Evaluation of a Commercially Available Microconcentric Nebulizer for Inductively Coupled Plasma Mass Spectrometry Journal of Analytical Atomic Spectrometry FRANK VANHAECKE MIRJA VAN HOLDERBEKE LUC MOENS AND RICHARD DAMS Laboratory of Analytical Chemistry Ghent University Institute for Nuclear Sciences Proeftuinstraat 86 B-9000 Ghent Belgium The performance of a commercially available microconcentric nebulizer (MCN-100 CETAC Technologies USA) operated at flow rates ranging from dO.001 up to 0.65 ml min-' was evaluated using a Perkin-Elmer Sciex ELAN 5000 ICP-mass spectrometer. The observations made were compared with those for the standard GemTip cross-flow nebulizer. Registration of signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate) at different rf powers and at different sample uptake rates allowed firstly a systematic optimization of the operation parameters and secondly a comparison of the signal behaviour observed when using both types of nebulizer.The stability of the MCN-100 was evaluated at different sample uptake rates and the Occurrence of memory effects was checked for a number of elements. Also the level and the behaviour of oxide and doubly charged ions was studied. Furthermore the susceptibility to matrix effects was investigated using synthetic matrices of different origin (acid organic and high salt content) and it was demonstrated that generally matrix effects observed with both nebulizers are comparable and the MCN-100 can be used with NaCl concentrations up to 4 g I-' without capillary blocking.Finally it is illustrated that the MCN-100 can be applied at sample flow rates of <5 p1 min-' as are encountered when coupling capillary zone electrophoresis to ICP-MS for elemental speciation studies. Keywords Inductively coupled plasma mass spectrometry; microconcentric nebulizer; cross-ow nebulizer Since its commercial introduction in 1983 inductively coupled plasma mass spectrometry (ICP-MS) has aroused great inter- est and during the past decade it has evolved into a powerful and widely applied technique for the determination of trace and ultra-trace elements in a variety of matrices. This success mainly results from its extremely low limits of detection multi- element capabilities and the possibility of obtaining isotopic information on the elements determined.Aqueous solutions make up the majority of samples analysed with ICP-MS such that in its standard configuration the technique is equipped with a pneumatic nebulizer for sample introduction. In spite of their low cost instrumental simplicity high sample throughput and good stability these 'standard' concentric or cross-flow nebulizers also show important draw- backs ( i ) a low transport efficiency (in combination with a spray chamber typically 1-2Y0);~ ( i i ) the need for relatively large sample amounts (2 1 ml); (iii) the simultaneous introduc- tion of analyte and matrix into the plasma giving rise to both spectral and non-spectral interferences; and (iv) the limitation to liquid samples. As a result alternative sample introduction systems have been developed throughout the years and are still an important topic of current research. For the intro- duction of liquid samples into an ICP among others ultrasonic thermospray nebuli~ation,~,~ high-pressure nebulization,lO." direct injection n e b u l i ~ a t i o n ~ .' ~ - ~ ~ oscillating capillary nebulization,16 monodisperse dried microparticulate inje~tion"~'~ and electrothermal v a p o r i z a t i ~ n ' ~ - ~ ~ have already been used. Application of these alternative sample introduction devices often allows an improvement of the transport efficiency and may offer other advantages over 'standard' pneumatic nebulization although often at the cost of the advantages of the latter system such as stability simplicity and low cost. Recently a microconcentric (pneumatic) nebulizer (MCN- 100 CETAC Technologies Omaha NE USA) has become commercially available. The price of this device is only slightly higher than that of the 'standard' pneumatic nebulizers (price difference of the order of a factor of two) and is hence much lower than that for other alternative nebulizers such as a direct injection nebulizer or an ultrasonic nebulizer.The MCN-100 is suitable for the introduction of low sample flow rates in an ICP and therefore may be of particular interest in cases where only limited sample amounts are available. As a result of the low flow rates (e.g. 1 pl min-l) involved when coupling capillary zone electrophoresis (CZE) as a separation method to ICP-MS as a detector this microconcentric nebul- izer also seems attractive to interface both components of this elemental speciation tool currently studied in our laboratory. The present article represents the results of a systematic study of the features of the MCN-100 with the emphasis on operation at low sample uptake rates. EXPERIMENTAL Instrumentation The instrument used is a Perkin-Elmer Sciex Elan 5000 ICP- mass spectrometer.In its standard configuration this instru- ment is equipped with a multi-channel peristaltic pump (Minipuls-3) a GemTip cross-flow nebulizer a Perkin-Elmer Type I1 spray chamber made of Ryton drained by the same peristaltic pump and a Perkin-Elmer corrosion-resistant torch Table 1 the Perkin-Elmer Sciex Elan 5000 ICP-mass spectrometer* Instrumental conditions and measurement parameters for Rf power/W Gas flow rates/l min-' Plasma Auxiliary Nebulizer Ion sampling depth/mm (from load coil) Sampling cone Skimmer Data acquisition mode Dwell time/ms Points per spectral peak Sweeps per reading 1300 15 1.2 t 10 Nickel 1.0 mm aperture diameter Nickel 0.75 mm aperture diameter Peak hop 35 5 20 ~~ * Unless otherwise indicated.t Variable/optimized in order to obtain maximum signal intensity. Journal of Analytical Atomic Spectrometry August 1996 Vol. 11 (543-548) 543- Nebulizer RESULTS AND DISCUSSION Signal Behaviour The effect of several instrumental parameters (nebulizer gas flow rate rf power sample uptake rate) on the M' signal intensity was systematically investigated. Signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate) were recorded at different rf powers for both the MCN-100 and the standard cross-flow nebulizer.For elements with a relatively high second ionization potential and a low oxide bond strength (not likely to form doubly charged and/or oxide ions32) increasing the rf power leads to a higher maximum intensity and a shift of the signal behaviour plot towards higher gas flow rates for both nebulizers. This is illustrated for Co using the MCN-100 in Fig. 2(a). The first aspect is a result of more energy being available in the plasma while the second aspect can be explained as follows. On increasing the rf power the plasma becomes more energetic such that a shorter residence time in the ICP is sufficient for ionization. As a result the zone of maximum M+ density is located more upstream in the plasma (closer to the induction coil) such that a higher nebulizer gas flow rate is required to push this zone in the direction of the sampler in order to obtain an efficient extraction of M' ions into the interface r e g i ~ n .~ ~ - ~ Although also the noise level (measured at a mass-to-charge ratio of 250) was observed to slightly increase on enhancing the rf power the signal-to-noise ratio exhibits exactly the same behaviour as the signal intensity. Figs. 2(u b and c ) represent the signal behaviour plots of U+ U2' and UO' respectively. Also for the U2+ and the UO' ion the signal behaviour plot shifts to higher nebulizer gas flow rates on increasing the rf power. Hence also the zones of maximum U2' and UO+ density are transposed to a more upstream position in the plasma on increasing the rf power.As for the Co' ion for the U2' ion this shift is accompanied by a significant increase in the maximum signal intensity. For the UO+ ion on the other hand this shift is not accompanied by an increase of the maximum signal intensity. The latter effect is probably to be attributed to a faster desolvation of the sample droplets being introduced into the plasma and a more pronounced dis- sociation of oxide species as a result of more energy being available at a higher rf power. Finally also for the U' ion an increase of the rf power to values exceeding 1000 W no longer leads to an increase in the maximum signal intensity. This more complex behaviour is probably a result of a competition between the singly charged doubly charged and oxide ions.The Co' signal intensity is plotted as a function of the sample uptake rate for both the MCN-100 and the standard cross-flow nebulizer in Fig. 3. It can be seen that for the MCN-100 in the range studied the signal intensity continu- ously increases with increasing sample uptake rate. For the standard cross-flow nebulizer however the signal intensity only increases with the sample uptake rate up to a value of about 1 ml min-' and thereafter the M' signal intensity remains more or less constant. The behaviour observed for the standard pneumatic nebulizer may be a result of several phenomena firstly the increased nebulizer mass transport may be compensated by a lower nebulizer transport efficiency causing the net mass transport to be roughly ~ o n s t a n t ; ~ ~ .~ ~ . secondly at higher sample uptake rates the droplet size distribution may become less favourable causing a larger fraction of the sample to be removed by the spray chamber; and finally since on increasing the sample uptake rate the (UO' U') ratio was observed to increase continuously intro- duction of larger amounts of solvent in the ICP may also lead to a reduction of the ionization efficiency. Observation of the slopes of the curves for the MCN-100 indicates that the same behaviour could probably also be observed for the latter nebulizer but in the sample uptake range studied (up to 625 sapphire nebulizer Orifice / Peek union (1) Fig. 1 Schematic representation of the MCN-100 microconcentric nebulizer (taken from the MCN-100 manual CETAC Technologies USA).( 1 ) PEEK union used to connect the sample tubing to the polyimide nebulizer capillary. (2) Nebulizer gas port used to connect the nebulizer gas supply from the ICP-MS instrument to the MCN with standard alumina injector. The instrumental settings and the measurement parameters used are briefly summarized in Table 1. The microconcentric nebulizer investigated (MCN-100 CETAC Technologies) is schematically represented in Fig. 1 (taken from the MCN-100 manual). Without any further modification this nebulizer can be mounted onto the standard spray chamber. The sample was transported to the standard cross-flow nebulizer using PVC pump tubing with an internal diameter of 0.635mm and to the MCN-100 using PVC pump tubing with an internal diameter of 0.127 mm for sample uptake rates from <1 to 87 p1 min-' 0.254mm for sample uptake rates from 3 to 155 pl min-' or 0.381 mm for sample uptake rates from 13 to 650 p1 min-' (CETAC Technologies).Sample uptake rates were determined by weighing of the amount of Millipore Milli-Q water pumped in a given time interval e.g. 10 min (carried out before and after each experiment). Standards All standard solutions used (50 or 100 pg 1-') were prepared from commercially available stock solutions (1 g 1-') by dilution with 0.14 mol 1-' HN03 (14 mol 1-' HNO purified by sub-boiling distillation diluted with Millipore Milli-Q water). In order to study the features of the MCN-100 signal behaviour plots were recorded for a number of elements spread across the mass range (Be Co In Ba and U) and the level and behaviour of doubly charged and oxide ions was evaluated using the signals of U2+ (m/z 119) and UO' (m/z 254) respectively. In addition Se was also monitored since it is the subject of the elemental speciation studies planned with CZE.In the study of matrix effects Pt was also included since earlier experience showed that severe memory effects necessitating demounting and cleaning of the sample interface may occur for this element.31 The stability of sample introduction using the MCN-100 was studied using the ion signal intensities of 6 elements showing an ionization potential from 5.79 V (In) up to 9.81 V (As) and a mass number from 9 (Be) up to 158 (Gd). Finally the synthetic matrices of different origin (acid organic and high salt content) were prepared using reagents of the highest purity available (pro analisi grade) at our laboratory H2S04 EtOH and NaCl.HNO (0.14 moll-') was throughout considered as the reference matrix. 544 Journal of Analytical Atomic Spectrometry August 19968 Vol. 1 10.4 0.6 0.8 I 1.2 25 I 20 I5 10 0 0.4 0.6 0.8 I 1.2 0.4 0.6 0.8 I 1.2 50 0.4 0.6 0.8 I 1.2 Nebulizer gas flow ratel 1 m i d Fig. 2 Signal behaviour plots at different rf powers observed for sample introduction with the MCN-100 at a sample uptake rate of 45 p1 min-' (a) 5 9 c O + ; (b) 238u+ . ( ) 238U2+ * a nd (d) 238UO+ 600 I 600 2 g ." B 2. 8 .9 4 +O Y 300 200 100 0 0 0.2 0.4 0.6 1 300 *MCN 0.254 IIIIII (Y,) 0.8 1 1.2 1.4 1.6 Sample uptake ratelm1 mid Fig. 3 59C0+ signal intensity (100 pg 1-') as a function of the sample uptake rate for the MCN-100 and the standard cross-flow nebulizer (optimum nebulizer gas flow rate; rf power 1300 W; sample tubing used indicated in the figure) p1 min-') the signal intensity only shows a continuous enhancement on increasing the sample uptake rate.Fig. 4 represents the variation in the Co' signal intensity as a function of the nebulizer gas flow rate at different sample uptake rates for both the MCN-100 and the standard cross- flow nebulizer. It is clear that the sample introduction is more efficient for the MCN-100 since at comparable sample uptake rates the intensity observed for the MCN-100 is significantly higher than that for the standard cross-flow nebulizer (1.7 x higher at about 350 pl min-' and about 2.9 x higher at about 50 pl min-I).It was also observed that this difference in transport efficiency between both nebulizers continuously increases with decreasing sample uptake rate. Finally even at sub-pl min-l sample uptake rates the MCN-100 still gives rise t o a stable ion signal while for the standard cross-flow nebulizer operation at sample uptake rates <30 pl min-l is GCF 350 p1 min" *MCN 50 p1 m i d +MCN 0.6 p1 min" 0.4 0.6 0.8 1 1.2 Nebulizer gas flow ratell m i d Fig. 4 Signal behaviour plots for 100 pg 1-' Co at different sample uptake rates observed for sample introduction with the MCN-100 and the standard cross-flow nebulizer at an rf power of 1300 W not possible (signal intensity at sample uptake rates <30 pl min-l barely exceeding the blank level).The latter obser- vation is of course of great importance in the framework of application of the MCN-100 as an interface between capillary zone electrophoresis and ICP-MS. Figures of merit Results for the short term stability (RSD for 10 consecutive measurements total measurement time of 30 min) are presented in Table2. Typically for sample uptake rates between 6 and 80p1 min-' these RSDs are observed to be <l%. These results seem to be slightly better than those observed in day- to-day operation with the standard cross-flow nebulizer at a sample uptake rate of about 1 ml min-l (typically 1-2% RSD). An investigation was also carried out to discover whether the improved nebulization efficiency would have a beneficial effect on the isotope ratio precision.However isotope Journal of Analytical Atomic Spectrometry August 1996 Vol. 11 545Table 2 data acquisition mode 1 data point per peak 100 ms dwell time per data point) Temporal stability (30 min measurement time n= 10) of the MCN-100 at different sample uptake rates (rf power 1250 W peak hop Sample uptake RSD (Yo) RSD (Yo) RSD (Yo) RSD (Yo) RSD (Yo) RSD (Yo) rate/pl min-' 9Be 59c~ 7 5 A ~ 82Se 115~n "'Gd 6 0.85 0.73 0.85 0.81 1.04 0.84 10 0.59 0.62 0.96 1.67 0.86 0.89 20 0.80 0.8 1 0.65 0.36 0.69 0.8 1 40 0.8 1 0.46 0.62 0.60 0.44 3.73 80 0.98 0.91 0.86 2.95 0.80 0.53 ratio measurements carried out both at a typical sample uptake rate for each nebulizer (about 370 p1 min-' for the MCN-100 and about 1 ml min-' for the standard cross-flow nebulizer) and at about 37 pl min-' for both nebulizers in both cases using the same measurement parameters did not show a great improvement in isotope ratio precision for the MCN-100 (Table 3).Finally the occurrence of memory effects was tested by monitoring the signal intensities of 82Se lg5Pt and 238U after nebulization of 50 pg 1-' standard solutions. For all three elements rinsing of the sample introduction system with 0.14 mol 1-l HN03 for only a few minutes sufficed to bring the blank level back to the original value. For Pt this was quite surprising when compared to earlier experiences with both a VG PlasmaQuad I and a Finnigan MAT Element in their standard configuration. The absence of severe memory effects for Pt (observed for both nebulizers investigated) in the present study probably results from the fact that no glass components are used in the sample introduction system.MO' and M2' levels and behaviour Fig. 5(u) represents the variation of the (UO' U') ratio as a function of the nebulizer gas flow rate at different sample uptake rates. As can be expected on the basis of the transport efficiencies for comparable sample uptake rates the oxide levels observed with the MCN-100 are somewhat higher than those observed with the standard cross-flow nebulizer the higher analyte transport efficiency of the MCN-100 is of course accompanied by a higher solvent transport efficiency such that the plasma temperature with the MCN-100 is somewhat lower,37 leading to slightly higher (MO' M') ratios. Also with increasing sample uptake rates the oxide level increases as a result of more water being introduced into the pla~rna.~~-~O 0.4 0.5 0.6 0.7 0.8 0.9 1 0.04 0 I!! ._ B 5 e +^ ? 0.004 Q CF 350 pI mi" *MCN 50 plmin-' +MCN 0.6 plmin-' I .. . . 0.4 0.5 0.6 0.7 0.8 0.9 I Nebulizer gas flow rate11 mid Fig. 5 [UO+ U'] (a) and [U2+ U'] (b) ratios as a function of the nebulizer gas flow rate at different sample uptake rates observed for sample introduction with the MCN-100 and the standard cross-flow nebulizer at an rf power of 1300 W Table 3 Isotope ratio precision (RSDYO n= 10) obtained using the MCN-100 and the standard cross flow nebulizer for sample introduction (100 pg 1-l Pb peak hop data acquisition mode 10 ms dwell time per data point) (a) 1000 W rfpower 5 data points per peak and 455 sweeps per replicate Nebulizer (sample uptake rate) 204Pb/208Pb (%) MCN-100 (370 pl min-') Standard (1 ml min- l) MCN-100 (37 pl min-l) Standard (37 pl min-') 0.61 0.38 0.83 0.82 zo6Pb/208Pb (%) 0.33 0.19 0.46 0.30 207Pb/208Pb (Yo) 0.21 0.13 0.46 0.35 (b) 1300 W rfpower 5 data points per peak and 455 sweeps per replicate Nebulizer (sample uptake rate) 204Pb/208Pb (Yo) MCN-100 (370 pl min-') Standard (1 ml min-') MCN-100 (37 pl min-') Standard (37 pl min-') 0.62 0.28 0.95 0.94 (c) 1000 W rfpower 1 data point per peak and 2275 sweeps per replicnte Nebulizer (sample uptake rate) 204Pb/208Pb (%) MCN-100 ( 3 7 0 ~ 1 min-') Standard (1 ml min-') MCN-100 (37 pl min-') Standard (37 p1 min-') 0.49 0.48 0.68 0.89 206Pb/208Pb (%) 0.17 0.14 0.25 0.27 206Pb/208Pb (%) 0.15 0.21 0.21 0.29 207Pb/208Pb (Yo) 0.18 0.14 0.13 0.26 207Pb/208Pb (Yo) 0.16 0.23 0.28 0.31 546 Journal of Analytical Atomic Spectrometry August 1996 1'01.11Of course the variation of the (U2+ U') ratio with the carrier gas flow rate at different sample uptake rates [Fig.5(b)] can be explained in an analogous way at comparable sample uptake rates the level of doubly charged ions is somewhat lower with the MCN-100 than with the standard cross-flow nebulizer and an increase in the sample uptake rate leads to a cooler plasma (more water being introduced into the plasma) and hence a lower level of doubly charged ions. Matrix eflects The effect of a concomitant matrix on the signal intensity was studied using synthetic matrices of different origin acid (0.5 mol I-' H,SO,) organic (2.5% EtOH) and high salt content (4 g 1-' NaC1).Throughout these experiments 0.14 mol 1-' HN03 was considered as the reference matrix. Since it is known that matrix effects (matrix-induced signal suppression or enhancement) may be the result of both a displacement of the zone of maximum M+ density and a reduction or enhance- ment of the maximum signal intensity attainable (at an opti- mum position of the zone of maximum M+ density with respect to the sampling cone),35 the effects of a concomitant matrix can be studied most appropriately by registration of signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate). The introduction of 0.5 moll-' H2S04 leads to a suppression of the maximum "'In' signal intensity for both the MCN-100 and the standard cross-flow nebulizer (Fig.6). For both the MCN-100 and the standard cross-flow nebulizer the introduc- tion of 4 g 1-' NaCl leads to a suppression of the maximum signal intensity which is not accompanied by an important shift of the position of the zone of maximum "'In+ density (Fig. 6). This signal suppression can probably be attributed to 0.4 0.6 0.8 1 1.2 250 I I 1 F/ f 0.14 rnol I-' HNOJ flandard cross-flow nebulizer A * 0.5 rnol I-' H$04 .E 2oo 0.4 0.6 0.8 1 1.2 1.4 1.6 Nebulizer gas flow ratell m i d Fig.6 Signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate) observed for 1OOpg 1-' In in 0.14 mol 1-' HN03 0.5 rnol 1-' H2S04 2.5% EtOH and 4 g 1-1 NaCl for sample introduction using (a) the MCN-100 and (b) the standard cross-flow nebulizer at a sample uptake rate of about 5Opl min-' and an rf power of 1300 W a shift in the ionization eq~ilibrium,~' ambipolar diffusion effects4' and/or nebulizer effects.Most important however is that in spite of the small internal diameter continuous nebuliz- ation of solutions containing up to 4 g 1-l NaCl for longer periods (up to 20 min) does not cause clogging of the capillary of the MCN-100. The introduction of 2.5% EtOH leads to a slight increase in the maximum "'In' signal intensity and a shift of the signal behaviour plot to lower nebulizer gas flow rates (Fig. 6). Both observations are to be attributed to a higher nebulization efficiency observed on addition of EtOH to the sample solu- t i o n ~ .~ ~ ~ ~ With increasing nebulization efficiency not only does the amount of analyte introduced into the ICP increase (increase in maximum signal intensity) but also the amount of solvent. Hence the plasma becomes somewhat cooler,37 such that a longer residence time is required for complete ionization. Therefore a lower nebulizer gas flow rate is required for an optimum position of the zone of maximum "'In' density with respect to the sampling cone. Finally the introduction of 2.5% EtOH (and to a lesser extent also 0.5 mol I-' H2S04) was observed to lead to an exceptional increase in the "Se + signal intensity compared with the reference matrix (Fig. 7). This effect is not typical for the MCN-100 [compare Fig. 7(u) with Fig. 7(b)] and has also been observed by several other research g r o ~ p s .~ ~ - ~ ~ It is discussed in more detail elsewhere.48 The effect described cannot be attributed to an improvement in the nebulization efficiency (as could be seen by comparison of the magnitude of the enhancement effects for In and Se respectively) but probably results from energy transfer reactions from C+ to neutral Se within the plasma enhancing the sensitivity for the latter element.49 0.4 0.6 0.8 1 1.2 50 3 30 v (b) standard cross-flow nebulizer 8 0.5 moll-' b S 0 4 * 2.5% EtOH 0.4 0.6 0.8 1 1.2 1.4 I .6 Nebulizer gas flow rate/l min-' Fig.7 Signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate) observed for 1OOpg 1-' Se in 0.14 mol 1-' HNO 0.5 moll-' H2S04 2.5% EtOH and 4 g 1-' NaCl for sample introduction using (a) the MCN-100 and (b) the standard cross-flow nebulizer at a sample uptake rate of about 50 p1 min-' and an rf power of 1300 W Journal of Analytical Atomic Spectrometry August 1996 Vol.11 547CONCLUSIONS The MCN- 100 is a recently commercially introduced microcon- centric nebulizer which is available at a price only slightly higher than that of the standard cross-flow nebulizer. At sample uptake rates 3 30 pl rnin-l the characteristics (e.g. M+ signal behaviour level and behaviour of oxide and doubly charged ions stability isotope ratio precision and matrix effects) of both nebulizers are comparable although a limited improvement in transport efficiency can be observed for the MCN-100. Since in spite of the limited diameter of the capillary nebulization of solutions containing up to 4 g 1-1 NaCl does not lead to clogging the standard cross-flow nebulizer may be replaced by the MCN-100 for routine oper- ation although the benefit obtained at these sample flow rates is somewhat limited.At flow rates <30 pl min-l however with the MCN-100 stable introduction of sample solution into the ICP is still guaranteed (RSDs d 1 %) while the standard cross-flow nebul- izer does not allow operation at these uptake rates. 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S. and Praphairaksit N. 2996 Winter Conference on Plasma Specrochemistry (Fort Lauderdale) Abstracts Book p. 189 ThP1. Paper 6/01 7820 K Received March 13 1996 Accepted May 16 1996 548 Journal of Analytical Atomic Spectrometry August 19968 Vol. 1 1