Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > The Measurement of UV and Visible Absorption > The Multi-Wavelength Dispersive Spectrometer > Page 18

(4)

 

or ln (IT) = ln (Io) - kcl

 

where (Io) is the intensity of the light entering the cell,
(l) is the path length of the cell,
and (k) is the molar extinction coefficient of the solute for the specific wavelength of the UV light.

 

If equation (1) is put in the form,

(5)

then (k') is termed the molar extinction coefficient.

 

Differentiating equation (5),

It is seen that the sensitivity of the sensor as measured by the transmitted light will be directly proportional to the value of the extinction coefficient (k) and the path length of the sensor cell (l). It follows, that to increase the sensitivity of the system for a given substance having a given extinction coefficient (k), (l) should be increased. However, in a flow-through sensor cell of a GC/spectrometer combination, the extent to which the path length can be increased is limited, as the total volume of the sensor cell must be restricted to ensure minimum peak dispersion and only a small fraction of a peak can be allowed to exist in the cell at any one time (4). To restrict peak dispersion and maintain a small sensor volume, it follows that the radius of the sensor cell must also be reduced as (l) is increased. This results in less light falling on the photo–cell which, in turn, will reduce the signal–to–noise ratio and, thus, the sensor sensitivity, or minimum detectable concentration. Consequently, increasing the sensor sensitivity by increasing the path length has limitations and a well–designed cell involves a careful compromise between cell radius and length to provide the maximum sensitivity. Most modern UV spectrometer sensor's have path lengths that range between 1 and 10 mm and internal radii that range from about 0.5 to 2 mm. From equation (2),

It is seen that the sensitivity of the sensor as measured by the transmitted light will be directly proportional to the value of the extinction coefficient (k) and the path length of the sensor cell (l). It follows, that to increase the sensitivity of the system for a given substance having a given extinction coefficient (k), (l) should be increased. However, in a flow-through sensor cell of a GC/spectrometer combination, the extent to which the path length can be increased is limited, as the total volume of the sensor cell must be restricted to ensure minimum peak dispersion and only a small fraction of a peak can be allowed to exist in the cell at any one time (4). To restrict peak dispersion and maintain a small sensor volume, it follows that the radius of the sensor cell must also be reduced as (l) is increased. This results in less light falling on the photo–cell which, in turn, will reduce the signal–to–noise ratio and, thus, the sensor sensitivity, or minimum detectable concentration. Consequently, increasing the sensor sensitivity by increasing the path length has limitations and a well–designed cell involves a careful compromise between cell radius and length to provide the maximum sensitivity. Most modern UV spectrometer sensor's have path lengths that range between 1 and 10 mm and internal radii that range from about 0.5 to 2 mm. From equation (2),

 

Where (A) is termed the absorbance?

 

Now (DA) is commonly employed to define the sensor sensitivity where the value of (DA) is the change in absorbance that provides a signal-to-noise ratio of two.

 

Thus

 

Where, (Dc) is the sensor concentration sensitivity or minimum detectable concentration.

 

Thus

 

The UV spectrometer used for monitoring the eluent from a GC column employs a source that provides light over a wide range of wavelengths and consequently, with the aid of an appropriate optical scanning system, absorption spectra of any substance eluted from a gas chromatograph can be obtained for identification purposes. The actual procedure, however, differs with the type of spectrometer being used.

 

 

There are two basic types of UV spectrometer, the dispersion spectrometer and the diode array spectrometer, the latter being the more popular for use in conjunction with the gas chromatograph. Both types require a broad emission light source such as the deuterium or the xenon lamps the use of the deuterium lamp being the most widespread. The two types of spectrometers have important differences. In the dispersive instrument the light is dispersed before it enters the sensor cell and thus virtually monochromatic light passes through the sensor. However, if the incident light can excite the solute and cause fluorescence at another wavelength, then the light falling on the photo cell will contain that incident light that has been transmitted through the cell together with any fluorescent light that may have been generated in the cell. Consequently the light monitored by the photo cell will not be solely monochromatic but may contain light of another wavelength, which, if present, could impair the linear nature of the response. In most cases this effect would be negligible but with some substances the effect could be quite significant. To acquire a spectrum from a dispersive spectrometer, when used in conjunction with a gas chromatograph, the vapor must be held in the sensor cell while the spectrum is taken. This will require the chromatographic development to be arrested otherwise the column eluent must be passed to waste during the scanning process with the possible loss of sample.

 

The diode array spectrometer operates in quite a different manner. Light of all wavelengths generated by the deuterium lamp is transmitted through the cell and the transmitted light is then dispersed over an array of diodes. In this way, the absorption at small discrete groups of wavelengths is continuously monitored at each diode. However, the light falling on a discrete diode may not be solely that transmitted through the cell from the source, but may contain light resulting from fluorescence excited by light of a shorter wavelength. In this case, the situation is exacerbated by the fact that the cell contents are exposed to all the light emitted from the lamp and so fluorescence is more likely. In general, this means that under some circumstances measurement of transmitted light may also contain fluorescent light and as a consequence the absorption spectrum obtained for a substance may be slightly degraded from the true absorption curve.

 

The ideal spectrometer would be a combination of both the dispersion and a diode array instrument. This system would allow a monochromatic light beam to pass through the detector and then the transmitted beam would itself be dispersed again onto a diode array. Only that diode corresponding to the wavelength of the incident light would be used for monitoring the transmission. In this way any fluorescent light would strike other diodes, the true absorption would be measured and accurate absorption spectra could be obtained. In addition fluorescent spectra can be simultaneously produced if desired.

 

The Multi-Wavelength Dispersive Spectrometer

 

A diagram of the multi–wavelength dispersive spectrometer is shown in figure 11.

 

 

Figure 11. The Multi-Wave Length UV Dispersive Spectrometer