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Production of Propylene Glycol From Glycerol

Glycerol is a by-product obtained during the production of biodiesel. As the biodiesel production is increasing rapidly, the crude glycerol generated from the transesterification of vegetables oils has been generated in a great quantity. About 10% of crude glycerol will be formed during the synthesis of biodiesel from triglycerides. Products obtained from glycerol can be used in food, pharmaceuticals, polymer, agricultural, cosmetics, resins functional fluid plastics etc. Increasing the production of biodiesel, excess of glycerol has been formed, causing market prices to fall; this would become a cheaper feedstock in the chemical synthesis. For this reason, it is essential a new technology for conversion of glycerol into valuable chemicals to make biodiesel production a cost effective process.
Glycerol can be converted into higher value products such as 1,2-propanediol (propylene glycol) and 1,3-propanediol from reduction of glycerol.
Propylene glycol 1,2-propanediol is an important product chemical derived from propylene oxide. Bio-routes enable reduction to 1,3-propanediol, as an important monomer which has potential effectiveness in a manufacture of cyclic compounds and polyurethanes, and production of polyester fibres.1,3-propanediol is the organic compound with the formula CH2(CH2OH)2 and its colourless viscous liquid that is miscible with water. They are also used in liquid detergents, wetter flavouring and fragrances, cosmetics, precursor in chemical and pharmaceutical industry, painting and animal food.
1,2-propanediol has an annual global demand estimated at between 1.18 and 1.58 billion tonnes1. By early 2007 it was selling at around US$1.8 per kg, with a 4% annual growth in market size2.
Either 1,2-propanediol or 1,3-propanediol can be produced by selective dehyroxylation of glycerol through chemical hydrogenolysis or by biocatalyst reduction. Researchers hope commercial production of 1,2propanediol is turning excess glycerol into an advantage for the biodiesel industry.
Processes: 1. Hydration of acrolein
1,3-propanediol is currently produced by the hydration of acrolein to ?-hydroxypropionaldehyde, which yields 1,3-Propanediol upon hydrogenation. In this process the yield is low and also acrolein is dangerous hazardous chemical. There is a low yield in the first step of the process is because acrolein has a large tendency to polymerize through self-condensation, the hydration reaction has to compete with acrolein self-condensation to produce the desired ?-hydroxypropionaldehyde.
Due to the low efficiency and hazardous chemical nature of acrolein process, researchers have been interested for an alternative method to produce 1,3-propanediol. An alternative way is producing 1,3-propanediol from glycerol. Since glycerol has been derived from biomass it has been attractive process as it can be useful way of reduction of petroleum in the future.
2. Selective dehydroxylation
The production of 1,3-propanediol from glycerol through selective dehydroxylation the scheme is to selectively convert the middle hydroxyl group of glycerol into tosyloxyl group. Once it has been converted then to eliminate the transformed grouped by catalytic hydrogenolysis. The tosyloxyl group is a better leaving group than hydroxyl group and is easier to replace with a hydride ion. The conversion of glycerol to1,3-propanediol is done in three steps: acetalization, tosylation, and detosyloxylation. Glycerol dehydroxylation process attracts the attention of investigators for the fermentation process.
The first step (acetalization), in the conversion of glycerol to 1,3-propanediol is to acetalize the glycerol with benzaldehyde. The purpose of this step is to protect the first and third hydroxyl groups of glycerol. This is because that only the 2nd group can be tosylated in the second step and then removed in the third step.
The last step of the conversion is a detosyloxylation reaction or a hydrolysis reaction. The tosylated central hydroxyl group removes in the detosyloxylation reaction. The protection removes on first and final hydroxyl groups in the hydrolysis reaction. This last step converts to 1,3-Propanediol. It also regenerates the group protection reagent benzaldehyde, which can be recycled back to the acetalization reactor for reuse in the first-step conversion4.
As it shown in figure 1 the detosyloxylation reaction is basically involves hydrogenolysis reaction. The reaction is done with hydrogen molecular in the presence of a transition metal catalyst. The tosylate has not been hydrogenolysed catalytically with hydrogen molecule as a reducing agent. Currently hydrogenolysis of tosylates is affected with lithium hydride. These reagents are too expensive to use industrial scale. The feasibility of catalytically hydrogenolysis the tosylate is focus of the current research.
3. Hydrogenolysis process:
The above diagram shows the conversion of glycerol to glycols. In the presence of hydrogen and metallic catalysts, glycerol can be hydrogenated to 1,2-propanediol, 1,3-propanediol, or ethylene glycol.
This glycol production by hydrogenolysis is a process used is economically and environmentally attractive compared to their production from petroleum derivatives.
Hydogenolysis of glycerol are used from supported metal catalysts from transition metals. For this reaction supported catalyst such as Ruthenium, Platinum, Rhodium, and Palladium are used. Addition of solid acid to metal catalysts enhances the conversion and selectivity of reaction5. Solid acid catalyst contributes the main role in conversion of glycerol hydrogenolysis. It is found that Ruthenium based catalysts exhibit better activity than other metals for this reaction5. However, Ruthenium gives excessive C-C bond cleavage which leads to degrative products.
In hydogenolysis of glycerol to get 1,2-propanediol it requires selective cleavages of C-O bond without cleavage of C-C bond. For this reason, copper based catalysts are better catalysts in comparison to other transition metal catalysts. The copper based catalyst is active under mild reaction conditions and does need a separate solid acid catalyst. Studies shows that copper chromite catalyst is a good selectivity and conversion for propylene glycol under mild reaction conditions particularly at low H2 pressures.
The figure below shows using copper chromite catalyst shows the highest selectivity for 1,2-propanediol with higher conversion compare to different catalyst at temperature 200oC and at pressure 13.8bar.
In a reactive distiallation glycerol can be hydrogenolysis over copper chromite catalyst at less than 10 bar and 200OC in a reactive distillation.
The aim for this process is to produce propylene glycol in pure condition. The reactive distillation process now achieves greater than 99.8% purity, which means the product can be used both as industrial feedstock and as antifreeze2.
The practical advantages of the reactive distillation approach are:
Low water content of the feed
low pressure (200psi)
High selectivity (>90%)
Low catalyst cost.
Using a two-step reaction process under mild reaction, the reaction pathway proceeds through acetol (hydoxyacetone) intermediate.
The first step: relatively pure acetol is produced from glycerol at 0.65bar pressure and 200oC in the presence of copper chromite catalyst.
The second step: using a copper catalyst again similar to the first step, the acetol is further hydrogenated to 1,2-propanediol at 200oC and 13.8 bar hydrogen pressure. This allows 1,2-propanediol in 90% yield and at considerably lower cost than starting from petroleum.
The selectivity to propylene glycol decreases if temperature is above 200oC due to excessive hydrogenolysis of the 1,2-propanediol.
The reaction is conducted in two step, because major problems can occur when the reaction is conducted in a single step are the catalyst becomes coated with oligomers and its difficult to achieve above 80% selectivity for 1,2-propanediol. However in two steps, by combining the reaction and separation steps, 1,2- propanediol yield is 99% and the catalyst life cycle is significantly extended. Water and acetol are simultaneously removed from the reaction mixture during the heating step as they are formed, the lower pressure used in the first of the two step prolongs catalyst life. Further reduction of acetol water feed with hydrogen over a similar copper chromite catalyst at 13.8bar and 185oC allows 1,2-propanediol selectivity greater than 95% and 99% conversion.
Acetol formed as an intermediate is the advantage of this new process as it acetol is an important monomer used in industry to manufacture polyols. When this produced from petroleum it costs as little as $1 per kg, opening up even more potential applications and markets for glycerol8. The second advantage further purification is not required when using the copper chromite catalyst to convert crude glycerol, whereas supported noble metal catalysts are easily poisoned by contamination for example chlorides.
The disadvantage of this process is the use of high pressure and temperature as it is expensive to use high pressure equipment and also increases the capital cost of the process. An additional disadvantage is copper chromite based catalyst are undesirable for the environmental aspects as chromium is toxic. For this reason, research has studied using Cu-ZnO catalyst at high pressure instead of copper chromite catalysts. However the greatest selectivity (100%) for 1,2-propanediol obtained by hydrogenolyisis of an aqueous solution of glycerol in the presence of CuO-ZnO catalysts gives a low yield. Copper chromite catalyst has much better selectivity and conversion compare to CuO-ZnO catalysts.
There are a number of routes to produced propylene glycol from renewable feedstock. The most common is the hydrogenolysis process in presence of a metal catalyst. However this important reaction at the moment is limited in the laboratory scale.
New glycerol hydrogenolysis processes developed by Davy, soon to be commercialised indication suggest that the process will give high purity propylene glycol, suitable for all applications. In this process under relativity moderate conditions glycerol is reacted with hydrogen over a heterogeneous copper catalyst. The glycerol recycle stream is vaporised in a recalculating stream of hydrogen, typically from pressure-swing adsorption unit. High purity propylene glycol recovers from the refining scheme, where as the water produced in the reaction is to be passed to a biological treatment plant. High quality small by products stream can be used as functional fluids or as solvents. By products are removed by distillation and glycerol conversion is around 99%. High selectivity to the desired product is the advantage of the Davy process.
4. Fermentation process:
There are number different way to produce 1,3-propanediol. For example glycerol production by hydrogenolysis in presence of a metal catalyst and also by the hydration of acrolein to ?-hydroxypropionaldehyde, which yields to 1,3-Propanediol. Even though it is possible to produce 1,3-propanediol by these methods, they are expensive and are environmental pollutants.
Glycerol can serve as a feedstock for the fermentative production of 1,3-propanediol and its production by fermentation appears to be a reasonable alternative to chemical synthesis. Bacterial strains are able to convert glycerol into 1,3-propanediol and are found in the species of Lactobacillus, Citrobacter, Klebsiella, and , Clostridum.
In a two-step enzyme-catalysed reaction sequence glycerol is converted to 1,3-propanediol(PDO). These equations are shown in figure 7.
In the first step: dehydrates the catalyses conversion of glycerol to 3-hydroxy-propionaldehyde (3-HPA) and water, equation 1.
In the second step: 3-HPA is reduced to 1,3-propanediol by a pyridine nucleotide: NAD oxidoreductase to yield 1,3-propanediol, a dead end cellular metabolite.
The overall reaction consumes a reducing equivalent in the form of a cofactor, reduced beta-nicotinamide adenine dinucleoride(NADH), which is oxidised to nicotinamide adenine dinucleotide (NAD )9, equation 3.
The genes are responsible for the conversion of glycerol to 1,3-propanediol. Hetrologous genes in Escherichia coil for example from Cirobcater and klebsiella have shown to convert glycerol to 1,3-propanediol. From all these bacteria, Klebsiella pneumoniae is the most interesting because of their yield, productivity, efficient conversion to 1,3-propanediol and resistance to both reagents and products.
The technical and economic aspect of this process is attractive for this fermentation process. This technique uses immobilisation instead of freely suspended cells which causes an increase in productivity. One of the disadvantages of this process is its low theoretical yield. Another disadvantage is that the process is substrate-inhibited. The bacteria used in the fermentation are generally not able to stand a glycerol concentration above 17. Thus, both the productivity and product concentration are low.

ECG and Pulse Oximetry: History and Types

In this chapter, we will discuss the history of ECG and pulse oximetry, the timeline and variations through time of the concepts used. We all also discuss the types of pulse oximetry and the electronics used with their requirements.
1.1 History of ECG The history of ECG is very wide, dating back to the 1600 with William Gilbert (that introduced the electrica concept for objects holding static electricity) (1).The most important founders of the electrocardiogram concept were Emil Reymond and Willem Einthoven. In 1843, Emil Reymond was the founder of the electrocardiograph concept by using a galvanometer to state that muscular contraction has action potentials. He also identified the types of waves by using the P, Q, R, and S waves. His studies inspired many physicians to continue and develop his work further. The evolution of concepts continued until the discovery of P, Q, R, S and T waves by Willem Einthoven in 1895. Einthoven also invented a modified galvanometer and used in for electrocardiogram recording. As a reward for his work, he won a Noble price in 1924 for inventing the electrocardiograph (1).
As stated before, the history of ECG is very wide, therefore we will limit the observation to the movement done between 1843 and 1942 as shown in the following table:
Table 1: ECG Timeline
Year Scientist Concept
1842 Carlo Matteucci heart beat is accompanied by electric current
1843 Emil Dubois-Reymond Muscular contraction is accompanied by action potential.
Test carlo’s concept on animals successfully
1856 Koelliker , Muller Record of the action potential concept
1869 Alexander Muirhead Might have recorded a human electrocardiogram
1872 Gabriel Lippmann Capillary Electrometer invented
1876 Marey EJ Electrical activity of animal recorded by the electrometer
1878 John Sanderson , Frederick Page Electrical current of the heart is recorded
Divide into two phases (later known as QRS and T)
1887 Augustus Waller First human electrocardiogram is published
1890 GJ Burch Arithmetic correction of the electrometer
1891 William Bayliss , Edward Starling Capillary electrometer improved
Discovery of deflections (later known as P,QRS,T) and delay (later know as PR interval)
1893 Willem Einthoven The term electrocardiogram introduced
1895 Deflections P,Q,R,S and T distinguished
1897 Clement Ader Galvanometer invented( Amplification system for the lines of telegraph )
1901 Willem Einthoven Galvanometer modified for ECG use
1902 ECG records using galvanometer published
1903 Commercial production of galvanometer discussed
1905 Telecardigram invented (transmission of ECG signal by telephone)
1906 Normal and abnormal ECG record published
Introduction of the U wave
1908 Edward Schafer First purchase of Einthoven’s galvanometer
1910 Walter James, Horatio Williams Electrocardiography reviewed for the first time in America
1911 Thomas Lewis Publication of a book about heart beat mechanism
1912 Willem Einthoven Description of the Einthoven triangle (formed for the leads)
1920 Hubert Mann Derivation of mono-cardiogram (later known as vector-cardiogram)
1924 Willem Einthoven Nobel price won for the electrocardiograph invention
1928 Ernstine, Levine Introduction of vacuum-tubes for ECG amplification
Frank Sanborn First portable ECG invented
1932 Charles Wolferth and Francis Wood Description of the chest leads use in the coronary occlusion
1938 American heart and cardiac British association Standard positions of chest leads defined and added (V1 to V6)
1942 Emanuel Goldberge Addition of aVR, aVL and AVF to previous model
Final ECG model used today
1.2: History of pulse oximetry
The revolutionary paper by Comroe and Botelho was the founder movement that stated the need for a better method for the detection of hypoxaemia later known as pulse oximetry. The paper clearly underlined the unreliability of the cyanosis method currently used for the detection of arterial hypoxaemia. This was done by showing that if the oxygen saturation is reduced to 75% the cyanosis could not be detected. Another paper written by Lundsgaard and Van Slyke enhanced the movement. The paper showed the factors that enhance the cyanosis such as 5mg reduced hemoglobin per 100 ml capillary blood. The paper also showed that the subject, environmental factors and the tester affects greatly the detection of cyanosis. As a result, many type of instrumentation were developed to detect the presence of hypoxaemia. However, these devices were inaccurate due to the inability to detect the difference between arterial oxygen saturation and the arterial venous and capillary blood. This separation remains a problem until the microprocessor era where the separation was finally realizable.
Pulse oximetry started as a simple monitoring technique and evolved through 15 years to become mandatory with every anaesthetic. It has the ability to detect the difference between arterial blood and venous capillary blood due to the pulsatile characteristics of the arterial blood and the smooth flowing of the capillary blood. The pulse oximetry became mandatory in anaesthetic due to the many characteristic such as:
having a safety monitor
showing the amount of oxygenation in the patient and the circulation of the blood
having an non-invasive nature
having no morbidity
low running cost
low capital cost
On the other hand, pulse oximetry has been imposed to some unjust criticism as in the case of any new technology. As a result, pulse oximetry has been accused of morbidity despite being a non-invasive technique; it has been accused of causing tissue damage to the tissues adjacent to the probe. As a result, the Medical Devices Agency in England issued a safety action bulletin that contained a historical background, mode of operation, calibration problems, the characteristics of clinical uses and the technique limitation.
1.2.1 Hewlett-Packard ear oximeter
Johann Heinrich Lambert was the founder of the correlation that exists between the absorbant and the amount of light absorbed in 1760. His ideas were developed later on by August Beer in 1851. However, the first real adoption of pulse oximetry was the ear oximeter founded by Hewlett-Packard. The concept used in this oximeter is based on an incandescent source combined with narrowband interference filters to transmit eight different wavelengths. Fiberoptics are used to lead the transmitted light from pinna to the detector. The calculation of the arterial oxygen saturation is based on the eight wavelengths absorption. In order to approximate the arterial saturation .this calculation is based on an approximation of overall absorption. The ear is heated causing vasodilation and the capillary blow flow to increase. That phenomenon leads to the approximation of the arterial saturation. The main problem of the device was the constant need for calibration due to the large and hard to handle probe-head. However, this technique was the only technique that allows continuous measurement of oxygen saturation; therefore this technique was the founder of pulse oximetry
1.2.2 Prototype pulse oximeter
The founder configuration of pulse oximeter or the prototype used a light source and two bundles of fibers. The light source is made of halogen incandescent lamp to transmit the broad band energy to a fingertip probe. This transmission was done using a glass fiber bundle. Another bundle of fibers were used to return the transmitted energy to the apparatus. This returning energy is divided into two paths at the apparatus: one passing through a 650nm centered filter interface having a narrow bandwidth, and the second path passing through an 805 nm centered filter centered, that point is isopiestic hemoglobin. Then, a semiconductor sensor is used to detect the appropriate energy at the wavelengths passed through each filter. Finally, an analogue calculation is used to find the appropriate value of the oxygen saturation. This is clearly shown in the figure bellow.
This primary prototype had many disadvantages such as:
Having a heavy probe
Having an hard to manage Fiberoptics cable
Having an inaccurate filters letting some undesired wavelengths to pass through the tissues of the fingers
Having a biohazard on the finger, in some cases the finger could burn
Not fully respecting the beer-Lambert law
Insensitivity with low pulse pressure
Having a tendency to change in the analogue electronics part
1.2.3 Traditional pulse oximeter
The current pulse oximeter uses light – emitting diodes with a semiconductor photo detector to generate two wavelengths of 660 nm and 940 nm. Therefore this design provides a small and efficient probe to be attached to the ear or the finger and a small cable to connect the probe and the main unit. However, the pulse oximeter used with a magnetic resonance scanner has a different design. The main unit contains all the electronic components and optical fibers are used to transmit the light energy to and from the patient
1.2.4 Complete history of pulse oximetry
Beer’Lambert law in 1851
Discovery of oxygen carrier in blood as a form of pigment by Georg Gabriel Stokes in 1864
Purification of the pigment and naming it hemoglobin by Felix Hoppe in 1864
Detailed study of the reflection spectra of the hemoglobin and the finger by Karl von Veirordt in 1876
Detailed study of the absorption spectra by Carl Gustav Hufner in 1887’90
Measure of the oxygen saturation in fish using spectroscopy by August Krough and I Leicht in 1919
Study of the light transmitted throughout human tissues using quantitative spectrophotometry by Ludwig Nicolai in 1931
Measurement of the oxygen saturation of blood through laboratory tubes Kurt Kramer in 1934
Measurement of the spectrum of concentrated hemolysed and non-hemolysed blood by David Drabkin and James Harold Austin in 1935
Continuous monitoring of oxygenation is achieved by passing red and infrared light throughout the finger web by JR Squires in 1940. This was done by creating bloodless area of calibration by compression of tissues
Revolutionary change in the concept of oximeter leading to the development of the Millikan oximeter by Glen Alan Millikan in 1940-42
Creation of Wood’s ear oximeter by Earl Wood in 1948’50
Ability to differentiate between hemoglobin, carboxyhemoglobin and methemoglobin by the creation of ‘CO-oximeter’ in 1960
Creation of the ear oximeter having eight wavelengths by Robert Shaw in 1964
Marketing of the newly created ear oximeter by Hewlett-Packard in 1970
Separation of the arteries absorption from the tissues absorption using the pulsatile nature of the absorption signal by Takuo Aoyagi in 1971
Development of prototype pulse oximeter containing luminous light source , filters and analogue electronics by Aoyagi in 1974
Commercialization of the pulse oximeter in 1975
Chapter II: Pulse Oximetry Characteristics The pulse oximeter separates the variation of oxygenation absorbance of the human boundary. The pulse oximeter uses the reflection from the skin and tissues or the transmission through the human boundary to perform spectrophotometry. The most common used technique is the transmission technique, but the reflection technique is also used in intrapartum monitoring.
2.1 Transmission pulse oximetry
The human parts that must be chosen as extremity are the earlobe, toe, noise or typically the finger. The chosen part should have a short optical path length to have a translucent nature at the wavelengths used. The wavelengths used should have the range of 600 nm to 1300 nm and in the same range of the absorption spectrum due to the fact that each spices of hemoglobin have a unique absorption as shown in the figure bellow.
As a result from the formulas we can show that the minimum number of used wavelengths should be greater or equal to the number of unknowns. As a result the commonly used pulse oximetry uses two wavelengths for the two unknowns’ oxygenated hemoglobin and deoxygenated hemoglobin. In addition, the wavelengths used must be monochromatic and have a low cost. In the design, a sensitive detector must be used to prevent high levels energy that causes tissue damage from passing through. Thus, there is a need to separate the saturation value for arterial hemoglobin. In order to separate the saturation, computing power is used for arterial hemoglobin saturation extraction.
In addition to that, spectrophotometry requires the use of a laser due to the requirement of a single wavelength/color source as energy source. Therefore two lasers are used each having a different wavelength in order to transmit the energy to the patient boundary using optical fibers. Due to the presence of the laser, the pulse oximetry will have a high cost, a fragile nature and requires safety implications.
However, the fiber optic cables were rejected in the later designs after the discovery of the possibility of the use of LED as an energy source. As a result, the overheating of the tissues problem was removed and the narrowband filters were removed from the design thus reducing the cost and fragility of the design. In addition, the number of photodector was reduced to a single device due to the possibility of switching the LEDs on and off quickly.
2.2 LEDs
Energy sources used in pulse oximetry are monochromatic ideally with the option of using the expensive semiconductor lasers. Early pulse oximeter used similar wavelengths of 660 nm for red light and 940 nm for near infrared. Therefore, LEDs of 660nm and 940 nm were used in these designs. However, modern devices used additional wavelengths.
Doped Material Wavelength Light
Ga.28In.72As.6P.4 1250 nm Infrared
Ga 1100 nm
GaAsSi 940 nm
GaAs 900 nm
GaAIAs 880 nm
GaAIAs 810 nm Near Infrared
GaP:ZnO GaAs.6P.4 780 to 622 nm Red
GaAs.35P.65 622 to 597 nm Orange
GaAs.14P.86 597 to 577 nm Yellow
GaP:N 755 to 492 nm Green
GaAs-phosphor (ZnS, SiC) 492 to 455 nm Blue
GaN 455 to 390 nm Violet
GaN GaS2 455 to 350 nm Ultraviolet
Standard pulse oximetry have the isobestic point (805 nm) at which there are two wavelength concentrated at each side. As stated earlier, two wavelengths of 940 nm (infrared) and 660nm. The absorption spectra are flat at 940nm allowing the calibration to be immune to the variations in the peak wavelength. In addition to that, the difference between the absorption of reduced hemoglobin and the absorption of oxygenated hemoglobin at 660nm is large ,causing a flat curve and allowing the detection of changes in absorption caused by small changes in oxygen saturation .
2.3 Probe
The probe of a pulse oximeter consists of light – emitting diodes as energy source having a perpendicular output through the extremity towards a semiconductor photo-detector. The mechanical design prevent mispositioning that cause errors in calibration
2.3.1 Differential Amplifiers in the probe
Nowadays differential amplifier techniques are being used in the plethysmograph signal to enhance the common mode electrical and magnetic noise reduction.
The amplification is done between the conductor signal and the current pathway. This amplification is performed to prevent the electromagnetic interference (EMI) from affecting the probe or the lead. Due to the fact that, a small voltage signal cause the voltage generated by the EMI to be greater than the signal itself.
Two identical conductors from the detector to an amplifier are feed through the differential amplifier. The resulting output will be the absolute value of the signal from conductor 1 minus the signal from conductor 2. The advantage of using such a differential amplifier is that the induced voltage from the EMI will be two identical signals that will cancel each others.
The energy output of the photo detector must be immune to the variation in the finger’s thickness, leading to a variable energy output from the LEDs. This criterion requires detectable and unsaturated energy levels that reach the semiconductor. In the other hand, the current passing through the LED must be varied to allow the variation in the intensity of the output over several orders of magnitude. This variation is necessary to prevent high level of energy from passing through the tissues, causing heat damage.
2.3.2 LED in the probe
LED used in pulse oximetry have a bandwidth between 10 and 50 nm and a 15 nm centre wavelength’s variation.
On the other hand, variations in the driving current cause errors at the red LED but doesn’t have any effect on the near infrared LED. These facts are related to the absorption spectra; it is flat near infrared region and steep in near the red region as shown in figure 3. This will lead to an increasing inaccuracy in pulse oximeter as the oxygen saturation decreases. This problem can be solved by two different ways:
1. Selection of LED having an acceptable range of errors in the center wavelengths.
2. Measurement and calibration of center wavelengths into actual wavelength
The calibration is usually performed by the use of a fixed resistor attached to the connector of the probe lead. This resistor will automatically set the probe’s wavelength to the one of the red LED.
2.4 Photo-detector
In pulse oximetry, a single photo-detector made of silicon photodiode is positioned perpendicularly to the LED in order to detect the energy from both LEDs. Due to the fact that semiconductors are sensitive to external energy and light, general semiconductors have their size increased. However, Semiconductor photo-detectors having their sensitivity varying with wavelength, take advantage of the limited photosensitivity to limits the choice of device and the scope of wavelengths. The silicon photodiode is characterized by the direct correlation between the output and the incident light and its wide dynamic range. On the other hand, phototransistors have more electrical noise, but more sensitivity than photodiodes.
The electrically screened flexible cable carries the LED’s power and the small signal from the photo-detector. The cables also have the function of temperature detection of the probe and the skin using conductors. Finally, in order to be immune to the mechanical artifacts caused by movement, the cable must be flexible and light.
2.5 Electronics
2.5.1 Electronics circuitry
Pulse oximetry makes use of different electronics circuitry for different purposes:
Amplifies the signal coming from the photo detector
Separates the plethysmograph signals into red signals and infrared signals.
Switching and controlling the current of the LED.
Setting the gain of the signals to be equivalent to the other signal
Divide the signal into arterial signal and other signals
Convert the infrared signals and the red signals into digital signals using AD conversion.
Computation of the ratio red to infrared.
Eliminates artifacts
Compute the value of oxygen saturation
Display of the computed values
Managing the alarms settings
The absorption of energy from the LED to the photo-detector creates the signal in the red and the infrared channels. This absorption is the assembly of different absorptions from various sources such as arterial blood and its pulsation, venous blood and tissues.
The initial amplification stage is implemented by analog electronics, whereas calculation of spo2 stage is implemented with a microprocessor, the photo-detector signal is treated by electronics or microprocessors. The output signal from the analog part is processed by an ADC to be suitable for the digital part or the microprocessor.
2.5.2 Amplification stage
The amplification is processed in different stages:
The low amplitude photo-detector signal is amplified.
The LEDs are energized in an alternating sequence with a short delay in between to allow the measurement of external light.
The amplified signal is decomposed into three signals: red, infrared, and dark signal.
The electronic filters remove the 1 KHz high-frequency switching, making the signal continuous and having different wavelength.
The dark signal is subtracted from the DC levels to prevent problems from the energy source.
The DC components of the infrared signal is equalized to the DC components of the red signal by changing the amplitude of a photo-plethysmograph signal .
The red to infrared ratio is calculated from the amplitudes of the AC components.
2.5.3 Conventional Spo2 calculation methods
Earlier pulse oximetry used one of two methods to calculate the spo2 values. The first method is solving simultaneous Beer’Lambert law equations. However, this method have many limitations such as one unknown, absence of scattering and turbidity, and the need for the path length to be constant. Due to the many limitations, this method is considered inaccurate and therefore rejected. The second and common method uses the red to infrared ration with a look up table to find the spo2 values.
The thickness and size of the finger varies from one person to another, thus the optical density will also vary from one patient to another. However, the saturation of the semiconductor does not depend on the characteristics of the patient but only on the intensity of light. In order to have the same saturation, the same amount of light is applied to the patient regardless of the size and age. This can cause serious heat damage for children. The prevention of this problem is another microprocessor’s role. The microprocessor implements a correction factor that controls the LED current and synchronizes the LEDs intensities. The resulting current should be the minimum amount of light energy allowing the calculation of pulse oximetry while not damaging the tissue
2.6 Elimination of artifacts
The intact calculated saturation values include the real values with some invalid values created by artifacts. Therefore, statistical averaging methods are used in order to remove these artifacts
2.6.1 Mechanical movement artifacts
The mechanical movement artifacts are processed with the Nellcor algorithm. The Nellcor algorithm consists of the following steps:
Divide the output signal from the differential amplification stage into pulses.
Check the pulses for motion artifacts
If the pulses do not contain motion artifacts, compare the identified pulse to the normal pulse.
If the pulse contains motion artifacts, higher standards for the quality of the light motion signal are applied. The resulting pulse should be compared to the normal pulse
If the pulse is not identical to the normal pulse, that pulse is rejected
If the pulse is identical to the normal pulse, check if characteristics of the indentified pulse are physiologically possible
If the characteristics of the identified pulse are not physiologically possible , that pulse is rejected
If the characteristics of the identified pulse are physiologically possible, the pulse is compared to the average of the preceding pulses
If the pulse is not equal to the average of the preceding pulses, that pulse is rejected
If the pulse is equal to the average of the preceding pulses, the pulse is divided at dicrotic notch . Then the whole pulse or the main component is selected for the calculation.
Then, a filter based on confidence assessment is implemented
Finally, the SpO2 value is calculated