Analytical Methods in Environmental Chemistry Questions and Answers

Answer: B

The stationary phase in reverse phase chromatography is non-polar.

In normal phase chromatography the stationary phase is polar, such as bare silica, and the mobile phase non-polar. Reverse phase inverts this: silica is bonded with hydrocarbon chains, typically C18 (octadecyl) or C8, to make a non-polar surface, and a polar mobile phase such as water mixed with methanol or acetonitrile is used. Non-polar compounds are held longer and polar ones elute first.

A polar stationary phase defines normal phase, so 'polar', 'both of these' and 'none of these' are wrong.

Answer: D

The error is 8·0%.

The relative error is the difference between the measured and true values as a percentage of the true value:

\[ \begin{aligned} \text{error} &= \frac{54 - 50}{50} \times 100 \\ &= \frac{4}{50} \times 100 = 8\% \end{aligned} \]

The other values come from slips in the arithmetic: 0·8% is the answer divided by ten, and 0·4% and 1·6% match no correct use of the 4 ppm difference against the 50 ppm standard.

Answer: B

0·1040 has four significant figures.

The rules:

• Leading zeros, before the first non-zero digit, only fix the decimal place and are not significant. The 0 before the decimal point does not count.
• Zeros between non-zero digits are significant: the 0 between 1 and 4 counts.
• Trailing zeros after the decimal point are significant: the final 0 counts.

So the significant digits are 1, 0, 4 and 0. Three would wrongly drop the trailing zero, and five or six would wrongly count the leading zero.

Answer: C

A mass spectrometer separates ions by their mass to charge ratio (m/z).

Ions are accelerated and then deflected by a magnetic or electric field, or sent through a quadrupole or a flight tube. The path or flight time of each ion depends on its mass divided by its charge, so the instrument sorts ions by m/z and the spectrum is plotted against m/z.

Mass alone, or molecular weight, cannot be the basis, because an ion of mass 100 carrying two charges behaves like an ion of mass 50 carrying one. Charge alone does not separate ions either, since most ions carry a single charge.

Answer: B

The olfactory perception threshold is the dilution at which 50% of the jury perceives the odor.

In dynamic dilution olfactometry, an odorous sample is mixed with odour-free air or water in steps, and a panel (jury) of trained assessors judges at each step whether they can smell it. The detection threshold is defined statistically as the concentration at which half of the panel detects the odour, and the dilution factor at that point gives the odour concentration (for air, in odour units per cubic metre under EN 13725).

A 25% or 75% point is not the defined threshold, and requiring all of the jury would push the threshold up to the least sensitive assessor.

Answer: C

The fuel gas in flame atomic absorption spectrometry is acetylene.

In flame AAS the sample is sprayed into a flame that atomises it. The usual flame burns acetylene with air at about 2300 °C; for elements that form refractory oxides, such as aluminium, acetylene is burnt with nitrous oxide, which gives a hotter flame of about 2700 °C.

In both cases acetylene is the fuel. Air and nitrous oxide are the oxidants that support its combustion, not fuels. Nitrogen does not burn; it is used as an inert gas, for example as a purge gas.

Answer: B

Mass spectrometers determine the relative mass of atoms (and molecules).

A mass spectrometer ionises a sample, accelerates the ions and separates them in electric or magnetic fields according to their mass-to-charge ratio. The detector records how many ions arrive at each ratio, giving the relative masses of the atoms or molecules present and their abundances, for example the isotopes of an element.

It does not measure bulk properties such as density, the acidity of a sample (a pH measurement) or the absorption of light, which is the job of a spectrophotometer.

Answer: C

A photomultiplier tube is used as the detector in the atomic absorption spectrometer.

In atomic absorption spectrometry, light from a hollow cathode lamp passes through the atomised sample, and the light that is not absorbed is measured by a photomultiplier tube, which converts weak light into an amplified electrical signal. The absorbance gives the concentration of the metal.

An ion chromatograph normally uses a conductivity detector, a gas chromatograph uses detectors such as flame ionisation or thermal conductivity, and a TOC analyser measures the carbon dioxide formed, usually with a non-dispersive infrared detector.

Answer: B

By Beer's law, transmitted intensity falls in proportion to Concentration.

The combined Beer-Lambert law says absorbance is proportional to both the path length and the concentration of the absorbing species:

\[ A = \log \frac{I_{0}}{I} = \varepsilon\, l\, c \]

Historically, Lambert's law deals with the path length (distance through the sample) and Beer's law with the concentration. That is the basis of colorimetric and spectrophotometric analysis of pollutants: absorbance is read and converted to concentration from a calibration curve.

Distance is Lambert's variable, so it is the trap here. Volume and time do not enter either law.

Answer: B

The suppressor's role is to reduce the baseline conductivity.

In ion chromatography the eluent is itself an electrolyte, such as sodium carbonate or hydroxide, so its background conductivity would swamp the small signal of the sample ions. The suppressor, placed between the column and the conductivity detector, exchanges the eluent's cations for H⁺, turning the eluent into weakly conducting carbonic acid or water while turning the analyte anions into their highly conducting acids. Background falls and the signal rises.

The suppressor does not buffer the eluent's pH, raise its concentration or deal with metal interference; it acts after the separation, purely to improve detection.