Benchtop Inductively Coupled Plasma Mass Spectrometer

The HP 4500 is the first benchtop 1CP-MS. it has a new type of optics system that results in a very low random background and high sensitivity, making analysis down to the subnanogram-peMiter (parts-per-trillion) level feasible. It can be equipped with HP's ShieldTorch system, which reduces interference from polyatomic ions, by Yoko Kishi

Inductively coupled plasma mass spectrometry OOP-MS) is an analytical technique that performs elemental analysis with excellent sensitivity and high sample throughput, The ICP-MS instrument employs a plasma (ICP) as the ionization source and a mass spectrometer (MS) analyzer to detect the ions produced. It can simultaneously measure most elements in the periodic table and determine analyte concentration down to the subnanogram-per-Iiter or part-per-trillion (ppt) level. It etui perform qualitative, semiquantitative, and quantitative analysis and compute isotopic ratios.

The schematic diagram of an ICP-MS instrument is shown in Fig. 1. Basically, liquid samples are introduced by a peristaltic pump to the nebulizer where a sample aerosol is formed. A double-pass spray chamber ensures that a consistent aerosol is introduced to the plasma. Argon (Ar) gas is introduced through a series of concentric quartz tubes, known as the ICP torch. The torch is located in the center of an RF coil, through which 27.12-MHz RF energv is passed. The intense RF field causes collisions between the Ar atoms, generating a high-energy plasma. The sample aerosol is instantaneously decomposed in the plasma (plasma temperature1 is in the order of 6,000 to 10,00OK) to form analyte atoms, which are simultaneously ionized. The ions produced are extracted from I he plasma into I he mass spectrometer region, which is held at high vacuum (typically 10^ Ton1, 10 4 Pa), The vacuum is maintained by differential pumping.

The analyte ions are extracted through a pair of orifices, approximately I mm in diameter, known as the sampling cone and the skimmer cone. The analyte ions are then focused by a series of ion lenses into a quadrupole mass analyzer which separates the ions based on their mass/ charge ratio (m/z). The term quadrupole is used because the mass analyzer is essentially four parallel molybdenum rods to which a combination of RF and dc voltages is applied. The combination of these vollages allows the analyzer to transmit only ions ora specific mass/charge ratio. Finally, the ions are measured using an electron multiplier, and data at all masses is collected by a counter. The mass spectrum generated is extremely simple. Each elemental isotope appears at a different mass (e.g. -1A1 wot!id appear at 27 amu) with a peak intensity directly proportional to the initial concentration of that isotope. The system also provides isotopic ratio information.

New Benchtop ICP-MS

The HP 4500 is the world's first benchtop ICP-MS (see Fig. 2). The reduction in instrument size is dramatic: the size of the previous model is 1550 by 900 by 1450 mm, while t hat of the

ICR Torch r Delector

Plasma Gas I Auxiliary Gas

Spray Chamber

Carrier Gas

Nebulizer

ICR Torch r Delector

Plasma Gas I Auxiliary Gas

Spray Chamber

Carrier Gas

Nebulizer

Fig. I. HP 4500 ECP-MSschematic diagram.

Sample

Sample

Fig. I. HP 4500 ECP-MSschematic diagram.

HP 45011 is 1 101) by (500 by .".sl: mm. Previous generations of iCP-MS instruments had requirements—space, utilities, and environment—that dictated that a special room be dedicated for the instrument. Installing an ICP-MS could he particularly difficult. since major construction changes were often required,

The HP -15111 i is smaller and lighter so that it can he installed on an existing bench. The layout of the instrument is designed to make user interaction with the sample introduction system, the interfaces, and the ion lenses routine. All parts eati he accessed from the front and connected or disconnected easily. These and other new features and technology introduced and used by the HP 1500 help to make ICP-MS a more routine and therefore a more accessible technique.

Ion Lens System

The configuration of the ion lens system is one of the key design issues because il directly affects lhe ion transmission efficiency of an II Tp-MS system. Various ion lens Configurations were produced and evaluated to determine the optimum configuration and operating conditions for the HP4600. Ion I raj eel ories through each ion lens system were predicted mathematically.

Tin» IIP 1500 is equipped with a new type of Optics system, as shown in Fig. 3a. The otnega lens consists of a pair of crescent-shaped lenses that resemble the Greek letter [J.

The optics system contains two onrega lenses, the omega-t-;uid omega- lenses, which bend the ion beam, allowing the quadrupole and detector to be mounted off-axis. This prevents photons from reaching the detector (which would increase random background noise), and also focuses the ions very efficiently. The result is a very low random background anil high sensitivity, making ultratrace analysis down lo the subuunogram-per-Iiler level feasible. In contrast, other K'P-MS systems employ a photon stop lens system as shown in Fig. 3b,1 Ions are defocused after extraction into the main vacuum chamber and then refocused, while photons are blocked by ibe photon slop With this design, some ions inevitably collide with the photon stop and are lost, so overall (ransmission is reduced.

An example of ion trajectory mapping for the opiics system of Fig. 3a is shown in Fig, 1. In ibis example, the initial ion energy was estimated at 10 eV and the space-charge effect2 was ignored. The broad trace in the center shows the ion trajectories for the tens v oltage settings shown. Stalling from the left, the lenses and their voltages are: skimmer cone (no voltage), extraction lens ! (-IfiOY I. extraction lens 2 (-70V). einzel lens I ( l(K)Yr), einZel lens 2 (SV>, ein/.el lens 3 (-KHIY), omega bias Jens (-35 V); 6niega+ lens (4V), omega- lens (-5V). quadrupole focus and plate bias lenses [-I0V). The einzel lenses are a traditional electrostatic lens system in which the voltage on the center lens is different from the voltage on the other two lenses.

From

Sampling Cone

Skimmer Cone

Extraction Lensas

Quadrupole

Detector

Einzel Lenses

Quadrupole

Detector

From

Dniegs Lenses

Sampling Skimmer

Cone Cone

P h old n Stop

Quadrupole

Detector

Quadrupole

(u) HP 45(ltl omeBa lens system, tb) PhoKM) .stop system

August tlltfï Hewlfll Packard ,[ijiirii;i] 7;t

Fitf. i. Example of inn trajectory mapping. Dual-Mode Detection System

The dynamic range of the ICP-MS system is extended from six to eight orders of magnitude in the HP 4500 by a newly developed dual-mode detection system. The electron multiplier used in the dual-mode system is a discrete dynode type operated in both pulse Count and analog modes.

The block diagram of the dual-mode system is shown in Fig. 5. When an ion enters the electron multiplier, il tills the first dynode and a shower of electrons is generated. These electrons hit the next dynode, generating more electrons. Finally. Ilie pulse generated is detected by the collector. This small signal is amplified and a measurable pulse signal is obtained. At this point, the output signal from the amplifier contains both electrical noise and the pulse signal. After the amplifier, the electrical noise is eliminated by a discriminator circuit and pulse signals higher than Ilie discriminator voltage are converted to an ideal pulse shape. This pulse is measured as one count

At very high analyte concentrations (>l mg/1 in the sample solution), detector saturation occurs, so I he dual-mode system is automatically switched to analog mode and the ion current is measured. The ion Current is converted to a frequency by a vol rage-to-frequency converter and measured as counts per second.

The dual-mode detector system extends the maximum working range of the instrument up to approximately 100 mg/l. The appropriate mode for each isotope is selected automatically by the IIP ChemStation operating software, and dualmode data is acquired simultaneously, which is another first for ICP-MS, The great benefit is that samples containing a range of anal.vtes al different concentration levels can be analyzed in a single analysis.

Witliout dual-mode operation, dilution, preconcentration, or other complicated sample preparation and steps would be involved, it is inevitable that as the process for sample preparation gets more complex, an increasing number of errors and contamination will occur. Contamination during sample preparation is always of concern when analyzing elements ai I race levels.

The Shield Torch System

Although the ICP-MS generates essentially monatomic, positively charged analyte ions, there are still several polyatomic ions such as Art), ArC, and ArH, which arise mainly from the combination of the argon gas used lo generate the plasma with oxygen, carbon, and hydrogen from the air and the samples. The main interferences are shown in Table 1.

Table I

Typical Interferences in ICP-MS

Analyte

m/z

Inte de rant

K

39

WH

Ca

40

40Ar

Ca

44

l^Oy

Cr

52

40M&C

Fe

5G

"'ArlüO

The IIP 4500 can be equipped with HP's proprietary technology called the ShieldTorch system, which reduces interference from polyatomic ions.3 The electrical model of the

JluA

Ion Deflector

Collector

Ions

Electron Multiplier

"Hir

Discriminator

Counter

Pulse

V/f Convener

Counter

Anatag

Fig, 5. Blor k diagram ct" the HP 45011 dual-merle detection system.

Matching Network

RF Coil Pctenti a I

Matching Network

RF Source

Sampling Cone

Ion Energy

Plasma Poten us I

RF Coil Pctenti a I

Torch

Sample fn

Plasma

Shield Plate

RF Coif

Sampling Cone

Plasma Poten us I

plasma and the interface region is shown in Fig. (I. When the plasma is coupled with the RF coil inductively, the plasma has only a slight dc potential. However, there is eapaeitive coupling between the plasma and the RF coil, which creates a positive plasma potential oscillating at the radio frequency of the plasma source.

Within the plasma, positive ions and electrons exist, since I he plasma temperature is high ( 6,000 to 10.000K): The numbers of positive ions and electrons arc essentially equal, so the plasma is electrically neutral. Since the sampling cone is cooled by water, the plasma temperature decreases dramatically when the plasma comes close to the cone. Positive ions and electrons do not exist any more and the neutral Aratom becomes dominant, creating a "sheath" between the interface and the plasma. Since the plasma potential is grounded to the interface and the vacuum chamber through I he sheath, it acts as a condenser and the charge buildup around the sampling cone results in the formation of a discharge inside lite first vacuum stage, commonly called the seeandary discharge. The secondary discharge ionizes molecules such as Art >, Aril, and At Ar inside the first vacuum stage, giving rise to interferences with anaiyte ions al the same nominal mass.

Fig. 6. Electrical model of the plasma and the interface region.

When the ShieldTorch system is used, a shield plate is inserted between the torch and the RF coil, eliminating the eapaeitive coupling between the plasma and the RF coil so that the plasma potential is effectively reduced to zero As a result, there is no longer a secondary discharge and polyatomic ions are not ionized behind the sampling cone. To reduce the polyatomic ions even further, the plasma temperature is reduced, since these polyatomic ions are also generated in ihe plasma itself. By lowering the plasma temperature, the ShieldTorch system reduces these interferences dramatically, resulting in improved detection limits down to or ppt levels for elements such as Fe, Pa, and K—typically three orders of magnitude better than without the ShieldTorch system. Typical spectra with and without the ShieldTorch system are shown in Fig. 7.

HP ChemStation Operating Software

The HP ChemStation operating soft w are Is easy to learn and use. All instrument parameters are controlled via the HP ChemStation, unlike traditional I CP-MS systems which were completely manual before the introduction of the HP 4500. An example screen from the HP ChemStation is shown

40 50

Mass lamu)

40 50

Mass lamu)

Fig- 7. Typical speclra of rie-loo ionized water (a) with and (b) without the ShieldTorch system.

Auhu-si tiff)? Hewtett Packard Journal 75

Fifj. 8. Example screen fireini the HP 450t) Che inStiil ion in Fig. 8—tj lis is the iiistrument control screen. A single click of I he mouse starts ihe entire system, while system status is displayed in real time.

The HP ('hemStation automates day-1o-day operation by employing a suite of autotuning routines. Autotuning automatically optimizes the sensitivity, background level, and mass resolution and performs mass calibration. In the tuning screen, the user can select the tuning actions to be performed and Ihe target values for sensitivity, oxide and doubly charged ions, and background Three masses [typically one each at low, middle, and high mass) are simultaneously adjusted using a proprietary algorithm based on the simplex method.1 Each ion lens voltage is changed to increase the signal of the element that has the weakest relative response (ratio of actual signal to target value) among the three masses until all the signals satisfy the target values. This allows less experienced operators to operate the instrument to its full potential.

Applications

The HP 1500 [CP-MS offers high-through)mi multielement analysis with ng/l (ppt) or better detection limits, very small sample volume requirements, robustness, and ease of use. Therefore, the application areas for the HP 4500 are very wide, from the semiconductor industry in which the concentration of analytes is extremely low. to the environmental, geological, and clinical fields in which high-matrix or "dirty" samples are analyzed.

Semiconductor Sample Analysis. The trend towards pattern miniaturization and ultra large-scale integration (I'LSI) in semiconductor devices requires the lowering of the level of metallic impurities present. In recognition of the need for higher-purity chemicals to meet the needs of submierometer device production, the SEMI Process Chemicals Committee has proposed several grades for each chemical.

Hydrogen peroxide, 1 LOv, is widely used to remove metallic, organic, and particulate contaminants from wafer surfaces during the semiconductor manufacturing process. The 1M G must be of exlremely high purity to avoid contamination of the wafer surface by I he cleaning solution itself. The specification for Hg03 (30-32%) in the SEMI Tier C Guidelines (the quality needed to produce K's whose critical dimensions lie in the range of 0.09 to 0.2 um or greater) stipulates that the maximum concentration of impurities should be 11)0 ng/l (ppt) for a suite of 18 metals. Table II shows the results of a{juantNative purity analysis of Hal)o (30%).

Until now. recovery data presented lo the Process Chemicals Committee by member companies has involved the use of ICP-MS followed by graphite furnace atomic absorption spectroscopy (G FAAS) for Caand Fe. The HP 4500 with the ShiehlTorch system can determine even Fe, K, and ( a at low ppt levels not normally possible by quadrupole ICP-MS because of interferences from polyatomic ions and isobars such as Ail), Aril and Ar.

Table II also shows the recovery results at the 50 ng/l (ppi) level The recoveries of all of the elements were well within SEMI Tier C Guidelines, which stipulate that recovery data must be obtained showing 75 to 12596 recoveries for all metals.

Environmental Sample Analysis. Concerns regarding safe levels of contaminants in ihe environment, particularly heavy metals, continue to grow. The requirement for analysis of more elements at ever-decreasing concentrations is exposing the limitations of currently used analytical techniques. ICP-MS is the only technique that offers the improvements in sensitivity that will be demanded in the near future. ICP-MS is approved for several environmental analytical methods including those developed by the U.S. Environmental Protection Agency (EPA).

Table II

Quantitative Results for Hydrogen Peroxide (30%)

Table II

Quantitative Results for Hydrogen Peroxide (30%)

Concentration

Detection

Recovery

Element

(ng/l)

Limit (ng/lP

1%)

B

188

4

96

Na

4-7

0.5

102

Mg

8

2

97

Al

9

3

102

K

not detected

0.02

L01

Ca

:J4

4

109

Ii

14

2

98

Cr

2

!

101

Mn

1.3

O.i

102

Fe

9

1

101)

Ni

e.5

0.6

98

Gu

2.0

0.4

102

Zn

S

1

101

As

12

0.7

115

$n

4.4

0.5

102

Sb

3.1

0,5

104

Au

7

2

100

Pb

3.4

0.3

98

Fig. fl demonstrates the qualitative spectrum of river water Standard reference materia] (SIJiS-3). A lar ge number of elements, ranging from lithium (U) at low mass to uranium (U) at high mass can be clearly observ ed, even though the total analysis time was only 100 seconds. Table III shows IIP 4500 IGP-MSquantitative results, which are in excellent agreement with the certified values. The dual-mode detection system allows the user to quantitate the analytes from a few tens ofng/1 (ppt) to the mg/I (pptn) level.

Table III

Quantitative Results for River Water

Certified

Measured

Concentration

Concentration

Element

(H9/1)

(pg/1, N = 3)

Be

0.005 ±0.001

0.0051 ± 0.0004

Na

2300 ±200

2260 ±30

Mg

1600 ±200

1450± 10

Al

31 ±3

32.3 ±0.5

K

700 ±100

700±30

Ca

6000 ±400

5720± 10

V

0.3 ±0.02

0.303 ±0.004

Cr

0.3 ±0.04

0.303 ± 0.003

Mn

3.9 ±0.3

3.70 ±0.07

Fe

100±2

98.7 ±0.5

Co

0.027 ±0.003

0.0288 ± 0.0002

Ni

0.83 ±0.08

0.769 ±0.003

Cu

1.35 ±0.07

1.39 ±0.02

Zn

1.04 ±0.09

1.01 ±0.00

is

0.72 ±0.05

0.697 ±0.007

Sr

28.1*

30.1 ±0.2

Mo

0.19 ±0.01

0.193 ±0.005

Cd

6.013 ±0.002

0.0125±0.0002

Sb

0.12±0.(H

0.127±0.001

Ba

1-3.4 i 0.6

13.3 ±0 1

Pb

0.068 ±0.007

0.060 ±0.003

U

0.045*

0.0413 ±0.0008

' Net certified. information value onlv N is the number el repetitions

' Net certified. information value onlv N is the number el repetitions

5000

2500

0 200

Mn Fe

Hb Sr

30 40

110 120 130 140

150 ISO 170 Bare Earth Eiefitems

180 130

210 Z2D

230 240 Mass tan iu]

250 260

Fig. 9. (.JualiliitJv- spectrum of river wider rd'ereni e niiiterial.

Augtisi t fNEiT [h'wk'it-Parkaril Journal 77

Clinical Sample Analysis. The determination of toxic elements such as mercury (Hg), lead (Pb) and cadmium (Cd) in humans has heen a critical issue in the field of clinical chemistry from the toxicology viewpoint. In addition, since recent biomedical research lias shown that some elements at trace levels have specific functions in the biochemistry of living organisms, lite determination of trace element concentra-lions in human beings has also become a major issue in Ihe field of nutritional study. As a result, the analysis of toxic elements and also many trace elements in biological samples is required. The analyte concentration range is large, ranging from the trace levels normally found in the body t o the high levels resulting from industrial exposure. Since medical treatment regimes for hospital patients depend on the analytical results reported, the analysis of biomedical samples is critical. Therefore, the need for fast, and reliable analytical methods and instrumentation is paramount.

Table IV shows the HP 4500 ICP-MS quantitative results Tor human hail' standard reference material (NIES No. 5) which was decomposed by a microwave sample preparation system. The concentrations of II elements analyzed were in good agreement for all the elements that had certified values (there is no certified value for As).

Table IV

Quantitative Results for Human Hair

HP ChemStation

Certified

Measured

Detection

Concentration

Concentration

Limit

Element

lpg/g)

Ifig/al

<ng/g>

Ai

240*

220 ±6

0.003

Cr

1.4 ±0.2

1.72 ±0.07

0.004

Mn

5.2 ±0.3

5.47 ±0.13

0,001

Fe

225 ±9

219 ±5

0.9

Ni

1.8 ±0.1

1.87 ±0.06

0.004

Cu

16.3 ± 1.2

16.7 ± 0.6

0.002

Zn

169 ±10

171 ±4

0.004

As

**

0.18 ±0.02

0.02

Se

1.4*

2.4 ±0.3

0.004

Cd

0.2 ±0.03

0.21 ±0.03

0.0002

Hg

4,4 ± 0.4

4.52 ±0.15

0.003

Pb

6.0*

5.98 ±0.11

0.0007

" Not certified, information value only " Not certified.

" Not certified, information value only " Not certified.

Solid Sample Analysis. Solutions and liquids are the normal sample types measured by ICP-MS. Solid samples are normally digested using mineral acids and analyzed as solutions. However, solid samples such as glass can be analyzed directly using the laser ablation system. The schematic diagram of this system is shown in Fig. 10. A sample is placed in the sample cell and ablated by the beam from a Nd:YAG laser operating at 26(5 rati. The fine aerosol generated is carried directly to the plasma by Ar carrier gas. Fig. 11 shows qualitative data for glass standard reference material (NIST 614). Group 1 and 2 elements, transition metals, rare earth elements, and actinides can be clearly seen from a two-minute

HP ChemStation

Power Supply

Sample Cell

Fig. 10. Schematic diagram of laser ablation system.

Power Supply

Sample Cell

Fig. 10. Schematic diagram of laser ablation system.

analysis, even though the concentration of most elements was at the mg/kg (ppm ) level or lower in the glass.

hi addition to the bulk analysis capability shown, ibis technique also has the capability to analyze sample features and inclusions as small as 10 pm in diameter.

Speciation Analysis. Organotin compounds have been widely-used for a variety of commercial applications. Trialkyll.in compounds have been used for antifouling paints for ships and fish traps. Dialkyltin has been used for polynterizat ion catalysts. Currently, there is growing concern about their effects on the environment. Methods to determine the species of tin (Sn) and the total amount of Sn present are required, since the toxicity of organotin compounds varies widely with t he number and types of organic groups attached to the Sn atom. The combination of IC P-MS and chromatography has the ability to perform speciation analysis with high selectivity and sensitivity. Fig. 12 shows a chromatogram of six organotin compounds obtained by the IIP 4500 ICP-MS combined with the IIP 1050 liquid chromatograph. Each organotin compottnd was separated clearly within a total run time of 20 minutes. Detection limits obtained were 24 to 51 pg as Sn.

Summary

Tire IIP 4500 ICP-MS offers high sensitivity, low background, a wide dynamic range, and t he reduction of polyatomic ions, even though its benchtop size is only one fifth the size of the previous model. It is designed for routine use, easy operation, and easy maintenance. With these features, the HP 4500 is ideal for a wide range of applications in the semiconductor industry, environmental studies, laboratory research, plant quality control, and other areas.

Acknowledgments

The author wishes to acknowledge all of i he different teams who contributed to the development of the HP 4500. Thanks to Don Potter and Toshiaki Matsuda for renewing this paper and providing many helpful suggestions.

Time ls)

Flg. 12. (UiroiTiaingr.im of six organoi ii i Compounds triiwi hyliin (TMT), rilmelhyltin (DHT). diplienylt.m (l)IT), triphnnyitin (TI'T). djbutyltin (I)liT). (.nliiit.ylt.tii (TUT),

Fig. 11. Qualitative* spei l min of gl Li.ss reference mal arial.

Keferences

1. A.L (Jray, "TheOrigins, Realizaticin, and Performance of It P-M5 Systems,'Apjilica I¡uns of Inducticrty CoupUid Plasma Muss Sper-troscopy, A.Ii. Iiatp and AL. Gray, edltore, Blackie and San Ltd., ¡1)81).

Z K.K Jan is. A.Ii Gray, and H.H. tlouk, iltindlxink of ¡»flucti imtff f 'üupltil Plus um Mass Speetrometry, Blackie Academic and Professional, 1991

:i, K. Sakata imd K. Kawahala, "Keduction of fundamental polyatomie iuns in induelively coupled plasma massspeelrometry," Spectra-rlrinürn Arta, Vol. 49H, no. Kl. 1994, pp. 1027-1038. 1. M.A. Sharaf, D.L. lllnian, and H.H. Kowalski, < 'lirmanirtrirs. John Witey & Sons, Im ., 1986.

Antust 1907 llculeu 1'fU.-kSBt-Journal 79

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