Editing Micellar electrokinetic chromatography

{{short description|Chromatography technique}}
[[File:MEKC.gif|thumb|300px|Distribution of analytes (A) in micellar electrokinetic chromatography based on their hydrophobicity.]]
'''Micellar electrokinetic chromatography''' ('''MEKC''') is a [[chromatography]] technique used in [[analytical chemistry]]. It is a modification of [[capillary electrophoresis]] (CE), extending its functionality to neutral analytes,<ref>{{cite journal |last1=Hancu |first1=Gabriel |last2=Rusu |first2=Aura |last3=Simon |first3=Brigitta |last4=Mircia |first4=Eleonora |last5=Gyeresi |first5=Arpad |title=Principles of Micellar Electrokinetic Capillary Chromatography Applied in Pharmaceutical Analysis |journal=Advanced Pharmaceutical Bulletin |date=2013 |volume=3 |issue=1 |pages=1–8 |doi=10.5681/apb.2013.001 |pmid=24312804 |pmc=3846027|ref=US National Library of Medicine}}</ref> where the samples are separated by differential partitioning between [[micelles]] (pseudo-stationary phase) and a surrounding aqueous [[buffer solution]] (mobile phase).<ref>{{cite journal | last1 = Terabe | first1 = S. | last2 = Otsuka | first2 = K. | last3 = Ichikawa | first3 = K. | last4 = Tsuchiya | first4 = A. | last5 = Ando | first5 = T. | year = 1984 | title =  Electrokinetic separations with micellar solutions and open-tubular capillaries| journal = Anal. Chem. | volume = 56 | pages = 111–113 | doi=10.1021/ac00265a031}}</ref>

The basic set-up and detection methods used for MEKC are the same as those used in CE.  The difference is that the solution contains a [[surfactant]] at a [[concentration]] that is greater than the [[critical micelle concentration]] (CMC).  Above this concentration, surfactant [[monomers]] are in [[chemical equilibrium|equilibrium]] with micelles.

In most applications, MEKC is performed in open capillaries under [[alkaline]] conditions to generate a strong [[electro-osmosis|electroosmotic flow]].  [[Sodium dodecyl sulfate]] (SDS) is the most commonly used surfactant in MEKC applications. The anionic character of the sulfate groups of SDS causes the surfactant and micelles to have [[electrophoresis|electrophoretic mobility]] that is counter to the direction of the strong [[electroosmotic flow]].  As a result, the surfactant monomers and micelles migrate quite slowly, though their net movement is still toward the [[cathode]].<ref>Baker, D.R. "Capillary Electrophoresis" John Wiley & Sons, Inc.: New York, 1995.</ref> During a MEKC separation, [[analyte]]s distribute themselves between the [[hydrophobe|hydrophobic]] interior of the micelle and [[hydrophile|hydrophilic]] buffer solution as shown in ''figure 1''. 

Analytes that are [[soluble|insoluble]] in the interior of micelles should migrate at the electroosmotic flow velocity, <math>u_o</math>, and be detected at the retention time of the buffer, <math>t_M</math>.  Analytes that solubilize completely within the micelles (analytes that are highly hydrophobic) should migrate at the micelle velocity, <math>u_c</math>, and [[elution|elute]] at the final elution time, <math>t_c</math>.<ref name="AC1984_113">{{cite journal | last1 = Terabe | first1 = S. | last2 = Otsuka | first2 = K. | last3 = Ichikawa | first3 = K. | last4 = Tsuchiya | first4 = A. | last5 = Ando | first5 = T. | year = 1984 | title =  Electrokinetic separations with micellar solutions and open-tubular capillaries| journal = Anal. Chem. | volume = 56 | page = 113 | doi=10.1021/ac00265a031 }}</ref>

== Theory ==
The micelle velocity is defined by:

:<math>u_c= u_p+u_o</math>

where <math>u_p</math> is the electrophoretic velocity of a micelle.<ref name="AC1984_113"/>

The retention time of a given sample should depend on the capacity factor, <math>k^1</math>:

:<math>k^1=\frac{n_c}{n_w}</math>

where <math>n_c</math>  is the total number of [[mole (unit)|moles]] of solute in the micelle and <math>n_w</math> is the total moles in the aqueous phase.<ref name="AC1984_113"/>  The retention time of a solute should then be within the range:

:<math> t_M\le t_r\le t_c </math>

Charged analytes have a more complex interaction in the capillary because they exhibit electrophoretic mobility, engage in [[electrostatics|electrostatic]] interactions with the micelle, and participate in hydrophobic partitioning.<ref name="Cunico1998">Cunico,R.L.; Goodin, K.M.; Wehr,T. "Basic HPLC and CE of Biomolecules" Bay Bioanalytical Laboratory: Richmond, CA, '''1998'''.</ref>

The fraction of the sample in the aqueous phase, <math>R</math>, is given by:

:<math> R= \frac{u_s-u_c}{u_o-u_c}</math>

where <math>u_s</math> is the migration velocity of the solute.<ref name="AC1984_113"/>  The value <math>R</math> can also be expressed in terms of the capacity factor:

:<math> R=\frac{1}{1+k^1}</math>

Using the relationship between velocity, tube length from the injection end to the detector cell (<math>L</math>), and retention time, <math>u_{o}= L/t_M</math>, <math>u_{c}= L/t_{c}</math> and <math>u_s = L/t_r</math>, a relationship between the capacity factor and retention times can be formulated:<ref name="Cunico1998"/>

:<math> k^1=\frac{t_r-t_M}{t_M(1-(t_r/t_c))}</math>

The extra term enclosed in parentheses accounts for the partial mobility of the hydrophobic phase in MEKC.<ref name="Cunico1998"/>  This equation resembles an expression derived for <math>k^1</math> in conventional [[packed bed]] chromatography:

:<math> k=\frac{t_r-t_M}{t_M}</math>

A rearrangement of the previous equation can be used to write an expression for the retention factor:<ref>{{cite journal | last1 = Foley | first1 = J.P. | year = 1990 | title =  Optimization of micellar electrokinetic chromatography| journal = Anal. Chem. | volume = 62 | issue = 13| pages = 1302–1308 | doi=10.1021/ac00212a019}}</ref>

:<math>t_r =\left ( \frac{1+k^1}{1+(t_M/t_c)k^1} \right )t_M</math>

From this equation it can be seen that all analytes that partition strongly into the micellar phase (where <math>k^1</math> is essentially ∞) migrate at the same time, <math>t_c</math>.  In conventional chromatography, separation of similar compounds can be improved by [[gradient]] elution. In MEKC, however, techniques must be used to extend the elution range to separate strongly retained analytes.<ref name="Cunico1998"/>

Elution ranges can be extended by several techniques including the use of [[organic compound|organic]] modifiers, [[cyclodextrin]]s, and mixed micelle systems.  Short-chain alcohols or [[acetonitrile]] can be used as organic modifiers that decrease <math>t_M</math> and <math>k^1</math> to improve the resolution of analytes that co-elute with the micellar phase.  These agents, however, may alter the level of the EOF.  Cyclodextrins are cyclic [[polysaccharide]]s that form inclusion complexes that can cause competitive hydrophobic partitioning of the analyte.  Since analyte-cyclodextrin complexes are neutral, they will migrate toward the cathode at a higher velocity than that of the negatively charged micelles.  Mixed micelle systems, such as the one formed by combining SDS with the non-ionic surfactant Brij-35, can also be used to alter the selectivity of MEKC.<ref name="Cunico1998"/>

==Applications==
The simplicity and efficiency of MEKC have made it an attractive technique for a variety of applications.  Further improvements can be made to the selectivity of MEKC by adding [[chirality (chemistry)|chiral]] selectors or chiral surfactants to the system.  Unfortunately, this technique is not suitable for protein analysis because proteins are generally too large to partition into a surfactant micelle and tend to bind to surfactant [[monomer]]s to form SDS-protein complexes.<ref>Skoog, D.A.; Holler, F.J.; Nieman, T.A.  "Principles of Instrumental Analysis, 5th ed." Saunders College Publishing: Philadelphia, 1998.</ref>

Recent applications of MEKC include the analysis of uncharged [[pesticide]]s,<ref>{{cite journal | last1 = Carretero | first1 = A.S. | last2 = Cruces-Blanco | first2 = C. | last3 = Ramirez | first3 = S.C. | last4 = Pancorbo | first4 = A.C. | last5 = Gutierrez | first5 = A.F. | year = 2004 | title =  Application of Micellar Electrokinetic Capillary Chromatography to the Analysis of Uncharged Pesticides of Environmental Impact| journal = J. Agric. Food Chem. | volume = 52 | issue = 19| pages = 5791–5795 | doi=10.1021/jf040074k| pmid = 15366822 }}</ref> essential and branched-chain [[amino acid]]s in nutraceutical products,<ref>{{cite journal | last1 = Cavazza | first1 = A. | last2 = Corradini | first2 = C. | last3 = Lauria | first3 = A. | last4 = Nicoletti | first4 = I. | year = 2000 | title =  Rapid Analysis of Essential and Branched-Chain Amino Acids in Nutraceutical Products by Micellar Electrokinetic Capillary Chromatography| journal = J. Agric. Food Chem. | volume = 48 | issue = 8| pages = 3324–3329 | doi=10.1021/jf991368m| pmid = 10956110 | hdl = 11381/2441649 | hdl-access = free }}</ref> [[hydrocarbon]] and alcohol contents of the [[marjoram]] herb.<ref>{{cite journal | last1 = Rodrigues | first1 = M.R.A. | last2 = Caramao | first2 = E.B. | last3 = Arce | first3 = L. | last4 = Rios | first4 = A. | last5 = Valcarcel | first5 = M. | year = 2002 | title =  Determination of Monoterpene Hydrocarbons and Alcohols in Majorana hortensis Moench by Micellar Electrokinetic Capillary Chromatographic| doi = 10.1021/jf011667n | journal = J. Agric. Food Chem. | volume = 50 | issue = 15| pages = 4215–4220 | pmid = 12105948 }}</ref>

MEKC has also been targeted for its potential to be used in combinatorial chemical analysis.    The advent of [[combinatorial chemistry]] has enabled [[medicinal chemistry|medicinal chemists]] to synthesize and identify large numbers of potential [[medication|drugs]] in relatively short periods of time. Small sample and [[solvent]] requirements and the high resolving power of MEKC have enabled this technique to be used to quickly analyze a large number of compounds with good resolution.

Traditional methods of analysis, like [[high-performance liquid chromatography]] (HPLC), can be used to identify the purity of a combinatorial library, but assays need to be rapid with good resolution for all components to provide useful information for the chemist.<ref>{{cite journal | last1 = Simms | first1 = P.J. | last2 = Jeffries | first2 = C.T. | last3 = Huang | first3 = Y. | last4 = Zhang | first4 = L. | last5 = Arrhenius | first5 = T. | last6 = Nadzan | first6 = A.M. | year = 2001 | title =  Analysis of Combinatorial Chemistry Samples by Micellar Electrokinetic Chromatography| journal = J. Comb. Chem. | volume = 3 | issue = 5| pages = 427–433 | doi=10.1021/cc000093g| pmid = 11549360 }}</ref>   The introduction of surfactant to traditional capillary electrophoresis instrumentation has dramatically expanded the scope of analytes that can be separated by capillary electrophoresis.

MEKC can also be used in routine quality control of antibiotics in pharmaceuticals or feedstuffs.<ref>{{cite journal | last1 = Injac | first1 = R. | last2 = Kočevar | first2 = N. | last3 = Kreft | first3 = S. | year = 2007 | title = Precision of micellar electrokinetic capillary chromatography in the determination of seven antibiotics in pharmaceuticals and feedstuffs | journal = Analytica Chimica Acta | volume = 594 | issue = 1| pages = 119–127 | doi=10.1016/j.aca.2007.05.003| pmid = 17560393 }}</ref>

== References ==
{{reflist}}

== Sources ==
* Kealey, D.;Haines P.J.; instant notes, Analytical Chemistry page 182-188

{{chromatography}}
[[Category:Chromatography]]