Method and device for on-column injection of a liquid sample into small diameter columns

A liquid sample in a syringe can be injected into a capillary column with inner diameter less than 200 microns without inserting the injection needle of the syringe into the column. The end of a modified injection needle according to this invention has a tubular opening with inner diameter sufficiently large so that the intake end of the column can be inserted inside, thereby establishing an annular duct between the needle and the column. A carrier gas is caused to flow through this duct while the plunger on the syringe is pressed to squeeze the liquid into the needle, thus forcing the liquid into the capillary column.

IN THE BACKGROUND 
This invention relates generally to a method and device for introducing a 
liquid sample into a small diameter column and more particularly to a 
method and device for on-column injection of a liquid sample into a 
capillary column with inside diameter less than 200 microns. 
Columns of increasingly smaller diameters have been used for gas 
chromatography. The advantages in using small diameter columns for trace 
analysis have been described, for example, by J. Hinshaw (5th 
International Capillary Symposium, Riva Del Garda, 1984). Wall-coated open 
tubular columns with internal diameters of 100 microns are now 
commercially available. Flame based detectors such as flame ionization 
detectors and flame photometric detectors generally provide acceptable 
performance when using these narrow diameter columns. A new design for an 
electron capture detector compatible with columns with internal diameters 
of 100 microns has been disclosed recently by G. Wells and R. Simon (High 
Res. Chrom. & Chrom. Comm. 6 (1983) 427 and 651) while the use of the 
split-splitless injection techniques and cold on-column injection with 
such columns was discussed by Onuska (J. of Chromatogr., 289 (1984) 207). 
Although the advantages of cold sample introduction into the column are 
well known in terms of mass discrimination and inertness, the conventional 
method of using a thin needle to place the sample inside a capillary 
column has the disadvantage of being limited to columns of inside 
diameters of about 200 microns or greater since the outer diameter of the 
needle must necessarily be smaller than the inside diameter of the 
capillary column. The inside diameter of such a thin needle would be too 
small and hence impractical. Moreover, a needle with outside diameter not 
much smaller than the inside diameter of the capillary column may easily 
scratch the inside of the column when placed directly inside. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide a method and device 
for on-column injection of a liquid sample into a capillary column with 
inside diameter less than 200 microns. 
It is another object of this invention to provide a method of sample 
introduction into a capillary column without the need for connectors or 
glued joints on the column or for inserting a needle inside the column.

DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention which relates to the on-column injection 
of a liquid sample into a capillary column, the liquid sample is initially 
inside a syringe of a conventional type equipped with a plunger. It is 
desired, however, to obviate the need of thrusting the syringe needle into 
the interior of the capillary column. For this reason, a modified type of 
needle is used with a conventional syringe according to this invention. 
These is shown in FIG. 1 an axial cross-sectional view of such a needle 10 
comprising a sheath 12 of stainless steel with inner and outer diameters 
about 0.18 mm and 0.64 mm, respectively, and an uncoated fused silica tube 
14 of inner and outer diameters 0.10 mm and 0.16 mm, respectively, sealed 
into the sheath 12. The silica tube 12 extends to the top (proximate end) 
of the sheath 12 where the needle 10 joins the syringe (not shown) while 
the silica tube 12 is terminated at the distal end approximately 8 mm from 
the bottom. In other words, the needle 10 has therethrough a narrower 
passageway 16 defined by the silica tube 12 and then an exit end section 
18 which defines a wider passageway extending a predetermined distance 
along the length of the needle 10 at the distal end. 
Referring next to FIGS. 2(a)-(d), there is schematically shown how to use 
according to the present invention a conventional syringe with a needle of 
the type shown in FIG. 1. FIGS. 2(a)-(d) show the method as a series of 
operations and, for this reason, same parts are assigned same reference 
numerals throughout. As shown in FIG. 2(a), a liquid sample 20 to be 
injected is initially inside the syringe 22 of a conventional type 
equipped with a plunger 24 and a needle 25 of a type described in FIG. 1. 
The syringe 22 is so positioned with respect to the capillary column 30 
that not only will the needle 25 and the column 30 be mutually coaxial but 
also the inlet 32 of the column 30 is inside the exit end section 34 of 
the needle 25, forming an annular duct 35 between the outer wall of the 
capillary column 30 and the inner wall of the needle 25 inside the exit 
end section 34. In the next step, the plunger 24 is pressed as shown in 
FIG. 2(b) so as to push the liquid 20 into the needle 25. A portion of the 
liquid 20, upon reaching the exit end section 34 with larger inner 
diameter, will move into the column 30, in part aided by the capillary 
effect, with the remaining portion moving inside the annular duct 35. 
In step 3 which is illustrated in FIG. 2(c), a carrier gas such as helium 
is introduced through the annular duct 35 (shown symbolically by arrows) 
in order to push back the liquid 20 from flowing down and out of the 
annular duct 35. The plunger 24 remains pressed in the meantime and this 
forces the trapped liquid 20 to move back and into the capillary column 
30. In practice, step 2 (FIG. 2(b)) and step 3 (FIG. 2(c)) may be started 
simultaneously. FIG. 2(d) shows how the liquid 20 is injected into the 
capillary column 30 as desired, although a small portion of the liquid 20 
is trapped by the pressure of the carrier gas above the exit end section 
34. Since the internal cross-sectional area of the needle 25 is very 
small, this means that only a very small fraction of the liquid initially 
in the syringe may thus be trapped inside the needle. 
For the purpose of proper positioning of the syringe with respect to the 
capillary column as explained above, it is convenient to use an insert 
device, or a needle-alignment means as shown, for example, in FIG. 3 in an 
axial cross-sectional form. According to this embodiment, the insert 40 
may be an aluminum piece 42 with a cylindrical hole 43 of inner diameter 
about 0.7 mm completely penetrating it, making funnel-like conical 
surfaces 44 at both ends. A stainless steel tube 45 of inner diameter 
about 0.25 mm is pressed on the inner surface of the hole 43, extending 
about one-half of entire thickness. For the positioning of the needle 
prior to the operation of FIGS. 2(a)-(d), the column is positioned from 
below and the needle is lowered from the above as shown in FIG. 3. 
The use of smaller diameter columns provides better resolution in addition 
to lower detection limits because the column bleed noise is less. A 
experiment using a 1075 split injector instead of an on-column injector 
showed that there is no apparent loss in resolution caused by the mode of 
injection; the lower column bleed noise of these smaller diameter columns 
allows lower detection limits when detectors of other types are used such 
as a flame ionization detector. 
The invention has been described above in terms of only one embodiment but 
the disclosure given above is intended to be illustrative and hence to be 
construed broadly. For example, FIGS. 1 and 3 are intended to be schematic 
diagrams. Dimensions and materials of various parts of the injection 
needle and the insert means may be freely varied. Regarding the insert 
means 40 of FIG. 3, in particular, the disclosed method of forming a hole 
having two sections with different inner diameters is not to be considered 
as a limitation. The only requirement regarding the inner diameters of the 
two sections is that the larger inner diameter must be large enough to 
admit the needle while the smaller diameter must be large enough for the 
capillary column to pass through but not for the needle. The scope of this 
invention is limited only by the following claims.