摘要:

Comparison of characteristics and limits of detection of pneumatic micronebulizers and a conventional nebulizer operating at low uptake rates in ICP-AES† Invited Lecture Jose�-Luis Todolý�,*‡a Vicente Hernandis,a Antonio Canalsa and Jean-Michel Mermetb aDepartamento de Quý�mica Analý�tica, Universidad de Alicante, 03071 Alicante, Spain. E-mail: jose.todoli@ua.es bLaboratoire des Sciences Analytiques (UMR 5619), Universite� Claude Bernard-Lyon, F-69622 Villeurbanne Cedex, France Received 21st January 1999, Accepted 4th March 1999 Three micronebulizers, the high-eYciency nebulizer (HEN), the microconcentric nebulizer (MCN) and the micromist (MM), were compared with a conventional pneumatic concentric nebulizer working at low liquid flow rates in ICP-AES.The gas back-pressure, the free liquid aspiration rate, the drop size distribution of primary and tertiary aerosols, the solvent and analyte transport rates, the emission intensity and the limits of detection were measured.The solvent evaporation inside the spray chamber proved to be a very important transport phenomenon when working at very low liquid flow rates. The micronebulizers produced finer primary aerosols, higher solution transport rates through the spray chamber and higher sensitivities than the conventional pneumatic concentric nebulizer. The HEN used in this work provided slightly lower ICP-AES limits of detection than the other two micronebulizers, but at the expense of a higher back-pressure. at low liquid flow rates, thus improving the nebulizer perform- Introduction ance in terms of surface generation.The HEN and MM are Nebulization of solutions is by far the most common means made of glass, whereas the MCN is manufactured of an of sample introduction in inductively coupled plasma atomic HF-resistant polymer. emission spectrometry (ICP-AES). Normally, the ICP-AES Liu et al.6,7 successfully used the HEN to analyze four conventional liquid sample introduction systems consist of a certified samples by ICP-AES and ICP-MS.This nebulizer did nebulizer, mainly of the pneumatic concentric type, a spray not suVer from tip blocking when working with biological and chamber and an injector tube.1 plant reference materials. Because of its characteristics, the Nowadays, the analysis of very small volumes of sample HEN has been used as an interface between liquid separation solutions is becoming one of the key research subjects in techniques and ICP-MS.9,10,14 Pergantis et al.9 employed the HEN with a microscale FIA-HPLC system and ICP-MS for atomic spectroscopy.This is explained by the great number of the determination of arsenic in very low sample volumes. areas in which the sample size may be limited: semiconductor, Good reproducibility was observed even for 0.5 ml sample clinical, geological, on-chip technology,2 etc. In addition, volumes.3,9 Because of its small dead volume, less band coupling ICP-AES with separation methods may require low broadening was observed in capillary electrophoresis with the liquid uptake rates.HEN than a conventional pneumatic nebulizer.10,11 Generally, although conventional nebulizers can be used at The MCN was satisfactorily applied to the determination rates under 1 ml min-1, their design is not optimized for this of rare earth elements in wine by ICP-MS,17 showing that the purpose, as regards either the dead volume or the liquid and matrix eVects were less severe than for a conventional pneu- gas injection areas.There is, therefore, a need for nebulizers matic nebulizer, which was explained by the lower liquid flow devoted to work eYciently at rates as low as 10 ml min-1.3–34 rates employed with the MCN. This nebulizer can withstand Among them, there are pneumatic micronebulizers that are NaCl solutions without tip blocking.16,18,22 The MCN was usually employed in conjunction with a spray chamber, such also applied, with good results, to As and Se speciation by as the high-eYciency nebulizer (HEN),3–15 the microconcentric employing a microbore column in ICP-MS.23 A recent paper25 nebulizer (MCN)16–26 and, more recently, the micromist described the features of the MCN coupled to a cyclonic spray (MM).27 All are concentric nebulizers and the main diVerence chamber as a capillary electrophoresis–ICP-MS interface.with respect to the conventional nebulizers is a conspicuous The MCN has also been adapted to a desolvation system.reduction in their critical dimensions (i.e., gas and/or liquid In this case, the aerosol is first heated by means of a convection/ exit cross-sectional areas and liquid capillary wall thickness). conduction mechanism and the generated vapor is then This reduction allows a more eYcient gas–liquid interaction removed by means of a porous membrane.3,20,26,28,29 This assembly is known as the MCN6000 and evidence was given that matrix (acids) eVects were reduced both in ICP-AES20 †Presented at the 1999 European Winter Conference on Plasma and ICP-MS.29 Spectrochemistry, Pau, France, January 10–15, 1999.No results on the characterization of the behavior of the ‡Work performed while on leave from the Universidad de Alicante, Spain. MM were found in the literature. J. Anal. At. Spectrom., 1999, 14, 1289–1295 1289One should also mention that two micronebulizer designs Filtration was eVected by forcing the solution through a 1.2 mm pore size filter (Millipore, Bedford, MA, USA).have been described in which the spray chamber was avoided (i.e., the aerosol was directly injected into the plasma): the The gas flow rate was controlled by means of a mass flow controller (5857 TR Series, Brooks Instruments, Veenendaal, direct injection nebulizer20,26,30–34 and the direct injection highe Yciency nebulizer (DIHEN).3,12,13 This leads, obviously, to The Netherlands). The optimized value in terms of the ICP-AES SBR was 0.70 l min-1 for all the nebulizers.an analyte transport eYciency close to 100%. Because of this, under the same conditions, the sensitivity is expected to Aerosol data were obtained by using a Model 2600c Fraunhofer laser diVraction system (Malvern Instruments, increase and the wash-out times to decrease with respect to the other nebulizers. However, as more solvent load is also Malvern, Worcestershire, UK) equipped with a lens of 63 mm focal length that allowed the measurement of drop diameters introduced with the concomitant analyte, the deterioration of the plasma excitation properties (in ICP-AES) and/or the in the range 1.2–118 mm. The software was the B.0D version.A model-independent algorithm was used to calculate the drop increase in the spectral interferences (in ICP-MS) must be taken into consideration when these nebulizers are to be size distribution from the energy data. In order to minimize the eVect of solvent evaporation on the characteristics of the selected.Together with the pneumatic micronebulizers mentioned aerosols generated by the nebulizer (primary aerosol ), the nebulizer tip was placed 5 mm away from the beam axis. Note above, some others have been proposed based on diVerent principles, such as the micro-ultrasonic nebulizer35 and the that the beam was around 10 mm in diameter. The aerosols emerging from the spray chamber (i.e. tertiary aerosols) were oscillating capillary nebulizer.14,36 According to previously published work, the main advantage measured by placing the outlet of the spray chamber 5 mm away from the laser beam center.of the micronebulizers is that they give rise to limits of detection (LODs) similar to those obtained with the conven- Unless stated otherwise, the surface mean diameter (D3,2) was selected to describe the aerosol mean size. A set of three tional nebulizers, but at liquid flow rates 10–40 times lower.3,4,6,12,15 Obviously, this is the result of increased trans- replicates was performed in each case.For primary aerosols, the D3,2 precision (RSD) was seen to be dependent on the port eYciency4 and/or enhanced signal stability.8,15 Low sample consumption rates also mean a decrease in the waste liquid flow rate (Ql) employed; the lower was Ql, the higher were the RSD valueently, the RSD changed from management costs.33 So far, no systematic comparison has been performed <1 to around 6% on decreasing Ql from 600 to 20 ml min-1.This was clearly a problem resulting from the very low liquid between these diVerent pneumatic concentric micronebulizers (PCMNs) in ICP-AES. Therefore, the aim of the present study volume of the aerosol. When it is too low (i.e. several microliters per minute), small variations in the background was to evaluate the behavior of these pneumatic micronebulizers (i.e., HEN, MCN and MM) in ICP-AES with aqueous may lead to significant shifts in the Malvern energy distribution and, consequently, in the aerosol drop size distribution, thus solutions, by reference to a conventional nebulizer.To this end, the drop size distribution of the aerosol, the solvent and giving rise to a degradation of the precision.19 The solvent transport rate (Stot) was measured by means of analyte transport rates, the signal-to-background ratio (SBR) and the LODs were determined. a direct method, i.e., by the adsorption of the aerosol in a Utube filled with silica gel during a 10 min period.37 By weighing it before and after the aerosol tube exposure, the Stot values Experimental were easily derived.The analyte transport rates were obtained by a direct Three diVerent pneumatic concentric micronebulizers were used in conjunction with a double pass spray chamber: a high- method, i.e., by collecting the aerosol on a glass-fiber filter (type A/E, 47 mm diameter, 0.3 mm pore size; Gelman Sciences, eYciency nebulizer (HEN) (Meinhard, Santa Ana, CA, USA), an MCN-100, microconcentric nebulizer (MCN) (CETAC Ann Arbor, MI, USA) placed above the spray chamber.38 A 500 mg ml-1 Mn solution was nebulized.The Mn retained Technologies, Omaha, NE, USA) and an AR30-1-FM005 micromist (MM) (Glass Expansion, Camberwell, Victoria, after a period of 10 min was extracted by washing the filters with 1.0% (m/m) hot nitric acid. The total solution volume Australia).A TR-30-A3 conventional pneumatic concentric nebulizer (PN) (Meinhard) was selected for comparison. was adjusted to 100 ml in a calibrated flask. Finally, the Mn concentration in each solution was determined by flame atomic Table 1 gives their relevant dimensions. In order to compare the behaviors of the nebulizers absorption spectrometry (FAAS). A Perkin-Elmer Optima 3000 DV ICP-AES system was employed, the same Ryton double-pass Scott-type spray chamber (120 ml inner volume) was used to transport and used in the axial viewing mode.Table 2 gives the experimental conditions. This instrument includes a 40 MHz free-running filter the aerosol towards the torch; this is the most widely employed spray chamber in ICP-AES. generator, a polychromator with an e� chelle grating of 79 lines mm-1 with a blaze angle of 63.4° and a cross-dispersing For the PN, HEN, MCN and MM, the liquid flow was varied in the range 5–160 ml min-1 by means of a peristaltic element for the UV range and a prism for the visible and a segmented-array charge-coupled device (SCD) detector that pump (Perkin-Elmer, Norwalk, CT, USA).Tygon capillaries of 0.25 mm id were used. allowed the simultaneous measurement of several line intensities in addition to the background signals. The resolution of Aqueous sample solutions were filtered in order to prevent capillary blocking especially with the HEN and the MCN. Table 2 Instrumental conditions of the ICP-AES spectrometer Table 1 Characteristics of the nebulizers used Gas outlet Liquid Capillary Rf power 1.2 kW Outer gas flow rate 15 l min-1 cross-sectional capillary inner wall Nebulizer area/mm2 diameter/mm thickness/mm Intermediate gas flow rate 0.5 l min-1 Nebulizer gas flow rate Variable Integration time 20 ms PN 0.028 0.40 0.06 HEN 0.011 0.10 0.03 Sampling time 1000 ms Injector id 2 mm MCN 0.017 0.10 0.03 MM 0.025 0.14 0.05 Plasma viewing mode Axial 1290 J.Anal. At.Spectrom., 1999, 14, 1289–1295Table 3 Elements, lines and Esum values a given gas flow rate the pressure is expected to follow the order: HEN>MCN>MM. The results agreed with the Element l/nm Esum/eVa expected trends. Thus, to keep the gas flow rate at 0.7 l min-1, the measured back-pressures were 120 psig (850 kPa), 36 psig Cr II 205.55 12.77 (250 kPa) and 20 psig (140 kPa) for the HEN, MCN and Mn II 257.61 12.25 Mg II 280.27 12.07 MM, respectively. For the conventional nebulizer the back- Sr II 421.55 8.64 pressure was 15 psig (110 kPa).According to these results, at K I 766.49 1.62 a given liquid and gas flow rate, the amount of energy available Li I 670.78 1.85 for the nebulization was larger for the HEN than for the Mg I 285.21 4.30 remaining nebulizers. Zn I 213.86 5.80 Pneumatic concentric nebulizers are normally able to aEsum=ionization potential+excitation potential. aspirate a solution at a given flow rate without any external pumping device.At a 0.7 l min-1 gas flow rate (Qg) the free liquid uptake rates were 40 and 90 ml min-1 for the HEN and the system was kept at its normal value, i.e., each pixel was MM, respectively. Although this magnitude was not measured considered as a whole. for the MCN, recent studies23 reported that the free liquid A standard solution containing Cr, Zn, Mn, Mg, Sr, Li and uptake rate for an MCN lies between 30 and 50 ml min-1 K (1 mg ml-1) was used. Table 3 lists the characteristics of the depending on the gas flow rate.The conventional nebulizer, lines employed. in turn, gave a liquid uptake rate of 160 ml min-1. These results can also be accounted for by considering that the Results and discussion smaller the inner diameter of the liquid capillary, the lower is the free aspiration rate (Table 1).41 Design considerations Fig. 1 displays the images of the tips corresponding to the Primary drop size distributions three PCMNs, showing that the HEN, MCN and MM are Few studies dealing with the drop size characterization of the diVerent in design.Thus, for the HEN [Fig. 1(a)], both liquid aerosols generated by the PCMN have been published3–6,12,19 and gas outlets are located on the same plane. For the MCN and they are mainly devoted to the comparison of the PCMN [Fig. 1(c)], the liquid capillary ends outside the nebulizer. In with conventional nebulizers. Fig. 2 shows that for the three the case of the MM, Fig. 1(b) indicates that the sample micronebulizers the primary aerosols become coarser (D3,2 capillary exhibits a recess with respect to the nebulizer tip. increases) as Ql increases. This widely described behavior is According to the pictures shown in Fig. 1, the gas–liquid typical for pneumatic concentric nebulizers and it is due to interaction for the HEN takes place just at the exit of the the increase in the energy per unit mass ratio, thus enhancing nebulizer. For the MCN this interaction is produced at a given the generation of liquid surface (i.e., finer aerosols).4–6,42–44 distance from the gas exit [approximate measurements indi- Fig. 2 also shows that the three micronebulizers generated cated that the portion of liquid capillary depicted in Fig. 1(c) primary aerosols with lower D3,2 values (i.e., finer) than the was around 400 mm in length]. In the case of the MM the conventional nebulizer. This is reasonably the combined liquid is subject to the action of the high velocity gas stream result of their smaller gas and liquid sections and thinner wall inside the nebulizer. As a result of these diVerences, it can be of the liquid capillary, that make the liquid–gas interaction predicted that the gas–liquid interaction will be more eYcient more eYcient.40,44 One should highlight the importance of the for the MM than for the MCN.This assessment is supported capillary diameter on the characteristics of the aerosol by the fact that for the MCN the aerosol generation is generated by comparing the data for PN and MM.produced when the gas stream has lost a fraction of its initial Among the micronebulizers, the HEN generated the finest kinetic energy.39,40 From this point of view, the HEN primary aerosols (i.e., lowest D3,2, Fig. 2). These expected represents an intermediate situation. data were the result ofhe higher gas back-pressure (i.e., Besides these considerations, the back-pressure is a very kinetic energy) for the HEN.40,42–44 However, the aerosols useful parameter to evaluate the potential nebulizer performgenerated by the MM are slightly finer than those generated ance.According to the gas section areas given in Table 1, for by the MCN. The reason for this behavior is probably that, as has already been pointed out for the MCN, the aerosol Fig. 2 Primary aerosol mean surface diameter (D3,2) versus liquid flow Fig. 1 Images of the nebulizer tip. (a) High-eYciency nebulizer; rate, Ql.(A) High-eYciency nebulizer; (B) micromist; (C) microconcentric nebulizer; (D) conventional pneumatic concentric nebulizer. (b) micromist; (c) microconcentric nebulizer; (d) conventional pneumatic concentric nebulizer. Qg=0.8 l min-1. J. Anal. At. Spectrom., 1999, 14, 1289–1295 1291Fig. 4 Volume drop size distribution curves for the tertiary aerosols. Fig. 3 Volume drop size distribution curves for the primary aerosols. (A) High-eYciency nebulizer; (B) micromist; (C) microconcentric (A) High-eYciency nebulizer; (B) micromist; (C) microconcentric nebulizer; (D) conventional pneumatic concentric nebulizer. Ql= nebulizer; (D) conventional pneumatic concentric nebulizer.Ql= 0.6 ml min-1; Qg=0.8 l min-1. 0.6 ml min-1; Qg=0.8 l min-1. comparison between the diVerent nebulizers led to similar conclusions as for primary aerosols. Again, the micronebulizers generation takes place at a given distance from the gas outlet. In this zone, the initial amount of kinetic energy available to generated finer tertiary aerosols than the conventional nebulizer.Liu and Montaser5 achieved a similar conclusion for the generate the aerosol is reduced because of the expansion of the gas stream produced at the nebulizer gas vent. Therefore, HEN in comparison with concentric and cross-flow standard nebulizers. On comparing the three micronebulizers, the HEN the amount of energy useful to spread out the liquid bulk is reduced.Hence the gas–liquid interaction is less eYcient, thus gave rise to the finest tertiary aerosols (Fig. 4). Droplets with diameters larger than 8 mm have been reported giving rise to coarser aerosols than expected according to the back-pressure values. not to be useful for analytical purposes. Olesik and Fister46 found that these droplets were not completely vaporized in Fig. 3 shows the volume drop size distribution curves, presented as the variation of the percentage of liquid volume the plasma measurement zone, leading to large plasma perturbation and noise.According to the data in Fig. 4, the percent- contained in droplets under a given drop diameter versus the drop diameter, D (i.e., undersize representation) for the four ages of liquid volume contained in droplets larger than 8 mm (V8) were 3, 11, 16 and 26% for the HEN, MM, MCN and nebulizers, showing the same trends as in Fig. 2. Hence, under the conditions employed in Fig. 3, the percentage of the aerosol PN, respectively.Another useful parameter supplied by the particle sizer is volume contained in droplets with diameters under 9.6 mm was 86, 63, 58 and 41% for the HEN, MM, MCN and PN, the so-called volume concentration (VC). This parameter is defined as the fraction of the active laser beam probe volume respectively. The results obtained for the HEN agree with previously published work by Olesik et al.4 Note that diVerent that is occupied by droplets.45 Therefore, this variable gives an indication of the amount of solution that is transported liquid and gas conditions are employed.Summarizing these results, the reduced dimensions of the sample capillary and out of the spray chamber. The examination of the VC for the tertiary aerosols indicated that (i ) VC increases on increasing the gas outlet with respect to a conventional nebulizer allow the micronebulizers to work eYciently at very low Ql values. the liquid flow rate (for the HEN, VC increased from 0.02 to 0.05% when Ql increased from 0.1 to 0.6 ml min-1) and (ii) the HEN provides the highest VC values (0.05, 0.04, 0.03 and Tertiary drop size distributions 0.02% for the HEN, MM, MCN and PN, respectively).The behavior of the tertiary aerosols as the liquid flow rate On considering simultaneously the results for V8 and VC, a was varied was close to that shown by the primary aerosols lower background noise for the PCMNs than for the MN (i.e., the lower the Ql value, the finer the tertiary aerosols). could be expected.This assessment is based on the fact that Nevertheless, the relative variations of the tertiary aerosol the absolute volume of the tertiary aerosol contained in characteristics as Ql was varied were less pronounced than droplets larger than 8 mm (i.e., proportional to V8×VC) was those of the primary aerosols because of the processes taking lower for the former. Hence, it can be easily derived that, at place inside the spray chamber (i.e., drop evaporation, coagu- 0.6 ml min-1 and 0.8 l min-1, this parameter took values of lation, losses of the coarsest droplets, etc.).These phenomena 0.15, 0.44, 0.48 and 0.52 for the HEN, MM, MCN and PN, tend to dampen the diVerences between the statistical param- respectively. eters of the tertiary aerosols with respect to those found for primary aerosols. In addition, several problems linked to the Solvent and analyte transport rates diYculty of measuring very low aerosol volumes appeared.4 Hence the examination of the D3,2 values did not provide clear In order to gain more insight into the processes taking place along the aerosol path inside the spray chamber, the total evidence of the Ql eVect, because no significant variations were found on decreasing this parameter. In these cases, some mass of solvent, Stot, and analyte, Wtot, transport rates were measured.additional aerosol parameters could be examined, e.g., the median of the volume drop size distribution (D50).45 Thus, for Fig. 5 shows the eVect of Ql on (a) Stot and (b) Wtot for the four nebulizers. The variations of these transport param- instance, at a 0.8 l min-1 gas flow rate, the D50 values of the tertiary aerosols generated by the HEN changed from 2.7 to eters with the liquid flow rate were concomitant with those of VC. 3.0 mm when the liquid flow was varied from 100 to 600 ml min-1. These results agreed with previously published On considering the diVerent nebulizers, for a given flow rate, the values of Stot and Wtot agreed well with the tertiary work.5 Fig. 4 shows plots of the volume drop size distributions of aerosol VC data discussed above. Hence, the HEN aVorded the highest transport values since it generated the finest the tertiary aerosols for the HEN, MCN, MM and PN. The 1292 J. Anal. At. Spectrom., 1999, 14, 1289–1295duced chiefly from the aerosol at locations near the nebulizer tip, 100% solvent transport eYciency would be expected at the lowest Ql.In fact, an almost 100% solvent transport eYciency for the HEN at 10 ml min-1 was obtained (Table 4). Nonetheless, at 120 ml min-1, the amount of water required to saturate the argon stream represents only between 12 and 18% of the solution volume nebulized. Note that the contribution of the evaporation from the chamber walls was neglected. The solvent transport eYciencies for the micronebulizers at the lowest liquid flow rates were fairly high (92% in the case of HEN at 10 ml min-1).Other researchers12,21 have found transport eYciencies in the range 50–100% for liquid flow rates below 20 ml min-1. Solvent transport eYciencies of up to 50% have also been reported for an MCN coupled to a double-pass spray chamber at 100 ml min-1 by Woller et al.23 for a solution of ammonium malonate. In this case, an indirect method was applied to measure Stot. As regards the conventional pneumatic nebulizer, few reports about the solvent transport at low liquid flow rates have been published.The results given in Table 4 are in agreement with previously published work.2,49 Thus, for a conventional nebulizer–double-pass spray chamber combination, Hettipathirana and Davey49 found 12% for es at Ql= 100 ml min-1. The same trends were observed for the analyte transport eYciency, en (Table 4). At 10 ml min-1, en reached values of Fig. 5 Solvent [(a) Stot] and analyte [(b) Wtot] transport rates versus 55, 43, 42 and 23% for the HEN, MM, MCN and PN, liquid flow rate, Ql, for a double-pass spray chamber.(A) Highrespectively. Liu et al.6 reported that, for the HEN operated eYciency nebulizer; (B) micromist; (C) microconcentric nebulizer; at 1.0 l min-1 and coupled to a double-pass spray chamber, (D) conventional pneumatic concentric nebulizer. Qg=0.7 l min-1. the analyte transport eYciency dropped from 50 to around 7% on increasing Ql from 11 to 220 ml min-1.Olesik et al.4 Table 4 Solvent (es) and analyte (en) transport eYciencies (Qg= found en values of 20% using the same set-up at 50 ml min-1. 0.7 l min-1) From the data presented in Table 4, it can be observed that Ql/ml min-1 Parameter HEN MM MCN PN the PN gives rise to transport eYciencies around half those obtained for the HEN. Olesik and co-workers48,50 found 10 es (%) 92 87 83 80 smaller diVerences between the analyte transport eYciencies 20 64 62 60 55 for an HEN and a PN (TR-30).In this case the former 40 41 36 34 33 nebulizer aVorded en values around 1.5 times higher than the 80 25 23 20 20 latter in the best of the cases.48 120 20 19 18 15 On comparing the es results with those for en, interesting 10 en (%) 55 43 42 23 conclusions can be drawn. First, as expected, es>en in all the 20 34 28 25 16 cases. Second, the relative improvement of es on switching 40 19 15 17 10 80 11 9 8 6 from PN to any of the PCMN was lower than that of en. 120 8 7 6 5 Thus, the solvent ratio (es)HEN/(es)PN was 1.2 at 10 ml min-1, whereas the analyte ratio (en)HEN/(en)PN reached a value of 2.4. These results can also be explained by considering that a large fraction of the solvent was transported through the spray primary aerosols and more solution mass was allowed to leave the spray chamber.38,43 chamber as a vapor. In addition, it is also apparent that the solvent mass evaporated from the inner walls of the spray The solvent transport eYciency (es) values in Table 4 indicate that es is a function of the liquid flow employed.The lower is chamber was not negligible. This factor is expected to be less significant as the liquid flow rate is increased. Therefore, it Ql, the higher are the es values.3,6,13,21 Several reasons could be put forward to explain this general behavior. The number can be concluded that the solvent evaporation inside the spray chamber is one of the predominating aerosol transport phen- of particles decreases as Ql drops.In this way, there will be less probability for the droplets to coagulate, thus generating omena when very low Ql are employed. As a consequence, parameters such as the environment temperature can modify larger droplets that could be more easily removed from the aerosol stream.47,48 In addition to this eVect, the solvent the values of both Wtot and Stot. The great importance of the solvent evaporation could also evaporation from the aerosol inside the spray chamber was, in relative terms, more significant as the liquid flow rate explain some data presented in a previous paper.19 Thus, for nitric and sulfuric acid solutions, a decrease in the emission diminished.This facilitated the aerosol transport through the spray chamber. It has been indicated that, at 25 °C, 20–30 mg signal with respect to plain water solutions was observed, this eVect being more pronounced at low (several tens of ml min-1) of water per liter of argon are required to saturate the gas stream.4,48 In our studies, a 0.7 l min-1 argon flow rate was than at high Ql (around 1 ml min-1). The extent of the solvent evaporation for acids is lower than for plain water since acid employed.Therefore, 14–21 ml of water could be evaporated per minute. The dotted line in Fig. 5(a) represents this value. solutions have a lower vapor pressure.51 Because of this, the analyte transport is expected to be more eYcient for water On comparing this line with the values of Stot, it can be concluded that a very important fraction of the solvent leaving than for acids. The (Wtot)acid/(Wtot)water ratio is expected to decrease as the liquid flow rate is reduced to a value for which the spray chamber is present in vapor form.Indeed, at the lowest values of Ql the tertiary aerosol was certainly not the solvent evaporation becomes one of the predominant aerosol transport phenomena. saturated with water.Assuming that the evaporation is pro- J. Anal. At. Spectrom., 1999, 14, 1289–1295 1293The LODs were calculated by employing the 3sb criterion, where sb is the standard deviation for 10 replicates of the blank. In general terms, the background noise produced by the PCMNs was lower than that produced by the PN, consistent with their smaller amounts of solution in droplets larger than 8 mm. Table 5 gives the LOD for the four nebulizers. As a result of the increased sensitivities and reduced background noise, the PCMNs achieved lower LOD values than the conventional nebulizer.Among the micronebulizers, the HEN gave the lowest LODs, since it exhibited the highest SBR values. The (LOD)HEN/(LOD)PN ratio was within the range 2.5–5. The background equivalent concentration values (BEC) Fig. 6 ICP-AES signal-to-background ratio (Mn 1 mg ml-1) versus Ql followed a similar trend. Table 5 indicates that the lowest BEC when using a double-pass spray chamber.(A) High-eYciency was for the HEN, followed by the MM and MCN and then nebulizer; (B) micromist; (C) microconcentric nebulizer; (D) conven- by the conventional nebulizer. These results can be accounted tional pneumatic concentric nebulizer. Qg=0.7 l min-1. for by the slightly lower background signals and the higher analyte emission signals obtained by the three PCMNs with ICP-AES analytical figures of merit respect to the PN. As expected, there is a correlation between the figures of merit and the data on aerosol characterization and solution trans- Conclusions port.As can be seen in Fig. 6, an increase in Ql led to increases From the data obtained in the present study, it can be in the SBR values. The rate of growth of the SBR became concluded that the use of micronebulizers as liquid sample smaller as the liquid flow rate increased. The comments made introduction systems for ICP-AES leads to good results when in the previous section about the ease of transport of an the sample volume or the solution rate is the limiting factor aerosol as the droplet density decreases should be applicable to perform the analysis.However, the conventional pneumatic here, also. This behavior was the same for the diVerent concentric nebulizer operated at several tens of microliters per elements and conditions tested, although, from Fig. 6, it minute has proved to be more useful than expected, since appears that a slight signal decrease was observed for the analyte transport eYciencies as good as 20% were reached.HEN at the highest liquid flow rate (i.e. 160 ml min-1). The Nevertheless, the high dead volume is still a limitation to deterioration of the plasma conditions as a result of the high apply the PN to the analysis of very low liquid sample volumes. solvent plasma load may be invoked to account for it. In Solvent evaporation was shown to be one of the major order to verify this hypothesis, the Mg II 280.27 nm to Mg I processes inside the spray chamber when working at low liquid 285.13 nm intensity ratio was calculated, as suggested else- flow rates.where.52,53 This parameter revealed that actually the plasma The best nebulizer in terms of primary aerosol features, conditions had slightly deteriorated. The ratio decreased from solution transport and ICP-AES analytical behavior was the 6.0 to 5.5 when Ql was increased from 10 to 160 ml min-1. high-eYciency nebulizer.Nevertheless, the high back-pressure Under the same conditions, the three micronebulizers required for the HEN may be a limitation for some aVorded higher SBR values than the conventional nebulizer. commercially available ICP-AES systems. This result was in agreement with the transport results (Fig. 5 The use of pneumatic concentric micronebulizers used in and Table 4). Furthermore, the signal improvement correthe direct injection mode (DIN), or incorporating desolvation sponded more or less to that of the analyte transport, i.e.at units (MCN6000), would further increase the ICP-AES sensi- 10 ml min-1 the (SBR)HEN/(SBR)PN ratio for Mn was 2.8, tivities and decrease the LODs. Nevertheless, the DIN has whereas the transport ratio was 2.4. The slight diVerences some important limitations (i.e., prone to blocking, high cost, between these two ratios could be explained by the smaller diYcult to use). More work is required in order to optimize mean sizes of the tertiary aerosols obtained when using the these systems further.One recent example is the DIHEN HEN, thus leading to a more eYcient analyte excitation developed by Montaser and co-workers.3,12,13 A comparison process. This explanation has also been suggested recently.48 among these two direct injection nebulizers would be very As happened for Wtot, the lower the Ql value, the higher was interesting. Also, a study to the response of the DIHEN to the improvement factor provided by the PCMNs.the matrix (acids, salts, etc.) eVects would be advisable. Table 5 Limits of detection (LOD) and background equivalent The authors thank Perkin-Elmer for the loan of the Optima concentrations (BEC) (Qg=0.7 l min-1; Q1=10 ml min-1) 3000 DV ICP-AES system. They are also grateful to Spin France and CETAC Technologies for the availability of the Element Parameter HEN MM MCN PN MCN. The MM was provided by courtesy of Glass Expansion Cr LOD/ng ml-1 12 17 15 30 (Switzerland).J.L.T. also thanks the European Commission Zn 10 12 14 26 for a Marie Curie Grant (Project No. ERBFMBICT961632). K 9 15 18 35 Mn 0.9 1.0 1.5 3.0 Mg II 0.6 1.4 2.2 4.0 References Sr 0.3 0.2 0.4 0.8 Li 0.4 0.8 0.5 1.3 1 Sample Introduction in Atomic Spectroscopy, ed. J. Sneddon, Elsevier, New York, 1990, p. 1. Cr BEC/ng ml-1 219 347 520 803 2 G. M. Hieftje, J. Anal. At. Spectrom., 1996, 11, 613. Zn 194 297 454 722 3 A. Montaser, M. G. Minnich, J. McLean, H.Liu, J. A. Caruso K 1444 4553 6141 9242 and C. W. McLeod, in Inductively Coupled Plasma Mass Mn 56 85 113 178 Spectrometry, ed. A. Montaser, Wiley-VCH, New York, 1998, Mg II 28 43 58 125 p. 83. Sr 31 52 70 204 4 J. W. Olesik, J. A. Kinzer and B. Harkleroad, Anal. Chem., 1994, Li 70 128 182 349 66, 2022. 1294 J. Anal. At. Spectrom., 1999, 14, 1289–12955 H. Liu and A. Montaser, Anal. Chem., 1994, 66, 3233. 29 CETAC Technologies Applications Bulletin, 003MCN1196U, CETAC Technologies, Omaha, NE, 1997. 6 H. Liu, R. H. CliVord, S. P. Dolan and A. Montaser, Spectrochim. 30 K. E. Lawrence, G. W. Rice and V. A. Fassel, Anal. Chem., 1984, Acta, Part B, 1996, 51, 27. 56, 289. 7 H. Liu, A. Montaser, S. P. Dolan and R. S. Schwartz, J. Anal. At. 31 D. R. Wiederin and R. S. Houk, Appl. Spectrosc., 1991, 45, 1408. Spectrom., 1996, 11, 307. 32 S. C. K. Shum, S. K. Johnson, H. Pang and R. S. Houk, Appl. 8 S. H. Nam, J. S. Lim and A. Montaser, J. Anal. At. Spectrom., Spectrosc., 1993, 47, 575. 1994, 9, 1357. 33 J. S. Crain and J. T. Kiely, J. Anal. At. Spectrom., 1996, 11, 525. 9 S. A. Pergantis, E. M. Heithmar and T. A. Hinners, Anal. Chem., 34 D. R. Wiederin, F. G. Smith and R. S. Houk, Anal. Chem., 1991, 1995, 67, 4530. 63, 219. 10 J. A. Kinzer, J. W. Olesik and S. V. Olesik, Anal. Chem., 1996, 35 M. A. Tarr, G. Zhu and R. F. Browner, Anal. Chem., 1993, 65, 68, 3250. 1689. 11 Q. Lu, S. M. Bird and R. M. Barnes, Anal. Chem., 1995, 7, 2949. 36 L. Wang, S. W. May, R. F. Browner and S. H. Pollock, J. Anal. 12 J. A. McLean, H. Zhang and A. Montaser, Anal. Chem., 1998, At. Spectrom., 1996, 11, 1137. 70, 1012. 37 D. D. Smith and R. F. Browner, Anal. Chem., 1982, 54, 533. 13 J. A. McLean, M. G. Minnich, L. A. Iacone, H. Liu and 38 R. F. Browner, A. Canals and V. Hernandis, Spectrochim. Acta, A. Montaser, J. Anal. At. Spectrom., 1998, 13, 829. Part B, 1992, 47, 659. 14 C. B’Hymer, K. L. Sutton and J. Caruso, J. Anal. At. Spectrom., 39 A.H. Lefebvre, Atomization and Sprays, Hemisphere, New York, 1998, 13, 855. 1989. 15 Meinhard Associates Web Site, http://www.meinhard.com, 40 B. L. Sharp, J. Anal. At. Spectrom., 1988, 3, 613. December 1998. 41 A. Kundt and E.Warburg, Ann. Phys., 1975, 155, 337. 16 M. Vanhaecke, M. Van Holderbeke, L. Moens and R. Dams, 42 A. Canals, V. Hernandis and R. F. Browner, J. Anal. At. Spectrom., 1990, 5, 61. J. Anal. At. Spectrom., 1996, 11, 543. 43 J. L. Todolý�, A. Canals and V. Hernandis, Spectrochim. Acta, Part 17 S. Augagneur, B. Medina, J. Szupnar and R. Lobinski, J. Anal. B, 1993, 48, 373. At. Spectrom., 1996, 11, 713. 44 D. E. Nixon, Spectrochim. Acta, Part B, 1993, 48, 447. 18 C. Dubuisson, E. Poussel and J. M. Mermet, J. Anal. At. 45 2600 Laser DiVraction User Manual, Malvern Instruments, Spectrom., 1997, 12, 281. Malvern, 1987. 19 J. L. Todolý�, J. M. Mermet, A. Canals and V. Hernandis, J. Anal. 46 J. W. Olesik and J. C. Fister, Spectrochim. Acta, Part B, 1991, At. Spectrom., 1998, 13, 55. 46, 869. 20 J. L. Todolý� and J. M. Mermet, J. Anal. At. Spectrom., 1998, 47 A. Canals, V. Hernandis and R. F. Browner, Spectrochim. Acta, 13, 727. Part B, 1990, 45, 591. 21 T. D. Hettipathirana and D. E. Davey, J. Anal. At. Spectrom., 48 J. W. Olesik, presented at the 1999 European Winter Conference 1998, 13, 483. on Plasma Spectrochemistry, Pau, France, 10–15 January, 1999. 22 M. De Wit and R. Blust, J. Anal. At. Spectrom., 1998, 13, 515. 49 T. D. Hettipathirana and D. E. Davey, Appl. Spectrosc., 1996, 23 A. Woller, H. Garraud, J. Boisson, A. M. Dorthe, P. Fodor and 50, 1015. O. F. X. Donard, J. Anal. At. Spectrom., 1998, 13, 141. 50 J. W. Olesik, J. A. Kinzer, J. Hartshorne and A. JeVery, presented 24 E. Debrah, S. A. Beres, T. J. Gluodenis, Jr., R. J. Thomas and at the 1998 Winter Conference on Plasma Spectrochemistry, E. R. Denoyer, At. Spectrosc., 1995, 16, 197. Scottsdale, AZ, January 5–10, 1998. 25 K. A. Taylor, B. L. Sharp, D. J. Lewis and H. M. Crews, J. Anal. 51 I. I. Stewart and J. W. Olesik, J. Anal. At. Spectrom., 1998, 13, At. Spectrom., 1998, 13, 1095. 1249. 26 CETAC Technologies Web Site, http://www.cetac.com, 52 J. M. Mermet, Spectrochim. Acta, Part B, 1989, 44, 1109. November 1998. 53 E. Poussel, J. M. Mermet and O. Samuel, Spectrochim. Acta, Part B, 1993, 48, 743. 27 Glass Expansion Web Site, http://www.glassex.com.au, November 1998. 28 R. I. Botto and J. J. Zhu, J. Anal. At. Spectrom., 1996, 11, 675. Paper 9/00598F J. Anal. At. Spectrom., 1999, 14, 1289–129

ISSN:0267-9477    

DOI:10.1039/a900598f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Potential and limitations of capillary electrophoresis inductively coupled plasma mass spectrometry† Invited Lecture Bernhard Michalke GSF National Research Center for Environment and Health, Institute for Ecological Chemistry, Ingolstaedter Landstr. 1, D-85764 Neuherberg, Germany. E-mail: bernhard.michalke@gsf.de Received 18th January 1999, Accepted 6th April 1999 The hyphenation interface of CE to ICP-MS was characterised with respect to capillary position, gas flow rates and positioning of the total interface to the ICP-MS system.It turned out that absolutely optimised and reproducible operation is possible when all variable parameters are adjustable by micrometer screws. Analysis of a laboratory reference sample gave identical results (detection limit, element concentration, time of detection) after each set-up of the system during several years. After optimising the interface, the potential of the hyphenated system was elucidated. The hyphenated system was characterised by high separation capabilities, low detection limits and a broad field of application.Here, application was extended especially to iodine speciation. Detection limits of 0.04–1.2 mg L-1, depending on the specific iodine species, were achieved. As the operation of the hyphenated system was totally reproducible according to micrometer screw drives, possible problems were found to be not typically ‘hyphenation problems’, but problems of capillary interactions and distorted separation.Wrong CE conditions were demonstrated to result in sample sticking to the capillary wall, lack of separation/focusing or decreased species stability.These problems were demonstrated by means of Sb speciation. of the technique will be elucidated in routine work. As a basis, Introduction methodical improvements have already been published by Capillary electrophoresis (CE) has continued to mature in Michalke and Schramel (1998).14 There, the potential of recent years into a useful and reproducible tool in separation distinguishing between the separation and the detection step investigations and in element speciation.The advantages of in combination with extended capillaries was ruled out. An CE include rapid analysis, low sample requirements, high increase in detection sensitivity, improved separation capabiliseparation eYciency and low operating costs.1 Especially the ties and introduction of capillary isoelectric focusing (cIEF) separation potential of CE,2 the superior detection capability coupled with ICP-MS was demonstrated.Here the potential of ICP-MS and the diversity of potential applications1,2 bring of CE-ICP-MS in iodine speciation is described as an example. CE-ICP-MS coupling to the fore in element speciation, where Problems are also experienced. During operation for several typically the separation of species is followed by element years with various set-ups it turned out that the coupling itself selective detection.was running reproducibly without remarkable problems. The The key to the successful coupling of CE with ICP-MS is observed problems were more related to the conflicting situthe interface.3 Two major requirements for interfacing the two ation of improving concentration detection limits by increased instruments have been described recently:3–5 the closing of the sample volumes and stacking procedures on the one hand and electrical circuit from CE and optimised nebulisation eYciency preserving species stability and separation eYciency on the and mass transport into the ICP-MS. Several successful other.Therefore, experiments will be described to demonstrate approaches have been described for setting up such an some of these problems and their solutions. interface, mostly working along similar technical solutions based on pneumatic nebulisation systems.3,5–12 Other attempts Experimental have used an ultrasonic nebulisation (USN) device.8,13 However, to the best of the author’s knowledge, most of these Chemicals and reagents studies involved the analysis of standard solutions and only a Stock standard solutions of iodide and iodate were prepared few demonstrated an application to low concentration real by dissolving appropriate amounts of KI and KIO3 (both samples. Also, a detailed description and operation of these from Merck, Darmstadt, Germany) in ultra-pure water systems have not been published in most cases.(18MV water) obtained from a Milli-Q system (Millipore, Therefore in this study we first checked the reproducible Bedford, MA, USA). Thyroxine (T4-hormone, Sigma, St. working of the hyphenation interface after each new hyphen- Louis, MO, USA) was dissolved in 50 mM NaOH, as it is ation session by investigating routinely the nebuliser suction soluble only in alkaline solvents.15 The laboratory reference and the performance on a laboratory reference sample (the sample was an aqueous extract of a soil, prepared as described hyphenated system is installed for ca. 1.5–2 weeks about previously,16 spiked with Na2PtCl6·6H2O (final concentration every 2–3 months). Subsequently, the potential and problems 20 mg L-1). Stock standard solutions of Pt were prepared by dissolving 5 mg of Na2PtCl6·6H2O (Aldrich, Steinheim, Germany) in 100 mL of 18MV water. This stock standard †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999.solution was kept frozen until use. J. Anal. At. Spectrom., 1999, 14, 1297–1302 1297Stock standard solutions of Sb(III) (100 mg L-1) and Sb(V) to a nut by a screw and a silicone-rubber seal. Moving this nut also moved the CE capillary. Reliable closing of the CE (100 mg L-1) were prepared by dissolving the appropriate amount of potassium antimonyl tartrate trihydrate (Aldrich; electrical circuit during nebulisation was provided by a coaxial sheath flow (10 mL h-1 buVer) around the CE capillary. ACS grade) and of potassium hexahydroxyantimonate (Fluka, Buchs, Switzerland; analytical-reagent grade) in 18MV water.The nebuliser fitted tightly into a laboratory made spray chamber which was specially designed considering the nebulis- Trimethylantimony dichloride [(CH3)3SbCl2] was donated by Professor W. Cullen (Vancouver Canada) having been syn- ation characteristics of the CE-ICP-MS nebuliser. An additional Ar–H2 gas stream was directed to the torch of the thesised according to published methods.17,18 A stock standard solution of this compound was prepared by dissolution in ICP-MS and ‘coated’ the inner surface of the spray chamber.This avoided aerosol condensation on the walls of the chamber water. All stock standard solutions were stored in the dark at 4 °C. and the aerosol was transported completely into the plasma. The very small amount of aerosol coming from the CE Working standard solutions were prepared daily by appropriate dilution with 18MV water.capillary was transported more eYciently to the Ar plasma. Further, the addition of an Ar–H2 gas stream influenced the CE buVers plasma and optimised ionisation.21 Therefore, detection limits and signal stability (especially at low concentrations) were CE methods with buVers and stacking electrolytes are listed improved. A height adjustable platform (with micrometer in Table 1. A stock standard solution of 100 mM screw) was set below the interface for optimising its position Na2HPO4–NaH2PO4 (pH 5.6) was prepared by taking approrelative to the ICP-MS.priate amounts of both salts and dissolving in 18MV water. NaOH and acetic acid (analytical-reagent grade) were obtained Element selective determination from Merck and used after appropriate dilution. Borate buVer (pH 8.3) (Bio-Rad, Munich, Germany) and acidic phosphate The ICP-MS (ELAN 5000, Perkin-Elmer, SCIEX, Thornhill, buVer (pH 2.3) (Bio-Rad) were used as received.If necessary, ON, Canada) was used in the graphic mode with the laboratory lower concentrations of the buVers were achieved by dilution made nebuliser fitting into a laboratory made spray chamber with 18MV water. Na2CO3 (50 mM, pH 11.6) was prepared (id corresponding to od of the nebuliser). Antimony was by dissolving the appropriate amount in 18MV water. This determined at m/z 121 and iodine at m/z 127. In the laboratory solution was used as a stacking electrolyte or, after dilution reference sample platinum was monitored at m/z 195.with 18MV water, as a background electrolyte (20 mM). The Instrumental parameters were taken from ref. 22 with the terminating electrolyte was prepared by mixing the background following modifications: rf power, 1200 W; nebuliser gas, Ar electrolyte with acetonitrile (1+1 v/v).14,19 at 0.885 L min-1; auxiliary gas for spray chamber, Ar–H2 at Acetonitrile, acetic acid, Na2CO3, NaH2PO4 and Na2HPO4 0.2 L min-1.The total gas flow rate (nebuliser gas+auxiliary were obtained from Merck and Ar and Ar/H2 from Messer gas) was 1.085 L min-1, again being in the optimum range Griesheim (Munich, Germany). Capillaries were bought either already found in previous investigations.5,10–12 The dwell time from CS-Chromatographie Service, Langerwehe, Germany was set to 50 ms. (uncoated) or from Bio-Rad (coated). For total element quantification without CE separation, conventional ICP-MS was used.Parameters were set in analogy Samples with ref. 22 or 23 for iodine measurements. Single standards or standard mixtures were used as samples for determining detection times, resolution, species stability Quality control and concentration detection limits (DL). DL were determined Estimation of a possible suction driven flow by nebulisation according to IUPAC recommendations (3s criterion) using gas stream. A liquid flow within the capillary, produced by the standard additions method.As a real sample, an LMW suction from the nebulisation gas stream, might alter the human milk fraction was analysed for iodine speciation. The separation. Therefore, the occurrence of such an alteration milk samples were voluntarily given by women from the was estimated according to ref. 10. First the capillary was Augsburg area, Germany. The milk (fifth lactation day) from filled with buVer and the electric current was determined at five donors was pooled and worked up as described 20 kV.Then the capillary inlet was kept in the air while the previously.20 nebulisation gas was on during 40 min. In the case of suction Capillary zone electrophoresis flow, air should intrude into the capillary. Subsequently the electrical circuit along the capillary was checked again when A Biofocus 3000 capillary electrophoresis system (Bio-Rad) the nebulisation gas was oV and the capillary inlet dipped into was used.The temperature was set to 20 °C for sample/buVer the inlet vial again. If suction flow occurred an air bubble carousels and for the total capillary up to the nebuliser. must interrupt the electrical circuit. Operation. Before each run, the capillary was purged with Reproducibility. The laboratory reference sample was 18 MV water (180 s, 8 bar) and background electrolyte (see analysed several times during each hyphenation session to Table 1) (180 s, 8 bar). The separation methods used diVerent check the performance of detection time, peak area, signal buVers and stacking electrolytes.Table 1 gives an overview of intensity and the sensitivity of the hyphenated system, includ- the CE conditions. Based on previous experience,5 separation ing the calculated DL. These data were compared with earlier was distinguished from the detection step: after 10 min of results for this sample (‘chart over time’ according to ref. 24). separation inlet buVer was forced into the capillary (pressure driven, 8 bar, 130 s), moving separated species to the ICP-MS system.This resulted in a flow rate of ca. 1.5 mL min-1. Results Interfacing and nebulisation. The CE nebuliser was Quality control aspects laboratory made and especially designed for the requirements of a CE-ICP-MS interface.5 Special care was focused on an Suction flow. The experiment (measuring electrical current) indicated that there was no detectable suction flow.The electric exact and optimised positioning of the capillary end and of the total interface to the ICP-MS. The CE capillary was fixed current again reached the normal value after the experiment. 1298 J. Anal. At. Spectrom., 1999, 14, 1297–1302Table 1 Parameters and conditions for CE separations Element Stacking Stacking Inlet High of interest/ Capillary: electrolyte 1 Background electrolyte 2 buVer voltage, method No. Sample Injection cm×mm id (pre-injection) electrolyte (post- separation time injection) A Platinum/ Laboratory reference 15 s, 8 bar 150×50, 100 mM, pH 2.3 20 mM, — 20 mM, 12 kV, method: sample: soil extract+ uncoated phosphate buVer, pH 5.6, pH 5.6, 10 min Pt 1 Na2PtCl6·6H2O, 20 mg L-1 148-5010, Bio-Rad, NaH2PO4– NaH2PO4– 15 s, 8 bar Na2HPO4 Na2HPO4 B Iodine/ Iodide+thyroxine 15 s, 8 bar 150×50, 50 mM, pH 5.6, 2% acetic acid 10% SDS, NaOH, -18 kV, method: A uncoated NaH2PO4–Na2HPO4, 3 s, 8 bar 20 mM 8 min (Fig. 3) 15 s, 8 bar C Iodine/ Iodide+iodate+thyroxine 15 s, 8 bar 150×50, NaOH, 20 mM, 2% acetic acid 10% SDS, Borate -18 kV, method: B or coated 15 s, 8 bar 3 s, 8 bar buVer, pH 8 min (Fig. 1 and 2) human milk fraction 8.3, 25 mM (Bio-Rad 148-5023) D Antimony/ Sb(III) tartrate 15 s, 8 bar 150×50, 100 mM, pH 5.6, 20 mM, — 20 mM, -18 kV, method: Sb 1 coated NaH2PO4–Na2HPO4, pH 5.6, pH 5.6, 10 min (Fig. 4, top) 15 s, 8 bar NaH2PO4– NaH2PO4– Na2HPO4 Na2HPO4 E Antimony/ Sb(III) tartrate 15 s, 8 bar 150×50, 50 mM, pH 11.6, 20 mM, — Na2 CO3, -18 kV, method: Sb 2 coated Na2CO3, pH 11.6, 20 mM+acetonitrile 10 min (Fig. 4 middle) 7 s, 8 bar Na2CO3 F Antimony/ Sb(V), Sb(III) tartrate, 15 s, 8 bar 150×50, 20 mM, pH 14, 20 mM, — 1% acetic -18 kV, method: Sb 3 trimethylantimony dichloride coated NaOH, pH 5.6, acid, 10 min (Fig. 4 15 s, 8 bar NaH2PO4– pH2 bottom) Na2HPO4 J. Anal. At. Spectrom., 1999, 14, 1297–1302 1299Reproducibility. This was checked by the analysis of a laboratory reference sample and calculation of the DL.The detection limit for this sample during six independent setups was 1.08±0.28 mg Pt L-1. The concentration of the determined species was 21.1±1.8 mg Pt L-1 (target value 20 mg L-1) and the detection time of the species was at 18±1 s. This clearly shows reproducibility of the hyphenated system in operation and sensitivity. The absolute detection limit was compared with that for HPLC-USN-ICP-MS coupling. Although both systems showed diVerent concentration DL for the laboratory reference sample (HPLC-USN-ICP-MS, 25 ng L-1, 20mL injection; CE-ICP-MS, 1 mg L-1, 500 nL injection), the absolute DL were identical at 0.5 pg of Pt.Hence, the nebulisation eYciency of the CE-ICP-MS interface was comparable to that of the USN. Fig. 2 An electropherogram (127I ) of an LMW fraction from human milk [native (solid line) or with thyroxine addition (broken line)]. For Potential of CE-ICP-MS graphical reasons only the broken trace is printed with an oVset.Iodide and thyroxine were identified and quantified by standard All experiments again were based on the diVerentiation of the additions. The total iodine content was 40.5 mg L-1, iodide 14 mg L-1 separation step from detection. This special kind of operation and thyroxine 27.3 mg L-1. combined with sample stacking improved the detection limits typically by a factor of 500, as already indicated by Michalke and Schramel.14 The advantages described there were applied to iodine speciation successfully.Fig. 1 shows standard applications for checking separation and determination of DL from the iodine species. The three investigated iodine species can be separated clearly. The inorganic species show very sharp peaks and very low detection limits. Thyroxine, however, shows (a still acceptable) peak broadening. Iodide (1 mg L-1) showed a DL (3s) of 0.04 mg L-1. This is similar to iodate (0.3 mg L-1), having a DL (3s) of 0.05 mg L-1.The DL (3s) of thyroxine (10 mg L-1) is 1.2 mg L-1. To check the applicability of this method to real samples, the LMW fraction of human milk was analysed and peaks were identified by the standard additions method. This is demonstrated in Fig. 2. Two peaks are seen, identified as iodide (18.2 s) and T4-hormone (thyroxine, 26.1 s). Iodide appears insignificantly later in this sample compared with standard solution, whereas thyroxine shows a marked Fig. 3 Sticking of thyroxine to the capillary.The CE conditions are migration time shift from 32.9 to 26.1 s. Identifications and not suitable for focusing of the T4-hormone. Whereas iodide is focused quantifications of the species were performed by standard and separated well at ca. 10 s, thyroxine sticks to the capillary. Hence additions. Iodide had a concentration of 14±0.2 mg L-1 (n= the peak height is low (only 10% compared with iodide; see 10-fold 3) and thyroxine of 27.3±3.0 mg L-1 (n=3). The total iodine ‘zoom’) and the signal is noisy.Accidentally, parts of the sticking concentration in this fraction was determined as 40.5 mg L-1, thyroxine are removed and monitored as late peaks. These artefacts fitting to the sum of the determined species. mimic further iodine species. Problems with CE-ICP-MS a good peak shape and suYcient sensitivity. However, the Sticking to capillary. Fig. 3 shows an electropherogram of T4-hormone is obviously sticking to the capillary under the iodide and thyroxine, 80 mg L-1 each.Iodide is detected with applied conditions. It is not really mobilised. The thyroxine signal extends from ca. 15 to 70 s, whereas the 127I trace is very noisy and the peak height reaches only 10% compared with iodide (shown clearly by the 10-fold ‘zoom’). Further, ‘ghost peaks’ are coming oV the capillary repeatedly, even at detection times later than the maximum detection time (the maximum detection time is the time needed for a compound to be flushed from the inlet through the whole capillary without being retained on the capillary wall ).Species stability and absent separation. Fig. 4 shows an example of absent separation/focusing and a lack of species stability. At the top, moderate separation conditions without stacking are chosen (Table 1, D), possibly preserving the species but failing completely in focusing the Sb species. Species appear as a low and broad hump between 10 and 35 s. Fig. 1 An electropherogram (127I ) of iodide (1 mg L-1), iodate Subsequently, conditions were chosen (Table 1, E) that pro- (0.3 mg L-1) and thyroxine (10 mg L-1).The iodine species are well vided a focusing and separation of, e.g., Sb(V) and Sb(OH)3 separated. The detection limits are 0.04, 0.05 and 0.8 mg L-1, respect- (not shown). However, it turned out (Fig. 4, middle) that ively, calculated according to IUPAC recommendations. CE parameters are described in Table 1, C. antimony(III) tartrate was destabilised under these conditions 1300 J.Anal. At. Spectrom., 1999, 14, 1297–1302Potential of the technique When the interface is working reliably no specific ‘coupling problems’ occur and investigations can concentrate on the broad potential of this technique, such as low concentration detection limits, diVerent CE separation technologies (e.g., cIEF-ICP-MS14) combined with element specific detection methods. Further, there are no stationary phases which can impair species stability25 Application to real samples, e.g., eluates of tunnel dust and soil (platinum26,27), sewage and fouling sludges (antimony, arsenic28,29 ) and serum and human milk (selenium11,12) were demonstrated earlier.This series of applications is diVerent to other reports of CE-ICP-MS hyphenation, as mostly only instrumental characterisations of the system using standard or artificial solutions were described (e.g., standard solutions of several cationic species in refs. 3, 6 and 7 and standard solutions of cationic species or Cd–metallothionein in ref. 30). Kinzer et al.,9 however, reported an application to drinking water. This paper completes our application series for complex (real ) matrices with iodine speciation. Low detection limits were achieved and the first real samples were investigated. The very low DL are explained by iodine being a monoisotope (the total elemental information is detected at one specific mass 127I ), which is not interfered with, e.g., by polyatomic interferences (low noise, in contrast to 75As, another monoisotope being easily/heavily interfered with29).Although the investigated human milk fraction had a high organic load31 and low total iodine concentration, quantitative speciation was possible. Quantitative speciation is generally accepted as a fundamental part of speciation.1,24,32,33 The presence of two iodine species, their amounts and one of the two species being iodide fits well with preliminary (unpublished) results by our group from SEC-ICP-MS experiments.Problems Fig. 4 Top: an electropherogram of antimony(III) tartrate using neu- The construction of the interface providing variable parameters tral focusing conditions (Table 1, D). There is no focusing/separation (capillary position, interface position, gas flow rates, make-up eYciency. Middle: an electropherogram of antimony(III) tartrate using buVer) being reproducibly controlled by micrometer devices alkaline focusing conditions (Table 1, E).The focusing/separation avoided specific ‘hyphenation problems’. Most problems aris- eYciency is increased, but species stability is violated. There are three peaks for only one compound. Bottom: an electropherogram of ing during all the hyphenation experiments could finally be antimony(V), antimony(III) tartrate (marked with an asterisk) and related to the attempt to decrease the DL to real concentrations trimethylantimony dichloride using a neutral background electrolyte when using (partly inadequate) stacking and separation but an alkaline stacking buVer (Table 1, F).The focusing/separation conditions. eYciency is suYcient and the species stability is not violated. The Any diYculties that occurred were related to chemical three species (1 mg Sb L-1, each) are separated well without interactions of samples, electrolytes and the capillary or detec- degradation. tor interferences.29 This is not surprising as species stability can easily be impaired by ‘wrong’ CE conditions, predominantly complexing electrolytes, inadequate pH, etc.3,10,27,20 A and two new Sb species were generated.These newly generated serious problem is a total or partial sticking of compounds to species (11.4 and 15.5 s) were successfully separated from the the capillary. In this case quantifications are typically wrong original species at 21.4 s. Only the combination of neutral and, most critically, ‘pseudo-species’ are detected, e.g., see (moderate) separation conditions with an alkaline stacking Fig. 3. Such peaks may mimic species within a sample, but are electrolyte (Table 1, F) was successful in focusing/separation only artefacts. As species identification is often performed by and still preserving species stability. This is shown in Fig. 4 comparing migration times of standards and samples, such (bottom). Here antimony(III ) tartrate is focused without artefacts may accidentally appear at a specific migration time degradation and clearly separated from other Sb species.and thus may be ‘identified’ as a certain species. The well known migration time variations according to diVerences in Discussion ionic strength of buVers or samples34–36 are a further problem for species identification (cf.,migration time variations between Quality control Fig. 1 and 2). Standard additions help to overcome this In several ‘hyphenation sessions’ over several years it was uncertainty.37 However, new generation of species during proved that the interface introduced in ref. 5 and modified in analysis is also recognised as a serious source of ref. 12 was operating stably and reproducibly. This was especi- error.3,10,27,28,30 This is also demonstrated in Fig. 4. Although ally shown by analysing a laboratory reference sample during the separation eYciency is increased in Fig. 4 (middle) comeach hyphenation session and determination of species pared with Fig. 4 (top), decomposition of the investigated Sb concentrations, detection limits and detection times. An unde- species becomes obvious. There are three peaks monitored for sirable suction flow during the separation step was proved to only one component. The alkaline conditions may be the be absent by experiments in analogy with previous reason.28,38 Fig. 4 reflects clearly the conflicting situation for improving separation eYciency and decreased species stability investigations.5,10,11 J.Anal. At. Spectrom., 1999, 14, 1297–1302 130110 B. Michalke and P. Schramel, J. Chromatogr. A, 1996, 750, 51. by stacking. Species preserving conditions here cannot provide 11 B. Michalke and P. Schramel, J. Chromatogr. A, 1998, 807, 71. focusing of this species (others were suYciently focused28) and 12 B. Michalke and P. Schramel, Electrophoresis, 1998, 19, 270. more eYcient separation conditions destroy the component. 13 Q. Lu and R.M. Barnes, Microchem. J., 1996, 54, 129. Only a compromise gives clear separation/focusing and species 14 B. Michalke and P. Schramel, Analusis, 1998, 26, M51. stability, as shown in Fig. 4 (bottom). IneYcient focusing has 15 Ro�mpp Chemie Lexikon, ed. J. Falbe and M. Regitz, Georg Thieme, Stuttgart, New York, 1991. been reported to be due to high conductivity of the sample29 16 S. Lustig, S. Zang, B. Michalke, P. Schramel and W. Beck, Sci. and/or low conductivity of the background electrolyte.3 Total Environ., 1996, 188, 195.There are also problems for the detection part. These 17 M. Dood, S. L. Grundy, K. J. Reimer and W. R. Cullen, Appl. problems can be summarised as variation in detection times Organomet. Chem., 1992, 6, 207. according to sample/buVer viscosity11 and interfered mass 18 G. Koelb, K. Kalcher and K. J. Irgolic, J. Autom. Chem., 1993, signals due to polyatomic or isobaric interferences.14,29,39 15, 37. 19 Z. K. Shihabi, Paper presented at the 4th Go� ttinger Capillary Detection limits of the CE-ICP-MS system are just acceptable Electrophoresis Symposium, Go� ttingen, 22–23 October, 1997.or still too high for several real samples. Hence, the demand 20 B. Michalke and P. Schramel, Biol. Trace Elem. Res., 1997, 59, 45. for coupling to more sensitive detectors, e.g., high-resolution 21 P. Schramel, R. Fischer, A. Wolf and S. Hasse, ICP Info. Newsl., ICP-MS, has already been expressed.40 1981, 6/8, 401. 22 P.Schramel, I. Wendler and S. Lustig, Fresenius’ J. Anal. Chem., 1995, 353, 115. Conclusion 23 P. Schramel and S. Hasse, Mikrochim. Acta, 1994, 116, 205. Exact and reproducible adjustments of each parameter at the 24 Ph. Quevauviller, E. A. Maier and B. Griepink, in Element Speciation in Bioorganic Chemistry, ed. S. Caroli, Wiley, New interface are necessary. Then specific ‘hyphenation problems’ York, 1996, pp. 195–222. do not really exist. Operation is stable and reproducible over 25 J.Harms and G. Schwedt, Fresenius’ J. Anal. Chem., 1994, 350, 93. long time periods. Problems arise from the need to decrease 26 S. Lustig, B. Michalke, W. Beck and P. Schramel, Fresenius’ detection limits to real concentrations. This is done by sample J. Anal. Chem., 1998, 360, 18. stacking. The variability in sample stacking has great potential 27 B. Michalke, S. Lustig and P. Schramel, Electrophoresis, 1997, on the one hand but is a potential danger for species stability 18, 196. 28 B. Michalke and P. Schramel, J. Chromatogr. A, 1999, 834, 341. on the other. Nevertheless, solutions were described for several 29 B. Michalke and P. Schramel, Electrophoresis, 1998, 19, 2220. applications concerning elements and matrices. 30 V. Majidi and N. J. Miller-Ihli, Analyst, 1998, 123, 803. 31 B. Michalke and S. Lustig, in Jahresbericht 96, GSF– References Forschungszentrum fu� r Umwelt und Gesundheit, Neuherberg, 1997, pp. 18–24. 1 R. M. Barnes, Fresenius’ J. Anal. Chem., 1998, 361, 246. 32 First International Conference in Biomedical, Nutritional and 2 K. Sutton, R. M. C. Sutton and J. A. Caruso, J. Chromatogr. A, Environmental Sciences, GSF–Forschungszentrum fu� r Umwelt 1997, 789, 85. und Gesundheit, Neuherberg, 4–7 May, 1998. 3 J. W. Olesik, J. A. Kinzer and S. V. Olesik, Anal. Chem., 1995, 33 B. Michalke, Fresenius’ J. Anal. Chem., 1999, 363, 439. 67, 1. 34 Z. Deyl and R. Struzinsky, J. Chromatogr. A, 1991, 569, 63. 4 M. J. Tomlinson, L. Lin and J. A. Caruso, Analyst, 1995, 120, 583. 35 G. Bondoux, P. Jandik and R. W. Jones, J. Chromatogr. A, 1992, 5 B. Michalke and P. Schramel, Fresenius’ J. Anal. Chem., 1997, 602, 79. 357, 594. 36 A. Schmutz and W. Thormann, Electrophoresis, 1994, 15, 51. 6 Y. Liu, V. Lopez-Avila, J. J. Zhu, D. R. Wiederin and 37 B. Michalke, Fresenius’ J. Anal. Chem., 1995, 351, 670. W. F. Beckert, Anal. Chem., 1995, 67, 2020. 38 J. Lintschinger, personal communication. 7 A. Tangen, R. Trones, T. Greibrokk and W. Lund, J. Anal. At. 39 ICP-MS Interferenz Tabelle, Finnigan MAT, Bremen, 1995. Spectrom., 1997, 12, 667. 40 R. Lobinsky, Appl. Spectrosc., 1997, 51, 260A. 8 Q. Lu, S. M. Bird and R. M. Barnes, Anal. Chem., 1995, 67, 2949. 9 J. A. Kinzer, J. W. Olesik and S. V. Olesik, Anal. Chem., 1996, 68, 3250. Paper 9/00495E 1302 J. Anal. At. Spectrom., 1999, 14, 1297&

ISSN:0267-9477    

DOI:10.1039/a900495e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Qualitative and quantitative determination of tetraethyllead in fuel using low pressure ICP-MS† Invited Lecture Gavin O’Connor,‡ Les Ebdon and E. Hywel Evans* University of Plymouth, Department of Environmental Sciences, Drake Circus, Plymouth, UK PL4 8AA Received 28th January 1999, Accepted 30th April 1999 A low pressure inductively coupled plasma sustained at 6 W, with 7 ml min-1 of helium and 1.8 ml min-1 of isobutane, has been optimised for the determination of tetraethyllead, with a detection limit of 7 pg.A chromatogram of a standard reference fuel (NBS SRM 1637 II ) yielded a single peak which was nominally identified as tetraethyllead. The mass spectrum was compared to an electron impact library spectrum, and the identity of the tetraethyllead species in the reference material was confirmed. Quantitative analysis of the fuel yielded a tetraethyllead concentration of 13.1±0.9 mg ml-1 as Pb, which was within the certified range of 12.9±0.07 mg ml-1.polar ice caps and high mountain glaciers, has formed the Introduction basis for the archiving of historical automobile pollution, and The coupling of element specific detectors to chromatographic the study has shown clear links between the increased use of systems has greatly enhanced the ability of analysts to provide leaded petrol and organolead species being found in the trace level information on a wide variety of organometallic environment.1 Also highlighted in this study was the wide species.However, these highly specific and selective detectors variety of species derived from the natural breakdown of have a number of unattractive features. First, much of the tetraalkyllead compounds. Organolead species have also been instrumentation used for trace level multielement analysis has found in wine samples, and this serves as an excellent example high capital and running costs. These running costs can be of how such species enter the human food chain.2 Hence, such justified when performing multielement analyses on aqueous studies can provide invaluable information on the fate of samples that are directly introduced into the instrument, polluting and naturally occurring trace metals in the environyielding analysis times of less than 1 min.However, the ment, the use of which will be essential for future environmenaddition of a chromatographic separation technique greatly tal impact assessments. increases the analysis time per sample, making the speciation Inductively coupled plasmas, operated at reduced pressure of trace metals a costly exercise.Also, by using such detection and sustained with argon, have recently been used for the systems much of the chemical information available to the production of atomic3–5 and molecular6 mass spectra, using analyst is lost because the metal species are totally atomised. gaseous and vapour sample introduction. Evans et al.6 have Hence, a ready supply of certified reference compounds and investigated the use of a low pressure (LP) helium ICP, at standards of known organometallic species is required to powers between 4 and 40 W and 1 mbar pressure, for the enable species identification by comparison of retention time.production of mass spectra similar to those given by an EI This is disadvantageous because the analysis of individual source, for a series of organometallic and halogenated species standards is a time consuming and costly aVair, and for many introduced by gas chromatography (GC).On increasing the organometallic species such standards simply do not exist. power and pressure of this source it was possible to increase Therefore, any instrumentation capable of providing both the degree of fragmentation until at 150 W and 10 mbar quantitative trace level element specific information and qualipressure total fragmentation occurred. Recently, a specially tative identification of unknown species, without having to designed LP-ICP instrument has been assembled7 and investi- rely on the running of standards but with reduced capital and gated for use as a tuneable source8 to provide both atomic running costs, would be welcomed.and molecular mass spectra. Glow discharges9 have also been For many years alkyllead compounds have been added to used with GC sample introduction for the speciation of a petrol as anti-knocking agents.1 While this practice has series of organotin10,11 and organolead12 compounds, with the decreased in recent years, organolead species are still prevalent production of molecular fragment ions from the analyte com- in the environment. Many organolead species are neurotoxins, pounds, but with poor sensitivity for molecular fragments.which can enter the body orally and trans-dermally, and therefore are of interest for clinical reasons. The determination From these studies it has become obvious that low pressure of organolead species by GC-MIP-AES, in core samples from plasma sources are capable of being operated in a tuneable mode.The analysis of organolead species has been previously †Presented at the 1999 European Winter Conference on Plasma performed by low pressure inductively coupled plasma mass Spectrochemistry, Pau, France, January 10–15, 1999. spectrometry.6 In this instance a 1 l min-1 argon LP-ICP was ‡Present address: LGC, Queens Road, Teddington, Middlesex, UK TW11 0LY.used and only atomic information was obtained. The main J. Anal. At. Spectrom., 1999, 14, 1303–1306 1303aim of this paper is to assess the ability of LP-ICP-MS to provide further species information, enabling unequivocal identification of the organometallic species in combination with chromatographic separation but without solely resorting to a comparison of retention time. Experimental A detailed description of the design and optimisation of the GC-LP-ICP-MS, used in this study, has been given previously.7 In brief, a Hewlett-Packard Mass Selective Detector (MSD) was modified to enable it to analyse and detect ions from the LP-ICP.This was achieved by using a custom made ion sampling interface. The low pressure plasma was sustained using a modified rf generator and matching network, in a 140 mm long quartz tube of 1/2 od, with a 1/4 od side arm Fig. 1 The eVect of column head pressure (kPa) on the signal intensity to which a calibration vial containing perfluorotributylamine for tetraethyllead.(PFTBA) was attached. The quartz plasma torch was connected to the ion sampling interface via a low pressure sampling set to full-scan mode, continuously scanning a mass range cone (Machine Shop, University of Plymouth), which was between 100 and 500 m/z. The resulting spectrum was then machined from aluminium, had a 2 mm orifice and an Ultracompared with a library spectrum obtained from a Hewlett- torr fitting for a 1/2 pipe.This enabled a vacuum seal to be Packard 5970 GC-MS workstation. formed between the low pressure torch and the ion sampling interface. The reagent gases were added to the plasma gas via the side arm tube of the quartz torch. The amount of gas Results and discussion added was controlled using a scaled needle valve (Edwards, Optimisation Crawley, West Sussex, UK). A standard of tetraethyllead (99% Aldrich Chemicals, The column head pressure (i.e, helium carrier gas flow and Gillingham, UK) were freshly prepared by weight in pentane hence the pressure in the torch) was optimised first.Such an (Rathburn Chemicals, Scotland, UK). A 15 ng ml-1 solution optimisation was essential because the analyte eluted close to was then used to optimise the instrumental parameters, with the solvent front, which has been shown to aVect the fragmenthree separate injections being made at each instrument setting. tation of the analyte.8 The eVect of column head pressure on The optimised instrumental parameters used for the analysis a 15 ng on-column injection of tetraethyllead is shown in of the alkyllead species in a standard reference fuel are shown Fig. 1. The optimum head pressure, yielding the highest total in Table 1. The instrument response was first calibrated for ion count, was 60 kPa. This resulted in a column carrier gas tetraethyllead using the three most intense fragment ions in flow of 4 ml min-1 at 120 °C. Also shown in Fig. 1 are the the mass spectrum, at 208, 237 and 295 m/z.The concentration relative intensities of the fragment ions, which changed little of lead in a reference fuel (NBS SRM 1637 II ) was then with increasing flow rate. This suggested that the increase in determined by direct injection of 1 ml of reference material gas flow did not cause further fragmentation of the analyte as onto the column of the gas chromatograph. has been previously observed for the halobenzene species,8 A full-scan mass spectrum of the tetraethyllead standard possibly due to the slightly increased pressure in the torch at and reference fuel was obtained using the same plasma and higher flows.GC conditions as above; however, the mass spectrometer was The eVect of plasma forward power on analyte signal is shown in Fig. 2. In a previous study, an increase in the power caused a decrease in the molecular ion signal and an increase Table 1 Operating conditions for the analysis of tetraethyllead using low pressure inductively coupled plasma mass spectrometry in the atomic ion signal for the halobenzenes.8 However, in this case, the reduction in the intensity of the molecular Mass spectrometer— fragment ions at 237 and 295 m/z was not accompanied by a Quantitative analysis 208, 237 and 295 corresponding increase in the atomic ion signal for Pb at 208 selected ions monitored (m/z) m/z; rather, the relative intensities of the three ion signals Qualitative analysis 100 to 500 remained more or less constant.One possible explanation is scan range (m/z) Plasma— that the ionisation process is not dominated by electron impact, Forward power/W 6 and that other ionisation mechanisms play a major role. The Reflected power/W 0 observation that the halogens, with higher ionisation poten- Helium make-up gas 3 flow rate/ml min-1 Isobutane flow rate/ml min-1 1.8 Pressure (Torr)— Torch 0.2 Interface 4×10-2 Analyser <10-6 Gas chromatograph— Injector Cold on-column Column DB1, 30 m length, 0.32 mm od, 0.25 mm film thickness Carrier gas Helium Head pressure/kPa 60 Carrier flow/ml min-1 4 Injection volume/ml 1 Oven temperature/ °C 120 isothermal Fig. 2 The eVect of plasma forward power (W) on the signal intensity Transfer line temperature/ °C 120 for tetraethyllead. 1304 J. Anal. At. Spectrom., 1999, 14, 1303–1306Table 2 Figures of merit for tetraethyllead calibration using low pressure inductively coupled plasma mass spectrometry m/z monitored TIC 208 237 295 Linear range studied 102 102 102 102 Slope (counts ng-1) 23833 4860 9726 8245 R2 0.9996 0.9999 1.000 1.000 Limit of detection/nga 0.007 0.02 0.07 0.07 RSD (%)b 7.0 1.3 5.2 4.7 a3 standard deviations of 10 readings taken at the blank level.b4 readings at 0.7 ng on-column. Fig. 3 The eVect of isobutane flow rate (ml min-1) on the signal Table 3 Determination of tetraethyllead in standard reference fuel, intensity for tetraethyllead. NBS SRM 1637 II, by low pressure inductively coupled plasma mass spectrometry tials, are more easily ionised than lead suggests this possibility; Determined Determined Certified however, more detailed investigations are necessary to confirm concentration as concentration as value for or reject this hypothesis. Hence, it is possible that higher tetraethyllead/mg ml-1 lead/mg ml-1 lead/mg ml-1 power caused greater fragmentation of the compound, but the atomic species were not ionised in the plasma. 20.4±1.6 13.1±0.9 12.9±0.07 The eVect of increasing reagent gas concentration is shown in Fig. 3. No atomic ions nor molecular fragment ions were observed in the absence of isobutane reagent gas, which again a threshold analyte concentration; however, the use of a suggests that Pb was not ionised in the plasma under these reagent gas in this and previous studies8 results in the integrity operating conditions. The lowest gas flow of 0.18 ml min-1 of the mass spectrum being maintained right down to the resulted in the most intense ion signals, with both the absolute detection limits of 20–70 pg for the fragment ions.and relative intensities of the fragment ions studied remaining Results for the determination of tetraethyllead in NBS SRM constant at higher flows. A molecular ion at m/z 324 was not 1637 II reference fuel are shown in Table 3. The fuel has been observed for tetraethyllead, unlike the halobenzene species, certified for total lead; however, previous studies using for which very intense molecular ions have been observed.8 GC-ICP-MS detected only one lead species in the reference The addition of a helium make-up gas to the plasma was fuel.14 The determined concentration of lead was in good essential to maintain good plasma stability and to keep the agreement with the certified value.plasma sustained while the solvent eluted from the GC column. A total ion chromatogram for the analysis of the standard The eVect of the added helium on the analyte signal is shown reference material is shown in Fig. 5(a). The lighter, hydro- in Fig. 4. An almost linear decrease in sensitivity was observed carbon, fraction of the petrol eluted rather quickly (<1 min) with increased helium gas flow, and the relative intensities of from the DB1 GC column, leaving the tetraethyllead well the atomic and fragment ions remained constant. This was separated from these fractions. The presence of tetraethyllead again contrary to the previous study,8 in which an increase in was verified by the mass spectrum of the chromatographic the make-up gas resulted in an increase in the signal for the peak [Fig. 5(b)]. The molecular ion for tetraethyllead at 324 atomic ions of Cl, Br and I. m/z was not observed [Fig. 5(b)]; however, the peaks at 295, 267 and 237 m/z were consistent with the loss of successive Analysis of fuel ethyl groups (29 m/z) from the molecule, and the lead isotopes Calibration data and analytical figures of merit for the were observed between 204 and 208 m/z.The mass spectrum fragment ions studied, using the optimised plasma conditions, for a 10 ng injection of a tetraethyllead standard is shown in are shown in Table 2. The calculated limit of detection using Fig. 6(a) and the EI library spectrum for tetraethyllead is shown the total ion chromatogram was 7 pg, which is similar to the in Fig. 6(b). Comparison of the spectrum unequivocally identlimit of detection for a mass selective detector operating with an electron impact ionization source.13 It is important to note that similar detection limits were achieved for all three atomic and fragment ions at m/z 208, 237 and 295.It has not always been possible to achieve a fragmentation mass spectrum below Fig. 5 1 ml on-column injection of NBS SRM 1637 II; (a) total ion Fig. 4 The eVect of helium make-up flow (ml min-1) on the signal intensity for tetraethyllead.chromatogram; (b) mass spectrum. J. Anal. At. Spectrom., 1999, 14, 1303–1306 1305Acknowledgements The authors would like to thank BP International (Sunbury Group) for their kind donation of the HP 5970 MSD, the NuYeld foundation for the provision of an instrument development grant, and the University of Plymouth for financial support of G. O’C. References 1 R. Lobinski, Analyst, 1995, 120, 615. 2 F. C. Adams, LC-GC Int., 1994, 7, 694. 3 E. H. Evans and J. A. Caruso, J.Anal. At. Spectrom., 1993, 8, 427. 4 T. M. Castillano, J. J. Giglio, E. H. Evans and J. A. Caruso, J. Anal. At. Spectrom., 1994, 9, 1335. 5 X. Yan, T. Tanaka and H. Kawaguchi, Appl. Spectrosc., 1996, 50, 2, 182. Fig. 6 Mass spectrum for a standard solution of tetraethyllead 6 E. H. Evans, W. Pretorius, L. Ebdon and S. Rowland, Anal. obtained using: (a) LP-ICP-MS; (b) EI source library. Chem., 1994, 66, 3400. 7 G. O’Connor, L. Ebdon, E. H. Evans, H. Ding, L. K. Olson and J. A. Caruso, J. Anal. At. Spectrom., 1996, 11, 1151. 8 G. O’Connor, L. Ebdon and E. H. Evans, J. Anal. At. Spectrom., ifies the lead species in the standard reference material as 1997, 12, 1263. tetraethyllead. 9 L. K. Olson, M. Belkin and J. A. Caruso, J. Anal. At. Spectrom., 1996, 11, 491. 10 J. W. Waggoner, M. Belkin, K. Sutton, H. Fannin and J. Caruso, Conclusions J. Anal. At. Spectrom., 1998, 13, 879. 11 M. A. Belkin, L. K. Olson and J. A. Caruso, J. Anal. At. Low pressure ICP-MS was used for the qualitative and Spectrom., 1997, 12, 1255. quantitative determination of tetraethyllead in fuel. The mol- 12 T. Go� recki, M. Belkin, J. Caruso and J. Pawliszyn, Anal. ecular mass spectrum for tetraethyllead was very similar to Commun., 1997, 34, 275. 13 HP5970 Mass Selective Detector, Hardware Manual, Publication that of an EI source library spectrum. The LP-ICP could not No. 05970–90049, Hewlett Packard, Sunnyvale, California, USA. be used to obtain atomic mass spectra, as was previously 14 A. W. Kim, PhD Thesis, University of Plymouth, 1993. observed for the halide species. It is possible to obtain atomic spectra for tetraethyllead with an alternative low pressure Paper 9/00776H plasma source,6 but the prototype instrument used for this study could not be operated at high enough gas flows for this to be achieved. 1306 J. Anal. At. Spectrom., 1999, 14, 1303–13

ISSN:0267-9477    

DOI:10.1039/a900776h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

In situ propylation using sodium tetrapropylborate as a fast and simplified sample preparation for the speciation analysis of organolead compounds using GC-MIP-AES† Monika Heisterkamp and Freddy C. Adams* University of Antwerp, Micro and Trace Analysis Centre, Universiteitsplein 1, 2610 Wilrijk, Belgium Received 17th February 1999, Accepted 21st April 1999 A fast and simple sample preparation procedure for the speciation of organolead compounds is described. Separation and detection of the diVerent species was carried out using gas chromatography hyphenated to microwave-induced plasma atomic emission spectrometry.Derivatization was performed using in situ propylation with sodium tetrapropylborate in an acetic acid–sodium acetate buVer medium of pH 4.5 with simultaneous extraction of the propylated species into hexane. Detection limits (as Pb) ranging from 77 fg (0.15 ng kg-1) to 102 fg (0.21 ng kg-1) and relative standard deviations between 4 and 6% were achieved.Accuracy of the procedure was confirmed by analysis of a certified reference material (CRM 605, Road Dust), where the desorption of the diVerent species from the matrix by acid leaching was also integrated in the derivatization and extraction step. The optimized sample preparation procedure was applied to the analysis of alpine snow sampled at the Mont-Blanc in France. raphy (LC) to atomic absorption spectrometry (AAS),4–7 Introduction inductively coupled plasma (ICP-AES)8,9 or microwave- Tetraalkyllead compounds have been used as anti-knocking induced plasma atomic emission spectrometry (MIP-AES)10–12 agents in leaded petrol to prevent premature ignition of the or inductively coupled plasma mass spectrometry air–gasoline mixture for more than 75 years.Their use is (ICP-MS).13–16 restricted or even banned in many Western countries, but only The ionic organolead species must be derivatized in order a few attempts were made to curtail the use of lead additives to make them suitable for GC separation, because the polar in developing countries where the consumption of organolead compounds have to be converted into volatile, thermally stable is still expanding.Hence, it is still the most important burden species retaining the species-specific information of the native of organolead compounds in the environment.1 These com- organometal. Generally, two diVerent methods can be pounds enter the atmosphere due to incomplete combustion employed, those based on alkylation with appropriate or evaporation during production, storage and distribution of Grignard reagents and those applying in situ derivatization both anti-knocking agents and leaded gasoline. Once released using tetraalkylborates or tetrahydroborates.The kind of into the atmosphere, these species can be transported over derivatization determines the whole sample preparation progreat distances as free molecules or adsorbed on particles, so cedure, because Grignard alkylation has to be performed in that they can even be detected in remote areas such as an organic solvent, whereas in situ alkylation can be carried Greenland.2,3 These species (tetramethyllead [TeML: out in an aqueous medium.Grignard alkylation is a well- (CH3)4Pb], tetraethyllead [TeEL: (C2H5)4Pb] or mixed tetra- accepted method, because many diVerent Grignard reagents (ethylmethyl )lead) decompose via their ionic tri- and dialkyl- are commercially available to perform many diVerent types of lead compounds into inorganic lead.The ionic alkyllead is alkylation for nearly all organometallic species. Organolead washed out from the atmosphere by precipitation and compounds are mostly propylated17,18 or butylated19,20 using transported into the hydrosphere. appropriate Grignard reagents; moreover, pentylation21 and It is well known that organolead species are much more phenylation22 are also applied. The main drawback of this toxic than inorganic lead compounds due to their higher derivatization method is the fact that Grignard reagents are liposolubility, whereas the toxicity of the former increases with destroyed by water and the reaction therefore demands an the number of alkyl groups bound to one lead atom.Therefore, aqueous-free medium, which implies a multi-step and highly it is important to develop analytical procedures for the specitime consuming sample preparation procedure.Separation ation analysis of these compounds, able to determine not only from the matrix, extraction into an organic solvent after the total concentration of an element in a sample but also complexation, alkylation by a Grignard reagent, destruction able to identify and quantify the diVerent species in which it of the excess of this reagent and a possible clean-up step are occurs. Hyphenated techniques are most commonly applied the subsequent steps of such a sample preparation procedure; for speciation analysis combining powerful separation methods all of them can be subject to sources of error.with sensitive and element-specific detection systems. This is In recent years, alkylation with tetraalkylborates has become mostly performed by coupling gas (GC) or liquid chromatogpopular for speciation analysis of organometallic compounds due to the product’s solubility and stability in aqueous media. As a consequence, sample preparation can be simplified drasti- †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999.cally, because derivatization can be performed in situ, meaning J. Anal. At. Spectrom., 1999, 14, 1307–1311 1307Table 1 GC-MIP-AES operating conditions directly in the aqueous medium. Even if complex matrices such as soils or dust are analyzed, the analytes can be separated Injector parameters— from the matrix, derivatized and extracted into an organic Injection port Cooled injection system in solvent simultaneously, making sample preparation a relatively on-column mode fast, one-step procedure. The commercial availability of Injection liner Deactivated silica, 18 mm Tenax (60–80 mesh) sodium tetraethylborate (NaBEt4) made in situ ethylation an Injection program 15 °C�2 °C s-1�20 °C (60 s)� attractive alternative for the speciation analysis of organotin 12 °C s-1�270 °C (240 s) and organomercury.For organolead speciation, it is only Injection volume 5 ml feasible for methylated species,23–25 as both ethyl- and inor- Splitting ratio Splitless ganic lead would form the same derivatization product, namely GC parameters— TeEL, with a loss of species-specific information.The use of Column HP-1: 25 m×32.0 mm id×0.17 mm tetrabutylammonium tetrabutylborate ([Bu4N]+[BBu4]-)26,27 film thickness or sodium tetrapropylborate (NaBPr4)28 allows the speciation Column head pressure 130 kPa analysis of all relevant organolead species, namely trimethyl- Oven program 45 °C (1 min)�30 °Cmin-1�270 °C Purge valve On (0.9 min)�oV (1.6 min)�on lead (TML: (CH3)3Pb+), dimethyllead (DML: (CH3)2Pb2+), Transfer line HP-1 column triethyllead (TEL: (C2H5)3Pb+) and diethyllead (DEL: Transfer line temperature 270 °C (C2H5)2Pb2+).MIP-AES parameters— This paper describes the optimization of the sample Wavelength 405.783 nm; 261.418 nm preparation procedure using in situ derivatization with NaBPr4 Helium make-up gas flow 300 ml min-1 (measured at the for the speciation analysis of organolead compounds in water cavity vent) and road dust.The diVerent species were derivatized and Scavenger gases: simultaneously extracted into hexane. The acid leaching H2 pressure 90 psi (621 kPa) required for the analysis of solid samples was carried out in O2 pressure 20 psi (138 kPa) Spectrometer purge flow 1.5 l min-1 of nitrogen the same step. The method developed is illustrated by the Solvent vent-oV program On (2.4 min)�oV (3.7 min)�on analysis of a certified reference material (CRM 605, Road Column–detector coupling Column-to-cavity Dust) and of alpine snow samples containing a very low Cavity temperature 270 °C concentration of the organolead compounds.Experimental ranging from pH 2.0–10.5 were adjusted b amounts of ammonia solution (25%). Apparatus Trimethyllead and triethyllead chloride stock solutions In order to be able to inject more than 1–2 ml of the organic (1000 mg ml-1) were prepared by dissolving appropriate extract onto the capillary column, a cooled injection system, amounts of the respective alkyllead salts (obtained from Alfa, Model KAS 503 (Gerstel, Mu�lheim a.d.Ruhr, Germany), Karlsruhe, Germany) in water. Dimethyllead and diethyllead containing a programmed temperature vaporization (PTV) stock solutions (500 mg ml-1) were synthesized by reaction of was employed. The sample is introduced into a cooled injection appropriate amounts of trimethyllead and triethyllead chloride port liner, consisting of a deactivated silica tube (93 mm × solutions, respectively, with iodine monochlorine (1 mol l-1).29 1.25 mm id×2 mm od) which is partially filled with Tenax Mixed working standard solutions were prepared daily from (18 mm, 60–80 mesh).This system allows the injection of up the stock solutions by a series of dilutions with water. to 50 ml by evaporating the solvent prior to releasing the analytes onto the column by means of an appropriate injection Procedure program.Separation of the analytes was performed using a Analysis of snow samples. A 30–50 ml volume of snow HP Model 5890 Series II gas chromatograph (Hewlettsample was placed in an extraction vessel and buVered to Packard, Palo Alto, CA, USA), equipped with an HP-1 pH 4.5 with 1 ml of acetic acid–sodium acetate (HAc–NaAc) capillary column. The diVerent organolead species were buVer solution.In order to mask the inorganic lead, 0.5 ml of detected by means of a HP Model 5921 A atomic emission a 0.1 mol l-1 aqueous solution of the disodium salt of ethylene- detector (AED). The working conditions of the PTV, GC and diaminetetraacetic acid dihydrate (EDTA) was added. Then, AED systems are shown in Table 1. A Model IEC Centra CL ml of the tetrapropylborate solution and 0.5 ml of hexane centrifuge from the International Equipment Company were added.The mixture was shaken manually for 5 min and (Dunstable, UK) was used for the analysis of the road dust. set aside to enable phase separation. The organic phase was sampled and stored at -20 °C in the dark until analysis. Reagents De-ionized water further purified with a Milli-Q system Analysis of Road Dust (CRM 605). About 0.4 g of the urban (Millipore, El Paso, TX, USA) was used throughout. All dust was accurately weighed and placed in a centrifugation reagents used for derivatization were of analytical-reagent vessel together with 10 ml of HAc–NaAc buVer solution and grade and purchased from Merck (Darmstadt, Germany) 0.5 ml of EDTA as a masking agent.After addition of 0.5 ml unless stated otherwise. of the tetrapropylborate solution and 0.5 ml of hexane, the Sodium tetrapropylborate was synthesized by the mixture was shaken manually for 5 min and centrifuged for Laboratory for Organic Chemistry, Ghent University (Gent, 3 min at 4000 rev min-1 to enable phase separation.The Belgium), according to the method described by De Smaele organic phase was sampled and stored at -20 °C in the dark et al.28 A 1.0% m/v aqueous solution was prepared daily. until analysis. The diVerent acetate buVer solutions (0.1 mol l-1) were made by dissolving 13.6 g of sodium acetate trihydrate in 1 l Results and discussion of water and the pH values ranging from pH 4.0–5.0 were adjusted with the appropriate amount of concentrated acetic Optimization of the derivatization conditions acid.For preparation of the diVerent citric acid–ammonia buVer solutions (0.1 mol l-1), 21.0 g of citric acid mono- Using GC separation, the ionic organolead compounds have to be derivatized in order to make them volatile. The derivatiz- hydrate were dissolved in 1 l of water and the pH values 1308 J. Anal. At. Spectrom., 1999, 14, 1307–1311program must be applied. By using the oven program shown in Table 1, the species of interest could be baseline separated within 6 min.By using temperature programmed evaporation, in addition to the analytes also the impurities in the reagents are preconcentrated on the packed liner. Therefore, the amount of borate added for derivatization of the analytes should be limited to a minimum to avoid carbon accumulation in the discharge tube, shortening its lifetime drastically and increasing the baseline noise. In order to investigate the eVects of carbon interferences caused by impurities in the NaBPr4 on the sensitivity of the method it is necessary to measure a leadand a carbon-specific wavelength within the same chromatographic run.The AED is equipped with a photodiode-array spectrophotometer, allowing simultaneous determination of up to four elements, if their emission is in a wavelength range of 20 nm in the visible and 40 nm in the ultraviolet region of Fig. 1 Influence of the reaction medium on the recovery of the diVerent the spectrum.30 The software controlling data acquisition organolead species; HCi: 0.1 mol l-1 citric acid, HAc: 0.1 mol l-1 allows the definition of recipes to build up the diVerent acetic acid, NaAc: sodium acetate.analytical lines for a given element. It is possible to measure lead-specific emissions at 405.783 and 261.418 nm, the latter ation reaction very strongly depends on the pH value, which wavelength being 2.4 times less sensitive than the former. was carefully optimized by propylation of standard solutions Carbon can be measured at diVerent wavelengths, but only buVered at pH values ranging from pH 2 to 10.5.The influence the line at 247.857 nm is useful for simultaneous determina- of the reaction medium on the recovery of the diVerent tion of both lead- and carbon-containing compounds. organolead species is illustrated in Fig. 1. A maximum response Therefore, the less sensitive lead-specific line at 261.418 nm is obtained at pH 4.5 using HAc–NaAc buVer solution.was selected for investigating the interferences caused by the By using NaBPr4, sample preparation can be both simplified derivatization reagent. and hastened, as all the diVerent steps can be performed Fig. 3 shows a chromatogram of a standard containing the simultaneously. In order to make the whole procedure as fast diVerent propylated organolead species measured simul- as possible, the eVect of the reaction time on the recovery of taneously at the carbon- and lead-specific wavelengths.Two the diVerent organolead species was investigated. As can be carbon-containing compounds were detected in the standard seen from Fig. 2, a maximum derivatization yield was achieved at high concentrations. One compound elutes between the after a reaction time of 5 min for all species. Contrarily to De TML and DML peaks with a retention time of 3.36 min and Smaele et al.,28 who observed a much slower derivatization another between the DML and TEL peaks at a retention time reaction for TEL in an HAc–NaAc buVer medium at pH 4.0, of 4.13 min.These two compounds are well separated from the reaction times of all organolead species were comparable. the organolead species and scarcely aVect the baseline of the The eVect of the amount of NaBPr4 on the recovery of the chromatogram obtained by measuring at the lead specific diVerent species was examined by extraction and derivatization wavelength. These and other carbon-containing compounds, of spiked tap water buVered at pH 4.5 with a HAc–NaAc on the other hand, are also preconcentrated on the liner, if buVer using diVerent amounts (100-1000 ml ) of a 1.0% solution volumes greater than 1 ml are injected using a temperature of NaBPr4.Tap water was chosen as a sample in order to programmed injection procedure. By applying this injection minimize the amount of NaBPr4 that would quantitatively technique, the analytes of interest are adsorbed on the liner derivatize the organolead compounds in a real matrix.For a packing, but the carbon-containing compounds are held on maximum derivatization yield, addition of 0.5 ml of NaBPr4 the Tenax also, so that they are subsequently released onto solution was necessary for the analysis of water samps as the column. Therefore, these species reach the detector in high matrix components and other trace elements present in the concentrations, where they severely damage or even destroy sample also react with the borate.the discharge tube by causing a white precipitate to form on its walls. Hence, the maximum amount that could be injected Optimization of the injection and GC conditions without damaging the discharge tube was 0.5 mg of borate, In order to shorten the time needed for the chromatographic which corresponds to 5 ml of the organic extract. separation of the diVerent organolead species, a suitable oven Fig. 3 Chromatogram of a mixture of propylated organolead stan- Fig. 2 EVect of the reaction time on the recovery of the diVerent dards (3 pg as Pb of each species) with simultaneous determination of lead (261.418 nm) and carbon (247.857 nm). organolead species. J. Anal. At. Spectrom., 1999, 14, 1307–1311 1309Table 2 Analytical characteristics of the method TML DML TEL DEL Slope 6.2±0.1 7.8±0.2 5.9±0.1 6.5±0.1 Intercept 0.5±0.6 0.6±1.2 0.1±0.7 0.2±0.3 Correlation coeYcient 0.999 0.998 0.996 0.992 Absolute detection limit/fg 96 77 102 93 Detection limit/pg g-1 0.19 0.15 0.21 0.18 Reproducibility (%) 4.7 3.7 5.6 4.7 Recovery (%) 95±3 98± 1 98± 3 92± 4 Analytical characteristics Fig. 4 Chromatogram of a typical snow sample from the Mont-Blanc Calibration graphs and detection limits. For calibration, a (405.783 nm). series of spiked standards were derivatized using NaBPr4 and simultaneously extracted into hexane at five diVerent concentration levels to give absolute injected amounts ranging depths below 90 m).A chromatogram of a snow sample is between 0 and 10 pg (as Pb) based on a 5 ml injection volume. shown in Fig. 4. From these data detection limits were calculated, defined as three times the standard deviation of the background of an Conclusions injection of 5 ml of a blank solution obtained by propylation and extraction of a non-spiked standard solution. The figures A fast and simplified sample preparation procedure including of merit of the calibration graphs, and the absolute and relative in situ propylation using sodium tetrapropylborate was optimdetection limits for the diVerent organolead compounds are ized for the speciation analysis of organolead compounds in listed in Table 2.alpine snow samples. Derivatization of the diVerent species and extraction of the propylated species could be performed Precision, recovery and accuracy. The precision of the simultaneously within a one-step procedure. Sample prepmethod was evaluated by five successive injections of 5 ml of aration, separation and detection of the organolead commixed propylated standard solutions containing 2.5 pg (as Pb) pounds could be performed within 15 min.Moreover, in of each species. Reproducibilities near 5% could be achieved contrast to procedures for the speciation analysis of organolead as shown in Table 2. based on in situ ethylation that can only discriminate between Recovery experiments were performed by propylation and methylated species, this method can be applied to the detection extraction of spiked snow and comparing the results with of all relevant organolead species.The procedure was applied those obtained from the analysis of the propylated standards. successfully to the determination of diVerent lead species at All the organolead species could be nearly quantitatively the pg kg-1 level in alpine snow. More detailed investigation recovered (Table 2) due to the simple matrix of the alpine of the snow can provide a closer insight into organolead snow.Incomplete derivatization or extraction of the diVerent contamination originating from European sites, as it can be propylated organolead species can explain the minimal losses. used as an archive of environmental pollution in the same way Accuracy of the in situ propylation was confirmed by as wine, peat or polar snow. determination of TML in a certified reference material (CRM 605, Road Dust) purchased from the Standards, Measurements Acknowledgement & Testing Programme (SM&T) of the European Union.The determined TML concentration of 7.4±0.4 mg kg-1 A research grant by the F.W.O.–Vlaanderen, Belgium, to (mean±standard deviation) is in good agreement with the M.H. is gratefully acknowledged. The authors thank Professor certified TML content of 7.9±1.2 mg kg-1. Pat Sandra and Jos Vandyck (University of Ghent, Belgium) for synthesis of the sodium tetrapropylborate and Katja van Analysis of alpine snow de Velde (University of Grenoble, France) for providing the snow samples.The optimized method was applied to the determination of organolead compounds in snow samples, originating from a glacier in the heart of the Mont Blanc mountain at an altitude References of 4500 m. The analyzed snow was sampled at diVerent depths 1 C. N. Hewitt and R. M. Harrison, in Organometallic Compounds between 20 and 100 m, which can be correlated to a period in in the Environment, ed.P. J. Craig, Longman, Harlow, 1986, the past reflecting the years 1991–1956. Dating of the snow pp. 160–179. was performed at the University of Grenoble, France. It 2 R. £obin� ski, Analyst, 1995, 120, 615. combines known reference levels from atmospheric nuclear 3 F. C. Adams, M. Heisterkamp, J. P. Candelone, F. Laturnus, K. van de Velde and C. F. Boutron, Analyst, 1998, 123, 767. weapon tests and the Chernobyl accident on the one hand and 4 W. M. R.Dirckx, M. B. de la Calle, M. Ceulemans and a glaciological ice flow model on the other hand.31 F. C. Adams, J. Chromatogr. A, 1994, 683, 51. The analyses were performed by triplicate injections of the 5 A. M. Naghmush, K. Pyrzynska and M. Trojanowicz, Talanta, organic phase and precisions between 5 and 15% could be 1995, 42, 851. achieved, which is excellent at such low concentration levels. 6 K. Bergmann and B. Neidhard, Fresenius’ J. Anal. Chem., 1996, Only methylated species were found in the alpine snow with 356, 57. 7 L. Ebdon, S. Hill and R. W.Ward, Analyst, 1987, 112, 1. one sample correlated to the year 1974 also containing 8 P. Uden, J. Chromatogr. A, 1995, 703, 393. 0.57±0.05 ng kg-1 (as Pb) DEL. Concentrations of TML and 9 G. Lespes, M. Pontin-Goutier and A. Astruc, Environ. Sci. DML ranged from 0.23 to 3.38 ng kg-1 and from 0.25 to Technol., 1992, 13, 207. 3.04 ng kg-1 (as Pb), respectively. The highest organolead 10 M. Paneli, E. Rosenberg, M.Grasserbauer, M. Ceulemans and concentrations occurred in the snow sampled at a depth of F.C. Adams, Fresenius’ J. Anal. Chem., 1997, 357, 756. 39 m (corresponding to the year 1988). No organolead was 11 R. £obin� ski and F. C. Adams, J. Anal. At. Spectrom., 1992, 7, 987. found in the snow dated for the period before 1962 (sampling 1310 J. Anal. At. Spectrom., 1999, 14, 1307–131112 I. A. Pereiro and R. �©obin¢¥ ski, J. Anal. At. Spectrom., 1997, 23 C. Witte, J. Szpunar-Lobinska, R. �©obin¢¥ ski and F. C. Adams, Appl. Organomet. Chem., 1994, 8, 621. 12, 1381. 24 A. Wasik, I. R. Pereiro and R. �©obin¢¥ ski, Spectrochim. Acta, Part 13 T. De Smaele, L. Moens, R. Dams and P. Sandra, Fresenius¡� B, 1998, 53, 867. J. Anal. Chem., 1996, 354, 778. 25 M. Ceulemans and F. C. Adams, J. Anal. At. Spectrom., 1996, 14 M. Heisterkamp, T. De Smaele, J. P. Candelone, L. Moens, 11, 201. R. Dams and F. C. Adams, J. Anal. At. Spectrom., 1997, 12, 1077. 26 M. Heisterkamp and F. C. Adams, Fresenius¡� J. Anal. Chem., 15 L. Ebdon, S. J. Hill and C. Rivas, Spectrochim. Acta, Part B, 1998, 1998, 362, 489. 53, 289. 27 K. Bergmann and B. Neidhart, Fresenius¡� J. Anal. Chem., 1996, 16 A. Prange and E. Jantzen, J. Anal. At. Spectrom., 1995, 10, 105. 356, 57. 17 R. �©obin¢¥ ski and F. C. Adams, Anal. Chim. Acta, 1992, 262, 285. 28 T. De Smaele, L. Moens, R. Dams, P. Sandra, J. Van der Eycken 18 Y. Wang, A. B. Turnbull and R. M. Harrison, Appl. Organomet. and J. Vandyck, J. Chromatogr. A, 1998, 793, 99. Chem., 1997, cock and A. Slater, Analyst, 1975, 100, 422. 19 B. Pons, A. Carrera and C. Ner©¥¢¥n, J. Chromatogr. B, 1998, 716, 30 J. J. Sullivan and B. D. Quimby, Anal. Chem., 1990, 62, 1034. 139. 31 C. Vincent, M. Vallon, F. Pinglot, M. Funk and L. Reynaud, 20 M. A. R. Abdel-Moati, Environ. Pollut., 1996, 91, 97. J. Glaciol., 1997, 43, 513. 21 V. Minganti, R. Capelli and R. De Pelligrini, Fresenius¡� J. Anal. Chem., 1995, 351, 471. 22 D. S. Forsyth and W. D. Marshall, Anal. Chem., 1983, 55, 2132. Paper 9/01340G J. Anal. At. Spectrom., 1999, 14, 1307.1311 13

ISSN:0267-9477    

DOI:10.1039/a901340g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Non-spectroscopic suppression of zinc in ICP-MS in a candidate biological reference material (IAEA 392 Algae)† M. J. Campbell* and A. To� rve�nyi International Atomic Energy Agency, Agency’s Laboratories at Seibersdorf and Vienna, A-2224 Seibersdorf, Austria. E-mail: M.Campbell@IAEA.Org Received 1st March 1999, Accepted 30th April 1999 ICP-MS was used for the simultaneous determination of nine analytes in a candidate reference material, IAEA 392 Algae (Scenedesmus obliquus), following microwave digestion with nitric acid and dilution to volume with de-ionised water.For eight analytes, the results showed reasonable agreement with consensus mean results obtained from a world-wide intercomparison exercise on this material. However, the value obtained for Zn was 55.4% of the consensus mean result (69.6 versus 125.6 mg g-1, respectively). Analysis of the same digest solutions by flame AAS gave results which were in good agreement with the consensus mean (127.1 versus 125.6 mg g-1), indicating that all the Zn was in solution.When the method of standard additions was employed, the result was overestimated by #17% by ICP-MS. A 1+9 aqueous dilution of the digests gave an acceptable result for Zn (122.0 mg g-1). Evidence is presented to demonstrate that the underestimation of Zn by external calibration ICP-MS is due to a non-spectroscopic suppression of the Zn in the digest solution caused by nitric acid. accordance with the latest international guidelines.2–4 Introduction Nevertheless, the consensus mean value often lies close to the Microwave digestion in sealed Teflon containers using nitric mean value derived from an expert group and represents a acid followed by ICP-MS is probably one of the most versatile robust estimate of the ‘true’ value.methods available to the analytical chemist for the analysis of It was decided that IAEA 392 would be certified, for up to biological materials. It has the advantages of higher analytical 17 analytes, in accordance with the recommendations of an throughput and reduced contamination risk over conventional expert group5 on the basis of results obtained from four invited hot-plate digestion methods.It has been widely applied to a laboratories and the IAEA’s Seibersdorf laboratory. This large range of materials and (with some minor exceptions) is approach was intended to identify the practical limits of the characterised by accurate and precise results.This method was application of current guidelines relating to quality measures, recently used by this laboratory in the certification campaign uncertainty quantification and traceability for the certification of an IAEA candidate reference material (IAEA 392 Algae); of the trace element contents of a natural matrix reference however, the apparent Zn recovery was seriously underesti- material. The techniques used were neutron activation analysis mated by ICP-MS giving only 55.4% of the consensus mean (NAA), X-ray methods, electrochemical methods, thermal value, derived from a preliminary statistical evaluation of a ionisation mass spectrometry, AAS, ICP-OES and ICP-MS.world-wide intercomparison exercise on this material.1 If the The results of the certification campaign are currently being digests were diluted 1+9 in de-ionised water, an improved evaluated and, if appropriate, certified values for the trace recovery was obtained.The usually robust method of standard element content of the material will be published later this additions failed to compensate completely for the eVect in the year. Consequently, it should be stressed at this juncture that undiluted digests, resulting in an overestimation of the result. the values cited in this paper for the trace element contents of Evidence is presented to demonstrate that the underestimation this material do not constitute reference values. of Zn by external calibration ICP-MS is due to a non- The results for Zn from the preliminary statistical evaluation spectroscopic suppression of the Zn in the digest solution of the 1996 world-wide intercomparison exercise on IAEA 392 caused by nitric acid.obtained from NAA, AAS and ICP-MS are presented in Fig. 1 which shows the classical ‘S’-shape format with the horizontal part of the ‘S’ indicating values that agree with the consensus Intercomparison study mean. The 95% confidence interval is very narrow due to the The Analytical Quality Control Service (AQCS) of the large number of laboratories which contributed to the data set International Atomic Energy Agency (IAEA) organised a (126 accepted out of 142 submitted laboratory means).From world-wide intercomparison exercise to determine trace Fig. 1, it appears that the consensus mean is slightly overelements in an algae material [IAEA 392 Algae (Scenedesmus estimated with respect to two of the three techniques illustrated. obliquus 208)] during 1996.A wide range of analytes (38 in The means and 95% confidence intervals for the individual total ) were determined with varying degrees of precision and techniques are: AAS, 129.9±4.3; ICP-MS, 120.7±4.5; and accuracy.1 The IAEA is currently reviewing its policy relating NAA, 122.1±5.1 mg g-1 Zn, respectively. The overall consensus to such exercises which are no longer suYcient to characterise mean is 125.6±2.5 mg g-1 Zn. On the basis of these results, it a matrix as a reference material of high metrological value in is apparent that the Zn value obtained for IAEA 392 by ICP-MS in-house (69.6±7.8 mg g-1), during the certification study mentioned above, is a significant underestimate of the †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999.‘true’ value. J. Anal. At. Spectrom., 1999, 14, 1313–1316 1313Fig. 1 Results from selected techniques used in the intercomparison of IAEA 392 Algae.Table 1 Comparison of results for the determination of Zn in various reference materials by ICP-MS Matrix This work/ 95% confidence Reference value/ 95% confidence mg g-1 interval mg g-1 interval Cabbage (IAEA 359) 23.2 7.0 38.9 — Algae (IAEA 392) 69.6 7.8 125.6 28 Oyster Tissue (NIST SRM 1566a) 531.2 11.9 830 57 Spinach (NIST SRM 1570a) 52.1 13.1 82 3.0 River Sediment (NIST SRM 1645) 1670 — 1720 170 then the vessels are sealed and tightened using a torque wrench Experimental prior to digestion.As a further confirmation of the adequacy All ICP-MS measurements were made using an Elan 6000 of the digestion procedure, a suite of eight other analytes was (Perkin-Elmer SCIEX, Concord, Ontario, Canada) and a determined in the algae material and the results, presented in Perkin-Elmer AAnalyst 800 (Perkin-Elmer, Norwalk, CT, Table 2, confirm that its performance was generally acceptable. USA) was used for the atomic absorption measurements.Both instruments were operated in their standard configurations. Standard additions For ICP-MS, data were accumulated in peak jump mode for In order to investigate the discrepancy observed between the the analytes of interest, with a 2 s integration per peak. The ICP-MS results for Zn in IAEA 392 and the consensus mean AAS measurements for Zn were performed using an air– value from the world-wide intercomparison, a variant of the acetylene flame at a wavelength of 213.9 nm with a slit-width technique of standard additions was used.Nine sub-samples of 0.7 nm with deuterium background correction. of algae (#100 mg: average 104.8 mg, RSD 3.5%) were taken In order to investigate whether or not the suppression of for digestion: three sub-samples were left unaltered, three had Zn observed in IAEA 392 for ICP-MS analysis was a matrixappropriate spike additions made prior to an overnight cold specific problem, a number of other biological matrices and a digestion stage in nitric acid and the last three were spiked river sediment sample were analysed for their Zn content following microwave digestion when the solutions were made following microwave digestion (Table 1). For the work up to volume (50 ml ).This enabled the influence of the spiking reported here, 100 mg of samplee digested and made up to a final volume of 50 ml (undiluted); in some cases, an Table 2 Results for the analysis of IAEA 392 Algae by ICP-MS aliquot of 1 ml was further diluted 1+9 in de-ionised water compared with the intercomparison consensus mean values (Millipore, Bedford, MA, USA; Milli-Q 18.5 MV).A cocktail of Sc, Y and In was added to all solutions for internal Element This work/ 95% confidence Nominal value/ 2sa standardisation purposes. The results are presented in Table 1. mg g-1 interval mg g-1 Chromium 4.32 0.15 4.37 3.2 Microwave digestion Cobalt 2.99 0.09 3.27 0.8 A simple, robust, microwave digestion program was developed Copper 19.1 1.19 22.4 15 Iron 468 16 491 156 to give quantitative recoveries for a range of analytes in Lead 0.67 0.10 0.55 0.3 various biological reference materials.In order to minimise Magnesium 1970 160 2314 740 polyatomic interferences, the only reagents used were 5 ml of Manganese 60.2 1.49 66.4 15.8 concentrated nitric acid (Merck, Darmstadt, Germany) and Nickel 0.55 0.13 0.64 0.48 de-ionised water (Millipore, Milli-Q).A Milestone microwave Zinc 69.6 7.8 125.6 28 oven (MLS 1200 mega, Milestone S.R.L., Sorisole, Italy) was aBased on the standard deviation of all (statistically) accepted results used. In this system, samples are placed in Teflon liners which for the intercomparison. are supported inside a resin sleeve, reagents are added and 1314 J. Anal. At. Spectrom., 1999, 14, 1313–1316chronology to be studied and gave a total of six results (three values each calculated against the pre- and post-spiking standard additions responses, respectively).The undiluted samples were also analysed by flame AAS. A typical result for both techniques is plotted in Fig. 2 and the results are given in Table 3. In an unrelated study, it had been noted that the apparent Zn recovery improved when the samples were diluted, so the solutions were diluted 1+9 and re-analysed at a later date (Fig. 3). Note, in Fig. 2 and 3, the y-axis is expressed as ‘equivalent concentration’ by dividing the signal response for AAS and ICP-MS by the appropriate sensitivity factors such that both techniques can be represented on a single graph (Fig. 2). Influence of acid strength The influence of acid strength is illustrated in Fig. 4. A series of solutions were prepared containing fixed amounts of Sc, Y, In and Zn in 0, 1, 3, 5 and 10% v/v HNO3, respectively. This Fig. 3 Determination of Zn in IAEA 392 by standard additions at eVect was further studied by digesting IAEA 392 using a half 1+9 dilution by ICP-MS.and a fifth of the normal volume of nitric acid (5 ml ) and making the solutions up as before (Table 3). Results and discussion When low analyte recoveries are encountered, it is usually assumed that the discrepancy is due either to (i) incomplete dissolution or (ii) loss of the analyte either as a volatile component or to the walls of the reaction vessel. The results Fig. 4 Influence of nitric acid concentration on Sc, Y, In and Zn response.shown in Table 1 indicate that the combination of the digestion procedure used here, with quantification by ICP-MS, leads to a systematic underestimation of Zn in biological matrices. For NIST SRM 1645 River Sediment, an acceptable value was found, but in this case a modified method was used involving mixed acid attack and a much larger dilution (vide infra). For the biological matrices, the usual reasons for low recovery can be dismissed since AAS analysis of the same digest solutions Fig. 2 Determination of Zn in IAEA 392 by standard additions using ICP-MS and AAS. gave correct results, indicating that all the Zn was indeed in Table 3 Summary of standard additions results for Zn in IAEA 392 Algae before and after dilution compared with the results from aqueous calibration Regression coeYcient Slope Replicate/ Replicate 2/ Replicate 3/ mg g-1 mg g-1 mg g-1 Undiluted— Pre-spiked 0.9941 1130.4 147.81 145.56 146.13 Post-spiked 0.9980 1118.3 147.10 148.27 148.88 Versus aqueous — — 74.96 74.13 73.82 calibration Diluted 1+9— Pre-spiked 0.9988 203.6 124.78 126.98 121.29 Post-spiked 0.9953 205.0 119.94 122.15 116.60 Versus aqueous — — 93.6 91.0 95.3 calibrationa Versus aqueous 117.10 119.30 113.84 calibrationb aOriginal standard set also diluted 1+9, i.e., 0.1% HNO3.bCalculated from original calibration, i.e., 1% HNO3. J. Anal. At. Spectrom., 1999, 14, 1313–1316 1315solution (Fig. 2). Furthermore, the method used gives good analytical response of In. These eVects are consistent with an recoveries for eight other analytes on the basis of ICP-MS eVective reduction in the energy of the plasma in the central results (Table 2).Zn is underestimated by ICP-MS against an analyte channel. An eVect of reducing the nebuliser gas flow aqueous calibration, despite the use of internal standards, is to increase the residence time in the plasma which, to some giving roughly half of the expected concentration.extent, oVsets the reduction in plasma energy. This idea can The influence of the chronology of the Zn spike incorpor- be visualised using the ‘zone model’ of Vanhaecke et al.10 ation for standard additions was studied in this work since which relates the spatial occurrence of the maximum region Campbell et al.6 showed that the chronology of the addition of M+ density to operational parameters of the instrument of an isotopically enriched spike had a strong influence on the and the mass of the analyte.It can be calculated that the final result for the determination of Hg in a biological CRM. addition of nitrogen from nitric acid to the plasma is two However, the results of the pre- and post-digestion spiking orders of magnitude lower than that used by Wang et al.;9 study (Table 3) demonstrate that such an eVect does not play however, the influence of the time taken for evaporation and a role here. dissociation of the acid may play a role.Furthermore, the Standard additions (to independent digests) to the undiluted ionisation potential of Zn (9.4 V) is significantly higher than algae solution give better agreement between the ICP-MS those of the analytes used in their study and, therefore, would result and the consensus value for Zn, but lead to an overestim- be expected to be more sensitive to changes in the available ation by about 17% whereas 1+9 dilution of the digest energy of the plasma than easily ionised analytes.This could solution provides results which are in good agreement with explain why a correct result was obtained for NIST SRM the consensus value. If none of the nitric acid used for the 1645 River Sediment in this work since the total dilution factor digestion process was lost or consumed, the maximum acid (2500) is large enough to negate any eVect of nitrogen on Zn. content in the final solution would be 10%. The observation Indeed, the residual acid concentration was so low that the that simple dilution of the digest solutions gave higher apparent solutions were made up to 1% HNO3, to avoid the risk of Zn recoveries provided strong circumstantial evidence that loss of analytes from solution; consequently, the acid strength acid strength was involved in the suppression.In one digestion matched that of the external calibration. set, the acid content was reduced to 50 and 20% of its original A similar eVect of nitric acid concentration on sensitivity level and the results obtained (versus external aqueous stan- was observed for As in biological matrices11 which resulted in dards containing 1% v/v HNO3) were 63.4 and 93.3 mg g-1, erroneously high As values.Both Zn and As have relatively respectively, which is in agreement with the trend depicted in high first ionisation potentials (9.4 and 9.8 V, respectively) Fig. 4. However, for the latter digest, the solution was col- which may make them more sensitive to ‘spatial eVects’ within oured, indicating that the digestion process was not complete.the plasma. Confirmation that the eVect described in this The influence of acid strength was studied by monitoring the paper is not limited to ICP-MS was provided by Marichy sensitivity for a suite of analytes at constant concentration et al.,12 who reported a #15% decrease in optical emission (50 mg l-1) over a range of acid strengths. Clear evidence is intensities for Co II as the acid strength was increased from 0 shown (Fig. 4) which demonstrates significant non- to 1% HNO3; Co has a first ionisation potential of 7.9 V. spectroscopic suppression of Zn at nitric acid concentration From the work presented here, it is clear that Zn results levels where other analytes (Sc, Y and In) are largely unaVec- will only be reliable if the concentration of nitric acid is kept ted, rendering internal standardisation ineVective for Zn. AAS as low as possible and held constant.A similar eVect is results do not show evidence of any influence of nitric acid on predicted for analytes with high first ionisation potentials. Zn recovery from aqueous solutions and provide accurate results for the determination of Zn in IAEA 392 from the References same solutions. Stewart and Olesik7 studied the influence of nitric acid 1 Preliminary Statistical Evaluation of the Intercomparison Run concentration and nebuliser gas flows on aerosol transport IAEA 390, IAEA/AL/101 P, 1998, International Atomic Energy rates into an ICP and noted a sharp decrease in analyte signal Agency, Vienna, Austria.as the acid concentration increased from 0 to 2%, depending 2 Programme Performance Assessment External Reviewers’ Report on the Consultants’ Meeting on Analytical Quality Control Services, upon operating conditions. We observed this decline in signal IAEA/AL/114, 1998, International Atomic Energy Agency, between 0 and 2% HNO3 for Zn, but not for Sc, Y or In.Vienna, Austria. However, they report that the eVect is not so pronounced 3 Report of the Consultant’s Meeting on Traceability of IAEA-AQCS under ‘robust plasma conditions7’ which correspond to the Reference Materials to SI-Units, IAEA/AL/105, 1997, conditions we were using. In an earlier paper,8 they note that International Atomic Energy Agency, Vienna, Austria. analyte signal depression ( lasting from 5 to 25 min) may occur 4 ILAC Requirements for Accreditation of Certifiers of Reference as acid strength is increased, until the steady state is Materials (draft), 1997, International Standards Organisation, Geneva, Switzerland.re-established. This is attributed to competing evaporational 5 Report on the Consultants’ Meeting on Good Analytical Practice in processes between acid aerosol droplets and coalesced droplets the Analysis of IAEA Materials for Certification, 1996, in on the spray chamber walls and presumably aVects all analytes preparation. in a similar manner. In our work, the original digest solutions 6 M. J. Campbell, G. Vermeir, Ph. Quevauviller and R. Dams, are produced in a batch process and have similar acidity levels; J. Anal. At. Spectrom., 1992, 7, 617. they incorporate internal standards which should minimise 7 I. I. Stewart and J. W. Olesik, J. Anal. At. Spectrom., 1998, 13, 1249. this eVect, so it is unlikely that it is responsible for the Zn 8 I. I. Stewart and J. W. Olesik, J. Anal. At. Spectrom., 1998,13, 843. suppression observed. 9 J. Wang, E. H. Evans and J. Caruso, J. Anal. At. Spectrom., 1992, The observed behaviour of Zn may be due to the high local 7, 929. concentration of nitrogen atoms in the plasma (due to the 10 F. Vanhaecke, R. Dams and C. Vandecasteele, J. Anal. At. dissociation of the nitric acid). Wang et al.9 studied the eVects Spectrom., 1993, 8, 433. of introducing nitrogen into an argon ICP used for mass 11 M. J. Campbell, C. Demesmay and M. Olle�, J. Anal. At. spectrometry. They observed that when 3% of the nebuliser Spectrom., 1994, 9, 1379. 12 M. Marichy, M. Mermet and J. M. Mermet, Spectrochim. Acta, gas flow was made up of nitrogen, the central analyte channel Part B, 1990, 45, 1195. in the plasma broadened, the optimum M+ signal shifted to lower nebuliser gas flow rate, certain polyatomic species were significantly reduced and a slight decline was observed for the Paper 9/01639B 1316 J. Anal. At. Spectrom., 1999, 14, 1313–13

ISSN:0267-9477    

DOI:10.1039/a901639b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

An alternative GC-ICP-MS interface design for trace element speciation† Marý�a Montes Bayo�n, Manuel Gutie�rrez Camblor, J. Ignacio Garcý�a Alonso* and Alfredo Sanz-Medel Department of Physical and Analytical Chemistry, University of Oviedo, Julia�n Claverý�a 8, 33006 Oviedo, Spain. E-mail: JIGA@sauron.quimica.uniovi.es Received 4th March 1999, Accepted 21st May 1999 The design, construction and performance characteristics of an easily removable interface to couple a gas chromatograph (GC) to an inductively coupled plasma mass spectrometer (ICP-MS) for trace element speciation is described.The interface is based on the use of a heated metallic tube where the last part of the capillary GC column is inserted. This metallic tube is connected to a metallic T-piece where a flow of argon carrier gas is introduced perpendicularly and externally to the metallic tube, creating a high velocity flow of intermediate sheath gas, which prevents condensation of the analytes eluting from the column on the walls of the T-piece or on the connection tubing to the ICP-MS torch.A flexible non-heated PTFE tube is used to connect the exit of the T-piece to the ICP-MS torch. This new interface has been applied to the separation and detection of diVerent organometallic compounds of Se, S, Pb, Hg and Sn. Introduction However, this coupling is not so straightforward. There are Organometallic species may be found in our environment several limitations, probably the most important one being either because they are naturally formed there or because they that the analytes have to be maintained in gaseous form during have been introduced by anthropogenic contributions.In transportation from the GC to the ICP-MS, avoiding any general terms, these species are more toxic than their inorganic condensation eVect at the interface. Apart from this fact, the salts (except in the case of organoarsenic compounds) particu- eZuent from the gas chromatograph (a few ml per minute) larly because of their combined hydrophobic and lypophilic requires an aerosol carrier (make up) gas to achieve suYcient characteristics, which make them capable of entering biological flow to get the analytes into the central channel of the plasma.cycles with detrimental consequences.1,2 Therefore the need to For these reasons, most of the analytical literature concerning perform organometallic speciation studies has become a topic GC-ICP-MS addresses the construction and development of of growing importance over the last few years.3 appropriate interfaces able to provide reliable and reproducible The most common strategy to carry out the separation and results to carry out trace element speciation in environmental determination of diVerent organometallic species is the hyphen- samples.Nevertheless, some problems still remain as none of ation of a powerful separation technique such as high-perform- the interfaces described in the literature are commercially ance liquid chromatography (HPLC),4 gas chromatography available.This is due to the fact that most existing transfer (GC),5,6 supercritical fluid chromatography (SFC)7 or capil- lines are based on metallic tubes that take the chromatographic lary electrophoresis (CE)8 with element-specific detection tech- column from the oven all the way to the ion source (the niques such as atomic absorption (AAS), atomic emission ICP-MS torch or the plasma itself ) in a manner similar to (MIP-AES and ICP-AES) or mass spectrometry (ICP-MS).that used traditionally for the GC-MS coupling using an The choice of an adequate separation technique is determined electron impact ion source. These interfaces are therefore rigid by the physicochemical properties of the analyte (volatility, and have to be heated over the whole length in order to charge, polarity), whereas that of the detection technique is prevent peak broadening due to condensation of the analytes. determined by the analyte’s level in the sample and by the This fact makes the coupling and decoupling of the GC to the availability of a suitable detector. ICP-MS diYcult and time consuming.In trying to avoid these The combination of capillary GC with an ICP-MS has major drawbacks of the existing interfaces, a novel transfer become an ideal methodology for the speciation of organomet- line has been designed by our research group, based on allic compounds in complex environmental samples, which previous experience using cryogenic trapping devices coupled requires the high resolving power of GC and the sensitivity to the ICP-MS.19 and specificity detection provided by the ICP-MS.9–11 The first The present work describes a new flexible and easily investigations on the coupling of a GC to the ICP-MS were removable interface to couple a GC to an ICP-MS.This carried out using packed columns.12,13 A real breakthrough of transfer line has been evaluated for the speciation of diVerent this coupling appeared when capillary columns became com- organometallic species containing tin, mercury, lead, sulfur mercially available.The first publication on this subject was and selenium of environmental and biological interest. by the group of Ebdon and co-workers14 and many other applications of capillary GC coupled to ICP-MS have been Experimental published since.15–18 Reagents and solutions Monobutyltin trichloride (MBTCl3, 95% purity), dibutyltin dichloride (DBTCl2, 97% purity) and tributyltin chloride †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999.(TBTCl, 96% purity) were obtained from Sigma–Aldrich J. Anal. At. Spectrom., 1999, 14, 1317–1322 1317(Bornem, Belgium). Stock solutions were prepared in methanol cury, lead and selenium. For the separation of the enantiomeric forms of D,L-methionine the column used was a Chiralsil-Val (HPLC grade, Romil Chemicals, Cambridge, UK).Mercury (as nitrate, 1000 ppm) was purchased from Merck (Darmstadt, (100% L-valine tertbutylamide) from Machery–Nagel. The injection temperature was maintained at 250 °C and the injec- Germany) and methylmercury chloride (98% purity) and ethylmercury chloride (97% purity) were obtained from ICN tion volume was 1 ml for injection of all the species. A splitless time of 0.5 min was used for all the experiments.The chromato- Biochemicals (Cleveland, OH, USA). Selenomethionine (98% purity) and selenoethionine (97% purity) were obtained from graphic gas flow rate was 4 ml min-1 of helium (Air Liquide, Oviedo, Spain, purity 99.999%) and the pressure at the head Sigma–Aldrich. Methionine was obtained as a derivatised standard solution from the chiral column manufacturers of the column was 100 KPa. The transfer line was located on the top of the gas chromatograph, replacing the standard FID Machery–Nagel (Du� ren, Germany).Sodium tetraethylborate (NaBEt4, 2%) (Strem Chemicals, detector in the commercial instrument. The ICP-MS was an HP Model 4500 (Hewlett Packard, Bischeheim, France), was prepared in 0.1% NaOH solution (obtained from Merck) and diluted in Milli-Q water (Millipore, Yokogawa Analytical Systems, Kyoto, Japan) connected to the GC via the transfer-line proposed here (Spanish patent Molsheim, France). The organotin compounds were dissolved with glacial acetic acid (Merck) and the derivatisation was application number P-9801923).In this design, the PTFE tubing used to transfer the analytes to the ICP-MS is directly done at pH 4–5.5. To obtain buVer solutions with pH values between 4 and 5.5, appropriate volumes of acetic acid and inserted into the plasma torch. Using this design, the automatic torch positioning routine of the HP 4500 during the ignition sodium acetate (Merck) solutions were mixed together.The derivatives were finally extracted in hexane (Romil Chemicals, of the plasma is not disturbed. The final interface design will be extensively described under Results and Discussion. The HPLC grade). Butyl magnesium chloride was obtained from metallic connections were from Swagelok (Swagelok, OH, USA). The operating conditions for the GC and the ICP-MS Sigma–Aldrich. DiethyldithDTC) (Merck) was used for complexation of organomercury compounds and are summarised in Table 1.All data were acquired under the time resolved analysis programme and evaluated using the then they were extracted in toluene (Romil Chemicals, HPLC grade). chromatographic software provided with the instrument. The derivatisation of amino acids was carried out as described by other workers20 using 4 M HCl (Merck) in propan-2-ol (Romil ). A mixture of trifluoroacetic acid anhy- Procedures dride (Fluka, Buch, Germany) and dichloromethane (Romil ) The derivatisation of the organotin compounds was were also used.20 accomplished by diluting the methanolic standards in glacial acetic acid.The pH was adjusted by adding 3.5 ml of sodium Instrumental acetate (1 M) to obtain a final value of 4–5.5. Then, 1 ml of sodium tetraethylborate (2%) was added to the solution and The gas chromatograph used for packed columns was an HP Model 5720 A (Avondale, PA, USA) equipped with an the derivatised species extracted into hexane and injected into the GC-ICP-MS system.on-column injector and the packed column was a Cromosorb W HP (mesh 100–120, coated with 10% OV 210, 1.83 m length Organomercury compounds were dissolved in Milli-Q water and then complexed using 1 ml of 0.5 M DDTC at pH 9.2, and 2 mm id) purchased from Hewlett Packard. Capillary GC studies were carried out using a Hewlett achieved by adding 1 ml of a borate buVer. These complexed species were extracted in 2 ml of toluene; 1 ml of the toluene Packard gas chromatograph Model 6890 equipped with split/ splitless injection.An HP-5 column (5% phenyl, 95% polydime- layer was placed in a three-armed 25 ml flask placed in an ice bath and then the derivatisation was accomplished using 1 ml thylsiloxane, 30 m length×0.25 mm id) was used to perform the separation of the organometallic compounds of tin, mer- of butyl magnesium bromide. The mixture was left to stand Table 1 Operating conditions of the GC-ICP-MS Species containing Sn, Hg, Pb, Se S Injector parameters– Injection mode Split/splitless Split/splitless Splitless time 0.5 min 0.5 min Injection volume 1 ml 1ml Splitting ratio 1515 1515 Injection temperature 250 °C 180 °C GC parameters— Column HP-5 L-Chirasil Val (30 m×0.25 mm×0.25 mm) (25 m×0.25 mm) Carrier gas/inlet pressure He, 100 psi He, 130 psi Injector temperature 250 °C 200 °C GC program 50 °C (0.5 min) to 250 °C 60° C to 130 °C at 2°Cmin-1 (0.5 min) at 30 °Cmin-1 Transfer line dimensions 80 cm length, 1.5 mm id 80 cm length, 1.5 mm id Transfer line temperature 250 °C 200 °C ICP-MS parameters— Isotopes 118,120Sn, 202Hg, 208Pb, 82Se 34S Rf power 1300W 1300W Sampling depth 7 mm 5.8 mm Carrier gas flow rate 1.0 l min-1 1.5 l min-1 Intermediate gas flow rate 1.0 l min-1 1.0 l min-1 Outer gas flow rate 15 l min-1 15 l min-1 Dwell time 0.1 s per m/z 0.1 s per m/z 1318 J.Anal. At. Spectrom., 1999, 14, 1317–1322for 5 min to complete the reaction and 0.5 M sufuric acid was then added to decompose the excess of Grignard reagent.The derivatisation of Se amino acids was carried out as described elsewhere:20 first by esterification of the carboxylic group with 4 M HCl in propan-2-ol in closed reaction vials at 110 °C for 50 min and, after evaporation under nitrogen at 40 °C, acylation was performed with a mixture of trifluoroacetic acid anhydride and dichloromethane at room temperature. The excess of reagent was evaporated under nitrogen and the residue was re-dissolved in dichloromethane to be injected directly into the GC-ICP-MS.Results and discussion Interface design and optimisation Previous work in our laboratory on the coupling of a hydride generation–cryogenic trapping system to the ICP-MS for butyltin speciation,19,21 following the design extensively used by Randall et al.,22 showed that a simple glass piece, such as that shown in Fig. 1, connected to the end of the U-shaped glass chromatographic column, assured the transport of the butyltin hydrides from the glass column to the ICP-MS.This Fig. 2 Glass connection employed for coupling the GC (with packed was provided with a high velocity Ar carrier make up gas columns) to the ICP-MS; a nichrome wire is used to heat the external to the eluent from the chromatographic column which glass piece. prevented the condensation of the analytes on the walls of the transfer tube to the ICP-MS torch. Peak profiles for MBT and DBT hydrides in that design19,21 proved to be satisfactory, but pronounced tailing was observed for TBT hydride.This former design of Fig. 1 was then evaluated as an interface for GC-ICP-MS in the HP 5720 A chromatograph, equipped with a 2 m long packed chromatographic column for the separation of MBT, DBT and TBT after ethylation using Et4BNa. The glass piece was set on the top of the GC oven as shown in Fig. 2, using a small piece of PTFE tubing to fix the end of the chromatographic column inside the glass piece. In order to prevent condensation of the analytes, this glass piece was heated using a nichrome wire.After adequate optimisation of GC and ICP-MS experimental parameters, a mixture of MBT, DBT and TBT containing 50 pg of each compound as Sn was injected. Fig. 3 shows a typical Fig. 3 GC-ICP-MS separation of 50 pg of the ethyl derivatives of MBT, DBT and TBT using the heated glass connection shown in chromatogram obtained with such an interface.Fig. 2 and a packed column. Using an HP-5 capillary column in a new HP 6890 gas chromatograph, the peak profiles improved further when the glass piece of Fig. 2 was replaced by a metallic Swagelok 1/4 in standard T-piece, externally heated by a metallic heating block. As shown in Fig. 4 the peak profiles for MBT and DBT ethyl derivatives now appear fairly satisfactory. However, TBT still showed a broad peak profile. Further modifications followed and the interface which is shown in Fig. 5 was finally designed and constructed. It consists Fig. 4 Peak profile corresponding to MBT, DBT and TBT by GC-ICP-MS using a capillary column and a metallic connection Fig. 1 Glass connection used for the coupling of the cryogenic trapping heated by means of a variable electric resistance (100 pg injected of each compound as Sn). to the ICP-MS for organotin compounds.19 J. Anal. At. Spectrom., 1999, 14, 1317–1322 1319Fig. 6 Peak profiles for MBT, DBT and TBT for 15 pg injection of each ethyl derivative using the final interface design between the GC and the ICP-MS.given temperature programme. Absence of tailing of these peaks is also apparent in Fig. 6. Optimisation of temperature of heating block When the temperature of the metallic block (Fig. 5) was not high enough, the width of the chromatographic peaks increased. Separated species can condense, both on the column or on the walls of the T-piece at the interface, at too low temperatures.Therefore a study of the influence of the tem- Fig. 5 Final interface design (Spanish patent application number P-9801923). perature of the metallic block on the peak profiles of the investigated species was carried out. Fig. 7 shows the observed baseline peak width plotted of a metallic T-piece (1/4 in od Swagelok, each side arm) against the temperature of the metallic block for MBT, DBT connected to a concentric assembly of a copper tube (8.91 cm and TBT as ethyl derivatives.No significant variations could length and 0.15 cm id) and an internal stainless steel tube be found for the MBT and DBT peak widths using tempera- (0.06 cm id) where the last 9 cm of the chromatographic tures above 175 °C. Nevertheless, the ethyl derivative of TBT column are placed and protected against breakage. The copper showed a better performance and narrower peak profile using tube is heated by an external metallic block containing an temperatures above 200 °C.Also, the retention time for TBT electric heater and a temperature sensor, both controlled by changed slightly on increasing the temperature of the heating the GC and thermally isolated with industrial pipe lagging. block up to 200 °C (indicating condensation eVects inside the One cm of the copper tubing passing through the metallic chromatographic column rather than on the walls of the block for heating, is inserted into the oven wall of the GC. interface). In order to avoid decomposition of the coating of The chromatographic column exiting the GC reaches the top the capillary column in the transfer line and to ensure that no of the copper tubing and is immobilised inside of the tube matrix compounds could condense on the column walls when assembly by using a female Swagelok adapter (see Fig. 5).The real samples were analysed, a transfer line temperature of copper tubing is fixed to the metallic block by means of an 250 °C was chosen for speciation of organotin compounds.ordinary screw drilled in its wall. Temperatures higher than 280 °C were not tested to avoid The additional Ar carrier make up flow is introduced melting of the PTFE tubing. through the side-arm of the T-piece and flows through a gap between the copper tube and the T-piece and externally to the Argon carrier make up gas flow rate GC efluent. This design allowed us to obtain a high velocity The peak profiles and the sensitivity achieved from these fast intermediate sheathing Ar flow to transport the analytes from transient signals are strongly dependent on the make up Ar the GC to the ICP-MS, avoiding condensation eVects on the flow rate employed to transport the analytes exiting the column walls of the interface or in the connecting tubing. It has to be to the plasma.In Fig. 8, it is possible to observe that this noted that no additional heating of the Ar make up gas was parameter seemed to aVect MBT, DBT and TBT in a similar necessary to achieve good peak profiles.The other end of the T-piece was connected to a reducing union (1/4 to 1/8 in, Swagelok not shown in Fig. 5) where a 1.5 mm id PTFE tubing was directly attached and inserted into the central channel of the ICP-MS torch for ca. 3 cm (never reaching the tip of the torch) using a rubber O-ring and standard glass olives to obtain a gas tight connection. For MBT, DBT and TBT as the ethyl derivatives, it was observed that, with this design, the length of the PTFE tubing to connect the GC to the ICP-MS did not aVect the chromatographic peak profiles. Therefore, for better handling of the system, a final length of 80 cm was used. Fig. 6 shows some illustrative peak profiles obtained under optimum conditions for the separation of a mixture of 15 pg (as Sn) of each of the ethylated MBT, DBT and TBT. As can be observed, all three compounds showed very narrow peak profiles (peak widths at 10% of the height of 0.8, 1.6 and 2.1 s, respectively), similar Fig. 7 Baseline peak widths for MBT, DBT and TBT plotted against the temperature of the metallic block. to those obtained for conventional GC detectors under the 1320 J. Anal. At. Spectrom., 1999, 14, 1317–1322for MBT and DBT). The isotope ratio values obtained were independent of the species under study, but they did not agree with the theoretical ratio value due to mass discrimination eVects. For isotope dilution analysis the mass discrimination eVects will have to be corrected for using natural tin standards.Calibration graphs, based on peak area measurements were carried out for solution mixtures of increasing concentrations of the three tin compounds (from 0.1 to 103 ng ml-1) with adequate regression coeYcients as shown in the last line of Table 2. Potential of interface for other trace element speciation problems Fig. 8 The eVect of the Ar make-up flow rate on the signal of the As the proposed interface design is the subject of a Spanish organotin compounds (evaluated on 100 pg injection of each patent (application number P-9801923) it has been also compound) at 7 mm sampling depth and 1300W forward power.evaluated for other important speciation problems in the environmental and biological fields. way. The optimum value found, for the three of them, was As regards environmental applications, Fig. 9 shows the 1.0 l min-1 using a sampling depth of 7 mm and a forward chromatogram corresponding to the separation of 2 pg of the power of 1300 W.It has to be also pointed out that no butyl derivatives of mercury, methylmercury and ethylmercury significant variations on the retention times of the studied in a standard solution after Grignard derivatisation. As can analytes were found when increasing the Ar make up from 0.8 be observed, good chromatographic profiles and sensitivities to 1.3 l min-1. This result was expected from the fact that in were also obtained for these compounds using the chromatothe 80 cm long PTFE tubing of 1.5 mm id used, in the transfer graphic conditions detailed in Table 1.Similarly, Fig. 10 shows of the analytes from the GC to the ICP-MS it takes only 0.1 s the response for the methylethyllead compounds from a diluted from the GC to the torch at 1.0 l min-1. petrol (dilution factor 1+10000 in hexane) monitoring the Using a 1.0 l min-1Ar gas flow rate, the integration time lead 208 isotope using the same set of conditions.As a necessary for good definition of the peak profile was also conclusion, it seems that the simultaneous speciation of organoptimised by measuring the m/z 118 and 120 tin isotopes. It omercury, organolead and organotin compounds would be is well documented that too short integration times provide non-adequate peak profiles due to the low precision of the measurements. On the other hand, if integration times are too long the number of acquisition points in the ICP-MS to define a chromatographic peak become too low.In this vein, an integration time of 0.1 s per m/z was found to provide satisfactory results in terms of precision and peak definition. All final optimised operating conditions for tin speciation are summarised in Table 1. Analytical performance characteristics for organotin speciation The analytical performance characteristics of the optimised GC-ICP-MS coupled system for tin speciation are listed in Table 2.Detection limits of 50, 70 and 100 fg for MBT, DBT and TBT, respectively (as Sn), were obtained (defined as three times the standard deviation of the background noise of a Fig. 9 Peak profiles corresponding to 2 pg injection of the butyl blank). In order to examine the reproducibility, 1 ml of a derivatives of: 1, Methyl mercury; 2, ethyl mercury; 3, inorganic 10 ng ml-1 solution of a mixture of butyl compounds, injected mercury after Grignard butylation. manually five times, produced relative standard deviations of 1.5, 2 and 5% for MBT, DBT and TBT, respectively, measured as peak area at m/z 120.Precisions obtained for the 1185120 peak area ratio (n=5) were between 0.4 and 1.6% with the best results for MBT and worsening precision with increasing peak width of the corresponding species (precision for TBT was always worse than Table 2 Analytical characteristics found for MBT, DBT and TBT as ethyl derivatives Parameter MBT DBT TBT Retention time/min 4.58 5.42 6.16 %RSDret.time 0.15 0.16 0.14 LOD (as Sn)/fg 50 70 100 Peak area, %RSD (5 injections) 1.5 2 5 Isotope ratio 1185120 0.705 0.708 0.708 (over 20 pg) (theor.value 0.743) Fig. 10 Chromatogram of methyl ethyl derivatives of lead in a petrol Isotope ratio, %RSD 0.4 0.7 1.6 (dilution factor 1+10000, approximately 12 pg of total Pb injected): Correlation coeYcient (r2) 0.9991 0.9993 0.9995 1, tetramethyllead; 2, trimethylethyllead; 3, diethyldimethyllead; 4, (0.1–103 ng g-1) monomethyltriethyllead, 5, tetraethyllead. J.Anal. At. Spectrom., 1999, 14, 1317–1322 1321Conclusions The construction of a flexible and easily removable interface for the coupling of a capillary gas chromatograph with an inductively coupled plasma mass spectrometer has been successfully accomplished. The main advantage of the proposed interface is that the two instruments can be used separately and when both the GC and the ICP-MS have to be used they can be connected in less than 5 min.Another feature of the proposed design is that the GC column does not need to be taken all the way to the ICP-MS torch. Only the last 9 cm of the column are out of the GC oven and kept hot by means of the metallic block. Also, the use of flexible PTFE tubing to connect the exit of the T-piece to the ICP-MS torch allows the free movement (x–y–z) of the torch box and the fast coupling and decoupling Fig. 11 Peak profiles for: 1, Se-methionine; 2, Se-ethionine of both instruments.Also the use of a non-heated Ar make corresponding to 500 pg injection of each compound as Se. up gas to transport the analytes from the GC to the ICP-MS is very convenient and provides an intermediate gas flow which seems to avoid condensation eVects in the PTFE tube connecting the GC to the ICP-MS. Further studies have to be carried out in order to assess the whole potential of the interface design for diVerent organometallic species, but the results are promising for the speciation of Sn, Pb, Se, S and Hg.References 1 A. Prange and E. Jantzen, J. Anal. At. Spectrom., 1995, 10, 105. 2 R. Lobinsky, Appl. Spectrosc., 1997, 51, 260A. 3 G. K. Zoorob, J. W. McKiernan and J. A. Caruso, Mikrochim. Acta, 1998, 128, 145. 4 J. J. Thompson and R. S. Houk, Anal. Chem., 1986, 58, 2541. 5 M. Heisterkamp, T. De Smaele, J. P. Candelone, L. Moens and F. C. Adams, J. Anal. At. Spectrom., 1997, 12, 1077. 6 J. Feldmann, J. Anal. At.Spectrom., 1997, 9, 1069. 7 N. P. Vela and J. A. Caruso, J. Chromatogr., 1993, 641, 337. 8 J. W. Olesik, J. A. Kinzer and S. V. Olesik, Anal. Chem., 1995, 67, 1. Fig. 12 Chiral separation of L- and D- methionine derivatives 9 H. Hintelmann, R. D. Evans and J. Y. Villeneuve, J. Anal. At. monitored as sulfur at m/z 34. Spectrom., 1995, 9, 619. 10 T. De Smaele, P. Verrept, L. Moens and R. Dams, Spectrochim. Acta, Part B, 1995, 11, 1409. 11 L. Moens, T. De Smaele, R. Dams, P.Van Den Broeck and P. Sandra, Anal. Chem., 1997, 69, 1604. possible in a single chromatographic run with a single set of 12 N. S. Chong and R. S. Houk, Appl. Spectrosc., 1987, 41, 66. conditions using the proposed GC-ICP-MS coupling. 13 G. R. Peters and D. Beauchemin, J. Anal. At. Spectrom., 1992, Further applications were tested for the separation of Se 7, 965. 14 A. W. Kim, M. E. Foulkes, L. Ebdon, S. J. Hill, R. L. Patience, and S amino acids. Fig. 11 shows the chromatographic profile A.G. Barwise and S. J. Rowland, J. Anal. At. Spectrom., 1992, obtained for the volatile derivatives of Se-methionine and 7, 1147. Se-ethionine using the proposed procedures and analysed using 15 W. J. Pretorius, L. Ebdon and S. J. Rowland, J. Chromatogr., the same column and similar chromatographic conditions to 1993, 646, 369. those of Sn, Hg and Pb (Table 1). The chromatogram corre- 16 T. De Smaele, L. Moens, R. Dams, P. Sandra, J. Vandereycken sponds to a 500 pg injection of each compound (as Se) and and J.Vandyck, J. Chromatogr. A, 1998, 793, 99. 17 S. M. Gallus and K. G. Heumann, J. Anal. At. Spectrom., 1996, detected at m/z 82. 11, 887. Finally, using a chiral column (L-Chirasil Val, containing 18 J. Poehlman, B. W. Pack and G. M. Hieftje, Am. Lab., 1998, L-valine tert-butylamide as the stationary phase) enantiomeric 30, C50. separation of amino acids was possible. In fact good optical 19 E. Segovia Garcia, J. I. Garcý�a Alonso and A. Sanz-Medel, resolution of D- and L-methionine by monitoring sulfur at m/z J. Mass. Spectrom., 1997, 5, 542. 34 can be achieved as demonstrated in Fig. 12. Of course, in 20 M. B. De La Calle-Guntin� as, C. Brunori, R. Scerbo, S. Chiavarini, P. Quevauviller, F. Adams and R. Morabito, J. Anal. At. this chiral separation the interaction of the analytes with the Spectrom., 1997, 9, 1041. column requires a slow ramp of approximately 1.5 °Cmin-1 21 E. Segovia Garcia, J. I. Garcý�a Alonso and A. Sanz-Medel, in and so the peaks showed a broader profile than those obtained Applications of Plasma Source Mass Spectrometry, ed. G. Holland before in the case of Sn, Hg, Pb or Se. A similar separation and S. Tanner, Royal Society of Chemistry, Cambridge, 1997. of D–L selenoamino acids using the interface developed here p. 182. can be found elsewhere.23 22 L. Randall, O. F. X. Donard and J. H. Weber, Anal. Chim. Acta., 1986, 184, 197. Although further improvements can be expected in terms of 23 S. Pe�rez Me�ndez, M. Montes Bayo� n, E. Blanco Gonza�lez and sensitivity and chromatographic resolution for amino acids, A. Sanz-Medel, J. Anal. At. Spectrom., in the press. these studies open new avenues in trace element speciation research on biological material by combining the excellent Paper 9/01760G capabilities of the GC with the extremely sensitive and selective detection of the ICP-MS. 1322 J. Anal. At. Spectrom., 1999, 14, 1

ISSN:0267-9477    

DOI:10.1039/a901760g

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Species-selective determination of cobalamin analogues by reversedphase HPLC with ICP-MS detection† Alexei Makarov and Joanna Szpunar* CNRS, EP132, Helioparc Pau Pyre�ne�es, 2 av. Pr. Angot, F-64053 Pau Ce� de�x 9, France. E-mail: joanna.szpunar@univ-pau.fr Received 22nd January 1999, Accepted 12th April 1999 The coupling of reversed-phase HPLC with ICP-MS was optimized for the species-selective determination of cobalt complexes with macrocyclic ligands (vitamin B12 and its analogues).The use of high eYciency microcolumns (33–50 mm) with porous and non-porous small diameter (1.5–2 mm) stationary phases was evaluated and compared with conventional (5 mm packing) HPLC. High concentrations of organic solvent (up to 50% of acetonitrile or methanol ) could be introduced into the ICP using aMeinhard-type nebulizer and a cooled spray chamber. The choice of the column was dictated by the compromise among the analysis time, sensitivity and separation eYciency required.The best results in terms of resolution and sensitivity were obtained using a 15 cm×4.6 mm id C8 column. Absolute detection limits of 80–160 pg (4–8 ng ml-1 in the injected solution) were achieved. The method developed was applied to the determination of the active component in pharmaceutical cobalamin preparations and for monitoring its degradation. the resolution of mixtures of compounds having very similar Introduction structures, such as macrocycles with slightly diVerent func- Cobalt is an essential nutrient and a component of vitamin tional groups.The coupling of RP-HPLC with ICP-MS, B12 (cyanocobalamin).1,2 The status of the latter is an import- however, has attracted less attention than size-exclusion chroant parameter that needs to be evaluated in a number of matography,19 apparently because of the need for long runs clinical samples, such as blood plasma, breast milk, foodstuVs using mobile phases rich in an organic solvent diYcult to and pharmaceutical preparations.Methods for performing handle in ICP-MS. Attempts to alleviate this shortcoming routine analyses for vitamin B12 include the determination of included the use of a cooled spray chamber20–22 or an aerosol the total cobalt by AAS3 or chemiluminescence,4 and a radio- desolvation unit.23–26 immunoassay.5 The classical microbiological assay by Recently, the use of microbore RP-HPLC with post-column Ochromonas malhamensis provides the most reliable, sensitive eZuent dilution prior to direct-injection nebulization (DIN) and specific protozoan method.6 These techniques are not into an ICP-MS was proposed as a method for species-selective cyanocobalamin specific; not only are other cobalamins such analysis of cobalamin analogues.27 The method allowed a gain as adenosylcobalamin (coenzyme B12, 5¾-desoxyadenosyl ), in terms of sensitivity of two orders of magnitude over the methylcobalamin (CH3), hydroxocobalamin (OH) and aquo- previous methods.However, the detection limits were still cobalamin (H2O) co-determined with cyanocobalamin, but above values attainable with radioisotope dilution, the duralso their potentially harmful analogues devoid of enzymatic ation of the chromatographic run exceeded 30 min and activity, such as cobamides and cobinamides. expensive equipment was required. Selectivity in terms of cobalamin species is usually obtained The objective of this work was to develop a method for the by the separation of cobalamin, cobamide and cobinamide species-selective determination of cobalamin analogues by analogues by reversed-phase HPLC.This is followed by the ICP-MS using reversed-phase chromatography and the stano V-line determination of cobalt in the relevant fraction of the dard sample introduction system. Particular attention was chromatographic eluate.7 Despite the attractive figures of given to reducing the duration of the chromatographic run merit, this kind of methodology is too tedious to allow and decreasing the concentration of organic solvent by measurements on a routine basis.Other approaches to speci- employing microparticle microcolumn chromatography, and ation determination of cobalamin analogues included electros- to decreasing the detection limits. pray tandem mass spectrometry8 and HPLC with UV–VIS detection,9–12 atomic absorption (AAS)13,14 or ICP emission spectrometry15 or using a nitroxide radical trap.16 The detec- Experimental tion limits of the on-line methods are at the level of Apparatus several mg ml-1 (as compound), which is insuYcient for many practical applications.The ICP mass spectrometer used in this work was an ELAN Inductively coupled plasma mass spectrometry (ICP-MS) is 6000 (Perkin-Elmer SCIEX, Concord, Ontario, Canada). The an established sensitive and element-selective detection method sample introduction system consisted of a Meinhard nebulizer in liquid chromatography.17,18 Reversed-phase (RP) chroma- combined with a cooled spray chamber (Perkin-Elmer, tography oVers an attractive separation mechanism allowing Norwalk, CT, USA).A Minipuls 3 peristaltic pump (Gilson, Villiers-le-Bel, France) was used to provide make-up flow and also for draining the spray chamber. All the connections were †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999. made of PEEK tubing (0.17 mm id).J. Anal. At. Spectrom., 1999, 14, 1323–1327 1323For HPLC-ICP-MS experiments using the Meinhard identified by comparison of the retention time of a peak in the sample chromatogram with that of a standard compound. nebulizer with the cooled-spray chamber, an HP Series 1100 pump (Hewlett-Packard, Avondale, PA, USA) using a ICP-MS conditions. The rf power used was 0.9 kW. The Rheodyne (Cotati, CA, USA) Model 7010 injection valve with nebulizer gas flow rate was set at 0.6 l min-1.The X–Y position a 20ml injection loop was used. HPLC experiments with of the ICP torch was adjusted first to maximize the signal for microparticle microcolumns were performed using an ABI cobalt. ICP-MS measurement conditions were optimized daily 140C microbore syringe pump and an ABI Model 112A using a standard built-in software procedure. A tuning solution injection module (Applied Biosystems, Foster City, CA, USA) containing 10 mg ml-1 Co2+ in 60% methanol was used.The with a 1 ml sample loop. An ABI Model 785A absorbance optimum nebulizer gas flow rate was 0.6 l min-1 (1.05 l min-1 detector equipped with a microbore cell was used for HPLC for cross-flow nebulizer). No changes in the signal intensity experiments with UV detection. HPLC-ICP-MS results were were observed up to at least 60% methanol. The spray chamber processed using Turbochrom 4 software (Perkin-Elmer). temperature was 4 °C. Three types of HPLC columns were used: (1) C8, 150 mm×4.6 mm id, 5 mm particle size (Vydac, Hesperia, CA, USA), (2) NPS ODS-II, 33 mm×4.6 mm id, 1.5 mm film Results and discussion particle size (Micra, Northbrook, IL, USA); and (3) TSKgel The HPLC column eYciency is known to increase with Super ODS, 50 mm×4.6 mm id, 2 mm particle size increasing homogeneity of the packing and decreasing packing (TosoHaas, Stuttgart, Germany).particle size.28 Consequently, the smaller and the more homogeneous the packing, the shorter the column can be at a Reagents, standards and samples constant resolution, and the shorter the duration of the Analytical-reagent grade ammonium acetate and acetic acid chromatographic run.Reversed-phase chromatography using were purchased from Aldrich (St. Quentin Fallavier, France). microparticle stationary phases thus represents an attractive Methanol and acetonitrile (Sigma–Aldrich) were of LC grade. potential as a sample introduction technique for ICP-MS, Milli-Q water (18MV) (Millipore, Bedford, MA) was used.allowing a decrease in the cost of speciation analysis. Cyanocobalamin (vitamin B12, CN-Cbl), hydroxocobala- Replacing the conventional 5 mm packing by a 3 mm packing min (vitamin B12a, OH-Cbl), 5¾-deoxyadenosylcobalamin has been demonstrated to allow the rapid speciation determi- (coenzyme B12, Ado-Cbl ), and methylcobalamilco- nation of arsenic by RP-HPLC-ICP-MS.29 The possibility of enzyme B12, Me-Cbl ) were obtained from Sigma (St.Louis, the fast separation of metallothionein–cadmium complexes30 MO, USA). Stock standard solutions were prepared from the and cobalamin analogues31 on columns with a 2 mm porous commercial products by dissolving 25 mg of each of them in stationary phase and a 1.5 mm non-porous packing was evoked 25 ml of distilled water under dim light. They were stored in but no applications of microparticle (1–2 mm) chromatography dark bottles at 4 °C.Working standard solutions were obtained with element-selective detection have been reported. by dilution immediately before the measurements and kept in In this work, we attempted to optimize the coupling of the dark.RP-HPLC with ICP-MS detection with particular attention to The buVer solutions were prepared by dissolving 25 mM post-column peak broadening in the spray chamber and to its ammonium acetate in water (or in 50% organic solvent) and eVect on the chromatographic resolution. Spectrophotometric adjusting the pH to 4.0 with acetic acid.detection was used as a reference because of the minimum post-column peak broadening when a microbore cell is used. Instrumental conditions The optimization was aimed at reducing the analysis time and reducing the organic solvent content in the mobile phase while HPLC conditions. To avoid pressure limitations, acetonitrile retaining the baseline separation of the cobalamin analogues. was used as organic solvent in experiments with the 1.5 mm column and methanol in the other cases.Gradient elution was Choice of the separation conditions used in all separations specific for each column. BuVer A was 25 mM ammonium acetate in water (pH 4.0), buVer B was Analytical column (4.6 mm id ) with conventional (5 mm) packing. The major naturally occurring cobalamins have been 25 mM ammonium acetate in 50% methanol–water (pH 4.0) and buVer C was 25 mM ammonia acetate in 50% aceto- reported to be separated isocratically within 40 min using 30% acetonitrile in water.11 Since acetonitrile at this concentration nitrile–water (pH 4.0).The solutions were de-gassed in an ultrasonic bath. The HPLC conditions are summarized in is poorly tolerated by the plasma, attempts were made to optimize the separation with methanol. Fig. 1(a) shows that it Table 1. The length of the peak tubing connecting the column with the nebulizer did not exceed 20 cm. A calibration curve is possible to separate the cobalamin analogues isocratically using a 25% methanol mobile phase but the time of such was established by the injection of a freshly prepared mixture of hydroxocobalamin, cyanocobalamin, adenosylcobalamin analysis is relatively long (30 min).Shorter runs with the baseline resolution preserved turned out to be impossible in and methylcobalamin in amounts of 0.05, 0.2, 1.0, 2.0 and 5.0 mg ml-1 (each). Compounds in the analysed samples were the isocratic mode. Gradient separation which started at 10% Table 1 Optimum chromatographic conditions Parameter Column type, 4. 6 mm i.d. MICRA NPS II Toso ODS Vydac C8 (33 mm, 1.5 mm) (50 mm, 2 mm) (150 mm, 5 mm) Injection volume/ml 1 5 20 Flow rate/ml min-1 1.0 1.0 1.0 Initial buVer composition (I ) 93% A–7% C 90% A–10% B 80% A–20% B Final buVer composition (F) 83% A–17% C 20% A–80% B 100% B Programme 100% I (1 min) to 100% I to 100% F 100% I to 100% F within 5 min 100% F within 1.5 min within 5 min then 100% F for 1 min Duration of run/min 2.5 5 6 1324 J.Anal. At. Spectrom., 1999, 14, 1323–1327Fig. 2 Gradient HPLC separation of four naturally occurring cobalamins (1 ml, each standard at 2 mg ml-1) at pH 4.0. Gradient: 0–5 min, 20–80% B. TSKgel Super ODS column; bold line, ICP-MS detection, light line, spectrophotometric detection. Peaks 1=hydroxocobalamin; 2=cyanocobalamin; 3=adenosylcobalamin; 4=methylcobalamin. Fig. 3 shows that, indeed, a baseline separation of the cobalamin species is possible within 2 min not only with spectrophotometric detection but also with ICP-MS detection.However, the peak broadening by the spray chamber is significant and the resolution with ICP-MS detection is poorer. It should be noted that methanol-containing mobile phases cannot be used because of excessive pressure build-up on the 1.5 mm columns. An acetonitrile buVer was therefore optimized. Separations were obtained for gradients between 3.5 and 8.5% of acetonitrile; these concentrations are lower than for Fig. 1 HPLC separation of four naturally occurring cobalamins. (a) Isocratic elution (20 ml, each standard at 2 mg ml-1) at 25% methanol methanol. They are also lower in comparison with the concen- (50% B), pH 4.0. Vydac C8 column (4.6 mm id ); ICP-MS detection. trations necessary to obtain baseline separation on an (b) Gradient elution (20 ml, each standard at 2 mg ml-1) at pH 4.0. analytical (5 mm) column. Gradient: 0–5 min, 40–100% B.Vydac C8 column (4.6 mm id); ICP-MS detection. Peaks: 1=hydroxocobalamin; 2=cyanocobalamin; Figures of merit 3=adenosylcobalamin; 4=methylcobalamin. Table 2 summarizes the detection limits for the columns investigated with spectrophotometric and ICP-MS detection. methanol (20% B) was optimized to allow the separation of Values reported elsewhere27 for the coupling of microbore the analyte compounds within 7 min [Fig. 1(b)]. Higher HPLC with ICP-MS using a direct injection nebulizer (DIN) concentrations of methanol in the starting buVer resulted are also given for the purpose of comparison.in a lack of resolution between cyanocobalamin and The lowest detection limits are oVered by analytical adenosylcobalamin. RP-HPLC. Despite the fact that ICP-MS detects only Co, which forms 5% of the total molecule, the detection limits Microcolumns (33–50 mm) with reduced particle size obtained by HPLC-ICP-MS are about 500 times lower than (1.5–2 mm) packing. Replacing the conventional stationary those with spectrophotometric detection. In comparison with phase of porous 5 mm C18 particles by a packing of the previously developed microbore HPLC-DIN-ICP-MS, a 2.29±0.27 mm particles allowed a reduction in the column gain in concentration detection limits of 2–8-fold is observed.length required to carry out the separation to 50 mm. The gradient of the mobile phase for the fastest separation was optimized, similarly as in the case of the analytical column, at 10–40% methanol (20–80% B).Fig. 2 compares chromatograms obtained with ICP-MS (bold line) and spectrophotometric detection (dashed line) under the above conditions. The interface between HPLC and ICP-MS is responsible for considerable peak broadening and a distorted peak shape. Consequently, hardly any shortening of the duration of the chromatographic run could be achieved in comparison with the conventional analytical 5 mm column with ICP-MS detection.A decrease in the duration of the chromatographic run can also be achieved by employing a non-porous packing of a smaller diameter. The advantage of a non-porous packing is the absence of pores, which eliminates all intra-particle diVusion. The sensitivity is significantly improved, but at the Fig. 3 Gradient HPLC separation of four naturally occurring cobala- expense of the sample load. A column length of 33 mm filled mins (1 ml, each standard at 2 mg ml-1) at pH 4.0.Gradient: 0–1 min, with 1.50±0.17 mm particles should oVer, according to the LC 7–17% C. Micra NPS II column; bold line, ICP-MS detection, light theory,28 a similar number of theoretical plates to a 50 mm line, spectrophotometric detection. Peaks: 1=hydroxocobalamin; 2= cyanocobalamin; 3=adenosylcobalamin; 4=methylcobalamin. column with a 2 mm packing. J. Anal. At. Spectrom., 1999, 14, 1323–1327 1325Table 2 Comparison of detection limits (defined as three times the standard deviation of the baseline noise) for cobalamin analogues by RP-HPLC-ICP-MS using diVerent columns and diVerent interfaces ICP-MS with Meinhard nebulizer and cooled spray chamberc Spectrophotometric DIN-ICP-MS:b,c detectiona,b (l=550 nm): Micra NPS TosoHaas ODS Vydac C8 microbore (1 mm id) microbore (1 mm id) (3.3 cm, 1.5 mm) (50 mm, 2 mm) (150 mm, 5 mm) (150 mm, 5 mm) (150 mm, 5 mm) Compound ADLd ng DL/mg ml-1 ADL/ng DL/mg ml-1 ADL/ng DL/mg ml-1 ADL/ng DL/mg ml-1 ADL/ng DL/mg ml-1 OH-Cbl 0.01 0.01 0.04 0.04 0.14 0.007 0.10 0.02 2.9 2.9 CN-Cbl 0.008 0.008 0.03 0.03 0.08 0.004 0.25 0.05 1.2 1.2 Ado-Cbl 0.007 0.007 0.06 0.06 0.10 0.005 0.05 0.01 2.9 2.9 Me-Cbl 0.014 0.014 0.05 0.05 0.16 0.008 0.15 0.03 2.9 2.9 aInvestigated range: 0–100 mg ml-1.bFrom ref. 27. cInvestigated range: 0–5 mg ml-1. dADL=absolute detection limit. The absolute detection limit (0.1 ng) of cobalamin remains the same and matches the detection limits normally determined oV-line by radioisotope dilution.HPLC-ICP-MS is thus a potentially attractive technique for the evaluation of the serum cobalamin status. Microcolumns show a generally poorer analytical performance. With spectrophotometric detection (no postcolumn dead volume), the loss of sensitivity due to the very limited amount of sample injected (1 ml for the non-porous 1.5 mm column and 5 ml for the porous 2 mm column) is compensated in the peak height quantification mode by the decrease in peak width.In the case of ICP-MS detection, as can be seen in Fig. 2, the same absolute amount of analyte injected leads to a two fold smaller signal than with a conventional analytical column. In terms of the detectable analyte concentration this means an eight fold decrease in sensitivity in comparison with the analytical column. For the non-porous 1.5 mm, 33 mm long, columns, the absolute detection limits are similar to those in conventional analytical chromatography.In terms of the detectable analyte concentration, however, a 20-fold loss is observed because the peak broadening in the spray chamber eliminates the gain in sensitivity owing to the decreased peak width observed when microparticle packings were used. Calibration curves are linear over three decades of concentration (0.02–20 mg ml-1) and the analytical precision (standard deviation of five consecutive injections) is 1–2% with UV and 3–6% with ICP-MS detection in all cases.Fig. 4 Chromatograms of a pharmaceutical preparation (diluted ) at Analysis of pharmaceutical preparations pH 4.0. Vydac C8 column (4.6 mm id ); ICP-MS detection. (a) Peaks: 1=Co2+; 2=unknown; 3=hydroxycobalamin; 4=cyanocobalamin; Two commercial pharmaceutical preparations with a known 5=adenosylcobalamin; 6=methylcobalamin. (b) Peaks: 1=Co2+; content of hydroxycobalamin were analyzed by analytical 2=unknown; 3=hydroxycobalamin; 4=cyanocobalamin; 5=aden- (4.6 mm id column) HPLC-ICP-MS.The chromatograms are osylcobalamin. shown in Fig. 4(a) and (b). The chromatogram of the first preparation shows a major signal corresponding to hydroxoco- Conclusions balamin and three others identified as cyano-, adenosyl- and Analytical (4.6 mm id column) RP-HPLC interfaced with methylcobalamins. The concentration determined by using the ICP-MS via a Meinhard nebulizer and a cooled spray chamber calibration curve was 57±5 mg ml-1, compared with the value oVers a fast and sensitive technique for the determination of 60 mg ml-1 given by the manufacturer of the preparation.cobalamin analogues. The method developed is ca. 500-fold The chromatogram of the second solution contains a peak more sensitive than HPLC with spectrophotometric detection of Co2+ eluting with the dead volume and three other signals and finds application when the latter fails because of lack of with retention times of two of them corresponding to hydrosensitivity. The advantages in terms of speed of using smaller xocobalamin and adenosylcobalamin.Peak 2 corresponds to particle size microcolumns are negligible when ICP-MS a compound that could not be identified. The concentration detection is used because of the loss of eYciency due to the determined from the calibration curve was 1.1±0.1 mg ml-1 post-column dead volume (spray chamber). for hydroxocobalamin and 0.5±0.1 mg ml-1 for adenosylcobalamin. The concentration of hydroxocobalamin found was in References good agreement with the value of 1 mg ml-1 given for for this compound by the manufacturer; no independent measurement 1 B-12, Vol. 2, Biochemistry and Medicine, ed. D. Dolphin, Wiley, New York, 1982. was available for adenosylcobalamin. 1326 J. Anal. At. Spectrom., 1999, 14, 1323–13272 P. Gimsing and E. Nexo, in The Cobalamins, Churchill 18 P. C. Uden, J. Chromatogr. A, 1995, 703, 393. 19 A. Makarov and J. Szpunar, Analusis, 1998, 26, M44. Livingstone, Edinburgh, 1983, p. 1. 20 N. P. Vela and J. A. Caruso, J. Anal. At. Spectrom., 1993, 8, 787. 3 K. Akatsuka and I. Atsuya, Fresenius¡� Z. Anal. Chem., 1989, 335, 21 H. Ding, L. K. Olson and J. A. Caruso, Spectrochim. Acta, Part B, 200. 1996, 51, 1801. 4 W. Qin, Z. J. Zhang and H. J. Liu, Anal. Chim. Acta, 1997, 22 Z. Zhao, W. B. Jones, K. Tepperman, J. G. Dorsey and 357, 127. R. C. Elder, J. Pharm. Biomed. Anal., 1992, 10, 279. 5 F. Watanabe, Y. Nakano, E. Stupperich, K. Ushikoshi, 23 H.J. Yang, S. J. Jiang, Y. J. Yang and C. J. Hwang, Anal. Chim. S. Ushikoshi, I. Ushikoshi and S. Kitaoka, Anal. Chem., 1993, Acta, 1995, 312, 141. 65, 657. 24 N. Jakubowski, C. Thomas, D. Stu¡§wer, I. DettlaV and J. Schram, 6 US Pharmacopeia, XXII Revision, US Pharmacopeial Convention, J. Anal. At. Spectrom., 1996, 11, 1023. Rockville, MD, 1989. 25 C. M. Andrle, N. Jakubowski and J. A. C. Broekaert, 7 D. Lambert, C. Adjalla, F. Felden, S. Benhayoun, J. P. Nicolas Spectrochim. Acta, Part B, 1997, 52, 189. and J. L. Gueant, J. Chromatogr., 1992, 608, 311. 26 B. Fairman, T. Catterick, B. Wheals and E. Polinina, 8 H. Chassaigne and R. �©obin¢¥ ski, Analyst, 1998, 123, 131. J. Chromatogr. A, 1997, 758, 85. 9 C. C. Jansen and J. P. De Kleijn, J. Chromatogr. Sci., 1990, 28, 42. 27 H. Chassaigne and R. �©obin¢¥ ski, Anal. Chim. Acta, 1998, 359, 227. 10 H. Iwase, J. Chromatogr., 1992, 590, 359. 28 R. P. W. Scott, High Performance Liquid Chromatography: 11 D. W. Jacobsen, R. Green, E. V. Quadros and Y. D. Montejano, Principles and Methods in Biotechnology, ed. E. D. Katz, Wiley, Anal. Biochem., 1982, 120, 394. Chichester, 1996, pp. 25.94. 12 J. Dalbacke and I. Dahlquist, J. Chromatogr., 1991, 54, 383. 29 S. A. Pergantis, E. M. Heithmar and T. A. Hinners, Analyst, 1997, 13 P. Vin. as, N. Campillo, I. Lopez-Garcia and M. Hernandez- 122, 1063. Cordoba, Anal. Chim. Acta, 1996, 318, 319. 30 H. Chassaigne and R. �©obin¢¥ ski, Fresenius J. Anal. Chem., 1999, 14 P. Vin. as, N. Campillo, I. Lopez-Garcia and M. Hernandez- 363, 522. Cordoba, Chromatographia, 1996, 42, 566. 31 R. �©obin¢¥ ski, H. Chassaigne, I. Rodriguez, A. Wasik and J. Szpunar, J. Anal. At. Spectrom., 1998, 13, 859. 15 M. Morita, T. Uehiro and K. Fuwa, Anal. Chem., 1980, 52, 349. 16 R. J. Kelly, Anal. Commun., 1998, 35, 257. 17 L. Moens, Fresenius¡� J. Anal. Chem., 1997, 359, 309. Paper 9/00633H J. Anal. At. Spectrom., 1999, 14, 13

ISSN:0267-9477    

DOI:10.1039/a900633h

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Determination of element species at trace levels using capillary electrophoresis–inductively coupled plasma sector field mass spectrometry† Andreas Prange* and Dirk Schaumlo�Vel GKSS Research Centre, Institute of Physical and Chemical Analysis, Max-Planck-Strasse, D-21502 Geesthacht, Germany Received 4th March 1999, Accepted 27th April 1999 The development and analytical characterization of a coupled capillary electrophoresis–inductively coupled plasma sector field mass spectrometer (CE-ICP-SFMS) system for the simultaneous determination of diVerent species of As, Sb, Se and Te at trace levels are described.The species are separated by CE using a 55 cm×75 mm id fused silica capillary column. A newly developed interface with a special, low dead volume spray chamber allows the optimization of the fluid mechanical properties, thus preventing a suction eVect between the nebulizer and the CE capillary. The ICP-SFMS instrument runs in the low resolution mode and is equipped with a grounded platinum electrode at the ICP torch in order to achieve the highest possible sensitivity.The CE-ICP-SFMS system developed provides stable electrophoretic conditions and allows reproducible separations to be obtained with RSDs below 3% for the migration times and below 8% for the peak areas. Excellent peak shapes down to 4 s and short analysis times of a few minutes are other important features of this combined instrument. Detection limits in the low mg L-1 region for compounds and in the picogram to femtogram range for the isotopes are achieved from standard solutions.The key to the successful realization of a coupled system is Introduction the design of the interface. This is especially true for In environmental chemistry, biomedical science and toxic- GC-ICP-MS and for CE-ICP-MS, whereas HPLC-ICP-MS ology, the identification and quantification of element coupling is relatively easy to achieve, because the flow rates in species such as metals in diVerent oxidation states, organomet- HPLC fit well with the nebulization flow rate in ICP-MS.allic or metal–protein compounds are becoming increasingly The prerequisites for a CE-ICP-MS interface are to provide important. It has been noted that the distribution, bioavail- an electrical connection, to adapt the flow rate in CE to the ability, accumulation and toxicological properties of heavy flow rate of the nebulizer and to prevent a suction eVect metals are strongly dependent on the chemical binding forms between the nebulizer and the CE capillary.Several attempts in which they occur in natural compartments.1 to develop interfaces for CE-ICP-MS have been described over Chromatographic instruments, e.g. equipped with a UV the last few years. These constructions were mostly based on detector, which could be used for the determination of such commercial nebulizers such as pneumatic,19–24 direct element species in compounds, are often not sensitive enough.injection25 or ultrasonic nebulizers.26,27 The analytical strategy adopted here for the determination of We describe here the development and analytical element species at low concentration levels is therefore to characterization of a new, on-line coupled CE-ICP-MS system. combine a chromatographic method, especially suited and A recently developed novel and robust interface28 is used selected according to the chemical properties of the species to which fulfils the requirements mentioned above and which, via be analyzed, with a highly sensitive element specific method. optimization of the interface design, overcomes the great Inductively coupled plasma mass spectrometry (ICP-MS) is drawback of suction flow inside the CE capillary.Using one of the most sensitive element specific techniques, providing standard solutions, the analytical performance of the new also multi-element capabilities, and has proved to be well coupled system has been characterized with respect to repeatsuited as a metal specific detector in on-line coupled systems2–5 ability, detection limits and linear range for cationic and involving chromatographic methods such as gas chromatogra- anionic compounds of arsenic, antimony, selenium and phy (GC),6–8 high performance liquid chromatography tellurium which can be separated in a single run.The use of a (HPLC)9–11 and capillary electrophoresis (CE).12 guard electrode at the ICP torch in order to enhance sensitivity More recently, CE has also been used for metal speci- is also discussed.ation.13–15 The field of application is similiar to that of HPLC, but CE has several advantages such as high resolving power, Experimental small sample volume requirement, minimal buVer consumption and high sample throughput. In addition, CE is especially Instrumental design suitable for the separation of biological macromolecules, e.g. A schematic diagram of the instrumental set-up containing proteins.16–18 the CE, the interface and the ICP-MS components is shown in Fig. 1. A Hewlett-Packard (Avondale, PA, USA) 3D CE system containing fused silica capillaries with length 55 cm, id †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999. of 75 mm and od of 150 mm is used at a voltage of 30 kV. The J. Anal. At. Spectrom., 1999, 14, 1329–1332 1329properties of the interface prevents a suction eVect between the nebulizer and the CE capillary.The main diVerence between the newly constructed nebulizer and the MCN 100 is that the nebulizer capillary has been changed to a narrower id. In this way the flow resistance of the nebulizer capillary has been adapted in order to minimize the negative pressure which occurs at the orifice of the nebulizer, along the nebulizer capillary. Hence a minimized negative pressure at the end of the CE capillary is achieved and any laminar flow inside the CE capillary is avoided. This was verified by experiments such as a comparison between CE-ICP-MS and CE-UV.Further details are described elswhere.29 Data acquisition and evaluation. The transient signals are acquired by the ELEMENT software (Finnigan MAT) and exported as ASCII files. When imported into ORIGIN (Microcal Software, Northampton, MA, USA), the signals of Fig. 1 Schematic diagram of the CE-ICP-SFMS set-up. an electrophorogram can be integrated.These values are exported to EXCEL (Microsoft, Redmond, WA, USA) for further calculations. interface is based on a modified microconcentric nebulizer (MCN 100, CETAC, Omaha, NE, USA) with a special, low dead volume spray chamber. An inductively coupled plasma Chemicals and materials sector field mass spectrometric (ICP-SFMS) system Analytical-reagent grade chemicals were used unless specified (ELEMENT, Finnigan MAT, Bremen, Germany) is used as otherwise.an element specific detector. Arsenite standard solution (1000 mg L-1 As) was prepared by dissolution of diarsenic trioxide (As2O3,Merck, Darmstadt, Instrumental parameters Germany) in 30% NaOH (Merck, Suprapur) and acidifying The main experimental parameters are summarized in Table 1. with 6 mol L-1 HCl (Merck, Suprapur). Standard solutions of monomethylarsonic acid (MMA), dimethylarsinic acid CE. The sample is injected hydrodynamically at 300 mbar s (DMA), phenylarsonic acid, arsenocholine and arsenobetaine (50 mbar for 6 s), which corresponds to a sample volume of (all solutions of 1000 mg L-1 As) were prepared by dissolving 41 nL.The capillary is rinsed with buVer for 1 min at 50 mbar MMA(disodium salt, hexahydrate), arsenocholine and arsenobefore each run and additionally with 0.1 mol L-1 NaOH and betaine (all purchased from Argus Chemicals, Vernio, Italy), water for 5 min before changing from high to low concen- DMA (sodium salt, trihydrate, Merck) and phenylarsonic acid tration samples.To prevent buVer depletion, the buVer is (Merck) in ultrapure (18MV) water (Millipore, Bedford, changed completely at least every fifth run. MA, USA). Arsenate, cesium, indium and rubidium standard solutions ICP-MS. The ICP-SFMS system is run in the low resolution (1000 mg L-1 As, Cs, In or Rb in 0.5 mol L-1 HNO3) were mode (R=300) and fitted with a grounded platinumlectrode purchased from Merck. Two antimony(V) stock standard at the ICP torch to achieve the highest sensitivity possible.solutions (100 and 1000 mg L-1 Sb) were prepared by dissolv- The fast scanning magnet of the mass spectrometer, having a ing potassium hexahydroxyantimonate (Riedel-de-Hae�n, scan duration of about 500 ms, allows very sharp and narrow Hannover, Germany) and potassium antimonate (98% Alfa, CE signals to be acquired. Karlsruhe, Germany) in water. Selenite and selenate (1000 mg L-1 Se) standard solutions were prepared by dissolv- CE-ICP-MS interface.The nebulizer works in the self- ing sodium selenite pentahydrate (Merck) and sodium selenate aspiration mode at an optimized flow rate of around decahydrate (98%, Aldrich, Milwaukee, WI, USA) in water. 6 mL min-1. A make-up liquid (1% m/v HNO3; 1 mg L-1 In) Tellurite and tellurate (1000 mg L-1 Te) standard solutions provides the electrical connection and adapts the flow rate of were prepared by dissolving potassium tellurite (>90%, Sigma, the electro-osmotic flow (EOF) inside the CE capillary to the St.Louis, MO, USA) and telluric acid (99%, Fluka, Buchs, flow rate of the nebulizer. Optimization of the fluid mechanical Switzerland) in water. All standard solutions were stored at 10 °C in darkness. The stability of the standards was checked with regard to total Table 1 Experimental conditions of the CE-ICP-SFMS set-up elemental content by ICP-MS and regarding species transformation by HPLC-ICP-MS; this system has been described pre- Capillary electrophoresis— BuVer solution 20 mmol L-1 CAPS, pH 10, viously in detail.9 Over a period of several months there was 40 mmol L-1 b-cyclodextrin as modifier, no decrease in the total elemental contents and no formation 4 mg L-1 Cs as marker of diVerent species.Standard solutions of lower concentrations Voltage 30 kV were prepared by appropriate dilution with water. Injection Hydrodynamic, 300 mbar s Nitric acid (Merck, Suprapur) was cleaned by sub-boiling in Make-up liquid 1.5% m/v nitric acid, a quartz apparatus and diluted with water. 3-(Cyclo- 1 mg L-1 In as marker hexylamino)propane-1-sulfonic acid (CAPS) buVer solution ICP-SFMS— (pH 10, 25 mmol L-1) was purchased from Microsolv CE Cool gas 14 L min-1 (Scientific Resources, Eatontown, NJ, USA) and b-cyclodextrin Auxiliary gas 0.95 L min-1 Nebulizer gas 0.92–0.96 L min-1 (ultra-pure) from Brand-Nu (Meriden, CT, USA). Power 1075W Fused silica capillaries of 75 mm id and 150 mm od were Internal standard Rb 20 mg L-1 purchased from Thermo Separation Products (TSP, Egelsbach, Guard electrode Yes Germany) and pre-conditioned by rinsing with 1 mol L-1 Isotopes 75As, 82Se, 85Rb, 115In, 121Sb, 130Te, 133Cs NaOH (Hewlett-Packard) for 5 min before use. 1330 J. Anal. At. Spectrom., 1999, 14, 1329–1332etaine, 3=arsenite, 4=dimethylarsinic acid, 5=phenylarsonic acid, 6=monomethylarsonic acid, 7=arsenate, 8=selenite, 9= selenate, 10=antimonate, 11=tellurite and 12=tellurate.The element concentrations of the diVerent species are 100 mg L-1 for arsenic, antimony and tellurium and 1000 mg L-1 for selenium. The separation was carried out in 20 mmol L-1 CAPS buVer solution; 40 mmol L-1 b-cyclodextrin were added as a modifier to achieve a better separation between arsenite and dimethylarsinic acid. As make-up liquid, dilute nitric acid was used to prevent too much salt from entering the plasma. The diVerence in pH is not critical because, contrary to capillary isoelectric focusing, the low pH is at the cathode and the high pH at the anode.Hence the EOF migrates towards the cathode and interface. Intrusion of acid into the capillary, which would cause a pH gradient, cannot occur during an electrophoretic run. Therefore, the diVerence in pH of the buVer and the make-up liquid should not have any eVect on separation, EOF and focusing. In summary, it can be demonstrated that this CE-ICP-MS Fig. 2 Simultaneous separation of 12 species of four elements. coupling is able to achieve a clear separation of the diVerent Concentration of the elements: As, Sb, Te 100 mg L-1 each, Se species with sharp peaks in a short analysis time.Owing to the 1000 mg L-1. properties of the fast scanning mass spectrometer, simultaneous determination of species of diVerent elements is possible. The analytical performance of the CE-ICP-MS system was characterized by checking the stability of the CE-ICP-MS setup and by quantifying the linearity, repeatability and detection limits for the separation of the standard solution.Stability of CE-ICP-MS coupling The stability of the CE-ICP-MS set-up was checked by monitoring the electrical current, the EOF and the nebulization during an electrophoretic separation. Fig. 3 shows that at a default voltage of 30 kV a stable electrical current can be observed, which indicates that the interface provides a stable and continuous electrical contact.This is necessary for reproducible electrophoretic separations. With the ICP-MS system it is possible to observe the EOF directly by adding 4 mg CsL-1 to the CE Fig. 3 Monitoring of current, EOF (133Cs) and make-up flow (115In) during electrophoresis at 30 kV with 20 mmol L-1 CAPS buVer at buVer and monitoring the 133Cs signal. The EOF is also stable, pH 10. which indicates stable analyte transport from the CE capillary into the plasma. The nebulization monitored by 115In is also stable and independent of the electrophoresis.Results and discussion In order to evaluate the analytical performance of the CE-ICP-MS coupling, separations of a test solution containing Linearity 12 species of the elements arsenic, antimony, tellurium and The linearity was checked over a range of three orders of selenium were carried out, while the isotopes 75As, 121Sb, 130Te magnitude by injecting samples which contain element concen- and 82Se were monitored by the mass spectrometer.An example trations between 2 and 2500 mg L-1 As, Sb or Te and 20 and of an electropherogram is presented in Fig. 2. The separated species in ascending order are 1=arsenocholine, 2=arsenob- 25 000 mg L-1 Se. Table 2 presents the linearity expressed by r Table 2 Analytical data for the CE-ICP-SFMS coupling obtained with standard solutions. LOD calculated from the calibration graphs Repeatability (n=8) (%)a Rel. LOD/ Abs. LOD/ Linearity Linear range/ Species mg L-1 metal fg metal (r) mg L-1 metal Peak area Migration time Arsenocholine 0.4 17 0.99929 2–2500 2.9 1.2 Arsenobetaine 6.0 250 0.99943 10–2500 7.9 1.5 Arsenite 1.3 54 0.99767 10–2500 7.0 2.2 Dimethylarsinic acid 1.7 79 0.99518 10–1000 3.6 2.4 Phenylarsonic acid 0.5 19 0.99791 2–2500 3.9 2.9 Monomethylarsonic acid 0.6 24 0.99521 2–2500 3.4 2.5 Arsenate 0.3 14 0.99924 2–2500 5.0 2.7 Selenite 4.2 174 0.99943 20–10000 7.4 2.4 Selenate 9.5 395 0.99954 20–25000 4.3 2.6 Antimonate 0.5 19 0.99793 2–100 3.9 2.5 Tellurite 1.0 39 0.99945 2–2500 5.7 2.8 Tellurate 0.8 34 0.99729 2–1000 6.0 1.7 aConcentration of the elements: As, Sb, Te 100 mg L-1 each, Se 1000 mg L-1.J. Anal. At. Spectrom., 1999, 14, 1329–1332 1331Conclusions It can be stated that our initial eVort to develop a system with CE coupled with ICP-SFMS has resulted in (i) a suitable coupling device for quantitative multi-element and multicompound analysis at trace levels, (ii) a high separation eYciency of the capillary electrophoresis due to the properties of a newly developed interface, (iii) excellent peak shapes in short analysis time and (iv) good linearity, good precision of peak area and migration times and low detection limits.The application of the system to environmental samples such as interstitial water and to biomedical samples such as metallothioneins is under investigation and will be published separately. Fig. 4 Enhancement of the sensitivity by using a guard electrode at the ICP torch within the CE-ICP-SFMS device.References 1 G. Schwedt, Chem. Unserer Zeit, 1997, 31, 183. 2 K. Sutton, R. M. C. Sutton and J. A. Caruso, J. Chromatogr. A, 1997, 789, 85. 3 R. Lobinski, Appl. Spectrosc., 1997, 51, 261. and the linear range of each species. With the exception of 4 G. K. Zoor, J. W. McKiernan and J. A. Caruso, Mikrochim. antimony, the coupling system shows good linearity over a Acta, 1998, 128, 145. range of 2–3 orders of magnitude. 5 J. Szpunar-Lobinska, C. Witte, R. Lobinski and F. C. Adams, Fresenius’ J. Anal. Chem., 1995, 351, 351. 6 E. Jantzen and A. Prange, Fresenius’ J. Anal. Chem., 1995, 353, 28. 7 T. De Smaele, P. Verrept, L. Moens and R. Dams, Spectrochim. Repeatability Acta, Part B, 1995, 50, 1409. Table 2 gives the relative standard deviation (RSD) of the 8 A. Prange and E. Jantzen, J. Anal. At. Spectrom., 1995, 10, 105. 9 T. Lindemann, A. Prange, W. Dannecker and B. Neidhart, migration times and the peak areas for eight injections of a Fresenius’ J.Anal. Chem., in the press. test sample containing seven arsenic, two tellurium and one 10 E. H. Larsen, G. Pritzl and S. Hansen, J. Anal. At. Spectrom., antimony species with element concentrations of 100 mg L-1 1993, 8, 557. and two selenium species with 1000 mg L-1 Se. The RSD of 11 G. Zoorob, M. Tomlinson, J. Wang and J. A. Caruso, J. Anal. At. the migration time ranges between 1.2 and 2.9% and of the Spectrom., 1995, 10, 853.peak area between 2.9 and 7.9%. 12 R. M. Barnes, Fresenius’ J. Anal. Chem., 1998, 361, 246. 13 E. Dabek-Zlotorzynska, E. P. C. Lai and A. R. Timerbeav, Anal. Chim. Acta, 1998, 359, 1. 14 D. Schlegel, J. Mattusch and R. Wennrich, Fresenius’ J. Anal. Detection limits Chem., 1996, 354, 535. 15 K. Van den Broeck and C. Vandecasteele, Mikrochim Acta, 1998, In Table 2, the detection limits of the 12 species considered 128, 79. are also presented.The detection limits were calculated by the 16 A. Theobald and L. Dunemann, J. High Resolut. Chromatogr., calibration graph method.30 This involves measuring 10 1996, 19, 608. samples having concentrations in the range from near the 17 J. H. Beattie, M. P. Richards and R. Self, J. Chromatogr., 1993, estimated detection limit of 2 mg L-1 up to a 10 times higher 632, 127. 18 R. Lehmann, H. M. Liebich and W. Voelter, J. Capillary concentration of 20 mg L-1. These measurements were Electrophor., 1996, 3, 89. repeated three times.The detection limits with respect to the 19 J. W. Olesik, J. A. Kinzer and S. V. Olesik, Anal. Chem., 1995, elements are around 1 mg L-1 with the exception of arseno- 65, 1. betaine (6.0 mg L-1 As), selenite (4.2 mg L-1 Se) and selenate 20 Q. Lu, S. M. Bird and R. M. Barnes, Anal. Chem., 1995, 67, 2949. (9.5 mg L-1 Se). 21 B. Michalke and P. Schramel, Fresenius’ J. Anal. Chem., 1997, 357, 594. 22 K. A. Taylor, B. L. Sharp, D.J. Lewis and H. M. Crews, J. Anal. Guard electrode. A grounded platinum electrode At. Spectrom., 1998, 13, 1095. (GuardElectrode, Finnigan MAT) was inserted between the 23 V. Majidi and J. Miller-Ihli, Analyst, 1998, 123, 803. quartz ICP torch and the rf load coil to prevent capacitive 24 K. L. Sutton, C. B’Hymer and J. A. Caruso, J. Anal. At. coupling from the load coil into the ICP. The enhancement of Spectrom., 1998, 13, 885. sensitivity by using this device was tested by measuring the 25 Y. Liu, V. Lopez-Avila, D. R. Wiederin and W. F. Becker, Anal. signal (85Rb) from a 100 mg L-1 Rb test solution with and Chem., 1995, 68, 2020. 26 P. W. Kirlew,M. T.W. Castilliano and J. A. Caruso, Spectrochim. without the guard electrode (Fig. 4). The increase in sensitivity Acta, Part B, 1998, 53, 221. by a factor of two was much lower than the expected factor 27 Q. Lu and R. M. Barnes, Microchem. J., 1996, 54, 129. of up to 10 (according to the specifications published by 28 D. Schaumlo�Vel and A. Prange, German Pat., 198 41 288.6, 1998. Finnigan MAT in 1998). This latter value was observed with 29 D. Schaumlo�Vel and A. Prange, Fresenius’ J. Anal. Chem., in sample introduction systems such as a Meinhard nebulizer and the press. Scott spray chamber, which produce a wet aerosol. On the 30 DIN 32 645, Nachweis-, Erfassungs- und Bestimmungsgrenze, Deutsches Institut fu� r Normung e.V., Beuth Verlag, Berlin, 1994. other hand, the smaller increase in sensitivity found here indicates that the nebulizer of the interface generates a very Paper 9/01749F dry aerosol. 1332 J. Anal. At. Spectrom., 1999, 14, 1329&ndas

ISSN:0267-9477    

DOI:10.1039/a901749f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Selenomethionine chiral speciation in yeast and parenteral solutions by chiral phase capillary gas chromatography-ICP-MS† S. Pe�rez Me�ndez, M. Montes Bayo�n, E. Blanco Gonza�lez and A. Sanz-Medel* Department of Physical and Analytical Chemistry. University of Oviedo, C/ Julia�n Claverý�a 8, 33006-Oviedo, Spain. E-mail: asm@sauron.quimica.uniovi.es Received 29th March 1999, Accepted 2nd June 1999 Chiral resolution and speciation of DL-selenomethionine enantiomers by capillary gas chromatography (GC) as Ntrifluoroacetyl (TFA)-O-isopropyl derivatives using an L-valine-tert-butylamide modified polydimethylsiloxane chiral stationary phase (Chirasil-L-Val ) were investigated.Good resolution was achieved using a temperature program from 100 °C (held for 3 min) to 160 °C at 1.5 °Cmin-1 and He as carrier gas. Very good selectivity and excellent detection limits of 0.25 mg L-1 (0.1 mg L-1 as Se) for each enantiomer were obtained by on-line coupling of the chiral capillary column with selenium-specific detection by inductively coupled plasma mass spectrometry (ICP-MS).Experimental parameter optimization is described in detail. This optimised GC-ICP-MS method was successfully applied to the determination of the optical purity of L-selenomethionine in commercial samples, to the determination of the enantiomeric ratio of selenomethionine in parenteral solutions used for human use and also to chiral Se speciation in selenized yeast. metabolism of Se by microorganisms (e.g., yeast) are topics Introduction of great analytical interest.Selenium is nowdays widely recognized as both a toxic and However, very few papers on this problem have appeared an essential element depending on its concentration.1 The gap so far. The enantiomeric resolution of selenomethionine by between toxic and essential levels of Se in humans is narrow2 reversed phase HPLC with UV detection after derivatization because when ingested at levels only 3–5 times higher than with a chiral reagent has been reported by Hansen and those required for optimum nutrition, detrimental eVects on Poulsen,21 while Vespale and co-workers22,23 used vancomycin health are noticed.3 Diseases related to Se deficiency such as as a chiral selector for the optical resolution of diVerent seleno- Keshan and Kaschin–Beck diseases, hypertension, infertility, containing derivatized amino acids by capillary electrophoresis arthritis, ageing and cataracts4–7 seem to have caused more (CE), again using UV detection.Both methods suVer from problems than selenium toxicity.8,9 Therefore, dietary Se sup- inadequate sensitivity and selectivity for application to complex plementation has become very popular for the prevention of real samples. In fact, no applications of chiral speciation to such diseases.5–7 Prophylactic Se supplementation has also real samples have been reported.21–23 In a previous paper,24 been recommended for clinical cancer chemoprevention.10,11 we described a method for the chiral separation of However, the nutritional bioavailability,7,12–14 toxicity7,13,15 DL-selenomethionine enantiomers derivatized with naphand cancer chemopreventive activity14,16 of Se are not only thalene-2,3-dicarboxaldehyde (NDA) by reversed phase influenced by the total trace element concentration but also HPLC on a chiral b-cyclodextrin column.Detection was by its chemical form. Among the numerous forms of Se able accomplished by molecular fluorescence of the NDA derivato be used for Se supplementation, selenomethinonine has tives and compared with Se-specific detection by on-line been suggested to be less toxic than inorganic Se forms.17 microwave digestion assisted-hydride generation-ICP-MS Selenomethionine also has an increased absorption compared (HG-ICP-MS). Fluorimetric detection proved to be superior with the inorganic salts selenite and selenate.18 In addition, in terms of sensitivity but it was unselective for real complex selenium ingested as free selenomethionine or selenized yeast samples such as selenium enriched yeast in which more than has been shown to be more bioavailable than selenite in the 20 selenium-containing species have been reported to be prelactating rats.19 Hence selenomethinonine is commonly used sent.25,26 HG-ICP-MS, however, provided suYcient selectivity as a source of selenium in several nutritional supplements and to allow the detection of DL-selenomethionine in such samples.parenteral solutions. Unfortunately, precise quantification of the enantiomers of On the other hand, selenomethionine is chiral in nature and DL-selenomethionine in these samples was not possible owing it is known that individual enantiomers of chiral compounds to the poor detection limit achieved. Therefore, there is still a usually play diVerent roles in living organisms. In fact, it has lack of adequate methodologies to deal with the separation been reported that L-selenocystine is much more toxic in rats and determination of DL-selenomethionine enantiomers in real that the D-form.20 Therefore, optical resolution of selenome- complex samples.thionine into its D- and L-enantiomers, the determination of Over the last two decades, chiral stationary phases in enantiomeric ratios of DL-selenomethionine in diVerent sel- capillary gas chromatography (GC) have undergone exciting enium solutions used for human comsumption and chiral developments,27,28 particularly for ‘optical purity’ determispeciation of selenium in selenoamino acids produced by the nations in amino acids.Fused silica capillary columns coated with L-valine-tert-butylamide (Chirasil-L-Val ) have been shown to be very suitable for the optical resolution of N- †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999. trifluoroacetyl (TFA)-O-alkyl derivatives of several amino acid J.Anal. At. Spectrom., 1999, 14, 1333–1337 1333enantiomers.29 Unfortunately, an unselective flame ionization Derivatization procedures detector (FID) is the usual detector used.29 N-Trifluoroacetyl-O-isopropylselenomethionine esters of In the present work, we investigated for the first time the standards were synthesized as follows. Amounts of 0.3 mg of analytical potential of coupling chiral GC, on a Chirasil-Lracemic selenomethionine or L-selenomethionine samples and Val column, with selenium-specific detection by ICP-MS for 250 mL of propan-2-ol–acetyl chloride (8+2) were heated in a the enantiomeric resolution and determination of DL-selenomeclosed vial at 100 °C for 1 h.After the esterification reaction, thionine optical isomers. The Se specificity and high sensitivity the solvents were removed in a stream of nitrogen, 200 mL of of ICP-MS combined with the high chiral resolution power dichloromethane and 100 mL of trifluoroacetic anhydride were aVorded by the capillary column allowed the determination of added and the mixture was heated at 100 °C for 1 h.The the optical purity of L-selenomethionine in commercial solvents were then removed in a stream of nitrogen and the samples, the determination of the enantiomeric ratio of selenoresidue was dissolved in 2 mL of dichloromethane. Aliquots methionine in parenteral solutions for human use and the of 1 mL of such solution were injected into the GC column.chiral speciation of Se in more complex nutritional samples Similarly, aliquots of 200 mL of the solutions obtained as such as ‘selenized yeast’. described above from the selenium-enriched yeast and serum infusion samples were mixed with 300 mL of concentrated HCl and 500 mL of propan-2-ol and the mixture was heated in a Experimental closed vial at 100 °C for 1 h. The solution was evaporated in a stream of nitrogen, 200 mL of dichloromethane and 100 mL Apparatus of trifluoroacetic anhydride were added and the mixture was GC-FID analysis was performed using a Hewlett-Packard heated at 100 °C for 1 h.Solvents were removed again in a (Avondale, PA, USA) Model 5890 gas chromatograph stream of nitrogen and the residue was dissolved in 2 mL of equipped with an FID and a fused silica Chirasil-L-Val colmethane and used for the GC analysis as above. (25 m×0.25 mm id) (Macherey–Nagel, Du� ren, Germany).Data were acquired with Hewlett-Packard 3365 Chemstation Results and discussion software. GC-ICP-MS analysis was carried out with the same Enantiomeric separation chromatographic column in a Hewlett-Packard Model 6890 Careful optimization of the relevant operating conditions for gas chromatograph coupled to a Hewlett-Packard 4500 inducthe chiral phase capillary GC separation of DL-selenomethion- tively coupled plasma mass spectrometer (Hewlett-Packard, ine enantiomers using conventional FID was first carried out.Yokogawa Analytical Systems, Tokyo, Japan) througth a Parameters such as the temperature program of the column, laboratory-built interface which has been described in detail the injector temperature, the injection mode and the carrier elsewhere.30 It mainly consist in a metallic T-piece connected gas (He) flow rate were investigated. Then the parameters to a copper tube where the chromatographic column is placed. aVecting the performance of interface between the GC and This copper tube is inserted in a metallic block heated by ICP-MS systems were also optimized, including the length of means of an electric heater and controlled by the GC.The Tthe transfer line, the interface temperature and the make-up piece allows the insertion of an Ar gas sheathing flow (make-up sheathing gas flow (Ar). Finally, the optimum conditions for flow) orthogonal to the GC eZuent. This flow transports the specific on-line detection of Se by GC-ICP-MS were estab- analytes from the end of the GC column, via a PTFE tube lished.All these optimizations were aimed at achieving simul- (1.5 mm id) of about 40 cm length, to the nebulizer of the taneously the satisfactory optical resolution of the DL- ICP-MS for sample introduction of volatile analytes. selenomethionine derivatives and also maximum sensitivity of selenium detection. In addition, the performance of the ICP- Reagents and samples MS instrument was optimized independently of the GC system for ion lens voltages using a standard solution containing Li Racemic DL-selenomethionine and L-selenomethionine were (m/z=7), Y (m/z=89) and Tl (m/z=205) at 10 ng g-1 to purchased from Sigma (St.Louis, MO, USA), protease from obtain maximum sensitivity on the Y signal. Merck (Darmstdt, Germany), acetyl chloride from Aldrich The operating conditions selected after optimization for the (St. Louis, MO, USA), trifluoroacetic anhydride from Fluka enantiomeric GC separation were as follows: an initial oven (Buchs, Switzerland) and propan-2-ol and dichloromethane temperature of 100 °C was maintained for 3 min, then increased from Teknockroma (Barcelona, Spain).to 160 °C at a rate of 1.5 °Cmin-1 and the final oven tempera- All other chemicals and solvents were of analytical-reagent ture was maintained for 1 min. Injections were made in the grade and ultrapure Milli-Q water (18 MV cm) (Millipore, split/splitless mode (splitting ratio 1540, valve time 0.3 min) Bedford, MA, USA) was used throughout. Selenium-enriched and the injector temperature was 250 °C, the carrier gas being yeast and parenteral solution samples were kindly provided He at a pressure of 140 kPa.A typical GC-FID trace obtained from Dr. M. Potin-Gautier (University of Pau, France) and for a standard racemic mixture of DL-selenomethionine (total Dr. P. Bra�tter (Hahn-Meitner-Institut, Berlin, Germany), Se content 20 mg L-1), derivatized as described and analyzed respectively.as detailed above (Table 1), is shown in Fig. 1(A). As can be seen, very good resolution was achieved between the two Sample preparation enantiomers (Rs=2.5). Fig. 1(B) shows the chromatogram obtained when the same Selenium-enriched yeast samples were treated by enzymatic hydrolysis using the method reported by Gilon et al.:31 yeast chiral GC separation was coupled to Se-specific ICP-MS detection, through an interface design proposed by our (200 mg) and protease (20 mg) were added to 5 mL of water in a polypropylene centrifuge tube and shaken in the dark for research group.30 As can be seen, using the chromatographic conditions detailed in Table 1 and a temperature of the metallic 24 h; the solution was then centrifuged for 30 min at 3000g.The supernatant solution was removed and filtered through a block of 250 °C, band broadening took place, ruining the resolution between the D- and L-selenomethionine derivatives, 0.45 mm membrane. Aliquots of such filtrate solutions were used for derivatization.but without any significant variation of the previously observed values for retention times. In order to decrease this post- The parenteral solution sample was diluted (1540 v/v) with ultrapure water before taking an aliquot to be derivatized. column band broadening interface, the argon make-up flow 1334 J. Anal. At. Spectrom., 1999, 14, 1333–1337Table 1 Operating conditions for the hybrid GC-ICP-MS system for the enantiomeric separation of DL-selenomethionine Injector parameters— Injector port Split/splitless Injection volume 1 mL Splitting ratio 1540 Injection temperature 250 °C GC parameters— Column Chirasil-L-Val (25 m×0.25 mm id) Temperature program 100 °C (3 min); 1.5 °Cmin-1 to 160 °C Carrier gas and inlet pressure He, 130 kPa Interface temperature 250 °C Transfer line length 45 cm ICP-MS parameters— Isotopes 78Se, 82Se Rf power 1300W Fig. 2 Optimised GC-ICP-MS of a racemic mixture of DL-selenome- Sampling depth 5.8 mm thionine (as N-TFA-O-isopropyl derivatives). Experimental conditions Make-up gas flow rate 1.5 L min-1 are given in Table 1. Intermediate gas flow rate 1.0 L min-1 External gas flow rate 15 L min-1 Dwell time 0.1 s per mass ation could be almost completely achieved with this hybrid system (Rs=0.96). rate and the length of the PTFE tubing connecting the T- Analytical performance characteristics piece, where the GC column is inserted, to the ICP-MS were The precision of the total analytical method (derivatization optimized.It was observed that an increase in the Ar make-up procedure+chromatographic determination) in terms of RSD flow rate provided a noticeable improvement in the observed (n=5) was 15% for the D-selenomethionine enantiomer and band broadening while optimization of the length of the tubing 12% for the L-selenomethionine enantiomer at a concentration was critical.The optimum operating conditions finally selected of 50 mg L-1 for each enantiomer. These precisions seems to for the GC-ICP-MS system are summarized in Table 1. A be adequate for the application of the method to real samples. typical chromatogram obtained under such optimum working The calibration graphs, obtained from GC-ICP-MS analysis conditions for a standard racemic mixture of DL-selenomeof standard solutions of racemic DL-selenomethionine deriva- thionine derivatives (total Se content 6 mg L-1) is presented tives of increasing concentration showed good linearity over in Fig. 2. The results show that selenomethionine chiral specithe concentration range studied (0–100 mg L-1 of selenomethionine). The calibration curve for the D-enantiomer was best described by the equation y=32461x+1525 and that for the L-enantiomer by y=28884x+4548 ( y being the measured peak area and x the selenomethionine concentration in mg L-1). Good correlation coeYcients (r2=0.998) for both enantiomers were observed.The developed GC-ICP-MS method proved to be very sensitive as the detection limits (calculated as the concentration for a net signal equivalent to three times the background noise in the chromatogram) were found to be 0.25 mg L-1 of selenomethionine (0.1 mg L-1 as Se) for each enantiomer, with an injection volume of 1 mL. Detection limits using the GC-FID method were only estimated (owing to the elevated drift of the baseline) for comparison and turned out to be around 2–3 mg L-1.It is worth nothing that the detection limits obtained by the proposed GC-ICP-MS method were about 20 times lower than the values obtained by chiral HPLC with fluorescence detection and around 700 times lower than those observed by chiral HPG-ICP-MS methods previously developed in our laboratory.24 Chiral speciation of Se in selenomethionine in some real samples The applicability of the developed chiral phase capillary GC-ICP-MS methodology for the enantiomeric resolution and determination of D- and L-selenomethionine enantiomers in real samples was investigated.The following samples were analyzed: a commercial ‘pure’ L-selenomethionine sample, a parenteral solution used for human nutrition and a seleniumenriched yeast used as a nutritional supplement for human consumption. All these samples were treated and derivatized before analysis as described in the Experimental section. In all Fig. 1 GC of a racemic mixture of DL-selenomethionine (20 mg L-1, cases, standards together with reagent blanks were run in as Se) injected as N-TFA-O-isopropyl derivatives: (A) FID; (B) ICP-MS detection. Experimental conditions are given in the text. parallel. The signals from the blanks were always negligible, J. Anal. At. Spectrom., 1999, 14, 1333–1337 1335chloride (3.6 mg), copper DL-hydrogenaspartate (5.05 mg), manganese DL-hydrogenaspartate (2.18 mg), sodium fluoride (1.26 mg), selenomethionine (0.12 mg) and sodium molybdate (43.5 mg).Fig. 4 clearly indicates that selenomethionine in the analyzed parenteral solution sample is present as a racemate (information not given on the commercial label ). In our previous work using HPLC, this chiral speciation of Se was not possible owing to the elevated content of hydrogenaspartate in the parenteral solution sample (in HPLC with fluorescence detection, the peak corresponding to the derivatized aminoacid hydrogenaspartate overlapped the NDA selenomethionine derivative peak,24 and using HG-ICP-MS for the low sensitivity of this hypenated HPLC-HG-ICP-MS methodology provided the detection of Se in the 1540 diluted pareteral solution sample24).Chiral speciation of Se in selenized yeast. The enantiomeric resolution and determination of selenomethionine enantiomers Fig. 3 GC-ICP-MS of a commercial sample of ‘pure’ L-selenomethionby the proposed methodology was finally applied to a complex ine derivatized as above.sample of commercial selenium enriched yeast. This sample is a Saccharomyces cerevisiae yeast brought up in a sodium so the amount of DL-selenomethionine in the sample was selenite-rich medium. After pasteurization, the yeast is used as estimated from the respective peak areas of the corresponding a source of selenium and sold in pharmacies as a nutritional analyte in the sample and standard. supplement for human consumption. Recents reports31 have suggested that selenomethionine is Commercial L-selenomethionine purity.Fig. 3 shows the the predominant selenium species in selenium-enriched yeast GC-ICP-MS trace obtained for the derivatized ‘commercially (about 40% of the total selenium). The problem, however, is pure L-selenomethionine’ sample. As can be seen the presence open to debate because, as stated before, more than 20 of D-selenomethionine in the sample is apparent, in agreement selenium-containing species have been reported to appear in with our previous analysis of this sample by chiral HPLC with selenized yeast, including selenomethionine, inorganic selboth fluorimetric and HG-ICP-MS detection.24 The relative enium, selenocysteine and metylselenocysteine.25,26 Fig. 5 level of D-selenomethionine in such a commercial sample was shows the GC-ICP-MS trace obtained for an enzymatic calculated to be ca. 8% of the total DL-selenomethionine hydrolyzate of the enriched yeast sample provided by Dr.M. content, which is assumed to be 100%. Simple calculations Potin-Gautier (enzymatic hydrolysis extraction eYciency based on the extreme selectivity and sensitivity of the developed 92%31). As can be seen, a selenium peak eluting at the expected methodology demonstrate that impurity levels of the D-enanti- retention time (32.5 min) of the L-selenomethionine enantiomer omers of less than 0.1% could be detected in the commercial (Fig. 2) is apparent. However, another selenium peak eluting product.at the retention time (31 min) of the D-selenomethionine enantiomer is also visible. From the areas of the selenomethion- Serum infusion selenomethionine: chiral discrimination. The ine peaks, the latter amounts to about 15% of the total DLchromatogram for the GC-ICP-MS chiral analysis of a par- selenomethionine content in the sample. Although selenoamino enteral solution sample (diluted 1540 with ultrapure water acids of natural origin should possess the L-configuration,20 it before derivatization) is presented in Fig. 4. This sample was seems that D-amino acids are ubiquitous and common constitulabeled as containing (per 30 mL) magnesium L-hydrogenas- ents of fermented foods and beverages32 as a consequence of partate (4.87 g), zinc DL-hydrogenaspartate (15.82 mg), iron Fig. 5 Direct GC-ICP-MS of a ‘selenized yeast’ (after enzymatic Fig. 4 GC-ICP-MS of a serum infusion solution (after dilution 1540 and derivatization as above). hydrolysis and derivatization as above). 1336 J. Anal. At. Spectrom., 1999, 14, 1333–1337the action of microorganisms (bacteria, yeast). Moreover, it References has been reported that dietary commercial yeast can be a 1 M. SimonoV and G. SimonoV, Le Selenium et la Vie, Masson, potential source of D-amino acids in foodstuVs.32 Paris, 1991. In the light of the above results, it is not surprising to 2 W. N. Choy, P. R. Henika, C. C. Willhite and A. F. Tarantal, observe the presence of D-selenomethionine in the selenium- Environ.Mol. Mutagen., 1993, 21, 73. enriched yeast analyzed for Se species in this work (Fig. 5). 3 G. Yang, L. Gu, L. Zhou and R. Yin, in Selenium in Biology and Fig. 5 also shows the presence of many other selenium- Medicine, ed. A.Wendel, Springer-Verlag, Berlin, 1988, p. 223. 4 L. Fisbein, in Metals and Their Compounds in the Environmental. containing species in the enriched yeast. Occurrence, Analysis and Biological Relevance, ed.E. Merian, VCH Publishers, New York, 1991, p. 1153. 5 J. Arnaud, V. Imbault-Huart and A. Favier, in Selenium in Conclusion Medicine and Biology, ed. J. E. Neve, Walter de Gruyter and Co., GC with a fused-silica capillary column coated with L-valine- Berlin, 1988, p. 125. 6 A. D. Salbe and O. A. Levander, J. Nutr., 1990, 120, 200. tert-butylamide (Chirasil-L-Val ), as the chiral stationary 7 L. H. Foster and S. Sumar, Crit. Rev. Food Sci. Nutr., 1997, phase, has been shown to allow the enantiomeric resolution 37, 211.of N-TFA-O-isopropyl derivatives of DL-selenomethionine 8 M. T. Lo and E. J. Sandi, Environ. Pathol. Toxicol., 1980, 4, 193. enantiomers. 9 O. E. Olson, J. Am. Coll. Toxicol., 1986, 5, 45. The use of a simple new interface developed in our 10 G. F. Combs and S. B. Combs, The Role of Selenium in Nutrition, laboratory30 to couple such chiral GC separation to ICP-MS Academic Press, Orlando, FL, 1986. 11 N. V. Dimtrow and D.E. Velrey, J. Am. Coll. Toxicol., 1986, 5, 95. selenium-specific detection has proved to provide a very selec- 12 S. J. Fairweather-Tait, Eur. J. Clin. Nutr., 1997, 51, S20. tive and sensitive method allowing the enantiomeric separation 13 C. C. Willhite, W. C. Hawkes, S. T. Omaye, W. N. Choy, and chiral speciation of selenium in selenomethionine. D. N. Cox and M. Cukierski, J. Food Chem. Toxicol., 1992, 30, Detection limits around 250 ng L-1 (ppt) (100 ng L-1 as 903.selenium) can be achieved. This means a detectability for Se 14 A. J. Butler, C. D. Thomson, P. D. Whanger and M. F. Robinson, chiral speciation of around 700 times better than that obtained Am. J. Clin. Nutr., 1991, 53, 748. 15 G. H. Heinz, L. J. HoVman, L. J. LeCaptain, Arch. Environ. in our previously work by chiral HPLC-HG-ICP-MS.24 Such Contam. Toxicol., 1996, 30, 93. detection limits are also better than those reported by several 16 C. Ip and H. Gonter, in Cancer Chemoprevention, ed. workers using conventional (achiral ) HPLC-ICP-MS33–35 and L.Wattenberg, M. Lipkin, C. W. Brone and G. F. Kellof, CRC those reported by De la Calle-Guntin�as et al.36 using conven- Press, Boca Raton, FL, 1992, p. 479. tional (achiral ) GC-MIP-AES d GC-MS. It should be 17 M. A. Beilstein and P. D. Whanger, J. Nutr., 1986, 116, 1701. pointed out that chiral separation is a diYcult separation 18 P. B. Moser-Veillon, A. R. Mangels, K. Y. Patterson and C. Veillon, Analyst, 1992, 117, 3.process achieved in this work which was not aimed at in the 19 A. M. Smith and M. F. Picciano, J. Nutr., 1987, 117, 725. other work. 20 P. A. Mc Adam and O. A. Levander, Nutr. Res., 1987, 7, 601. The method has also a precision in the range reported so 21 S. M. Hansen and M. N. Poulsen, Acta Pharm. Nord., 1991, 3, 95. far using GC with a derivatization step.36 Therefore, to our 22 R. Vespale, H. Corstein, H. A. H. Billiet, J. Frank and knowledge, this hybrid GC-ICP-MS technique seems to pro- K.Ch. A. M. Luyben, Anal. Chem., 1995, 19, 3223. vide one of the more sensitive and selective methods reported 23 R. Vespale, H. A. H. Billiet, J. Frank and K. Ch. A. M. Luyben, J. High Resolut. Chromatogr., 1996, 19, 137. for Se speciation in amino acids and an excellent approach to 24 S. Pe�rez Me�ndez, E. Blanco Gonza�lez, M. L. Ferna�ndez Sa�nchez chiral selenium speciation both in parenteral solutions and, and A. J. Sanz-Medel, J.Anal. At. Spectrom., 1998, 13, 893. what is more diYcult, in complex biological samples. 25 S. Bird, G. Honghong, P. C. Uden, J. F. Tyson, E. Block and Application of this coupled method to diVerent Se E. J. Denoyer, J. Chromatogr. A, 1997, 789, 349. compounds has been demonstrated with the determination of 26 S. Bird, P. C. Uden, J. F. Tyson, E. Block and E. J. Denoyer, the ‘optical purity’ (percentage of D-enantiomer relative to the J. Anal. At. Spectrom., 1997, 12, 785. 27 V.Schuring, J. Chromatogr. A, 1994, 666, 111. total amount of amino acid) of L-selenomethionine in a 28 I. Abe, N. Fijimoto, T. Nishiyama, K. Terada and T. Nakahara, commercial ‘pure’ L-selenomethionine sample and in a par- J. Chromatogr. A, 1996, 722, 221. enteral solution used for Se supplementation in humans. 29 H. Bru�ckner and M. Lu�pke, Chromatographia, 1991, 31, 123. The applicability of the proposed technique appears to be 30 M. Montes Bayo� n, M. Gutie�rrez Camblor, J. I. Garcý�a Alonso particularly important for tackling the diYcult problem of and A. Sanz-Medel, J. Anal. Atom. Spectrom., 1999, 14, 1317. chiral speciation of Se in real biological matrices; in fact, the 31 N. Gilon, M. Potin Gautier and M. Astruc, J. Chromatogr. A, 1996, 750, 327. very high sensitivity and the specificity for Se aVorded by 32 H. Bru�ckner, M. Langer, M. Lu�pke, H. T. Westhauser, ICP-MS detection allows the direct chiral speciation of this J. Chromatogr. A, 1995, 697, 229. semi-metal in the extremely complex mixture of aminoacids 33 M. A. Quijano, A. Gutierrez, M. C. Pe�rez-Conde, C. Ca�mara, and selenoamino acids25,26 resulting from hydrolysis of J. Anal. At. Spectrom., 1996, 11, 407. ‘selenized yeast’ (Fig. 5). 34 H. M. Crews, P. A. Clarke, J. Lewis, L. M. Owen, P. R. Strutt and A. Izquierdo, J. Anal. At. Spectrom., 1996, 11, 1171. 35 R. Olivas, O. F. X. Donard, N. Gilon and M. Potin-Gautier, Acknowledgements J. Anal. At. Spectrom., 1996, 11, 1177. 36 B. De la Calle-Guntin� as, C. Brunori, R. Scerbo, S. Chiavarini, Support via a grant to S. Pe�rez Me�ndez from the Ministerio P. Quevauviller, F. Adams and R. Morabito, J. Anal. At. de Educacio�n y Cultura (Spain) is gratefully acknowledged. Spectrom., 1997, 12, 1047. Thanks are extended to Dr. M. Potin-Gautier (University of Pau, France) and to Dr. P. Bra�tter (Hahn-Meitner-Institut, Berlin, Germany) for providing the real samples. Paper 9/02524C J. Anal. At. Spe

ISSN:0267-9477    

DOI:10.1039/a902524c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Application of capillary electrophoresis-electrospray ionisation mass spectrometry to arsenic speciation† O. Schramel,* B. Michalke and A. Kettrup GSF-National Research Center for Environment and Health, Institute for Ecological Chemistry, Neuherberg, Germany. E-mail: oliver.schramel@gsf.de Received 18th January 1999, Accepted 22nd March 1999 The on-line coupling of capillary electrophoresis (CE) and electrospray ionisation mass spectrometry (ESI-MS) is a very useful tool for speciation analysis.This hyphenated technique provides elemental (isotopic pattern, if the element is not monoisotopic) as well as structural (molecular mass and/or fragmentation) information on an unknown species. Owing to several properties (high separation eYciency, low ‘flow rates’), CE is best suited as the separation device for this coupling. The geometrical dimensions of both systems require the use of rather long CE capillaries (up to 100 cm), which leads to long total analysis times.The application of pressure along the capillary during or after the CE separation shortens the total analysis time dramatically. The eVects of this ‘pressure mobilisation’ on detection limits, peak shape and resolution are discussed in detail. The technique was applied to the speciation of arsenic. A CE method was developed, providing the separation of six arsenic species of interest in a single run {arsenite [As(III )], arsenate [As(V)], methylarsonic acid (MMA), dimethylarsinic acid (DMA) arsenobetaine (AsB) and arsenocholine (AsC)}.The method used an acidic electrolyte system (ammonium acetate–acetic acid) for pH stacking. With the exception of As(III) and MMA, the arsenic species were baselineseparated from each other. Detection limits were calculated as 60–480 mg L-1 for the arsenic species. The only exception was arsenite, As(III), with a detection limit of 50 mg L-1. The method was applied to standard mixtures and urine samples.sinic acid (DMA) are much less toxic. The quaternary 1 Introduction arsonium compounds arsenobetaine (AsB) and arsenocholine The determination of arsenic has been the subject of much (AsC) are non-toxic. Without consumption of seafood the analytical work for many years. Since the toxicity, bioavail- typical distribution of arsenic species in human urine is 10–15% ability and transport of arsenic are dependent on the species inorganic arsenic, 10–15% MMA and 60–80% DMA.15 After present, powerful analytical methods are necessary to dis- consumption of seafood containing large amounts of tinguish between the diVerent species.The most frequently organoarsenicals [AsB, AsC and dimethylarsinylribosides used technique is the combination of HPLC coupled to an (arsenosugars)], the main excreted species is AsB.16 element-specific detection system, such as AAS, ICP-AES and In this paper, we present a CE-ESI-MS method for the ICP-MS. A comprehensive review of the methods used over simultaneous determination of six commonly encountered the last decade has been given by Goessler et al.1 In recent arsenic species [As(III), As(V), MMA, DMA, AsB, AsC].The years, capillary electrophoresis (CE) has been demonstrated method was applied to standard mixtures. In addition, the to be a powerful separation technique for arsenic.2–6 Despite possibility of identification of the arsenic compounds in human the advantages of high selectivity and sensitivity, the element- urine with and without spiking experiments was investigated.specific detection systems do not provide molecular infor- Additionally, the eVects of ‘pressure mobilisation’ (applimation; instead, species identification is based on chromato- cation of pressure along the capillary during or after the CE graphic retention times compared with those of available separation) on detection limits, peak shape and resolution are standard substances. For this reason, the soft molecular detec- discussed in detail. This ‘new’ detection mode shortens the tion techniques of electrospray-MS (ESI-MS) or ionspray-MS total analysis time dramatically.(IS-MS) are the techniques of choice, because they oVer both elemental (isotopic pattern and/or mass-to-charge ratio of the positively charged element itself, if the ESI-MS is operated in 2 Experimental the element mode7,8) and structural (molecular mass and/or 2.1 Chemicals and reagents fragmentation information).Numerous reports about extensive ESI-MS studies on uncomplexed metal ions in solution,9,10 Stock standard solutions of As(III) (10 000 mg L-1 As) and metal complexes11,12 and organometallic species13,14 have been As(V) (10 000 mg L-1 As) were prepared by dissolving the published, since Kebarle and co-workers first demonstrated appropriate amount of sodium meta-arsenite (Sigma, this possibility with ESI-MS in 1990.7,8 Deisenhofen, Germany, 97%) or disodium hydrogenarsenate The two inorganic arsenic species arsenite, As(III), and heptahydrate (Fluka, Deisenhofen, Germany, puriss.p.a.) in arsenate, As(V), are the most toxic species, while the methyl- water. Methylarsonic acid (MMA), dimethylarsinic acid ated forms monomethylarsonic acid (MMA) and dimethylar- (DMA), arsenobetaine (AsB) and arsenocholine (AsC) were kindly donated by Service Central d’Analyse (CNRS, Vernaison, France) in the form of aqueous stock standard †Presented at the 1999 European Winter Conference on Plasma Spectrochemistry, Pau, France, January 10–15, 1999.solutions each containing 400 mg L-1 As. All stock solutions J. Anal. At. Spectrom., 1999, 14, 1339–1342 1339were stored in the dark at 4 °C. Working standard solutions Level II: 270.0 mg L-1. Values for the single arsenic species were not determined. The given concentrations do not reflect of lower concentrations were prepared daily by appropriate dilution with water. the total arsenic content, but only the inorganic arsenic [As(III) and As(V)] and its metabolites (MMA and DMA).AsB, AsC All chemicals used for the preparation of electrospray sheath liquids and CE electrolytes were of analytical-reagent grade and arsenosugars (dimethylarsinylribosides) were not included, because they do not form volatile hydrides. They can only be or higher purity. Water was first purified by normal de-ionisation and then by a special cartridge ion-exchange determined in urine after complete digestion.19 Spiking of the samples was performed by adding a small unit (Milli-Q water purification system). amount (max. 2.5% of the total sample volume) of an aqueous 2.2 Instrumentation standard solution containing the appropriate amount of the arsenic compounds. 2.2.1 CE-ESI-MS hyphenation. A BioFocus 3000 capillary electrophoresis system (Bio-Rad, Munich, Germany) was used as the CE device. The temperature was set to 15 °C for the 3 Results and discussion sample carousel (air cooling) and capillary ( liquid cooling) 3.1 Analysis of standard solutions during all experiments.The uncoated CE capillary (Bio-Rad) was 95 cm×50 mm id in size. The CZE method used an acidic The pH of the background electrolyte influences the migration electrolyte system ( leading electrolyte: 30 mM ammonium time of the analytes as well as the separation power. The formate adjusted to pH 6.0 with acetic acid; terminating eVects of pH can be attributed to its influence on the bulk electrolyte: 25% acetic acid) for pH stacking of the arsenic electro-osmotic flow (EOF) and the electrophoretic flow of species.Prior to sample injection (pressure: 75 psi, x s), the the arsenic species. capillary was purged with water (45 s, 8 bar), 25% acetic acid At the beginning of the CE separation, the capillary was (90 s, 8 bar) and leading electrolyte (150 s, 8 bar; i.e. the completely filled with ammonium formate (30 mM, pH 6.0).capillary was completely filled with leading electrolyte). After When a positive high voltage was applied (+25 kV), the sample injection, the capillary was dipped into the terminating positively charged AsC and the fairly positive AsB (pKa=4.7) electrolyte (25% acetic acid) and the voltage was set to migrated towards the cathode, as is clear from Fig. 1 and 2. +25 kV. At pH 6.0 the EOF rapidly moved the undissociated DMA On-line ESI-MS detection was performed with a TSQ 700 (pKa=6.2) towards the cathode.The negatively chargedMMA triple-quadrupole mass spectrometer and an electrospray ionis- (pKa1=3.6) also moved towards the cathode, because the ation unit (both from Finnigan-MAT, Bremen, Germany). EOF is higher than the electrophoretic mobility. With increas- Special care was focused on the exact positioning of the CE ing separation time a pH gradient was formed within the capillary inside the ESI probe tip, which is an important capillary [anode: 25% acetic acid (pH 3.0), cathode: 30 mM prerequisite for stable and reproducible ionisation conditions.ammonium formate (pH 6.0)]. With decreasing pH the degree As has been described earlier,14 a modified CE-MS interface of deprotonation of MMA decreases, resulting in a lower kit, which allows an infinitely variable adjustment of the electrophoretic mobility and faster movement to the cathode. capillary, was used for this purpose. A syringe infusion pump As(III ), which is neutral under the chosen pH values (pKa= (Harvard Apparatus, South Natick, MA, USA) delivered the 9.3), should be detected together with DMA2 (driven by the sheath liquid (MeOH–H2O, 95+5 v/v, plus 10 mMammonium EOF).However, its detection together with MMA and the formate; flow rate: 3 mL min-1). ESI conditions were as fol- broad, unsymmetrical peak (see Fig. 1 and 2) were an indilows: spray potential: 5.6 kV, spray current: 64–68 mA, tem- cation of capillary wall eVects.As(V), which is negative under perature of the heated capillary: 200 °C, sheath gas (nitrogen) flow rate: 100 mL min-1. The MS instrument was operated in the single ion monitoring mode, acquiring data for m/z=107 [positively charged As(III) species, probably AsO2+], 139 [DMA+H]+, 141 [MMA+H]+, 157 [(arsenic acid monomethyl ester)+H]+, 165 [AsC+] and 179 [AsB+H]+ with 3 scans s-1. 2.2.2 Pressure-assisted detection mode. The ESI-MS detection of the arsenic species was performed (i) with and (ii) without pressure assistance.Pressure assistance means that the separated molecule bands were transferred to the ESI-MS system by pressing the inlet buVer (350 mbar or 8 bar) into the capillary after 10 min of normal CE separation. The CE separation voltage remained switched on during the pressuredriven detection step, in order to prevent band broadening due to wall eVects. This two-step procedure has been presented elsewhere17 and was derived from Michalke and Schramel,18 who used it for extensive CE-ICP-MS speciation experiments. 2.2.3 Real samples. Two lyophilised urine control samples, ClinRepA Urine Control Level I and Level II (Recipe Chemicals+Instruments, Munich, Germany), were used for speciation analysis. These lyophilised controls are based on human urine. Prior to analysis the samples were reconstituted Fig. 1 CE-ESI-MS electropherogram of the six arsenic species in an with Milli-Q water (only half the amount recommended by aqueous standard solution containing 25 mg L-1 of MMA, DMA, the manufacturer) according to given descriptions and filtered AsB and AsC, 100 mg L-1 of As(V) and 1000 mg L-1 of As(III).through a 0.45 mm filter. The reference values for arsenic Detection was performed without pressure mobilisation. Peaks for [determined directly without digestion by using hydride gener- m/z 107 (12 min) and m/z 157 (16 min) were artefacts from AsC and AsB. For experimental parameters see Section 2.2.1.ation AAS (HGAAS)] were as follows: Level I: 131.0 mg L-1, 1340 J. Anal. At. Spectrom., 1999, 14, 1339–1342about 58 to 18 min). The CE separation voltage remained switched on during the pressure-driven detection step. (ii) Peak symmetries of the separated species were improved. There are three reasons for this. Firstly, the time of interaction of slowly migrating species with the CE capillary inner walls (uncoated silica capillary) was shortened. Secondly, the band broadening eVects of the diVerent ionic mobilities of the analyte and buVer ions were reduced.Thirdly, the influence of the sheath liquid composition on the migration times and peak shapes of the analytes21 was reduced. This could be observed best for the slowly migrating species As(V) and As(III ) in Fig. 1, showing broad, unsymmetrical peaks. (iii) The detection limits (3s) were improved, because of an increasing signal-to-noise ratio due to the introduction of a higher sample volume per unit time and more symmetrical peak shapes (maximum intensity is higher).(iv) Almost no loss of resolution. With the exception of As(III) and MMA, the arsenic species were baseline separated, independent of the mode of detection (see Fig. 1 and 2). If the migration times of two analytes are too similar, baseline separation is not possible, when using low pressure mobilisation. This has been demonstrated for the separation of seleno-DL-methionine and seleno-DL-cystine.17 There are two reasons for this.Firstly, the separated molecule bands were pushed together again and Fig. 2 CE-ESI-MS electropherogram of the six arsenic species in an aqueous standard solution (same concentrations as in Fig. 1). secondly, the dwell time of the MS instrument was not short Detection was performed after 10 min of CE separation applying low enough to distinguish between the two species. pressure (350 mbar) along the capillary. Peaks for m/z 107 (13.5 min) In some cases the application of high pressure (8 bar) led and m/z 157 (14.8 min) were artefacts from AsC and AsB.For to dramatic deteriorations in the ESI-MS detection (not experimental parameters see Sections 2.2.1 and 2.2.2. shown). (i) The total analysis time was further reduced (about 11 min), but the resolution decreased. With the exception of AsC, the peaks of the remaining species overlapped. (ii) The the chosen pH values (pKa1=2.3, pKa2=6.9), had the highest detection limits worsened.In order to handle the higher electrophoretic mobility towards the anode. For this reason it amounts of electrolyte/sample solution, the ESI-MS operating was detected last. parameters had to be changed (higher electrospray voltage, With the exception of As(III) and MMA, baseline separation higher sheath gas flow rates). This improved the electrospray could be achieved within 10 min. The detection limits (3s) stability at the cost of reduced sensitivity (noisy baseline).22 were calculated as: DMA (60 mg L-1), AsB (110 mg L-1), AsC (170 mg L-1), MMA (480 mg L-1), As(V) (1.6 mg L-1) and As(III ) (51 mg L-1).The absolute detection limits for DMA, 3.3 Analysis of urine samples AsB and AsC were the lowest, because these species already Persons not exposed to excess arsenic have total arsenic existed as positively charged ions in solution. DMA and AsB concentrations in urine in the range 3–20 mg L-1 As,2 while were detected as positively charged protonated molecular ions.an arsenic-rich diet (water and/or food) can lead to concen- AsC was detected as [(CH3)3AsKCH2KCH2KOH]+. The trations up to 600 mg L-1 As.1 One aim of this work was to remaining three species had to be charged during the ESI investigate the feasibility of this method for the determination process, explaining their higher detection limits. MMA was of inorganic and organometallic arsenic species in urine detected as [MMA+H]+, As(V) as positively charged prosamples.Owing to the poor absolute detection limits (DL), tonated arsenic acid methyl ester [CH3OAsO(OH)2+H]+ and the inorganic arsenic species As(III) (DL=51 mg L-1) cannot As(III ) as an unidentified positively charged species (m/z= be identified and quantified in natural urine samples. As(V) 107), which is most likely AsO2+. The formation of positively (DL=1.6 mg L-1) and MMA (DL=480 mg L-1) can only be charged metal hydroxides (Ba, Pb) and oxides, and also detected by using the standard additions method.oxide–water clusters (Y), was also discussed by Corr and Fig. 3 shows a CE-ESI-MS electropherogram of the urine Anacleto.20 sample ClinRepA Urine Control Level II (total arsenic content: 270 mg L-1), having 1 mg L-1 of AsB added. Several state- 3.2 Detection mode ments can be made: (i) The ESI-MS detection of the diVerent arsenic species is unspecific, which is obvious from the spectra. Usually, only a small CE capillary (ca. 10–25 cm) is necessary for complete separation, leading to short total analysis times. Arsenic is monoisotopic; hence, certain urine matrix compounds, which accidentally have the same mass-to-charge Owing to the geometrical dimensions of CE and ESI-MS, long capillaries (up to 100 cm) are required for coupling. In order ratio, produce artefacts (marked with an arrow in Fig. 3). Therefore, a diVerentiation between signals produced from to shorten the total analysis time, pressure is applied along the capillary during or after the CE separation. matrix compounds or arsenic species is only possible by standard additions, MS-MS experiments and/or comparison There are two diVerent modes of ‘pressure mobilisation’: (1) low pressure mobilisation with 350 mbar (Fig. 2) and (2) of migration times. Only AsC gave a signal in the unspiked urine sample (not shown), proved by standard additions (plus high pressure mobilisation with 8 bar (not shown). Both techniques were compared with normal CE separation/ 1 mg L-1). The remaining species could not be detected.(ii) The ionisation in the ESI process depends on the matrix. Even detection (Fig. 1). As can be seen in Fig. 2, several advantages (in comparison when adding standard solutions at concentrations clearly higher than the calculated detection limits (2 mg L-1 each), with Fig. 1) could be observed, when applying low pressure (350 mbar) along the capillary after 10 min of CE separation: neither MMA nor DMA could be detected. Only after the addition of 10 mg L-1 of each species were satisfactory signals (i) Reduction of the total analysis time by a factor of 3 (from J.Anal. At. Spectrom., 1999, 14, 1339–1342 1341addition, the detection limits and peak shapes could be improved. The resolution remained virtually the same. The application of high pressure worsened the detection limits dramatically and led to a loss of resolution. Arsenic speciation in urine samples using CE-ESI-MS only gave satisfactorily results for AsC.This compound was the only species that could be detected without the use of the standard additions method. The remaining species were strongly disturbed by the formation of artefacts, ionisation problems due to matrix compounds and high baseline noise. The detection of AsB in urine samples with high AsB contents should be possible; however, owing to a lack of suitable samples, this has not been proved.The addition of AsB in the range of its detection limit gave the corresponding signals. References 1 W. Goessler, C. Schlagenhaufen, D. Kuehnelt, H. Greschonig and K. I. Irgolic, Appl. Organomet. Chem., 1997, 11, 327. 2 H. Greschonig, M. G. Schmid and G. Gu� bitz, Fresenius’ J. Anal. Chem., 1998, 362, 218. 3 A. R. Timerbaev, Electrophoresis, 1997, 18, 185. 4 Y. M. Huang and C. W. Whang, Electrophoresis, 1998, 19, 2140. 5 B. Michalke and P. Schramel, Electrophoresis, 1998, 19, 2220. Fig. 3 CE-ESI-MS electropherogram of a human urine sample con- 6 Y. Liu, V. Lopez-Avila, J. J. Zhu, D. R. Wiederin and taining 270 mg L-1 of total arsenic. Additionally, the sample was W. F. Beckert, Anal. Chem., 1995, 67, 2020. spiked with 1 mg L-1 of AsB. The peaks marked with an arrow were 7 A. T. Blades, P. Jayaweera, M. G. Ikonomou and P. Kebarle, Int. caused by matrix compounds and did not come from the arsenic J. Mass Spectrom. Ion Processes, 1990, 101, 325. species. 8 A. T. Blades, P. Jayaweera, M. G. Ikonomou and P. Kebarle, Int. J. Mass Spectrom. Ion Processes, 1990, 102, 251. 9 G. R. Agnes and G. Horlick, Appl. Spectrosc., 1995, 49, 324. obtained. One explanation is that the large number of matrix 10 T. G. Huggins and J. D. Henion, Electrophoresis, 1993, 14, 531. compounds disturb the ionisation process during the detec- 11 Y. Xu, X. Zhang and A. L. Yergey, J. Am. Soc. Mass Spectrom., tion of MMA and DMA. These compounds (ionic or neutral 1996, 7, 25.species) compete for the available charge in the ESI process, 12 R. Guevremont, K. W. M. Siu, J. C. Y. Le Blanc and S. S. Berman, J. Am. Soc. Mass Spectrom., 1992, 3, 216. thus lowering the maximum sensitivity.21,23 (iii) Some of the 13 H. Chassaigne and R. £obinski, Analyst, 1998, 123, 131. masses had high baseline noise (see e.g. m/z=107 between 11 14 O. Schramel, B. Michalke and A. Kettrup, J. Chromatogr. A, and 13 min), probably caused by matrix compounds, which 1998, 819, 231. strongly interfered with the capillary’s inner walls and, 15 M. Vahter, Clin. Chem., 1994, 40, 679. therefore, were not detected as peaks but as broad plateaus. 16 X. C. Le, W. R. Cullen and K. J. Reimer, Clin. Chem., 1994, 40, 617. 17 O. Schramel, B. Michalke and A. Kettrup, Fresenius’ J. Anal. 4 Conclusion Chem., 1999, 363, 452. 18 B. Michalke and P. Schramel, Fresenius’ J. Anal. Chem., 1997, A CE-ESI-MS method was developed, which allowed the 357, 594. separation and detection of six inorganic and organometallic 19 P. Schramel and S. Hasse, Fresenius’ J. Anal. Chem., 1993, 346, arsenic species in a single run. With the exception of MMA 794. and As(III ), all of the species were baseline-separated. The 20 J. J. Corr and J. F. Anacleto, Anal. Chem., 1996, 68, 2155. 21 J. Cai and J. Henion, J. Chromatogr. A, 1995, 703, 667. absolute detection limits were calculated to be between 0.06 22 R. D. Smith, J. A. Loo, C. G. Edmonds, C. J. Barinaga and and 0.48 mg L-1. The only exception was arsenite, As(III), H. R. Udseth, Anal. Chem., 1990, 62, 882. with a detection limit of 50 mg L-1. 23 D. C. Gale and R. D. Smith, Rapid Commun. Mass Spectrom., The low pressure mobilisation (350 mbar) of the separated 1993, 7, 1017. molecule bands led to a dramatic reduction of the total analysis time (in comparison with normal operating conditions). In Paper 9/00494G 1342 J. Anal. At. Spectrom., 1999, 14, 1339–13

ISSN:0267-9477    

DOI:10.1039/a900494g

出版商:RSC    

年代:1999

数据来源: RSC