Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B
j ourna l homepage: www.e lsev ie r .com/ locate /sab
Considerations about the detection efficiency in inductively coupled plasmamass spectrometry☆
Kay NiemaxDepartment of Analytical Chemistry and Reference Materials, Federal Institute for Materials Research and Testing (BAM), Richard-Willstaetter-Strasse 11, 12489 Berlin, Germany
☆ This paper is dedicated to Gary M. Hieftje, on the ocrecognition of his boundless contributions to spectrosco
Article history:Received 14 May 2012Accepted 21 June 2012Available online 5 July 2012
Keywords:ICP-MSDetection efficiencyMicrodroplets
Experimental investigations of analyte atomization, ionization and diffusion processes in the inductivelycoupled plasma applying single droplet introduction and optical emission spectroscopy provide hints howto improve the detection efficiency of inductively coupled plasma mass spectrometry. It is discussed howthe flow, amount and type of injector gas, the size of droplets injected, the analyte mass, and the samplerinterface of the mass spectrometer determine the position of analyte atomization and ionization as well asthe magnitude of radial analyte ion diffusion at the interface of the mass spectrometer applied.
1 Both assumptions are made for a first, simplified discussion. It is well known that
1. Introduction
In the last 45 years Prof. Hieftje and his co-workers have pub-lished numerous groundbreaking papers in analytical spectroscopy.Among those are many important publications dealing with sampleintroduction into analytical atomizers, such as flames and plasmas,and investigations of the physical and chemical processes in theatomizers. In a particular group of papers [1–9] single particle andsingle reproducible microdroplet injection into the analytical flamesand electrothermal atomizers were applied and demonstrated asexcellent probes to study atomization processes with temporal andspatial resolution by optical spectroscopy.
These early papers initiated an increasing number of publicationsby other research groups investigating desolvation, atomization andionization processes in the widely used inductively coupled plasma(ICP) also applying single particle and microdroplet introduction[10–29]. A comprehensive study by end- and side-on optical emissionspectroscopy (OES) applying monodisperse microdroplets was pub-lished by our laboratory recently [29]. It was shown that the spatialpositions where analyte atomization and ionization start and areaccomplished depend on the injector gas flow rate, the dropletdiameter, and the amount of analyte.
On the basis of our recent paper and earlier experimental studiesof other research groups the most significant experimental parame-ters will be discussed which have to be taken into account for improv-ing the detection efficiency of ICP-MS which is currently at its best onthe order of 10−4–10−6 for most commercial instruments and needsto be improved particularly for detection of nanoparticles by ICP-MS[28,30–34]. The discussion will be assisted by taking into account
casion of his 70th birthday, inpy and analytical chemistry.
rights reserved.
theoretical and experimental results on gas flow and temperatureconditions in the ICP [35], numerical simulations of an ICP withmass spectrometer interface [36], and theoretical investigations ofthe impact of He admixed to the ICP injector gas [37], all publishedby the PLASMANT group at the University of Antwerp (Belgium).
2. ICP-MS detection efficiency and its dependence on spatial atom-ization, ionization and diffusion
Fig. 1 displays the longitudinal section through the axis of an ICPshowing the spatial radial plasma temperature condition withoutthe sampler of a mass spectrometer as calculated by the PLASMANTgroup [35] using the dimensions and operation conditions of the ICParrangement used in our laboratory [24,27,29,35]. Let us assumethat a particle or a very small droplet of aqueous element solution istransported through the injector (on the left side of Fig. 1) into theICP, and the particle or the analyte in the droplet, after evaporationof the water, starts to atomize at the position marked by the lightblue (grey) star. If we further assume that the Ar gas velocity andthe plasma temperature are constant across the torch,1 the analyteatoms will diffuse and ionize forming spherical clouds whose diame-ters linearly increase downstream the plasma.2 It means that theanalyte ion clouds will have a certain diameter at the positionwhere the sampler of the mass spectrometer is introduced inICP-MS. In this simple picture the ion cloud diameter at the samplerorifice will be different if the spatial position of atomization can be
the gas velocity and the plasma temperature in ICP's are spatially dependent (see,e.g., [35] and discussion below).
2 Note that the diameters of ion clouds depend on the diffusion coefficient of the el-ement which is dependent on plasma temperature.
Fig. 1. Simplified picture showing the dependence of the ion cloud diameter on the po-sition of atomization in the ICP for two different cases. Gas velocity and plasma temper-ature are assumed to be constant across the torch and the influence of the cooledsampler and the pressure drop in the region of the orifice [36] are neglected. Thetemperature distribution is taken form theoretical results of the PLASMANT group atUniversity of Antwerp [35].
66 K. Niemax / Spectrochimica Acta Part B 76 (2012) 65–69
changed. For example, the ion cloud diameter can be reduced bymoving the position of atomization downstream the ICP (blue(black) star in Fig. 1). In that particular case, we can expect thatmore ions will enter the mass spectrometer and the detection effi-ciency of ICP-MS is increased.
The recent paper on atomization, ionization and diffusion in theICP studied by end- and side-on OES [29] also pointed out that the de-tection efficiency of ICP-MS is dependent on the element measured.For example, side-on measurements of the alkaline earth ionresonance lines of Be, Mg, Ca, Sr, and Ba revealed that the ratiosof ion cloud diameters were about 1 : 0.73 : 0.68 : 0.59 : 0.55 just+0.4 cm above the top of the torch. These measurements andside-on observation of other elements clearly showed the mass de-pendence of the diffusion. As a consequence, the MS detection effi-ciency is element dependent, i.e., the element with the higher masswill be more efficiently detected in ICP-MS, as sketched in Fig. 2.The experimental evidence for the element dependent detection effi-ciency, as was already mentioned in [29], can be found, e.g., in [38]where the ion detection efficiencies of many elements between 7Liand 238U were measured by laser ablation ICP-MS. The detectionefficiencies ranged from ~10−7 for 7Li to ~3×10−5 for 238U. Smallvariations found from element to element are correlated with theionization energies of the analytes measured.
It has to be stressed again that the simplified pictures explaining thespatial dependence of the ion cloud diameter in Fig. 1 and the element
Fig. 2. Simplified picture of the dependence of the ion cloud diameter on analyte diffu-sion. The blue (black) and the light blue (grey) clouds display the different diffusions ofa heavy and light element, respectively. Gas velocity and plasma temperature areassumed to be constant across the torch and the influence of the cooled sampler andthe pressure drop in the region of the orifice [36] are neglected. The temperaturedistribution is taken form theoretical results of the PLASMANT group at University ofAntwerp [35].
dependent diffusion in Fig. 2 have to be modified because the ICP tem-perature and the gas velocity are not constant in the volume relevant foratomization, diffusion and ionization, and the temperature dependenceof atomic diffusion was neglected. Furthermore, the introduction of acooled sampler into the plasma and the pressure drop through theorifice of the sampler will change the plasma temperature and the gasvelocity near the sampler, respectively [36].
The spatial variation of the axial gas velocity for a gas flow rate of0.25 L/min through an injector with 1 mm diameter is plotted in theupper part of Fig. 3. The data are taken from the ICP simulation of thePLASMANT group at the University of Antwerp [35]. It has to be notedthat the velocity conditions shown are comparable to the conditionsin a ICP torch with 2 mm injector and a four times larger injectorgas flow rate (1 L/min). Injectors with inner diameters of about2 mm and flow rates of about 1 L/min are usually found in commer-cial ICP instruments.
At the particular condition (1 mm injector and 0.25 L/min) theaxial gas velocity is increasing from about 6 m/s behind the injectortip to about 21 m/s just above the torch. Beyond that position thevelocity is decreasing again.
For the sake of clarity, the variation of the gas velocity in depen-dence on the radius is only given for the position of the highest axialgas velocity in the upper part of Fig. 3. One can see that there is asignificant change of the velocity with increasing radius. The changeof the radial gas velocity is even larger if the injector flow rateincreases. Radial gas velocities at other locations can be found in [35].
As a result of the increasing axial gas velocity the analyte cloudswill no longer grow linearly as shown in Figs. 1 and 2. The expansionwill rather look like displayed in the simplified bottom picture ofFig. 3. Furthermore, the decrease of the radial gas velocity withincreasing distance from the axis will also cause a deviation fromspherical symmetry if the analyte clouds are relatively large. Suchdeviations from symmetry are not sketched in Fig. 3.
The velocity of the plasma gas transporting the atoms and ions ofthe analyte is also affected by the pressure drop through the orifice ofthe sampler. There is a dramatic increase of the gas velocity towards
Fig. 3. Top: Axial and radial gas velocities of the Ar ICP considered applying an injectorflow rate of 0.25 L/min. The radial gas velocities are given for one position just abovethe torch only. Gas velocities and plasma temperatures are taken from theoreticalresults of the PLASMANT group at University of Antwerp [35]. Bottom: Simplifiedpicture showing the influence of the varying down stream velocity on the size of theanalyte ion cloud. The decrease of the radial gas velocity towards the torch wall wasnot taken into account.
67K. Niemax / Spectrochimica Acta Part B 76 (2012) 65–69
the orifice as shown in recent calculations by the PLASMANT group atthe University of Antwerp [36]. The increase of the analyte transportwill deform the analyte ion cloud if it is getting closer to the sampler.This is shown in an exaggerated way in Fig. 4. A small orifice will cer-tainly not have a strong effect on the shape of the cloud. However,larger orifices, which would require a pump with large pump rate inthe first vacuum stage of the MS, will certainly cause significant de-formations of the clouds and a larger number of analyte ions aresucked into the mass spectrometer, increasing the detection efficien-cy of ICP-MS.
It has to be noted that the temperature distribution in Fig. 4, as inall preceding figures, was taken from an ICP without sampler. Thepresence of a cold sampler and the increasing gas velocity due tothe pressure drop through the orifice are changing the temperaturedistribution and the analyte ion trajectories in particular near theinterface. The influence of a sampler at 10 mm above the load coil,having a 1 mm orifice, and creating a pressure drop of 133.3 Pa ontemperature and gas velocity are presented in [36].
3. Parameters determining analyte atomization, diffusion,ionization and the ICP-MS detection efficiency
The experimental parameters determining the location of atomi-zation in the ICP and the spatial distribution of diffused analyte ionsat positions where interfaces are typically placed in ICP-MS havebeen studied recently by OES with end- and side-on observation ofthe plasma [29]. The ICP torch used was taken from the prototypeof a commercial ICP-MS instrument (Model Element 1, ThermoFinnigan). The inner diameter of the tapered torch injector was1 mmwhich is about a factor of two smaller than in most commercialICP instruments. The ICP was operated at 1 kW with a plasma gasflow rate of 16 L/min and an auxiliary gas flow of 1 L/min. The injec-tor gas flow rate was varied between ~0.22 and ~1.0 L/min.
Droplets of aqueous element standard solution were reproduciblygenerated by a commercial droplet dispenser with a frequency oftypically 10 Hz and transported by Ar through the injector into theICP. The droplet diameter could be adjusted to fixed diameters inthe range 30 to about 56 μm. The relative standard deviation of thedroplet diameter once fixed was better than 1%.
The emission intensities of atomic and ionic lines were measuredend- as well as side-on applying monochromators, photomultipliers,and fast data storage devices. Further details can be taken from [29]and references given therein.
3.1. Injector gas flow rate and inner diameter of injector
In order to study the influence of the injector gas flow rate on theposition of atomization, 50 μm droplets of 50 μg/mL Ca solution wereintroduced by flow rates between 0.25 L/min and 0.5 L/min and the
Fig. 4. Simplified picture indicating the influence of the pressure drop at the orifice ofthe MS sampler on the size and shape of the ion diffusion cloud. The temperaturedistribution in the ICP is taken from theoretical results of the PLASMANT group atUniversity of Antwerp [35] derived for the ICP without sampler and pressure drop inthe region of the orifice [36].
emission of the Ca II resonance line was measured side-on. At0.25 l/min, the first side-on emission signal was measured about20 mm below the top of the ICP torch, i.e., the process of dropletdesolvation and the following start of analyte atomization and ioniza-tion occurred deeply upstream in the torch at the mentioned experi-mental conditions. The analyte atomization process was stronglyshifted downstream when the injector gas flow rate was increased.For example, the position was about 5 mm above the top of thetorch when the gas flow rate was doubled (0.5 L/min). The reasonfor the shift of ~25 mm was not only the higher transport velocityof the injector gas in the ICP axis but in particular the relative largedownstream shift of the plasma boundary with increased injectorgas flow rate and the lowering of the maximum axial temperaturedue to the two times larger cold gas volume injected. These effectscan nicely be seen in Fig. 5 displaying the spatial temperature distri-bution at 0.25 and 0.5 L/min in the ICP used as calculated by the col-leagues of the PLASMANT group in Antwerp [35]. It has to be notedthat the actual plasma temperature at droplet injection will also be af-fected by the evaporation of the droplets which is locally cooling theplasma [23]. On the other hand, the heat conduction in the plasmamight be changed due to the evaporated liquid of the droplets. Forexample, hydrogen atoms and ions from aqueous solution dropletswill diffuse rapidly all over the plasma (see, e.g., [23,29]) and changethe spatial temperature profile of the ICP.
The diameters of the analyte ion clouds have to be as small as pos-sible at the position of the sampler if the aim is high detection effi-ciency in ICP-MS (see discussion in chapter 2 and Fig. 1). This canbe achieved by shifting the complete atomization and ionization ofthe analyte downstream as much as possible. For example, the tem-perature distribution at 0.5 L/min would generate smaller ion cloudsat the sampler of the mass spectrometer than for the conditions at0.25 L/min. However, the maximum temperature at 0.5 L/min islower than at 0.25 L/min which is reducing the atomization rate aswell as the ionization of the analytes. Furthermore, it may happenthat particularly larger analyte masses may be incompletely atomizedby the lower plasma temperature over a shorter distance at highertransport velocity. This would also reduce the ion density at the sam-pler and thus lower the detection efficiency of ICP-MS.
It would be interesting to optimize the ICP-MS detection efficiencyby varying the inner diameter of the injector tube. Note that the injec-tor gas velocity is the same at 0.25 L/min with ~0.71 mm injector asin a 1 mm injector at 0.5 L/min. However, the volume of cold trans-port gas per time unit is two times smaller than with 1 mm injector.The axial temperature would be higher with a 0.71 mm than with a1 mm injector which is good for atomization and ionization. On theother hand, the central plasma boundary where atomization startswould be much less shifted upstream than shown in Fig. 5 wherethe injector diameter was 1 mm at 0.25 L/min. In any case, theoptimization of analyte transport velocity, plasma temperature, andanalyte–plasma interaction length for the analyte detection efficiencyin ICP-MS requires careful experimental investigations which shouldbe supported by numerical simulation.
0.25 L/min
0.50 L/min
Fig. 5. Radial temperature profile in the Ar ICP obtained with 1 mm injector diameterand 0.25 and 0.5 L/min injector gas flow rates. Numerical simulations by thePLASMANT group at University of Antwerp [35].
68 K. Niemax / Spectrochimica Acta Part B 76 (2012) 65–69
3.2. Droplet penetration depth as a function of droplet size
Larger droplets of analyte solution penetrate deeper into the ICPthan smaller droplets before they are desolvated and atomization ofthe analyte residuum can start. It means that the size of the analyteion cloud from larger droplets is smaller at the position of the sampler(see Fig. 1). This effect was demonstrated with 49 and 30 μm dropletsof Ca solution in [29]. For example, 0.4 mm above the top of the torch,the Ca ion cloud from 49 μm droplets was about six times smallerthan that from 30 μm droplets. It has to be noted that the droplet di-ameters of 49 and 30 μmwere measured directly behind the nozzle ofthe droplet generator. The actual sizes of the droplets entering the ICPwere certainly smaller because of desolvation during the transport.However, it can be stated that the detection efficiency in ICP-MS is de-pendent on the droplet size if wet aerosols are introduced. As largerthe droplet as better is the detection efficiency if there is complete at-omization of the analyte at the position of the sampler of the massspectrometer.
3.3. Atomization of dry aerosol particles
Compared to analyte droplet introduction, dry aerosol particles as,for example, produced by laser ablation (LA) of solid samples will im-mediately start to evaporate if they penetrate into the ICP. Conse-quently, analyte diffusion begins further upstream in the ICP andthe ion cloud will be larger at the position of the sampler than withanalyte droplets decreasing the analyte ion detection efficiency (seeFig. 1). In order to increase the ion detection efficiency of LA-ICP-MSthe plasma boundary position has to be shifted downstream, e.g., byincreasing the injector flow rate and using a small injector diameter(see chapter 3.1).
It should be noted here that the position of dry particle atomiza-tion will be affected by water vapour if particles are introduced tothe ICP by particle suspension nebulisation followed by desolvation.Therefore, analyte atomization of desolvated droplets will be differ-ent from particles produced by laser ablation, unless the water vapouris precipitated before analyte introduction.
3.4. Analyte mass dependence
Let us assume two droplets of the same size but different analyteconcentration (analyte mass) or two particles of the same elementalcomposition but different mass are introduced into the ICP. The posi-tions where atomization starts in the ICP will be the same. However,larger analyte masses require more energy for atomization, i.e., theatomization process will last longer and the relative increase of freeatoms and ions around the atomizing analyte is slower than for small-er masses. Measurements of the excitation temperature for dropletswith different analyte mass have shown stronger local plasma coolingand slower temperature increase when droplets with larger concen-trations were injected [23]. Therefore, the relative ion concentrationin the axial region at the position of the samples will be higher ifthe atomization is delayed by a larger [23] analyte mass.
The delayed atomization of a trace element in the presence of amatrix element was shown recently in [29]. Monodisperse dropletsgenerated from a solution with low Ca concentration were introducedinto the ICP and the Ca II line emission was measured end-on. The CaII emission was significantly delayed with respect to the onset of Siline emission when SiO2 particles with diameters between 0.83 and1.55 μm were included in the Ca solution droplets.
3.5. Impact of type of injector gas
Another possibility to change the ion detection efficiency inICP-MS is the use of different plasma gases. Due to economic and per-formance reasons the common plasma gas is Ar [39]. However, other
gases can be admixed to the injector gas which can have a strong in-fluence on the plasma conditions in the axial region of the ICP whereanalyte atomization and ionization occur. Due to the low flow rate,the gas consumption of the injector gas is small and even more ex-pensive gases than Ar can be admixed. For example, He is a populargas in laser ablation. It provides better ablation characteristics[40–42] and higher ICP-MS detection efficiencies (see, e.g., [33]).
Recently, the PLASMANT group at University of Antwerp hasshown that He or mixed He/Ar gas flows through the injector of theICP are increasing the plasma temperature [37]. Therefore, higher in-jector gas flow rates can be applied which shift the plasma boundarytowards the sampler of the mass spectrometer without lowering theplasma temperature so much as in the case of pure Ar (see, e.g.,Fig. 5). The result is a smaller analyte ion cloud at the positionof the sampler of the mass spectrometer and an increased detectionefficiency in comparison to pure Ar as injector gas.
It is likely that admixture of other gases than He will also have animpact on the detection efficiency in ICP-MS. However, this has to becarefully studied experimentally as well as theoretically by modelling.
3.6. Influence of orifice diameter and pressure drop at the MS sampler
As discussed at the end of chapter 2 the diameter of the orifice andthe pressure drop at the sampler has influence on plasma tempera-ture and gas velocity [36], and, therefore, also on the ion detection ef-ficiency in ICP-MS. For example, Taylor and Farnsworth [43] haverecently shown that the transmission of analyte ions is growing con-siderably if the orifice diameter is increasing at constant pressuredrop. It would be interesting to apply even larger pressure dropsthan applied in [36] to study the influence of very high gas flowrates through the orifice on the ICP temperature and gas velocitynear to the sampler and on the detection efficiency.
3.7. Complete analyte atomization and equilibrium of ionization andrecombination
As long as there is no completed atomization of analyte masses inthe ICP, equilibrium between ionization and recombination processescannot be expected. Side-on OES measurements with monodisperseanalyte solution droplets revealed temporally shifted intensitypeaks of the ionic lines with respect to the atomic lines [29] whenthe ionization rate was still larger than the recombination rate.Downstream the ICP the shift between the atomic and ionic side-onemission vanished more and more indicating equilibration of analyteionization and ionic recombination. Note that non-equilibrium condi-tions result in lower ion detection efficiency in ICP-MS.
Here it has to be stressed again that all considerations made aboveconcerning the dependence of ICP-MS detection efficiency on injectorgas flow and inner diameter of the injector, size of analyte droplets,analyte mass, and type of injector gas were made under the assump-tion of ionization/recombination equilibrium.
4. Conclusion
The detection efficiency in ICP-MS is best when the diffusion of ana-lyte ions at the position of the MS sampler is small. This can be accom-plished by shifting the location where analyte atomization starts and iscompleted with respect to the sampler downstream the ICP. Further-more, one should keep the plasma temperature in the analyte–plasmainteraction region as high as possible to guaranteemost effective atom-ization and ionization. Both a downstream shift of the atomization andhigh temperature can be achieved, e.g., using injectors with small innerdiameters and high injector flow rates. For a general optimization sev-eral experimental parameters, such as the injector flow rate and gasmixture, the inner diameter of the injector tube, the size of the analytedroplets (if wet aerosols are introduced), the samplemass, the diffusion
69K. Niemax / Spectrochimica Acta Part B 76 (2012) 65–69
rates of the elements studied, the position of the sampler of the massspectrometer as well as the diameter and pressure drop at the MS ori-fice have to be taken into account. However, the difficulty is that the pa-rameters are dependent on each other, i.e., almost all other parametersare affected in a complex way if one of the parameters is changed andneed to be optimized again. Therefore, it is difficult to find experimen-tally the global optimum in a reasonable time varying all parametersmentioned. The optimization problem needs the support by numericalsimulation of the ICP with sampler interface taking into account allimportant physical processes involved.
Acknowledgements
The author thanks Prof. Annemie Bogaerts of University of Antwerp(Belgium) for providing the temperature plots of the ICP used as back-ground in all figures. Financial support of the project “Investigation ofmatrix effects in liquid analyses by ICP-OES/MS applyingmonodispersedroplet injection” by theDeutsche Forschungsgemeinschaft is gratefullyacknowledged.
References
[1] G.J. Bastiaans, G.M. Hieftje, Vaporization of individual solute particles in flamespectrometry, Anal. Chem. 46 (1974) 901–910.
[2] C.B. Boss, G.M. Hieftje, Calculation of the velocity of a desolvating aerosol dropletin an analytical flame, Anal. Chem. 49 (1977) 2112–2114.
[3] C.B. Boss, G.M. Hieftje, Theoretical studies of spatial distributions of atomssurrounding an individual solute particle vaporizing in an analytical flame,Anal. Chem. 51 (1979) 895–901.
[4] C.B. Boss, G.M. Hieftje, Experimental studies of spatial distributions of atomssurrounding an individual solute particle vaporizing in an analytical flame,Anal. Chem. 51 (1979) 1897–1905.
[5] R.E. Russo, G.M. Hieftje, An accurate model for sample droplet acceleration in anair–acetylene flame, Anal. Chim. Acta 118 (1980) 293–299.
[6] R.E. Russo, R. Withnell, G.M. Hieftje, Simple and inexpensive design for an isolat-ed droplet generator useful in studies of atomization in flames, Appl. Spectrosc.35 (1981) 531–536.
[7] B.D. Bleasdell, E.P. Wittig, G.M. Hieftje, An approximate model for the liberationand ionization of atoms from individual solute particles in flame spectrometry,Spectrochim. Acta Part B 36 (1981) 205–213.
[8] J.G. Shabushnig, G.M. Hieftje, Microdrop sample application in electrothermalatomization for atomic absorption spectrometry, Anal. Chim. Acta 148 (1983)181–192.
[10] J.W. Olesik, S.E. Hobbs, Monodisperse dried microparticulate injector: a new toolfor studying fundamental processes in inductively coupled plasmas, Anal. Chem.66 (1994) 3371–3378.
[11] J.W. Olesik, Investigating the fate of individual sample droplets in inductivelycoupled plasmas, Appl. Spectrosc. 51 (1997) A158–175A.
[12] S.E. Hobbs, J.W. Olesik, Influence of incompletely desolvated droplets andvaporizing particles on chemical matrix effects in inductively coupled plasmaspectrometry: time gated optical emission and laser induced fluorescence mea-surements, Spectrochim. Acta Part B 52 (1997) 353–367.
[13] J.W. Olesik, J.A. Kinzer, G.J. McGowan, Observation of atom and ion cloudsproduced from single droplets of sample in inductively coupled plasmas by opti-cal emission and laser-induced fluorescence imaging, Appl. Spectrosc. 51 (1997)607–616.
[14] J.W. Olesik, J.A. Kinzer, Measurement of monodisperse droplet desolvation in aninductively coupled plasma using droplet size dependent peaks in Mie scatteringintensity, Spectrochim. Acta Part B 61 (2006) 696–704.
[15] A.C. Lazar, P.B. Farnsworth, Matrix effect studies in the inductively coupledplasma with monodisperse droplets. Part I: the influence of matrix on the verticalanalyte emission profile, Appl. Spectrosc. 53 (1999) 457–464.
[16] A.C. Lazar, P.B. Farnsworth, Matrix effect studies in the inductively coupledplasma with monodisperse droplets. Part II: the influence of matrix on spatiallyintegrated ion density, Appl. Spectrosc. 53 (1999) 465–470.
[17] A.A. Mills, J.H. Macedone, P.B. Farnsworth, High resolution imaging of barium ionsand atoms near the sampling cone of an inductively coupled plasma mass spec-trometer, Spectrochim. Acta Part B 61 (2006) 1039–1049.
[18] R.S. Houk, R.K. Winge, X. Chen, High speed photographic study of wet dropletsand solid particles in the inductively coupled plasma, J. Anal. At. Spectrom. 12(1997) 1139–1148.
[19] D.B. Aeschliman, S.J. Bajic, D.P. Baldwin, R.S. Houk, High-speed digital photo-graphic study of an inductively coupled plasma during laser ablation: comparisonof dried solution aerosols from a microconcentric nebulizer and solid particlesfrom laser ablation, J. Anal. At. Spectrom. 18 (2003) 1008–1014.
[20] D.C. Perdian, S.J. Bajic, D.P. Baldwin, R.S. Houk, Time-resolved studies of particleeffects in laser ablation inductively coupled plasma-mass spectrometry – Part 1,J. Anal. At. Spectrom. 23 (2009) 325–335.
[21] D.C. Perdian, S.J. Bajic, D.P. Baldwin, R.S. Houk, Time-resolved studies of particleeffects in laser ablation inductively coupled plasma-mass spectrometry – Part 2,J. Anal. At. Spectrom. 23 (2009) 336–341.
[22] K. Jorabchi, R.G. Brennan, J.A. Levine, A. Montaser, Interferometric droplet imag-ing for in situ aerosol characterization in an inductively coupled plasma, J. Anal.At. Spectrom. 21 (2006) 839–846.
[23] S. Groh, C.C. Garcia, A. Murtazin, V. Horvatic, K. Niemax, Local effects of atomizinganalyte droplets on the plasma parameters of the inductively coupled plasma,Spectrochim. Acta Part B 64 (2009) 247–254.
[24] C.C. Garcia, A. Murtazin, S. Groh, M. Becker, K. Niemax, Characterization of parti-cles made by desolvation of monodisperse microdroplets of analyte solutions andparticle suspensions for nanoparticle characterization in inductively coupledplasma spectrometry, Spectrochim. Acta Part B 65 (2010) 80–85.
[25] C.C. Garcia, S. Groh, A. Murtazin, V. Horvatic, K. Niemax, Characterization of singleAu and SiO2 nano- and microparticles by ICP-OES using monodisperse droplets ofstandard solutions for calibration, J. Anal. At. Spectrom. 25 (2010) 645–653.
[26] S. Groh, P. Diwakar, C.C. Garcia, A. Murtazin, D.W. Hahn, K. Niemax, 100% efficientsub-nanoliter sample introduction in laser induced breakdown spectroscopy andinductively coupled plasma spectrometry: implications for ultralow sample vol-umes, Anal. Chem. 82 (2010) 2568–2573.
[27] A. Murtazin, S. Groh, K. Niemax, Measurement of element mass distributions inparticle ensembles applying ICP-OES, J. Anal. At. Spectrom. 25 (2010) 1395–1401.
[28] S. Gschwind, L. Flamigni, J. Koch, O. Borovinskaya, S. Groh, K. Niemax, D. Günther,Capabilities of inductively coupled plasma mass spectrometry for the detection ofnanoparticles carried by monodisperse micro-droplets, J. Anal. At. Spectrom. 26(2011) 1166–1174.
[29] A. Murtazin, S. Groh, K. Niemax, Investigation of sample introduction- andplasma-related matrix effects in inductively coupled plasma spectrometry applyingsingle analyte droplet and particle injection, Spectrochim. Acta Part B 67 (2012) 3–16.
[30] C. Degueldre, P.Y. Favarger, Thorium colloid analysis by single particle inductivelycoupled plasma-mass spectrometry, Talanta 62 (2004) 1051–1054.
[31] C. Degueldre, P.Y. Favarger, C. Bitea, Zirconia colloid analysis by single particleinductively coupled plasma-mass spectrometry, Anal. Chim. Acta 518 (2004) 137–142.
[32] C. Degueldre, P.Y. Favarger, R. Rosse, S.Wold, Uranium colloid analysis by single par-ticle inductively coupled plasma-mass spectrometry, Talanta 68 (2006) 623–628.
[33] L. Laborda, J. Jimenez-Lamana, E. Bolea, J.R. Castillo, Selective identification, char-acterization and determination of dissolved silver(I) and silver nanoparticlesbased on single particle detection by inductively coupled plasma mass spectrom-etry, J. Anal. At. Spectrom. 26 (2011) 1362–11371.
[34] B. Franze, I. Strenge, C. Engelhard, Single particle inductively coupled plasmamass spectrometry: evaluation of three different pneumatic and piezo-basedsample introduction systems for the characterization of silver nanoparticles,J. Anal. At. Spectrom. 27 (2012) 1074–1083.
[35] H. Lindner, A. Murtazin, S. Groh, K. Niemax, A. Bogaerts, Simulation and experi-mental studies on plasma temperature, flow velocity and injector diametereffects for an inductively coupled plasma, Anal. Chem. 83 (2011) 9260–9266.
[36] M. Aghaei, H. Lindner, A. Bogaerts, Effect of a mass spectrometer interface on induc-tively coupled plasma characteristics: a computational study, J. Anal. At. Spectrom.27 (2012) 604–610.
[37] H. Lindner, A. Bogaerts, Multi-element model for the simulation of inductivelycoupled plasmas: effects of helium addition to the central gas stream, Spectrochim.Acta Part B 66 (2011) 421–431.
[38] M. Wälle, J. Koch, L. Flamigni, S. Heiroth, T. Lippert, W. Hartung, D. Günther,Detection efficiencies in nano- and femtosecond laser ablation inductivelycoupled plasma mass spectrometry, Spectrochim. Acta Part B 64 (2009) 109–112.
[39] A. Montaser, Inductively Coupled Plasma Mass Spectrometry, Wiley-Vch, 1998.[40] J. Koch, D. Günther, Review of the state-of-the-art of laser ablation inductively
coupled plasma mass spectrometry, Appl. Spectrosc. 65 (2011) 155A–162A.[41] R.E. Russo, X. Mao, H. Liu, J. Gonzalez, S.S. Mao, Laser ablation in analytical
chemistry—a review, Talanta 57 (2002) 425–451.[42] C.C. Garcia, H. Lindner, K. Niemax, Laser ablation inductively coupled plasma mass
spectrometry - current shortcomings, practical suggestions for improving perfor-mance, and experiments to guide future development, J. Anal. At. Spectrom. 24(2009) 14–26.
[43] N. Taylor, P.B. Farnsworth, Experimental characterization of the effect of skimmercone design on shock formation and ion transmission efficiency in the vacuum in-terface of an inductively coupled plasma mass spectrometer, Spectrochim. ActaPart B 69 (2012) 2–8.
