase coating procedures
useful for metal capillaries seldom yield high efficiency
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glass capillary columns, or columns which are satisfac-
torily stable. To overcome these drawbacks, glass
surface modifications and treatments have been exten-
sively studied. Among these are surface roughening
procedures which theoretically should yield greatly
enhanced column efficiency and stationary phase film
stability.
Among the closest of these prior art methods to
the invention are those which use hydrogen fluoride
reagent to attack the glass surface. In this respect,
both gaseous ~nd aqueous hydrogen fluoride have been
used for surface roughening of glass capillary columns.
Aqueous hydrogen fluoride, however, does not, as used
in typical procedures, apply a matte-etch -to boro-
silicate glass, and treatment of flint or soft glasscapillaries by either gaseous or aqueous hydrogen
fluoride is extremely difficult to control. These
procedures, therefore, are not currently in widesprèad
use.
A more commonly applied method is that which
relies on the decomposition of ammonium bifluoride (or,
e.g., a fluorinated ether) which is deposited in the
capillary, the ends of the capillary sealed, and the
capillary subsequently heated -to ~50C to generate
hydrogen fluoride gas. This procedure causes what is
referred to as "silica whiskerl' formations on the inner
surface of -the glass. The major shortcomings of this
method remain a lack of satisfactory uniformity, fre-
~uently in the form of too severe an etch for chromato-
graphy purposes. In addition, the high temperaturereaction with hydrogen fluoride gas poses severe
potential hazards and must be approached with extreme
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caxe. This latter procedure, for more details, is
described in some depth by Onuska, J. of Chrom., 142
(1977), pp. 117-125.
Yet another surface roughening procedure for glass
capillaries for chromatography uses 10-20% aqueous
potassium difluoride reactant which is added to the
capillary bore, followed by rinsing with water to
dissolve the reaction product (K2SiF~), thus producing
an eroded or roughened surface texture. The resultant
etch is often gradient in nature, however, and thus not
satisfactorily uniform along the length of the column.
A new technique has now been discovered for
improved etching of glass ~borosilicate and soft glass)
capillaries for chromatography which overcomes the
deficiencies of the prior art. It is both much safer
to practice, being conducted typically a~ ambient
temperatures, and also is highly controllable. It thus
results in an extremely uniform and reproducible matte~
etching of the inner surface of the capillary producing
a surface with more ideal properties for depositing a
stationary phase.
More specifically, the present invention resides
in a method for etching the inner surface of a glass
capillary column for chromatography, and in preparation
for deactivating and coating the inner surface of the
capillary with stationary phase, comprising the steps
o~:
(a) flowing through the bore of the capillary
a liquid etching solution of ammonium bifluoride dis-
solved in an organic solvent,
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(b) continuing step (a) for a sufficient time
to produce a frosted surface,
(c~ thereafter adding a rinse solution of a
liquid organic solvent to the bore of the capillary,
(d) without segmenting the flow of the rinse
solution to the capillary, adding thereto water, thereby
rinsing the capillary with a solution of the organic
solvent mixed with water, and
(e) without segmenting the flow of the rinse
solution to the capillary, reducing or terminating the
addition of organic solven~t to produce a final liquid
rinse of the capillary wherein the rinse solution is an
effective amount of water sufficient to essentially
completely remove the etchant reaction product adhering
to the wall of the capillary to produce a uniformly and
finely etched surface~
Glass column capillaries for chromatography, which
are prepared by the inventive process, are generally
between about 0.1 0.8 mm I.D. and from about 10~100
meters in length. They are suitably made o~ borosilicate
(Pyrex~) or soda-lime (soft or 1int) capillary glass
stock.
A very important aspect of the invention in
achieving a uniform matte-etch of the inner surface of
these capillaries is the use of step (a) of the inven-
tive process in which a self-limiting reaction of the
glass is obtained by the selection of the proper etching
reagent and its solvent. Because the reaction is of a
self-limiting nature, it can be controlled and made
highly uniform for the entire length of the bore of the
capillary. The critical properties of the organic
solvent used in the invention is the solubility of a
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reactive amount of ammonium bifluoride, and relative
insolubility to the reaction product (NH4)2SiF6; and
for the rinsing steps, the further property of being
water miscible. The organic solvent is selected from
methanol, ethanol, or mixtures of methanol and ethanol,
or mixtures of the oryanic solvent and water. Most
preferably the organic solvent is methanol having
little or no dissolved water present.
Other solvents having the above recited critical
properties could be mixed with methanol, or substituted
in the alternative to effectively practice the invention.
Such alternative oxganic solvents are intended to be
covered within the broad scope of the term organic
solvent as herein defined. In addition, the organic
solvents used herein may also incorporate small amounts
of water in order to increase their polarity for
dissolving a reactive amount of ammonium bifluoride~
Thereafter, the invention is particularly described in
reference to using methanol as the most preferred
organic solvent.
The liquid etching solution is prepared by
dissolving preferably a saturated amount of ammonium
bifluoride (NH4HF2) in dry methanol to prepare a
solukion of about 4% (w/v) ammonium bifluoride in
methanol.
The etching solution is added, e.g., from a
suitable reservoir from which it is pushed by nitrogen
or other inert gas through the bore of the capillary.
Preferably ambient temperatures are used, although
elevated temperatures below the boiling poin-t of the
methanol carrier may be alternatively employed. Gen-
erally, about 3-7 column volumes of etching solution
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are pushed slowly through the capillary for a prolonged
period, e.g., 5-20 hours at flow rates of typically
about 1-10 ml/hr.
Etching by this process produces as the reaction
product (NH4)2SiF6 which is insoluble in methanol and
adheres to the glass sur~ace, thereby limiting further
reaction with the etching solution; and thus limiting
the depth of the reacted layer. Upon completion of
step (a), the capillary exhibits a highly uniform and
frosted surface over the entire length of the capillary
bore (visible only in the dry capillary).
~ ollowing the etching step, the capillaLy is
disconnected from the reservoir of etching solution,
and connected preferably to a li~uid chromatographic
pump to complete the remaining rinsing steps (c), (d),
and (e). Apparatus for practicing these steps comprises
preferably a mixing vessel suitably of about 40 ml
volume. The mixing vessel is connected by a switching
valve to reservoirs of water and methanol, respectively.
In practicing step (c) of the process, the mixing
vessel is filled with methanol which is pumped slowly,
suitably at a rate of about 0.5-1.0 ml/min, thxough the
capillary bore. Typically, about 3-10 column volumes
of dry methanol are passed through the capillary for an
initial rinse period of from 0.5 to 3 hours. The
methanol removes essentially all residual ammonium
bifluoride reactant without detrimentally disturbing
the adhered layer of (NH4)2SiF6. Without segmenting
the flow of the rinse solution to the bore of the
capillary, the switching valve is manually turned to
add water to the mixing vessel and terminate the flow
of methanol. A mixture of methanol and water is thus
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produced within the mixing vessel which gradually is
diluted to pure water as the vessel is emptied and
additional water is added. The water/methanol rinse
(and later essentially pure water rinse) gradually
dissolves and dislodges the (NH4)2SiF6 reaction
product. The slow graduation of flow from methanol to
water in the rinse avoids capillary plugging with
dislodged ~H4)2SiF6 particles-
Generally about 3-10 column volumes of the
methanol/water mixture, followed by 3-10 columns of
pure water are pumped slowly through the column to
obtain essentially complete removal of the (N~I4)2SiF6
reaction product. Following completion of this step,
and drying with purified nitrogen, a uniform fine
matte-etch is evident in the form of an easily
perceptible opagueness. Although the etch is very fine
structured, no problems with air entrainment have been
encountered (as with whisker columns) making the columns
of the present invention particularly suitable for all
standard coating procedures.
The final rinse step with essentially water is
preferentially practiced using pure water to insure
khat the (NH4)2SiF6 reaction product is entirely removed
from the capillary surface. The term "essentially
water" means in the broadest sense that there is a suf-
ficient amount of water in the rinse to accomplish this
removal. The rinse solution if other than pure water
may contain ingredients other than as would detrimentally
prevent this extraction from being carried to essential
completeness.
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The term "without segmenting" means the avoidance
of detrimentally introducing air bubbles into the rinse
solutions which would cause capillary plugging. In
this respect, particles of (N~4)2SiF6 dislodge and move
forward with the rinse solution. If an air bubble is
pxesent, the particles can collect and eventually plug
the capillary. Plugging can also occur if the transi-
tion from methanol to water is made too abruptly.
Upon completion of the etching process, the capil-
lary columns may be pxepared for storage by rinsingwith methanol, followed by methylene chloride, and
drying with purified nitrogen. The column ends are
then flame sealed to protect the column for prolonged
storage. Alternatively, the etched columns may be
coated and used immediately for chromatography by
deactivating and coating the inner bore of the capillary
with stationary phase.
Various applicable coating procedures are well
known and usually preceded by acid leaching of the
etched surface of the capillary with heated, concen~
trated hydrochloric acid, for a prolonged period of
time, e.g., as described by ~rob et al., Chromatographia,
lO, 181-187; followed by surface deactivation and
coating with stationary phase. Highly efficient and
thermally stable capillary columns are surface deac-ti-
vated and coated by using apolar and mixed apolar/polar
stationary phase coatings.
Example I
A borosilicate capillary is first flushed with
5-column volumes of methanol followed by methylene
chloride and then dried with purified nitrogen. Five
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column volumes of an etching solution of methanol
saturated with ammonium bifluoride (NH4HF2) are very
slowly pushed through the column such that the total
contact time is about 8 to 12 hours. The column is
then rinsed with about 5-column volumes of methanol and
about 5-column volum~s of water/methanol solution
according to the gradient rinse procedure described. A
final rinse of about 5-columns of water is used. The
rinse solutions are added slowly and without interrup
tion over a prolonged rinse cycle of from l to 4 hours.
After drying, an etched surface is evident in the
form of an easily perceptible opaqueness. A SEM photo-
graph reveals the etch to be extremely uniform and fine
structured~ The depth of the etch ranges from 0.1 to
0.2 micron with a distance of about 10 microns between
"peaks".
Exam~le II
A borosilicate capillary is first flushed with
5-column volumes of m~thanol followed by methylene
chloride and then dried with purified nitrogen. Five;
column volumes of an etching solution of ethanol
containing 10 percent by weight water saturated with
ammonium bifluoride (NH4HF2) are very slowly pushed
through the column such that the total contact time is
about 8 to 12 hours. The column is then rinsed wlth
about 5-column volumes of ethanol containing lO percent
by weight water and a~out 5-column volumes of water/-
ethanol solution according to the gradient rinse
procedure described. A final rinse of about 5-columns
o~ water is used. The rinse solutions are added slowly
and without interruption over a prolonged rinse cycle
of from i to 4 hours.
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After dxying, an etched surface is evident in the
form Or an easily perceptihle opaqueness.
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