Investigations into the role of a premixed oxygen-hydrogen flame in flame emission spectrometry

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Authors Gutzler, David Eugene, 1949-

Publisher The University of Arizona.

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INVESTIGATIONS INTO THE ROLE OF A PREMIXED OXYGEN-HYDROGEN FLAME IN FLAME EMISSION SPECTROMETRY

byDavid Eugene Gutzler

A Thesis Submitted to the Faculty of theDEPARTMENT OF CHEMISTRY

In Partial Fulfillment of the Requirements For the Degree ofMASTER OF SCIENCE

In the Graduate CollegeTHE UNIVERSITY OF. ARIZONA

1 9 7 3

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library„

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR This thesis has been approved bn the date shown below:

M. BONNER DENTON Assistant Professor of Chemistry

Date

ACKNOWLEDGMENT S

The author expresses his sincere thanks to Dr. M. Bonner Denton for all the help and enthusiastic support he has given at The University of Arizona.

The author wishes to express his appreciation to all of the Chemistry faculty, especially those in the Analytical Division, for all of their assistance during his graduate studies.

TABLE OF CONTENTS

PageLIST OF ILLUSTRATIONS viLIST OF TABLES . . . . . . . . . '-.o . . . . . . . . . . ix

5̂1* e « o o o o o o o o o o o o o o o o o o o o o 5d.CHAPTER

I. INTRODUCTION . . . . . . . . . . . . . . . . . . 1Flame Emission Spectrometry . . . . . . . . 1The Premixed 02"-'̂ 2 ■EP̂ ame « « » . . . . « . , 3

II. SYSTEM DESCRIPTION FOR COMPARISON STUDIES . . . 5Measurement System . . . . . . . . . . . . . 5Ultrasonic Nebulizer . . . . . . . . . . . . 5Rurners . . . . . . . . . . . . . . . . . . 1.3.Radio Frequency Power . . . . . . . . . . . 14System Operation . . . . . . . . . . . . . . 16

III. COMPARISON OF DETECTION LIMITS FOR THEPREMIXED 02-H2 FLAME . . . . . . . . . . . . . 18

Flame Background Emission Spectra . . . . . 18Turbulent Oxygen-Hydrogen Flame . . . . . . 28Burner and Nebulization Characteristics

of the Premixed Oxygeri-HydrogenFlame . . . . . . . . . . . . . . . . . . 3̂ ^

Premixed Flames of an^ N20-C2H2 « « 30IV. DESCRIPTION AND RESULTS FOR AN OPTIMIZED

o 2-h 2 FLAME EMISSION SYSTEM . . . . . . . . . . 36Measurement System . . . . . . . . . . . . . 36Lock-In Amplifier . . . . . . . . . . . . . 36System Operation . . . . . . . . . . . . . . 38Detection Limits . . . . . . . . . . . . . . 39

IV

VTABLE OF CONTENTS— Continued

PageV 0 ANALYTICAL ADVANTAGES OF THE PREMIXED

O2 -H2 FLAME 0 0 0 0 0 0 .o o o o o e o o o e o 43Detection Limits . 0 0 0 0 0 0 = 0 0 0 0 = 0 43Standard Reference Material, OrchardLeaves 0 0 0 0 0 0 = 0 0 0 0 0 0 0 0 0 0 0 3

Analytical Curves = 0 0 0 0 = 0 = 0 . 0 0 = 44Chemical Interferences = = = = = = = = = . = 50

VIo SMALL VOLUMES OF SAMPLE BY ATOMIC EMISSION = = „ 63Background = = = = « = = = , = = = . = « = = 63Small Samples by Flame Emission = = = = , = 63

VII. RECOMMENDATIONS FOR FUTURE INVESTIGATIONS . . . 69Ultrasonic Nebulizer = . . = = . = = . . . . 69Pulse Ultrasonic Nebulizer-Burner = . = . . 70Atomic Fluorescence . . . . . . o . o . . . 71

LIST OF ILLUSTRATIONS

Figure Pagelo Experimental System for Comparison Studies . . . 62= Schematic Diagram of Additional Time

Constants in the Log/Linear CurrentModule (Reference 31) . . . . . . . . . . . . 7

3. The Ultrasonic Nebulizer System . . . . . . . . 84. Internal Details of the Base of the

Ultrasonic Nebulizer . . . . . . . . . . . . . 95. Diagram of 13-Hole Burner . . . . . . . . . . . 12.6. Background Emission Spectra of Premixed

0̂ "™*Ĥ Flame . . . . . . . . . . . . . . . . . 3.7. Background Emission Spectra of Turbulent

0̂ 2*™Ĥ 2 Flame . . . . . . . . . . . . . . . . . 208. Background Emission Spectra of Premixed

N20-C2H2 Flame (Slit Width is 300 Microns) . . 219. Background Emission Spectra of Premixed

0 2“C2h2 Flame (Slit Width is 300 Microns) . . 2210. Background Emission Spectra of Premixed

N20-C2H2 Flame((Slit Width is 100 Microns) . . 2311. Background Emission Spectra of Premixed

02™C2h2 Flame (Slit Width is 100 Microns) . . 24

12. Influence of Flame Background Intensity andFlicker on Minimum Detectable Atomic Concentration and Optimum Slit Width(Reference 5 2 2 7

13. Experimental Configuration for anOptimized Flame Emission System . . . . . . . 37

vi

viiLIST OF ILLUSTRATIONS— Continued

Figure14. Operating Instructions for Lock-In

Amplifier (Reference 55) . . . . . . . . .15. Calibration Curve for Ca in Standard

Reference Material (Analyzed at 200 ppb Level) o o e e o © o o o o o © o © <

16. Calibration Curve for Ca in Standard Reference Material (Analyzed at 500 ppt Level) . . o o o o o o o o o o o c

17. Calibration Curve for Mn in Standard Reference Material (Analyzed at 100 ppb Level) . . . . o o o o © o o o o

18. Calibration Curve for Mg in Standard Reference Material (Analyzed at .6 ppm Level) . . . . . . o o o o o o o o c

19. Analytical Working Curves for In, Tl, and Mg e o e o o o o o o e o o o o

20. Analytical Working Curves for Ca, Cr, La, and Ni o o o o o o o c o o o o o

21. Analytical Working Curves for Sr, Ba, M n , and Cu . © o e o © e o o o o o o

22. The Effect of P O ^ on the CalciumEmission Signal (0.5 |=tg/ml) . . . . . .

23. The Effect of Al̂ ~*" on the CalciumEmission Signal (0.5 |ig/ml) . . . . . .

3—■24. The Interference of PO^ on the CalciumEmission Signal (0.5 (ig/ml) as Viewed in Different Heights of the ■(->2 ~ ^ 2 ’̂-*-ame

25. The Effect of Operating the Nebulizerat Different Frequencies Upon the Interference of PO 3- on the Calcium Emission Signal (0.5 fig/ml) . .

Page

40

45

46

47

48

51

52

53

55

56

59

61

VXX1

LIST OF ILLUSTRATIONS— ContinuedFigure26o The Effect of Operating the Nebulizer at

Different Frequencies Upon the Interference o% Al^t 0n the Calcium Emission Signal (0.5 ^g/ml) . . . . . . .

27. Detailed View of Pulse UltrasonicNebulizer-Burner . . . . . . . . . . . . .

Page

62

65

LIST OF TABLES

Tablel o

2«,

3.

4.

5o

60

7.

80

9.

PageOperating Parameters for the BC-191

Transmitter and Modified TU-5B Tuning Unit (16) Using the UltrasonicMebUlrZOr ne o o © e o o o o o e o o o o o o o 15

Operating Parameters for the HeathLog/Linear Current Module „ . , „ . . „ » „ « 16

Comparison of the Detection Limits for the Turbulent 02“H2 Fbame with Pneumatic Nebulization Versus the Premixed O2 -H2 Flame with Ultrasonic Nebulization Utilizing the Configurationf 1 g* © 1 o o © © o o © e o o o o o o © o o © 2 9

Comparison of Detection Limits for the Premixed Oxygen-Hydrogen Flame UsingPneumatic and Ultrasonic Nebulization © © © © 31

Detection Limits for a 13-Hole and 7-Hole Laminar Flow Burner with a PremixedOxygen-Hydrogen Flame © . © © » » » » . © » » . 32

Detection Limits for Three Premixed Flames Supported by the 13-Hole Burner Using Ultrasonic Nebulizationin Configuration 1 . . . 3 4

Operating Conditions for the Lock-Injo 11 f i or* o © © © © © © © © © © ©.© © © © © © 3 9

Comparison of Reported Detection Limits in Flame Emission (53) Versus the Results Observed in This Study Utilizing the Optimized ExperimentalConfiguration Shown in Fig, 13 „ 42

Analysis of Standard Reference Material,1571 Orchard Leaves 44

ix

X

Table10.

11.

LIST OF TABLES— ContinuedPage

Operating Parameters for the BC-191 Transmitter and TU-5B Tuning Unit (#58150) Using the Pulse UltrasonicU e b u l r Z e r e o o o o e o o o o e o o o e o o o 8 0

Comparison of Detection Limits for the Ultrasonic Nebulizer with 50 ml Sample Volume and “the Pulse UltrasonicNebulizer with 200 |«ll Sample Volume „ » , , , 68

ABSTRACT

Investigations are described into the use of alaminar flow premixed oxygen-hydrogen flame combined withultrasonic nebulization. The premixed flame, as studied byflame emission spectrometry, gives improved detection limitsapproaching three orders of magnitude when compared to aconventional turbulent oxygen-hydrogen flame.

In a comparison with two other premixed flames, thepremixed oxygen-hydrogen flame gave the lowest detection

olimits for elements with wavelengths longer than 4000 A.Data are presented demonstrating the effect of two classical interferences in the premixed flame. An analysis of a standard indicates that the premixed oxygen-hydrogen flame has a useful role in trace analysis =

The combination of the laminar flow premixed oxygen- hydrogen flame with pulse ultrasonic nebulization makes it possible to perform flame emission analysis on a small volume of sample (200 pi) without any loss in sensitivity.

xi

CHAPTER I

INTRODUCTION

Flame Emission SpectrometrySimple combustion flames are now one of the most

useful and convenient devices available to the analyticalchemist for the determination of trace concentrations ofmetals in solution. This usefulness stems, for the mostpart, from the efficient manner in which a flame may beused to convert an aerosol of a sample in solution intofree atoms of the element(s) to be determined. Once freeatoms are formed, they may be readily detected at thetrace level by their atomic absorption, emission, orfluorescence spectrum, .

Flame emission spectrometry has enjoyed widespreaduse for years as a trace level analytical technique for anumber of elements which can be efficiently excited by theenergy available within a flame, Winefordner, Svoboda, andCline (71) have presented an excellent theoretical treatmentindicating that superior detection limits for flame emissionshould be possible as compared to absorption or fluorescence

ofor elements with resonance lines longer than 4000 A. They report that for high temperature flames the flame flicker intensity is the major contributor to the total noise.

1

Parsons, McCarthy, and Winefordner (52) describe the effect of the monochromator slit width and the product of the flame flicker background intensity on the minimum detectable atomic concentration. They conclude that as the product of flame flicker and flame background decreases, the optimum slit width will increase allowing for a smaller minimum detectable atomic concentration.

Many tradeoffs must be considered when choosing a gas mixture to support the flame; including the temperature produced, level of background emission, the resulting chemical environment, and the availability of a burner design capable of safe stable operation with the desired gas mixture.

The chemical environment within the flame influences the production of free atoms. This is most important in the case of elements which form relatively stable monoxides at ordinary flame temperatures. Changes in the chemical environment as a result of changes in flame stoichiometry produce large differences in the efficiency of free atom production. Chemical interferences are the result of a reduction in the number of free atoms available for excitation due to the formation of relatively stable molecular species. The use of high temperature flames favors the disassociation of these compounds and minimizes the chemical interferences. Higher temperatures can, however, increase the formation of ionized species, resulting

in a decreased ground state atomic population. Thus, it is often necessary to compromise between the optimum excitation of atomic lines, the optimum disassociation of chemical compounds, and minimal ionization. Depending on the type of application for the flame, some of these parameters will be more important than others,

The Premixed ^l&^eInitially in flame emission spectrometry, total

consumption burners producing turbulent flames achieved the greatest popularity as a result of their sensitivity, modest cost, safety, and availability. In recent years, however, premixed laminar flow flames have enjoyed increasing popularity due to their lower flame flicker, reduced rise velocities, increased dimensional stability, and improved homogeneity (12, 25, 26, 53, 54). Particularly encouraging results have been obtained by investigations employing premixed nitrous oxide-acetylene flames (12, 53, 54). In general, the improved sensitivity obtained with this flame arises from its high temperature when compared with other common premixed flames such as air-acetylene and air- hydro gen , and its highly reducing nature which promotes atomization efficiency, particularly in the case of elements forming stable oxides. Unfortunately, the nitrous oxide- acetylene flame produces a rather intense background continuum. In contrast, the premixed oxygen-hydrogen flame

possesses a very low background emission and a flametemperature only approximately 150 degrees cooler. Theadvantage of the simple and low flame background emission isimportant. Since the reactions in a hydrogen flame give asimple flame composition, the flame background emission isrelatively clean with the only major radiation resulting

ofrom OH bands at 2811, 3064, and 3428 A. With the

and °2_C2H2 f-*-ames/ C 2 > CN' an|3 CH radicals are formed, emitting over the whole region of the UV-visible spectrum. The intense flame background contributes noise which causes a degradation in detection limits and in precision of analysis.

Due to its high burning velocity, the premixed laminar oxygen-hydrogen flame cannot be supported on conventional commercial premixed burners. Suddendorf and Denton (65) have shown, however, that it is possible to safely operate a premixed laminar flow oxygen-hydrogen flame on a capillary type burner if the capillary ports are of a sufficiently small diameter.

The purpose of this study is to compare the performance as studied by emission spectrometry of a premixed laminar flow oxygen-hydrogen flame with the premixed flames of nitrous oxide-acetylene and oxygen-acetylene, and with a turbulent oxygen-hydrogen flame. Ultrasonic nebulization is used in combination with the premixed flames to introduce the sample into the flame.

CHAPTER II

SYSTEM DESCRIPTION FOR COMPARISON STUDIES

Measurement System Basically, two experimental configurations were

employed in this study« One experimental system was used for a comparison study of premixed flames and a turbulent flame. The second configuration was employed to increase the sensitivity of the premixed laminar oxygen-hydrogen flame. The configuration for the comparison studies (Fig, 1) consists of a Heath EU 700 monochromator, an EU 701-93 Photometric Module powered by a Heath EU-42A High Voltage Power Supply, a Log/Linear Current Module EU-20-28 and Recorder EU-20B (Heath Company, Benton Harbor, Mich. 49022), and using a 1P28A photomultiplier. Adjustable time constants were implemented by introducing switch selectable capacitors between 0.001 and 0.1 uF in parallel with the feedback resistors (Fig. 2).

Ultrasonic Nebulizer The ultrasonic nebulizer (Fig. 3) is available from

Denco Research (Tucson, Arizona 85715). The details of the base of the ultrasonic nebulizer are illustrated in Fig. 4. The ultrasonic system consists of a BC-191 highpower radio frequency source which drives the piezoceramic transducer

HEATH E U -7 0 0 HEATH EU-20IV MONOCHROMATOR CURRENT SYSTEM

FUEL

CONFIGURATION

ULTRASONIC ' NEBULIZER

■itOXIDIZER

ULTRASONICPOWER

GENERATOR

Fig. 1. Experimental System for Comparison Studies

7

.01

.QOS

TO LOG DIODEREV

IN P U T LOG

TO LOG DECADE C0UNTER(R22,23)

REV

I9B K

498KeT.04998 K

1ST3 M • " “^ 1 ---hooT p^AA-

.00220 MLINEAR SPAN

AMPS SOMr—AAA-J 1|--

4 0 0 p f

.00//00M

Z O O M

-■--II--100 p f

C A P A C I T O R VA LUES A R E IN M IC R O F A R A D S

U N L E S S O T H E R W IS E I N D I C A T E D500 M

39 p f

Fig. 2. Schematic Diagram of Additional Time Constants in the Log/Linear Current Module (Reference 31)

8

Aerosol Dispersion to ’ Burner

Wash Water

C3CO

Conical Glass Pipe

Carrier

Gas

CDCB

V ring

to Drain__< tzH T ra nsducerAssembly

R FExcitation

10 cm

Fig. 3. The Ultrasonic Nebulizer System

Drain

Teflo n

located in the base of the nebulizer» The radio frequency power source is composed of an army surplus BC-191 transmitter (Farnsworth Television and Radio Corporation, Fort Wayne, Ind.) and a TU-5B tuning,unit which are powered by a RA-34J power supply (Radio Receptor Company, Philadelphia, Pa.). This RF source is capable of delivering up to 200 watts of power at the operating frequency of 1 MHz. The frequency of this system can be varied over a range to obtain the transducer resonance frequency required for maximum production of aerosol. The one inch piezoceramic transducer is composed of C 600 barium titanate fabricated with a coaxial electrode geometry. The surface exposed to analyte solutions is glaze coated to provide the required inert chemical properties.

Conventionally, pneumatic nebulizers are used to convert the solution into aerosol. An ideal nebulizer would convert all the aspirated liquid to very fine drops, all of which would reach the flame and be completely vaporized.With most pneumatic nebulizers, only 5-15 per cent of the aspirated solution reaches the flame (69) and a wide range of droplet size is produced (13). In addition to pneumatic nebulization, other forms of energies can be used to transform a solution into small droplets suitable for supplying the flame, such as centrifugal nebulization (9), electrostatic nebulization (62), vibrating reeds (19, 56,73) and capillaries (32, 45), and ultrasonic nebulization

(34, 40, 44, 64). All of these means have been used to supply a flame with an analytical sample. In ultrasonic nebulization, a piezoceramic transducer such as barium titanate converts radio frequency energy into acoustic energy. When a liquid is exposed to the high amplitude acoustic energy, it is dispersed in a fine mist. The size of the droplets is dependent on the frequency of oscillation. At higher frequencies, the drop sizes will be smaller (61, 63). The smaller the droplet size, the easier it will be for the flame to desolvate the droplet. With an increase in the number of desolvated droplets, there will be an increase, in the amount of atomic vapor, resulting in a greater sensitivity.

The method of ultrasonic nebulization used by Denton (16) shows efficient use of sample solution and small drop size. This design of ultrasonic nebulization has been combined with the two flame techniques of atomic absorption and atomic fluorescence by Denton and Malmstadt (17, 18) and significant improvements have been observed.

BurnersA Beckman total consumption burner (No. 4020,

Beckman Instruments, Fullerton, Ca. 92634) was compared.to a multihole burner (Fig. 5) designed for this study. The multihole burner has thirteen holes which are 0.56 mm in diameter and was used to support the premixed O^-H^ flame.

12

^ ------ 6.35 cm

/flTN

7 or 13 holes 0.56 mm

Fig. 5. Diagram of 13-Hole Burner — Cooling fins addadditional safety in prevention of flashback by facilitating cooling of the burner head.

13The burner head is 2 cm thick and 6.35 cm in diameter. Themulti-hole burner is surrounded with cooling fins providing

2a radiating area of 133.3 cm . A second burner was built which had only seven holes with the same hole diameter.A third burner was built for the °2-C2H2 f-'Lameo hadthirteen holes but the diameter of the holes was reduced to 0.45 mm. While neither burner flashed back under normal operating conditions, adequate safety protection in the form of pop-off plugs was incorporated in all three burners.

The 13-hole burner used for the premixed °2“H2 flame demonstrated that it can tolerate very concentrated solutions by showing no salt buildup and a wide range of linear response with no memory effects.

The flow rates for the Beckman total consumption burner were : 3.6 l/min and : 10.2-12.0 l/min. For the multi-hole burners and the laminar premixed flames, the flow rates were : 2.3 l/min and : 5.4-6.2 l/min; ^ 0 :2.1 l/min; : 0.7-1.0 f/min; 0^% 3.2 £/min; and :1.8-2.3 .6/min. With the ultrasonic nebulizer, solutions were nebulized at the rate of 0.235 ml/min.

The rate of solution converted to aerosol was measured using a cold trap which consisted of a tube and a dry ice acetone slurry in a Dewar flask. Glass wool fills up the inside of the test tube. The test tube and glass wool are weighed and then put into the dry ice-acetone slurry for five minutes. The tube is stoppered« Two holes

14are in the stopper for the incoming aerosol and the escaping oxygen. The ultrasonic nebulizer is turned on for two minutes. The gain in weight of the cold trap indicates the amount of solution converted to aerosol.

Radio Frequency PowerTo obtain radio frequency power, the radio frequency

unit is connected to the power supply with the appropriate cables. The Filament, Control, and Plate circuit breakers are turned on. The AC/DC switch located in the transmitter should be in the AC position. The Tone/Vo ice/C.W. switch is set at C.W. Turning on the main power switch will cause the green light to come on. The filament control is now set to 10.5 volts. After 30 seconds, depress the plate start button. The red light will come on indicating that voltage is now supplied to the plates in the transmitter.The other operating parameters for the various switches are shown in Table 1.

A 40-50 ml sample is poured into the ultrasonic nebulizer. The level of the liquid must be kept below the oxygen inlet. The transmitter and the tuning unit for the ultrasonic nebulizer must be tuned to produce the desired amount of aerosol. Set the band change switch at position 3 or 4, then adjust the antenna coupling switch until some oscillations are visible in the liquid sample in the ultrasonic nebulizer. Move the M.0o tuning dial until the

15Table 1. Operating Parameters for the BC-191 Transmitter

and Modified TU-5B Tuning Unit (16) Using the Ultrasonic Nebulizer

Band Change SwitchA3

M.O. Tuning B

1800

P.A. Tuning C

90

Ant. Coupling Switch

D3

Ant. Ind. Tuning

M6

Ant. Circuit Switch

N4

Ant. Cap, Tuning

Ant. Ind. Switch

O0 1

16largest fountain of liquid is produced which does not have a quantity of large drops. Then, increase the antenna circuit switch until the desired amount of aerosol is produced.

System Operation Stock solutions of 1000 ppm were made following the

direction of Dean and Rains (14). All other solutions were prepared by successive dilutions from the stock solutions using deionized water. For the elements with ionization potentials below 7.5 eV (53), potassium chloride at 1000 (Jg/ml was added as an ionization suppressant. Some normal operating parameters for the log-linear current module are listed in Table 2.

Table 2. Operating Parameters for the Heath Log/Linear Current Module

Normal % T on Lin-I % T modeZero Position 1 Time Constant middle00ioi—!Xin

positionLinear Span, Amps.

In attempting to find the detection limit for an element, it is first necessary to find the right conditions that maximize the signal-to-noice ratio. In this study, the voltage of the photomultiplier was set at 700 volts. Initially, the resonance line for the element must be found

17in the literature and the monochromator is adjusted to thisparticular wavelength and the proper photomultiplierinstalled. The slit width is initially set at 150 |i;however, it will have to be optimized later. A solution ofthe metal ion is aspirated into the flame. This solutionshould be well above the supposed detection limit. The gasflow rates, flame position, and slit width are optimized toprovide a maximum signal-to-noise ratio. Now that theexperimental system has been optimized, it will be possibleto determine the detection limit. The detection limit willbe the concentration of the aspirated solution of the metalion which gives an emission signal twice that of the peak-to-peak background signal. This value can be obtained by

—9increasing the sensitivity, sometimes as low as 2 x 10 amps, and using a long time constant. It is good experimental procedure upon finding the detection limit to aspirate a solution of solvent before and after the solution which gives the- detection limit. This will make it clear that there hhs not been any instrumental drift to give a false detection limit.

CHAPTER III

COMPARISON OF DETECTION LIMITS FOR THE PREMIXED 02-H2 FLAME

Flame Background Emission Spectra In selecting a flame for analysis by flame emission

spectrometry, it is important to examine the background emission of the flame. The presence of bands of , CH, CN, OH, and O ^ d e p e n d i n g on the flame, may cause problems when attempting to select the proper resonance line of an element. In atomic emission spectrometry, the phototube shot noise coming from the flickering in the flame background intensity are dominant. It is essential when attempting to determine the detection limit of an element in a particular flame that the signal-to-noise ratio be optimized.

The flame background emission spectra of the premixed\

oxygen-hydrogen flame is compared to the background spectra of a turbulent oxygen-hydrogen flame and the premixed flames of nitrous oxide-acetylene and oxygen-acetylene in Figs.6-11. It should be noted that for a given sensitivity and slit width, the flame background emission of the premixed oxygen-hydrogen flame is much less than the other three flames. The intensity of the background emission and flame flicker has been reduced with the premixed oxygen-hydrogen

2000 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0

Fig. 6 . Background Emission Spectra of Premixed O 2 -H2 Flame — Full-scale sensitivity is 2 x 1 0 “ 7 amps and the slit width is 300 microns.

HVO

2000

Fig. 7. Background Emission Spectra of Turbulent O 2 -H2 Flame — Full scale sensitivity is 2 x 10“7 amps and the slit width is 300 microns.

roo

— I— 2000

. 8.3000 ' 40*00 50*00 6000

Background Emission Spectra of Premixed N 2 O-C2 H2 Flame (Slit Width is 300 Microns) — Full scale sensitivity is 2 x 10“ ̂ amps.

I 1 I I--------------- — I—2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0

Fig. 9. Background Emission Spectra of Premixed O 2-C2 H2 Flame (Slit Width is 300 Microns) — Full scale sensitivity is 2 x 10“ ̂ amps.

rv>N)

2000-- 1--3000 4000 5000

1—6000

Fig. 10. Background Emission Spectra of Premixed N2O-C2 H2 Flame (Slit Width is100 Microns) — Full scale sensitivity is 2 x 10“^ amps NO

CO

2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0

Fig. 1 1 . Background Emission Spectra of Premixed °2“c2^2 Flame (Slit Width is 100 Microns) — Full scale sensitivity is 2 x 10 amps.

N)

25flame. The principal reaction and possible side reactions in the oxygen-hydrogen flame (15) are:

2 H 2 + °2 — > 2 H 0

H + 02 ^ OH + 0

0 + H 2 ^ OH + H

OH + H 2 ^ H20 + H

Since the reactions in a hydrogen flame give a simple flamecomposition, the flame background emission is relativelyclean with the only major radiation resulting from OH bands

oat 2811, 3064, and 3428 A. The disadvantage of the premixed nitrous oxide-acetylene and oxygen-acetylene flames is the relatively intense background which is emitted within certain regions. This background spectrum is the result of radicals and molecules which are present in the flame gases in large quantities. Typical reactions in the oxygen- acetylene flame (15) are:

C 2 H2 + 5/2 0 2 -> 2C02 + H O

co2 —>■ CO + ^ o2

h2o - > h2 + ^ o2

H20 ^ H2 + OH

Some of the reactions happening in the nitrous oxide-acetylene flame are (7):

Parsons et al. (52) describe the effect of flamebackground intensity and flicker on the minimum detectable atomic concentration and the optimum slit width. Their results indicate that the flame background flicker noise at large slit width values is the most important source of noise. As the product of flame background flicker and background intensity increases , the optimum slit width shifts to smaller values for a minimum detectable atomic concentration. This effect is shown in Fig. 12. Therefore, in atomic emission it is important to optimize the monochromator slit width.

In this study, the average value for the slit width for the premixed oxygen-hydrogen flame was 250 p and the amplifier gain was 2 x 10 ̂ amperes. For the premixed flames of nitrous oxide-acetylene and oxygen-acetylene, however, the slit has to be narrowed to 1 0 0 |i and the amplifier gain reduced to 2 x 10 ® amperes. It is necessary to make a compromise between the intensity of the signal versus the intensity of the background noise to properly maximize the signal-to-noise ratio.

LOG

MIN

IMU

M

DE

TEC

TAB

LE

AT

OM

IC

CO

NC

EN

TRA

TIO

N

ato

ms

/cm

^21

Fig.

/ 3

2

1 0 2 5 0 5 0 0 7 5 0 1 0 0 0S L I T W I D T H microns

A — f - i * 1.5 x 10'9 B — f - i = 1.5 x I 0'10 C — f i = 1.5 x 10 ' "

f = f l a m e b a c k g r o u n d f l i c k e r f a c t o r

i = f l a m e b a c k g r o u n d i n t e n s i t y

12. Influence of Flame Background Intensity andFlicker on Minimum Detectable Atomic Concentration and Optimum Slit Width (Reference 52)

28

Turbulent Oxyqen-Hydroqen Flame In flame emission spectrometry, the early investi

gators used total consumption burners producing turbulent flames. The turbulent oxygen-hydrogen flame in a total consumption aspirator-burner has been widely used in flame emission analysis (28-30, 46, 58, 67, 76). A comparison between the oxygen-hydrogen flame background emission observed for the total consumption burner and the premixed laminar flow burner has been illustrated in Figs. 6 and 7. The laminar premixed oxygen-hydrogen flame is very stable and exhibits much less flame flicker. In the case of thepremixed flame,, the peak-to-peak noise has been reduced by a

ofactor of 5 and in the region of 4000-6000 A, and the background emission intensity has decreased to about one- third of that observed in the turbulent flame. This lower flame background improves the signal-to-noise ratio and allows the use of higher amplifier gain and/or a larger slit width, improving the monochromator throughput.

Table 3 compares the detection limits for several elements using the Beckman total consumption burner with pneumatic nebulization and the premixed laminar multi-hole burner with ultrasonic nebulization. The limit of detection was defined as that concentration resulting in a signal-to- noise ratio of 2. The average increase in sensitivity for the eleven elements is 300. A remarkable improvement in detection limits is exhibited for B a , Ca, In, La, Sr, and Tl.

29Table 3. Comparison of the Detection Limits for the

Turbulent O 2 -H2 Flame with Pneumatic Nebulization Versus the Premixed O2 -H2 Flame with Ultrasonic Nebulization Utilizing the Configuration of Fig. 1

H 2 Flow Rates forTurbulent Laminar Premixed Laminar

0 ° 2 “ h 2 ° 2 “ h 2 FlameLine (A) ((ig/ml) (Ug/ml) (l/min)

Ba 5535 ' 0.25 0 . 0 0 0 1 6 . 0Ca 4227 0.005 0 . 0 0 0 0 1 5.9Cr 4254 0.05 0.005 5.9Cu 3274 0.5 0 . 1 5.6In 4511 0.003 0 . 0 0 0 1 5.9La 4418 (LaO) 0 . 1 0.003 5.8Mg 2852 0.25 0 . 0 1 5.5Mn 4031 0 . 0 1 0.0025 6 . 2Ni 3415 0.25 0.05 5.6Sr 4607 0.005 0 . 0 0 0 1 6 . 2T1 5351 0.03 0 . 0 0 1 5.4

30The increase in sensitivity is attributed to the lower background emission of the premixed laminar flame and the smaller droplet sizes produced by ultrasonic nebulization.

Burner and Nebulization Characteristics of the Premixed Oxygen-Hydrogen 71ame

It is also possible to compare pneumatic versus ultrasonic nebulization into the premixed oxygen-hydrogen flame. This is accomplished by using the Beckman total consumption aspirator-burner for pneumatic nebulization.The solution is aspirated into a cylindrical chamber and then swept by oxygen into the laminar flow burner. The results for several elements are shown in Table 4. In examining the data it is possible to see the influence of both ultrasonic nebulization and the premixed laminar flow flame on the increased sensitivity over the turbulent oxygen-hydrogen flame.

It was decided to determine if the number of holes in the laminar flow multi-hole burner would have an effect on the sensitivity of the premixed oxygen-hydrogen flame with ultrasonic nebulization. The results are displayed in Table 5. No definite conclusion could be drawn from the experimental work.

Premixed Flames of °2“C2H2 an

Table 4. Comparison of Detection Limits for the Premixed Oxygen-Hydrogen FlameUsing Pneumatic and Ultrasonic Nebulization

Element Line (A)Turbulent

° 2 - H 2

Laminar O2-H2 with Pneumatic Nebulization

Laminar O2 -H2 with Ultrasonic Nebulization

Ba 5535 0.25 Ug/ml 0 . 0 1 Ug/ml 0 . 0 0 0 1 Ug/mlCa 4227 0.005 0.0005 0 . 0 0 0 0 1La 4418 (LaO) 0 . 1 0 . 0 1 0.003Sr 4607 0.005 0.0005 0 . 0 0 0 1

32Table 5. Detection Limits for a 13-Hole and 7-Hole Laminar

Flow Burner with a Premixed Oxygen-Hydrogen Flame

Element Line (A) 7-Hole Burner 13-Hole Burner

Ba 5535 0 . 0 0 0 1 (ig/ml 0 . 0 0 0 1 t-lg/mlCa 4227 0 . 0 0 0 1 0 . 0 0 0 0 1Cr 4254. 0 . 0 1 0.005Cu 3274 0 . 1 0 . 1Mn 4031 0.0025 0.005Ni 3415 0 . 1 0.05

comparisons about the effectiveness of a particular flame difficult. In the studies with the N^O-C^H^ flame (41, 54) it was quite safe to support the flame with a slot burner because of the flame's low burning velocity of 320 cm/sec (15). To support the premixed oxygen-acetylene flame, however, which was a burning velocity of 1 1 0 0 cm/sec, the slot opening of the burner had to be reduced to 0.025 cm (26). With a still higher burning velocity of 2000 cm/sec, the premixed oxygen-hydrogen flame is difficult to safely support on a slot burner. In this study, however, the burner design makes it possible to examine the capabilities of these flames under identical conditions with the same introducing aerosol.

33The detection limits for the premixed oxygen-

hydro gen , nitrous oxide-acetylene, and oxygen-acetylene flames are given in Table 6 . All three flames were supported by a 13-hole burner of the design shown in Fig. 5 and operated in the comparison configuration (Fig. 1 ). Each of the flames has its own characteristics that make it useful for flame emission analysis. The oxygen-acetylene is the hottest and will have a greater efficiency for the production of free atoms. However, it also can produce higher populations in upper excited states. The nitrous oxide- acetylene flame is important because of its high temperature and reducing nature, making it a desirable flame for elements forming stable oxides. The premixed laminar oxygen-hydrogen flame offers a high temperature, a much lower flame background emission, and a vast reduction in flame flicker which can result in an improved signal-to-noise ratio. The improved detection limits with the premixed oxygen-hydrogen flame indicate that the low flame background emission for oxygen-hydrogen flame is significant. Even though this flame is slightly cooler than the ° 2 -C2 H 2 ^^ame an

Table 6. Detection Limits for Three Premixed Flames Supported by the 13-HoleBurner Using Ultrasonic Nebulization in Configuration 1

Laminar°2 -C 2 H 2

LaminarN 2 °-C 2 H2

Laminar°2 - H 2

Line (A)Detection

Limit (Ug/ml)

Slits

35gives the lowest detection limits the majority of the time except in the case of elements whose most sensitive emission lines fall in the region of the OH background bands.

While a very thorough study conducted by Moss'holder, Fassel, and Kniseley (50) indicates reduced atomization efficiency in the premixed oxygen-hydrogen flame, when compared with the fuel-rich nitrous oxide-acetylene flame, the improved detection limits observed in these studies is attributed to the greatly reduced background emission and decreased flame flicker allowing for the use of wider monochromator slit widths and higher amplifier gain.

CHAPTER IV

DESCRIPTION AND RESULTS FOR AN OPTIMIZED 02 -H2 FLAME EMISSION SYSTEM

Measurement System In an attempt to achieve the lowest detection limits

for the premixed oxygen-hydrogen flame, a second, more complicated configuration was used (Fig. 13). This experimental system included a parabolic mirror and lens to improve light gathering efficiency, a light chopper (Model 125, Princeton Applied Research, Princeton, N. J. 08540), a photometric preamplifier, and a lock-in amplifier (Model 184 and 126, Princeton Applied Research).

Lock-In Amplifier In atomic emission spectrometry, the major

contributors to noise are the shot noise at the photo-\

detector output and flame flicker noise (71). To help eliminate the shot noise and direct current drift, a chopper, a photometric preamplifier, and a lock-in amplifier were added to the experimental system. The lock- in amplifier discriminates against noise, not only on the basis of frequency as does a tuned amplifier, but also according to phase. Thus, the only signals including noise

36

HEATH E U -7 0 0 PAR 126 HEATH EU-20VPARIES MONOCHROMATOR LOC K - IN RECORDERCHOPPER WITH 164 PREAMP

PARABOLICMIRROR

LENSFUEL

CONFIGURATION

ULTRASONIC ' NEBULIZER

OXIDIZER

ULTRASONICPOWER

GENERATOR

Fig. 13. Experimental Configuration for an Optimized Flame Emission System

38that will be amplified are those having a specified frequency and phase with respect to a reference waveform.

The main signal enters the lock-in amplifier through the low noise preamplifier. The signal then enters the phase sensitive detector and is synchronously rectified (full wave rectification) by the reference signal which has been converted into a square wave. When the two signals are in phase, a maximum output will be obtained.

After the phase sensitive detector, a low pass RC filter reduces the white noise that remains. The bandwidth can be made extremely narrow, but in many cases a compromise must be achieved between the signal bandwidth and the rejection of white noise. The output is a clean reproduction of the very small original signal voltage. The average DC output drives the panel meter which is proportional to the radiant signal input.

System Operation In this experimental configuration (Fig. 13), the

parabolic mirror was focused on the monochromator slit by turning off the room's lights and focusing the image of the flame on a white paper towel which covered the slit. This arrangement was then checked by aspirating a sample and slightly moving the mirror until the largest signal was realized. Each element in the flame was first optimized without the chopper and lock-in amplifier. After finding

39that the system was operating properly and low detection limits were observable, the chopper and lock-in amplifier were added. Two quartz slides were taped to cover the window of the chopper due to the breeze caused by the chopper giving an unstable flame. The chopper was operated at 333 Hz. The lock-in amplifier was optimized according to the directions in Figure 14. A set of normal operating parameters are listed in Table 7.

Table 7. Operating Conditions for the Lock-In Amplifier

Signal Channel Reference Channel

Sensitivity 500 uv Mode ExternalL.F. Rolloff 100 Hz Freq(Hz) x 1 0 0H.F. Rolloff IK Hz Phase 0 °Time Constant 1 0 sec. Function PSD output x 100Normal-Hold Switch Normal

Photometric Preamplifier 10 ^ Amps/Volt

Detection Limits A comprehensive list of detection limits for flame

emission spectrometry is reported by Pickett and Koirtyohann (53). They used a premixed N^O-C^H^ flame supported by a slot burner. The comparison between their results and the

Step One. Choose an appropriate amp I If Ier.

Step Two. Be sure the rear panel 115/ 230 V switch Is in the proper position. Connect the Model 126 to the power line. Turn the Power switch on.

Step Three. Connect the signal source from the experiment to the preamp 11f ier(s).

Step Four. Select a mode of operation for the Reference Channel. Base your choice on what is to "" control the frequency.

Step Five. Apply the reference drive to The experiment. The reference drive is obtained from the Reference Channel Output connector on the front panel when using the INT/VCO mode. In the external modes, the experiment can be driven by either the experimental reference source or by the Model 126 output, whichever is most suitable.

External f/2. Use this mode to measure the second harmonic mode of a signal. Connect the external reference signal source to the Reference Channel Input connector. The oscillator in the Model 126, and thus the Phase

r— — Sensitive Detector, will operate at twice the frequency of the external reference within the limits In the above table after the Unlock lamp has extinguished.

Externa I. Connect the external reference Channel Input connector. Set the range switch to a position which wi11 cover the frequency of the external reference according to the following table and wait for the Unlock lamp to extinguish.Band (Frequency Range) Maximum Time

XI (0.2 Hz to 21 Hz) 15 min.XIO (2 Hz to 210 Hz) 2 mi n.

XI00 (20 Hz to 2.1 kHz) 10 sec.XI K (200 Hz to 21 kHz) 2 sec.

XI0K (2.1 kHz to 2 10 kHz) 2 sec.

INT/VCO. Set the frequency by means of ihe front panel digital dials and range switch for Internal operation, set up as for Internal except add or subtract to the set frequency by feeding a plus or minus voltage to the VCO Input on the rear panel.

Step Six. Set the two Rolloff controls to bracket the frequency of the signal to be measured.

Step Seven. Preliminary adjustment of the Sensitivity switch: Set theFunction switch to ACVM. Set the Time Constant controls to 330 ms, 6 dB/octave, and the small toggle switch to Normal. Be sure the Zero Offset switch Is off (center position). Place the Sensitivity switch to the most counterclockwise position which does not cause overload.

Step Eight. Turn the Function switch to the XI position. Adjust the phase controls for a maximum positive meter deflection. Change the Time Constant switch to as large a setting as required to reduce the motor fluctuations to a tolerable level. If the meter deflection is too small after adjusting the phase controls, attempt to increase the Sensitivity setting without Incurring overload. If the meter indication is still too low, use the Function switch to increase the output gain to cither XI0 or XI00 as necessary.

Fig. 14. Operating Instructions for Lock-In Amplifier (Reference 55)o

41detection limits for the premixed laminar °2 “ H 2 ^lame operated in the comparison study mode (Fig. 1) are shown in Table 8 . It should be noted that for most of the elements studied, the premixed ° 2 “ H 2 f^ame gives lower detection limits.

In an attempt to achieve the best possible sensitivity, the experimental configuration with the lock-in amplifier was employed (Fig. 13). While it is somewhat more complicated to optimize, the additional light gathering ability and increased time constant resulted in improved limits of detection for most elements as displayed in the last column of Table 8 .

42Table 8 . Comparison of Reported Detection Limits in Flame

Emission (53) Versus the Results Observed in This Study Utilizing the Optimized Experimental Configuration Shown in Fig. 13

Line (A)

Laminar N 2O—C 2 H2

(Reference 53) (Ug/ml)

LaminarOo—Ho(Fig. 1) (Ug/ml)

Laminar0 2 -I^2 (Fig. 13)(Ug/ml)

Ba 5535 0 . 0 0 1 0 . 0 0 0 1 0.000005Ca 4227 0 . 0 0 0 1 0 . 0 0 0 0 1 0.00000025Cr 4254 0.005 0.005 0 . 0 0 0 1Cu 3274 0 . 0 1 0 . 1 0 . 1In 4511 0 . 0 0 2 0 . 0 0 0 1 0 . 0 0 0 0 0 1La 4418 (LaO) 0 . 1 0.003 0 . 0 0 0 1Mg 2852 0.005 0 . 0 1 0.005Mn 4031 0.005 0.0025 0 . 0 0 1Ni 3415 0.03 0.05 0 . 0 1Sr 4607 0 . 0 0 0 1 0 . 0 0 0 1 0 . 0 0 0 0 1T1 5351 0 . 0 2 0 . 0 0 1 0 . 0 0 0 0 2

CHAPTER V

ANALYTICAL ADVANTAGES OF THE PREMIXED 0 2 -H2 FLAME

Detection Limits In the previous chapter the detection limits for the

premixed laminar oxygen-hydrogen flame were compared to detection limits for several other flames commonly used in flame emission spectrometry» The increased sensitivity of the premixed oxygen-hydrogen flame indicates it will be a sensitive source for emission spectrochemical trace analysis.

Standard Reference Material,' Orchard LeavesIn order to evaluate the accuracy of the results

obtained with the premixed laminar oxygen-hydrogen flame, an analysis of a standard was performed. The standard was a National Bureau of Standards Standard Reference Material 1571, Orchard Leaves. Three constituents of the standard, Ca, Mn, and Mg had been previously used in the study of the detection limits for the premixed oxygen-hydrogen.flame.

A 0.2610 gram sample of the Standard Reference Material was dried in a muffle furnace at 90°C for 24 hours. Then the sample was ashed in a low temperature asher. The

43

44sample was transferred to a 250 ml volumetric flask and diluted with deionized water.

The results of the analysis of the standard are reported in Table 9.

Table 9. Analysis of Standard Reference Material, 1571 Orchard Leaves

ElementReportedValue

ExperimentalValue

Level at Which Analysis Performed

Ca 2.09 + .03 wt % 2.09 wt % 2 0 0 ppbCa 2.09 + .03 wt % 2 . 1 1 wt % 500 pptMn 91 4 Mg/g 92 pg/g 1 0 0 ppbMg 0.62 + . 0 2 wt % 0.65 wt % 6 ppm

The analysis of the Standard Reference Material was performed by first making a calibration curve for the desired element. Then the original sample was diluted until it was in the range of the calibration curve. The four calibration curves are shown in Figs. 15-18.

Analytical Curves It is important to find out in which concentration

range the analytical working curve is linear. The analytical curve is usually defined as a log-log plot of the photodetector signal versus the concentration of sample

V)CLEocoiO"x-J

CDCD

0.5

0 0.1 0.2 0.3 0.4 0.5Ca CONCENTRATION ug /m l

Fig. 15. Calibration Curve for Ca in Standard Reference Material (Analyzed at200 ppb Level)in

5.0

2.5

to| 2.0 o

aiOxU<CDCO

0.5

0 0./ 0.2 0.3 0.4 0.5Mn CONCENTRATION u g / m l

Fig. 17. Calibration Curve for Mn in Standard Reference Material (Analyzed at100 ppb Level)

COa En

coiOx

<5:CDCO

.0

.5

2.0/.5

/.0

0.5

0 84 /2Mg CONCENTRATION ug/ml

Fig. 18. Calibration Curve for Mg in Standard Reference Material (Analyzed at6 ppm Level)

ftoo

49solution aspirated into the flame. After the linear range of the analytical curve is found, then future analysis can be safely performed in this range.

It is well known that the shape of growth curves is the predominant factor in affecting the shape of analytical curves (77). The growth curves are log-log plots of some function of integrated intensity of emission versus some function of atomic concentration. There are a number of factors causing the analytical curves to have shapes differing from the theoretical straight lines. Some of these factors include self-absorption, ionization, compound formation, variation in solution flow rate and atomization efficiency, entrance optics, and the effect of measuring spectral line multiplets (68).

An important factor that can influence the linearity of the analytical curves is self-absorption. Self- absorption occurs as photons emitted by atoms in the interior of a flame may have a chance to be reabsorbed by other atoms of the same species. The energy of the reabsorbed photons is thus partially converted into kinetic energy, and the outgoing radiation power of the flame is weakened. The linear relationships between line intensity and atomic concentration breaks down.

Evaluation of any deviation from the linear relationship may be obtained by construction of an analytical curve where the emission of a molecular band rather than an

atomic line is observed (72). In this way an analytical curve is obtained which is independent of self-absorption over a wide range of concentration. A molecular band emission analytical curve has unit slope and in general deviations from this result from changes in aspirator yield, solution flow rate, or incomplete solute vaporization. Therefore, curves of this type should always be made when inadequacies in the experimental apparatus are suspected. One inconvenience of the method is that not all elements give rise to molecular band emission flames.

The analytical curves for the elements examined in this study are displayed in Figs. 19-21. The majority of the elements express a linear relationship over the concentration range studied. Several of the elements give a linear analytical curve over seven orders of magnitude.

Chemical InterferencesSeveral types of interferences may inhibit the use

of a particular flame for a given analysis. There are physical, spectral, and chemical interferences that may occur in a flame. Physical interferences may be defined as those effects which are caused by a physical property of the sample solution or which alter one of the physical processes involved in flame measurements. Spectral interferences are any radiations which coincide with or overlap that of the desired element, whether it stems from

7 ^

10 -5

v>o.E° I 0 - 7

-J

2 ioCO

-9

/o -II

b0 .001

1. / 10

CONCENTRATION p. p.m.

J

1000

Fig. 20. Analytical Working Curves for Ca, Cr # La, and Ni — 0 = Ca, [] = Cr,0 = La, and A = Ni.

LnIV)

SIG

NA

L am

ps.

- 5

- 9

-II

.001 1000CONCENTRATION p.p.m.

Fig. 21. Analytical Working Curves for Sr, Ba, Mn, and Cu — ®= Sr # □ = Ba,Q = Mn, and A = Cu.

OJ

54the sample or from the excitation source. Chemical interferences will be studied in detail.

A serious limitation when applying emission spectrometry to real world samples can result from chemical interferences. This interference is caused by other elements and ions interfering with the emission of the element to be determined. Many ways have been proposed to counteract chemical interferences based on the estimation or elimination of the interference.

Particular difficulties are encountered in the determination of Ca in the presence of phosphate or aluminum; both markedly lower the emission of Ca at itsmain resonance line (1, 4, 5, 21, 47, 57).

The premixed °2-H2 ^ ame exhibits the chemical interference of the depression of the Ca signal by phosphate and aluminum as illustrated in Figs. 22 and 23.The interference of phosphate is postulated as follows: during the dehydration of the aerosol droplet, the Ca has every opportunity to react with. P at the combining ratio to form a compound of high thermal stability, Ca2 P2 ° 7 or C a ^ P O ^ ^ (24). The interference of aluminum is due to thereaction of the aluminum with the alkaline earth oxidesforming very nonvolatile aluminates, and as a consequence, the signals of the alkaline earth elements decrease with increasing aluminum concentration.

RELA

TIVE

IN

TEN

SIT

Y

T T i nTTTT

100

RELA

TIVE

IN

TEN

SIT

Y 100

nil i no I 5 1 0 fOO

MOLE RATIO A l / C a

Fig. 23. The Effect of Al^+ on the Calcium Emission Signal (0.5 |Jg/ml) — A = Oo-Ho flame, O = OpHo flame + La. □ = NpO-CpHp flame, and (D = 02-H2 flame + EDTA. incn

57If the premixed 02 “ H 2 flame is to be used for

analysis of Ca, the interference of phosphate and aluminum must be eliminated. Elimination of some chemical interferences has been accomplished with the use of EDTA (66). There has been considerable discussion of the effect of EDTA on this chemical interference. Some propose that chelates are formed so that the nonvolatile compound cannot be formed. Others suggest that the organic compounds are built into the solid particles during crystallization and cause their explosive disintegration on heating in the flame (3). No attempt will be made here to decide how the EDTA eliminates the interference.

The elimination of Ca depressions due to the presence of EDTA is shown in Figs. 22 and 23. One disadvantage of EDTA is the enhancement of the Ca emission signal.

Another attempt to eliminate chemical interferences was described by Dinnin (20). For the elimination of depressing effects based on the formation of nonvolatile compounds, Dinnin proposed the so-called "releasing agent" effect. This is based on the observation that after addition of a releasing element in sufficient amounts, the signal of the alkaline earth element in the flame in absence of the interfering elements is recovered. La and Sr are the most commonly used releasing agents. The proposed mechanism is that the releasing element forms a less soluble compound with the interfering element and during evaporation

58of the spray droplets, this compound crystallizes preferentially, leaving the required element unbound*

The effect upon the calcium emission signal due to the addition of lanthanum is displayed in Figs. 22 and 23. Several advantages of lanthanum are observed. Lanthanum contributes very little emission to the calcium signal and does not give a yellow flame as in the case when using the disodium salt of EDTA. Lanthanum restores the calcium signal to within a few per cent of the value for calcium without any interference. The premixed N 2 O-C2 H2 flame has been recommended for use if the depression of the Ca signal due to phosphate or aluminum exists. However, this study shows that the premixed flame gives lower detectionlimits than the premixed ^€>-€2 ^ flame. In the presence of the chemical interference, if lanthanum has been added, premixed ° 2 ~ H 2 î a s s l̂o w n to ke as satisfactory as the

flame.It has been previously demonstrated that the effect

of the chemical interference varies with height of observation above the burner (27, 59), This was also shown to be the case for the premixed ® 2 ~ ^ 2 ^^ame (Fig. 24).Due to its increased residence time in the flame, the nonvolitile compound will have more time to decompose.However, it should be noted that as a higher position in the flame is observed, the emission of the calcium signal decreases.

100xKCo

£g 5 0

K

UJq:

1000 I 5 1 0MOLE RATIO PO4 /CQ

Fig. 24. The Interference of P04 " on the Calcium Emission Signal (0.5 ug/ml) as Viewed in Different Heights of the O2 -H2 Flame — □ = 12 mm,0 = 18 mm, and A = 26 mm. Ul

KD

60Denton (16) has presented a detailed account on the

effect of operation frequency of an ultrasonic nebulizer and the resulting droplet size. The effect of the smaller drop size when operating at 3 MHz on the depression of the Ca signal is shown in Figs. 25 and 26. The decrease in droplet size should lead to a more complete decomposition of the calcium phosphorus compound. More decomposition leaves more unbound calcium available for excitation by the flame and an increase in calcium emission.

>_ 1 0 0Kcog2: 5 0uj>

uUjQ:

5 10 100MOLE RATIO P 0 4/C q

Fig. 25. The Effect of Operating the Nebulizer at Different Frequencies Uponthe Interference of PO^3- on the Calcium Emission Signal (0.5 i-ig/ml)_0 = 3 MHz, and 0 = 1 MHz. Higher frequencies give rise to smaller drops (16). O'!H

x 100KCOluh.S 50s

K-JUjQZ

0 I 5 1 0 1 0 0MOLE RATIO A l /C q

Fig. 26. The Effect of Operating the Nebulizer at Different Frequencies Uponthe Interference of Al3+ on the Calcium Emission Signal (0.5 Ug/ml) —□ = 3 MHz , and 0 = 1 MHz. Higher frequencies give rise to smaller drops (16).

cnN)

CHAPTER VI

SMALL VOLUMES OF SAMPLE BY ATOMIC EMISSION

BackgroundBeing able to properly analyze small volumes of

sample has grown increasingly important in recent years in environmental and clinical analysis» Most investigators have used atomic absorption spectrometry in working with this problem. A number of flame-related "sampling boat"(37, 38) and flame-less techniques— filaments (8, 74, 75), ribbons (22, 35), and graphite rods (6, 49) have been employed. While such nonflame techniques have shown excellent results, in many cases various interference problems can limit their use in varying real world problems (2). In a recent investigation, Korte, Moyers, and Denton (43) have demonstrated that a pulse ultrasonic nebulizer- burner system could efficiently handle small volumes of sample with improved sensitivity.

Small Samples by Flame Emission Only a limited amount of work has been presented

describing techniques suitable for performing atomic emission analysis on small volumes of sample (11, 33, 39, 51, 60). If the pulse ultrasonic nebulizer burner system of Korte, Moyers, and Denton is modified and combined with

63 -

64the premixed ° 2 ~ H 2 flame described in this study, it should be possible to analyze small volumes of sample by atomic emission spectrometry.

The pulse ultrasonic nebulizer-burner system is illustrated in Fig. 27. There are 25 holes (0.50 mm in diameter) in the burner head which are sufficiently small enough to support a premixed oxygen-hydrogen flame. The burner head can be removed and another head inserted so that it is possible to support several flames with the same burner. It was necessary to operate both the oxygen and hydrogen at increased flow rates as compared to the gas flow rates for the 13-hole burner. The oxygen and hydrogen flow rates are 3.2 -6/min and 9.0 ^/min, respectively. If there is not a sufficient amount of oxygen in the burner, the flame will flashback when the hydrogen is turned off.The flame looks very similar to the flame supported on the 13-hole burner.

The parameters previously described must be optimized to give a maximum signal-to-noise ratio. Thus, it is necessary to adjust the gas flow rates, flame position, and slit width. It is somewhat more difficult to optimize this experimental system because the emission from the sample is only a spike and it is not possible to adjust a certain parameter and immediately be able to observe its effect upon the emission signal. Therefore, it is necessary to perform a systematic study of the influence of the different

65

1 0 .5 cm >

2 5 holes

0 . 5 0 mm

Oxidizer

^ Tef lon Solution Cup

Glass Coated

Piezoceramic Trans ducerspring

contact

rt H

3 MHz R F

Fig. 27. Detailed View of Pulse Ultrasonic Nebulizer-Burner

66parameters. The parameters will have to be changed a small increment at a time to notice a change in the signal. This must be performed until all the experimental parameters have been optimized.

The piezoceramic transducer was operated at an overtone of 3 MHz. To supply the proper radio frequency power, a second tuning unit had to be used. The operating conditions are listed in Table 10.

Table 10. Operating Parameters for the BC-191 Transmitter and TU-5B Tuning Unit (#53150) Using the Pulse Ultrasonic Nebulizer

Band ChangeSwitch M.O. Tuning P.A. Tuning

A B C4 2460 85

Ant. Coupling Ant. Ind. Ant. CircuitSwitch Tuning Switch

D M N5 6 3

Ant. Cap. Ant. IndTuning Switch

O0 1

67The proper frequency of excitation for a particular

transducer is initially determined by operating the transducer outside of the burner.

A 0.2 ml or smaller sample is injected on the face of the transducer with an Eppendorf microsyringe (Brinkman Instruments, Westbury, N. Y. 11590). The frequency is adjusted to provide maximum operation of fog without generation of large droplets. Production of the very large droplets must be avoided to prevent burner port clogging and to insure reproducible aerosol generation. Once the frequency is determined for a certain transducer, no further adjustment is necessary.

A timer is used to activate the radio frequency power to the transducer for a given amount of time. The details of the timer are described by Korte (42).

For analysis, a sample is injected into the sample cup. The assembly is then inserted into the bottom of the burner. The timer is then activated and the aerosol is swept into the flame.

The results of this initial investigation are given in Table 11. One element does not definitely prove the system works, but it does give a good indication that there is a good chance for success.

Both ultrasonic nebulizers and burners were operated in the comparison study configuration (Fig. 1). This study is a good indication that it should be possible to perform

68Table 11. Comparison of Detection Limits for the Ultrasonic

Nebulizer with 50 ml Sample Volume and the Pulse Ultrasonic Nebulizer with 200 |ll Sample Volume

Ultrasonic Pulse Ultrasonico Nebulization NebulizationElement Line (A) 50 ml Volume 200 M-l Volume

Ca 4227 0.00001 Ug/ml 0.00001 |ag/ml

flame emission analysis on a small volume of sample (200 y,l or less) using pulse ultrasonic nebulizer-burner system with the premixed laminar oxygen-hydrogen flame.

CHAPTER VII

RECOMMENDATIONS FOR FUTURE INVESTIGATIONS

Ultrasonic Nebulizer It has been previously described how the effect of

chemical interferences decreases with increasing height in the flame. However,, when the flame is viewed in these higher positions, there is a loss in sensitivity. In some cases where it is impractical to perform trace analysis because of the chemical interferences, it might be worthwhile to view the upper portions of the flame and not have to add a releasing agent. However, to use this procedure it would be necessary to examine how much sensitivity is lost as the influence of chemical interferences is reduced. It would be important to find the exact height in the flame which gives maximum sensitivity and reduction in chemical interferences. But, if it is found that one sacrifices too much sensitivity and therefore is not able to perform the analysis, a releasing agent such as lanthanum should be added to the samples.

Another experiment which might be worthwhile is attempting to increase the amount of aerosol from the ultrasonic nebulizer into the flame. It was found in this study that a maximum amount of aerosol cooled the flame

69

70and so was undesirable. However, if it were possible to usea heating device to help desolvate the aerosol on its path

' )

into the flame, an increased amount of aerosol could be advantageous. A possible method is to wrap the heating tape around the hose carrying the aerosol into the burner.A preliminary investigation of this method was tried with limited success. A more sophisticated procedure would use a microwave cavity to help desolvate the aerosol in the base of the burner. If it became possible to introduce more solution into the flame, there would be an increase in the number of atoms in the higher portions of the flame and therefore increase the sensitivity of this position in the flame.

Pulse Ultrasonic Nebulizer-Burner Initial investigations indicate that the pulse ultra

sonic nebulizer-burner will be useful in atomic emission spectrometry. This burner is significant because it only requires a small sample volume (200 ^1) and in combination with the premixed oxygen-hydrogen flame, low detection limits are possible. In this study, the pulse ultrasonic burner showed identical sensitivity to that of the ultrasonic nebulizer in which the sample volume is approximately 50 ml. In future work with the pulse ultrasonic burner, it will be important to determine the optimum sample size and nebulization time. One would like to use as small a

71sample volume as possible. It is necessary, however, to find the sample volume which will give the greatest sensitivity o The nebulization time is also important. The sample must be nebulized long enough to allow all of the sample to be converted to aerosol. Optimum sample volume and nebulization time can be checked by examining some reproducibility data. For future work, it might be convenient to build a shutter into the bottom of the pulse ultrasonic burner so the flame will not have to be extinguished each time the sample solution is changed.

Atomic FluorescenceIn atomic fluorescence spectrometry, flames serve

as simple, convenient media for converting metal salts into the gaseous phase. The intensity of fluorescence emitted by metal atoms in flames is, however, strongly dependent on the type of flame used. The important characteristics of the flame are the temperature, type of flow, and flame composition.

The optimum temperature is a compromise between being high enough to give efficient vaporization and dis- association of the sample and yet not so high as to cause unnecessarily high noise levels due to thermal emissions from the flame.

Though little used for atomic fluorescence spectrometry, the laminar flow premixed flame has important

12

advantages over turbulent flames. Noise levels are lower in the laminar flames. Another advantage of laminar flames is that for many elements, the extent of compound formation varies along the length of the flame and the optimum part of the flame may be selected to obtain the highest free atom concentration.

The composition of the flame is important because it has two effects on the intensity of fluorescence. The first, which is unique to atomic fluorescence, is in determining what fraction of the absorbed radiation is remitted as fluorescence. The second effect of flame composition is that, together with the flame temperature, it determines the extent of compound formation and hence, the concentration of free atoms in the flame.

The intensity of atomic fluorescence depends upon the intensity of exciting radiation; the ratio of the excitation source band half-width to the absorption line half-width, the diameter of the absorption cell, the atomic concentration of the absorbing atoms, the efficiency of converting absorbed energy to fluorescence energy, and the spectral parameters such as the frequency of the absorption line.

Total consumption aspirator burners, familiar to workers in atomic emission spectrometry, have been used in most of the atomic fluorescence investigations, despite their relatively poor aspiration efficiency and droplet

73dispersion distribution (10, 23, 36, 48, 70). The total consumption aspirator-burners result in flames of small size and in a noise component due to scattering of 1exciting light from unevaporated solvent and analyte particles in the flame gases. In attempting to determine detection limits by atomic fluorescence, a major limiting factor is the noise caused by random scattering of the radiation due to water droplets in the flame. The limit of detection as well as the reproducibility of the method could be improved by means of more efficient atomization.

The laminar flow premixed oxygen-hydrogen flame in combination with ultrasonic nebulization might possibly serve as a convenient source for atomic fluorescence analysis. Denton and Malmstadt (18) found improved sensitivity with an argon-hydrogen-entrained air flame and ultrasonic nebulization. They attributed this improvement to reduced light scattering and a lower flame background emission. The laminar premixed oxygen-hydrogen flame with ultrasonic nebulization should exhibit little light scattering and it has a reasonably low flame background emission spectra. In addition it has a flame temperature about 1500° hotter than the argon-hydrogen-entrained air flame. With these considerations, the premixed oxygen- hydrogen flame might prove to be very useful in atomic fluorescence spectrometry.

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