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Atomic spectrometry update—X-rayfluorescence spectrometry Philip J.Potts,a Andrew T.Ellis,b Peter Kregsamer,c Christina Streli,cChristine Vanhoof,d Margaret West*e and Peter Wobrauschek cReceived3rd August2006First published as an Advance Article on the web11th September2006DOI:10.1039/b611269m1Introduction and reviews2Instrumentation2.1General instrumentation and excitation sources2.2Detectors3Spectrum analysis,matrix correction and calibration procedures3.1Spectrum analysis3.2Matrix correction and calibration procedures4X-ray optics and microfluorescence5Synchrotron radiation6TXRF6.1Applications7Portable and mobile XRF8On-line XRF9Applications9.1Sample preparation and preconcentration techniques 9.2Geological and industrial minerals9.3Environmental9.3.1Atmospheric particulate matter9.3.2Other environmental9.4Archaeological,cultural heritage and forensic9.5Industrial9.6Clinical and biological9.7Thinfilms and coatings9.8Chemical state,speciation and crystal characterisation 10Abbreviations11ReferencesThis annual review of X-rayfluorescence covers developments over the period2005–2006in instrumentation and detectors, matrix correction and spectrum analysis procedures,X-ray optics and microfluorescence,synchrotron XRF,TXRF,porta-ble XRF and on-line applications,as assessed from the published literature.The review also covers a survey of applications, including sample preparation,geological,environmental,ar-chaeological,forensic,biological,clinical,thinfilms,chemical state and speciation studies.The trend set in previous years continues with large numbers of papers published in the areas of environmental,industrial and biological/clinical applications, followed this year by a significant contribution in the area of archaeological science and cultural heritage.Notable contribu-tions this year are in areas of topical societal interest,including the rapid throughput screening for combatorial materials.Inter-est continues in nanotechnological applications as well as the related micro-analytical instrumentation that permits profiling and mapping of samples,which was previously undertaken by non-XRF techniques.1Introduction and reviewsThis review continues the series of Atomic Spectrometry Updates in X-ray Fluorescence Spectrometry,1this one cover-ing developments as assessed from the published literature over the period2005–6.Interest has continued in the use of XRF in characterising reference materials.Sturgeon et al.2described the certification of afish otolith reference material,based on data provided by a wide range of techniques,including XRF.This RM was designed for laboratories requiring quality assurance of mea-surements of the bulk elemental composition of otoliths and other marine aragonites.Eight new geochemical reference materials for XRF major and trace analysis were prepared by Lozano and Bernal,3comprising a lateritic soil,dolomite, limestone,andesite,three different syentites,and a gabbro,all from Mexico.Provisional data were provided for24major and trace elements,FeO and LOI.Kurahashi et al.4developed a method that will be applicable to the certification of reference materials,based on monochromatic X-ray excitation XRF. The method was evaluated by the determination of trace elements in sediment certified reference materials.An important aspect of all analytical measurements is the estimation of measurement uncertainty.Morgenstern et al.5 evaluated the Eurochem Guide‘Quantifying uncertainty in analytical measurement’specifically in thefield of XRF ana-lysis,taking account of the strong influence of the sample matrix on the analytical response.Their approach focused on the use of the intensity of the Compton scatter tube line as an internal standard to assess the effect of matrix.In order to simplify use of this review,contributions are divided into clearly labelled sections.However,each year contributions are published that are challenging to categorise. This year,the accolade goes to Rozic et al.6who described the use of EDXRF in determining the elemental composition of ash remaining after burning writing,copying and computer printing papers.The results of this burning issue showed that paper from one manufacturer contained significantly higher concentrations of Pb,Sr and Zr and laser printer papera Department of Earth Sciences,The Open University,Walton Hall,Milton Keynes,UK MK76AAb Oxford Instruments Analytical Ltd.,Halifax Road,High Wycombe,Buckinghamshire,UK HP123SEc TU Wien,Atominstitut der O¨sterreichischen Universita¨ten,Stadionallee2,A-1020Vienna,Austriad VITO,Environmental Measurements,Boeretang200,B-2400Mol,Belgiume West X-ray Solutions,405Whirlowdale Road,Sheffield,UK S119NFASU REVIEW /jaas|Journal of Analytical Atomic Spectrometrycontained higher concentrations of Co,Cu,Fe,Mn and Ti than non-printer paper.2Instrumentation2.1General instrumentation and excitation sourcesThe increasing threat of the use of chemical warfare(CW) agents,not only in military activities but also against civilians by terrorists,has concentrated minds(and funding)on high throughput screening for combinatorial materials.Havrilla and Miller7developed commercial m-XRF instrumentation for high throughput screening of elemental composition of a broad spectrum of CW agents at the meso-scale.The techni-que was demonstrated by screening an oligopeptide library for selective binding of the degradation products of the nerve agent VX.The technique is non-destructive,requires minimal sample preparation or special tags for analysis and the screen-ing time was reported to be dependent upon the desired sensitivity.Luo et al.8developed a high throughput X-ray characterisation system that could screen the structure and composition of CW agent libraries using EDXRD,XRF and X-ray photoluminescence.Their system comprised an X-ray source,a polycapillary X-ray lens,one or two X-ray energy detectors and afibre optic spectrometer plus associated software.Developments continue to be reported for pyroelectric X-ray sources,initially introduced as an ultra-portable X-ray generator for use in imaging and materials analysis.Geuther and Danon9showed how a paired-crystal pyroelectric source could be used to excite the K shells of both thorium and lead. Teams at Kyoto University,Japan10,11built on the earlier work of Terasawa by constructing a compact emission instru-ment for the production of soft X-rays through the application of a high voltage to an insulator.The X-ray intensity increased with increasing high voltage,demonstrating that X-rays could be produced without supplying electrons from afilament. Readers who understand the problem of Bremsstrahlung background interference due to electron–specimen interaction in a SEM,will be pleased to know that Bjeoumikhov et al.12 reported a reduction in background interference of two orders of magnitude by combining a micro-focus X-ray tube with efficient polycapillary optics that focused the emitted X-rays onto spots between30and100m m in diameter.In this way they were able to perform localfluorescence X-ray analysis at the same point where the electron beam hit the sample and gave examples for the application of this confocal scheme for EPMA and m XRF.Other instrumentation in this review period included the development of an atmospheric electron X-ray spectrometer by Urgiles et al.13as a surface analysis tool for samples in ambient atmospheres.Moderate to high spectral resolution was reported through the use of a thin electron-transmissive SiN membrane to isolate the vacuum of a miniature20keV electron probe,obviating the need for the sample to be drawn into the probe vacuum.The equipment was tested using NIST and USGS traceable metal and mineral reference materials giving good agreement with certified composition for samples in atmospheric pressures up to90Torr.Grazing-exit XRF and m XRF methods were applied to chemical microchips by Tsuji et al.14The analysing region was designed so that a sample solution could be dried and concentrated in an area corre-sponding to the size of the primary X-ray beam.In GE-XRF mode,the technique was reported to be feasible for trace element analysis and preliminary results in m XRF mode were encouraging.Bruker15announced the launch of a new series of analysing crystals that could double the intensity of selected low atomic number elements.Coupled with the S4Pioneer spectrometer,the XS series of crystals have been specially designed to improve the intensity performance of Be,B,C,F, Mg,N,Na and O.2.2DetectorsSemiconductor detectors based upon silicon PIN or silicon drift devices remain the mainstay of EDXRF spectrometry and developments in each of these two types of detector were reported during the review period.Pantazis16,17reported im-pressive energy resolution of around135eV with peaking times in the range12–20m s for a silicon PIN diode detector combined with a new digital pulse processor.The pulse processor was equipped with peaking times in the wide range of0.8to102m s and with software that could automatically optimise the peaking time for energy resolution,which is a valuable asset for users inexperienced in thefield.The im-portance of a number of factors to reliable EDXRF measure-ments was recognised by Paepen and co-workers18including not only energy resolution and throughput but also of stability of spectrometer gain,sensitivity and resolution over time and at varying throughputs.The authors characterised their so-called X-PIPS detector,which was a fully depleted,0.5mm thick silicon PIN with a passivated implanted front contact. Prior to some design changes,the authors reported a sensitiv-ity decrease of5.5percent per day,which is clearly unaccep-table for practical use.The new design showed no significant sensitivity change with time and only a very small gain change of0.13eV at5.9keV,which correlated with diurnal tempera-ture change.The best energy resolution obtained for this silicon PIN detector was182eV at5.9keV and a maximum throughput of7kcps.The energy resolution increased to 215eV at a maximum throughput of31kcps.An extremely impressive60mmo annular silicon drift detector(SDD) comprising twelve5mmo devices was used for m-PIXE map-ping by Doyle et al.19on the nuclear microprobe at Sandia National laboratory.Very high solid angles of up to1.09sr were achieved by virtue of the close coupling of the detector to the sample and the proton beam passing through the central aperture in the annular detector array.Twelve discrete nucleonics channels were used for data acquisition and the typical energy resolution at5.9keV of195eV at60kcps total was reported for the summation of the twelve spectra.Count rates of1Mcps were used routinely and an energy resolution of o200eV was maintained,which made for high-quality maps and reduced dwell and sample exposure times,so mini-mising sample damage.The same type of12-channel SDD was also used by Longoni and co-workers20in combination with a microfocus X-ray tube and polycapillary X-ray optic in acoaxial m-XRF system that was used for an interesting variety of archaeometric,industrial and biological X-ray mapping studies.The average energy resolution at5.9keV of the twelve SDD elements was155eV with a shaping time of0.5l s.The only negative characteristic of this impressive detector array was a peak shift at8.04keV of around40eV over a count rate range of10–180kcps.A new coaxial SDD detector array comprising four15mm2‘‘teardrop’’SDDs was also reported by this team.The energy resolution at5.9keV of the four SDDs was impressive at130.4,130.45,132.08and132.3eV, which made this device much more attractive than the12-element SDD array.In addition,the four SDD‘‘cloverleaf’’design had an equivalent solid angle to the SDD-12and needed only four electronics channels.The energy resolution increased to220eV at300kcps and an impressive P/B ratio of 5000:1was obtained,making this detector very attractive for high rate X-ray mapping when combined with a central microfocus X-ray tube and an optic to give an intense coaxial excitation beam.An interesting16-element SDD array was also described by Marisaldi et al.,21who used the device as a photodetector for light from an overlayer of CsI(Tl)scintilla-tor material.The device was under development for deploy-ment as a space-borne gamma ray detector.In another rather exotic set up,a silicon PIN diode X-ray detector was used by Hohl and colleagues22to investigate the electron density and temperature in an electron cyclotron resonance ion source plasma.A review of the past20years of developments in cryogenic X-ray detectors was presented by Previtali,23who indicated the great challenge for the future of making much larger detectors than are currently available.Further improvements in the NIST cryogenic microcalorimeter were described by Jach and colleagues24who were able to reduce energy scale drift/ peak shift from410eV per hour to1–2eV per hour.This improved performance resulted in the observation of XRF line widths r12eV over an extended six hour counting time.A new,optimisedfiltering algorithm was reported by a team at Leicester,UK25to improve energy and position resolution in transition edge sensor cryogenic detectors.In thefield of superconducting tunnel junction(STJ)cryogenic detectors, Shafranjuk et al.26used a double barrier Josephson junction design that was expected to yield a10–100fold improvement in charge amplification and lead to much higher sensitivity and better energy resolution.An STJ with aluminium trapping layers and a novel photosensitive polyimide insulator layer was shown by Yoshida et al.27to be easier to fabricate and yielded an encouraging energy resolution at5.9keV of86eV. Activity was disappointingly low during the review period in thefield of pixellated X-ray detectors.Kruger28presented a comprehensive review of two-dimensional X-ray detectors in which he identified the trends to further improve sensor-to-electronics interconnects and signal processing systems and also the way in which detectors for particle physics and medical imaging were already diverging.A straightforward optical CCD coupled to an inexpensive plastic scintillating fibre array was reported by Naseri et al.29for use in X-ray imaging while Tate and colleagues30used simple,commer-cially available components in a similar arrangement compris-ing a1024by1024pixel CCD camera optically coupled to an X-ray sensitive phosphor screen with a standard35mm camera lens.Although relatively simple systems,their use is directed at medical imaging and they are of little interest for practical two-dimensional EDXRF system measurements. Commercially available large area avalanche photodiodes operated at room temperature orÀ201C were reported31to yield a best energy resolution of6.4percent at5.9keV and a low energy threshold down to0.5keV.A fast signal collection time of 1.9ns(FWHM)and low energy sensitivity were particular advantages and when used in conjunction with a scintillating crystal these detectors were also suitable for high energy X-ray and gamma ray detection.Turning now to detector materials other than silicon,Sun et al.32briefly reviewed the state of the art in thin epitaxial gallium arsenide detectors and went on to show how recently available thicker and larger area layers could be used for X-ray imaging—albeit focusing primarily on medical imaging.A wet chemical etching technique was described33for the checking of defects in gallium arsenide array detector material also des-tined for use in medical X-ray imaging applications.A new type of photovoltaic X-ray detector based upon epitaxial gallium arsenide structures was described.34This type of detector was also planned to be used in X-ray detectors: however,only electrical characteristics were reported and not X-ray detection performance.Further work towards room temperature detectors was reported and included carrier trap-ping defects in thallium bromide semiconductor detectors,35 growth and properties of thick epilayers of cadmium tell-uride36and the fabrication and performance of chemical vapour deposition(CVD)diamond-based X-ray detectors. There were only two reports of gas-filled detectors during the review period,thefirst of which described a two-dimen-sional gas microstrip detector operated at room temperature.37 Using a P10gas mixture an energy resolution of12.3percent was obtained at5.9keV with a working count rate up to 1kcps.A sealed gas proportional scintillation counter(GPSC) was described by Goganov and Schultz38for use in industrial EDXRF systems.The combination of a xenon gasfill and a 50m m beryllium entrance window yielded good efficiency over the whole energy range of 1.5to25keV and the energy resolution was reported to be18percent for Al K(1.5keV), 9percent at5.9keV and4.2percent for Ag K a at22.2keV and a maximum count rate of50kcps was indicated.The detector was claimed to be robust and even though fully sealed,the authors claimed a working lifetime for the detector in excess of five years and no evidence of performance degradation with total number of detected photons.3Spectrum analysis,matrix correction and calibration procedures3.1Spectrum analysisIn an interesting and comprehensive paper on peakfitting in EDXRF spectrometry Greaves et al.39established that there was an optimum number of channels in the peak for minimis-ing the precision of determining the peak centroid.A compre-hensive theoretical treatise was presented andfitting of both theoretical and empirical spectra established that the optimumnumber of channels for the peak to establish the peak centroid was2.5–3channels for the FWHM value.Values greater than this led to a significant degradation in quality of thefitted peak position.The challenge of modelling the background from sample diffraction peaks in EDXRF spectra was taken on by Verkhovodov,40who used the new concept of a virtual pri-mary radiation source to establish the excitation spectrum of both characteristic and Bremsstrahlung radiation.The author then showed how Bragg diffraction in a polycrystalline sample gave rise to significant background in the detected energy dispersive spectrum.A detailed study of the background in Johansson optics WDXRFfixed channels led to a novel digitalfiltration method for reducing detected spectrum background.41The authors con-nected a digital recording oscilloscope to the detector output between the shaping amplifier and the discriminator unit to sample the output in real time.An algorithm was then developed to apply a digitalfilter to the pulse amplitude distribution,which resulted in a reduction by a factor of3in spectrum background for a number of cases.This amplitude distributionfilter was shown theoretically and experimentally to be a very attractive option compared with the use of a simple threshold and window discriminator,although the new approach may not be so simple and universally applicable as that of conventional discriminators. The knowledge and characterisation of XRF detector effi-ciency is key to optimised spectrometer design and accurate standardless analysis.Although perhaps of most interest to the microanalysis community,the careful characterisation by Schulze and Procop42of commercially available low-energy ED detector windows will be of keen interest to some in the XRFfield.The entrance windows typically comprise a grid-supported,aluminised polymerfilm and knowledge of the efficiency in the o1keV energy range is often complicated by a lack of exact composition and dimensional data for these windows.The use of SR from a BESSY2beamline ensured that a calculable and highly characterised primary X-ray spectrum was available and the use of tuneable,monochromatised SR from a second beam line enabled careful high resolution measurements of detector efficiency to be made against a reference detector.By using these state of the art SR techniques the authors were able to fully characterise the detector windows and the measurement methods and reported data will be invaluable to those carrying out XRF measurements in the challenging sub-1keV energy range.At the opposite end of the EDXRF spectrum,Yilmaz and co-workers43studied the escape of Ge K X-rays,Compton scattered incident radiation and photoelectrons following excitation by81keV incident photons. Good agreement for Ge K and Compton escape was found between the authors’experimental and MC simulation data and with one other published MC simulation study.The observed and calculated photoelectron escape probabilities differed greatly,and although the authors indicated some factors that could influence this observation,there was no conclusion to fully understanding this significant escape mechanism.3.2Matrix correction and calibration proceduresThe use of MC simulation to predict the complete EDXRF spectrum or the X-ray tube spectrum continued during the review period.Although directed at X-ray tubes for X-ray radiography,the MC method described by Gallardo et al.44 could doubtless be applied to lower energies typical of XRF spectrometry.Rather than making direct measurements of the tube output spectrum,which is clearly the optimum approach, the authors used the901Compton scattered spectrum from a Perspex scatter block measured by a HPGe detector.A full description was given of the mathematical and computational approach adopted by the team and good agreement was obtained between the calculated and observed scatter spectra, and where discrepancies were seen the authors proposed the likely reasons for them.A description of the development by Gardner and colleagues at North Carolina State University of the full MC library least squares code was given by Gardner and Guo.45This comprehensive package has been developed over many years and benchmark results were presented for two typical stainless steel samples to demonstrate the effec-tiveness of the package in calculating the full EDXRF spec-trum from these complex samples.Although in thefield of medical imaging radiography,the use by Song et al.46of so-called angular response functions that were themselves pre-computed by MC simulation reduced the overall simulation time by factors of up to1000when compared with a full MC simulation method.The authors’proposed technique may be useful in XRF for studying the effects of various instrument configurations such as collimators,baffles and X-ray collection or focusing optics.Fundamental parameter(FP)based XRF analysis is widely used for bulk analysis and increasingly for thinfilm analysis. Kolbe and colleagues47at PTB,Berlin,described an impress-ive reference-free FP method for accurate measurement of single and dual layer metal nanolayers.Samples comprised copper and nickel layers on silicon with a thickness in the range5–50nm,for which no suitable reference materials existed.X-ray reflectometry was used as the comparison method to check results from the fully characterised and absolutely calibrated XRF instrumentation.The use of such an approach yielded results from XRF that were statistically indistinguishable from those from X-ray reflectometry,there-by establishing the PTB XRF method as an accurate and absolute one for the quantitative measurement of the thickness and composition of these analytically challenging nanolayers.A study by Han et al.48showed that FP XRF analysis was able to fully correct for matrix effects in several Japanese iron and steel,NIST glass and Chinese geological reference materials. In addition,the authors established,unsurprisingly,that further improvements in accuracy could be obtained using simple samples as similar standards when fully matrix-matched calibration samples were unavailable.A paper by Boto and Murphy49described how the method of standard additions used in XRD could be improved when analyte concentrations were greater than5%m/m.The authors also recommended their method for XRF but equal accuracy and much greater simplicity would doubtless be obtained through the use of one of the many FP algorithms available nowadays. The incorporation in quantitative analysis routines of cor-rection for enhancement and secondaryfluorescence is necessary in most cases,but is particularly important for certain sample types and instrument configurations.Karydas50calculated andmeasured the self-element enhancement such as that for a K line of molybdenum enhancing the molybdenum L series emission from a pure molybdenum sample.In the case of monochromatic excitation the self-element K on L enhance-ment could account for20–25percent of the signal for elements in the Z range38–46.As might be anticipated,the higher the energy of the monochromatic beam used for primary excitation,the greater the effect.Similar self-enhance-ment was reported for L–L and L–M interactions,albeit with reducedfluorescence intensities.The author reported generally good agreement between calculated and measured self-en-hancement effects but the lack of reliable fundamental para-meters for L–M enhancement calculations led the author to recommend the use of an experimental approach to estimating those effects.The contribution of scattering effects to the measuredfluorescence intensities of analyte lines from low atomic number,highly-scattering matrix samples was investi-gated by Han et al.51In their FP approach the authors took into account coherent and incoherent scatter within the fused bead samples.The enhancement due to additional excitation by scattered radiation was several percent in many cases and 410%in the case of the Pb L.series,which would clearly lead to substantial errors if the FP calculations did not include these effects.The accuracy of measurement,for an example, was shown to improve from an average relative error of2.75% using NRLXRF with no inclusion of corrections for enhance-ment due to scatter,to0.93%using their augmented FP algorithm.A similar study was reported by Bos and Vrielink,52 who also showed significant improvements for low-Z matrices when scattering enhancement effects were taken into account in their FP program,which was applicable to polychromatic excitation and complex samples.The interesting case for selective excitation of the light elements(C,F,N and O)by L and K lines of the matrix element was studied by Dukhanin and Pavlinsky.53The analyte was0.1%m/m in a metal matrix of99.9%m/m,which is an uncommon analytical situation but served well to evaluate the magnitude of the effects.Initial studies were reported from an iron matrix in which Fe K and Fe Lfluorescence lines were considered for their direct en-hancement effect and the additional contribution for Fe K–L internal conversions was also included.X-ray tubes with a tungsten target and either a300or a75m m beryllium window were operated at40kV for this work,which showed that for an iron matrix the contribution to the observedfluorescence signal of the three enhancement mechanisms was3.8,11.4,23 and34%relative for C,N,O and F,respectively,when the 300m m beryllium window was used.With the thin,75m m, beryllium window direct excitation improved and the enhance-ment by the matrix lines reduced to10.2,16.5and22.6% relative for N,O and F,respectively.The major contribution to the enhancement was,as would be expected,from the Fe L series,and in the case of excitation of F Kfluorescence and the 300m m beryllium window this enhancement alone contributed 26%relative in the total of34%relative.The magnitudes of the other enhancements by Fe K lines or Fe K–L internal cascade transitions were similar to each other and a factor 5–10times less than for enhancement by the Fe L lines.The authors also considered and tabulated the enhancement effects for the same analytes in a matrix of nickel and of zinc,showing a similar domination of the enhancement by the matrix L line emissions.It was also clear from this study how large was the effect on thefluorescence intensity of the excitation from photo-and Auger electrons,which contributed29–90percent of the signal for these low-Z analytes when using a300m m beryllium window.In a second paper by these two authors and others,54the contribution of photo-and Auger electron ex-citation to C K X-rayfluorescence intensity in high Z matrices was studied.Extensive tabulations of the calculated effects were presented for both rhodium and chromium target X-ray tubes equipped with75m m beryllium windows and matrix concentrations of99.9%m/m.Those tables showed the very high contribution to C K excitation made by these effects of up to around90%relative.Without these effects being taken into account the FP calculations would,in one case quoted,be a factor of 5.6times wrong!Good agreement was reported between calculated and experimental data on C in eight carbon compounds with elements covering the Z range of 3–82including Li2CO3,MgCO3and Pb(CH3COO)2. Finally,it was a disappointing year in terms of numbers of publications on chemometric and other techniques.In the sole paper on chemometrics,Serneels et al.55described their so-called continuum regression(CR)method,which combined elements from ordinary least squares,partial least squares and principal component regression but yielded better prediction results than any of these techniques used alone.Problems of robustness were reported with CR for data sets that were not normally distributed or where there were outliers.The authors’proposed technique of‘‘robust continuum regression’’was claimed to provide the robustness required to solve real analy-tical problems and its advantages were shown by improved data from XRF analysis of hydrometallurgical samples.Sitko56 described a new algorithm that was derived from the Sherman equation and which successfully corrected absorption and enhancement effects in samples of intermediate thickness by means of theoretical influence coefficients.In previous work enhancement had often been ignored in order to simplify the approach but this author included it and applied the calcula-tions to all matrix elements.To simplify the calculations and avoid the need for an iterative solution,this author treated the matrix coefficients as constant for any given mass thickness, although this approach also required matrix diversity to be limited.The author demonstrated the validity of this approach with results from geological CRMs and ceramic samples.4X-ray optics and microfluorescence Investigations into the use of polycapillary optics continue to appear in the literature with Sun and Ding57presenting a study of the energy dependence of the focal spot size,transmission efficiency and output focal distance of a polycapillary X-ray lens measured over a broad energy range.Measurements were made using a low current micro-focus(50m m)Mo X-ray tube and an axial scanning method was devised to determine the energy dependence of the output focal nger et al.58 studied the polychromatic X-ray excitation of Kossel patterns by an X-ray tube coupled to a focusing polycapillary lens, emphasising the advantages offered by the polycapillary device. Bjeoumikhov and colleagues59reviewed the opportunities。