Inorganic Contaminants: Trace Metals Flashcards
True/False: organic produce is not permitted to use any synthetic pesticides or chemicals
False; list of approved synthetic substances for crop production
Meat from a seal and an arctic char (fish) are harvested from the same waters. How would the trace metal levels compare in each?
Seal would have higher levels (higher trophic level on food chain)
What are some sources of possible trace metal contamination? (3)
- pesticides
- Food processing equipment (contact materials)
- Environment
What trace metal is of particular concern in environmental contamination, especially in seafood? What causes the high levels?
Methylmercury
Bioaccumulation + biomagnification (concentration increases up the food chain)
What trace metal is of concern in organic produce? Why?
copper
approved use of copper sulfate as pesticide/algicide
How might food processing equipment cause trace metal contamination?
acidic or salty foods + high velocity -> erosion and corrosion of equipment (leech metals into food)
What properties of metals are important to consider and why? (4)
- Effect of WATER + pH on speciation -> affects chem behaviour, toxicity
- Volatility -> behaviour, possible losses
- Binding to Ligands -> speciation in food (bind with components), behaviour in body, bioavailability
- Atomic absorption/emission spectra -> affect analytical methods
Metals can be present in a variety of forms, known as ____ _____.
metal speciation
In Canada, the safe limits of metals in foods is outlined in the:
Food and Drug regulations
the toxicity of ____ depends on its charged form. Which is more toxic?
Chromium
Cr (VI) is much more toxic than Cr (III)
Is speciation relevant in terms of trace metal analysis? Does it have any implications on health?
analysis: No, do not differentiate between species
health: Some species may be less toxic than others
What differences in exposure lead to different adverse health effects? (2)
Dose (higher dose -> greater risk/severity)
Frequency (acute vs chronic will have different effects)
____ arsenic is extremely toxic, while ____ arsenic is less toxic.
inorganic; organic
The speciation of the metal will affect: (3)
environmental fate
food chemistry
toxicity
The 2 general types of ashing:
Dry
Wet
After initial preparatory steps (drying, homogenize, etc), what is the general first step in trace metal analysis
Ashing (removal of organic matrix)
What trace metal is known to be volatile?
Mercury
The organic matrix must be completely destroyed to prevent: (3)
- interference (signals/noise)
- deposition inside instrument
- complexation with metals
3 general steps for trace metal analysis:
- sample prep
- Destroy organic matter
- instrumental analysis
Dry ashing is also known as:
thermal decomposition
Should conventional wet ashing be done in an open or closed system?
Open
Closed system will cause explosion!
Wet ashing is also known as:
acid digestion/oxidation
What is the procedure for dry ashing? What agents may be added and why?
complete combustion in furnace at 450-550 over several hours
Can add oxidizing agents (Mg nitrate, Mg oxide) to speed up process
What acid is a common reagent for wet ashing, either alone or with others?
Why is it good to use? (2)
HNO3
Good oxidizer; does not impact analytical tools
Which wet ashing reagent is highly poisonous? In what case must it be used?
HF Silica digestion (sand)
When doing conventional wet ashing, what is used to provide heat? Where should the experiment take place?
Hot plate or block heater
Under fume hood (noxious fumes)
In wet ashing, the organic matrix is ____ through use of ____. In dry ashing, the organic matrix is ____ through use of ____.
wet: oxidized; strong acid
dry: combusted; heat
Why might peroxide be included with nitric acid for digestion?
H2O2 reconvert reduced nitrate species back to NO3 (re-oxidize)
No impact on analytical instrument
Why are sulfuric acid or HCl not used alone as reagents for wet ashing?
Non-oxidizing; cause interferences
What reagent can be combined with HNO3 for wet ashing but rarely used alone due to its explosive nature?
perchloric acid (HClO4) (strong oxidizing agent)
What are the advantages of microwave assisted digestion? (3)
- much faster
- contained system
- less manual labour
After the process of wet ashing, what is the appearance of the sample? what does it contain?
Clear solution solubilized cations (metals) in acid
Is the vessel for microwave ashing an open or closed system?
Can be either.
Open: open focused systems (under atmospheric pressure)
Closed: pressurized closed vessel (digestion bomb)
What is “aqua regia”
1:3 mix of HNO3 and HCl (use for wet ashing)
What is the process of microwave assisted digestion?
Sample + acid added to Teflon vessel, placed in microwave
Microwaves used to generate heat directly in solution
what parameters would give the least time for ashing?
Microwave assisted wet ashing, in a digestion bomb (closed vessel)
metals are (soluble/insoluble) in acid
highly soluble
What are the two instrumental analyses used to quantify trace metals?
atomic spectroscopy (emission, absorption, fluorescence) mass spectrometry
Describe the heating curve in microwave assisted digestion, and when the sample is safe to remove.
Heating ramp (linear) -> plateau -> cooling (logarithmic) Must wait until after cooling curve
Compare emission vs absorption
Absorption: measure amount of energy ABSORBED by particle to become “excited” - go from ground state to higher energy level
Emission: energy RELEASED by particle returning to ground state from excited state
Advantages of the “digestion bomb” method:
faster
closed system -> no volatile loss
What are the current methods of atomic spectroscopy for trace metal analysis? (4)
flame AAS
graphite furnace AAS
ICP-OES (Inductively coupled plasma - optical emission spectroscopy)
MP-AES (Microwave plasma - atomic emission spectroscopy)
What energy can be used to cause excitation of particles?
thermal or electrical
How is the sample flamed?
drawn up from sample via capillary action into chamber, with injected OXIDANT and FUEL
Name the basic parts of an atomic spectroscopy instrument, and their purposes.
Flame/Plasma -> atomize (also excitate)
Monochromator (or polychromator, for emission) -> filter out other wavelengths (tuned for that specific wavelengh to be measured)
Detector -> detect signal (connected to computer)
*lamp (for absorption/fluorescence): shine through sample, measure DECREASE IN SIGNAL (absorption) or EMISSION from absorbed light (fluorescence)
What is the method of MS used for trace metal analysis?
ICP-MS
What other method can be used to atomize the sample in AAS? How does it work?
Graphite furnace
sample contained in small graphite tube
high voltage -> high heat (1000s degrees)
sample atomized, light passed through
Describe ICP steps:
What analyses is it applicable for?
sample nebulized (fine spray) into plasma In plasma: molecules broken down into atoms - recombine in plasma, emit certain wavelengths -> emission spectroscopy - or become CHARGED IONS (lose electrons), can analyze mass -> mass spectrometry
How do you create high temperature plasma?
Microwaves
MIP
A benefit of the graphite furnace method is it has higher ___ ___ than flame atomization
atomization efficiency
List the analytical methods for trace metals in decreasing order of LOD:
Flame AA, ICP emission, GFAA, ICP-MS
The purpose of plasma in trace metal analyses is to: (3)
- atomize
- radiation source (for AES or OES)
- ionization source (MS)
What element is usually used as a plasma source and why?
Argon; inert gas
How do flame and graphite furnace AA differ in LOD?
Flame has higher LOD (Less sensitive)
True/False: ICP-MS can confidently detect levels as low as parts per trillion for some analytes
True
The two types of plasma atomizations are:
ICP, MIP
The LOD depends on: (2)
analyte
analytical method/instrument
What method can be used to make analysis of metals like As, Pb, Se, or Sn easier?
HYDRIDE GENERATION
convert to hydrides -> more volatile
What are the 3 main problems with trace metal analysis?
contamination
matrix effect
volatile metals (losses) vs non-volatile oxides (can’t atomize)
what reagents are used for hydride generation?
H, NaBH4
Where can contamination come from? (4) How can we help prevent it?
- residue on equipment/tools: wash with acid prior to use
- environment: avoid exposure to open air
- reagents: use high purity (analytical grade)
- Water: ultra pure (milli Q) water
Describe the process of cold vapor atomic absorption spectroscopy: (5)
- REACTION: mercury sample oxidized by nitric/sulfuric acid, reduced by tin chloride
- Air pump -> stream of inert gas carries sample through system
- water vapor + mercury travel into drying tube (agent removes water)
- mercury vapor travel to QUARTZ chamber
- measure absorbance of gas
How is ultra pure water different from distilled water? (3)
Deionized
No trace of metals
known resistivity/other properties
What property related to matrix effect should be adjusted when preparing a calibration curve? why?
pH: should have similar pH as matrix of sample (adjust % acid)
pH affect speciation and signal -> affect calibration curve (need adjustment so that curve is valid for your sample)
____ is problematic because of its high vapor pressure, making it incompatible with flame methods - so what is used instead?
mercury
cold vapor AAS
