Air-argon inductively coupled plasma for organic solution analysis: spectral characteristics and analytical performances

S ectrochrmico Acta Vol. 47B. P&d m Great B&h

No

12, pp

1353-1360.

05868547/92 $5.00 + 00 Per,qmon Press Ltd

1992

Air-argon inductively caupled plasma for organic solution analysis: spectral characteristics and analytical performances Y. Q. TANG,* Y. P. Du,t

J: C. SHAO, C. LIU, W. TAO and M. H. ZHU

Analysis and Research Centre, East China Institute of Chemical Technology, Shanghai 200237, People’s Republic of China (Received 31 January

1992; accepted 9 June 1992)

low power air-argon inductively coupled plasma (ICP) obtained on a commercial 40.68 MHz ICP spectrometer has been used for organic solution analysis. Spectroscopic studies show that in air-argon ICP, the molecular bands from hydrocarbons of organic samples such as CN and C2 are largely depressed or even eliminated. The effects of operating conditions such as air ratio and ICP power on the signal-tobackground ratio of atomic and ionic lines for organic solution in air-argon ICP are similar to those for aqueous solution. Detection limits for some elements in methyl-isobutyl ketone (MIBK) solution have also been measured and compared with those obtained by argon ICP. For most atomic lines and some ionic lines, the detection limits in air-argon ICP are better than those in pure argon ICP. A 50% air-argon ICP has been applied to the determination of metallic elements in waste oil diluted with xylene and in nickel naphthenate diluted with MIBK. The analytical results are satisfactory. Abstract-A

1. INTR~DuC~~N IN RECENTyears, many authors have shown interest in the study and application of low power, molecular gas inductively coupled plasmas (ICP) or mixed gas ICPs [l-17]. MEYERand BARNESdescribed the operation of air and nitrogen ICPs as well as their uses in spectrochemical analysis of aqueous solutions and fine powder [5]. MEYER and THOMPSON have also measured the detection limits for 19 elements in air and oxygen ICPs [6]. CHOOT and HORLICK systematically studied the spectral characteristics and analytical performances of Ar-N2, Ar-02, Ar-air and Ar-He mixed gas ICP [&ll]. The developments of analytical molecular gas and mixed gas ICPs were well reviewed by MONTASER[15] and 0~~s and SOMMER[14]. However, although most molecular gas ICPs were used for aqueous solution analysis, only few [7, 131 were applied to organic solutions. On the other hand, direct analysis of organic solutions is generally carried out using argon ICPs [l&26]. The introduction of oxygen into the injection gas was suggested [24-261 to avoid carbon deposition on the injector tip and some spectral interferences. Considering the natural presence of oxygen in air-argon ICP, we assume that the air-argon ICP should give good performance for organic solution analysis. In this paper, the spectral characteristics and analytical performances of an air-argon ICP loaded with organic solutions are studied. The application of 50% air-argon ICP to the analysis of waste oil diluted with xylene and nickel naphthenate diluted with methyl-isobutyl ketone (MIBK) is also briefly described.

2. EXPERIMENTAL The experiments were performed with a Baird PS/6 Spectrovac ICP system which was equipped with a l-m polychromator and a 0.5-m Czerny-Turner monochromator. The plasma generator operated at a frequency of 40.68 MHz with a maximum power of 1.5 kW. A conventional Fassel torch was used. A Meinhard nebulizer. a double pass spray chamber and a Gilson peristaltic pump were used for sample introduction. Air was supplied by a Hitachi air compressor through a T-joint and was mixed with argon in a buffer bottle before being sent to the plasma. An argon * Author to whom correspondence should be addressed. t Present address: Department of Chemical Engineering, Qiqihaer Institute of Light Industry, Qiqihaer 161006, People’s Republic of China. 1353

1354

Y. Q.

Table 1. Instrumental

TANG

parameters

et al.

and operating conditions

ICP: frequency 40.68 MHz; incident power 1.2-1.4 kW; reflected power < 5 W; outer gas Row 15 I min-‘; air ratio in outer gas O-NO%; intermediate gas Row 1-2 I min-‘; nebulizer gas flow 0.6 1 min-‘; observation height lo-14 mm; sample uptake rate 0.7 ml min-I. Monochromator: 0.5-m Czerny-Turner

Polychromator: l-m Pachen-Runge

type; 1800 grooves mm-‘; reciprocal linear dispersion 0.55 nm mm-’ (2nd); entrance slit 15 pm; exit slit 20 km.

type; 1440 grooves mm-‘; reciprocal linear dispersion 0.664 nm mm-’ (1st); entrance slit 50 km; exit slit 80 km; 40 channels.

ICP was first obtained under normal operating conditions. An air flow was then gradually introduced into the outer gas and the impedance of the matching box was adjusted to obtain minimum reflected power (< 5 W). The ICP operating conditions and the optical parameters are listed in Table 1. Although a 100% air sustained ICP could be obtained, most of the studies were conducted with a 50% air-argon ICP due to the limited power of our ICP generator. The spectral studies were mainly carried out with a monochromator while the analytical applications

CN

(b) CN CN

(d) / “1 L-iw..-__i I 3

350

366

374

362

0

khn)

Fig. 1. Spectra between 350 and 390 nm in ICPs with different air ratio. (a): 0% (1.2 kW); (b): 10% (1.2 kW); (c): 50% (1.2 kW); (d): 100% (1.4 kW).

Organic solution analysis by air-argon ICP

13.55

(b)

CN

440

460

460

500

520

A (nm)

Fig. 2. Spectra between 420 and 520 nm in ICPs with different air ratio. (a): 0% (1.2 kW); (b): 10% (1.2 kW); (c): 50% (1.2 kW); (d): 100% (1.4 kW).

were performed with a polychromator. All measurements were controlled by an IBM PY2 computer with the software PLASMACOMP IV. Analytical-reagent grade ethanol, MIBK and xylene were used as the solvents for preparing both standards and sample solutions. The stock standard solutions were obtained by dissolving analytical-reagent grade inorganic salts in ethanol. They were diluted to a suitable concentration with MIBK or with xylene before each experiment. The organic samples were directly diluted with MIBK or with xylene to get 0.2% (m/v) solutions. The ratio of ethanol in both standards and sample solutions was always 15% to achieve matrix matching. In the case of waste oil analysis, a conventional method for transferring the waste oil sample into aqueous solution [27] was also used. A suitable amount of waste oil was weighed and put in a porcelain crucible, concentrated H2S04 was slowly added and mild heating was applied to carbonize the oil completely. The crucible was then heated at 525°C in a furnace for about 3 h and was cooled to room temperature. A volume of 0.5 ml 1:l HCl and 0.5 ml HN03 were added to achieve complete dissolution. An aqueous solution was then obtained by diluting with deionized water. This solution was analysed by the argon ICP to get reference data for the trace elements in waste oil.

3. RESULTS AND DISCUSSION

When an MIBK solution was fed to an argon ICP, the plasma expanded and the central channel became very luminous due to the emission of carbon bands. Additionally,

1356

Y. Q. TANG etal. 230000

T; 0 'r

2000 27000

(b)

I

(d)

I

'; 9 h

1000 12000

3 0 4

500 1300

'; 9 %

0 452 X (nm)

Fig. 3. Interferences

of molecular bands to Ba II 455.403 nm in ICPs with different air ratio. (a): 0%; (b): 10%; (c): 20%; (d): 50%.

the thermal emission of graphite particles produced continuous background. In comparison, an air-argon ICP with MIBK was smaller in volume. It was also less luminous because carbon was oxidized. Moreover, the carbon deposition, often occurring in the case of the argon ICP, disappeared in the air-argon ICP. By scanning with the monochromator we have investigated some molecular bands under various operating conditions. Figure 1 shows the spectra in the region 350-390 nm. In the argon ICP, the CN bands caused by an organic solution are very weak. When a small amount of air is introduced into the outer gas, the presence of nitrogen greatly increases the intensity of the CN bands. In a 10% air-argon ICP, the CN bands are about 300 times more intense than those in an argon ICP. However, when the air/argon ratio becomes higher, the intensity of the CN bands declines sharply. In a 50% air-argon ICP, the CN bands are just at a level slightly above that found in an argon ICP (Fig. l(c)). This phenomenon can be explained by the oxidation of CN molecules

1357

Organic solution analysis by air-argon ICP Table 2. Spectral

interferences in 50% air-argon loaded with MIBK Interference

Line (nm)

Ag 1 & 1 Al I Al I Ba II Ca II Ca II co II co I cu II cu I Fe II Fe I Mg II Mg II Mg I Mg I Na I Ni II Ni I Ni I Sr II Sr II Zn I Zn I

328.068 520.9cn 309.271 396.152 455.403 393.366 396.847 238.892 345.351 224.700 324.754 238.204 438.355 279.553 280.270 285.213 518.362 588.995 221.647 232.003 352.454 407.771 421.552 213.856 481.050

ICP

3.8 6.0 4.0 3.1 7.9 9.3 9.2 13.5 4.0 16.0 3.8 13.1 4.3 12.1 12.1 4.3 5.1 2.1 14.3 5.3 3.5 8.7 8.6 5.8 6.7

no yes serious (NJ no no no yes ;O) serioi’(NO) yes FO) no no no no no seriot’(NO) no no no yes no no

owing to the large amount of oxygen in air. In a pure air ICP, the bands completely disappear in the background. In the spectral range 430-520 nm (Fig. 2) the introduction of MIBK produces very intense Cz molecular bands in an argon ICP. When 10% air is introduced, the C, bands disappear but the CN bands become very strong. The formation of CN molecules and the oxidation of carbon in an air-argon ICP decrease the number of Cz molecules. The CN bands also disappear when a higher air/argon ratio is applied. The spectral interferences of molecular bands C, and CN to Ba II 455.403 nm are shown in Fig. 3. In the argon ICP, the molecular bands are so strong that the Ba II 455.403 nm line is totally buried in C2 and CN emissions. With an increase of the air/argon ratio in the outer gas, all molecular bands decrease and the line gradually appears. When the air content attains 50%, the spectral interferences of the molecular band on the line can be ignored. The interferences of molecular bands on 27 prominent analytical lines of 12 elements for an MIBK solution in a 50% air-argon ICP are listed in Table 2. Most lines are free from interference except those coincident with N2 and NO bands. The influences of the operating parameters such as incident power, observation height and air/argon ratio on the signal-to-background ratio (SBR) of analytical lines have been studied. The results are generally similar to those obtained with an aqueous solution in an air ICP [12] or an N,-argon ICP [2]. Under optimum operating conditions, we have determined the detection limits (3 a)* of 12 analytical lines in an MIBK solution with a 50% air-argon ICP and compared * These detection limits were obtained using the operating software provided to the ICP spectrometer by the Baird Corp. According to Refs [28] and [29], the detection limits derived from SBR and relative standard deviation of the background (RSDB) are more representative and will provide far more information for the readers. Our further work will use this new approach.

Y. Q. TANG et al.

1358

Table 3. Detection limits in MIBK solution (ng ml-*) 13 ml Line (nm)

50% air ICP

328.068 455.403 493.409 393.366 396.847 345.351 324.754 438.355 279.553 280.270 285.213 213.856

Ag I Ba II Ba II Ca II Ca II co I cu I Fe I Mg II Mg II Mg I Zn I

2.9 0.5 0.3 0.3 5.1 12.6 1.3 16.4 11 2.8 2.6 7.6

Argon ICP 7.5 2.2 2.3 5.5 9.4 38 3.8 122 0.2 1.6 5.7 15.5

them with those obtained with our argon ICP of similar power (Table 3). Most detection limits obtained with the 50% air-argon ICP are better than those obtained with a pure argon ICP except for the results for the two ionic lines Mg II 279.553 nm and Mg II 280.270 nm, which have relatively high excitation energy. Obviously, the improvement of the detection limits for some lines in the air-argon ICP is due to the reduction of background from molecular bands emission. We did not compare our results with other works because different authors had different operating conditions and ICPs; therefore the results could be quite different. On the basis of the above studies, the 50% air-argon ICP has been applied to the analysis of an MIBK solution of nickel naphthenate and xylene solution of waste oil. Through careful examination, a number of analytical lines (available on the polychromator) free from interferences were chosen. They were Mg II 279.551 nm, Fe II 259.936 nm, Al II 308.21 nm, Ca II 317. 993 nm and Cu I 324.754 nm. A set of compromise working conditions (1.2 kW, 6 mm) was also established for the analysis. Since the solutions were made by directly diluting the samples with ethanol and the corresponding solvents, the effect of ethanol on line intensities was studied (Fig. 4). When the ethanol/solvent ratio is between 10 and 20%, the variation in line intensity is negligible. Therefore, the amount of ethanol in the standard and sample solutions was fixed at 15%. Under the compromise conditions, the dynamic range of the calibration curves was over four orders of magnitude. Four nickel naphthenate samples and one waste oil I (ad.) 800 600

. -

400

,~ .

200

0

5 Ethanol

15

10 ratio

20

(%)

Fig. 4. Effect of ethanol on analytical lines. 0: Ca II 317.933 nm; V: Mg II 279.553 nm; X: Ni 1361.939 nm; n : Fe II 259.936 nm.

Organic solution analysis by air-argon ICP

1359

Table 4. Analytical results and recovery tests for trace elements in MIBK solution of nickel naphthenate (ug ml-i) Ni 1

mean + s 2

mean + s 3

mean f s 4

mean 2 s

1.0 1.0 100

0.13 0.13 0.13 0.13

0.23 0.21 0.21 0.22?0.01

1.00 1.05 1.02 1.02?0.03

18.7 18.4 18.9 18.720.25

0.21 0.20 0.20 0.20-e0.01

0.16 0.17 0.17 0.16eO.02

0.64 0.66 0.66 0.63kO.04

3.69 3.37 3.65 3.57fO.17

0.13 0.12 0.12 0.12?0.01

0.27 0.29 0.26 0.27?0.02

0.25 0.27 0.20 0.24kO.04

4.63 4.67 4.39 4.56kO.15

0.29 0.28 0.28 0.28?0.01

0.21 0.23 0.20 0.21*0.02

0.27 0.30 0.22 0.2620.04

Mg

Fe

5.0 4.2 84

Table 5. Determination Ni Detected

Mean + s Reference Relative error (%)

Ca

32.9 32.1 33.4 32.8kO.66

Ni Added: Detected: Recovery (%)

Fe

Mg

10.0 11.0 110

0.2 0.2 100

1.0 0.9 90

2.0 1.8 90

0.2 0.2 100

1.0 0.9 90

Ca 2.0 1.8 90

0.2 0.2 100

1.0 0.9 90

2.0 1.9 95

of trace elements in xylene solution of waste oil (kg ml-l) Mg

Fe

Al

Ca

cu

29.5 27.8 26.7 28.021.4 27.2

29.5 32.2 30.8 30.821.4 33.3

35.3 36.1 38.5 36.621.7 38.7

87.3 74.3 83.8 81.8a6.7 76.8

91.1 89.8 87.2 89.4k2.0 89.3

42.6 47.8 49.4 46.623.6 44.0

+2.9

-7.5

-5.4

+6.1

+0.11

+5.9

sample were analysed, each in triplicate. The results are listed in Tables 4 and 5. Recovery tests (see also Table 6) and reference values are also listed in the tables to check the reliability of the results. Generally speaking, the results are satisfactory.

4. CONCLUSIONS Compared with an argon ICP, an air-argon ICP has some advantages for organic solution analysis: there is no carbon deposition on the injector; the spectral interferences caused by molecular bands from hydrocarbons are largely depressed or even eliminated; and for many analytical lines, the detection limits are lower than those in an argon ICP. Therefore, this kind of ICP is suitable for direct elemental determinations in organic solutions. A further study of the combination of the air-argon ICP with normal-phase (organic mobile phase) high performance liquid chromatography (HPLC) is in progress.

1360

Y. Q. TANGet al. Table 6. Recovery tests for trace elements in xylene solution of waste oil

Element

Added (CLgg-‘)

Detected (ug g-‘)

Recovery (%)

Ni

1.0 5.0 10.0

0.91 4.3 9.5

91 86 95

Mg

0.5 2.5 5.0

0.48 2.1 5.0

96 84 100

Fe

1.0 5.0 10.0

0.95 4.2 8.8

95 84 88

Al

1.0 5.0 10.0

Ca

1.0 5.0 10.0

cu

0.50 2.5 5.0

1.0 5.0 11.7

100 100 117

0.83 4.6 8.4

83 92 84

0.53 2.1 5.2

106 84 104

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