Anal. Chem. 1904, 56,289-292
289
CORRESPONDENCE Direct Liquid Sample Introduction for Flow Injection Analysis and Liquid Chromatography with Inductively Coupled Argon Plasma Spectrometric Detection Sir: The coupling of flow injection analysis (FIA) or high-performance liquid chromatography (HPLC) techniques to inductively coupled plasma atomic emission spectrometry (ICP-AES) offers new and attractive approaches for the determination of elemental concentrations in a wide variety of sample matrices. Several advantages of FIA over continuous flow methods for sample introduction into the ICP have been discussed by Greenfield (1,2). One of the most attractive features that FIA offers is a rapid and precise means of automating sample introduction into an ICP for simultaneous, multielement analysis at the trace, minor, and major constituent level with minimal sample consumption. The utilization of the ICP as a detector for HPLC retains most of the advantages of FIA-ICP, while providing the analyst with a powerful and versatile means of compound separation (1-8). This added dimension becomes particularly important when metal speciation is of primary interest, rather than total metal content. T o date, the coupling of FIA and HPLC to the ICP has only been accomplished using conventional cross-flow, concentric, or Babington-type pneumatic nebulizers (1-8). Limits of detection under these conditions have generally been observed to be poorer when compared to conventional continuous sample flow conditions. These limitations have been attributed to the large dead-volume and the sample losses associated with conventional nebulizers and band broadening of eluents from FIA transfer tubing or HPLC columns prior to entering the nebulizer unit (9, 10). In an effort t o resolve these difficulties, we have developed a microconcentric nebulizer which is inserted directly into the tip of a conventional sample introduction tube of an ICP torch. Preliminary data on the potential utility of direct liquid sample introduction into the ICP are presented. EXPERIMENTAL SECTION Microconcentric Nebulizer. A schematic diagram of the nebulizer and torch assembly used in this study is shown in Figure 1. Both the torch and nebulizer were fabricated at the Ames Laboratory. A magnified view of the microconcentric nebulizer tip is shown in Figure 2. The liquid samples [A] were introduced through a 0.19 mm 0.d. by 0.05 mm i.d. fused-silica,inner capillary tube [B] (Spectran Corp., Sturbridge, MA). The argon nebulizer gas flow [C] was directed through a 0.70 mm 0.d. by 0.50 mm i.d. fused-silica outer capillary tube [D] that sheathed the inner capillary tube [B]. The tip of the nebulizer gas capillary tube was tapered down to a 0.25 mm orifice as shown in Figure 2. The 0.03-mm annular spacing between the tubes at the nebulizer tip created a high-velocity nebulizing gas flow capable of producing an aerosol for direct introduction into the axial channel of the plasma. A 1.6 mm 0.d. by 0.8 mm i.d. ceramic insulating tube [E] (Ventron Corp., Alfa Products, Beverly, MA) was inserted over the outer capillary tube to straighten and hold the nebulizer rigid. The gap between the outer capillary tube and ceramic was sealed with an epoxy resin. Auxiliary nebulizer argon flow [F] was carried between the normal sample introduction tube [GI and the ceramic insulating tube. FIA/HPLC-ICP System. Standard FIA and HPLC techniques and equipment were used throughout this study. Solutions
Table I . Plasma Operating Conditions and Detection Facilities plasma: HF generator power plasma Ar flow rate auxiliary plasma Ar flow rate nebulizer Ar flow ratea auxiliary nebulizer Ar flow rateb vertical observation height monochromator focal length grating slit widths amplifier strip chart recorder
Plasma-Therm, Inc., Kresson, NJ Model HFS-5000D 1.8 kW forward power 17 L/min 0 . 5 L/min 0.2 L/min 0.6 L/min 16 mm
McPherson Model 2051 l m 2400 grooves/mm blazed for 250 nm 20 pm Model 417, Keithly Instruments, Inc., Cleveland, OH Model 250-1, Curken Scientific, Inc., Danbury, CT
a Defined as the argon flow in the microconcentric Defined as the argon flow in the annular nebulizer. space between the microconcentric nebulizer and the normal sample introduction tube.
were continuously drawn from a solvent reservoir through a high-pressure, single piston pump (Model 112, Beckman Instruments, Inc., Berkeley, CA) into a syringe-loading sample loop injector (Model 7125, Rheodyne, Inc., Cotati, CA). Sample volumes of 10 ILLwere used for the FIA mode, and 5 ILLvolumes for the HPLC mode. A 2.5-cm precolumn packed with XAD-1 resin fines was inserted immediately before the sample injector in the FIA mode to establish a required minimum pressure (approximately 300 psi) in the pump for proper flow regulation. Analyte solutions injected into the sample loops were transported by the solvent stream through a 12 cm length of 0.25 mm i.d. stainlesssteel tubing lined with 0.19 mm 0.d. by 0.10 mm i.d. fused silica capillary for FIA, or through a 1 mm i.d. X 50 cm CI8 microbore column (HRSM-50-C18,C-M Laboratories,Nutley, NJ) for HPLC. Effluents from the columns passed directly into the inner capillary tube of the nebulizer assembly t o the base of the plasma where direct nebulization into the plasma occurred. The dead volume between the ends of the FIA transfer tubing or HPLC columns to the nebulizer tip was approximately 1.5 pL. The plasma operating conditions and detection facilities are summarized in Table I. Reagents. Reference solutions of Mg, Mn, Cd, Cr, As, Se, Hg, Sr, Co, Ba, and Pb were prepared from 1000 ppm stock solutions (Fisher Scientific Co., Fair Lawn, NJ). Reference solutions containing a Cr(II1) and Cr(V1) mixture were prepared from chromium(II1)chloride and sodium chromate, respectively (J. T. Baker, Phillipsburg, NJ), for HPLC speciation studies. Reference solutions containing sodium arsenate and sodium arsenite (Fisher), benzenearsonic acid (Eastman Kodak Co., Rochester, NY), and methanearsonic acid (Sigma, St. Louis, MO) were prepared for As speciation studies. All chemicals were used without further purification. Sodium 1-pentanesulfonate (PIC B-5) and tetra-
0003-2700/84/0356-0289$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2. FEBRUARY 1984
290
1___1_______1____-_____I________________------.-------.---------_._--__
Table 11, Detection Limits for FIA-ICP limits of detection analytical wavelength, nm
element
microconcentric nebulization relative (ppb) absolute (ng)
279.6 257.6 214.4 205.6 193.7 226.4 196.0 194.2 407.8 455.4 238.9
Mg
Mn Cd Cr As
Pb Se Hg Sr Ba co
0.003 0.016 0.049 0.27 1.2 0.64 1.4 0.69 0.007 0.025 0.27 NEBULIZER ORIFICE
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,1
SAMPLE
TUBE (6rnm OD1
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k-PLASblA
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1
1 i~;;b
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4I
,
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I
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GAS
GRAPHITE FERRULES
AUXILIARY F 7 P L A S M A GAS
