Sample Modulator Cell for Gas Chromatography

This invention relates generally to a method of improving response in chromatographic analysis and more particularly to a sample modulator cell for synchronous detection with improved signal-to-noise ratio.

Two problems that limit the utility of detectors in gas chromatography are drift and noise. Examples of detectors that are subject to drift are the thermal conductivity detector (TDC) and the thermionic specific detector (TSD). The drift in the TCD is caused by small changes in the temperature of the cell wall. The drift in the TSD is caused by small changes occurring on the surface of the bead. These changes are both thermal and chemical. Examples of detectors in which the dominant noise source is within the detector cell are the flame photometric detector (FPD), flame ionization detector (FID) and the TSD. The noise in the FPD is caused by random low frequency fluctuations in the luminosity of the background flame. The TSD likewise suffers from fluctuations in the nature of the bead surface.

When the drift and noise sources are inherent to the detector and not to the electronics which controls the detector or amplifies the signal therefrom, it has been known to minimize or eliminate these problems by AC techniques. U.S. Pat. No. 3,740,154 issued to J. A. Green, for example, discloses a flame photometer whereby the gas to be analyzed is modulated in a modulator equipped with a flexible sinusoidally oscillating membrane. Since the sample gas comes into direct contact with this membrane, this modulator cannot be used for certain samples, such as caustic ones and those at a very high temperature. Fluid control systems not requiring a flexible membrane of the aforementioned type have been disclosed, for example, in U.S. Pat. No. 3,357,233 issued to L. B. Roof, and U.S. Pat. No. 4,309,898 issued to R. L. Horton, but both these devices are fluidic in nature and require a stream which is greater by orders of magnitude in order to properly operate.

U.S. Pats. Nos. 4,316,381 and 4,316,382 issued to T. A. Woodruff disclose a modulated detector which is provided with a storage volume for modulation in the fluid path so that the measurements are made only at times in which the flow has stopped. This is a static measurement, not a continuous dynamic process which allow shorter response times.

It is therefore an object of the present invention to provide a sample modulator for improving the signal-to-noise ratio of a chromatographic detector by synchronous detection.

It is another object of the present invention to provide a sample modulator with a means for alternately supplying a sample fluid into two inlets which means can operate effectively at flow rates substantially below those required for fluidic flow control systems.

It is still another object of the present invention to provide a sample modulator which allows continuous dynamic measurements.

It is a further object of the present invention to provide a sample modulator equipped with a valve means for alternately supplying a sample fluid into two inlets, said valve means not coming into direct contact with said sample fluid.

It is a still further object of the present invention to provide a sample modulator which allows all of the sample from a gas chramotographic column to be utilized in a detector.

FIG. 1 shows schematically a sample modulator cell according to the present invention.

FIG. 2 shows another embodiment of sample modulator cell of the present invention.

FIG. 3(a) is a sketch of the unmodulated profile of a chromatograph pulse.

FIG. 3(b) is sketch of the modulated profile as received through a first channel.

FIG. 3(c) is a sketch of the modulated profile as received through a second channel having 180.degree. longer phase length.

FIG. 3(d) is a sketch of the modulated profile as received by combining both channels.

FIG. 3(e) is a sketch of the modulated profile as enhanced by a compression pulse.

FIG. 4 is a chromatogram obtained by a modulator cell of FIG. 2.

There is shown schematically in FIG. 1 the horizontal view of a sample modulator cell for a gas chromatographic detector embodying the present invention. A gas for controlling the direction of sample flow, which may hereinafter be referred as diverting gas, is flow-controlled by the combined action of a pressure regulator 11 and a flow restriction 12 of any known types and enters a diverting valve 15 which may be any mechanical device whose action is to alternately divert the incoming flow into an upper path 16 and a lower path 17. The upper and lower paths 16 and 17 open into a vertical duct 20 at points 21 and 22, respectively, the point 21 being above point 22. The sample to be analyzed enters the cell through an inlet 25 which opens into the duct 20, forming a tee 26 at a point between points 21 and 22. When the sample from the inlet 25 reaches the tee 26, it is alternately forced out of an upper vent 31 and a lower vent 32 by means of the gas diverting flowing through the lower path 17 and the upper path 16, respectively. In other words, during that phase of the alternate action of the diverting valve 15 when the diverting gas is diverted into the upper path 16, the sample from the inlet 25 is forced to flow downward inside the duct 20 and through the lower vent 32 as the gas movements are shown by the dotted arrows, while during the other phase of the alternate action of the diverting valve 15 when the carrier gas is diverted into the lower path 17, the sample from the inlet 25 is forced to flow upward inside the duct 20 and through the upper vent 31 as the gas movements are shown by the solid arrows. The duty cycle of the diverting valve 15 need not be evenly divided, nor is it necessary that a sample-containing gas should leave the upper vent 31 and the lower vent 32 respectively 50% of the time during each cycle. For high-frequency operations, it may be found advangtageous to reduce this ratio in view of the peak spreading.

In one mode of operation, the upper vent 31 may be connected, for example, to a fused silica detector insert of an indium-sensitized FPD (not shown). A balance load 35 on the lower vent 32 may be adjusted to equal the load downstream of the upper vent 31 due to the detector so that the net gas flow through each vent 31 and 32 will be the same and, secondarily, that a nearly constant flow of gas, either sample-containing or pure diverting gas, is always flowing through both vents 31 and 32 at all times of the operation. The modulated sample flow into the detector is converted to an electrical signal by a suitable transducer (not shown) and synchronously detected by any method known in the art.

Another mode of operating the modulator cell of the type shown in FIG. 1 is illustrated in FIG. 2. This mode is for remedying the disadvantage of the aforementioned operation wherein one-half of the sample leaving the gas chromatographic column is diverted away and not utilized in the detector. In order to utilize all of the sample and yet to achieve modulation so as to increase the peak response obtained by the modulator cell, the two passageways 116 and 117 from a diverting valve (not shown) and a sample inlet 125 open into a circular duct 140 at points 121, 122 and 126, respectively. An exit passageway 145 also opens into the circular duct 140 at point 148 and connects the latter to an exit port 150. For the convenience of explanation, the right- and left-hand parts of the duct 140 from the point 126 to point 148 moving clockwise and counter-clockwise will be called channels A and B, respectively. Point 148 is located so that channel B is twice as long as channel A. This is for the purpose of creating a 180-degree phase difference in the modulated sample flow through channels A and B as will be explained below.

In operating the modulator cell of FIG. 2, the diverting valve performs the same function as described above so that the carrier gas enters the duct 140 alternately at points 121 and 122. If a sample having a profile generally of the shape shown by Curve (a) of FIG. 3 is injected at 126 with a proper pressure, it will be alternately diverted into channels A and B by the motion of the carrier gas. The profiles of the modulated sample flow in channels A and B are shown by Curves (b) and (c) of FIG. 3 for a low frequency situation for the purpose of illustrating the mode of operation. As described above, the length of the channels A and B are such that there is introduced therein a phase difference of 180 degrees so that the sample mass flow measured at point 148 will be as shown by Curve (d) of FIG. 3 with peaks twice as high as those in either Curve (b) or (c). This desirable result is obtained because the half of the sample which leaves the cell through the lower vent (32 of FIG. 1) is not wasted, but is also utilized in the detector. Alternatively, if a periodic flow of gas such as a compression pulse of either controlled gas or fuel gas is applied through an entrance port 149 either at point 148 as shown in FIG. 2 or alternatively somewhere between points 148 and 150, the mass flow measured at point 150 will increase due to the increased gas flow. The phase of such compression pulse must of course be such that the sample portion of the wave form (FIG. 3(d)) is inside the exit passageway, or between points 148 and 150 when it is applied. For example, if the flow at point 148 is doubled when the compression pulse is applied, the same mass will pass through the exit port 150 in half the time so that the peak mass flow will have doubled and the flow profile will be as shown in FIG. 3(e). FIG. 4 shows the modulated sample profile for butane eluted from a capillary column at frequency of 5 Hz without the use of the aforementioned compression pulse through the entrance port 149. The dotted curve is the response from a FID to the modulated sample. The doubling in peak response obtained by the phase shifted modulator cell of FIG. 2 is illustrated.

The present invention has been disclosed above in terms of only a few examples, but they are intended to be illustrative rather than limiting, and the disclosure therefore should be broadly construed. For example, FIGS. 1 and 2 are merely schematic and do not represent dimensional relationships which are preferably. Thus, the duct 140 need not be exactly circular and the directions in which the passageways 116 and 117 and the inlet 125 open into the duct 140 need not necessarily be as depicted in FIG. 2. FIG. 1 need not necessarily be considered a horizontal view; the apparatus may be placed in any orientation with respect to the vertical in spite of the use above of expressions "upper", "lower" and "vertical". The diverting gas may, or may not be the same as the carrier gas used in gas chromatography. The scope of the present invention is limited only by the following claims.