Modern Sample Preparation for Chromatography

446 Pages·2014·15.726 MB·English

by  Serban C. Moldoveanu, Victor David

MODERN SAMPLE
PREPARATION FOR
CHROMATOGRAPHY
S M
ERBAN OLDOVEANU
R.J.Reynolds,TobaccoCo.,Winston-Salem,NC,USA
V D
ICTOR AVID
UniversityofBucharest,Bucharest,Romania
AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON(cid:129)NEWYORK(cid:129)OXFORD
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Preface
In the last 10 to 15 years, considerable prog- was taken in a previous book published by
ress has been made in chromatography, in Elsevier in 2002 by the authors of present book
particularbytheintroductionofnewseparation andhavingthetitleSamplePreparationinChroma-
media, higher pressure instruments for HPLC tography.Thepresentbookcanbeconsideredan
(UPLC), as well as by the introduction of more updateoftheolderbook,withmostofthemate-
sensitivedetectorswithdevelopmentsintandem rial rewritten in a simpler and shorter manner
mass spectrometry for both LC (LC/MS/MS) and with a significant reduction of the descrip-
and GC (GC/MS/MS). This progress required tion of derivatization procedures for various
further development and changes in sample classes of molecules. The new book is focused
preparation for chromatography, which have on true sample preparation procedures and
been achieved by the introduction of new tech- does not include descriptions of techniques that
niques and the introduction of new materials are basically stand-alone analytical methods,
with better performance in sample preparation such as pyrolysis-GC/MS, bidimensional chro-
practice. The capturing of this progress in matography, or other hyphenated chromato-
a new book dedicated to sample preparation graphic techniques. These stand-alone
for chromatography was one of the main goals analyticalproceduresarewelldescribedindedi-
ofthepresentmaterial.Morerecently,insample cated monographs and cannot be seen just as
preparation for chromatography there is the sample preparations. A problem related to
tendency to use simpler techniques than in the a book on sample preparation for chromatog-
past,theanalysisdependingmoreontheperfor- raphyistheextenttowhichspecificapplications
mance of the chromatographic equipment and aredescribed.Virtuallyeveryanalyticalmethod
less on processing of the sample. The idea of includesasamplepreparationstep,andanenor-
“minimal” sample preparation appears more mousnumberofapplicationsofsampleprepara-
and more attractive, since sample preparation tions are reported in the literature. The details
is typically labor intensive and requires well- necessaryforapplyingsamplepreparationsteps
trained operators. The concept of minimal can be easily obtained from the original source,
samplepreparationputspressureondeveloping and their inclusion in a book was considered of
methods that are simpler and more efficient. littleusefulness,aswellasimpossibletocapture
Some ways to achieve this goal are discussed comprehensively. For this reason, only a few
in this book. An additional objective of the examples of applications used for illustrating
book is to present sample preparation for chro- different procedures were included in the book.
matographyasacoherentsubjectandtoinclude Thebookisintendedtobeaddressedtochemists
the description of the new developments as of a wide range of levels of specialization, from
a logical progress in the field. This approach both industry and academic sectors. It is
xi
xii
PREFACE
intendedtoenhancetheunderstandingofprinci- Sivaraj, for their contribution to the publication
ples and techniques usedin sample preparation of this book. Also, the authors express their
for chromatography that can help in obtaining thanks to Mr. Paul Braxton, Mrs. Crystal Byrd,
reliable results in chemical analyses and in Dr. Anthony Gerardi, and Mrs. Carol Moldo-
making the correct decision related to the veanu for reviewing the manuscript and sug-
studied system. gesting valuable corrections.
The authors wish to thank the editorial team
from Elsevier, Ms. Jill Cetel and Ms. Anitha
C H A P T E R
1
Preliminaries to Sample Preparation
O U T L I N E
1.1 CollectionofInformationandPlanningfor Sensitivity andLimit ofDetection 16
a Chromatographic Analysis 3 PracticalLODandLOQ(PLODandPLOQ)21
General Comments 3 Validation ofan Analytical Method 21
Chromatographic Core Analysis 4
1.3 Sampling 23
Collectionof Information Regarding the
General Comments 23
Analysis 4
SamplingTechniques 24
Planning the Analysis 7
Proper Sampling 25
1.2 The Evaluation of Analytical Data 8 Some Comments on Samplingof Gases 28
General Aspects 8 Some Comments on Samplingof Liquids 29
Precisionand Accuracy inQuantitative Some Comments on Samplingof Solids 30
Chemical Analysis 9 Some Comments on Samplingof Mixed-Phase
Propagation ofUncertainty 13 Materials 30
Comparisonof Two Procedures Regarding Some Comments on Handling,Preservation,
Accuracy 14 and Storage ofSamples 30
Comparisonof Two Procedures Regarding
References 31
Precision 16
1.1 COLLECTION OF
indicatedassamplepreparation.Themodification
INFORMATION AND PLANNING
isnecessarybecauseitiscommonthatthechem-
FOR A CHROMATOGRAPHIC
icalanalysisisnotpossibleoritisnotconvenient
ANALYSIS
withoutapreliminaryprocessingofthesample.
Samples can be considered as made from two
General Comments
distinct parts, the analytes and the matrix. The
Chemical and/or physical modification of a analytes are the molecular species of interest in
sampletomakeitamenableforachemicalanal- thesample,andthematrixistherestofthesam-
ysisortoimprovetheperformanceofanalysisis ple components. Sample preparation can be
ModernSamplePreparationforChromatography
http://dx.doi.org/10.1016/B978-0-444-54319-6.00001-3 3 Copyright©2015ElsevierB.V.Allrightsreserved.
4
1. PRELIMINARIESTOSAMPLEPREPARATION
performed for sample dissolution, matrix modi- All these qualities made chromatography the
fication and/or simplification, analytes concen- most utilized procedure of chemical analysis.
tration, preliminary separation of the analytes For this reason, an enormous body of informa-
fromthesample,chemicalchangesoftheanaly- tion is available regarding chromatographic
tes. All these operations are performed before methods of analysis, including dedicated
the core analysis with the purpose to improve scientific journals, books, Website articles. The
the analysis results. detailsofchromatographicseparationandmea-
The steps in a chemical analysis can be surementareimportantforthesampleprepara-
summarized as follows: information collection tion step, since sample preparation must be
/ planning / sample collection / sample designed to make the sample amenable for
preparation / coreanalysis / dataprocessing thatspecificmethodofanalysis.Forthisreason,
/resultsinterpretation.Asshown,thechemical the description of the analytical method in-
analysisdesignedtocharacterize(identify,quan- cludes sample preparation as an intrinsic part
titate, etc.) the analytes typically involves some of the procedure.
steps related to information and some to opera- It is common that the sample injected in a
tions. The core analysis considered in this book chromatographic instrument has already been
ischromatography. subject to specific modifications (sample prepa-
ration) that are necessary to have the analytes
Chromatographic Core Analysis amenable for the chromatographic analysis.
Thisprocessingtransformstheinitialrawsample
Chromatography comprises several tech- into a processed sample (see e.g., Ref. 1). After a
niques used for the separation of different sample preparation step, the processed sample is
molecular species from a mixture. The chro- used for the core chromatographic procedure
matographic separations are performed for ofanalysis.Thetransformationoftherawsample
both laboratory and industrial applications. In into the processed sample is the subject of this
thelaboratory,chromatographycanbeapplied book.
foranalyticalpurposesorforotherapplications
(e.g., preparative). In analytical chromatog-
Collection of Information Regarding the
raphytheseparationiscoupledwithadetection
Analysis
capability for the separated components. Some
chromatographic techniques have a very high The collection of information for a chemical
separation power and can be coupled with analysis should include (1) the purpose of anal-
exceptionally high sensitive detectors. High ysis, (2) the nature of material to be analyzed,
performance liquid chromatography (HPLC), (3) sampling and sample characteristics, (4) the
for example, allows the separation and quanti- analytestobemeasured,(5)methodsofanalysis
tation of a wide variety of compounds even in (possibilities for sample preparation and core
very complex mixtures such as biological or analytical methods available for the requested
other natural samples. Gas chromatography analysis), (6) data processing, (7) available
(GC), in particular when coupled with mass instrumentation, materials, and expertise in the
spectrometric detection (GC/MS), is used for laboratory, (8) available funding and other po-
the analysis of volatile compounds allowing tentialrestrictions, (9) relation with results from
both quantitation and compound identification previous work, results from other laboratories
even in samples with many components. In or databases, and (10) certainty of the informa-
addition, both HPLC and GC techniques can tion about the sample, the method of analysis,
be applied on very small amounts of sample. and other resources. The sources of information
I.GENERALCONCEPTSINSAMPLEPREPARATION
5
1.1 COLLECTIONOFINFORMATIONANDPLANNINGFORACHROMATOGRAPHICANALYSIS
include(1) theprovider ofthe sample(s)/collec- should beanalyzedor onlya specific part
tor of the sample(s), (2) the recipient of the re- (surface,solublecomponent,selectedpoints,
sults (possibly the same as the sample etc.). The informationon the purpose of
provider), (3) public knowledge related to the analysis also helpsto decide whether a
sample and analysis (literature, web informa- specific protocol must be followed during
tion,etc.),and(4)private(andpersonal)knowl- the analysis or that no regulations are
edge on the analysis. The step of information imposed.Some analyses are requiredto be
collection for a chromatographic analysis does nondestructive, and in certain cases the
not differ in principle from that for a general analysis is done in conjunction with
analysis, with the exception that the analytical preparative purposes, which alsoshould be
method selection is restricted to a chromato- known. Awide range of requests can
graphic one. The steps during information bemadefor ananalysis, and samples must
collection are not necessarily taken in a specific beanalyzed fornumerous reasons. In
sequence, and in most cases the iteration of the industrialenvironmentssuch purposes may
processisnecessaryforbringinganuniformun- includeofficialor legalrequirements,
derstanding. The correlation between different assessing the quality of raw materials,
pieces of information is very important. Some processcontrol or troubleshooting, assuring
of the collected information can be certain, but the quality of finished products, research,
some can be vague or uncertain. This aspect reverse engineering, ordevelopment
qualifyingtheinformationmustalwaysbetaken purposes. Samplesare frequently analyzed
into consideration during the planning of the for health-related purposes (e.g., medical
analysis. Several comments on each step analyses,analysis of pharmaceuticals,
regarding information collection are further analysis of metabolites), for evaluating
made: environmental issues, for forensicpurposes,
forexploratoryreasons,andforfundamental
1. Theinformation regarding the purposeof research. Depending on theanalysis reason,
the analysis is very important. The analysis specific decisions are made about the
shouldbegearedtowardansweringspecific analyticalprocess.
questions, even if the analysis is only 2. The informationregarding the type of
exploratory or the goal of performing the sampleshouldtellifitisofaknowntypeora
analysis is vague. From the purposeof new type. The physical state of the sample
analysis, it must beknown whether (homogeneous, nonhomogeneous, solid,
qualitative, quantitative, or bothtypes of liquid,gas) is also important. Details about
results are expected from the analysis, or if the nature of the sample matrix (organicor
special types of analysis such as that of inorganic material,biological,
enantiomersor of compounds structure are environmental, composite,etc.) must be
projected. The list of analytes (single or obtained.Theknowledge about the matrix
multiple component analysis) or class of composition(andabouttheanalytes)willbe
analytes that must be analyzed (if known), important fordeciding if anew analytical
the required precision of the results (in methodisneededorifonealreadyavailable
particular for quantitative analysis), the canbe used or adapted. Tothisinformation
further use of theresults, and the rapidity should beadded the knowledge about the
withwhich the results must bedelivered amountof sample available(largequantity,
must beknown before startingan analysis. small quantity, readily available,unique,
Also,it must beknown if the whole sample etc.),thevalueofthesample,theoriginofthe
I.GENERALCONCEPTSINSAMPLEPREPARATION
6
1. PRELIMINARIESTOSAMPLEPREPARATION
sample, sample thermal stability and level of analytesisvery importantin
perishability, safety concerns about the decidingabouttherequiredsensitivityofthe
sample. Also, the number of samplesto be analyticalmethod as well as the sample
analyzed atonepoint or in anextended enrichment approach tobe used.
periodoftimemustbeknown.Thiswillhelp 5. The collection of information on methods
to decideif a routine analysis will be of analysis is a very important step. The
necessary or a unique or limited time method of analysis can be considered as
analysis shouldbeused. dividedintotwodifferentsteps,thesample
3. Informationabout the sampling processis preparation and the core analytical
also very useful, in particularindicating the procedure. However, it is common that the
sample homogeneity,the age of the sample, information on an analytical method
potential contamination.In some cases the includes together the details about an
sample must bereturned to the provider appropriate sample preparation, the core
aftera small amount has been usedfor chromatographic separation, and the
analysis and this should also be known. measurementprocedure.Whensampleand
Knowledgeregardingotheranalysesalready the analytes are of a known type, it is
performed on the sample isalways common that a good method of analysis is
important, and occasionally it isusefulto available. The scientific literature (printed
have informationabout otheranalyses that or on the web) contains an enormous
are planned to beperformed later on the number of analytical methods. Separate
sample.Incasethatverylittleinformationis sample preparation procedures, core
availableabout the sample, preliminary analytical separations, as well as
analyses shouldbesometimes performed. measurements are also available in the
Thispreliminaryanalysis can bequalitative literature. In many cases, the sample
or semiquantitative. For example,a GC/MS preparation is adjusted to match up with
analysis (if possible) may providevaluable the analytical chromatographic separation
qualitative information foran unknown and measurement, but it is not uncommon
sample. that the same sample preparation can be
4. Theinformation about the analytes to be used for more than one type of analysis. In
measured is another important component some cases, a reported analytical method
in planning ananalysis. This includesthe canbedirectlyappliedtoacertainanalysis.
nature of the analytesor atleastthe class of However, it is common that a direct
the analytes (inorganic,organic, functional application without any change is not
groups in organic compounds, ionic possible. In this case, one or more reported
character,etc.).Ifthisinformationismissing, analytical methods should be compared
it must beknown at least if the analytes are and a new technique can be developed.
smallmoleculesorpolymericones.Incaseof A third possibility exists, when no such
small analyte molecules, data regarding analysis as the one of interest is known,
volatility, solubility, and reactivity are very and an entirely new method should be
useful.For macromolecules, a general developed. In this case, the information
characterizationistypicallynecessary.Other about similar methods is still very useful.
dataregardingtheanalytesarehelpful,such The selection or development of an
as informationon the estimated level of analytical method must take into account,
analytesinthesample(trace,mediumlevels, besides the nature of the analytes and the
majorconstituent).Theaspectregardingthe matrix of the sample a number of other
I.GENERALCONCEPTSINSAMPLEPREPARATION
7
1.1 COLLECTIONOFINFORMATIONANDPLANNINGFORACHROMATOGRAPHICANALYSIS
elements such as the availability of 8. The availablefunding and other potential
instruments,theexpertiseinthelaboratory, restrictions should also be known before
and the available funding. starting the planning for a specific analysis.
6. Data processing is acommon step in The performance of some analyses is a very
chromatographicanalysis.Thismayinclude simple task,whileotheranalyses require
integration ofthe areas of chromatographic considerable resources.
peaks(areas are proportional to the amount 9. Therelationwithresultsfromprevious
ofanalytesinjectedintothechromatographic analyses,resultsfromotherlaboratoriesor
column), qualitative identification of databasesalsoshouldbecollectedaspartof
analytes (e.g., when amassspectrometer is preliminaryinformationforananalysis.Even
usedas detector),statistical processing of whenasimpleanalysisisrequired,the
datawhen an array of results is available. comparisonofthenewresultswithprevious
Specific electronic data processing onesmustbepartoftheprocess.Forthis
capabilitiesare commonly presentin reason,informationaboutotherresultsonthe
modern chromatographic equipment (e.g., sametypeofsampleoronsimilarmaterials
forpeak areaintegration, formassspectra shouldbepartofinitialinformation.
identification). Other capabilities are 10. The research regarding availabledataon
availablesuchasseparatestatisticalsoftware sample characterization and analytical
packages. Informationabout the data procedures issometimes necessary for
processing needs and availability shouldbe selecting the most promising pathfora
collected before starting planning foran chemicalanalysis. Only reliable information
analysis. shouldbeusedfortheselectionofachemical
7. Instrumentation availability and expertise analysis,since uncertain informationcan be
in the laboratory are also factors that misleading and may produce unreliable
must be considered in planning a specific results.
analysis. For this reason, information must
be collected about what it is available.
Planning the Analysis
Information about the existent instruments
capability is important, for example, Once as much as possible information is
related to potential changing of sensitivity collected, the next step is the planning for the
of instruments. Potential purchase of new analysis. This includes (1) planning for sam-
instrumentation must be considered in pling(sometimesnotpossiblewhenthesample
specific cases. In addition to instruments, already has been collected), (2) selection of a
information should be collected about the specific core analytical procedure, (3) selection
materials available, indicating, for of a sample preparation plan, and (4) plans for
example, the need for preliminary data processing. As seen in this list, the plan-
purchases of chromatographic columns, ning for sample preparation is usually done af-
reagents, and standards. Related to the ter the selection of a specific core analytical
safety concerns, appropriate laboratory procedure, although the sample preparation
conditions must be assured for the operations are done before the core analysis.
analysis of certain samples. The expertise This is necessary since the sample preparation
available in the laboratory to perform the must be adjusted based on the requirements of
analysis is also an important factor, the core analytical procedure and not the other
including information about the need of way around. Sample preparation can be a
potential training. time-consuming step, requires well-trained
I.GENERALCONCEPTSINSAMPLEPREPARATION
8
1. PRELIMINARIESTOSAMPLEPREPARATION
operators, can contribute to the decrease of
1.2 THE EVALUATION
analytical precision, and therefore should be
OF ANALYTICAL DATA
utilized only when it is beneficial for the chro-
matographic analysis. For this reason, “mini-
General Aspects
mal” sample preparation appears more and
more attractive for the chromatographic prac- Sample preparation is performed before the
tice. The concept of minimal sample prepara- core analysis with the purpose to improve the
tion requires that sample preparation methods analysis results. This improvement requires
still utilized should be more efficient. somemeanstobeevaluated.Besidesdescriptors
The information collection and planning are such as the number of operations, higher
of informational nature and do not necessarily throughput for the laboratory, and lower cost
involve any real operation (unless preliminary of a sample preparation procedure, the quality
analyses are performed). The other steps of the analytical results must be evaluated and
(except the understanding of the meaning of potentially compared with other procedures
results) are of operational nature. The decision used for achieving the same goal. Criteria for
toperformsamplepreparationshouldbetaken the evaluation of the results from an analytical
after evaluating reported analytical methods method, as well as procedures for comparing
for the analytes of interest and the capabilities the results from different methods are further
in the laboratory where the analysis will be discussed in this section.
performed. In some cases, the decision of the Chemicalanalysismaybedesignedtoprovide
need for sample preparation or for modifica- qualitative information (e.g., compound nature,
tions to sample preparation may be taken structure,presenceoforganicfunctionalgroups)
only after attempts to perform the analysis orquantitativeinformationrelatedtotheamount
without sample preparation. More details ortheconcentrationofoneormorecomponents
regardingthecriteriaforthechoiceofaspecific in the sample. Both qualitative and quantitative
sample preparation are discussed further in informationmustbeaccurateandrepresentative
Section 2.6. Also, since sample preparation forthematerialanalyzed.Thecharacterizationof
may be time-consuming, and the addition of qualitative information depends on the purpose
steps along the analysis may represent sources of analysis and the procedure utilized to obtain
of additional errors, a compromise should be it. For quantitative information the results are
made in some instances between using for typically evaluated using statistical concepts.
analysis a very clean processed sample or a Thisispossiblebecauseonthesametypeofsam-
sample that just gives appropriate results. pleitiscommontoperformmorethanonemea-
Advancesinchromatographicinstrumentation, surement. The most common characteristics for
inparticularrelatedtotheenhancedseparation quantitativedataaretheprecisionandtheaccu-
capability of specific chromatographic col- racy. Precision is used to describe the reproduc-
umns,mayallowlessneedforsamplecleanup. ibility of the results obtained in identical
On the other hand, the increased requirements fashionandcanbedefinedastheagreementbe-
for higher sensitivity in chemical analyses may tweenthenumericalvaluesoftwoormoremea-
require more sample preparation. This book is surements. Accuracy denotes the nearness of the
dedicated to the discussion of sample prepara- resultstothetruevalueofthemeasuredquantity
tion procedures, while the presentation of (ifknown)ortoitsgenerallyacceptedvalue.The
sample collection, core analysis, and data pro- comparison of different analytical methods and
cessing are only tangentially discussed as implicitlyof differentsamplepreparationproce-
they are related to sample preparation. dures is frequently done using precision and
I.GENERALCONCEPTSINSAMPLEPREPARATION