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Liquid secondary ion mass spectrometry with a focussed primary ion source
71
International Journal of Mass Spectrometry and Ion Processes, 61 (1984) 71-79 EIsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
LIQUID SECONDARY ION MASS SPECTROMETRY FOCUSSED PRIMARY ION SOURCE
R.G. STOLL,
D.J. HARVAN
WITH A
and J.R. I-IASS
Laboratory for Molecular Biophysics, National Institute of Environmental P. 0. Box 12233, Research Triangle Park, NC 2 7709 (U.S.A.)
Health
Sciences,
(First received 5 March 1984; in revised form 23 April 1984)
ABSTRACT Experiments are described, which indicatetheadvantage of focussing the primary ion beam to a spot smaller than the sample diameter in liquid secondary ion mass spectrometry (SIMS}. This results in lowering the detection limit for many substances below the nanomole level and prolongs the useful measuring time for a given amount of sample. A focussed SIMS source can be built small enough to fit into existing mass spectrometers without major modifications.
INTRODUCTION
Secondary ion mass spectrometry (SIMS) is of growing interest for the analysis of thermally labile compounds. In this ionization technique, [M + H]+ or [M + alkali]+ or [M - H] - ions are produced by bombardment of the sample surface with energetic ions or neutral atoms (“fast atom bombardment”, FAB) (I,21 of Ar, K, Xe, Cs, and various metals [3-61. By increasing the mass of the primary particle, a significant increase of the secondary ion current has been reported. Destruction of the sample within the measuring time can be avoided by keeping the primary ion current density of the SIMS source below lOi atoms s-i cmm2 (-c 10e8 atoms cme2), meeting the conditions of static SIMS [7] or by dissolving the sample in a liquid matrix (liquid SIMS, FAB) [$I, so that rapid replenishment of the surface occurs. Using a liquid matrix like glycerol, high primary and secondary ion currents can be obtained, making this method convenient for conventional organic mass spectrometers. The construction and application of a source-mounted primary ion gun similar to that reported by Rudat [9], but with a defined focussed beam of small size are reported here. The advantages of this source over saddle field-type sources used in many liquid SIMS applications are its small size, requiring no expensive modification of the mass spectrometer, and ,the 0168-1176/84/$03.00
0 1984 Elsevier Science Publishers B.V.
72
well-defined ion beam which irradiates only that portion of the sample where secondary ions have a reasonable chance to be accepted by the mass spectrometer. EXPERIMENTAL
All experiments were performed on a double focussing mass spectrometer (ZAB 2F, VG Analytical Ltd., Altrincham, Ct. Britain) equipped with a field desorption ion source. The experimental set up is shown in Fig. 1. For SIMS, the cathode slit and its ceramic support were removed. The ion source block was replaced by four supporting stainless steel rods, 1, holding the extraction and focussing plates of the ion source, thus providing a better vacuum inside
Fig. 1. SIMS source with focussing primary ion source. 1, Extrkztion and focussing plates of the secondary ion source; 2, sample holder; 3, Capillaritron primary ion source; 4, isolating ceramic support; 5, extracting and focussing elements of the primary ion source.
73
the source. The field desorption probe was used to support the sample holder, 2. The source was operated by the electron impact ion source electronic circuits with the repeller supply connected to the probe. The sample holder was a polished 2 mm stainless steel rod cut approximately 60 o to the axis of the mass spectrometer. The primary ion source assembly, 3, was mounted directly on top of the ion source, parallel to the entrance slit. The angle of 90 O between the primary ion source and the optical axis of the mass spectrometer was chosen because of the simplicity of construction. The primary ion source consists of a Capillaritron ion source [lo] and a two-element focussing lens system. The Capillaritron is a 25 pm fine bore tungsten capillary with a concentric counter electrode. Ions are formed by passing gas through the capillary and applying a voltage difference of several kV between the capillary and the counter electrode, causing a gas discharge. A 10 cm glass capillary in the gas supply line prevented arcs from the Capillaritron to the system ground through the gas line. Xenon was supplied as the gas and its flow was adjusted to produce a discharge current of 0.5-1.5 PA at a potential difference of 7 kV between the capillary and the counter electrode. The Capillaritron was mounted on the ion source via an isolating ceramic block, 4. The counter electrode and the focussing electrodes were made of stainless steel and were mounted on ceramic washers inside the source, 5. The distance between the Capillaritron and the counter electrode was 7 mm, between the counter electrode and the focussing electrodes 2.25 mm each, and between the last focussing electrode and the optical axis of the mass spectrometer 2.7 mm. The counter electrode and the focussing electrodes had a diameter of 1.5 mm and 5 mm, respectively. The whole primary source assembly, including gas supply line and electrical connections, was shorter than 40 mm, measured from the optical axis of the mass spectrometer. The primary ion source used the following electric supplies. The Capillaritron was connected to a 15 kV high voltage power supply (2707R, Brandenburg Ltd., Thornton Heath, Gt. Britain) via a 40 Ma resistor chain; the counter electrode and the focussing electrode close to the sample were connected to the source potential of ,the mass spectrometer; the the second focussing electrode was operated with a 10 kV high voltage power supply (B410 J. Fluke Mfg. Co., Mountlake Terrace, WA). The peptides methionin-enkephalin (Sigma, St. Louis, MO), bombesin (Bachem Torrance, CA) and P-endorphin (gift of C.H. Li, University of California, San Francisco, CA), and the nucleotide guanosine monophosphate (Sigma, St. Louis, MO) were chosen as test compounds. They were dissolved in methanol or water and applied to 1 ~1 of glycerol with 0.5% oxalic acid on the sample holder. A solution of cesium iodide in water was applied to the probe to produce Cs(CsI)n+ clusters, which were used for calibration.
74 RESULTS
AND
DISCUSSION
The actual shape of the primary ion beam was examined on the sample holder by placing a metal sheet covered with a tantalum pentoxide refractory film, where the sample normally resides. After 10 min of operation of the ion source, the interference color of the film changed due to sputtering of the oxide layer [ll]. The spot size was very dependent on the applied focussing voltage. With 6 kV acceleration energy of the mass spectrometer, 13 kV on the capillary and 6500 V on the focussing electrode, an‘ oval spot 0.4 X 1.0 mm on the tilted sample holder could be achieved. The spot was oriented in the direction of the entrance slit. With a discharge current of 0.5 to 1.5 PA, the current fluctuations of the gun were a few nA, probably caused by fluctuations of the gas supply. Phosphorescent materials on the sample holder did not show the actual beam shape. Secondary electrons produced on various surfaces caused an intense phosphorescence on the whole sample holder, making the observation of the ion beam shape impossible. The glycerol cluster at 369 daltons was used to measure the effect of different focussing voltages on the secondary ion signal intensity. The focussing voltage was changed from the source potential of 6000 V up to 7000 V with a discharge current of 900 nA. A sharp maximum was observed
100
6500
7000
V focus
Fig. 2. Intensity of the glycerol cluster at 369 daltons as function of the focussing voItage at the primary ion source. Source potential 6 kV; primary ion source potential 13 kV with respect to ground; discharge current 1 pA.
at 6450 V, which also represents the smallest spot size that could be made by the existing gun (Fig. 2). Without focussing elements, a similar secondary ion current could be achieved only by increasing the discharge current to higher than 5-8 PA. This indicates a significant increase of current density on the sample surface within the acceptance of the mass spectrometer by focussing the primary ion beam. According to our experience, the high discharge current in the unfocussed mode quickly destroys the Capillaritron tip. After less than 100 h of operation, the orifice of the Capillaritron tip opened up to a diameter of more than 50 pm, making a stable discharge difficult to maintain. The isolating ceramic insulator that holds the Capillaritron contaminated even faster, requiring frequent cleaning. In the focussed mode, with discharge currents below 2 PA, the wear on the tip was negligible and the whole gun required no maintenance for more than 100 h of operation, except for occasional cleaning of the isolating ceramics. The peptide methionin-enkephalin and the nucleotide guanosine monophosphate were used to test the detection sensitivity of this SIMS source by measuring the time dependence of the ion signal of the protonated molecular ion with various amounts of the sample. With 100 ng of methionin-enkephalin, the protonated molecular ion could be measured with the focussed and the unfocussed primary ion gun. The results are shown in Fig. 3.
I-8 : : I I I I I
I I I
1 \
IO
\
\
20
30
Time Lmin.1 Fig. 3. Intensity of the protonated molecular parent ion from 100 ng of methionin-enkephalin versus time with an unfocussed primary ion beam of 9 PA (- - - - - -) and a focussed ) with 7 kV acceleration voltage. primary ion beam of 1 pA (-
76
Using the unfocussed primary ion gun, the protonated molecular ion signal decreased rapidly, thus limiting the useful measuring time to a. few minutes. Using the focussed primary ,ion gun with its focussing voltage adjusted to the most intense secondary ion signal, this signal could be measured for 30 mm. During this time, the intensity of the glycerol cluster signals decreased and the signal corresponding to the protonated molecular ion increased, so that the signal-to-noise ratio was enhanced during the measurement (Fig. 4). This behavior was found with all substances tested. After approximately 30 min, the signal of the glycerol at 185 daltons and of the protonated molecular parent ion went down below the noise level, regardless of the amount of sample applied to the glycerol. The complete evaporation of the glycerol probably stopped further ion production from the sample. With the focussed ion gun, the detection limit was a factor 5-10 below that of the unfocussed gun. With less than 20 picomole (10 ng) of methionin-enkephalin, the protonated molecular ion could be detected for more than 25 mm with a signal-to-noise ratio of at least 4. Figure 5 shows the mass spectrum of 10 ng ( < 20 picomole) of this substance after 22 min of irradiation. The glycerol clusters above 400 daltons are no longer detectable and the protonated molecular ion of the sample is clearly visible above the uniform background. The detection limit for the protonated molecular ion of
20
IO Time
30
Cmin.3
Fig, 4. Intensity of the glycerol cluster at 553 daltons as a function of time with a focussed primary ion beam of I PA.
77
guanosine monophosphate was less. than 500 ng and the ‘ion current lasted for more than 20 min. The polypeptide bombesin (1618 daltons) could be detected with less than 1 lug of sample (Fig, 6). One microgram (less than 0.3 nanomole) of the polypeptide /3-endorphin was sufficient to be detected as its protonated molecular ion at 3461 daltons (3 kV acceleration potential, 15 00 mass resolution). The long desorption time was an important advantage when making measurements with compounds of molecular weights above 1500 daltons. Then the reduced acceleration potential required careful tuning of the mass spectrometer before acquiring the mass spectrum. After 10 mm of irradiation, the intensity of the ion signals of the glycerol clusters decreased, resulting in mass spectra with relatively intense sample ion signals. The reason for the prolonged desorption time and slightly higher sensitivity may be due to concentrating the primary ion beam on the region of the sample surface where the secondary ions formed are accepted by the mass analyzer. Under the given conditions, the diameter of the sample holder is greater than the diameter of the primary beam. The sample covering the whole sample holder is only sputtered by the primary beam where secondary ions can be accepted by the mass analyzer, minimizing the sample loss. The
350
400
450
500
550
600
650
700
M/Z
Fig. 5. SIMS spectrum of 17 picomole (10 ng) of methionin-enkephalin in a glycerol-oxalic acid matrix after 22 min of bombardment with a focussed primary ion beam of 1 PA and 7 kV energy.
78 IOOT
BOMBESIN
50: 7 .y - h’ *,-’ i” . .. ” ll 1 +* 650
__, _ L. .,_i,h
, I.._..
750
800
650
950
too0
to50
II00
II50
t 550
1600
1660
50-
loo900
”I.. I.
700
_, _..,
50 E
1500
I700
I f50
M/Z Fig. 6. SIMS spectrum of 1 c(g of the polypeptide bombesin in a glycerol-oxalic with a focussed primary ion beam of 1 PA and 7 keV energy.
acid matrix
sample can flow and diffuse from the sides into the small sputtering zone as long as the liquid matrix is present. This results in a prolonged ion formation time, limited just by the complete evaporation of the liquid glycerol matrix. However, with unfocussed FAB sources, the primary ion beam diameter is larger than the sample diameter. This results in the formation of secondary ions which cannot be accepted by the mass analyser but which are lost in the source. When sodium iodide was placed on the sample holder by evaporation of an aqueous solution to give a solid matrix, no difference was noted between a focussed and unfocussed primary ion beam. Thus, under conditions in which sample transport is limited, the replacement of sputtered material from the extremes of the sample holder cannot occur and no advantage was realized by focussing the primary ion beam. CONCLUSIONS
Using a simple focussed primary ion beam in liquid SIMS, the detection limit for various substances can be reduced to the picomole range and the
79
desorption time for molecular parent ions can be prolonged significantly. These results indicate that low detection- limits in liquid SIMS can be achieved by closely matching the secondary ion emittance and the acceptance of the mass analyzer, thus limiting the loss of .secondary ions in the ion source to a minimum. A further increase of sensitivity can be achieved by using higher mass primary particles such as heavy metals. Experiments in this direction are planned_ ACKNOWLEDGEMENT
We thank B.L. Bentz for the suggestion pentoxide refractory film.
and supply of the tantalum
REFERENCES 1 M. Barber, R.S. Bordoli, R.D. Sedgwick and A.N. Tyler, Nature (London), 293 (1981) 270. 2 M. Barber, R.S. Bordoli, R.D. Sedgwick and A.N. Tyler, J. Chem. Commun., (1981) 325. 3 D.F. Hunt and W.M. Bone, 29th Annu. Conf. Mass Spectrom. Allied Topics, Minneapolis, 1981, Am. Sot. Mass Spectrom., 1981, p. 357. 4 B.T. Chait and K.G. Standing, Int. J. Mass Spectrom. Ion Phys., 41 (1981) 185. 5 R. Stall, U. Schade, F.W. Roellgen, U. Giessman and D. Barofsky, Int. J. Mass Spectrom. Ion Phys., 43 (1982) 227. 6 D.F. Barofsky, U. Giessman, A.E. Bell and L.W. Swanson, Anal. Chem., 55 (1983) 1318. 7 A. Benninghoven, Surf. Sci., 28 (1971) 541. 8 W. Aberth, K.M. Straub and AL. Burlingame, Anal. Chem., 54 (1982) 2029. 9 M.A. Rudat, Anal. Chem., 54 (1982) 1917. 10 J.F. Mahoney, J. Perel and A.T. Forrester, Appl. Phys. Lett., 38 (1981) 320. 11 B.L. Bentz and P.G. Gale, Anal. Chem., 55 (1983) 1434.
International Journal of Mass Spectrometry and Ion Processes, 61 (1984) 71-79 EIsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
LIQUID SECONDARY ION MASS SPECTROMETRY FOCUSSED PRIMARY ION SOURCE
R.G. STOLL,
D.J. HARVAN
WITH A
and J.R. I-IASS
Laboratory for Molecular Biophysics, National Institute of Environmental P. 0. Box 12233, Research Triangle Park, NC 2 7709 (U.S.A.)
Health
Sciences,
(First received 5 March 1984; in revised form 23 April 1984)
ABSTRACT Experiments are described, which indicatetheadvantage of focussing the primary ion beam to a spot smaller than the sample diameter in liquid secondary ion mass spectrometry (SIMS}. This results in lowering the detection limit for many substances below the nanomole level and prolongs the useful measuring time for a given amount of sample. A focussed SIMS source can be built small enough to fit into existing mass spectrometers without major modifications.
INTRODUCTION
Secondary ion mass spectrometry (SIMS) is of growing interest for the analysis of thermally labile compounds. In this ionization technique, [M + H]+ or [M + alkali]+ or [M - H] - ions are produced by bombardment of the sample surface with energetic ions or neutral atoms (“fast atom bombardment”, FAB) (I,21 of Ar, K, Xe, Cs, and various metals [3-61. By increasing the mass of the primary particle, a significant increase of the secondary ion current has been reported. Destruction of the sample within the measuring time can be avoided by keeping the primary ion current density of the SIMS source below lOi atoms s-i cmm2 (-c 10e8 atoms cme2), meeting the conditions of static SIMS [7] or by dissolving the sample in a liquid matrix (liquid SIMS, FAB) [$I, so that rapid replenishment of the surface occurs. Using a liquid matrix like glycerol, high primary and secondary ion currents can be obtained, making this method convenient for conventional organic mass spectrometers. The construction and application of a source-mounted primary ion gun similar to that reported by Rudat [9], but with a defined focussed beam of small size are reported here. The advantages of this source over saddle field-type sources used in many liquid SIMS applications are its small size, requiring no expensive modification of the mass spectrometer, and ,the 0168-1176/84/$03.00
0 1984 Elsevier Science Publishers B.V.
72
well-defined ion beam which irradiates only that portion of the sample where secondary ions have a reasonable chance to be accepted by the mass spectrometer. EXPERIMENTAL
All experiments were performed on a double focussing mass spectrometer (ZAB 2F, VG Analytical Ltd., Altrincham, Ct. Britain) equipped with a field desorption ion source. The experimental set up is shown in Fig. 1. For SIMS, the cathode slit and its ceramic support were removed. The ion source block was replaced by four supporting stainless steel rods, 1, holding the extraction and focussing plates of the ion source, thus providing a better vacuum inside
Fig. 1. SIMS source with focussing primary ion source. 1, Extrkztion and focussing plates of the secondary ion source; 2, sample holder; 3, Capillaritron primary ion source; 4, isolating ceramic support; 5, extracting and focussing elements of the primary ion source.
73
the source. The field desorption probe was used to support the sample holder, 2. The source was operated by the electron impact ion source electronic circuits with the repeller supply connected to the probe. The sample holder was a polished 2 mm stainless steel rod cut approximately 60 o to the axis of the mass spectrometer. The primary ion source assembly, 3, was mounted directly on top of the ion source, parallel to the entrance slit. The angle of 90 O between the primary ion source and the optical axis of the mass spectrometer was chosen because of the simplicity of construction. The primary ion source consists of a Capillaritron ion source [lo] and a two-element focussing lens system. The Capillaritron is a 25 pm fine bore tungsten capillary with a concentric counter electrode. Ions are formed by passing gas through the capillary and applying a voltage difference of several kV between the capillary and the counter electrode, causing a gas discharge. A 10 cm glass capillary in the gas supply line prevented arcs from the Capillaritron to the system ground through the gas line. Xenon was supplied as the gas and its flow was adjusted to produce a discharge current of 0.5-1.5 PA at a potential difference of 7 kV between the capillary and the counter electrode. The Capillaritron was mounted on the ion source via an isolating ceramic block, 4. The counter electrode and the focussing electrodes were made of stainless steel and were mounted on ceramic washers inside the source, 5. The distance between the Capillaritron and the counter electrode was 7 mm, between the counter electrode and the focussing electrodes 2.25 mm each, and between the last focussing electrode and the optical axis of the mass spectrometer 2.7 mm. The counter electrode and the focussing electrodes had a diameter of 1.5 mm and 5 mm, respectively. The whole primary source assembly, including gas supply line and electrical connections, was shorter than 40 mm, measured from the optical axis of the mass spectrometer. The primary ion source used the following electric supplies. The Capillaritron was connected to a 15 kV high voltage power supply (2707R, Brandenburg Ltd., Thornton Heath, Gt. Britain) via a 40 Ma resistor chain; the counter electrode and the focussing electrode close to the sample were connected to the source potential of ,the mass spectrometer; the the second focussing electrode was operated with a 10 kV high voltage power supply (B410 J. Fluke Mfg. Co., Mountlake Terrace, WA). The peptides methionin-enkephalin (Sigma, St. Louis, MO), bombesin (Bachem Torrance, CA) and P-endorphin (gift of C.H. Li, University of California, San Francisco, CA), and the nucleotide guanosine monophosphate (Sigma, St. Louis, MO) were chosen as test compounds. They were dissolved in methanol or water and applied to 1 ~1 of glycerol with 0.5% oxalic acid on the sample holder. A solution of cesium iodide in water was applied to the probe to produce Cs(CsI)n+ clusters, which were used for calibration.
74 RESULTS
AND
DISCUSSION
The actual shape of the primary ion beam was examined on the sample holder by placing a metal sheet covered with a tantalum pentoxide refractory film, where the sample normally resides. After 10 min of operation of the ion source, the interference color of the film changed due to sputtering of the oxide layer [ll]. The spot size was very dependent on the applied focussing voltage. With 6 kV acceleration energy of the mass spectrometer, 13 kV on the capillary and 6500 V on the focussing electrode, an‘ oval spot 0.4 X 1.0 mm on the tilted sample holder could be achieved. The spot was oriented in the direction of the entrance slit. With a discharge current of 0.5 to 1.5 PA, the current fluctuations of the gun were a few nA, probably caused by fluctuations of the gas supply. Phosphorescent materials on the sample holder did not show the actual beam shape. Secondary electrons produced on various surfaces caused an intense phosphorescence on the whole sample holder, making the observation of the ion beam shape impossible. The glycerol cluster at 369 daltons was used to measure the effect of different focussing voltages on the secondary ion signal intensity. The focussing voltage was changed from the source potential of 6000 V up to 7000 V with a discharge current of 900 nA. A sharp maximum was observed
100
6500
7000
V focus
Fig. 2. Intensity of the glycerol cluster at 369 daltons as function of the focussing voItage at the primary ion source. Source potential 6 kV; primary ion source potential 13 kV with respect to ground; discharge current 1 pA.
at 6450 V, which also represents the smallest spot size that could be made by the existing gun (Fig. 2). Without focussing elements, a similar secondary ion current could be achieved only by increasing the discharge current to higher than 5-8 PA. This indicates a significant increase of current density on the sample surface within the acceptance of the mass spectrometer by focussing the primary ion beam. According to our experience, the high discharge current in the unfocussed mode quickly destroys the Capillaritron tip. After less than 100 h of operation, the orifice of the Capillaritron tip opened up to a diameter of more than 50 pm, making a stable discharge difficult to maintain. The isolating ceramic insulator that holds the Capillaritron contaminated even faster, requiring frequent cleaning. In the focussed mode, with discharge currents below 2 PA, the wear on the tip was negligible and the whole gun required no maintenance for more than 100 h of operation, except for occasional cleaning of the isolating ceramics. The peptide methionin-enkephalin and the nucleotide guanosine monophosphate were used to test the detection sensitivity of this SIMS source by measuring the time dependence of the ion signal of the protonated molecular ion with various amounts of the sample. With 100 ng of methionin-enkephalin, the protonated molecular ion could be measured with the focussed and the unfocussed primary ion gun. The results are shown in Fig. 3.
I-8 : : I I I I I
I I I
1 \
IO
\
\
20
30
Time Lmin.1 Fig. 3. Intensity of the protonated molecular parent ion from 100 ng of methionin-enkephalin versus time with an unfocussed primary ion beam of 9 PA (- - - - - -) and a focussed ) with 7 kV acceleration voltage. primary ion beam of 1 pA (-
76
Using the unfocussed primary ion gun, the protonated molecular ion signal decreased rapidly, thus limiting the useful measuring time to a. few minutes. Using the focussed primary ,ion gun with its focussing voltage adjusted to the most intense secondary ion signal, this signal could be measured for 30 mm. During this time, the intensity of the glycerol cluster signals decreased and the signal corresponding to the protonated molecular ion increased, so that the signal-to-noise ratio was enhanced during the measurement (Fig. 4). This behavior was found with all substances tested. After approximately 30 min, the signal of the glycerol at 185 daltons and of the protonated molecular parent ion went down below the noise level, regardless of the amount of sample applied to the glycerol. The complete evaporation of the glycerol probably stopped further ion production from the sample. With the focussed ion gun, the detection limit was a factor 5-10 below that of the unfocussed gun. With less than 20 picomole (10 ng) of methionin-enkephalin, the protonated molecular ion could be detected for more than 25 mm with a signal-to-noise ratio of at least 4. Figure 5 shows the mass spectrum of 10 ng ( < 20 picomole) of this substance after 22 min of irradiation. The glycerol clusters above 400 daltons are no longer detectable and the protonated molecular ion of the sample is clearly visible above the uniform background. The detection limit for the protonated molecular ion of
20
IO Time
30
Cmin.3
Fig, 4. Intensity of the glycerol cluster at 553 daltons as a function of time with a focussed primary ion beam of I PA.
77
guanosine monophosphate was less. than 500 ng and the ‘ion current lasted for more than 20 min. The polypeptide bombesin (1618 daltons) could be detected with less than 1 lug of sample (Fig, 6). One microgram (less than 0.3 nanomole) of the polypeptide /3-endorphin was sufficient to be detected as its protonated molecular ion at 3461 daltons (3 kV acceleration potential, 15 00 mass resolution). The long desorption time was an important advantage when making measurements with compounds of molecular weights above 1500 daltons. Then the reduced acceleration potential required careful tuning of the mass spectrometer before acquiring the mass spectrum. After 10 mm of irradiation, the intensity of the ion signals of the glycerol clusters decreased, resulting in mass spectra with relatively intense sample ion signals. The reason for the prolonged desorption time and slightly higher sensitivity may be due to concentrating the primary ion beam on the region of the sample surface where the secondary ions formed are accepted by the mass analyzer. Under the given conditions, the diameter of the sample holder is greater than the diameter of the primary beam. The sample covering the whole sample holder is only sputtered by the primary beam where secondary ions can be accepted by the mass analyzer, minimizing the sample loss. The
350
400
450
500
550
600
650
700
M/Z
Fig. 5. SIMS spectrum of 17 picomole (10 ng) of methionin-enkephalin in a glycerol-oxalic acid matrix after 22 min of bombardment with a focussed primary ion beam of 1 PA and 7 kV energy.
78 IOOT
BOMBESIN
50: 7 .y - h’ *,-’ i” . .. ” ll 1 +* 650
__, _ L. .,_i,h
, I.._..
750
800
650
950
too0
to50
II00
II50
t 550
1600
1660
50-
loo900
”I.. I.
700
_, _..,
50 E
1500
I700
I f50
M/Z Fig. 6. SIMS spectrum of 1 c(g of the polypeptide bombesin in a glycerol-oxalic with a focussed primary ion beam of 1 PA and 7 keV energy.
acid matrix
sample can flow and diffuse from the sides into the small sputtering zone as long as the liquid matrix is present. This results in a prolonged ion formation time, limited just by the complete evaporation of the liquid glycerol matrix. However, with unfocussed FAB sources, the primary ion beam diameter is larger than the sample diameter. This results in the formation of secondary ions which cannot be accepted by the mass analyser but which are lost in the source. When sodium iodide was placed on the sample holder by evaporation of an aqueous solution to give a solid matrix, no difference was noted between a focussed and unfocussed primary ion beam. Thus, under conditions in which sample transport is limited, the replacement of sputtered material from the extremes of the sample holder cannot occur and no advantage was realized by focussing the primary ion beam. CONCLUSIONS
Using a simple focussed primary ion beam in liquid SIMS, the detection limit for various substances can be reduced to the picomole range and the
79
desorption time for molecular parent ions can be prolonged significantly. These results indicate that low detection- limits in liquid SIMS can be achieved by closely matching the secondary ion emittance and the acceptance of the mass analyzer, thus limiting the loss of .secondary ions in the ion source to a minimum. A further increase of sensitivity can be achieved by using higher mass primary particles such as heavy metals. Experiments in this direction are planned_ ACKNOWLEDGEMENT
We thank B.L. Bentz for the suggestion pentoxide refractory film.
and supply of the tantalum
REFERENCES 1 M. Barber, R.S. Bordoli, R.D. Sedgwick and A.N. Tyler, Nature (London), 293 (1981) 270. 2 M. Barber, R.S. Bordoli, R.D. Sedgwick and A.N. Tyler, J. Chem. Commun., (1981) 325. 3 D.F. Hunt and W.M. Bone, 29th Annu. Conf. Mass Spectrom. Allied Topics, Minneapolis, 1981, Am. Sot. Mass Spectrom., 1981, p. 357. 4 B.T. Chait and K.G. Standing, Int. J. Mass Spectrom. Ion Phys., 41 (1981) 185. 5 R. Stall, U. Schade, F.W. Roellgen, U. Giessman and D. Barofsky, Int. J. Mass Spectrom. Ion Phys., 43 (1982) 227. 6 D.F. Barofsky, U. Giessman, A.E. Bell and L.W. Swanson, Anal. Chem., 55 (1983) 1318. 7 A. Benninghoven, Surf. Sci., 28 (1971) 541. 8 W. Aberth, K.M. Straub and AL. Burlingame, Anal. Chem., 54 (1982) 2029. 9 M.A. Rudat, Anal. Chem., 54 (1982) 1917. 10 J.F. Mahoney, J. Perel and A.T. Forrester, Appl. Phys. Lett., 38 (1981) 320. 11 B.L. Bentz and P.G. Gale, Anal. Chem., 55 (1983) 1434.