Characterization and Optimization of a Direct Injection Nebulizer for Introduction of Organic Solvents and Volatile Analyte Species into an Inductively Coupled Plasma

T H O M A S W. AVERY,* C H I T R A C H A K R A B A R T Y , t and J O S E P H J . T H O M P S O N : ~ Department of Chemistry, Ball State University, Muncie, Indiana 47306

A direct injection nebulizer (DIN) is evaluated as a means of introducing organic solvents and volatile analyte species into an inductively coupled plasma atomic emission spectrometer (ICP-AES). The DIN used in this work differs in several important aspects from a commercially available DIN, after which it is patterned. When used for flow injection analysis, the DIN exhibits a long-term precision of about 4% RSD while nebu- lizing pure organic solvents. Under normal operating conditions, detec- tion limits are generally an order of magnitude worse in organic solvents than in water, but they can usually be improved by using higher forward powers and viewing higher in the plasma. Measurements of the atom/ ion line intensity ratios of copper under various conditions indicate that a large but nearly constant degree of plasma cooling exists when organic solvents are nebulized. When compared to a Meinhurd nebulizer-ICP system, the DIN-ICP system gives more uniform response towards dif- ferent chemical species of the same element, even when great differences in volatility exist between the species. Index Headings: Inductively coupled plasma; Sample introduction; Di- rect injection nebulizer.

I N T R O D U C T I O N

Atomic emission spectrometers have recently been used with good success as element-specific detectors for high- performance liquid chromatography (HPLC) or for flow injection analysis (FIA). Examples include inductively coupled plasma, 1-3 direct-current plasma, 4,5 and micro- wave-induced plasma atomic emission spectrometers s (ICP-AES, DCP-AES, MIP-AES) and the ICP-mass spectrometer (ICP-MS). 7,s Because these detectors are quite sensitive and respond only to the presence of the element of interest, they are ideal for applications in- volving speciation of trace elements in complex matri- ces.9, lo

For HPLC, sample introduction into element-specific detectors has been identified as a problem. 11 Normally, the efl]uent from the HPLC column is transferred di- rectly into the inlet of a concentric, cross-flow, or ultra- sonic nebulizer with a short length of tubing. However, the spray chamber used in these conventional nebulizers contributes additional dead volume to the system as the small liquid volume of analyte (usually < 100/~L) forms an aerosol and expands into the large volume of the spray

Received 25 May 1990; revision received 12 July 1990. * Present address: Upjohn Company, 4921 Bldg. 41 Stop 17, 7000 Port-

age Road, Kalamazoo, MI 49001. t Present address: Depar tment of Biochemistry, Indiana University

School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46223. * Author to whom correspondence should be sent. Present address:

Ross Laboratories, 625 Cleveland Ave., 54, Columbus, OH 43215.

chamber. This dead volume can be significant compared to the dead volume already existing in the transfer tub- ing, and may degrade detection limits. Also, when a sam- ple contains species of the same element that have widely different boiling points, the vapor in the spray chamber can become selectively enriched in the more volatile spe- cies. 12'13 If this happens, the signal for volatile species will be enhanced relative to nonvolatile species, and a dif- ferent calibration curve must then be made for each spe- cies appearing in a chromatogram--a time-consuming process. Furthermore, most organic solvents commonly used as mobile phases for HPLC extinguish the plasma when introduced by conventional nebulizers, unless the liquid flow rate is quite small (less than 0.2 mL rain-l), or unless desolvation is employed.

Alternative nebulizers have been developed specifi- cally for liquid chromatographic applications. Caruso and co-workers described a glass-frit nebulizer that was op- timized for microcolumn HPLC. s This device nebulized pure organic solvents, but it also contained a small spray chamber that could, in principle, degrade detection lim- its and enhance signals for volatile species. Jinno e t al . ~4 constructed a cross-flow nebulizer that had no conven- tional spray chamber; the nebulized solution entered di- rectly at the base of the ICP torch. The authors found considerable variation in the response to various carbon- containing species when using this system as a carbon- specific detector for microcolumn HPLC. Koropchak and Winn ~5 demonstrated that a thermospray nebulizer sim- ilar to one used in LC-MS sample introduction could also be used for ICP-AES. A condenser was installed prior to the ICP to prevent the elevated solvent tem- peratures from producing excessive vapor loading in the plasma. For arsenic species, relative response factors were found to be species dependent? s Mazzo e t a [ . 17 developed a nebulizer specifically for the DCP, which has a small spray chamber and good mechanical efficiency. However, no data were reported on the behavior of this nebulizer when organic solvents were used. Gustavsson and his co- worker ls,19 developed two related nebulizers, one with a jet separator and the other with a membrane separator, which provided high analyte transport efficiency with minimal solvent loading of the plasma. However, they did not report any studies examining the use of these devices with microliter solution volumes.

In 1984, Lawrence e t al . developed and characterized the first direct injection nebulizer (DIN). 2° This micro- concentric nebulizer fit into the central aerosol tube of a conventional ICP torch. Solutions were nebulized di-

1690 Volume 44, Number 10, 1990 ooo3-7o2s/9o/.lo.lo9o$2.oo/o APPLIED SPECTROSCOPY © 1990 Society for Applied Spectroscopy

TABLE I. Components of a modified direct injection nebulizer.

Supplier's part No. Description Supplier

SS-102-1 ¼6-in. nut SS-400-R-2 1A-%-in. reducer SS-100-3 ¼6-in. union tee T-400-SET 1A-in. Teflon® ferrules T-100-SET ¼8-in. Teflon ® ferrules SS-100-SET ¼6-in. stainless set SS-200-R-1 Vs-~-in. reducer Custom DIN torch

Indianapolis Valve & Fitting, Indian-

apolis, IN (Swagelok parts)

Precision Glassblowing, Engelwood, CO

062463 0.05-ram i.d., 0.20-mm Scientific Glass, Austin, o.d. GC capillary tub- TX ing

160-2320 0.32-mm i.d., 0.50-ram J&W Scientific, Folsom, o.d. GC capillary tub- CA ing

ORX132116 6-in. (¼6-in. o.d., ~ - in . Omega Engineering, i.d.) Omegatite 450 Stamford, CT ceramic insulators

97066 30-cm length of ~8-in. Alltech Associates, Deer- o.d. stainless tubing field, IL

rectly at the base of the plasma; dead volume was min- imized because there was no spray chamber, separator, or wide-bore transfer tubing used. Detection limits for microliter volumes of analytes were therefore superior to those for most other types of nebulizers, with the added advantage that the plasma could tolerate low- volume flow rates of pure organic solvents. In other stud- ies, the DIN was applied to flow injection analysis of trace elements and the elemental speciation of extracts of coal and crude oil. 21,22

In this paper, we investigate some characteristics of the DIN, not previously reported, that are perhaps unique to this total consumption nebulizer. Species of osmium, phosphorus, and sulfur, having widely different boiling points, can be quantitated by reference to a single stan- dard of these elements. We show qualitatively how the excitation temperature in the plasma varies as different operating conditions are employed, and measure the de- tection limits and long-term precision of the nebulizer in various organic solvents. Also, we present a modified design of the DIN that makes it easier to rebuild and to obtain reproducible behavior while nebulizing organic solvents.

EXPERIMENTAL

Repair of a Commercially Available DIN. Two types of direct injection nebulizers were used in this study. The first was purchased from Cetac Technologies, Inc. (Omaha, NE) and was used as received except for the outer capillary tubing, which we replaced after it had been chipped. The replacement outer capillary was ta- pered to the 0.25-mm i.d. necessary for efficient nebu- lization by a process of drawing out a 0.53-ram-i.d. piece of glass tubing in an oxygen-propane flame while spin- ning it evenly on a lathe. 23 The tube was then cut at the narrowest section and sanded until it had an inner di- ameter of about 0.25 mm. This tapered outer capillary was then cemented onto a new ceramic support. Only about one out of every five nebulizers worked after the outer capillary had been replaced and the DIN reassem-

TABLE II. Typical DIN-ICP-AES operating conditions.

Plasma argon flow rate Auxiliary flow rate Auxiliary nebulizer gas flow rate Nebulizer gas flow rate Vertical observation height Slit width Forward power Pump flow rate

16 L min -~ 2 L min -1 0.8 L rain -1 0.2-0.3 L rain -1

12 mm above load coil 0.02 nm 1.5 kW

120 #L min -1 (FIA) 0.8 mL rain ~ (HPLC) with 15% split

bled. However, the only alternative was to send the entire nebulizer back to the manufacturer for repair. This type of DIN was used to collect the data on phosphorus com- pounds.

Construction of a Modified DIN. Instead of tapering the outer capillary tubing to a diameter of 0.25 ram, we used a short, straight length of 0.32-mm-i.d., 0.50-mm- o.d. capillary tubing. As a result, the DIN now operated under a significant backpressure in order to force argon gas between the inner and outer capillary tubes. Teflon ® ferrules were used in place of graphite ferrules for better sealing under the 60 psi operating pressure. Since we could not obtain reducing Teflon ® ferrules, a ~s-in. tee (bored out to Y6~-in. o.d. through the length of the tee) had to be substituted for the original ~-in. tee. A com- plete list of components of the modified DIN is given in Table I, along with supplier and part number. The only part not commercially available is the union that at- taches the H-in. ICP torch stem to the 1/~6-in. tee. This is made from a Swagelok ~-in.-~-in. reducer and a ~6-in. Swagelok nut. The ~-in. pipe from the reducer is cut off and discarded and then the ~s-in. nut is welded back- to-back with the remaining section of the reducer. A detailed procedure for assembling the DIN can be ob- tained from the authors. The reader should refer to the original publication 2° for a diagram of the DIN.

FIA-ICP System. The pumping system consisted of an Isco Model 2350 pump (Isco, Inc., Lincoln, NE) and a Valco injector (Model C6W-HC, Valco Instruments Co. Inc., Houston, TX). For FIA, a short length of stainless steel tubing lined with 0.05-mm-i.d. capillary tubing was needed to provide the small backpressure necessary to stabilize the plasma as the injection valve was turned, and to decrease noise in the baseline. For HPLC, a tee and a backpressure device were used to split 15% of the mobile phase to the DIN. The total dead volume between the injection valve and the tip of the DIN in our system was calculated to be 30-35 #L. This was a larger dead volume than the 13 #L reported in a previous publication ~o and could have been reduced.

The Meinhard nebulizer used for comparison studies was obtained from Precision Glassblowing (Englewood, CO), and had a flush-mounted tip. Typical operating conditions for the DIN-ICP (Perkin-Elmer, Model 5500) are given in Table II. Before an at tempt to detect sulfur was made, the monochromator was purged with argon for at least a half-hour.

Separation of Phosphorus Compounds. The only ex- periment that used HPLC was the separation of the var- ious phosphorus species. For that separation a 25-cm Cls column (5-~tm particle size, Isco) was employed with a guard column. The mobile phase was 50/50 methanol

APPLIED SPECTROSCOPY 1691

4 0 0 0

Z m 3000. Z

Z

9 2000

W W > 1000

o o.15

COMMERCIAL D I N

/

+ / + ~ + ~ .A- -A- -~- -A

, I

+~ MODIFIED DIN I I

' l I ' l ' I

0.18 0.21 0.24 0.27 0 . 3 0

NEBULIZER GAS FLOW RATE (L min -1)

FIG. 1. Effect of nebulizer gas flow rate on analyte signal for the commercial DIN (0.25-ram i.d. tapered outer capillary) and the mod- ified DIN (0.32-ram i.d. straight capillary). Ordinates represent relative peak heights for 10-/~L injections of 1 mg L -~ zinc at a pump flow rate of 120 uL rain -1.

water at a flow rate of 0.8 mL min -~. Phosphorus emission was monitored at 214.9 nm.

Measurement of Copper Atom/Ion Intensity Ratios. This experiment required that pump and plasma con- ditions be held constant over a four-hour time period. A solvent flow rate of 80 #L rain -~ was chosen to ensure stable operation of the plasma during nebulization of all of the organic solvents. A moderate photomultiplier tube gain of 725 V was chosen and held constant during the experiment. The wavelength was optimized at 324.7 nm [Cu(I)] by injecting 200 ~L of 10 mg L -~ copper and scanning the monochromator at 0.1 nm min -1. The loop was replaced with a 40-#L loop, and duplicate injections of the same aqueous standard of copper were made. The average net peak height was recorded as a function of observation height for the atom line in water. The mono- chromator was then slewed to the ion line (224.7 nm), the wavelength setting was reoptimized with the use of 200-~L injections, and peak heights from duplicate 40- ~L injections of the aqueous copper standard were again measured at the same gain. After the ion line intensities were measured at the different observation heights, the mobile phase was slowly changed over to methanol, and then we repeated the whole process, starting with the ion line in methanol and injecting 10 mg L -1 copper standards prepared by diluting 1 mL of an aqueous cop- per standard with methanol. The weak ion line intensity in organic solvents made it difficult to optimize on this line in isopropanol. The gain was temporarily raised to about 900 V in order to optimize wavelength and reset to 725 V before the intensity measurements were initi- ated. All the values in Tables IV, V and VI were deter- mined by dividing net intensities.

Reagents. Standard solutions of As, Ba, Co, Cd, Cr, Cu, Fe, Mn, P, Pb, Pt, S, Se, and Zn were prepared from 1000-ppm stock solutions from either Fisher Scientific Company (Fair Lawn, NJ) or Spex Industries, Inc. (Ed- ison, NJ). All aqueous solutions were made with water from a Milli-Q Plus water purification system (Millipore Corp., Bedford, MA). All organic solvents, except ace- tone, were HPLC-grade and were obtained either from Burdick & Jackson (Baxter Healthcaxe Corporation, Muskegon, MI), Fisher Scientific, or Alltech Associates,

TABLE III. DIN relative detection limits (3a, ~g L- ') in water and organic solvents (200-~tL injections).

Wave- length

Element" (nm) Water b MeOH i-PrOH ACN

As(I) 193.7 20 (50) 200 200 . . . Cd(II) 214.4 6 (3) 80 60 20 Co(II) 238.9 5 (6) 50 40 70 Cr(II) 205.6 4 (6) 50 40 40 Cu(I) 324.8 4 80 60 140 Fe(II) 238.2 5 (1) 50 50 70 Mn(II) 257.6 3 (1) 20 40 100 P(I) 214.9 30 300 300 500 Pb(II) 220.4 40 (40) 400 300 300 Pt(I) 214.4 70 400 400 .. . Se(I) 196.0 30 (80) 300 300 600 Zn(I) 213.9 2 (2) 60 60 20

"Atom lines (I) and ion lines (II) are indicated. b Literature values (Refs. 20 and 21) using the original DIN are shown

in parentheses.

Inc. (Deerfield, IL). The dibenzothiophene (98%), thia- naphthene (97 % ), 1,2-benzodiphenylene sulfide (99 % ), hexamethylphosphoramide (99 % ), and osmium tetroxide (99.8%) were obtained from Aldrich Chemical Co. (Mil- waukee, WI). Osmium tetroxide is volatile and very tox- ic. This chemical should always be handled with gloves, in a hood. Trimethylphosphate and triethylphosphate were from Strem Chemicals, Inc. (Newburyport, MA). Thiophene and benzenethiol were obtained from East- man Chemicals (Rochester, NY), and dimethyl sulfoxide from Fisher Scientific.

RESULTS AND DISCUSSION

Initial Characterization of DIN. In order to initiate our studies of the direct injection nebulizer, we elected to reproduce some of the experiments in Refs. 20 and 21 using the DIN with the tapered outer capillary. The rel- ative standard deviation (RSD) of ten replicate injections of 1 mg L -1 Zn (in the FIA mode) was found to be 1- 2 %. The optimal liquid flow rate for aqueous solutions was 120/~L rain -1, and the optimal sample loop size was 50/~L (i.e., the smallest sample loop that gave a peak height equal to that obtained under continous flow con- ditions). Nebulizer gas flow rate was found to be a critical parameter in DIN operation; if the gas flow rates were too low, the DIN would not nebulize properly, whereas if they were too high, analyte residence time in the plas- ma was decreased, yielding lower signals. Thus, in Fig. 1, a maximum signal for zinc in aqueous solutions was obtained at nebulizer gas flow rates in the range from 170 to 190 mL rain -1, and dropped off sharply above 200 mL rain -1. Detection limits in pure water were evaluated and were similar to those reported in Table III for the modified DIN and to values reported previously in the literature. 2°,2~ These experiments were valuable in them- selves because they confirmed the only other published figures of merit for a direct injection nebulizer.

In the course of performing these experiments, we ex- perienced some difficulty in the daily operation of the DIN. For instance, the inner capillary tip (positioned about 7 mm below the load coil) degraded from the heat of the plasma, the dimensions of the outer capillary or- ifice increased due to chipping, and the graphite ferrules

1692 Volume 44, Number 10, 1990

2000

1600

1200

800

400

O

200o1 t ,,001 i 1200- I / n b ~ I

/ hX'.[] . . . . [ ] .-~ ,

,oo] x' i [ ] cu o t . . . . . . . , . . . . _ ~

0 10 20 30 40 50

+ Cu 2000

/~, (a) I n c a 1600

[] Ba 1200

Cd 800

• Mn 400

• Co 0 10 20 30 40 50

2000 + Zrt

1600 P

1200

[] Ca 8O0

40O

0

/ / ~ (b) + Mn

z~ Ca

f / ~ O Ba

1'o 2'o 3'0-;0%0

(d) * Zn

~ P / " o Ca

~ [] Cu

, , v - T - 10 20 30 40 50

OBSERVATION HEIGHT (mm)

FIG. 2. Effect of observation height on analyte signal: (a) ion lines with the DIN, (b) ion lines with the Meinhard, (c) atom lines with the DIN, and (d) atom lines with Meinhard.

used in attaching the torch and HPLC tubing degraded after a few tightenings, causing the inner capillary to slip. Most nebulizers that we reconstructed (with tapered outer capillaries) had difficulty operating with 100 % or- ganic solvents (i.e., the plasma was extinguished), prob- ably because larger droplets were produced as the outer capillary eroded. Also, unstable plasma operation was observed while ion pairing reagents were being nebulized at concentrations above 0.01 M. These observations were contrary to data in Ref. 22, which showed chromatograms taken with a mobile phase of 100 % pyridine, and inter- ference data in Ref. 21 obtained at levels up to 0.3 M NaC1. Repair of the commercial DIN was not trivial (see experimental section) and is an acquired art.

The distances between the ends of the outer and inner capillary and the diameter of both capillary tubes are critical and must be maintained in order to achieve re- producible results. Because the DIN had to be repaired frequently, we were concerned that each new nebulizer would have slightly different characteristics because of irreproducible orifice sizes at the tip of the tapered outer capillary. Therefore, the DIN was modified to make it more durable and easier to construct and to give it more precise inter-laboratory figures of merit. The modified DIN was used in all of the remaining work except for the experiments with phosphorus compounds.

Experiments with Modified DIN. The modified DIN produced a fine, conical plume of mist for every nebulizer constructed. 24 In fact, once we gained experience in their fabrication, it was unnecessary to test each DIN prior to installation inside the torch box. A 2-3 mm separation between the outer and inner capillary tips was found to be optimal. It was also found that the length of the outer capillary tube should not exceed 3~ in. because of the excessive backpressures (> 90 psi) required to force argon between the inner and outer capillary tubes at the nec- essary flow rate (about 300 mL min-~). We had tried to exactly match conditions in the commercial device by using 0.25-mm-i.d. tubing for the outer capillary, which is also commercially available, but the resulting argon flow rate was much too low, regardless of the length of the tubing or the backpressure.

TABLE IV. Copper atom/ion intensity ratios in various solvents and at different observation heights."

Mein- DIN/ Height hard/ DIN/ iso- DIN/ DIN/ (ram) water water propanol methanol acetone b

5 10.9 4.72 11.6 16.4 19.8 [711/36] 10 6.30 3.75 11.0 17.4 17.4 [1200/69] 15 3.69 4.88 18.8 33.8 29.7 [1187/40] 20 3.46 8.35 63.4 53.5 82.5 [1402/17] 25 4.41 20.1 155 107 153 [1682/11] 30 9.92 42.0 360 " - 321 [1924/6] 35 33.8 180 . . . . . . . . . [2076]

E value ° 13.1 13.1 22.5 47.2 264

"Injection of 40 #L of 10 mg L -1 Cu, 80 #L rain -1, 1.5 kW. b For acetone only, the individual atom and ion line intensities (net)

are shown in brackets. ° The evaporation factors (E, explained in text) are given for each

solvent. The units are #m3/s.

The modified DIN behaves nearly identically to the commercial device. One difference observed is that the nebulizer gas flow rate has less effect on analyte signal and is normally higher (Fig. 1), presumably because the 0.32-mm nebulizer orifice is larger than the 0.25-mm or- ifice. The fiat shape of the curve in Fig. 1 indicates that it is easier to "optimize" the nebulizer gas flow rate with the modified DIN; in practice we simply set the outlet pressure of the gas tank to 60 psi and turned the needle valve on the flowmeter fully open. We could not obtain flow rates higher than about 300 mL min -1 unless the regulator pressure was increased above 60 psi. Because of the 60 psi pressure, all nebulizer gas connections and the DIN itself must be leak-tight (hence the switch to Teflon ® ferrules). The linear gas flow rate at the tip of the nebulizer (assuming a volume flow rate of 300 mL min -1) is calculated to be 102 m s -1, smaller than the 120 m s -~ reported for the original DIN. 2°

Figure 2 shows how analyte signal changes as the ob- servation height is varied. Data are included for both our Meinhard nebulizer and the DIN, to aid in comparison. For the DIN, the ion lines of most elements peak low in the plasma and have very low intensities above 20 mm (Fig. 2a). It appears that the elements with lower first ionization energies (Ba, Ca) have a broader distribution of intensities with respect to observation height than do elements whose ions are more difficult to form (Mn, Co, Cd). Ion line intensity distributions measured with the Meinhard follow similar trends but are broader and peak higher in the plasma (Fig. 2b) relative to the DIN. With the DIN, atom line intensities also peak low in the plas- ma, but tend to level off after 20 mm (Ca and Cu, Fig. 2c). Atom lines of zinc and some other elements with high ionization energies, such as phosphorus and osmium (not shown), seem to peak at approximately the same location but tail off quickly. In organic solvents, atom line intensities usually peak higher in the plasma (see Table IV). Atom line intensities measured with the Mein- hard-ICP (Fig. 2d) again have a broader distribution and can sometimes peak very low in the plasma (e.g., Ca). Thus, it appears that viewing height is a more important parameter to optimize with the DIN as opposed to a Meinhard nebulizer, especially if ion lines are being mea- sured.

The long-term reproducibility of analyte signal with

APPLIED SPECTROSCOPY 1693

the DIN and the DIN's ability to nebulize high-salt so- lutions were next investigated. In one experiment, 20 ~L of a 1 mg L -1 zinc standard was injected repeatedly over a two-hour period into both aqueous and organic mobile phases. The RSD among the 24 peak heights (one in- jection every 5 min for 2 h) was 5.5 % in pure H20 and 4.2 % in pure methanol. When 0.1 M NaC1 was nebulized, a white buildup of salt could be seen at the tip of the inner capillary after about ten minutes, and the signal from a 1 mg L -~ zinc standard decreased gradually there- after (Fig. 3). Although the plasma continued to run, nebulization was sporadic. After an hour, the intense orange-yellow color due to sodium emission in the plasma occasionally became greatly diminished, as did the nor- mal hissing noise the DIN makes while nebulizing prop- erly. In Fig. 3, the overall RSD for the peaks injected into the NaC1 mobile phase was 27 % over 2 h, and about 8% over a ten-minute period. Therefore, 0.1 M repre- sents an upper limit in salt concentration that can be continuously nebulized by the DIN, without the effects of clogging.

Detection limits (3a) for several elements in water and in pure organic solvents are listed in Table III. These detection limits were taken under operating conditions that are typical of those used during aqueous solution nebulization (1.5 kW, 15-20 mm observation height). No at tempt was made to optimize plasma conditions or to try alternate wavelengths in order to improve the detec- tion limits in organic solvents, which are almost uni- formly an order of magnitude worse than those measured in pure water. Detection limits in organic solvents for some of these elements can be improved dramatically by using higher forward powers and by viewing much higher in the plasma. For instance, a detection limit of 10 ~g L -~ for copper was obtained in methanol with the use of 2.25 kW forward power and viewing 35 mm above the load coil.

There are several reasons why the detection limits may be poorer in organic solvents. There may be spectral overlap with background band emissions from carbon- containing molecular species produced from the organic solvents. However, Boom and Browner have tabulated the wavelengths of organic species band emission in the 200-563 nm region 2s and found only a few possible cases of spectral overlap with the elements studied in this pa- per. There may be a different distribution of aerosol particle sizes when organic solvents are nebulized (this has not yet been measured for the DIN), but one would expect to produce smaller particles (and hence better detection limits) when nebulizing organic solvents, which generally have lower viscosities and surface tensions. The overall background level does change substantially as organics are added, but the level can either decrease (with methanol) or increase (with most other solvents tested). Also, the noise in the background is sometimes better when one is nebulizing organic solvents as opposed to water, as demonstrated by the experiments on long- term precision. Thus, noise and/or increased background levels do not explain the poor detection limits in organic solvents.

Solvent loading has been indentified as a major con- tributor to plasma instability and poor detection limits. 25 Indeed, if a condenser is used to remove most of the

1694 Volume 44, Number 10, 1990

z

0 20 40 60 80 100 120

TIME (rain)

FIG. 3. Flow injection peaks for 40/~L of 1 mg L -~ zinc in a matrix of 0.1 M NaC1. One injection was made every five minutes for two hours.

solvent, some workers have reported detection limits in organic solvents that are nearly the same as those in water. 1,2,25 Other workers 6,26 who have not employed de- solvation experience approximately the same tenfold worsening of detection limits, in addition to soot buildup that might extinguish the plasma. The DIN operates at flow rates ten times less than those of conventional neb- ulizers, but it is 50-100 times as efficient at transporting the solvent to the plasma. Therefore, the DIN delivers approximately 5-10 times as much solvent to the plasma as does a conventional nebulizer, and this additional sol- vent load would likely decrease the excitation temper- ature in the ICP and diminish detection limits. However, with the DIN, solvent load on the plasma should be the same regardless of whether aqueous or organic solvents are nebulized. Calculations show that, if water and meth- anol are introduced into the plasma at the same flow rate, approximately the same amount of energy is needed to break all the chemical bonds in either solvent. Thus, the large solvent loading does not entirely explain the difference in detection limits observed in water and or- ganic solvents.

To observe whether organic solvents were cooling the ICP when using the DIN, we compared the net intensity of the atom and ion lines of copper under different con- ditions. These ratios can be used (with some assump- tions) as a qualitative probe of plasma excitation tem- perature. 25,27 As shown in Table IV, the atom line is relatively more intense in organic solvents than in water, which probably indicates a decrease in excitation tem- perature due to solvent interactions with the plasma. The evaporation factors (E) for the solvents listed are 13.1, 22.5, 47.2, and 264 ~mS/s for water, isopropanol, meth- anol, and acetone, respectivelyY 5 The factor E is related to the extent of solvent evaporation from aerosol droplets and roughly correlates with the tolerance of the ICP towards a particular solvent25--the ICP generally has less tolerance for solvents with high values of E. Table IV shows that there is little correlation between the evap- oration factor of the organic solvent and the value of the ratio, indicating a more uniform degree of solvent loading than is seen with spray-chamber type nebulizers. 25

The significance of the above is as follows: Some work- ers using spray chamber-type nebulizers state that gra-

TABLE V. Copper atom/ion intensity ratios at various forward powers (DIN)."

Power Cu(I) Cu(II) Ratio 1.25 617 13 47.5 1.50 788 23 34.3 1.75 916 35 26.2 2.00 1020 46 22.2 2.25 1143 57 20.1

Conditions: 100% methanol at 80 ~L min -1,15 mm observation height, 40 uL injection of 10 mg L -~ Cu.

dient HPLC-ICP-AES is not very feasible because of the changes in emission background that occur as the mobile phase is changed. It is likely that this is partly due to variations in the solvent loading of the plasma caused by differences in the volatility of the two solvents. Be- cause the DIN is a total consumption nebulizer, the rate at which any organic solvent is introduced into the plas- ma will remain constant, which, in turn, would impart a relatively constant degree of solvent loading and exci- tation characteristics to the plasma. Thus, gradient HPLC should be more feasible with the use of the DIN, at least when only organic solvents are employed. We have per- formed some preliminary experiments that seem to ver- ify this reasoning. As the mobile phase is gradually changed from 100% methanol to 100% isopropanol, there is very little change in the chromatographic baseline, even at a relatively high photomultplier gain of 800 V. However, if the gradient is from 100% water to 100% methanol, the baseline does change by a factor of 2 to 3. Other means of performing gradient separations with atomic emission detectors have been investigated, in- cluding the utilization of thermal gradient HPLC 28 or the use of DCPs, 4,5 which are generally more tolerant of changes in solvent composition.

Another trend seen in Table IV is that, for a given solvent, atom/ion intensity ratios generally increase with observation height. This result correlates with the trends seen in Fig. 2a and 2c, in which it was seen that ion intensities peak very low in the plasma, and atom line intensity generally levels off at higher observation heights. For clarity, the individual atom and ion line intensities have been omitted from Table IV, except for acetone; the trends for the other organic solvents are very similar. Note that, when organic solvents are being nebulized, analyte atomic emission reaches a maximum very high in the plasma. For the Meinhard nebulizer operating in water, the ratios are comparatively constant from a 10- 30 mm observation height, which is also consistent with the trends observed in Fig. 2b and 2d.

At a given observation height, the atom/ion intensity ratios decrease substantially with increases in forward power, as shown in Table V. This result is expected be- cause increasing the forward power increases the exci- tation and ionization abilities of the plasma. At the 15- ram observation height used in this experiment, the atom line was roughly twice as intense at 2.25 kW as it was at 1.25 kW, indicating that detection limits in organic sol- vents might be improved for the DIN by using higher forward powers. Note the data that Tables IV and V have in common; i.e., the intensity ratios in methanol at 1.5 kW and a 15-mm viewing height are almost identical, even though these ratios were obtained on different days

TABLE VI. Copper atom/ion intensity ratios at various liquid flow rates (DIN). a

Flow rate Cu(I) /Cu(II) Cu(I) /Cu(II) (~L min -1) in water in acetone

60 4.90 21.1 80 5.04 31.5

100 5.63 38.7 120 5.66 48.9 140 6.22 47.3

a Conditions: 10 mg L -1 Cu, 100 ~L injection, 15 m m observation height, 1.5 kW; same gain as in Tables IV and V.

and with a different DIN. Thus, the use of three signif- icant figures in these tables is justified, and it appears that our new method of DIN construction is successful in producing nebulizers that all have very similar figures of merit.

Table VI shows the variation in copper atom/ion line intensity ratios as the mobile phase flow rate is changed. One would expect solvent loading to increase as the rate of liquid introduced into the plasma is increased, and this is what is observed. It is also seen that changes in the flow rate of acetone have a greater effect on plasma excitation characteristics than do changes in the flow rate of water. Thus, it may be important to optimize liquid flow rates to the DIN when some of the more volatile organic mobile phases are used in chromatography.

In a final characterization of the DIN, matrix effects were compared in pure water and water/methanol so- lutions. Figure 4 shows the relative change in calcium ion or atom line intensity as sodium was added to the solution a t various concentrations. Because of increased organic content, one might expect matrix effects to be more severe in the methanol solution; however, there is no discernible difference seen in the extent of interfer- ence caused by concomitant sodium ions in the two so-

I - -

£ Z

Z _o CO

U J

U J _>

400

300

200

100

0

Water /

. . . . . . . . I . . . . . . . . I . . . . . . . . I ' ' •

10 100 1000 10000

400

300

200 +/ / "~'

0 . . . . . . . . i . . . . . . . . . , t . . . . . . . . 10 100 1000 10000

[] Ca(I) ~ Ca(ll) O Ca(I) + Ca(I) 10mm 10mm 20mm 20mm

SODIUM CONCENTRATION (mg L -1)

FIG. 4. Effect of sodium concentration on calcium signal in (a) water and (b) 50% methanol at two different observation heights with the use of the DIN.

APPLIED SPECTROSCOPY 1695

TABLE Vll. Comparison of signal response of three phosphorus spe- 3500 cies using the D I N vs. the Meinhard nebulizer?

DIN- DIN- Meinhard Meinhard Compound bp (°C) FIA HPLC FIA HPLC

T M P 197 +0.12 +5.1 -18 .6 -19 .9 T E P 215 +0.36 +0.46 -10 .7 -18 .0 HMPA 231 +0.30 not detm -14 .5 not detm

"The numbers listed are percent differences in peak height (FIA) or peak area (HPLC) between the indicated compound and a phosphoric acid standard.

lutions. The data shown in Fig. 4b follow a rather erratic pattern, particularly for the calcium atom line at a 10- ram observation height. This result might be at least partly explained by the fact that clogging of the DIN was more severe in the methanol solution, judging from re-measurement of emission intensities of the initial standards after the 10,000 mg L -1 NaC1 had been neb- ulized. Strangely, in both the water and methanol so- lutions, both the atom and ion line intensities increased with an increase in concomitant sodium. The cause of this behavior is unknown. Clearly, there is much more to be understood about the behavior of the DIN.

In conclusion, there is a clear difference in behavior observed when one is nebulizing water vs. organic sol- vents. The evident change in the spatial distribution of analyte atoms that occurs when organic solvents are neb- ulized could explain the poor detection limits measured low in the plasma, but it is not clear why the excitation temperature is being lowered by the organic solvents and not also by aqueous solutions. The DIN may prove ad- vantageous for theoretical studies of "organic" ICPB be- cause processes occurring in a spray chamber do not need to be considered.

DIN/ ICP Response as a Function of Chemical Species. Several reports indicate that ICP-nebulizer systems ex- hibit uniform sensitivity for different species of the same element2 -5,2s,~ However, when the species compared have widely different volatilities, there is almost always a dif- ference in the system response. This observation has been attributed to selective enrichment of the vapor phase in the more volatile components, SO that species with lower boiling points produce more signal per mole of element than do species with higher boiling points. The difference in volatility of osmium species has, in fact, been exploited to enhance the ICP sensitivity for that element, is Likewise, the detection of sulfite by HPLC- ICP-AES is reported to be enhanced by a factor of 100 by formation of a volatile sulfur species, a° The disad- vantage of nonuniform nebulizer/detector response for speciation studies is that a separate calibration curve would be needed for each species appearing in the chro- matogram. It would thus be difficult to quantitate un- known peaks or those known species for which a standard was unavailable.

Since the DIN has no spray chamber, and the com- ponents in solution are introduced into the plasma di- rectly and at high linear velocity, selective volatilization should not occur, and truly uniform peak heights or areas should be obtained for a given mass of element regardless of its chemical form. This hypothesis was tested with both types of DINs described above. To obtain the data in Table VII, we injected three phosphorus compounds

>-

09 Z UJ I-- Z

Z o

w w _>

3 0 0 0 I

2500

2000 '

1500"

1000

(a)

• . . i . . . . i . . . . i . . . . i . . . . 0.18

- + - ACID

- ~ - TEP

- o - HMPA

• " o " TMP

0.20 0.21 0.23 0.24 0.26

3500

3000

2500

2000

(b)

1000

0.55 0.59 0.63 0.67 0.71 0.75

- + - A C I D

- 4 - TEP

- o - HMPA

""C3"" TMP

NEBULIZER GAS FLOW RATE (L min -1)

FIG. 5. Response to four phosphorus species (each 10 mg L -1 as P) as a function of nebulizer gas flow rate for the (a) DIN and (b) Meinhard nebulizer.

(200 #L of 10 mg L -1 as elemental P) of moderately different volatilities into the DIN and Meinhard nebu- lizers. The compounds were triethyl phosphate (TEP), trimethyl phosphate (TMP), and hexamethyl phosphor- amide (HMPA). Each compound was injected three times to check the peak height reproducibility for each species. The mean peak height was then compared to a phos- phoric acid standard; the percent differences are listed in the table. Whereas peak heights were clearly different with the Meinhard nebulizer (e.g., T M P differed from phosphoric acid by - 1 9 % ) , they were nearly the same for all four compounds with the DIN. The maximum difference found was between phosphoric acid and TEP, amounting to only 0.4 % difference. This experiment was performed with the commercial (tapered outer capillary) DIN.

Figure 5a shows that this response for phosphorus was uniform regardless of the nebulizer gas flow rate, which is the most critical parameter affecting DIN performance characteristics. The data shown in Fig. 5 represent net response--generally, when either type of DIN was used, the background did not depend upon gas flow rate (with- in a factor of two), unless the DIN was operating at a flow rate near the minimum necessary for nebulization. In contrast, the Meinhard nebulizer exhibited the most difference in the response to the four phosphorus com- pounds at the same nebulizer gas flow rate that gave optimal sensitivity for phosphorus (Fig. 5b). Note that the most volatile compound, TMP, produced the least signal with the Meinhard. This indicates that other fac- tors besides boiling point may affect the response of the nebulizer/detector, u,25

These same phosphorus compounds were separated by HPLC, and the peak a r e a s of each peak were compared (Table VII). For HPLC, peak areas were used rather than peak heights because the late-eluting T E P peak was con-

1696 Volume 44, Number 10, 1990

12 ' OSMIUM (VIII) OSMIUM (VIII)

IN WATER IN 1 M NaOH (a) 9-

o

N -- 3' z I- _z 0 z 0 0 300 600 900

¢n 12 OSMIUM (VIII)

w IN WATER w (b) > 9-

i~ OSMIUM (VIII)

- 25ZJ 3-

.

0" / I

0 200 600 400

TIME (s)

FIG. 6. Replicate flow injection peaks for two different osmium species: (a) 100 #L of 1 mg L -1 osmium (VIII) with the use of the DIN; (b) 100 ~L of 20 mg L -1 osmium(VIII) with the use of the Meinhard.

siderably broader and shorter than the HMPA and TMP peaks. For the Meinhard nebulizer, peak areas differed by as much as 20 % from the phosphoric acid standard whereas, when the DIN was used, the areas differed by no more than 5 %, even when gradient elution was em- ployed (not shown).

A similar experiment was carried out with two different chemical forms of Os(VIII), one of which is considerably more volatile than the other. The volatile form (OsO4, boiling point of the pure solid = 130°C) was made by diluting a freshly prepared aqueous stock solution of osmium tetroxide (approximately 5200 mg L -1 in Os) with pure water. The nonvolatile HOsOs- was obtained by adding the same volume of stock solution to a volu- metric flask and diluting with 1 M NaOH. 13 Figure 6 compares the response of the DIN and Meinhard neb- ulizers to these two osmium solutions. The difference in performance between the two nebulizers was more ob- vious than it was during the phosphorus study. The vol- atile Os04 produced about four times more signal than the nonvolatile form when the Meinhard nebulizer was used (Fig. 6b), whereas the DIN exhibited virtually no difference (Fig. 6a). For the Meinhard nebulizer, the dif- ference in signal between both forms of osmium was found to depend on the concentration of NaOH and the concentration of osmium. As the concentration of NaOH in the HOs05- solution became more dilute, the signal obtained from the solution became more intense, indi- cating a greater equilibrium concentration of volatile 0s04 present in the same solution. Figure 7 shows the differ- ence in sensitivity (i.e., slope of the calibration curve) between osmium species that results when the Meinhard is used. Only one calibration curve would be necessary to quantitate any form of osmium with the DIN.

When determining sulfur in organic matrices, Elle- bracht e t a l . 12 found that different sulfur compounds in one solvent yielded substantially different sensitivities

~->" 2000 ~ O *

z uJ 1500' SMIUM (VIII) I -- _z I ~ i N WATER -- OSMIUM (VIII) z O IN 1 M NaOH co 1000 '

I11

W

-> 500, _ J u l

0 0 1 2 3 4 5 6

OSMIUM CONCENTRATION (mg L -1)

FIG. 7. Calibration curves for osmium species with the use of the Meinhard nebulizer.

for sulfur when using a DCP with a conventional neb- ulizer. Also, the same sulfur compound dissolved in dif- ferent solvents yielded different sensitivities. Fabec and Ruschak, 26 using an ICP, noted enhanced emission in- tensities from volatile sulfur compounds, particularly mercaptans, dissolved in organic solvents. Thus, the presence of mercaptans in crude oils could significantly affect the accuracy of total sulfur determinations made by atomic emission spectroscopy.

The DIN-ICP-AES response factors of six sulfur-con- taining compounds dissolved in o-xylene are given in Table VIII. These compounds include a mercaptan and some sulfur species present in crude oily s Within the precision of this experiment, all six compounds produced equivalent responses in the detector, even though the volatilities of the solutes were substantially different. The modified DIN nebulized 100% o-xylene for over three hours in this experiment, and the long-term pre- cision was reasonable--RSDs of less than 10 % in peak height were obtained for every compound except di- methyl sulfoxide, which for some unknown reason yield- ed abnormally broad flow injection peaks. Detection lim- its for sulfur were poor because the torch box was not purged with argon. Thus, variations in the extent of ox- ygen absorption of the sulfur emission over a three-hour period might have affected the long-term precision in this experiment.

CONCLUSIONS

The modified DIN has characteristics similar to the DIN originally reported in the literature, but is easier to build and repair, which should invite more researchers to try this method of sample introduction for HPLC or FIA-ICP. A major advantage of the DIN is its ability to nebulize 100% organic solvents for long periods of time. Because the DIN has no spray chamber, dead volume is diminished and absolute detection limits for microliter sample volumes are the same or better than those for most other nebulizers having spray chambers. 21 The lack of a spray chamber also ensures uniform response of the detector/nebulizer to different species of the same ele- ment, even when the species have greatly different boil- ing points. Gradient HPLC should be more straightfor- ward with the DIN because solvent loading is held

APPLIED SPECTROSCOPY 1697

TABLE VIII. DIN-ICP response factors for some sulfur species dis- solved in o-xylene?

% of thiophene Compound bp (°C) response b

Thiophene 84 100 (± 9) Benzenethiol 169 90 (_+ 8) Dimethylsulfoxide 189 115 (± 16) Thianaphthene 221 88 (_+ 5) Dibenzothiophene 332 91 (±6) 1,2-benzodiphenyl sulfide large 90 (±8)

a Conditions: 40 ~L of 1000 mg L -1 (as S) standards were injected into 100% o-xylene. Peak height responses were normalized to the con- centration of the thiophene standard. Flow rate: 80 #L rain -~. Wave- length: 182.6 nm.

b Uncertainties listed are the standard deviations of peak heights from 6-9 injections over a 3-h period.

relatively constant as the mobile phase is changed. A major disadvantage of the DIN is the extent of plasma cooling that occurs while organic solvents are being neb- ulized. This problem degrades detection limits for most of the elements by an order of magnitude, and it is not possible to reduce the organic solvent load by adding a condenser or by modifying a spray chamber. Long-term precision is a problem when one is nebulizing solutions with salt concentrations near 0.1 M. Better performance might be attained by using higher-power ICPs. Also, the DIN might be suitably adapted to HPLC-ICP-MS be- cause molecular or atomic ions from organic solvents normally do not interfere with analyte ions, particularly high-mass analytes, and detection limits are an order of magnitude better than for HPLC-ICP-AES. s,1°

ACKNOWLEDGMENTS

This project was funded in part by a grant from Research Corpo- ration. We also thank the Indiana Academy of Science, the Ball State Office of Research and Supported Programs, and the Ball State De- partment of Chemistry for their financial support. We acknowledge the following undergraduate students, who have helped in various as- pects of this project: Darren Bowman, Christy Campbell, Stephanie Chesney, Jeffrey Stadick, and Esther Surface. Discussions with Kim- berly LaFreniere and Art D'Silva concerning the nebulizer were very helpful.

1. D. W. Hausler and L. T. Taylor, Anal. Chem. 53, 1223 (1981). 2. D. W. Hausler and L. T. Taylor, Anal. Chem. 53, 1227 (1981). 3. D. R. Heine, M. B. Denton, and T. D. Schlabach, J. Chromatogr.

Sci. 23, 454 (1985). 4. W. R. Biggs, J. C. Fetzer, and R. J. Brown, Anal. Chem. 59, 2798

(1987). 5. W. R. Biggs, J. T. Gano, and R. J. Brown, Anal. Chem. 56, 2653

(1984). 6. M. Ibrahim, W. Nisamaneepong, and J. Caruso, J. Chromatogr.

Sci. 23, 144 (1985). 7. H. Suyani, D. Heitkemper, J. Creed, and J. Caruso, Appl. Spectrosc.

43, 962 (1989). 8. J. J. Thompson and R. S. Houk, Anal. Chem. 58, 2541 (1986). 9. W. A. J. De Waal, F. J. M. J. Maessen, and J. C. Kraak, J. Chromat.

407, 253 (1987). 10. D. Beauchemin, M. E. Bednas, S. S. Berman, J. W. McLaren, K.

W. M. Siu, and R. E. Sturgeon, Anal. Chem. 60, 2209 (1988). 11. R. F. Browner and A. W. Boom, Anal. Chem. 56, 786A (1984). 12. S. R. Ellebracht, C. M. Fairless, and S. E. Manahan, Anal. Chem.

50, 1649 (1978). 13. K.D. Summerhays, P. J. Lamothe, and T. L. Fries, Appl. Spectrosc.

37, 25 (1983). 14. K. Jinno, S. Nakanishi, and C. Fujimoto, Anal. Chem. 57, 2229

(1985). 15. J. A. Koropchak and D. H. Winn, Anal. Chem. 58, 2561 (1986). 16. S. B. Roychowdhury and J. K. Koropchak, Anal. Chem. 62, 484

(1990). 17. D. J. Mazzo, W. G. Elliott, P. C. Uden, and R. M. Barnes, Appl.

Spectrosc. 38, 585 (1984). 18. K. Biickstr5m and A. Gustavsson, Spectrochim. Acta 44B, 1041

(1989). 19. A. Gustavsson, Spectrochim. Acta 42B, 111 (1987). 20. K. E. Lawrence, G. W. Rice, and V. A. Fassel, Anal. Chem. 56, 289

(1984). 21. K. E. LaFreniere, G. W. Rice, and V. A. Fassel, Spectrochim. Acta

40B, 1495 (1985). 22. K. E. LaFreniere, V. A. Fassel, and D. E. Eckels, Anal. Chem. 59,

879 (1987). 23. T. W. Avery, M.S. Thesis, Ball State University, Muncie, Indiana

(1988). 24. C. Chakrabarti, M.S. Thesis, Ball State University, Muncie, In-

diana (1990). 25. A. W. Boorn and R. F. Browner, Anal. Chem. 54, 1402 (1982). 26. J. L. Fabec and M. L. Ruschak, Anal. Chem. 57, 1853 (1985). 27. J. M. Mermet, Spectrochim. Acta 44B, 1109 (1989). 28. W. R. Biggs and J. C. Fetzer, Anal. Chem. 61, 236 (1989). 29. M. Morita and T. Uehiro, Anal. Chem. 53, 1997 (1981). 30. D. R. Migneault, Anal. Chem. 61, 272 (1989).

1698 Volume 44, Number 10, 1990