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Instrumental Analysis

by: Annamae Beatty

Instrumental Analysis CHE 331

Annamae Beatty

GPA 3.95

David Nabirahni

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David Nabirahni
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This 33 page Class Notes was uploaded by Annamae Beatty on Wednesday September 30, 2015. The Class Notes belongs to CHE 331 at Pace University - New York taught by David Nabirahni in Fall. Since its upload, it has received 10 views. For similar materials see /class/217114/che-331-pace-university-new-york in Chemistry at Pace University - New York.


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Date Created: 09/30/15
CHE 331 Chapter 10 Atomic Emission Spectroscopy History and Theory of Atomic Absorption Spectroscopy As the name implies atomic absorption is the absorption of light by free atoms An atomic absorption spectrophotometer is an instrument that uses this principle to analyze the concentration of metals in solution The versatility of atomic absorption an analytical technique Instrumental technique has led to the development of commercial instruments In all a total of 68 metals can be analyzed Advantages of AA Determination of 68 metals Ability to make ppb determinations on major components of a sample Precision of measurements by ame are better than 1 rsd There are few other instrumental methods that olfer this precision so easily AA analysis is subject to little interference Most interference that occurs have been well studied and documented Sample preparation is simple often involving only dissolution in an acid Instrument easy to tune and operate Kirchoff and Bunsen39s Experiment Between 1859 to 1861 Gustav Kirchoff Prussian physicist with his colleague Robert Tunsen a German chemist at the University of Heidelberg demonstrated that every element gives off a characteristic color when heated in incandescence The apparatus used for their classic experiment is shown here Applying this new research tool they discovered the element cesium and rubidium Kirchoff Absorbance amp Emission Line Kirchoff and Bunsen not only identified various characteristic spectra but they established the relationship between the emission spectra and the absorption spectra thus explaining the presence of the dark lines in the solar spectra Ground State Atom With that brief history of the development of the atomic absorption procedure and Varian atomic absorption instruments we will now examine the atomic theory that explains how an atomic absorption signal is generated In order to understand the atomic absorption process one must first understand the structure of the atom and its orbitals The atom consists of the central core or nucleus made up of positively charged protons and neutral neutrons Surrounding the nucleus in precisely de ned energy orbitals are the electrons All neutral atoms have an equal number of protons in the nucleus This means that each element has a unique number of electrons and protons The outermost electrons are known as the valence electrons and atomic spectroscopy involves energy changes in these valence electrons Beer Lambert Law The relationship that converts the intensity of the light beam to concentration is called the Beer Lambert Law or simply Beer39 s law Beer s Law states that the absorbance A is equal to the molar absorptivity or extinction coefficient a times the path length over which the measurement is made b times the concentration of the analyte c For a given set of conditions the molar absorptivity a is a constant The path length of the determination b is also a constant Therefore the absorbance is equal to a constant times the concentration A abc Kc where A absorbance a absorptivity constant b sample thickness path length c concentration K a constant If this expression is plotted a curve of absorbance versus concentration is drawn Beer39s Law predicts that a straight line will result In practice we nd that deviation from the linear calibration is observed at higher concentrations Normal Absorbance The important thing to remember in the use of Beer s Law is that A refers to absorbance not absorption Absorbance is defined by the equation A log lol where A absorbance lo the initial intensity I the intensity after absorption Calibration The concentration of the unknown is determined by comparing the samples with a series of standards AA is always a comparative technique where the determination is performed using freshly prepared matrix matched standards Flame Emission and Atomic Absorption Spectroscopy The following are the 3 main types of Flame Emission and Atomic Absorption Spectroscopy a Atomic Emission with thermal excitation AES b Atomic Absorption with optical photon unit AAS c Atomic Florescence AFS All of the following methods use the same or similar steps 1 Atomization Breakdown of the molecule into its atomic components in the gas phase Aerosolgt DesolvationgtVaporizationgt Atomization 2 Excitation Thermal excitation for ABS and Optical excitation for AAS andor AFS 3 Measurement Absorption AAS Emission AFS amp AES A powerful technique of measurement is the ICPAES which stands for Inductively Coupled PlasmaAtomic Emission Spectroscopy In terms of simplicity Atomic Emission Spectroscopy AES is the most complex because of the atomization part which is a function of temperature Furthermore in terms of cost AES is the most expensive and in terms of efficiency and precision AES is also the most efficient and precise In terms of sensitivity AES is the least sensitive Simplicity AASgtAFSgtAES Cost AESgtAASgtAFS Sensitivity AFSgtAASgtAFS It should be noted that in AES one would like the excited state of the elements to be populated by the electrons ATOMIC EMISSIVE SPECTROMETRY WITH PLASMAS Atomic emissive spectrometry AES can be performed where the ame is replaced with either a plasma or electrodes A plasma is an electrical conducting gaseous mixture containing a significant concentration of cations and electrons The concentration of the two are such that the net charge approaches zero Argon plasmas are used most often for non ame AES The high temperatures that are achieved in argon plasmas cause more efficient excitation of atoms and ions than is achieved with ames As a result the intensities of the emitted lines are greater and more spectral lines are observed Three types of hightemperature plasmas are encountered and these are l the inductively coupled plasma ICP 2 the direct current plasma DCP and the microwave induced plasma MIP The most important of these plasmas is the inductively coupled plasma ICP The Inductively Coupled Plasma Source The gure below is a shematic of a typical inductively coupled plasma source called a torch It consists of three concentric quartz tubes through which streams of argon gas ow Depending upon the torch design the total rate of argon consumption is 5 to 20 lmin Surrounding the top of this tube is a watercooled induction coil that is powered by a radio frequency generator which is capable of producing 05 to 2kW of power at about 27 or 4lMHz Radiofrequency induction coil Sample aerosol or vapor in argon l inductively coupled plasma From 39 101 A pica I Figure W nissiou Cumright V A Fassel Science 1978 202 185 With pen I 1978 by the American Assnciatlon far the Advancement afScmnca The wavelength selector for an instrument that uses a plasma is a narrowband pass monochromator The wavelength of the monochromator as well as the other functions of the spectrometer are generally controlled by a microcomputer Various detectors can be used including photomultiplier tubes and diode arrays Several wavelengths can be simultaneously monitored or the wavelengths can be sequentially scanned The readout devices that are used with the spectrometers include cathoderay tubes recorders and line printers Qualitative analysis is done using AES in the same manner in which it is done using FES The spectrum of the analyte is obtained and compared with the atomic and ionic spectra of possible elements in the analyte Generally an element is considered to be in the analyte if at least three intense lines can b matched with those from the spectrum of a known element Quantitative analysis with a plasma can be done using either an atomic or an ionic line Ionic lines are chosen for most analyses because they are usually more intense at the temperatures of plasmas than are the atomic lines Interference that is encountered with plasmas can be grouped into the same categories as those that were encountered with AAS Chemical interference owing to refractory compounds Is rarely a problem because plasmas have high temperatures Spectral interference is more plentiful when plasmas are used because an increased number of atomic and ionic lines are possible at the higher temperatures of plasmas Plasma temperatures of plasmas Plasma temperatures are in the approximate range from 6000 to 10000K AES WITH ELECTRICAL DISCHARGES An electrical discharge between two electrodes can be used to atomize or ionize a sample and to excite the resulting atoms or ions The sample can be contained in or coated on one or both of the electrodes or the electrodes can be made from the analyte The second electrode which does not contain the analyte is the counter electrode Electrical discharges can be used to assay nearly all metals and metalloids Approximately 72 elements can be determined using electrical discharges For analyses of solutions and gases the use of plasmas is generally preferred although electrical discharge can be used Solid samples are usually assayed with the aid of electrical discharges Typically it is possible to assay about 30 elements in a single sample in less than half an hour using electrical discharges To record the spectrum of a sample normally requires less than a minute ELECTRODES FOR AES The electrodes that are used for the various forms of AES are usually constructed from graphite Graphite is a good choice for an electrode material because it is conductive and does not spectrally interfere with the assay of most metals and metalloids In special cases metallic electrodes often copper or electrodes that are fabricated from the analyte are used Regardless of the type of electrodes that are used a portion of each of the electrodes is consumed during the electrical discharge The electrode material should be chosen so as not to spectrally interference during the analysis Sketches of several common forms of graphite electrodes are shown in Figure The cylindrical graphite electrodes typically have a diameter of 62mm and a length of 38mm Electrical discharge occurs at the pointed end of the counter electrodes where the strength of the electrical eld is maximum Several types of sample electrodes are available The pointed electrode can be a graphite rod on which the sample solution is coated and allowed to dry before analysis It is also the usual design when the electrode is constructed from the analyte Electrodes of that design are often used for steel or other metal samples The electrode is a graphitecup electrode The sample usually a powder is placed in the cup in the top of the electrode A drill bit is used I v Smpk m m to form the cup in the electrode Often the neck of the electrode below the 5mm vams cup is narrowed in order to minimize conduction of heat away from the cup during the electrical discharge In some electrodes the neck is of the same diameter as the remainder of the electrode A porouscup electrode is shown in Fig 73f It is used for solutions Several milliliters of the solution are placed inside the electrode The sample cavity in the electrode is prepared by drilling a hole to within about 3mm of the end of the graphite rod The solution slowly seeps through the bottom of the electrode The counter electrode is placed below the porouscup electrode The rotatingdisk electrode Fig 73 g is also used for solutions The disk which is about 13 cm in diameter is mounted on an aXle and dipped into the sample solution As the disk is rotated a lm of the solution is carried to the top of the disk The counter electrode is placed above the rotating disk at the top of the electrode In the rotatingplatform electrode Fig 73h the sample solution is placed on the top of the disk and allowed to drive The disk is rotated during the assay Both forms of electrodes are typically rotated at between 5 and 3910 revolutions per minute rmin DC ARC Electrical atomization ionization and subsequent excitation of the sample can be accomplished with either spark or are discharges Commercial instruments often contain two or more of the electrical excitative sources Of the several common types arcs and sparks the de arc is the simplest It uses a de potential that is between 10 and 50 V to cause an electrical discharge that corresponds to a current of between 1 and 5 A to ow between the counter and the sample electrode Fig 74 The temperature generated by the electrical discharge is about 4000 C at the anode and about 200C at the cathode Between the electrodes the temperature is in the 4000 to 7000 C range The sample electrode can be either the cathode or anode but generally it is the anode Temperatures that are achieved with the de arc are hotter than those achieved with most ames The excitation of the sample is attributable to the combination of the high temperature and the electrical energy between the electrodes Because different elements are vaporized and excited at different times it is necessary to use the arc until the entire sample has been vaporized minm In most AC IL fr 5 r1251an i FIJllwn39n quot 1nstruments the I 39 i er 5quotquot MKM 1quot 39 quot dc arc 1s started 1 Eluclmdcs Q by apply1ng a hi ghpotential spark the electrodes After the arc has been started the spark can either be shut off or allowed to continue The de arc yields intense emissive lines and consequently is often used for qualitative analysis Because the de arc wanders across the surfaces of the two electrodes and ickers the intensities of the emissive lines are not particularly stable ie the output signal from the de arc is noisy Another problem that is encountered with the de arc is the formation of gaseous cyanogen CN2 by chemical reaction of carbon from the electrodes with nitrogen from the air Cyanogen emits broadband radiation between about 360 and 420 nm that can interfere with many assays The problem can be eliminated by blanketing the electrode tips and the space between the electrodes with argon or a mixture of argon 70 to 80 percent and oxygen 20 to 3930 percent The exclusion of nitrogen prevents formation of cyanogen A Stallwood jet is a quartz enclosure that is placed around the electrodes and through which the protective gas is passed The gas passes upward over the sample electrode In addition to excluding nitrogen the protective gas decreases wandering of the arc The enclosure is constructed from quartz to permit emitted radiation to eXit from the chamber AC ARC An ac is similar to a dc arc expect the discharge between the electrodes is not continuous The cathode and anode alternate after each halfcycle of the applied ac potential Typically the potential supply operates at 60 Hz which results in a polarity reversal of the electrodes at a rate of 120 times each second During the discharge in each halfcycle the current is continuous as in the dc arc The discharge must be restarted each time the polarity of the electrodes is switched Because the potential that is required to start a discharge is greater than that necessary to maintain a discharge the ac potentials that are used with ac arcs are greater than the de potentials that are required to sustain a de arc The use of a potential between 2000 and 000 V usually results in a current between 1 and 5A The ac arc effectively samples the analyte during each discharge between the electrodes Uneven sampling that is characteristic of the de arc is prevented with a resulting increase in reproducibility The sensitivity of the ac arc is less than that of the de arc Sample solutions that are assayed using the ac arc are usually coated on the surface of the sample electrode and allowed to evaporate to dryness before the assay Copper electrodes as well as graphite electrodes can be used with an ac arc SPARK The spark excitative source uses ac power an LC circuit and a spark gap that is operated by a synchronous motor to cause a spark to jump between the electrodes The spark gap operates in a manner similar to that of the spark gap Variablu Distributor of Unljnbk indu lur mm an automobile 1 if 39 39 Its function is to I quot Vnr abk EIrctrmlcg i L pm Wm 3quot ensure that the Ar 15m 393 39 39 k WU g P39HF spar Jumps 15a 2213 939 1ran nrnm r Auxiliary between the EparlL Eng electrodes only gure 39J S air3min diagram mfa Fluesncr ciruujt thn1 is um 1a thruspark 51qu when the potential that is stored in the capacitor in the ac circuit is at a maximum The motor rotation is synchronized to the frequency of alternation of the current A sketch of a simple circuit Feussner circuit that can be used for a spark source is shown in Fig 75 Several variations of the circuit are in use in different instruments The potential after the stepup transformer in the circuit is between 10000 and 50000 V with a highvoltage source and about 1000 V with a medium voltage source The spark is active for periods between 10 and 100ps and typically discharges at a rate of 120 to 180 times each second Heating effects on the electrodes are minimized by the cooling that occurs between sparks That leads to less fractional distillation of the sample from the electrode than is observed with the dc arc The time required to obtain a spectrum with a spark is about 10s The spark generally yields the most reproducible results and the highest precision of all of the spark and arc discharges It is not as sensitive however as the de arc Minimum concentrations that can be assayed with a spark are about 001 percent for solid metallic samples and about 1 VtgmL for solutions Solid metallic samples are usually machined into a rod for use as the sample electrode Normally the counter electrode is a pointed graphite or silver rod Powders are pressed into pellets and inserted in the of the sample electrode Liquids often are assayed with the aid of a porouscup electrode LASER MICROPROBE The laser microprobe uses a laser to vaporize a small section on the surface of a sample The vaporized sample passes between two ac spark electrodes that excite the sample The resulting emissive spectrum is recorded as with the other AES methods The laser microprobe is ideally suited for examination of small areas on a surface A microscope is used to focus the beam from the laser onto an area that is roughly 10 to 50vim in diameter Often a pulsed laser is used The electrodes are held in place about 25m above the surface The laser is fired between the electrodes The two electrodes are sharply pointed rods that serve to control the location of the electric field during the discharges A sketch of a laser microprobe WAVELENGTH SELECTION AND DETECTION FOR AES Arc and spark instruments normally contain non scanning monochromators Either a series of slits is cut in the focal plane of the monochromator and a photomultiplier tube is placed behind each slit that corresponds to the wavelength of a line that is to be measured or one or more photographic plates or pieces of film are placed on the focal of the monochromator The instrument is a spectrometer if a photomultiplier tube or other photon detectors are used It is a spectrograph if the detector is a photographic plate or film Commercial spectrometers can contain as many as 90 exit slits The analyst chooses the exit slits that correspond to the spectral lines that are to be measured and places a detector behind each chosen slit For many analyses between 20 and 35 detectors are simultaneously used at different slits to simultaneously assay one element for each detector Each detector is termed a channel A spectrometer of that design is a direct reader or a directreading spectrometer If the chosen slits are too close together to permit placement of a detector behind each mirrors can be used behind the slits to re ect the radiation to the detectors In a spectrograph the entire spectrum of the sample is simultaneously recorded Each spectral line forms an image in the shape of the entrance slit to the monochromator on the film Generally the entrance slit and the images are narrow rectangles Measurement of the intensity of a particular spectral line is a requirement for quantitative analysis Intensity measurements with lms and plates are not as easily accomplished as they are with photomultiplier tubes After development of the lm or plate each spectral line appears as a black image on the developed photograph or a light image on the negative The intensity of the spectral line is proportional to the amount of darkening on the developed lm or to the lack of darkening on the negative The amount of darkening is measured with a densitometer A densitometer focuses radiation on the image of each line and uses a photomultiplier tube or other detector to measure the amount of radiation that is transmitted through or re ected by the image The measurement is similar to the percent transmittance measurement in a spectrophotometer The measured percent transmittance for each image is generally not directly proportional to the concentration of the assayed element Working curves are used to determine the concentration of a particular element in a sample About 16 spectra can be recorded on a roll of 5mm lm and 40 spectra on a 10 25cm photographic plate The densitometer that is used for most measurements is a microphotometercomparator It uses a tungsten lament lamp as the source of radiation Microphotometercomparators contain a slit that can be adjusted by the operator and a photon detector such as a photomultiplier tube that functions well the visible region QUALITATIVE ANALYSIS WITH ARC AND SPARK AES Qualitative analysis is performed by comparing the wavelengths of the intense lines from the sample with those for known elements It is generally agreed that at least three intense lines of a sample must be matched within a known element in order to conclude that the sample contains the element Normally a de arc is the source of choice for qualitative analysis because it produces intense spectral lines Other arcs and sparks also can be used In order to assign wavelengths to the developed images on a photographic plate or lm it is helpfull to obtain the spectrum of a reference element that has lines of known wavelengths near the lines from the sample Iron is often used as lines of known wavelengths near the lines from the sample Iron is often used as the reference element because it emits a multitude of lines A I I 39I the entire ibl region The spectrum of the reference element is obtained on the same photographic plate as that used for the sample in order to prevent possible changes in alignment during insertion of new plate About 72 elements can be qualitatively and quantitatively assayed with arc spark AES QUANTITATIVE ANALYSIS WITH ARC AND SPARK AES With direct readers quantitative analysis is straightforward A channel is assigned for each element The measured intensity of the spectral line is used with a working curve to quantitate the element in the sample The wavelengths must be carefully chosen to prevent spectral interference Typically precision obtained with direct readers are in the range of i03 to 3 percent When a photographic plate or lm is used as the detector the precision is not as good as that achieved with direct readers In order to obtain accurate and precise results all of the experimental conditions must be carefully controlled Variables such as exposure time film type and developing conditions particularly are important Automated development of the film or plat is advisable whenever possible in order to minimize changes in the development process With careful control of conditions errors between 1 and 10 percent can be achieved using photographic detection Regardless of the type of detection used for the assay the precision of the results can be improved by matrixmatching the standards with the sample Use of the internalstandard method also improves precision Usually a working curve is prepared by plotting the ratio or logarithm of the ratio of intensity of the standards line to the internal standards line as a function of the logarithm of the concentration of the standard The corresponding ratio for the analyte is obtained and the concentration determined from the working curve In many cases the precision and accuracy of an analysis of a compound that contains organic components can be increased by washing the sample prior to the assay Normally the sample is placed in a platinum or silica crucible and heated in a muf e furnace to 50039C Ashing can also be done in a lowtemperature oxygen plasma The temperature should be sufficiently high to remove all traces of any organic matrix of the analyte but it cannot be high enough to vaporize the assayed elements The ashing process is similar to that performed in furnace cells during atomic absorption spectrophotometry Internet References wwwanachemumusejumpstationhtm wwwanachemumusecgijumpstationexeAtomicSpectroscopy wwwanachemumusecgijumpstationexeOpticalMolecularSpectroscopy wwwminyositsrmiteduaurcmfamstheoryhtml http sciencewidener edusubftirintroiithtml httpwwwsa s0rg httpwwwchemswcom httpwww wimedin vim Md 1 39 htm httpnercdg0rg httpwwwanalyticoncom wwwlcgmagcom wwwlcmscom wwwdqfctun1pt QOF Chromahtm1 wwwssgchemutaseduau wwwyah00 39 39 39 J 39 39 www0nlinegccom httpwwwaur0ra instr com httpwwwchemu eduNitl34177598spectroscopyaeshtm httpwwwr0hansdsuedustaffdrjackmchemistrychemlinldanalyticanalyt1htm1 httpwwwcofcedudeav0rj521jpd521htm httpwww wimedin vim Md 1 htm Chemistry 331 Chapter 2 Electrical Componentsquot and Circuits The purpose ofthis chapter is to discuss basicdirectrcurrent dc Circuit components in preparation for the two following chapters that deal with integrated circuits and microcomputers in instruments for chemicalanalysis 2A DIECT CURRENT CIRCUITS AND MEASUREMENTS Some basic direct current circuits and how they are used in making current voltage and resistance measurementswill be considered The general definition ofa circuit is a closed path that may be followed by an electric current A galVanometer is a device with a rotating indicator that will rotate from its equilibrium position when a current passes through it A galvanom ete r has a negligible resistance I I I I I R I I I I Figure 1 Ampermeter An ampermeter ammeter is a galvanometer with a calibrated current scale for its indicator and a bypass resistor called a shunt for a xed fraction ofthe current shown in Figure 1 Many ammeters haVe several selectable shunts which provide their corresponding current meter ranges Typically ammeters can be found with calibrated ranges of1 microA for full scale deflection up to 1000 A for full scale de ection and in multiples of 10 between these extremes Figure 2 Voltmeter A voltmeter shown in Figure 2 is just a calibrated galvanometer withra series resistor so that39the total resistance of the path is increased The galvanometer range is calibrated for the current lg passing through it This scale is adjusted to display the potential difference between points A and B voltage by substituting Vg values for lg on the scale where Vg lg Rg and Rg is the total resistance of the voltmeter Voltmeters may have more than one calibrated scale which can be selected by changing the resistance Rg Current in a circuit is the ow of th e positive charge from a high potential to a low potential Meters are labeled to indicate the proper direction of current ow through them A reverse ow of DC current may destroy a meter Electrical charge will not move through a conducting path unless there is a potential difference between the ends ofthe conductors All materials resist the ow of current through them requiring work to be done to move the charge through the material The source of energy in a circuit which proVides the energy to move the charge through the circuit can be a battery photocell or some other power supply An electrical circuit is a circuitous path of wire and devices A schematic drawing of a real circuit utilizes the symbols shown in Figure 3 I mii Eattery or DC Galvanometer Power Supply Ampemeter Variable DC Supply voltmeter AC Voltage Supply Resistor Wire I I Capacitor o ere fmm Connectlon Inductor Circuit Symbols Switch 39 I Diode Figure 3 An example Figure 4 shows a circuit with a DC power supply in a series with a resistor a parallel branch with a resistor and voltmeter and an ammeter AAAA VVVV Figure 4 Example of an Electric Circuit BASIC ELECTRIC CIRCUIT The flashlight is an example of a basic electric circuit It contains a source of electrical energy the dry cells in the flashlight a load the bulb that changes the electrical energy into a more useful form of energy light and a switch to control the energy delivered to the load A load is any device through which an electrical current ows and which changes this electrical energy into a more useful form The following are common examples of loads A light bulb changes electrical energy to light energy An electric motor changes electrical energy into mechanical energy A speaker in a radio changes electrical energy into sound A source is the device that furnishes the electrical energy used by the load It may be a simple dry cell as in a flashlight a storage battery as in an automobile or a power supply such as a battery charger A switch permits control of the electrical device by interrupting the current delivered to the load 51 DRYCEL LI BAT 39 E A DEENERGIZED FLASHLIGHT BULB 051 L B ENERGIZE D FIGURE 31 Schematic of a Flashlight Schematic ofa Basic Circuit the Flashlight Laws of Electricity Ohm s law describes the relationship among potential resistance and current in a resistive series circuit In a series circuit all circuit elements are connected in sequence along a unique path head to tail as are the battery and three resistors shown in Figure 21 Ohm s Law may be written as V IR Where V is the potential difference in volts between two points in a circuit R is the resistance between the two points in ohms and l is the resulting current in amperes diagrams for determining resistance and voltage in a basic circuit respectively 1 quotI saw u l p ma R1 FIGURE 32 Determining Resistance in a Basic Circuit E ll l I ll l I5A 3145 FIGURE 33 Determining Voltage in a Basic Circuit Using Ohms39s Law the resistance ofa circuit can be determined knowing only the voltage and the current in the circuit In any equation if all the variables parameters are known except one that unknown can be found For example using Ohm39s Law if current I and voltage E are known you can determine resistance R the only parameter not known Basic formula I E The formula may also bl expressed as EIxR orR I Increase in Resistance Constant Voltage 2 A steady increase in resistance in a circuit with constant voltage produces a progressively not a straightline if graphed weaker current 0 123455 Resistance Increase in Voltage Constant Resis39ta nee In simplerterms Ohm s Law means an 1 A steady increase in voltage in a circuit with A Current constant resistance produces a constant linear rise in m current 0 1 2 3 4 5 5 Voltage TECHNICAL DEFINITION ALERT Ohm39s Law is a formulation ofthe relationship of voltage current and resistance expressed as Where V15me Vonage measured m vo ts 1 rs the Current measured m amberes R rsme resrstance measured rn Ohms Therefore Vuts Amps umes Resrszance Ohms Law 15 usedto Camu ate a rmssmgva ue m a orcmt 12v BA I39I39ERY mslsea wuo L 12 Amps ofCurrent 1n Ohm v 12 x1 v 12 Volts 12V 1r we knew the battery was supphng 12 vow of pressure vo tage and there was a resrstwe bad of 1 Ohm b1aced m serres me curremwoub be 1211 12Amps 12A V w 12A men the Resrstance Womd be R12I12R1W 1W Usmg me rrrstrorrnu1a rrorn above we determme the Vonage 12V Note R em emb era battery is not measured in amperagezas is commonly belieVed with beginners to electronics The battery suppliesthje pressure that creates the flOW Qur r e nt in av39givencircuit The ampe ra39ge rating in a battery is quotHow long the battery will last39for one hour While driving a circuit oflth39at39 amperagequot it is measured in AmperagesHours 8103 1000mm would last for391 hour in acne amp circuit 1 000mAh isr1A for39one hour An easy way to remember the formulas is by using this diagram To determine a missing value cover it with your nger The horizontal line in the middle means to divide the two remaining values The quotXquot in the bottom section of the circle means to multiply the remaining values If you are calculating voltage cover it and you have IX R left V times R o If you are calculating amperage cover it and you have V divided by R le lVR If you are calculating resistance cover it and you have V divide by I left RVl Note The letter E is sometimes used instead ofV for voltage Kirchhoffs Law Kirchhoff s current law states that the algebraic sum of currents around any point in a circuit is zero Kirchhoffs voltage law states that the algebraic sum of the voltages around a closed electrical loop is zero Kirchhoffs Voltage Law Kirchhoffs Voltage Law or Kirchhoffs Loop A I B Rule is a result of the electrostatic eld being conservative It states that the total voltage around a closed loop must be zero lfthis 39I were not the case then when we travel around a closed loop the voltages would be D 39l C inde nitequot so Figure 1 Around a closed loop ZV0 the total voltage should be zero In Figure 1 the total voltage around loop 1 should sum to zero as does the total voltage in loop2 Furthermore the loop which consists of the outer part of the circuit the path AB CD should also sum to zero We can adopt the convention that potential gains ie going from lower to higher potential such as with an emf source is taken to be positive Potential losses such as across a resistor will then be negative However as long as you are consistent in doing your problems you should be able to choose whichever convention you like It is a good idea to adopt the convention used in your class PoWer Law The power law states that the power in watts dissipated in a resistive element is given by the product of the current in amperes and the potential difference across the resistance in volts And substituting Ohm s law gives P I2R V2R Basic Direct Current Circuits The Schematic Dia ram The schematic dia ram consists ofidealized circuit elements each of which represents some property ofthe actual circuit The Fiqure shows some common circuit elements encountered in DC circuits A twoterminal network is a circuit that has only two points of interest say A and B L V I a b c Figure Common circuit elements encountered in DC circuits a ideal voltage source b ideal current source and c resistor Two types of basic dc circuits will be described series resistive circuits and parallel resistive circuits Series Circuits Figure 21 shows a basic series circuit which consists of a I R VI Iquotml B 4 R v1 In V V R liq I 3 Ili1ilzllgh V8VV3V3 R Ry Rg39 R battew a switch and three resistors in series Figure 21 Principles of Instrumental Analysis The current is the same at all points in a series circuit that is Application of Kirchhost voltage law to the circuit in Figure 21 yields V V1 V2 V3 The total resistance R5 of a series circuit is equal to the sum of the resistances of the individual components R5R1R2R3 Parallel Circuits Figure 22 shows a parallel dc circuit A W I I 2 a I v l tlt 1lt tlt T 1 I 11 g R1 12 32 3 gtR3 12 3 h Figure 22 Principles of Instrumental Analysis Applying Kirchhoff s current law we obtain t123 Applying Kirchho39ff s voltage law to this cirCuit gives three independent equations V I391R1 V 2R2 V 3Rr3 Substitution and division by V giVes 1 Rp 111R2 1Rr3 Since the conductance G of a resistor R is given by G 139R GpG1G2G3 Conductances are additive in 39a parallel circuit rather than the resistance In conclusion the most important things to remember about the differences between resistors in series and parallel are as follows Resistors in series have the same current and Resistors in parallel have the same volta39ge ZB SEMICONDUCTOR DIODES OBJECTIVES Learning objectives are stated at the beginning of each chapter Th ese learning objectives serve as a preview ofthe information you are expected to learn i39n the chapter The comprehensive Check questions are based on the objectives The learning obj ectiVes39are listed below Upon completion ofthis chapter you should be able to do the following State in terms of energy bands the differences between a conductor an insulator and a semiconductor Explain the electron and the hole ow theory in semiconductors and how the semiconductor is affected by doping Define the term quotdiodequot and give a brief description of its construction and operation Explain how the diode can be used as a halfwave recti er and as a switch Identify the diode by its symbology alphanumerical designation and color code List the precautions that must be taken when working with diodes and describe the different ways to test them A diode is a nonlinear device that has greater conductance in one direction than in another Useful diodes are manufactured by forming adjacent ntype and ptype regions within a single germanium or silicon crystal the interface between these regions istermed a pn junction Figure 23a is a cross section of one type of pn junction which is formed by diffusing an eXcess of a ptype impurity such as indium into a minute silicon chip that has been doped with an ntype impurity such as antimOny A junctiOn ofthi s kind permits movement of holes from the p region into the n region and movement of electrons in the in the reverse direction As holes and electrons diffuse in the opposite direction a region is created that isdeple ted of mobile charge carriers and thus has very high resistance This region is referred to as the depletion region Because there is a separation of39charge across the depletion region a potential difference develops across the region that causes a migration of holes and electrons in theopposite direction The current that results from the diffusion of holes and electrons is balanced by the current produced by migration ofthe carriers in the electric eld thus there is no net current The magnitude of potential difference acr0ss the depleted region depends upon the composition ofthe materials used in the pn junction For silicon diodes the potentialdifference is about 06V and for germanium it is about 03V When a positive potential is applied across a pn ju39nction there is little resistance to current in the direction of the ptype to the ntype material On the other hand the pn junction offers a high resistance to the ow of holes in the opposite direction and is called a current recti er Figure 23b illustrates the symbol for a diode The arrow points in the direction of low resistance to p0sitive current The triangular portion of the diode symbol may be imagined to point in the directiOn of current in a conducting diode Figure 230 shows the mechaniSm of conduction of charge when the p region is made positive with respect to the n region by application ofa potential this process is called forward biasing The holes in the p region and the excess electrons in the n region move underthe in uence of the electric field toward the junction where they combine and annihilate each other The negative terminal of the battery injects new electrons into the n region which can then continue the conduction process the positive terminal extracts electrons from the p region creating new holes that are free to migrate towards the pn junction Figure 23d shows when the diode isreversebiased and the majority carriers in each region drift away from the junction to form the depletion layer which contains few charges Only the small concentration of minority carriers present in each region drifts toward the junction and creates a current Figure 1 Ampermeter An amp ermeter ammeter is a galvanometer with a calibrated current scale for its indicator and a bypass resistor called a shunt for a xed fraction ofthe current shown in Figure 1 Many ammeters haVe several selectable shunts which provide their corresponding current meter ranges Typically ammeters can be found with calibrated ranges of1 microA for full scale deflection up to 1000 A for full scale de ection and in multiples of 10 between these extremes Figure 2 Voltmeter A voltmeter shown in Figure 2 is just a calibrated galvanometer With a series resistor so that the total resistance of the path39is increased The galvanometer range is calibrated for the current lg passing through it This scale is adjusted to display the potential difference between points A and B voltage by substituting Vg values for lg on the scale where Vg lg R9 and Rg is the total resistance of the voltmeter Voltmeters may have more than one calibrated scale which can be selected by changing the resistance Rg Current in a circuit is the ow ofthe positive charge from a high potential to a low potential Meters are labeled to indicate the proper direction of current ow through them A reverse ow of DC current may destroy a meter Electrical charge will not move through a conducting path unless there is a potential difference between the ends ofthe conductors All materials resist the ow of current through them requiring work to be done to move the charge through the material The source ofenergy in a circuit which provides the energy to move the charge through the circuit can be a battew photocell orisome other power s upply An electrical circuit is a circuitous path of wire and devices A schematic drawing of a real circuit utilizes the symbols shown in Figure 3 Types of39electrical devi ces and their uses Bar Code Devices 196 companies Devices such as sCanners and verifiers used to decode read the bar codes stamped on products Batteries and Accessories 39639 companies Devices39that convert stored energy into electrical current the two main types are chemical batteries and physical batteries such as solar cells nuclear energy and thermal batteries Connectors 714 companies Components used to conduct and transfer signals electrical Optical rf etc or power from one cable to another Data Input Devices 208 companies Devices such as a keyboard or mouse used to interact with other devices or computers for the purpose of in puttin g data Electrical and Electronics Fasteners and Hardware 95 cOmpanies Small components and hardware for electrical and electronic applications Electrical Distribution and Protection E ui ment 391 668 com anies Equipment used to distribute power and protect other equipments and systems from current or voltage surges Electrical Testing Eguipment 200 companies Electrical testing instruments for current leakage and insulation resistance measurements Enclosures 2785 companies Used for enclosing or containing electrical electronic or mechanical c0mponents or to provide protection for their operators Fans and Electronic Cooling 765 companies Devices and equipment used to regulate temperature by removing heat from electricaland electronic components Fu39ses 169 companies FUses protect electrical devices frOm overciurrents and short circuits that occur in improperly Operating circuits Industrial Counters and Timers 234 com anies Industrial countersand industrial timers are used in a variety ofapplications including process timing process control and unit counting Magnets 194 companies A magnet is simply any material capable of attracting iron and producing a magnetic eld outside itself either naturally or induced Meters Readouts and Indicators 776 companies Any type of equipment used to display information in various formats including digital readouts indicator lights or panel meters Motors 818 companies All types of rotary and linear motors including AC DC servo stepper induction hydraulic pneumatic motors Passive Electronic Components 2211 companies Passive electronic compon ents such as resistors inductors and capacitors that do not require power to operate Power Generation and Storage 605 companies Products and accessories related to power generation and storage Power Supplies and ConditiOners 1713 companies Devices that produce constant voltage and stabilize voltage levels and signals Relays an d Relay Accessories 326 companies Relays are electromechanical switches in which the variation of current in one electric circuit controls the ow of electricity in another circuit Surge Sup pressors184 companies Electrical devices used to det ectand control high voltage or current surges in order to protect equipments or systems Switches 582 companies DeVices used to route signals by allowing or preventing the signal ow when in closed or open position Transformers 516 companies Transtrmers transfer electrical energy from one electric circuit to another typically by the principles of electromagnetic induction Transformer types include potential current stepup stepdown distribution and others VWes Cables and Accessories 1816 companies V res cables and ac cessorie39suSed to transmit electrical power 0r signals REFERENCES Direct Current Circuits httppneumaphysualbertacagingrichphys395notesnode2html Field effect transistors FETs as transducers in electrochemical sensorsquot httpwwwchpwedupldybkocsrgisfetchemfethtml Skoog Holler and Nieman Principles of Instrumental Analysis 395 ed Orlando Harcourt Brace amp Co 1998 Shul ga AA KoudelkaHep M de Rooij NF Netch iporouk Ll Glucose sensitive enzyme eld effect transistor using potassium ferricyanide as an oxidizing substratequot Analytical Chemistm 15 Jan 1994 Thompson JM Smith SC Cramb R Hutton iC39linic al39evaluation of sodium ion selective eld effect transistors for whole bloodassay Annals of Clinical Biochemistm 31 Jan 1994


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