Forensic Mid-semester Short Answer Practice Questions Answers

8/17/2019 Forensic Mid-semester Short Answer Practice Questions Answers

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CHEM 2404 Forensic and Environmental Chemistry

After you have entered an answer for a question, click !ave Answer"#After you have answered all questions that you can, click !u$mit %ui&"

'()E* +ou can try this assessment as many times as you wish rior to the date of thecut-off# (nly your most recent attemt will $e marked#

.n the followin/ choose the one otion which $est answers the question#

# )he oeration of an ioni&ation detector can $est $e descri$ed as*

A# )he measurement of the li/ht emitted when ions or e1cited atoms ormolecules return to their /round state after e1citation $y incidentradiation in a crystal#

An ioni&ation detector measures current, not li/ht#

# .ncident radiation causes /as molecules at low ressure to mi/rateunder the influence of an electric field#

)he molecules must $e ionised to mi/rate under the influence of anelectric field in an ioni&ation detector 3mi/ration of diolar moleculesunder the influence of an electric field is somethin/ comletely unrelatedto ioni&ation detectors can $e imortant in electrohoresis5#

C# Measurement of the current which results when ion airs, /enerated ina /as $y incident radiation, mi/rate to char/ed electrodes#

Correct

6# A current is /enerated when 7i8 ions and electrons doed in anultraure silicon or /ermanium crystal, are searated $y the interactionwith a γ -ray and move throu/h the donor and accetor levels of the

crystal#

'either lithium nor lithium ions are involved in an ioni&ation detector they are found in lithium-drifted detectors 3in which they are in silicon- or /ermanium- semiconductors5

2# 9hich alication of 'eutron Activation Analysis would $e most likely toena$le an analyst to determine the relative amounts of As2(: and As2(; in a

samle<



A# .nstrumental 'eutron Activation Analysis## Chemical 'eutron Activation Analysis#C# =adiochemical 'eutron Activation Analysis#

.nstrumental neutron activation analysis rovides no seciation

information, so is inaroriate# !ince the question doesn>t state thenucleide to which As is converted, it is not ossi$le to decide whether thatnucleide has more than one o1idation state, nor, if it does, how thoseo1idation states are to $e searated, so there>s insufficient information tochoose =adiochemical 'eutron Activation Analysis# However, searation

rocesses for arsenic o1idation states are well known 3or the e1istence oftwo o1idation states would not have $een esta$lished5, so, on theinformation /iven, Chemical 'eutron Activation Analysis is the onlysuita$le answer#

:# 9hich statement $est descri$es 'eutron Activation Analysis<

A# A neutron collides with an atom roducin/ a γ -ray and the chan/e inneutron flu1 $efore and after the collision is measured#

.ncorrect the ener/y of the delayed ?-ray is measured#

# A neutron collides with an atom roducin/ a γ -ray which is measured#

.ncorrect this would descri$e @romt ?-ray 'eutron Activation Analysis#

'eutron Activation Analysis normally measures the delayed ?-ray from thedau/hter nucleus, formed $y decay of the comound nucleus, itself formed

$y the interaction of the neutrons with the tar/et nucleus#

C# A neutron collides with an atom roducin/ an e1cited nucleus which

decays to a dau/hter nucleus, which, in turn, decays emittin/ a γ -ray,the ener/y of which is measured#

Correct#

6# A neutron collides with an atom roducin/ an electron which ismeasured#

?-rays, not electrons, are measured 3electrons are measured in Au/ersectroscoy, $ut this is an entirely different technique5#

4# )he 7ithium-drifted ermanium Be37i5 detector is maintained at lowtemerature $ecause*

A# etter resolution is o$tained at low temerature#



@ossi$ly true, $ut not the reason, since there>s no su//estion it couldn’t $eused at, e#/#, room temerature5

## Hi/her sensitivity is o$tained at lower temerature#

Also ossi$ly true, $ut, similarly not the reason#

C# @hotons ass further throu/h a crystal if the atoms of the crystal aremovin/ slowly#

'ot true and in any case, deends on the ener/y of the hotons,unsecified in this answer# )hese devices measure secific hotons#

6# !o as to sto recom$ination of 7i8 ions and electrons and loss oflithium#

Correct#

E# @hoto/enerated 7i8 ions will move faster to an electrode if the motionsof the atoms of the crystal host are slowed#

A/ain, even if true, there is nothin/ to su//est here that the measurementcouldn’t $e made at, e#/#, room temerature#

;# A standard containin/ ;#D µ/ of antimony 3!$5 and a $ullet samle wei/hin/

:2; m/ were irradiated simultaneously in a nuclear reactor# )he standard andsamle were removed from the irradiation source# )he standard was counted

4#:0 minutes after the end of the irradiation and the 24!$ countin/ rate was

determined to $e #: 1 0: cm# )hen the samle was counted ;#; minutes

after the end of the irradiation and had a 24!$ countin/ rate of 2#: 1 0: cm# 9hat is the concentration 3in arts er million5 of !$ in the samle< For 24!$, the half life is #;4 minutes#

R

R

W e

W e

std

sam

std - T std

sam- T

sam

=( )

( )

λ

λ

where*

R countin/ rates of standard 3 Rstd5 and samle 3 Rsam5

W mass of the element in the standard3W std5 and in the samle3W std5

) elased time since irradiation

λ 0#GD:τ2



here*λ 0#GD:τ2 0#GD:#;4 0#4;

6-5-

5-6-

sam

0.45x5.15-

std

4.300.45x-6-

3

3

sam

T-

sam

std

T-

std

sam

std

10x6.79194319.3

10x2.3309

10x2.3309

(0.0985)

(0.1444)10x15.9 .43193

)()(10x15.9 =

10x2.13107.31x

)(

)( =

x

x x

e x

e

eW

eW

R

R

λ

λ

i#e#, in the ::: 1 0-: / $ullet samle, there were G#DD 1 0-G / !$,

i#e#, I!$J 2#0:DG 1 0

-;

// 20#:DG 20#4m

G# Fluorescence techniques are of value in forensic chemistry rimarily $ecause*

A# )here are no interferences from other molecules#

)here is nothin/ in the theory of fluorescence to su//est other moleculeswill nor fluoresce in the resence of a tar/et molecule, so this 3desira$le asit may $e5 is incorrect#

# )he wavelen/ths of the incident and o$served radiation are different#

Correct a characteristic of fluorescence techniques is that the ener/ies ofthe incident and e1itin/ hotons are different#

C# eer>s 7aw is o$eyed under all circumstances#

.ncorrect# 'othin/ in the derivation of eer>s 7aw makes this e1cetion for fluorescence techniques#

6# )he fluorescence of rhodamine G is solvent deendent#

Althou/h this is true, it is a secific e1amle, not necessarily of relevanceto forensic chemistry in /eneral, and so, not an advanta/e secific tofluorescence techniques#

# .t is susected that a erson has $een oisoned $y thallium acetate# )halliumin the $lood serum was determined $y atomic a$sortion sectroscoy, asfollows*



)wo searate two hundred microlitre 3200 µ75 aliquots of serum 3 µ7 0-G

75 were reared# )o only ('E of them was added ten microlitres 30 µ75 of

a 0#020 M solution of )l'(:# Each of the two solutions was then diluted to

#00 m7 and assayed $y AA!# )he two solutions /ave si/nals of 4#K and

;;#D ar$itrary units#9hich of the followin/ $est corresonds to the concentration of thallium 3as

µM5 in the serum#

A flow dia/ram for the analysis is reroduced $elow*

y mole Tl+ in

200 L

y mole Tl+ in

200 L

10 L

spie

210 L o! sample

"it#

x + y mole o! Tl+ 200 L o! sample

"it#

y mole Tl+

dil$te to 1 ml

"#i%# t#en

%ontains

x + y mole Tl+

dil$te to 1mL

"#i%# t#en

%ontains

y mole Tl+

ives a readin/ of ;;#D units

ives a readin/ of 4#K units



F.=!)* calculate the num$er of moles of )l8 in the sike

)he sike is 0 µ7 of a 0#020 M solution of )l'(:#

!ince µ7 0L G 7, the sike is 0 1 0L G 7 of a 0#02 M solution of )l'(:#

.f there are 0#020 mole of )l8 in 7)here will $e x mole of )l8 in 0 1 0L G 7

And so x 30 1 0L G530#0205 2 1 0L mole of )l8#

)his 2 1 0L mole of )l8 is resonsi$le for the increase in the si/nal of thesiked samle#)he increase in the si/nal is ;;#D 4#K 4# units!o

2 1 0L

mole of )l

8

/ives 4# units

!EC('6* use this value to calculate the num$er of moles of )l8 in the samle

y mole of )l8 in the samle /ave a resonse of 4#K units2 1 0L mole of )l8 /ives 4# units

!o, y 34#K4#53 2 1 0L 5 ;#D2D 1 0L mole )l8#

)his is the num$er of mole of )l8 in m7

)he m7 was a dilution of 200 µ7, i#e#, of 200 1 0L 7

!o the ;#D2D 1 0L mole )l8 came from the 200 1 0L 7

)H.=6* calculate the I)l8J in µM.f there are ;#D2D 1 0L mole )l8 in 200 1 0L 7)here will $e z mole )l8 in 7!o, 3200 1 0L 53;#D2D 1 0L 5 2#DG4; 1 0L : mole )l8 er 7And,

I)l8J 2#DG4; 1 0L : M 2DG4#; µM#

K# )he rincile $ehind Marsh>s test for arsenic is*

A# Nolatile AsH: is formed and can $e reciitated in the resence ofcoer#

=ecall that Marsh>s test involves the conversion of the arseniccomound to As2(;, and addition of the As2(; to a susension of &inc#Acid is added to this susension, /eneratin/ hydro/en# )he hydro/enreduces the As2(; to AsH:# )he AsH: is volatile and is allowed toescae alon/ a /lass tu$e# )he /lass tu$e is assed throu/h a 3e#/#,5

unsen flame and the heat of the flame reduced the AsH: to elementalAs3 s5, which is deosited as a shiny mirror on the walls of the tu$e#



n3 s5 O n283aq5 8 2e2H83aq5 8 2e O H23 g 5

n3 s5 8 2H83aq5 O n283aq5 8 H23 g 5

As2(;3 s5 8 KH23 g 5 O 2AsH:3 g 5P 8 ;H2(

AsH:3 g 5 8 heat O As3 s5 8 :2H23 g 5P

)here is no mention of coer in this sequence of reactions# !o thisanswer is incorrect 3however, coer will interfere with the Marshtest5#

# Arsenic comounds are reciitated $y H2! as yellow oriment, whichcan $e easily detected#

)his descri$es the test that Marsh erformed at the trial of Qohn o/le#.t was the failure of the test, the acquittal of o/le, and his su$sequentconfession, which insired Marsh to develo a more relia$le test forarsenic#

C# inc reduces arsenic comounds to AsH:#

As outlined in the chemistry a$ove, &inc is resent as a reducin/ a/ent,and AsH: is /enerated, $ut &inc does not directly reduce the arseniccomounds# =ather, &inc reduces the acid to H2, which reduces the

arsenic comounds#

6# Nolatile AsH:, roduced from arsenic o1ides, can $e thermallyde/raded to elemental arsenic#

Correct#

D# )he amount of an element in a samle su$Rected to 'eutron ActivationAnalysis can $e calculated from the e1ression for first order radioactivedecay, assumin/*

A# nothin/ else

)his is correct try it if you>re of mathematical $ent# .f you don>t likemaths, this answer can $e determined $y eliminatin/ the others#

)he mathematical solution is*

For a samle of concentration 3in this case, mass, since it>s a solid5, a,decayin/ $y a first-order rocess, with a rate constant, k ,



!o,

Assume the num$er of counts from an analyte 3 R5 is roortional to themass of the analyte, i#e#,

R ∝ a,.f the roortionality constant for that element is, say, S, then,

R Sa)hus, after time ),

R Sa0e-k )

!o, if the initial mass of standard is W std, and the standard has a

countin/ rate, Rstd, after a time, ),)hen, Rstd S3W std5initial e-k )

.f the initial mass of the samle is W samle, and the standard has acountin/ rate, Rsamle, after a time, )>,)hen

Rsamle S3W samle5initial e-k )>

!o,

)his is the same as the equation in the notes, with k T 3whenevaluated at when a a02, i#e#, when t t½ U, k 0#GD:U5#

# )hat the measurement is made within three half-lives of irradiatin/ thesamle#

)here is nothin/ in the formula for determinin/ the amount of theanalyte element that imoses this requirement# .t is erhas a ractical

oint, $ecause if too many half-lives have assed, there won>t $e muchradiation to measure, $ut it>s not required in the derivation#

6# )hat the mass of the analyte element in the standard is less than that inthe unknown samle#

)here is nothin/ in the formula for determinin/ the amount of theanalyte element that imoses this requirement# )he formula has onlythe ratio of the masses, which, unless one is &ero 3which would violatea $oundary condition and so can>t aly5, must always $e finite#

E# )hat the mass of the analyte element in the standard is more than thatin the unknown samle#

)here is nothin/ in the formula for determinin/ the amount of theanalyte element that imoses this requirement# )he formula has only



the ratio of the masses, which, unless one is &ero 3which would violatea $oundary condition and so can>t aly5, must always $e finite#

A# nothin/ else

0# 'eutron Activation Analysis relies on the detection of γ -rays rather than a articles or electrons $ecause*

A# γ -rays carry no char/e, whereas α-articles and electrons are char/ed#

)rue, $ut not the essential reason ?-rays are used#

# )he ro$a$ility of a$sortion of a hoton $y a matri1 decreases with

increasin/ ener/y of the hoton#

Correct# )his is a rather technical way of sayin/ that ?-rays have more enetratin/ ower than do V-articles or electrons#

C# α-articles and electrons move too slowly for enou/h to $e counted in

a reasona$le time#

.f the V-articles and electrons can>t escae from the samle, $ecausetheir enetratin/ owers are insufficient, then they won>t $e counted,

irresective of how fast they move# ?-rays are ener/etic enou/h toescae the matri1 of the samle and so are the radiation of choice#

6# .f α-articles and electrons collide they can form He atoms, whichwon>t $e detected $y an ioni&ation cham$er#

)rue, $ut the technique would use one or the other, not $oth to/ether#

# )he technique of W-ray Fluorescence is $est descri$ed as*

A# An incident W-ray eRects an electron from an atom and the ener/y of

the electron is measured#

)he incident W-ray does eRect an electron 3from the core shell5 $utwhat is measured is the ener/y of the W-ray emitted as a consequenceof an outer shell electron movin/ to the core#

# An incident W-ray eRects an electron from an atom and the W-ray roduced when an outer-shell electron fills the electron hole" ismeasured#

Correct#



F# An incident W-ray eRects an electron from an atom and the W-ray roduced when an outer-shell electron fills the electron hole" eRectsan electron, which is measured#

)his is a descrition of Au/er sectroscoy, not W-ray Fluorescence#

6# An incident W-ray eRects an electron from an atom and measurement of the intensities of the incident and transmitted W-rays allows thecharacteristic a$sortion co-efficient of the analyte to $e determined#

)his is a com$ination of the action of the incident W-ray in W-rayFluorescence and the unrelated determination of the a$sortion co-efficient of the analyte# Moreover, the descrition is one of comarin/intensities of W-rays, not ener/ies, as in W-ray Fluorescence#

2# Atomic a$sortion sectroscoy relies on*

A# A molecule asirated into a flame a$sor$in/ li/ht at ener/iescharacteristic of its constituent atoms#

Atomic a$sortion sectroscoy is, as the name su//ests, thesectroscoy of atoms, not molecules#

# Measurin/ the emission of radiation as a samle, atomised in a flamerela1es to the /round state from a thermally romoted e1cited state#

)his is a $etter descrition of flame hotometry 3or flame emissionsectroscoy5 than of atomic a$sortion sectroscoy#

C# )he a$sortion of li/ht $y a samle atomised in a flame and in itsthermally romoted e1cited state#

Atomic a$sortion sectroscoy involves a$sortion of li/ht from theatom in its /round state, not in its e1cited state#

6# 6etectin/ the a$sortion of ener/y $y /as hase atoms in their /round

states#

Correct#

:# )he hollow cathode lam widely used in atomic a$sortion sectrometryoerates $y*

A# A ure metal cathode is heated to vaorise metal atoms in an e1citedelectronic state, and the atoms collide with /as in the lam emittin/radiation characteristic of that metal#



)he atoms are not vaorised from the cathode, $ut are suttered off $ycollisions of e1ited /as ions with the cathode#

# .nert /as ions, roduced $y collisions with hi/h ener/y electronsemitted $y a metal cathode, are accelerated to the cathode, sutterin/

from its surface metal atoms which, followin/ /as hase collisionswith electrons or ions, rela1 to their /round state, emittin/ radiationcharacteristic of that metal#

Correct#

C# Hi/h ener/y electrons are accelerated $y a hi/h volta/e $etween ananode and a metal cathode, sutterin/ from the cathode surface metalatoms, which, followin/ romotion to an e1cited electronic state $ycollisions with /as hase atoms or ions, rela1 to their /round state,

emittin/ radiation characteristic of that metal#

)he atoms do not leave the cathode as a result of the imact ofelectrons, $ut are suttered off as a result of collisions of /as ionsaccelerated towards the cathode# )his is closer to a descrition of theCoolid/e hot cathode tu$e, e1cet that electrons are /enerated $yheatin/ a tun/sten cathode and travel to the anode 3at which W-rays are

roduced5, and the tu$e is at a reasona$ly hi/h vacuum 3so there aren>t/as hase atoms nor ions5#

6# @assin/ li/ht throu/h the flame and analyte, with a choer"

eriodically $lockin/ and un$lockin/ the li/ht, such that su$traction of the si/nal from the flame from that of the flame and analyte allows theconcentration of the analyte to $e determined#

)his is a descrition of the means $y which the radiation emitted $y thelam can $e detected in the resence of the si/nal from the flame,rather than a descrition of the /eneration of radiation $y the lam#

Forensic Mid-semester Short Answer Practice Questions Answers

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