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Clinical Chemistry > Week 5 > Flashcards

Week 5 Flashcards

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1
Q

What are the electrical measuring systems we use at Michener

A

pH meter
Siemens RAPIDPoint 500 (Blood gas) analyzer
Vitros 350

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2
Q

what is polarography

A

-measuring current that flows through an electrochemical cell and the electrical potential between two electrodes as a constant voltage is applied
-the current is produced by a reduction of analyte in the sample
-amount of current is proportional to the concentration
-part of oxidation reduction reaction
-also known as amperometry

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3
Q

what is electrical potential

A

force that causes electrons to move in a conductor and is measured in volts

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4
Q

What are the advantages of polarography

A

-can detect low volume samples
-Pharmaceutical uses measuring Antibiotics, hormones, vitamins,
- rapid
-Cost effective

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5
Q

What is ion selective electrodes ISE

A

-selection for desired ion measurement
-ONLY ONE TYPE SHOULD BE DETECTED
-other ions are different because their ionic radius is different
-different types of polymeric materials as simple membranes to separate ions or a liquid membrane system for ion complexing

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6
Q

what are the advantages of ISE

A

-less expensive
-not destructive
-not contaminating
-short response time
-not affected by colored compounds or turbidity

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7
Q

What are glass electrodes made up of

A

-thin layer of alkali metal silicates acting as sieves for ions while producing chemically active binding sites in the glass
-composition of glass membranes determines what ion it is selective for

example
-H+ sensitive glass electrode (pH meter)
-Na+ sensitive glass electrode

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8
Q

What is a H+ (ph) glass electrode

A

-contains Ag wire in 0.1 HCl solution with AGCl tip
-a potential difference is created across a membrane that is measured
-pH electrode most common for H ion activity
-needs a glass membrane

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9
Q

What is a combination electrode

A

example: Galvanic cell
-when the indicator and reference electrodes are in one small probe
-Ag/AgCl internal reference covered in glass with pH sensitive tip. The Ag/AgCl are stable and dont change over time because of temperature
-Loss of electrolyte due to evaporation will not change the potential
-connected to voltmeter

-The outer glass is filled with KCl and has a little pore at the tip

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10
Q

What is Calomel HgCl

A

-mercury covered with Calomel -HgCl which will come in contact with Kcl
-As long as the Cl doesnt change the potential of the half cell is constant
-the activity of the Cl is set to a fixed value by saturating the solution with chloride salt like KCl

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11
Q

What is a pH meter

A

-gives H concentration in a solution using
Actual molar concentration of H ions
Dissociation constant of the acid (pKa)
Temperature

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12
Q

Issues with glass electrodes

A
  • buffer standard accuracy
    -junction drift must be corrected every 2 hours because of the potential at the pore
    -whenever the H concentration is low and Na is high there can be a saline error or alkaline error which gives a low pH.
    -temperature must be same during calibration and pH measuring
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13
Q

What is a solid state electrode

A

-measure of anions
-crystals on the membrane to provide ions
-silver salts common
-membrane of the Cl electrode will have AgCl and silicone
-used for Cl, Br, and I

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14
Q

What are liquid ion exchange electrodes

A

-has membrane with ion binder (carrier) dissolved in inert solvent (plasticizer) in PVC
-the carrier is called ionophore because it will selectively bind to ion of interest to produce electrical potential

Example:
K+ electrode: valinomycin (antibiotic) binds K+ in the centre of a ring of oxygen atoms
Ca2+ electrode: organo-phosphate salt in PVC

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15
Q

What is a compound gas electrode

A

-has a SECOND membrane
-the first separates what you want to test from the solution and it goes through a reaction and the second membrane measures the produce of the reaction

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16
Q

what is a gas sensing electrode for PCO2

A

Severinghaus CO2 electrode
-type of pH electrode good for pt monitoring
-CO2 will go through a Teflon membrane and alters the pH of a bicarb buffer solution and the H concentration is measured at the glass electrode
CO2 + H2O <—–> H2CO3 <—–> H+ + HCO3-

-there is a lower pH which is proportional to the CO2 in specimen - blood gas analyzer

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17
Q

what is a gas sensing electrode for PO2

A

-called a Clark electrode and is based on polographic techniques
-a platinum cathode surrounded by a tubular silver anode
-the O2 in a sample will diffuse through a gas permeable polypropylene membrane and be reduced at the platinum cathode with a 4 electron gain
O2 + 2 H2O + 4e- —–> 4OH-
Current produced proportional to partial pressure of O2 in sample (PO2)

-blood gas analyzer

18
Q

What is an enzyme electrode

A

-measures non ionic compounds like glucose and urea
-the ions generated in an enzyme reaction are proportional to the concentration of analyte

urease immobilized in a membrane over an ammonium (NH4+) sensing electrode
urease
urea ————-> NH4+ + CO2

-NH4+ produced is measured by glass electrode & is proportional to concentration of urea
-NH4 can cause falsely high K
Example: enzyme glucose oxidase is used on an electrode to measure glucose

19
Q

What are some membrane properties of electrodes

A

Selectivity
-membrane must have a high degree of selectivty for ion to be tested for example a Vanco membrane is more selective for K then Na

Lifetime
-FINITE life because of contamination, membrane parts loss- machine fatigue

Stability
-the junction potential will be changed because of protein decomposition due to blood and plasma causing varying electrical signal and loss of membrane components

Response time
-as the electrode gets older the response time increases and accuracy/precision decreases

20
Q

What are some electrode configurations

A
  1. Stationary-pH meters
  2. Flow-Through
    -membrane forms part of the wall of the “flowcell” that keeps multiple electrodes
    -used on Siemens RAPIDPoint 500 measures pH, pCO2, pO2, Na+, K+ Cl- , Ca++
  3. Dry-Slide- used on Vitros (potentiometric slide)
    measures Na+ K+ Cl-
21
Q

What is in a Stationary Electrode Configuration

A

-simple electrode system; e.g. pH meter
-electrode is lowered into the test solution

hand held ones like from sofina

22
Q

What are temperature and Protein contamination disadvantages for electrodes

A

Temperature
Nernst equation is temperature dependent
need fixed & constant temp. e.g. pre-heater

Protein contamination
-leads to response delay, changing selectivity and electrode sensitivity
-needs PM to deproteinize and condition electrodes
auto alert for Siemens RAPIDpoint

23
Q

What are standardization and interfering substances disadvantages for electrodes

A

Standardization
-a standard should be the same as the unknowns pH, ionic strength, protein content
-must be wiped after use because otherwise there will a protein residue on the membrane causing residual junction potential

Interfering substances
-can cause false reading
Heparin can falsely decrease ionized Ca

24
Q

What are electrode poisoning and ph change disadvantages for electrodes

A

Electrode poisoning
-when Electrodes are filled with incorrect filling solutions, deteriorating filling solutions
-when you use Harsh cleaning agents, disinfectants on an electrode it can cause damage to ISE membranes or denature enzymes on membranes

Specimen: pH changes
Some ions very sensitive because of competitive binding to proteins in serum e.g. ionized Ca++

25
Q

how to avoid electrode fatigue

A

maintenance

26
Q

What is the Electrolyte exclusion effect

A

-when electrolytes are excluded from the part of total plasma volume occupied by solids like lipids
-electrolytes are not distributed throughout the plasma and are trapped in the water phase
-when we do results they are usually whole plasma volume

when you have indirect measuring
-you measure an aliquot that has been prediluted by the analyzer which increases the exclusion effect
-results falsely decreased

so your fatty absorbs the electrolytes making the result low then you do indirect methods and the diluting makes the result even lower.

in normal states its ignored because we assume that water fraction is constant but in disease if your water goes down the solids increase - lipemia

measuring directly is not an issue because you measure without a dilution as they occur in your body

27
Q

how do we calibrate electrochemical intruments

A

-establish a electrode slope that you can use to calculate pt results
-standards need to have similar composition as pt samples
-the electrode can act different between liquid calibrator vs whole blood - MATRIX EFFECT

Change in millivolts is proportional to concentration

28
Q

Advantages of Instruments Using EMS…why we use them!

A

-small sample
-analyze whole blood without centrifugation
-short TAT
-very selective and sensitive - picomoles
-can measure many at a time
-precise
-easy to work
-can self calibrate
-automatic flow cell cleaning

29
Q

Colorimetric determination how is it done on Vitros 350

A

-has 4 layers
-measures the concentration or anaylte activity ly light reflectance
-light intensity is proportional (not directly) to color formation

End point: color, reflectance density = concentration and reflectance is measured at end of reading

Two point rate: two reflectance reading done and rate of change is related to concentration

Rate: many readings taken as reaction occurs
-rate of color change in slide related to enzyme activity

30
Q

Glucose determination on Vitros 350

A

-pt sample put on slide and then it goes through to under layers

B-D-glucose + O2 + H20 D-gluconic acid +H2o2
Oxidation of glucose is catalyzed by glucose oxidase to form hydrogen peroxide and gluconate

-peroxidase catalyzes oxidative coupling and produces a dye whos intensity is measured by light

2H2O2 + “dye precursors” red dye

31
Q

Potentiometric determination on Vitros 350

A

-used for electrolyte determination by measuring potential difference between sample and reference electrode
-paper bridge with two hole, two ISE and electrical contact areas
-when the specimen and ESF are put into the two holes they move toward each other and form a stable junction
-measure the difference between the two half cells which is the conversion to analyte concentration

32
Q

What are sodium slides on the Vitros

A

-multilayered slide with two ISE each with methyl monensin (ionophore for Na), reference layer and Ag/AgCl coated on a polyester support

-put one drop of pt sample and one of reference fluid and both migrate to center of bridge forming a liquid junction connecting reference and sample electrode
-there is a electrochemical response to Na creating a potential difference between the two electrodes

33
Q

what is a Dry slide for K in Vitros Potentiometric Slide

A

-consists of ion selective membrane - Valinomycin with a KCl reference layer, Ag/AgCl layer

34
Q

What the Major cations of extracellular fluid

A

Sodium
-regulated by kidneys
-low levels caused by increased excretion, diarrhea, Addisons (not enough Cort or ALD) and renal disease
-high due to dehydration, brain injury, diabetic come,

K
-electrolyte imbalance, cardiac arrhythmais, muscle weakness, renal failure
-must monitor in ketoacidosis in diabetes
-low levels occur in vomiting, diarrhea, alcoholism, folic acid deficiency
-high K due to renal failure, hemolysis, trauma, Addisons acute starvation

35
Q

What is the Rapidpoint 500

A

-POCT
-blood gas, lytes, metabolites, Hb total and derivatives
-ABG, VBG, CBPG

36
Q

What does the Rapidpoint 500 use for measuring

A

-potentiometry, amperometry and conductance to measure sample concentration
-signal porpotional to concentration
-highly selective scanners
-uses thickfilm technology and solidstate design
-need small volume
-needs no maintenance , no replacing fill solutions just replace the cartridge

37
Q

what can be measured on Rapidpoint 500

A

pH- Expresses H+ activity; H+ activity reflects the acid-base balance within blood
Determines acid-base disorders caused by several pathologic conditions (renal, gastro inadequacy)

pCO2 -CO2 is a product of normal metabolism, is released into the blood where it is transported to the kidneys and lungs for excretion
Change reflect respiratory status

pO2-O2 is essential for cell and tissue metabolism in the body; transported to cells by cardiopulmonary system
Measures efficiency of pulmonary gas exchange
Evaluates degree of hypoxemia (deficiency of oxygen in arterial blood)

38
Q

what is the technology inside the rapidpoint

A

lamp light goes through lens and filter, trasmitted through fibre optic cable, mirrors will focus and direct light into sample chamber, through the cable to the polychromator, light will be diverted to photodiode, the polychromator will seperate light into wavelengths and measure at each wavelength. It will be converted to electrical reading .

Wave length calibrator has neon lamp which will emit a stable emission spectrum to test polychromator alignment

39
Q

What is Amperometry and Conductance

A

Amperometry -applying voltage to an electrode and then measuring the current generated

Conductance is the readiness with which a conducting substance transmits electrical current

40
Q

What is a Proboscis in vitros 350

A

aspirates the sample and transports them to the metering station

Clinical Chemistry (8 decks)

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