COMMUNICATION

Improving sensitivity for CE-ICP-MS using multicapillary parallel

separation{

Vahid Majidi,*a Johanna QvarnstroÈm,b Qiang Tu,b Wolfgang Frechb and Yngvar Thomassenc

aLos Alamos National Laboratory, Los Alamos, New Mexico 87545, USAbDepartment of Chemistry, Analytical Chemistry, UmeaÊ University, 901 87 UmeaÊ, SwedencNational Institute of Occupational Health, P.O. Box 8149DEP, N-0033 Oslo, Norway

Received 27th July 1999, Accepted 15th September 1999

A multicapillary, capillary electrophoresis inductively coupled plasma mass spectrometry interface is described.

This interface allows for higher sample loading to improve the overall sensitivity and analyte detection limits

without sacri®cing the separation ef®ciencies. The results obtained with this parallel system are presented for a

cross ¯ow nebulizer. A comparison of single capillary electrophoresis for both DIHEN and cross ¯ow

nebulizers is presented.

Introduction

The strong need to identify speci®c compounds and distribu-tion of elements has increased the use of separationtechnologies with spectroscopic detection.1±4 Speci®cally,capillary electrophoresis has become the method of choicefor separation when speciation applications are considered.5,6

Because of the lack of a stationary phase and with anoperational pH range well within the stability region of manyspecies, CE is one of the few technologies that seem to preservethe original chemical information during the separation stage.In spite of ¯ow rate incompatibilities and limited samplevolumes, inductively coupled plasma mass spectrometry hasemerged as the method of choice for detection of metalloidsand metal-containing species separated by CE.7±9

The major drawback, when using CE for chemical specia-tion, is the limited amount of sample utilized during theanalysis. Because the sample loading in CE is typically less than100 nl, even with the low detection limit of ICP-MS, the analyteconcentration within the sample must be relatively large(w10 mg l21).9 Although this may not pose a great problemfor chemical speciation in many environmental samples,evaluation of metal species in clinical samples at physiologicalconcentrations is not practical.

One approach to alleviate the stringent requirement forhigher analyte concentration is to use more ef®cient nebuliza-tion schemes for sample introduction into ICP. Severalresearchers have utilized high ef®ciency nebulizers for plasmabased applications.10±12 In most instances a gain of 10±100 caneasily be obtained when high ef®ciency nebulization isemployed. Nonetheless, this gain is still at least an order ofmagnitude lower than what is really needed for clinicalapplications (e.g., measurement of manganese bound totransferrin). Introduction of larger sample volumes into CEis not a viable solution as peak broadening and poor resolutiondue to column overloading deteriorates the quality ofinformation gained through separation. Using larger diametercapillaries can yield higher column loading; however, at best afactor for 2±5 larger sample amounts are usable beforeproblems associated with larger column diameters fullymanifest themselves. The main problem with large column

diameters is the need for large separation currents, which willcause heating of the run buffer, in turn leading to poorseparation ef®ciencies. Therefore, column loading cannot beincreased effectively through the use of a single capillary, hencea multicapillary system seems to provide the appropriatesolution for sample loading. Currently, monolithic multi-capillaries are commercially available. Unfortunately thesesystems are not appropriate for CE applications because theuniformity of internal diameter for each individual capillarycannot be veri®ed throughout the length of the column(electron micrographs show signi®cant capillary±capillaryvariation of internal diameter). Furthermore, because ofmonolithic construction the heat transfer from each capillaryto the ambient environment is extremely poor and subse-quently the separation conditions are not constant throughoutall capillaries (i.e., the capillaries at the center of the pack runhotter than the capillaries on the outer perimeter).

In this paper we describe a unique approach which takesadvantage of a parallel separation scheme to realisticallyimprove the analytical sensitivity and detection limits. Severalindividual capillaries with identical internal diameters andlengths are placed in a loose bundle. Because the capillaries arenot physically in contact throughout the bundle, the heattransfer to ambient atmosphere is not hindered. The terminalend of these capillaries (cathodic end) is placed into a cross ¯ownebulizer. For comparison a direct injection high ef®ciencynebulizer (DIHEN) is also used for single capillary separationto provide high ef®ciency nebulization without the need for aspray chamber. Therefore, nearly 100% sample utilization isrealized without adverse effects derived from the spraychamber (i.e., analyte±chamber interactions, band broadening,sample loss, etc.).

Experimental

Separation capillaries consisted of 75.0 cm long fused silicacolumns (from the same spool) with an internal diameter of75 mm and an external diameter of 375 mm (PolymicroTechnologies, Inc., Phoenix, AZ, USA). Prior to their use,capillaries were conditioned by ¯ushing with 0.1 M NaOH for30 min, followed by washing with water and buffer. The CEsystem was designed in-house using a Spellman CZE1000,(Spellman High Voltage Electronics, Hauppauge, NY, USA){US Government Copyright.

J. Anal. At. Spectrom., 1999, 14, 1933±1935 1933

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power supply. The power supply can be operated in voltage(30 kV maximum) or current controlled (300 mA maximum)mode. The CE-ICP-MS interface was assembled according tothe procedure published previously.13 The CE performance wastested for one and several parallel capillaries using a number ofcations. A cross ¯ow nebulizer connected to a double-passScott spray chamber (provided with the ICP-MS instrument)was used for most experiments. A DIHEN nebulizer(J. E. Meinhard Associates, Inc., Santa Ana, CA, USA) wasused to assess the sensitivity improvements for single capillaryCE separation.

The test sample contained 100 mg l21 each of La, Mn, Ba andCs in Milli-Q (Millipore, Bedford, MA, USA) water, unlessotherwise noted. During sample introduction the nebulizer gas¯ow was set to 0.4 l min21 and the samples were introducedelectrokinetically for 10 s at 21 kV. The length of the capillarieswas adjusted so that each cathodic terminal (at the nebulizer)and each anodic end (sample cup) were at the same position.Furthermore, both ends of the capillary bundle were held at thesame elevation to minimize the ¯ow through the capillariesbecause of pressure difference. The position of the capillarieswas not changed during sample introduction; instead, thesample cups were moved for injection and for introduction ofthe run buffer. For each run sample cups were ®lled with afresh, 100 mg l21 mixture of cations. The buffer solution wasreplenished at least every third run. Between every run, with themulti-capillary setup, buffer was ¯ushed (0.25 ml min21)through the columns for 1 min. The total volume of eachcapillary was 3.3 ml. After each day the capillaries were ¯ushedwith and placed in water and in the morning of the followingday they were washed with 0.1M NaOH, water and buffer asdescribed above.

The separation buffer was a 20 mM ammonium acetatesolution with a pH of 6.5. This solution was made fresh daily orevery other day. The buffer was ®ltered through a 0.45 mm ®lterand de-gassed in an ultrasonic bath for 10 min. The samplesolution was made up daily in acid washed bottles from a1 mg l21 stock solution.

The quadrupole-based ICP-MS used in these studies was aPerkin-Elmer ELAN-6000 (SCIEX, Thornhill, Ontario,Canada). During the operation of CE-ICP-MS the nebulizergas (Ar) ¯ow was set at the optimized value of 1.05 l min21 andthe sheath buffer ¯ow rate was 0.4 ml min21. The voltage wasset to 21 kV and the current was about 30 mA for eachcapillary. One MS scan, measuring all elements using the peakhopping mode, was completed within 1.4 s.

Results and discussion

It is important to emphasize that the current work ispreliminary in nature and is designed to evaluate the potentialuse of a parallel separation scheme for high sample loadingresulting in low detection limits. As such, at this stage, we didnot attempt to use massively parallel bundles to maximize theloading. Because of the restricted inlet on DIHEN only onecapillary with an external diameter of 375 mm could ®t withinthe nebulizer. Therefore, a cross-¯ow nebulizer was used toevaluate the multicapillary concept. However, in future work,capillaries with internal diameter of 75 mm and externaldiameter of 150 mm will be used to maximize the sampleloading, speci®cally for use with DIHEN. Using the 150 mmexternal diameter capillaries, as many as eleven parallelcolumns may be used for separation. The schematic diagramof parallel capillary interface with a cross-¯ow nebulizer isshown in Fig. 1.

To illustrate the system performance and establish abenchmark, the CE-ICP-MS was initially set up with a singlecapillary using a cross-¯ow nebulizer. After sample injection,the analytes were separated and introduced into the ICP-

MS. All four analytes were separated and are illustrated inFig. 2. The migration time from the capillary for all analytes isbetween 3 and 5 min. Although the concentration of allelements is the same, the individual response of the ICP-MS isnot identical. Subsequently, Cs and Ba have the largest peakheights and peak areas, while smaller peaks are obtained forMn and La. The cations migrate in the capillary in the order ofCs, Ba, Mn and La. The size and charge of the cation±acetatecomplex formed during the separation dictate this migrationorder. To establish a benchmark, we may consider themigration time and peak height for Cs, which are 3.2 minand 330 000 cps, respectively.

The results for an injection of the sample on a three-capillaryparallel separation device are shown in Fig. 3. Several points ofinterest can be highlighted for this separation. First, themigration time and the migration orders are similar, withinexperimental error, for three parallel capillaries and the single

Fig. 1 Schematic diagram for multicapillary interface to a nebulizer.

Fig. 2 Separation and detection of four cations with a single capillaryCE-ICP-MS.

Fig. 3 Separation and detection of four cations with a multicapillaryCE-ICP-MS.

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capillary system. Second, the signal intensity for all analytes ishigher when parallel capillaries are used. More speci®cally,considering our benchmark, Cs, for the parallel separationscheme the migration time and signal intensity are 3.2 min and1 000 000 cps, respectively. The fact that only a single peak isobserved for each element and that the signal intensity is threetimes larger than the single capillary clearly indicate thesuccessful operation of this device.

Although all capillaries were taken from the same spool andwere conditioned in the same manner, occasionally themigration time may be different amongst individual capillaries.Reconditioning and careful adjustment of capillaries to exactlythe same height in the sample cup is essential. Furthermore, toverify that each capillary behaves nearly the same as all othercapillaries, the run current for each capillary was measured inorder to ®nd columns that would match very closely (thisprocedure took only a few minutes for each capillary tested).With three such identi®ed capillaries it was possible to obtainpeaks with identical retention times for the elements studied.However, lack of attention to the above mentioned operationalparameters will yield poor separation conditions and ultimatelylead to broadened peaks or peak splitting. This manifestationof separation artifacts in unmatched capillaries is shown inFig. 4. In this ®gure, we can note that two of the capillaries arefunctioning in an identical manner, while the third capillary hasslightly different separation characteristics. Nonetheless, thepeak areas of all runs were constant, regardless of the artifacts.Good separation results were obtained when the sheath ¯owrate of the make-up buffer was at 0.4 ml min21.

The advantages of using a DIHEN are clearly shown inFig. 5. In this experiment, a 100 mg l21 solution of Cs, Ba andLa mixture is separated on a 75 mm internal diameter CEcolumn. The optimized nebulizer sheath ¯ow rate and the Ar¯ow rate were experimentally determined to be 10 ml min21 and0.25 l min21, respectively. Under these conditions a slightnegative pressure at the cathodic end of the capillary results inshorter migration times (due to a slight laminar ¯ow).Therefore, to minimize this laminar ¯ow caused by aspirationduring the sample injection period, the makeup buffer sheath¯ow rate was increased to 100 ml min21 and the nebulizer Ar¯ow rate was intentionally stopped. From the ®rst glance onecan note that the peaks are much more symmetrical whenDIHEN is used because band broadening due to analyteresidence time in the spray chamber is eliminated. Second,while La gave a broad peak when the spray chamber was used,with DIHEN a sharp and symmetrical peak is obtained. Thisindicates that La strongly interacts with the spray chamberwalls and, as such, the CE resolution for La analysis will beadversely affected. Lastly, because of the lower amount ofmake up buffer used during the separation, the backgroundcounts are extremely small for the analytes studied. This lowerbackground translated into a better signal to noise ratio andultimately results in lower detection limits. A comparison ofidentical samples run on both DIHEN and cross-¯ow nebulizershow that DIHEN is twice as sensitive. The signal-to-noiseratio is also better by a factor of two for DIHEN (over cross-¯ow). Future work will be speci®cally aimed at using multi-capillary separation with DIHEN nebulizers.

Conclusion

We have clearly demonstrated in this work that use of parallelseparation schemes with capillary electrophoresis and ICP-MS

detection can enhance the sensitivity for a given analysis. Whencomparing a multicapillary with a single capillary run the peakarea was increased proportionally to the number of capillaries.This was also true for the peak height with completelyoverlapping peaks. With three capillaries and a cross-¯ownebulizer the detection limits (36noise) for Cs, Ba, Mn and Laare of 33, 100, 100 and 500 ng l21, respectively. The concept ofmulticapillary electrophoresis could also be applied to otherinterfaces.

Acknowledgements

One of the authors (V. M.) received funding from ADAPTprogram at Los Alamos National Laboratory for travel toNorway and Sweden to initiate this collaborative work.

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Paper 9/06100B

Fig. 4 Separation and detection of four cations with a poorly matchedmulticapillary CE-ICP-MS.

Fig. 5 Separation and detection of three cations with a DIHENnebulizer CE-ICP-MS.

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