Thesefindingsdon't necessarily mean that other researchers' results were wrong, Wightman says. "Previous measurements used microdialysis," he explains. "Because of the nature of the measurements, you collect data over a 5-minute period, essentially averaging it." A big spike that falls off quickly might look like a sustained release. But with the rapid measurements of cyclic voltammetry, it's easy to tell the difference. Wightman says the task now is to find out what dopamine really does. His lab will study a variety of novelty behaviors—such as an animal's reaction to being startled—to see if dopamine is present. Somebody also has to pinpoint the neurotransmitter involved in addiction. Wightman says he has a few ideas. And with any luck, the culprit will be one he can detect with cyclic voltammetry. Elizabeth Zubritsky
dent on pH or electroosmotic flow, or on whether you use cationic or anionic micelles, and if all that's involved is adding salt to the sample, it seems that it should be applicable to most MCE systems." Landers describes the stacked micelles as "magnets for hydrophobic analytes". "By stacking the micelles at the front of the sample plug," he says, "you essentially create a super-concentrated plug of magnets that migrates through the sample plug and effectively collects all of the analytes into a very sharp zone." The width of the individual analyte peaks depends their affinity for the micelles. Palmer points out that the affinity of all of the analytes increases with micelle concentration "Our mechanism is efficient because stack the micelles before they interact with the analyte " In this system sodium chloride was used to adjust the conductivity of the sample matrix But Palmer says there is a caveat about the
MCE stacks up Field-amplified stacking, in which the separation buffer has a much higher ionic strength than the sample matrix, is a common method of reducing the width of the sample zone and concentrating analytes in CE. Sample stacking has also been used with micellar CE (MCE), but most of the existing methods are "finicky"—requiring a particular combination of micelle type, sample pH, electroosmotic flow rate, and separation-mode polarity. In addition, the difference in ionic strength between separation buffer and sample matrix is often achieved by diluting the sample, which compromises sensitivity. A remarkably simple method described by James P. Landers, James Palmer, and Nicole J. Munro of the University of Pittsburgh has no such problems {Anall Chem. 1999, 71,1679-87). .nstead of stacking the analytes in the sample matrix, they cause the micelles to stack by adding salt (in this case NaCl) to the sample so that the conductivity of the sample is 2-4 times that of the separation buffer. The stacking of the micelles leads, in turn, to sample stacking but without the need for sample dilution. Infield-amplifiedstacking, the species to be stacked starts out in a low-conductivity region where it has a higher velocity because it carries all the charge. The analytes are slowed upon reaching the region of higher conductivity, which stacks the analyte (or micelles in this case) into a very narrow zone. In normal field-amplified
TXT/"*
The proposed mechanism of high-salt stacking. (A) Conventional field-amplified stacking with a low ionic strength sample matrix. (B) High-salt stacking with a sample matrix of ionic strength greater than the separation buffer.
stacking, the stacking phenomenon occurs with the analytes in the sample matrix. In this method, the stacking occurs with the micelles in the separation buffer outside the sample matrix. "What we describe in this paper is really counterintuitive," says Landers. "We actually increase the ionic strength of the sample to get the opposite effect [of standard field-amplilied stacking]. The advantage of doing this is that it's always easy to add something to your sample. You essentially can keep the sample concentration intact and just add salt. You now get a stacking effect that will enhance detection sensitivity as opposed to diminishing it." They demonstrated this "universal concept" with normal MCE, meaning that the system includes an anionic micelle (sodium cholate), electroosmoticflow,and normal polarity (cathode on the detector side). However, they believe that their method is not constrained to these conditions. "I don't think you can say that [the method will work with every buffer system] until you try every buffer system," says Landers. "The bottom line is that if it's not depen-
are nonmicellar systems that have a lot of negative chare-ps on them like his-hly sulfated 1H t ' i K h'th
complexing agents have pretty high mobilities, he says. You need the ionic component of your sample matrix to have a higher ionic mobility than your micellar carner. We knew it intuitively, but we p,oved it with highly sulfated B-cyclodextrins, which could not be stacked with any conn i
•
i
i
i
/
,
i
.
i
-.i
i
•
w
if SU
W
1C
"ji IT' as a mucu u-lgu er anodic morality, were we nai y arae to stacK tne suuatea y-cyclodextnns. Landers predicts that this method will make some biological fluids, which tend to be high in ionic strength, easier to work with. "In the past, some biological samples have been a nightmare to work with by CE because the high ionic strength was counterproductive. Now, if a micellar system is in the separation mode, the naturally occurring high ionic strength in those fluids may aid you in stacking and thus improve the detection sensitivity." Celia Henry
Analytical Chemistry News & Features, May 1, 1999 311 A
