Multifunctional detector

A detector assembly includes means for determining the fluorescence characteristics, the ultraviolet absorbance characteristics and the electrical conductivity characteristics of a single sample of an eluate of a chromatographic separating column.

The present invention generally relates to a detector especially useful in 
liquid chromatography and, in particular, relates to a detector having 
multiple detection functions. 
In liquid chromatography, a sample solution is passed through a separating 
column which is designed to partition the sample into its constituents 
such that the constituents are serially eluted. The eluent of the column 
is analyzed to characterize the constituents as they elute. 
Generally, the characterization of liquid chromatography eluents is 
performed by determining the absorption, the fluorescence or the 
conductivity properties of the eluent. Presently, each of these properties 
requires a different discrete detector mechanism. For example, to measure 
light absorption, the detector mechanism must direct radiation through the 
eluate and detect that amount of radiation which passes therethrough. To 
measure fluorescence, the detector must include a source of radiation to 
initiate the fluorescence and a sensor, usually positioned at a right 
angle to the initiating radiation, for receiving the fluorescence. The 
measurement of the conductivity of an eluate requires at least two spaced 
apart electrodes in contact with the eluent. 
Due to the inherent physical configurations and component requirements, 
conventional liquid chromatography detectors measure only one of the above 
properties. Hence, in order to investigate more than a single property of 
an eluate it has been necessary to either arrange multiple detectors in 
series or perform multiple separations with the same sample but with 
different detectors. 
In the first approach, i.e., the use of multiple detectors in series, the 
major factor inherently present which reduces the accuracy and reliability 
of the results is that the band dispersion of the eluate is always 
increased as it is passed through each detector. 
The major difficulty in exploring the second approach is, or course, the 
time and expense required to perform the identical separation two, three 
or more different times. 
SUMMARY OF THE INVENTION 
Accordingly, it is one object of the present invention to provide a 
detector useful in liquid chromatography which can detect a plurality of 
properties of an eluate. 
This object is accomplished, at least in part, by a housing block having at 
least two different property characterization elements operatively 
associated with a single sample cell therein. 
Other objects and advantages will become apparent to those skilled in the 
art from the following detailed description read in conjunction with the 
appended claims and the drawing attached hereto.

DETAILED DESCRIPTION OF THE INVENTION 
A detector assembly, generally indicated at 10 in the drawing and embodying 
the principles of the present invention, includes a housing block 12 
having a sample cell 14 defined therein. The detector assembly 10 also 
includes a source of radiation 16, means 18 for measuring the electrical 
conductivity of a liquid passing through the sample cell 14, means 20 for 
detecting radiation passing through the sample cell 14, and means 22 for 
detecting fluorescence radiation occurring within the sample cell 14. The 
detector assembly 10 is also provided with sample inlet and outlet 
conduits, 24 and 26, respectively, for passing a sample fluid through the 
sample cell 14. 
In a preferred embodiment, the detector assembly 10 includes the source of 
light radiation 16 in a first opening 28 near one end 30 of the housing 
block 12, a first photodetector 32 disposed inwardly of the other end 36 
of the block 12, a second photodetector 38 in a third opening 40 in one 
sidewall 42 of the block 12, and first and second spaced apart electrodes, 
44 and 46 respectively, extending through a second sidewall 48 of the 
block 12 and adapted to be contacted by liquid passing through the sample 
cell 14. 
In addition, the housing block 12 includes a bore 50 therein for receiving 
the sample cell 14 and having a first optical window 52 at one end 54 of 
the sample cell 14 and a second optical window 56 at the other end 58 of 
the sample cell 14. Preferably, the second optical window 56 is shaped to 
disperse light passing therethrough from the sample cell 14 over more of 
the surface 60 of the first photodetector 32. 
In the preferred embodiment, the first and second electrodes, 44 and 46 
respectively, are hollow and serve as the inlet and outlet conduits, 24 
and 26 respectively, and each is sandwiched between a pair of seals 62 
which form a fluid tight seal about the ends of the sample cell 14 as well 
as electrically isolate the electrodes, 44 and 46. 
As shown in FIG. 1, the second optical window 56 is securely positioned by 
means of a retaining washer 64 which is urged against the second window 56 
by a pair of spring washers 66 held in place via a lens retaining nut 68. 
The first photodetector 32 is fixedly positioned within the lens retaining 
nut 68 by means of a detector retaining nut 70. In this fashion the 
spacing between the second optical window 56 and the first photodetector 
32 remains constant. As a consequence, the detector measurement 
characteristics of the first photodetector 32, i.e., which primarily 
depends upon both the spacing between the second window 56 and the surface 
60 of the photodetector 32 and the specific surface area of the 
photodetector used, remains constant for a given photodetector. 
The detector assembly 10 is clearly advantageous since any one of a 
plurality of properties can be measured without interchanging the 
detector. Additionally, since more than one property can be measured 
simultaneously, all properties are measured on exactly the same sample. 
A system, generally indicated at 72 in FIG. 2 and specifically adapted to 
function with the detector assembly 10, for measuring a plurality of 
properties includes a high voltage power supply 74 for energizing the 
source of light radiation 16 thereof. The first and second electrodes, 44 
and 46 respectively, are connected to a measuring circuit 76 which, in the 
preferred embodiment, includes a Wheatstone bridge type circuit for 
accurately measuring the impedance between the electrodes, 44 and 46. 
The first and second photodetectors, 32 and 38 respectively, of the 
detector assembly 10 are electrically connected to a signal amplifier 
circuit 78 which provides a representative output signal to a recording 
mechanism 80. Preferably, the recording mechanism 80 includes, as at least 
one mode thereof, a strip chart recorder for providing a chromatogram. 
Another major advantage of the detector assembly 10 is the ease by which it 
can be fabricated and assembled. This advantage is more fully discussed 
hereinafter with respect to a specific embodiment shown in FIGS. 3 and 4. 
A 2.5 cm by 2.5 cm by 3 cm long housing block 12 of opaque chemically 
resistant material, such as, for example, Delrin.RTM., a registered 
trademark of the duPont Corporation, is initially machined as shown in 
FIG. 3. Specifically, the lamp receiving first opening 28 is formed with a 
diameter of about 8 mm and axially located about 6 mm from the one end 30 
of the block 12. A 3 mm.times.1.5 mm slot 82 is formed in the third 
opening 40 and opens into the bore 50 which passes through the center of 
the block 12. Preferably, the bore 50 is about 4.5 mm in diameter. A 2 mm 
long internal opening 84 of about 3 mm diameter is formed between the bore 
50 and the first opening 28 to form a passage through which light 
radiation passes into the bore 50. The third opening 40, having a diameter 
of about 1 cm, is formed in the one sidewall 42 generally symmetrical 
about the slot 82. Preferably, the third opening 40 is axially 
perpendicular to the bore 50. This is sized to receive the second 
photodetector 38, i.e., which measures the fluorescence radiation. 
A second opening 34, for receiving the lens retaining nut 68 and the first 
photodetector 32 is formed in the block 12 from the other end 36 to a 
depth of about 8 mm. Preferably, the first opening 28 is provided with an 
internal thread 86, for example, 3/8-24 threads per inch, and the second 
opening 34 is provided with internal threads 88, for example, 1/2-20 
threads per inch. In order to simplify the final assembly of the 
electrically conductive inlet and outlet conduits 24 and 26, respectively, 
a 0.8 mm slot 90 is formed in the block 12 from the other end 36 and 
extends thereinto to the end of the bore 50, i.e., a distance on the order 
of about 1.9 cm. 
Referring now more specifically to FIG. 4, the final assembly of the 
detector assembly 10 is more fully discussed hereinafter. 
As shown, the first optical window 52, in the form of a flat surfaced 
quartz lens, is inserted into the bore 50 from the other end 36 of the 
block 12. Adjacent that lens is a 0.125 mm thick polytetrafluroethelene 
(PTFE) seal 62 followed by the mobile phase inlet conduit 24. As an 
alternative, the seals 62 can also be formed from other chemically 
resistant materials such as that known as KAPTON.RTM., a registered 
trademark of duPont Corporation. The mobile phase inlet conduit 24 
includes a washer-like end 92 having a 0.75 mm diameter opening 94 
therethrough. A second PTFE seal 62 is inserted and thereafter the sample 
cell 14 is inserted adjacent the second seal 62. Preferably, the sample 
cell 14 is a glass disk having an outside diameter of about 4.5 mm and an 
opening 96 therethrough which is 0.75 mm diameter by 2 mm long. For 
reasons well known in the art, such as a sample cell 14 introduces very 
little band dispersion and is a relatively low volume cell. Thereafter, a 
third PTFE seal 62 and the mobile phase outlet conduit 26, also having a 
washer-like end 98 and an opening 100 therethrough, are inserted and 
followed by a fourth PTFE seal 62. This arrangement completely seals the 
bore 50 and prevents fluid leakage through the slot 90. Further, this 
assembly 10 seals the electrodes, 44 and 46, and prevents sample leakage 
thereabout. Finally, the second window 56, preferably having a concave 
surface 102 distal the light source 16, is inserted and retained in place 
by retaining washer 64. In order to secure, and more exactly position the 
second window 56 with respect to the first photodetector 32, the second 
window 64, and hence the assembly within the bore 50, is retained in place 
by the pair of opposing spring washers 66 biased inwardly by means of the 
lens retaining thread 105 which mates with the internal thread 88 of the 
second opening 34. 
In one particular embodiment, the sample cell 14 is opaque to the radiated 
light, with the consequence that light is only passed through the opening 
96 into the cell 14, but transparent to the fluorescent radiation. For 
example, if the source of radiation 16 transmits in the approximate 
wavelength band of 180 nm to 280 nm and the fluorescent radiation of 
interest is greater than about 350 nm, such as the conditions when 
characterizing quinine sulfate, a borosilicate glass could be used for the 
sample cell 14. Of course, other light sources, such as, for example, 
light emitting diodes, infrared, could also be used with a corresponding 
selection of material for the sample cell 14. 
Referring back to FIG. 1, the first photodetector 32 is then secured within 
the lens retaining nut 68 by means of a detector retaining nut 70 having 
openings 104 therethrough for the electrical leads 106 thereof to be 
connected to the amplifier circuit 78 external the block 12. In the 
preferred embodiment, the electrically conductive inlet and outlet 
conduits, 24 and 26 respectively, are formed from stainless steel tubing 
having an inside diameter of about 0.25 mm and an outside diameter on the 
order of about 0.5 mm. The washer-like disk is provided at the ends 
thereof to not only ensure electrical contact between the conductive 
conduits, 24 and 26, and the fluid flowing therethrough, but also provides 
a surface against which the seals 62 can create a fluid tight seal for 
both the sample cell 14 and the electrodes 44 and 46. 
The detector assembly 10 described herein not only provides the 
chromatographic advantages of low volume and low dispersion, but 
additionally provides major advantages in the selection and 
characterization of an eluate from a liquid chromatography separating 
column. As will be understood from the previous discussion relating to the 
system 72, shown in FIG. 2, any one of three properties can be measured 
either individually, in pairs or all three simultaneously. Such 
availability of measuring possibilities on the identical amount of sample 
fluid provides reliable determinations and characterizations of eluents 
from separating columns. 
In designing a trifunctional detector of the type disclosed, there are 
numerous considerations, some of which interact with each other, that must 
be accounted for in the final design. Ideally, the sample cell should be 
small in volume and preferably less than 3 .mu.L although larger sized 
cells will function but not as efficiently. The cell must be sufficiently 
long to provide an adequate signal according to Beer's Law. As a 
consequence of these two criteria, the cross-sectional area of the cell 
must be very small. In order that the radiation intensity is sufficient to 
be detected as intensity is controlled by the inverse square of the 
distance, the radiation source must be located as close as possible to the 
face of the cell. It also follows for this reason that the radiation 
sensor should be as close as possible to the other face of the cell. As 
the cross-sectional area of the radiation passing through the sample is 
small, it is desirable to have a divergent lens to distribute the 
radiation passing through the sample over the surface of the radiation 
sensor. 
In order for the trifunctional detector to be able to measure sample 
conductivity, the electrodes must be sufficiently far apart so as to 
permit conductivity measurement. The sample cell and the rest of the 
detector must also be made in a manner so that the electrodes are 
electrically isolated from each other. Also, since conductivity must be 
measured over the length of the sample cell, the electrodes must be 
designed so that they do not interfere with the radiation passing through 
the sample. 
The cell of the trifunctional detector must also be designed so that its 
walls are transparent to florescent light from the sample therein. The 
fluorescence sensor must be located as close as possible to the cell to 
avoid loss of sensitivity. It is also desirable to be able to locate a 
filter between the cell and the fluorescence sensor to filter the incident 
radiation and transmit the flourescent radiation. 
The detector must be designed in a manner permitting disassembly thereof to 
permit cleaning of the parts. When assembled, however, the detector must 
have a liquid tight seal at the typical pressure of a liquid 
chromatograph. 
The present invention has been described herein in relationship to a 
particular specific embodiment which embodiment is exemplary only as other 
assemblies and configurations will become apparent to those skilled in the 
art from reading this description. Hence, the present invention is deemed 
limited only by the appended claims and the reasonable interpretation 
thereof.