Plasma Interface Element2

Transcription

1 Plasma Interface Element2

2 Interface (backside) & Extraction Lens

3 Extraction Lens (-2000 volts)

4 ION OPTICS IF this positive potential is great enough, a discharge from the plasma into the orifice is observed called secondary discharge ( ARCing ) host of bad effects including - orifice (cone) erosion - multiply charged ions - variance in the kinetic energy of the ions

5 Ion Focusing Purpose - deliver ions to mass analyzer focuses and directs ion beam

6 ION LENS (TRANSFER LENS) STACK

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9 ION LENS (TRANSFER LENS) STACK The Transfer Lens system is used to: 1. extract the ions entering the analyzer section through the orifices of the cones with very high velocity; i.e. supersonic speed; 2. focus the divergent ion beam onto the target (Entrance Slit), 3. correct the direction of the beam to the target (Entrance Slit) 4. accelerate the ions to their full speed by applying high voltage 5. shape the ion beam into a flat shape so as to make it through the Entrance slit. The ions are subsequently travelling with the desired velocity through the focus point (Entrance Slit) and start to diverge again slightly. In order to control the ion beam during the travel path through the Magnetic Field and the Electric Field, the system controls the rotation of the beam and focuses again to the next focus point.

10 The proportion of ions sampled from the plasma that make it through to the detector is actually extremely small (1 part in 10 6 to 10 8 ), therefore, a high ion gain detection system is required if low detection limits are to be achieved

11 Ion Focusing General Assumptions 1. All ions are free particles with positive charge 2. Density of ion beam is not great enough to induce repulsion ( space charge effects) 3. Presence of ions does not change electrostatic fields 4. Vacuum conditions are adequate to give ions necessary mean free path 5. Ions originate with a constant kinetic energy

12 Ion Optics In an ideal world... All ions leaving the plasma would have the same kinetic energy regardless of m/z ratio This would result in uniform ion transmission through the lens stack

13 Ion Optics In the real world... All ions are essentially accelerated to the supersonic velocity of Ar in the supersonic jet This results in a range of kinetic energy that is a function of mass

14 ION ENERGY vs m/z MAX. ION KE (ev) m/z

15 Ion Optics What does this mean? A single set of ion lens settings will not be appropriate for all elements Therefore, must COMPROMISE (unless looking at a narrow mass range)

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17 Space charge effects The mutual repulsion of ions of like (similar) charge limits the total number of ions that can be compressed into a beam of given size

18 Space Charge Effects Plasma - ion flux is balanced by the electron flux - essentially neutral Supersonic jet - ion flux is balanced by the electron flux - essentially neutral Lens stack - electrons are repelled by negative potential - ion beam gains positive charge

19 Space Charge Effects Remember our assumptions - density of ion beam not great enough to induce space charge effects ASSUMPTION NOT MET -further reason for non-ideal behavior and different response across the mass range

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21 Question: what elements are most likely to be effected by space charge effects?

22 Space Charge Effects Space charge effect is most strongly felt by lighter mass elements The space charge force (positive-positive repulsion) acts on all ions equally

23 Space Charge Effects Recall most of the ions present are Ar = mass 40 Elements lighter than mass 40 are going to undergo space charge effects even if there is no other matrix element!

24 Question: why are the low mass elements effected by space charge effects to a greater degree?

25 Space Charge Effects Heavy elements are effected less than light elements Heavy matrices cause more problems than light matrices Best case scenario = analysis of uranium in water Worst case scenario = analysis of Li in organicrich solution

26 ESA Detector ELEMENT SCANNING HIGH RES ICP-MS DEVICE Entrance slit Quad lenses Magnet & flight tube Extraction lenses Skimmer Sampler ICP Neb & Spray chamber

27 Attom HIGH RES- ICP-MS INSTRUMENT

28 Detectors The purpose of an electron multiplier is to detect every ion of the selected mass that has passed through the energy (mass) filter of a mass spectrometer. The basic physical process that allows an electron multiplier to operate is referred to as secondary electron emission. When an ion or electron strikes a surface it can cause electrons located within the outer layers of atoms to be released. The number of secondary electrons released depends on the type of incident primary particle, its energy, and characteristic of the incident surface. In general, there are two basic types of electron multipliers commonly used in mass spectrometric analysis: these are discrete-dynode and continuousdynode electron multipliers. The Element 2 and AttoM HR-ICP-MS instruments both contain a discretedynode electron multiplier (manufactured by ETP Electron Multipliers, Australia).

29 Detectors Discrete-dynode electron multipliers amplify the secondary electron emission process by using an array of electrodes referred to as dynodes. Ions hitting the first dynode cause secondary electrons to be emitted from the surface. The optics of the dynodes focuses these secondary electrons onto the next dynode of the array, which in turn emits even more secondary electrons from its surface than the first dynode. Consequently, a cascade of electrons is produced between successive dynodes, with each dynode increasing the number of electrons in the cascade by a factor of 2 to 3. This process is allowed to continue until the cascade of electrons reaches the output electrode where the signal is extracted. A typical discrete-dynode electron multiplier has between 12 and 24 dynodes and is used with an operating gain of between 10 4 and 10 8.

30 Detectors For a new (unused) electron multiplier, the gain is achieved with a lower applied voltage (~1800 volts). With time and usage, the surfaces of the dynodes slowly become covered with contaminants from the high vacuum system, which results in a decrease of their secondary electron emission capacity (and consequently drop in gain ). Thus, the operating high voltage applied to the electron multipliers must be periodically increased in order to maintain the required multiplier gain.

31 ELECTRON MULTIPLIER V ANALOG OUT, GATE GAIN ~ V PULSE COUNTING OUTPUT GAIN ~ 10 8

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33 Detectors discrete dynode electron multiplier

34 Detector cont d Single detector with two stages- Upper ANALOG stage for high intensity signals (>5x10 6 counts per second- cps) Lower PULSE counting stage for low intensity signals (<100,000 cps) Both stages can be used in the range of 100,000 to 5x10 6 cps However, the two stages need to be calibrated against one another

35 Detector Calibration This procedure converts the analog signal to an effective count rate (cps) in order to plot intensity on the same scale from the ppb to ppm range During the calibration, the lens voltage is adjusted to attenuate the ion beam in order to obtain points in the cross calibration region Only one solution is required for this calibration procedure

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37 Detector Calibration - AttoM The detector system of the AttoM comprises up to three different stages: Pulse counting electron multiplier, attenuated pulse-counting multiplier and Faraday (not available on our instrument) The instrument continuously monitors beam intensity and performs automatic switchover between detector modes as required. The crossover between the modes should be calibrated on a regular basis, depending on the applications and the required precision.

38 Sample Introduction System Liquid Peristaltic pump or automated sampling system Nebulizer Spray chamber Plasma torch

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40 Sample Introduction System Peristaltic Pump Pump liquid sample towards nebulizer and plasma torch

41 Sample Introduction System NEBULIZER Its function is to mix the liquid sample with the nebulizer gas (Argron) to produce a fine sample aerosol for introduction to the plasma discharge area.

42 Sample Introduction System NEBULIZERS 3 main categories Pneumatic - concentric, cross flow, Babington, v-groove, Cone Spray Ultrasonic Direct insertion

43 Aerosol production Liquid sample aspirated either in free aspiration mode or with a peristaltic pump -usually made of glass or various kinds of polymers (for highly corrosive liquids/samples) Ar gas (0.6 to 1.2 L/min)

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45 Elemental Scientific Inc. MicroFlow PFA Nebulizer 100% Teflon Self-aspiration: 20 µl/min 50 µl/min 100 µl/min 400 µl/min

46 Concentric nebulizers

47 Concentric nebulizers Self-actuating Solutions are drawn up by the pressure drop generated as the nebulizer gas passes through the orifice also referred to as freerunning or self-aspiration. Thus, a peristaltic pump is not necessarily required. Advantage- Generally, the ion signal is much more stable

48 Concentric nebulizers Disadvantages cannot handle high total dissolved salts (TDS % m/v solids); i.e. 250 mg sample dissolved in 100 g of solution samples with different viscosities will have different flow rates liquid uptake tied to nebulizer gas flow cannot easily increase flow for different samples

49 Optimal Ar gas flow rate

50 Concentric nebulizers However.. Can use concentric nebulizer in conjunction with peristaltic pump - more commonly used than selfaspirating mode viscosity effects are reduced liquid uptake is metered by pump rate pump rate can be changed for each sample Disadvantages - Can occasionally lead to poorer precision due to pulsing of the flow

51 Ion signal pulsing

52 Spray Chamber Its function is to eliminate all droplets with the exception of those that are the correct size and velocity for introduction into the plasma since plasma discharge is inefficient at dissociating large droplets (>10 micron- 1x10-6 metres). The latter are eliminated by gravity and exit through a drain tube. An aerosol with a diameter of ~1-5 microns is considered to have an ideal diameter for introduction into the plasma. Its secondary purpose is to smooth out pulses that occur during the nebulization process

53 SPRAY CHAMBER & SOLVENT REMOVAL Aerosol out Coolant Drain Fig. 21. Cooled spray chambers for solvent removal. a) cooled double pass Scott chamber b) Cyclone chamber, side and top views. In both chambers, most of the large droplets are deposited at the bends, while fine droplets pass out to the plasma.

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55 Example of wet plasma introduction system -Spray chamber is cooled to 5 C in order to provide thermal stability, minimize the amount of sample entering the plasma, and reducing the quantity of oxide species. -Oxide species (M + O - vs. M + ) should be kept to below 3% of total ion signal; If not, then the plasma has not been correctly fine-tuned.

56 Desolvating Introduction System- Dry Plasma (e.g. DSN-100, Nu Instruments)

57 DSN-100 The main purpose of the heated spray chamber and PTFE membrane is to drive-off water from the sample, and thus reducing the overall size of the aerosols (hence the term dry plasma). This action results in increasing the instrument s sensitivity ; i.e. the DSN-100 introduction system yields ~10 times more ion signal compared to a meinhard nebulizer + cyclonic spray chamber (wet plasma) introduction system.

58 DSN-100 Sample aspiration occurs in free aspiration mode, typically at a rate of 50 to 100 microlitres per minute, with a Meinhard (micromist) nebulizer The nebulized sample (fine aerosols) are introduced into a heated (110ºC) spray chamber, where it is vapourized The vapourized sample is then carried into a heated (110ºC), semi-porous PTFE membrane wall and then transported away by an external gas stream (membrane gas flow)

59 Torches 2 main types: fixed and demountable fixed: 1 piece demountable: injector tube removable

60 Torches typically constructed of quartz glass injector tubes can be made of a variety of materials alumina tubes for HF solutions injector tubes can have varying diameters to accept a range of TDS solutions

61 Sample Analysis Design Solution Mode

62 Sample Analysis Design Step I Sample preparation The quality of your data will only be as good as the quality of your sample i.e. did you adequately prepare your sample in the clean lab? With respect to the destruction of matrices for samples requiring digestion Did you adequately spike samples with the correct internal standard? Sample handling protocol is extremely important, e.g. weighing

63 Sample Analysis Design Solid Samples Analyze in solid state via LA-ICP-MS? Analyze in solid state via SIMS secondary ion mass spectrometry? Convert into a glass bead and analyze via XRF x-ray fluoresence? Take powder and digest into solution with acids?

64 Sample Analysis Design- Liquid (aqueous) samples run as-is? filter then run? dilute then run? acidify, dilute, then run?

65 Sample Analysis Design Method of sample preparation also depends upon the elements of interest e.g. don t analyze your samples in a hydrofluoric acid medium if you wish to measure Si abundances why? Elemental concentration determinations at ultra-trace level (ppb, ppt) are very susceptible to contamination during sample preparation and therefore should be conducted in clean laboratory environments

66 Sample Analysis Design Clean room environment Laboratory clean room is a facility in which the concentration of airborne particles is controlled to specified limits. Eliminating sub-micron airborne contamination is a control-driven process since contaminants are generated by people, process, facilities and equipment. Hence, sub-micron particles must be continually removed from the air.

67 Sample Analysis Design Clean room environment Typical office building air contains from 500,000 to 1,000,000 particles (0.5 microns or larger) per cubic foot of air. A Class 100 cleanroom is designed to never allow more than 100 particles (0.5 microns or larger) per cubic foot of air. Class 1000 and Class 10,000 cleanrooms are designed to limit particles to 1000 and 10,000, respectively

68 Sample Analysis Design Clean room environment A human hair is about microns in diameter. A particle 200 times smaller (0.5 micron) than the human hair can cause major disaster in a clean room. Human hair typically concentrates elements/contaminants such as Pb!

69 Facilities: Sample Analysis Design - Sources of contamination Walls, floors and ceilings Paint and coatings Construction material (sheet rock, saw dust etc.) Air conditioning debris Room air and vapors Spills and leaks

70 People: Sample Analysis Design - Sources of contamination Skin flakes and oil Cosmetics and perfume Spittle Clothing debris (lint, fibers etc.) Hair

71 Sample Analysis Design - Contamination Control HEPA (High Efficiency Particulate Air Filter) - Extremely important for reducing contamination - filter particles as small as 0.3 microns with a 99.97% minimum particle-collective efficiency. Clean room design requires air flow dynamics to be the least disruptive as possible laminar flow Cleaning!

72 Sample Analysis Design - Contamination Control Purity of reagents, acids, cleanliness of digestion vessels, sample bottles, etc can dramatically effect background levels and data quality If possible, use highest purity commercial acids At the minimum - sometimes need to further process reagents e.g. acid distillation in our laboratory Digestion vessels made of disposable fluorinated polymers (teflon, PFTE, PFA, etc) Solutions stored in polypropylene or equivalent

73 Sample Analysis Design Concentration terminology Concentrations are typically expressed as either µg/g, or µg/ml (1 µg - microgram = 1 x 10-6 grams), or ppm, ppb, ppt ppm = parts per million = 1 x 10-6 g/g = 1 µg/g (µg = microgram) ppb = parts per billion = 1 x 10-9 g/g = 1 ng/g (ng = nanogram) ppt = parts per trillion = 1 x g/g = 1 pg/g (pg = picogram)

74 Sample Analysis Design Concentration terminology E.g. Zircon ZrSiO 4 ZrO 2 = 67.2 wt% SiO 2 = 32.8 wt% Atomic mass of Zr= Atomic mass of Si= Atomic mass of O= % Zr in ZrO 2 = /( ( *2)) = 74 % Si in SiO 2 = /( ( *2)) = 46.7

75 Sample Analysis Design Concentration terminology If 100% = 1,000,000 ppm, then 74% Zr (out of 67.2 wt%) = 49.73% or 497,300 ppm (or µg/g) 46.7% Si (out of 32.8 wt%) = 15.32% or 153,200 ppm (or µg/g) If you are asked to weigh out g of zircon, then the amounts of total Zr and Si you would have are: Zr = 497,300 µg/g x g = 4.97 µg Si = 153,200 µg/g x g = 1.53 µg

76 Sample Analysis Design Concentration terminology However, you are asked to prepare a solution of this zircon sample for ICP-MS analysis in solution mode; then what would be the minimum dilution factor required given that the maximal amount of ion signal intensity allowed is 50 x 10 6 cps and the yield for both elements in medium resolution mode is ~60,000 cps/ppb?

77 Sample Analysis Design Concentration terminology Maximum allowable concentration is = Max. count rate/ yield = 50 x 10 6 cps/ 60,000 cps/ppb = 833 ppb (ng/g) 4.97 µg of Zr needs to be diluted into? ml of 5% HNO µg = 4970 ng/833 ng/g = ~5.97 g (ml) of 5% HNO 3