Tài liệu HPLC for Pharmaceutical Scientists 2007 (Part 17) doc

The analysis time, t a, is the time it takes for all sample components to elute off a column at a certain flow rate and is given by 17-1 where L is the column length, u is the linear flow

of 30 minutes and a total of 12 injections, a run time of 6 hours would berequired to cover system suitability, calibration, and sample analysis If the runtime were 5 minutes, only 1 hour would be required for the analysis With theadvent of commercial chromatographic porous media of less than 5µm andmore recently in the 1- to 2-µm range, analyses times of less than 1–2 minuteshave been demonstrated Hundreds of samples which required days can now

be analyzed in less than a day This chapter will focus on how to optimize cratic and gradient methods for speed without sacrificing resolution In addi-tion, the implication on selection of column dimensions and media particlesize on the speed of methods development will also be discussed

iso-Reducing chromatographic media particle size allows the number of retical plates per second to be increased However, due to the resolution

theo-765

HPLC for Pharmaceutical Scientists, Edited by Yuri Kazakevich and Rosario LoBrutto Copyright © 2007 by John Wiley & Sons, Inc.

dependence on N1/2, doubling of N will only increase resolution by 21/2 As cussed below, a reduction in particle size can lead to a pressure limitation due

dis-to the inverse dependence of pressure drop dis-to the square of the particle eter and the maximum operating pressure of the chromatograph The key tooptimizing speed is to maximize selectivity,α Maximizing selectivity for thecritical separation pairs will allow the shortest column lengths and highestmobile-phase linear velocity Short columns, 3–10 cm packed with particles inthe 1- to 3-µm range, provide high-speed analyses while maintaining reason-able pressure drop Due to the fast analysis time of these short columns,method development time can also be shortened Multiple columns can berapidly screened for optimizing selectivity Short columns are especially usefulwhen the components to be separated are known However, when dealing withcomplex samples with unknown components such as forced decomposition orbiological samples, using longer columns may be more judicious to achieveoptimum separation of critical components After selectivity optimization,the method can be optimized for speed by reducing column length The dis-cussion in this chapter will focus on optimizing speed of analysis and not onselectivity The reader is referred to Chapters 4 and 8 on how to optimize selectivity

diam-17.2 BASIC THEORY

To understand how to optimize a separation for speed, it is worth revisitingsome of the theoretical concepts developed earlier in this text The analysis

time, t a, is the time it takes for all sample components to elute off a column at

a certain flow rate and is given by

(17-1)

where L is the column length, u is the linear flow velocity of the mobile phase, and k is the retention factor of the latest-eluting peak Notice here some

obvious ways to increase the speed of analysis: The length of the column can

be shortened, mobile phase can be pumped at a faster flow velocity, and onecan ensure that the retention of sample components is not prohibitively long.Once any of these approaches are attempted, however, it is quickly seen thatother important parameters of the separation are affected, principally the res-olution and the column backpressure These parameters must be consideredwhen enhancing the speed of analysis Ideally, the analyst would like to max-imize both resolution and speed of analysis, while remaining within the pres-sure capabilities of the instrument.What is discovered, though, is the inevitableexistence of a trade-off between resolution, analysis time, and backpressure.Resolution can be enhanced if more time is allowed; conversely, analysis time can be shortened, but at the expense of resolution In addition, both

a= (1+ )

resolution and speed are limited by the constraints of the instrumentation.The interrelationship between these factors will be considered, starting with the most important parameter describing the quality of our separation—resolution.

17.2.1 Resolution and Analysis Time

The practical goal of most separations is not to achieve the greatest resolutionpossible, but rather to obtain sufficient resolution to separate all components

in the shortest amount of time To optimize for speed, the starting condition

is that there is a minimum resolution requirement for the separation tion is a function of three parameters: column efficiency, or theoretical plates

Resolu-(N), selectivity (a), and the retention factor (k):

(17-2)

Selectivity and retention are influenced by the choice of column chemistryand the mobile phase and gradient conditions Due to the trade-off betweenresolution and analysis time, any “excess” resolution that can be generatedbeyond the minimum requirement can theoretically be traded for shorteranalysis times In this regard, the power of selectivity cannot be underesti-mated, especially when a is close to 1 For example, Karger et al [1] haveshown that an increase in a from 1.05 to 1.10 can result in more than a three-fold reduction in analysis time High selectivity also lessens the required the-oretical plate count necessary to resolve all components, which allows use

of a shorter column to speed up the analysis Consequently, choosing a column or using mobile-phase conditions that produce a high relative selec-tivity between critical peak pairs can be very advantageous for achieving fastmethods In addition, resolution as well as analysis time depends on the reten-

tion factor For isocratic conditions, the optimum k for resolution and speed

occurs in the range of 1–10 [1] For samples containing many components orwith analytes of wide-ranging polarity, gradient elution must then be used toachieve reasonable analysis times Optimizing selectivity and retention so as

to maximize resolution and minimize analysis time in gradient separations isdiscussed further in Section 17.6

Beyond these two parameters, the minimum resolution that must beachieved will require a certain number of theoretical plates, which can be

expressed in terms of the column length and plate height, H, as

(17-3)

From this equation, column efficiency scales directly with column length and

is inversely proportional to the plate height Solving this equation for L and

11

2 2

aa

substituting into equation (17-1) results in a useful expression that moreclearly relates analysis time to the quality of the separation:

higher plate count and resolution However, resolution increases not with N,

but with , meaning the gain in resolution from lengthening the column willalways be proportionally less than the price paid in time Consequently, forfast analyses, columns no longer than that which gives the minimum theo-retical plates to adequately resolve all peaks should be used

Note also that t a varies with the ratio H/u Equation (17-3) shows that

reduc-ing the plate height is one way to obtain higher theoretical plates withoutincreasing the column length Now it is seen that for a fixed plate number (theplates needed to achieve the resolution requirement), decreasing the plateheight will shorten analysis times by allowing use of a shorter column As dis-cussed in the next section, though, plate height is dependent on the linearvelocity Thus, when optimizing for speed, the two must be considered

together The goal, then, is not just to reduce H, but to minimize H/u This will favor both high resolution and short analysis times Minimizing H/u, then,

encompasses the heart of what is desired in a fast HPLC method—greatest

resolution per unit of time

Exploring this concept a little further, knowing that H = L/N and u = L/t0,

substituting in these relationships results in

more effectively describe the criteria of resolution per unit time that are

desired to be maximized (actually, N/t is proportional to resolution squared

per time); unfortunately, they are not widely used in the literature, and for thesake of continuity will not be used in this discussion The following sectionswill look at what influences plate height and velocity and how best to mini-

mize H/u.

H u

t N

17.2.2 Plate Height and Band-Broadening

Plate height is a measure of peak-broadening and column performance:Reducing or eliminating sources of band-broadening should be a main goalwhen choosing columns and instrumentation, and otherwise developingmethods Plate height can also be described in terms of its dependence on the

linear flow velocity, u, by the van Deemter equation [4]:

in more detail

The A term of the van Deemter equation is independent of the

mobile-phase linear velocity and describes the broadening that occurs due to the tiple flow paths present within the column Since these paths are of differentlengths, molecules will travel different distances depending on what flow paths

mul-they experience For a column bed of randomly packed particles, the A term

is proportional to the particle diameter, dp, and to a factor λ related to the

The B term describes broadening due to axial molecular diffusion and is

inversely proportional to the linear velocity In other words, the faster ananalyte zone migrates through the column, the less broadening due to diffu-

sion it will experience The B term coefficient is given by

(17-8)

where D Mis the diffusion coefficient of the analyte in the mobile phase, and γ

is the tortuosity or obstruction factor, accounting for the obstruction to sion presented by the packing material

diffu-The C term, or resistance to mass transfer term, is a complex

agglomera-tion of all broadening that becomes worse with increasing flow velocity

Multiple contributions to the C term have been described; however, for the

purposes of this discussion the focus will only be on the relationships relevant

to improving resolution per unit time In general, C is related to the diffusion coefficient D of the analyte in the medium through which mass transfer is taking place, and it is also related to the square of the distance d over which

it occurs Fast diffusion and short diffusional distances aid mass transfer and

reduce band-spreading; hence, the C term takes the form

(17-9)

For example, for the mass transfer in the bulk mobile phase between

stationary-phase particles, D becomes the diffusion of the analyte in the bulk mobile phase, D M , and d becomes the distance between particles, which is roughly proportional to the particle diameter, d p The mobile-phase C term expression C Mcan therefore be approximated as

(17-10)

When looking at the individual plate height equations, some important

rela-tionships are noticed The B term worsens at slower flow velocities and with faster molecular diffusion In contrast, C-term broadening worsens at faster

velocities, but improves with faster molecular diffusion These opposing nomena are what cause the van Deemter curve to possess a minimum plate

phe-height at some intermediate velocity (the optimum velocity, uopt) It can also

be seen from Figure 17-1 that the increase in plate height is more abrupt at

the low velocity end of the curve (where broadening is dominated by the B term) than it is at the high velocity side (where the C term is dominant) Since

conditions that favor speed are desired, operating at velocities greater than

D M p M

the optimum velocity without significantly sacrificing efficiency is geous.

advanta-Although the B and C terms exhibit opposite relationships with analyte fusion, the C-term relationship is mainly of interest because resistance to mass

dif-transfer is the dominant form of band-spreading at the faster velocities thatare desired Equations (17-9) and (17-10) imply that speeding up diffusion willincrease mass transfer and help decrease plate height The Wilke–Chang equa-tion [9] shows that diffusivity is directly proportional to temperature andinversely proportional to viscosity:

(17-11)

where T is temperature, h is the solvent viscosity, V1 is the molar volume of

the solute, M2is the molecular weight of the solvent, and Y2is a solvent ciation factor Since mobile-phase composition largely dictates the selectivity

asso-of our separation, varying the viscosity asso-of the mobile phase directly by theselection of solvents may not be an option Raising the temperature of themobile phase, then, is the most effective way to speed up diffusion It also hasthe added benefit of lowering the mobile-phase viscosity, thereby increasingdiffusion indirectly This all serves to reduce the plate height at faster veloci-ties As shall be seen in the next section, raising the temperature also speeds

up the analysis by lowering the pressure drop across the column

The plate height relationships also show that the A term is dependent on the particle diameter, and the mobile-phase C-term is dependent on the par-

ticle diameter squared Reducing the diameter of the packing material is fore a powerful approach for reducing plate height The minimum attainable

there-plate height for a column, Hmin—that is, the there-plate height occurring at the optimum velocity uopt—will be proportional to d p When operating at veloci-

ties greater than uopt, the quadratic dependence of C on d p means that thereduction in plate height is especially significant This makes sense, since masstransfer will improve as the distances molecules must travel become smaller.That is, smaller particles result in smaller interparticle spaces and thus shorter

diffusional distances By using a smaller particle size, the slope of the C-term

side of the van Deemter curve will decrease dramatically, allowing operation

at higher velocities without having to sacrifice as much in resolution compared

to larger particles This is illustrated in Figure 17-2, which shows the mance of columns packed with 1.7-, 3-, and 5-µm particles Smaller plate

perfor-heights and higher velocities are made possible, thus considerably reducing

H /u As a result, one should aim to keep the particle diameter as small as

possible

Since the goal is to reduce analysis time by minimizing H/u while holding

N constant (at the minimum required plate count), the approximation can

be made that H ∝ d p , and therefore N ∝ L/d p This means as the particle diameter is reduced, the column length must also be reduced proportionally

Holding L/d p constant while both length and particle size are decreased istherefore one of the most effective means of achieving fast separations This

is the motivation seen in the evolution of chromatography columns over thelast few decades Where once the 25-cm column packed with 5-µm particleswas the standard workhorse analytical column, now 10- and 15-cm columnspacked with 3-µm particles are used As column technology continues

to improve, even shorter columns packed with particles <3 µm are being introduced

To illustrate more clearly the effect of these variables on analysis time,reduced parameters can be used for the plate height and velocity Reducedparameters effectively normalize the plate height and velocity for the particlediameter and the diffusion coefficient to produce dimensionless parametersthat allow comparison of different columns and separation conditions Thereduced plate height and reduced velocity are expressed, respectively, as

= 2 (1+ )

D p M

The benefit of reducing the particle diameter on separation time is mostevident here It is also seen that increasing diffusion will speed up the analysis.

Now that the factors affecting plate height have been examined, it is time

to turn to the effect of linear velocity and the limitation of pressure on theresolution per unit time

17.2.3 Flow Velocity and Column Backpressure

It is known that increasing the linear flow velocity of the mobile phase will

lead to faster separations But since H is dependent on u, what velocity is needed to maximize the resolution per unit time (minimize H/u)? Using the van Deemter equation, H/u may be expressed as

(17-15)

From this equation, H/u approaches its minimum value of C as u becomes

large In other words, the separation should be performed at the fastest ity possible (Note also that this represents mathematically what was presented

veloc-in the previous section; that is, veloc-in the case of optimizveloc-ing for speed, the

sepa-ration is dominated by the C-term.) This doesn’t mean that the resolution itself will improve—on the contrary, since H generally increases with velocity when

u > uopt, resolution will worsen—but that the resolution per unit time is

improv-ing Again, since the quality of the separation must not be sacrificed, the speed

of analysis can be improved only to the point where resolution can no longer

be sacrificed

Of course, the ability to increase u depends on the pressure capabilities of

the instrument, since pressure is directly proportional to velocity:

(17-16)

where∆P is the pressure drop across the column, η is viscosity, and φ is the

flow resistance factor Thus the speed of analysis is limited by the maximumpressure capability of the instrument As a result, the most should be made ofthe pressure available by reducing the pressure drop across the column asmuch as possible

Decreasing the column length lowers the pressure requirement tionally, allowing use of the available pressure to gain an advantage in speed.Column efficiency, however, drops with use of a shorter column and at fastervelocities Care must therefore be taken to ensure that resolution betweenpeaks is not lost when decreasing analysis time in this manner

propor-Lowering the viscosity of the mobile phase is another way to lessen therequired pressure This may be accomplished by raising the column tempera-

∆P uL

dp

= ηφ2

H u

A u

B

= + 2 +

ture Increasing temperature has the double advantage of allowing use of ahigher flow velocity and speeding up diffusion, both of which appear in thedenominator of equation (17-14) This is a strong motivator for the use of tem-perature above ambient conditions in order to speed up the separation Ofcourse, sample degradation, the boiling point of our mobile phase, stability

of the stationary phase, and the capability of the column heater limit themaximum temperature that can be used Temperatures up to about 70°C areconsidered routine; beyond that, columns and heaters specifically designed forhigh-temperature chromatography are needed Much research has been done

in the area of elevated-temperature chromatography, where interesting bilities arise, such as the use of temperature gradients and purely aqueousmobile phases [10] Chapter 18 elaborates on the use of temperature in chro-matography for pharmaceutical applications

possi-The velocity we can obtain at a given pressure will also be limited by theresistance to flow presented by the column, known as the specific column permeability In equation (17-16) the permeability is broken up into its twomain components: the flow resistance parameter,φ, and the particle diameter

squared, d p2, and can be expressed as

(17-17)

whereε is the interstitial porosity of the column (i.e., the fraction of the totalcolumn volume occupied by the interparticle space), usually about 0.4 Theflow resistance parameter is given by

(17-18)

and is purely a function of the porosity of the column—that is, the packingdensity Its value is essentially fixed for a given column and out of the analyst’scontrol The quantity f/e, represented by the symbol Φ, has a value around

1000 for well-packed columns [11]

Reducing the particle diameter can be a powerful way to gain speed in arations On the other hand, equation (17-16) shows an inverse quadratic rela-tionship of pressure to the particle diameter This strong dependence meansthat an enormous price in pressure is paid for reducing the particle diameter.However, it was stated previously that when reducing the particle diameter

sep-the column length can be reduced as well to keep L/d pconstant Since sure scales with column length, this eases the pressure requirement But even

pres-keeping L/d p constant, the pressure will still go up with 1/d p Eventually, theupper pressure limit of the pump will be reached and it won’t be possible to

further reduce d p without either a proportionally greater reduction in L, which

reduces the efficiency, or a relatively smaller linear velocity, which cuts back

on speed Because uopt increases in proportion to 1/d, the maximum pressure

of the system may not be able to reach a velocity beyond the optimum, andthe plate height may suffer Finally, as the column length becomes ever shorter,the column volume becomes smaller relative to the volume of the tubing,injector, and detector In this circumstance, extra-column band-broadeningbecomes a significant issue (see Section 17.7 of this chapter) With standardcommercial pumps having an upper pressure limit of ∼400 bar (∼6000 psi) andcolumns now being produced with particles <2 µm in size, that practical limithas now been reached This demonstrates that the upper pressure limit of theinstrument becomes the limiting factor for the resolution per unit time andspeed of analysis that may be obtained.

There are a few approaches to the problem of the pressure limitation thathave gained in popularity in recent years Monolithic columns replace the bed of packed particles entirely and instead use a “monolithic” polymerized

porous structure synthesized in situ These columns are able to produce

equiv-alent efficiencies of packed particle columns but with much lower flow tances, enabling much higher velocities Another approach is to developinstrumentation capable of much higher pressures than is possible with con-ventional systems This technique is termed ultra-high-pressure liquid chro-matography (UHPLC) With the pressure maximum now much higher, thelimitations on particle size and column length can consequently be extendedeven further, allowing even greater resolution per unit time and speed ofanalysis The subjects of monolithic columns and UHPLC are covered in thenext two sections of this chapter

resis-17.3 MONOLITHIC COLUMNS

Both the speed and efficiency attainable by an HPLC column are ultimatelylimited by the maximum pressure capabilities of the instrument In the case of particle-packed columns, decreasing the particle diameter leads toimproved efficiency and speed; however, because DP is proportional to 1/dp2,the price paid in pressure will always be proportionally greater than the gain in column performance Monolithic columns are a viable alternative toparticle-packed columns as a means to achieving efficient separations whileovercoming the pressure limitation due to their comparably higher perme-abilities (lower flow resistances) [12] High efficiencies together with lowerpressure drop make monolithic columns attractive options for fast HPLC

17.3.1 Physical Properties and Preparation of Monolithic Columns

A monolithic column is a single interconnected skeletal stationary-phasesupport structure consisting of large-flow through-pores (1–3µm) The key tohigh permeability is that this network support structure can be made in such

a way that the ratio of the through-pore size to the skeleton size is muchgreater than can ever be obtained by a column packed with individual spher-

ical particles While the interstitial porosity of spherical particle-packedcolumns is typically ∼0.4, monolithic columns exhibit external porosities of

∼0.6–0.7 When the intraparticle pores in spherical particles and mesopores inthe monolithic skeleton are included, the total porosities are on the order of

∼0.65–0.75 for particulate columns and on the order ∼0.80–0.90 for monolithiccolumns The presence of the mesopores (10–25 nm) supplies sufficient surfacearea for retention, around 300 m2/g, which is comparable to most porous silicaparticles [13]

Monolithic columns are generally prepared by the in situ polymerization of

either organic or inorganic monomers to form the skeletal support Organicpolymer monoliths are produced by nucleation and growth, followed by aggre-gation to form the network structure Control of the polymerization kineticsdetermine the size of the macro- and mesopores A main drawback of polymermonoliths, however, is that polymers tend to swell or shrink in the presence

of organic solvent, which leads to poor chromatographic performance and alack of mechanical stability under pressure-driven flow Monolithic silicacolumns are prepared using a sol–gel method by hydrolytic polymerization ofalkoxysilanes to form the skeleton Physical features such as through-pore sizeand skeletal size can be more precisely controlled in the preparation of silicamonoliths In addition, the chemical and mechanical stability of silica mono-liths is better than polymeric columns However, due to shrinkage during solid-

ification, silica monoliths cannot be prepared in situ, but must first be prepared

in a mold, and then removed and encased in PEEK tubing before bonding ofstationary phase takes place [13, 14]

17.3.2 Chromatographic Properties and Applications of

Monolithic Columns

In addition to higher permeabilities, another advantage of monoliths isimproved (that is, decreased) mass transfer broadening In packed columns,flow occurs through the interstitial spaces between particles, while mobile-phase transport inside the pores occurs predominantly by diffusion By contrast, due to the high porosity of monolithic columns, a much greater percentage of mobile phase transport is accomplished by flow Where diffu-sion does occur, in the mesopores, the shorter diffusion path lengths afforded

by the small skeleton sizes aids in mass transfer This is especially true for largemolecules such as proteins that have small diffusion coefficients Accordingly,

a silica-based monolith (Chromolith, Merck, Darmstadt, Germany) strates efficiencies comparable to a column of identical dimensions packedwith 3-µm particles, while exhibiting backpressures comparable to that of 11-µm particles [15] Wu et al [16] performed van Deemter analysis oncolumns packed with 3- and 5-µm particles and on a commercially availablemonolithic column (Figure 17-3) The minimum plate height of the monolithiccolumn was similar to the 3-µm particle column; however, the slope of the

demon-high-velocity, C-term side of the plot was lower, enabling faster velocities.

Silica monoliths have been applied to peptide separations [17, 18] as well as

to small-molecule pharmaceutical development samples [16, 19]

Monolithic columns do have disadvantages Although very high flow ratesare used to speed up separations, this generates a considerable amount ofsolvent waste for ≥4.6-mm-bore columns The number of phases and columnsizes is very limited at present, as is the number of commercial manufactur-ers Also, the technology of particle-packed columns is not static, but contin-ues to improve as well Monolithic columns have not yet demonstrated the performance capabilities exhibited by sub-2-µm particles and UHPLC.However, advances in monolithic column technology in the years to comepromise to bridge that gap

17.4 ULTRA-HIGH-PRESSURE LIQUID CHROMATOGRAPHY

The increase in resolution and speed of analysis afforded by reducing the ticle diameter has resulted in a trend of using smaller particles in shortercolumns Columns packed with particles less than 2µm in size, however, chal-lenge the pressure capabilities of conventional HPLC instrument technology,which operate up to ∼400 bar Since chromatographers generally shouldoperate at or above the optimum flow velocity for a given column, evenextremely short columns with these particles reach the system pressure limitsbefore their full benefits can be realized A straightforward approach to takefull advantage of sub-2-µm particles is to develop instrumentation capable ofthe requisite pressures

par-In 1997, the laboratory of James Jorgenson at the University of North Carolina [20, 21] was the first to demonstrate this approach by introducing

ULTRA-HIGH-PRESSURE LIQUID CHROMATOGRAPHY 777

Figure 17-3 van Deemter curves for packed (YMC C18) and monolithic (Chromolith)4.6-× 100-mm columns (Reprinted from reference 16, with permission from Elsevier.)

UHPLC They utilized long (>50 cm) fused silica capillary columns packedwith 1.5-µm nonporous silica reversed-phase particles and pressures as high

as 4100 bar to achieve greater than 200,000 theoretical plates The tremendousimprovement in performance that was demonstrated over conventionalcolumns (i.e., stainless steel tubes 3–4.6 mm in diameter packed with particles3–5µm in size) generated significant interest in this technique A number ofacademic research labs have subsequently conducted research using UHPLCwith nonporous particles, notably the laboratories of Milton Lee (BrighamYoung University) [22–29] and Luis Colón (SUNY—Buffalo) [30, 31] Thepractical challenges and limitations of the technique have largely limited itsuse to research environments However, the recent development of porous sta-tionary-phase material in the sub-2-µm range [32, 33] and the introduction in

2004 of the first commercial instrumentation are steps toward making UHPLC

a viable tool for pharmaceutical analysis

Two pressure regimes have been described: very-high pressure LC(VHPLC), for the pressure range of about 400–1500 bar, and ultra-high pres-sure LC (UHPLC), for pressures >1500 bar [20, 21, 34] This naming conven-tion is not strictly adhered to, however, and it is often common to refer toanything above the conventional HPLC pressure limit of 400 bar as UHPLC.Ultra-high-pressure LC will find its greatest utility for complex samples con-taining dozens or even hundreds of components (e.g., samples of biologicalnature) where extremely high resolving power is needed For such applica-tions, long, highly efficient columns packed with micron-sized particles run atultra-high pressures are desirable Very-high-pressure LC is well-suited forapplications where it is not so much high resolution that is needed, but fastanalysis times Samples containing less than 15–20 peaks, such as those encoun-tered in pharmaceutical development (e.g., small-molecule pharmaceuticalcompounds and related impurities and degradation products), can be sepa-rated in a matter of seconds to minutes using short columns with particles 1–

2µm in size at pressures moderately higher than conventional HPLC.Although these columns offer only a marginal improvement in efficiency overconventional HPLC columns, their advantage lies in speed of analysis due tosmaller particles, shorter column lengths, and higher pumping pressures (seeSection 17.2 of this chapter) Of course, VHPLC used with longer columns canprovide an improvement in efficiency for the separation of complex samples

as well Due to the challenges of constructing ultra-high-pressure tation and manufacturing porous micron-sized stationary-phase materials, thelogical first step is a chromatographic system in the very-high-pressure realmusing 1.5- to 2-µm particles This will allow a significant gain in speed and areasonable improvement in separation power of analytical HPLC methods.Commercial products are now available that meet these goals With continu-ing research and advances in instrument and column technology, it is hopedthe goal of a commercial ultra-high-pressure LC system can be realized in thenear future

instrumen-17.4.1 Instrument Considerations when Using Ultra-High Pressures

A number of concerns arise when using ultra-high pressures in phy The most obvious is the engineering challenge associated with operating

chromatogra-at such pressures The pumps, pump seals, tubing, connections, valves, columns,and other hardware must be able to reliably withstand and operate at the pres-sures required Careful consideration must be given to the pressure limitations

of instrument components and the design of the system This necessitates at aminimum a comprehensive improvement in existing HPLC technology andmay require altogether new designs of instrument components, such as pumps,injectors and autosamplers, columns, and detectors

For this reason, initially all UHPLC systems were custom-made ments Jorgenson described a constant pressure isocratic system consisting of

instru-a Hinstru-askel® air-driven pneumatic pump and high-pressure tubing and fittingscapable of pressures up to 7000 bar [20] A stainless steel static-split injectionvalve and column fittings were designed and constructed in-house Slurry-packed fused silica capillary columns were prepared in-house as well Similarsystems with lower pressure capabilities were constructed in other labs [22,30] A largely custom-made constant flow gradient system with a pressure limit

of 5000 bar has also been described [21] While these instruments have beensuccessful in an academic research environment, they lack the ruggedness, reli-ability, and ease of use required in an industrial setting Toward that aim, Tolley

et al [34] modified a commercially available pump to achieve pressures inexcess of 1200 bar for use with capillary columns 22 cm long packed with 1.5-µm nonporous particles Finally, commercially available systems withupper pressure limits of 1000 bar have been introduced Although this repre-sents a moderate increase over the conventional HPLC pressure limit whencompared to the systems just described, it allows use of sub-2-µm particles in

a system capable of meeting the rigorous requirements for use in the maceutical industry

phar-Special consideration must be given to the injection valve The moving partsand sample handling requirements make sample introduction at ultra-highpressures a challenge A number of parameters must be considered: pressurelimitation, injection volume range, injection accuracy (i.e., delivery of mass oncolumn), precision (i.e., peak area reproducibility), linearity of response versusinjection volume, injection cycle time, and finally the amount of broadening tothe sample plug caused by the injector The first static-split UHPLC injectorsaccomplished an injection by applying pressure over several seconds to intro-duce a small plug of sample onto the head of the column The actual injectionvolume is difficult to determine and reproducibility is poor, precluding use ofthis injector for quantitative analysis A number of commercially availableinjectors capable of high pressures have been evaluated with UHPLC Onesuch system was a novel pressure-balanced rotary injection valve from ValcoInstruments that was employed by Wu et al [24] It operated at pressures upULTRA-HIGH-PRESSURE LIQUID CHROMATOGRAPHY 779

to 1200 bar and demonstrated superior peak area reproducibility (<2% RSD,

n = 5) compared to the custom-built static-split injectors Recently, an actuated needle valve injection system, also from Valco, rated to withstand

air-2700 bar and capable of injection cycle times of <2 seconds was evaluated pendently by both Anspach et al [35] and Xiang et al [36] Injection repro-ducibility of ≤1.5% RSD (n = 5) was obtained at 2000 bar for injection volumes

inde-of 1–2.5µL [35] Given the challenges of performing injections under pressure conditions, it is strongly recommended that the injection performance

high-of a high-pressure instrument is evaluated before developing methods forhighly accurate quantitative analyses Doing this during performance qualifi-cation (PQ) of the instrument or an initial vendor evaluation before purchas-ing is best Precision (peak area, peak height, and retention time), accuracy(recovery of mass injected on column), and linearity are parameters thatshould be investigated In order to routinely meet system suitability criteria,

it is recommended that an instrument be able to achieve ≤1% RSD (n = 6)for peak area and peak height precision

Due to the high efficiency afforded by sub-2-µm particles, it is crucial thatextra-column broadening be kept to a minimum [37] With fused silica capil-lary columns it is possible to introduce sample directly onto the head of thecolumn and to perform on-column detection, thereby essentially eliminatingextra-column sources of broadening This is usually not the case with moreconventional column dimensions and instrumentation The tubing, connec-tions, the injector, and the detector flow cell all add extra-column volume tothe system which will contribute to analyte band-spreading Extra-columnbroadening and instrument requirements for highly efficient columns is dis-cussed further in Section 17.7.3

Extremely narrow peak widths will present challenges for detection Thedata acquisition rate must be sufficiently fast to sample enough data points toaccurately define the peak.A good rule of thumb is to acquire at least 15 pointsper peak Peaks on the order of 1 second wide will also challenge the cycletimes of scanning mass spectrometers For fast, highly efficient chromatogra-phy, a time-of-flight (TOF) mass analyzer may be the best option for very fastdetection Detector requirements for fast LC is covered in more detail inSection 17.7.2

Currently the availability of columns packed with sub-2-µm phase material is rather limited This situation will likely change as the use ofUHPLC proliferates and the demand for such columns is more widespread.Until recently, the only stationary phase materials available in the sub-2-µmregime were nonporous silica particles They are much easier to synthesize inthis size range than are porous materials, and their mechanical strength allowsthem to be used at pressures up to 7000 bar [38] They also have the advan-tage of greater efficiency than porous particles of equivalent size due to thelack of additional band-broadening contributions presented by the pores Allthe initial work demonstrating the principles of UHPLC was consequentlyperformed on nonporous materials Their obvious drawback, however, is their

stationary-poor loading capacity—up to 100 times less sample may be loaded onto porous particles compared to porous particles This drastically limits the sen-sitivity obtainable using such columns and may require alternate detectionschemes or derivitization of the sample Also, very small injection volumes thatchallenge the capabilities of the injector may be required As a result, the use

non-of conventional columns packed with nonporous particles has been limited tospecific applications in pharmaceutical analysis such as protein and peptideseparations, where sample volumes are often very small and the slower mol-ecular diffusivities make the absence of pores especially beneficial due to the

decreased C-term band-broadening The development of high-quality porous

particles 1–2µm in size for use with elevated pressures has therefore been anecessary and critical advancement for UHPLC [32, 33] The superior loadingcapacity of these materials makes them practical for most pharmaceuticalanalyses Such columns are still susceptible to the crushing of particles at highpressures, which will manifest itself as rising backpressure due to plugging ofthe column Another problem that may arise is the compression of the packedbed inside the column, leaving a void at the column head which will result indistorted peak shapes Care should be taken not to operate a column at a pres-sure higher than that at which the column was packed To avoid this, columnsshould be packed at pressures several hundred bar greater than the maximumpressure at which it is to be used

Finally, the safety of UHPLC must also be considered Rupture or failure

of seals, tubing, and fittings can present a potential danger to the user Withproper instrument design and normal safety precautions, however, UHPLCcan be safe to use [38] This is especially true of commercial instruments, whichare no more dangerous than any other HPLC

17.4.2 Chromatographic Effects of Ultra-High Pressures

Another concern is the potential for frictional heating inside the column.Forcing mobile phase through the column at such pressures will generate heatthat may cause a significant rise in the temperature of the mobile phase [40,41] As heat is dissipated from the column walls, axial and radial temperaturegradients will form within the column The resulting differences in analyte diffusivity and retention within the column will lead to additional band-broadening.The power (heat) generated by flow through a packed bed is equal

to the product of the flow rate (F) and the pressure drop across the column

(∆P):

(17-19)

Table 17-1 shows the power generated by pumping mobile phase through 100-mm long columns of various diameters packed with 1.5-µm particles at alinear velocity of 3 mm/s and a pressure of ∼900 bar One can see that at thispressure a standard-bore 4.6-mm-i.d column generates 3.0 W of heat For

P= ∆F P

ULTRA-HIGH-PRESSURE LIQUID CHROMATOGRAPHY 781

comparison, consider conditions typically encountered in conventional HPLC.

A 4.6- × 100-mm column packed with 3-µm particles operating at 1 mL/min(corresponding to 1.5 mm/sec) will require 170 bar and generates only 0.19 W

of heat Columns larger than 2.1 mm in diameter would therefore be able for pressures and conditions outlined in Table 17-1 As the particle size

undesir-is reduced even further—to 1.0µm, for example—or as the column is ened, the operating pressures become correspondingly greater and even moreheat is generated This pushes the largest usable column diameter to under 1.0 mm Thus, the frictional heating effect serves as a strong motivator for theuse of capillary columns in UHPLC Patel et al [42] investigated the effect offlow-induced heating in capillary columns up to 150µm in diameter packedwith 1.0-µm particles and found negligible effects on column efficiencies.Indeed, the vast majority of academic research in UHPLC has been performed

length-in fused silica capillaries less than 100µm in diameter

Thought must also be given to the possible chromatographic effects arisingfrom changes in the retention factor of analytes and the compressibility of themobile phase as a function of pressure It has been shown that retention factorsfor small molecules increase linearly with pressure [42] This increase is mod-erate, however, and does not significantly affect analysis time Because of thecompressibility of the mobile phase at ultra-high pressures, a situation that isfamiliar in gas chromatography results:The volumetric flow rate will be greater

at the outlet of the column than at the inlet as the compressed mobile phaseexpands at lower pressures The magnitude of this difference will vary depend-ing on the solvent composition and pressures used In practice, this means themeasured flow rate at the outlet of the column will be greater than the flowrate to which the pump piston is set Other than some theoretical considera-tions, the changes of retention factor and flow rate with pressure will have noadverse effects on a chromatographic run One consequence has been noted,however, for UHPLC systems that perform injections while the column is off-line of the pump or otherwise at atmospheric pressure [43] In such systems,sample is introduced onto the head of the column and then pressure is sub-sequently applied to the column to start the run When this occurs, the mobile

TABLE 17-1 Power Generated Due to Frictional Heating of the Mobile Phase at a Linear Velocity of ~3 mm/sec in Columns of Varying Diameter Packed with 1.5-mm Particlesa

Column Dimensions Flow Rate Power Generated4.6× 100 mm 2.0 mL/min 3.0 W3.0× 100 mm 0.85 mL/min 1.3 W2.1× 100 mm 0.41 mL/min 0.60 W1.0× 100 mm 92µL/min 130 mW0.30× 100 mm 8.5µL/min 13 mW

aIn all cases, column backpressure is ∼900 bar A solvent viscosity of 1.0 cP was used for all calculations.

phase inside the column becomes rapidly compressed in volume This pression causes a surge in velocity at the head of the column, which contributessignificantly to broadening of the sample band An injector that performsinjections while the column is pressurized must be used in order to circum-vent this problem.

com-The use of ultra-high pressure in LC was found to have beneficial effects

on protein recovery [44] By using pressures >1600 bar, protein recovery wasenhanced and carry-over from run to run was reduced and in some cases elim-inated While the mechanism of recovery is not known, it was postulated thatultra-high pressures improve desorption from the stationary phase surface bycausing partial unfolding or deaggregation of the proteins

17.4.3 UHPLC Applications

Isocratic separation of test compounds is a useful way to demonstrate the formance of a system Basic chromatographic characteristics, such as theoret-ical plates, are easily measured and can be compared to what is expected fromtheory and to performance of other chromatographic systems Figure 17-4 is

per-a UHPLC chromper-atogrper-am obtper-ained under isocrper-atic conditions on per-a 43-cm-longcapillary column packed with 1.0-µm nonporous C18 particles (Eichrom ULTRA-HIGH-PRESSURE LIQUID CHROMATOGRAPHY 783

Figure 17-4 Chromatogram obtained on a column packed with 1.0-µm nonporous ticles at a run pressure of 3000 bar (Reprinted from reference 37, with permission.)

par-Scientific, Darien, IL) Five compounds—ascorbic acid (dead time marker),hydroquinone, resorcinol, catechol, and 4-methyl catechol—were eluted with

a 10/90 (v/v) acetonitrile/water mobile phase containing 0.1% TFA and weredetected with amperometric detection (+1.0 V versus Ag/AgCl) The chro-matogram was obtained near the optimum linear velocity at a run pressure of

3000 bar All compounds eluted in less than 8 minutes, with efficiencies rangingfrom a low of 244,000 plates for 4-methyl catechol to as high as 330,000 platesfor hydroquinone These correspond to about 570,000 and 770,000 plates/m,respectively—much higher than the 150,000 plates/m typically seen with con-ventional columns

The potential for fast gradient separations is shown in Figure 17-5 A series

of phenones was separated by an extremely fast gradient in the very-high sure regime, at about 750 bar (11,000 psi) This separation was accomplished

pres-in less than 1 mpres-inute and was performed on a commercially available highpressure instrument and column packed with 1.7-µm porous bridged-ethylhybrid C18 particles All peaks are less than 2 seconds wide and are baselineresolved The data acquisition rate was set at 20 pts/sec

A more complex gradient UHPLC separation of a tryptic digest of theprotein bovine serum albumin is shown in Figure 17-6 This sample containshundreds of peptide fragments and requires a separation method with largepeak capacity The sample was run with gradient elution using constant-flowpumps at 3600 bar on a 38-cm-long capillary packed with 1.0-µm nonporousC18 particles The peptides from the digest were tagged with the fluorophoretetramethylrhodamine isothiocyanate (TRITC) and detected by laser-inducedfluorescence Since it is not valid to calculate theoretical plates under mobile-phase gradient conditions, peak capacity is used as an alternative measure ofthe separating power of a system Peak capacity is defined as the total number

Figure 17-5 Gradient separation of, in order of elution, acetophenone, propiophenone,

n-butyrophenone, valerophenone, hexanophenone, heptanophenone, and none, performed on a Waters Acquity UPLCTMinstrument Column: 2.1 × 100 mm,1.7-µm ACQUITY BEH C18 Gradient: 50–90% acetonitrile in 1.0 minutes Columntemperature 35°C

octanophe-of peaks separable with unit resolution in a given separation space In the matogram shown, the peak capacity between 48 and 168 minutes is approxi-mately 500, with an average peak width of 14.5 seconds This is significantlyhigher than the peak capacities of conventional HPLC columns packed with5-µm particles, which tend to be below 200 for similar samples.

chro-Low flow rates and narrow peak widths make capillary UHPLC larly suitable for coupling with mass spectrometry via nanoelectrospray ion-ization [23, 45] Tolley et al [34] have used very-high pressures of around 1000bar (15,000 psi) to separate bovine serum albumin digests on columns packedwith 1.5-µm nonporous reversed-phase particles by gradient elution, withquadrupole/time-of-flight (Q-TOF) tandem mass spectrometry for detection.Figure 17-7 is a base peak index (BPI) chromatogram of 12.5 fmol of a BSAdigest on a 150-µm × 22-cm column packed with 1.5-µm nonporous C18 par-ticles A 30-minute gradient from 1 to 45% acetonitrile in water with 0.5%formic acid was used A 20-fold enhancement in sensitivity over nanoelectro-spray MS/MS was observed

particu-17.4.4 Method Transfer Considerations

The significant gains in speed and resolution are strong motivators to transferexisting HPLC methods to a commercial ultra-high-pressure instrument Thiscan be accomplished fairly easily with a few simple steps First, a UHPLCcolumn that is appropriate for the separation must be selected One with selec-tivity similar to that of the original column is preferred Choice of column

ULTRA-HIGH-PRESSURE LIQUID CHROMATOGRAPHY 785

Figure 17-6 UHPLC gradient separation of a tryptic digest of bovine serum albumin.

A peak capacity of 500 was obtained between 48 and 168 minutes (Reprinted fromreference 37, with permission.)

diameter, length, and particle size should be made in accordance with the ciples discussed in this chapter and with the overall goal of the method trans-fer (i.e., to speed up the existing separation or to obtain greater resolution ofcomponents).

prin-Second, the method should be scaled geometrically to account for ences in column dimensions The scaling equations can be found in Section17.7.4 Once these parameters are scaled, the flow rate can then be increased

differ-to speed up the separation With an increase in flow rate the gradient timesmust again be adjusted proportionally (e.g., doubling the flow rate requiresgradient times to be halved) The optimum flow velocity for the separationmust be kept in mind, however A column with smaller stationary-phase par-ticles will have a higher optimum velocity, so the new scaled method may not

be at optimum conditions The molecular weight of the analyte also plays arole: Large molecules will have a lower optimum velocity than small mole-cules due to slower diffusion in the mobile phase At this point the methodmay be further optimized using standard method development strategies

17.5 SEPARATIONS ON CHIPS

The miniaturization of chemical analysis systems has grown considerably

in recent years due to the promise of faster analyses, the ability to analyze

Figure 17-7 Base peak index chromatogram of BSA digest on 150-µm × 22-cm columnpacked with 1.5-µm nonporous C18 silica particles (Reprinted from reference 34, withpermission from American Chemical Society.)

very small sample volumes, less reagent consumption, and exciting ties such as multiplexing and interfacing of multiple analytical techniques.These miniaturized systems are described by a number of terms such as

possibili-“lab-on-a-chip,” “micro-total analysis system (µTAS),” or “microfluidic”devices Devices have been developed for numerous and varied applicationsincorporating many aspects of sample analysis: sample preparation, fluidichandling and manipulation, reactions or derivitization, separation, and detec-tion The focus here is on those systems that incorporate a separations stepon-chip

Lab-on-a-chip systems typically consist of microfabricated fluid nels photolithographically patterned onto a silicon or glass substrate Morerecently, polymeric substrates such as poly(dimethylsiloxane) (PDMS) orpoly(methyl methacrylate) (PMMA) have been used [46, 47] The channeldimensions range from tens of micrometers in width and depth to less than

chan-a micrometer Trchan-ansport of fluid is chan-accomplished most commonly by troosmosis, and less so by pressure-induced pumping, which usually requiresinterfacing the chip to a benchtop pump The difficulties in construction

elec-of valves on-chip is another limitation for pressure-based systems quently, capillary electrophoresis is the easiest and most common separationprinciple employed on a chip Separation channels packed with stationary-phase particles as well as porous monoliths have been prepared to performchromatography and electrochromatography While benchtop separationinstruments typically analyze samples one at a time, multiple parallel separation channels may be patterned onto a chip to simultaneously analyzedozens of samples for increased sample throughput and lab productivity.Detection may be performed on-chip via fluorescence or electrochemicaldetection, for example, or the chip may be interfaced to a mass spectrometer,which is particularly amenable to the low flow rates of chip-based separa-tions [48]

Conse-While a wide range of opportunities exist, such as environmental, clinical,and trace analysis, the principal application for labs-on-a-chip is in the analy-sis of biological samples The miniaturized dimensions allow extremely smallsample volumes to be analyzed, and a microchip format can allow chemicalreaction, mixing, sample manipulation, and multiplexing to be performed.Single-cell analysis, immunoassays, protein and peptide separations, DNAanalysis and sequencing, and polymerase chain reactions have all been per-formed on microchip devices [48]

There are still significant technical hurdles that must be overcome formicrochips to develop into an accepted and widespread technique The unre-liability of electroosmotic pumping and other microfluidic processes con-tributes to the lack of robustness of these systems The miniaturization andintegration of other components, such as pumps, valves, and detection schemes,onto the chip is another essential step for future development The reader isreferred to the reviews cited in the references for research performed withmicrochips [46–48]