least squares linear equations fitted t o points before and after the end point. The first four titrations were done at a constant temperature of 25.0 "C. The remaining titrations were performed at ambient temperature. The shorter time allowed for mixing and the absence of temperature control did not affect the accuracy. Also, the different moisture contents of the solutions did not appear t o affect accuracy, but none of the titrations were run with solutions wet enough so that the picric acid formed its characteristic yellow picrate with the base water. Except in a few cases, errors less than 3 % were ob-
tained and the average error was +1.02% and the standard deviation of the method was 2.12 %. Titrations below about 10-3M PiOH become difficult because the dielectric constant does not change enough. Upper concentration limits are determined by the solubility of the acid, base, and especially the salt.
RECEIVED for review September 7,1971. Accepted December 7,1971.
Sample Injection Port for High Pressure Chromatography Brian Pearce and W. L1. Thomas The British Petroleum Company Limited, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex THEMETHODS which have been employed hitherto for introducing small samples of liquids into high-pressure chromatographic columns fall into three main categories. 1. The sample is inserted onto the column at atmospheric pressure, following which the column is connected into the system and high-pressure carrier fluid passed through. This method suffers from the disadvantage that the column has to be depressurized before each sample is introduced. There is also inevitably some loss in precision in measuring retention data. 2. The sample is introduced by means of a sampling loop in conjunction with a by-pass system. This necessitates at least three valves for isolating the sampling loop to permit further samples to be introduced. This method is rather cumbersome and is not very precise. Also the multiplicity of valves increases the complexity and dead volume of the equipment and makes leaks more likely. 3. The sample is inserted by means of pneumatic highpressure valves of suitable design. These have to be welldesigned and robustly constructed and they are accordingly rather costly. The method most widely used for introducing samples in conventional gas chromatography at atmospheric pressure i s the hypodermic syringe/septum technique. There is no doubt that this is a much more elegant method than those described above. The apparatus is simple and cheap, and it is eminently suitable for introducing very small and reproducible amounts of liquid. Unfortunately, the ordinary type of septum cannot be used at pressures much above 200 psig because the rubber septum pad does not have the requisite strength to withstand the pressure. Attempts have been made in the past to overcome this difficulty by backing the septum with a metal disk having a central hole to allow the needle of a hypodermic syringe to be inserted. However, it has been reported that the septa normally used in gas chromatography are extruded through orifices as small as 1 mm at high pressure (I, 2). In the present paper, a septum of novel design is described which was constructed during recent investigations at BP Research Centre, Sunbury, on super-critical fluid chromatography. This device, which has been employed extensively since that time and has given very satisfactory service enables microliter samples of liquid to be injected onto a column
Scott, D. W. J. Blackburn, and T. Wilkins, J . Gus Chromurogr., 5 , 183 (1967). (2) H. N. M. Stewart, R. Amos, and S. G. Perry, J . Clzromatogr., 38, 209 (1968). (1) R . P. W.
Table I. Pressure Drop and Thrust across Septum Pad 0.71 Diameter of hole mm 0.51 Pressure drop 1 bar (ga) 69 138 206 69 138 206 across pad \ psig 1000 2000 3000 1000 2000 3000 Thrustonunsupported! Newton 1 . 3 2.7 4 . 0 2.8 5 . 5 8 . 3 area of pad \ lbf 0 . 3 0 . 6 0.9 0.6 1 . 2 1 . 9 ~~
operating at pressures up to 213 bar (3100 psi) and temperatures up to 190 "C using an ordinary hypodermic syringe. In view of its potentiality in high-pressure chromatography, a Patent Application has been made covering the design of the injection port (3). EXPERIMENTAL
Design Considerations. Ideally an injection port of the type envisaged should satisfy the following requirements : It should provide on-column injection; it should be possible to use ordinary hypodermic syringes of the type used in GLC (e.g., Hamilton 10-pl syringes) to penetrate the septum pad; neither the injection port nor the syringe should present any safety hazard to operating personnel; and it should be easily serviced. The factors limiting the life of a septum are the surface area pierced by the needle, the applied force on the unsupported area, and the pressure drop across the septum when a leak has developed. In order to prolong the life of a septum as much as possible, it is essential that the same hole should be used every time a needle is inserted. It is known this can be achieved by placing a metal disk on the low-pressure side of the septum having a central hole just large enough to accommodate the syringe needle. The disk performs another important function also, oiz.,it supports the flexible septum against the high pressure in the column. The hole should be as small as possible to limit the forces acting on the unsupported area and keep the area of rubber septum perforated by the needle to a minimum. Calculated thrusts for various operating pressures and two sizes of hole are tabulated in Table I. The actual force on the unsupported area is very small. Since the internal diameter of the syringe needle is considerably smaller than the diameter of the hole, the force acting on the syringe plunger is even less than that indicated. Accordingly it is considered practicable to use an ordinary Hamilton syringe for sample injection. The pressure drop across the septum, and hence the forces acting on the septum, may be reduced in the new design by employing a second septum pad, provided, like the first pad, with a backing disk. If so desired, a multiple septum port of this type can be constructed. ( 3 ) UK Patent Application No. 43311/69.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
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N E E D L E INLET
Table 11. Septum Life
LOCKING B U S H
Number of injec-
WALL OF INJECTION PORT BACKING DISKS
SEPTUM P A D S CARRIER INLET
Septum Single -
Syringe Hamilton IC-bl
Double
Hamilton 10-pl (glass) SGE-1-pl (steel in steel)
l a ) MACHINED OUT OF A BLOCK OF METAL
Double NEEDLE INLET
(glass)
Ranges of operating tions pressures before bar (pa) psig failure 41 to 152 600 to 2200 204 110 to 206
1600 to 3000
249
124 to 206
1800 to 3000
87
LOCKING BUSH REDUCING S L E E V E WALL OF INJECTION PORT
BACKING DISKS
SEPTUM PADS CARRIER INLET
C O M B I N E D N EEDLEGUIDE AND SEAT FOR INJECTOR ASSEMBLY
( b l ADAPTATION OF AUTOCLAVE ENGINEERING ‘ T ’ UNION
Figure 1. Injection port
In order to simplify construction, the septum pads and disks may be housed in a standard high pressure “tee” drilled out as described later to provide a good fit for the various parts. The vertical limb of the “tee” may conveniently be used as the carrier fluid inlet while the horizontal limbs are used to c m nect to the column and to accommodate the injection port. With regard t o the syringe, it is convenient to fit a stop on the plunger guides to limit the outward movement of the plunger within the bore. The 2-inch (5-cm) needle is long enough to reach the end of the column and provide on-column injection. Description of Injection Port. The double septum injection port is shown in Figure l(a). The septum pads which are identical with those used in ordinary gas chromatography, are made of silicone rubber and are 0.928 cm in diameter and 0.312 cm thick. Each septum pad is backed by a stainless steel disk of the same diameter and thickness. The 0.051-cm diameter holes are drilled axially through each disk, and those holes are just large enough for 0.045-cm diameter standard hypodermic syringe needles to be inserted without undue difficulty. The septa and disks are located in a cylindrical chamber 0.932 cm in diameter and approximately 1.27 cm deep drilled out of a block of metal such as stainless steel. Alternatively, a n Autoclave Engineering 0.635 cm internal high pressure tee [Figure l(b)], or Ermeto 0.312 cm internal tee can be adapted for use. The disks and septa are retained in place by a locking nut which has a central hole of the same diameter as those in the disks. Easier insertion of the hypodermic syringe needle is ensured by chamfering the holes in the upper faces of the locking nut and backing disks, as shown in Figure 1. The components of the port are assembled as follows. The locking nut, backing disks, and septa are aligned and assembled in order, on a syringe needle. This assembly is inserted into the body of the tee, the locking nut is tightened by hand t o compress the septa, and the needle is withdrawn. This technique ensures correct alignment of the holes in the disks. The injection port is pressurized with carrier fluid, and soap solution is used to detect any gas leaks. These are easily cured by tightening the locking nut slightly. When the port is gastight, surplus soap solution is wiped away. 1108
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Table 111. Operating Conditions
Carrier fluid Carbon dioxide Nitrous oxide Argon Arcton 12 (dichlorodifluorornethane)
Upper pressure limit bar (gal Psig 230 3360 138 2000 152 2200 138
2000
Upper temperature limit, “C 150 150 50 190
It is advisable to use a Hamilton Kel-F plunger guide with Hamilton 10-pl syringes. This guide serves a twofold purpose: first, it prevents bending of the small diameter plunger and second, a metal bush fitted to the free end of one of the guide rods acts as a stop and limits the outward movement of the plunger in the barrel. Method of Operation. At pressures below 180 bar (2500 psi) samples are injected by means of a syringe through the injection port just as in conventional gas chromatography. Above this pressure, the plunger guide is positioned and secured. The syringe needle is pushed through the septa until its tip rests on the column packing. At the higher pressures, a small pair of pliers is useful for inserting the needle through the injectionport. The sample is injectedjn the usual manner. Leaks which develop over a period of time can be taken up by tightening the locking nut slightly. Eventually after prolonged use, these leaks cannot be cured in this way and it then becomes necessary t o replace the septa. DISCUSSION
Septum Performance. Although it has been found possible t o use a n injection port containing only one septum pad at quite high pressures, a double septum is to be preferred, especially at the higher pressures, since far less attention is required to maintain it leak-proof, A double septum also has a n appreciably longer life than a single septum as shown in Table I1 (It is to be noted that the double septum was used at much higher pressures than the single septum.) The SGE 1-p1 syringe has a larger needle than the Hamilton 10-p1 syringe and requires a backing disk with a larger hole (0.71 mm diameter instead of 0.51-mm diameter). Undoubtedly the much shorter life of the double septum using the SGE syringe is due t o the use of a larger needle which makes a larger perforation in the septum pad. This is borne out by the fact that the septa were extruded slowly over a period of time from the 0.71-mm diameter port but not from the 0.51-m diameter port. The double septum has been tested with various super critical carrier fluids using a Hamilton 10-pl syringe for injection under the conditions specified in Table 111. None of these carrier fluids had any deleterious effect o n the silicone rubber septa under the conditions employed. Also it should be stated that the temperatures and pressures
quoted above do not necessarily represent the maximum practical limits for the double septum, although as the pressures and temperatures are increased, a shorter septum life is to be expected. Finally it should be mentioned that in our experience an average of 80 injections may be achieved per syringe before failure occurs.
ACKNOWLEDGMENT
Permission to publish this paper has been given by The British Petroleum Company Limited. Thanks are also due to A. F. Harding for assistance with construction of the injectionport. RECEIVED for review December 7, 1971. Accepted February 14, 1972.
Versatile Low Cost Laboratory Integrator Donald R. Kendalll Atomics International, A Division of North American Rockwell Corporation, Canoga Park, Calif. 91304
VARIOUSTECHNIQUES have been applied to the integration of electronic signals in the analytical chemistry laboratory. These techniques include ball and disk mechanical, low inertia motor, electrochemical, analog to digital conversion followed by counting, and operational amplifier. The first four types of integrators above were evaluated by Sawyer and Barr for use in gas chromatography ( I ) . These four types of integrators have certain disadvantages, such as high cost, indirect readout, insufficient accuracy and precision and/or non-applicability to all types of laboratory signals. Operational amplifier integrators for short term integrations have also been described (2, 3). A low drift, chopper-stabilized operational amplifier used for the integration of long term signals is described by Harrar and Behrin (4). The state of the art of fabrication of solid state operational amplifiers has advanced to the point where integrators of excellent precision and accuracy can be produced at a relatively low cost. Operational amplifier integrators also possess the advantage of a direct readout which can, if desired, be easily adjusted to represent concentration units.
Figure 1. Operational amplifier integrator model circuit
Figure 2. Operational amplifier integrator circuit showing bias currents
THEORY
An operational amplifier integrator model circuit schematic diagram is shown in Figure 1. The output of the circuit E,,,, is related to the input Ei,,by Equation 1 . 1
r t
,
El
By using the operational amplifier ideality assumption, = E1,it can be shown that
r t
iRedt
+ RC
lt
E,, dt (1)
The first term of Equation 1 represents the desired output while the second through fourth terms represent output error created by integration of input bias current to the inverting input ib-, current leakage through the integrating capacitor iRc, and integration of the offset voltage E,,, respectively. The two bias currents for the two input transistors iband ibt are similar in magnitude. The error caused by integration of ib- can be significantly decreased by causing ib+ to flow through a resistance. Figure 2 shows an operational amplifier integrator circuit with grounded inputs in which such a resistance, R2, is included, and in which an EOcaused only by bias currents is considered. Present address, Allied Chemical Company, P. 0. Box 2204, Idaho Falls, Idaho 83401. (1) D. T.Sawyer and J. K. Barr, ANAL.CHEM., 34, 1213 (1962). (2) J. R. Barnes and H. L. Pardue, ibid., 38, 156 (1966). (3) H. V. Malmstadt and C. G. Enke, “Electronics for Scientists,” W. A . Benjamin, New York, N. Y . , 1963, p 356. (4) J. E. Harrar and E. Behrin, ANAL.CHEM., 39, 1230 (1967).
Eo = i b f Rz
+ CR1 Rz
-
s
ib+ dt
-C
s
ib- dt
(2)
By making R1 = R1 in Figure 2 and noting that the offset, = ib- - ib+, Equation 2 simplifies to Equation 3.
or difference current io.
EO = ib’R2
’s
-C
io, dt
(3)
With a value for C of 1 pF and bias current magnitudes of a few picoamperes, the first term of Equation 3 becomes negligible compared to the second term after a small portion of the integration period has passed. The equation for EO due to the integration of bias currents then becomes EO =
‘s
-C
io, dt which is significantly less than the EO =
‘S
- --C
ib- dt obtained in the absence of Rz. This holds true for both short term integrations where Rz would, for example, equal l o 3 ohms, and long term integrations where R2 would, for example, equal 106 ohms. The error caused by the integrating capacitor leakage resistance can be made negligible by employing a high quality dielectric material such as polystyrene, in the capacitor. With a 1-pF polystyrene integrating capacitor (10l2 ohms ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
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