Etched tungsten foils as emitters in field ionisation

415

. ftiermt!iomd 0

Eiscvicr

.I ournd

of Mass

Specfrometry

2nd Ion Ph_v&q

19 (1976)

415417

Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Short commtication

Etched tungsten foils as emitters in field ionisation R- E- ARDREY, D. 6. HIBBERT AXD A. J. B. ROBERTSON Department of Chemi&r__r,King’s ColIege, Strand. London CVCZR 2L.S (GL Britain) P_ J_ DERRICL,

MM-A_ BALDWIN,

J_ T_ HILL

AND

R_ P. MORGAN

Depwrmenr of Chcmtirr_t-.Unircrsify College, -29 Gordon Srreef, Londcm WCIH

OAJ

(Gr. Britain)

tReceiwxi 4 July 1975)

The increasing interest in field ionisation and field desorption has led us to reopen early investigations [I] into sharp edge emitters, and we describe here an etched tungsten foil which may be superior in many respects to razor blades_ However, foils made in this particular way may not be suitable for field ionisation kinetics studies because of uneveness of their sharp edges. i3ching rechique The results reported here were obtained using 50~rn thick tuneten foils electrochemically etched in 10 oA (w/w) potassium hydroxide solution. Two procedures were adopted: (a) a “fast etchin,a_’ technique in which the foiIs were subjected to alternating currents of high densities &cater than 6 A cm-’ at 8 V a-c.) for a short time (I 5 s), and (b) a “slow etching-’ technique, when smaller current densities (O-6 A cm-’ at 2 V a-c.) were used for a longer time (300 s)_ The Iatter technique is similar to that used previously [I ]_ “Fast etched” foils had an uneven edge with noticeably greater etching at the comers of the foil_ Ion cm-remfionz foils A foil etched by method (a) gave twice the total ion current as that obtained from a foil produced by method (b)_ The etched foiis were tested both in an MS-9 mass spectrometer [2] and in a separate apparatus for measuring the totai observed current from the positively charged foil in the presence of a sekcted gas at a known pressure_ With acetone at a nominal pressure of a few tenths of a millipascai (about iO-6 torr) in the mass spectrometer, the total ion current emitted from a freshly etched foil was about 10m7A at a foil - cathode separation of 0.4 mm and

pl&-Lz 70

90

m/it

E~1_Tkfiefdioaiationmasspmnua

of n-hcpcanc obmimcd with an etched tungsten foil.

Iaccusitis for DI& i= 100have been multipficd by IOO-

with a potential difference of 8 kV between the foil and the cathode. Under similar conditions a fresh Schick razor blade gave a total current of about IO-*10-9A. Figure I shows the mass spectrum obtained from n-heptane using an etched tungsten foil- Speier et al_ [3] have determined a relationship between the field strength at a tip emitter and the relative intensity of nt/e29 in the mass spectrum of n-heptane, as measured with that tip_ Extending this relationship to sharp edges, we find from the data in fig_ 1 that the average field strength at the ede of the tungsten foil is l-4 - IO9Vcm-’ which compares with the value 0-9. I09Vm-’ obtained for a Schick razor blade- Conditioning [4] tungsten foils with either acetone or benzonitrife enhanced the total emission about IO times, It has been pointed out [5] that tungsten foils tend to support the development of protrusions more effictiveiy than stainless steel razor blades. It seems that etched tungsten foils may be Suptioi to razor blades for most analyticaJ applications of field ionisation mass spectrometry- The conditioning techniques deve!oped by Beckey et al, 161 for tungsten wires may be effective for tungsten foi4 thus allowing them to be used for producing field desorption mass spectra_ However, it seems from a few experiments reported by Heitbaum [7] that suitabk sharp edges can be used for field desorption without prior conditioning

ACKNO\VLEDCGEMENTS

We express our gratitude to the Ramsay Memorial Trust for the award of a General FelIowship (to P_J_D_),to the Science ResearchCouncil for a maintenance grant (RPM_) and a research grant, and to the British PetroIeum Co_ for a Research Studentship (to D_B_H_)_We gratefully acknowledge the financiai sup port of the Central Rezarch Fund, University of London, We are indebted to the

417 Schick Safety Razor Company cooperation_

(Milford,

Connecticut,

U.S.A_)

for vaIuabIe

1 P_ k Blenkinsop, B_ E Job. D_ F_ Braitsford, C_ M. Cross and A_ J_ B. Robertson, Int_ f_ Afass Specrrom. lun P&-s_. 1 (1968) 421-

3 4 5 6 7

(1975) 393F_ Speicr. H- 3. Heinen and H- D_ Beckey, Messfccfinik, 6 (1972) 147P_ J- Derrick and A_ J- B- Robertson, fnr. J_ Muss Specmm_ km Pip_. P- J- Derrick. Ph-D- Thesis, University of London. 1969_ H_ D_ Betkey. E Hilt and H_ R Schulten, J_ P&s_ E, 6 (1973) 1043_ J_ Heitbaum, Dtkuss_ Farada_t-Sot.. 56 (1973) 305.

10 (1973) 315.