摘要:

Surface Characterisation of Laser Modified Human Tooth Enamel Using Laser Microprobe Mass Spectrometry and Scanning Electron Microscopy ALISON CHEW, DAVID E. SYKES AND ANN E. WADDILOVE* Institute of Surface Science and T echnology, L oughborough University, L oughborough, L eicestershire, UK L E11 3TU A technique in preventative dentistry currently under remove and ionise material from the sample surface, the resulting ions being detected in a time of flight mass spec- development is the modification of the tooth surface by the fusion of glassy metal oxides into the enamel surface using a trometer.The advantages of using LMMS include: (a) the availability of chemical species information in addition to CO2 laser. This could provide a protective coating to the tooth which would be applied in the mouth. Such a radical treatment elemental analysis; (b) the small analysis area (1–2 mm diameter) enables localised variations in chemistry to be detected; and is intended to produce major changes in chemistry at the tooth surface and characterisation of these changes is a necessary (c) the sampling depth (typically 0.25–0.5 mm) makes it ideal for the characterisation of thin surface coatings.part of the development of the technology. LMMS and SEM have been used, as part of a feasibility study, to characterise The technique has been used extensively to examine biologically- related samples, using both thin sections and bulk coatings deposited with laser assisted fusion on the surface of human tooth enamel, in particular the chemical and samples. Examples of its use include investigation of the localisation of trace elements in relation to the structural topographical changes taking place.The results demonstrate that the analytical approach adopted provides useful qualitative properties of insect mandibles,2 localisation of trace elements in human tissue samples3 and analysis of wood preservatives information about the physical and chemical changes taking place.They highlight the fact that although deposition of the in ancient oak from the ‘Mary Rose’.4 SEM was used to examine the topographical changes to the coating is patchy, areas are present where the coating has apparently fused to the tooth enamel. Changes in the sample surface. chemistry of the tooth enamel subjected to laser irradiation were identified, both in the absence of coating precursors and EXPERIMENTAL with precursor materials present.Treated samples were supplied by Dr. B. Patel of the Institute Keywords: L aser microprobe mass spectrometry; human tooth of Dental Surgery. They consisted of freshly extracted human enamel; laser modification; surface coating; scanning electron molar teeth which had then been subjected to continuous wave microscopy CO2 laser irradiation with and without the presence of metal oxide precursors on the tooth surface. The metal oxide precur- The focus of modern dentistry is being aimed increasingly at sors were in the form of aqueous metal alkoxide solutions prevention of decay rather than repair of damage.Alongside (based on Si Al Zr Na) and aluminosilicate glass/ceramic the promotion of good oral hygiene and a low sugar diet to powders which were painted onto the tooth surface. The laser prevent build-up of plaque and reduce the incidence of tooth beam was rastered over an area 7 by 9 mm. decay, methods for modification of the tooth surface to protect LMMS was carried out using a LIMA 2A instrument the enamel and reduce bacterial colonisation are being devel- (Cambridge Mass Spectrometry).Instrumental details can be oped. These include the development of coatings to be applied found elsewhere.5 The samples were supplied and stored in an to the teeth in situ, either as a partial film to cover and protect aqueous medium and had to be dried by pumping under rough areas which are particularly susceptible to decay1 or to alter vacuum overnight before analysis.No further sample prepthe chemical nature of the enamel. aration was required. The ionic activity of the tooth surface is in dynamic equilib- SEM images of the treated surfaces were obtained using a rium with the oral environment. Under certain circumstances, Cambridge Stereoscan 340 instrument. The microscopy was influenced by factors such as tooth geometry and pH, the rates carried out after the LMMS analysis had been completed as of mineral loss (predominantly calcium and phosphate ions) it was necessary to gold coat the samples to reduce sample far exceed those of the mineralisation process.This normally charging. leads to the undermining of the enamel and dentine structure and caries ensues. The physicochemical modification of tooth RESULTS AND DISCUSSION surfaces by laser assisted fusion and the incorporation of metal oxides into the surface is one approach that may help to Laser irradiation of the natural tooth surface produces local melting of the tooth enamel.This can be seen in Fig. 1, an prevent this mineral loss. In the present feasibility study, LMMS and SEM were used SEM micrograph showing the edge of an irradiated area. The region of localised melting is easily distinguished from the non- to monitor the changes produced in freshly extracted human molar teeth which had been subjected to continuous wave CO2 irradiated natural tooth surface.The scan lines of the laser beam are clearly visible in the irradiated area. The crack laser irradiation with and without the presence of metal oxide precursors on the tooth surface. The desired eVect of the laser running from top to bottom of the image is believed to be associated with shrinkage of the tooth resulting from loss of treatment was to incorporate the oxides into the enamel surface. LMMS is a technique which provides qualitative elemental water during the sample drying process.LMMS analysis of a non-irradiated surface showed the and chemical species information from localised areas on the surface of a solid sample. It utilises a pulsed UV laser to elements Na and Ca in positive ion spectra [Fig. 2(a)]. In Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1101–1103) 1101Fig. 1 SEM micrograph of a partially irradiated natural tooth surface showing the boundary between irradiated and non-irradiated regions. Fig. 3 Positive ion LMMS spectrum of irradiated natural tooth surface. CaOH is absent. Negative ion spectra showed no significant diVerences between the two areas. Similar results were obtained from other samples when irradiated and non-irradiated areas were compared. These observations suggest that the laser irradiation process produces a physical change in the tooth surface through a process of localised melting and that a chemical change also takes place resulting from the loss of water and/or hydroxide from the hydroxyapatite. Fig. 4 is an SEM micrograph showing the edge of an irradiated area of tooth surface which had been pre-treated Fig. 2 (a) Positive ion LMMS spectrum of non-irradiated natural with a precursor containing Na, Zr, Al and Si. Again the tooth surface. (b) Negative ion LMMS spectrum of non-irradiated surface shows local melting and the precursor has been incor- natural tooth surface. porated into the enamel or formed a glassy layer on top. LMMS analysis shows the presence of Na, Mg, Al, Si and Zr along with major peaks owing to Ca in the positive ion spectra.addition, molecular fragments are present at m/z=56 (CaO), 57 (CaOH), 59 (CaF) and 96 (Ca2O). A typical negative ion The presence of Ca in the spectra suggests that either the coating is very thin and the underlying enamel is being spectrum is shown in Fig. 2(b) with peaks at m/z=16 (O), 26 (CN), 42 (CNO), 63 (PO2) and 79 (PO3). Other minor peaks sampled, or else there is intermixing of elements from the coating and the enamel in the glassy layer. in the spectrum can be assigned to CnHm molecular fragments.These features are generally consistent with the chemical A similarly prepared sample showed Na, Mg, Al, K and Zr in addition to Ca in the non-irradiated area, but only Na, K composition of human tooth enamel, which is mostly calcium hydroxyapatite with small amounts of other trace elements and Ca were found in the irradiated area. This suggests that either the coating was uneven or that it may have been and organic material.1 Positive ion spectra from an irradiated surface adjacent to removed rather than incorporated by the laser treatment.As with the uncoated irradiated sample, the relative intensity of that described above showed a diVerence in the molecular structure obtained. This is demonstrated by Fig. 3. Peaks for the CaOH fragment is greater in the non-irradiated region of the surface than in the irradiated region.Na, Ca, CaF and Ca2O are all present but that owing to 1102 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 4 SEM micrograph of a tooth coated with metal oxide precursor and partially irradiated, showing the boundary between irradiated and non-irradiated areas. Further evidence for incomplete treatment of the tooth which may not be desirable, for example, loss of hydroxide from the enamel. surface was found in a sample pre-treated with a precursor based on mica.Large regional diVerences in composition were Further development of the treatment method is required. However, this work has demonstrated the feasibility of the seen from point to point. SEM analysis showed regions covered with mica platelets and regions devoid of mica on the surface analytical approach in identifying the physico-chemical changes taking place as a result of laser irradiation. of the sample not exposed to the radiation. In the area exposed to the laser, regions where the mica has fused onto or into the surface and regions devoid of a glassy layer were found.The authors thank Dr. Bipin Patel (Institute of Dental Surgery) These results demonstrate that further development of the for providing the samples, Mark Bewick and David Anderton treatment method is needed. Although the LMMS measure- for their project work and John Bates for the SEM images. ments are not quantitative and hence the degree of incorporation of precursor into the tooth surface cannot be determined, REFERENCES its patchy distribution has been demonstrated and chemical changes occurring as a result of the laser irradiation highlighted. 1 Cole, A. S., and Eastoe, J. E., Biochemistry and Oral Biology, Wright, London, 2nd edn., 1988. It must be noted, however, that both LMMS and SEM are 2 McClement, J. G., Ph.D. Thesis, University of Southampton, vacuum-based technologies and hence undesirable physical UK, 1995. changes such as cracking of the tooth surface can occur as a 3 Iancu, T. C., Perl, D. P., Sternlieb, I., Lerner, A., Leshinsky, E., result of the necessary sample drying. Kolodny, E. H., Hsu, A., and Good, P. F., Biometals, 1996, 9, 57. Whilst this treatment shows promise in that elemental and 4 Finney, R. W., and Jones, A. M., Stud. Conserv., 1993, 38, 36. chemical changes are induced in the hydroxyapatite, the 5 Southon, M. J., Witt, M. C., Harris, A., Wallach, E. R., and Myatt, J., Vacuum, 1984, 34, 903. measurements have shown that: (a) there is a need to improve the degree of coverage of the precursor if a complete barrier Paper 7/00086C layer is to be produced; and (b) in addition to incorporating Received January 3, 1997 metal oxide compounds into the tooth surface, the laser irradiation is producing chemical changes in the hydroxyapatite Accepted March 14, 1997 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1103

ISSN:0267-9477    

DOI:10.1039/a700086c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Contribution of Mass Resolution to Secondary Ion Mass Spectrometry Microscopy Imaging in Biological Microanalysis CATHERINE FOURRE� *a , J. CLERCb AND P. FRAGUc aL aboratoire de Biophysique et Me� decine Nucle�aire, CHU Bice�tre, 78 Rue du Ge�ne�ral L eclerc, 94275, L e Kremlin Bice�tre, France bService deMe� decine Nucle�aire, Hopital Necker, Paris cService deMe� decine Nucle�aire, Institut Gustave Roussy, V illejuif, France This paper presents recent developments in nuclear medical element to provide an in vivo functional image.Internal radiotherapy has been proposed as an alternative to conven- applications of secondary ion mass spectrometry (SIMS) microscopy. This technique is the only method (with laser tional external radiotherapy in cancer treatment. It consists of administering tumor-specific drugs which can be labeled with microprobe mass spectrometry) potentially able to map all the elements of the periodic table, including stable and radioactive radioactive isotopes mainly emitting b particles, such as iodine-131.In both diagnosis and therapy, the in situ disinte- isotopes. We have demonstrated with our microscope ( lateral resolution 0.5 mm; mass resolution 300–10 000) that this gration of the radiopharmaceuticals causes radiobiological injuries which can be tumoricidal. The radiation delivered to imaging technique can localize radioligand biodistributions on tissue sections if a mass resolution above 2000 is used.We the target is influenced by the radioligand biodistribution. Because it is impossible to shield surrounding tissues, knowl- chose three radioactive isotopes: technetium-99 m (99mTc), strontium-89 (89Sr) and bromine-76 (76Br). The 99mTc is edge of how these radioligands are distributed in human tissue is an essential prerequisite for dosimetry studies in nuclear introduced into the cell by a lipophilic ligand. The 89Sr is metabolised and integrated into bone matrix because it is a medicine. In this work, we chose three radioactive isotopes: technetium-99 m (99mTc), strontium-89 (89Sr) and bromine-76 calcium analog.The 76Br targets adrenals because it is bound to a ligand which has chemical analogies with endogenous (76Br). The 99mTc is introduced into the cell by a lipophilic ligand. The 89Sr is metabolised and integrated into bone matrix bioamines. Our data confirm and extend to other models the suggestion that SIMS should constitute a pertinent approach because it is a calcium analog.The 76Br targets adrenals because it is bound to a ligand which has chemical analogies to cellular dosimetries. with endogenous bioamines. Keywords: Secondary ion mass spectrometry microscopy; high The study of the distribution of radioactive elements in mass resolution; technetium-99 m distribution; strontium-89 tissue is diYcult because they are present in small quantities distribution; bromine-76 distribution; nuclear medicine; cellular and their manipulation gives rise to many problems.All dosimetry isotopes of the same element have the same number of electrons and therefore show identical chemical behavior. Distributions of radioactive elements in tissue can thus be determined by Secondary ion mass spectrometry (SIMS) microscopy, first studying the distribution of corresponding stable isotopes: 88Sr developed in the early sixties by Castaing and Slodzian,1 is a and 79Br or 81Br can be localized instead of 89Sr and 76Br.surface analytical technique commonly used for elemental 99Tc can be localized instead of 99mTc because it is a daughter localization in geochemistry, metallurgy and electronics, but element with little specific activity. We defined the working less so in biological sciences.2–5 SIMS microscopy is the only conditions of SIMS microscopy for these particular method (with laser microprobe mass spectrometry) capable of applications. mapping all the elements of the periodic table in tissue sections, including stable and radioactive isotopes.The first biological applications were developed by the group of Pierre Galle in EXPERIMENTAL the seventies with the first generation SIMS microscope: the SIMS microscopy Cameca SMI 300.6 Its mass resolution in imaging mode was too low to yield specific signals from polyatomic ions of low Principle element concentrations. In 1980, a new generation of SIMS Fig. 1 shows the principle of SIMS microscopy.The instrument microscopes, such as the IMS 3–5F, was designed specifically used was the Cameca IMS 3F (Cameca, Courbevoie, France) for imaging with a mass resolution of 200–40 000. With such fitted with an O2+ or Cs+ primary ion source. This instrument a SIMS microscope the field of medical applications was was especially designed to provide images with high mass enlarged to include the detection of small amounts of chemical resolution. The lateral resolution was 0.5 mm.The analysed elements in tissue samples.3–5 The principal contribution of area varied from 1.5 to 400 mm in diameter. Our working SIMS microscopy to biological microanalysis has been the conditions used 60 and 150 mm diameter areas. imaging of physiological elements such as thyroid iodine7,8 and cytoplasmic calcium9 and the mapping of the distribution of 15N and 14N compounds in plants.10 In the present study, Sample analysis we succeed in localizing radiopharmaceuticals, which allows assessment of the absorbed dose at tissue level.Here we With the IMS 3F, section flatness and adherence to gold holders, provided by resin-embedded material, are essential for determine the conditions for the detection of radioactive isotopes used to label ligands as applied to diagnosis and analysis. Section flatness avoids relief eVects and minimizes charge eVects, at least in microscope mode.11 Whatever the radiotherapy in nuclear medicine. Molecules used in diagnosis were composed of a specific target ligand carrying a radioactive fixation procedure used (chemical or cryotechnic), semi-thin Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1105–1110) 1105processing system was used to obtain high speed signal integration which improved the S/N. Image integration times were optimized for the quality of the image but were not representative of local element concentration. On-line and oV-line superimpositions in pseudocolors of two and three ion images, respectively, were used to obtain a composite image which was easier to interpret.Direct ion imaging (O2+ or Cs+) was performed with a primary beam illuminating areas of 60 and 150 mm in diameter on the sample surface. Radioelement Study T echnetium Technetium-99 m, a gamma emitter, is an artificial nuclide obtained from molybdenum disintegration with an Mo/Tc generator. It has a 6 h physical half-life and decays principally by gamma emission to 99Tc (physical half-life: 2.13×105 years).Technetium used for labeling in nuclear medicine is a mixture of 99mTc and 99Tc, whose proportions depend on the time elapsed between two elutions. Whereas the 99mTc content in nuclear medicine should be as large as possible in order to be detected by the gamma camera, this is not necessary in SIMS because both 99mTc and diexcited 99Tc can be visualized in the same signal. We report here our tissue distribution study of Fig. 1 Diagram of SIMS microscope, the Cameca IMS 3F. 99mTc, combined with a ligand, hexamethylpropyleneaminoxime (HM-PAO).17 This lipophilic complex labels isolated sections (3 mm) were cut and laid on ultrapure gold holders leukocytes, for the exploration of infectious sites in humans by for SIMS analysis. The intensity of the primary ion beam, scintigraphy.18 which is generally about 1–5 mA in metallurgy, must be adapted to biological specimens. A low primary ion beam intensity is necessary (O2+, 50–200 nA for the strontium study and Cs+, Strontium 5–30 nA for bromine and technetium).Strontium-89, a beta emitter, is a calcium analog and produced in the (n,c) reactor. Its physical half-life is 50.5 d and it decays Mass spectra principally by b particle emission (Emax=1.44 MeV) to In SIMS the mass resolution is expressed by M/DM (M= yttrium-89 m. Strontium chloride is used in painful bone studallest diVerential mass that can metastases associated with prostatic cancer.19 Selective uptake be distinguished, i.e., the width of that peak to 20% of peak of 89Sr in osteoblastic areas surrounding primary osteogenic intensity).When the mass resolution is high, the sensitivity of and metastatic areas was first demonstrated by autoradiodetection decreases because mass resolution is obtained by graphy. Because the chemical contains a high proportion of opening or closing mass spectrometer slits. In practice, the stable strontium chloride, predominantly 88Sr, we could use mass resolution can be adjusted from 200 to 10 000.We studied this stable isotope to localise 89Sr in bone by SIMS microscopy. mass spectra at several resolutions. During sample sputtering some ionized atoms are emitted as polyatomic ions which are specific to the matrix but are also a source of artefactual Bromine signals. The number of nucleons of polyatomic ions is identical to that of a single element with the same nominal mass; Bromine-76 is a positron emitter with a physical decay of however, due to a mass defect, the polyatomic ions are heavier 16.2 h.In this section we describe our eVort to localize a (for mass<Ag) and do not follow the same trajectory in the bromine radioisotope in metabromobenzylguanidine (mBBG). spectrometer like single ions. The presence of these many We hypothesized that this novel carrier would be of great polyatomic species interferes with the correct interpretation of interest in metabolic therapy owing to its two highly energetic masses present at low intensity.So, when studying biological b+ emissions. Metaiodobenzylguanidine (mIBG) is a drug tissue, reference samples of the same nature, as described which has chemical analogies with the endogenous bioamines, below, should be used to determine the minimum mass reso- and is specifically taken up by many neural crest tumors, such lution required to eliminate polyatomic ion interferences.12,13 as neuroblastoma and pheochromocytoma.20,21 The use of Mass spectra were obtained on areas of 60 mm in diameter.iodine-131 as a means of radioactive bombardment has given rise to criticism because this beta emitter has a rather short path length in water (about 815 mm). Indeed, we have shown, Elemental imaging using an experimental animal model of a human neuroblastoma xenograph, that the mIBG is distributed in a highly non- Ion imaging is essential in biological applications because it is the only method capable of indicating the element microheter- uniform pattern within tumors.22,23 This may underirradiate those areas of the tumor which are not correctly targeted by ogeneity present in tissues and cells.In order to improve image acquisition with low primary ion-beam intensity and achieve mIBG. Possible alternative halogenations indicated that the physical properties of bromine-76 would be of diagnostic and a high S/N in high mass resolution conditions, and at low elemental concentration, a digital imaging system had to be therapeutic interest.Since SIMS microscopy can detect both bromine isotopes, we examined whether mBBG could be designed to allow image processing adapted to specific biological problems.14,15 In our system16 a sensitive SIT video camera specifically imaged within a mouse adrenal natural target of mIBG. (LHESA, Pontoise, France, SIT 4036) linked to an image 1106 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Protocol T echnetium Before localizing 99Tc in cell sections, two 99Tc samples were necessary to identify the technetium peak. The first sample (99TcO4-NH4+ standard) served to set up the mass spectrometer, and the second (99Tc reference sample) to identify the polyatomic ions. They were prepared according to the technique described previously.17 Briefly, the standard containing 18.5 MBq (71 mg of 99Tc) of 99TcO4-NH4+ was directly embedded in methacrylate resin.For the reference sample, human albumin macroaggregates (AMA) were labeled with pertechnetate solution (24 GBq of 99mTc corresponding to 1.63 mg of 99Tc+99mTc) and leukocytes with [99mTc-99Tc]HM-PAO (1.1 GBq of 99mTc corresponding to 75 ng of 99Tc+99mTc). After chemical fixation and dehydration in ethanol, specimens were embedded in methacrylate resin (Historesin, Pharmacia, Uppsala, Sweden). Negative controls were prepared in the same manner but the pertechnetate solution was replaced by NaCl (0.9%).Strontium In order to set up the 88Sr mass in the mass spectrometer, 20 mg of 88SrCl were prepared in the same manner as the 99Tc standard. Five patients were treated intravenously with 89Sr chloride 160 MBq (50 mg of 88Sr+0.147 mg of 89Sr) at the Institut Gustave Roussy (IGR Villejuif, France). The 89Sr chloride was obtained from Amersham International (CardiV, UK), as a donation. Bone biopsies were obtained from these patients after strontium treatment.Physical procedures (cryotechniques) were used for sample processing. Biopsy samples were cryofixed by ultra rapid immersion (5000 K s-1) in liquid Fig. 2 99Tc detection: matrix influence. The mass spectra obtained propane, subcooled (77 K) by liquid nitrogen, cryosubstituted from the 99TcO4-NH4+ sample allowed us to set the mass spectrometer in acetone (183 K) and then embedded in Lowicryl K11M at 98.906 u, M/DM>2000 (A). Mass spectra of 99Tc bound to albumin (Chemische Werke Lowi, Germany) at 213 K.The ideal nega- macroaggregates were obtained with variable mass resolutions: (B) low tive control would have been a bone biopsy without 88Sr. For mass resolution, M/DM=300, (C) high mass resolution, M/DM= ethical reasons we chose a piece of bone derived from surgery. 3500. No 99Tc signal was detected on AMA without 99Tc (D). sample was used to determine the mass resolution required to Bromine eliminate polyatomic ion interferences because the 99Tc reference has a matrix similar to that of the biological sample.Stable mBBG had to be synthesized for SIMS experiments, Fig. 2 (B–D) shows mass spectra obtained for a mass resolution because the weight of the injected amounts of the radioactive ranging from 300 to >2000 and the change in the specificity brominated analog (76Br-mBBG) was below the detection of the 99Tc- signal as a function of the mass resolution. 99Tc- sensitivity of the SIMS microscope.Chemical synthesis of could not be separated from polyatomic ions with a mass radioactive mBBG has been reported elsewhere, and can be resolution of 300 (Fig. 2B); 3500 (Fig. 2C) seemed to be more achieved directly or by isotopic exchange with the stable iodine than adequate to separate major neighboring polyatomic ions of mIBG.24 in this macroaggregate matrix. Furthermore, the Tc specific For SIMS analysis, 100 mg of stable mBBG was injected peak was not observed in the negative control sample (Fig. 2D).intraperitoneally into a mouse. At 24 h, the mouse was killed Fig. 3 shows the mass spectra obtained with sections from and the adrenals were removed and prepared for SIMS analythe leukocyte pellets. A mass resolution of about 3000 (Fig. 3A) sis. Since the problem was to determine the physical conditions was inadequate for leukocytes labeled with [99Tc]HM-PAO for SIMS detection, we used the chemical sample processing because additional peaks, which were not present in described for the technetium sample, although we know that [99Tc]AMA (Fig. 2C), appeared in this cell matrix. These new this preparation could not prevent drug diVusion from its peaks were clearly visible with a mass resolution of about 5000 specific sites of uptake. The mBBG biodistribution was delin- (Fig. 3B). This was further improved with a mass resolution eated by the detection of 79Br- or 81Br-, because bromine of about 6000 (Fig. 3C) and no new polyatomic ions were consists of two stable isotopes of similar abundance (50.57 and generated. Thus a mass resolution above 5000 guaranteed 49.43%, respectively). signal specificity for this cell matrix. RESULTS 99 Tc- imaging T echnetium Fig. 4 shows the role of mass resolution in imaging with a low (300) or high mass resolution (>2000). Only this latter mass Fig. 2A shows mass spectra obtained with a 99TcO4-NH4+ standard used in order to set up the 99Tc mass in the mass resolution gives the specific image of technetium (Fig. 4B) by elimination of the interference images of the polyatomic ions spectrometer under caesium bombardment, which appears more eYcient than that of an oxygen beam. The 99Tc reference 12C81H3-. Fig. 5 shows the mapping of a section of embedded Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1107Fig. 3 Mass spectra of 99Tc introduced by HM-PAO in leucocytes. At M/DM=3100 (A) the 99Tc and polyatomic ions constitute a composite peak.At M/DM=4675 (B) the 99Tc and polyatomic ion peaks are well separated and a higher mass resolution (M/DM=5610) Fig. 5 SIMS of 99Tc introduced by HM-PAO in leukocytes. is not necessary (C). Computerized image of technetium (green) in leukocyte sections. By image processing it is possible to superimpose this image on those of cytoplasmic sulfur ( blue) and nuclear phosphorus (red) to produce a composite one. With the technique, the nuclear technetium appears leukocytes labeled with [99Tc]HM-PAO.Fig. 5A shows the white (blue+red+green) and the cytoplasmic one appears turquoise 99Tc- image of the same image field. Superimposing the sulfur (blue+green). Sharp arrows show 99Tc in the parts of the cytoplasm and phosphorus images permitted the analysis of the cellular section without nuclear material. Large arrows show 99Tc in the distribution of 99Tc (Fig. 5B). Technetium labeling varied from nuclear section. Image field: 60 mm.one cell to another: some cells were heavily labeled, while others were not; 99Tc was present in all labeled cells in both Bromine the nucleus and the cytoplasm, as clearly shown in the parts of the section of the cytoplasm devoid of nuclear material. MBBG was detected easily by imaging of the bromine-79 negative secondary ions. This demonstrated that mBBG is eVectively taken up by the adrenals. In a first series of Strontium experiments, we worked with a low mass resolution to enhance detection sensitivity.In these conditions, we obtained a first Fig. 6 shows mass spectra obtained with a 88SrCl standard (A) and sections of bone biopsy with 88Sr (B) and without 88Sr (C) image (Fig. 9A) made up of two types of distribution: 79Br and polyatomic ions. The corresponding mass spectrum (Fig. 8A) sections. A mass resolution of 7000 separated the specific 88Sr- peak from polyatomic ions. Fig. 7 shows the superimposition indicated that the mass resolution was of the order of 560.In a second series of experiments, we worked at a high mass of two elemental images: 88Sr- in red and 40Ca- in green. The calcium image displays the bone structure. Strontium has the resolution, which displayed (Fig. 9B) only the second diVuse distribution. The associated spectrum (Fig. 8B) showed three same distribution as calcium but some areas showed a higher density. peaks, corresponding to 79Br- (78.9184 u), the intense polya- Fig. 4 Mass resolution and SIMS imaging specificity of 99Tc bound to albumin macroaggregates. Imaging was performed with variable mass resolution: (A) image obtained with low mass resolution (M/DM=300) and corresponding to a combination of 99Tc and 99(12C81H3). Images obtained with high mass resolution (M/DM>2000) which allows the 99Tc signal (B) to be separated from the 99(12C81H3) cluster ion (C). Image field: 60 mm. 1108 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 8 Mass spectra of 79Br-mBBG mapping in mouse adrenal were obtained with variable mass resolutions: low mass resolution M/DM= 560 (A), high mass resolution M/DM=4500 (B). Fig. 6 Mass spectra of 88Sr. The mass spectra obtained from the 88SrCl sample allowed us to set the mass spectrometer at 87.9056 u (A). Mass spectra of 88Sr included into bone were separated from polyatomic ions with a mass resolution of 7000 (B). No 88Sr signal was detected on a negative control (C).Fig. 7 88Sr imaging in human bone biopsy. Computerized images of strontium (red) and calcium (green) are superimposed. The distribution of strontium in bone is not equivalent to that of calcium. Image field: 60 mm. tomic ions of phosphorus in 31P16O3- (78.9585 u) and, probably, 12C431P- (78.9738 u). The mass resolution of the image was found to be over 4500. Fig. 9 79Br-mBBG mapping in mice adrenal. Imaging was performed with variable mass resolution: (A) image obtained with low mass DISCUSSION resolution and corresponding to 79Br and 31P16O3-; (B) images obtained with high mass resolution which allows the 79Br signal to be A knowledge of the distribution of radionuclides in tissue is separated from the cluster ion.Image field: 60 mm. essential for dosimetric studies, especially in metabolic radiotherapy. Unfortunately, the calculation of the absorbed dose is erroneously based on the assumption that the radioligand One of the main problems in SIMS microscopy is signal specificity.The complexity of the organic matrix leads to the is homogeneously distributed throughout the target. Since the absorbed dose must be calculated at tissue level, knowledge of emission of a large number of cluster ions. The large number of combinations of carbon, hydrogen, nitrogen, oxygen and the biodistribution of the radioligand is essential. Although numerous chemical elements have been detected phosphorus atoms possible in hydrocarbons gives rise to numerous mass interferences in biological matrix analysis. On by SIMS microscopy in biological specimens, technetium and strontium in bone had never been studied with this technique.the other hand, some of these polyatomic species are very helpful, because their imaging indicates the histological struc- We succeeded in detecting and localizing these elements in cell sections that arise from this biological matrix by using high ture of the tissue (e.g., the polyatomic 12C14N- at mass 26 because proteins are enriched with this species25) and they mass resolution images.Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1109facilitate the detection of some isotopes, such as 14C at mass the method of choice for the evaluation of absorbed dose despite its limited lateral resolution, because it is able to 28 (14C14N-)25 or 15N at mass 27 (12C15N-).26 However, a pinpoint and evaluate local element concentrations.22,28 major drawback is that most of these cluster ions are the However, the development of a SIMS sub-micron probe, the source of artefactual signals, especially for atomic mass nanoSIMS,29 should improve the lateral resolution, to 50 nm, elements higher than 12, a signal which can interfere with the selectivity and sensitivity.These new features will greatly selected element. There is no problem when the element being facilitate the detection of radionuclides used in metabolic examined is present at high concentration, as in induced or radiotherapy.spontaneous bio-accumulation, because the imaging study can then be performed with a low mass resolution (M/DM=300). The authors thank Ms Ingrid Ku�chenthal for preparing the So in physiopathological studies the images of low concenmanuscript. trations of physiological elements or radioligand were erroneous because they require high mass resolution con- REFERENCES ditions. When working with a mass resolution >2000, the mass of iodine (126.907) can be clearly distinguished from that 1 Castaing, R., and Slodzian, G., J.Microsc., 1962, 395. of a phosphorus cluster ion (126.980). The distribution of the 2 Galle, P., J. Nucl. Med., 1982, 23, 52. latter, which is mainly associated with the cell nucleus and 3 Burns, M. S., Ultramicroscopy, 1988, 24, 269. 4 Chandra, S., and Morrisson, G.,Methods Enzymol., 1988, 158, 157. cytoplasmic phosphorylated proteins, is predominant when the 5 Fragu, P., Clerc, J., Brianc�on, C., Fourre�, C., Jeusset, J., and cellular iodine concentration is low, as for example in goitrous Halpern, S., Micron, 1994, 25, 361.thyroid tissue.7 In another example, a mass resolution of 2000 6 Galle, P., Ann. Phys. Biol. Med., 1970, 42, 83. is adequate for 14C-/13C1H-/12C1H2- separation,26 whereas 7 Fragu, P., Brianc�on, C., Noe�l, M., and Halpern, S., J. Clin. a mass resolution>6000 is necessary for the 12C15N-/13C14N- Endocrino 1989, 69, 304.separation.10 This high mass resolution is indispensable for the 8 Brianc�on, C., Halpern, S., Telenczak, P., and Fragu, P., Endocrinology, 1990, 127, 1502. study of the distribution of radioligands in nuclear medicine 9 Stelly, N., Halpern, S., Nicolas, G., Fragu, P., and Adoutte, A., because these elements are used at a small mass quantity. J. Cell Sci., 1995, 108, 1895. The theoretical mass resolution for the specific signal detec- 10 Grignon, N., Halpern, S., Gojon, A., and Fragu, P., Biol.Cell., tion of 79Br–mBBG is M/DM>1968; this avoids any inter- 1992, 74, 143. ference with the polyatomic 31P16O3-. However, since this ion 11 Hallegeot, P., Girod, C., and Levi-Setti, R., Scan. Microsc., 1990, is strongly emissive in phosphorus-rich biological tissues, a 4, 605. 12 Burns, M. S., Anal. Chem., 1981, 53, 2149. practical working mass resolution of over 4000 appears advis- 13 Truchet, M., The`se de doctorat d’e�tat. Universite� Paris VI, 1982.able. This clearly aVects detection sensitivity by a factor of 5 14 Bryan, S. R., Woodward, W. S., GriYs, D. P., and Linton, R. W., to 10. The detection of 81Br–mBBG is a good alternative, J. Microsc., 1985, 138, 15. because at this mass level (80.9164 u), the secondary 15 Cavellier, J. F., Berry, J. P., Escaig, F., and Boumati, P., ion 12C314N31P- (80.9768 u) is rather easy to eliminate J. Microsc., 1989, 154, 31. 16 Olivo, J. C., Kahn, E., Halpern, S., Brianc�on, C., Fragu, P., and (M/DM>1338) and its overall presence in biological samples Di Paola, R., J. Microsc., 1989, 56, 105.is of moderate intensity. Anyway, the possibility of a coupled 17 Fourre�, C., Halpern, S., Jeusset, J., Clerc, J., and Fragu, P., SIMS detection made either on bromine-79 or on bromine-81 J. Nucl. Med., 1992, 33, 2162. always oVers a good internal control for detection specificity. 18 Peters, A. H., Osman, S., Henderson, B. L., Kelly, J. D., Dampure SIMS microscopy demonstrated the heterogeneity of the inter- H.J., Hawker, R. J., Hodgson H. J., Neirinckx, R. D., and and intracellular distribution of 99Tc in leukocytes. A mass Lavender, J. P., L ancet, 1986, 25, 46. 19 Pecher, C., Univ. Calif. Publ. Pharmacology, 1942, 11, 117. resolution of about 2000 was suYcient to avoid major polya- 20 Shapiro, B., J. Nucl. Biol. Med., 1991, 35, 357. tomic ion interferences with the 99Tc reference sample. This 21 Smets, L. A., and Rutgers, M., J.Nucl. Biol. Med., 1991, 35, 191. was not the case for leukocytes labeled with [99Tc]HM-PAO. 22 Telenczak, P., Ricard, M., Halpern, S., and Fragu, P., in Sims VII, A mass resolution of about 5000 was needed to circumvent ed. Benninghoven, A., Wiley, Chichester, 1990. these interferences because the chemical composition of the 23 Clerc, J., Halpern, S., Fourre�, C., Omri, B., Jeusset, J., and cell is more complex than that of albumin aggregates. Fragu, P., J. Nucl. Med., 1993, 34, 1565. 24 Loc’H, Ch., Mardon, K., Valette, H., Brutesco, C., Merlet, P., Furthermore, mass spectra obtained from leukocytes labeled Syrota, A., and Mazie`re, B., Nucl. Med. Biol., 1994, 21(1), 49. with HM-PAO showed polyatomic ions which were not present 25 Quettier, A., and Quintana, C., C. R. Acad. Sci., Se�r. D, 1979, in cells incubated with NaCl. The new polyatomic ions intro- 289, 433. duced by the HM-PAO solution were therefore exogenous in 26 Hindie, E., Hallegeot, P., Chabala, J. M., Thorne, N. A., origin. The nuclear and cytoplasmic distribution of 99Tc in Coulomb, B., Levi-Setti, R., and Galle, P., Scanning Microsc., 1988, 2, 1821. leukocytes provided by SIMS microscopy verified the exact 27 Fourre�, C., Pe�tiet, A., and Colas-Linhart, N., Cell. Mol. Biol., position of Tc in the cell. Our data show clearly that SIMS 1996, 42(3), 385. microscopy can pinpoint the distribution of the 99Tc introduced 28 Jeusset, J., Stelly, N., Brianc�on, C., Halpern, S., Roshani, M., and by the radioligand in tissue and complement the images Fragu, P., J. Microsc., 1995, 179, 314. obtained by microautoradiography.27 In the same manner, the 29 Slodzian, G., Daigne, B., Girard, F., Boust, F., and Hillion, F., high mass resolution of the IMS 3F revealed that strontium C. R. Acad. Sci., Se�r. III, 1990, 311, 57. follows the distribution pattern of calcium, but it is also present in areas devoid of calcium which could correspond to metast- Paper 7/00777I asic osteoblastic bone. Received February 3, 1997 Accepted June 5, 1997 In conclusion, our data indicate that SIMS microscopy is 1110 Journal of Analytical Atomic Spectrometry,

ISSN:0267-9477    

DOI:10.1039/a700777i

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Bismuth in Serum and Urine by Direct Injection Nebulization Inductively Coupled Plasma Mass Spectrometry HONGYAN LIa , BERNIE M. KEOHANEb , HONGZHE SUNa AND PETER J. SADLER*a aDepartment of Chemistry, University of Edinburgh, King’s Buildings, WestMains Road, Edinburgh, EH9 3JJ UK. E-mail: p.j.sadler@ed.ac.uk bDepartment of Chemistry, Birkbeck College, University of L ondon, Gordon House and Christopher Ingold L aboratories, 29 Gordon Square, L ondon, WC1H 0PP UK A sensitive method for the determination of Bi by direct Hg,15 Bi12,16,17 and Au18, owing to the large dead volume of the sample introduction system.injection nebulization inductively coupled plasma mass spectrometry (DIN-ICP-MS) in biological fluids is described. A microconcentric nebulizer, termed a direct injection nebulizer (DIN), which is placed inside the injector tube of the ICP The detection limit for Bi is 9.7 ng l-1 (ca. 46 pmol dm-3 ) with DIN compared with 17 ng l-1 (ca. 81 pmol dm-3) with torch, and eliminates the need for a spray chamber, was first described by LaFreniere and co-workers.19–21 It allows faster conventional pneumatic nebulization (PN). The absolute amount detectable by DIN-ICP-MS is about two orders of sample introduction and faster wash-out times, improving studies of memory-prone elements such as Hg.22 Reduced peak magnitude lower for DIN compared with PN (0.019 and 1.70 pg, respectively). Sample wash-out times were greatly broadening during flow injection has been observed due to the low dead volume of the DIN.Such a nebulizer has been reduced using DIN owing to minimization of the memory eVect for Bi. Using Tl as an internal standard, good coupled with ICP-AES,23 and with microwave-induced plasma mass spectrometry (MIP-MS),24 and Wiederin et al. have calibrations were obtained for Bi standards in 0.14 mol dm-3 nitric acid, serum and urine with comparable linearity between developed an improved DIN coupled with ICP-MS.25 DINICP- MS has previously been successfully used for the determi- the matrices, and these were used for the determination of Bi in serum and urine samples from animals dosed with the nation of Pt and Au in metalloproteins, injecting 20 ml of sample to minimize blockage of the nebulizer tip and sample antiulcer compound RBC (ranitidine bismuth citrate).This method is potentially useful in studies of the metabolism and cone.26 In the present paper, results are reported for the optimization biodistribution of Bi-containing drugs. of DIN-ICP-MS for the determination of Bi in serum and Keywords: Bismuth; direct injection nebulization; inductively urine matrices.The determination of Bi in rat serum and urine coupled plasma mass spectrometry; serum; urine samples from rats orally-dosed with RBC, a new antiulcer compound,3,4 was also investigated. Bismuth has been associated with medicine for more than 200 years. Clinical interest in the application of bismuth com- EXPERIMENTAL pounds has increased with the discovery of Helicobacter pylori Instrumentation and the usefulness of bismuth compounds in the treatment of infection caused by this microorganism. Bismuth subsalicylate A VG PlasmaQuad PQ2 ICP-MS instrument (VG Elemental, Winsford, Cheshire, UK) was used coupled with a CETAC (BSS; Pepto-Bismol) and colloidal bismuth subcitrate (CBS; De-Nol) have also been found to be eVective for the treatment Microneb 2000 direct injection nebulizer (CETAC Technologies, Omaha, NE, USA), which consisted of a micro- of peptic ulcers and to have a bactericidal eVect on Helicobacter pylori.1,2 Recently, a new antiulcer compound, ranitidine concentric nebulizer constructed inside the injector tube of the torch.A detailed description of the DIN is available else- bismuth citrate (RBC, developed by GlaxoWellcome),3,4 which combines the antisecretory action of ranitidine with the where.25 The carrier liquid was transported through a capillary made from fused silica (60 mm id×0.4 m long) directly into mucosal protectant and the bactericidal properties of bismuth, has been granted a product licence in the UK.Despite the the plasma, thereby avoiding the need for a spray chamber. Samples were injected into the carrier stream via an injection medicinal interest, the metabolism of bismuth drugs is still not well understood. This is partially due to the lack of suitable valve constructed from polyether ether ketone (PEEK) material to provide a metal-free system.Transport of liquid required techniques to detect bismuth at trace or ultra-trace levels (sub-mg l-1). a backpressure of 17×105 Pa provided by a gas displacement pump (GDP). Although ETAAS has been used to determine bismuth in serum, blood5–7 and tissues,8 this method is hampered by the The DIN-ICP-MS system was optimised for Bi sensitivity on a daily basis using a 500 ml sample loop which gave a occurrence of matrix eVects and the need for pretreatment of samples prior to analysis.9 Since 1983, ICP-MS has been steady-state signal for approximately 5 min.The instrument operating conditions are listed in Table 1. The nickel cones commercially available for multi-element determinations at low and sub-mg l-1 levels in biological materials.10–12 Total showed no signs of erosion when water was nebulized through the DIN (as other workers have found25) so they were used dissolved solids must be less than 0.2% to avoid blockage of the nebulizer tip and sampling cone.This is usually achieved for the present work. A 20 ml loop was used for all experiments to minimize the amount of protein injected. by sample dilution with 0.14 mol dm-3 nitric acid12,13 or alternative diluents.14 Long rinse times have been observed, The timings of the load and injection sequences for DIN were adjusted to minimize peak tailing. A 10 s injection time particularly for analytes with known memory eVects such as Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1111–1114) 1111Table 1 DIN-ICP-MS operating conditions Control rat serum was labelled as S(C) and serum samples from two rats dosed with RBC (200 mg kg-1, 1 h) as S(Bi1) VG PlasmaQuad PQ2 ICP-MS and S(Bi2). Similar procedures were used for rat urine samples, ICP torch CETAC B-23 and were labelled as U(C), control and U(Bi) and UF(Bi) for Argon flow rates— fed and fasted rats, respectively, dosed with RBC.These *Outer gas 14 l min-1 samples were diluted with 0.14 mol dm-3 nitric acid before *Intermediate gas 0.7 l min-1 analysis. *Aerosol carrier gas 0.15 l min-1 GDP-DIN parameters— *DIN nebulizer gas (argon) 5.4×105 Pa Reagents pressure High-purity water was obtained with an Elga UHQ water *GDP pressure 17×105 Pa *Liquid flow rate 100 ml min-1 purification system (resistivity of 18 MV cm). The Bi and Tl standard solutions (10 000 mg ml-1 in 5% nitric acid) were Plasma forward power 1350 W supplied by Aldrich (Gillingham, Dorset, UK). Concentrated Reflected power <10 W *Sampling position 10 mm from front of load nitric acid (67%, Suprapur grade) was from Merck coil (Lutterworth, Leicestershire, UK).The Bi and Tl standard ICP-MS interface— solutions were freshly prepared prior to analysis with 0.14 mol Sampler 1.00 mm Nicone dm-3 nitric acid as diluent. Skimmer 0.75 mm Nicone Analyser pressure 2×10-4 Pa RESULTS * These values were adjusted daily to maximize ion signal intensity.Detection Limits A comparison was made between the detection limits for Bi in 0.14 mol dm-3 nitric acid using DIN and conventional pneu- was adequate for all samples as the steady signal was reached matic nebulization (PN). The detection limit, calculated as after 3 s and remained for 10 s before tailing oV. The load three times the standard deviation of the signal produced from time, and also the rinse time, was set at 60 s for Bi aqueous ten replicates of a 0.14 mol dm-3 nitric acid blank solution, standards to allow the signal to drop to background levels was determined to be 9.7 ng l-1 (0.019 pg) for DIN and 17 ng prior to the next sample injection.Biological samples required l-1 (1.70 pg) using conventional PN. The LODs for concen- rinse times of 120 s to reach background levels, probably tration are comparable with PN but the absolute amounts are because of memory eVects from RBC or adsorption of proteins nearly two orders of magnitude lower owing to the small and salts in the sample introduction system. A summary of volume injected (20 ml compared with 1 ml for PN).the instrumental and operating parameters is given in Table 1. Memory EVects Data Acquisition Three diVerent Bi standard solutions (10, 50 and 100 mg l-1 in Time resolved analysis (TRA) software (VG Elemental) was 0.14 mol dm-3 nitric acid) were introduced into the plasma used for continuous element monitoring during analysis. Shortby either PN or DIN with flow rates of 1 ml min-1 and 100 ml term stability tests with DIN-ICP-MS were conducted in the min-1 for PN and DIN, respectively, and 20 ml of the standard peak-jumping mode using a 160 ms dwell time, three measurewere injected into the plasma, to compare rinse times for Bi. ments per peak and 30 s acquisition time, which gave RSDs The profiles produced from both modes of sample introduction of less than 1% for ten replicates of a 10 mg l-1 multiinto the ICP-MS instrument are shown in Fig. 1. In Fig. 1(a), element standard. the standard was introduced after 50 s and the steady-state Data were acquired in the peak-jumping mode and for each signal was reached after a further 60 s. Long wash-out times scan, data were stored as a unique time slice. Both Bi (m/z were necessary, particularly for the 100 mg l-1 standard which 209) and Tl as the internal standard (m/z 205) were measured using a 0.5 s time slice, 10 ms dwell time and 3 measurements per peak.A 20 ml loop was used for all sample injections in order to minimize amounts of proteins and salts injected and avoid blockage of the sampling cones and nebulizer tip. The timings of the load and injection sequences for DIN were adjusted to minimize peak tailing. The injection time was set to 10 s, and the load time was 60 s for aqueous Bi standards and 120 s for biological samples, with three injections per sample.Sample Collection and Preparation Rat blood samples (supplied by GlaxoWellcome) were taken with a poly(propylene) intravenous catheter mounted on a metallic needle and collected in higher-purity quartz tubes to avoid contamination. Samples of 20 ml of blood were collected and no anticoagulant was added. Serum was separated by centrifugation for 30 min at 3500 rev min-1 and 4 °C and decanted into polyethylene screw-cap containers. Afterwards, the serum samples were stored at #-20 °C in a freezer until required for analysis. Before analysis, the samples were Fig. 1 (a) Bi wash-out times for PN at three diVerent concendefrosted, and then a known volume was transferred into a tration levels, and (b) Bi wash-out times for DIN at three diVerent concentration levels. polyethylene calibrated flask using a polyethylene pipette. 1112 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12took 300 s to reach background levels.Introduction and wash- Bismuth in Urine and Serum Samples out times were greatly reduced with DIN as shown in Fig. 1(b). Samples of serum and urine were obtained from rats which A steady-state signal was reached after 3 s and the wash-out had received oral doses of RBC of 200 mg kg-1 body mass, time for all Bi standards was less than 30 s. which is an elevated, non-therapeutic dose. The samples were diluted with 0.14 mol dm-3 nitric acid to achieve a Bi concentration within that of the calibration curve.The Bi concen- Standard Calibration Curves trations obtained for diVerent rat serum and urine samples DIN-ICP-MS responses for diVerent concentrations of Bi were before and after dosage of RBC are given in Table 3. measured and the calibrations for Bi in 0.14 mol dm-3 nitric Background levels of Bi from the control samples are 0.54 mg acid, serum and urine (dilution factor five) were prepared. The l-1 (0.0026 mM) for serum and about 0.56 mg l-1 (0.0027 mM) correlation coeYcient for the calibration in 0.14 mol dm-3 for urine.nitric acid was 0.9998, and the RSDs ranged from 1.6% at 20 mg l-1 to 0.2% at 100 mg l-1. The slopes for the three calibrations varied, with a marked decrease in sensitivity for DISCUSSION the urine samples which suggested that the Bi signal was Recently, ICP-MS has become an important technique for the suppressed because of matrix eVects. Serum and urine samples determination of total elemental concentrations in body fluids were therefore spiked with Tl (10 mg l-1) and varying concento monitor metabolism in vivo and study the bioavailability of trations of Bi (20, 40 and 80 mg l-1) and the Bi concentrations elements.27 were determined by DIN-ICP-MS. With an internal standard The determination of Bi in biological samples using present, the measured concentrations are close to the expected ICP-MS and conventional nebulization has previously been values for both serum and urine samples (Table 2).The calireported. 12,16 Owing to an extensive memory eVect for Bi with bration curves for Bi in 0.14 mol dm-3 nitric acid, serum and PN, a long rinse time is needed and it normally takes at least urine are shown in Fig. 2. 10 min before an acceptable background level is obtained after the determination of a Bi standard solution with a concentration 100 mg l-1.16 In the present paper, for the first time, Table 2 Accuracy of Bi determinations for serum and urine samples the determination of Bi in biological samples by ICP-MS using with addition of 10 mg l-1 Tl as an internal standard for three DIN is reported.The important improvement with DIN is replicates (mean±s) that the memory eVect of Bi is dramatically reduced since the Bi concentration/mg l-1 DIN fits directly into the torch, replacing the injection tube Recovery and avoiding the use of a spray chamber and aerosol transfer Sample Expected Measured (%) tubing.With DIN, the rinse time was 60 s for Bi standards Urine 20 18.25±0.99 91.3 and 120 s for biological samples. This allowed the determi- 40 35.54±0.76 88.9 nation of Bi in these samples to be made more rapidly and 80 71.50±1.46 89.4 accurately. Serum 20 17.28±0.98 86.4 For PN, sample volumes of 2–3 ml are required, but with 40 36.09±1.41 90.2 DIN this was reduced to 20 ml without loss of sensitivity.15 80 72.89±1.32 91.1 The reduction of sample size decreased matrix eVects and also reduced the possibility of blockages of the sampling cone and nebulizer tip.The sampling eYciency of DIN is much higher than for conventional sample introduction, since the nebulizer tip is placed only a few millimeters from the base of the plasma. The aerosol has a small cross-sectional area when it enters the plasma, which ensures that nearly all of the aerosol is injected into the axial channel.22,25 After elevated, non-therapeutic, oral dosage with the Bi antiulcer compound RBC, high levels of Bi were observed in both urine and serum, with significantly higher levels in urine. It has been reported that an increase of Bi in human whole blood is observed after intake of Bi antiulcer drugs (CBS).28,29 Thiol-containing ligands such as glutathione bind strongly to Bi and are probably involved in its transport, for example through the membranes of red blood cells.30 Recently it has been found that Bi can bind strongly to human serum transferrin, an iron transport protein (80 KDa),31 although the data given in the present paper suggest that the Bi levels are not high enough to saturate transferrin (#35 mM in serum)32 even Fig. 2 Plots of concentration versus intensity ratio for Bi: # in after elevated doses of Bi. However, metal metabolism can 0.14 mol dm-3 nitric acid; D in rat urine; and % in rat serum with Tl added as an internal standard. vary between diVerent animal species. Table 3 Concentrations of Bi in urine and serum samples before and after oral dosage of the rat with RBC Serum* Urine* Bi concentration S(C) S(Bi1) S(Bi2) U(C1) U(C2) U(C3) U(Bi1) U(Bi2) UF(Bi) mg l-1 0.54 3532 2340 0.42 0.63 0.56 11411 17660 20064 mM 0.0026 16.9 11.2 0.0021 0.003 0.0027 54.6 84.5 96.0 *Sample labels: C control, Bi after dosing, S serum and U urine; numbers indicate samples from diVerent animals (see under Experimental). Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 111310 McLaren, J. W., Beauchemin, D., and Berman, S.S., Anal. Chem., ICP-MS has previously been used for on-line monitoring 1987, 59, 610. studies of trace element species in protein matrices separated 11 Gelinas, Y., Youla, M., Beliveau, R., Schmit, J. P., and Ferraris, by HPLC.33,34 It is possible to couple DIN-ICP-MS with J., Anal. Chim. Acta, 1992, 269, 115. microcolumn chromatography which operates at the same 12 Vanhoe, H., Dams, R., and Versieck, J., J. Anal. At. Spectrom., flow rate as DIN (100 ml min-1).This approach is likely 1994, 9, 23. 13 Mulligan, K. J., Davidson, T. M., and Caruso, J. A., J. Anal. At. to be useful in studies of the distribution of Bi and other Spectrom., 1990, 5, 301. metallo-drugs in biological samples. 14 Delves, H. T., and Campbell, M. J., J. Anal. At. Spectrom., 1988, 3, 343. 15 Powell, M. J., Quan, E. S. K., Boomer, D. W., and Wiederin, CONCLUSION D. R., Anal. Chem., 1992, 64, 2253. 16 Vanhoe, H., Versieck, J., Vanballenberghe, L., and Dams, R., Clin.It has been shown that DIN-ICP-MS is a sensitive method Chim. Acta, 1993, 219, 79. for the determination of Bi in biological fluids with a detection 17 Williams, C. A., Abou-Shakra, F. R., and Ward, N. I., Analyst, limit for absolute amounts about two orders of magnitude 1995, 120, 341. lower than conventional PN. The use of DIN greatly reduced 18 Mauras, Y., Premel-Cabic, S., and Allian, P., Clin. Chim. Acta, memory eVects. Bismuth antiulcer drugs are widely used and 1993, 218, 201. 19 LaFreniere, K. E., Fassel, V. A., and Eckels, D. E., Anal. Chem., it has been demonstrated that Bi in biofluids can readily be 1987, 59, 879. determined using DIN-ICP-MS. Since the required flow rates 20 LaFreniere, K. E., Rice, G. W., and Fassel, V. A., Anal. Chem., are compatible with microcolumn chromatography, it should 1984, 56, 289. be possible to extend this work to speciation studies. It is 21 LaFreniere, K. E., Rice, G. W., and Fassel, V. A., Spectrochim.likely that such work will considerably improve knowledge of Acta, Part B, 1985, 40, 1495. 22 Powell, M. J., Quan, E. S. K., Boomer, D. W., and Wiederin, the biochemical pharmacology of bismuth. D. R., Anal. Chem., 1992, 64, 2253. 23 Gjerde, D. T., Wiederin, D. R., Smith, F. G., and Mattson, B. M., The authors thank GlaxoWellcome and the EPSRC for their J. Chromatogr., 1993, 640, 73. support of this work, the Committee of Vice-Chancellors and 24 Giglio, J. J., Wang, J., and Caruso, J.A., Appl. Spectrosc., 1995, Principals for an Overseas Research Student Award (to H.L.), 49, 314. 25 Wiederin, D. R., Smith, F. G., and Houk, R. S., Anal. Chem., Drs. W. Jenner and M. Dunne (GlaxoWellcome) for supplying 1991, 63, 219. urine and serum samples, and Drs. G. Klinkert and C. Donaghy 26 Christodoulou, J., Kashani, M., Keohane, B. M., and Sadler, P. J., for helpful comments. J. Anal. At. Spectrom., 1996, 11, 1031. 27 Taylor, A., Branch, S., Crews, H.M., Halls, D. J., and White, M., J. Anal. At. Spectrom., 1996, 11, 103R. REFERENCES 28 Hespe, W., Stall, H. J. M., and Hall, D. W. R., L ancet, 1988, 2, 1258. 29 Froomes, P. R. A., Wan, A. T., Keech, A. C., McNeil, J. J., and 1 Marshall, B. J., Am. J. Gastroenterol., 1991, 86, 16. McLean, A. J., Eur. J. Clin. Pharmacol., 1989, 37, 533. 2 WagstaV, A. J., Benfield, P., and Monk, J. P., Drugs, 1988, 36, 132. 30 Sadler, P. J., Sun, H., and Li, H., Chem. Eur. J., 1996, 2, 701. 3 McColm, A. 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Spectrosc., 1991, 12, 77. Accepted April 9, 1997 1114 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a700269f

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Comparison of Inductively Coupled Plasma Atomic Emission Spectrometry and Inductively Coupled Plasma Mass Spectrometry With Quantitative Neutron Capture Radiography for the Determination of Boron in Biological Samples From Cancer Therapy† THOMAS U. PROBST*a , NATALIA G. BERRYMANa , PETER LEMMENb , LOTHAR WEISSFLOCHc , THOMAS AUBERGERd , DETLEV GABELe , JO� RGEN CARLSSONf AND BO� RJE LARSSONg aInstitute of Radiochemistry, T echnical University of Munich, Walther-Meißner-Str. 3, D-85747 Garching, Germany bInstitute of Organic Chemistry and Biochemistry, T echnical University of Munich, L ichtenbergstr. 4, D-85747 Garching, Germany cClinic and Polyclinic for Radiotherapy and Radiological Oncology, Klinikum Rechts der Isar, T echnical University of Munich, Ismaninger Str. 22, D-81675 Mu�nchen, Germany dClinic for Radiotherapy, L eopold-Franzens University of Innsbruck, Anich Str. 35, A-6020 Innsbruck, Austria eDepartment of Chemistry, University of Bremen, P.O. Box 330440, D-28334 Bremen, Germany fDivision of Physical Biology, Department of Radiation Sciences, Uppsala University, P.O.Box 535, S-75121, Uppsala, Sweden gPaul-Scherrer Institute, CH-5232 V illigen, Switzerland A boron-containing compound was administered to tumor- metabolism of administered boron compounds can be deduced from the course of the time-dependent boron concentrations bearing mice and the time-dependent concentration was determined by ICP-AES, ICP-MS and quantitative neutron in tissues and individual organs.Generally, quantitative boron determinations in biological samples can be carried out by capture radiography (QNCR). The atomic spectrometric procedures were optimized to minimize analyte losses during flame and electrothermal (ET) atomic absorption spectrometry (AAS), spectrophotometry, potentiometric methods, titrimetry, microwave digestion. Non-spectroscopic interferences were investigated in detail and appropriate internal standards were inductively coupled plasma (ICP) and direct current plasma (DCP) atomic emission spectrometry (AES), ICP mass spec- established for ICP-MS.The best detection limits achieved for boron were 30 ng ml-1 by ICP-AES and 0.3 ng ml-1 by trometry (ICP-MS), prompt gamma neutron activation analysis (PGNAA), neutron activation mass spectrometry (NA-MS) ICP-MS. The respective determination limits in the digested and diluted tissue samples were 0.4 mg ml-1 and 3 ng ml-1. and thermal ionization mass spectrometry (TIMS).25,26 Boron determination by widely used methods such as AAS and The results of the spectrometric determinations were confirmed by QNCR of whole-body cryosections of six mice.A brief ETAAS25,27 or by electrothermal vaporization (ETV)-ICPAES28 –31 and ETV-ICP-MS32,33 is generally impaired by boron description of the metabolism of the boron compound is given. carbide formation in the AAS flame or in the graphite furnace Keywords: Inductively coupled plasma mass spectrometry; which reduces the repeatability of the results.Therefore, inductively coupled plasma atomic emission spectrometry; rapid and sensitive determination of boron in biological quantitative neutron capture radiography; boron neutron samples is often carried out by ICP-AES,11,14,25,31,34–39 capture therapy; cancer therapy; microwave digestion; rac- ICP-MS11,14,25,26,31,40–43 and laser ablation-ICP-MS 1-(9-o-carboranyl)nonyl-2-methylglycero-3-phosphocholine (LA-ICP-MS).44,45 The boron distribution in solid samples can be visulized by imaging techniques such as ion microscopy imaging by secondary ion mass spectrometry (SIMS), electron At present, boron neutron capture therapy (BNCT) is a major field for investigations1–16 and for clinical studies.17–22 In this energy loss spectroscopy (EELS), 11B-nuclear magnetic resonance (NMR), 1H-observed and 10B-edited spin-echo NMR binary cancer therapy, a non-cytotoxic compound is administered which is selectively enriched in the tumor.18,23 During diVerence spectroscopy, magnetic resonance imaging (MRI), prompt gamma spectroscopy, positron emission tomography subsequent irradiation with thermal neutrons, 10B captures a neutron and emits high energy 42 He and 73 Li particles (Fig. 1).24 of 18F-labelled substances (PET) and quantitative neutron capture radiography (QNCR).25 High resolution images can The total kinetic energy of the emitted particles (2.79 MeV) is rapidly dissipated within a radius of 10 mm.Since this average be obtained by in vivo investigations of human and animal organs by NMR and MRI techniques.13,25,46,47 pathlength is approximately the size of a cell, it is possible to destroy a tumor selectively without aVecting the surrounding In in vitro experiments, phospho-ether lipids have been shown to accumulate selectively in tumors.14,48–50 To ensure a tissues. In theory, a single incident of neutron capture is suYcient to destroy a cancer cell.In practice, a concentration of 30 mg of 10B per gram of tumor, which is equivalent to 108–109 atoms of 10B per cell,11 is required to destroy the tumor. Information on the biodistribution, accumulation and † Presented in part at the 5th International Conference on Plasma Fig. 1 10B neutron capture reaction (nuclear cross section: 3837 barn; Source Mass Spectrometry, University of Durham, 16–20 September, 1996. natural abundance of 10B: 19.8%).Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1115–1122) 1115high boron content per molecule an ether lipid substituted Microwave Digestion with a closo-carborane was chosen for our in vivo experiments. All samples of the mouse tissues were stored at -10 °C and The compound rac-1-(9-o-carboranyl)nonyl-2-methylglyceroat 4 °C before analysis, respectively. The samples were thawed 3-phosphocholine (B-Et-11-OMe) (Fig. 2) was admistered to at room temperature, then 60–800 mg were weighed into pre- C3H mice with an AT17 mammary carcinoma and to BALB/c cleaned Teflon vessels of 150 ml volume.Large samples were mice with an OTS-64 osteosarcoma.11,14,51 Groups of five divided into two parts and these were analysed separately to animals were sacrificed at fixed intervals between 0.5 and 48 h check the repeatability of the entire procedure. After addition after intraperitonal (i.p.) administration of 50 mg of of 1 ml of de-ionized water, 3 ml of 65% v/v nitric acid and B-Et-11-OMe per kg mouse in a 0.9% NaCl solution.14,52,53 1 ml of 85% m/m phosphoric acid54 (Aldrich, ACS reagent), The blood, blood plasma, tumor, liver, kidneys and brain, and the samples were digested in an MLS 1200 mega apparatus a sample of the thigh muscle, were removed and acid-digested (MLS, Mikrowellen-Labor-Systeme, Leutkirch, Germany) for in a microwave oven.37,54 30 min.The microwave programme consists of 20 min heating For our investigations, the boron concentrations in the at 250 W, 5 min ventilation, and 10 min heating at 250 W.biological samples were determined by ICP-AES, ICP-MS of The closed sample containers were cooled to room temperature individual organs, and by QNCR of whole-body cryosections. and then placed in an ice-bath for 10 min to minimize analyte ICP-AES and ICP-MS analyses were optimized to give minilosses. The resulting homogeneous colourless solutions were mal analyte losses during acid digestion and sample prediluted with 2% v/v nitric acid to a total volume of 10–25 ml.treatment and to minimize the standard deviations. To achieve accurate results, an appropriate internal standard for the determination of boron by ICP-MS was selected and the non- ICP-AES spectral interferences aVecting the ICP-AES and ICP-MS A sequentially operating Plasma 40/400 ICP-AES instrument signals were studied in detail. The results of the three indepen- (Perkin-Elmer, U� berlingen, Germany) equipped with a Ryton dent analytical methods and their detection limits are spray chamber, a quartz injector tube in the Fassel torch and compared.an AS-90 autosampler was employed for the determination of boron. The sample aerosols were produced using a cross-flow nebulizer. Rh, at a concentration of 10 mg ml-1, was used as an internal standard. The operating ns are summarized EXPERIMENTAL in Table 1. Purification of Solvents and Preparation of Standards ICP-MS Nitric acid (65% v/v) (Merck, Darmstadt, Germany, pro analysi grade) was purified by repeated sub-boiling distillation in a An Elan 5000 quadrupole ICP-MS instrument (Perkin-Elmer Berghof BSP 929 sub-boiling apparatus (Berghof SCIEX, U� berlingen, Germany) equipped with an AS-90 auto- Laborprodukte, Eningen, Germany).The nitric acid was sampler, platinum sampler and skimmer cones, a quartz injecdiluted with de-ionized water to 2% v/v and stored in a FEP tor tube in the Fassel torch, a Meinhard type C nebulizer and bottle.The de-ionized water (18.2 MV cm) was purified first a Scott-type glass chamber was used. Boron was determined using a Millipore Milligard Cartridge water purification set using the lines at 10 and 11 u. Be or Rh (10 ng ml-1) was used and subsequently by a Millipore Milli-Q Plus (Millipore, as an internal standard. The highest linear counting rates of Bedford, MA, USA) system. Element standards were prepared the mass detector are limited to 2.5×106 counts s-1.Signals daily in 2% v/v nitric acid by mass-controlled dilutions of with an intensity smaller than 150 mV are discriminated. The 1000 mg ml-1 AAS standard solutions (Aldrich, Milwaukee, operating conditions are summarized in Table 2. WI, USA) of the individual elements. Carry-over and memory eVects of boron were investigated by addition of the detergent Quantitative Neutron Capture Radiography Triton X-100 (Merck; pro analysi grade) or of sodium fluoride (Merck; suprapur grade) to the diluted samples and calibration The mice were sacrificed by inhalation of diethyl ether at 4, 6, samples, respectively. 8, 12, 24 and 48 h post-application of B-Et-11-OMe. They The blank concentrations of boron in all the employed were then frozen at -69 °C and transported from Munich to solutions and vessels were checked. Pre-cleaned poly(propy- Bremen (Germany) where they were shaved, frozen at -72 °C lene) tubes were used for sample storage in the autosamplers.and embedded in 1.57% carboxymethylcellulose. Sagittal sec- All digested and diluted samples were analysed by ICP-AES tions of 50 mm thickness were cut by a cryotome at -23 °C and ICP-MS using external calibration and internal stan- and then mounted on boron-free tape. Eight boron standards dardization. To reduce errors due to the high matrix content consisting of human blood spiked with 10B-enriched (95%) of the samples, the acids employed to digest the tissue samples were added in the appropriate amounts to the external cali- Table 1 ICP-AES operating conditions bration standards. A 2% v/v nitric acid solution containing 50 ml of Triton X-100 was added for rinsing in ICP-AES ICP system— and ICP-MS.Sample uptake/ml min-1 1.1 Washing time/s 160 Plasma power/W 1083 Ar flow rates: nebulizer/l min-1 2 auxiliary/l min-1 0.945 plasma/l min-1 15 No. of replicates 3 Emission spectrometer— Wavelengths/nm: Rh 246.104; 251.103 B 249.773; 249.678 B 208.959; 208.893 Fig. 2 Structural formula of the closo-carborane B-Et-11-OMe. 1116 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Table 2 ICP-MS operating conditions carborane, are described in detail elsewhere.14,16,53 The synthesis of the borone-containing ether lipid is described in ref. 51. ICP system— Plasma power/W 1050 Sample uptake/ml min-1 0.8 RESULTS AND DISCUSSION Washing time/s 160 Microwave Digestion Ar flowrates: nebulizer/l min-1 0.890 A high pressure microwave digestion procedure was applied auxiliary/l min-1 0.7 to decompose the biological matrix as well as the injected plasma/l min-1 11 carborane compound and thus to obtain clear and homo- Mass spectrometer— CEM voltage/kV 3.82 geneous solutions of the samples.31,34,35,37,54,56–62 Deflector voltage/kV 3.54 To investigate to what extent the carborane compound is Running vacuum/Torr 6.66×10-6 destroyed by microwave digestion, its synthetic precursor, Basic vacuum/Torr 4.66×10-7 1-benzyloxy-2-methoxy-3-(9-o-carboranyl)nonyloxypropane, Ion lens settings was digested. 11B-NMR investigations of the resulting solution B, P, E1, S2 45, 43, 23, 45 indicate that it is quantitatively transformed into the volatile Peak scan parameters— compound boric acid. By the addition of phosporic acid before No. of replicates 1 the digestion, very soluble boron-phosphate anions Sweeps per reading 10 [B(HPO4)2]- of low volatility are formed.54 When spiked Dwell time/ms 30 Points per peak 1 samples of bovine liver were digested with the optimized Analysis time/s 72 microwave procedure, clear, homogeneous solutions were obtained.No losses of boron were observed. The DOC contents of digested liver samples are in the range 3.3–5.1% C (relative to the sample mass), which is equivalent to 3.0 mg ml-1 C in boron at concentrations between 0.1 and 25.0 mg g-1 were also the diluted solutions. Severe non-spectroscopic interferences prepared.The sections and standards were freeze-dried at are to be expected if the appropriate precautions, such as -23 °C for 24 h and fixed on three a-sensitive films (CR 39, dilution, matrix-matching or standard additions calibration, 500 mm, Pershore Moulding, Worcestershire, UK). At the are not applied. Studsvik Science Research Laboratory in Sweden, the samples were introduced into polyethylene bags. After evacuating the Memory and Carry-over EVects in ICP-AES and ICP-MS bags, the samples were exposed to a thermal neutron flux of Analysis 3.45×1012 n cm-2 s-1 for 35 min at the Studsvik research reactor.Finally, the films were etched with 6.25 M NaOH at Boron determinations by ICP-AES and ICP-MS can suVer 63 °C for 2.5 h. Five areas per organ of a field size of from severe carry-over and memory eVects,41,42,63,64 since 0.25×0.25 mm2 were scanned by a Quantimet 720 picture boron accumulates in the transporting tubing, on the quartz analyser at the Department of Biomedical Radiation Sciences, glass and also on the first stop lens of the ion optics and even Uppsala University, Sweden.Neutron capture radiography is in the quadrupole mass analyser. The memory eVect of a described in detail elsewhere.25,55 50 ng ml-1 boron solution was investigated using the detergent Triton X-100 and sodium fluoride (0.5 mg ml-1),42 respectively. On raising the initial boron concentration to 500 ng ml-1, the Dissolved Organic Carbon (DOC) washing time is extended to about 800 s if an NaF solution is The digested samples were diluted to 40 ml and de-gassed by used.The wash-out time was reduced from 260 s without an bubbling nitrogen through the solution. To obtain a blank, additive to 180 s using NaF, and to 160 s using Triton X-100. the complete procedure was carried out without tissue. A Thus, Triton X-100 was added to the rinsing solution for both 500 ml volume of hydrochloric acid (30% v/v, Merck; suprapur ICP-AES and ICP-MS analyses.grade) was added to all the solutions. The DOC content was measured by catalytic thermal oxidation and subsequent IR Background Equivalent Levels and Detection Limits detection of the CO2 vibration bands (TOCOR 2, Maikhak, Hamburg, Germany). To detect memory eVects, acidified water The determination of boron by ICP-MS is impaired by backwas measured after each sample. The DOC values were ground levels of boron in the solvents employed.41 The boron determined by external calibration using potassium hydrogen- blank concentrations of the investigated tissues are lower than, phthalate (C8H5KO4) as a standard.The determination limit or equal to, 2 ng ml-1. The count rates on mass 10 u vary is 0.5 mg ml-1 C. Each sample was measured twice. between 85 and 270 counts s-1 for de-ionized water and nitric acid (2% v/v); those on mass 11 u vary between 430 and 1450 counts s-1. Using a 10 ng ml-1 solution of boron, the back- 11B-NMR ground equivalents were calculated to be 1.5 ng ml-1 in The precursor, 1-benzyloxy-2-methoxy-3-(9-o-carboranyl)non- de-ionized water, 5 ng ml-1 in 10% v/v nitric acid and yloxypropane, of the administered compound was acid- 11 ng ml-1 in 10% v/v hydrochloric acid.The detection limits, digeed in a microwave oven. D2O (10%) was added to a calculated according to the 3s criterion, were 30 ng ml-1 boron sample of the resulting acid solution to obtain a lock.The 11B- for ICP-AES and 0.3 ng ml-1 for ICP-MS. The determination NMR spectrum was measured by means of a Bruker AM 360 limits in the digested and diluted tissue samples were spectrometer (Rheinstetten, Germany) at 115.5 MHz. The sing- 0.4 mg ml-1 and 3 ng ml-1, respectively. let at d=22.15 ppm (versus external standard BF3 Et2O) was identified as originating from boric acid by addition of auth- Interferences AVecting ICP-AES and ICP-MS entic material. The determination of boron by ICP-AES can be aVected by spectral interferents, such as Fe, Al and Ni.64–67 When blank Tumor Models and Synthesis of B-Et-11-OMe solutions containing interferences at a concentration of 10 mg ml-1 were investigated, a signal superposition due to The isotransplantation of the AT17 mammary carcinoma and the OTS-64 osteosarcoma, and the administration of the the tailing of the Fe signal was observed on the boron line at Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1117249.773 nm. The other boron lines at 249.678, 208.959 and concentration of the matrix element. The value of the first ionization potential of the matrix element has virtually no 208.893 nm were not aVected and the other possible interferents caused no signal enhancements. Since the maximum Fe concen- eVect. This is especially true for matrix components at a concentration 10 mg ml-1 (K, Na, P, S). The count rates of tration in our tissue samples was determined to be 5 mg ml-1 and no Fe interference was observed at this concentration, the boron in these solutions are reduced to less than 20% of the original value, regardless of the fact that the first ionization two most intense lines (249.773 and 249.678 nm) were employed for the ICP-AES investigations.potential varies between 4.34 V (K) and 10.49 V (P). For solutions with a lower matrix concentration, only a slight In ICP-MS, carbon (10 mg ml-1 as potassium hydrogenphthalate) and Be (50 ng ml-1) were investigated as possible influence on the boron count rates is observed, even though the first ionization potential varies between 5.39 V (Li) and spectral interferents.For 11B, severe peak tailing eVects which result from high concentrations of 12C were expected.26,63 17.42 V (F). The only exception is the solution containing potassium hydrogenphthalate.70 In both ICP-AES and Whereas no interference due to the peak tailing of Be was observed at 10 u, the peak tailing of carbon caused a slight ICP-MS an exceptionally high suppression of the count rates to values of less than 20% of the original value is observed.enhancement of the count rate at 11 u. This eVect was also observed during the experimental runs, when large tissue The severe matrix eVects in ICP-MS demonstrate the need for an internal standard. Eight internal standards were investi- samples were analysed. The most common matrix elements in biological samples68,69 gated.Their first ionization potentials are given in parentheses: 9Be (9.3 V), 71Ga (6.0 V), 72Ge (7.9 V), 89Y (6.4 V), 93Nb (6.9 and the maximum concentrations expected in our tissue samples are given in Table 3. To investigate the matrix eVects V), 103Rh (7.5 V), 115In (5.8 V) and 133Cs (3.9 V). To find the optimum internal standard, the experiments conducted to of both analytical methods, the count rate of a solution containing 30 ng ml-1 boron and a single matrix element or investigate the matrix eVects were repeated using solutions containing 30 ng ml-1 boron, 30 ng ml-1 of the internal stan- solvent was measured alternately to a standard containing 30 ng ml-1 boron in de-ionized water.To account for drift, dards and the individual matrix elements in the concentrations given above. Again, to account for drift, the count rates in the count rates were normalized to that of the standard. The results are shown in Fig. 3. On the left-hand side of the figure, Fig. 4 are given relative to that of a 30 ng ml-1 boron standard. As expected, 9Be is the best internal standard for boron the eVects of the solvents are given, then the matrix elements follow in order of decreasing concentration from left to right. (ionization potential: 8.3 V) since both their atomic masses and first ionization potentials are very similar.42 However, the As reported by other workers, the observed matrix eVects are generally more pronounced in ICP-MS than in ICP-AES.fact that 103Rh is a very close second and that 115In is by far the worst internal standard shows that the choice of internal The observed matrix eVects are a function of the applied standard must be checked for every application and that generalizations according to considerations of mass and first Table 3 Interferents investigated for matrix eVects ionization potential can lead to false results. Compound or element Concentration Solvent Solute Correlation of ICP-AES With ICP-MS for the Determination HNO3 2% — — of Boron HCl 2% — — K 15mgg-1H2O KCl A total of 385 tissue samples were digested according to the Na 10 mg g-1 H2O NaCl, NaNO3 optimized microwave procedure and analysed by both ICPP 10mgg-1H2O KH2PO4 AES and ICP-MS.After elimination of outliers, the results S 10mgg-1H2O K2SO4 obtained by the individual atomic masses and the individual Mg 1 mg g-1 HNO3 Mg wavelengths and the mean results of ICP-AES and ICP-MS Fe 1 mg g-1 HCl (1%) FeCl3 were compared.The regression parameters are given in Table 4. Ca 300 mg g-1 HNO3 CaCO3 Mn 300 mg g-1 HNO3 Mn The correlation of the boron concentrations in 385 tissue Cu 200 mg g-1 HNO3 Cu(NO3)2 samples obtained by ICP-AES and ICP-MS is shown in Fig. 5. Zn 150 mg/g-1 HCl (1%) Zn Correlation between the two techniques is observed in the C 10mg g-1 H2O Potassium concentration range 0.01–70 mg ml-1.11 To avoid the interhydrogenphthalate ference due to carbon, the given boron concentration was Li 10 mg g-1 HCl (1%) Li2CO3 Fig. 4 Influence of mineral acids and various salt solutions on the Fig. 3 Influence of mineral acids and various salt solutions on the intensity of the count rates of boron and the internal standards Be and Rh in ICP-MS. intensity of the count rates of boron in ICP-AES and ICP-MS. 1118 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Table 4 Correlation of ICP-AES and ICP-MS results; N: number of samples; r2: square of Pearson’s correlation coeYcient Correlation N r2 Slope Intercept ICP-MS (10 u)/ICP-AES (249.678 nm) 385 0.9701 0.863 537 ICP-MS (11 u)/ICP-MS (10 u) 385 0.9972 1.105 -17 ICP-AES (249.773 nm)/ICP-AES (249.678 nm) 385 0.9996 1.005 2 Within-run precision of ICP-AES 63 0.9973 1.011 471 Within-run precision of ICP-MS 63 0.9871 0.986 379 Between-run precision of ICP-AES 31 0.9490 1.077 2342 Between-run precision of ICP-MS 63 0.9339 0.857 4200 Fig. 6 Correlation of the boron concentrations in tissue samples Fig. 5 Correlation of the boron concentrations in 385 tissue samples determined by ICP-MS at 10 and 11 u. determined by ICP-AES (249.678 nm) and ICP-MS (10 u). than those determined in the first run. Since several weeks had determined at 10 u. It can be seen that the results agree well elapsed between the two runs, it is possible that this eVect is at concentrations below approximately 15 mg g-1 tissue. Above due to evaporation of tissue water.However, since only the this value, the results obtained by ICP-MS are generally up kidney samples are aVected it is more probable that sample to 10% lower than those obtained by ICP-AES. Since the inhomogeneity is the main cause for the deviation of the mass of the liver samples and thus their carbon content was results. As can be seen in the a-track etches [Fig. 8(b)], the much higher than that of the other tissues, it is probable that kidneys have the most inhomogeneous boron distribution of the matrix-matching of the standards for the external caliall the tissue types investigated.bration was insuYcient for these solutions. The correlation of the ICP-AES results at the two diVerent wavelengths is very good. The deviation from the optimum Quantitative Neutron Capture Radiography (QNCR) slope value of unity and an optimum intercept value of zero is marginal. This is further indication that, for our investi- Quantitative neutron capture radiography was carried out to obtain additional information on the metabolism of gations, the determination of boron by ICP-AES is not interfered with by Fe.The ICP-MS results obtained at 10 and 11 u B-Et-11-OMe in the mice and to confirm the results of the applied atomic spectrometric methods. Whole-body cryosec- also agree well (Fig. 6). Whereas the correlation is very good for the kidney, muscle and tumor samples, in the liver samples tions of six mice were investigated and five fields per organ of 0.25×0.25 mm size were analysed (Figs. 7 and 8). The results the values determined at 11 u are significantly higher than those determined at 10 u. This is probably due to the tailing obtained from a single animal were compared with the boron concentrations determined by ICP-AES and ICP-MS as the of the carbon peak at 12 u, since the average mass of the liver samples was four times that of the other samples. It should be mean of five animals. One of the main problems of QNCR is calibration.The possible to avoid this problem by further diluting the digested liver samples. calibration graphs are obtained by spiking samples with known concentrations of boron and irradiating the standard samples For 63 liver, kidney and tumor samples, the volume of the solution was suYciently large such that the analysis could be on the same sensitive sheet as the tissue samples. Since the neutron flux is not identical for the various sheets, diVerent repeated with the same solutions by both ICP-AES and ICP-MS.The correlation between the two techniques is good, calibration graphs are obtained for each irradiation. In addition, the calibration graphs are not linear (Fig. 9); they but again the results obtained by ICP-AES are more reproducible than those obtained by ICP-MS (correlation coeYcients: are corrected for outliers. At high boron concentrations, overlapping a-tracks result in decreased count rates, whereas at ICP-AES: 0.9973; ICP-MS: 0.9871).No significant influence from either the concentration or the tissue type was observed. low concentrations, tracks caused by (n,a)-, e.g., 6Li(n,a)3H, and other (n,p)-reactions, e.g., 14 7N(n,p) 14 6C, interfere with the To check the reproducibility of the complete analytical procedure, 31 large samples were divided and then separately determination of boron. The determination limit of boron by QNCR is calculated to be 80 ng g-1. digested and analysed.The most severe problems arise when re-analysing the kidney samples. The values obtained by both A compilation of the results obtained by three diVerent analytical methods is given in Table 5. Whereas the results of ICP-AES and ICP-MS are much higher in the second run Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1119(a) (b) tumor intestines Fig. 7 (a) Cryosection of a mouse sacrificed 4 h after i.p. injection of 50 mg kg-1 B-Et-11-OMe; (b) a-track etch of the cryosection shown in (a). The radiograph has been inverted and areas with a boron concentration greater than 25 mg g-1 tissue are colored.The color scheme changes from red to blue with increasing boron content. The diagram in the upper left-hand corner gives the approximate position of the cryosection. (a) (b) kidney intestines stomach liver Fig. 8 (a) Cryosection of a mouse sacrificed 6 h after i.p. injection of 50 mg kg-1 B-Et-11-OMe; (b) a-track etch of the cryosection shown in (a). the atomic spectrometric determinations can be compared inhomogeneous distribution of boron in the individual organs are averaged.In contrast, radiography only evaluates five small without any problems, it is diYcult to compare these values with those obtained by QNCR. The spectrometric methods areas of a single organ. The inhomogeneity of the distribution of boron, particularly in the sub-structures of the kidneys, can give the mean boron concentration of five complete organs so that both variations in the metabolism of the animals and the clearly be seen in Fig. 8(b). Even in seemingly homogeneous 1120 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 9 Calibration graphs obtained by three irradiations in QNCR. Table 5 Comparison of the boron concentrations in mouse tissues obtained by QNCR with those obtained by ICP-AES and ICP-MS. All concentrations are given in mg g-1 Time of Range obtained by Organ sacrifice/h QNCR ICP-AES and ICP-MS Tumor 4 3.2±0.7 1.0–1.7 Liver 4 19–29* 28–52 Cortex of the kidney 4 25±2 2.3–8.5† Pelvis of the kidney 4 37±1 2.3–8.5† Liver 6 7.1±0.8 28–40 Kidneys 6 14±4 3.4–10 Kidneys 8 0.5±0.1 4.7–7.6 Liver 12 1.0±0.1 1.1–3.8 * Range obtained from two cryosections of the same animal. † Mean of the whole kidney.Fig. 10 (a) Time-dependence of the boron concentration in blood and tumor. (b) Time-dependence of the boron concentration in liver and kidney. organs, such as the liver, a large discrepancy between two determinations is observed.For example, the liver of the mouse was found, which is much lower than that necessary for BNCT sacrificed 4 h after administration of B-Et-11-OMe was ana- (approximately 30–35 mg kg-1), but boron was homolysed in two cryosections 200 mm apart. The boron concen- geneously distributed within the tumor section, as shown in trations were determined to be 19 and 29 mg g-1. Considering Fig. 7(b). The ratio of the boron concentration in the tumor the diVerences in the analytical procedures, the results of the to that in blood rises from 0.1 at 0.5 h to a maximum of 1.3 atomic spectrometric and the radiographic methods agree well.at 12 h after administration. This indicates that the boron compound is only bound extracellularly in the tumor and not absorbed.11 To find the reasons for the bad resorption of the Metabolism of B-Et-11-OMe in Mice boron compound, light scattering measurements were per- The time-dependent boron concentrations determined by ICP- formed with B-Et-11-OMe solutions in 0.9% NaCl.The results AES and ICP-MS (Fig. 10) as well as the a-track etches were showed that the boron compound is very hydrophobic and used to deduce the metabolism of B-Et-11-OMe in tumor tends to form vesicles and micelles in the serum.14 It should bearing mice. As can be seen in Fig. 8(b), most of the boron be possible to modify the boron-containing ether lipid structurcompound is quickly metabolised in the liver, i.e., a maximum ally to enhance the resorption of the injected compound.value of 80.2 mg kg-1 at 1.5 h, where it is attached to bilary Further work in this direction is in progress. acids and excreted with the faeces via the intestines [Fig. 7(b)]. The attachment of boron to the bilary acids can be proved by We thank M. Bannasch and M. Hug for their analytical the radiographs of the mouse sacrificed 12 h after adminis- assistance.Special thanks to F. Baumga�rtner for his support tration. Since boron is incompletely separated from the bile of this work. This work was supported by the programme acids in the intestines, the a-tracks can be seen to be following ‘Dosisreduzierung bei der Reaktorneutronentherapie’ of the the biological pathway of the resorbed bile acids.16,53 A lesser Bayerisches Staatsministerium fu� r Landesentwicklung und part of the boron compound is excreted by the kidneys.The Umweltfragen and by the Penguin Foundation of the Henkel boron concentration in the kidneys is approximately constant KG a.A. Company, Du� sseldorf. between 0.5 and 8 h after i.p. administrations, i.e., 5–7 mg kg-1 tissue. Boron was not found in the brain and very rarely in REFERENCES the muscles of the animals. The determined boron concentrations in whole blood and blood plasma show that most of 1 Dencker, L., Larsson, B., Olander, K., Ullberg, S., and Yokota, M., Pharmacol.T oxicol., 1981, 45, 141. the boron (up to 152) is to be found in the plasma.16,53 2 Dencker, L., Larsson, B., Olander, K., and Ullberg, S., col. 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Chem., 1995, 33 Wei, W.-C., Chen, C.-J., and Yang, M.-H., J. Anal. At. Spectrom., 351, 745. 1995, 10; 955. 34 Tamat, S. R., Moore, D. E., and Allen, B. J., Anal. Chem., 1987, Paper 7/00445A 59, 2161. Received January 20, 1997 35 Tamat, S. R., Moore, D. E., and Allen, B. J., Pigment Cell Res., 1989, 2, 281. Accepted May 19, 1997 1122 Journal of Analytical Atomic Spectrometry,

ISSN:0267-9477    

DOI:10.1039/a700445a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Silicon in Biological Tissue by Electrothermal Atomic Absorption Spectrometry Using Slurry Sampling of Original and Pre-ashed Samples M. HORNUNG AND V. KRIVAN* Sektion Analytik und Ho� chstreinigung, Universita�t Ulm, D-89069 Ulm, Germany Two methods for the determination of silicon in biological step and the use of complex modifier mixtures represent serious tissue by electrothermal atomic absorption spectrometry using sources of contamination limiting the performance of the the slurry sampling technique are described.In one of them, a method. slurry of the tissue and, in the second, a slurry of the ash The above contamination problems can be avoided by direct obtained by separate thermal pre-treatment in an ashing analysis of solid samples. However, the dc arc atomic emission furnace was introduced into the graphite furnace. The second spectroscopy used by Indraprasit et al.2 as well as direct method proved to be superior regarding the elimination of neutron activation methods reported by Velandia and matrix interferences.Optimum sensitivity was obtained by Perkson26 and Ward and Mason27 suVer from low achievable using a mixture of palladium nitrate and magnesium nitrate as LOD. Guzzi et al.28 reported a radiochemical neutron actimodifier. The silicon contents determined were between about vation (RNAA) method ensuring a low contamination risk 3 and 14 mg g-1 and they were compared with results obtained too, but it requires a complex step for the selective separation by other methods and laboratories.The limits of detection of of the indicator radionuclide 31Si from all other radionuclides the direct method and the method involving pre-ashing were produced by irradiation of the sample. found to be 0.2 and 0.03 mg g-1, respectively. In recent years, the slurry sampling technique has increasingly been applied to the analysis of various sample types, Keywords: Silicon determination; biological tissue; slurry including biological matrices, by ETAAS.29,30 DiVerent aspects sampling; electrothermal atomic absorption spectrometry determining the performance of slurry ETAAS were studied comprehensively by Miller-Ihli.31–34 Considering the problems In recent years, the role of trace concentrations of silicon in connected with silicon determination by solution methods, biological systems, especially in the human body, has become especially the high risk of contamination and loss by volatilof increasing research interest.1 Some investigations have ization, slurry sampling ETAAS seemed to us to be very shown that patients with chronic renal failure have elevated promising because it avoids the sample digestion step and silicon levels in blood plasma and various tissues as well as at the same time oVers the advantages of a solution technique decreased silicon excretion through urine.2–9 In the brain tissue regarding sample introduction. of patients with Alzheimer’s disease, silicon has been found in In this work, two slurry sampling techniques for the determithe form of aluminosilicates in senile plaques and colocalised nation of silicon in biological tissue by ETAAS have been with aluminium in neurofibrillar tangles.10–14 In both cases, developed.The first uses slurries of powdered tissues while the however, the mode of silicon action has not yet been clarified. second technique involves ashing of the samples and prep- Furthermore, silicones are frequently used as breast prostheses.aration of slurries of the remaining ash. The results obtained Patients with such implants show increased concentrations of by these two methods are compared with those obtained by silicon in blood15,16 and in tissues.17 In view of these obser- other methods within an interlaboratory collaborative study. vations, there exists an urgent need for reliable methods for the determination of silicon levels in biological fluids and tissues.Several methods have been reported for the determination EXPERIMENTAL of silicon in serum and urine18–23 and in biological tissues.2,24–28 Krushevska and Barnes24 determined silicon in food by ICP- Instrumentation AES. After sample dissolution using HNO3, H2O2 and HF, A Perkin-Elmer (U� berlingen, Germany) Model 4100 ZL atomic the surplus acid was neutralized by addition of water-soluble absorption spectrometer, equipped with a THGA graphite tertiary amines in order to eliminate the attack on the quartz furnace, an AS-70 autosampler and a USS-100 slurry sampler, parts of the ICP-AES apparatus and to minimize the contamiwas used.Background correction was performed using the nation risk. They achieved an LOD for silicon of 75 ng g-1 longitudinal inverse Zeeman eVect. Perkin-Elmer THGA and checked this method by comparing the results with those graphite tubes with and without end caps were used. obtained in their laboratory by ICP-AES after a complex Slurry preparation was carried out in a clean bench sample fusion with LiBO2.Zhuoer25 proposed an ETAAS PBO/H13/6 (Meissner & Wurst, Stuttgart, Germany). method for the determination of silicon in bones and soft Suspensions were pre-treated in a Sonorex RK 255H ultrasonic tissues involving sample digestion with concentrated nitric bath (Bandelin Electronic, Berlin, Germany). For the determi- acid. As chemical modifier, a mixture of La(NO3)3, CaCl2, nation of sampling eYciency, an electronic micro-balance NH4H2PO4 and Na4EDTA was used for soft tissue digests, (Sartorius, Go� ttingen, Germany) was used.Sample ashing was whereas La(NO3)3 and tartaric acid disodium salt were applied performed using 15 ml nickel crucibles and an ashing furnace to the analysis of bone digests. However, due to the extraordinarily high overall concentration of silicon, both the digestion (Heraeus, Karlsruhe, Germany). Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1123–1130) 1123Samples and Reagents throughout. Calibration standards were prepared by dilution of a stock solution (1000 mg l-1 Si in 5 mol l-1 aqueous The pork liver samples L1 and L2 and the pork fillet sample NaOH, Merck, Darmstadt, Germany) with 5% HNO3. The F1 were prepared and distributed to the participants in the nitric acid (65%, pro analysi, Merck) was purified by subinterlaboratory collaborative study by Novartis (Basel, boiling distillation.Magnesium, palladium and calcium nitrates Switzerland). Samples (1 kg each) of meat minced by a butcher were of Suprapur quality (Merck). Triton X-100 (Merck) was were divided into two 0.5 kg portions, spread onto two stainless used as a surfactant for preparation of slurries of original steel trays and moistened using 20 ml of high-purity water for samples. samples L1 and F1 and using a freshly shaken mixture of 2 ml of polysiloxane spike in toluene and 18 ml of water for sample L2.After 3 days of freeze-drying using the Edwards Procedures ‘Supermodulo’, 18 l ice capacity (delivered by N. Zivy & Cie., Preliminary procedure for direct slurry sampling Oberwil, Switzerland), the two portions of each sample were combined and pre-cut into small pieces using the Cut-O-Mat, For the preparation of slurries, between 20 and 70 mg of pork Type H4 (Kneubu� hler AG, Luzern, Switzerland). Then they liver (samples L1 and L2), and between 20 and 60 mg of pork were powdered in a Retsch-Mill, type MS03, which has milling fillet (sample F1) were mixed with 10 ml of 5% nitric acid components made of ZrO2 (Retsch KG, Haan, Germany).The containing 0.004% Triton X-100 in a 15 ml polystyrene vessel. particle size distribution of the samples is shown in Fig. 1. It Before use, the vessels were rinsed twice with 5% nitric acid was determined by the Fraunhofer diVraction method by and ultrapure water and dried under a clean bench.The Novartis. suspensions were pre-treated in an ultrasonic bath for about Ultrapure water, obtained by using the Milli-Q System 30 s to accelerate wetting of the particles. (Millipore GmbH, Neu-Isenburg, Germany), was used The beakers containing the slurries were used directly for autosampling. Prior to pipetting, homogenization was performed by ultrasonic agitation with the USS-100 for 20 s at about 7 W. Fitres of the aqueous modifier solution (containing 20 mg of palladium as nitrate, and 20 mg of magnesium nitrate) and 20 ml of the sample slurry were introduced sequentially into the atomizer. For calibration using the standard additions technique, the slurries were spiked twice sequentially with 1 mg of silicon (10 ml of a standard solution containing 100 mg l-1 of silicon).The optimized temperature programme and the instrumental parameters used are summarized in Table 1. Developed procedure based on sample pre-ashing The nickel crucibles (15 ml ) used for sample ashing were cleaned by standing them for about 30 min in concentrated hydrofluoric acid. Then they were rinsed three times with Fig. 1 Particle size distribution of the samples analysed. ultrapure water, kept for 15 min in nitric acid (151) and rinsed Table 1 Temperature programmes and instrumental parameters used in the ETAAS analysis of untreated and pre-ashed samples T emperature programme 1 used for slurries of untreated samples— Ramp Hold Argon flow/ Step Temperature/°C time/s time/s ml min-1 Drying 110 1 60 250 130 60 60 250 Charring 300 60 10 50 450 60 10 50 750 60 10 50 1300 10 20 250 Atomization 2400 0 5 0 Cleaning 2600 1 5 250 T emperature programme 2 used for slurries of pre-ashed samples and for standard silicon solutions— Ramp Hold Argon flow/ Step Temperature/°C time/s time/s ml min-1 Drying 110 1 20 250 130 5 30 250 Charring 1100 10 20 250 Atomization 2400 0 5 0 Cleaning 2600 1 5 250 Instrumental parameters— Wavelength 251.6 nm Slit width 0.2 nm Source Hollow cathode lamp Lamp current 40 mA Read 5 s Signal mode Peak area Sample volume 20 ml Modifier volume 5 ml 1124 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12again three times with ultrapure water. Finally, they were dried subsequently. The maximum slurry concentration applicable for the samples L1 and L2 was about 0.7% m/v and for the under a clean bench. Portions of about 200–500 mg of the samples L1 and F1, sample F1 about 0.6% m/v.It is well known that in analyses involving slurry sampling and 200 mg of the sample L2, were placed in the crucibles and mixed carefully with 10% m/m Mg(NO3)2 6H2O. After being the particle size may aVect both the accuracy and the precision. As can be seen in Fig. 1, the particle size of the samples covered with a nickel lid, the crucibles were heated in an ashing furnace using the temperature programme given in investigated is distributed over a wide range.Therefore, the sampling eYciency had to be investigated thoroughly. For this Table 2. The duration of the final step depends on the sample portion, ranging from 10 min for 200 mg to 25 min for 500 mg. purpose, 20 ml of the slurry were pipetted 25 times by the autosampler into pre-freeze-dried and weighed autosampler The resulting ash (20–50 mg) was transferred quantitatively into a 15 ml polystyrene vessel and mixed with 10 ml of 5% cups. Then the cups were freeze-dried for 3 days and weighed again.The sampling eYciency was calculated by comparison nitric acid. The suspensions were pre-treated in an ultrasonic bath for about 5 min to disintegrate large particle agglomerates. of the sample mass weighed with that corresponding to sample mass in 500 ml slurry. For a kind of ‘blank control’, empty The beakers containing the slurries were used directly for autosampling. Homogenization was performed by magnetic cups as well as cups manually charged with 500 ml of the slurry medium and about 3 mg of sample, respectively, were processed stirring.The aqueous modifier solution (5 ml containing 20 mg of palladium as nitrate) and the sample slurry were introduced simultaneously. It was found that the mass of the empty cups and the cups charged with sample was constant. The sampling sequentially into the graphite furnace. The volume of the sample slurry injected was 20 ml for the samples L1 and F1 eYciency was calculated from the diVerence in the mass of the cups loaded with the slurry and with the pure slurry medium.and, due to its higher silicon content, 10 ml for the sample L2. The optimized temperature programme and the instrumental For the samples L1, L2 and F1, sampling eYciencies of 101±2, 99±1 and 97±6%, respectively, were determined from six parameters used are summarized in Table 1. Before use, the polystyrene vessels were cleaned by the replicates. Thus, it is rather surprising that, in spite of the extraordinarily broad particle size distribution of the materials procedure described above for the first method.To clean the magnetic stirring bars, they were kept for about 30 min in (see Fig. 1) and the relatively small id of the Perkin-Elmer standard pipette of approximately 300 mm, excellent sampling concentrated hydrofluoric acid and then rinsed twice with 5% nitric acid and ultrapure water. Finally, they were dried under eYciencies were obtained.A possible reason might be a decomposition eVect of the nitric acid leading to reduction of a clean bench. the particle size. The much broader particle size distribution of the sample F1 is obviously the reason for the higher standard RESULTS AND DISCUSSION deviation of the sampling eYciency compared with the two other samples. Optimization of the Procedures Poor sensitivity was obtained when silicon was atomized Direct slurry sampling from the sample without addition of a chemical modifier.Calcium nitrate,21,35 magnesium nitrate,36 palladium as Neither water nor 5% nitric acid was suitable for preparation nitrate,37,38 as well as its metallic form39,40 and a mixture of of slurries of untreated samples, because the particles were not palladium nitrate and magnesium nitrate41 have been used as wetted suYciently. For this reason, the suitability of several modifiers for silicon in various matrices. From a comparison suspension media with the sample L2 (containing 1.2 ng Si in of the integrated absorbances and signal shapes in Fig. 2, the 20 ml of slurry) was tested. Using ethanol, dioxane and a superiority of a mixture of palladium and magnesium nitrate mixture of 5% nitric acid and 0.004% Triton X-100 as suspenas a chemical modifier for the determination of silicon in sion media, blank values of 0.010, 0.022 and 0.008, respectively, biological tissue is apparent. In this work, palladium pre- and blank corrected integrated absorbances of 0.079, 0.076 reduced in the graphite tube at 1000 °C and palladium nitrate and 0.094, respectively, were obtained.The increase in the were tested. As there was no significant diVerence in the integrated absorbance using the mixture of nitric acid and sensitivity for silicon obtained with pre-reduced palladium and Triton X-100 as suspension medium can be explained by the with palladium nitrate, the latter was chosen for subsequent attack of the acid on the tissue, i.e., the partial digestion of the use as it is superior with respect to time consumption. sample reduces matrix eVects.For aqueous silicon standard, addition of magnesium nitrate Within a concentration of nitric acid of 1–5%, the integrated to palladium nitrate did not influence the sensitivity for silicon. absorbances showed no significant diVerences. However, the However, in processing sample slurries, the mixture of 20 mg maximum applicable slurry concentration for the sample L2 of palladium and 20 mg of magnesium nitrate proved to be the was reduced by about 30% when the acid concentration was optimum modifier.The addition of magnesium nitrate decreased from 5 to 3%. In order to achieve lower LOD, the improved the peak shape and increased the sensitivity by mixture of 5% nitric acid and 0.004% Triton X-100 was used about 20%. Using end-capped tubes, a further improvement of the sensitivity for silicon by a factor of about 2.5 could be Table 2 Temperature programme for ashing the samples in an achieved for both the aqueous standards and the sample ashing furnace slurries as compared with tubes without end caps.The silicon standard solutions could be processed by using Ramp Hold the short temperature programme 2 (see Table 1). However, Temperature/°C time/min time/min applying this short temperature programme to sample slurries 100 10 10 led to a considerable reduction of the silicon absorbance signal 150 10 10 compared with that obtained by a longer step-wise pyrolysis, 200 10 45 although application of short one-step pyrolysis was reported 230 10 45 for the determination of various elements in biological tissues 270 10 45 300 10 45 by slurry sampling ETAAS.42 We assume that the observed 350 10 10 signal depression using the short programme 2 is caused by 400 10 10 losses of silicon during the violent pyrolysis process and its 450 10 10 incomplete atomization.Fig. 3 shows the dependence of the 700 45 10–25 integrated absorbances of silicon for the samples L1 and F1 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1125Fig. 2 Influence of various chemical modifiers on the absorption signal of silicon (black line) and the background signal (grey line) in processing 96 mg of sample L2 as slurry (1.4 ng Si) (a) without modifier, (b) with 20 mg calcium nitrate, (c) with 20 mg magnesium nitrate, (d) with 20 mg palladium (as nitrate), (e) with 20 mg pre-reduced palladium and ( f ) with 20 mg palladium (as nitrate) and 20 mg magnesium nitrate. In all cases, the respective blank value was subtracted from the integrated absorbance value.PA=peak area; PH=peak height. temperature programme could be elaborated. Therefore, the influence of oxygen as an ashing aid on the pyrolysis behaviour of this material was tested. However, including an oxygen ashing step at 300 and 450 °C (ramp 1 s, hold 30 s) in temperature programme 1 led to no improvement of the integrated absorbance (see Table 4).The oxygen ashing at 450 °C caused a significant worsening of the absorption signals after five runs (see Fig. 4), indicating that an unfavourable modification of the platform surface was taking place. Therefore, a further increase of the oxygen ashing temperature was not considered appropriate. When programme 1 was applied, the graphite tubes could be used for up to about 150 atomization cycles.The increase of the id of the end-caps led to a decrease of sensitivity, indicating the end of the graphite tubes’ lifetime. Fig. 3 Dependence of the integrated absorbances of silicon in the samples L1 (0.5 ng Si) and F1 (1.0 ng Si) on the mode of pyrolysis. The capitals A–F stand for the applied temperature programmes listed Procedure Based on External Ashing of Samples in Table 3. Because of the limited application of the direct slurry sampling discussed above to liver samples, eVorts were directed to the on the mode of pyrolysis.It can be seen that the use of temperature programmes with more and more stepwise pyro- development of a method applicable to determination of silicon in all biological tissues. The addition of magnesium nitrate to lysis led for sample L1 (sample L2 behaved similarly) to a stabilization of the integrated absorbance, indicating the the sample before ashing was necessary to obtain ash quality suitable for preparation of slurries; otherwise the ash was too achievement of optimized pyrolysis conditions.Thus, the eVects causing interferences in the determination of silicon could be coarse-grained and stable slurries could not be obtained. When slurries were prepared from the ashed samples, in contrast to minimized by appropriate drying and pyrolysis of the sample and by reduction of the argon flow during the pyrolysis. For slurry sampling applied directly to the powdered tissue, the presence of Triton X-100 in the suspension medium had no the thermal pre-treatment of the liver samples, the temperature programme D in Table 3 and Fig. 3 (identical with programme influence on either the sensitivity or the reproducibility. Consequently, for the preparation of these slurries, 5% HNO3 1 in Table 1) proved to be the optimum one, with respect to the sensitivity as well as to the analysis time. However, with alone was used. Furthermore, the mode of slurry homogenization had only a slight influence on the sampling error.With the sample F1, no satisfactory stabilization of the integrated absorbance could be achieved within a meaningful pro- magnetic stirring the integrated absorbances were about 5% higher compared with ultrasonic agitation, presumably due to longation of the pyrolysis, and therefore no well optimized 1126 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Table 3 Temperature programmes used in optimization of the experimental conditions for ETAAS determination of silicon T emperature programme A As in temperature programme 2, Table 1 T emperature programmes B and C— Ramp Hold Argon flow/ Step Temperature/°C time/s* time/s* ml min-1 Drying 110 1 30/60 250 130 30/60 30/60 250 Charring 300 30/60 10 250 450 30/60 10 250 750 30/60 10 250 1300 10 20 250 Atomization 2400 0 5 0 Cleaning 2600 1 5 250 * Temperature programme B, 30 s, and C, 60 s.T emperature programme D As in temperature programme 1, Table 1 T emperature programmes E and F— Ramp Hold Argon flow/ Step Temperature/°C time/s* time/s* ml min-1 Drying 110 1 60/99 250 130 60/99 60/99 250 Charring 150 60/99 10 50 250 60/99 10 50 350 60/99 10 50 450 60/99 10 50 550 60/99 10 50 700 60/99 10 50 850 60/99 10 50 1300 10 20 250 Atomization 2400 0 5 0 Cleaning 2600 1 5 250 * Temperature programme E, 60 s, and F, 99 s.Table 4 Integrated absorbances for processing 100 mg of F1 slurry (1.0 ng of Si) without and with oxygen ashing (n=4) Mode of treatment Temperature programme 1 +O2 at 300 °C +O2 at 450 °C Integrated absorbance 0.022±0.002 0.024±0.003 0.022±0.002 more continuous slurry mixing during pipetting and crushing medium is evidently 100%.Thus, due to the leaching eVect, the actual sampling eYciency for all slurries was higher than of the coarse ash particles by the magnetic stirring bar. The slurry sampling eYciency with magnetic stirring was 99%. Applicability to both tissue materials investigated seems to determined for all three samples by comparing the amount of sample actually dispensed with the calculated value.For this be the main advantage of the slurry sampling technique using ashed samples in comparison with that using untreated purpose, 30 consecutive pipetting and drying steps were performed with a slurry of known concentration and the weight samples. For chemical modification, 20 mg of palladium (as nitrate) diVerence between the empty tube and the tube containing the dry sample was determined by using a micro-balance.For four per atomization were used. Addition of magnesium nitrate to the modifier solution was not necessary because this reagent replicates of the samples L1, L2 and F1, sampling eYciencies of 92±3, 92±4 and 99±5%, respectively, were obtained. This was added to the sample in the sample ashing step. The same temperature programme was used for the sample ash slurries procedure was applicable to the pre-ashed samples as these, in contrast with the untreated samples, did not decompose at the and the matrix-free standard solution (see Table 1).For slurries of pre-ashed samples, it was not necessary to use the long drying temperature used. However, due to the leaching eVect, the actual sampling temperature programme as it was for slurries of the untreated samples as the ashing of the samples was a separate analysis eYciency related to the analyte for the samples L1 and L2 is much higher than that determined by the above procedure.stage. With the short temperature programme, the diameter of the orifice in the end caps of the tubes started to increase only For the determination of the leaching factor, sample ash slurries were prepared and the particles were allowed to sink after about 450 runs. Thus, omission of ashing the tissue samples from the temperature programme by processing on the ground. From the determination of silicon in the liquid fraction and of the total silicon in the slurry, it was found that pre-ashed samples considerably increased the lifetime of the graphite tube as compared with processing untreated samples. 87±6, 84±4 and 89±7% (n=4) for samples L1, L2 and F1, respectively, were extracted from the ash particles into the liquid phase. From the partial eYciency for the particles and Calibration and Sample Analysis the liquid phase and the percentage distribution of silicon in these two phases, the total eYciency was calculated.The Both calibration via a calibration curve using aqueous standards as well as via standard additions was tested. Although sampling eYciency of silicon extracted in the suspension Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1127Fig. 6 Absorption signals of silicon for (a) L2 slurry (1.5 ng of Si) of untreated sample: (A) without addition of silicon standard, (B) spiked with 2.0 ng of silicon, and (C), spiked with 4.0 ng of silicon; and (b) L2 slurry (3.1 ng of Si) of pre-ashed sample: (A) without addition of silicon standard, (B) spiked with 1.0 ng of silicon, and (C) spiked with 2.0 ng of silicon.In all cases, the respective blank value was subtracted from the integrated absorbance value. PA=peak area. Table 5 Silicon concentrations (mg g-1) in the samples L1, L2 and F1 obtained by slurry ETAAS Untreated samples Pre-ashed samples (n=6) (n=4) L1 3.3±0.4 2.5±0.2 L2 14±2 14.2±0.7 F1 5±1 10±4 For the liver sample L2, the results of the two methods are in excellent agreement and for the liver sample L1, taking into Fig. 4 Silicon absorption signals (black line) and background signals account the low silicon content, the agreement can be con- (grey line) obtained in processing 100 mg of sample F1 as slurry (1.0 ng sidered good. For the 12.5 mg g-1 of silicon spiked to the Si); (a) with temperature programme 2 and (b) with temperature sample L1 to prepare the sample L2 (see the section Samples programme 2 extended by ashing with oxygen at 450 °C, fifth replicate.and Reagents), recoveries of 11±2 and 12±1 mg g-1 were PA=peak area; PH=peak height. obtained using untreated and pre-ashed samples, respectively, for slurry sampling ETAAS. Thus, these results are in satisfacthe charring curves (see Fig. 5) and the absorption signals (see tory accordance, too. However, the contents of silicon in the Fig. 6) indicate similar behaviour of silicon in aqueous standard sample F1 determined by using the two slurry sampling solution and in unspiked and spiked slurries of untreated and techniques diVer considerably.Obviously, for the kind of tissue pre-ashed samples, the characteristic masses of silicon atomized represented by sample F1, the temperature programme 1 in from aqueous solution (47±3 pg) and from a slurry of the Table 1 as well as the programmes E and F in Table 3 do not sample L2 used as an example (56±2 pg) diVer significantly.provide suYcient thermal pre-treatment of this material prior Therefore, the method of standard additions had to be used to the atomization step. Thus, with this programme, the matrix for the calibration. interferences are not suYciently eliminated. The results pre- The silicon content in all three samples determined by the sented in Fig. 3 support this assumption: whereas within the two slurry sampling ETAAS methods is compared in Table 5. programme modes D–F, no further increase of the absorbance was achieved with the sample L1, a still increasing absorbance could be observed with the sample F1. Besides this, for the sample F1, according to the ratio of the concentrations determined in the pre-ashed samples L1 and F1 (see Table 6), a higher absorbance is to be expected under interference-free conditions than the maximum one in Fig. 3. We did not attempt further optimization of the temperature programme which would allow elimination of the matrix interferences, as such a programme, if at all possible, would be very complicated and time-consuming and it would cause a further shortening of tube lifetime.Therefore, the technique involving sample preashing seems to be generally preferable, as for both types of samples under investigation, the interferences in the determination of silicon can be suYciently minimized. We assume that this technique might be applicable also to analysis of other types of biological tissue for silicon whereby the same tempera- Fig. 5 Charring curves of silicon obtained: (A) for aqueous silicon ture programmes for sample ashing and sample analysis can standard (2.0 ng of Si); (B) for slurry of the untreated sample L2 be used. (2.0 ng); (C) for slurry of the untreated sample L2 spiked with aqueous In Table 6, the results obtained for all three tissue materials silicon standard (3.0 ng); (D) for slurry of the pre-ashed sample L2 by slurry sampling ETAAS using pre-ashed samples are com- (3.1 ng); (E) for slurry of the pre-ashed sample L2 spiked with aqueous pared with those obtained in an interlaboratory collaborative silicon standard (4.1 ng) [for (A), (B) and (C), 20 mg of Pd–20 mg of study organized by J.Pavel43 using direct wavelength dispersive Mg(NO3)2, and for (D) and (E), 20 mg of Pd were added per atomization as chemical modifiers]. X-ray fluorescence (WDXRF), solution ETAAS and ICP-AES, 1128 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Table 6 Contents of silicon in the samples L1, L2 and F1 obtained by ETAAS using slurry of ash and by diVerent methods in other laboratories Sample Silicon concentration/mg g-1 Silicon concentration/mg g-1 This work Other methods and laboratories Other methods and laboratories Ash slurry ETAAS Direct WDXRF Sol-ETAAS Sol-ICP-AES 1 2 3 4 5 6 7 89 10 11 12 13 14 L1 2.5±0.2 7.7±0.8 5.8±0.8 7.7±1.3 9.9 9.9 4.4 2.6 4.7 1.7±0.6 5.0 <5 2.0±0.5 2.2 [2.3, 2.7] [7, 9] [5, 7] [6, 9] [9, 13] [4.0, 4.7] [1.1, 2.5] [1, 5] [1.5, 2.5] [2.1, 2.3] L2 14.2±0.7 20.5±2.9 17.0±0.1 17±2 22.4 21.2 11.1 4.1 7.7 11.9±0.7 14.3 12 11±2 12.3 [13.5, 15.1] [18, 25] [17, 17] [15, 18] [20, 23] [11.0, 11.2] [11.0, 12.7] [11, 13] [9.8, 13.5] [11.2, 13.3] F1 10±4 13±3 12±8. 11±7 9.8 11.2 24.4 9.4 15.1 3.5±1.9 14.5 8 16±3 14 [5.3, 13.2] [9, 16] [7, 28] [<5, 28] [8, 13] [1.9, 5.8] [7, 18] [14, 20] [7, 20] In brackets: [extreme values]. Methods: 1 slurry sampling ETAAS of pre-ashed samples.Sample size 200–500 mg (n=4). 2–5 direct analysis by WDXRF. Sample size for 3: ~5 g (n=6), for 4: ~100 mg (n=4), for 5: ~2.5 g (n=3) and 6: ~4 g. 6 digestion in an open PTFE vessel with HNO3–H2O2, followed by ETAAS. Sample size ~40 mg. 7 pressurized microwave digestion in a PFA-bomb with HNO3, followed by ETAAS. Sample size ~350 mg (n=2). 8 extraction of silicon using tetramethylammonium hydroxide, followed by ETAAS after dilution with water.Sample size ~100 mg. 9 digestion in a capped PTFE-tube at 120 °C with HNO3–HF (251), followed by ETAAS. Sample size ~200 mg. 10 pressurized microwave digestion in a closed PFA-bomb with HNO3–H2O2, followed by ETAAS. Sample size 60–100 mg (n=6). 11 fusion with LiBO2, followed by ICP-AES as described24 for low-silicon food samples. Sample size ~4 g. 12 pressurized microwave digestion in a PFA-bomb with HNO3–H2O2, followed by ICP-AES. Sample size ~500 mg. 13 pressurized digestion in a PTFE-bomb at ~180 °C with HNO3–HF (151), followed by ICP-AES.Sample size ~120 mg (n=3). 14 fusion with NaOH suprapur (500 °C), followed by ICP-AES. Sample size ~5 g (n=2). both involving diVerent sample handling procedures in 12 for silicon is presently impossible but preparation for its future use is in progress. laboratories. From this comparison, it is evident that the determination of silicon in biological tissue represents one of the most diYcult analytical tasks which still cannot be solved Limits of Detection satisfactorily.Even the results obtained by only one of the two LOD were calculated as three times the standard deviation of types of solution methods, i.e., ETAAS and ICP-AES, show the replicates from the blank measurements. For this purpose, considerable disagreement. For example, the silicon contents in the direct slurry sampling technique, the suspension medium determined in the individual laboratories by solution ETAAS was processed, and in the ash slurry technique, the whole in the samples L1, L2 and F1 were in the range 1.7–9.9, procedure including pre-ashing was performed with 25 mg of 4.1–21.2 and 3.5–24.4, respectively.Only the results obtained magnesium nitrate. For the direct and the ash slurry sampling, for the samples L2 and F1 by WDXRF and for the sample L2 a blank value of 0.8±0.07 mg g-1 (n=10, assuming a sample by ICP-AES show reasonable consistency. Nevertheless, for amount for slurry of 70 mg) and 0.1±0.01 mg g-1 (n=8, the samples L2 and F1, the results of WDXRF and ICP-AES, assumed sample amount: 500 mg), was obtained leading to respectively, are in good agreement with our result.For the LOD of 0.2 and 0.03 mg g-1, respectively. samples L2 and F1, the agreement with the results of WDXRF and of ICP-AES, respectively, is within 60%. However, in no one case could satisfactory agreement of our results with the CONCLUSION results of one method and laboratory for all three samples be Slurry sampling ETAAS applied to untreated and to pre-ashed achieved.The highest degree of agreement was achieved with samples seems to be an advantageous and promising method lab 11 and lab 14 using sample fusion with LiBO218 and with for the determination of silicon in biological tissue materials. NaOH, respectively. Unfortunately, the method could not Compared with methods involving sample digestion with acids, provide any more exact results for the sample L1, because the an essential minimization of the risk of contamination and content was near to the LOD.For the samples L2 and F1, the volatilization loss seems to be the main advantage of the two agreement was within 20%. The agreement of our results with developed slurry sampling techniques. Ash slurry sampling is those of laboratory 14 was very good for the samples L1 and superior to direct slurry sampling regarding LOD, sample L2 and it was within 40% for the sample F1.From the homogeneity, precision and its applicability to both biological minimum and maximum values of the replicates, it can be seen materials processed in this work. However, owing to the lack that for the sample F1, the replicates of all methods and of a biological standard reference material with a certified laboratories show a much larger scatter compared with the silicon content as well as of a reliable reference method, the samples L1 and L2. accuracy check could be performed only to a limited degree.Thus, a conclusive comparison, which would provide a more Thus, further improvement of the state of development of the reliable judgement of the degree of accuracy of our results, is methods for determination of trace silicon in biological tissue presently barely possible. An essential improvement of this is necessary. unfavourable situation could be expected from an RNAA method. However, it requires the availability of a reactor The authors thank J.Pavel for making available the results of providing an extremely high ratio of thermal neutron flux to other methods and for fruitful discussion. fast neutron flux which would minimize the primary interference reaction induced on phosphorus by fast neutrons. In REFERENCES addition, sensitive detection of the indicator radionuclide 31Si can be achieved exclusively by non-specific counting of its beta 1 Cavic-Vlasak, B. A., Thompson, M., and Smith, D. C., Analyst, radiation.This makes selective separation of the indicator 1996, 121, 53R. radionuclide in the radiochemically pure form necessary. For 2 Indraprasit, S., Alexander, G. V., and Gonick, H. C., J. Chronic Dis., 1974, 27, 135. these reasons, application of RNAA to analysis of the samples Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 11293 Mauras, Y., Riberi, P., Cartier, F., and Allain, P., Biomedicine, 23 Zhuoer, H., Spectrochim. Acta, Part B, 1995, 50, 1383. 24 Krushevska, A. P., and Barnes, R. M., J. Anal. At. Spectrom., 1980, 33, 228. 1994, 9, 981. 4 Adler, A. J., and Berlyne, G. M., Nephron, 1986, 44, 36. 25 Zhuoer, H., J. Anal. At. Spectrom., 1994, 9, 11. 5 Hosokawa, S., and Yoshida, O., Int. Urol. Nephrol., 1991, 23, 281. 26 Velandia, J. A., and Perkson, A. K., J. Radioanal. Chem., 1974, 6 Gitelman, H. J., Alderman, F. R., and Perry, S. J., Am. J. Kidney 20, 715. Dis., 1992, 10, 140. 27 Ward, N. I., and Mason, J.A., J. Radioanal. Chem., 1987, 113, 515. 7 Marco-Franco, J. E., Torres, V. E., Nixon, D. E., Wilson, D. M., 28 Guzzi, G., Pietra, R., and Sabbioni, E., J. Radioanal. Chem., 1976, James, E. M., Bergstrath, E. J., and McCarthy, J. T., Clin. 34, 35. Nephrol., 1991, 35, 52. 29 Miller-Ihli, N. J., Anal. Chem., 1992, 64, 964A. 8 Wrobel, K., Blanco Gonzalez, E., and Sanz-Medel, A., J. Anal. 30 Bendicho, C., and de Loos-Vollebregt, M. T. C., J. Anal. At. At. Spectrom., 1993, 8, 915. 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ISSN:0267-9477    

DOI:10.1039/a701313b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Images of the Direct Sample Insertion Process in an Inductively Coupled Plasma CAMERON D. SKINNER AND ERIC D. SALIN* Department of Chemistry, McGill University,Montreal, Quebec, Canada, H3A 2K6 An inductively coupled plasma was imaged with a charge deposition11 and the use of vaporization enhancing gases in coupled device camera through interference filters during direct the DSI probe.15 These methods of sample preparation and sample insertion. Images were acquired throughout the treatment do not necessarily require a probe in the form of a insertion and revealed how the analyte behaves in the plasma.cup and may benefit from an examination of probe design. Graphite cups, graphite tubes and wire loops were used as The key to a better understanding of plasma behavior during sample carrying probes. The diameter of the graphite cup has the DSI process is to image the entire plasma simultaneously. a dramatic eVect on the plasma and calcium emission. We define the behavior of the analyte in the plasma, in the Narrower cups disturb the plasma the least and keep the context of this study, as the apparent position of the analyte calcium within the center of the plasma.The use of a carrier in the plasma as measured by the imaging system. The vaporizgas through a graphite tube and hollow stem cup shows that a ation of the sample provides a transient signal so that it is darker central channel is established. The use of a central gas diYcult to observe the changes that take place in the plasma in combination with Freon-12 yields a performance without a high speed imaging system.A spectrometer may be comparable to that of wire loops. In the case of a deep and used to image a small portion of the plasma, and if one is narrow graphite cup the analyte appears to emerge into the persistent enough the entire plasma may be mapped by this plasma along the walls of the cup. method. A more eYcient prospect is to image the plasma with a two-dimensional detector in combination with an imaging Keywords: Inductively coupled plasma; direct sample insertion; spectrometer.The advantages of this method are the wide imaging; Abel inversion choice of wavelengths and high spectral resolution.16 The disadvantages are low light throughput and the potential for Our laboratory has been interested for many years in method- image overlap from adjacent images. A relatively inexpensive ologies that increase the sensitivity of atomic analysis, with alternative to these two methods involves using interference much of the research focusing on direct sample insertion (DSI) filters in conjunction with a high speed imaging detector.This as a method of introducing samples into the inductively method provides high light throughput and allows short coupled plasma (ICP).1–4 The DSI technique was extensively integration times while providing moderate spectral isolation. reviewed in 1990 by Karanassios and Horlick.5 This technique The detector may be based on one of several technologies increases the sensitivity by vaporizing the sample in a short but charge coupled devices (CCDs) are nearly ideal because of time period, producing high analyte concentrations in the their high quantum eYciency and low noise.A more complete plasma. Some types of samples, e.g., geological materials, or description of how CCDs (and charge injection devices) operate large samples can produce visible disturbances in the plasma and their applications to spectroscopy was given by Bilhorn as the sample vaporizes from the probe.6,7 These include and co-workers.17,18 changes in the shape of the plasma, emissions from the entire CCDs may be operated in several modes.One of the modes plasma, bright and variable emissions above the cup and that is of particular interest is the frame transfer mode, in unstable plasma operation. Large disturbances of the plasma which an image is acquired on half of the CCD array.The are undesirable and the eVects of probe type and geometry on charge that has accumulated is then transferred to a masked the plasma are poorly understood. portion of the array and the exposed area begins to collect a DSI probe designs are varied but probes are generally new image. During the relatively long exposure time, the fabricated from graphite or refractory metals.8 The design of charge in the masked region can be digitized with high the probe has a significant eVect on the intensity, duration and precision.This process repeats for each image. noise of the signal that is observed and also memory eVects.9 Once the image has been acquired, it may be examined Wire loops have been used to hold small volumes of liquid directly to indicate the spatial behavior of the analyte and that may be dried and introduced into the plasma. These plasma or it may be processed to extract information about probes heat very quickly and produce signals that may last the analyte by performing background subtraction.Additional for only a few tenths of a second.1 Thin walled and thin information can be obtained by deconvolving the image to stemmed graphite cups produce signals that rival wire loops obtain radial intensity profiles. The deconvolution of lateral for speed of vaporization while being able to hold larger intensity measurements to yield radial intensity profiles can be amounts of sample.4,10 These graphite cups can also be used done by the Abel inversion.19 Several researchers have utilized as a target for spray depositing a volume of sample much this method to determine the radial intensities of the ICP.20–31 larger than the capacity of the cup.11 Others have also found The Abel inversion is a special case of computed tomography that the thickness of the cup walls has an important impact where the symmetry of the plasma can be used to advantage.32 on the signal that is observed.12 The graphite probe can also The Abel inversion makes several assumptions about the be treated as a sacrificial sample holder and the sample and source, namely circular symmetry, an optically thin source and probe can be consumed with oxygen.13 In these cases the a well known center.The first two assumptions have been design of the probe has been based on common sense and partially addressed by modifications to the algorithm.28,33 The experience.Recently, we have begun to investigate other possthird assumption is usually fulfilled by examination of the ible probe designs because of our interest in preconcentration of analyte species directly on to activated charcoal,14 spray data. The Abel inversion has an additional shortcoming of Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1131–1138) 1131being highly sensitive to noise in the input data. The eVect The controller was equipped with a 16 bit A/D converter with a maximum readout rate of 100 kHz.The controller and of the noise can be moderated by smoothing of the data23,28 or fitting the data to one or more polynomials.19,22,29,31 software package provide full control of all camera functions such as thermoelectric cooling of the detector head, exposure The method that was used in this series of experiments is based on the method of Scheeline and Walters.33 Each row of time, shutter control, readout rate and on chip binning.Esco Products (Oak Ridge, NJ, USA) interference filters data in the image is considered to be an image of a small cross-section of the plasma. The plasma is divided into a series (390 mm Part No. S903900; 420 mm Part No. 5904200) were used for spectral isolation. The filters have a 25.4 mm (1 in) of rings where each ring is the width of a pixel from the image. The intensity that is measured for any given pixel is the sum od and 10 nm bandpass (FWHM) at the 390 and 420 nm center wavelengths.With these filters it was possible to observe of the contributions from each ring. The center and the edges of the image are determined and the areas of the segments of the 393.3 nm Ca II and the 422.6 nm Ca I emission lines. All images were acquired using a Nikon AF Micro 60 mm lens. the circle are calculated. The radial intensity is calculated from the outside of the image towards the center. The radial This lens has 50% of maximum transmission at 390 nm and 80% of maximum at 420 nm.The interference filters were intensity of the outermost ring is the measured intensity divided by the area of the segment of the circle from which the attached to the lens with a custom built adapter ring that could also accommodate optical density filters (neutral density radiation originates. The next radial intensity is calculated by subtracting the contribution from the outer radius and again filter set, Part No. 03FSQ 011; Melles Griot, Irving, CA, USA) as needed.For all of the experiments the F/n of the lens was 32. dividing by the area of the segment. This process continues until the center radial intensity is calculated. Each time the The camera was operated with a 5 ms exposure with the shutter locked open during the experiment. The images were measured intensity is corrected for contributions from the outer radii. Fig. 1 illustrates this method for determining the acquired using an active area of 150×256 pixels on the CCD chip at 16 bits resolution. This enabled an image to be acquired intensity at any given radius.The generalized equation is approximately every 0.38 s. Each experiment comprised a sequence of 45 images (a movie) that recorded the entire insertion process. The beginning of the experiment was trig- Ir=ir/2Ar, r-2 . j=R j=r+1 IjAr,j gered manually. Background movies were acquired with empty probes. where Ir is the radial intensity, ir is the measured lateral intensity at radius r, R is the outermost pixel from the centre of the image and Ar, r is half of the area of a particular segment of the circle that represents the plasma cross-section. DSI Probe Designs To gain a greater understanding of how DSI probes behave The probes used for this series of experiments were constructed in the plasma, we imaged the plasma with a CCD camera.from graphite rod using techniques previously described.15 Some of the images were subjected to Abel inversion to reveal Several types were used (Bay Carbon, Bay City, MI, USA, greater information about the characteristics of emission pat- Part No.S-8 HD for 3 mm probes; SGL Carbon Group, Speer terns of calcium as it leaves the DSI probe. Canada, St. Laurent, Canada, Part No. 580-.375 for 5 mm probes, Part No. 580-500 for 8 mm probes) and are illustrated in Fig. 2. The leftmost cup is commonly used in our laboratory EXPERIMENTAL for introducing liquid samples.Three diVerent cup diameters Camera (dimension a, Fig. 2) were investigated; the large diameter was 8.3 mm, the standard diameter was 5.2 mm and the narrow A Princeton Instruments Canada (Stittsville, Canada) frame transfer camera with a 512×1024 element CCD array and a diameter was 3.1 mm. These three cups were all 6.3 mm deep (dimension b). Two cups of longer length were also used (b= ST-138 controller was used for this series of experiments. The camera system was interfaced to a generic EISA-based 486 PC 12 mm) with the standard and narrow diameters.The second type of cup illustrated is a hollow stem cup which has a using the cabling and interface card supplied by Princeton Instruments. Images were acquired using the Windows-based standard diameter but a stem that has been enlarged to allow for the 1.6 mm diameter hollow stem. The third type is a WinView 1.3A software supplied by Princeton Instruments. A6,6 Pixel no. Radial intensity I6 = i6/2 A6,6 6 5 4 3 2 1 A5,5 A4,4 A3,3 A3,4 A3,5 A3,6 A2,2 A1,1 A1,2 A1,3 A1,4 A1,5 A1,6 I3 = i3/2 A3,3–2(I4 A3,4 + I5 A3,5 + I6 A3,6) I r = ir /2 Ar,r –2 S I j Ar,j j = R j = r + 1 Fig. 1 Radial intensities (Ir) for any given radius (r) are calculated from the observed intensity (ir) and the area (Ar,r) of the plasma that generates that measurement. For the inner radii the measured intensity must be corrected for the contribution from the outer radii of the plasma. 1132 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 2 The four types of probes used in the experiments. The three on the left are made from graphite. The fourth is a tungsten wire loop. hollow tube with an internal diameter of 2.4 mm. A small piece Fig. 3 The area of the plasma imaged by the camera and the torch of reticulated vitreous carbon foam (3 mm long, porosity 100; and DSI cup. Energy Research and Generation, Oakland, CA, USA) was placed within the tube to hold the sample.The fourth type of probe used was a 0.5 mm diameter tungsten wire loop of 3 mm the intensity (I) that was obtained from the image at the diameter.1 normal viewing height for a spectrometer [16 mm above top In all cases 5 ml of 100 ppm calcium was introduced on to of the load coil (ATOLC)]. The beginning of each experiment the probes with an Eppendorf pipette. The liquid was dried was triggered manually and consequently synchronization of by heating the probe inductively in the plasma coil. When the the images for background subtraction had to be done manuhollow stem cup was used the carrier gas was introduced ally.In some experiments there is a slight synchronization through the stem at a flow rate of 290 ml min-1. For the error that results in imperfect background correction. The experiments with the graphite tube a reduced flow rate of eVect of this error is most pronounced for the images of the 180 ml min-1 was used, otherwise the foam would be launched actual insertion where the plasma rapidly changes brightness.through the plasma. Freon-12 (1000 ppm in argon) (Matheson For the images prior to and after the insertion the error is Gas Products, Montreal, Canada) was the carrier gas. negligible because the diVerence from frame to frame of the background is small. Our experience with DSI has shown that the signals are Overview of the Method of Calculating Radial Intensities reproducible (3–5% RSD) when simple aqueous standards are The file generated by the camera software was read into used.This was also found to be true with the images collected Matlab 4.0 for Windows 3.1 (The MathWorks, Natick, MA, with the camera. For a given set of experimental conditions USA). An image was chosen for the center finding routine the variation in intensity, from movie to movie, at any particubased on high calcium signals with no or minimal saturation lar time during the insertion is about 10%.The larger variaof the detector. Some images were saturated for a small area bility is due to the problem of manual synchronization of the directly above the cup. The center of mass determined for this beginning of each experiment. small area may not be correct but will not aVect the accuracy Calcium was chosen as an analyte for two reasons: it has of the radial calculations in the unsaturated regions. Each of an atomic and ionic emission line in the spectral range of the the 256 cross-sectional images of the plasma was smoothed camera and it is suYciently refractory that the signal can be using a ten point moving average.The center of the image was observed over several seconds. Initially we believed that the found by finding the center of mass for each of the smoothed two lines would provide complementary information about cross-sections. The center determined from this image was the plasma, but the images are strikingly similar and provide used for all of the images in the movie. The images that the the same information.radial intensities were to be calculated for were then also Fig. 5 shows some of the images that were taken with the smoothed and the radial intensities were calculated using the three diVerent diameter graphite DSI cups. Only a few of algorithm described in the Introduction. Each half of the image the many images in each movie can be presented here owing was calculated separately and the two halves were combined to space constraints.The images in Fig. 5 were taken with the to yield a radial intensity map of the plasma. 390 nm filter at 1.25 kW. All of the images have been corrected for background so the images show net calcium emission. The eVect of the synchronization error for background correction RESULTS AND DISCUSSION can be seen in the first image from the medium diameter cup Fig. 3 illustrates the area of the plasma that was imaged by in Fig. 5. The first image for each size of cup is immediately the camera.Included is the region of the plasma directly above prior to, or during, the insertion of the cup and is assigned the DSI probe to well above the normal analytical viewing time 0. There is some inaccuracy in the timing because the zone. Many of the images that are shown have distortions due beginning of image acquisition was triggered manually. We to the bonnet as well as the top of the torch. All of the images estimate that the maximum error is equal to the image presented in this paper utilize false coloring that scales the acquisition time of 0.4 s.The second image in each sequence intensity to a color and should not be confused with the actual colors emitted by the plasma. The color scale that is used for the images is illustrated in Fig. 4. The numerical values of the minimum and maximum will be presented with the images since the scaling may change from image to image. In the instances where an optical density filter was used the scales Fig. 4 Color scale used for images. Black is the minimum value and white is the maximum. were corrected for the filters’ attenuation. Some figures include Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1133Fig. 5 Images from diVerent sizes of DSI cups. Note that the width of the plume is proportional to the size of the cup. 1134 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 6 Images taken from the other types of DSI probes studied. Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1135is immediately after insertion (t=0.4 s) and shows the beginning (note that the times for the frames are not the same). It seems that the combination of low probe mass and the use of gas of the calcium emission. The third image is taken at the time of greatest emission intensity during the experiment. The fourth through the tube produces rapid and eYcient vaporization.In the case of the tube the cooling eVect of the gas retards the image is taken prior to the removal of the probe from the plasma. vaporization maximum until 1.4 s. With the hollow stem cup the larger diameter of the graphite cup allows the sample to The narrow cup (3.1 mm) yields the greatest intensity (I) when compared with the medium and large diameter cups (5.2 heat more rapidly and the maximum emission is observed earlier (0.76 s) than with the hollow tube.and 8.3 mm) for the region in the plasma where analytical measurements are routinely made. The dramatic increase in As was the case with the three diVerent diameters of cups, the analyte is largely dispersed by the time it reaches the area signal from the large to the narrow diameter cup is probably due to three sources: the narrower cups heat more quickly, in the plasma where analytical measurements would normally be made; however, the graphite tube is best able to keep the reach a higher temperature and release more calcium into the plasma per unit time;10,34 the smaller cup also places a smaller analyte within a central channel, probably because the absence of a cup decreases turbulences.One interesting point that load on the plasma, allowing greater eYciency of excitation; and the narrower cup keeps the calcium confined to a narrower should be made is that the intensity of emission just above the tip of the tube is much higher than that of any other probe.plume in the center of the plasma. The first two observations are consistent with work reported by Barnett et al.34 regarding This is shown in Fig. 7, where the intensity is 37 000 about 2 mm above the top of the tube. The other types of probes do the rate of heating and heat loss as a function of cup geometry. Many other workers have determined that the thickness of the not display this intense emission directly above the probe. Unfortunately, the intensity drops oV very quickly above the supporting stem is important to the emission versus time profile.9 In general, the thicker the stem or the cup walls the top of the tube.To see if inserting the tip of the tube close to the observation point would yield a significantly larger signal, lower is the rate of vaporization. Umemoto and Kubota10 found that when the internal volume of the cup was held a longer (33 mm) hollow tube probe was constructed. Fig. 8 shows the results that were obtained when the viewing height constant and the wall thickness was increased that the time of maximum emission was retarded and that the maximum cup of the spectrometer was set to 16.5 mm ATOLC and the insertion depth was increased.The results are disappointing temperature was reduced. A narrow plume is advantageous because it increases the since there is only a gradual loss of signal as the probe is inserted deeper into the plasma. There may be two explanations analyte signal intensity in the viewing zone and minimizes the impact of the sample on the plasma.If large amounts of sample for the apparently conflicting results in Figs. 7 and 8: the large emission above the top of the tube observed with the camera are allowed to mingle with the plasma, interferences become possible. With liquid nebulization the majority of the sample may be due to broadband emission that the filter allows through or the vaporization and excitation conditions are is constrained to a narrow central channel, thus limiting the eVect of the sample on plasma behavior.35 Within the torch favorable.The background calcium 393 nm data from the the narrow cup is the most eVective cup design at keeping the calcium in a narrow plume. Additionally, we found that the narrow cups disturb the plasma the least. The medium diameter cup causes the plasma to fluctuate on insertion but the plasma stabilizes quickly. The large cup was diYcult to insert into the plasma without the plasma being extinguished.The images reveal that the plasma is most adversely aVected by the large diameter cup, the emission intensity being very low. The integrated intensity in the region above the torch is 4.3×108, 1.7×108 and 8.8×106 counts for the narrow, medium and large diameter cups, respectively (from the brightest image). The increase in diameter spreads the calcium over a greater volume of the plasma, Fig. 7 Emission intensity as a function of the viewing height above as can be seen from the images.Additionally, the integrated the top of the graphite tube. The emission of calcium is much higher intensity shows that the excitation conditions are poorer for just above the tube and rapidly falls oV. Note the distortions due to cups with larger diameters. the top of the torch and the bonnet. These data were taken from time Fig. 6 shows some of the results that were obtained with 1.52 s in Fig. 6 (the image from time 1.14 s was unusable because of sample probes that may be considered ‘high performance’.detector saturation). These probes heat and vaporize the sample very rapidly. All of the images were acquired through the 420 nm filter at 1.25 kW forward power. The tungsten wire loop produces the shortest transient signals and is shown here as a benchmark. Calcium readily forms refractory carbides on graphite but the use of Freon greatly improves the volatility.15 For the experiments comparing graphite probes with the wire loop, a carrier gas, introduced through the stem of the graphite probe, of argon enriched with Freon-12 (1000 ppm) was used.15 The enhanced volatility of the calcium yields signals that are comparable to those of more volatile species.From Fig. 6, it can be seen that the wire loop has the shortest transient of the three types of probes tested. The emission intensity (I) dropped to 1200 at 1.52 s and 170 at 1.9 s (not shown). The intensity from the hollow stem graphite cup is 480 by 12.2 s, the last frame with the cup in the plasma.Fig. 8 Normalized signal intensity as a function of insertion depth of The hollow tube’s intensity is 130 by 4.2 s, indicating that the hollow stem tube. The expected large emission near the top of the tube was not observed. tube has a performance comparable to that of the wire loop 1136 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12spectrometer were examined for any evidence of broadband confined to the center of the plasma.It also shows that the plasma changes within the torch. There is a slight decrease in emission, but none was observed. Hence it seems probable that a position of the tube low in the plasma leads to eYcient the emission from the right-hand side of the plasma. This is likely due to poor centering of the hollow tube in the plasma. vaporization and excitation. When the tube is inserted to greater depths, the eYciency of the excitation is reduced One of the primary reasons for embarking on this study was to gain a greater understanding of how the analyte moves because the tip of the tube is no longer in the hot base region of the plasma.Additionally, there is more of the tube in the through the plasma. In many cases direct study of the images has revealed how the probes alter the behavior of the analyte plasma, which probably reduces the amount of energy available for vaporization and excitation.in the plasma. In some cases, calculation of the radial intensities can suggest where the calcium is in the plasma. We have been The use of gas through the DSI probe creates a central channel in the plasma similar to the central channel found curious about where the analyte emerges from the graphite cup. This question has been partially answered by calculating with liquid nebulization. The gas flow needed to establish a central channel is much less in the case of the DSI probe the radial intensities.A deep and narrow diameter cup (12×3.1 mm, dimensions because the probe is inserted into the plasma and does not have to overcome the magnetohydrodynamic resistance that b and a, respectively, in Fig. 2) was inserted such that the base was at a normal position (-6 mm ATOLC) and the top of the is encountered with liquid nebulization.36 Fig. 9 shows a contour plot of the diVerence when gas is and is not used (i.e., an cup was visible to the camera.The top of the cup is most easily seen in the fourth image in Fig. 10. Some of the images image of when the gas was flowing was subtracted from when it was not) with the empty hollow tube probe. The contour (390 nm filter) are shown in Fig. 10. The scaling of the intensities has been adjusted to show the presence of two lobes of plot clearly shows that the central channel is darker and higher intensity directly above the walls of the cup. The radial intensities were calculated for these images and revealed that in the case of the long DSI cup the material that is vaporized from the cup emerges along the walls and not as a plume from the center of the cup (Fig. 11) assuming that the excitation conditions are constant across the plasma in the region above the cup. Similar images were obtained from the calcium 422.6 and aluminium 394.4 nm atom lines. The length of this particular cup may allow the formation of a stable vortex above (and perhaps within) the cup that reduces mixing of the calcium in the plasma.As can be seen from Fig. 10, the annular emission is stable for nearly 8 mm above the cup. When the radial Fig. 9 Contour plot of the diVerence between when gas and no gas is used with the hollow tube probe. The values on the axes are the Fig. 11 Radial intensities calculated 2.8 mm above top of cup from pixel number. The distortions from the torch and bonnet can be seen on the plot (lower 25 pixels); an outline of the torch has been added.Fig. 10. The cup is 3 mm wide and the two maxima are 3.3 mm apart. Fig. 10 Images of a deep and narrow DSI cup inserted to +6 mm ATOLC. The intensity scaling has been adjusted in each image for clarity. The top of the cup is visible in the first image as the blue line of calcium emission. The calcium emerges along the walls of the cup and maintains an annular plume for approximately 7 mm. These images are background corrected and the area within the torch has been enlarged.Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 113711 Rattray, R., Min� oso, J., and Salin, E. D., J. Anal. At. Spectrom., intensity maps from standard length cups (6.3 mm) were exam- 1993, 8, 1033. ined (both medium and narrow diameter), no evidence of this 12 Umemoto, M., and Kubota, M., Spectrochim. Acta, Part B, 1991, annular emission was found. It seems probable that the shorter 46, 1275. cups disturb the flow of the gases in the plasma so that there 13 Liu, X.R., and Horlick, G., J. Anal. At. Spectrom., 1994, 9, 833. is rapid mixing of the calcium as it emerges from the cup. 14 Cazagou, M., Blaise, J., Skinner, C. D., and Salin, E. D., Appl. Spectrosc., in the press. 15 Skinner, C. D., and Salin, E. D., J. Anal. At. Spectrom., 12, 725. CONCLUSIONS 16 Olesik, J. W., and Hieftje, G. M., Anal. Chem., 1985, 57, 2049. 17 Bilhorn, R. B., Sweedler, J. V., Epperson, P. M., and Denton, Imaging of the plasma during the insertion of DSI probes has M.B., Appl. Spectrosc., 1987, 41, 7, 1114. revealed how the analyte behaves in the plasma. For a given 18 Bilhorn, R. B., Epperson, P. M., Sweedler, J. V., and Denton, amount of calcium narrower graphite cups yield higher emis- M. B., Appl. Spectrosc., 1987, 41, 1125. sion intensities. The narrow cups are better able to keep the 19 Cremers, C. J., and Birkebak, R. C., Appl. Opt., 1966, 5, 1057. 20 Galley, P. J., Glick, M., and Hieftje, G.M., Spectrochim. Acta, analyte in the center of the plasma. This is especially true of Part B, 1993, 48, 769. the region within the torch where extensive mixing may 21 Monnig, C. A., Gebhart, B. D., Marshall, K. A., and Hieftje, increase the likelihood of interferences. The use of gases G. M., Spectrochim. Acta, Part B, 1990, 45, 3 261. through the center of the probe establishes a central channel 22 Babis, J., Pilon, M. J., and Denton, M. B., Appl. Spectrosc., 1990, that reduces the background of the plasma and entrains the 44, 1280.analyte in a central channel. In the case of a long and narrow 23 Olesik, J. W., Den, S., and Bradley, K. R., Appl. Spectrosc., 1989, 43, 924. DSI graphite cup, the calcium emerges along the walls of the 24 Hauser, P. C., and Blades, M. W., Appl. Spectrosc., 1988, 42, 595. cup. There is rapid mixing in the plasma that reduces the 25 Walters, P. E., Gunter, W. H., and Zeeman, P. B., Spectrochim. eVectiveness of narrow cup design and the use of carrier gases.Acta, Part B, 1986, 41, 133. 26 Choot, E. H., and Horlick, G., Spectrochim. Acta, Part B, 1986, The authors acknowledge the generosity of Princeton 41, 935. Instruments Canada and Hugh Garvey for the loan of the 27 Niebergall, K., Brauer, H., and Dittrich, K., Spectrochim. Acta, CCD camera and associated controllers and software. We also Part B, 1984, 39, 1225. 28 Blades, M. W., Appl. Spectrosc., 1983, 37, 371. thank NSERC for funding under the Strategic Grants Program. 29 Choi, B. S., and Kim, H., Appl. Spectrosc., 1982, 36, 71. 30 Blades, M. W., and Horlick, G., Appl. Spectrosc., 1980, 34, 696. 31 Kornblum, G. R., and De Galan, L., Spectrochim. Acta, Part B, REFERENCES 1977, 32, 71. 1 Sing, R. L. A., and Salin, E. D., Anal. Chem., 1989, 61, 163. 32 Monnig, C. A., Marshall, K. A., Rayson, G. D., and Hieftje, 2 Blain, L., Salin, E. D., and Boomer, D. W., J. Anal. At. Spectrom., G. M., Spectrochim. Acta, Part B, 1988, 43, 1217. 1989, 4, 721. 33 Scheeline, A., and Walters, J. P., Anal. Chem., 1976, 48, 1519. 3 Moss, P., and Salin, E. D., Appl. Spectrosc., 1991, 45, 1581. 34 Barnett, N. W., Cope, M. J., Kirkbright, G. F., and Taobi, 4 Rattray, R., and Salin, E. D., J. Anal. At. Spectrom., 1995, 10, 829. A. A. H., Spectrochim. Acta, Part B, 1984, 39, 343. 5 Karanassios, V., and Horlick, G., Spectrochim. Acta Rev., 1990, 35 Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 13, 89. ed. Montaser, A., and Golightly, D. W., VCH, New York, 6 Blain, L., and Salin, E. D., Spectrochim. Acta, Part B, 1992, 47, 399. 1987, p. 141. 7 Zaray, G., Broekaert, J. A. C., and Leis, F., Spectrochim. Acta, 36 Inductively Coupled Plasmas in Analytical Atomic Spectrometry, Part B, 1988, 43, 241. ed. Montaser, A., and Golightly, D. W., VCH, New York, 8 Karanassios, V., and Horlick, G., Spectrochim. Acta, Part B, 1989, 1987, p. 142. 44, 1361. 9 Karanassios, V., Horlick, G., and Abdullah, M., Spectrhim. Paper 7/02038D Acta, Part B, 1989, 45, 105. ReceivedMarch 24, 1997 10 Umemoto, M., and Kubota, M., Spectrochim. Acta, Part B, 1989, 44, 713. Accepted July 14, 1997 1138 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a702038d

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

High Speed Photographic Study of Wet Droplets and Solid Particles in the Inductively Coupled Plasma R. S. HOUK*, ROYCE K. WINGE AND XIAOSHAN CHEN† Ames L aboratory—US Department of Energy, Department of Chemistry, Iowa State University, Ames IA 50011, USA Motion pictures of the ICP were taken at 4000 frames s-1 . eVorts at analysis of solids as slurries using solutions for calibration. An intact wet droplet containing yttrium causes the formation of a pale red cloud juxtaposed on the usual ambient emission structure of the plasma.Most of these droplet clouds are EXPERIMENTAL shaped like an oval or a comet. A few droplets produce small, bright spheres followed by faint, wispy streaks that point ICP Conditions downstream. Such a spherical cloud is caused by some rapid Standard operating conditions are given in Table 1. A event such as explosion of a droplet in the final stages of Meinhard concentric pneumatic nebulizer was used with a solvent evaporation.The faint streaks are some residue, Scott-type double pass spray chamber operated at room perhaps small solid particles. In a particular frame, a number temperature. In some experiments, the wet aerosol from the of these faint streaks protrude from the tip of the initial spray chamber was injected into the plasma directly. In other radiation zone (IRZ) into the normal analytical zone (NAZ). cases, the aerosol from the double-pass spray chamber was When wet droplets are introduced, that portion of the analyte dried into particles with a conventional desolvation system, that travels through the center of the plasma passes through i.e., the aerosol stream was heated to 140 °C and then cooled three distinct regions (i.e., the IRZ, the streaks, then the to 0 °C in a water-cooled condenser.This desolvation system NAZ), rather than directly from the IRZ to the NAZ. Groups was similar to that described by Fassel and Bear.13 The plasma of two or three droplets tend to appear together in the same was operated horizontally in the orientation usually used for time interval (#0.12 ms) in the plasma.These droplet clouds ICP-MS and for ICP-AES with axial viewing. are not seen, and the particle streaks are much less evident, when the solvent is removed before the aerosol is injected into the plasma. Aqueous slurries of Y2O3 in various particle sizes Samples (0.1 or 3 mm mean diameter) produce white streaks along the Yttrium was chosen because the red emission from neutral Y center line of the plasma, which are attributed to individual atoms and YO molecules can be distinguished easily from the solid particles.These observations also support the general blue emission from Y+. The yttrium was introduced in several precept that calibration of the response for slurries using diVerent forms: (a) 5000 ppm Y dissolved in 2.5% aqueous aqueous solution standards is best accomplished by keeping the HNO3; (b) 0.1 mm diameter colloidal Y2O3 suspended in H2O particle loading such that each wet droplet contains no more at 100 000 ppm Y; (c) 3.2 mm diameter Y2O3 suspension in than one solid slurry particle.H2O at 10 000 ppm Y; and (d) 8.5 mm diameter Y2O3 suspended Keywords: Inductively coupled plasma; inductively coupled in H2O at 10 000 ppm Y. plasma mass spectrometry; nebulization; sample introduction; The slurry particle sizes cited above were the mean diameters slurry; noise; particles; droplets given by the supplier.The reader should note that the particles comprising each slurry actually consist of a distribution of sizes. These numerical values are merely used to distinguish The ICP is a very eVective source of atoms and ions for the diVerent slurries in the subsequent discussion. The 3.2 mm analytical atomic spectrometry. To the human eye, it appears diameter slurry was examined under an optical microscope. to be a stable, steady-state source. However, several important dynamic phenomena are evident when the plasma is studied with suYcient time resolution.Along these lines, there is Table 1 Experimental facilities and operating conditions substantial interest in the deleterious eVects of wet droplets Torch Fassel type,12 horizontal, outer tube 18 mm id, and solid particles in the ICP. A variety of optical and jet injector with tapered tip, 1.5 mm id MS studies by Cicerone and Farnsworth1 and Olesik and Nebulizer Meinhard concentric co-workers2–9 have described transient eVects caused by such Type TR–30–C3, droplets and particles passing through the axial channel of the Scott-type double-pass spray chamber12 ICP.Our previous studies emphasized the oscillations in the ICP: Model HFS 2500D size and shape of the plasma that arise when the hot, flowing Plasma Therm (now RF Plasma Products) Power 1.3 kW plasma gas interacts with the surrounding atmosphere.10,11 Frequency 27.12 MHz These earlier photographic studies also showed evidence of Ar flow rates: vapor clouds surrounding intact droplets or particles.The Outer gas 14 l min-1 present work examines these phenomena in more detail and Intermediate gas 0.4 l min-1 illustrates ways to distinguish droplets from particles. This Aerosol gas 1.0 l min-1 paper shows that the droplets can be removed by desolvating Camera: Fastax Model WF3 Film Eastman 7297 color negative, 16 mm the aerosol and also provides insights pertinent to ongoing Framing rate 4000 frames s-1 Lens Wollensak Fastax-Raptar, 75 mm focal length with 10 mm extension † Present address: Metropolitan Water District of Southern tube California, 700 N.Moreno Ave., La Verne, CA 91750–3399, USA. Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1139–1148) 1139Individual particles in a range of sizes from 1 to 4 mm were brighter ‘head.’ In the next frame [Fig. 1(d)], the comet is replaced by a faint white streak that is probably produced by apparent, as were some larger agglomerates of various sizes and shapes.Addition of Triton X-100 at 0.5% v/v did not a solid residue from the wet droplet. The streak is surrounded by a spherical, blue region, the upper boundary of which greatly aVect the size or the number of these agglomerates. The suspensions were agitated during nebulization with a appears as a bulge on the upper side of the blue NAZ. This spherical blue zone represents a high local density of excited magnetic stirrer.A special, high-solids nebulizer, as commonly used for analysis of slurries, was not necessary. The usual Y+ that emit in the blue. The length scale on the photograph is calibrated from the diYculties of plugging of the nebulizer and settling of the particles were not problems in this work, because each photo- width of the plasma at the mouth of the torch, i.e., at the far right edge of the photograph. The width of the plasma at this graphic experiment lasted only a few seconds, as discussed below.For these reasons, a dispersant was not used when the position is approximately 16 mm. Thus, the diameter of the blue cloud in Fig. 1(d) is #3 mm, and its center is about 6 mm ICP photographs were taken. In our opinion, a dispersant was not necessary in this case, in agreement with the observations downstream from the center of the pale red ‘head’ of the comet in Fig. 1(c). Our blue Y+ cloud is slightly wider than the 2 mm of Broekaert and co-workers14,15 for alumina slurries.The slurries were introduced either as wet droplets or as dry, false color image shown by Olesik for a Sr+ cloud that was also #6 mm downstream from the point of initial formation desolvated particles. of Sr+, i.e., 13 mm from the load coil (Fig. 2 of ref. 9). Photographic Conditions Velocity of Droplet Clouds The high-speed camera was operated with rolls of film that were 30 m long. Each roll consisted of 4000 frames.The time The total elapsed time between frames is 1/4000 frames s-1= required to accelerate the camera to the nominal framing rate 0.25 ms. A prominent droplet cloud that is visible during the (4000 frames s-1) consumed approximately half the roll. Each entirety of two successive frames is selected; the cloud shown roll therefore lasted about 1.5 s. The viewing field of the camera in Fig. 1(b) and (c) is suitable. From the beginning of Fig. 1( b) was approximately 45 mm along the torch axis.to the beginning of Fig. 1(c), this droplet travels approximately 7.5 mm, so the flow velocity is (7.5 mm/0.25 ms)=30 m s-1 . Previous measurements of flow velocity in the axial channel Processing of Films and Selection of Prints find a range of 20–30 m s-1, in reasonable agreement with this The prints shown below were selected from viewing of individ- estimate.1,11 Note that the exposure time of a single frame is ual frames from the processed films. The films were also approximately half of the elapsed time between frames because transferred to video cassettes and viewed as full sequences with of the masking of the rotating prism inside the camera.Thus, a video editor. The prints shown in this paper were selected the exposure time of a given frame is approximately 0.12 ms. to represent clearly the various diVerent types of clouds or streaks caused by discrete droplets or particles. Shapes of Droplet Clouds The oval [Fig. 1(b)] and comet [Fig. 1(c)] are the two most RESULTS AND DISCUSSION common shapes for droplet clouds in the ICP. The comet Ambient Emission Structure of ICP shape is caused by a medium-sized droplet that loses a substantial amount of solvent and thus shrinks noticeably The transient events from intact droplets and particles are during the exposure time of a given frame (#0.12 ms). The superimposed on the usual emission structure of the ICP. oval in Fig. 1(b) is simply the cloud caused by the early stages Nebulized solutions containing yttrium produce the familiar of vaporization of a large wet droplet. pale red initial radiation zone (IRZ), blue normal analytical The sequence shown in Fig. 2 illustrates the other general zone (NAZ),16 and deep red tail plume. Presumably, this shape seen for droplet clouds. First, a very large wet droplet emission structure results from vaporization, atomization, exci- ( labelled 1) emerges from the IRZ [Fig. 2(a)]. This droplet is tation and ionization of yttrium from relatively small droplets so large that the corresponding cloud does not shrink notice- and particles that dissociate and atomize properly in the ably in a single frame.In the next frame [Fig. 2(b)], droplet 1 plasma. travels downstream and shrinks considerably, to the point This ambient emission structure is depicted in Fig. 1(a). The where its vapor cloud begins to taper. Another large droplet plasma flows from right to left out of the torch. The torch (2) leaves the IRZ.In Fig. 2(c), droplet 1 moves still further protrudes through a circular hole cut through an aluminum downstream and decomposes into a faint white particle track. shielding box surrounding the load coil. This shielding box The faint white streaks in Figs. 1(d) and 2(c) are probably blocks the bright white emission from the induction region caused by some residue remaining after decomposition of the and greatly facilitates photography of detail downstream in bulk of the solid particle.Alternatively, the streaks could be the analyte zones. emission from neutral Y atoms that have been created from the last solid residue. In either case, these residual streaks are Red Vapor Clouds and Particle Tracks FromWet Droplets of not very wide, which shows that the atoms from them are Solution Aerosols quickly converted into Y+ as they diVuse outwards from the residue into the plasma. The size and shape of the NAZ change with the audio plasma fluctuation, as noted previously.10,11 In roughly 60% of the Next, we return to Fig. 2(c). Droplet 2 shrinks and produces a bright, spherical, pale red cloud that lasts only briefly, hence frames, transient, pale red emission clouds are superimposed on the ambient IRZ and NAZ. Examples are shown in a its image is not elongated. A faint streak trails this small red sphere on the downstream side, i.e., pointing towards the tail sequence of successive frames in Fig. 1( b)–(d). In Fig 1( b), a large droplet leaves the IRZ displaced oV-center. Cooler con- of the plasma. Meanwhile, another large droplet (3) leaves the IRZ on-center, while droplet 4 can be seen inside the axial ditions surrounding this droplet produce the oval-shaped, pale red cloud above and to the left of the tip of the IRZ in channel oV-center. In Fig. 2(d), droplets 1 and 2 are gone, and droplets 3 and 4 both evolve into bright, spherical clouds Fig. 1( b). This droplet moves further downstream and forms a tapered, comet-shaped cloud in Fig. 1(c). This ‘comet’ cloud followed by faint streaks. These pale red spheres with streaks pointing downstream displays a noticeable gap between the diVuse ‘tail’ and the 1140 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12(a) (b) (c) (d) Fig. 1 Sequence of four consecutive frames during introduction of wet aerosols, 5000 ppm Y. The plasma is horizontal and flows from right to left. The induction region and most of the torch are blocked from view by a shielding box.In this and subsequent photographs, the width of the plasma at the far right is approximately 16 mm, and time begins at the top and progresses to the bottom. (a) Ambient emission structure; (b) a large droplet is seen above the center line; (c) the droplet shrinks into a comet-like shape; (d) the droplet vaporizes and is replaced by a thin white streak. There is a spherical cloud of blue Y+ around the streak in (d).Note also the thin, wispy region to the left of the tip of the IRZ in each frame. are the third and least common general shape for droplet ignition of precipitates in classical gravimetric analytical procedures. 17 In a study with isolated droplets in analytical flames, clouds mentioned in the previous section. This shape is attributed to rapid decomposition, perhaps even explosion, of a Bastiaans and Hieftje18 proposed a similar explosive process as one possible mechanism for the decomposition of analyte particle or a droplet in the final drying stages.For these clouds to be spherical, the time frame of this ‘explosion’ must be fast particles that have dried on the outside and entrapped solvent on the inside. Our observations diVer from those of Bastiaans relative to the exposure time (0.12 ms), otherwise the clouds would be elongated. Gradual atomization of the residue then and Hieftje in that we see evidence of solid particles or residues after the explosion, whereas they saw evidence of a smaller causes the faint white streak.Observation of this succession of events in consecutive frames is unusual; generally, a given wet droplet after explosion of the crust. The isolated droplets used by Bastiaans and Hieftje were #60 mm in diameter, i.e., droplet is seen in only one frame. The precise causes of these ‘explosions’ are unclear. One much larger than any of the droplets used in this study, which could explain why we did not see wet droplets after the possible source is as follows.Suppose a particularly large droplet dries from the outside in. This drying process creates explosions. Childers and Hieftje18 report a morphological investigation a solid crust with water trapped inside. The trapped water overheats, boils suddenly and ruptures the crust. Such of collected particles from a flame that is also pertinent to this work. In their study, samples of KCl were introduced as ‘explosions’ of occluded solvent occur at times during the Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1141(a) (b) (c) (d) Fig. 2 Another sequence of four consecutive frames during introduction of wet aerosols, 5000 ppm Y. (a) A very large droplet (1) leaves the IRZ; (b) droplet 1 moves downstream and shrinks into a comet shape, while another large droplet (2) leaves the IRZ; (c) droplet 1 has become a faint white streak, droplet 2 dries and produces a small, spherical pale red cloud followed by a faint streak, and droplets 3 and 4 emerge from the IRZ; (d) droplets 3 and 4 also decompose into small, spherical clouds and streaks.uniform, wet droplets of diameter 67 mm. Solid particles were though all droplets were the same size! A larger variation of possible morphologies probably arises from the polydisperse collected on either a MgO-coated slide or the sample stub of a scanning electron microprobe for examination. Solid particles aerosol produced by a conventional nebulizer, such as that used in this paper and in most analytical work.of a variety of sizes and shapes were found. Some were regular cubic crystals of KCl, especially if the collection point was The ‘explosions’ of some of the Y(NO3)3 particles could also be related to recent observations concerning possible near the onset of vaporization of the droplets. Other particles were agglomerates of many microparticulates or crystals that atomization mechanisms of metal nitrates in electrothermal atomic absorption spectrometry.Holcombe and co- had melted partially. Finally, some of the collected particles were the residues of hollow shells that had been shattered, workers19–21 believe the main mechanism for formation of gaseous metal oxides first involves the formation of a solid either in the flame or on impact with the collection surface.18 These hollow residues are thought to be analogous to the metal oxide: explosions seen in this work and described in this section.Our observations and those of Hieftje and co-workers M(NO3)2(s)�MO(s)+2NO2(g)+DO2(g) (1) illustrate the following important point: for a given sample composition, all droplets and particles need not dry and decom- The solid metal oxide then decomposes into MO (g), often by an explosive shattering of the solid MO. L’vov and pose in precisely the same way. In Hieftje’s work, diVerent morphologies were seen at a particular sampling position even Novichikhin22 argue that gaseous metal oxide can be formed 1142 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12directly from the nitrate: For example, the shadow stop in Perkin-Elmer SCIEX instruments collects solids, minimizes formation of deposits on the M(NO3)2(s)�MO(g)+2NO2(g)+DO2(g) (2) subsequent ion lenses, and thus helps reduce drift and improve stability. If either mechanism occurs inside an isolated microparticulate in an ICP, our view is that the gas liberated will shatter the These photographs also show that the common practice of specifying the sampling position in ICP-MS, or the observation microparticulate and cause an ‘explosion’ such as those shown in Figs. 2(c) and (d). Neither Holcombe nor L’vov studied position in ICP-AES, relative to the tip of the IRZ is at best a time-averaged approximation. For wet aerosols, the position yttrium nitrate, which is the material used in this work. At any rate, the photographs in Figs. 1 and 2 show that, for of the tip of the IRZ varies both axially and radially on a millisecond time scale, especially when large droplets pass yttrium dissolved in nitrate solutions, there are at least two diVerent processes by which the final stages of drying lead to through the plasma. For dry particles, variations in the particle size and number density also contribute to instabilities in the formation of solid particles and then free atoms in the ICP.Most droplets evaporate smoothly, but a minority undergo position of the tip of the IRZ. these abrupt ‘explosions.’ Lateral Position and ‘Bunching’ of Droplets Overall Depiction of Drying and Vaporization Processes Examination of the entire film shows that the larger droplets often occur together as bunches of several droplets. Such a Fig. 3 shows our interpretation of these photographs pertinent bunch of three droplet clouds is shown in Fig. 2(c). Montaser to the fate of droplets in the ICP.We assume that a tapered, and co-workers23,24 observed a similar bunching of large elongated red vapor cloud represents the path of a spherical droplets from scattering measurements of the droplet stream cloud that shrinks as it moves axially during the exposure. leaving the spray chamber. Four such spherical clouds are shown in Fig. 3; each of these The red clouds are most frequent along the central axis of four clouds is produced by the same droplet. For some droplets, the plasma. Some droplets are occasionally seen oV-center (e.g., a spherical cloud from an explosion forms next, followed by a Fig. 1), but they tend to be smaller and disappear faster than faint streak and a spherical blue cloud of Y+. Most droplets those along the center line. These oV-center droplets have dry and vaporize smoothly without the explosion passed through the axial channel close to the induction region. Inside the load coil, this outer annular region of the axial Analytical Implications of Droplet Clouds and Particle Tracks channel is hotter than the center line.Thus, the large wet droplets dry and vaporize more eVectively if their path through Individual, isolated white streaks such as those shown in the axial channel is oV-center. The greater frequency of large Figs. 1(d) and 2(c) are seen only occasionally when solutions droplets on-center could also mean that the larger droplets are nebulized. However, most of the photographs show a fuzzy are enriched along the center axis of the gas flow out of the region beginning at the tip of the IRZ and extending downtorch injector.stream into the NAZ, e.g., the faint grey zone at the left of the This argument is complicated by the fact that the photo- tip of the IRZ in Figs. 1( b) and 2(d). This region is presumably graphic process integrates the emission through the thickness caused by juxtaposition of a number of such faint streaks, of the plasma.Suppose the plasma is sliced into a collection none of which is suYciently prominent to stand out individuof segments in a manner analogous to the Abel inversion ally. Although the ICP is often considered to have two analyte procedure. A slice from the camera through the plasma along zones, the IRZ and the NAZ,16 in fact there are three distinct the central axis is the thickest slice and thus contains the most regions, as noted in our first photographic study of noise droplets, even if the droplets are actually distributed uniformly behavior.10 The analyte that remains on-center first passes with respect to the radius.through the IRZ, then the grey, transition zone, and finally What are the emitting species responsible for the red vapor the NAZ. clouds? These photographs probably cannot distinguish In ICP-MS, the sampler is usually positioned just downbetween Y (I) atomic lines and YO bands, both of which are stream from the tip of the IRZ, i.e., right in the grey, streaky in the red part of the spectrum.Both neutral atoms and oxides zone. In most ICP-MS experiments, therefore, the solid residues are probably present in the vicinity of wet droplets, and the that cause the streaks pass directly into the sampler, as do the red emission from the vapor clouds shown in Fig. 1 is probably large wet droplets [e.g., Fig. 2(a)]! It is, therefore, not suprising a mixture of both Y (I) lines and YO bands. that solids deposit on the skimmer, photon stop or ion lens.Desolvated Solution Aerosols In a total of at least 20 000 frames from five separate films, large red droplet clouds are never seen when the aerosol is desolvated, i.e., by drying the droplets at 140 °C and then condensing the solvent. This observation proves that the red clouds are indeed from wet droplets, as proposed in our previous work. A variety of adverse eVects such as oxide ions in ICP-MS and noise in ICP-AES and ICP-MS have been blamed on fluctuations in analyte density caused by passage of these droplets.This present work shows that desolvation is a simple way to remove them. Also, the red emission from the IRZ is much less prominent, the tip of the IRZ is more stable with time, and the grey streaky zone is much less evident when the solvent is removed by desolvation. The removal of the streaky region has an additional advantage for ICP-MS. The sampling process in ICP-MS is highly localized spatially, i.e., the analyte ions that pass through the Fig. 3 Drawing depicting evaporation of solvent from droplet, followed by explosion, atomization and ionization of residue. skimmer originate from a small zone just in front of the Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 sampler. If a solution aerosol is desolvated, very few streaks from gaseous YO vaporized from the Y2O3 particle. This supports our hypothesis that these streaks are caused mainly are present, and the analyte atomizes in a narrower range of axial positions.Several workers report significant improve- by emission from hot, solid particles. ments in sensitivity (i.e., analyte signal per unit concentration) in ICP-MS when the aerosol is dried,25–29 especially for double Behavior of 0.1 mm Slurry Particles focusing mass spectrometers.30,31 Peters and Beauchemin29 also showed that noise can be reduced to some extent by pre- When the 0.1 mm Y2O3 particles are injected as wet droplets, the usual ‘ambient’ emission structure is observed.A single, evaporating the aerosol before it is injected into the ICP. Both these sensitivity enhancements and noise reductions are prob- thin white streak is also seen along the entire length of the plasma (Fig. 5a). Apparently, many more particles are now ably due, at least partly, to the removal of the droplet clouds and the grey streaky region when the aerosol is desolvated present than is the case for the larger 3 mm particles, so the individual tracks appear to be one continuous streak.This or dried. streak is seen only along the central axis; it is never displaced radially. Also, red vapor clouds are seen only near the tip Particle Tracks From Desolvated Y2O3 Slurries of the IRZ, and they occur only briefly and occasionally. Desolvation of these slurry particles does not greatly change The various Y2O3 slurries described in the Experimental section produce diVerent behavior than the yttrium solution the appearance of the plasma.The infrequent red vapor clouds from wet droplets are removed completely when the aerosol is described above. The slurries are desolvated in this experiment so that the dry, solid particles can be distinguished from wet desolvated, as expected. As shown in Fig. 5b, the central streak is slightly wider and droplets. With desolvated slurries, the background IRZ and NAZ are visible but less intense than when an yttrium solution more diVuse when the droplets are desolvated.The reasons why the central streak is narrower when the slurry particles is nebulized. The finer slurries (0.1 and 3.2 mm particle diameter) produce faint, wispy, white streaks through the center of the are introduced in wet droplets are not clear. We suggest the following explanation. Suppose the larger, heavier droplets plasma. For the 0.1 mm slurry, many such streaks combine into a more or less continuous white line down the central axis of remain tightly localized along the center line.It is possible that the gas flow velocity out of the torch injector is suYciently the plasma. For the 3.2 mm slurry, individual streaks can be discerned (Fig. 4). The slurry with the largest particles (8.5 mm) high to enrich large droplets along the center line. The same eVect is used in the jet separator interface for gas produces only ambient yttrium emission, with no streaks. In this last case, the plasma looks much like that from a desolvated chromatography–MS,34 in the particle beam interface for liquid chromatography–MS,35 and in other applications of momen- aqueous solution of yttrium.The white streaks from the particles in Fig. 4 are clearly tum separators. Desolvated particles contain the same amount of analyte but no solvent. They are thus much lighter than evident but are much less prominent than the red vapor clouds discussed previously. The shape and orientation of the white wet droplets and spread out more extensively as they leave the injector of the torch.streaks from the 3 mm slurry are also interesting. They occur at uniform intervals only along the center line of the plasma, Wet aerosols containing these 0.1 mm slurries produce many fewer red emission clouds than the yttrium solution, even they tend to be tilted relative to the long axis of the plasma, and they also curl noticeably at the ends. This tilt and curled though there is more total yttrium in the slurry.Those droplet clouds that are seen exist only in individual frames and do not shape could result if the solid particles swirl slightly with the gas flow in the plasma, as suggested by French.32 Such a swirl persist downstream in the NAZ, even though the nebulizer and spray chamber are the same as those used to produce the eVect could be important in experiments with the monodisperse dried microparticulate injector (MDMI), where the particle droplet clouds shown in Figs. 1 and 2. Each of these droplets contains many Y2O3 particles, as described in the next section. stream is highly localized and must be aimed precisely at the sampling orifice of the MS instrument.6–9,33 Very little yttrium dissolves into the surrounding water, because the slurry is not acidified and Y2O3 is not appreciably soluble In the sequence chosen in Fig. 4, one streak (labelled by the letter ‘A’) stands out as being brighter than the others. This at pH#7 [for Y(OH)3, Ksp#10-23].36 Apparently, the yttrium must be in solution in a wet droplet to produce red vapor clouds brighter streak could be an agglomeration of several slurry particles or two separate particles that happen to pass through that persist in the NAZ.Also, no ‘explosions’ are seen from any of the slurries. the plasma nearly together. This particle streak appears in each frame in Fig. 4 and survives all the way down the central The arguments in the previous paragraph indicate an important facet of the red clouds from wet solution droplets described axis of the plasma.Apparently, at least some of the Y2O3 particles do not atomize completely in the plasma. Brighter previously. Many of these clouds are detached from the IRZ and are surrounded by the blue emission from the NAZ. streaks such as this are unusual; most frames show a succession of uniformly-spaced streaks of similar intensity. It is possible Examples are droplets 1 and 2 in Fig. 2(a) and (b). Such detached clouds are seen only when the yttrium is dissolved that the streaks come solely from agglomerates, not from individual 3 mm particles. However, the observation that most in the droplet. Thus, these isolated red clouds consist primarily of Y (I) and/or YO emission from the yttrium contained within streaks have the same size and intensity argues that the particles either did not agglomerate severely in the liquid phase that droplet, not from Y or YO formed from ‘ambient’ Y+ ions that have been cooled by passage of the droplet.Otherwise, before the sample was nebulized and introduced into the plasma, or that the agglomerates were broken up by the isolated red vapor clouds would also be seen when wet slurries are introduced, as these droplets would cool the ‘ambient’ Y+ nebulization and desolvation processes. Otherwise, the streaks would be of various sizes. into neutral Y and/or YO. Near the central axis all particle streaks are surrounded by a faint transition zone of diameter 2 mm or less.Within a Estimate of Number of Solid Particles per Droplet for Slurries short distance from the axis, bright blue emission from excited Y+ is observed. Apparently, ionization occurs rapidly after Goodall et al.37 provide general guidelines on the subject of slurry analysis that are relevant to this work. For accurate atomization, because the vapor clouds bear the blue color of excited Y+, not the red of neutral Y or YO emission.analysis of slurries using dissolved aqueous standards, the main criterion is that the particle concentration should corre- It is also interesting that the fine streaks shown in Fig. 4 are not red, as would be the case if they were caused by emission spond to no more than one solid particle per wet droplet (on 1144 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 4 Four consecutive frames during introduction of Y2O3 slurry containing 3 mm particles at 10 000 ppm Y.The solvent was removed by desolvation in this experiment. Note the discrete white tracks through the center line of the plasma. The letter ‘A’ marks the passage of a particularly large particle, or possibly two particles in close proximity. average). Generally, the solid should be ground into particles diVerent degrees that depend on the chemical and physical properties of the particles and the slurry medium. Even at the no more than 3 mm in diameter to satisfy this requirement. Denser solids require more thorough grinding into finer relatively high solid load (100 000 ppm Y) used for the colloidal slurry in the present work, the #400 solid particles in a typical particles still.For comparisons with these criteria, calculated numbers of (2.5 mm) droplet would coalesce into a single dry particle only #0.7 mm in diameter, which is still under the maximum size particles per wet droplet are shown in Table 2 for the three slurries used in this work.For the colloidal slurry (0.1 mm of 1.2 mm determined by the criteria of Goodall et al.37 for Y2O3 (density=5.0 g cm-3 39). diameter particles), the calculations show that 10 mm wet droplets each contain #30 000 solid particles. However, only Calculated particle loadings per droplet for the other Y2O3 slurries are also shown in Table 2. These data suggest that few a few of these large wet droplets reach the plasma. Most droplets that leave the spray chamber used in this work are in wet droplets that are small enough to escape the spray chamber (i.e., <5 mm diameter) contain solid particles in the sizes used the size range 0.5–5 mm, with maximum transmission in the 2–3 mm range.38 Even so, these ‘average’ droplets of 2.5 mm (3.2 or 8.5 mm).In other words, only 1% of the 5 mm wet droplets contain a 3.2 mm solid particle, while only 0.05% of diameter still contain #400 of the 0.1 mm diameter particles.The fate of these multi-particle droplets is unclear. Do they such droplets contain an 8.5 mm particle. This low value for occupancy (i.e., low value for solid particles per wet droplet) dry into many discrete 0.1 mm particles, do they agglomerate in the solution, or is an agglomerate formed as the droplets explains why the 8.5 mm slurry produced few visible streaks. The data in Table 2 also show that the probability for occu- dry in the plasma? All three processes probably occur to Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1145and experience higher temperatures along the outer edge of the axial channel, i.e., they pass closer to the hot induction region than droplets that stay along the center line. These observations are pertinent to the dictum of Goodall et al.37 that calibration of the analyte response from a slurry using external calibration with an aqueous solution works best if the particle size and total slurry level are adjusted so that each wet droplet contains no more than one slurry particle.In this case, each droplet dries into one slurry particle, whose behavior in the plasma is not greatly diVerent from that of a solution residue. Suppose a spray chamber is used that transmits droplets primarily in the size range 3–10 mm, with a maximum at 5 mm. The estimated solid particle sizes shown in Table 3 indicate that wet droplets (3–10 mm) containing dissolved yttrium nitrate or oxide should dry into solid particles in the range 0.4–2.2 mm.Slurries are usually ground to particle sizes of 1–3 mm (depending on density), which would be expected to vaporize and atomize in a similar fashion as the calibration solution. The particles in the 3 mm Y2O3 slurry are too large for single occupancy. We also suggest that the standard additions protocol should work well when combined with slurry nebulization and the single occupancy criterion. If each droplet contains one particle, the solution phase elements can ‘condense’ onto this particle as the droplet dries.Thus, the elements from the calibration solution are released into the plasma in more or less the same way and time as the atomized elements from the solid slurry particle. Finally, we call the reader’s attention to the observation that the 0.1 and 3 mm slurries, either dry or wet, produce a noticeable line or set of discrete tracks along the axis of the plasma (Figs. 4 and 5).These tracks persist for the entire length of the NAZ, even for the 0.1 mm slurry, which suggests that the particles are not fully atomized, although suYcient yttrium is atomized and ionized to provide the usual ambient a b emission structure. Fig. 5 Drawings contrasting appearance of ICP during introduction This observation supports those of several others concerning of 0.1 mm Y2O3 slurry particles. a, Particles introduced as wet droplets; atomization of slurries of refractory solids.Raemaekers et al. b, particles introduced as dry, desolvated particles. The dried slurries (Fig. 6 of ref. 15) found that the Al emission sensitivity from produce a wider line of particle tracks through the center of the plasma. Also, the wet droplets produce very few large red droplet an Al2O3 slurry (mp=2045, bp=2980 °C)39 was close to but clouds. 2–10% below that from a solution of Al, with better agreement at lower aerosol gas flow rate. Ebdon and co-workers40,41 introduced wet MgO particles (mp=2852, bp=3600 °C)39 with Table 2 Calculated values for number of solid Y2O3 particles per a mean size of 2 mm and a maximum size of 4 mm.They found droplet wet of given size, slurry nebulization that these particles were atomized with an eYciency of 80–85%. No. of solid particles The yttrium oxide particles (mp=2410 °C, bp=?) 39 used in Solid Y per wet droplet of specified size this work are in this same size range, are also refractory, and particle concentration thus would not be expected to atomize fully in an ICP.We Size/mm (ppm) 0.5 mm 2.5 mm 5.0 mm 10mm suggest the un-atomized fraction travels through the plasma 0.1 100 000 3 400 3000 30 000 3.2 10 000 1×10-5 1×10-3 0.01 0.08 8.5 10 000 5×10-7 6×10-5 5×10-4 4×10-3 Table 3 Calculated values for diameter of solid particle of yttrium oxide or nitrate at 10 000 ppm in solution produced by drying wet droplets of particular sizes pancy of a droplet by multiple particles decreases rapidly with Estimated diameter of dry particle increasing particle size.Thus, very few droplets would be (mm) for Diameter of wet expected to contain more than one 3.2 mm particle, so the droplet/mm Y2O3* Y(NO3)3† white streaks in Fig. 4 are probably caused by individual particles rather than agglomerates formed in the plasma. 0.5 0.07 0.11 For the larger slurry particles, most of the droplets that 1 0.14 0.22 leave the spray chamber contain no Y2O3. Nevertheless, the 2 0.27 0.45 ICP still shows the usual background structure of red IRZ 3 0.41 0.68 5 0.68 1.1 followed by blue NAZ.Thus, some yttrium is atomized 10 1.4 2.2 upstream in the plasma to provide the necessary Y atoms and 20 2.7 4.5 ions. Presumably, this ambient yttrium comes from relatively 30 4.1 6.8 small droplets that beat the odds by containing an Y2O3 50 6.8 11.3 particle. These ‘lucky’ droplets that contain yttrium then dry early enough for at least some atomization of yttrium.Some * Density=5.0 g cm-3, ref. 39. † Density=2.7 g cm-3, ref. 39. of these ‘lucky’ droplets are also sprayed outwards radially 1146 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12inside the white axial streak(s) depicted in Figs. 4 and 5. As CONCLUSIONS noted above, these streaks would pass straight into the The photographs presented here illustrate several points sampling orifice of an ICP-MS device. pertinent to practical analysis with the ICP: 1. Desolvation removes the deleterious eVects of wet droplets.Comparisons With Olesik’s Studies 2. There are two general ways that wet droplets dry into solid particles: (a) by gradual removal of the solvent, and (b) Many of the phenomena from these photographs agree closely by gradual drying followed by rapid explosion of a residue. with observations made by Olesik and co-workers,2–9 who use 3. Wet droplets that are injected oV-axis are dried more time-resolved emission, fluorescence, scattering and MS to eVectively than those that pass through the plasma on-center.study the fate of wet droplets and the subsequent solid particles 4. When wet droplets are introduced, the position of the in the ICP. In the subsequent discussion, we refer mainly to IRZ varies with time. Also, there is a third analyte zone along our observations made during introduction of wet aerosols. the central axis consisting of thin streaks caused by the residues 1.A wet droplet ‘cools’ a zone 1–2 mm in diameter, which of the wet droplets. These faint streaks could be due to small corresponds roughly to the width of the droplet clouds shown solid particles formed as the droplets dry, as suggested by in Fig. 1. Olesik and co-workers. These streaks are most abundant right 2. Large red clouds from wet droplets are much more in the usual sampling position used for ICP-MS. numerous than the fine streaks from particles. 5. Introduction of yttrium as Y2O3 particles in slurries 3.Small droplets desolvate and atomize upstream in the produces the usual ambient emission structure with intact solid axial channel, probably inside the section enclosed by the particles localized along the center line of the plasma. These induction region, and provide the ambient emission structure slurry particles behave similarly to, but not the same as, of the ICP. dissolved solutes. Y2O3 particles of 3 mm diameter can be seen 4. Most droplet clouds or particle tracks are seen on-center to survive intact all the way down the central axis of the near the IRZ at high aerosol gas flow rate.plasma. These particles are larger than the calculated size of 5. Solid particles survive only a short distance after the 1.2 mm for the single occupancy criterion for Y2O3 particles in droplet dries. The free neutral atoms are then excited and 10 mm wet droplets at 1% total slurry loading. ionized rapidly. 6. Blue Y+ emission is formed quickly after the solid par- 6.There is no exact ‘vaporization position’ for solid particles ticles decompose, as if a significant fraction of the excited Y+ from polydisperse droplets. Instead, vaporization occurs over is formed directly from neutral Y in a single elementary step. a finite range of axial positions. 7. The larger wet droplets often appear together in bunches There are two important diVerences in the observations of two or three droplets, as suggested by Montaser and described in this work compared with those of Olesik and co-workers.23,24 co-workers.First are the ‘explosions’ shown in Fig. 1 and Olesik42,43 has recently published two new papers that are discussed above. Second is the basic cause of the red droplet directly relevant to the fates of droplets and particles in the clouds. Olesik’s studies with dual nebulizers indicate that many ICP. The first paper is mainly a review of previous measure- of the droplet clouds represent emission from ytttrium that is ments and describes many of the observations cited above.already present as atoms and ions and is cooled by the water For example, neutral atoms of Sr and Y are very rapidly evaporating from the droplet. If this is the main cause of converted into excited ions, i.e., in 100 to 150 ms. Strontium droplet clouds, introduction of wet slurries would show about atom emission produces a cloud that becomes only #1 mm the same number of isolated red droplet clouds as the nebulized diameter until it disappears as the neutral Sr is converted into solutions, and the number of isolated red clouds would be Sr+ ions.The size of this Sr atom emission cloud is similar to about the same for any of the diVerent size slurries used, since that of the ‘explosions’ shown for yttrium nitrate in Fig. 2. The the same nebulizer and spray chamber are used for either Sr+ emission clouds shown are of similar size as those reported sample.As indicated previously, we see red droplet clouds in the present work.42 from slurries only well upstream in the axial channel near the The second paper shows new results obtained with mono- tip of the IRZ. disperse droplets that have been dried partially to approxi- This diVerence of opinion may be partly due to diVerences mately 13 mm diameter. Such droplets produce fluorescence in the droplet size distribution transmitted by the spray clouds from excited Sr+ in its lowest electronic state that are chambers used.The size measurements for tertiary aerosols of similar size as the emission clouds from excited Sr+ i.e. 3, (i.e., those droplets leaving the spray chamber) reported by to 4 mm diameter depending on the distance from initial Olesik and Fister3 show a substantial number of droplets production of neutral Sr atoms. The fluorescence signal from between 10 and 20 mm, whereas our previous measurements Sr atom falls to zero only 50 ms after it reaches its maximum. show very few droplets above 10 mm diameter.38 Very large The total lifetime of the Sr atom cloud is only #70 ms, which droplets above 10 mm, if present in greater numbers in Olesik’s is fast enough to cause the ‘explosion’ clouds shown above to plasma than in ours, could account for their view that many be spherical rather than elongated.43 It is possible that the droplet clouds represent emission from material already atom- ‘explosions’ shown in the present work are merely small clouds ized but cooled by passage of the droplet through the plasma, of emission from neutral Y atoms.This explanation does not as opposed to our clear evidence that an isolated droplet cloud account for the fact that these clouds are seen in only a small is caused by emission only from yttrium within that particular fraction of the frames. Also, the total lifetime of the Sr atom droplet. However, aerosol size distributions measured with cloud measured by Olesik is #70 ms,43 which should be long diVerent devices may not be closely comparable.enough to cause the ‘explosion’ clouds shown in the present Another possible diVerence between our measurements and work to be elongated rather than spherical. those of Olesik and co-workers may be related to the axial location of the droplet clouds. Our photographs are primarily sensitive to red clouds that are physically separate from the Ames Laboratory is operated by Iowa State University time-averaged red emission of the IRZ.Olesik’s work shows for the US Department of Energy under Contract No. that the droplet clouds are most abundant near the tip of the W-7405-Eng-82. This research was supported by the OYce of IRZ, and it is possible that we miss most of the droplet clouds that are not completely separated from the ambient IRZ. Basic Energy Sciences. The authors are grateful to John Olesik, Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 114722 L’vov, B. V., and Novichikhin, A. Spectrochim. Acta, Part B, 1995, Barry French and Gary Hieftje for valuable discussions con- 50, 1427. cerning the interpretation of these observations. 23 CliVord, R. H., Sohal, P., Liu, H., and Montaser, A., Spectrochim. Acta, Part B, 1992, 47, 1107. 24 McLean, J. A., HuV, R. A., and Montaser, A., paper presented at REFERENCES the Federation of Anal. Chem. Spectrosc. Socs. Conference, Kansas City, MO, September 1996, Paper No. 494. 1 Cicerone, M. T., and Farnsworth, P. B., Spectrochim. Acta, Part 25 Lam, J. W., and McLaren, J. W., J. Anal. At. Spectrom., 1990, B, 1989, 44, 897. 5, 419. 2 Olesik, J. W., Smith, L. J., and Williamsen, E. J., Anal. Chem., 26 Tsukahara, R., and Kubota, M., Spectrochim. Acta, Part B, 1990, 1989, 61, 2002. 45, 581. 3 Olesik, J. W., and Fister, J. C., III, Spectrochim. Acta, Part B, 27 Jakubowski, N., Feldmann, I., and Stuewer, D., Spectrochim. 1991, 46, 851 and 869.Acta, Part B, 1992, 47, 107. 4 Hobbs, S. E., and Olesik, J. W., Anal. Chem., 1992, 64, 274. 28 Jakubowski, N., Feldmann, I., and Stuewer, D., J. Anal. At. 5 Hobbs, S. E., and Olesik, J. W., Spectrochim. Acta, Part B, 1993, Spectrom., 1993, 8, 969. 48, 817. 29 Peters, G. R., and Beauchemin, D., Spectrochim. Acta, Part B, 6 Olesik, J. W., and Hobbs, S. E., Anal. Chem., 1994, 66, 3371. 1993, 48, 1481. 7 Dziewatkoski, M. P., Daniels, L. B., and Olesik, J. W., Anal. 30 Hutton, R.C., personal communication, 1995. Chem., 1996, 68, 1101. 31 Wiederin, D. R., paper presented at the Federation of Anal. 8 Olesik, J. W., and Dziewatkoski, M. P., J. Am. Soc. Mass Chem. Spectrosc. Socs. Conference, Kansas City, MO, September Spectrom., 1996, 7, 362. 1996, Paper No. 490. 9 Olesik, J. W., Anal. Chem., 1996, 68, 469A. 32 French, J. B., personal communication, 1995. 10 Winge, R. K., Eckels, D. E., DeKalb, E. L., and Fassel, V. A., 33 French, J. B., Etkin, B., and Jong, R., Anal. Chem., 1994, 66, 685. J. Anal. At. Spectrom., 1988, 3, 849. 34 Watson, J. T., Introduction to Mass Spectrometry, Raven, New 11 Winge, R. K., Crain, J. S., and Houk, R. S., J. Anal. At. Spectrom., York, 1985, p. 131. 1991, 6, 601. 35 Willoughby, R. C., and Browner, R. F., Anal. Chem., 1984, 56, 2626. 12 Scott, R. H., Fassel, V. A., Kniseley, R. N., and Nixon, D. E., 36 Vickery, R. C., T he Chemistry of Yttrium and Scandium, Pergamon, Anal. Chem., 1974, 46, 75. New York, 1960, p. 52. 13 Fassel, V. A., and Bear, B. R., Spectrochim. Acta, Part B, 1986, 37 Goodall, P., Foulkes, M. E., and Ebdon, L., Spectrochim. Acta, 41, 1089. Part B, 1993, 48, 1563. 14 Van Borm, W. A. H., Broekaert, J. A. C., Klockenkamper, R., 38 Wiederin, D. R., and Houk, R. S., Appl. Spectrosc., 1991, 45, 1408. Tscho� pel, P., and Adams, F. C., Spectrochim. Acta, Part B, 1991, 39 CRC Handbook of Chemistry and Physics, ed. Weast, R. C., CRC 46, 1033. Press, Cleveland, OH, 52nd edn., 1971, pp. B-64, B-106 and B-153. 15 Raemaekers, B., Graule, T., Broekaert, J. A. C., Adams, F., and 40 Ebdon, L., Foulkes, M. E., and Hill, S., J. Anal. At. Spectrom., Tscho� pel, P., Spectrochim. Acta, Part B, 1988, 43, 923. 1990, 5, 67. 16 Koirtyohann, S. R., Jones, J. S., and Yates, D. A., Anal. Chem., 41 O’Hanlon, K., Ebdon, L., and Foulkes, M., J. Anal. At. Spectrom., 1980, 52, 1965. 1996, 11, 427. 17 Diehl, H., and Smith, G. F., Quantitative Analysis, Wiley, New 42 Olesik, J. W., Appl. Spectrosc., 1997, 51, 158A York, 1952, p. 32. 43 Olesik, J. W., Kinzer, J. A., and McGowan, G. J., Appl. Spectrosc., 18 Bastiaans, G. J., and Hieftje, G. M., Anal. Chem., 1974, 46, 901; 1997, 51, 607. Childers, A. G., and Hieftje, G. M., Appl. Spectrosc., 1986, 40, 939. 19 Bass, D. A., and Holcombe, J. A., Anal. Chem., 1987, 59, 974. 20 Jackson, J. G., Novichkin, A., Fonseca, R. W., and Holcombe, Paper 6/07579G J. A., Spectrochim. Acta, Part B, 1995, 50, 1423. Received November 7, 1996 21 Jackson, J. G., Fonseca, R. W., and Holcombe, J. A., Spectrochim. Acta, Part B, 1995, 50, 1449. Accepted April 30, 1997 1148 Journal of Analytical Atomic Spectrometry, October 1997, Vol

ISSN:0267-9477    

DOI:10.1039/a607579g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Reducing the Energy Distribution in a Plasma-source Sector-field Mass Spectrometer Interface THOMAS W. BURGOYNE, GARY M. HIEFTJE* AND RONALD A. HITES Department of Chemistry and School of Public and Environmental AVairs, Indiana University, Bloomington, IN 47405, USA. E-mail: hieftje@indiana.edu Ion-beam kinetic-energy distributions were measured with At Indiana University, we have constructed a plasma-source several sector-field plasma-source mass spectrometer interface mass spectrograph for simultaneous multi-element detecconfigurations. The original interface design produced relative tion.14,15 Because the interface to the mass spectrograph was energy distributions of 23% (with an ICP source) and 13% built first, it was coupled to a single-focusing mass spectrometer (with a GD source).These relative energy distributions were for characterization. Kinetic-energy measurements were comindependent of the interface potential. Accelerating more pleted on the ion beam, with both a dc GD and an ICP evenly and over a shorter distance (with a grid installed ) in the source.These preliminary experiments led to several instrumensecond vacuum stage dramatically reduced the relative energy tal modifications intended to reduce the relative energy distribution but also the signal level. The addition of an ICP distribution. torch shield also helped to reduce the relative energy distribution, presumably by lowering the plasma oVset voltage.EXPERIMENTAL New ion optics were designed with the distance between the Description of Original Interface apertures leading to the second and third vacuum stages decreased (to roughly 20 mm) and with the majority of the In our sector-PSMS interface (see Fig. 1), ions are accelerated ion-beam acceleration moved from the second to a short by floating the source and applying the accelerating potential distance into the third vacuum region. This configuration to the first vacuum stage (separated from the rest of the reduced the relative ion-beam energy distribution to roughly instrument with a 1 cm Teflon spacer).This arrangement 5% for both sources but also resulted in some loss of signal. ensures that large potential diVerences occur at either moderate vacuum (below 10-3 Torr; 1 Torr=133.3 Pa) or at atmospheric Keywords: Plasma-source sector-field mass spectrometry; pressure, to eliminate unwanted discharge. As a result, it is inductively coupled plasma; glow discharge; ion-beam energy; retarding-field energy analysis necessary to operate the first-stage roughing pump at the accelerating potential.Also, PVC wire-reinforced PVC tubing (McMaster-Carr, Chicago, IL, USA) (not metal wire-reinforced The shape and energy characteristics of the initial ion beam tubing) is used to eliminate a discharge between the first are critical determinants of focusing in sector-field MS. Ideally, vacuum stage and the pump. A 6 kV, 470 pF capacitor is one would want a monoenergetic ion beam with little or no attached between the sampling plate and the torch housing angular aberration; however, this goal is diYcult to achieve in (this capacitor is not needed when a ‘torch shield’16,17 is used, plasma-source MS (PSMS).The necessary series of diVerential see below). The capacitor (or the torch shield) capacitively pressure stages, combined with the requirement to accelerate couples the first vacuum stage to the rest of the instrument the ion beam to several keV, complicates the task.Of course, and eliminates discharge in the second pumping stage. With electric sectors in double focusing mass spectrometers can this capacitor in place, rf currents can pass the interface compensate, in part, for a kinetic-energy distribution (and without generating an appreciable rf oVset voltage; yet the additionally filter out ions of unwanted energies). However, a desired dc accelerating voltage is maintained.broad range of energies cannot be fully compensated, and The aperture diameters in the sampler and skimmer cones filtering ion energies ultimately results in a loss of signal. are 0.7 and 0.5 mm, respectively. The distance between the tip Early studies by Olivares and Houk1 and Fulford and of the sampling plate and skimmer tip is 10 mm. An electrically Douglas2 were aimed at characterizing the kinetic-energy isolated lens tunnel is located behind the skimmer (lens-tunnel distributions in PSMS instruments. Both studies were conducaperture= 1.0 mm, distance between skimmer aperture and ted with quadrupole mass spectrometers (in which the accelerlens- tunnel aperture=60 mm).This tunnel acts as both a lens ation of the ion beam is nominal) and an ICP source. The and as a pressure barrier between the second and third vacuum beam energy and distribution were determined by retardingstages (maintained by 300 and 230 l s-1 turbo pumps, respect- field energy analysis.3 This method of energy analysis on PSMS instruments has been used by others.4–6 The ion-beam energy ively).Three cylindrical lenses are located within the lens has been characterized also with a rf-powered GD source on tunnel. a double-focusing PSMS instrument.7 Energy was measured For this study, the interface was connected to a slightly by closing the slit behind the electric sector and, at a constant modified single-focusing Varian MAT CH7 mass analyzer. The magnetic field strength, scanning the acceleration voltage and angle and radius of deflection are 90° and 214 mm, respectively.measuring the subsequent ion transmission.8 Unfortunately, The entrance slit width was 400 mm and the exit slit width was this method does not take into account energy discrimination 100 mm. All spectra shown below were obtained with an within the ion optics and therefore does not measure the true acceleration potential of 3 kV and with the instrument optimenergy profile of the sampled ion beam.Though information ized solely for resolution. has been published concerning some characteristics of plasmasource sector-field mass spectrometers,7,9–13 no information Ion-beam Energy Measurement exists concerning ion-beam energy distributions generated from Ion-beam energies were determined by retarding-field energy the interface of these instruments, even though they have been in use for some time. analysis.3 A simple energy analyzer was constructed which Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1149–1153) 1149Fig. 1 Scale drawing of the original sector-field, ICP-MS interface. In the following figures, this lens tunnel and cylindrical lens system will be designated as ‘original ion optics’. Energy analyzing Faraday cup is pictured twice its size. Shaded areas represent insulator (Teflon or Macor) and diagonally ruled areas represent stainless steel (type 304).See text for design specifications. consists of a retarding field (grid) in front of an ion collector (Faraday cup). Ions of energy greater than the retarding field pass through the grid to the collector and produce a current; ions of energy less than the retarding field are repelled and do Fig. 2 Example ion-beam energy measurement with interface at not make it to the collector. Gradually increasing the retarding 500 V. (a) EVect on signal as grid potential is increased. (b) Derivative field results in a drop in the collected current.The derivative of signal vs. grid potential plot. Energy distribution is measured at FWHM. of the current versus retarding-potential curve results in a peak whose full width at half maximum yields the desired energy distribution. Although this method of energy analysis is con- respectively. Deionized water was introduced at 1 ml min-1 venient and simple, it has shortcomings that include errors and nebulized with a glass-concentric nebulizer and a Scottfrom a nonuniform retarding potential and scattering from the type spray chamber at room temperature.A 100 ppm Zr grids.18,19 solution in dilute HNO3 was used to evaluate resolution (see The retarding-field energy-analyzing Faraday cup (see Fig. 1) below). Unless stated otherwise, all lenses (including the lens was located roughly 190 mm from the lens-tunnel aperture. tunnel) were maintained at ground potential. The Faraday cup consists of an outer grounded shell housing The dc GD source was similar to that used in a previous a brass collector (10.6 mm id by 15.7 mm long) behind two study.20 The GD sample was machinable brass (60–63% Cu, grids (2 mm apart, 2.76 lines mm-1, Buckbee-Mears, St.Paul, 2.5–3.7% Pb, 0.35% Fe and remainder Zn) and was positioned MN, USA). Grid 1 is maintained at ground potential (to 4 mm from the skimmer orifice. The GD was maintained at minimize fringing fields from grid 2), and grid 2 is electrically 900 V and 10 mA.Common first-, second- and third-stage isolated in order to apply the retarding-field potential. vacuum pressures were 0.35, 1×10-4 and 7×10-7 Torr, The ion-beam energy measurement was computer-controlled respectively. The GD was used to eliminate rf from the ICP by means of a programmable high-voltage power supply as a possible source of the energy distribution. All other (Spellman, Plainview, NY, USA) (for the retarding potential) experimental conditions are identical to those used with the and a picoammeter (Keithley Instruments, Cleveland, OH, ICP.USA) (for the ion current). All data-collection and analysis programs were written in LabVIEW (National Instruments, Instrumental Conditions Austin, TX, USA). Approximately 100 data points were collected at a resolution of roughly 4 V/point; an average scan The beam energy was measured with four diVerent instrument configurations. Measurements were first performed with the took 70 s.The derivative of the collector current was smoothed (5 point moving average smooth) and plotted against the initial instrumental design described above. In the second arrangement, a grid was positioned 10 mm in front of the retarding potential. The energy distribution (full width at half maximum) was manually determined and recorded. Each third-stage orifice (see second-stage grid in Fig. 1). This grid was used to provide a shorter and more uniform accelerating beam-energy measurement experiment was conducted approximately six times and made at a variety of accelerating potentials field in the second stage.In the third set-up, a torch shield16,17 (with the ICP) was installed to eliminate an orifice-linked from 40 to 2500 V. Fig. 2 is an example of an ion-beam energy measurement with the accelerating potential set at 500 V. discharge and to reduce the ICP oVset voltage. The secondstage grid behind the skimmer was in place during this During all of the ion-beam energy measurements, the forward rf power to the ICP was maintained at 1.35 kW, with experiment.Lastly, an entirely new ion-optic configuration (see Fig. 3) was designed based on the experimental results with the reflected power approximately 5 W. The distance from the top of the load coil to the sampling plate was 10 mm. The the previous instrumental conditions. Although the reasons for the design in Fig. 3 are presented load coil was top grounded (coil closest to the sampler was grounded) to the torch housing.The outer, intermediate and later, a brief description of the new ion-optic arrangement is appropriate here. The new lens tunnel incorporates a third- central-channel argon flow rates were 14.0, 1.06 and 1.00 l min-1, respectively. Typical first-, second- and third- stage aperture (aperture diameter=0.5 mm) placed 24 mm behind the skimmer orifice. The smaller third-stage aperture stage vacuum pressures were 0.8, 4×10-4 and 1×10-6 Torr, 1150 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Fig. 3 Improved lens tunnel with quadrupole doublet. In the following figure captions, this configuration is designated as ‘new ion optics’. Material designation same as in Fig. 1. See text for design specifications. reduced the ion-beam throughput but improved the vacuum within the lens-tunnel area. The beam is then accelerated between two grids, 4.76 mm apart. The subsequent ion lenses consist of an einzel lens (first and third lens grounded) with an inner diameter of 19 mm and a thickness of 5 mm.Next is a quadrupole doublet, also with an inner diameter of 19 mm. Each quadrupole rod has a radius of 10.9 mm and is 31.8 mm long. Fringing fields are reduced between each of the quadrupoles by insertion of a 3.18 mm wide shunt between the Fig. 4 EVect of interface potential on relative energy spread with GD (squares) and ICP (circles) sources for the original ion-optic design (a) quadrupoles. All of the optics are electrically isolated and and the original ion-optic design with second-stage grid (b).Note separated by sapphire balls (2.38 mm diameter, Swiss Jewel diVerent vertical scales in (a) and (b). Dashed line represents the Company, Philadelphia, PA, USA). average relative energy distribution over all interface potentials. Each data point is the average of several ion-beam energy measurements and the error bars represent the standard deviation of that average.RESULTS AND DISCUSSION See Fig. 1 for reference. Ion-beam Energy Comparison ment. Ion-trajectory simulations confirmed that fewer ions The initial interface arrangement resulted in a 23±5% relative enter the third-stage orifice with the second-stage grid in place. energy distribution with the ICP source and a 13±0.2% Trajectory simulations showed that, with the second-stage grid relative energy distribution with the GD source [see Fig. 4(a)]. installed, any focusing eVect in the second stage is lost and These energy measurements were made with and without iononly ions with a small angle of divergence pass through the beam focusing and resulted in no observable diVerence in the third-stage aperture. outcome. Obviously, these values are too high (even given the Adding a torch shield16,17 to the ICP source (with the large standard deviation) to be overcome by focusing or second-stage grid installed) further reduced the relative energy filtering (filtering would result in an unacceptably large loss in distribution from 6.2±0.6% to 5.2±0.8% (see Fig. 5). The signal). The source of the measurement variance is instrumental Student t-test indicated that addition of the torch shield made drift over the time period required for taking the measurements. a statistically significant diVerence. Eliminating the plasma This drift tends to be more dramatic at lower beam currents. oVset voltage and, as a consequence, the secondary discharge Our preliminary hypothesis was that the energy distribution is probably the reason for this reduction. originated in the second pumping region of the interface.With the original ion-optic arrangement, all of the acceleration occurs in the second pumping region, where the pressure is moderately high, and the sampled beam is probably neither a pure ion beam nor a true plasma. Ions may make the transition from ‘plasma’ to ion at diVerent locations in the second pumping region.Accelerating in this region and in an uneven fashion (curved equipotential lines formed from the diVerence in potential between the skimmer and the lens tunnel) possibly resulted in the large energy spread. Collisions in this relatively high pressure region could also add to the energy spread. To remove this origin of the energy distribution, we added a second-stage grid (see Fig. 1) which delayed acceleration of the ion beam over parallel equipotential lines.Addition of this grid reduced the relative energy distribution dramatically to 6.2±0.6% with the ICP source and 4.5±0.5% with the GD source [see Fig. 4(b)]. Unfortunately, the grid reduced the Fig. 5 EVect of interface potential on relative energy spread with ICP signal level as well; the ion beam had to be focused onto the source. Original ion optics with second-stage grid without torch Faraday collector to produce enough current to perform the shielded (triangles) and with torch shielded (circles).Each data point energy analysis. The signal was so small that, in fact, it was is the average of several ion-beam energy measurements and the error bars represent the standard deviation of that average. not possible to generate a mass spectrum with this arrange- Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1151were made to a distant vacuum feedthrough, making assembly diYcult. Also, portions of these unshielded wires might have interfered with the ion beam and caused defocusing.An additional electrical feedthrough port was added to the outer case of the second vacuum stage; it is not drawn in Fig. 1. This new ion-optic design reduced the relative energy distribution to 5.2±0.2% with the GD source and 5.0±0.5% with the ICP source (see Fig. 6). For these measurements, the potential diVerence between the interface and the lens-tunnel cap was maintained at 200 V for the GD and the ICP sources (see below for details).Unlike with the second-stage grid, the ion signal was not seriously compromised with the new lens system. Spectral Comparison Fig. 6 EVect of interface potential on relative energy spread with GD Mass-spectral scans taken with the initial and new ion-optic (squares) and ICP (circles) sources for new ion optics (torch shielded). designs also demonstrate the diVerence in energy distribution. Each data point is the average of several ion-beam energy measurements and the error bars represent the standard deviation of that All spectra have been optimized for resolution, at times comproaverage.See Fig. 3 for new ion-optic design. mising signal. In addition, all spectra have been normalized in amplitude to compensate for an observed change in the electron With the above experiments in mind, a new ion-optic system multiplier gain over time (as is evident from the diVerence in was designed. The goals of the new system were: (a) to reduce signal-to-noise ratios). Figs. 7(a) and (b) show GD mass spectra the amount of acceleration that occurs in the second vacuum of lead with the original ion optics and the new ion optics. On region (where collisions are frequent) and instead to have the the basis of curve-fitting techniques, the calculated baseline majority of the acceleration occur in the third vacuum region resolution with the GD source is 210 with the original ion and over an even, short distance.Also, we wanted to reduce optics and 250 with the new ion optics. The improvement in the length of time the ions spend in the second vacuum region. resolution is slight and is probably due to a substantial amount Again, it was assumed that the majority of the energy distri- of energy discrimination that occurs in the original ion optics. bution originated in the second pumping stage. (b) To eliminate Figs. 8(a) and (b) are ICP mass spectra of Zr and display a Macor. Signal instability with the original ion-optic design more dramatic improvement in resolution. The calculated basewas attributed to ceramics exposed to the ion beam.(c) To line resolution with the ICP source is about 20 with the original improve focusing with an einzel lens and quadrupole doublet. ion optics but 140 with the new ion optics. The calculated The original ion-optic system consisted of three cylindrical energy distribution from the measured resolution does not lenses. To shape the beam more eVectively onto a slit, a pair match the measured energy distribution, indicating that some of quadrupole lenses was employed.The einzel lens is used to energy discrimination within the ion optics is occurring. help focus the beam along the y-direction (in and out of the plane of Figs. 1 or 3). (d) To improve electrical feedthroughs Throughput Comparison for lenses (hence the electrical feedthrough port in Fig. 3). The original design had no such option and electrical connections As stated above, signals obtained with the diVerent ion-optic arrangements could not be compared directly because of the variation in electron multiplier gain.However, a rough signal comparison with the ICP source is useful. With the interface at 500 V and all lenses grounded, the original ion-optic design produced a signal at the Faraday cup of 3×10-10 A. The signal at the Faraday cup with the new ion-optic design was 3×10-12 A with the interface at 500 V, with the lens tunnel cap at 300 V and with all other lenses at ground potential.Part of this 100-fold diVerence can be attributed to the reduced aperture diameter of the lens tunnel (1 mm vs. 0.5 mm diameter). However, there is still roughly a factor of 25 diVerence in signal between the two ion-optic designs. Recently, it was discovered that there was poor alignment among the first-, second- and third-stage apertures. A diVerence in alignment between the first and second lens tunnel designs is the most likely reason for the apparent drop in signal.Energy Distribution Origin Characteristics In an attempt to determine the origin of the energy distribution, we assume that the energy distribution is somehow a result of the acceleration potential applied to the first-stage vacuum chamber; these comparatively large energy distributions are not experienced in grounded PSMS interfaces.1,2 It is unlikely that the distribution originates in the first vacuum region or ion source of the instrument since neither the GD nor ICP source generates this wide an energy distribution.Additionally, the energy distribution is a function of the interface potential; Fig. 7 dc GD mass spectra of lead from machinable brass with the original ion optics (a) and with the new ion optics (b). no potential diVerence arises in the first pressure region. 1152 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12CONCLUSIONS Ion-beam relative energy distributions from several sector-field PSMS interface configurations were measured and were reduced from 23% (ICP source) and 13% (GD source) to roughly 5% for both sources.A beam with a narrow energy distribution could be obtained by reducing the distance between the second- and third-stage pumping regions and by accelerating the beam in the third vacuum region and over a short distance. It seems likely that the beam extracted from the second stage has both plasma and ion-beam characteristics.Accelerating the ‘plasma-beam’ unevenly and over a relatively long distance (as in our original design) results in a large energy distribution. By the third vacuum region, the sampled beam probably consists principally of ions, so accelerating it evenly over a short distance results in a dramatically reduced energy distribution with only a slight loss in signal. A similar technique is employed in the Finnigan high-resolution mass spectrometer.12 The addition of a torch shield also helped reduce the energy distribution.The authors thank Gangqiang Li for informative discussions on ion physics. David Solyom aided in the development of this instrument. The instrument was constructed by the Mechanical Instrument Services and Electronic Instrument Services at Indiana University, whose support was invaluable. The improved lens tunnel with quadrupole doublet was made Fig. 8 ICP mass spectra of 100 ppm zirconium with the original ion by Delbert Allgood and John Dorsett.Supported (in part) by optics (no second-stage grid or torch shield) (a) and with the new ion the National Institutes of Health through grant GM-53560 optics (b). and by the US Department of Energy through grant 87ER-60530. Therefore, the source is limited to the second or third vacuum region. REFERENCES The relative energy distribution is independent of the accelerating potential for any of the ion-optic configurations presented 1 Olivares, J.A., and Houk, R. S., Appl. Spectrosc., 1985, 39, 1070. here. In Fig. 6, the second vacuum stage acceleration was held 2 Fulford, J. E., and Douglas, D. J., Appl. Spectrosc., 1986, 40, 971. 3 Simpson, J. A., Rev. Sci. Instrum., 1961, 32, 1283. constant and overall acceleration was changed in the third 4 Chambers, D. M., and Hieftje, G. M., Spectrochim. Acta, Part B, stage. The relative energy distribution remains constant and is 1991, 46, 761. not a function of the third-stage acceleration potential, for 5 Tanner, S.D., J. Anal. At. Spectrom., 1993, 8, 891. both the GD and ICP. The energy distribution was measured 6 Cable, P. R., and Marcus, R. K., Appl. Spectrosc., 1995, 49, 917. with the same interface configuration as in Fig. 6 but with the 7 Duckworth, D. C., Donohue, D. L., Smith, D. H., Lewis, T. A., and Marcus, R. K., Anal. Chem., 1993, 65, 2478. interface held at 1000 V and with diVerent accelerating voltages 8 Strobel, F. H., and Adams, J., J.Am. Soc. Mass Spectrom., 1995, applied in the second vacuum region (see Fig. 9). The relative 6, 1232. energy distribution remains fairly constant and is also not a 9 Bradshaw, N., Hall, E. F. H., and Sanderson, N. E., J. Anal. At. function of the second-stage acceleration potential, for both Spectrom., 1989, 4, 801. the GD and ICP. From this, we infer that the important factor 10 Kim, C.-K., Seki, R., Morita, S., Yamasaki, S., Tsumura, A., Takaku, Y., Igarashi, Y., and Yamamoto, M., J.Anal. At. in the energy distribution is the overall acceleration potential Spectrom., 1991, 6, 205. and not the individual acceleration steps in each vacuum stage. 11 Feldmann, I., Tittes, W., Jakubowski, N., Stuewer, D., and Giessmann, U., J. Anal. At. Spectrom., 1994, 9, 1007. 12 Giebmann, U., and Greb, U., Fresenius’ J. Anal. Chem., 1994, 350, 186. 13 Halliday, A. N., Lee, D., Christensen, J. N., Walder, A. J., Freedman, P. A., Jones, C. E., Hall, C. M., Yi, W., and Teagle, D., Int. J. Mass Spectrom. Ion Processes, 1995, 146–147, 21. 14 Burgoyne, T. W., Hieftje, G. M., and Hites, R. A., Proceedings of the 44th ASMS Conference on Mass Spectrometry and Allied T opics, Portland, OR, May 12–16, 1996, WPC46, American Society for Mass Spectrometry, Santa Fe, NM. 15 Burgoyne, T. W., Hieftje, G. M., and Hites, R. A., J. Am. Soc. Mass Spectrom., 1997, 8, 307. 16 Gray, A. L., J. Anal. At. Spectrom., 1986, 1, 247. 17 Sakata, K., and Kawabata, K., Spectrochim. Acta, Part B, 1994, 49, 1027. 18 Wei, P. S. P., and Kuppermann, A., Rev. Sci. Instrum., 1969, 40, 783. 19 Enloe, C. L., and Shell, J. R., Rev. Sci. Instrum., 1992, 63, 1788. 20 Myers, D. P., Heintz, M. J., Mahoney, P. P., and Hieftje, G. M., Appl. Spectrosc., 1994, 48, 1337. Fig. 9 EVect of second-stage acceleration voltage on relative energy spread with GD (squares) and ICP (circles) sources. Large error bars Paper 6/07861C (standard deviations) on some of the measurements are due to low Received November 20, 1996 signal levels. The signal was consistently greatest at a second vacuum Accepted April 15, 1997 stage acceleration voltage of 75 V with the ICP and a second vacuum stage acceleration voltage of 150 V with the GD. Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1153

ISSN:0267-9477    

DOI:10.1039/a607861c

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Speciation of Chromium by Direct Coupling of Ion Exchange Chromatography With Inductively Coupled Plasma Mass Spectrometry CARSTEN BARNOWSKIa , NORBERT JAKUBOWSKI *a , DIETMAR STUEWERa AND JOSE� A. C. BROEKAERTb aInstitut fu� r Spektrochemie und Angewandte Spektroskopie, Postfach 10 13 52, D-44013 Dortmund, Germany bUniversita�t Dortmund, Fachbereich Chemie, D-44221 Dortmund, Germany Ion exchange chromatography has been coupled with tion spectrometry (AAS),3,4 inductively coupled plasma atomic emission spectrometry (ICP-AES)5,6 and inductively coupled inductively coupled plasma mass spectrometry (ICP-MS) for the element speciation of chromium.Hydraulic high pressure plasma mass spectrometry (ICP-MS)7 are especially promising. Special sample introduction techniques with high eYciency, nebulization was used for sample introduction with high aerosol eYciency. It was the particular aim of this work to like thermospray nebulization (TSN),8 ultrasonic nebulization (USN),9 hydraulic high pressure nebulization (HHPN)10 and exclusively use nitric acid for elution in order to reduce interferences in ICP-MS.Therefore, an anion exchange pre- direct injection nebulization (DIN)11 have been applied successfully in the speciation of trace levels of Cr by ICP-AES column IonPac-AG5 (Dionex, Idstein, Germany) with a length of 50 mm and an id of 4 mm has been operated as a and ICP-MS.5,11–14 Application of chromatographic separation techniques in mixed-mode column, which shows cation exchange capabilities too.The column was operated at a flow rate of 1.2 ml min-1 combination with ICP-MS was described in an early paper by Roehl et al.7 Later on, ion pairing chromatography15 and and samples were injected by use of a sample loop with a volume of 100 ml. Both CrVI anions and CrIII cations could be reversed phase chromatography,16 as well as chromatography with microbore columns,17,18 with activated alumina19 and retained on the column, and optimum results concerning chromatographic and mass spectrometric conditions were with ion chromatography, were also applied successfully.20–23 In the application of ICP-MS in the speciation of Cr, a finally achieved with discontinuous elution in two steps by injection of 0.3 M (pH 0.5) HNO3 for CrVI and subsequently of major limitation generally arises from the occurrence of nonspectral as well as spectral interferences.Especially in the case 1.0 M (pH 0) HNO3 for CrIII.Detection limits just above 0.1 mg l-1 could be realized. of Cr speciation, the latter may be a severe limitation, when elements like carbon or chlorine are present in the sample or Keywords: Chromium; speciation; ion exchange the eluent. From this point of view an LC technique is desirable chromatography; inductively coupled plasma; mass which permits the separation of the species of interest from spectrometry; hydraulic high pressure nebulization the matrix eluted in a dead volume and for which eluents compatible with ICP-MS can be applied.Thus, Kalcher et al.24 applied HHPN in combination with reversed-phase ion-pair Chromium is a ubiquitous element, not only through its chromatography with tetraethylammonium nitrate, for which occurrence in nature but also due to many anthropogenic water can be used as an eluent. Caruso et al.8,17 used a mixed- influences resulting from its widespread industrial applications. mode column with a home-made DIN system17 and also with In particular in environmental studies, its analytical determia home-made TSN system,8 for which lithium hydroxide could nation is inevitably connected with the problem of speciation, be used as an eluent.However, in their work 2,6- because in its two main oxidation states, CrIII and CrVI, it pyridinedicarboxylic acid (PDCA) was also used as a chelating diVers significantly in biological and toxicological behaviour: agent for CrIII in the eluent.Absolute detection limits of 3 pg CrIII is essential for the maintenance of the glucose tolerance for both species were reported. Very recently, Manninen et al.25 factor in the human body, whereas CrVI is known to be toxic proposed a two-column approach with ion exchange chroma- and carcinogenic.1 The toxic nature of the CrVI ions, on the tography for on-line determination of cationic CrIII and anionic one hand, must be attributed to their high oxidation potential CrVI with nitric acid as eluent and a conventional pneumatic and their relatively small size, which enables them to penetrate nebulizer for sample introduction.The chromatographic pro- cell membranes. The inoVensive nature of CrIII ions, on the cedure described in this work is very similar to this two- other hand, results from the fact that in the biotic environment column approach, but with the advantage of significantly CrIII usually appears in aquo-hydroxo complexes of the form reduced expenditure (one pre-column) and analysis time.[Cr(H2O)n(OH)6n]n-3, and their size excludes them almost Additionally for this investigation, a HHPN system was used entirely from penetrating cell membranes. to further improve LOD, as was done in the work of Kalcher This ambivalent nature is the reason why the development and co-workers,24 who used ion pairing chromatography for of speciation techniques with suYcient selectivity and high chromatographic separation.For comparison, a survey of sensitivity for Cr is a permanent challenge in analytical chemis- LOD, which were found in the literature for diVerent LC try. The selective reaction with diphenyl carbazide with subtechniques coupled with ICP-MS, is given in Table 1. sequent photometric detection is often used for determination In previous investigations, we studied the speciation of Cr of the hexavalent species in routine analysis.2 Selective techby ion pairing chromatography13 and also by complexation niques for speciation of Cr based mainly on solvent extraction, and solid phase extraction26 in combination with reversed- coprecipitation and electrochemical separation have been phase chromatography, HHPN and ICP-MS.In both cases applied so far. On-line selective determination of both species LOD were restricted by spectral interferences due to the can be achieved particularly successfully by chromatographic techniques coupled with atomic spectrometry.Atomic absorp- formation of ArC from the organic solvents used for elution, Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1155–1161) 1155Table 1 Detection limits (mg l-1) for CrIII and CrVI with diVerent filter with a pore diameter of 5 mm. Before introduction to the ICP-MS techniques (IC=ion chromatography) ICP torch, the aerosol produced passes through a home-made desolvation system, which comprises a heating stage and two Technique CrIII CrVI Reference cooling stages at diVerent temperatures. This permits eVective IC–ICP-MS — 1.0 7 drying of the aerosol, so that the solvent loading to the plasma IC–ICP-MS 0.3 0.5 25 is eVectively reduced. Finally, the aerosol is fed to the plasma HPLC–ICP-MS 0.4 1 22 torch via Tygon tubing.Operating conditions for the chromato- HPLC–DIN–ICP-MS 1.2 1.1 17 graphic and the sample introduction systems are listed in HPLC–DIN–ICP-MS 0.18 0.18 23 HPLC–HHPN–ICP-MS 0.6 1.8 15 Table 2.HPLC–HHPN–ICP-MS 0.2 0.1 26 As the ICP-MS system, a laboratory-developed instrument HPLC–HHPN–ICP-MS 0.3 0.3 24 with a 40 MHz ICP and a quadrupole mass analyser was used. HPLC–TS–ICP-MS 2.5 2.3 8 Details of its design and operation have been presented in HPLC–HHPN–ICP-MS 0.1 0.2 this work previous work, from which the optimum operational conditions were taken.27 The ICP operation parameters are included in Table 2. Careful optimization of the torch position, the ion although oxygen addition was consequently applied.In the optical lens system and the aerosol gas flow was performed present work we have therefore investigated the speciation of after every reactivation of the system or any change of the Cr with an ion exchange column, which can be modified to operation conditions. A dynode secondary electron multiplier show cation as well as anion exchangbilities and which in analogue mode was used for ion detection. can be eluted with nitric acid.In order to compare the performance (figures of merit, validation) of the speciation procedure with that of a commer- EXPERIMENTAL cial instrument, comparative measurements with a PQ2+ (VG Elemental, Winsford, UK) were made. ICP-MS Instrumentation For data acquisition for the isotopes 35Cl, 52Cr, 53Cr, 55Mn In a continuation of previous work,13 HHPN was chosen as and 103Rh, the peak jumping mode was chosen with an the sample introduction technique, because of its high nebuliz- integration time of 100 ms per isotope.The ratio of 52Cr and ation eYciency. A sketch of the experimental arrangement is 53Cr was always used to check for spectral interference by presented in Fig. 1. A dual-piston HPLC pump (Knauer, 40Ar12C, and for all chromatograms shown the isotope 52Cr Berlin, Germany) is used to force the carrier solution through was used throughout this investigation unless stated otherwise. the chromatographic system to the nebulizer. The special Ti For data evaluation, the raw data were exported to a PC using pumping head, developed for the needs of elemental analysis, Excel 5 and Origin 3.5 calculation programs.The peak area and the metal-free six-port injection valves (Knauer) minimise of the chromatographic signals was used as the analytical contamination. Three sample loops are provided, one with a signal. volume of 100 ml for sample injection and two with volumes of 2 and 5 ml, respectively, for eluent injection.They are Chromatography mounted on the injection valves to avoid contact between strong acids and the pumping head. The combination of both As the column for anion exchange chromatography, an IonPacloop volumes served to inject the solvents for re-conditioning AG5 (Dionex, Idstein, Germany) was used. This is a low cost of the column. As carrier liquid, 0.01 M nitric acid prepared guard column with a length of 50 mm and an id of 4 mm, with double-distilled water was used.Sample loops and con- filled with ion exchange resin with a particle size of 15 mm. necting capillaries between all parts are made of polyether The particles consist of poly(styrene–divinylbenzene) spheres ether ketone (PEEK), which is inert and pressure-resistant, as substrate, to which aminated latex particles are attached on and an inert syringe is used for sample introduction. the surface by sulfonic acid groups. These latex particles have The system for sample introduction by HHPN has already a diameter of approximately 0.1 mm and they are the carriers been described in detail.13 For the experiments performed, a for the actual anion exchange function by (-NR3)+ groups.Pt–Ir nozzle with a diameter of 15 mm was used. The nozzle According to the manufacturer, the column is designed for holder made from Ti is attached to the ground plate of the anion exchange with a special selectivity for anions of higher spray chamber which is made from polytetrafluoroethylene (PTFE).To prevent clogging, the nozzle is protected by a Ti Table 2 Compilation of operating parameters HHPN system— Sample uptake rate 1.2 ml min-1 Sample loop volume 0.1 ml Pressure 4.5 MPa Nozzle diameter 15 mm Desolvation system— Heating temperature 150 °C Cooling temperature, first stage 0 °C Cooling temperature, second stage -5 °C ICP-MS system I (self-developed)— Power 1200 W Aerosol gas flow rate 0.75 l min-1 Outer gas flow rate 25 l min-1 ICP-MS system II (VG PQ 2+)— Power 1350 W Reflected power <5W Aerosol gas flow rate 0.75 l min-1 Fig. 1 Experimental arrangement. 1: eluent reservoir; 2: double piston Intermediate gas flow rate 0.5 l min-1 HPLC pump; 3: sample loops; 4: chromatographic column; 5: HHPN Outer gas flow rate 15 l min-1 unit; 6: drain; 7: desolvation system, heating stage; 8 (a,b): desolvation Sampling distance 10 mm system, cooling stages; 9: ICP torch; 10: MS instrument. 1156 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12valences like oxoanions. The column material is stable over present. The latter has strong oxidizing properties, so that speciation of CrVI at a pH<1 should be avoided. Only the a pH range from 0 to 14. As eluent a mixture of NaHCO3–Na2CO3 is recommended. trication and the monoanion can be retained on the column. These considerations suggested that the chromatographic In order to realize the desired exclusion of carbon compounds, this recommendation for anion exchange was not separation of both species seemed promising at low pH values.followed, but nitric acid was chosen for conditioning of the column as well as for elution. This eliminated the crucial spectral interference by 40Ar12C+ at mass 52. The choice of Column Conditioning nitric acid is possible as a result of the robustness of the column material, which allows a wide range of pH values for With respect to the problem of interference in ICP-MS, the use of nitrate will generally be preferable.Therefore we have the sample and the eluent. Samples were usually injected without any pre-treatment to avoid conversion of the species brought the column into the nitrate form by adding 10 ml of 1 M HNO3 with subsequent flushing by double-distilled H2O and change of the equilibrium between the species. until a pH value of 7 was finally reached again. The eVectiveness of this procedure was checked by the injection of 100 ml of a Chemicals solution containing 10 mg l-1 of CrVI with H2O as carrier liquid.Subsequently, a volume of 5 ml of 1 M HNO3 was All solutions used were prepared with double-distilled water and sub-boiled nitric acid. Standards for CrIII and CrVI as injected from the third sample loop (cf. Fig. 1). This acid concentration proved to be optimum in providing, on the one CrO42- were prepared from stock solutions (Merck, Darmstadt, Germany) with a total Cr concentration of hand, complete recovery and on the other, preservation of the stability of the column material. 1000 mg l-1 on a day-to-day basis to guarantee stability, especially of the CrVI solution. For blank measurements double- After each chromatographic cycle, the column was re-conditioned. A variation in the volume of 1 M HNO3 applied distilled water was used. The eluent for the chromatographic separation consisted of sub-boiled nitric acid diluted to diVer- for regeneration showed a strong influence on the resulting chromatographic signal.With an increasing volume of nitric ent concentrations by double-distilled water. Acid concentrations above 1 M were generally excluded in order to preserve acid, the peaks become more uniform and display less scatter around the basic peak profile. A lower relative standard the integrity of the column material and the exchange resin. For the study of the elution of cations and anions from the deviation (RSD) of the peak area intensity values is obtained as well, whereas the increase of the total intensity is only weak.column, additional standard solutions were prepared for Mn, Rh and Cl (Merck, Darmstadt, Germany). For optimization Saturation is reached with a volume of 7 ml of nitric acid, and this value was used in all further measurements for recon- of the ion yields, a standard solution with indium at a concentration of 1 mg l-1 was used. As standard reference ditioning of the column. This serves simultaneously for cleaning, because otherwise blank values of Cr may become apparent material for total chromium, the NIST sample 1643a (Trace Elements in Water) was used.A further sample could be as a result of an accumulation of this element from the carrier solution on the column. obtained from the European Commission, Standards, Measurements and Testing Programme (formerly BCR) with the advantage that both chromium species are certified independently. It is available as a lyophilized sample, which for Elution stability reasons has been prepared from a CrIII–CrVI solution in a HCO3-–H2CO3 buVer.For analytical application the The most important step for optimization of the analytical procedure is the choice of the pH value, which proved to be solid standard was dissolved in a reconstituted HCO3-–H2CO3 buVer at a pH of 6.4 as recommended by the supplier. of basic importance for both the signal intensity and the chromatographic resolution. This influence was studied for anion exchange of CrVI and cation exchange of CrIII in indepen- RESULTS AND DISCUSSION dent measurements during which a volume of 100 ml from a solution containing 10 mg l-1 of the actual species was injected.The choice of the column and the chromatographic working conditions is based on the following considerations. The sub- The resulting signal was registered over 5 min. Selected examples of the chromatograms at varying pH for both species strate material for ion exchange resins is basically the same for anion exchange as for cation exchange. The aminated latex are presented in Fig. 2. The results of the measurements are summarized in Fig. 3 for the case of peak area integration with particles are attached only to the sulfonic groups for anion exchange, but in the case of cations the sulfonic groups on the baseline correction for both species. substrate themselves are already carriers of the exchange function. It may therefore be supposed that in the case of the anion exchange material the big latex particles will introduce Anion exchange a significant steric impediment which prevents full coverage of the sulfonic groups.If adequately conditioned, a certain For CrVI the pH of the eluent was varied between 1.3 and 0. With a retention time below 3 min the elution for all investi- exchange capacity will be preserved in anion exchange operation for cations also, so that a mixed bed property of the gated pH values is very fast and it can be shortened to about two minutes with decreasing pH value.The peaks in the column is realized in favour of the analytical performance, keeping in mind that the sulfonic acid groups are fully ionized chromatogram are broad and asymmetric with a certain tailing. At decreasing pH both peak form and intensity are improved at a pH>2, whereas the quaternary ammonium exchange sites are ionized over the whole pH range. significantly, which is demonstrated for the latter in Fig. 3. When varying the pH value from pH 2.3 to 0, the intensity The application of nitric acid as the eluent shifts the working range of the recommended eluent–buVer system from a pH increases strongly but comes to saturation below a pH of 1.0. This dependence reflects incomplete elution, as was confirmed range of 9–11 to more acidic values. This, of course, has severe consequences for the speciation analysis itself. In an eluent by checking the Cr signal during the regenerating cycle with 1 M HNO3.In order to realize complete elution and corre- with pH<4, CrIII exists only as the trivalent species [Cr(H2O)6]3+, whereas at a pH<1, for CrVI, both the mono- sponding maximum intensity for CrVI, a pH<0.5 should be chosen. valent oxoanion HCrO4- and the neutral H2CrO4 may be Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1157Fig. 4 Influence of nitric acid concentration (A: pH 0; B: pH 0.05; C: pH 0.1) on the separation of Cr species (10 mg l-1 each) by continuous elution; constant retention for CrVI, varying retention for CrIII.of each. Nitric acid was injected from the 5 ml loop (cf. Fig. 1) at pH values in the range 0.1–0.0. The chromatograms in Fig. 4 demonstrate that separation of the species is possible for all three pH values used. The peaks for CrVI appear at a constant retention time and with an identical profile, whereas Fig. 2 Retention of chromium species with varying concentrations of both vary for CrIII, with a certain gain in intensity at lower nitric acid in the eluent; (a): CrVI (A: pH 0; B: pH 0.7; C: pH 1; D: pH values.As a compromise, a pH of about 0.1 can be chosen. pH 1.3); (b): CrIII (A: pH 0; B: pH 0.05; C: pH 0.1; D: pH 0.13). This permits suYcient chromatographic resolution, and then the signal intensities for both species diVer only by about 10%. To avoid a possible misunderstanding, it should be emphasized that both species are retained on the column and separated during elution by one eluent under compromise conditions.Injecting standard solutions of the single species ensures that no species conversion takes place on the column at the extreme conditions chosen; either for CrVI or for CrIII. Furthermore, no Cr signal was observed in the reconditioning cycle as well as in the solution within the dead volume of the column. It should be mentioned that in the case of real samples control of the eZuent within the dead volume is an important means of checking for losses as they may occur for instance in the presence of organometallic Cr compounds.25 Fig. 3 Influence of the acid concentration in the eluent on the analytical signals of CrIII (A) and CrVI (B) (10 mg l-1 each). Though basically possible, this mode of operation is not recommended for practical applications. On the one hand, at these compromise conditions the best performance for CrIII is Cation exchange not obtained, and on the other, CrVI can also exist in the For the cationic species CrIII, the retention times are similar strongly oxidizing form of H2CrO4, so that the risk of column to those obtained for CrVI, but the peaks are narrower and damage becomes too high.nearly symmetrical. As represented in Fig. 3, the range for the The best elution for both species according to the results in pH of the eluent was restricted to 0.3–0, which is only a very Fig. 3 is possible when working in two steps with diVerent pH small part of the range that can be covered in the case of CrVI.values. Therefore, three injection loops with the necessary As before, the completeness of the elution was checked after switching facilities are provided in the final experimental each run by recording the Cr signal during the regeneration arrangement (Fig. 1). The first loop serves for sample injection; with 1 M HNO3. No signal at all is observed for CrIII above a the second loop is used to elute the CrVI with HNO3 with a pH of 0.5, and the total amount of Cr is found in the pH value of 0.5; and the third loop is used to elute the CrIII regeneration cycle.With a pH<0.25 the elution becomes with HNO3 with a pH value of 0. In this two-step procedure, increasingly complete and more reproducible with an RSD of again a mixture of both species with a concentration of about 5% in this range. Accordingly, the signal strongly 10 mg l-1 of each was injected.The results are shown in Fig. 5 depends on the pH value of the eluent and reaches a maximum and the switching times for the second and the third loop are at a pH of 0, which is the tolerance limit for the column material. This shows that the aYnity of the CrIII cation for the stationary phase is stronger than the aYnity of the CrVI anion, and the highest tolerable acid concentration is required for elution. Generally, it can be concluded from these measurements that the column material can be utilized as a cation exchanger for retention of CrIII.Mixed bed operation The combined results for the species in Fig. 3 show that both may be eluted at the same pH value, if this is chosen in the small interval of overlap. A combination of both cationic and Fig. 5 Combined chromatograms from independent determinations anionic exchange capabilities accordingly permits a mixed bed of the Cr species (10 mg l-1 each) with diVerent nitric acid concenoperation of the column.Therefore, we investigated the elution trations (A: CrVI solution; B: CrIII solution; injection #1: pH 0.5; injection #2: pH 0). of both species for a mixture with a concentration of 10 mg l-1 1158 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12marked. It can be seen that optimum conditions for the separation of the species are realized and that the intensity maxima of the independent procedures are obtained. Indeed, the chromatogram obtained for a mixture of the species is identical to a superposition of the independent ones for both species.This again indicates that no disturbances from a conversion of the species by redox reactions occur. The chromatographic resolution is excellent despite a certain extension of the measurement time. However, if necessary this may be reduced by choice of the elution conditions. Fig. 7 Chromatograms for the Cr species (B; 10 mg l-1 each) and As a final step in the development of the chromatographic chloride ions (A; 50 mg l-1), which are not retained on the column.procedure, the flow rate was optimized. With the HPLC pump used here, constant operation was possible with flow rates in fact oVers the unique advantage that the procedure can be within the range 0.5 to 2.0 ml min-1, which coincides with the adjusted to cope with chloride ions in each case. The same interval for optimum operation of the HHPN sample introducstrategy could be used to cope with interferences from carbon- tion system.The influence of the flow rate on the signal ate and hydrogencarbonate ions, which will be discussed in intensity for both species is represented in Fig. 6. A weak more detail later on. maximum appears for both species at about 1.2 ml min-1, so that this flow rate was finally chosen as a standard value. Higher flow rates may, of course, be useful to reduce the Multielement capability analysis time with only a moderate loss of sensitivity of For a preliminary exploitation of the mixed bed properties of about 10%.the column, the speciation of Cr was performed with a multiele- Finally, it should be remembered that the working conditions ment solution containing both species and some additionally for the column in our procedure are not in agreement with selected elements with a concentration of 10 mg l-1 of each. the recommendations given by the manufacturer, but, neverthe- The results obtained with the two-step elution procedure less, the reproducibility was satisfactory over hundreds of outlined above are represented in Fig. 8.Obviously there are injections. considerable diVerences in the elution of cations. It can be seen that Rh passes through the column with hardly any Analytical Aspects retention, whereas Mn requires the 1 M HNO3 of the second elution step as is the case for CrIII, but with diVerent retention Interferences times. As a general trend, we can conclude from our measure- With respect to application to real samples, possible inter- ments that monovalent cations are eluted by the carrier ferences from other elements must be considered.The most whereas concentrated acids are required for multivalent cations prominent spectral interference is expected for solutions con- in each case. taining carbon or chloride ions, whereas for the latter chroma- Though preliminary, these results are promising for practical tographic interferences may also be expected. Indeed, an excess applications.It may be suggested that not only problems of Cl- ions may influence the exchange processes by competi- arising from spectral and non-spectral interferences can be tive reactions, particularly for CrVI. Furthermore, the solved to a certain extent by adequate optimization of the 35Cl16O1H+ ion is well known to be a significant spectral elution conditions, but also that internal standards can be interferent in the determination of Cr through its main isotope applied with elution conditions similar to those of the species of mass 52.For a solution containing both Cr species at a under investigation. This should be investigated in more detail concentration of 10 mg l-1 of each and an excess of Cl- ions in future work. at a concentration of 50 mg l-1, the results are represented in Fig. 7. The measurements demonstrate that chloride ions, in Figures of merit contrast with the Cr oxo-anions, pass through the column with some retention, but the 0.01 M HNO3 carrier itself is For the intensity measurements of the chromatographic peaks already suYcient for elution.In comparison to the peak form an RSD of about 5% was obtained, even for a whole working of the Cr species, the Cl-interferent signal is very broad due day, for the solutions with only one species as well as for to weak elution. The intensity and retention time of the solutions with a mixture of both. chromium species are not aVected, and there is no significant Calibration was performed by standard addition.The results contribution to the Cr signals as can be seen from the base of the calibration measurements with the commercial ICP-MS line. In the case of an even higher Cl- concentration, the decay instrument showed linearity for a concentration range from 1 of the Cl signal may last even longer, so that an injection of to 100 mg l-1, and it is preserved over this range, as can be nitric acid for elution of CrVI must be delayed in order to seen from the regression coeYcient (CrIII: 0.9988; CrVI: 0.9979). avoid disturbances from the tailing of the Cl- peak; but this Fig. 8 Multiple ion detection profiles for selected elements with a Fig. 6 Influence of the eluent flow rate on the analytical signals of concentration of 10 mg l-1 each in a chromatographic run (A: Rh; B: Cl; C: Cr; D: Mn). the species CrIII (A) and CrVI (B). Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1159The slope of the calibration line shows that the sensitivity for CrIII of 6.4 (y-axis intercept: 10) is higher than the one for CrVI of 3.4 ( y-axis intercept: 2.8) by a factor of about two. The relative detection limits calculated with respect to the 3s criterion are 0.1 mg l-1 for CrIII and 0.2 mg l-1 for CrVI, corresponding to absolute detection limits of 10 and 20 pg. These detection limits represent an improvement by about one order of magnitude in comparison with those in previous work with the same nebulization system, but with reversed-phase ionpair chromatography for species separation.15 The improvement may therefore be attributed entirely to the actual chroma- Fig. 9 Analysis of a fresh tap water sample: A, CrVI; B, CrIII. tographic procedure. On the other hand, the detection limits are comparable to those obtained by complexation of both analysed. The samples were tapped from the drinking water species and reversed-phase chromatography, whereas in comline of our institute and were analysed without any delay.The parison with reversed-phase ion-pair chromatography a certain concentration of total chromium was determined to be about improvement must be attributed to an additional pre- 3 mg l-1, and it appeared almost exclusively as CrVI (cf. Fig. 9). concentration step.26 This can be explained by the ozone cleaning of the drinking Further improvements may be possible either by using water and the resulting oxidation of CrIII to CrVI.It is gradient elution techniques or by preconcentration of the remarkable that CrVI is not reduced to CrIII by any organic investigated species from higher sample loop volumes, which components of the water sample, so that further investigations should be investigated in future work. concerning the stability of CrVI in environmental systems might be worthwhile. Validation Analytical validation of the developed speciation technique CONCLUSION again requires the availability of suitable standard CRM.For The procedure for the speciation of chromium developed in a first check we used a certified water sample (NIST 1643c), this work uses LC coupled with ICP-MS, with HHPN as a which with respect to its trace constituents is generally comparhigh- eYciency technique for sample introduction, and it is able with fresh water. The total chromium concentration in based on the modification of a commercial chromatographic this sample is certified as 19.0±0.6 mg l-1 without any reference column for anion exchange with respect to a special analytical to oxidation states and the sample is acidified to a pH of 0.3.task. This modification has made it possible exclusively to use Although this pH value is unusual for ion pairing chromatogranitric acid for elution, which keeps the risk of interferences in phy, the sample can be injected directly without sample ICP-MS to a minimum in comparison with any other possible preparation.The analysis of this sample by our technique eluent. This is particularly important for the determination of showed that the total chromium appears exclusively as CrIII, Cr for which interferences from ArC+ as well as from ClOH as no signal was observed for CrVI. Using the standard and ClO ions may become crucial. The procedure elaborated additions method, the total chromium content was determined in this work oVers the additional advantage that these interas 16.7±2.1 mg l-1, which within the standard deviation agrees ferences stemming from the presence of Cl can be controlled with the certified value.by adequate choice of the elution conditions. A further sample is available from the European Modification of the column has permitted simultaneous Commission, Standards, Measurements and Testing Proretention of both anions and cations on the same column. gramme (formerly BCR), but now with the advantage that Exploiting this peculiar feature, a two-step procedure has been both chromium species are certified independently.However, developed which allows independent optimization of the elu- first experiments with this sample material gave rise to some tion conditions for both CrIII and CrVI. Moreover, it could be problems, because the elaborated working conditions did not shown that this type of mixed bed operation is a promising permit reliable and accurate determination of the Cr content approach for the development of multielement speciation produe to an 40Ar12C spectral interference generated from the cedures based on the experience explained above, and that carbonate and hydrogencarbonate ions from the buVer used.commercial chromatographic columns can be modified to cope In particular, peak splitting appeared which must be attributed with the peculiar problems of ICP-MS as one of the most to co-elution of buVer ions together with the chromium species.powerful element determination techniques. In order to cope with this problem, the buVer ion concentration was reduced by dilution (1550). Nevertheless, species trans- The work has been supported financially by the Ministerium formation remained unlikely, especially since the time between fu� r Wissenschaft und Forschung des Landes Nordrheinsample preparation and analytical measurement was always Westfalen and by the Bundesministerium fu� r Bildung, kept as short as possible.Furthermore, the acid elution was Wissenschaft, Forschung und Technologie. delayed by about 3 min, so that the buVer ions could pass through the column while the chromium species were retained. After complete removal of the buVer ions the chromium species REFERENCES were eluted in the two step procedure as outlined above. 1 De Flora, S., and Wetterhahn, K. E., L ife Chem. Reports, 1989, This modification of the procedure permitted reliable deter- 7, 169. mination of both species.When applying standard addition 2 Normausschuß Wasserwesen im DIN, DEV, 1987, DIN 38405 for quantitation, the concentration of CrIII was determined as (D24), 1. 3 Sperling, M., Xu, S., and Welz, B., Anal. Chem., 1992, 64, 3101. 14.0±2.5 mg l-1 and the concentration of CrVI as 24.4± 4 Posta, J., Berndt, H., Luo, S., and Schaldach, G., Anal. Chem., 2.1 mg l-1. This is in satisfactory agreement with the certified 1993, 65, 2590. values of CrIII as 16.1±3.1 mg l-1 (recovery: 87.0%) and of CrVI 5 Roychowdhury, S.B., and Koropchak, J. A., Anal. Chem., 1990, as 24.2±2.8 mg l-1 (recovery: 100.8%), confirming the 62, 484. reliability of the procedure for the speciation of Cr in typical 6 Krull, I. S., Bushee, D., Savage, R. N., Schleicher, R. G., and water samples. Smith, S. B., Jr., Anal. L ett., 1982, 15, 267. 7 Roehl, R., and Alforque, M. M., At. Spectrosc., 1990, 11, 210. Finally, as an example of a real sample, tap water was 1160 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 128 Tomlinson, M. J., and Caruso, J. A., Anal. Chim. Acta, 1996, 322, 1. 20 Urasa, I. T., and Nam, S. H., J. Chromatogr. Sci., 1989, 27, 30. 21 Beere, H. G., and Jones, P., Anal. Chim. Acta, 1994, 293, 237. 9 Wang, S.-R., and Jiang, S.-J., J. Chin. Chem. Soc. (T aipei), 1991, 38, 327. 22 Byrdy, F. A., Olson, L. K., Vela, N. P., and Caruso, J. A., J. Chromatogr. (A), 1995, 712, 311. 10 Luo, S. K., and Berndt, H., Spectrochim. Acta, Part B, 1994, 49, 485. 11 LaFreniere, K. E., Fassel, V. A., and Eckels, D. E., Anal. Chem., 23 Powell, M. J., Boomer, D. W., and Wiederin, D. R., Anal. Chem., 1995, 67, 2474. 1987, 59, 879. 12 Shum, S. C. K., Neddersen, R., and Houk, R. S., Analyst, 1992, 24 Lintschinger, J., Kalcher, K., Go� ssler, W., Ko� lbl, G., and Novic, M., Fresenius’ J. Anal. Chem., 1995, 351, 604. 117, 577. 13 Berndt, H., Fresenius’ J. Anal. Chem., 1988, 331, 321. 25 Pantsar-Kallio, M., and Manninen, P. K. G., Anal. Chim. Acta, 1996, 318, 335. 14 Houk, R. S., and Jiang, S. J., Chromatogr. L ibr., 1991, 47, 101. 15 Jakubowski, N., Jepkens, B., Stuewer, D., and Berndt, H., J. Anal. 26 Andrle, C. M., Jakubowski, N., and Broekaert, J. A. C., Spectrochim. Acta, Part B, 1997, 52, 189. At. Spectrom., 1994, 9, 193. 16 Jen, J.-F., Ou-Yang, G.-L., Chen, C.-S., and Yang, S.-M., Analyst, 27 Thomas, C., Jakubowski, N., Stuewer, D., and Broekaert, J. A. C., J. Anal. At. Spectrom., 1995, 10, 583. 1993, 118, 1281. 17 Zoorob, G., Tomlinson, M., Wang, J., and Caruso, J., J. Anal. At. Spectrom., 1995, 10, 853. Paper 7/02120H 18 Gjerde, D. T., Wiederin, D. R., Smith, F. G., and Mattson, B. M., ReceivedMarch 27, 1997 J. Chromatogr., 1993, 640, 73. 19 Cox, A. G., and McLeod, C. W., Anal. Chim. Acta, 1986, 179, 487. Accepted June 17, 1997 Journal of Analytical Atomic Spectrometry, October 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a702120h

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

On-line Dilution for ICP-MS With a Flow Injection Recirculating Loop Manifold JULIAN F. TYSON*a , HONGHONG GEa AND ERIC R. DENOYERb aDepartment of Chemistry, Box 34510, University of Massachusetts, Amherst,MA 01003-34510, USA bT he Perkin-Elmer Corporation, 761Main Ave., Norwalk, CT 06859-0215, USA A FI manifold consisting of a recirculating loop has been In this work a recirculating loop manifold was coupled to a plasma source mass spectrometer and used (a) to provide coupled to a plasma source mass spectrometer to provide successive on-line dilutions.Part of the loop is injected into the data for calibration curves with a single standard and (b) to reduce matrix eVects with successive dilutions, a requirement carrier stream followed by dilution, within the loop, of the remaining part with the carrier solution. For a manifold with a of EPA ICP-MS methods 200.8 and 6020M. Matrix eVects generally depend on the total matrix concentration rather than calculated dilution factor of 2.02 (based on the volume ratio), 10 successive injections gave a mean of 1.99 and a 95% relative concentration of analyte and matrix, and therefore, in general, matrix eVects may be removed by successive dilution.confidence interval of ±0.065 for the ratio of successive peak heights. The between-run precision for a particular peak height The performance of several mixing devices for increasing the speed of the dilution process was evaluated. These devices ranged from 1.7 to 3.2% relative standard deviation (RSD).In a study of the decay of the concentration oscillations in the included a simple split and confluence network to provide concentration damping by destructive interference. recirculating loop, it was found that the reciprocal of the time to achieve uniform concentrations decreased linearly with increasing flow rate and decreasing loop volume. The dilution EXPERIMENTAL behaviors of 19 elements were studied. Of these, nine (Ag, Ba, Cr, Cu, Ni, Pb, Sb, Tl and U) could be diluted from 100 ppb Principle of the Recirculating Loop Dilution Manifold by three orders of magnitude with a precision of 5% RSD or A schematic diagram of a recirculating loop manifold conbetter, six (As, Cd, Co, Th, V, Zn) could be diluted over the structed with two six-port rotary valves is shown in Fig. 1. same range with precisions between 5 and 10% RSD, and four The sample is loaded into the loop when both valves are in elements (Be, Mo, Se and Hg) displayed a systematic the ‘load’ position (solid line) and the pump, P, is on.When decrease in the dilution factor which was interpreted as valve 2 is switched to the closed position (dashed line), a closed retention of these elements within the loop. The influence of a loop is formed in which the sample solution circulates. When wine matrix on the determination of Ce was removed by five valve 1 is switched to the inject position (dashed line), the successive dilutions with a factor of 3.04 per injection for a sample in the ‘bottom’ portion of the loop (efgh) will be total dilution factor of 260.For a total loop volume of 1–2 ml, injected into the carrier stream, pumped from C to D and rapid damping of the concentration oscillations could be transported to the detector. This operation fills the ‘bottom’ produced by the destructive interference produced by a two-line portion of the loop with carrier. When valve 1 is switched network (split and confluence) with tube lengths of 50 and back to the ‘load’ position, the sample in the ‘upper’ portion 25 cm.of the loop (abcd) mixes with the carrier in the ‘lower’ portion Keywords: Inductively coupled plasma mass spectrometry; flow of the loop until a homogeneous diluted solution is obtained. injection; dilution; recirculating loop; successive dilutions; Valve 1 is then again switched into the inject position which calibration; wine introduces the diluted sample to the detector.Successive dilution can thus be performed by repeatedly actuating valve 1. The dilution factor (d) of the manifold for one actuation of the Dilution is used in analytical laboratories to prepare cali- valve is the ratio of the volume of the total loop (abcefghd) to bration standards, to bring sample concentrations within that of the volume of the ‘upper’ portion of the loop. The instrumental working ranges and/or to reduce matrix eVects. dilution factor for n successive actuations is dn.Thus dilutions There is, therefore, a need for automated on-line dilution by large factors can be obtained by repetitive actuation of techniques. Flow injection (FI) oVers several ways to valve 1. For example, if d=3, dilution by a factor of over 2000 implement on-line dilution, including gradient dilution,1,2 con- is obtained after six injections. tinuous dilution, 3,4 with manifolds incorporating mixing chambers, 5 variable tube dimensions,6 merging zones,7,8 networks9 and variable injection volume dilution.10–13 Most of these techniques are based on the dynamic dispersion characteristics of the FI process and thus are based on the requirements of precise timing of a particular operation (such as valve switching) and the requirement of precise pumping. For those systems based on the inherent dispersion coeYcient of some particular manifold configuration, precise fluid delivery is still required as the dispersion coeYcient is a function of flow rate.It is diYcult to ensure good day-to-day reproducibility in flow delivery when peristaltic pumps are used. A recirculating loop FI manifold, which was first introduced for atomic spectrometry applications by Tyson et al.,14 provides a new method for on-line dilution. The dilution factor is determined by the Fig. 1 Schematic diagram of a recirculating loop manifold: S, sample; volumes of specified parts of the manifold. Such volumes are C, carrier; P, peristaltic pump; D, detector; V1 and V2 six-port rotary valves.invariant parameters of a FI system. Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 (1163–1167) 1163Table 3 FIAS program for successive dilutions Spectrometers The plasma source mass spectrometer used was a Perkin- ICP-MS Time/ P1 P2 Electronic Step reading s (rpm) (rpm) V1 V2 valve Elmer (Norwalk, CT, USA) SCIEX model ELAN-5000. The parameter settings used are shown in Table 1.A Hewlett- 1 no 20–60 60 120 recirculate closed load Packard (Avondale, PA, USA) model HP8452A diode array 2 no 20–60 60 120 recirculate closed load 3 yes 5 120 0 inject closed load spectrophotometer was used to study the processes of mixing 4 yes 25 120 0 inject closed inject in the loop. FI Manifolds for a successively diluted sample. In step 1 of Table 2, valve 1 was in the recirculate position and valve 2 was open (solid The recirculating loop manifold for ICP-MS, shown in Fig. 2, lines). Pump 1 was for the carrier introduction and pump 2 was constructed from a manually operated two-way, six-port was for the sample loading. In step 2, pump 2 was stopped for valve (Rheodyne, Cotati, CA, USA) and a Perkin-Elmer 5 s and the six-port valve (V2) was turned to the closed FIAS-200 FI analyzer fitted with two four-channel peristaltic position (dashed line). A closed loop, filled with concentrated pumps and a two-layer, eight-port rotary valve.The upper sample or standard, was formed at this stage. Valve 2 was loop consisted of 35 cm of Tygon tubing of id 1.14 mm, retained in the closed position (dashed line) for the rest of the together with 25 cm of PTFE tubing of id 0.76 mm. The lower experiment. In step 3, the eight-port valve (V1) was turned loop consisted of 110 cm of 0.76 mm id PTFE tubing. The counter clockwise to the inject position (dashed line), allowing FIAS-200 instrument was remotely controlled by the the sample in the bottom portion of the loop (23hgfe67) to be ELAN-5000 software.Internal standards or spike standards flushed out by the carrier. The signal collection process was were introduced to the flow system by a six-port rotary started at the beginning of this step. The holes of the eightinjection valve (P.S. Analytical, Orpington, Kent, UK) which port valve are not equally spaced, the distance between ports was remotely controlled by the FIAS-200 unit via a built-in 4 and 5, and 1 and 8, being larger than the distances between electronic switch closure.The volume of the internal or spike the other ports. So in the inject position, ports 4 and 8 did standard injected was 156 ml. not line up. After 5 s, the six-port injection valve (shown as St The FIAS-200 programs are shown in Tables 2 and 3. in Fig. 2) was turned to the inject position (step 4), then the Table 2 contains the program for measurement of an undiluted internal standard or spike standard was merged with the sample (the first injection) and Table 3 contains the program sample at the confluence point, mixed in a 100 cm long coiled Teflon open-tubular reactor (id 0.76 mm) and finally flushed Table 1 ELAN-5000 operating conditions and data acquisition into the plasma for 25 s.parameters The program for the successive dilutions is shown in Table 3. In steps 1 and 2, V1 was in the recirculate position (solid line). Forward rf power 1000 W Pump 1 was set at 60 rpm (2 ml min-1) and pump 2 was at Plasma Ar flow 15 l min-1 Auxiliary Ar flow 0.8 l min-1 120 rpm (4 ml min-1). The sample remaining in the ‘upper’ Carrier (nebulizer) Ar flow 0.75–0.95 l min-1 portion of the loop (abcd) was mixed with the carrier in the Sample and skimmer cones Ni ‘bottom’ portion of the loop (efgh).The recirculating time for Data acquisition transient multichannel analysis both steps could be varied according to the speed of the mixing Scan mode peak hop transient process, which depended on the flow rate, loop volume, mixers Measurement one MCA channel used and expected precision.For example, for a manifold with (spectral peak average) Dwell time 40 ms 1.45 ml total loop volume and 3.38 dilution factor, a precision of 4.5% RSD was achieved with a 40 s recirculating time at a recirculating flow rate of 4 ml min-1. The manifold for the study of the mixing processes within the recirculating loop is shown in Fig. 3.The valves were both manually operated six-port rotary valves (Rheodyne model 5020). The flow cell (Pye Unicam, Cambridge, UK) of pathlength 1.0 cm and volume 18 ml, was mounted in the HP8452A diode array spectrophotometer. The performance of a variety of mixing devices, including wide-bore tubing, mixing chambers,15 split and confluence networks, was studied. Both the flow cell and the mixing devices were located in the recirculating loop. Fig. 2 The recirculating loop manifold built incorporating the FIAS-200 unit: P1 and P2 are peristaltic pumps on the FIAS-200 unit; V1 is an eight-port, two-layer rotary valve on the FIAS-200 unit; V2 is a manually operated two-way, six-port valve; C, carrier; S, sample; W, waste, ICP-MS, detector; St: standard solution (spike or internal standard).Table 2 FIAS program for the first peak in the successive dilutions ICP-MS Time/ P1 P2 Electronic Step reading s (rpm) (rpm) V1 V2 valve 1 no 30 60 120 recirculate open load Fig. 3 The recirculating loop manifold for the study of the mixing 2 no 5 0 0 recirculate closed load process: S, sample; C, carrier; P, peristaltic pump; W, waste; V1 and 3 yes 5 120 0 inject closed load V2, six-port rotary valves; U, flow cell in visible spectrometer; mx, 4 yes 25 120 0 inject closed inject mixing devices. 1164 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12Reagents a wine sample. The manifold used was the same as in the internal calibration construction.The original wine sample Standard solutions of metals for the ICP-MS experiments were was loaded into the loop and the portion in the bottom loop prepared from 1000 ppm plasma grade standards obtained was injected into the carrier and merged with 156 ml of a from Johnson Matthey (Wayne, PA, USA). Nitric acid, 1%, 43.7 ppb Ce standard solution. The signal intensities for each (purified by a quartz sub-boiling system) in distilled deionized successive diluted peak were recorded as I1 to In.When 1% water was used as diluent and carrier. Vendange wine with an nitric acid was used as the spike standard, a series of diluted alcohol content of 13.2% was used as the sample and was peak intensities were recorded (as B1 to Bn) corresponding to purchased from a local liquor store. Tartrazine (Eastman signals for successively diluted wine samples. When 1% nitric Kodak, Rochester, NY, USA) was used in the visible absorpacid was used as the sample and Ce as the spike standard, tion experiments as a tracer to follow the mixing processes in peak intensity was recorded as I0, corresponding to the signal the recirculating loop. for 156 ml of a 43.7 ppb Ce standard without any matrix eVect.The recovery of Ce from each successive diluted wine sample Development of the System was calculated as (Ii-Bi)/I0×100%. The analytical performance of the recirculating loop was examined for successive dilutions of a 91.5 ppb cerium standard Improvement of the Mixing EYciency solution.Ten successive dilutions were carried out which diluted the 91.5 ppb Ce standard solution to 0.089 ppb. The The mixing between sample and diluent in the recirculating accuracy of the dilution procedure was evaluated by comparing loop was studied by visible absorption spectrometry. A solution the observed dilution factor with the calculated dilution factor. of 10 ppm tartrazine was used as the sample and distilled The observed dilution factor was the ratio of the ICP-MS water as the diluent.The spectrometer was set at 426 nm where signals for two successive dilutions and the calculated dilution tartrazine has a strong absorption peak. The kinetics program factor was the ratio of the volume of the total loop to that of was used, which displayed absorbance as a function of time. the upper portion of the loop. The loop volume was determined The lower loop consisted of 135 cm of 0.76 mm id PTFE by a photometric procedure using tartrazine as the solute.The tubing. The upper loop consisted of 13.5 cm of Tygon tubing entire loop was filled with a 49.5 ppm tartrazine solution. The of id 2.06 mm, 10 cm of 0.76 mm id tubing and an 18 ml flow solution in the bottom loop (or the whole loop) was then cell. Parameters which aVect the mixing process were investiwashed out with distilled water into a 25 ml calibrated flask, gated. These included recirculating flow rate, loop volume and, and diluted to the mark.The absorbance of the diluted solution inserted into the upper loop, several mixing devices to speed was measured and the concentration determined from a cali- up the mixing of the sample and the diluent. These mixing bration curve. The ‘upper’ loop volume was calculated as the devices included a well stirred chamber (1 ml volume), a short diVerence between the whole loop volume and the ‘bottom’ fat tube (1.2, 2.8 or 5.6 cm long and 4.55 mm id) and a splitloop volume.confluence device, shown in Fig. 4, consisting of two open The reproducibility of the process was evaluated by running tubular reactors (0.76 mm id) of diVerent lengths connected by three series of successive dilutions. The calculated dilution two 120° three-way Omnifit connectors. Several experiments factor and the total loop volume of the manifold used were were performed with the same relative lengths (251) of tubing, 3.04 and 1.27 ml, respectively.The recirculating time for the for which the length of the shorter arm was 12.5, 25, 50, 75 mixing process was set at 120 s. or 100 cm. The dilution behavior of elements which can be determined by EPA 200.8 (a method to determine trace elements in water and waste by ICP-MS) was examined. The concentration range RESULTS AND DISCUSSION covered for each element was from about 100 ppb to the total Accuracy and Reproducibility recoverable detection limit (TRDL, the detection limit for a procedure to recover the analyte in all chemical forms) specified The peak intensities, observed dilution factors and calculated in the method protocol.For example, the TRDL of Ag is concentrations for 10 successive dilutions of the 91.5 ppb Ce specified as 0.1 ppb. The TRDL values specified cover the solution are given in Table 4. The average observed dilution range from 0.1 to 7.9 ppb. The loop parameters were the same factor was 1.99, with 4.6% RSD. The calculated dilution factor as above.was 2.02 (the ratio of the total loop volume of 0.97 ml to that External calibration curves for Ce and Ba were constructed of the upper loop volume of 0.48 ml). Good agreement was simultaneously from a single standard containing 91.5 ppb of achieved between the calculated and the observed dilution each element. The dilution factor of the manifold was 2.40 and factor with no diVerence at the 95% confidence level. the total loop volume was 0.84 ml.The recirculating time for The reproducibility of the dilution process for three repeated the mixing process was set at 120 s. series of successive dilutions is shown in Table 5. The reproduc- An internal calibration curve for Ce was also constructed ibility of dilution factors between diVerent series was as good with a single standard. The internal standard Tb was intro- as that of the successive dilution factors in the same series. duced to the flow system by the six-port valve which was The apparently lower between-run RSD is probably not remotely controlled by the FIAS-200 so that the manifold significant. operated in a merging zones mode.The concentrations of Ce and Tb were 43.4 and 43.7 ppb, respectively. The dilution factor of the recirculating loop was 3.04 and the total loop volume was 1.27 ml. The recirculating time was set at 120 s. The injection of the Tb internal standard was delayed by 5 s following the actuation of the eight-port valve.Fig. 4 A split-confluence device. The back-pressures of the two branches are diVerent because of the diVerences in lengths and so the Recovery of Cerium From a Wine Sample With Successive flow rate is slower in the longer branch. Regardless of the flow rates, the concentrations in the two lines are the same, but the oscillations Dilutions become progressively out of phase during passage through the device. The reduction of matrix eVects by successive dilution was also The lengths are chosen so that destructive interference occurs at the confluence point.studied by examining the recovery of a cerium standard from Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1165Table 4 Results for ten successive dilutions of a Ce standard PeakNo. Intensity/counts s-1 di* Concentration (ppb) 1 3.47×105 91.5 2 1.70×105 2.04 45.8 3 8.62×104 1.97 22.9 4 4.44×104 1.94 11.4 5 2.18×104 2.04 5.72 6 1.17×104 1.86 2.86 7 5.42×103 2.16 1.43 8 2.75×103 1.97 0.715 9 1.47×103 1.87 0.358 10 759 1.94 0.179 11 368 2.06 0.089 Fig. 5 The overlap of Ce and Tb FIA peaks produced by the merging * Dilution factor based on ratio of successive peaks. zones manifold for internal calibration. Table 5 Reproducibility of successive Ce dilutions dence intervals about the intercepts for all three lines included the origins. Peak ratio dilution factor Run number d1,2* d2,3 d3,4 d4,5 daverage RSD (%) Recovery of Cerium From a Wine Sample With Successive I 2.82 2.92 3.05 2.93 2.93 3.2 Dilutions II 2.80 3.00 2.92 3.03 2.94 3.5 In EPA ICP-MS methods, dilution is often needed to reduce III 2.91 3.11 2.99 2.96 2.90 2.8 matrix eVects.For example, as stated in EPA 200.8, section 10.6: dI-II average 2.84 3.01 2.99 2.97 — — RSDI-III (%) 2.1 3.2 2.2 1.7 — — ‘If the spike is not recovered within the specified limits, the sample must be diluted and reanalyzed to compensate for the * Dilution factors based on the ratio of successive peaks as indicated matrix eVect’.The specified limits are 75–125%. The reduction by the appropriate subscripts. of matrix eVects by successive dilution was demonstrated by a study of the recovery of Ce from a wine sample. The results are shown in Fig. 6. It was found that wine had an enhancement Dilution Behavior of Various Elements eVect in the determination of Ce. The recovery of Ce from the concentrated wine was 129%, which decreased to 110% with Of the 19 elements examined, most exhibited linear dilution three successive dilutions, and finally to 104% with two more behavior with good precision from about 100 ppb to the EPA successive dilutions.The final value is well within the limits 200.8 TRDL. The RSDs of observed dilution factors were less specified in EPA method 200.8. than 5% for the elements 107Ag, 52Cr, 63Cu, 60Ni, 121Sb, (206+207+208)Pb, 137Ba, 238U and 205Tl. Less than 10% RSD was achieved for the elements 59Co, Improvements in the Mixing EYciency Cd[111C-0.073(108C-0.712×106C)], 232Th, 66Zn, 75As and A typical concentration oscillation, tracked by the absorbance V[51C-3.127(53C-0.11352C)], where C represents the counts of the tartrazine solution, is shown in Fig. 7. It was found that s-1 at the specified mass to charge ratio, if diluted from about the concentration oscillation decay time, t (time in s taken for 100 ppb to the TRDL. Less than 5% RSD was obtained for the oscillation amplitude to decrease to 1% of the final these elements when diluted to about 5 ppb.The dilution absorbance signal ), decreased linearly with (a) increasing recir- factors of selenium, beryllium, molybdenum and mercury (not culating flow rate, Q, (over the range 1.00–4.00 ml min-1) and covered by the EPA 200.8 method) kept decreasing with (b) decreasing loop volume, Vi, (over the range 1.30–2.30 ml). successive dilutions. This may have been due to adsorption of The equations of the two lines were as follows (unweighted these elements within the loop, as long-tailed FI profiles were least squares fit), 1/t=4.48×10-4+8.0×10-4 Q (for which observed.the correlation coeYcient was 0.999) and 1/t= 7.8×10-3-2.63×10-3 Vi (for which the correlation coeYcient External and Internal Calibration Curves From a Single was also 0.999). Standard The oscillation decay times for solutions of diVerent concentration were found to be the same for absorbances between External calibration curves for 140Ce and 137Ba were con- 0.1 and 0.9.When the absorbance was too low, the oscillation structed simultaneously. Linear calibration curves were obtained. An unweighted least squares fitting procedure gave equations for the two lines as follows: 140Ce, y=162+7660x and for 137Ba, y=-84+888x where x is the calculated concentration in ppb, assuming the dilution factor was 2.40 (the calculated dilution factor of the manifold), and y is the observed intensity for each dilution in units of counts s-1.The correlation coeYcient of each line was 1.000. A linear internal calibration curve was also constructed for Ce with Tb as the internal standard. The overlap of the two FIA peaks is shown in Fig. 5, from which it can be seen that the Tb and Ce solutions were well mixed and reached the plasma at the same time. The equation of this line was (unweighted least squares fit), y=-0.0003+0.0252x, where x is the calculated concentration of Ce in ppb and y is the intensity ratio of 140Ce to Fig. 6 The recovery of Ce from a wine sample as a function of successive dilutions. 159Tb. The correlation coeYcient was 0.9997. The 95% confi- 1166 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12CONCLUSIONS A recirculating loop manifold can provide accurate and precise on-line dilution suitable for use with ICP-MS. The manifold can be used to obtain calibration data from a single concentrated standard, and to reduce matrix eVects by successive dilutions. The provision and maintenance of the ELAN-5000 and FIAS-200 instruments by Perkin-Elmer is gratefully Fig. 7 Tartrazine concentration oscillations in a recirculating loop. acknowledged. The upper loop consisted of 35 cm of Tygon tubing with 25 cm of 0.76 mm id PTFE tubing. The lower loop consisted of 220 cm of 0.76 mm id PTFE tubing and an 18 ml flow cell. REFERENCES 1 Olsen, S., Ruzicka, J., and Hansen, E. H., Anal. Chim. Acta, 1982, decayed rapidly into the noise.All three mixing devices speeded 136, 101. up the mixing process, but it was found that the mixing 2 Yang, J., Ma, C., Zhang, S., and Shen, Z., Anal. Chim. Acta, 1990, chamber caused problems with air bubbles. The split- 235, 323. confluence mixer was more eYcient than the closed loop alone 3 Tyson, J. F., and Appleton, J. M. H., T alanta, 1984, 31, 9. 4 Tyson, J. F., Appleton, J. M. H., and Idris, A. B., Anal. Chim. or the loop containing the short, fat tube. The oscillation decay Acta, 1983, 145, 159.times were 90, 62 and 52 s for the simple loop, the loop plus 5 Beinrohr, E., Csemi, P., and Tyson, J. F., J. Anal. At. Spectrom., 1.2 cm of short, fat tube (the most eYcient of the three reactors 1991, 6, 307. evaluated) and the split-confluence device with lengths 25 and 6 Tyson, J. F., Mariara, J. R., and Appleton, J. M. H., J. Anal. At. 50 cm, respectively. Because of the diVerent coil lengths in the Spectrom., 1986, 1, 273.split and confluence devices, the solutions in the two branches 7 Zagatto, E. A. G., Krug, F. J., Bergamin, H. F., Jorgensen, S. S., and Reis, B. F., Anal. Chim. Acta, 1979, 104, 279. had diVerent concentration oscillation profiles at the merging 8 Mindegaard, J., Anal. Chim. Acta, 1979, 104, 185. point. The addition of the two out-of-phase concentration 9 Tyson, J. F., and Bysouth, S. R., J. Anal. At. Spectrom., 1988, 3, 211. oscillation profiles resulted in a damped oscillation profile. 10 Bergamin, H. Fo., and Pessenda, L. C. R., Anal. Chim. Acta, 1981, Both the absolute and relative lengths of the two branches will 123, 221. aVect the mixing process. A ratio of 152 was found the best 11 Sherwood, R. A., Rocks, B. F., and Riley, C., Analyst, 1985, for the relative length. The optimal absolute length of the 110, 493. 12 de la Guardia, M., Morales-Rubio, A., Carbonell, V., Salvador, A., branches will vary with the total loop volume. Lengths of 50 Burguera, J. L., and Burguera, M., Fresenius’ J. Anal. Chem., and 25 cm coils (id 0.76 mm) were found to be suitable for a 1993, 345, 579. loop of total volume of 1–2 ml. This split-confluence mixer 13 Fang, Z., Welz, B., and Sperling, M., Anal. Chem., 1993, 65, 1682. was used in the manifold coupled to the plasma source mass 14 Tyson, J. F., Bysouth, S. R., Grzeszczyk, E. A., and Debrah, E., spectrometer. For a mixing time of 40 s, an RSD of 4.5% was Anal. Chim. Acta, 1992, 261, 75. obtained with three successive dilutions of 43.7 ppb Ce to 15 Tyson, J. F., Analyst, 1987, 112, 523. 16 Vanderslice, J. T., Rosenfeld, A. G., and Beecher, G. R., Anal. 1.1 ppb, and for a mixing time of 60 s an RSD of 2.8% with Chim. Acta, 1986, 179, 119. four dilutions to 0.33 ppb was obtained. The total loop volume of the manifold was 1.45 ml. Paper 6/08601B A theoretical treatment of the mixing in a recirculating loop Received December 24, 1996 should be possible, but is considered beyond the scope of this Accepted May 27, 1997 publication. A suitable basis for such a treatment might be calculations of the residence times for which the volume occupied by the sample bolus became multiples of the loop volume. A useful starting point for these calculations would be the description of laminar flow in closed circular conduits provided by Vanderslice et al.16 Journal of Analytical Atomic Spectrometry, October 1997, Vol. 12 1167

ISSN:0267-9477    

DOI:10.1039/a608601b

出版商:RSC    

年代:1997

数据来源: RSC