Anal. Chem. 1966. 58,2864-2867
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CORRESPONDENCE Introduction of Gaseous Hydrides into an Inductively Coupled Plasma Mass Spectrometer Sir: Inductively coupled plasma (ICP) source mass spectrometry is a relatively new technique for elemental analysis. Superior limits of detection have been established over conventional inductively coupled plasma atomic emission spectrometry (ICP-AES) instruments (1). Present methods for estimating the concentrations of hydride-forming elements are wide and varied (2-5). Among the techniques used are colorimetry, dc arc spectrography, and atomic absorption spectrophotometry. Methods have been developed for the analysis of water, fish, soils, and sediments using the hydride technique and inductively coupled plasma atomic emission spectrometry (6,7).Reasonable limits of detection have been reached for many elements along with simultaneous multielemental capability. However, there are still instances where even lower limits of detection are desirable. Hydrides are generated by the continuous mixing of sample and reagent solutions in the hydride generator. Sodium borohydride is added to the acidified aqueous solution containing the hydride-forming elements, the hydrides are formed by reduction and are evolved from the solution in a gaseous form. Mercury is reduced to its volatile elemental form in a similar manner. The analytes of interest, along with a portion of the hydrogen that is produced during the reaction, are swept by argon carrier gas directly into the ICP. Once in the ICP, analyte ions are introduced into the mass spectrometer (1). The signal obtained is directly proportional to the concentration of analyte ions in the plasma. With detection limits already a factor of at least 10 better than the optical spectrometers while using a nebulizer, hydride generation should give 2 orders of magnitude greater sensitivity with the ICP-MS. Along with this advantage the ICP-MS unit has fast sequential scanning ability so multielement analysis and isotope ratio determinations can be made. Elements such as arsenic, selenium, and mercury in surface and drinking water can be toxic to human and animal life (8). One of our tasks a t the Ministry of the Environment is to detect and estimate the concentrations of these elements in a variety of sample types at ultratrace levels. Antimony, tellurium, and bismuth are also of concern but were chosen to be included in this study primarily to demonstrate the range of applicability of the technique. EXPERIMENTAL SECTION Apparatus. The hydride generator used in this study is a Plasma Therm Model PTL/083/071 manufactured at Kangley Bridge Rd., London, England. The hydride is generated in a specially designed mixing chamber on the side of the instrument which minimizes irregularitiesin argon carrier gas flow rate. The proportioning pump is a Wataon-Marlowperistaltic pump followed by a rotary valve which allows for adequate wash times. Flow rates for sodium borohydride, HCl, and sample were set at 3.8 mL/min, 8.0 mL/min, and 10 mL/min, respectively. Silicon pumping tubes were used at an inside diameter recommended by the manufacturer. Figure 1shows a layout of the apparatus. The ICP-MS system used is an ELAN Model 250 elemental analyzer supplied by Sciex, Thornhill, Ontario, Canada. The ion source consists of a modified Plasma Therm Model 2500 control box with a conventional 27 MHz rf generator. The mass spec-
trometer contains a quadrupole ma98 fiiter capable of a m m range to m / z 300 with a pulse counting channel electron multiplier for ion detection. The quartz torch used is a Fassel type torch with an 2.0 mm bore injector. The sample introduction line is made of Teflon with a 3.0 mm i.d. Reagents. The sodium borohydride solution was made by dissolving 10.0 g of NaBH, in 1 L of 0.1 N NaOH. The 3.5 M HC1 solution was prepared by adding 580 mL of concentrated HC1 to 2.0 L of double distilled water. Standards were prepared in our laboratory by conventional means. ICP-MS Determination. The hydride generation system was started and allowed to equilibrate for 15 min. The plasma was then ignited and argon, hydrogen, and sample from the hydride generator were slowly adjusted from zero to the running setting of 1.2 L/min. The rotary valve on the hydride generator was automatically flushed several times between samples. The interface to the plasma was then engaged and m / z ranges were scanned in the regions of interest. Optimum conditions were determined by a univariate search. The wash time was varied from 30 s to 2 min. Five 2-min integrations are made with each standard. RESULTS AND DISCUSSION A substantial gain in sensitivity can be achieved when using the hydride generator. Figure 2 shows the sensitivity increase that was achieved when using a hydride generator in place of a nebulizer for sample introduction. Note that the ordinate is a log scale. The figure shows the ion intensities that were observed during a mass scan while running a multielement standard solution containing the analytes of interest a t a concentration of 0.1 CLgImL. The same plasma and mass spectrometer operating conditions were used for both nebulizer and hydride generator. Selenium has an isobaric interference from the Arz+ peak a t its most abundant isotope of m / z 80, so determinations were made at the less abundant isotope of mlz 78. I t is interesting to note that P b has no significant increase in sensitivity when reduced to its hydride with this generator. This may be due to higher concentration of HC1 needed with this particular generator that is hindering lead reduction (9). Selection of Optimal Operating Conditions. Previous studies have been conducted both in hydride generation optimization (6) and instrument optimization (10) and a simplex technique has been described (11). In this study a combination of the factors that we considered most critical were chosen for optimization. Optimum sensitivity was obtained by monitoring signal strength while running a 0.1 pg/mL multielement solution. (a) Forward Power. The range of forward power used was 800-1500 W. Power below and above these values would tend to cause plasma instability. Higher power gave a higher background due to photon noise which is also seen using nebulization. Figure 3 shows signal strength in ions per second plotted against the power range. The instrument seems to become more sensitive to selenium as the power level increases. A power level of 1180 W was selected. ( b )Ion Focusing Lens E (Photon Stop). Previous studies have shown that all four ion lens settings affect the signal
0 1988 American Chemical Society 0003-2700/86/0358-2884$01.50/0
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986 3.8 ml min
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1 Glass Interface Vessel
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Argon Gas
I H, + Hydride
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Drain WASTE
3. Pneumatic Valve Switch ON Sampling
Figure 1. Hydride generator setup for ICP use.
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Figure 2. Comparison of nebulizer vs. hydride generator, 0.1 mg/L multielement standard: black, nebulizer; white, hydride generator.
strength of the analyte ion (11). However the B lens seems to be most critical in terms of not only sensitivity but also precision (11). A range of 20-90 units in steps of 10 was chosen for the experiment. The units are arbitrary digipot values. Allowable digipot settings range from 00 to 99 (f10 V dc). The effect of varying voltage on the B lens over the range chosen seems to be mass discriminatory with the heavier ions having greater signal strength a t the higher setting. Here a compromise was made and a setting of 65 or +6.6 V was chosen. (e) Carrier Gas Flow Rate. The affect of varying carrier gas flow rate is similar to changing the flow rate on a nebulizer. Elements have different degrees of ionization depending on what area of the analytical zone in a plasma is viewed or, in the case of ICP-MS, sampled. Movement of this analytical zone will have a marked affect on sensitivity. By changing the carrier gas flow rate, one in effect changes the position of the analytical zone and hence area sampled. This particular hydride generator has sodium, possibly from the borohydride reagent, carried along in the gas stream. One can detect a definite initial radiation zone in the plasma which can be seen to move on adjustment of the carrier gas flow rate. The presence of Na may be undesirable because of possible interference with ion production. Figure 4 shows the effect of various flow rates on signal strength. Previous studies (11)have shown that a good compromise between the maximum sensitivity and acceptable levels of interferences can be achieved by sampling the ICP approximately 5 mm above the tip of the initial radiation zone observed while aspirating a 1000 mg/L Y solution. The initial radiation zone in the plasma for position viewing in this case is an advantage. Settings below 1.1L/min show a streamer that is set further back in the plasma and produces a lower signal strength. Settings above 1.3 L/min show the streamer tip actually entering the sample orifice producing greater signal strength. Experience in the past has shown that the greater signal produced in this way is not really an ad-
800
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FORWARD POWER (WATTS) Figure 3. Forward power vs. ion intensity.
vantage when practical analysis has to be performed (11). A flow rate of 1.2 L/min produced an initial radiation zone that was approximately 1 mm to 2 mm from the sampling orifice and was selected. ( d ) Sodium Borohydride Concentration. The sodium borohydride concentration was varied from 0.1% to 3 % . At
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
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io7
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GAS FLOW RATE (L/MIN) rate vs. ion intensity.
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MOLARITY Flgure 6.
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As Se H€! Sb Bi Te
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Acid molarity vs. ion intensity.
Table I. Comparison of Detection Limits (ng/mL)
element
I
v
H9
hydride” AA 1.0 1.0 0.01 1.0 1.0
nebulizerb hydridec ICP-AES ICP-AES 110 70 120 90 90 70
0.8 0.8 1.0 0.8 1.0
nebulizerd ICP-MS 1.0 6.0 3.0 0.5 0.3 1.0
hydrided ICP-MS 0.005 0.02 0.4
0.004 0.02 0.1
Vijan et al. (9) Hb detection limit based on clean waters. Detection limit based on 2a. *Limits of detection obtained on instrumentation from our laboratory. cThompson et al. ( 6 ) . Detection limits obtained this study. C
0
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CONCENTRATION OF NaBH,(%) Flgure 5. Sodium
borohydride concentration vs. ion intensity.
concentrations of 1.5% or over, adverse effects on the plasma were seen. Along with a more intense initial radiation zone, plasma flickering and unstable reflected power were observed causing poor precision on element determinations. Figure 5 shows the effect of varying reagent concentration on signal strength. A satisfactory compromise between plasma stability and sensitivity was obtained a t the 1% level. ( e ) Acid Molarity. The effect of varying reagent acid concentration can be seen in Figure 6. Optimum acid concentration was chosen at 3.5 M. A mercury sensitivity increase
can be seen a t lower reagent concentration. More experimentation will be done in the future to determine optimum parameters for P b and Hg. Linear Dynamic Range. All elements exhibit a linear range of 3 to 4 orders of magnitude. All curves are linear from the detection limit to at least 1.0 mg/L with correlation coefficients of better than 0.99. Tellurium may be nonlinear above 1.0 mg/L, but we have not examined this closely. Higher concentrations of these elements will initiate a safety device in the ICP-MS called a “throttle” that will protect the detector from ion count rates that exceed 5.0 X lo6 counts/s. This unfortunately limits us to concentrations for these elements to under 1.0 mg/L. Detection Limits. Table I shows the detection limits obtained with hydride generation and nebulization both with ICP-MS and ICP-AES instrumentation. Detection limits were obtained in this study by the formula 3 X std dev of the background signal detection limit = slope of the calibration curve Values reported in this fashion are theoretical and not likely to be obtained in actual sample analysis due to higher blanks, instrument drift, and shorter peak integration times to make speed of analysis practical. A good estimate of actual detection
Anal. Chem. 1988. 58. 2867-2869
limits would be to multiply theoretical limits obtained by a factor of a t least 5. Precision. Major limitations on precision include variations in carrier gas flow rates and proper analytical zone sampling. With this hydride generator setup, precision obtained on standards and samples is in the order of 2-5% relative standard deviation over several standardizations. Memory Effects. As in many introduction devices as well as the hydride generator, mercury has a memory effect. The introduction device may not be the only factor. The interface sampler is made of nickel and the ac focusing rods are gold plated. Both of these form an amalgam with Hg. More experimental work must be done to determine the actual causes. Memory effects resulted in analysis times for Hg in this study of 5 min per sample.
CONCLUSION The results of this paper indicate that further development of this technique may yield a viable simultaneous multielemental analysis method with detecting power far in advance of existing methodology. However many factors must be considered. High sample volume uptake and reagent use are definite disadvantages when using this particular hydride generator. Memory effects are pronounced for mercury and must be reduced to make analysis practical. Finally, this technique is limited by the chemistry of hydride generation, which is subject to chemical, kinetic, and thermodynamic interferences. More study must be done into interferences caused from high concentrations of other ionic species such
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as iron and copper which may affect reagent reducing power (12). Registry No. NaBH,, 16940-66-2;As,7440-38-2;Se, 7782-49-2; Hg, 7439-97-6; Sb, 7440-36-0; Bi, 7440-69-9; Te, 13494-80-9.
LITERATURE CITED Douglas, D. J.; Quan, E. S. K.; Smith, R. G. Spectrochim. Acta, Part B 1983, 388, 39-46. Bowen, H. J. M. Trace Nements in Biochemistry; Academic Press: New York. 1966. Gutzeit, M. Pharm. Ztg. 1979, 2 4 , 263. Stanton, R. E. €con. Geol. 1984, 59, 1599. Stanton, R. E.; MacDonaid, A. J. Trans.-lnst. Min. Metall. 1982. 71, 517. Thompson, M.; Pahlavanpour, B.; Waiton, S. J. Anavst (London) 1978, 103, 563. Gouiden, P. D.; Anthony, D. H.; Austen, K. D. Anal. Chem. 1981, 5 3 , 2027-2029. Ontario Drlflking Water ObJectives Handbook; Ontario Ministry of the Environment: Toronto, Ontario, 1963. Vijan, P. N.; Wood, G. R. At. Absorpt. Newsl. 1974, 13, 33. Horiick, G.; Tan, S. H.; Vaughan, M. A.; Rose, C. A. Spectrochlm. Acta, Part B 1985, 408,1555-1572. Boomer, D. W.; Powell, M. J. "Analysis of Acid Rain Samples by ICP/MS". Paper presented at C.I.C. conference Kingston, Ontario, abstract no. AN-AL-3, June 1965. Thompson, M.; Pahlavanpour, B.; Waiton, S. J.; Klrkbright, G. F. Analyst (London) 1978, 103, 705-713.
M. J. Powell* D. W. Boomer R. J. McVicars Ontario Ministry of the Environment Rexdale, Ontario, Canada M9W 5L1 RECEIVED for review March 18, 1986. Accepted July 8,1986.
Effects of Heavy Atom Containing Surfactants in the Room Temperature Phosphorescence of Carbaryl Sir: Room temperature phosphorescence (RTP) can be observed from compounds adsorbed on a solid substrate, such as filter paper (1-5). the physical and chemical interaction between the substrate and phosphor provide a rigid medium that allows the observation of phosphorescence at room temperature. While the roughness and irregularity of the surface of the paper can be advantageous since it protects the phosphor from oxygen quenching (5), the numerous and relatively large interstices between cellulose fibers allow for extensive penetration of the molecules (heavy atoms and lumiphors) into the bulk of the paper, as observed from surface studies of filter paper done through X-ray photoelectron spectroscopy (6). We have modified the surface of filter paper Whatman No. 1 by the use of surface-active agents (or more briefly, surfactants). Surfactants are amphipathic molecules that can be anionic, cationic, or nonionic in nature. They have a long nonpolar hydrocarbon end and a polar end that is expected to interact strongly with the hydroxyl groups of the cellulose polymer. Although micelles, which are aggregates of surfactant molecules, have been used with much success to observe RTP in solution ( 7 - I O ) , there is no report in the literature on the use of surfactants on solid substrate RTP. Recently, Alak et al. (11)published their results on the effect of surfactant spray reagents on the fluorescence densitometry of polycylic aromatic hydrocarbons and dansylated amino acids. They observed luminescence enhancements with the use of the surfactant sprays when silica and alumina TLC plates were used, but no effect or a modest decrease in luminescence when reverse phase, cellulose, or polyamide plates were used. 0003-2700/86/0356-2867$01.50/0
Qualitative and semiquantitative analyses of the surface of filter papers treated with the heavy atom ion inorganic salts (T1N03 and AgN03) and the surfactant salts (TlDS and AgDS) are done through X-ray photoelectron spectroscopy (XPS) analysis (6). XPS has provided us with information concerning the relative amounts of heavy atom ions and luminescence compounds present on the surface of treated papers and on the extent of penetration of the compounds into the bulk of the filter paper (12). The results obtained from the surface analysis are correlated with the observed effects of the surfactants on the phosphorescence of carbaryl.
EXPERIMENTAL SECTION Apparatus. Phosphorescence spectra were obtained with a Model LS-5 luminescence spectrometer (Perkin-Elmer,Norwalk, CT) coupled to a model 3600 data station. A delay time (after the light pulse) of 0.1 ms and an observation time (gate time) of 9.0 ms are used for the measurement of the phosphorescence of carbaryl. A laboratory-built sample compartment, previously described (12),was used in all experiments. X-ray photoelectron spectroscopic determinationswere obtained with a Kratos XSAM 800 spectrometer. A detailed description of the instrumental operating conditions and experimental procedures for the XPS analysis of treated and untreated filter papers was given in a previous paper (6). Infrared absorption and atomic absorption measurements were performed with a Nicolet 7199 FTIR and a Perkin-Elmer 303 atomic absorption spectrometer, respectively. Procedure. The dodecyl sulfate salts of thallium(I), TlDS, and of silver (I), AgDS, were prepared by using a procedure s i m i i to that published by Humphrey-Baker et al. (13). Qualitatively the purity of the batches of TlDS and AgDS were verified employing infrared spectrometry (in the range of 4000 to 400 cm-' and using the Nujol mull technique). The weak intensity peaks 0 1986 American Chemical Society
