JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, SEPTEMBER 1992, VOL. 7 79 1

Drift Diagnostics in Inductively Coupled Plasma Atomic Emission Spectrometry* Plenary Lecture

Martine Carre,? Emmanuelle Poussel and Jean-Michel Mermet Laboratoire des Sciences Analytiques, University of Lyon I, 69622 Villeurbanne Cedex, France

A simple experiment is described to carry out drift diagnostics in inductively coupled plasma atomic emission spectrometry. It is based on the behaviour of the Ba II 455.403, Zn II 206.200 and Ar I 404.442 nm line intensities as a function of the power, the solution uptake rate and the carrier gas flow rate. Additional information on the atomization and ionization conditions is obtained from the Mg II 280.270 nm to Mg I 285.213 nm line intensity ratio. Methods to verify the presence of a drift over a limited period of time are discussed. Several examples illustrate the feasibility of these diagnostics. A conclusion is drawn about the selection of internal standards for drift correction. Keywords: Inductively coupled plasma; atomic emission spectrometry; drift

Inductively coupled plasma atomic emission spectrometry (IPC-AES) is widely used for routine analysis. Among the figures of merit of an analytical method, accuracy and precision are of prime concern. Most users of commercially available ICP-AES systems obtain satisfactory short-term precision. When expressed as the relative standard devia- tion (RSD) of the replicate fluctuations, precision is usually less than 0.5% and in the range 0.5-l0/o, with and without internal standardization, respectively. However, the long- term stability, e g . , over a period of 4 h, is often less satisfactory because of drift phenomena.

Many causes of drift have been reported in ICP-AES.' They can be classified into three categories: (i) change in the energy transfer from the plasma to the sample; (ii) variation in the efficiency of the sample production and transport; and (iii) degradation of the line intensity measurement.

Change in the energy transfer can originate in a variation in the forward power, a modification of the shape of the coil and a variation in the gas flow rates, in particular in the carrier gas flow rate. Change in the sample introduction efficiency can be related to a variation in the carrier gas flow rate or the solution uptake rate, a partial blocking of the pneumatic nebulizer and a change in the temperature of the spray chamber and the solution. Degradation of the line intensity measurement is usually linked to either a thermal drift of the optical components of the dispersive system or an opacity of the collimating system.

A consequence of drift is the need for frequent, time- consuming recalibrations. Where drift is observed between two calibration procedures, there is no general rule about the way to correct the data for the change in the calibration graph, as drift is not necessarily a simple function of time. Another consequence of drift is observed with sequential ICP systems. If the selection of many elements and several replicates results in a long sequence, then a degradation of the RSD is observed when drift occurs between the first and the last replicates.

Most publications have dealt with internal standardiza- t i o n * ~ ~ and computational drift c o r r e ~ t i o n . ~ ~ ~ ~ ~ ~ ~ Lorber et aL4 and Ramsey and Thompson' have described the generalized form of internal reference method (GIRM) and

Surprisingly little work has been reported on

*Presented at the 1992 Winter Conference on Plasma Spectro-

tPresent address: Air Liquide, Centre de Recherche Claude chemistry, San Diego, CA, USA, January 6-1 1, 1992.

Delorme, Les Loges en Josas, France.

the parameter-related internal standard method (PRISM), respectively. These methods were developed to overcome the difficulty of matching the behaviour of a single internal standard to that of every analytical line. The GIRM was based on the use of four internal standards, two atomic lines and two ionic lines, whereas the PRISM made use of only two internal standards, one atomic line and one ionic line. The influence of four operating parameters, power, plasma and carrier gas flow rates and solution uptake rate was studied to develop the GIRM, whereas only two para- meters, power and uptake rate, were used for the PRISM. Although significant improvements in stability were ob- tained, there was no clear explanation of the origins of the drift. Computational methods can certainly correct for drift to a substantial extent, but it seems to be more logical to suppress 'the causes of drift. There is, therefore, a need for drift diagnostics. The purpose of this work was to describe such diagnostics that can be used on any commercially available sequential ICP system. These diagnostics should permit the ICP user: (i) to explain the main origins of the drift; (ii) to provide data on the magnitude of the drift; (iii) to test any improvement; (iv) to make a fair comparison between ICP systems or components; and ( v ) to select adequate internal standards for drift correction.

Test Elements for Drift Diagnostics Most of the time, drift is related to changes in the energy transfer and in the efficiency of the sample introduction system. Simulation of these changes can be performed by modifying the forward power, the carrier gas flow rate and the peristaltic pump rate. The selection of the test elements must consider the influence of these ICP operating para- meters on the line intensities. A line with a low sum of excitation and ionization energies will not be sensitive to a change in the power. The Ba I1 455.403 nm line was therefore selected as the analytical line with the lowest energy sum. In contrast, a line such as the Zn I1 206.200 nm line will be very sensitive to the power as its energy sum is one of the highest. However, both lines will exhibit similar behaviour when there is a change in the nebulizer efficiency. The Ar I 404.442 nm line was also selected to follow the response of the plasma to changes in the operating parameters. The Mg I1 280.270 nm:Mg I 285.2 13 nm ratio was also used to verify any variation in the atomization and ionization effi~iency.~ This ratio is sensitive to any change in the power and the carrier gas flow rate, in particular when

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792 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, SEPTEMBER 1992, VOL. 7

Table 1 Line selection (nm) for drift diagnostics, excitation energy (E,,,), ionization energy (Eion) and sum of ionization and excitation energies (Eion +EeXc), expressed in eV

Element Eion Eexc Eion+Eexc Ba I1 455.403 5.21 2.72 7.93 Zn I1 206.200 9.39 6.01 15.40 ArI 404.442 MgI 285.213 - 4.35 Mg I1 280.270 7.65 4.42 12.07

- 14.69 - -

the value of the ratio is low, e.g., less than 6.9 The characteristics of these lines are summarized in Table 1. Usually, the concentration was around 1 mg dm-3 for Ba and Mg and 10 mg dm-3 for Zn. Background correction was performed for each line.

Influence of the ICP Operating Parameters Three parameters were studied to verify the behaviour of the Ar, Zn and Ba test lines: (i) the power (change in the energy transfer); (ii) the peristaltic pump rate (variation in the amount of aerosol); and (iii) the carrier gas flow rate (change in both the energy transfer and the amount of aerosol). The experiments were carried out at or near the values that are normally used for routine analysis. Various commercially available ICP systems were used for these experiments and are mentioned in the text.

Under conventional operating conditions, the behaviour of each of the three test lines against the power was found to be almost independent of the ICP system. A typical result is presented in Fig. I , based on the use of a Perkin-Elmer 5500 ICP system. An increase in the power produced an increase of a similar magnitude in both the Ar and Zn line intensities. As mentioned above, there was no significant change in the Ba line intensity. A decrease in the peristaltic pump rate led to a decrease of the same magnitude in both the Zn and Ba line intensities, irrespective of the ICP system. This can be easily understood as less sample was reaching the plasma. In contrast, an increase in the Ar line intensity was observed as a result of a slight increase in the temperature (Fig. 2). A Philips 8060 ICP system was used to obtain the results given in Fig. 2. The negative correlation between the Ar line intensity and the Zn or Ba line intensity as a function of the nebulizer efficiency was also reported by Ivaldi and Slavin.lo

It was convenient to summarize the intensity behaviour of each line against the power (P) and the amount of aerosol (Q) using the symbols '+', '-' and '=' (Table 2). The

160 r I

1.1 1.2 1.3 1.4 40 1 .o

PowerIkW

Fig. 1 Influence of the power on the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities. The signals were normalized to 100 for a power of 1.2 kW. A Perkin-Elmer 5500 ICP system was used for this experiment

300 b

0' I 1

200 400 600 800 1000 Relative variation of the uptake rate

1 I

Fig. 2 Influence of the peristaltic pump rate on the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities. The signals were normalized to 100 for the maximum pump rate, i.e., 2 cm3 min-'. A Philips PV 8060 ICP system was used for this experiment

Table 2 Behaviour of the: Ar, Zn and Ba lines with a change in the power ( P ) and the amount of aerosol (Q); + indicates an increase, - a decrease and = no significant change

Parameter Ar Zn Ba + + = - + + + -

- - P+ P - - -

- Q+ Q-

symbol '+' indicates that the slope of the intensity as a function of P or Q is positive, '-' that the slope is negative, and ',=' that there is no significant change near the central value 'of P or Q. Based on the use of these symbols, the comparison of the variations of each line intensity with time permits an unambiguous assignment of the parameter that caused the change. For instance, a decrease in the Ar and Zn line intensities and no change in the Ba line intensity lead to the symbols -, - and =, respectively. From Table 2, this set of symbols indicates a change in power with time.

More information can be obtained from the variation of the carrier gas flow rate, although its influence is more complex than that of the power and the solution uptake rate. In each instance, an increase in the carrier gas flow rate leads to a decrease in the Ar line intensity. This is due to a slight cooling of the plasma. The behaviour of the Zn and Ba line intensities depends on some ICP parameters, in particular on the observation height. In the instance where the residence time is short, i.e., a carrier gas flow rate of 1 dm3 min-' and an injector i.d. of 1.5 mm," the Ba and Zn line intensities exhibit a different spatial distribution as a function of observation height above the load coil. The peak intensity of the Zn line is above that of the Ba line (Fig. 3). An increase in the carrier gas flow rate corresponds not only to a change in the amount of aerosol but also to a shift in the spatial distribution of the two lines. The behaviour of both the Zn and Ba line intensities depends on the observation height used for the experiment. In case A (Fig. 3), the observation height was selected as a com- promise between the various ionic lines. An increase in the carrier gas flow rate leads to a decrease in the Zn line intensity in contrast to an increase in the Ba line intensity. A typical example (Perkin-Elmer 5500 ICP system) is given in Fig. 4. The behaviour of the three lines in case A is summarized in Table 3 in a similar manner to that used in Table 2.

In case B (Fig. 3), the observation height was selected to optimize the intensities of low-energy lines such as the Ba I1 455 nm line. Therefore, any substantial change in the

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JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY, SEPTEMBER 1992, VOL. 7 793

I

Observation height - Fig. 3 Schematic variation of the Zn I1 206 and Ba I1 455 nm line intensities as a function of the observation height above the load coil. A, B and C indicate the positions of the various observation heights used for experiment as described in the text

140 1

40 ' I 1 J 0.75 0.8 0.85 0.9 0.95

Carrier gas flow rate/dm3 min-'

Fig. 4 Influence of the carrier gas flow rate on the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities. The observation height corresponds to positions A in Fig. 3. The signals were normalized to 100 for a carrier gas flow rate of 0.85 dm3 min-I. A Perkin-Elmer 5500 ICP system was used for this experiment

Table 3 Behaviour of the: Ar, Zn and Ba lines with the carrier gas flow rate (D). The observation height corresponds to case A in Fig. 3

Parameter Ar Zn Ba + D+

D- + + - - -

carrier gas flow rate always corresponds to a decrease in the Ba line intensity. However, this change is negligible for a small variation in the carrier gas flow rate, in contrast to the changes in the Ar and Zn line intensities. Both the Ar and Zn line intensities decrease with the carrier gas flow rate. The behaviour of the three lines in case B is summarized in Table 4. An example (Philips PV 8060 ICP system) is given in Fig. 5. Under these conditions, the change due to the carrier gas flow rate is similar to that obtained with a variation of the power. In order to separate the two processes, it is therefore necessary to use a different observation height, such as that used in case A.

Table 4 Behaviour of the Ar, Zn and Ba lines with the carrier gas flow rate. The observation height corresponds to case B in Fig. 3

Parameter Ar Zn Ba D+ D - + + =

- - - -

300

.- *: .-n 200

" 0.9 1 .o 1.1 1.2 1.3

Carrier gas flow rate/dm3 min-'

Fig. 5 Influence of the carrier gas flow rate on the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities. The observation height corresponds to position B in Fig. 3. The signals were normalized to 100 for a carrier gas flow rate of 1.05 dm3 min-'. A Philips PV 8060 ICP system was used for this experiment

2oo fi

I 1 I I 0.5 0.6 0.7 0.8 0.9 1

Carrier gas flow rate/dm3 min-'

Fig. 6 Influence of the carrier gas flow rate on the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities. The observation height corresponds to position C in Fig. 3. The signals were normalized to 100 for a carrier gas flow rate of 0.75 dm3 min-'. A Spectroflame D ICP system was used for this experiment

Table 5 Behaviour of the Ar, Zn and Ba lines with the carrier gas flow rate. The observation height corresponds to case C in Fig. 3

Parameter Ar Zn Ba D+ - = + D- + = -

In case C (Fig. 3), the observation height was selected to optimize the intensities of high-energy lines such as the Zn I1 206 nm line. In this instance, the Zn line intensity remains unchanged for small variations of the carrier gas flow rate and the behaviour of the three lines is summarized in Table 5. An example is given in Fig. 6 (Spectroflame D ICP system).

In conclusion, the behaviour of the three lines can be predicted for a variation of the power and the peristaltic pump rate. However, in the instance of the influence of the carrier gas flow rate, it is necessary to carry out experiments as the results will depend on the observation height.

Observation of the Presence of Drift Before carrying out time-consuming experiments on drift diagnostics, it is preferable to verify the presence of drift, possibly over a limited period of time. In the instance where the drift exhibits the same sign, e.g., the signal drifts downwards with time, the slope of a linear regression of the data can be used successfully. Use of 10-20 replicates

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794 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, SEPTEMBER 1992, VOL. 7

Table 6 Behaviour of the Ar, Zn and Ba lines with the carrier gas flow rate. The spatial distributions of the Ba and Ar line intensities are similar

Parameter Ar Zn Ba D+ D- + = -

- - - - - -

is sufficient to verify that the slope is different from zero. The slope can be expressed as percent. per period of time, e.g., per 10 min.

Although the slope depends on the number of data points, one can estimate that a slope of 1% per time unit corresponds to a 0.3% RSD over the same period of time, If the subscripts ‘reg’ (for data after linear regression), ‘exp’ (for experimental data), ‘cor’ (for data corrected for drift) are used for the RSDs of the regression line, the experi- mental data and the data corrected for drift, respectively, then

RSDeXp2 = RSD,,,Z + RSD,,;

The RSD,,, value can be easily deduced from eqn. (1). An example is given in Fig. 7. Twenty replicates were used over a period of 14.5 min. The slope of the linear regression and the various RSDs are given in Table 7. It can be seen that the Ar drift is negligible in contrast to that of Zn and Ba. Moreover, the signal fluctuations are larger for both Zn and Ba. Taking into account the random distribution of the data, it would be difficult to observe such a drift just by inspection of the raw data. As the value of RSD,, is less than that of RSD,,,, the influence of the drift on RSD,,, is negligible over such a short period of time, although the magnitude of the drift is large. The conclusion would be different over a period of 4 h.

110 1

0 4 8 12 Tirne/min

Fig. 7 Variation of the: A, Ar 1404; B, Zn I1 206; and C, Ba I1 455 nm line intensities as a function of the replicate number to illustrate the role of short-term fluctuations compared with drift. The total measuring time was 14.5 min. The signals were normalized to 100 for the first replicate. A scale shift of + 5 and - 5 was applied to Ar and Ba, respectively. A linear regression is overlapped to each curve to give evidence of the presence of a drift. Slopes are given in Table 7

When a larger number of replicates is used or when the drift exhibits different directions of variation, use of the slope of a linear regression is no longer adequate. Visual inspection of the curve of the signal with time is not always helpful to separate the short- and long-term fluctuations. It is therefore necessary for a smoothing procedure to be carried out to minimize the short-term fluctuations. A smoothing procedure is equivalent to an increase in the integration time. Use of a moving average is a simple way of performing such a smoothing.’* If at least 100 data points are available, use of the average of nine successive data points is a good compromise. The average of data points 1 to 9 is computed, then 2 to 10, and similarly up to the last value. Results can be seen in Fig. 8 for the Zn line intensity. This procedure can be improved by weighting the data. Such a procedure was used13 for background smoothing. For a central value n, with a weight of 128, the weight will be 1 1 5, 86, 52 and 26 for the two (n- 1) and (n+ l), (n-2) and (n + 2), (n - 3) and (n + 3), and (n - 4) and (n + 4) values, respectively. The sum is then divided by 686. In Fig. 8, it can be seen that the short-term fluctuations are reduced compared with those observed in the absence of weighting. Visual inspection of the smoothed curves as in Fig. 8 permits the ICP user both to verify the presence of drift and to describe its shape.

An alternative to these methods was described in ref. 14. It is based on the use of the RSD of successive values, RSD,,,, and its comparison with the conventional RSD,,, based on the use of least-squares values. The least-squares standard deviation sexp is:

L J

The SD of successive values is:

n - I ’1.

s,,,= 1 1 (Xi+! -xJ*/2(n- 1) I l - (3)

where n is the number of replicates and 2 is the mean. For a large number of data, ssUc is practically independent of the shape of the curve in contrast to sexp. Without a drift, sex, should be equivalent to s,,,. When sex, is greater than ssuc, this should indicate the presence of a drift. The reliability of this method depends on the value of RSD,,, compared with that of RSD,,,. When drift is predominant over the short- term fluctuations (Fig. 9), i.e., RSD,,,>RSD,,,, the value of RSD,,, is significantly different from that of RSD,,, and can be used as evidence of the presence of drift (Table 8). In contrast, when the short-term fluctuations predominate over drift (Fig. 7), the value of RSD,,, is not significantly different from that of RSD,,, (Table 7).

Examples of Drift Diagnostics Thanks to the collaboration of many ICP users, a large amount of data has been gathered. It is not possible to provide a comprehensive list of results. The purpose of this

~~

Table 7 Example of observation of drift based on 20 replicates. The slope of a linear regression was used to estimate the drift. The total measuring time was 14.5 min. The RSD subscripts ‘exp’, ‘reg’ and ‘cor’ stand for the experimental data, the data after linear regression, and the data corrected for drift, respectively. The subscript ‘suc’ stands for the RSD obtained with the use of successive data

Slope/ Element % per 10 min RSD,,,(%) RSD,(%) RSD,,r(oh) RSD,,,(%)

Ar Z n Ba

0.29 1.49 1.21

0.54 0.12 0.53 0.54 2.24 0.66 2.14 2.05 2.63 0.58 2.56 2.1 1

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JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY, SEPTEMBER 1992, VOL. 7 795

Table 8 Example of observation of drift based on 20 replicates. The RSD of successive values was used to estimate the drift. The total measuring time was 19.5 min (subscripts as in Table 7)

Slope/ Element Oh per 10 rnin RSD,,,(%) RSD,(%) RSD,,,(%) RSD,,,(%)

Zn 4.23 2.52 2.48 0.46 0.44 Ba 2.03 1.34 1.21 0.57 0.40

105 1

- m C .$ 100

4 95

0, .- w

I

a

I 0 50 100 150 200

Replicate No.

Fig. 8 Variation of the Zn I1 206 nm line intensity as a function of the replicate number. The total measuring time was 100 min. The signal was normalized to 100 for the first replicate. Two smoothing procedures were applied, based on the moving average of nine values with (bold line) and without (normal line) a weighting procedure

c .p 103 0, .- w - Q, a

99 0 5 10 15 20 Ti me/m i n

Fig. 9 Variation of the: A, Zn I1 206; and B, Ba I1 455 nm line intensities with time (20 replicates). The signal was normalized to 100 for the first replicate. Short-term fluctuations were small compared with drift

105 .- w .- L Q

z 100 C cn fn .- P) .- 95 - P) a

1 I I 0 50 100 150 200 250

Ti me/mi n

90 '

Fig. 10 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 5 and - 5 was applied to Ar and Ba, respectively. The RSDs (O/O) were 0.6, 0.45 and 0.75, for Ar, Zn and Ba, respectively. The warm-up time was 1.5 h

I

0 50 100 150 200 2 90 L

Time/min D

Fig. 11 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba 11 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 5 , + 3 and -5 was applied to Ba, Ar and Zn, respectively. The RSDs (Yo) were 1.85, 2.1 and 1.4, for Ar, Zn and Ba, respectively. An increase in the sample introduction efficiency was observed up to 50 min, whereas a slight decrease in the energy transfer was observed after 50 rnin

section is rather to illustrate the potential of the method by means of selected examples based on the use of the Ar, Zn and Ba line intensities for drift diagnostics. Before giving examples of drift it is worth considering results in the absence of drift. Several experiments indicated that the RSDs of the three test lines can be less than 1% over a period of 4 h using commercially available ICP systems. In Fig. 10, an example based on the use of an ICP system without internal standardization is given. A warm-up time of 1.5 h was used and RSDs of 0.6, 0.45 and 0.75% were obtained for Ar, Zn and Ba, respectively.

In Fig. 1 1 results are given with RSDs of around 2%; RSDs of 1.85, 2.1 and 1.4% were obtained for Ar, Zn and Ba, respectively. These results are adequate for most applications. Nevertheless a drift was observed, which is summarized in Table 9. From Table 2 it can be deduced that a slight increase in the efficiency of sample introduc- tion was observed in the range 0-50 min, followed by a slight decrease in the power in the range 50-240 min.

Warm-up is a particular case of drift. It is illustrated in Fig. 12 where the warm-up time was around 40 min. The behaviour of the three lines is indicated in Table 10. The experiment was started 10 min after plasma ignition. The warm-up period corresponded to a drastic increase in the energy transfer. After 40 min, a slight decrease in the efficiency of the sample introduction system was observed with RSDs of 0.84, 1.23 and 0.84% for Ar, Zn and Ba, respectively.

Table 9 Behaviour of the Ar, Zn and Ba line intensities over a period of 4 h (Fig. 1 1 )

Timdmin Ar Zn Ba 0-50 - + +

- - - 50-240 -

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796 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, SEPTEMBER 1992, VOL. 7

113 1 h

0 50 100 150 200 250 Ti me/m i n

Fig. 12 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A warm-up time is observed up to 40 min after which a slight decrease in the sample introduction system efficiency is observed where the RSDs (O/O) were 0.84, 1.23 and 0.84, for Ar, Zn and Ba, respectively

115 I A /-

I 40 80 120 160 85 ‘

0 Ti me/mi n

Fig. 13 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. The drift is explained by a drastic change in the efficiency of the sample introduction system

Table 10 Behaviour of the Ar, Zn and Ba line intensities over a period of 4 h (Fig. 12)

Time/min Ar Zn Ba 0-40 + + =

- 40-240 + -

The magnitude of drift shown in Fig. 13 is large, but can be easily explained. Both the Zn and Ar line intensities exhibit the same decrease in contrast to the Ar line intensity. A drastic degradation of the efficiency of the sample introduction system was the main cause of drift.

A more complex case is the overlap of at least two causes of drift (Fig. 14). The behaviour of the three lines is summarized in Table 1 1 . The variation of the Zn line intensity was more important than that of Ar. An increase in the efficiency of energy transfer (Ar+, Zn+ and Ba=) and an increase in the sample introduction system (Ar-, Zn+ and Ba+) were observed. Additive effects for Zn and subtractive effects for Ar were therefore observed, which explains the stronger effect on the Zn line intensity. Use of the ionic to atomic line ratio of Mg (Fig. 15) clearly indicates an increase in energy transfer during the same time period as the ratio varied from 7.5 to 8.7. This ratio has to be compared with that obtained during a more stable period (Fig. 15).

190 C 0

.- 170

.- L m iE 150 0, v) a 130 >

.-

.- w

5 110 oc

B N I A -/-

20 40 60 80 100 I20 140 ’ Time/mi n

Fig. 14 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. There was no warm-up time. The drift is explained by the additive effects of an increase in the energy transfer and an increase in the efficiency of the sample introduction system

=I r” 70 40 80 120 160 Time/min

Fig. 15 Variation of the Mg I1 280 nm to Mg I 285 nm line intensity ratio with time. Curve B was obtained at the same time as curves in Fig. 14 whereas curve A was obtained after a 3 h warm-up time. Curve B illustrates the change in the energy transfer

120

110 0 .- *,

L .- p 100 - m 0, .- : go .- c -

80

.-0 50 100 150 200 250 Timelmin

Fig. 16 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 10 and - 10 was applied to Ba and Ar, respectively. The RSDs (O/o) were 4.03,4.14 and 1.07, for Ar, Zn and Ba, respectively. The large RSDs for Zn and Ar compared with that of Ba give evidence of a problem of power

Table 11 Behaviour of the Ar, Zn and Ba line intensities over a period of 140 min. Example of the addition of at least two causes of drift (Fig. 14)

Time/min Ar Zn Ba 0- 140 + ++ +

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120 I

c 100 0 .- c..

L .- 9 80 - (0 C (3, .- a 60 >

40< 20 0 40 80 120 160 200

Ti me/m i n

Fig. 17 Variation of the: A, Ar I 404; B, Zn I1 206; and C, Ba I1 455 nm line intensities with time. The signals were normalized to 100 for the first replicates. A scale shift of + 5 and - 5 was applied to Ba and Ar, respectively. The large drift for both Zn and Ba indicates a significant power problem

The test elements can also be used to predict a breakdown of the instrument while the ICP is in operation. An example is given in Fig. 16 where the drift was negligible. However, the RSDs of both the Ar and Zn lines was large (4.03 and 4.14%, respectively) compared with that of Ba ( 1.07%). There was clearly a problem with power. This was con- firmed later (Fig. 17). A drift was observed, related to a decrease in the energy transfer. Actually, the triode of the high-frequency oscillator failed at the end of the experiment.

Conclusions In the instance where the drift has one or two major causes, a simple experiment allows the ICP user to suggest explanations for this drift. It is also possible to indicate the magnitude of the drift for the sake of comparison. The only constraint is the need to study the variation of the three lines around the values normally used for routine analysis.

It is also worth knowing the cause of drift when an internal standardization procedure is applied. In the in- stance where the drift is assigned to a problem linked with the sample introduction system, most elements will exhibit the same behaviour. A single internal standard can there- fore be used to correct for drift and the use of sophisticated computational correction is not necessary. In contrast, when the energy transfer is involved in drift, the behaviour of the ionic lines will depend on their energy. Moreover, the behaviour of the atomic lines with change in the power is rather complex. Consequently, a single internal standard will not compensate for drift in every element. Use of computational methods such as GIRM4 or PRISM' can be helpful to solve this problem although the behaviour of atomic lines is not always predictable.

The use of optical feedback power regulation has been reported as a means of improving stability.6 The Ar line can

be used for this purpose. It was observed that the Ar line intensity can vary with the power and the efficiency of the sample introduction system. Therefore, in order to use the Ar line for power regulation, the stability of the sample introduction system must be sufficiently good to assume that no drift originates from this part of the system. An efficient way of obtaining this improvement is the use of a thermostated spray chamber. I s Association of a tempera- ture controlled spray chamber with optical feedback power regulation based on the use of an Ar line has been reported16 as providing improvement of a factor of 2 in the RSD, reaching an average of 0.15% RSD over a 3 h period.

Without correction, a long-term stability of better than 1 Yo RSD can be expected over a period of 4 h. A lower value can be obtained with correction. Unfortunately, this result cannot be observed for every commercially available ICP system. Our experience is that changes in energy transfer and efficiency of the sample introduction system are the most probable causes of drift. Another current drawback is too long a warm-up time. It is therefore necessary to undertake systematic studies on ICP systems to solve the drift problem, which is probably the last challenge in ICP-AES. It is hoped that the approach described in this paper will contribute to a better understanding of drift phenomena.

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Paper 2/0086 7J Received February 19, I992

Accepted June I , 1992

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