Toward on-chip X-ray analysis

Toward on-chip X-ray analysis

Eduardo D. Greaves*ac and Andreas Manzbc

Received 14th October 2004, Accepted 7th February 2005

First published as an Advance Article on the web 18th February 2005

DOI: 10.1039/b415836a

The possibility of performing chemical analysis and structure determinations with the use of

X-rays in a microfluidic chip environment is explored. Externally generated radiation,

radioisotope irradiation and on-chip generated X-rays were considered as excitation means for the

performance of sample analysis with the techniques of X-ray fluorescence and diffraction. The

absorption properties of chip-building materials by different radiation sources are reviewed and

data on absorption coefficients calculated, upon which recommendations for optimisations with

the use of various X-ray sources may be made. The capabilities and limitations of on-chip X-ray

analysis are placed in perspective by preliminary experimental results of diffraction, fluorescence

and on-chip X-ray generation experiments.

1. Introduction

Microfluidic chips or micro total analysis systems (mTAS) have

shown powerful advantages of miniaturisation for the perfor-

mance of analytical tasks.1–3 A variety of physico-chemical

phenomena such as synthesis, component separation, detec-

tion and measurement have been adapted to on-chip pro-

cedures, leading to process improvement. Moreover, diverse,

well-established analytical techniques have been used as

sensors or transducers of chip behaviour; however, the power-

ful measurement techniques of X-ray fluorescence (XRF) for

elemental analysis4,5 or of X-ray diffraction6 (XRD) for

structure determination or compound identification have not

been reported for this application. There are three possibilities

which can be envisaged for on-chip X-ray sample excitation. In

the first, an external, conventional X-ray generator could be

used to generate a suitably collimated beam in order to strike a

micro-channel containing the analytes. A second possibility is

to use a radioisotope source incorporated inside the chip for

excitation, and the third one is to generate the excitation

X-rays locally on the chip. Both analytical techniques, XRF

and XRD, could be useful for on-chip analysis. They have in

common the use of X-rays for probing material properties, but

they have very different source characteristics, experimental

geometries and instrumental requirements. An XRF signal

from a microfluidic channel might provide elemental informa-

tion on inorganic solutions or metalloprotein-bearing fluids.

An XRD signal from gaseous or liquid suspensions of

particulate solid samples might provide information for

particle identification, separation or extent of mixing of solid

phases in the mm range of particle sizes. Likewise the

appropriate conditions for protein crystallization might be

detected by a sensitive, real-time, XRD signal ‘‘on-chip’’

instead of the ‘‘post-process’’ detection recently reported.7

The most straightforward technique for sample excitation is

the use of an external X-ray generator.8 The traditional heavy

and bulky X-ray generator requiring an attached cooling water

facility seems to be inadequate for the miniaturisation process

afforded by chip-based processes. However, this seems a

natural forward development and recently reported work

shows the potential of the method.7 Recent advances have

resulted in the availability of very compact X-ray sources.9–11

They rely on several developments: (i) the phase out of the

heavy transformer-based high voltage power supply giving

way to compact switching high voltage power supplies; (ii)

the use of grounded-cathode X-ray tubes which imposes

less stringent requirements on the tube’s power supply; (iii)

advances in the production of low-power tubes, which are air-

cooled; (iv) production of point-source tubes with X-ray spot

sizes in the 50–250 mm range; and (v) the use of poly-capillary

focusing optics or mono-capillary tubes. These developments

have led to the availability of small portable systems,12,13

which are capable of delivering an intense beam of X-rays

confined to a small spot size14,15 of hundreds or tens of mm,

eminently appropriate for use in a mTAS chip.

For the second possibility miniature radioactive sources can

be fashioned with high specific activities in the shape of needles

or ‘‘point’’ geometries compatible with this application and

incorporated in a chip so as to efficiently irradiate the sample

channel. For instance, 109Cd sources are very convenient

quasi-monochromatic sources with emission lines of 22.2 and

24.9 keV produced by the silver Ka and Kb X-ray lines. These

sources are used for analysis by exciting, with reasonably high

efficiency, the K lines of elements beginning approximately at

atomic number 14 (silicon), up through the transition elements

to atomic number 44 (ruthenium) and, by exciting the L lines,

all the other heavier elements with somewhat reduced

sensitivity. The isotope activity is usually electrodeposited on

discs or on annular shaped plates of silver to produce the

commercial sources used for XRF analysis. Electrodeposition

of 109Cd on the tip of a thin silver wire could be used

to produce a very small excitation source appropriate for an

on-chip analyser. The thin wire source could be placed in an

appropriate channel in a chip and excite a sample fluid.

The third analysis possibility is to produce the X-rays

directly on the chip. This seems an improbable task as X-rays

are generated by the deceleration of an electron beam, with

energies of tens of keV, on to a metallic anode material, thus*[email protected]

PAPER www.rsc.org/loc | Lab on a Chip

382 | Lab Chip, 2005, 5, 382–391 This journal is � The Royal Society of Chemistry 2005

requiring the application of a high voltage on the chip. The

production of the electron beam usually requires a heated

filament cathode and the process must be carried out under

high vacuum conditions to prevent filament burnout and to

avoid a plasma discharge. Moreover, the low efficiency of the

X-ray generating process (y1%) requires a means of dissipat-

ing the majority of the energy expended as heat to effect

photon emission. However, advantageous scaling laws may

result in very small energy dissipation requirements if the

radiation source is on the chip at sufficient proximity to

the target sample. Hence, cold cathode emission, which is the

basis of a recently available, commercial X-ray source,16 may

suffice. Relatively high voltages have been applied on a chip to

produce gas discharges for optical spectroscopy applications17

and recent developments indicate that X-ray emission can take

place in a gas at low pressure,18 allowing the construction of a

miniature generator that uses less than 300 mW power.19

In this paper we explore the three excitation possibilities

mentioned above and show preliminary experimental results

that give an indication of the potential of these alternatives.

2. Theoretical

X-rays scattered from materials contain the complete struc-

tural and compositional information of the material. The

interaction is two-fold: either the inelastic, incoherent interac-

tion which produces the Compton and photoelectric effects,

the latter of which leads to X-ray fluorescence (XRF), which

provides the chemical composition, or an elastic, coherent

interaction which leads to diffraction phenomena and provides

determination of structural information or the degree of

crystallinity if long range order exists. All elements in the

periodic table can, in principle, be detected by XRF; however,

the low value of the energy of the X-rays emitted by the lighter

elements generally precludes, with present technology, detec-

tion of the first 10–12 lightest elements.20 This limitation

prevents the elemental analysis of organic material, save the

cases in which the analyte compound contains some heavy

metal or transition element in its structure. The coherently

scattered radiation contains the structural information in two

different modes: (i) in the spatial anisotropy of the scattered

radiation field or (ii) in the energy anisotropy of the scattered

radiation field. These two modes lead to different instrumenta-

tion, both of which can be used to determine crystal structure.

The most common methods of structural analysis make use of

monochromatic incident radiation and the spatial anisotropy

of the scattered radiation. This is the basis of the majority of

standard X-ray diffraction (XRD) equipment. The use of a

polychromatic source and of the energy anisotropy of the

coherently scattered radiation allows the so-called energy-

dispersive X-ray diffraction method (EDXRD).6

X-ray generation

X-rays generated by electrons striking matter are isotropic.

They propagate in free space, decreasing in intensity according

to the inverse square of the distance. Hence, well known,

significant intensity enhancement factors of the order of

(Ro/r)2 are obtained when the source–sample distance of

macroscopic dimensions, Ro, is reduced to microscopic, r,

distances. If the usual centimetre distances in conventional

X-ray analysis devices are reduced between source and sample

to typical distances on the scale of microfluidic chips, of the

order of tens or hundreds of micrometers, the expected

intensity enhancement is between three and four orders of

magnitude. These advantageous scaling factors suggest the

possibility of offsetting the necessarily low power of on-chip

X-ray generation for XRF analysis applications. Furthermore,

the use of specially designed radioactive excitation sources

with appropriately minute dimensions, such as micrometer

diameter needles of high specific surface activity, may lead to

useful sensitivities for element identification in reaction or

process diagnosis.

X-ray absorption

In order to carry out any form of X-ray analysis on a chip the

exciting radiation has to enter through the chip construction

material in order to reach and interact with the analyte in a

sample micro-well or micro-channel. Likewise, the scattered

radiation, which contains the sought-after analytical informa-

tion, has to leave the sample, traverse the construction

material, and go out of the chip in order to reach the detector.

Radiation absorption considerations play a decisive rolel in the

choice of the radiation to be used, in the geometry and in the

materials for on-chip X-ray analysis. Hence, it is important to

consider carefully the radiation absorption process in order to

minimise its adverse effects.

Upon traversing a linear distance x inside matter the

radiation intensity decreases in an exponential fashion, given

by the well-known relation (ref. 5, p. 16):

I 5 Io exp 2 [m x] (1)

where the incident intensity Io (photons cm22) is reduced to the

emerging intensity I, and m is the total linear absorption

coefficient for the material. m itself is a sensitive function of

several factors, among which these are important: the

radiation energy, the atomic weights of the elements contained

and the density of the absorbing material. For a compound

with several elements i of fractional concentration Ci in the

material the relation becomes:

I 5 Io (Si Ci exp 2 [mi x]) (2)

where mi is the absorption coefficient of element i, which is a

function of the radiation energy. The dependence of the

absorption coefficient m, for a given material composition and

density, is also an approximately decreasing exponential

function of the energy. The measured absorption coefficients21

are well described by log m versus log E curves that are

approximately linear, save for sharp discontinuities at the

X-ray absorption edges of the elements contained in the

absorbing material. Table 1 shows the relevant properties

for the several well-established chip-building materials. Fig. 1

shows the results of calculating the absorption coefficient for

the materials in the table. Calculations were made with the

‘‘utilities’’ provided in the QXAS-AXIL software package.22

This figure shows that borosilicate glass has the highest

absorption coefficient. PDMS is very similar on account of the

This journal is � The Royal Society of Chemistry 2005 Lab Chip, 2005, 5, 382–391 | 383

very similar silicon concentration (37%). Both of the plastic

materials have approximately identical absorption properties.

The calculation shows that above the silicon absorption edge

(1.838 keV) the polycarbonate plastic materials have, almost,

an order of magnitude lower absorption coefficients than glass

or PDMS.

Fig. 1 also shows the importance of the energy dependence

of the absorption coefficient. In the energy region of interest

this variable spans 3 orders of magnitude. If only this property

is taken into consideration, it is desirable to use the highest

possible energies for the excitation radiation and for the

outgoing radiation. Hence, for X-ray diffraction experiments

seeking structural information by the use of monochromatic

incident radiation, the use of a molybdenum anode X-ray tube

(with an emission energy of 17.48 keV) is much more desirable,

by one order of magnitude, than the use of the usual copper

anode tube (emission energy of 8.05 keV) or indeed the cobalt

(emission energy of 6.93 keV) or chromium (emission energy of

5.41 keV) anode X-ray tubes. Table 2 shows the wide range of

values which are obtained for the mass absorption coefficient

of various chip building materials at the characteristic energy

of the most common diffraction X-ray tubes.

Likewise, for energy-dispersive X-ray diffraction (EDXRD)

experiments that require sample irradiation with white light,

the use of particularly high voltage for excitation of the

continuous bremstrahlung is highly desirable. Also desirable in

this case, as discussed below, is the use of a low fixed angle

geometry for the energy-dispersive detector used in the

experiment. This leads to diffraction peaks located in the

high-energy region of the spectrum that will be much less

affected by material absorption.

In order to discuss geometrical effects let us consider Fig. 2,

where a path is shown for the incident radiation and for the

scattered radiation about a sample channel inside a chip. Chips

are three-dimensional, but generally exhibit a high aspect ratio.

Incident radiation and scattered radiation clearly have

minimum paths in the material at directions that are normal

to the plane of the chip or at small angles relative to the normal

direction. For a chip thickness t the length l of the path of the

radiation propagating at an angle h, measured from the plane

of the chip, is given by: l 5 t/sinh. This is a function that

rapidly increases for small angles, as shown in Fig. 2.

Using the data of the absorption coefficients as a function

of energy and eqn. (2), the X-ray transmission factor (I/Io)

was calculated for several chip-building materials, namely

borosilicate glass, polydimethylsiloxane (PDMS), polymethyl-

methacrylate (PMMA) and polycarbonate of bisphenol A. The

X-ray transmission factor was calculated at the characteristic

energy of the four most common diffraction X-ray tubes

Table 1 Some properties of chip-building materials

Material Formula %Density/g cm23

Borosilicate glass Si O2 80.5 2.23B2O2 12.9Na2O 3.8K2O 0.4Al2O3 2.2

Polydimethylsiloxane PDMS Si61O60C124H368 100 0.92Polymethyl methacrylate

PMMA (Plexiglas, Lucite)C5H8O 100 0.94

Polycarbonate of BisphenolA (Lexan)

O3 C16 H22 100 1

Fig. 1 Absorption coefficients as a function of X-ray energy (keV) for

several chip-building materials. The arrows indicate the characteristic

radiation energies of the most common X-ray tubes.

Table 2 Mass absorption coefficients calculated for chip-buildingmaterials at the Ka emission energy of the most common X-ray tubes

X-ray tubeanodematerial

X-rayenergy/keV

Mass absorption coefficient/cm g21

BS glass PDMS PMMA Polycarbonate

Mo 17.478 3.03 2.4 0.46 0.459Cu 8.047 30 24.4 4.98 4.98Co 6.93 47.7 38.1 7.88 7.89Cr 5.414 95.8 78.8 16.8 16.8

Fig. 2 The length of the path that radiation transverses as a function

of the incidence angle h for a material of unit thickness t 5 1. (h is

measured from the plane of the chip.)


mentioned above as a function of the length of the path or

‘‘thickness’’ that the radiation traverses in the material. Figs. 3

and 4 show representative results. Transmission factors for

these materials may be calculated for any thickness with the

absorption coefficients given in Table 3. Fig. 4 shows the

transmission factors calculated for the copper anode and

molybdenum anode X-ray tubes.

A very significant advantage due to the higher transmission

factors is obtained by use of the higher energy of the

characteristic radiation of the molybdenum anode X-ray tube,

i.e., at a thickness of 1 mm the transmission factor using a Mo

anode tube improves from 60% to 95% for PMMA, while the

corresponding improvement for glass is from 5% to 73%.

The values obtained demonstrate that the polycarbonate of

bisphenol A exhibits the lowest absorption coefficient and the

maximum transmission factor, making it the best chip-building

material from the radiation absorption point of view.

However, the values obtained for PMMA are very close and

the differences between the two materials are not significant.

On the other hand, glass and PDMS are not adequate due

to the high absorption, which would require extremely thin

entrance and exit windows for the radiation path. A notable

exception is the traditional use of thin-walled glass capillary

for Debye–Scherrer diffraction samples.

On-chip X-ray generation

The production of X-rays is the result of collisions of high-

energy electrons with matter. The conventional procedure for

producing an electron beam with the necessary energy to

produce X-rays is to apply a steady high voltage difference

between electrodes in high vacuum. An electrically heated

filament cathode at high negative potential acts as an electron

source, while a water-cooled anode at ground potential, where

the electrons strike, acts as the X-ray source. A high vacuum is

required for the dual purpose of avoiding filament burnout

and avoiding a plasma discharge between the electrodes which

otherwise would effectively short circuit the applied high

voltage. However, less conventional approaches to X-ray

production have been reported recently by Brownridge and

collaborators23,24 where the medium between cathode and

anode is a low-pressure gas in the range of 400 to 10 mTorr.

Furthermore, the source to produce the electron beam and

accelerate the electrons to the required high voltage is the

temperature cycling of pyroelectric crystals. Oriented single

crystals of lithium niobate (LiNbO3), lithium tantalate

(LiTaO3) and caesium nitrate (CsNO3) exhibit an electric

polarization parallel to the crystallographic z-axis. A positive

charge is developed on the crystal face perpendicular to the

Fig. 3 Calculated transmission factor (I/Io) for polycarbonate of

bisphenol A as a function of the material thickness for the X-ray

energy of different diffraction X-ray tubes.

Fig. 4 Calculated transmission factors for various chip-building

materials: A, for Cu-anode X-ray tubes of 8.05 keV radiation; and

B, for Mo-anode X-ray tubes of 17.5 keV radiation.

Table 3 Calculated values of the linear absorption coefficient of micro-fluidic chip-building materials at the characteristic energy of the mostcommon X-ray tubes

Molybdenum tube Copper tube Cobalt tube Chromium tube

Material Density/g cm22 X-ray energy/keV 17.478 8.047 6.93 5.414Borosilicate glass 2.23 m 5 3.03 30 46.7 95.8Polydimethyl siloxane (PDMS) 0.92 m 5 2.4 24.4 38.1 78.8Methyl methacrylate (MMA) 0.94 m 5 0.46 4.78 7.88 16.8Polycarbonate (Bisphenol A) 1 m 5 0.46 4.78 7.88 16.8


+z axis when the temperature of the crystal is below the Curie

temperature. Upon heating or cooling the crystal there is a

change in the surface charge. Experiments have shown that

weak electron beams with energies as high as 160 keV may be

obtained.25,26 By temperature cycling of the pyroelectric

crystal in the appropriate geometry, X-rays are produced as

electrons alternatively strike the crystal or an anode target

separated a few millimetres from the crystal.27 The minute size

and low power of these unconventional X-ray generators

suggest a possible use as local sources for XRF analysis in

microfluidic chips.

Examination of the discharge properties of low pressure

gases shows that high voltages can be applied without

producing a gas discharge.28–30 If the applied voltage is in

the range of or above several thousand volts, the generation of

X-rays is feasible. For this phenomenon to take place it is

necessary to operate in the left side of the Paschen curve that is

applicable to the particular combination of gas, electrode

geometry and gas pressure. Operating in this regime the gas

exhibits a high resistance and charge flow takes place by the

so-called Townsend dark current. The detailed mechanism by

which the X-rays are produced under the above-described

conditions have, to our knowledge, not been advanced.

However, the experimental results obtained by generating the

high voltage and weak electron beams by means of the

polarising or depolarising of pyroelectric crystals, as reported

by Brownridge and collaborators, suggest that a very high

impedance potential source in the tens of kilovolts range could

substitute the pyroelectric crystal. Hence, a properly designed

chip configuration, so that the required conditions for gas

conduction under Townsend dark current are met, may lead to

X-ray generation from a miniaturized device in a micro-fluidic

chip, as reported below.

3. Experimental

A system was built that contained an X-ray spectrometer,

facilities for performing fluorescence experiments and for on-

chip X-ray generation. Fig. 5 shows a diagram of the apparatus.

The X-ray spectrometer consisted of a planar, 5 mm thick,

HPGe detector (Canberra Industries Inc. GL0055PS) with

50 mm2 active area and a 50 mm thick Be window. Nominal

resolution is 145 eV at the Mn 5.9 keV line; NIM modular

electronics were used and a PC-based multichannel analyser

(Canberra Industries Inc. S100). The spectrometer signals were

analysed with the use of the AXIL-QXAS software.22

X-ray fluorescence

Isotope excited X-ray fluorescence (XRF) measurements were

carried out with the use of a small radioisotope source that

was incorporated in a purpose-designed chip. The chip was

fabricated in glass, PDMS and lead. Fig. 6 shows the layout of

the chip with the radioactive source, sample channel and the

lead mask. The chip was assembled over a 76 6 22 mm

microscope glass slide and the whole mounted flush on the

lead cap that is used as the protector for the delicate beryllium

window of the detector. As shown in the figure, a 45-degree

source–sample–detector geometry was used. The lead mask is

intended to shield the source radiation from directly reaching

the detector crystal. The total distance from the sample to the

detector beryllium window is estimated at 2.5 mm and the

source–sample distance about 1 mm. The detector that was

used has a relatively large distance between the beryllium

window and the detector crystal (5 mm) with a resulting loss in

the system sensitivity.

Samples consisted of strips of thin 0.1 mm thick metal foils

approximately 1 mm wide placed in the sample channel in

front of the 241Am source, as shown in Fig. 6

The radioactive source used31 was Am241 (see Table 4).

Sources of this isotope specifically designed for XRF are

normally fitted with a filter covering the source surface. This

Fig. 5 Diagram of the apparatus. A, A block diagram of the energy-

dispersive X-ray spectrometer and vacuum system. B, Glass chamber

detail.

Fig. 6 The XRF chip mounted on a glass slide. Detail: source–sample

geometry.

Table 4 Properties of the radioisotope used for XRF

Radioisotope 24195 Am

Half life (years) 458 (a decay to 23793 Np)

Activity (Bk) 3.3 6 104 (0.9 mCi)Physical size/mm 1.8 diameter disk 0.25 thickAlpha energy32/keV Intensity (%)5490 855440 13Photon energy32/keV Origin Relative intensity11.89 Np Ll 2.213.9 Np La 37.117.8 Np Lb 51.220.8 Np Lc 13.826.35 Np gamma 759.54 Np gamma 100


filter is designed to suppress all the Np X-rays shown in the

table and allow only the 59.5 keV gamma ray for excitation.

The use of such a filter produces a ‘‘clean’’ monochromatic

source, which generates simple fluorescence spectra free from

the Np lines. This in turn results in a continuous monotonic

sensitivity curve, or detection-limit curve, as a function of

atomic weight of the analysed element, which results in easy

quantification methods. However, the source used in the chip

had no such filter so that the full alpha, gamma and neptunium

X-ray spectrum of the source was used for excitation. Fig. 7

shows the photon spectrum of the source.

X-ray diffraction

Experiments were carried out to demonstrate the possibility

of performing X-ray diffraction analysis on polycrystalline

materials contained in a chip nanovial or micro-channel. For

that purpose an X-ray diffraction standard material was

placed in three chips selected or purpose built for the testing of

materials and evaluation of different geometrical configura-

tions. Corundum (a-Al2O3), with particle size about 1 mm

(BDH Laboratory Reagents), was used as the standard sample

material. In each chip three specimens were placed at different

concentrations by means of preparing a slurry in water with

different amounts of corundum. The slurry was pipetted

directly onto the chip. The chips were made of polycarbonate

of bisphenol, polydimethylsiloxane (PDMS) and borosilicate

(BS) glass. The polycarbonate chip is a commercial design

(DI2000 by Weidman Plastic Technology AG, CH-8640

Rapperswir, Switzerland) intended for PCR analysis. The

PDMS chip was a prototype intended for spore analysis.33 The

third chip was purpose made with PDMS and BS glass. In all

cases the corundum slurry was placed directly in the micro-

channel or nanovial built in the chip. The X-ray diffractometer

used (D8 Discover with GAADS, Bruker, Karlsruhe, Germany)

had a standard diffraction copper-anode X-ray tube fitted with

a Goebel mirror tuned to the Cu Ka line. This is followed by a

collimator so as to produce an intense, one mm diameter beam

on the sample. X-rays were detected with a low-noise two-

dimensional detector (GADDS 2-D detector). The detector

signals are stored, processed and displayed as a colour-coded

image representing the radiation intensity crossing each point

in the plane of the detector. The image consists of circular

patterns corresponding to the diffracted radiation cones

that intersect at a right angle the detector plane. Software

contained in the GAADS analysis program integrates, along

constant radii, the circular patterns in the image, values

with constant radius corresponding to diffraction angles. This

analysis procedure results in the standard intensity versus

angle diffraction pattern. This instrument produces a greatly

enhanced detection sensitivity, reducing collection time by

one or two orders of magnitude and making it particularly

appropriate for the measurement of very small samples or of

very weakly diffracting specimens. Experiments were carried

out in two geometries, in the usual reflection geometry and in

the transmission geometry.

On-chip X-ray generation

Measurements were carried out in a small glass vacuum

chamber as shown in Fig. 5. The vacuum system consisted of

a rotating vane mechanical pump (Edwards, RV5), which

evacuates the chamber while a leak valve (SwageLock,

BMG series) allows a controlled air intake to obtain the

desired air pressure. Pressure was monitored with a conduc-

tivity gauge (Edwards, PR10-K). The chamber contains

feedthroughs for connection to the high voltage source and

a thin 11 mm thick Al foil window to allow the exit of

fluorescent X-rays to the detector. Measurements were done

in the pressure range reported by Brownridge23–25 (namely

0.1 mTorr to 10 mTorr). An energy-dispersive high resolution

X-ray spectrometer, described above, was used to monitor the

radiation signals.

The high voltage supply to the chip (FUG HCN 7E-35000)

was connected in series to a high impedance resistor (6.5 GV)

to limit the current, avoid gas discharge and ensure operation

in the Townsend dark current regime.28 The potential and

current supplied by this unit were measured with a digital

voltmeter at the V-monitor and I-monitor points of the HV

supply via a previously obtained calibration curve.

A variety of microelectrode geometries were tested by con-

necting the device’s anode and cathode to the glass chamber

vacuum feed-through electrodes. Geometries tested included

three- and two-dimensional configurations. They were built

on 77 6 25 mm microscope glass slides cut to one half or

one third size. Some two-dimensional devices were made by

photolithographic techniques on chromium coated glass plates

(Nanofilm, Westlake Village, CA 91361, USA.) and then cut

to size. Epoxy resin (Araldite) was used to electrically isolate

the wire connectors from the glass feed through to the chip

inside the vacuum chamber. Epoxy was also used to isolate all

exposed connectors within the chip or device being tested.

The system geometry was accurately measured to be able to

estimate the photon flux generated in the chip. This estimation

was done for the chromium characteristic radiation produced

by the MKD chip assuming isotropic 4p generation. The

calculation included the X-ray attenuation due to the alumi-

nium 11 mm window, the beryllium detector window and the

air path. The HP Ge detector efficiency was assumed 100% at

the 5.4 keV energy of the Cr Ka line.

Fig. 7 241Am spectrum obtained with the X-ray spectrometer

showing the Np X-rays and the gamma rays used as excitation

radiation for the XRF experiments.


4. Results and discussion

X-ray fluorescence

Fig. 7 shows the 241Am source spectrum. By allowing the neptu-

nium X-rays to be used as excitation radiation, the XRF sensi-

tivity of the lower atomic weight elements is greatly enhanced.

The activity of the source used was extremely small 3.3 6104 Bq (0.9 mCi), three to four orders of magnitude smaller

than the activity of sources ordinarily used for XRF excitation.

This allowed its use under simplified controls in a laboratory

not designated for radioactive work, as required by radiation

protection rules. However, the fact that clearly measurable

XRF signals were obtained with such a small activity for the

excitation source is evidence of the enhanced signal that is

obtained with the miniaturisation afforded by a chip design.

A series of XRF spectra were obtained with the purpose of

establishing a sensitivity curve. Fig. 8 shows representative

spectra. Good signal to noise ratios were obtained, as is

apparent from this figure. Fig. 9 shows the system sensitivity

(counts s21 mCi21) as a function of atomic weight of the target

element.

Usually XRF sensitivity curves produced by monochro-

matic radioisotope sources are a continuous and smoothly

variable curve with a discontinuity in the region of the K to L

line transition. The discontinuous values shown by the

sensitivity in Fig. 9 show the effect of the unfiltered radiation

used for excitation. The various elements are selectively excited

according to the energy position of their absorption edges

relative to the energy of the various emission lines of the

unfiltered 241Am source shown in Fig. 7.

X-ray diffraction

In spite of the great sensitivity of the XRD diffractometer

used, no usable signals were obtained in the traditional

scattering geometry. However, experiments carried out by

mounting minute samples of similar mass (#0.4–0.04 mg )

as that contained in the chips in air showed a very clear

and strong diffraction pattern of corundum. These results

are due to absorption, in the construction materials, of the

copper Ka radiation used in the experiments, both absorp-

tion of the excitation radiation and of the diffracted radia-

tion in the path inside the chip. Experiments done in

transmission geometry in the polycarbonate of bisphenol

chip showed weak diffraction signals (see Fig. 10). In this

case the exciting radiation traverses 1 mm thickness to

reach the sample (normal incidence) and the diffracted

radiation traverses somewhat higher values, depending on

the scattering angle, as shown in Fig. 2, to exit the material

covering the sample.

X-ray generation

Several chip configurations were successful in producing

X-rays under a reduced gas pressure. Air was used in all

cases. In each case the pressure was fixed at a predetermined

value and the potential slowly increased while X-ray genera-

tion was monitored. Three current regimes were identified:

a low current regime (about 0.01–0.06 mA) where no radiation

is observed, a middle current regime (about 0.2–0.6 mA)

associated with X-ray production and a high current regime

(.2 mA) associated with gas breakdown and no radiation.

Under gas breakdown conditions the plasma produced in

the chamber has low resistance, hence the supplied high

voltage is mostly across the high value resistor at the output of

the source.

Fig. 11A shows the spectrum of the radiation produced by

one of the simplest three dimensional structures, a ‘‘parallel

plate’’ geometry. Two circular 10 and 12 mm diameter discs of

nickel and copper metal plates are separated by 4 mm. The

nickel plate was connected as the anode. The spectrum shows

the characteristic Ka and Kb lines of the nickel anode over the

bremsstrahlung.

Fig. 11B shows the spectrum of another three-dimensional

structure. In this case it is a ‘‘plate and needle’’ geometry, built

with a nickel plate as cathode and a sharpened copper wire

perpendicular to the plate as the anode. The electrode separa-

tion was approximately 3 mm. X-rays in this device were

detected beginning at about 13.6 kV up to 27 kV in a very

stable and repeatable manner. X-ray intensity increased withFig. 8 X-ray fluorescence spectra of various metals obtained with the

radioisotope-loaded chip described.

Fig. 9 Elemental sensitivity as a function of atomic weight for the

radioisotope-excited chip XRF system.


applied potential until breakdown at just over 27 kV. The

broadened unresolved characteristic peak is due to an ac noise

in the spectrometer amplifier baseline produced by the FUG

HCN 7E-35000 HV power supply.

A number of two-dimensional configurations were tried

with varying degrees of success. Fig. 11C shows an X-ray

spectrum produced by one of these geometries. This chip was

built on a 15 6 15 mm chromium plated glass with a ‘‘line and

point’’ geometry. Inter-electrode separation was 2.5 mm and

was connected with the ‘‘point’’ as the anode. As in previous

geometries, epoxy isolated all exposed metal connections save

for the intended anode and cathode.

Surprisingly high radiation intensities were recorded con-

sidering the low power dissipated in these devices. For the

MKD device, the spectrum shown in Fig. 11, a photon

intensity (4p at 5.4 keV) of 3.6 6 105 photons s21 was

calculated for the Ka line of chromium. For this device a

total integrated photon intensity of 3.6 6 106 photons s21

was calculated. The exposed chromium anode tip was

0.47 mm2, leading to an estimated surface flux of 3.8 6106 photons s21 mm22. The total integrated intensity of this

device is equivalent to a radioactive source of 3.6 6 106 Bq

(0.1 mCi). The power dissipated was estimated to be #4 mW.

Power was calculated with the applied or the spectrum-

measured cut-off potential. Both values closely coincided. The

current was measured at the HV supply. With several of the

devices tested, under certain conditions, a 0.1 mm thickness

glass cover slide was used in front of the detector window

in order to attenuate the radiation produced and prevent

saturation of the detector signal.

5. Conclusions

The fluorescence experiments performed resulted in a high

signal to noise ratio, which was attained with a source of

much less than optimum geometry or dimensions and with

an activity that was three orders of magnitude smaller

than what could be used or what is common practice

in X-ray fluorescence instrumentation. These results demon-

strate that very high sensitivity may be achieved by making

Fig. 10 X-ray diffraction patterns of (top to bottom) 0.2 mg, 0.04 mg and 0.001 mg corundum specimens contained in a polycarbonate of

bisphenol chip.

Fig. 11 A, X-ray spectra obtained in 100 s with parallel plate device

MK1. Air pressure was 15 mTorr, applied voltage 12.75 kV. Peak area:

75 3305 counts. Total integrated intensity: 358 326 counts. Power

dissipated #5 mW. B, X-ray spectra obtained in 100 s with chip design

MK5. Air pressure 10 mTorr, applied voltage 27 kV, peak shows

the characteristic (unresolved) Ka and Kb lines of the copper anode

over the bremstrahlung. Peak area: 73 170 counts. Total integrated

intensity: 233 662 counts. Power dissipated #19 mW. C, X-ray spectra

obtained in 20 s with chip design MKD. Air pressure 13 mTorr,

applied voltage 17.3 kV, peak shows the characteristic (unresolved)

Ka and Kb lines of the chromium anode over the bremsstrahlung.

Peak area: 8794 counts. Total integrated area: 40 561 counts. Power

dissipated #4 mW.


use of the scaling factors afforded by the minute chip

geometry and that further, significant improvements are

possible.

The results of the diffraction experiments done, seen in

the light of the X-ray absorption calculations contained

in Fig. 4, indicate that for on-chip X-ray diffraction it is

recommended: (i) the use of lower absorption coefficient

materials: (ii) that a transmission geometry must be used;

(iii) that any design must incorporate a very small thick-

ness of the chip if it is to be crossed by radiation (circa

100 mm); and (iv) the employment of high energy X-ray

radiation which results in smaller diffraction angles (i.e., a

molybdenum anode tube) in order to realise on-chip structure

analysis.

On-chip X-ray generation of very significant intensity has

been demonstrated in chip devices of various geometries

under reduced air pressure. Further development is required

to establish geometrical characteristics and other conditions

of sufficient stability to be able to use the phenomenon in

analytical applications. The detailed mechanism of the X-ray

production achieved is not quite clear and further work is

required.

X-ray fluorescence or diffraction detection limits depend

strongly on numerous factors pertaining to the particular

excitation geometry, radiation intensity, detector efficiency

and nature of the sample. It is unlikely that measurement

of trace element concentrations will be achieved by these

techniques in a chip. High and intermediate concentrations

should be possible: however, careful control of the relevant

factors will determine, in each case, how low a detection limit

will be reached.

Acknowledgements

This work was carried out during a sabbatical leave granted to

E.D.G. by the Universidad Simon Bolıvar, Caracas, Venezuela,

which is gratefully acknowledged. Also, the Deutsche

Forschungsgemeinschaft (DFG) of the Federal Republic of

Germany is gratefully acknowledged for the granting of a

Mercator Visiting Professorship. The authors are grateful to

Dr. Herbert Stori of Vienna University of Technology for

helpful discussions.

Eduardo D. Greaves*ac and Andreas Manzbc

aUniversidad Simon Bolıvar, Apartado 89000, Caracas 1080A,Venezuela. E-mail: [email protected] for Analytical Sciences, Bunsen-Kirchhoff-Strasse 11,D-44139 Dortmund, GermanycDepartmentt of Chemistry, Imperial College of London, London, UKSW7 2AZ

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Toward on-chip X-ray analysis