Agilent Evaluation of high intensity lamps for AAS
Contents
1. Authors Jonathan H Moffett J V Sullivan Evaluation of High Intensity Lamps for AAS Application Note Atomic Absorption Introduction The trend over the three decades since atomic absorption spectrometry AAS was introduced as a very successful analytical technique 1 has been to measure ever lower concentrations of trace metals This is especially true for the priority pollutant metals As Se Sb Pb Cd and Tl in the environment An improvement in light throughput of the AA spectrometer gives a more stable baseline and hence a lower limit of detection Three areas need to be considered e Monochromator e Photomultiplier tube e Light source Modern monochromator and photomultiplier tube designs have been refined to the point where further significant improvements are unlikely An important component the hollow cathode lamp HCL 2 can be modified to give better spectral characteris tics and hence better performance A new lamp design is described and characterized in this report The requirement is to increase the emission intensity of a HCL without broadening the emission line The characteristics of some arsenic and selenium hollow cathode lamps in particular can vary widely 3 which suggests their operating conditions are critical A broadened emission line generally results in lower absorbance poorer signal to noise ratio and greater calibration graph curvature Considerable research has gone towards dev2. 1983 C M M Smith and J M Harnly Spectrochim Acta 1994 49B 387 398 J V Sullivan Prog analyt atom Spectrosc 1981 4 311 340 F J Fernandez P J Morrisroe and J W Vollmer Pittsburgh Conference Paper 454 An improved Electrodeless Discharge Lamp design for AAS 1993 For More Information For more information on our products and services visit our Web site at www agilent com chem www agilent com chem Agilent shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing performance or use of this material Information descriptions and specifications in this publication are subject to change without notice Agilent Technologies Inc 1995 Printed in the USA November 1 2010 AA124 ni p Agilent Technologies
3. characteristic concentration and mass is that which is required to give 0 0044 absorbance 1 absorp tion and the smaller the concentration the better the sen sitivity The calibration graphs for arsenic and lead with the conventional lamps show marked curvature This means the characteristic concentrations of the con ventional and UltrAA lamps in Table 2 are not as different as the higher absorbances might suggest The performance of the UltrAA lamps overall is markedly better than that of the corresponding conventional lamps Table 2 Characteristic Concentrations ug L for Hollow Cathode Lamp of Different Designs Characteristic Concentration is Defined as Giving 0 0044 Absorbance Sample Volume was 20 uL Characteristic concentrations pg L W length Current UltrAA Conv Common Element nm mA lamp lamp anode lamp As 193 7 10 0 25 0 31 0 28 Pb 283 3 5 0 15 0 26 0 27 Se 196 0 15 0 49 2 20 0 75 STANDARD 3 050 0 50 UltrAA lamp seal Conventional HCL g q m a z ERDE j T T T F200 Time sec 74 60 Figure 2 These signal graphics for a 75 pg L Se standard demonstrate the enhanced sensitivity of the UltrAA lamp compared to a conventional lamp Calibration area Abworbance Caencentratian ug L 0 0 Comparison of arsenic peak area calibration graphs for UltrAA and conventional lamps Upper line is for UltrAA lower line is for conventional Figure 3 Calibration area 3 Q J o 7 Sigil najt B0 0
4. Figure 4 Comparison of selenium peak area calibration graphs for UltrAA and conventional lamps Upper line is for UltrAA lower line is for conventional Calibration area a g a 39 0 Concentration ug L Figure 5 Comparison of lead peak area calibration graphs for UItrAA and conventional lamps Upper line is UltrAA lower line is conventional Limit of detection is defined as the concentration of a solution which gives an absorbance equal to three times the standard deviation of the blank absorbance Thus limit of detection is a measure of noise and sensitivity The UltrAA lamps give a higher signal to noise ratio for all elements and produce a better limit of detection as shown in Table 3 Table 3 Limits of Detection ug L Measured Using Lamps of Various Types Fifteen Blank Readings Using a Sample Volume of 20 pL Were Made Limits of detection pg L W length Current UltrAA Conv Common Element nm mA lamp lamp anode lamp EDL 5 As 193 7 10 0 26 1 40 1 65 1 0 Pb 283 3 5 0 18 0 80 1 75 0 15 Se 196 0 15 0 29 3 10 0 67 0 8 The certified reference material results are shown in Table 4 The UltrAA lamps are capable of producing accurate results based on the certified values Table 4 Sample Results Using UltrAA Lamps for Arsenic Selenium and Lead Spread is Based on Sample Duplicate Results with Two Replicates Each Sample Volume was 10 pL Element Sample Found Certified Units Arsenic NIST 1643c 82 3 03 82 1 12 u
5. UltrAA lamp Experimental Instrumentation An Agilent SpectrAA 880Z atomic absorption spectrometer with Zeeman background correction fitted with a PSD 100 sampler was used The instrument was modified so both the UltrAA control module and a module for the commercial common anode lamps could be connected All lamps were new Atomization was from the wall of a pyrolytic graphite coated partition tube The inert gas was argon Instrument parameters are the default settings for each element The only changes were that the ash time was increased from one second to 10 seconds and 10 uL of chemical modifier was added to the sample Standards Calibration standards were diluted from 1000 mg L commer cial standards Merck in Type water with either dilute 0 1 M hydrochloric acid As Se or nitric acid Pb A work ing concentration of 50 pg L solution was prepared daily for As and Se Characteristic concentrations and limits of detection were determined using a 15 pg L solution The working concentra tion of lead was 20 ug L and a 10 ug L solution was used when calculating characteristic concentrations and limits of detection For the determinations a multi element standard Inorganic Ventures Lakewood NJ U S A diluted to 100 pg L was used A 10 ug L standard was included with the samples Samples The samples were certified reference materials and are sum marized in Table 1 They were chosen because they are supplied in liq
6. eloping more intense emission sources either by increasing the output from the hollow cathode lamp high inten sity or boosted hollow cathode lamps or by developing alternative emission sources to the hollow cathode lamp electrodeless discharge lamps EDL Agg Agilent Technologies The EDL works on a very different principle from the HCL The iodide salt of the element of interest is sealed with a noble gas at reduced pressure in a quartz tube Radiofrequency rf energy is used to form a plasma in the gas The temperature of this plasma evaporates some of the salt which then dissociates to give atoms The metal atoms are excited and emit light The light emission is significantly higher than that of a standard HCL but there are some significant practical disadvantages e The lamps are difficult to stabilize e Long warm up times are required e The lamp current must be optimized manually e A complex and expensive additional power supply is required The rf generator the EDL requires is usually external to the instrument The internal power supply for the conventional lamps is not used Two modifications to the conventional hollow cathode lamp design by Walsh and Sullivan increase the emission intensity by separating the sputtering process from the excitation process 4 within the lamp A boosting discharge passes a stream of electrons and ions through the sputtered atom cloud to increase the photon output by exciting more atoms A
7. g L CRM TMDW 79 8 0 9 80 0 0 4 ug L CRM TMF 96 9 0 5 100 0 0 5 mg L 10 pg L 10 2 0 4 10 0 0 1 ug L Selenium NIST 1643c 12 5 03 12 7 0 7 ug L CRM TMDW 9 4 0 1 10 0 0 1 ug L CRM TMF 94 0 4 10 0 0 1 mg L 10 pg L 9 7 0 1 10 0 0 1 ug L Lead NIST 1643c 36 7 0 0 35 3 0 9 ug L CRM TMDW 35 9 0 2 35 0 0 2 ug L CRM TMF 8 6 0 1 10 0 0 1 mg L 10 pg L 9 4 0 2 10 0 0 1 ug L Summary The UltrAA lamp design gives consistently better analytical performance than the corresponding conventional hollow cathode lamp The linearity of the calibration graphs provides indirect evidence that the emission line characteristics are improved The more intense emission line means there are more photons per unit time hence the noise associated with the random arrival of photons at the photomultiplier tube is less which in turn produces less background noise and better limits of detection The narrower emission line enhances the sensitivity In addition to the benefits of improved limits of detection and enhanced sensitivity the UltrAA lamps offer the same sim plicity and ease of use as the conventional SpectrAA hollow cathode lamps Acknowledgments The assistance of John Willis Don Anderson Eric Vanclay and Ken Selim is warmly acknowledged References 1 B J Russell J P Shelton and A Walsh Spectrochim Acta 1957 8 317 P A Bennett and E Rothery Introducing Atomic Absorption Analysis Varian
8. separate power supply provides the boosting current Two circuit configurations have been commercialized One config uration as modified by Lowe uses an anode common to both circuits In this design the boosting current passes directly through the region of the sputtering discharge and lowers the number of sputtered atoms available for excitation Hence each lamp has its own optimum boosting current which must be determined by the operator This is obviously not amenable to automation The other design commercialized as UltrAA is shown in Figure 1 UltrAA lamps have an electron source and a sec ondary anode in addition to the anode and cathode of a con ventional lamp The hollow cathode current is supplied by the instrument in the normal manner The boosting current is fixed This two anode lamp design has a number of advantages e The sputtering and excitation processes are separated e The same boosting current may be used for every lamp e The instrument can fully control the lamp s operation UltrAA lamps are installed in the lamp turret in the same way as are conventional SpectrAA lamps and have lamp recog nition so the instrument can automatically select the lamp and set the correct lamp current The external module supply ing the required boost current is able to power two lamps at the same time Boost anode Quartz window Glass shield Sa Discharge guide Cathode Boost filament Figure 1 Schematic of
9. uid form and no digestion Is required Table 1 Certified Reference Materials Used and Their Sources Certified reference materials National Institute of Standards and Technology Gaithersburg MD U S A SRM 1643c Trace elements in water High Purity Standards Charleston SC U S A CRM TMDW Trace metals in drinking water CRM TMF Trace metals in fish Modifiers Chemical modifiers were used to stabilize the volatile ele ments Reduced palladium 500 mg Pd L with 2 w v citric acid was used for arsenic and selenium Diammonium hydrogen orthophosphate 2 g 100 mL solution was used for lead Results and Discussions Typical signal graphics for the UltrAA lamp are compared with those from a conventional lamp in Figure 2 The performance of the UltrAA lamp is compared with that of the equivalent standard hollow cathode lamp and of the common anode design lamp for arsenic selenium and lead The arsenic selenium and lead calibration graph for peak area of the appropriate conventional lamp is overlaid with the corre sponding graph of the UltrAA lamp in Figures 3 4 and 5 respectively For all elements the slopes of the calibration graphs from the UltrAA lamps are greater than those from the corresponding conventional lamps Not only are the absorbances higher for the UltrAA lamps but the graphs are also more linear The explanation for this is presumably that the analytical emission line from the UltrAA lamps is nar rower The
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Agilent Evaluation of high intensity lamps for AAS