Characterization of a recycling nebulization system for inductively coupled plasma spectrometry—II. Matrix and memory effects

Sprefrochimica Aca. Vol. 418, No. II, pp. 1151-1155.1986. Riotedin Great Britain.

0

0584-8547/8653.00+ 0.00 1986 Pergamon Journals Ltd.

Characterization of a recycling nebulization system for inductively coupled plasma spectrometry-II. Matrix and memory effects CHEN ZICAI* and RAMON M. BARNES Department of Chemistry, GRC Towers, University of Massachusetts, Amherst, MA Olob3-0035, U.S.A. (Received 26 February 1986; in revised fom

9 June 1986)

Abstract-The matrix and memory effects in a recycling nebulization system for ICP spectrometry were investigated. The emission signals from Cu and Mn in the presence of different concentrations ofaluminium, sodium and magnesium chlorides were compared during long-term (40 min) nebulization. The experimental results indicate a complex matrix effect which may depend upon analyte concentration enhancement and altered nebulixation efficiencyor clogging. With water presaturatedcarriergas, the salt effects were minimized, and the emission signals of Cu and Mn were stable (l-2% RSD) during long-term nebulization in the presence of different matrices.

1. I~rR00oc~10N RECENTLY,four different recycling nebulization systems for atomic spectroscopy were designed [l-4]. One of them, a fixed cross flow nebulizer, was used for atomic absorption analysis [ 13. Other designs incorporating concentric nebulizers were applied with inductively coupled plasma atomic emission spectometry (ICP-AES). The nebulizer system described by HE and BARNES[3] has become commercially available. This recycling nebulization system provided good powers of detection and low sample solution consumption rate. For example, with one ml of sample, the solution could be nebulized for about an hour. Recently, an enhancement effect contributed by the evaporation of large aerosol droplets was demonstrated in the recycling nebulization system that could be minimized by presaturating the argon carrier gas with water [S]. HULMSTON and MCKILLOP [6] also observed an increase in the emission signal in the recirculating nebulizer, resulting from gradual evaporation of water from the sample and increased analyte concentration in the solution. The aim of this study is to examine some matrix and memory effects in a recycling nebulization system. 2. EXPERIMENTAL The apparatus and instrumentation are the same as reported earlier in the study of stability of the recycling nebulization system [S]. A recycling nebulization system manufactured by J. E. Meinhard Associates (Model RS-30X1) and an argon humidification chamber [S] made locally were attached in series to the ICP torch with about 30 cm of Tygon tubing. Wavelengths measured were Mn II 257.610 nm, Cu I 324.754 nm, Fe II 259.940 nm and Na I 330.237 nm. The test solution contained different matrices while the Mn and Cu concentrations were 1 pg/ml. Analytical grade aluminum, magnesium, and sodium chlorides were used to prepare the test solutions with major and minor ion ratios from 100 to 1 and 2000 to 1.

*On leave from Xinjiang Institute of Chemistry, Academia Sinica, Urumqi, Xinjiang, People’s Republic of China.

J. W. NOVOK, JR, D. E. LILLIE, A. W. BOURN and R. F. BROWNER,Anal. Chem. 52, 576 (1980). [2] P. HULMSTON, Analyst 108, 166 (1983). [3] Z. Z. HE and R. M. BARNES,Spectrochim. Acta 4OB, 11 (1985). [4] Z. Z. HE, S. K. CHAN and A. MONTASER, ICP Information News/et?. ll(ll), 23 (1985). [S] CHEN ZICAI and R. M. BARNES, Spectrochim. Acta 41B, 979 (1986). [6] P. HULMSTONand S. MCKILLOP, Analyst 110, 559 (1985). [l]

1151

1152

CHEN

ZICAI and RAMON M.

BARNES

3. EXPERIMENTALRESULTSAND DISCUSSION 3.1. Matrix e$ect 3.1.1. Copper emission signals. Copper (1 pug/ml)in different matrices was nebulized for 40 min while the emission signals were measured (Fig. 1). When distilled, deionized water was used, the emission signal continuously increased during nebulization because of the enhancement of solution concentration owing to the evaporation of water from large aerosol droplets in the recycling nebulization system chamber that do not pass into the ICP [S]. With a 100 pg/ml Na solution the Cu emission signal during the 40-min nebulization was more uniform, especially during the initial 13 min, although an overall signal increase of about 9 % occurred. When the concentration of Na was increased to 500 fig/ml, the long-term stability of Cu emission signals improved and only a slight signal drift of about 1 y0 was observed throughout the measurement period. Over the 40 min period, the relative standard deviations of the signals were 2.7 % for 100 pg Na/ml, 0.6 % for 500 pg Na/ml and 6.8 % for 2000 pg Na/ml. To explain the stabilization of the Cu signal in the presence of 100-500 pg Na/ml requires that one or more process be occurring. Some of the possibilities include (a) the amount of water vapor contributed per minute by large aerosol droplets that do not leave the nebulizer or by evaporation from the walls described in Ref. [S] is reduced by a Raoult’s law effect compared to the analyte solution without sodium, and (b) a shift in the sample uptake, nebulizer efficiency, aerosol size distribution [7], or other unidentified phenomenon occurs to offset the effect of added water vapor from large droplet evaporation. The net result of these possible effects is that the Cu emission signal becomes nearly stable during 40 min of nebulization in a 500 pg Na/ml solution. When the Na concentration was increased to 2000 pg/ml, the Cu emission signals became irregular and generally decreased by about 10-15 % over 40 min. This decreased signal could be caused by a change in the size distribution of the aerosol [7] or salt blockage in the tip of

0.4

I 0

I 8.00

I

I

I

1603

2400

32.00

Time

I 4000

(min)

Fig. 1. Copper(1 pg/ml)emission signals in (a) distilled, deionized water ( x ), (b) 100 pg/ml Na (o), (c) 500 pg/ml Na (0) and (d) 2000 pg/ml Na (0).

[7] R. K.

SKOGERBOE

and S. J. FREELAND, Appl. Spectrosc. 39,925 (1985).

Recycling nebulization system for ICP

1153

the concentric nebulizer in the presence of high sodium concentrations. The latter would cause the uptake rate of nebulizer to decrease, so that the emission signals would also decrease during long-term nebulization. Similar long-term drifts were observed with concentric nebulizers and salt rich solution [8,9]. The erratic nature of the drift suggests a physical effect like nebulizer fouling. 3.1.2. Sodium emission signals. To evaluate the effect of nebulizer changes with high salt concentration different solution concentrations of NaCl (100, 500 and 1000 pg/ml) were nebulized separately. The Na I 330.2 nm signals are shown in Fig. 2. Like the Cu signal in 100 pg Na/ml, the 100 pg/ml Na-signal over time increased slightly at essentially identical rates. In contrast, the Na emission at high concentrations decreased in time. Both the 500 and 1000 pg/ml signal depressions occur at approximately the same rate during the first 36 min as did the Cu signal in the 2000 pg Na/ml solution. Since a shift in the size distribution of sodium contained in the aqueous aerosol is not expected [7], a simpler nebulizer phenomenon was sought to rationalize these observations. Although the nebulizer employed was fabricated especially for high-salt containing solutions, signals from the 1000 pg/ml Na solution during 40 min suggest that a partial blockage of nebulizer tip may have occurred. The experiment with 500 pg/ml Na followed the experiment with 1000 pg Na/ml. The decreasing emission signals of the former could be explained by a residual blocking effect of 1000 pg/ml Na experiment. The initial Na intensities also did not increase linearly with the sodium concentration. 3.1.3. Manganese emission signals. When 1 pg/ml of Mn solutions with different matrices (1000 pg/ml Na, 1000 pg/ml and 400 pg/ml Al, and 2000 pg/ml Na) were nebulized separately for 40 min, the Mn II 257.6~nm emission signals were lower in magnitude than in the distilled, deionized water (Fig. 3). An enhancement with time of Mn emission signals from 1 pg/ml Mn in only deionized distilled water was expected [S], and an upward drift of about 18 % was observed. The decrease in Mn signal with time in the 1000 pg Na/ml (about 4 %) and the 2000 pg Na/ml (about 10 %) is consistent with the previous Cu and Na results. The rate of signal drop, however, is not as severe as observed for Cu. The decreased signal in the presence of 1000 pg/ml and 2000 pg/ml Na also may be due to partial blockage of the tip of nebulizer. However, the reason for the increased Mn signal drift by about 13 % in the 1000 &ml Na with 4OO&ml Al matrix is not clear. Additional experiments are needed to evaluate this observation.

0

800

1600

24.00

3200

I

4oLx,

Time (min)

Fig. 2. Sodium emission signals from 100 (o), 500 (+) and 1000 ( x ) pg/ml Na. [8] J. E. MEINHARD,Applications of Plasma EmissionSpectrochemistry (Ed. R. M. BARNES),p. 1. Heyden, Philadelphia (1979). [9] J. 0. BURMAN,Applications of Plasma Emission Spectrochemistry (Ed. R. M. BARNES),p. 15. Heyden, Philadelphia (1979).

&EN ZICAI and RAMON M. BARNES

1154 0.8 ,

02

0

I

I 8.00

I 16.co Time

I 24cx)

I Y!xJ

I 4000

(min)

Fig. 3. Emission signals from (1 pg/ml) Mn with different matrices, (distilled water (A), 1000 fig/ml Na (o), 2000 pg/ml Na ( x ), loo0 pg/ml Na and 400 pg/ml Al (+).

3.2. Water vapor presaturated argon

The carrier gas was pre~turat~ with water vapor before it entered the recycling nebulization system [S), which resulted in stabilized analyte signals for pure Cu, Fe and Mn solution over approximately 40 min. The stability of emission signals of 1 pg/ml Cu with different matrices also improved significantly (Fig. 4). The effect of evaporation of large aerosol droplets was eliminated and the nebulizer tip did not block, resulting in a constant uptake rate, Similar improvements were observed by BUR~WANwith a conventional concentric nebulizer and high salt con~ning solutions Ip]. The relative standard deviations of the Cu emission ranged between 1.09 and 1.86%. 3.3. Memory e#iect After 1 ml solution of 1 pg/ml Fe was nebulized for 60 min, the lower part of recycling nebulization system was washed using distilled, deionized water, and the nebulizer was operated for a few seconds. The procedure was repeated twice. Then 1 ml of fresh distills, deionized water was nebulized. The Fe 250.9~nm emission signals were measured during 2 successive 30-s periods are shown in Fig. 5. The increasing signal during the second 30-s period indicated that some memory effect persisted when the residual aerosol droplets that had condensed on the upper part of the chamber wall and the connecting tube between the

08

06 0

800

I600 Time

24a

3200

4000

bin)

Fig. 4. Emission signals of 1 &ml Cu with different matrix when thecarriergas was pm&mated with water vapor. (a) Na at 100 pg/ml (o), 500 gg/ml ( x ), 1000 ng/ml (i-) and 2000 pgglml (Ah (b) Na at 2ooOfig/ml in water (o), 200 pgcglmlMg ( x ) and 400 pg/mi Al (+). The mean values and signal precision (as RSD in %) were 100 pg Na/ml 0.795 nA (1.3.5x), 500 pg Na/ml 0.773 nA (1.86x), 1000 ng Na/ml 0.800 nA (1.07 %), 2000 pg Na/ml 0.799 nA (1.68 %), 2000 pg Na/ml and 200 pg Mg/mlO.804 nA (1.32x), and 2000 fig Na/ml and 400 pg Al/ml 0.864 nA (1.41%).

1155

Recycling nebulization system for ICP 03 0-30s

Fe

I ”

6.00

I 12.00

I

I

I600

24.00

moo

Time Is.1

Fig. 5. Memory effect of Fe in recycling nebulization system during the first (0) dnd second ( x ) 30-s washing periods, and the ICP background (+) when deionized distilled water was nebulized. The RSD value of background signal is 0.46 %.

chamber and the torch were washed into the water at the bottom of the system. After the lower part of the chamber was washed again, the signals of deionized distilled water returned to the background level and became stable during a 30 s nebulization period. To minimize memory effects, the length of the connecting tubing to the ICP should be as short as possible, and the tubing as well as the nebulizer should be rinsed between samples.

4. CONCLUSION The experimental results indicate that a matrix effect in the recycling nebulization system exists. However, when the argon carrier gas was presaturated with water vapor, the matrix effects were minimized, and long-term signal stability of l-2 % RSD could be achieved. By washing the chamber and nebulizing distilled, deionized water alternatively twice, the memory effect could be reduced. Without water saturated argon carrier, the tip of the concentric nebulizer apparently could become fouled when the concentration of the salt matrix is too high, although no salt deposit was observed. The improved copper and manganese signal stability in the presence of sodium compared to distilled water may have resulted from a reduction of the evaporation of water from large aerosol droplets in the chamber that, in turn, lowered the enrichment of analyte in the recyled solution [S]. Further signal suppression could result from decreased sampling rates or nebulizer efficiency, fouling of the nebulizer, change in the distribution of analyte contained in the aqueous aerosol, or alteration of the spatial distribution of ICP signals. Additional studies are needed to identify exactly the sources of deviation observed for the recycling nebulizer without water saturated argon carrier gas. By considering all of the experimental results in this and a companion paper [S], as well as the advantages of low sample consumption rate and good detection limit [3], the recycling nebulization system appears to be useful for atomic spectroscopy when the sample solution volume is limited or the collection of analyte signals extends over long periods. Acknowledgement-This

research was supported by the ICP Information Newsletter.