Refractometry

Refractometry is a technique that detects the difference in refractive indices between two media, one of which contains the gas sample of interest (e.g. air and anaesthetic vapour), the other which is of otherwise identical constitution (e.g. air only). The refractive index of a medium is a measure of the ratio of velocity of light in a vacuum to the velocity of light in that medium. The molecules of the medium delay the transit of the light wave through it; the delay depends on the number of molecules and hence on the density of the medium; thus refractive index for a gas medium depends on its concentration, pressure and temperature. When a light beam passes through parallel slits whose width is of the same order of magnitude as the wavelength of the light, interference patterns are produced by light waves arriving in phase (bright fringe) and 180° out of phase (dark fringe) with each other. When two such sets of fringes are formed from light passing through gas samples with differential velocities, they are displaced relative to each other and the displacement between them can be measured as a difference in refractive indices.

The refractometer is included, not because it is frequently used clinically as a gas analyzer, but because it is an accurate standard of gas analysis against which others are compared. All gasses and volatile agents can be quantified using this method, since all possess the physical property of refractive index. Laboratory refractometers are used by vaporizer manufacturers to calibrate vaporizers. A diagram of the portable version of the refractometer (the Riken refractometer) is shown in Fig. 15.3. Light from a small light source is collimated into a beam, which is split into two parallel beams by prisms. One beam is passed through the reference chamber containing, say, air, and the other is passed through the measuring chamber containing air and another gas/vapour species, for example sevoflurane, whose concentration is to be measured. The beams are reflected back through the chambers to enhance the effect. Each beam is passed through a slit, producing interference fringes. As discussed above, the effective path lengths of the two beams differ and two sets of fringes are produced. These can be realigned by a knob controlling a vernier scale and the amount of realignment is taken as a measure of refractive index change from the reference sample, and thus of concentration of the gas under test.

Figure 15.3 The principles of the portable refractometer.

Infrared absorption spectroscopy

The interatomic bond between dissimilar atoms of a molecule absorbs radiation in the infrared (IR) range. Thus, molecules such as nitrous oxide, carbon dioxide, water vapour and the volatile agents absorb IR light, but oxygen, nitrogen and helium do not. Polyatomic molecules of different species absorb IR radiation across a range of IR wavelengths and there is frequently overlap between species. However, different polyatomic species absorb maximally at characteristic wavelengths within the IR bandwidth, which means that it is possible to identify the gas molecule as well as quantify the gas concentration. The amount of absorption (A) of any radiation by any substance is governed by the Beer-Lambert Law, which links A to the intensity of the incident radiation I_i, the non-absorbed, transmitted radiation I_t, the extinction coefficient ε, the path length L and the concentration C of the absorbing substance, where:

Since CO₂ analysis is important in anaesthesia, such devices are available in most anaesthetic locations in modern operating rooms. Fig. 15.4 shows the principle of the Luft or non-dispersive type of IR gas analyzer, which uses a differential technique to minimize error. An IR light source emits radiation at between 1 and 15 µm. A ‘chopper’ wheel, with small windows containing optical filters at the rim, rotates in front of the light source at 25–100 Hz to prevent the gas sample from overheating; additionally the filters allow the passage of IR light of a narrower bandwidth than the source itself, to match the wavelength of maximum absorption of the gas under study. This window might contain a 4.26 µm filter for CO₂, a 3.3 µm filter for halothane, isoflurane and enflurane, and a 4.5 µm filter for N₂O. The light passes through this to two chambers, a reference chamber and a sample chamber; the sample chamber contains the same background gas as the reference chamber, but additionally contains the gas under analysis. The light, which is not absorbed (the transmitted light) in each sample cell, is passed to a pair of air-filled detector chambers, separated by a diaphragm. Because different amounts of IR light are absorbed in the reference and sample chambers, different amounts of light are, therefore, transmitted to the detector chambers, heating the air in them differentially. Because the chopper wheel produces an oscillating IR source, the changing air pressure in the two detector chambers causes the diaphragm separating them to oscillate. The diaphragm usually forms half of a capacitor and is, therefore, a component of electrical circuitry, which amplifies and processes the signal, to produce an output signal, which is proportional to the gas concentration.

Figure 15.4 The principles of the infrared spectrophotometer.

There are a number of sources of error in IR spectroscopy. Although CO₂, N₂O and CO absorb IR light maximally at 4.3, 4.5 and 4.7 µm respectively, there is considerable overlap in the absorption spectra of these gasses, particularly in the 3–5 µm range. There is also the phenomenon of collision broadening, where the presence of one gas may broaden the IR absorption spectrum of another. This effect is not only caused by IR absorbing agents themselves (such as CO₂ and N₂O) on the absorption spectra of other IR absorbing agents, but by other non-IR absorbing carrier gasses, such as helium, argon or hydrogen. It has been reported, for example, that in a gas mixture containing 79% helium in oxygen, an IR analyzer under-reads CO₂ values.⁴ Desflurane,⁵ cyclopropane,⁶ acetone and alcohol⁷ all produce errors in IR spectroscopy of respiratory gasses. There are electronic correction factors in the analyzers to allow for the presence of N₂O in the gas mix containing CO₂. Since water is a strong absorber of IR across the bandwidths of interest, it must be eliminated in the sampling process, in order to avoid some inaccuracy. If one wavelength is used for all volatile agent detection, then that agent must be selected manually, or the device will be inaccurate, if either the wrong agent is selected or if there is a mixture of agents present. Some analyzers use the 10–13 µm bandwidth to detect volatile agents, where there is less chance for interference between absorption spectra of the volatile agent and other gasses.

Infrared spectroscopy detects numbers of molecules in a gas sample and thus constitutes a partial pressure analyzer. Therefore, error can be introduced if the pressure of the gas sample changes or if there is ambient pressure change, against which the device is calibrated. If the gas sample pressure changes due to back pressure from a ventilator, or if there is significantly low pressure in a sampling tube, the partial pressure of the gas being analyzed will change, without there being a real change in the fractional concentration of the gas. Similarly, if the device is calibrated at sea level and subsequently used at altitude, there will be an error in calculating gas concentration. Therefore, calibration should be carried out frequently and under the conditions of intended use, or the device should be configured to display partial pressure rather than concentration.

The 90–95% response time of IR analyzers to a step change, including transit time and rise time, should be under 150 ms. Water vapour blocking the sampling tube sometimes causes the response time to increase. To deal with water vapour, most devices have either a water trap or use sample tubing, which absorbs water vapour. IR capnographs are accurate to about 0.1% in a range of CO₂ up to 10%.

A variant of the IR analyzer described above utilizes a combination of IR and photo-acoustic spectroscopy. The gasses under test are drawn through a small measurement chamber that is irradiated by an IR source. This IR source is first ‘chopped’ by a rapidly rotating wheel that has three concentric bands of holes that cause the IR energy passing through the holes to pulsate at three different sampling rates. Each part of the now divided beam passes through a separate narrow band optical filter allowing through only that wavelength that corresponds to the maximum absorption of the gas/vapour to be tested (i.e. CO₂, N₂O and volatile agent). The components of the IR light, now differentiated both by sampling frequency and wavelength, are absorbed by the gas molecules, which are caused to vibrate at different frequencies, leading to a set of audible, fluctuating pressure waves. The families of audible pressure waves are detectable by microphone. Fig. 15.5 shows the Bruel and Kjaer model of the photo-acoustic IR spectroscope. Note that, since oxygen does not absorb IR light, this device has a separate sampling channel that uses magneto-acoustic spectroscopy to measure oxygen (see the section on Paramagnetic oxygen analyzers).

Figure 15.5 The components of the Bruel and Kjaer photo-acoustic and magneto-acoustic spectrophotometer.

The signal from all gaseous components is taken from the microphone and filtered electronically into the four components of O₂, CO₂, N₂O and volatile agent. Separation and, therefore, gas identification occurs by audio frequency identification and quantification of the gas occurs by audio-amplitude. Note that the device does not distinguish between different volatile agents. The advantages of photo-acoustic IR spectroscopy over conventional IR spectroscopy include stability, zero drift, reduced need for calibration over prolonged periods and fast response time.

A more recent design of infrared absorption spectroscopy does away with the moving parts of the chopper wheel. Light from a broad-spectrum infrared source passes through a cuvette containing sampled gas. The Dräger ILCA (Fig. 15.6) then uses a prism as a beam splitter. On the four sides of the multichannel detection chamber are appropriate infrared filters and sensors for the maximal absorption wavelengths of each gas. By using two such modules in series the gas bench can quantify CO₂, N₂O and the individual volatile agents in the gas mixture.

Figure 15.6 Working principles of Dräger ILCA gas analyzer.

Mass spectrometry