practicum 2 fr

What is the purpose of tuning a GC-MS instrument?

To calibrate the instrument for accurate mass detection; different methods are used for different analyses (STUNE, ATUNE, Quick Tune).

What chemical is used to tune the GC-MS?

PFTBA (Perfluorotributylamine)

What three ions are monitored when tuning with PFTBA?

69, 219, and 502 m/z

What do ions before 69 m/z indicate?

Possible air leaks or contamination (18 m/z = water, 28 m/z = nitrogen dimers)

Why are library databases important in GC-MS and FTIR analysis?

They allow comparison of sample spectra to known references for compound identification.

Is FTIR better for mixtures or pure compounds? Why?

Pure compounds; mixtures have overlapping signals that interfere with interpretation.

What is the purpose of the Grob mixture in GC-MS?

To evaluate GC column properties such as polarity, inertness, and reactivity with acidic/basic compounds.

Name two compounds found in the Grob mixture.

(Accept any two) 2,6-dimethylphenol, methyl decanoate, methyl stearate

Why is MS better than TCD?

MS provides mass spectra for compound identification; TCD only gives retention time.

What components make up the total flow rate in GC?

Split flow + column flow + septum purge flow

What is the purpose of the glass liner in the GC injector?

To prevent sample from contacting metal parts and to keep the injector clean.

What does SIM stand for in GC-MS?

Selected Ion Monitoring

What is the main advantage of SIM?

Greater sensitivity and specificity for detecting target ions.

What is a disadvantage of SIM?

Requires prior knowledge of the target; other ions are ignored.

What does the Total Ion Chromatogram (TIC) display?

The total signal from all ions detected over time.

Which is more sensitive, TIC or SIM?

SIM – it shows higher area counts for selected ions.

What is the ATR crystal used for in FTIR?

Allows direct measurement of solid or liquid samples without preparation.

What is source attribution in forensic FTIR?

The process of linking an unknown sample to a known source by comparing spectra.

Does ethanol follow zero or first-order kinetics? What does that mean?

Zero-order; it's eliminated at a constant rate regardless of concentration.

How was BAC quantitation performed in lab?

By creating and using a calibration curve.

What is on the y-axis of the BAC calibration curve?

Ratio of ethanol/internal standard (IS) area counts.

How do you determine the BAC of an unknown from the calibration curve?

Use the calibration equation to solve for x (BAC) from the y value (ETOH/IS ratio).

What makes TQMS highly specific?

It monitors precursor to product ion transitions using SRM/MRM techniques.

What technique was used in lab instead of LC for TQMS?

Direct infusion

What is the difference between LC and direct infusion?

LC separates sample components; direct infusion does not (no separation).

What does the vacuum pump do in MS?

Maintains vacuum pressure inside the instrument for ion transmission.

Label the following TQMS components and know their locations:

ESI needle, Q1, Q2 (with gas line), Q3

Name the five GC-MS parts you should know.

GC inlet/injector, glass liner, GC column, ion source, quadrupole