determination of oh groups at plasma oxidised poly(propylene) by tfaa chemical derivatisation xps:...

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Determination of OH Groups at Plasma OxidisedPoly(propylene) by TFAA Chemical DerivatisationXPS: An Inter-laboratory Comparison

Thomas Gross,* Falko Pippig, Birgit Merz, Rolf Merz, Uwe Vohrer,Renate Mix, Hartmut Steffen, Wolfram Bremser, Wolfgang E. S. Unger

A first inter-laboratory comparison was conducted to demonstrate and document the capabilityof interested laboratories to measure the fraction of C�OH species on a plasma oxidisedpoly(propylene) sample by using a chemical derivatisation XPS approach. This report presentsthe results from that inter-laboratory comparison and includes the data received for themeasured values and their associated standard deviations of laboratory means, at a 95%confidence level. The measurements were guided by a protocol developed within the groupbefore and summarised shortly in the paper. Standard deviations thatmay characterise the state-of-the-art for the nominally simple and rather often practised case of TFAA chemical derivatisa-tion XPS of C�OH species on a plasma oxidised polyolefin surface are calculated following ISO5725-2:1994. The main conclusion is that the associated degree of equivalence reached by theparticipating laboratories in this comparison isstill low. Further research to improve chemicalderivatisation XPS protocols is mandatory.

T. Gross, W. E. S. UngerBAM Bundesanstalt fur Materialforschung und -prufung, VI.43‘‘Schicht- und Oberflachenanalytik’’, 12200 Berlin, GermanyE-mail: [email protected]. PippigFraunhofer-Institut fur Angewandte Polymerforschung (IAP),Wissenschaftspark Golm, Geiselbergstr. 69, 14476 Potsdam,GermanyB. Merz, R. MerzIFOS Institut fur Oberflachen- und Schichtanalytik GmbH,Trippstadter Straße 120, 67663 Kaiserslautern, GermanyU. VohrerFraunhofer-Institut fur Grenzflachen- und Bioverfahrenstechnik(IGB), Nobelstraße 12, 70569 Stuttgart, GermanyR. MixBAM Bundesanstalt fur Materialforschung und -prufung, VI.5‘‘Polymeroberflachen’’, 12200 Berlin, GermanyH. SteffenINP Greifswald e.V. (INP), Felix-Hausdorff-Str. 2, 17489 Greifswald,GermanyW. BremserBAM Bundesanstalt fur Materialforschung und -prufung, I.41‘‘Gasanalytik; Metrologie’’, 12200 Berlin, Germany

Plasma Process. Polym. 2010, 7, 494–503

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Introduction

Organic surfaces such as plasma modified polymers

frequently contain a variety of chemical species that are

indistinguishable to spectroscopic probes such as X-ray

photoelectron spectroscopy (XPS or ESCA). For instance, a

few elements, only C, H and O atoms, are present at the

surface of plasma oxidised polyolefins, but with many

combinations possible as carboxylic acid, ether, ester,

hydroxide, hydroperoxide, ketone, aldehyde, carbonate

groups and even more species. Those different groups

show specific and varied chemical reactivity. This feature is

exploited in chemical derivatisation XPS where a deriva-

tisation reaction is used to distinguish one species from the

co-existing others by an analysis of the reaction product

oftencarryinga labellingatom, for instanceF,notpresent in

the original surface composition.

Chemical derivatisation XPS has been found to be

successful in surface chemical analysis of polymers and a

considerably highnumber of examples had appeared in the

DOI: 10.1002/ppap.200900142

Determination of OH Groups at Plasma . . .

Scheme 1. Reaction of TFAA with OH functionalised C atoms.

literature over the last 30 years. One of the earliest papers is

byBriggs etal.[1]Anearly reviewonchemicalderivatisation

XPS had been published by Batich in 1988 where

derivatisation of C�OH species by exposure to trifluor-

oacetic anhydride, TFAA, is already considered.[2] Chemical

derivatisation XPS using awide range of reactions has been

thoroughly reviewed in a textbook by Briggs published in

1990.[3] Later on, in 1993, another comprehensive overview

has been published by Chilkoti and Ratner in a textbook

chapter.[4] Authors of ref.[2–4] already discussed rule sets for

proper application and limitations of the chemical deriva-

tisation XPS approach. However, despite the relative high

importance of the chemical derivatisation XPS approach,

neither inter-laboratory comparisons and international

standards nor studies dedicated to a metrological under-

pinning have been published so far.

The plasma community became interested to use

chemical derivatisation XPS in the 1990’s and an early

study of plasma oxidised poly(propylene) (PP) has been

published by Friedrich et al. in 1995.[5] In the mean time,

many papers have been published using the chemical

derivatisation XPS approach to determine surface concen-

trations of mostly but not exclusively C�OH and C�NH2

species at the surface of plasma modified polymers or

plasma deposited organic films. However, information on a

validation of a chemical derivatisation XPS approach

behind a study is scarcely found. So the state-of-the-art

of chemical derivatisation XPS is defined by in-house

protocols and nothing is known on how results from one

laboratory compare to those of another one. Furthermore,

there are some indications in the more recent literature

that, due to themore complex surface chemistry of plasma

functionalised samples, some of the derivatisation reac-

tions proved to be successful in chemical derivatisationXPS

of ‘‘classic’’ polymers do not work for those samples as

might be reasonably expected.[6,7]

The goal of the present inter-laboratory comparison

was to investigate the state-of-the-art of chemical

derivatisation XPS using TFAA for the determination of

the amount fraction of OH functionalised C atoms at

the surface of a plasma oxidised PP sample. In order to

reduce the variability of technical approaches across

the participating laboratories, a number of reasonable

constraints for the procedures to be applied had been

negotiated in the group before and distributed as a part

of the protocol of the comparison. The results obtained

for the measurements in different laboratories will be

displayed illustrating the degree of equivalence that can

be achieved today. Standard deviations calculated

following ISO 5725-2:1994[8] that may characterise the

state-of-the-art for the nominally simple and rather often

practisedcaseofTFAAchemicalderivatisationXPSofC�OH

species on a plasma oxidised polyolefin surface will be

presented.



Experimental Part

The fractionofOHfunctionalisedCatoms,xOH, onadividedplasma

oxidised PP test sample was determined in an inter-laboratory

comparison. To reach that goal, a chemical derivatisation reaction

of hydroxyl groupswith trifluoroacetic anhydride (TFAA) has been

used. Subsequently, the sample was analyzed by X-ray photoelec-

tron spectroscopy (XPS). This approach is known as chemical

derivatisation XPS. The derivatisation reaction of TFAA on plasma

oxidised PP is described by Scheme 1.

The measurand in this study is the amount fraction of OH

functionalised C atoms before TFAA labelling expressed as a

percentage, xOH, described by the equation:

xOH ¼ 100� nCOH=ðnCOH þ nCRÞ (1)

where nCOH is the amount of OH functionalised C atoms,

and nCR is the amount of all C atoms existing in bonds different

from C�OH.

The samples for the inter-laboratory comparison have been

prepared by BAM Division VI.5. In detail, an DIN A4 formatted PP

foil (Hoechst, Germany) was treated by an radio frequency

(13.56MHz) low pressure (p¼ 7.6 Pa) oxygen plasma at 100W

for 90 s. More details are given in ref.[9] Subsequently, the sample

was stirred in 12mL dry tetrahydrofuran (THF) and 3mL of 1M

diborane (Aldrich, Germany) solution under N2 atmosphere at

roomtemperaturefor18h.After this, the foilwasremovedfromthe

bath and immersed in an alkaline H2O2 solution of water and THF

for 2 h. Then the foilwaswashed inTHF, three times inwater and in

methanol thereafter. Finally, it was dried and stored formore than

five months to reach a status sufficiently stable for the use as a

test sample in the comparison. The wet chemical treatment was

applied to reduce a considerable number of C¼O and COOR species

(and eventually other oxidised carbons, too) formed by the

oxygen plasma treatment on the PP foil to C�OH species. It is

well known that plasma oxidised polymer surfaces suffer

from ageing phenomena. However, experimental data carefully

elaborated at BAMSubdivisionVI.5 revealed that the surface of the

PP foil treated and stored as describedabove is sufficiently stable in

terms of surface chemistry. Furthermore, low molecular weight

debris as produced during plasma modification are removed in

course of washing steps. The PP foil sample was divided into

�4�8 cm2 pieces (the test sample) and dispatched to the

participants.

The homogeneity of the divided test sample in terms of xOHwas carefully investigated by XPS. Results were evaluated

by appropriate statistical methods.[10] A relative inhomogeneity

of � 2.6% had been determined for xOH across the full DIN A4

foil surface.

www.plasma-polymers.org 495

T. Gross et al.

496

Selected Details of the Protocol of the Inter-laboratory

Comparison Given to the Participants

The protocol of the inter-laboratory comparison contained a

statement of the objectives, details of the material, sample

handling and details of issues to report including details of the

uncertainty calculation for 95% confidence. Derivatisation had to

be done in every participating lab immediately before analysis

with XPS. A gas phase TFAA derivatisation had to be applied. The

repeatability of measurement for a lab had to be determined by

repeating survey scanandhighly resolvedC1s spectra seven times,

each of themat a different position at the sample dispatchedwhen

possible.

Derivatisation Protocol

Apiece of the test samplehad tobe stored in a container that canbe

evacuated to �10mbar or even lower. Having reached that

pressure, the pumping line had to be closed. After that, the

containerwith the samplehad tobe linked to another line between

the container and a TFAA reservoir held at room temperature. By

doing this, the container was ventedwith TFAA vapor and the line

to the reservoir remained open. A test sample had to be exposed to

TFAA vapor for 15min. In a preliminary study by BAM VI.5, this

time of exposure had been found to be sufficient to reach

saturation. Subsequently, the TFAA line had to be disconnected

and the container connected to the pumping line again. The

container with the derivatised sample had to be pumped again

down top�10mbar andheld at this pressure for 30min in order to

remove non-reacted TFAA from the sample’s surface. Participants

were asked to use TFAA with purity better then 97%.

XPS Analysis and Data Evaluation

Using XPS two kinds of data sets had to be measured by the

participants. The first set called QEA (Quantitative Elemental

Analysis) had to be established by quantification of XPS survey

scans covering C 1s and F 1s photo peaks, the second one called PFA

(Peak FitAnalyses) by spectral interpretationof high resolutionC1s

spectra. The QEA data set had to be corrected for additional atoms

introduced by themarkermoiety obtained by the TFAA derivatisa-

tion reaction. Furthermore, information on used quantification

software, sensitivity factors, peak fit strategies and instrumental

parameters as excitation source, charge compensation, transmis-

sion function and times of exposure to X-rays had to be submitted

by participants.

Table 1. Participants of the inter-laboratory comparison. Numbering

Laboratory

1 Fraunhofer-Institut fur Angewandte Polymerforsch

2 Institut fur Oberflachen- und Schichtanalytik Gmb

3 Fraunhofer-Institut fur Grenzflachen- und Bioverfa

4 BAM Bundesanstalt fur Materialforschung und –p

5 BAM Bundesanstalt fur Materialforschung und –p

Oberflachenanalytik’’

6 Leibniz-Institut fur Plasmaforschung und Technolo



Participantswereaskedtodoseven independentmeasurements

and to evaluate means and standard deviations for QEA and PFA.

The Participants’ Measurement Procedures: Details

on Methods, Instrumentation and Selected

Experimental Details

Six laboratories submitted data (Table 1). All participants have

submitted results with means and standard deviations. Table 2

summarises information on the derivatisation procedures used by

participants. It is clear from Table 2 that the participants used

different derivatisation procedures in terms of size and material.

The base vacuum pressure achieved of those reaction vessels was

alsodifferent.Mostparticipants used15min for TFAAreactionas it

was requested in the protocol of the comparison. In difference to

that protocol, participant 5 tested a number of reaction times in

order to find the exposure time necessary for the derivatisation

reaction to be completed. Actually for the equipment, participant 5

used a longer reaction time, i.e., 30min TFAA exposure time, was

foundtobenecessary to reachsaturation (Figure1). All participants

evacuated the reaction vessel again for 30min after finishing

exposure to TFAA by closing the connection to the TFAA reservoir.

The transfer time from the derivatisation chamber to the vacuum

system of the spectrometer was approx. 10–15min in all cases.

Table3summarises informationonthe individualXPS instruments

used by participants. Experimental details as the X-ray excitation

they used, the X-ray source power, and whether or not they had

used charge neutralization for the XPS measurement are given.

Table 3 contains also information on the width of the binding

energy acquisition windows participants used for the measure-

mentofbothXPS survey scansandhigh resolutionXPSC1s spectra.

Finally, thedistancebetween theanalyzed spots on the test sample

theyused is specified.Another point of interest is that participant 5

characterised thephotonfluxat the sample in arbitraryunits using

a parallel sample current measurement with a grounded sputter-

cleaned silver sample of ca. 1 cm2. Though the X-ray intensity

distribution across the X-ray spot at the sample’s surface is

unknown, the irradiatedarea is surely smaller than the1 cm2of the

silver sample. The sample current determined in that way is

unequivocally correlated to the photon flux at the area viewed by

the spectrometer. XPS data displayed in Figure 2a–c are obtained

with the X-ray gun powered at 10mA, 15 kV providing a sample

current of 0.50 nA. Participant 5 considered X-ray degradation and

optimised acquisition times in order to minimise it. The time of

exposure to X-rays was reduced by setting the width of the BE

follows date of submission of data.

Identifier

ung, Potsdam-Golm IAP

H, Kaiserslautern IFOS

hrenstechnik, Stuttgart IGB

rufung, BAM VI.5 ‘‘Polymeroberflachen’’ BAM VI.5

rufung, BAM VI.43 ‘‘Schicht- und BAM VI.43

gie (INP Greifswald e.V.), Greifswald INP

DOI: 10.1002/ppap.200900142


Table 2. Summary of derivatisation procedures used by participants.

Participant Reaction vessel Pressure in

reaction vessel

before

reaction

with TFAA

Reaction

time

with

TFAA

Pumping

down

time after

completed

reaction

Transfer time

from the

derivatisation

chamber to the

vacuum system of

the spectrometer

Pa min min min

1 stainless steel chamber (0.75 L) 10 15 30 �15

2 glass desiccator < 1 000 15 30 �15

3 glass chamber 170 15 30 �10

4 glass desiccator �1000 15 30 �15

5 stainless steel chamber (approx. 0.3 L)a) < 100 30b) 30 �15

6 stainless steel chamber (15 L) 230 15 30 �15

a)Test sample was divided into two pieces and derivatisation was run in two batches; b)Determined by preceding saturation experiment

(Figure 1).

window used for survey scans for QEA to 250–720 eV resulting in

288 s acquisition time. The PFA C1s spectrum acquisition time

was optimised using a dwell time of 140 s. Table 4 summarises

information on the software used by the participants, the

background subtractionmethod for QEA, the C 1s and F 1s relative

sensitivity factors (RSF) for QEA, the background subtraction

method for PFA, and the peak shape models for PFA.

To provide an example the QEA and PFA procedures used by

participant5are explained in somemoredetail. TheQEAprocedure

was carried out with the help of the CasaXPS software, version

2.3.12, by using linear backgrounds and RSFs taken from theKratos

element library. The QEA data set was corrected for additional

atoms introduced by the marker moiety originating from TFAA

derivatisation. Because the test sample’s surface contains both OH

functionalisedandotheroxygen functionalisedCatomsaswell the

reaction Scheme 1 needs to be modified to Scheme 2.

Consequently, participant 5 determined the value for y by:

Schbot

Plasma

� 2010

y ¼ 1=3 � IF1s=RSFF1s (2)

where IF1s is the XPS intensity of F1s determined from the survey

scan after TFAA derivatisation (Figure 2a), and RSFF1s is the relative

sensitivity factor of F1s (Table 4), and the value for w by

w ¼ IC1s=RSFC1s � 2y (3)

where IC1s is the XPS intensity of C1s determined from the same

survey scan (Figure 2a), and RSFC1s is the relative sensitivity factor

of C1s (Table 4).

eme 2. Reaction of TFAA with test sample surface containingh OH functionalised and other oxygen functionalised C atoms.

Process. Polym. 2010, 7, 494–503

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Because

Figurea

xOH ¼ 100� y=w (4)

using Equations (2) and (3) the measurand xOH was finally

calculated as:

xOH ¼ 100�IF1s=RSFF1s3�IC1s=RSFC1s � 2�IF1s=RSFF1s

(5)

The PFA procedure was also carried out using the CasaXPS

software. A Shirley backgroundwas removed in a BEwindow from

282.6 to 295.1 eV. Six Gaussian/Lorentzian product function

components (Figure 2b) were used for fitting of the C 1s spectrum

of the derivatised sample where the CHx BE was constrained to

285.0 eV, the BEs of the components C�OR, C¼O and COOR were

re 1. XPS atomic fractions of O, C and F as a function of TFAAction time obtained by participant 5.


T. Gross et al.

Table 3. Summary of XPS analysis conditions used by participants

Participant Instrument X-ray

Excitationa)

Charge

Neutralization

XPS survey

scan

High resolution

XPS C1s spectra

Distance

between

the analyzed

spots on

the test

sample

X-ray

source

power

BE

acquisition

window

X-ray

exposure

time

X-ray

source

power

BE

acquisition

window

X-ray

exposure

time

eV s eV s mm

1 Kratos

AXIS165

Almono Yes 300W

(15 kV, 20mA)

0–1 200 270 300W

(15 kV, 20mA)

282–296 440 �9

2 Kratos

NOVA

Almono Yes 75W

(10 kV, 7.5mA)

0–1 100 900 150W

(15 kV, 10mA)

280.5–300.5 900 10

3 Kratos AXIS

Ultra DLD

Almono Yes 130W

(10 kV, 13mA)

0–1 350 780 130W

(10 kV, 13mA)

279–302.4 960 �8-9

4 SPECS

SAGE 100

Mg No 199R/�1W

(11 kV, 18mA)

�5–800 240 220W

(11 kV, 20mA)

271–310 400 �10

5 Kratos AXIS

Ultra DLD

Almono Yes 150W

(15 kV, 10mA)

250–720 288 150W

(15 kV, 10mA)

281–303 140 �12

6 Kratos

AXIS Ultra

Almono Yes 150W

(15 kV, 10mA)

0–1 200 180 225W

(15 kV, 15mA)

279–302.1 120 3

a)Almono, monochromated Al X-rays; Mg, unmonochromated Mg X-rays.

498

constrained to 286.0–286.5 eV, 287.0–287.8 eV, and 288.8–289.2 eV

BEwindows, respectively. The areas of the two fluorine containing

componentsCF3�COandCF3�COwere constrained tobeequal and

their BEs were unconstrained. Finally, a highly resolved F1s

spectrumwasacquired (Figure2c) revealingonlyonecomponentat

689.0 eV BE consistent with F in a CF3�CO moiety.

Applying the PFA procedure, the measurand xOH was deter-

mined by the equation:

Plasma

� 2010

xOH ¼ 100� ICF3�CO=ðICHx þ IC�OR þ IC¼O þ ICOORÞ (6)

where Ii are the intensities of CHx, C�OR, C¼O, COOR and CF3�CO

components determined by peak fitting the highly resolved C 1s

spectrum displayed in Figure 2b.

Results and Discussion

The data as they were submitted for the measurement of

xOH by the participants are summarised in Table 5. The

results of statistical evaluation of QEA and PFA data in

Table 5 obtained following the protocol of the inter-

laboratory comparison are displayed in Figures 3 and 4.

Inspection of Figures 3 and 4 reveals a dominant scatter of

data between the participating laboratories for both data

sets,QEAandPFA. Thereare systematic deviationsbetween

Process. Polym. 2010, 7, 494–503

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the labs. This is not unexpected and inevitable for the

implementation of such complex procedures in different

laboratories. No consolidation level is observed, which is

usually to be expected in close vicinity of the consensus

value. Furthermore, these results suggest systematic

deviations between results obtained by both the QEA

and PFA procedures. In order to separate the influences of

two factors, namely the impact of the different imple-

mentations of QEA or PFA in the participating labs and the

different procedures QEA and PFA a two-factorial analysis

of variance (ANOVA) has been carried out revealing high

significance of both factors. The conclusion is that, at the

given state, there is no compatibility between results

neither for procedures QEA and PFA nor for an individual

implementation of such a method in different labs.

Moreover, the Youden plot displayed in Figure 5 reveals

similar and significant differences for results obtained by

both procedures QEA and PFA across all participating labs.

Results of compatible analytical procedures would occur in

a Youdenplot as scattered closely around the diagonal y¼ x

line. This is not true in the given comparison. The PFA

procedure consistently results in lower (by�33%) xOH data

as QEA.

Following ISO 5725-2,[8] the mean of laboratory means,

the repeatability standard deviation sr, the between-

DOI: 10.1002/ppap.200900142


Figure 2. a) XPS spectrum measured by participant 5 after 30 minTFAA derivatisation of the test sample (survey spectrum used forQEA); b) XPS spectrum measured by participant 5 after 30 minTFAA derivatisation of the test sample (highly resolved C1sspectrum used for PFA with six components); c) XPS spectrummeasured by participant 5 after 30 min TFAA derivatisation of thetest sample (highly resolved F1s spectrum).



laboratory standard deviation sL and the reproducibility

standard deviation sR were calculated for all data sets

(Table 6). The evaluation of inter-laboratory comparisons

according to ISO 5725-2 relies on, and requires approxi-

matelyGaussiandistributeddatasetswithoutoutliers. This

has been successfully proved for this comparison by

searching for outliers using Dixon, Grubbs and Nalimov

tests (for absence of outliers) and a Kolmogorov-Smirnov

test for the verification of an approximate Gaussian

distribution, including skewness and kurtosis tests. The

repeatability standard deviation sr represents the scatter of

measurement results under repeatability conditions (same

laboratory, same instrument, and same operator). The

between-laboratory standard deviation sL characterises the

variability of results between laboratories. The reproduci-

bility standard deviation sR covers both components of

scatter mentioned before. It characterises the scatter of

measurement results under reproducibility conditions

(different laboratories, different instruments, and different

operators). In the given case, sR is rather high, in relative

terms in the order of 33%, and being rather similar for both

procedures of XPS analysis, QEA and PFA. This high level of

sR points to the fact that comparable results have not been

submitted by the participating laboratories. The relative

repeatability standard deviation sr is higher for the PFA

procedure pointing to additional sources of uncertainty

that will be related to the peak fit analysis itself. A

contribution of the test sample’s inhomogeneity (deter-

mined to be� 0.026 � xOH) can be neglected because it is far

below 1/3 of sR.[11]

Discussion

In the inter-laboratory comparison presented in this paper,

the relative reproducibility standard deviation sR has been

found to be as high as 33% for both procedures (QEA and

PFA) used to determine xOH by chemical derivatisation XPS

using TFAA. The conclusion is that the associated degree of

equivalence reached by the participating laboratories in

this comparison is low.Therearedifferentpotential reasons

for this observation.

i) T
he stability of the test sample is insufficient. In
contradiction to experiences on stability of this kind of

plasma modified PP samples acquired in BAM VI.5

(6month should be sufficient to stabilise the surface), it

has been discussed in the group that the test sample

might have lost OH-groups during storage and

shipment. However, careful inspection of data sets

submitted by participating labs and their dates of

measurement revealed that IAP, IFOS, IGB and BAM

VI.43 did their measurements within only 4 d but their

resulting xOH means varied between 3.6%–5.7% and


T. Gross et al.

Table 4. Summary of QEA and PFA procedures used by participants

Participant Software QEA PFA

Background-

subtraction

method

RSFa) for

C 1s

RSFa) for

F 1s

Background-

subtraction

method

Peak

funktionb)

1 Kratosc) Vision 2 Linear 0.278 1.000 Linear G

2 CasaXPSd)

Version 2.3.13 U Poly Tougaard 0.278 1.000 U Poly Tougaard GL

3 Kratosc) Vision 2.2.7 Beta Linear 0.278 1.000 Linear GL

4 Specslabe) Linear 1.00 4.26 Linear GL

5 CasaXPSd)

Version 2.3.12 Linear 0.278 1.000 Shirley GL

6 CasaXPSd) Version 2.3.14dev29 Linear 0.278 1.000 Linear GL

a)Relative sensitivity factor;b)G, Gauss function; GL, Gauss – Lorentz function;c)Kratos Analytical Ltd (http://surface.kratos.com/);d)Casa

Software Ltd (http://www.casaxps.com/);e)SPECS GmbH (http://www.specs.de/).

500Plasm

� 20

5.6%–8.6% applying procedures PFA and QEA, respec-

tively. Therefore, the stability of test sample cannot be

the reason for the observed scatter.

ii) T
he derivatisation procedures established in the
different labs lacks full control. Most probably satura-

tion has not been reached in every participating lab.

Figure 1 suggests that using the derivatisation

procedure of participant 5 saturation was definitely

not reached after 15min of derivatisation reaction

with TFAA as stated in the protocol of the comparison

but after 30min. By using this enhanced derivatisation

time, participant 5 obtained the highest values for xOH.

iii) T
he derivatised sample may undergo beam damage in
terms of losses of F and CF3-species during XPS

analysis. Some of the participating laboratories sub-

mitted data on beam damage observed with the

derivatised test sample. Participant 5 measured the

beam damage for xOH when determined by the QEA

procedure. Decay for the measurand xOH from 9.7%

after 308 s X-ray exposure to 7.9% after 1 492 s was

measured. Participant 2 reported a decay of xOHdetermined with the PFA procedure by 23% after

45min exposure compared to the xOH value deter-

mined after 15min. Participant 3 determined a xOHdecay rate of 0.02%/min for both the QEA and PFA

procedure assuming linear degradation vs. time of

exposure. Beam damage is probably rather different at

the participating laboratories and had not been

corrected for. Recently, a dedicated degradation study

revealed an option to control the X-ray flux on a

sample in XPS in terms of a sample current measured

on a silver sample at the same conditions of

a Process. Polym. 2010, 7, 494–503

10 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

analysis.[12] Based on this approach or, alternatively,

by using an appropriate polymer test sample a

standardised procedure should be developed to enable

a coverage of X-ray degradation issues in chemical

derivatisation XPS protocols.

Finally, the systematicdifferencesobserved forxOHwhen

determined by either QEA or PFA procedures has to be

considered. xOH by PFA has been found to be regularly only

at a level of �66% of the QEA value. This result might be

understood under the assumption of a rather thin layer of

material containing sites chemically reactive to TFAA. This

assumption is reasonable because the post-plasma treat-

ments of the test sample may result in an oxidised layer

with a thickness of only a few nanometres, which is in the

order or evenbelow the informationdepthof themethod in

the end. An alternative explanation could be that there

could be some hindrance of diffusion of TFAA molecules

down to a depth from which XPS information is obtained

within the reaction time. Since the reaction of TFAA with

C�OHisexpected toproceed rapidly, the conversion runsas

a front into thesurface.As longas this front is in the rangeof

the information depth a fraction of the material is not

converted.[13] Also, in that case the sample will be inhomo-

geneous in depth and the simple model used for QEA data

evaluation is not applicable. The quantification software

used by the participants in this inter-laboratory compari-

son is limited to the case of a homogeneous and isotropic

sample. Because the EAL of photoelectrons depends on

kinetic energy the F 1s EAL (kinetic energy KE �798 eV at

Al Ka excitation) is lower than that for C 1s (KE�1202 eV).

Therefore, the fraction of TFAA derivatisation products is

DOI: 10.1002/ppap.200900142


Table 5. Single measurement (Meas.) data as submitted by participants for the measurand xOH.

Procedure QEA

Laboratory Measurement

1

Measurement

2

Measurement

3

Measurement

4

Measurement

5

Measurement

6

Measurement

7

1 5.7 5.6 5.8 5.7 5.6 5.6 5.2

2 6.83 6.65 6.52 6.64 6.83 6.21 6.12

3 6.99 7.01 6.97 7.10 6.93 7.26 7.52

4 3.86 4.17 3.70 3.99 4.06 4.51 3.94

5 9.9 7.8 9.7 8.0 8.1 8.6 7.8

6 3.70 3.40 3.60 3.58 3.57 3.30 3.41

Procedure PFA

Laboratory Measurement

1

Measurement

2

Measurement

3

Measurement

4

Measurement

5

Measurement

6

Measurement

7

1 3.65 3.60 3.62 3.63 3.63 3.67 3.65

2 4.16 4.67 4.65 4.32 3.86 3.91 4.01

3 5.35 5.16 5.50 5.14 5.37 5.20 5.51

4 3.73 3.84 2.56 3.09 2.46 3.63 3.09

5 6.4 5.4 5.5 6.4 6.2 5.2 4.8

6 2.88 2.69 2.53 1.56 2.53 1.71 2.04

larger in the depth range sampled by F 1s photoemission

and xOH subsequently evaluated by the simple QEA pro-

ceduremight be overestimated. If the derivatisation occurs

in the topmonolayer, the analysis byQEA is over-estimated

roughly by a factor (1 202/798)0.8¼ 1.39. Using corrected

QEA data all results summarised in Table 5would fit rather

well the line with slope 1 in Figure 5. This result appears to

Figure 3. Laboratory means for xOH with expanded (k¼ 2) stan-dard deviations (diamonds with bars) and mean of the laboratorymeans (bold solid line) with its expanded (k¼ 2) standard devi-ation (dashed line) obtained after statistical evaluation of Quan-titative Elemental Analysis (QEA) data derived from XPS surveyscans covering C 1s and F 1s photo peaks.



confirm again that the functionalised atoms, and therefore

the �OH groups, are in the top-most atom layer of the test

sample. Onemay suspect that by using amore appropriate

quantification model taking an inhomogeneity of the test

sample correctly into account, the PFA andQEA approaches

might be found to be consistent within the measurement

and other uncertainties.

Figure 4. Laboratory means for xOH with expanded (k¼ 2) stan-dard deviations (diamonds with bars) and mean of the laboratorymeans (bold solid line) with its expanded (k¼ 2) standard devi-ation (dashed line) obtained after statistical evaluation of Peak FitAnalyses (PFA) data for highly resolved C 1s spectra.


T. Gross et al.

Figure 5. Youden-Plot for xOH data obtained by either QEA (Quan-titative Elemental Analysis by XPS) or PFA (Peak Fit Analyses ofhighly resolved XP C1s XP-spectra) in different labs. Darkdiamonds: experimental data points, grey line: regression graph,grey diamond: centre of gravity of data.

502

Using the PFA procedure to determine xOH for the test

sample that isassumedtobe inhomogeneous indepth, i.e., a

thin –OH functionalised layer on PP, C 1s components from

the non-oxidised PP substrate (which is not reactive to

TFAA) may substantially contribute to the C 1s spectrum.

Therefore, intensities of ICF3�CO and ICF3�CO C 1s compo-

nents characteristic for the derivatised layer on PPmight be

underestimated. Consequently a related xOH will be

underestimated, too.

The indications that a ‘‘thin overlayer model’’[14] is more

appropriate for the test sample suggests that the surpris-

ingly low values for the measurand obtained in the

comparison are due to the use of an oversimplified

quantification model. If we were able to establish a more

appropriate quantification model that takes into account

the existence of the OH-groups in a fraction of amonolayer

(which is another measurand as xOH defined in Equation 1)

much higher values would be the result. An increase by a

factor of ten is reasonable and those resultswould fit better

the expectation of readers concerned with, e.g., improve-

ments of wettability of polymers. All these difficulties

discussed here suggest that the test sample used in this

Table 6. Evaluation of QEA and PFA data sets following ISO 5725-2.Numbers are given in units of the amount fraction of OHfunctionalised C atoms before TFAA labelling expressed as apercentage, xOH.

Method Total mean sr sL sR

QEA 5.89 0.42 1.90 1.95

PFA 4.06 0.43 1.29 1.36



comparison was not fully optimal. For future comparisons

related to chemical derivatisation XPS, we have to find test

samples that definitely do not suffer from such in-depth

inhomogeneities at the scale of the information depth of

XPS. Recently, similar discrepancies of chemical derivatisa-

tion XPS results have been reported in a kinetic study of

TFAAderivatisationofPVAandPHEMAusing thesameQEA

and PFA procedures. They were also discussed in terms of

different F 1s and C 1s photoelectron EALs and samples

inhomogeneous in depth at the EAL scale.[13]

Another theoretical explanation for the differences

observed for xOH data measured with QEA and PFA could

that the relative sensitivity factors were incorrect and/or

the transmission function of the instrument not corrected

for. Some participants using KRATOS RSFs checked these

sensitivity factors by using an undefined Teflon sample

withresult that thedeviation ismaximum�10%.This isnot

enough to explain the difference of �33% for QEA and PFA

procedures. Transmission functions have been regularly

checked by the participants to be appropriate using

different test procedure.

Conclusion

It must be stated that the associated degree of equivalence

reached by the participating laboratories in this compar-

ison is low. Most obviously further action is necessary to

establish a validated derivatisation procedure, which can

easily be controlled in each lab. Attention should be paid to

a scrupulous determination of the exposure time required

for full derivatisation of analyzed surface of a sample. To

reach consistent results across different laboratories it

seems to be very useful to standardise the derivatisation

protocol including the hardware used more in detail.

Another conclusion is that for a validated chemical

derivatisation XPS protocol control and even correction of

X-ray degradation is mandatory. The quality of spectra

defined by acquisition times must be compromised to

minimise X-ray degradation. It can be concluded that a test

sample that seems to bemore correctly described by a ‘‘thin

overlayer model’’ is, at the given state-of-the-art, not

appropriate for an inter-laboratory comparison for testing

chemical derivatisation XPS. For the next comparison we

have to find test samples that really fit the simple

quantification model and are homogenous and isotropic

within the information depth of XPS. This should be

checked by the leading laboratory by using XPS (angle

resolved mode or by variable excitation energy) or by C60

sputter depth profiling before the next inter-laboratory

comparison is initiated. Another requirement for the

sample is that TFAA is able to reach all OH-sites across

this layer, i.e., limitations for TFAA diffusion must be ruled

out. Finally, in future inter-laboratory studiesusingfluorine

DOI: 10.1002/ppap.200900142


markers a reference sample of amaterial containingCandF

(e.g., PTFE) should be sent to participants to establish that

they can determine the correct composition for a bulk

homogenous sample.

Acknowledgements: The authors thank Prof. J. F. Friedrich of BAM,Dr. A. Hollander of Fraunhofer IAP and Dr. K. Schroder of INP forsupport and collaboration in the AK Plasma. Technical support byProf. H. Sitzmann and coworkers at Technische UniversitatKaiserslautern, J. Mayer of Fraunhofer IGB, E. Yegen andH. Hidde of BAM, and R. Ihrke of INP is gratefully acknowledged.

Received: September 10, 2009; Revised: January 7, 2009;Accepted: January 10, 2009; DOI: 10.1002/ppap.200900142

Keywords: chemical derivatisation; ESCA/XPS; inter-laboratorycomparison; photoelectron spectroscopy (PES); poly(propylene)(PP)

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[10] ISO Guide 35:2006, Reference materials - General and stat-istical principles for certification. ISO, Geneva.

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determination of oh groups at plasma oxidised poly(propylene) by tfaa chemical derivatisation xps:...

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