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The influence of the temperature of the rf work coil in inductively-coupled plasma atomic emission spectrometry
TECHNICAL NOTE
Tbe influence of the temperatureof the rf work coil in inductively-coupled plasma atomic emission spectrometry
(Received19 April 1983) 1. I ~~D~~IoN WITH THEincreasing
use of the i~ductjvely~up~~ plasma for practical analyses the reliability of the ICP is gaining in importance, A s~~j~~nt aspect of better rehabiiity is e.g. an improved long term stability, so tbat need for r~Iib~tions, which is now typically 44/day, can he diminished. Experiments carried out in our laboratory over a M-week period showed that the percentual standard deviation in the slope of calibration curves amounted to IO-15 % for six elements. To our disappointment careful adjustment of the normal ICP parameters and stabilization of the power and carrier gas flow resulted in a relative standard deviation of !5-10% [I], Our impression was that the temperature of the cooling water circulated in the rf work coil could play an important role. The aim of the present investigation is to measure the influence of the temperature of the cooling water through the rf work coil upon the net signal. Also a quahtative explanation for the observed in&ience
wifl be proposed.
2 EXPERIMENTAL M~~ern~~ are ear&d out with a torch unit which has been described earl& [23. The relevant details of the instruments and operating conditions are collected in Table 1. In our experiments the rf work coil is connected to a waterbath with a circulating pump. A cooling/heating unit with a capacity of 2 kW controls the temperature of the circulating water between P4O’C within f O.l”C. The temperature difference over the coil is measured to O.l”C with mercury thermometers inserted in the connecting tubes directly before and after the coil. The average of these two readings is taken as the temperature of the coil and the power uptake of the rf work coil is derived from the temperature increase of about 10°C and the flow of the cooling water of about 0.5 1mm-‘. After the temperature of the cooling/heating unit has been adjusted to the desired value it takes about an hour for the cooling water to reach a constant temperature.
Table 1. Apparatus and operating conditions Generator Torch Observation height Observation window Monochromator Nebulizer Nebulizer chamber Mass flow controller Sample gas Cooling gas Aux gas Rf-work coil
: Linn FS 4, output power 1413rt: 10 W stabilized to k5W : home made after BOUMANS~KORN~LUN [t] : 17 mm above coil : 0.04x2mm2 : Jobin Yvon JY38 PI computer (PDP f I/04) controlled : Meinhard, ail gfass, concentric (force fed) : Scott type, double walled : Applied Semiconductor Materials, type AFM 360 (sample intr~uction gas only) : l.O~O.O5Imin-’ : 22Imin-’ :: inner dia: 32 mm; 2.5 turn copper tubing; outside dii 6 mm; inner dia 3.5 mm
[l J G. R. KORNBLUM,J. SUEYERLVEIWKE, Y. MICHOTTE, A. KLOK, D. L. MAmarand L. DE GUN, Proc. 2nd Ini. Wrkshop on Trace Element Analytical Chemistry in Medicine and Siolcgy, 1983, p. i 161 Neuherberg, FRG. de Gruyter, Berlin. [2] G. R. KOKNBLUMand L, DE GAUN, ~~~~~~. Acta 29B, 249 (1974). 1363
Technical note
1364
3. RESULTS ANDDISCU~HON At several different temperaturepettings of the cooling water the net intensities of four spectral lines were measured. To simulate normal laboratory practice where a change in cooling water temperature would go unnoticed, the operating conditions were kept unchanged during these m~urements. The data for the elements are collected in Table 2. The results are shown in Fig. 1. The intensities are normalized to a temperature of 27°C to facilitate comparison of the different elements. For all transitions an approximately linear increase in the net line intensity with decreasing temperature is observed. The slope is element-dependent and varies from 3 “//“C for Cd II and Zn I to 2 “/,/“C for Mg I and 1.5 ‘;,/“C for Mg II. For a typical laboratory situation, where ordinary tapwater will be used to cool the rf coil, the tem~rature can easily vary over 3°C within one day which means a change of about 10 7; in net intensity for Zn I and Cd II and still 5 7; for the least sensitive magnesium line in Fig. 1. Over an extended period of time, say more than six months, the summer/winter temperature variation of tapwater might be as high as 15°C which will cause a net intensity variation of about 257;. Apparently, the rf coil temperature is an important source of long term instability in the ICP. To our knowledge this effect has not been reported previously in the iiteratureand is not mentioned in manuals of commercially available instruments. In practice an easy solution is to keep the cooling water temperature constant to within f 0S”C. From Fig. I it can be read that the remaining ins~biiity effect is only 1 “/, in the net signal, which is acceptable compared to other sources of drift. Naturally we were curious about the cause of the observedeffect. It might be speculated that a change in the rf coil temperature affects the spatial and notably the radial distribution of the intensity profiles. However, distributions measured at 10°C and 30°C for the rf coil were exactly the same and only differed in overall intensity in agreement with the data in Fig. 1. This suggests that the power input to the piasma changes. We checked this effect as follows. The power supplied by the generator (P._,) is partly dissipated in the plasma (P,), partly “sent” back as reflected power (P,,) and the remainder ISdissipated in the coil, so that:
The generator and reflected power are read from power meters supplied by the manufacturer. The power taken up by the coil is released to the cooling water and is measured ~lo~met~~lly. These three different power levels were measured simultaneously with the intensity. The results at five temperatures of the cooling water are collected in Table 3. It is seen that for a constant generator power the power dissipated in the coil is also constant, except for a statistically significant outlier at 16.8”C. Table 2. Spectroscopic data of the tines studied and concentrations of the relevant elements Element
Species
i Or@
Cd Mg Zil Mg
II II I I
214.438 279.553 213.856 285.213
Cone. (mgl-‘)
10 10 10 10
4X,
40,
W)
W
5.78 4.43 5.80 4.33
16.84 15.03 9.39 7.64
%K + Eion WI
22.62 19.46 15.19 10.99
Fig. 1. Influence of the temperature of the cooling water in the rf-coil upon the normalized net intensity for Zn I, Cd II, Mg II and Mg 1.Through the almost coinciding points for Zn I and Cd II o&y one line is drawn.
Technical note
1365
Table 3. Powers and Zn I intensities measured at different temperatures of the cooling water through the rf-working coil $
8
~
& 16.8 19.2 24.0 27.4 32.1
1413 1413 1413 1413 1413
218 273 276 276 281
156 160 166 172 188
(%j
(AL.)
1039 980 971 965 944
405 3% 324 304 255
However, the value of the reflected power increases steadily with the temperature of the cooling water. As a result, the power released in the plasma decreases by about 100W under the influence of a 16°C increase in temperature of the cooling water. Such a change in input power can easily explain the intensity variations presented in Fig. I. This is demonstrated in Fig. 2, where the input power to the plasma is deliberately varied by varying the generator power at two temperatures of the rf coil and the changes in net intensity of the Zn I fine recorded. Although the slope of the power-intensity relation is slightly larger at 10°Cthan at 3O”C,there is a perfect agreement with the curve derived from the data in the final two columns of Table 3. Therefore, the increase in line intensities observed when the temperature of the rfcoil drops (Fig 1)is indeed due to
0
a00
so0
Pin.W
loo0
Fig. 2. Net intensity for Zn I vs plasma power. (A) Constant temperature of the cooling water of 10°C. (B) Constant-temperatureof the cooling water of 30°C. (C) Variable temperature of the cooling water; data taken from Table 3.
Fig. 3. Variations of line intensities ys the temperature of the &coil without (solid lines) and with (dashed lines) m~im~tion of the reflected power; generator power constant at 1413 W.
1366
Technical note
the increase in input power to the plasma (Table 3). In turn, this increase in input power is the result of a decrease in reflected power. It should be- recalled at this point that throughout the experiments the operating conditions were deliberately left unchanged and, moreover, our equipment does not provide for automatic minimization or control of the reflected power. It might be thought that such a provision would overcome the problem, but the data presented in Fig. 3 show that such is not the case. In this figure the generator power is again kept constant at 1413 + SW, but upon each variation of the temperature of the rfcoil the reflected power is minimized..Although the intensity variations do become smaller than without minimization of the reflected power, they are still significant. For Cd II and Zn I the temperature sensitivity improves for Cd II from 3 %/“C to 1.5 %/“C and for Mg II from 1.5 %/“C to an almost, but not quite insignificant 0.5 %/“C. The different sensitivity for the four lines to changes in the coil temperature show no relation to their respective “hardness” [3]. A possible connection with their excitation potentials might seem plausible (see Table 2). More lines have to be measured, however, to draw a firm conclusion. 4. CONCLUSION The temperature of the cooling water circulating through the RF work coil has a sign&ant
effect
upon the line intensities in the ICP. For four transitions measured in this study the (negative) variation
changed from 3 %/“C for Cd II to 1.5 %/“C for Mg II when no provisions are taken to minimize the reflected power. However, even with such additional precautions there still remain variations between 0.5 and 1.5 %/“C. It is obviously much easier to control the temperature of the cooling water to within + 0.5”C. The ensuing influence on signal drift is then reduced to insignificant proportions. Laboratorium voor Analytische Scheikunde Jaffalaan 9 2628 BX Deut The Netherlands
[3] P. W. J. M.
BOUMANSand
M. C. H.
LUX-STEINER, Spectrochtm.Acta
G. R.
KORNBLUM, A. KLOK
and L. DEGAWN
37B, 97 (1982).
Tbe influence of the temperatureof the rf work coil in inductively-coupled plasma atomic emission spectrometry
(Received19 April 1983) 1. I ~~D~~IoN WITH THEincreasing
use of the i~ductjvely~up~~ plasma for practical analyses the reliability of the ICP is gaining in importance, A s~~j~~nt aspect of better rehabiiity is e.g. an improved long term stability, so tbat need for r~Iib~tions, which is now typically 44/day, can he diminished. Experiments carried out in our laboratory over a M-week period showed that the percentual standard deviation in the slope of calibration curves amounted to IO-15 % for six elements. To our disappointment careful adjustment of the normal ICP parameters and stabilization of the power and carrier gas flow resulted in a relative standard deviation of !5-10% [I], Our impression was that the temperature of the cooling water circulated in the rf work coil could play an important role. The aim of the present investigation is to measure the influence of the temperature of the cooling water through the rf work coil upon the net signal. Also a quahtative explanation for the observed in&ience
wifl be proposed.
2 EXPERIMENTAL M~~ern~~ are ear&d out with a torch unit which has been described earl& [23. The relevant details of the instruments and operating conditions are collected in Table 1. In our experiments the rf work coil is connected to a waterbath with a circulating pump. A cooling/heating unit with a capacity of 2 kW controls the temperature of the circulating water between P4O’C within f O.l”C. The temperature difference over the coil is measured to O.l”C with mercury thermometers inserted in the connecting tubes directly before and after the coil. The average of these two readings is taken as the temperature of the coil and the power uptake of the rf work coil is derived from the temperature increase of about 10°C and the flow of the cooling water of about 0.5 1mm-‘. After the temperature of the cooling/heating unit has been adjusted to the desired value it takes about an hour for the cooling water to reach a constant temperature.
Table 1. Apparatus and operating conditions Generator Torch Observation height Observation window Monochromator Nebulizer Nebulizer chamber Mass flow controller Sample gas Cooling gas Aux gas Rf-work coil
: Linn FS 4, output power 1413rt: 10 W stabilized to k5W : home made after BOUMANS~KORN~LUN [t] : 17 mm above coil : 0.04x2mm2 : Jobin Yvon JY38 PI computer (PDP f I/04) controlled : Meinhard, ail gfass, concentric (force fed) : Scott type, double walled : Applied Semiconductor Materials, type AFM 360 (sample intr~uction gas only) : l.O~O.O5Imin-’ : 22Imin-’ :: inner dia: 32 mm; 2.5 turn copper tubing; outside dii 6 mm; inner dia 3.5 mm
[l J G. R. KORNBLUM,J. SUEYERLVEIWKE, Y. MICHOTTE, A. KLOK, D. L. MAmarand L. DE GUN, Proc. 2nd Ini. Wrkshop on Trace Element Analytical Chemistry in Medicine and Siolcgy, 1983, p. i 161 Neuherberg, FRG. de Gruyter, Berlin. [2] G. R. KOKNBLUMand L, DE GAUN, ~~~~~~. Acta 29B, 249 (1974). 1363
Technical note
1364
3. RESULTS ANDDISCU~HON At several different temperaturepettings of the cooling water the net intensities of four spectral lines were measured. To simulate normal laboratory practice where a change in cooling water temperature would go unnoticed, the operating conditions were kept unchanged during these m~urements. The data for the elements are collected in Table 2. The results are shown in Fig. 1. The intensities are normalized to a temperature of 27°C to facilitate comparison of the different elements. For all transitions an approximately linear increase in the net line intensity with decreasing temperature is observed. The slope is element-dependent and varies from 3 “//“C for Cd II and Zn I to 2 “/,/“C for Mg I and 1.5 ‘;,/“C for Mg II. For a typical laboratory situation, where ordinary tapwater will be used to cool the rf coil, the tem~rature can easily vary over 3°C within one day which means a change of about 10 7; in net intensity for Zn I and Cd II and still 5 7; for the least sensitive magnesium line in Fig. 1. Over an extended period of time, say more than six months, the summer/winter temperature variation of tapwater might be as high as 15°C which will cause a net intensity variation of about 257;. Apparently, the rf coil temperature is an important source of long term instability in the ICP. To our knowledge this effect has not been reported previously in the iiteratureand is not mentioned in manuals of commercially available instruments. In practice an easy solution is to keep the cooling water temperature constant to within f 0S”C. From Fig. I it can be read that the remaining ins~biiity effect is only 1 “/, in the net signal, which is acceptable compared to other sources of drift. Naturally we were curious about the cause of the observedeffect. It might be speculated that a change in the rf coil temperature affects the spatial and notably the radial distribution of the intensity profiles. However, distributions measured at 10°C and 30°C for the rf coil were exactly the same and only differed in overall intensity in agreement with the data in Fig. 1. This suggests that the power input to the piasma changes. We checked this effect as follows. The power supplied by the generator (P._,) is partly dissipated in the plasma (P,), partly “sent” back as reflected power (P,,) and the remainder ISdissipated in the coil, so that:
The generator and reflected power are read from power meters supplied by the manufacturer. The power taken up by the coil is released to the cooling water and is measured ~lo~met~~lly. These three different power levels were measured simultaneously with the intensity. The results at five temperatures of the cooling water are collected in Table 3. It is seen that for a constant generator power the power dissipated in the coil is also constant, except for a statistically significant outlier at 16.8”C. Table 2. Spectroscopic data of the tines studied and concentrations of the relevant elements Element
Species
i Or@
Cd Mg Zil Mg
II II I I
214.438 279.553 213.856 285.213
Cone. (mgl-‘)
10 10 10 10
4X,
40,
W)
W
5.78 4.43 5.80 4.33
16.84 15.03 9.39 7.64
%K + Eion WI
22.62 19.46 15.19 10.99
Fig. 1. Influence of the temperature of the cooling water in the rf-coil upon the normalized net intensity for Zn I, Cd II, Mg II and Mg 1.Through the almost coinciding points for Zn I and Cd II o&y one line is drawn.
Technical note
1365
Table 3. Powers and Zn I intensities measured at different temperatures of the cooling water through the rf-working coil $
8
~
& 16.8 19.2 24.0 27.4 32.1
1413 1413 1413 1413 1413
218 273 276 276 281
156 160 166 172 188
(%j
(AL.)
1039 980 971 965 944
405 3% 324 304 255
However, the value of the reflected power increases steadily with the temperature of the cooling water. As a result, the power released in the plasma decreases by about 100W under the influence of a 16°C increase in temperature of the cooling water. Such a change in input power can easily explain the intensity variations presented in Fig. I. This is demonstrated in Fig. 2, where the input power to the plasma is deliberately varied by varying the generator power at two temperatures of the rf coil and the changes in net intensity of the Zn I fine recorded. Although the slope of the power-intensity relation is slightly larger at 10°Cthan at 3O”C,there is a perfect agreement with the curve derived from the data in the final two columns of Table 3. Therefore, the increase in line intensities observed when the temperature of the rfcoil drops (Fig 1)is indeed due to
0
a00
so0
Pin.W
loo0
Fig. 2. Net intensity for Zn I vs plasma power. (A) Constant temperature of the cooling water of 10°C. (B) Constant-temperatureof the cooling water of 30°C. (C) Variable temperature of the cooling water; data taken from Table 3.
Fig. 3. Variations of line intensities ys the temperature of the &coil without (solid lines) and with (dashed lines) m~im~tion of the reflected power; generator power constant at 1413 W.
1366
Technical note
the increase in input power to the plasma (Table 3). In turn, this increase in input power is the result of a decrease in reflected power. It should be- recalled at this point that throughout the experiments the operating conditions were deliberately left unchanged and, moreover, our equipment does not provide for automatic minimization or control of the reflected power. It might be thought that such a provision would overcome the problem, but the data presented in Fig. 3 show that such is not the case. In this figure the generator power is again kept constant at 1413 + SW, but upon each variation of the temperature of the rfcoil the reflected power is minimized..Although the intensity variations do become smaller than without minimization of the reflected power, they are still significant. For Cd II and Zn I the temperature sensitivity improves for Cd II from 3 %/“C to 1.5 %/“C and for Mg II from 1.5 %/“C to an almost, but not quite insignificant 0.5 %/“C. The different sensitivity for the four lines to changes in the coil temperature show no relation to their respective “hardness” [3]. A possible connection with their excitation potentials might seem plausible (see Table 2). More lines have to be measured, however, to draw a firm conclusion. 4. CONCLUSION The temperature of the cooling water circulating through the RF work coil has a sign&ant
effect
upon the line intensities in the ICP. For four transitions measured in this study the (negative) variation
changed from 3 %/“C for Cd II to 1.5 %/“C for Mg II when no provisions are taken to minimize the reflected power. However, even with such additional precautions there still remain variations between 0.5 and 1.5 %/“C. It is obviously much easier to control the temperature of the cooling water to within + 0.5”C. The ensuing influence on signal drift is then reduced to insignificant proportions. Laboratorium voor Analytische Scheikunde Jaffalaan 9 2628 BX Deut The Netherlands
[3] P. W. J. M.
BOUMANSand
M. C. H.
LUX-STEINER, Spectrochtm.Acta
G. R.
KORNBLUM, A. KLOK
and L. DEGAWN
37B, 97 (1982).