Future pathways for analytical separations - Analytical Chemistry (ACS

Todd Kajdan , Hernan Cortes , Krishna Kuppannan , Scott A. Young. Journal of ... Hernan J. Cortes , Bruce M. Bell , Curt D. Pfeiffer , Jeffrey D. Grah...
0 downloads 0 Views 6MB Size
J. Calvin Giddings Department of Chemistry University of Utah Salt Lake City, Utah 841 12

Future Pathways for Analytical Separations Separations employ such a variety of physical and chemictalforces, flows, phases, and geometrical designs that we can imagine a near infinity of pathways toward future technology. Yet, only a handful of techniques has become useful in present-day analytical chemistry, despite long and intensive effort. Optimistically, there are many important developments yet to be made, but obviously not each conceivable pathway will be productive. How do we choose the open pathways and avoid those that are dead ends? I t is this author’s opinion that the search for future separation technology can be both inspired and streamlined by going t? the roots of separative processes: finding out how and why these processes work, the parameters that limit them, and the measures that allow them to be compared. One source of perspelctive and guidance is the study of separation systematics-how separations, in general, originate and relate. Historically, different separation methods have been developed and refined by different cadres of scientists iwho rarely talked to one another to share insights and progress. By and large, methods have evolved independently (1).A shining exception is high performance liquid Chromatography (HPLC), which was initiated by extrapolating the theory of optimization of gas chromatography ( 2 4 ) .Without this extrapolation, it might have taken HPLC another decade to evolve, a t a great loss to science. Such relationships ought to be the rule, not the exception. Based on the talk, “New Directions in Separation Science,” presented at the symlposium, “New Directions in Analytical Chemistry,” at the 1981 Pittshur h Conference on Analytical Chemistry and Appyied Spectroscopy, Atlantic City, N.J., March 9,1981.

The search for future separation technology can be both inspired and streamlined by going to the roots of separative processes.

In a more specific vein, studies of GC and LC have shown that there are ceilings to separation performance that depend on certain physicochemical parameters such as viscosity and diffusivity, and operating limits such as the maximum pressure drop (5-8). Subsequent studies have similarly shown ceilings for electrophoresis and sedimentation (9, IO),isoelectric focusing and isopycnic sedimentation (10, l l ) ,and field-flow fractionation (12). Taking a systematic point of view, we conclude that the search for new areas of technological achievement in separations requires: pushing closer to the ceilings by careful experimental design, pushing back the ceilings by extending the limiting parameters, and finding new approaches with new ceilings beneath which new kinds of separations can be achieved. Opportunities and Constraints Imposed by Systematics There are some universal underpinnings of separations that bear on any global view. The treatment below borrows liberally from the author’s previous work (1,10). To start with, the act of separation requires that different components be

pulled apart, so there must always be a physical displacement through space. Therefore, transport processes underlie separations. The transport must, of course, have some elements that make it differential. There are two classes of displacement: bulk or flow displacement, in which components are carried along in their medium, and relative displacement through the surrounding medium. These displacement processes complement one another: The first is powerful but nonselective while the second is relatively weak in displacement power but provides selectivity. Since transport must be differential for sep’aration, relative displacement is needed in all separation processes. By contrast, flow displacement is optional. (Transport through evacuated chambers is a displacement class not considered here.) Focusing first on the key process of relative displacement, we note that the transport rate J per unit area along axis x is given by the expression (9):

0003-2700/81/035 1-945A$01.OO/O

0 1981 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981

945A

b

Flgure 1. The three types of chemical potential @*) profiles underlying the relative displacement of separation processes

where c is concentration, p* is the chemical potential driving transport (at least in isothermal systems), f is the friction coefficient, R is the gas constant, and T is the absolute temperature. The friction coefficient measures the drag force on molecules or particles undergoing relative transport through a medium. Our first global concltision stems directly from this equation. We see that relative transport rates are inversely proportional to the friction eoefficient f. Therefore, the time scale for any desired level of relative transport is proportional to f. Since the relative transport described by Equation 1underlies all separations, the time scale of separation is universally linked to the magnitude off. Separation speed, on the other hand, is inversely related to f, a point we shall illustrate in the next section. According to Stokes's law, f is directly proportional to viscosity ( 9 ) and particle radius ( a ) : f = 6rqa (2) Where this law is applicable, a reduction in carrier viscosity will hasten separation. This conclusion is almost universal in separations. Changes in the medium or in the temperature can be used to this end (IO). We note that friction coefficient f and diffusion coefficient D are inversely related hy the Planck-Einstein equation f = RTID (3) Hence, the time scale of separation, linked directly with f, is inversely related to D. Separation speed almost always increases with D . The chemical potential ( p * ) profile has two elements: a continuous (c) part resulting from the application of an external field, and a discontinuous (d) component resulting from the abrupt partitioning forces acting across an interface. Therefore, the p* S46A

Relative Displacement

I

I

Figure 2. Orientations and conditions of flow with respect to the relative displacement axis in separation systems. S = static. F = flow

Table 1. Grouping of Separation Methods In the Nine Categorles Based on Flow and Relative Displacement

sc S

(static)

Electrophoresis lscelectric focuslng Rata-zonal sedimentation lwpycnic sedimentation

sd Batch extraction Batch adsorption Batch uystallizatlon Batch sublimation Batch ion exchange Dialysis

Scd

ElecircdeposRion Electrostatic precipitation Electrolytic refining Electrodialysis Equilibrium

sedimentation

F(=) (parallel

flow)

R=)C Elubiation Countercurrent electrophoresis

F(+N F(+)

perpendlcular

flow

profile can assume three forms depending on the components employed (IO):c = continuous p* profile; d = discontinuous p* profile; and cd = combination p* profile. The three profiles outlined above are shown in Figure 1. (In an earlier classification, only two profiles were distinguished (I)). Since flow is optional in separation systems, we can have either static (S) systems or flow (F) systems. Furthermore, flow systems can be physically

ANALYTICAL CHEMISTRY, VOC. 53, NO. 8. JULY 1981

F(=)d

F(=)Cd

Filtration Ultrafiltration Reverse osmosis Pressure dialysis Zone mening

ForcBd-flow electrophoresis

F(+W

W)Cd

Chromatography Field-flow Countercurrent fractionation distribution Thermcgravitational Fractional distillation separation Adsorptive bubble separation

arranged such that the flow displacement is essentially perpendicular (+) to the relative displacement, or parallel (=) to it. The former, for example, is represented by chromatography, while the latter includes filtration techniques. The relationship of the three flow conditions (S, F(+), F(=)) to the relative displacement vector is shown in Figure 2. The systematic approach above leads us to the conclusion that there are nine ways in which relative dis-

Quality chromatography equipment and media The best results are obtained with a quality chromatography system and a1 quality system contains components-equipment, instruments and gels-functioning in a reliable and integrated manner. Consider the Pharmacia Liquid Chromatography System; a quality system with design and performanlce meeting these criteria.

The high quality UV-1 monitqr has a simple wavelength change over which enables you to monitor at ,254,280 or 405 nm You can add a separate UV-11214 optical unit to extend this choice to a fourth wavelength-214 nm. The Pharmacia UV-1 monitor; versatile and easy-to use. Dual path monitor UV-2 also available. CIRCLE 158 ON READER SER\/ICE CARD

Years of proven reliability make the P-3 the choice of peristaltic pumps. Use your P-3 to prepare linear, convex and concave gradients.

v1;

The Pharmacia programmable fraction collector FRAC-300 enables you to apply microprocessor technology to the collection of your samples. You can program the FRAC-300 to automatically alter the fraction number and size according to the characteristics of your separation and to collect the fractions in a wide range of vessels, including free standing beakers, in anv combination during a fractibnation.

-I

CIRCLE 163 ON READER SERVICE CARD

CIRCLE 159 ON READER SERVICE CAR

Gradient Mixer GM-I makes reproducible pH gradient and linear concentration gradients for ion exchange, affinity and hydrophobic interaction chromatography, density gradient centri ugation and gra electrophoresis. Chromatography media from Pharmacia all share the following mouerties-easv column packing, goovd how properties: high resolution and excellent reproducibility. Methodology booklets are available on request (gel filtration circle 160, ion exchange circle 161, affinity chromatography circle 162).

The accessory Chromatography Controller FRAC-CC allows you to automate valves, peristaltic uumus and gradiegt mixers in conjunction with the chrom'atographic profile to allow unattended sample application, equilibration of the column and buffer changes. A wide variety of gel filtration, ion exchange, and affinity chromatography applications are now routine laboratory procedures. The procedures place different demands on column design such that no one type is suitable for all applications. Pharmacia has a wide range of columns with different size, resistance and operational specifications that can meet nts. All are assembled, ready for use. CIRCLE 164 ON READER SERVICE CARD

Pharmacia Fine Chemicals Division of Pharrnacia Inc. 800 Centennial Avenue Piscataway, N.J. 08854

-

L

Pharmacia Fine Chemicals

ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981

947A

Table II. Performance Ceilings for Various Multicomponent

Separation Methods ssparating POW. N (no ~ l a l e s l "(peak caOacnY1

M e w and ties

kodp2AP

LC (Ilquid

~

chromatography) *?''IDrn

sopratkm S p e d hvt (OlaI%j/llrrm)

(M"2In v, 4

The equations show the magnitude of

Dm Ddp2(1 K )

+

Vnd"

potential gains stemming

F(+)d

from the stretching of limiting operating parameters, such as pressure or voltage. L

-

FFF (Iield-flow fractionation) F(+)cd

dG8't

ZrpV 28RT

Electrophoresis sc

Rate-zonal sedlmentatlon sc

M(l

Thermal diliusion

sc

- Vp)w2AP 48R

[

M(t

- ?pP)O2AP 648RT

]

~2

D M(1

%[

- Vp)w* RT

01 AT -

28 T

Isoelectric focusing

x

I-wmk sedlmentatlon sc Symbols: dD = particle diameter D = molecular diffusion mffi-

cient Dm = dibion mefficient in mobile phase E = electric field strength f = frictioncoefficient H = plate height 'h = capacity factor k, = permeability/dP2 8 = mean layer thickness L = channel length M = molecular weight P, = inlet pressure Po = outlet pressure r = radius in centrifuge R = gas constant

948A

+

T = temperature V = voltage drop V,, = maximum elution volume

V,," = minimum elution volume z;r = molar charge a = thermal diffusion factor Ar2 = r22

-

r,2

AT = temperaturedrop

y = obstruction factor q = viscosity w = radians& D = red& mass transfer term fw stationary phase 8 = dispersion cmfflcient =

8' = HIH (ideal) p = carrier density U = partial specific volume

ANALYTICAL CHEMISTRY, VOL. 53, NO. 8. JULY 1981

1 2

placement and flow displacement (or the lack of it) can be combined. These nine categories are shown in Table I, along with the distribution of some common separation methods among the nine categories. There are some important associations in Tahle I that are not commonly recognized. For example, cone melting is grouped in the F(=)d category with ultrafiltration. despite their considerahle differences in materials and applications. Nonetheless, when we examine the phenomenological nature and theory of zone melting and ultrafiltration. we find them to he almost identical. Understanding one should help in understanding the other. In general, the relationships suggested by Tables I and I1 should be invaluable in comparing methods, encouraging technological transfer between methodologies, and in suggesting new approaches. The ahove is merely an example of how systematic comparisons can provide useful relationships among methods. Relationships among steady-state separation methods (surh as isoelectric focusing) have been described elsewhere (13).

Opportunities and Constraints imposed by SpecificTheory The general mass transport process descrihed hy Equation 1, along with the thermodynamirs that determines the form of p ' , can be developed into sperific equations describing most multiromponent techniques. These equations provide the reilings noted earlier for separative performance in terms of physicochemical parameters and operating limits. These equations can he used M point out gaps between theoretical ceilings and prartical per. formance. Each significant gap represents an opportunity for inrreaaed

Though hidden in antiquity, dice probably evolved from prehistoric games of knucklebones. The use of dice for games developed first in the Orient, whilie dice similar to those in use today are depicted on Egyptian monuments and have been found in the ancient ruins of Thebes. Mark Antony was said to have wasted his time in Alexandria t)y playing dice. In 18th century England great fortunes were won and lost with the single roll of the dice. Today garnes of chance are still a major influence withiin our society. However, decisions regarding analytical instrumentation, although involving a degree of risk, should not be left to chance. When faced with decisions regarding plasma emission inst rumentation, consider Spectra-

Metria, the one company that provides an intelligent alternative to risk. The Spectraspan 111-B from SpectraMetrics is the first plasma emission spectrometer offering either DCP or ICP excitation sources. Furthermore, the Multiple Source Spectraspan System 111-B offers both DCP and ICP sources in a configuration which provides for simple switchover from one source to the other. Utilizing the high-resolution echelle spectrometer, the System 111-B offers true simultaneous multi-element analysis in either ICP or DCP mode. Many factors come into play while making instrumentation decisions. DON’T TAKE A CHANCE, CALL SPECTRAMETRICS ...WHERE YOU HAVE A CHOICE.

SpectraMetrics, Inc.

204 Andover Street

Andover, MA 01810 Telex 94-7134 A Subsidiary of Beckman Instruments, Inc. (617) 475-7015

CIRCLE 200 ON READER SERVICE CARD @SpectraMetrics,Inc., 1981

WhereThe Choice Is Yours

79 OVER 1000 MODELS

OVENS

Blue M has anticipated yourevery oven need with a choice of over 1000 models

. . . ranging from a $200 lab oven on up to a $20.000 fully automated inert gas

chamber with microprocessing control. Mechanical and gravity . . . sizes to 96 cu. 11. . . . temperatures to + 704% Batch . . . Life.Test . . . Utility . . . Burn-In . . . Drawer . . . High-powered . . . Vacuum,. , Drying - even FRICTION-AIREmt h e patented ekctric chambers with no heating elements. Most have Blue M control systems for superior performance - plus such other standard features as welded Stainless interiors, elapsed time meters, overtemperature protection. It's easily America's finest, most complete line. Choice. With Blue M it does mean selection. And with Blue M,choice also means highest quality. We offer you both. Is anything less worth your time? Where the choice is yours: Blue M Electric Company; Corporate Headquarters; Blue Island, Illinois 60406: Telephone: (3121 385-9000.

CIRCLE 27 ON READER SERVCE CARD

ieparative performance, hinging on thoughtful experimental design. The equations also show the magnitude of potential gains stemming from the 3tretching of limiting operating parameters, such as pressure or voltage. Furthermore, the equations show where little progress is to be expected, thus helping close off unproductive pathways. Table I1 shows performance ceilings for a group of contrasting techniques. Equations are shown for both separating power and speed. Separating power is expressed in terms of number of theoretical plates N (applicable even to electrophoresis (9)).and in terms of peak capacity n,the latter more generally useful because the plate concept does not extend well to such steady-state methods as isoelectric focusing (IO). Ceilings for separation speed are expressed as number of plates per unit time, Nlt. Table I1 has, of course, been simplified in the interest of clarity. and doea not begin to explain how each equation is to be used or the nature of any qualifications. But it captures the essence if not the details of each method's potential. The equations have been collected from the previously cited literature ( 5 7 - 1 1 ). Even casual inspection shows that there are many similarities among the equations in any of the columns of Table 11. This reflects the underlying systematics of separation, but space permits only a few points in this regard. We note that the terms 0 and P r e p resent the extent to which any dispersive process increases zone spreading. Ideally, 8 z 8' z 1. Parameter y plays the same role in the chromatographic equations, but is usually less than unity due to tortuous diffusion in packed columns. N is always inversely proportional to one of these parameters, while n is always inversely proportional to the square root of one of them. Reducing 8 or 8' is therefore a general goal for separations. Of significance, we note that every equation for separation speed Nlt is inversely proportional to friction coefficient f , or proportional to diffusivity D, which is one and the same thing according to Equation 3. This illustrates the validity of our earlier general conclusion that separation time is scaled with f and separation speed inversely with f and directly with D. (No separation speed equations are available for the steady-state methods where zones form by an unusual mechanism (11)).

Available \

~n USA and Canada l r ~ m

ORGANIC CHEMICALS CO.. INC.

-,A P 0 Box SGS

Columbia S C 29290 Phone

lWa1776 4990

CIRCLE 210 ON READER SERVICE CARD

950).

ANALYTICAL CHEMISTRY, VOL. 53. NO. 8. JVLY 1981

We note also that peak capacities are almost the same in the two complementary pairs: electrophoresis-isoelectric focusing and rate zonal sedimentation-isopycnic sedimentation

Your fluorometric assay capabilitiesare unlimited with the new Turner Model 112. HPLC. FIA. Therapeutic drug monitoring. Environmental testing. Even that new analytical procedureYou may be developing today,. .or tomorrow. A new instrument wlth 20 years of reliability. More scientists worldwide have chosen Turner Filter Fluorometersfor their double beam optical system stability, high sensitivity and exceptional reliability.The new Model 112 has all those Turner features plus more. New electronics for ease of operation. A digital readout permits more precise, error-freereadings. An integral BCD output provides computer and printer compatibility.

Excluslve QA Lamp. A front panel indicator lights when equilibrium is reached taking the guesswork out of your lab work. Built-in versatility, A new design filter holder allows filters to be changed in a few seconds. And there’s a number of accessoriesfor added versatility and to render it obsolescent proof-variable beam attenuator, continuous flow attachments, strip scanners, recorders, Stc. L~~ we}ght, lowmaintenance, lowcost, M~~~ the Model 112 anywhere.. .it’s portable and can even operate on a gasoline generator. It’sbuilt rugged and reliable. In fact, there’s no better filter fluorometer for the money. Contact your SequoiaTurner dealer for details or write:

SequoiaTurner Corporation -755Ravendale Drive, Mountain View, CA 94043 (415) 969-5533 Spectrophotometers Fluorometers Hematology Analyzers CIRCLE 192 ON READER SERVICE CARD

Theory shows many of the open and closed When You Install a New Column, Do You Install a Solution. . . or a Problem? dort chromatographers know the frus‘ration of installing a new column only :o f i n d out that the chromatograms are :errible.

Nhsre do you start?

3

I s your column OK?

Is the trouble somewhere

..

in

the instrument?. or Are your connections leaking?

It i s often difficult t o isolate the actual xoblem. and it’s extremely frustrating when you determine that the problem I your new column. You might eventu. illy f i n d that the column you just initalled is perfectly good. but is just not u i t e d t o your application. There is a way you can avoid this experience buy and use packinpr which have b n tested and approved specifically far four particular application. The technical staff a t Supelco continually deiigni and tests packingr for a wide range of sample analyses. and publishes technical information on performing there analyses. (Remember. Custom packingr made to your specifications sre n o t usually tested b y any company.) General purpose packing$ are tested b y our QC staff, however they sre tested for general purpose applications and may not d o the job for your application. When you use packings pretested far your applications, you know that the column is going to d o the j o b far you. 90 why should you buy one which might only cause

-

If you cannot find a packing which ha been lmled IO, your analysir. let us know. Our technical staff thrives on challenger. and IS waiting 10 SOIWYOU1 problam. S m e co has ,&I introouceo a new troubleshooting gu de w n c h wil s m . PIf y l e on your GC lab. If your chramatagramr .“st oon’l measure UP. there’s probao v a reason. Send to. day l o r your copy of or, new traublesnooting gdide. It can save you lime. money. an0 frwtratmns . . . and it’s FREE1

pathways toward future separation methods.

(IO,11).Therefore, in trying to expand separation technology in these directions, one should not waste time looking for the intrinsically superior system, hut should look for specific advantages in experimental design, selectivity, e k . More specifically now, the number of plates in electrophoresis has a ceiling proportional to voltage drop V (9, 10). With 0 1and V 2 lo4 volts, electrophoresis shows a potential for several million theoretical plates (10). This ceiling is difficult to approach because of the heating effect of the electrical current, which leads to uneven migration and thus to 0 >> 1. There are important opportunities here. In chromatography, N is limited by maximum pressure drop, as has been long understood. However, ceiling N values are so high (-107) that increasing speed N l t is a more practical goal. Although overall optimization is complex ( 8 ) ,this mainly involves reducing particle size d p In field-flow fractionation (FFF), ceilings on both N and N l t are determined by minimum possible solute layer thickness 1. Some of the limiting factors on 1 have been discussed (12). In theory, one ought to he able to use the thermal diffusion phenomenon in a static (S)mode, using temperature drop AT just like electrophoresis uses voltage drop V. Perhaps this is an overlooked opportunity. Therefore, an N equatibn is s h o w in Table I1 to assess the possibilities (10).When we plug in known values of a (usually